WECC VGITF White Paper - Draft - Western Electricity ... VGITF White... · Web viewThese tests include reactive power capability tests, dynamic response tests, and harmonics tests

Variable Generation Interconnection Lessons Learned and Best Practices in the Western

Interconnection

WECC Variable Generation Interconnection Task Force

February 15, 2016

155 North 400 West, Suite 200

Salt Lake City, Utah 84103-1114

White Paper Title 2

Disclaimer

WECC receives data used in its analyses from a wide variety of sources. WECC strives to source its data from reliable entities and undertakes reasonable efforts to validate the accuracy of the data used. WECC believes the data contained herein and used in its analyses is accurate and reliable. However, WECC disclaims any and all representations, guarantees, warranties, and liability for the information contained herein and any use thereof. Persons who use and rely on the information contained herein do so at their own risk.

W E S T E R N E L E C T R I C I T Y C O O R D I N A T I N G C O U N C I L

VGI Lessons Learned and Best Practices 3

Table of Contents

Disclaimer...........................................................................................................................................2

1 Executive Summary.......................................................................................................................5

2 Introduction..................................................................................................................................7

3 Variable Generation Characteristics..............................................................................................9

4 Reactive Power and Voltage Control.............................................................................................9

4.1 Lessons Learned..........................................................................................................................9

4.1.1 System Planning Study Issues........................................................................................10

4.1.2 Plant Modeling..............................................................................................................10

4.1.3 Voltage Control Mode...................................................................................................12

4.1.4 Reactive Power Coordination........................................................................................12

4.1.5 Dynamic Voltage Control..............................................................................................13

4.2 Best Practices............................................................................................................................14

4.2.1 Reacive Power Delivery to POI......................................................................................14

4.2.2 Static and Dynamic Reactive Power Sources.................................................................15

4.2.3 Voltage Control Operation............................................................................................16

4.2.4 Plant Response to Voltage Control Commands.............................................................16

5 Active Power and Frequency Control...........................................................................................17

5.1 Lessons Learned........................................................................................................................17

5.2 Best Practices............................................................................................................................17

6 System Studies............................................................................................................................18

6.1 Lessons Learned........................................................................................................................18

6.1.1 Modeling.......................................................................................................................18

6.1.2 Study Scenarios.............................................................................................................19

6.2 Best Practices............................................................................................................................19

6.2.1 Modeling.......................................................................................................................19

6.2.2 General Studies.............................................................................................................19

6.2.3 Voltage control and reactive power requirement studies.............................................20



6.2.4 Operating flexibility studies...........................................................................................20

6.2.5 Generation ramp studies...............................................................................................21

6.2.6 Other Studies................................................................................................................ 21

7 Commercial Operations...............................................................................................................21

7.1 Lessons Learned........................................................................................................................21

7.2 Best Practices............................................................................................................................22

7.2.1 Test reactive power capabilities....................................................................................22

7.2.2 Dynamic Response Tests and Performance Monitoring................................................23

7.2.3 Harmonics Test............................................................................................................. 24

8 Other Issues................................................................................................................................24

8.1 Harmonics.................................................................................................................................24

8.1.1 Harmonics Study...........................................................................................................24

8.1.2 Harmonics Test............................................................................................................. 24

8.1.3 Harmonics Requirements..............................................................................................25

8.2 Subsynchronous Resonance and Interactions...........................................................................26

9 Conclusions.................................................................................................................................27

10 References..................................................................................................................................27

Report Contributors..........................................................................................................................28



1 Executive Summary

Over the last ten years, variable generation penetration in the West Interconnection has increased steadily. According to Western Electricity Coordinating Council (WECC) 2015 State of the Interconnection Report published in June 2015, wind capacity has increased by more than 150% to 24.3 GW and solar capacity by 14 times to 8.4 GW. The focus of this white paper will be on wind and solar generation interconnected to the bulk transmission system. Wind and solar generation interconnected to the distribution system or other sources of variable generation are beyond the scope of this paper.

This white paper is intended to serve as a technical reference for WECC members when they plan and operate their transmission systems with variable generation connected. The lessons learned and best practices described in this document may pertain to the authors’ systems, not necessarily applicable to other entities’ systems. It is not intended to be a standard or policy to follow, rather a reference for establishing company standards or policies.

The main topics covered in this paper include reactive power and voltage control, active power and frequency control, system studies, and commercial operations.

Reactive Power and Voltage Control

Lessons Learned

System planning studies did not identify some operational issues that have since been observed with the variable generation power plants that did not have dynamic voltage control.

Good variable generation power plant models, both power flow and dynamic, are needed to determine if appropriate transmission reinforcements or additions are needed and to represent the actual performance during observed system events.

Having variable generation power plants operating in voltage control can increase the amount of generation allowed at an interconnection.

Reactive power coordination can be challenging between plants that have different voltage controls.

Dynamic voltage control with an appropriate reactive power range is beneficial for variable generation power plant owners.

Best Practices

A variable generation power plant must be designed to deliver the reactive power to the point of interconnection (POI) over the voltage range.

Reactive power capabilities can include dynamic and static reactive power sources. A power plant shall always operate in voltage control mode unless otherwise instructed by the

system operator.



A power plant shall be able to respond to the system operator commands to raise / lower its voltage schedule.

Active Power and Frequency Control

Lessons Learned

One of the issues was lack of frequency response capability from variable generators. This was particularly important during over-generation period normally at light load conditions.

Best Practices

WECC Balancing Authorities are at various stages regarding the requirements on frequency response capability. BPA requires new wind projects of greater than 50 MW to have the capability to respond to overfrequency and underfrequency (governor type) control and separate ramp rate control. CAISO is seeking stakeholder input on how best to meet the new requirements.

In general, we think the following recommendations [1] are good for WECC.

Require curtailment capability, but avoid requirements for excessively fast response. A 10%/second for rate of response to a step command to reduce power output is reasonable.

Require capability to limit rate of increase of power output. Encourage or mandate reduction of active power in response to high frequencies. Consider requiring the capability to provide increase of active power for low frequencies. This

presumably would be a rare occurrence, as the economic penalty associated with enabling these controls is high.

Consider requiring inertial response in the near future.

System Studies

Lessons Learned

In the early days of system studies, most dynamic models for wind and solar interconnection projects were user-defined models. These models, normally proprietary and distributed as object files, presented several challenges such as difficulty in model maintenance. Therefore, WECC has led a comprehensive effort to develop generic dynamic models for wind turbine generators (WTG) and Inverter based photovoltaic (PV) systems suitable for system analysis.

Best Practices

Wind/Solar power plant owners shall provide their plant models in accordance with WECC Wind/Solar Power Plant Power Flow Modeling Guidelines, WECC Wind/Solar Power Plant Dynamic Modeling Guidelines and WECC Data Preparation Manual. The dynamic models shall be WECC-approved models.

Power flow, voltage stability, and transient stability studies shall be performed to evaluate the impact of wind/solar integration on system performance as required by NERC TPL-001-4 Reliability Standards.



Other studies such as voltage/reactive power control requirements, operating flexibility, generation ramping, and electromagnetic transients may be required under certain circumstances.

Commercial Operations

Lessons Learned

Several incidences involving solar/wind generating plant tripping or oscillations occurred due to inappropriate control schemes.

Best Practices

Plant owners shall perform tests to validate the collective capability and performance for the entire solar/wind farm. These tests include reactive power capability tests, dynamic response tests, and harmonics tests

Other Issues

Harmonics and subsynchronous resonance/interactions shall be considered in certain system conditions.

2 Introduction

Twenty-nine states and the District of Columbia have Renewable Portfolio Standards (RPS) policies, and an additional eight states have nonbinding renewable portfolio goals.1 These state RPS targets range from 10% to 100% renewable energy integrated into the power grid from 2015 to 2045. Wind and solar generation are the most common types of variable renewable generation so far.

According to the WECC 2015 State of the Interconnection Report, published in June 2015,2 over the last decade, wind generation capacity has increased steadily in the Western Interconnection. A similar trend in solar capacity has occurred over the last five years. In 2014, utility scale solar capacity in the Western Interconnection was 14 times that in 2010. Figure 2-1 shows the installed variable generation capacity by year from 2010 to 2014.

1 http://www.eia.gov

2 https://www.wecc.biz/Reliability/2015%20SOTI%20Final.pdf



Figure 2-1: WECC Renewable Resources Nameplate Capacity by Year

The higher level of penetration of variable renewable generation has brought many challenges in planning and operations. This has resulted in various efforts from academia to industry. In 2012, North America Electric Reliability Corporation (NERC) published a report titled “2012 Special Assessment Interconnection Requirements for Variable Generation” [1], where a number of recommendations were made regarding NERC interconnection procedures and standards.

This white paper was developed at the request of the Western Electricity Coordinating Council (WECC) Planning Coordination Committee (PCC) with consideration of the NERC recommendations in the above report. The purposes of the white paper are to:

1) Identify lessons learned from areas that have high penetration of variable generation; and2) identify best practices from the owners, planners, and operators of the interconnection,

transmission, and variable generation facilities

The focus of the white paper is on wind and solar generation interconnected to the bulk transmission system. Other sources of variable generation such as wind and solar generation interconnected to the distribution system or distributed generation are beyond the scope of this paper.

This white paper is intended to serve as a technical reference for WECC members when they plan and operate their transmission systems with variable generation connected. The best practices and lessons



learned in this document may pertain to the authors’ systems, not necessarily applicable to other entities’ systems. So, use of the language “must” or “shall”, for example, may not be appropriate for all entities. It is not intended to be a standard or policy to follow. Rather, it is a reference for establishing company standards or policies.

3 Variable Generation Characteristics

Table 3-1 is a summary of wind and solar PV generation characteristics with respect to system operating requirements.

Table 3-1: Wind and Solar Generation Characteristics

Resource Name

Type Dispatchability Frequency Response

Inertia Voltage Support

Blackstart Capability

Wind 1 and 2 Limited Limited Some No No

3 and 4 Limited Yes No Some No

Solar PV Some Some No Some No

Thermal Yes Yes Yes Yes Yes

4 Reactive Power and Voltage Control

4.1 Lessons Learned

Prior to FERC Order 661-A, there was no process for determining dynamic voltage control and reactive power requirements for variable generation power plants. Early plants were only required to operate at the unity power factor at the point of interconnection. FERC issued Order 661-A in 2005, requiring a transmission planner to determine the power plant reactive power requirements on a case-by-case basis. In 2005, the amount of variable generation was small, and most of these power plants were interconnected at voltages of 115 kV or lower. The reliability implications of the power plants were mostly limited to a local area. At the time, the majority of wind generation technologies were induction generators. Induction generators do not have voltage control capabilities and needed additional investments in dynamic reactive devices such as STATCOMs to enable dynamic voltage control. FERC Order 661-A was appropriate at the time, allowing transmission planners to require voltage-control devices based on the system studies.



4.1.1 System Planning Study Issues

System planning study did not identify some operational issues that have since been observed with the variable generation power plants that did not have dynamic voltage control.

Some companies have experienced several operational challenges with early wind power plants, although the system studies did not identify such problems initially. Typical power flow studies cannot capture the full diversity of the operating conditions. For example; there were occurrences when wind generation controls did not respond as planned to wind ramps or system voltage changes. In other instances, wind power plant models were inaccurate in determining wind power plant reactive needs.

For example, BPA experienced operational issues at a 150-MW wind power plant connected to its 115-kV transmission system. This wind power plant has variable rotor resistance induction generators, known as type 2 machines. The plant was required to operate at unity power factor at the POI and was allowed to use switched shunt capacitors for reactive power control. Several events of dynamic voltage instability were observed at the plant. As system voltage was declining, the wind power plant was absorbing more reactive power from the transmission system, thereby further accelerating voltage decline. Such behavior is the exact opposite of voltage control. The power plant output was eventually curtailed to maintain stable grid voltages. The investigation concluded that the phenomenon was related to the controls used by type 2 wind turbine generators and that the dynamic reactive capability was needed at the plant to solve the voltage stability issues.

In this particular situation, the plant owner installed STATCOMs at the plant to provide dynamic voltage control. The plant operation has since been stable with all lines in service. However, another voltage instability event occurred during a line outage condition in June 2011 because the STATCOMs were sized only for operating conditions with all lines in service. In addition, BPA has also experienced several operational challenges at its Jones Canyon 230-kV wind generation hub. The hub included four 100-MW wind power plants, three of which had type 2 wind generators with no dynamic voltage control, and one had doubly-fed asynchronous generators (DFAG), also known as type 3 machines. Although type 3 machines are capable of providing dynamic voltage control, this particular plant was operating in the power factor mode. A voltage instability event occurred primarily due to lack of dynamic voltage controls.

4.1.2 Plant Modeling

Good variable generation power plant models, both power flow and dynamic, are needed to determine if appropriate transmission reinforcements or additions are needed and to represent the actual performance during observed system events.



BPA conducted “lessons learned” studies for the events described above. Model validation studies were performed first, and then “what if” scenarios were studied to determine the causes and solutions to the observed problems. In most cases, the models were unable to reproduce the observed phenomenon initially, and the wind power plant (WPP) model and parameters needed to be tuned after the fact.

The development of WPP models, particularly dynamic models, was lagging the deployment of wind generation in the Western Interconnection. The WECC Modeling and Validation Work Group approved “Wind Power Plant Power flow Modeling Guidelines” in 2008, making a very compelling argument for modeling the collector system equivalent and the wind generator pad-mounted transformer. The representation of the reactive power losses in the collector system is necessary to correctly size reactive compensation requirements during normal conditions and grid contingencies. BPA was one of the first adopters of the WECC guidelines by making appropriate modifications to power flow base cases and requesting wind power plant owners to provide as-built plant data. Because of these efforts, the WPPs connected to the BPA transmission system have adequate power flow models today.

Dynamic models are still under development. While several wind turbine manufacturers have been actively engaged from the beginning, other manufactures are only now starting to contribute to the development of generic wind generation models. Unfortunately, many of the operational issues occurred at the plants with inadequate models. Our attempts to validate plant responses had mixed results. In our opinion, the generic dynamic models used previously were deficient in representing WPP responses to frequency deviations in the grid, as well as power oscillations. Since then, the models have improved.

BPA participated in the project led by DOE’s National Renewable Energy Lab (NREL) and Utility Wind Integration Group (UWIG) on WPP dynamic model validation. BPA is in the process of deploying Phasor Measurement Units (PMUs) at WPPs. The PMU data will be used for model validation and performance monitoring.

Conventional transmission planning studies are done with wind generators at full power output. The studies generally assume that dispatchers have adequate time to switch reactive devices (shunt capacitors and reactors) to optimize the voltage profile in the power system. However, dispatchers may not be able to follow fast wind generation ramps, and the system can end up in a sub-optimal position. BPA worked with software vendors to develop time-sequence powerflow capabilities to address this modeling need. Time sequence power flow allows second-by-second simulation of wind ramps (over periods of tens of minutes), and has been used by BPA for analysis of wind events and for reactive power coordination at its Jones Canyon wind hub.



4.1.3 Voltage Control Mode

Having variable generation power plants operating in voltage control can increase the amount of generation allowed at an interconnection.

A power versus voltage “nose” curve is often used for assessment of voltage stability margins. With four Jones Canyon plants operating in power factor mode, the voltage stability limit was reached with 400 MW of wind generation with no dynamic voltage control. BPA increased the amount of interconnected wind generation to 600 MW in 2011 by integrating two more 100 MW WPPs. The two new plants had dynamic reactive capabilities and operated in voltage control mode. One plant with type 2 generators used a STATCOM to provide dynamic reactive capabilities, and another project had type 3 generators providing dynamic voltage controls. Both projects also used switched shunt capacitors to extend their reactive capability ranges. The Jones Canyon hub has operated at 600 MW with no voltage stability issues because of the dynamic voltage controls provided by the new projects.

4.1.4 Reactive Power Coordination

Reactive power coordination can be challenging between plants that have different voltage controls.

Having different voltage controls represents a technical challenge for the reactive power coordination among multiple variable generation power plants within a solar or wind hub, primarily at the sites where there is a mix of projects with and without dynamic voltage controls. This could result in reactive power circulating among the plants and therefore increase active power losses. From the voltage stability standpoint, one plant may have already used its dynamic resources to compensate for reactive losses in another plant. Should a disturbance occur, the former plant will not be able to provide voltage support. The solution from BPA was to have the variable generation power plants operate with reactive power output just sufficient to compensate the reactive losses through the collector system and transformers during normal system conditions, so that the dynamic reactive reserves can be used during a contingency. Using switchable capacitors is a good approach for supplying steady state reactive losses, maximizing dynamic reactive capabilities to respond to transient events. We also recommend that new plants control high side voltage with a reactive power droop to enable stable reactive power sharing among multiple projects in a site.

Here is an example from PacifiCorp. Prior to 2012, PacifiCorp had been experiencing difficulties coordinating the voltage control of several closely connected wind plants in eastern Wyoming power grid. In some cases, these plants shared a common POI or were regulating close-by plants. Before proper voltage control coordination was implemented, the plant regulators were fighting among themselves or “hunting”. This situation was a concern when an unacceptable local voltage performance was observed. In 2012, PacifiCorp also experienced a separate voltage performance



related problem in the eastern Wyoming wind corridor. PacifiCorp operators witnessed fluctuating voltages and high voltages within this high wind area when the wind farms were operated in radial conditions.

The problem was diagnosed as lack of reactive capacity and poor coordination of voltage control. The solution was 1) to determine appropriate voltage droop settings for the wind plants; 2) to determine appropriate line drop compensation for the synchronous condenser; and 3) to validate and refine shunt device passive control logic. The appropriate tuning allowed for reliable and robust regulation of the 230kV transmission system.

In summary, PacifiCorp

• Installed a 60MVA synchronous condenser at a 230kV substation which was electrically close to the neighboring wind plants

• Added 31.7 MVAr shunt reactor and 25 MVAr capacitor in two of those wind farms

• Retained wind plant voltage droops

• Applied line drop compensation of one 230kV POI transformer

• Reset the voltage control set points of the local switched shunt devices to accommodate the recommended passive coordinated control scheme

• Implemented condenser switched device coordination during starting between capacitors and reactors

• Implemented reactive reserve coordination scheme using capacitors and reactors for dynamic headroom of the condenser

In the end, the planned transmission upgrades and fine-tuned control strategies improved system performance and provided cohesive system benefits.

4.1.5 Dynamic Voltage Control

Dynamic voltage control with an appropriate reactive power range is beneficial for variable generation power plant owners.

We believe that it is also in the best interest of a wind power plant operator to operate under continuous voltage control. When Automatic Voltage Regulators were first developed for conventional generators, the generator owners wanted to have them primarily to prevent generator over-voltages that result from sudden load rejection. The same argument applies to the wind power plant generators today.

An event of high over-voltages was observed at Jones Canyon wind hub in February 2012. The risk of high voltages was known and the wind plant output was limited for loss of a line terminal. Taking



a line out of service for maintenance had left the WPP’s connected by a long transmission line with a lot of capacitive charging. One plant was not operating in voltage control mode, and other plants had their reactors undersized. As a result the substation voltages increased to near 260 kV, as the plant reactive power remained nearly the same. Had the plant been in voltage control mode, as planned, the over-voltages would have been avoided.

4.2 Best Practices

To properly determine the most appropriate voltage control requirements, the following questions need to be addressed:

• What reactive power capabilities are needed?o What is the relationship between the power plant reactive capabilities and system voltage?o What portion of the reactive capabilities needs to be dynamic and what portion can be

static?o How to coordinate dynamic and static resources within the plant?

• What voltage is regulated – Point of Interconnection, lower voltage buses, or wind/solar gener-ator terminals – and over what time frame?

• How fast the reactive power needs to be deployed?o How to cycle the reactive resources

• How long does the reactive power need to be sustained?• How to verify the voltage control and reactive power capabilities using staged tests and from

performance observation?

4.2.1 Reacive Power Delivery to POI

A variable generation power plant must be designed to deliver the reactive power to the POI over the voltage range.

The maximum reactive power boost shall be 33% (approximately 0.95 power factor lagging) of plant maximum active power capability (Pmax), measured at plant POI, being fully delivered at the minimum operating voltage.

The maximum reactive power buck is 33% (approximately 0.95 power factor leading) of plant maxi-mum active power capability (Pmax), measured at plant POI, being fully absorbed at the maximum op-erating voltage.

The operating voltage range varies from company to company. For instance, one company operates its system between 530 kV and 550 kV for the 500-kV system. One company operates its system between 236 kV and 242 kV for the 230-kV system. Another company uses 117 – 120 kV as the operating range for its 115-kV system.



Reactive power flows from higher voltage to lower voltage (on per unit basis, adjusted for transformer tap ratios). Because of distributed nature of wind and solar power plants, there is a concern of voltage rise or drop through the collector system as the wind turbine generators deliver or absorb reactive power. Transformer taps need to be selected to ensure acceptable voltage profile and deliverability of reactive power for design conditions described above.

These requirements are consistent with FERC LGIA-2003 Section 9.6.1.

4.2.2 Static and Dynamic Reactive Power Sources

Reactive power capabilities can include dynamic and static reactive power sources.

• If dynamic reactive power is provided by the wind/solar generator converters (vast majority of cases today): o the converter capabilities must be sized to provide at least 0.95 power factor, or +/–

33% of generator active power capability at the POIo additional shunt capacitors may be required to meet the design reactive power range

requirements at the POI defined in section 2.2.1 • If dynamic reactive power is provided by supplementary reactive devices, such as STATCOM

or SVC:o STATCOM / SVC must be sized to provide reactive power of +/– 33% of the plant maxi-

mum active power capability at the POI o short-term overload capabilities of STATCOMs can be counted if fast switching of shunt

capacitors is usedo mechanically switched shunts must be sized to compensate for reactive power losses in

the generator terminal step-up transformers, collector system and the main step up transformer at full power output

• Switched reactive devices must be sized in steps less than 33% of the dynamic reactive ca-pabilities in either boost or buck directions o switched capacitors need to include fast discharge capabilities, so they can be reinserted

in seconds• A power plant shall have controls to optimize coordination of dynamic and static reactive

resources to maximize the availability of dynamic reactive capabilities• If a power plant is limited in providing reactive power at a rate of change needed to the POI

(similar to synchronous machines), and the POI system has characteristics of fast voltage collapse for contingencies or needs dynamic reactive power to help dampen transients/os-cillations, the developer shall add one or more dynamic reactive power control devices such as STATCOM or SVC to supplement voltage control.



4.2.3 Voltage Control Operation

A power plant shall always operate in voltage control mode unless otherwise instructed by the system operator.

• Continuous voltage control must be enabled when a wind/solar power plant active power output is between 15%-100% of the total plant capacity, and can be turned off or operated in power factor control mode when the power plant output drops below 10%. The plant can be set to either voltage control mode or power factor mode when its output is between 10% and 15% of the total plant capacity.

• Reactive power droop shall be in 5% to 12% rangeo Reactive power droop is measured as change in POI voltage over change in plant reac-

tive power delivered to the point of interconnection. Change in voltage is in per unit on nominal voltage. Change in reactive power is in per unit on power plant maximum active power of the plant.

o The requirement can be met by either:- Regulating POI voltage with reactive power droop.- Regulating the lower class voltage with reactive line drop.

• Voltage control must be continuous, use of voltage control dead-band is not allowed• Voltage control must be sustained, use of slow reactive power reset function is not allowed• Voltage control response time needs to be optimized given the capabilities of wind/solar

generation technologies and coordination with other wind/solar power plants in the area. While fast voltage response is desirable, we need to make sure that such fast response does not result in oscillations or hunting within a power plant or between power plants within a solar/wind hub.

4.2.4 Plant Response to Voltage Control Commands

A power plant shall be able to respond to the system operator commands to raise / lower its voltage schedule.

• The system operator commands are sent as pulses, one second voltage pulse corresponding to voltage step of 0.1%.

• The full response to voltage raise / lower command shall be achieved within 30 seconds or less. Many power plants with current technologies can provide full response within 10 sec-onds.



5 Active Power and Frequency Control

5.1 Lessons Learned

On April 12, 2014 in HE 13, there were over-generation conditions due to excessive wind and solar generation in California Independent System Operator’s (CAISO) Balancing Authority Area (BAA). CAISO issued an Over-generation Notice at 12:00 pm on that day. Over-generation occurs when there is more internal generation and imports into a Balancing Area than load and exports. This normally occurs at light load conditions with high variable generation production. Before an over-generation event occurs, the system operator would exhaust all efforts to send dispatchable resources to their minimum operating levels and will have used all the decremental energy (DEC) bids available in the imbalance energy market. Consequently, regulation down could not be procured in one fifteen-minute interval because it would require generators to move up to have headroom first to provide the regulation down and this additional generation would exacerbate over-generation conditions.

CAISO included an assessment of frequency response during over-generation conditions in the annual transmission planning process. The purpose of the study was to evaluate potential over-generation within the CAISO BAA and its potential consequences. The study results of the 2014-2015 Transmission Planning Process indicated acceptable frequency response within WECC. Frequency response from WECC was above WECC Frequency Response Obligation, and the frequency nadir and settling frequency were acceptable. However, with high amount of renewable generation in the CAISO that doesn’t respond to frequency disturbances, response from CAISO was below its Frequency Response Obligation. Compared to the actual system performance during disturbances, the study results were optimistic since the actual frequency responses for the same disturbances were lower than the study indicated. It was found that this was caused by large headroom of the generation and inaccuracy of the dynamic models. Consequently, a few next steps have been identified such as conducting further model validation and exploring governor response from other sources.

One of the means of improving the system frequency response is to require new inverter-based generation to provide frequency response. The CAISO studies showed that if inverter-based units are providing frequency response, it may solve the deficiency observed in the studies. Modern inverters are capable of providing frequency response; however, inverters need to be oversized and there should be some margin in how inverter-based generation is dispatched.

5.2 Best Practices

For primary frequency response, the CAISO currently does not have a procurement target. With NERC Reliability Standard BAL-003-1 effective on December 1, 2016, the CAISO is seeking stakeholder input



on how best to meet the new requirement and on the frequency response capabilities of emerging technologies such as wind, solar, and energy storage devices. Other WECC BAs are also at various stages regarding the requirements on active power and frequency control.

BPA requires new wind projects (starting construction after 6/1/2011) greater than 50 MW to have the capability to respond to overfrequency and underfrequency (governor type) control and separate ramp rate control. BPA will develop policies to address older projects when the need is determined.

Outside of WECC, ERCOT requires wind or solar generators to provide primary frequency response.

In general, we think the following NERC recommendations [1] are still valid for WECC.

1) Require curtailment capability, but avoid requirements for excessively fast response. A 10%/second for rate of response to a step command to reduce power output is reasonable.

2) Require capability to limit rate of increase of power output.3) Encourage or mandate reduction of active power in response to high frequencies.4) Consider requiring the capability to provide increase of active power for low frequencies. This

presumably would be a rare occurrence, as the economic penalty associated with enabling these controls is high.

5) Consider requiring inertial response in the near future.

6 System Studies

6.1 Lessons Learned

6.1.1 Modeling

In the early days of system studies, most dynamic models for wind and solar interconnection projects were user defined models developed by manufactures and software vendors. These models were pro-prietary and generally distributed as object files. Use of these models in system studies in large inter-connected systems presented several challenges [4]:

• Difficult to certify quality assurance: With models developed by individual users and companies, it is difficult to investigate whether the model development and resulting code meets a satisfac-tory level of quality assurance.

• Difficult to gain insight on model behavior: Beyond analyzing model behavior through simula-tion results, it is sometimes necessary to follow the source code. Object files are the outcome of the original source code being compiled, and are not viewable by users — becoming a “black box” with contents hidden from the user. Thus, it is a challenge for model users to better un-derstand the model behavior.



• Difficult to maintain: To comply with multiple versions of different software, it is necessary to maintain numerous versions of the same model, and model maintenance procedures become demanding.

To resolve these issues, WECC has led a comprehensive effort to develop generic dynamic models for WTGs and Inverter-based PV systems suitable for system analysis.

The generic models satisfy the following requirements:

• Generic models are tunable to parameters to represent different WTGs or solar PV generators.• They are nonproprietary (model and data can be shared).• They form a standard library (not user-written).• They are cross-platform compatible.• They are fully documented block diagrams and descriptions, and default parameter sets.• They are validated against field tests, factory tests, or against electromagnetic transient models

that have themselves been validated against test data.

6.1.2 Study Scenarios

For any wind/solar power plant to be interconnected to the system, if it is anticipated that the inter-connection will have regional impact, major generators and transmission paths shall be evaluated in the study for the most stressed system operating conditions.

6.2 Best Practices

The following sets of studies shall be performed to determine the plan of service for a wind/solar power plant interconnection.

6.2.1 Modeling

Wind/Solar power plant owners shall provide wind/solar power plant models in accordance with WECC Wind/Solar Power Plant Power Flow Modeling Guidelines, WECC Wind/Solar Power Plant Dynamic Modeling Guidelines and WECC Data Preparation Manual. During the generation interconnection stage, wind/solar power plant owners shall coordinate with their vendors to ensure that the dynamic models are WECC-approved models, not user-defined models.

6.2.2 General Studies

Power flow, voltage stability, and transient stability studies shall be performed to evaluate the impact of wind/solar integration on system performance as required by NERC TPL-001-4 Reliability Standards [7].



With large amounts of solar/wind generation, Interconnection-wide system impact studies are neces-sary to evaluate their impact on Interconnection-wide stability issues, such as:

Interconnection frequency response, a study completed by General Electric and NREL [8] Frequency, damping, mode shapes of the inter-area modes of power oscillations Impact on System Operating Limits of voltage-limited and transient stability-limited paths

The most stressed system conditions should be covered by taking load and generation dispatch of the study area into consideration. For example, one company uses the following scenarios:

• On-peak summer case• Off-peak spring case with high-wind high hydro scenario• On-peak winter case

6.2.3 Voltage control and reactive power requirement studies

Usually voltage and reactive power controls at the POI are important interconnection requirements. Therefore, studies shall be conducted to evaluate if the wind/solar power plant is effective for voltage and reactive power control. For instance, some entities require that the wind/solar power plant have a reactive power compensation scheme, 1) sized to provide/control between a net 0.98 power factor bucking or leading and a net 0.95 power factor boosting or lagging at maximum generation output at the POI; and 2) the switching/control of the reactive power done in small enough increments to limit the change in reactive power production or absorption in steady state to steps of no more than 10% of the generated power. In this case, time series power flow can be used to verify the effectiveness of the voltage control scheme and reactive power device installation in the wind power plant.

Transient stability studies shall be done to verify if the dynamic reactive power control devices are ef-fective enough to eliminate fast transient voltage excursions. The simulation time shall be long enough to verify the effectiveness of static reactive power control devices as well.

6.2.4 Operating flexibility studies

Assessment shall be performed for wind/solar power plant performance and voltage control coordination under outage conditions. The studies will be done for a scenario when multiple lines can be taken out of service for maintenance. Studies will identify whether any operational restrictions need to be applied to the plant and corresponding system conditions and outages. Examples of operational restrictions include MW curtailment and MVAr control change. The studies shall be able to identify the required control change and its effectiveness.



6.2.5 Generation ramp studies

As applicable, wind/solar generation ramps shall be simulated using time-sequence power flow to as-sess the impact of wind/solar generation variability on the load customers in the area, specifically volt-age fluctuations and Load Tap Changer operations. Some utilities require that the wind/solar power plant must be capable of controlling power generation to be compliant with the system conditions. For instance, the production ramp-up limit, determined as a one-minute average value, or specified in terms of megawatts-per-minute, must not at any time exceed five percent (5%) per minute of the max-imum power of the wind power plant and production control must be capable of reducing output by at least fifty percent (50%) of then-current power production in less than two (2) minutes. These rules should be considered in setting up the time-sequence power flow for ramp studies.

6.2.6 Other Studies

A wind/solar power plant may be located in a remote area and connected to system through a long ra-dial line, or there may be some possibilities for a wind/solar power plant to connect and operate in this mode due to outages. Under these scenarios, in addition to conventional TPL studies, additional stud-ies shall be conducted to examine potential overvoltage when the line is energized to connect the power plant. This phenomenon is called Ferranti Effect. Due to low short circuit capacity and weak sys-tem nature of this scenario, the inrush current resulting from energizing transformers among the radial line could be troublesome for local customers and devices. Transient studies via ElectroMagnetic Tran-sients Program (EMTP)-type tools can be used to evaluate potential hazards and mitigation actions re-quired.

Contingencies may cause the wind/solar plant to be islanded with potential transient over-voltages. Fast and coordinated direct transfer trips may be required to mitigate the impact of subject contingen-cies.

7 Commercial Operations

7.1 Lessons Learned

In 2014, SCE noticed a 7 Hz oscillation during the noon period. The investigation indicated that one PV solar output was oscillating at the peak generating time of roughly 11 am to 3 pm, causing this 7 Hz os-cillation. As a result, a modification was made to the inverter control and the oscillation went away.

Another event in 2014, a three-phase fault at PG&E Midway substation with normal clearing caused approximately 261 MW of solar generation to be tripped in the SCE system. The solar generators seemed to not have the low-voltage ride through capabilities as required by the Interconnection Agreement. Further investigation of one of the plants discovered that the frequency calculation scheme of the inverter was not accurate, resulting in a false trip.



BPA observed a high frequency of 14 Hz power oscillations at one of its wind power plants when the plant output exceeded 90% of its capacity. Reactive power oscillations were 80 MVAr peak-to-peak at the 425-MW plant. The oscillation was detected by real-time applications that utilize PMU technology. The plant operator subsequently corrected controls to avoid future oscillations.

Wind farms equipped with a single voltage / var controller to regulate the POI voltage may be at risk of causing unacceptable voltages should the controller fail, particularly if the wind farm is connected to a weak system and supported with mechanically switched shunt var devices under such controls.

Wind farm designs and control schemes vary depending on the type of turbines and the interconnect-ing system. Well prepared commissioning test procedures and documentation would demonstrate the wind generating facility meets the intended capability and performance requirement such as var capa-bility (dynamic and static) and the plant control schemes.

7.2 Best Practices

Wind/solar plant operators shall perform the following tests no later than 180 days from the beginning of the commercial operation to certify the WPP voltage control and reactive power capabilities.

Wind/solar plant operators shall provide “as-build” model for the power plant, according to WECC Wind Power Plant Powerflow Modeling Guidelines and WECC Wind Power Plant Dynamic Modeling Guidelines. The model is the statement of the power plant dynamic performance and voltage control capabilities.

Wind farm plant and voltage control should take the impact of a single element failure (N-1 contin-gency) into consideration.

Plant owners are to have well prepared commissioning test procedures for review and acceptance by the interconnected utility to validate the collective capability and performance for the entire wind farm.

7.2.1 Test reactive power capabilities

This test can be used for compliance with NERC MOD-025 Reactive Capability Verification Reliability Standard.

Demonstrate that the plant can sustain its reactive power boost and buck capabilities for more than 15 minutes at a power output greater than 90%.

• The test can be done by raising the WPP voltage controller reference until reactive capability limits are reached. The test is suspended should another operational limitation reached first (e.g., due to high voltages), the limitation must be documented. The test needs to be coordi-



nated with the grid operator to ensure that there are no major outages in the area and to pro-vide a voltage profile favorable for the test.

• The test shall record active and reactive power during the test, voltages at point of interconnec-tion and the lower voltage class bus, status of shunt reactors and capacitors, and reactive power output from STATCOM if applicable

When an interconnection customer’s plant is required to meet certain power factor criteria at the POI such as between 95% bucking or leading to 95% boosting or lagging, start-up tests must be conducted beginning at 25% (if possible), 50%, 75%, and 100% of rated generator megawatt or real power load. This is to demonstrate that the switching and control of the reactive power can be done in small enough increments to limit the change in reactive power production or absorption in steady state of no more than 10% of the generated power. The capacity tests at the leading mode may be limited be-cause of operational limitations due to manufacturer’s design criteria or stator end iron heating con-cerns.

7.2.2 Dynamic Response Tests and Performance Monitoring

This test can be used for compliance with NERC MOD-026 Generator Excitation Verification Reliability Standard.

Provide evidence that the response time is within the required range and is consistent with the pro-vided model. The response can be validated for:

• changes in system voltages • steps in the WPP voltage reference• shunt capacitor switching within a plant

The tests shall provide the time recordings sampled at 20 times per second or faster for the following quantities:

• POI voltage• Medium voltage bus voltage• Controller voltage reference• Plant active and reactive power at the POI or voltage control point• Total dynamic reactive power provided by the plant including that by the installed devices such

as STATCOMs• Status of mechanically switched shunt capacitors and reactors

Dynamic test recordings shall be compared with those simulated via wind/solar power plant models.



PMUs or other monitoring devices with similar capabilities are to be used for dynamic performance monitoring. Details of PMU installation are described in [9]. PMU-based model validation can be used for compliance with NERC MOD-05, -026 and -027 Reliability Standards.

7.2.3 Harmonics Test

Harmonics test shall be done during a wind/solar power plant start-up. Refer to Section 6.1 Harmonics for details.

8 Other Issues

8.1 Harmonics

Harmonics is usually due to non-linear devices such as power electronic-based converters, which are used in wind/solar power plants. Harmonics can cause various problems including equipment over-heating and interference with sensitive load operation and communication systems. The amount of harmonics in a system is defined by the total harmonic distortion (THD).

8.1.1 Harmonics Study

It is common that a wind/solar farm connect wind/solar generators through underground medium voltage power cables. Therefore, the cable charging current and the capacitors existing at the terminal of a wind/solar generator result in certain amount of charging reactive power at a wind/solar power plant. When system or plant operating conditions change, such as shutdown of some wind generators, the charging reactive power could potentially match inductive reactive power and cause paralleled har-monic resonance, which could damage the devices in the wind/solar farm. When this risk exists, har-monics scan study using EMTP type tools shall be done under various operating conditions such as with different numbers of online/offline generators and/or energized/de-energized power cable collectors.

8.1.2 Harmonics Test

Harmonics test shall be done during a wind/solar power plant start-up.

The start-up wind/solar power plant harmonics test shall be done before the wind/solar power plant officially operates. Voltage and current harmonics from the generator shall also be measured and must fall within the required ranges. A power quality analyzer (provided by the Interconnection Customer) shall be used to monitor all three-phase currents, three-bus voltages, neutral current or generator neu-tral current, and an auxiliary contact from the Interconnection Customer’s generator breaker and also line breaker(s). The analyzer will have a minimum sample rate of 167 microseconds (128 points per cy-cle). The analyzer shall monitor the pre-breaker close conditions, the breaker closing, and the post-close conditions of the system.



8.1.3 Harmonics Requirements

IEEE Standard 519 shall be the minimum requirement for Interconnection Customer’s Generating Facil-ity plant to interconnect with the system. A Harmonics test should ensure voltage distortion and cur-rent distortion will meet the limits recommended in IEEE Standard 519.

The Reference of Voltage Distortion Limits and Current Distortion Limits from IEEE Standard 519:

Table 8-2: Voltage distortion limits

Bus voltage V at PCC Individual harmonic (%)

Total harmonic dis-tortion (THD) (%)

V ≤ 1.0 kV 5.0 8.01 kV < V ≤ 69 kV 3.0 5.069 kV < V ≤ 161 kV 1.5 2.5161 kV < V 1.0 1.5a

High-voltage systems can have up to 2.0% THD where the cause is an HVDC terminal whose effects will have attenuated at points in the network where future users may be connected.

Table 8-3: Current distortion limits for systems rated 120 V through 69 kV

Maximum harmonic current distortion in percent of ILIndividual harmonic order (odd harmonics)a, b

ISC/IL 3 ≤ h 11≤ h < 17 ≤ h < 23 ≤ h < 35 ≤ h ≤ TDD

< 20c 4.0 2.0 1.5 0.6 0.3 5.020 < 50 7.0 3.5 2.5 1.0 0.5 8.050 < 100

10.0 4.5 4.0 1.5 0.7 12.0100 < 1000

12.0 5.5 5.0 2.0 1.0 15.0> 1000 15.0 7.0 6.0 2.5 1.4 20.0

Common footnotes for Tables 8-2, 8-3, and 8-4:

a Even harmonics are limited to 25% of the odd harmonic limits above.

b Current distortions that result in a DC offset, e.g., half-wave converters, are not allowed.

c All power generation equipment is limited to these values of current distortion, regardless of actual Isc/IL.where

Isc = maximum short-circuit current at PCCIL = maximum demand load current (fundamental frequency component) at the PCC

under normal load operating conditions



Table 8-4: Current distortion limits for systems rated above 69 kV through 161 kV

Maximum harmonic current distortion in percent of IL

Individual harmonic order (odd harmonics) a, bIsc/IL 3≤ h

<1111≤ h < 17

17≤ h < 23

23 ≤ h < 35

35≤ h ≤50

TDD

< 20c 2.0 1.0 0.75 0.3 0.15 2.520 < 50 3.5 1.7

51.25 0.5 0.25 4.0

50 < 100

5.0 2.25 2.0

0.75

0.35 6.0100 < 1000

6.0 2.75

2.5 1.0 0.5 7.5> 1000 7.5 3.5 3.

01.25

0.7 10.0

Table 8-5: Current distortion limits for systems rated > 161 kV

Maximum harmonic current distortion in percent of ILIndividual harmonic order (odd harmonics)a, b

Isc/IL 3 ≤ h < 11

11 ≤ h < 17

17 ≤ h < 23

23 ≤ h < 35

35 ≤ h ≤ 50

TDD

< 25c 1.0 0.5 0.38 0.15 0.1 1.5

25 < 50 2.0 1.0 0.75 0.3 0.15 2.5≥ 50 3.0 1.5 1.15 0.45 0.22 3.75

8.2 Subsynchronous Resonance and Interactions

Subsynchronous Resonance (SSR) [5] is an electric power system condition where the electric network exchanges energy with a turbine generator at one or more of the natural frequencies of the combined system below the synchronous frequency of the system. A widely known SSR incident occurred at Mo-have Power Plant in the 1970s. SSR involves interaction between mechanical/torsional masses of a generator and a series capacitor. It is a well-understood phenomenon.

Subsynchronous Interactions (SSI) include Subsynchronous Torsional Interactions (SSTI) and Subsyn-chronous Control Interaction (SSCI). SSTI involves interactions between mechanical/torsional masses of a generator and a power electronic device such as a wind turbine. SSCI refers to interactions between a power electronic device such as a wind turbine and a series compensated system.

Although we have not experienced SSI issues in WECC, the incidence of SSI in 2009 at ERCOT also brought our attentions to this subject. Since then, a number of WECC members have done SSI studies for wind/solar power plants. So far, we have not found any issues in those studies.

To minimize the risks for SSI, we recommend the following best practices [1]:



• Modify controls of wind/solar turbine converter. This approach has already been demonstrated and proven at some wind plants.

• Avoid known grid configurations that cause subsynchronous interactions. This could involve transfer-tripping a wind/solar plant or bypassing a series capacitor if certain grid events occur.

• Add some damping in network for subsynchronous currents. This is most effective if installed at the series capacitor, but it could also be installed at a wind/solar plant.

9 Conclusions

We have summarized lessons learned and best practices based on experience from the WECC mem-bers regarding variable generation interconnection to the Bulk-Transmission System. Note that many of the issues are evolving as more variable generation is integrated into the existing generation mix, and new technologies and methodologies emerge. Our focus has been on planning and operations that reflect the latest developments and trends in the industry, particularly WECC.

10 References

[1] North America Electric Reliability Council, 2012 Special Assessment Interconnection Requirements for Variable Generation, September 2012

[2] WECC Wind Power Plant Power Flow Modeling Guidelines[3] WECC Wind Power Plant Dynamic Modeling Guidelines[4] WECC Variable Generation Planning Reference Book, May 14, 2013[5] IEEE SSR Working Group, Proposed Terms and Definitions for Subsynchronous Resonance, IEEE

Symposium on Countermeasures for Subsynchronous Resonance, IEEE Pub. 81TH0086-9-PWR, 1981, pp92-97.

[6] CAISO Frequency Response Issue Paper, August 7, 2015.[7] NERC TPL-001-4 Reliability Standard, http://www.nerc.com/files/TPL-001-4.pdf[8] Western Wind and Solar Integration Study Phase 3 – Frequency Response and Transient Stability,

NREL and GE, http://www.nrel.gov/docs/fy15osti/62906-ES.pdf[9] Technical Requirements for Interconnection to the BPA Transmission Grid, page 32, https://www.b-

pa.gov/transmission/Doing%20Business/Interconnection/Documents/tech_requirements_inter-connection.pdf



Report Contributors

WECC Variable Generation Interconnection Task Force (VGITF) Chair

George Zhou

WECC Support Staff

Nathan Powell

Contributors

Dmitry Kosterev, Ann Finley, Shengli Huang, Steven Pai, Song Wang, David Tovar, Irina Green, Jun Wen, Aldridge Madeleine, David Wang, Robert Easton, Eric Heredia, John Anasis, Jonathan Trejo, Roberto Favela, George Zhou

Reviewers

This report was reviewed by the WECC VGITF members and the WECC Planning Coordination Committee (PCC) members.
