Wind Generation Investigation Project

Wind Generation Investigation Project Investigation 8

Effect of Wind Generation on Small Signal Stability March 2008

Wind Generation Investigation Project Page 2 of 83 Effect of Wind Generation on Small Signal Stability

NOTICE

COPYRIGHT © 2007 TRANSPOWER New Zealand LIMITED

ALL RIGHTS RESERVED The information contained in the report is protected by copyright vested in Transpower New Zealand Limited (“Transpower”). The report is supplied in confidence to you solely for your information. No part of the report may be reproduced or transmitted in any form by any means including, without limitation, electronic, photocopying, recording, or otherwise, without the prior written permission of Transpower. No information embodied in the report which is not already in the public domain shall be communicated in any manner whatsoever to any third party without the prior written consent of Transpower.

Any breach of the above obligations may be restrained by legal proceedings seeking remedies including injunctions, damages and costs.

LIMITATION OF LIABILITY/DISCLAIMER OF WARRANTY Transpower make no representation or warranties with respect to the accuracy or completeness of the information contained in the report. Unless it is not lawfully permitted to do so, Transpower specifically disclaims any implied warranties of merchantability or fitness for any particular purpose and shall in no event be liable for, any loss of profit or any other commercial damage, including but not limited to special, incidental, consequential or other damages.

Version Control Version Date Change 1 30 July 2007 Draft for TSG information 2 31 March 2008 Final

Sign-off List Position Prepared By: Chandana Samarasinghe and David Vowles

Senior Investigations Engineer, Transpower NZ University of Adelaide

Reviewed By: Mike Gibbard and Graeme Ancell

University of Adelaide Manager- Investigations Group, Transpower NZ

Approved By: John Clarke

Senior Manager- Planning and Investigations


TABLE OF CONTENTS 1 EXECUTIVE SUMMARY ................................................................................................................................. 5

Introduction ................................................................................................................................5 Small Signal or Oscillatory Stability ...........................................................................................6 Investigation Eight......................................................................................................................6 Wind Generation Development Scenarios.................................................................................7 Wind generation investigation project approach........................................................................7 Wind generation dispatch ..........................................................................................................8 Conceptual overview -Potential impact of wind generation on the damping performance........8 Methodology...............................................................................................................................9 Caveats ....................................................................................................................................10 Discussion and Conclusions....................................................................................................12 Findings and recommendations................................................ Error! Bookmark not defined.

2 INTRODUCTION.......................................................................................................................................... 15 2.1 The Wind Generation Investigation Project ....................................................................15 2.2 Small signal stability ........................................................................................................15 2.3 Small Signal Stability Investigations................................................................................16

3 CONCEPTUAL OVERVIEW OF THE POTENTIAL FOR WIND GENERATION TO IMPACT ON THE DAMPING PERFORMANCE OF A POWER SYSTEM ................................................................................................................................. 17

4 ASSUMPTIONS AND APPROACH .................................................................................................................. 18 4.1 New Zealand power system............................................................................................18 4.2 Scope of Work.................................................................................................................21 4.3 Configuration of the existing network and committed projects .......................................21 4.4 Generation dispatch scenarios and load assumptions ...................................................21 4.5 Wind generation scenarios..............................................................................................22 4.6 Demand Forecast............................................................................................................23 4.7 Power system operation..................................................................................................23 4.8 Planned outages .............................................................................................................23 4.9 Modelling for the Analysis ...............................................................................................24

5 METHODOLOGY......................................................................................................................................... 27 6 CAVEATS ................................................................................................................................................. 29

6.1 Scope of studies..............................................................................................................29 6.2 Dynamic models of existing plant....................................................................................29 6.3 WEC models ...................................................................................................................30

7 DAMPING PERFORMANCE CRITERIA ............................................................................................................ 31 8 DAMPING PERFORMANCE OF THE EXISTING SYSTEM..................................................................................... 32

8.1 Introduction......................................................................................................................32 8.2 Performance of the North Island power system..............................................................32 8.3 Performance of the South Island Power System ............................................................35

9 OVERVIEW OF THE EFFECT OF WIND GENERATION ON THE ELECTRO-MECHANICAL DAMPING PERFORMANCE OF THE SYSTEM.................................................................................................................................................... 38 9.1 North Island .....................................................................................................................39 9.2 South Island ....................................................................................................................44

10 EFFECT OF THE TYPE OF WEC TECHNOLOGY ON THE DAMPING PERFORMANCE OF THE SYSTEM ........................ 49 11 EFFECT OF SYSTEM LOADING ON THE DAMPING PERFORMANCE OF THE SYSTEM ............................................ 52

11.1 Damping performance comparison between the light- and high-load cases with no additional wind generation ..............................................................................................52

11.2 Damping performance comparison between the light- and high-load cases with maximum wind generation ..............................................................................................55

11.3 Effect of wind generation on the damping performance of the system under light-load conditions ........................................................................................................................58

11.4 Summary of the effect of system load on the damping performance of the system.......61 12 EFFECT OF WIND FARM VOLTAGE CONTROL ON THE DAMPING PERFORMANCE OF THE SYSTEM....................... 62 13 NORTH ISLAND: EFFECT OF STRATFORD TO HUNTLY CONGESTION ON THE DAMPING OF THE TARANAKI MODE... 69 14 SOUTH ISLAND: EFFECT OF THE OUTAGE OF THE DOUBLE CIRCUIT 220 KV TRANSMISSION LINE BETWEEN

MANAPOURI AND NORTH MAKAREWA ON THE DAMPING OF THE MANAPOURI MODE......................................... 73 15 DISCUSSION AND CONCLUSIONS................................................................................................................. 76

15.1 Findings and Recommendations.....................................................................................77 16 REFERENCES............................................................................................................................................ 79 17 APPENDIX................................................................................................................................................. 80

17.1 Standard Site Abbreviations............................................................................................80


17.2 Generation Dispatch Assumptions..................................................................................81


Executive Summary

Introduction The Electricity Commission has initiated the Wind Generation Investigation Project (WGIP) to determine what changes to the Electricity Governance Rules and Regulations (EGRs) and industry arrangements will be necessary to accommodate the connection of large scale wind generation. The ‘Implications’ phase of the project is an investigation of the impacts of wind generation on the operation of the New Zealand power system and electricity market, for a specified set of wind generation development scenarios. Transpower has been engaged by the Electricity Commission to undertake investigations into areas where the connection of large scale wind generation may affect operation of the power system and electricity market. Nine areas where the variability of wind generation output or the technical capability of wind generation may adversely impact on the operation of the New Zealand power system and electricity market were identified. Each of these areas has been investigated to determine the likely impact under the defined scenarios and whether further analysis is required for the Options stage of the Project. Figure 1 shows the nine areas of investigation.

Figure 1: WGIP investigation areas

This report documents Investigation Eight which is concerned with the effects of wind generation on small signal stability. Issues related to large scale wind generation development that are found to be significant will be advanced to the next phase of the WGIP which considers options for addressing these issues.

Investigation 1 Effect of unpredictability of wind generation output on scheduling generation

Investigation 4 Effect of wind generation capability on steady state voltage management

Investigation 7 Effect of wind generation capability on power system transient stability

Investigation 2 Effect of variability of wind generation output on dispatch of generation

Investigation 5 Effect of wind generation capability on instantaneous reserves

Investigation 8 Effect of wind generation capability on oscillatory stability

Investigation 3 Effect of variability of wind generation output on asset loading

Investigation 6 Effect of wind generation capability on voltage stability

Investigation 9 Effect of wind generation capability on dynamic voltage stability

Variability of wind generation output

Wind generation technical capability

Scheduling and dispatch

Voltage and frequency management

Power system stability


The intent at this stage of the Wind Generation Investigation Project is to identify which of the nine areas where wind generation may affect the operation of the New Zealand power system and electricity market warrants detailed investigation. The approach is to take a worst case scenario (e.g. assume highest amounts of wind generation development scenario, most limited capability of wind generation) and determine if the impacts of wind generation are material. If the impacts are not material then no further investigation is required.

Small Signal or Oscillatory Stability Small signal stability is the ability of the interconnected synchronous machines of a power system to remain in synchronism when the power system is subjected to small disturbances. These small disturbances, such as normal variations of load and generation, are always occurring. Synchronous machines respond to these variations by varying rotor angle. These rotor angle variations need to be sufficiently damped out to prevent the continuation of oscillations. With no damping, such variations can instigate power flow variations where the reduction of load in a group of machines is accompanied by an increase in load of another group of machines that are at a considerable electrical distance from the first group of machines. These variations can escalate into large magnitudes, leading to the loss of synchronism. Small-signal stability, and more broadly the damping performance, of the power system are related to the damping of the electromechanical modes of oscillation. This oscillatory behaviour is associated fundamentally with (i) the variation in the electrical torque developed by synchronous machines as their rotor angles change; and (ii) the inertia of their rotors. The frequencies associated with these modes of oscillation are typically in the range from 0.5 to 3 Hz for the New Zealand system. Local modes typically involve the rotors of one, or a small number, of closely connected machines oscillating between themselves or together against the system. Inter-area modes, which generally have lower frequencies than local modes, involve the machines in one area of the system oscillating against machines in another area of the system. Typically, the impedance of the inter-area connection is high relative to the capacities of the machines in the respective areas. Small-signal stability requires that these modes are adequately damped and their damping depends on the system operating condition. Factors which affect damping include the presence of damping windings in the rotors of synchronous machines, the characteristics of machine excitation systems and their automatic voltage regulators (AVRs), supplementary excitation system controls known as power system stabilizers (PSSs), machine loading and excitation (leading or lagging reactive power), system load, the loading of key transmission lines, and the outage of transmission lines, etc.

Investigation Eight The objective of Investigation 8 is to provide an indication of the extent to which large scale wind generation in New Zealand might be expected to impact the damping performance of the North and South Island systems. The Impact of four wind power scenarios on the damping performance of the existing North Island (NIPS) and South Island (SIPS) power-systems was assessed. The assessment involved comparing the damping performance of the existing system with that of the corresponding wind power scenarios for the intact and contingency conditions. The investigation also included an assessment of the sensitivity of the damping performance to the following factors.

• The type of Wind Energy Converter (WEC) technology, including doubly-fed induction generators (DFIG), full scale frequency converter (FSFC) generators and conventional induction generator driven by an active-stall or passive stall turbine;


• If the wind farm is equipped with a well tuned continuously-acting voltage-control system, particularly if voltage control is achieved by means of a STATCOM;

• Variation in the power output of a wind farm; and;

• Level of system loading.

This investigation is not intended to search for power-transfer limits due to small-signal instability. Rather, it is intended to develop insight into the impact of wind generation on the damping performance of the system and the main factors associated with the wind generation scenarios that affect the performance.

Wind Generation Development Scenarios The Electricity Commission has developed four possible wind generation scenarios [1] (denoted scenarios A, B, C and D) which have been used in the nine investigations analyses. Wind generation development as in Scenario C was modelled in the base case for the small signal stability studies. Scenario C has the maximum wind penetration in aggregate as well as in the most relevant regions in both the North and South Islands, and will therefore be the most extreme case for examining impacts on small signal stability. The wind generation development scenarios developed by the Commission assume the connection of new wind generation on a regional basis but do not specify where in a region the new wind farms will be located. Future wind generation is assumed to connect into key regional nodes (e.g. Bunnythorpe in the Manawatu region) as shown in Table 1. It is also assumed that transmission upgrades have been undertaken where necessary to enable the new wind generation to connect to the power system at the major node.

Wind generation investigation project approach The Wind Generation Investigation Project has identified nine areas for investigation. The potential impact in each area has been assessed through preliminary analysis. The approach taken during the preliminary analysis was to determine for a worst case1 but credible scenario whether the impact of wind generation would result in significant problems for operation of the power system or electricity market during the next 10 years. If this scenario shows no significant effects then no further analysis is required. In keeping with the worst case but credible scenario, the following assumptions were made:

• The wind generation technology deployed is assumed to have minimum capability. For example, wind generation turbines are assumed to be doubly fed induction generators. (Note that the DFIG and FSFC technologies that have power electronic interfaces that control P & Q (or power-factor) tend to perform in a very similar manner from a small-signal point of view. The small-signal damping performance of the system with IG type wind turbines tended to be marginally better than that obtained with the DFIG or FSFC technologies).

• The wind generation development scenario with the greatest impacts on the area is chosen as the basis of the assessment.

• The displacement of other generation by wind generation will result in the worst case outcome for the area under investigation.

The assumptions specific to this investigation have been made so as to be consistent with the approach for the WGIP. 1 In terms of effect upon power system operation.


Wind generation dispatch The New Zealand wholesale market design includes offer-based merit order dispatch using locational marginal pricing (nodal pricing) to arrive at the overall lowest cost secure dispatch solution. Nodal pricing includes both losses and congestion. The model co-optimises the dispatch of energy and reserves. Reserves are procured sufficient to cover the loss of the single largest generating unit. The current electricity market arrangements in New Zealand require wind generators to offer their output at a price of $0 or $0.01 per MWh,2 which effectively results in wind generation being dispatched ahead of most other forms of generation. It is assumed, for the voltage stability analysis carried out in this investigation, that wind generation having limited or no reactive capability has displaced other generation that provides voltage support. Displaced generation could provide ancillary services (e.g. hydro units capable of operating in tail water depressed mode could provide voltage support). This has not been considered in the analysis as it is by no means certain that any particular hydro unit is capable or that the owners of the plant would wish to provide the service.

Potential impact of wind generation on the damping performance Wind Energy Converters (WECs) in large-scale commercial use today are asynchronously connected to the grid. Induction generators are typically employed to convert the mechanical power extracted by the windmill rotor from the wind to electrical power. These generators are either directly connected to the grid or interfaced wholly or partially to the grid by means of power electronic converters. The latter converters control various aspects of the WEC performance, primarily maximizing energy capture when the wind speed is less than its rated value. Although some wind turbine manufacturers currently employ variable-speed synchronous generators, they are asynchronously connected to the grid by full power back-to-back DC links. Since WECs in use today are not synchronously connected to the grid they will not in themselves cause electromechanical modes of oscillation. However, the introduction of large amounts of wind generation does have the potential to indirectly change the damping performance of the system by (i) significantly altering the dispatch of synchronous generation in order to accommodate wind generation; (ii) significantly altering the power flows in the transmission network; and (iii) interacting with synchronous machines to change the damping torques induced on their shafts. The first two of these factors are largely independent of the WEC technology. The third factor depends on the dynamic performance characteristics of the turbine generator and on other relatively fast acting wind farm controls (e.g. STATCOMs which may be installed for voltage control purposes.) Due to the indirect nature of the impact of existing wind generation technology on damping performance, it is expected that wind farms employing this technology will only affect those electromechanical modes which involve oscillations over significant areas of the system. The accommodation of very high levels of wind generation will most probably require a number of changes in the structure and operation of the system for reasons unrelated to small-signal stability. It is possible that such changes will have a consequential affect on the damping-performance of the system. It is also quite conceivable that WEC controls will evolve in the future to enable higher penetration of wind generation (e.g. the

2 Rules for offering and dispatch of wind generation were developed in 2004 in order to enable New Zealand’s

first grid-connected wind generator to be dispatched.


development of WEC controls to provide an inertial response to system frequency changes). Such developments may also impact on the damping performance of the system. An emerging WEC technology is to drive a synchronous generator at synchronous speed through a variable-ratio hydrodynamic gearbox [3]. With this approach the synchronous generator is directly connected to the grid, thereby avoiding the need for power electronic interfaces. Such technology would have a direct impact on the damping performance of the system and would give rise to new electromechanical modes. Therefore, it is necessary to continually reassess the implications of wind generation on the damping performance of the system as part of the on going planning and connection assessment processes.

Methodology This investigation assesses the small-signal dynamic and damping performance of the existing North Island (NIPS) and South Island (SIPS) power systems and the impact of four wind generation development scenarios. The small signal stability of a power system can be investigated by formation of the linearized model of the non linear equations describing the interconnected power system. The powerful techniques of analysis from linear systems theory can then be applied. These techniques provide the basis for small signal-stability analysis and are based on the theory that if the linearized system is stable at the operating point at which linearization is performed; the non-linear system is then stable at that operating point. Mudpack, an interactive software package for investigating the small signal dynamic performance of multi-machine power systems, was used in the studies. Mudpack software was developed by the University of Adelaide and uses conventional analytical techniques including eigen analysis, frequency response analysis, and time domain simulation to identify and assess the electro-mechanical modes of oscillation and damping performance of the system. The approach taken is to compare the damping performance of (i) a base case scenario which has no wind generation (except for existing wind farms) with (ii) a corresponding scenario in which wind generation is introduced to the system by displacing an equivalent amount of synchronous generation. The damping performance of the pre- and post-wind scenarios is compared, using the Mudpack software package [2], on the basis of the eigenvalues of a linearized model of the system. The work undertaken for this initially involved developing the small signal stability models of the North and South Island power systems and generic Mudpack models of wind turbines. Initial analysis was conducted, without modelling wind generation scenarios, to assess the small-signal and damping performance of the existing system for a set of base-case scenarios and a set of contingency conditions. The impact of wind generation development scenarios was then assessed, which involved comparing the damping performance of the existing system with that of the corresponding wind generation scenarios for the intact and contingency cases. Studies were extended to assess the sensitivity of system damping to various factors. The sensitivities analysed are listed below.

1. Sensitivity of system damping performance to the Wind Energy Converter (WEC) technology;

2. Sensitivity of system damping performance to whether or not selected wind farms are equipped with continuously acting voltage controllers;

3. Sensitivity of system damping performance to the level of system loading; and; 4. Sensitivity of system damping performance to the variation in the power output

from a wind farm.


To assess the impact that the various WEC technologies may have on the damping performance of the system, separate studies are conducted in which all wind generation is assumed to comprise WEC with (i) [DFIG] Doubly-Fed Induction-Generators in which the rotor voltage is controlled by a back-to-back DC link employing voltage-source-converters (VSCs); (ii) [FSFC] variable-speed generators connected to the grid by a Full Scale Frequency Converter back-to-back DC link employing VSCs; or (iii) [IG] Induction-Generators directly connected to the grid with either fixed-pitch turbine blades (i.e. passive-stall) or variable-pitch turbine blades (i.e. active-stall).

The damping of the least-damped electromechanical modes obtained with each of these WEC technologies is compared. In the case of the DFIG and FSFC technologies it is assumed that the power electronic converters control both the real (P) and reactive (Q) power output to a fixed value in the time frame of ~10 s considered in rotor-angle small-signal stability analysis. This assumption is consistent with the practice preferred by the major WEC suppliers. (The slow, discontinuous voltage-control systems provided by some manufacturers behave as constant Q devices in the short-term).

Although the assumption of constant PQ control represents the preferred default position of wind farm developers, it is necessary in some cases for wind farms to provide fast, continuously acting voltage control facilities to maintain adequate short-term voltage-stability margins within an area of the system to which they are connected. Sensitivity studies indicate that such voltage control facilities are unlikely to significantly affect the damping performance of the system.

Caveats Following is a consolidated list of caveats associated with the investigation. Where appropriate, these caveats are repeated in the relevant sections.

Scope of studies In this investigation, a limited number of comparative studies were made between a base case scenario without wind generation and corresponding scenarios in which wind generation displaces an equivalent amount of synchronous generation according to a prescribed merit order. These “base-line” studies were supplemented by a small number of sensitivity studies which examined, in a restricted manner, the impact on damping per-formance of

(i) alternative generation displacement scenarios; (ii) alternative WEC technologies; and (iii) the outage of key transmission lines.

This approach was expected to identify any significant adverse impact of wind generation on the damping performance of the system. However, it is recognized that there is a risk that this approach has not identifed some situations in which high levels of wind generation will result in significant adverse effects on the damping performance of the system. For reasons outlined below, an extensive investigation has not been made into the damping performance of the existing New Zealand system. Such an investigation would not only consider a wide range of loading and generation profiles but also a wide range of contingencies. Such an analysis would be desirable to ensure that plausible system operating conditions are identified for which the damping performance of the existing system is worst and would establish a benchmark against which the various wind generation scenarios could be assessed. Such studies would also provide a solid basis on which to develop hypotheses for adverse impacts of wind generation on the system: hypotheses which could then be tested through analysis. Although the rigorous analysis


suggested above has not been conducted, the following considerations will lead to reasonable deductions of the scenarios that should be analysed (i) the experience of Transpower; (ii) the network topology; (iii) the relative locations of load and generation; and (iv) the results of transient-stability analysis conducted by Transpower. However, as explained below, there are a number of concerns about the accuracy of the dynamic models of the existing plant and their controls (particularly excitation systems) which means that there are also concerns about the accuracy of the modal estimates derived from these models. Transpower intends, in the future, to undertake work to assess the accuracy of the model estimates derived from simulations and to enhance the accuracy of plant models as required. Thus an extensive analysis of the damping performance of the system is not worthwhile to conduct until such assessments have been undertaken.

Dynamic models of existing plant Synchronous machines and their controls The dynamic models of the North Island and South Island power systems used in the small -signal stability analysis are based on asset capability information provided by asset owners. The accuracy of the analysis depends to a great extent on the quality of the models. In many cases, the dynamic models have not been validated against recent test results. In addition, information for some parts of the dynamic models is not available (e.g. the dynamic characteristics of demand) and these elements have been modelled under a certain set of assumptions. There is uncertainty about the current settings and operational status of some of the power system stabilizers installed on the system. Thus, it is unclear if the settings of some power system stabilizers are accurately reflected in either the PSS/E or DIgSILENT models. These settings are of particular importance in determining the damping of the least damped electromechanical modes. Properly tuned PSSs can have a considerable impact on the damping performance of the system. In an attempt to minimize the risk of optimistic predictions in view of this uncertainty, all PSSs are removed from service in the studies, except for the PSS fitted to the Taranaki CCGT. Based on information provided by generating companies or similar, the latter PSS is represented in the studies with “as-commissioned” models and parameters. HVDC link The HVDC interconnection between the North and South Islands is represented as a load (sending end, usually SI) or negative-load (receiving-end, usually NI). This representation does not accurately represent the dynamic performance of the HVDC link. The significance of this modelling inaccuracy in determining the electromechanical modes of the system is unclear. WEC models Generic models have been applied for each class of WEC technology which has been investigated. These generic models do not represent the details of the controls of any particular WEC manufacturer. Whilst the use of such generic models in a scoping investigation is appropriate, it will be necessary to develop and implement models which reflect the actual behaviour of the particular WECs which are ultimately deployed in practice. It is emphasized that in small-signal analysis the details of control schemes which are implemented to ensure that the WECs remain connected in the event of significant voltage depressions are irrelevant. This is because under small disturbance conditions


the latter controls are inoperative and therefore should not be incorporated in small-signal models. The characteristics of fault ride-through schemes and their performance (which is one of the major points of difference between the various WEC manufacturers) do not need to be represented in small-signal models.

Discussion Although the New Zealand Electricity Governance Rules (EGR) [4] do not prescribe a damping performance standard, Transpower have recently established an internal requirement that electromechanical modes of oscillation should have a time-constant of less than 12 seconds, which corresponds to a damping-constant with magnitude greater than -0.083 Np/s and a 2% settling time shorter than 47 s. (It is understood that this criterion is based on that employed by National Grid in the UK. This criterion is significantly less stringent than that in the Australian National Electricity Rules [5] which require that, for simulations calibrated against past performance; the time-constant of the least damped electromechanical mode of oscillation is not more than 7.2 s after the most critical credible contingency event. The North and South Island components of the New Zealand power system are interconnected by a HVDC link which means that the damping performance of each Island can be analysed separately from the other. In the North Island (NI) base case used in this investigation, apart from local modes associated with relatively small stations at Karapiro and Mangapapa, the damping-constant of all electromechanical modes is better than -0.3 Np/s, which is a factor of 3.6 better than the Transpower damping-performance requirement. The damping-constant for the above stations are also well within the Transpower requirement and because of their highly localized nature are not expected to be affected to any significant extent by the introduction of wind generation. Based on considerations of the topology of the North Island system, it is apparent that the electromechanical mode of oscillation between the generation in the Taranaki region (and particularly the combined-cycle gas-turbine (CCGT)) and the system may become comparatively lightly damped when the total output from the Taranaki region is high and the total flow from the Wellington region northward to Auckland is high. An outage of one of the two circuits between Stratford and Huntly will tend to reduce the damping of the Taranaki Mode (TM) still further. The damping of the TM is an important consideration for scenarios which introduce large wind generation south of the Bunnythorpe hub. Analysis of the South Island (SI) base case reveals that the electromechanical mode of oscillation between the 850 MW Manapouri power station near the southern tip of the South Island and the system appears to be the least damped wide-area mode in the South Island. This “Manapouri Mode” (MM) has a frequency of about 6 rad/s (1 Hz) and a damping-constant of about -0.25 Np/s which corresponds to a time-constant of 4 sec and a 2% settling time of about 15 s, which is well within the Transpower requirements. The damping of the Manapouri mode is significantly degraded in the event of the outage of the double-circuit 220 kV transmission line from Manapouri to North Makarewa. (Transpower considers the simultaneous outage of both circuits on this transmission line to be a credible contingency.) Investigation 7 assessed the effects of wind generation upon power system transient stability. While Investigation 7 considered a different area of power system stability and used a different approach, some of the results from Investigation 7 are directly relevant to the results in Investigation 8. Investigation 7 used time domain analysis techniques to determine power system transient performance following large disturbances. Investigation 8 uses frequency domain analysis to determine the electromechanical modes of oscillation present in the power system. In Investigation 7, the transient analysis involving Manapouri generating units showed an oscillatory mode with a frequency of between 5 and 6 radians per second. Similarly, stability issues relating to the damping of the Taranaki mode were also indicated in Investigation 7.


Conclusions and recommendation A summary of the findings and recommendations of this investigation is given below. The damping-performance of both the North & South Island systems in the four

base line wind generation scenarios is comparable with that in the corresponding underlying no wind base-case.

For the North Island system, a scenario is identified in which high levels of wind

generation located at or south of Bunnythorpe may result in the degradation in damping of the Taranaki mode when the power output of the Taranaki area generation is high. The degradation in damping may approach the minimum acceptable level (i.e. time-constant of 12 seconds) in the event of an outage of the 220 kV transmission circuit between Stratford and Taumarunui.

For the South Island system, varying the synchronous generation which is

displaced to accommodate the wind generation has a relatively minor effect on the damping of the dominant Manapouri Mode.The degradation in damping of between 0.08 and 0.10 Np/s, which is observed following the outage of the double-circuit 220 kV transmission line from Manapouri to North Makarewa in the South Island, is similar in both the pre- and post-wind scenarios.

Sensitivity studies are conducted to provide an indication of the impact of system

load levels on the damping performance of the system. Light- and high-load study cases analysed with (i) only the existing wind generation in service; and (ii) maxi-mum wind penetration according to Scenario C. This analysis suggests that the damping performance of the system is not materially affected either by the level of system load or the amount of wind generation. It is noted that other factors which are being considered in other investigations within the WGIP may limit the maximum amount of wind generation to lower levels than those considered in these studies.

The damping performance of the system with DFIG and FSFC type WECs are prac-

tically identical. In the case of DFIG type, the drive-train, which couples the windmill rotor and generator, is relatively flexible and due to their constant power control characteristic, a relatively lightly damped shaft torsional mode results. These modes, although electromechanical in origin, are quite distinct from the electromechanical modes associated with synchronous machines. Typically, the shaft torsional mode must be actively damped with a stabilizer which modulates the electrical power output from the WEC. This provision and setting of this stabilizer (which is quite distinct from a power system stabilizer fitted to a synchronous generator) is the responsibility of the WEC manufacturer since the mode depends only on the characteristics of the WEC. The existence of the shaft torsional mode will typically manifest itself in the system as a power oscillation at the torsional modal frequency (of about 12 rad/s or 2 Hz) and will be particularly evident following a system fault. In the event that the wind farm is connected to a very weak part of the system these power oscillations can result in voltage oscillations of significant amplitude. In such cases care must be exercised to ensure that the voltage swings do not cause maloperation of voltage sensitive controls and protection either within the wind farm or neighbouring installations.

• Compared with DFIG & FSFC type WECs, the damping of the Manapouri Mode

tends to be slightly better when the WECs have fixed pitch blades with induction generators (IG) which are directly connected to the grid. The damping improvement may be associated with the fact that the IG wind turbines operate


with a (approx.) constant speed characteristic rather than constant power characteristic. It is for this reason that the shaft-torsional mode associated with the IG wind turbines tend to be better damped and have a significantly lower frequency than for the torsional mode associated with DFIG wind turbines. It should be noted that the marginal improvement in system damping observed with IG wind turbines is likely to be outweighed by other considerations, most im-portantly the fault ride through capability of the various WEC technologies.

There are some concerns about the accuracy of the dynamic models employed in

this investigation. Of particular concern is uncertainty about the current settings and operational status of some of the power system stabilizers (PSSs) installed on the system. Well tuned PSSs can have a considerable effect on system damping.

It is recommended that after plant models have been reviewed and appropriately revised and after a comprehensive analysis of the damping performance of NZ system is made using the updated models, the tentative conclusions obtained in this report be confirmed. It is understood that very few of the synchronous generators in the New Zealand system are equipped with in-service PSSs. If, despite the indicative findings of this investigation, it is discovered in the future that the damping performance of the system is degraded to an unacceptable extent by the introduction of a wind farm then there may be considerable scope for improving the damping performance of the system by fitting well-tuned PSSs to appropriate synchronous generators. Large scale wind generation (as envisaged in the wind generation development scenarios) does not appear to significantly affect small signal stability on the New Zealand power system. It is recommended that no further study is made at this stage under the scope of the WGIP and no actions in respect of the effects of wind generation on small signal stability need to be recommended by the WGIP.


1 Introduction

1.1 The Wind Generation Investigation Project The Electricity Commission of New Zealand has initiated the Wind Generation Investigation Project (WGIP) [1] to determine what changes will need to be made to the Electricity Governance Rules and Regulations (EGRs) and industry arrangements to accommodate the integration of a large volume of wind generation into the New Zealand power system. The project has four phases:

1. Development of potential wind generation development scenarios for New Zealand.

2. Assessment of the effects of the potential scenarios on the operation of the power system and electricity market.

3. Development of options to mitigate these effects. 4. Recommendation of changes to the rules and industry arrangements to

accommodate the connection of large scale wind generation to the New Zealand power system.

The current phase of the project (phase 2) is an investigation of the impacts on the operation of the New Zealand power system and electricity market for a specific set of wind generation development scenarios determined in phase 1 of the project. Transpower has been engaged by the Electricity Commission to undertake investigations into areas where the connection of large scale wind generation may affect operation of the power system and electricity market. Investigations into nine areas are being carried out, and include issues related to both variability of wind generation output and technical capability of wind generation. Significant issues related to large scale wind generation integration identified within any of the investigations will be put forward to the next phase of the WGIP which considers options for addressing these issues.

1.2 Small signal stability The small signal stability is the ability of the interconnected synchronous machines of a power system to remain in synchronism when the power system is subjected to small disturbances. These small disturbances always occur. Good examples are variations of load and generation. These variations are defined as ‘small disturbances’ because they are sufficiently small for the linearization of system equations. Such variations result in power flow variations in the transmission system along with associated rotor angle variations. These rotor angle variations need to be sufficiently damped out to prevent the continuation of oscillations. With no damping, such variations can instigate power flow variations where the reduction of load in a group of machines is accompanied by an increase in load of another group of machines that are at a considerable electrical distance from the first group of machines. These variations can escalate into large magnitudes leading to the loss of synchronism.

1.2.1 Modes of Oscillation Of particular importance to the analysis of rotor angle small signal stability are electromagnetic modes of oscillation (EMO) which involve the rotors of individual generators or of groups of generators oscillating or swinging against each other.


Electromechanical modes of oscillation can be subdivided into local and inter-area modes based on the origin and nature. Local Modes of Oscillation Local modes involve a single machine or a small group of closely connected machines swinging against other machines or machine groups in relatively close proximity. The frequency of local modes is normally in the range 5-12 rad/s (0.8 – 1.9 Hz). It is also common to refer to intra-plant modes as a sub-category of local modes. An intra-plant mode involves the machines within a power station swinging against each other. If there are N machines within a power station there will be N-1 intra plant modes. If the machines are identical and are identically loaded then the N-1 intra-plant modes will have identical values. Inter-area Modes of Oscillation System or inter-area modes represent oscillations between larger interconnected groups of generators within one region or area of a power system swinging as a coherent group against groups of machines in other areas of the system. Typically these oscillations occur between regions that are interconnected by tie-lines whose capacity is very low compared to the generating capacity of the regions they connect. Consequently, the frequency of inter-area modes is typically in the range 1-5 rad/s (0.16- 0.8 Hz). Small-signal stability requires that the above modes are adequately damped and their damping depends on the system operating condition. Factors which affect damping include the presence of damping windings in the rotors of synchronous machines, the characteristics of machine excitation systems and their automatic voltage regulators (AVRs), supplementary excitation system controls known as power system stabilizers (PSSs), machine loading and excitation (leading or lagging reactive power), system load, the loading of key transmission lines and the outage of transmission lines, etc.

1.3 Small Signal Stability Investigations The aim of this analysis is to determine how increasing amounts of wind generation integration will affect the capability of the power system to maintain synchronism following small disturbances. The System Operator plans to operate the power system maintaining dynamic stability such that electromechanical modes of power system oscillations following a small disturbance are avoided. A reduction in damping of the power system to withstand small scale disturbance may require solutions for improving system damping to enhance small-signal stability. Small signal stability can be enhanced by using power system stabilisers tuned to damp out oscillations. This investigation is intended to develop insight into the impact of wind generation on the damping performance of the system and the main factors associated with the wind generation scenarios that affect the performance. This investigation assesses the Impact of four wind power scenarios on the damping performance of the existing North Island (NIPS) and South Island (SIPS) power-systems. The assessment involves comparing the damping performance of the existing system with that of the corresponding wind power scenarios for the intact and contingency conditions.


2 Conceptual overview of the potential for wind generation to impact on the damping performance of a power system

Wind Energy Converters in large-scale commercial use today are asynchronously con-nected to the grid. Induction generators are typically employed to convert the mechanical power extracted by the windmill rotor from the wind to electrical power. These generators are either directly connected to the grid or interfaced wholly or partially to the grid by means of power electronic converters. The latter converters control various aspects of the WEC performance, primarily maximizing energy capture when the wind speed is less than its rated value. Although some wind turbine manufacturers currently employ variable-speed synchronous generators, they are asynchronously connected to the grid by full power back-to-back DC links. Since WECs in use today are not synchronously connected to the grid they will not themselves cause electromechanical modes of oscillation. However, the introduction of large amounts of wind generation does have the potential to indirectly change the damping performance of the system by (i) significantly altering the dispatch of synchronous generation in order to accommodate wind generation; (ii) significantly altering the power flows in the transmission network; and (iii) interacting with synchronous machines to change the damping torques induced on their shafts. The first two of these factors are largely independent of the WEC technology. The third factor depends on the dynamic performance characteristics of the turbine generator and on other relatively fast acting wind farm controls (e.g. STATCOMs which may be installed for voltage control purposes.) Due to the indirect nature of the impact of existing wind generation technology on damping performance, it is expected that wind farms employing this technology will only affect those electromechanical modes which involve oscillations over significant areas of the system. The accommodation of very high levels of wind generation will most probably require a number of changes in the structure and operation of the system for reasons unrelated to small-signal stability. It is possible that such changes will have a consequential affect on the damping-performance of the system. It is also quite conceivable that WEC controls will evolve in the future to enable higher penetration of wind generation (e.g. the development of WEC controls to provide an inertial response to system frequency changes). Such developments may also impact on the damping performance of the system. An emerging WEC technology is to drive a synchronous generator at synchronous speed through a variable-ratio hydrodynamic gearbox [3]. With this approach the synchronous generator is directly connected to the grid, thereby avoiding the need for power electronic interfaces. Such technology would have a direct impact on the damping performance of the system and would give rise to new electromechanical modes. Therefore, it is necessary to continually reassess the implications of wind generation on the damping performance of the system as part of the on going planning and connection assessment processes.


3 Assumptions and Approach

3.1 New Zealand power system

The New Zealand power system consists of two island power systems (North Island and South Island) connected by an HVDC link. Figure 2 and Figure 3 show the North Island and South Island power systems respectively.

For the purposes of analysis used in this investigation, it is assumed that the present capability and configuration of the grid will apply and that committed new assets (e.g. the Huntly E3P generating station) and committed grid upgrades have been commissioned.

It is assumed that there are no major upgrades to transmission capacity, and no new generating plant is commissioned beyond what is currently committed during the 10 year period considered by the wind generation development scenarios.


Figure 2: North Island power system


Figure 3: South Island power system


3.2 Scope of Work The impact of wind generation on the small signal stability of the power system was evaluated using a software package for analysing the small signal dynamic performance of a multi-machine power system. The software used was “Mudpack”, an interactive software package developed by the University of Adelaide. Mudpack software uses analytical techniques such as eigen analysis and frequency response analysis to identify the electro-mechanical modes of oscillation and damping performance of the system. Small signal stability models of the North and South Island power systems were used to assess the small-signal and damping performance of the existing system for certain base-case scenarios and contingency conditions. Small signal stability models of the North and South Island power systems for each of the wind generation development scenarios were developed. The performance of the power system under the wind generation development scenarios was compared with the performance of the existing system. Sensitivity studies assessed how various factors will affect the system damping. These are:

• Sensitivity of system damping performance to the Wind Energy Converter (WEC) technology

• Sensitivity of system damping performance to whether or not selected wind farms are equipped with continuously acting voltage controllers.

• Sensitivity of system damping performance to the system loading. • Sensitivity of system damping performance to the variation in the power output

from a wind farm.

3.3 Configuration of the existing network and committed projects

The New Zealand power system consists of two island power systems (North Island and South Island) connected by an HVDC link. Figure 2 and Figure 3 show the North Island and South Island power systems respectively.

For the purposes of analysis used in this investigation, it is assumed that the present capability and configuration of the grid will apply. Future wind generation is assumed to connect into key regional nodes (e.g. Bunnythorpe and Redclyffe – see Table 1). The wind generation development scenarios assume the connection of new wind generation on a regional basis but do not specify where in a region the new wind farms will be located. It is assumed that all wind generation in a region will connect into a major node in the region (e.g. Bunnythorpe in the Manawatu region) and that the necessary transmission upgrades have been made to enable the new wind generation to connect to the power system.

3.4 Generation dispatch scenarios and load assumptions

The New Zealand wholesale market design includes offer-based merit order dispatch using locational marginal pricing (nodal pricing) to arrive at the overall lowest cost secure dispatch solution. Nodal pricing includes both losses and congestion. The model co-optimises the dispatch of energy and reserves. Reserves are procured sufficient to cover the loss of the single largest generating unit.

Dispatch occurs every five minutes through formal dispatch instructions sent electronically. In New Zealand there is no Automatic Governor Control (AGC). All generation offered under the trading rules in Part G of the EGRs is dispatched through the offer process in real time.


The current electricity market arrangements in New Zealand require wind generators to offer their output at a price of $0 or $0.01 per MWh. This effectively results in wind generation being dispatched ahead of most other forms of generation, such that generation plant providing reactive support is displaced by minimum capability wind generation plant.

Analysis of system performance during peak demand periods using different generation and dispatch scenarios has been carried out. Peak Island loads and power factors are used for the studies. Winter and summer load (peaks and power factors) are used for winter and summer studies respectively.

3.5 Wind generation scenarios

The Electricity Commission has developed four possible wind generation scenarios, which are inputs to the analyses [1]. Wind generation development as in Scenario C was modelled in the base case for these studies. Scenario C has the maximum wind penetration in aggregate as well as in the selected regions in both the North and South Islands and will be the most extreme case for impacts on voltage stability.

The other wind generation development scenarios are expected to show a lesser impact on voltage stability. The results of studies for Scenario C for varying levels of wind generation penetration can be extended to the other scenarios. Separate studies for Scenarios A, B and D have not been conducted for assessing the power limits due to voltage instability. Table 1 shows a summary of the wind generation development scenarios developed by the Electricity Commission3, together with the assumed grid connection nodes. Island Region Grid Connection (for modelling

purposes) Scenario A

(high penetration, concentrated in the North Island)

Scenario B

(high penetration, diversified across the country)

Scenario C

(very high penetration, diversified across the country)

Scenario D

(low penetration, diversified across the country)

Northland Marsden 220 kV 100 MW 150 MW

Auckland Otahuhu 220 kV 100 MW 300 MW 30 MW

Waikato Huntly 220 kV 100 MW 50 MW 100 MW 30 MW

Hawkes Bay Redclyffe 220 kV 300 MW 150 MW 300 MW 30 MW

Wairarapa Masterton 110 kV 50 MW

Manawatu4 Bunnythorpe 220 kV 450 MW 350 MW 450 MW 250 MW

North Island

Wellington Wilton 220 kV 300 MW 150 MW 300 MW 30 MW

TOTAL North Island MW 1150 MW 950 MW 1600 MW 370 MW

Marlborough-Nelson

Blenheim 110 kV 50 MW 50 MW

Otago/South Canterbury

Timaru 220 kV 150 MW 300 MW

South Island

Southland Invercargill 220 kV 100 MW 100 MW 300 MW 50 MW

TOTAL South Island MW 100 MW 300 MW 650 MW 50 MW

Table 1: Summary of scenarios developed by the Electricity Commission

3 These scenarios were developed before the White Hills wind farm in Otago was committed. 4 This includes the existing 250 MW of wind generation (Te Apiti, Tararua I, II and III) located near

Bunnythorpe.


3.6 Demand Forecast Table 2 shows the regional peak demand forecast used in the analysis.

Region 2005 2010 2015 2020

Auckland and North Isthmus 2000 2250 2500 2750 Waikato 499 553 616 684 Hawkes Bay 300 325 350 375 Central North Island 317 350 389 431 Wellington 608 635 712 782 Nelson Marlborough 193 219 248 279 South Canterbury 92 100 108 117 Otago-Southland 1064 1111 1163 1216 North Island 4112 4665 5313 6020 South Island 2094 2275 2478 2692 New Zealand 6206 6940 7791 8712

Table 2: Regional and Aggregate Demand Forecasts (peak MW)

The peak demands may differ from those used in transmission planning in New Zealand (for example, the Electricity Commission’s Statement of Opportunities or Transpower’s Annual Planning Report). This is of little consequence, as this investigation is not an exercise in power system planning, rather a quick assessment of how wind generation development as envisaged in Scenario C is likely to affect management of steady state voltages.

3.7 Power system operation

The shunt capacitors connected to the grid are operated to meet set grid voltage profiles. These profiles are designed to maintain pre and post contingency voltages within a voltage range of ±10% as far as possible.

3.8 Planned outages

Planned transmission outages have not been specifically considered in these studies. Such outages only go ahead under satisfactory system conditions. Planned outages do not go ahead at times of system peak loading or at times of lightest system loading (e.g. early on Christmas morning). Outages tend to go ahead at times of lower loading. It is possible that there may be issues with steady state voltage management during some outages.

Outage planning considers how power system security will be maintained during outages in particular where concurrent outages are requested. Some outages will not be able to go ahead at the same time as other outages. Operational measures will be required during some outages to maintain power system security. These measures, such as load management and constraints on the dispatch of generation, are arranged with the affected parties prior to the outage taking place. Some outages may require generation to be shut down for the duration.

Planned generation maintenance is not explicitly considered in the studies. The effects of such outages can be estimated from the effects of displacement of generation in a region.


3.9 Modelling for the Analysis

3.9.1 System modelling The dynamic response of the power system depends on the dynamic response of the elements (e.g. generating plant, SVCs, HVDC links, motor loads) that make up the power system. These devices can have sophisticated control systems (e.g. automatic voltage regulators, governors and power system stabilisers). For the purposes of analysis, these devices and their control systems are represented by a non-linear system of differential and algebraic equations. High fidelity models within the bandwidth of electromechanical modes of oscillation from about 1 to 12 radians per second are required. The Mudpack software package which is used to conduct the small-signal stability analysis linearizes the non-linear models about the operating point defined by the initiating load flow. There are often limitations on the accuracy of models. Simplifications are often required to develop a workable model. Models need to be validated where possible against tests carried out on the assets. Knowledge of some components of the power system (e.g. the composition and characteristics of loads at grid exit points) is often limited.

3.9.2 Wind generation modelling Table 1 shows the Electricity Commission’s wind generation development scenarios. Each scenario has an amount of wind generation development envisaged for each region of the country. The location and size of new wind farms in each region are not specified. A variety of WEC technologies are commercially employed around the world. Since it is uncertain which of these alternatives will be adopted on a large scale in New Zealand it is considered desirable to assess the impact of a number of the possible alternatives on the damping performance of the New Zealand system. WEC technologies considered Therefore, three WEC technologies are considered in this investigation: 1. [DFIG] doubly fed induction generator with the rotor-voltage controlled by a back-to-back DC link employing voltage source converters (VSC). The grid side VSC is capable of both supplying and absorbing reactive power. The rotor is capable of variable speed operation and the turbine blades have controllable pitch angle. 2. [FSFC] Variable speed generator connected to the grid by means of a fully rated back-to-back VSC. The grid-side VSC is capable of both supplying and absorbing reactive power. 3. [IG] Directly-connected induction-generators with either fixed-pitch turbine blades (i.e. passive-stall) or variable-pitch turbine blades (i.e. active-stall). Since under small-distur-bance conditions the control of the pitch angle in active-stall turbines is slow-acting and/or discontinuous it is reasonable to represent active-stall turbines in the same manner as passive-stall turbines.


Wind farm connection arrangements The connection arrangements for the three types of WECs which have been assumed in this investigation are shown in Figure 4, Figure 5 and Figure 6. The following assumptions are made: 1.If a wind farm comprises N WECs, each with a rated power output of Prate, then the wind farm is represented in the studies by a single composite turbine with a rated power output of N x Prate. Appropriate scaling is applied to the WEC model parameters and associated wind farm collector network. 2. The WEC step-up transformers have an impedance of 8% on their rating. 3. The WEC step-up transformers are rated on the basis that the DFIG and FSFC type wind turbines have a leading and lagging power factor of 0.95 at rated power output. In the case of the IG type wind turbines the step-up transformers are rated on the basis of the assumption that fixed capacitors are connected to the induction generator terminals to compensate for the reactive-power consumed by the machines when operating at rated output. 4. The wind-farm collector transformer has an impedance of 12% on its MVA rating. The rating of this transformer is assumed to be N times the rating of an individual WEC transformer. It is assumed that only a single collector transformer is provided in each wind farm (i.e. no redundancy). 5. The wind farm collector transformer is equipped with an on-load tap-changer (OLTC). 6. The wind-farms are assumed to operate at unity power factor at the point-of-common-coupling (PCC). The PCC is assumed to be the 110 or 220 kV bus to which the wind farm is connected. This is based on the assumption (which is borne out in practice) that wind-farm developers will not provide fast, continuously-acting voltage-control capability unless absolutely necessary. 7. In the case of wind farms utilizing DFIG and FSFC type wind turbines the unity power factor requirement is satisfied by appropriately adjusting the WEC reactive-power output and collector transformer tap-setting to ensure that the wind-farm collector-network voltage is within acceptable bounds. It is assumed that on the relatively short time-scale of rotor-angle dynamic analysis that the reactive power output of the WEC is rapidly regu-lated to the initial steady-state value. 8. In the case of wind farms utilizing IG type wind turbines, besides the fixed capacitors provided at the machine terminals, a relatively small amount of fixed capacitance is provided on the wind-farm side of the collector transformer to compensate for net reactive losses in the transformers and collector network. 9. There is a zero impedance link between the PCC and the existing substation within the Transpower network to which the wind farm is connected.


Figure 4: Network connection of a DFIG type Wind Energy Converter (WEC) – figure should be

updated to indicate indusction generator as per Fig 6

Figure 5: Network connection of a FCSG type Wind Energy Converter (WEC)

Figure 6: Network connection of a IG type Wind Energy Converter (WEC)


4 Methodology This investigation assesses the small-signal dynamic and damping performance of the existing North Island (NIPS) and South Island (SIPS) power systems and the impact of four wind generation development scenarios. The small signal stability of a power system can be investigated by formation of the linearized model of the non linear equations describing the interconnected power system. Then the powerful techniques of analysis from linear systems theory can be applied. These techniques provide the basis of small signal-stability analysis and are based on the theory that if the linearized system is stable at the operating point at which linearization is performed; the non-linear system is then stable at that operating point. Mudpack, an interactive software package for investigating the small signal dynamic performance of multi-machine power systems was used in the studies. Mudpack software was developed by the University of Adelaide and uses conventional analytical techniques including eigen analysis, frequency response analysis, and time domain simulation to identify and assess the electro-mechanical modes of oscillation and damping performance of the system. The approach taken is to compare the damping performance of (i) a base case scenario which has no wind generation (except for existing wind farms) with (ii) a corresponding scenario in which wind generation is introduced to the system by displacing an equivalent amount of synchronous generation. The damping performance of the pre- and post-wind scenarios is compared, using the Mudpack software package [2], on the basis of the eigenvalues of a linearized model of the system. The work undertaken for this initially involved developing the small signal stability models of the North and South Island power systems and generic Mudpack models of wind turbines. Initial analysis was conducted, without modelling wind generation scenarios, to assess the small-signal and damping performance of the existing system for an agreed set of base-case scenarios and an agreed set of contingency conditions. Then the impact of wind generation development scenarios was assessed which involved comparing the damping performance of the existing system with that of the corresponding wind generation scenarios for the intact and contingency cases. Studies were extended to assess the sensitivity of system damping to various factors. The sensitivities analysed are listed below;

(a) Sensitivity of system damping performance to the Wind Energy Converter (WEC) technology

(b) Sensitivity of system damping performance to whether or not selected wind farms are equipped with continuously acting voltage controllers.

(c) Sensitivity of system damping performance to the system loading (d) Sensitivity of system damping performance to the variation in the power

output from a wind farm.

To assess the impact that the various WEC technologies may have on the damping performance of the system, separate studies are conducted in which all wind generation is assumed to comprise WEC with (i) [DFIG] Doubly-Fed Induction-Generators in which the rotor voltage is controlled by a back-to-back DC link employing voltage-source-converters (VSCs); (ii) [FSFC] variable-speed generators connected to the grid by a Full Scale Frequency Converter back-to-back DC link employing VSCs; or (iii) [IG] Induction-Generators directly connected to the grid with either fixed-pitch turbine blades (i.e. passive-stall) or variable-pitch turbine blades (i.e. active-stall).


The damping of the least-damped electromechanical modes obtained with each of these WEC technologies is compared. In the case of the DFIG and FSFC technologies it is assumed that the power electronic converters control both the real (P) and reactive (Q) power output to a fixed value in the time frame of ~10 s considered in rotor-angle small-signal stability analysis. This assumption is consistent with the practice preferred by the major WEC suppliers. (The slow, discontinuous voltage-control systems provided by some manufacturers behave as constant Q devices in the short-term).

Although the assumption of constant PQ control represents the preferred default position of wind farm developers, it is necessary in some cases for wind farms to provide fast, continuously acting voltage control facilities to maintain adequate short-term voltage-stability margins within an area of the system to which they are connected. Sensitivity studies indicate that such voltage control facilities are unlikely to significantly affect the damping performance of the system.


5 Caveats Following is a consolidated list of caveats associated with the investigation. Where appropriate, these caveats are repeated in the relevant sections.

5.1 Scope of studies In this investigation a limited number of comparative studies were made between a base case scenario without wind generation and corresponding scenarios in which wind generation displaces an equivalent amount of synchronous generation according to a prescribed merit order. These “base-line” studies were supplemented by a small number of sensitivity studies which examined, in a restricted manner, the impact on damping per-formance of (i) alternative generation displacement scenarios; (ii) alternative WEC technologies; and (iii) the outage of key transmission lines. It was considered that such an approach would identify any significant adverse impact of wind generation on the damping performance of the system. However, it was recognized that there is a risk that this approach would not identify some situations in which high levels of wind generation will result in significant adverse effects on the damping performance of the system. For reasons outlined below, an extensive investigation has not been made into the damping performance of the existing New Zealand system. Such an investigation would not only consider a wide range of loading and generation profiles but also a wide range of contingencies. Such an analysis would be desirable to ensure that plausible system operating conditions are identified for which the damping performance of the existing system is worst and would establish a benchmark against which the various wind generation scenarios could be assessed. Such studies would also provide a solid basis on which to develop hypotheses for adverse impacts of wind generation on the system: hypotheses which could then be tested through analysis. Although the rigorous analysis suggested above has not been conducted, the following considerations will lead to reasonable deductions of the scenarios that should be analysed (i) the experience of Transpower; (ii) the network topology; (iii) the relative locations of load and generation; and (iv) the results of transient-stability analysis conducted by Transpower. However, as explained below, there are a number of concerns about the accuracy of the dynamic models of the existing plant and their controls (particularly excitation systems) which means that there are also concerns about the accuracy of the modal estimates derived from these models. Transpower intends to undertake work to assess the accuracy of the modal estimates derived from simulations and to enhance the accuracy of plant models as required. Thus, it is not considered worthwhile to conduct an extensive analysis of the damping perform-ance of the system until issues concerning the accuracy of the simulation models have been resolved. In due course, it is desirable that after plant models have been reviewed and an extensive analysis of the damping performance of NZ system is made, that the tentative conclusions obtained in this report are confirmed.

5.2 Dynamic models of existing plant

5.2.1 Synchronous machines and their controls The dynamic models of the North Island and South Island power systems used in the small -signal stability analysis are based on asset capability information provided by asset


owners. The accuracy of the analysis depends to a great extent on the quality of the models. In many cases, the dynamic models have not been validated against recent test results. In addition, information for some parts of the dynamic models is not available (e.g. the dynamic characteristics of demand) and these elements have been modelled under a certain set of assumptions. There is uncertainty about the current settings and operational status of some of the power system stabilizers installed on the system. Thus, it is unclear if the settings of some power system stabilizers are accurately reflected in either the PSS/E or DIgSILENT models. These settings are of particular importance in determining the damping of the least damped electromechanical modes. Properly tuned PSSs can have a considerable impact on the damping performance of the system. In an attempt to minimize the risk of optimistic predictions in view of this uncertainty, all PSSs are removed from service in the studies, except for the PSS fitted to the Taranaki CCGT. Based on information provided by generating companies or similar, the latter PSS is represented in the studies with “as-commissioned” models and parameters.

5.2.2 HVDC link The HVDC interconnection between the North and South Islands is represented as a load (sending end, usually SI) or negative-load (receiving-end, usually NI). This representation does not accurately represent the dynamic performance of the HVDC link. The significance of this modelling inaccuracy in determining the electromechanical modes of the system is unclear.

5.2.3 WEC models Generic models have been applied for each class of WEC technology which has been investigated. These generic models do not represent the details of the controls of any particular WEC manufacturer. Whilst the use of such generic models in a scoping investigation is appropriate, it will be necessary to develop and implement models which reflect the actual behaviour of the particular WECs which are ultimately deployed in practice. It is emphasized that in small-signal analysis the details of control schemes which are implemented to ensure that the WECs remain connected in the event of significant voltage depressions are irrelevant. This is because under small disturbance conditions the latter controls are inoperative and therefore should not be incorporated in small-signal models. The characteristics of fault ride-through schemes and their performance (which is one of the major points of difference between the various WEC manufacturers) do not need to be represented in small-signal models.


6 Damping Performance Criteria Although the New Zealand Electricity Governance Rules (EGR) [4] do not prescribe a damping-performance standard, Transpower have recently established an internal requirement that electromechanical modes of oscillation should have a time-constant of less than 12 seconds, which corresponds to a damping-constant with magnitude greater than -0.083 Np/s and a 2% settling time shorter than 47 s. (This criterion is based on that employed by National Grid in the UK. This criterion is significantly less stringent than that in the Australian National Electricity Rules [5] which require that, for simulations calibrated against past performance; the time-constant of the least damped electromechanical mode of oscillation is not more than 7.2 s after the most critical credible contingency event.


7 Damping Performance of the Existing System

7.1 Introduction The damping performance of the existing North & South Island power systems is assessed for the base case scenarios. As stated earlier, very few machines in the New Zealand system are equipped with in-service power system stabilizers (PSSs) which are an indication that small-signal stability has not been recognized as a significant concern in the past.

7.2 Performance of the North Island power system Figure 7 shows the electromechanical modes of the North Island base case system with no additional wind generation, except for the existing wind power installed in the Manawatu region. This wind plant is represented as a negative constant-PQ load. All PSSs are removed from service in this study. The lightly damped mode of oscillation involves the Taranaki combined cycle gas turbine (CCGT) oscillating against the system. This mode is referred to as the “Taranaki Mode”. Without a PSS installed on the Taranaki CCGT the damping of this mode does not comply with the Transpower damping criterion of -0.083 Np/s. It is noted that the Taranaki generation region has an installed capacity of approximately 935 MW and is relatively weakly connected from Stratford to Huntly by two relatively long 220 kV transmission lines. The 220 kV connection between Stratford and Bunnythorpe connects the Taranaki region to the main 220 kV transmission corridor between Bunnythorpe and the North of the Island. However, in the event of very high South to North flow and high output from the Taranaki generation region congestion on the main 220 kV corridor will tend to force higher flows over the Stratford to Huntly corridor which may result in a degradation in the damping of the Taranaki Mode. The sensitivity of the damping of the Taranaki mode to such congestion and the associated impact of wind generation on the mode is examined in section 12. The effect of the PSSs fitted to the Taranaki and Huntly CCGTs on the damping of the electromechanical modes of the system is indicated in Figure 8. The damping of the Taranaki mode is increased by over 1 Np/s from -0.08 to -1.17 Np/s.


Figure 7: Electromechanical modes of the North Island base case system with no additional wind generation - All PSSs are removed from service


Figure 8: North Island - The effect of the PSSs installed on the Taranaki and Huntly CCGTs on the

damping of the electromechanical modes of the North Island base system.


7.3 Performance of the South Island Power System Figure 9 shows the electromechanical modes of the South Island base case system with no wind generation in-service and with no PSSs in-service. The electromechanical mode of oscillation between the Manapouri machines and the machines in the Otago region is the least damped. The effect of the PSSs installed on the Tekapo A and Manapouri machines is shown in Figure 10. The setting of the PSS parameters in the studies is determined on the basis of information available to Transpower. The damping of the Manapouri inter-area mode and the Tekapo local mode are both improved significantly by the PSSs. The damping of the Manapouri intra-plant modes are also improved considerably. The damping of the other electromechanical modes are practically unaffected by the Tekapo A and Manapouri PSSs.


Figure 9 : Electromechanical modes of the South Island base case system with no additional wind generation - All PSSs are removed from service


Figure 10 South Island - The effect of the PSSs installed on the Tekapo A and Manapouri

machines on the damping of the electromechanical modes of the south Island base system.


8 Overview of the effect of wind generation on the electro-mechanical damping performance of the system

The objective of this section is to obtain an overview of the effect of introducing wind generation on the electromechanical damping performance of the system. The electromechanical modes of the system in the base case scenario are graphically compared with the modes in each of the four wind generation scenarios. The comparisons are made for the North and South Island systems in sections 8.1 and 8.2 respectively. Since WECs in commercial use today are not synchronously connected to the grid they will not cause electromechanical modes of oscillation in their own right. However, the introduction of large amounts of wind generation does have the potential to indirectly change the electromechanical damping performance of the system by:

• significantly altering the dispatch of synchronous generation in order to accommodate wind generation;

• significantly altering the power flows in the transmission network and; • interacting with synchronous machines to change the damping torques induced on

their shafts. The first two of these factors are largely independent of the WEC technology and are the subject of this section. The third factor depends on the dynamic performance characteristics of the WECs and on other relatively fast acting wind farm controls (e.g. STATCOMs which may be installed for voltage control purposes.) These factors are considered in Section 9 (impact of WEC technology) and Section 11 (impact of wind farm voltage control systems). The wind farms in the following studies are assumed to employ DFIG type wind turbines. They are represented by the simplified generic model described in Section 3.9.1. The rationale for selecting DFIG technology for base case studies is;

(a) A limited number of comparative studies are conducted in this report which will show that, from a small-signal point of view, the damping performance is similar for the different types of WEC technologies considered (i.e. DFIG, FSFC & IG).

(b) In particular the WEC technologies that have power electronic interfaces that control P & Q (or power-factor) (i.e. DFIG & FSFC) tend to perform in a very similar manner from a small-signal point of view. (I.e. they appear to behave, on the time scale of electromechanical modes, as though they were negative constant PQ loads.)

(c) To the extent that the performance of the IG turbines was different to that of the DFIG & FSFC, the small-signal damping performance of the system with IG type WECs tends to be marginally better than that obtained with the other technologies considered.

(d) Of the DFIG & FSFC type WECs, DFIG seem to have enjoyed more commercial success, although this may change (e.g. GE is apparently pursuing a full converter design).


8.1 North Island Scenario A: 1150 MW of wind generation in the North Island Figure 11 compares the electromechanical modes of the system in the base case scenario (existing wind generation only) with those for Scenario A. No significant changes in the damping performance are indicated by this comparison. The Taranaki mode is unchanged. The mode of oscillation between the Huntly CCGT (Unit #5) and the Huntly steam units #1 to #4 which exists in the base case scenario disappears in Scenario A because the Huntly steam units are removed from service in Scenario A to accommodate the wind generation. Torsional modes of oscillation are introduced with a frequency of about 12.5 rad/s (~ 2 Hz). These modes are associated with the flexible shaft that couples the mill and generator via a gearbox. Due to the poor inherent damping of this mode additional damping is provided by means of an active stabilizer. Typically, such stabilizers act by modulating the electrical power output of the turbine so as to oppose changes in generator speed. Providing that (i) the wind farm is connected to a relatively strong network; and (ii) the amplitude of the power modulation is limited to a relatively small fraction of the wind farm capacity, then the stabilizer will have only a minor impact on the system. If, however, the wind farm is connected to a relatively weak network with high r/x ratio then the power oscillations caused by the stabilizer may result in significant voltage fluctuations. The dominant mode of the pitch control system has a damping ratio of about 0.6 and a frequency of about 0.1 Hz. Scenario B: 950 MW of wind generation dispersed across the North Island Figure 12 shows that there is no material change in the electromechanical damping performance of the system under this scenario. Scenario C: 1,600 MW of wind generation dispersed across the North Island. As revealed in Figure 13 no significant change in the electromechanical damping performance of the system is indicated following the introduction of 1500 MW of wind generation for the system loading and generation dispatch which has been considered in the analysis. Scenario D: 370 MW of wind generation in the North Island The small increase in wind generation under this scenario does not, based on the eigen analysis results in Figure 14, result in any material change in the electromechanical damping performance of the system when compared with the base case scenario.


Figure 11: North Island; Comparison between the electromechanical modes of the North Island

base case system and those of wind generation Scenario A in which 1150 MW of wind generation is dispersed across the island. The wind farms are represented using simplified DFIG wind turbine

models.



base case system and those of wind generation Scenario B in which 950 MW of wind generation is dispersed across the island. The wind farms are represented using simplified DFIG wind turbine

models.



base case system and those of wind generation Scenario C in which 1600 MW of wind generation is dispersed across the island. The wind farms are represented using simplified DFIG wind turbine

models.



base case system and those of wind generation Scenario D in which 370 MW of wind generation is dispersed across the island. The wind farms are represented using simplified DFIG wind turbine

models.


8.2 South Island Scenario A: 100 MW of wind generation in the South Island concentrated near Invercargill Figure 15 compares the electromechanical modes of the system in the base case scenario which has no wind generation with the modes in Scenario A. Apart from the introduction of the torsional mode associated the DFIGs there is no material change in the damping performance of the system in this scenario. The Manapouri Mode is practically unaffected by the introduction of the wind generation. Scenario B: 300 MW of wind generation dispersed across the South Island Figure 16 shows that the damping of the mode of oscillation between the Clyde machines and the Ohau C machines is relatively sensitive to the power output from the Clyde station. A reduction in the Clyde output of about 35 MW associated with the accommodation of the wind generation in this scenario resulted in an improvement in damping of about 0.175 Np/s. The Manapouri Mode is practically unaffected by the introduction of the wind generation. Scenario C: 650 MW of wind generation dispersed across the South Island The mode of oscillation between the Clyde and Ohau C machines which is present in the base case disappears in Scenario C (see Figure 17). This is because the three of the four Clyde machines are removed from service to accommodate the wind generation and the fourth Clyde machine is operating at an output of only 50 MW. There is degradation in damping of the mode of oscillation between the Benmore machines and other machines in the Otago and Southland regions of about of 0.06 Np/s from -1.02 to -0.96 Np/s. The changes in frequency of the Arnold (ALD) and Argyle (ARG) local modes appears to be due to significant changes in the excitation of these two machines. It should be noted that these machines are very small and deeply embedded in the system. The Manapouri Mode is practically unaffected by the introduction of the wind generation. Scenario D: 50 MW of wind generation concentrated at Invercargill The addition of this small amount of wind generation to the South Island has practically no impact on the damping performance of the system, as indicated in Figure 18.


Figure 15: South Island; Comparison between the electromechanical modes of the South Island

base case system and those of wind generation Scenario A in which 100 MW of wind generation is located near Invercargill. The wind farms are represented using simplified DFIG wind turbine

models.



base case system and those of wind generation Scenario B in which 300 MW of wind generation is dispersed across the Island. The wind farms are represented using simplified DFIG wind turbine

models.



base case system and those of wind generation Scenario C in which 650 MW of wind generation is dispersed across the Island. The wind farms are represented using simplified DFIG Wind turbine

models.



base case system and those of wind generation Scenario D in which 50 MW of wind generation is concentrated near Invercargill. The wind farms are represented using simplified DFIG WEC

models.


9 Effect of the type of WEC technology on the damping performance of the system

As indicated in Section 3.9.2, three types of WEC are considered in this investigation. The DFIG type WEC was employed in Section 8 to assess the impact of the four wind generation scenarios on the damping performance of the system. In this section a comparison is made between effect of the DFIG, FSFC and IG type WECs on the damping performance of the system. For wind generation Scenario C, the electromechanical modes of the North Island system are compared in Figure 19 for the three types of WEC model and are seen to be practically identical. It is also observed that the torsional modes of the DFIG and IG type WECs are significantly different, even though the inertia-constants and shaft-stiffness coefficients are similar. There is reason to believe that the difference is due to the fact that the conventional induction generator is essentially a constant-speed/variable-torque device whereas the DFIG is essentially a variable-speed/constant-power device. A similar comparison is made for the South Island system in Figure 20 with similar results concerning the torsional modes of oscillation associated with the DFIG and IG type WECs. Additionally, it is noted that the damping of the Manapouri Mode is about 0.09 Np/s better if the IG type WECs are employed compared to the DFIG and FSFC type WECs. It should be emphasized that the apparent beneficial effect of the IG type WECs turbines on the damping performance of the Manapouri Mode may well be overshadowed by concerns with voltage performance, fault ride-through capability and other factors. These issues are beyond the scope of this element of the investigation but are considered in other parts of the project.


Figure 19: North Island; Scenario C; Comparison of the effect of the DFIG (A), FSFC (B) and IG

(C) type wind turbine generator models on the electromechanical modes of the system.


Figure 20: South Island; Scenario C; Comparison of the effect of the DFIG (A), FSFC (B) and IG

(C) type WEC models on the electromechanical modes of the system.


10 Effect of System Loading on the Damping Perform-ance of the System

The limited number of studies is conducted to obtain an indication of the effect of system loading on the damping performance of the system. Studies are conducted for scenarios with (i) no wind generation (apart from that which currently exists); and (ii) maximum wind generation in Scenario C in Table 1.

10.1 Damping performance comparison between the light- and high-load cases with no additional wind generation

Comparisons are made between the damping performances of the system under light- and high-load conditions without any additional wind generation. These comparisons provide a basis to determine, for the maximum wind generation scenarios, the relative extent and significance of any differences between the damping performances of the system under light- and high-load conditions. Figure 21 compares, for the North Island system, the least damped electromechanical modes of the high- and light-load cases in the absence of any additional wind generation. There is a relatively small shift in a number of modes. A number of modes which are present in the high-load case are absent in the light-load scenario because the associated synchronous machines are off-line in the latter case. Overall, the damping performance of the light-load study system does not materially differ from that of the high-load study system. Figure 22 reveals a similar situation for the South Island system. However, it is worth noting in this case that the damping of the Manapouri Mode is somewhat improved in the light-load study case. The local mode of oscillation involving the Tekapo B machines (TKB) is worse in the light-load case. The number of modes in the high-load case which are absent in the light-load case due to their associated machines being off-line is particularly evident in Figure 22.


Figure 21: North Island, No Wind; Comparison between the electromechanical modes of the North

Island system under light (A) and high (B) load conditions in the absence of any additional wind generation.


Figure 22: South Island, No Wind; Comparison between the electromechanical modes of the South

Island system under light (A) and high (B) load conditions without an wind generation.


10.2 Damping performance comparison between the light- and high-load cases with maximum wind generation

Comparisons are made between the damping performances of the system under light- and high-load conditions with (near) maximum wind generation. Figure 23 compares, for the North Island system, the least damped electromechanical modes of the high- and light-load with wind generation represented according to Scenario C. The total amount of wind generation in the light-load case is 250 MW less than in the high-load case. The reduction is shared proportionately between the six wind generation sites. As a result the damping of the torsional modes of oscillation associated with drive-trains of the DFIG WECs is improved in the light-load case when compared with the high-load case. A large number of electromechanical modes which are present in the high-load case are absent from the light-load case because their associated machines have been removed from service. The system damping performance is satisfactory in both the high- and light-load study cases which have been investigated. A similar comparison is made in Figure 24 for the South Island system. It is noted that the damping of the Manapouri Mode is somewhat improved in the light-load study case and that the local mode of oscillation involving the Tekapo B machines (TKB) is worse in the light-load case. This behaviour was also observed in Section 10.1 where the comparison was also between the light- and high-load cases but with no wind generation. There is no indication, on the basis of the study cases which have been analysed, that the level of system load will have a material effect on the damping performance of the system.


Figure 23: North Island, Maximum Wind; Comparison between the electromechanical modes of the

North Island system under light (A) and high (B) load conditions with maximum wind generation. The wind farms are represented using simplified DFIG WEC models.


Figure 24: South Island, Maximum Wind; Comparison between the electromechanical modes of

the South Island system under light (A) and high (B) load conditions with maximum wind generation. The wind farms are represented using simplified DFIG WEC models.


10.3 Effect of wind generation on the damping performance of the system under light-load conditions

The damping performance of the system under light-load conditions is compared for cases in which (i) there is no additional wind generation; and (ii) maximum wind generation is represented. The results of the comparison are shown in Figure 25 and Figure 26 for the North and South Islands respectively. In the case of the North Island, the introduction of 1250 MW of additional wind generation results in a significant number of synchronous machines being removed from service. This results in many of the least damped electromechanical modes of oscillation which are present in the no wind case disappearing from the maximum wind case. There is no indication, on the basis of the study cases analysed, that the introduction of high levels of wind generation under light-load conditions will adversely affect the electromechanical damping performance of the North Island system. In the South Island system the introduction of 650 MW of additional wind generation under light-load conditions does not result in any significant change in the damping performance of the system.


Figure 25: North Island, light-load; Comparison between the electromechanical modes of the North

Island system under light-load conditions with (A) no additional wind generation; and (B) wind generation Scenario C in which 1250 MW of additional wind generation is dispersed across the

island. The wind farms are represented using simplified DFIG WEC models.


Figure 26: South Island, light-load; Comparison between the electromechanical modes of the

South Island system under light-load conditions with (A) no additional wind generation; and (B) wind generation Scenario C in which 650 MW of wind generation is dispersed across the island.

The wind farms are represented using simplified DFIG WEC models.


10.4 Summary of the effect of system load on the damping performance of the system

The study cases analysed in this section do not provide any indication that the level of system load (i.e. light- or high-load) will materially affect the damping performance of the system. That is, there is no indication that the operation of the system will need to be modified due to concerns about the electromechanical damping performance of the system under light-load conditions when compared with the performance under high-load conditions. These indications apply both to the situation with (i) the existing level of wind generation; and (ii) maximum wind generation under Scenario C. The indications in this section are based on a very limited number of study cases in which the dispatch ordering of synchronous generation is fixed. It may be the case that alternative generation profiles will modify network power flows and alter the damping performance of the system. There may be other factors, which are beyond the scope of this investigation on system damping performance, which will limit the amount of wind generation that can be dispatched under light-load conditions. These other factors are considered in other investigations associated with the WGIP.


11 Effect of Wind Farm Voltage Control on the Damping Performance of the System

The studies have been conducted on the basis that the wind farms are not provided with fast, continuously-acting voltage control systems (VCSs). This reflects the existing common practice for wind farms to be equipped either with either (i) no voltage control facilities (apart from tap-changing) or (ii) with slow, discontinuous voltage control achieved by switching capacitor banks or controlling the reactive power output from the WECs. As the proportion of wind generation increases there may be a need for wind farms to provide facilities for fast continuously acting voltage control in order to contribute to the requirement that system voltages be regulated in a stable manner within prescribed limits. A limited number of studies are conducted to provide an indication of impact that such VCSs may have on the damping performance of the North and South Island systems. For the purpose of these sensitivity studies it is assumed that the wind farms are equipped with DFIG-type WECs and that voltage control is achieved by means of a STATCOM connected to the wind farm side of the collector transformer, as indicated in Figure 27. It is assumed that the STATCOM is equipped with a fast, continuously acting automatic voltage regulator (AVR) to control the voltage of the bus to which the STATCOM is connected. The performance of the VCSs employed in these sensitivity studies is indicated by the voltage-step responses in Figure 28 and Figure 29. These step responses show, for the VCS of each wind farm, the response of the collector terminal voltage to a 1% step-change in the AVR voltage reference. The rise time is less than 0.1 s and the 2% settling time is less than 0.5 s. The steady-state error is less than 1%.

Figure 27: Network connection of a DFIG type WEC and incorporating a STATCOM for fast,

continuously acting voltage control.

Studies are conducted which compare the damping performance of the system with the wind farm VCSs (i) in-service; and (ii) out-of-service. The comparisons are for wind generation Scenario C (See Table 1). Figure 30 and Figure 31 shows that for the North Island system the VCSs have a negligible effect on the damping performance of system in either the high- or light-load study cases investigated. Similarly, Figure 32 and Figure 33 reveals that the VCS’s installed at the wind farms in the South Island have practically no effect on the damping performance in either the high- or light-load study cases investigated.


Thus, on the basis of the limited set of studies conducted, there is no indication that the widespread introduction of fast, continuously acting wind farm VCS’s will adversely affect the damping performance of the system. It is emphasized that detailed planning and design studies will be necessary prior to the actual implementation of a wind farm VCS. Such studies should take into consideration the damping performance of the system.

Figure 28: North Island, Scenario C: Step-responses of the voltage control systems connected to the Bunnythorpe (BPE), Huntly (HUN), Marsden (MDN), Otahuhu (OTA), Hawkes Bay (HB)and

Wilton(WIL) wind farm sites.


Figure 29: South Island, Scenario C: Step-responses of the voltage control systems connected to

the Blenheim (BLN), Timaru A (TIMA), Timaru B (TIMB) and Invercargill (INV) wind farm sites.


Figure 30: North Island, high system load: Effect of wind farm VCSs on the damping performance

of the North Island power system – (A) VCS out-of-service; (B) VCS in-service.


Figure 31: North Island, light system load: Effect of wind farm VCSs on the damping performance

of the North Island power system – (A) VCS out-of-service; (B) VCS in-service.


Figure 32: South Island, high system load: Effect of wind farm VCSs on the damping performance

of the South Island power system – (A) VCS out-of-service; (B) VCS in-service.


Figure 33: South Island, light system load: Effect of wind farm VCSs on the damping performance

of the South Island power system – (A) VCS out-of-service; (B) VCS in-service.


12 North Island: Effect of Stratford to Huntly congestion on the damping of the Taranaki Mode.

The damping of the electromechanical mode of oscillation between the machines in the Taranaki region and the remainder of the system is sensitive to the loading on the relatively high impedance transmission corridor between Stratford and Huntly. This mode of oscillation is referred to as the Taranaki Mode. Wind generation scenarios that influence the level of congestion on the Stratford to Huntly transmission corridor are considered in order to assess the potential impact of wind generation on the damping of the Taranaki Mode. The damping performance is assessed for both (i) the intact system; and (ii) the system with one of the two Stratford to Huntly 220 kV circuits out-of-service (i.e. the Stratford to Taumarunui 220 kV circuit). The base case for this sensitivity study comprises no wind generation in the North Island (apart from the existing wind generation installed in Manawatu region). The total generation in the Taranaki region is approximately 920 MW and the regional load is approximately 165 MW. The total Stratford-Huntly flow is 400 MW for the intact case (SN01) and 268 MW for the case with SFD-TMN OOS (SN02). The balance of the generation is exported southwards through Bunnythorpe. Wind generation is incorporated into the North Island system at or south of the Bunnythorpe substation in four equal increments of 150 MW: 1. Add 150 MW wind generation at Wilson; total wind generation 150 MW 2. Add 150 MW wind generation at Bunnythorpe; total 300 MW 3. Add 150 MW wind generation at Wilson; total 450 MW 4. Add 150 MW wind generation at Bunnythorpe; total 600 MW. Two scenarios for accommodating this increasing amount of wind generation are considered: 1. Wind generation displaces Huntly synchronous generation. As shown in Table 3 this scheme results in increasing flow on the Stratford to Huntly transmission corridor. 2. Wind generation displaces synchronous generation in the Taranaki region, thereby reducing the congestion between Stratford to Huntly.

Table 3: Power flow on the Stratford to Huntly transmission corridor.

As shown in Figure 34, accommodating increasing amounts of wind generation south of Bunnythorpe by displacing Huntly generation causes destabilization of the Taranaki electromechanical mode. This destabilization is associated with the increased loading of the high impedance transmission corridor between Stratford and Huntly. Conversely, if the


wind generation south of Bunnythorpe displaces Taranaki generation the loading on the Stratford to Huntly circuits is reduced and, as shown in Figure 35, the damping of the Taranaki Mode is improved. Congestion of the Stratford to Huntly corridor may be further compounded in the event that there is high import to the North Island from the HVDC link and high wind generation south of Bunnythorpe. It is therefore concluded that if the accommodation of increasing amounts of wind generation in the North Island results in increased congestion of the transmission corridor from Stratford to Huntly then the maintenance of adequate damping of the Taranaki Mode may become a limiting factor in the operation of the system. In the event that damping of the Taranaki Mode does become a constraining factor a range of remedial measures is possible, including the application of power system stabilizers on machines which participate significantly in the mode. Some machines may already be equipped with stabilizers, in which case they will need to be (re)tuned and commissioned. In other cases it may be necessary to consider retro-fitting PSSs.


Figure 34 North Island: The effect of congestion on the Stratford to Huntly 220 kV circuits on the

damping of the Taranaki Mode (Part A).

Scenario: High generation in the Taranaki region (approx. 950 MW). Increasing amounts of wind generation south of Bunnythorpe is accommodated by displacing Huntly

generation. This increases congestion on the Stratford to Huntly transmission corridor, thereby degrading the damping of the Taranaki Mode.A-E: System intact, wind generation increased in 150 MW increments from 0 to 600 MWF-J: As for A-E but with the Stratford

to Taumarunui 220 kV ckt. out of service.


Figure 35 North Island: The effect of congestion on the Stratford to Huntly 220 kV circuits on the

damping of the Taranaki Mode (Part B).

Scenario: High generation in the Taranaki region (approx. 950 MW in the no-wind cases), in-creasing amounts of wind generation south of Bunnythorpe is accommodated by displacing

Taranaki generation. This reduces congestion on the Stratford to Huntly transmission corridor, thereby improving the damping of the Taranaki Mode.A-E: System intact, wind generation increased in 150 MW increments from 0 to 600 MWF-J: As for A-E but with the Stratford to

Taumarunui 220 kV ckt. out of service.


13 South Island: Effect of the outage of the double circuit 220 kV transmission line between Manapouri and North Makarewa on the damping of the Manapouri Mode

The simultaneous forced outage of both circuits of the double-circuit transmission line between Manapouri and North Makarewa is considered a credible contingency event. Such an forced outage has occurred on several occasions in the past. The damping of the Manapouri Mode depends, in part, on the impedance of the connection between the Manapouri machines and the machines against which they oscillate in the central area of the South Island. Therefore, the double circuit outage is expected to reduce the damping of the Manapouri Mode. The electromechanical modes of the intact base case system with no wind generation (see Section 7.3) are compared in Figure 36 with those of the same system but with the outage of both circuits of the double-circuit transmission line between Manapouri and North Makarewa. The comparison reveals that the double circuit outage reduces the damping of the Manapouri Mode by 0.08 Np/s from -0.26 Np/s to -0.18 Np/s. A similar comparison is made in Figure 37 for the high-load study case which incorporates 650 MW of wind generation in accordance with Scenario C in Table 1. This comparison reveals that the degradation in damping of the Manapouri Mode is 0.10 Np/s, which is slightly greater than that in the no wind study case.


Figure 36 South Island, No wind generation: Effect of the double-circuit outage of the 220 kV

transmission line between Manapouri and North Makarewa on the damping of the Manapouri Mode in the high-load base case with no wind generation.


Figure 37: South Island, Wind generation Scenario C: Effect of the double-circuit outage of the

220 kV transmission line between Manapouri and North Makarewa on the damping of the Manapouri Mode in the high-load case including the connection of 650 MW of wind generation in

accordance with Scenario C.


14 Discussion and Conclusions

14.1 Small signal analysis Although the New Zealand Electricity Governance Rules (EGR) [4] do not prescribe a damping performance standard, Transpower have recently established an internal requirement that electromechanical modes of oscillation should have a time-constant of less than 12 seconds, which corresponds to a damping-constant with magnitude greater than -0.083 Np/s and a 2% settling time shorter than 47 s. (It is understood that this criterion is based on that employed by National Grid in the UK. This criterion is significantly less stringent than that in the Australian National Electricity Rules [5] which require that, for simulations calibrated against past performance, the time-constant of the least damped electromechanical mode of oscillation is not more than 7.2 s after the most critical credible contingency event. The North and South Island components of the New Zealand power system are interconnected by a HVDC link which means that the damping performance of each Island can be analysed separately from the other. In the North Island (NI) base case used in this investigation, apart from local modes associated with relatively small stations at Karapiro and Mangapapa, the damping-constant of all electromechanical modes is better than -0.3 Np/s, which is a factor of 3.6 better than the Transpower damping-performance requirement. The damping-constant for the above stations are also well within the Transpower requirement and because of their highly localized nature are not expected to be affected to any significant extent by the introduction of wind generation. Based on considerations of the topology of the North Island system and discussions with Transpower it is apparent that the electromechanical mode of oscillation between the generation in the Taranaki region (and particularly the combined-cycle gas-turbine (CCGT)) and the system may become comparatively lightly damped when the total output from the Taranaki region is high and the total flow from the Wellington region northward to Auckland is high. An outage of one of the two circuits between Stratford and Huntly will tend to reduce the damping of the Taranaki Mode (TM) still further. The damping of the TM is an important consideration for scenarios which introduce large wind generation south of the Bunnythorpe hub. Analysis of the South Island (SI) base case reveals that the electromechanical mode of oscillation between the 850 MW Manapouri power station near the southern tip of the South Island and the system appears to be the least damped wide-area mode in the South Island. This “Manapouri Mode” (MM) has a frequency of about 6 rad/s (1 Hz) and a damping-constant of about -0.25 Np/s which corresponds to a time-constant of 4 sec and a 2% settling time of about 15 s, which is well within the Transpower requirements. The damping of the Manapouri mode is significantly degraded in the event of the outage of the double-circuit 220 kV transmission line from Manapouri to North Makarewa. (Transpower considers the simultaneous outage of both circuits on this transmission line to be a credible contingency.)

14.2 Transient stability analysis Investigation 7 assessed the effects of wind generation upon power system transient stability. While Investigation 7 considered a different area of power system stability and used a different approach, some of the results from Investigation 7 are directly relevant to the results in Investigation 8. Investigation 7 used time domain analysis techniques to determine in the time domain power system transient performance following large disturbances. Investigation 8 uses frequency domain analysis to determine the electromechanical modes of oscillation present in the power system.


In Investigation 7, the transient analysis involving Manapouri generating units showed an oscillatory mode with a frequency of between 5 and 6 radians per second. Similarly, stability issues relating to the damping of the Taranaki mode were also indicated in Investigation 7.

14.3 Findings and Recommendations Summary of the findings and recommendations of this investigation is given below. The damping-performance of both the North & South Island systems in the four

base line wind generation scenarios is comparable with that in the corresponding underlying no wind base-case.For the North Island system, a scenario is identified in which high levels of wind generation located at or south of Bunnythorpe may result in the degradation in damping of the Taranaki mode when the power output of the Taranaki area generation is high. The degradation in damping may approach the minimum acceptable level (i.e. time-constant of 12 seconds) in the event of an outage of the 220 kV transmission circuit between Stratford and Taumarunui.

For the South Island system, varying the synchronous generation which is

displaced to accommodate the wind generation, has a relatively minor effect on the damping of the dominant Manapouri Mode. The degradation in damping of between 0.08 and 0.10 Np/s which is observed following the outage of the double-circuit 220 kV transmission line from Manapouri to North Makarewa in the South Island is similar in both the pre- and post-wind scenarios.

Sensitivity studies are conducted to provide an indication of the impact of system

load levels on the damping performance of the system. Light- and high-load study cases analysed with (i) only the existing wind generation in service; and (ii) maxi-mum wind penetration according to Scenario C. This analysis suggests that the damping performance of the system is not materially affected either by the level of system load or the amount of wind generation. It is noted that other factors which are being considered in other investigations within the WGIP may limit the maximum amount of wind generation to lower levels than those considered in these studies.

The damping performance of the system with DFIG and FSFC type WECs are prac-

tically identical. In the case of DFIG type, the drive-train which couples the windmill rotor and generator is relatively flexible and due, to their constant power control characteristic, a relatively lightly damped shaft torsional mode results. These modes, although electromechanical in origin, are quite distinct from the electromechanical modes associated with synchronous machines. Typically, the shaft torsional mode must be actively damped with a stabilizer which modulates the electrical power output from the WEC. This provision and setting of this stabilizer (which is quite distinct from a power system stabilizer fitted to a synchronous generator) is the responsibility of the WEC manufacturer since the mode depends only on the characteristics of the WEC. The existence of the shaft torsional mode will typically manifest itself in the system as a power oscillation at the torsional modal frequency (of about 12 rad/s or 2 Hz) and will be particularly evident following a system fault. In the event that the wind farm is connected to a very weak part of the system these power oscillations can result in voltage oscillations of significant amplitude. In such cases care must be exercised to ensure that the voltage swings do not cause maloperation of voltage sensitive controls and protection either within the wind farm or neighbouring installations.

Compared with DFIG & FSFC type WECs, the damping of the Manapouri Mode

tends to be slightly better when the WECs have fixed pitch blades with induction generators (IG) which are directly connected to the grid. The damping improvement


may be associated with the fact that the IG wind turbines operate with a (approx.) constant speed characteristic rather than constant power characteristic. It is for this reason that the shaft-torsional mode associated with the IG wind turbines tend to be better damped and have a significantly lower frequency than for the torsional mode associated with DFIG wind turbines. It should be noted that the marginal improvement in system damping observed with IG wind turbines is likely to be outweighed by other considerations, most importantly the fault ride through capability of the various WEC technologies.

There are some concerns about the accuracy of the dynamic models employed in

this investigation. Of particular concern is uncertainty about the current settings and operational status of some of the power system stabilizers (PSSs) installed on the system. Well tuned PSSs can have a considerable effect on system damping.

It is recommended that, after plant models have been reviewed and appropriately

revised and after a comprehensive analysis of the damping performance of NZ system is made using the updated models, the tentative conclusions obtained in this report be confirmed.

It is understood that very few of the synchronous generators in the New Zealand

system are equipped with in-service PSSs. If, despite the indicative findings of this investigation, it is discovered in the future that the damping performance of the system is degraded to an unacceptable extent by the introduction of a wind farm then there may be considerable scope for improving the damping performance of the system by fitting well-tuned PSSs to appropriate synchronous generators.

Large scale wind generation (as envisaged in the wind generation development

scenarios) does not appear to significantly affect small signal stability on the New Zealand power system. It is recommended that no further study is made at this stage under the scope of the WGIP and no actions in respect of the effects of wind generation on small signal stability need to be recommended by the WGIP.


15 References [1] Wind Generation Investigation Project Website – see

http://www.electricitycommission.govt.nz/opdev/comqual/windgen/wgip [2] D.J. Vowles & M.J. Gibbard, “Mudpack User Manual”, The University of Adelaide,

Version10R, May 2006. [3] CIGRE Technical Brochure on “Modeling And Dynamic Behavior Of Wind

Generation As It Relates To Power System Control And Dynamic Performance”, Working Group 601, of Study Committee C4, Final Report, January, 2007.

[4] (New Zealand) Electricity Governance Rules (EGR):

http://www.electricitycommission.govt.nz/rulesandregs/rules [5] National Electricity Rules (Australia)

http://www.aemc.gov.au/rules.php [6] Investigation of the Effect of Wind- Generation on the Damping Performance of

the New Zealand Power System, David Vowles and Mike Gibbard, The University of Adelaide, Chandana Samarasinghe and Graeme Ancell, Transpower NZ Ltd, Technical Report, October 2007

http://www.electricitycommission.govt.nz/opdev/comqual/windgen/wgip

http://www.electricitycommission.govt.nz/rulesandregs/rules

http://www.aemc.gov.au/rules.php


16 Appendix

16.1 Standard Site Abbreviations Table 4 shows a list of the site abbreviations used in the schematics.

Short Code

Site Short Code

Site Short Code

Site Short Code

Site

ABY Albury HEN Henderson NMA North Makarewa TGA Tauranga ADD Addington HEP Hepburn Road NPK National Park TIM Timaru ALB Albany HIN Hinuera NPL New Plymouth TKA Tekapo A ALD Arnold HKK Hokitika NSY Naseby TKB Tekapo B ANC Anchor Products HLY Huntly OAM Oamaru TKH Te Kaha ANI Aniwhenua HOR Hororata OHA Ohau A TKR Takapu Road APS Arthur’s Pass HPI Huapai OHB Ohau B TKU Tokaanu ARA Aratiatia HTI Hangatiki OHC Ohau C TMH Three Mile Hill ARG Argyle HUI Huirangi OHK Ohakuri TMI Te Matai ARI Arapuni HWA Hawera OKE Okere TMK Temuka ASB Ashburton HWB Halfway Bush OKI Ohaaki TMN Taumarunui ASY Ashley IGH Inangahua OKN Ohakune TMU Te Awamutu ATI Atiamuri INV Invercargill ONG Ongarue TNG Tangiwai AVI Aviemore ISL Islington OPI Opihi TOB Tokomaru Bay BAL Balclutha KAI Kaiapoi OPK Opunake TRK Tarukenga BDE Brydone KAW Kawerau OPU Opuha TUI Tuai BEN Benmore KEN Kensington ORO Orowaiti Tee TVT Treviot BLN Blenheim KIK Kikiwa OTA Otahuhu

Substation TWH Te Kowhai

BOB Bombay KIN Kinleith OTB Oteranga Bay TWI Tiwai BPE Bunnythorpe KKA Kaikoura OTC Otahuhu CC TWZ Twizel BRB Bream Bay KOE Kaikohe OTG Otahuhu Power

Station UHT Upper Hutt

BRK Brunswick KPI Kapuni OTI Otira UTK Upper Takaka BRR Branch River KPO Karapiro OWH Owhata WAA Whareroa BRY Bromley KPU Kopu PAK Pakuranga WAH Wahapo BWK Berwick KTA Kaitaia PAL Palmerston WAI Waiotahi CBG Cambridge KUM Kumara PAP Papanui WDV Woodville CLH Castle Hill KWA Kaiwharawhara PEN Penrose WEL Wellsford CML Cromwell LFD Lichfield PKE Poike WES Western Road COB Cobb LIV Livingstone PNI Pauatahanui WGN Wanganui COL Coleridge LTN Linton PPI Poihipi WHE Wheao CPK Central Park MAN Manapouri PRM Paraparaumu WHI Whirinaki CST Carrington Street MAT Matahina PTA Patea WHU Waihou CUL Culverden MCH Murchison RDF Redclyffe WIL Wilton CYD Clyde MDN Marsden ROB Robertson Street WIR Wiri DAR Dargaville MGM Mangamaire ROS Mount Roskill WKM Whakamaru DOB Dobson MHO Mangahao ROT Rotorua WKO Waikino DVK Dannevirke MLG Melling ROX Roxburgh WMG Waimangaroa EDG Edgecumbe MNG Mangere RPO Rangipo WPA Waipapa EDN Edendale MNI Motunui RTR Retaruke WPI Waipori FHL Fernhill MOK Mokai SBK Southbrook WPR Waipara FKN Frankton MOT Motueka SDN South Dunedin WPT Westport GFD Gracefield MPE Maungatapere SFD Stratford WPW Waipawa GIS Gisborne MPI Motupipi SPN Springston WRA Wairoa GLN Glenbrook MRA Moturoa STK Stoke WRK Wairakei GOR Gore MST Masterton STU Studholme WTK Waitaki GYM Greymouth MTI Maraetai SVL Silverdale WTU Whakatu GYT Greytown MTM Mt Maunganui SWN Southdown WVY Waverley HAM Hamilton MTN Marton TAK Takanini HAY Haywards MTO Maungaturoto TAP Te Apiti HBK High Bank MTR Mataroa TCC Taranaki

Combined Cycle

Table 4: Standard Site Abbreviations


16.2 Generation Dispatch Assumptions This appendix contains the generation dispatch assumptions for the evaluation of critical fault clearance times.

16.2.1 North Island

Name Type Region Installed Capacity MW total

Available Capacity MW

Base case Dispatch

Merit Order

Glenbrook Cogeneration Auckland 99 53 53 12 Otahuhu A GT Auckland 80 36 0 N/A Otahuhu CC CCGT Auckland 395 373 370 41 Southdown Cogeneration Auckland 116 112 112 16 Aniwhenua Hydro Bay of Plenty 25 25 20 28 Lloyd Mandeno / Kaimai Hydro Scheme

Hydro Bay of Plenty 15.4 15.4 0 29

Matahina Hydro Bay of Plenty 72 72 40 31 Ohaaki Geothermal Bay of Plenty 122 41 0 7 Poihipi Geothermal Bay of Plenty 55 42 38 8 Rotokawa Geothermal Bay of Plenty 31 31 30 9 Ruahihi Hydro Bay of Plenty 19 19 10 33 Trustpower – Wheao Hydro Bay of Plenty 25 25 24 36 Waikato – Aratiatia Hydro Bay of Plenty 90 84 70 20 Waikato – Atiamuri Hydro Bay of Plenty 84 80 70 21 Waikato – Ohakuri Hydro Bay of Plenty 112 112 90 25 Wairakei Geothermal Bay of Plenty 178 153 138 10 Wairakei 2 Geothermal Bay of Plenty 19 19 10 11 Mangahao Hydro Central North

Island 36 26 25 30

Tararua 1 & 2 Wind Central North Island

63 63 60 1

Te Apiti Windfarm Wind Central North Island

91 91 90 0

Tongariro – Rangipo Hydro Central North Island

120 120 110 34

Tongariro – Tokaanu Hydro Central North Island

240 240 200 35

Waikaremoana – Kaitawa Hydro Hawke Bay 32 32 18 37 Waikaremoana – Piripaua Hydro Hawke Bay 40 40 22 38 Waikaremoana – Tuai Hydro Hawke Bay 52 52 29 39 Whirinaki GT Hawke Bay 156 156 0 N/A Ngawha 1 & 2 Geothermal North Isthmus 20 20 11 6 Kapuni Cogeneration Taranaki 24.7 20 10 13 Kiwi Dairy, Hawera aka Whareroa

Cogeneration Taranaki 69 69 65 15

New Plymouth GT Taranaki 360 330 0 44 Patea Hydro Taranaki 31 28 16 32 TCC CCGT Taranaki 450 367 360 42 Huntly CCGT Waikato 373 373 373 40 Huntly (Units 1-4) Thermal Waikato 1000 1000 1000 45 Huntly P40 GT Waikato 50 50 40 43 Kinleith Cogeneration Waikato 40 40 19 14 Mokai Geothermal Waikato 82.1 55 27 4 Mokai 2 Geothermal Waikato 40 40 26 5 Te Awamutu Anchor Products

Cogeneration Waikato 64 22 25 17

Te Rapa Cogeneration Waikato 42 42 40 18




Base case Dispatch

Merit Order

Waikato – Arapuni Hydro Waikato 176 176 140 19 Waikato – Karapiro Hydro Waikato 96 96 94 22 Waikato - Maraetai A Hydro Waikato 180 180 100 23 Waikato - Maraetai B Hydro Waikato 180 180 100 24 Waikato – Waipapa Hydro Waikato 51 51 40 26 Waikato – Whakamaru Hydro Waikato 100 100 96 27

Table 5: Generation dispatch for North Island

The methodology for the dispatch is as follows:

First dispatch all wind generation at maximum capacity; Next dispatch all geothermal generation, as this is normally base load plant; Then dispatch all cogeneration plants, also usually base load; Next dispatch hydro generators starting with the Waikato River hydro scheme; The HVDC is dispatched next, as this is comprised of generation from South Island

hydro plants; Finally, the North Island thermal generation is dispatched, first CCGTs, then GTs

and lastly coal.

For all dispatch scenarios at least two units from the Huntly power station have been dispatched, with enough spare capacity to cover the spinning reserve requirement of 350 MW.

16.2.2 South Island



Base case Dispatch

Merit Order

Coleridge Hydro Canterbury 45 45 38 2 Highbank Hydro Canterbury 25 25 25 3 Cobb Hydro Nelson-

Marlborough 32 32 32 1

Wairua & Argyle Hydro Nelson-Marlborough

11 11 11 4

Clutha - Clyde Hydro Otago Southland 432 432 88 18 Clutha – Roxburgh Hydro Otago Southland 320 320 264 17 Manapouri Hydro Otago Southland 850 710 600 8 Paerau/Patearoa Hydro Otago southland 12 12 12 5 Teviot Hydro Otago Southland 15 15 15 6 Waipori Hydro Otago Southland 96 78 0 8 Waitaki – Aviemore Hydro South Canterbury 220 220 175 9 Waitaki – Benmore Hydro South Canterbury 540 540 480 10 Waitaki - Ohau A Hydro South Canterbury 264 264 212 11 Waitaki - Ohau B Hydro South Canterbury 212 212 170 12 Waitaki - Ohau C Hydro South Canterbury 212 212 170 13 Waitaki - Tekapo A Hydro South Canterbury 25.2 25.2 12 14 Waitaki - Tekapo B Hydro South Canterbury 160 146 128 15 Waitaki – Waitaki Hydro South Canterbury 90 90 70 16 Kumara/Dilmans Hydro West Coast 10 10 10 7

Table 6: Generation dispatch for South Island

The methodology for the dispatch is as follows:

First dispatch all wind generation at maximum capacity;


Next dispatch the hydro generation beginning with the smaller northern units, then Manapouri, followed by the Waitaki system and lastly the Clutha system;

For every dispatch scenario at least two Clyde power station units were dispatched, with enough spare capacity to cover the spinning reserve requirement of 120 MW.

Documents

Wind Generation Investigation Project