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Siemens Wind Power Pty Ltd 885 Mountain Highway,Bayswater, VIC 3153,
Australia
www.siemens.com/wind
Hornsdale 2 – Frequency ControlAncillary Service Trial Knowledge
Sharing Report
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Siemens Wind Power Pty Ltd 885 Mountain Highway,Bayswater, VIC 3153,
Australia
www.siemens.com/wind
Document History
Revision A Structure for commentRevision B Advanced DraftRevision C Editorial review and comments addressed
where possible
Document Review
Author Tristan Rayson-HIllAEMO reviewer Michael Bagot
Mark ThompsonNEOEN/GHD reviewer Jennie GaterARENA reviewer John DiesendorfReleased
References
Ref Document description Document name/Source[R 1] Hornsdale 2 Releasable User
GuideE-AU00020-R1-D-SP-20160331-HornsdaleWind Farm_RUG .pdf
[R 2] National Electricity Rules(Version 104)
http://www.aemc.gov.au/Energy-Rules/National-electricity-rules/Rules/CurrentRules
[R 3] Wind Power Supervisor usermanual
WPS_User_ Manual_V_312.pdf
[R 4] Hornsdale 2 Interim FCAS trialreport
Hornsdale 2 WF_FCAS Test results interimreport R0 20171113 Draft.pdf
[R 5] Model Description and Blockdiagrams
Technical_Description_of_SWTVD4_Reduced_Order_Model_Ver_03.pdf
[R 6] Market Ancillary ServiceSpecification V50
http://www.aemo.com.au/-/media/Files/Electricity/NEM/Security_and_Reliability/Ancillary_Services/Market-Ancillary-Service-Specification-V50--effective-30-July-2017.pdf
[R 7] Contingency FCAS frequencytrajectories supplied by email
Frequency contingency FCAS frequencytrajectories.xls”,
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from Ian Devaney (AEMO) toDaniel Gallagher (Siemens) byemail on 25 July 2017
[R 8] Frequency Injection Tool usermanual
WpsSignalInjection2Tool_User_Manual_V305.pdf
[R 9] Mainland Frequency OperatingStandards
https://www.aemc.gov.au/sites/default/files/content/c2716a96-e099-441d-9e46-8ac05d36f5a7/REL0065-The-Frequency-Operating-Standard-stage-one-final-for-publi.pdf
[R 10] WECC Wind Power Plant PowerFlow ModellingGuide
https://www.wecc.biz/Reliability/WECC%20Wind%20Plant%20Power%20Flow%20Modeling%20Guide.pdf
[R 11] DIgSILENT PowerFactoryTechnical Reference -Documentation AC VoltageSource ElmVac
TecRef_VoltageSource.pdf
[R 12] Hornsdale Stage 2 GeneratorPerformance Standard
Hornsdale Wind Farm 2 – GPS.pdf
[R 13] FCAS Verification Tool http://www.aemo.com.au/-/media/Files/Electricity/NEM/Security_and_Reliability/Ancillary_Services/EXTERNAL-MASS-50-FCAS-Verification-Toolv209.xlsx
[R 14] HPPP Functional Desription Functional Description - HPPP.pdf[R 15] Technical regulation 3.2.5 for
wind power plants above 11 kWhttps://en.energinet.dk/-/media/Energinet/El-RGD/El-PBU/Dokumenter/LVT-MDA---Tekniske-forskrifter/Engelske-tekniske-forskrifter/Engelske-tekniske-forskrifter---grid-connection/TR-3_2_5/Technical-regulation-325-for-wind-power-plants-above-11-kW--revision-4.PDF?la=en
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Siemens Wind Power Pty Ltd 885 Mountain Highway,Bayswater, VIC 3153,
Australia
www.siemens.com/wind
Table of Contents1. Executive Summary ........................................................................................................7
2. Introduction .....................................................................................................................7
3. Description of the FCAS Market Trial ..............................................................................9
3.1. Regulation Services .................................................................................................9
3.2. Fast Services ...........................................................................................................9
3.3. Slow Services .........................................................................................................10
3.4. Delayed Services ...................................................................................................10
4. FCAS Trial Objectives ...................................................................................................10
4.1. FCAS Test Plan .....................................................................................................11
4.1.1. Regulation Services ........................................................................................11
4.1.2. Contingency Services ......................................................................................13
4.2. FCAS modelling .....................................................................................................15
5. Changes Required for Hornsdale 2 to provide FCAS in the NEM ..................................15
5.1. Siemens Gamesa High Performance Park Pilot (HPPP) ........................................16
5.2. Changes required to control strategy to meet the requirements of the MarketAncillary Services Specification (MASS) ...........................................................................18
5.3. Technical and Communication Upgrades to receive AGC signals ..........................19
6. Testing Methodology .....................................................................................................21
6.1. Test Descriptions....................................................................................................21
6.1.1. Test 1 ..............................................................................................................22
6.1.2. Standard Ramps .............................................................................................23
6.1.3. Extreme Frequency Event Profiles ..................................................................24
6.1.4. Actual High Frequency Event ..........................................................................25
6.1.5. Actual Low Frequency Event ...........................................................................26
6.2. Test Matrix .............................................................................................................27
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6.3. Frequency Injection tool .........................................................................................27
6.4. Adaptation of the test profiles for frequency injection ..............................................28
7. Modelling methodology..................................................................................................30
7.1. Set up in Power Factory .........................................................................................31
7.2. Injection of frequency Profiles in to Power Factory .................................................32
8. Analysis .........................................................................................................................35
8.1. Comparison of Simulation and Test results ............................................................35
8.1.1. Differences between the field test setup and actual operation of the wind farm35
8.1.2. Issues impacting the ability for Hornsdale Stage to provide Fast raise FCAS ..35
8.1.3. Post Fault Reactive power control ...................................................................40
8.1.4. Zero order hold and the injection tool ..............................................................41
8.1.5. Frequency Injection Tool implementation ........................................................41
8.1.6. Operation ........................................................................................................43
8.1.7. Internet and Software Communication Delay...................................................44
8.1.8. Poling and wind farm delays ............................................................................44
8.2. N-1 Analysis ...........................................................................................................49
8.3. Speed of the FCAS response from HWF ................................................................49
8.4. Performance characteristics of HWF in providing FCAS in South Australia,specifically in instances when SA islands from the NEM ...................................................50
8.5. Learnings and observations such as how the definitions of the services might bevaried to better utilise wind generator capabilities in frequency control .............................50
8.6. Learnings as to whether FFR capabilities (beyond the current MASS specifications)might or might not be delivered by WTGs. ........................................................................53
9. Further work required beyond the trial to allow Hornsdale 2 to offer FCAS Services on apermanent basis ...................................................................................................................53
10. Conclusion .................................................................................................................53
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List of figures
Figure 1 - Frequency Droop..................................................................................................13Figure 2 - HPPP functional diagram .....................................................................................17Figure 3 - Step injection test .................................................................................................22Figure 4 - MASS standard frequency ramp for raise services ...............................................23Figure 5 - MASS standard frequency ramp for lower services ..............................................24Figure 6 – Extreme Frequency Event Profiles.......................................................................25Figure 7 - Actual High Frequency Event ...............................................................................26Figure 8 - Actual Low Frequency Event ................................................................................27Figure 9 - Hornsdale 2 rate of change of frequency withstand capability ..............................29Figure 10 - Power Factory Single Line Diagram representation of Hornsdale Stage 2 ..........31Figure 11 - Voltage source control model frame ...................................................................32Figure 12 - Voltage source controlled by external load flow controller [R 11] ........................33Figure 13 - block definition of the input signals .....................................................................34Figure 14 - FCAS assessment combined frequency and short circuit event .........................36Figure 15 - FCAS active power response .............................................................................37Figure 16 - Overview of wind turbine control relationships ....................................................38Figure 17 - FCAS response without combined short circuit event .........................................39Figure 18 - MVA optimisation ...............................................................................................40Figure 19 – Frequency Injection Tool Implementation ..........................................................41Figure 20 - Frequency Injection Tool Active ..........................................................................42Figure 21 - HPPP (Wind Farm Controller) Response to injection tool ...................................43Figure 22 - Time delays modelled in power factory ...............................................................45Figure 23 - effect of zero order hold (1 second sample rate) .................................................46Figure 24 – Effect of zero order hold (0.2 second sample rate) ............................................47Figure 25 - Example of a 1 second sample of a standard frequency ramp from Figure 4 ......48Figure 26 - Stepping response from power factory ...............................................................49Figure 27 - Spinning Reserve ...............................................................................................51Figure 28 - parameterisation of the Danish Grid Code frequency response ..........................52
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Siemens Wind Power Pty Ltd 885 Mountain Highway,Bayswater, VIC 3153,
Australia
www.siemens.com/wind
1. Executive SummaryThis report has been produced to share the experience of preparing for and conducting theAEMO Frequency Control Ancillary Services (FCAS) Trial. It details the steps taken toprepare the wind farm for provision of FCAS over and above what was anticipated in theoriginal project scope.
The report also details the steps taken to produce results from testing and simulationsrequired to satisfy AEMO that inverter connected wind turbines can successfully provideFCAS in accordance with the requirements of the Market Ancillary Services Specification(MASS). This is currently unique as it will be the first inverter connected generation toprovide frequency control ancillary services to the NEM.
Siemens Gamesa also detail some of the anomalies and work a round’s and fixes that wereimplemented to allow the trial to proceed and improvements that could be implemented in thefuture to allow Hornsdale 2 greater participation in FCAS.
Finally this reports details the lessons learnt that might provide insight for other participantshoping to offer wind farms for FCAS.
2. IntroductionThe Hornsdale 2 Wind farm comprises of 32 Siemens Wind Power Direct Drive 3.2 MW windturbines. Each wind turbine comprises of a permanent magnet generator and is connected tothe grid via a full scale converter and is therefore “inverter connected generation” for short.
The reason for converter/inverter connection is driven by three main considerations:
1. the desire for efficiency;2. the desire for grid code compliance especially with respect to voltage control; and3. desire to be able to operate in the widest possible range of conditions.
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Efficiency
The desire for efficiency is driven by wanting to extract as much aerodynamic energy aspossible. This requires that the blades rotate at the optimum speed to take advantage of theirparticular aerodynamic properties. This efficiency varies with wind speed and rotor speed.
Grid Code Compliance
Fundamentally the physical speed of an a.c. generator has to deliver an electrical output thatmatches the power system frequency of the grid. The varying optimum physical speed of awind generator rarely suits direct coupling to grid frequency. However a range offunctionalities can be implemented to deliver grid compliance and ancillary services if thegenerator and the grid are decoupled and recoupled at grid frequency by a converter system.These capabilities include voltage control, control during grid faults (often called fault ridethrough), and frequency control.
Range of Conditions
The goal of any wind turbine is to extract the maximum amount of wind energy at any givenwind speed notwithstanding the variable prevailing ambient conditions. This is best achievedby variable speed operation with smart converters.
Inverter Connected Generation and Inertia
In conventional synchronous machines the voltage and current waveforms are generated bya magnetically excited rotating mechanical shaft creating magnetic field and exciting thesurrounding generator windings. This gives the conventional synchronous generator a storeof kinetic energy in the rotating mass. The resistance to change of mechanical speed isinertia of the rotating machines whose speed is locked to the synchronous grid frequency.This energy is immediately available to resist grid frequency changes.
Variable speed wind generators also have rotating machines with inertia but their energy isnot automatically and instantly available as with synchronous machine inertia. With inverterconnected generation the current waveform is synthesised by the inverter using a feedbackloop sensed from the measured voltage waveform from the grid. This is often referred to asthe “phase lock loop (PLL)”. As the variable speed generator is effectively decoupled fromthe fluctuations of grid frequency, inverter connected generation is crudely referred to as“inertia-less”.
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Some grid operators insist on inverter connected generation “synthesising” inertia byaccessing the kinetic energy of the wind turbine rotor. Consequently the rotor slows.Subsequently some of the wind energy must be used to restore the wind turbine to itsoptimum speed, briefly reducing the energy output of the wind turbine.
3. Description of the FCAS Market TrialThe goal for Hornsdale 2 wind farm in the FCAS trial was to demonstrate participation ineach of the FCAS markets. The wind farm was set up to respond to grid frequency changesby raising active power in situations where the grid frequency was low and lowering activepower when grid frequency was high.
3.1. Regulation ServicesRegulation services are enabled to manage changes in frequency within the normaloperating frequency band following small deviations in the demand/generation balance withinthe five minute dispatch interval. These are controlled centrally by AEMO. AEMO monitorspower System Frequency and time error, and instructs generating units or loads enabled toprovide regulation services through the AGC system.
The AGC system allows AEMO to continually monitor System Frequency and send controlsignals to Ancillary Service Facilities providing regulation services so frequency is maintainedwithin the normal operating frequency band of 49.85 Hz to 50.15 Hz. These control signalsalter the megawatt (MW) output of the generating units or the consumption (MW) of the loadsto correct the demand/generation imbalance.
3.2. Fast ServicesThese services are designed to provide the initial support for system frequency after anevent that causes system frequency to deviate from the normal operating band. They aredesigned to raise or lower active power output to quickly bring the system frequency backinto its normal range. Once delivered the active power output is required to be sustained for60 seconds.
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3.3. Slow ServicesSlow services must be delivered over 60 seconds and then sustained for 5 minutes after thesystem frequency deviation. These services are designed to provide a sustained response toan event that causes a system frequency deviation.
3.4. Delayed ServicesDelayed services are to assist in frequency recovery over a longer period of time and mustbe activated after 5 minutes of the power system frequency outside the normal operatingband. The service must be sustained for 10 minutes after activation.
4. FCAS Trial ObjectivesThe overall objective of the trial is to:
· model, implement and test the capability of the Hornsdale 2 WF to be remotelycontrolled by AEMO and to provide all 8 Frequency Control Ancillary Services (FCAS)traded in the National Electricity Market,
· to register for the supply of those services in the NEM,· to successfully complete a 48 hour trial period of bidding and operating in the FCAS
markets.To also identify technical or regulatory barriers are identified that restrict the ability of theHornsdale 2 WF to comply with the Market Ancillary Services Specification (MASS) and fulfilall requirements to gain registration, the proponents are required to demonstrate that theyhave used their best endeavours to register and a more limited trial period will be agreed withAEMO. Any deficiencies in the current Rules, specifications or processes will be identifiedand documented.
Siemens Gamesa provided detailed modelling of the response characteristics of the windfarm and provided output information which both demonstrated a response compliant withthe MASS and provided benchmarks for the test program.
The overall objectives of, and the intended Outcomes for, the Activity are as follows:
· to model, implement and test the capability of Hornsdale 2 WF to be remotelycontrolled byAEMO to provide Frequency Control Ancillary Services (FCAS);
· to determine the types of FCAS for which the Hornsdale 2 WF can have itsgenerating units classified in accordance with NER 2.2.6;
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· to successfully complete a 48 hour trial of bidding and operating in the FCAS marketsfor which Hornsdale 2 WF can be registered, or where Hornsdale 2 WF cannot beclassified, successfully complete a market simulation trial.
· For the avoidance of doubt, successful completion of the trial means that theRecipient has fulfilled its obligations under the Detailed Test Plan;
· to determine the delayed response time and the accuracy of Hornsdale 2 WF'sresponse to the regulation set-point changes;
if technical or regulatory barriers are identified that restrict the ability of HWF2 Pty Ltd tocomply with the Market Ancillary Services Specification (MASS) or to fulfil all requirementsfor AEMO to classify Hornsdale 2 WF’s generating units as ancillary services generatingunits under clause 2.2.6 of the Rules, the Recipient will be required to demonstrate that it hasused its Reasonable Endeavours to fulfil the compliance and registration requirements and toprovide feedback facilitating a review of the MASS, if needed. Opportunities to amend theRules, specifications or processes to facilitate or enhance delivery of FCAS from wind farmsand other renewable energy sources will be identified and documented in such feedback;and to document and share the results of the Activity in accordance with the KnowledgeSharing Plan required under this Agreement.
4.1. FCAS Test Plan
4.1.1. Regulation ServicesThis test demonstrates the plant’s ability to follow AEMO’s AGC dispatch signals. Thepurpose of AGC is to enable the power plant to follow the active power set point dispatchedby AEMO at the end of every 4-sec time interval.
AEMO will conducted the test at three different wind resource intensity scenarios
(1) rated output;
(2) intermediate output (i.e. shoulder of power curve) with available power to increaseover the subsequent 15 mins; and
(3) intermediate output (i.e. shoulder of power curve) with available power to decreaseover the subsequent 15 mins.
Each test will provide appropriate selected, actual 4-second AGC signals that AEMO haspreviously sent to a regulation certified resource of similar nameplate capacity. It is proposedthat AEMO would measure the accuracy of a resource’s response to EMS signals during 15
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min intervals by calculating the ratio between the sum of total 4-sec set-point deviations andsum of AGC set points.
(1) Rated output
During a period where wind resource available is at a level equivalent to rated poweroutput, the plant would be instructed to operate within a real power range of 7 MW belowits rated power output capability. About 10-minutes of actual 4-second AGC signalswould then be fed into the plant’s controller and the plant’s response would be monitored.
(2) Intermediate output, increasing
During a period where wind resource available is at a level equivalent to the shoulder ofthe plants power curve and expected to increase, the plant would be instructed to operatewithin a real power range of 7 MW below a setpoint less than rated power as agreed withAEMO. About 20-minutes of actual 4-second AGC signals would then be fed into theplant’s controller and the plant’s response would be monitored.
(3) Intermediate output, decreasing
During a period where wind resource available is at a level equivalent to the shoulder ofthe plants power curve and expected to decrease, the plant would be instructed tooperate within a real power range of 7 MW below a setpoint less than rated power asagreed with AEMO. About 20-minutes of actual 4-second AGC signals would then be fedinto the plant’s controller and the plant’s response would be monitored
ExpectationDuring the test, AEMO will monitor the delayed response time of the plant (i.e. the timebetween the resource receiving a control signal indicating a change in set point and theinstant the resource’s MW output changes). AEMO will also monitor the accuracy ofplant’s response to the regulation set-point changes. The data from this test will be usedby AEMO to assess generation technology specific Regulation responses in the contextof broader obligations to maintain frequency in the NEM and potential changes that maybe necessary to AEMO’s internal systems for management of power system frequency.
CurtailmentIt is expected that the plant would be curtailed by 7 MW for about 45 (3x15 min) minutes.
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4.1.2. Contingency ServicesThe contingency frequency response capability would entail two separate test (1) a drooptest, and (2) a frequency response test.
The definition of implemented frequency droop control for a wind farm is the same as thatfor conventional generator:
%%
PowerDroopfrequencyD
=D
The plant should adjust its power output in accordance to the droop curve with symmetricdeadband shown inFigure 1 - Frequency Droop. The upper limit of the droop curve is theavailable plant power based on the current level of air density and ambient temperature.
Figure 1 - Frequency Droop
1. Frequency Droop Test ObjectiveThe objective of this test is to demonstrate that the plant can provide a response inaccordance with 5% and 3% droop settings through its governor-like control system. Theplant would be instructed to operate below its maximum capability during both tests.
Test ProcedureFor the first test, the plant would be instructed to operate at 14 MW below its maximumcapability. This test would be done using a 5% droop and a dead band of ± 0.05 Hz.
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AEMO would test the frequency droop capability of the plant by using an actual under-frequency events that have occurred in the NEM. The under-frequency event data set(approximately 10 minutes of data) would be fed into the plant’s controller and the plantresponse would then be monitored.
The frequency droop capability would be demonstrated using one actual high frequencytime series dataset provided by AEMO. Examples of under and over frequency eventtime series measured by AEMO are shown in Figure 7and Figure 8 respectively
The frequency event time series data will be used by the power plant controller to triggerthe droop response by the plant
The above test would be repeated with the plant at 14 MW below its maximum capability.This test would be done using a 3% droop and a dead band of ± 0.05 Hz.
Expectation
Through the action of the governor-like control system, the plant must respondautomatically in proportion to frequency deviations outside the dead band.
Curtailment
It is expected that the plant would be curtailed by 14 MW for approximately 60 minutes.
2. Capability to provide frequency response ObjectiveThe objective of this test is to demonstrate that the plant can provide contingencyfrequency response.
Test ProcedureThe plant would be instructed to operate 14 MW below its maximum capability beforeapplying a step-change of rapid frequency decline. An actual frequency event(approximately 10-minutes) would be fed into the plant’s controller and the plant’sresponse would be monitored. This test may require tuning to ensure the frequencyresponse is consistent with the requirements of the 6 second contingency servicesdefined in the MASS following the step-change in frequency.
The plant does not have headroom and can only reduce output in response to highfrequency deviation below scheduled frequency of 0.05 Hz. The test would entail feedingthe plant controller with a frequency greater than 0.05 Hz above scheduled frequency.
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Repeat the above test with the plant operating at 28 MW below its capability for a givenlevel of wind resource availability.
ExpectationThrough the action of the governor-like control system, the plant must respondautomatically in proportion to frequency deviations.
CurtailmentIt is expected that the plant would be curtailed by 14 MW for 60 minutes and 28 MW for60 minutes.
4.2. FCAS modellingThe model of the Hornsdale 2 wind farm used simulate the responses to the systemfrequency profiles is shown in section 6.1. Siemens Gamesa investigated using thesimulation program PSS/E which is widely but not exclusively used by AEMO to conductpower system simulations. However the simulation techniques used to produce the requiredresults have only become mature in the very latest versions of PSS/E and the uncertainty inthe models and engineering effort to run the way it was required meant that SiemensGamesa chose to run the simulations in Power Factory instead.
Power factory has the advantages that the communication between the grid model and thewind farm model are more visible and easier to manage.
The Siemens Gamesa Power Factory model and the Siemens Gamesa PSS/E model aredeveloped from the same control block diagrams [R 5] and the model behaviour is identical.
5. Changes Required for Hornsdale 2 to provide FCAS in theNEMHornsdale Stage 2 wind farm in its original form was not envisaged to participate infrequency control markets. However as with all modern wind farms it is equipped with acentralised controller that has the ability to coordinate the responses of the wind farm.
The Siemens Gamesa product that acts as a wind farm controller is called the HighPerformance Park Pilot. It is in this controller that the changes are required to beimplemented.
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5.1. Siemens Gamesa High Performance Park Pilot (HPPP)To meet the challenges of different grid codes, grid conditions and to achieve the mostrobust control outcomes Siemens Gamesa provide a highly configurable wind farm controller.The HPPP contains a group of flexible dynamic controls for active power, reactive power,voltage control and frequency control. Naturally the frequency control is the controller ofinterest for the FCAS trial.
Figure 2 shows the structure and the controllers that make up the HPPP.
The frequency control contains a specialised subset of active power controls designed toassist the grid maintain frequency within the normal operating band. The system frequencyreflects the balance between the active power generation and the active power consumptionin the power system. When providing service to assist in maintaining this balance, controllersare endevour to maintain this balance. Controllers are required to inject active power whenthe frequency falls and reduce active power when the frequency rises.
The HPPP senses the deviations in frequency and can be configured to respond quickly tochanges in frequency by injecting or reducing active power as required. The frequency signalis monitored continuously and provides an input into the closed loop frequency controlsystem. Ramp rates can also be configured to determine the speed of response.
The HPPP frequency controller supports various operating modes for responding tofrequency the mode chosen for the FCAS trial is “Frequency Sensitive Mode (FSM)”. In FSM,Hornsdale Stage 2 is capable of responding to under and over frequency events. Although torespond to under frequency events the wind farm needs to be operating at a generation levellower than the available power. For example, if 100 MW of power is available to ensure thatthe wind farm can provide 20 MW of FCAS the active power would need to be curtailed to 80MW.
The HPPP also has a spinning reserve function that provides a constant offset to theavailable generation.
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WTG2
WTG1
WTG3
GridActive Power
Controller
ELSPEC
Reactive PowerController
VoltageController
FrequencyController
WPSWPS DB WPS ModbusSlave Access
HPPP ModbusSlave Access
OPC Access IEC104 Access
Figure 2 - HPPP functional diagram
For the FCAS trial the controller of interest is the Frequency controller.
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5.2. Changes required to control strategy to meet therequirements of the Market Ancillary Services Specification(MASS)
The table below describes each of the parameters that can be used to adjust the HPPPactive power response to frequency events.
Table 1 - HPPP frequency control parameters
Parameter DescriptionMode Generic mode selection
0 = disabled1 = over frequency2 = under frequency3 = under and over frequency
Curve Selection 1 = curve 12 = curve 2
Target Frequency 50 HzRamp up rate Ramp rate limitation applied on changes in scheduled
power during frequency eventsRamp down rate Ramp rate limitation applied on changes in scheduled
power during frequency eventsPower ramp down rate Ramp rate limitation applied to the output of the
frequency functionPower ramp up rate Ramp rate limitation applied to the output of the
frequency functionFrequency high wait If the measured frequency exceeds this value the
output of the frequency function should not be allowedto increase until the frequency gets below FrequencyHigh Ramp up.
Frequency high ramp up Threshold frequency for allowing power reference toramp up after the frequency has exceeded the valuein Frequency High Wait.
Freeze frequency If the frequency goes from being smaller than to beinghigher than the freeze frequency, the output of thefrequency function is capped to the last measuredpower.
Freeze cancellation frequency When the frequency drops below the freezecancellation frequency value, the power is allowed toramp up again.
Curve mode 0 = Absolute generic1 = Droop generic2 = Danish grid code3 = Great Britain4 = Eire grid code5 = F4
Target frequency droop offset The scale at the target frequencyDefinition curve A point is defined by the values Power, Frequency
and Droop.
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These parameters do not include the other dedicated active power controls that can be seenin Figure 2.
For FCAS registration and trial the key parameters were
Table 2 - Updated HPPP parameters
Parameter DescriptionMode Generic mode selection
3 = under and over frequencyCurve Selection 1 = curve 1Target Frequency 50 HzCurve mode 1 = Droop genericTarget frequency droop offset 3.33 see also 4.1.2
5.3. Technical and Communication Upgrades to receive AGCsignals
The following technical changes were required for Hornsdale Wind Farm stage 2 to receivethe regulation FCAS signals from the AEMO Automatic Generation Control (AGC):
· programming the new databases and operational functionality;· update the wind farm remote terminal unit (RTU) to allow marshalling of the incoming
control signals);· update the ElectraNet Energy Management System (EMS) database;· Include the new functionality in the wind farm supervisory control and data acquisition
system (SCADA) including graphical functionality and database;· Update historian system and historian database;· simulation of all signals and factory acceptance testing; and· site acceptance testing and end to end testing of signals received from AEMO;
Given that this is a trial that was not envisaged by Siemens Gamesa or NEOEN, somefurther changes must be made if Hornsdale 2 wind farm is to participate in the FCAS marketson an ongoing basis.
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These changes are:
· Updates to the NEOEN control room energy management system (EMS) to allow theregulation services to be activated and the wind farm to start responding to AGCsignals;
· Changes would need to be made to the wind farm SCADA interface to allow thechange in mode within the HPPP that is currently only accessible by SiemensGamesa; and
· And a corresponding change to the NEOEN control room SCADA to send a signal tothe wind farm SCADA to enable contingency and regulation FCAS services.
In future these services will need to be considered as part of the scope of work in theexternal control systems in the conceptual design phase.
During the trial no ramp rate data or requirement was considered pre or post activation of thefrequency sensitive mode. These controls are not part of the frequency controller and can becontrolled via the active power control system. Siemens Gamesa notes that this was not partof the control methodology of the FCAS trial but could be implemented.
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6. Testing MethodologyNoting that these are pre-registration tests that were done to demonstrate the control systemengineering was capable of providing the services required, The testing has been brokendown into 5 separate tests:
· Step injections to verify frequency response dead-bands and the test signal;· Standard frequency ramp injections;· Extreme event ramp injections;· Actual under and over frequency event injections; and· Regulation up/down tests.
6.1. Test DescriptionsEach of the tests were conducted at three active power levels, 44 MW 74 MW and 88 MW todetermine the amount of contingency FCAS serviced that could be obtained provided thatthere was sufficient headroom. Each of the tests purposes and test profiles is presentedbelow.
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6.1.1. Test 1
Figure 3 - Step injection test
This test is designed to test the frequency dead-band settings as described in Figure 1. Theresponse to the test is also an indicator that the wind farm is in the correct operationalsetting.
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6.1.2. Standard RampsThis test is used to determine how much raise service is provided in the conditions defined inthe MASS. The ramp is defined by AEMO in Figure 4. This characteristic is used todetermine how much FCAS is offered in the market systems.
Figure 4 - MASS standard frequency ramp for raise services
This test is used to determine how much lower service is provided in the conditions definedin the MASS. The ramp is defined by AEMO in Appendix A and reproduced in Figure 5. Thischaracteristic is used to determine how much FCAS is offered in the market systems.
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Figure 5 - MASS standard frequency ramp for lower services
6.1.3. Extreme Frequency Event ProfilesThe objective of this test is to to assess the extent of of Hornsdale Stage 2 wind farm ability toprovide frequency support in response to extreme events.
Under frequency credible event and non-credible event profiles have been supplied by AEMOThe credible event profile has an initial rate of change of frequency (RoCoF) of -1Hz/s, thenon-credible event profile has an initial RoCoF of -3Hz/s. These test frequency profiles areapplied and the active power response recorded to assess the raise services.
Figure 6 displays these profiles in graphical form.
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Figure 6 – Extreme Frequency Event Profiles
6.1.4. Actual High Frequency EventThe purpose of this test is to examine the response to a real event and evaluate thefrequency support performance of the wind farm during a real event. This would also allow acomparison of performance against similarly enabled FCAS providers.
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Figure 7 - Actual High Frequency Event
6.1.5. Actual Low Frequency EventThe purpose of this test is to examine the response to a real event and evaluate thefrequency support performance of the wind farm during a real event. This would also allow acomparison of performance against similarly enabled FCAS providers.
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Figure 8 - Actual Low Frequency Event
6.2. Test MatrixFor each of the tests there were three different active power levels and a number of the testshad different frequency ramp requirements. The complete matrix of tests is shown in Table 3
Table 3 – test matrix
TestNo. Test Description Power Level1 Frequency Step 44 MW 74 MW 88 MW2 Standard Ramp (high) 44 MW 74 MW 88 MW3 Standard Ramp (low) 44 MW 74 MW 88 MW4 Extreme Frequency Event 1 Hz/s (high) 44 MW 74 MW 88 MW5 Extreme Frequency Event 1 Hz/s (low) 44 MW 74 MW 88 MW6 Extreme Frequency Event 3 Hz/s (high) 44 MW 74 MW 88 MW7 Extreme Frequency Event 3 Hz/s (low) 44 MW 74 MW 88 MW8 Actual Event (high) 44 MW 74 MW 88 MW9 Actual Event (low) 44 MW 74 MW 88 MW
6.3. Frequency Injection toolThe Wind Power Supervisor (WPS) frequency injection tool is a web based tool that wasoriginally intended for qualified technicians to check the performance and verify thefrequency controls and response were set up correctly. The tool has been extended beyondits original purpose for the Hornsdale 2 FCAS trial.
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The injection tool takes data that has been formatted in xml and mimics the frequency inputsignal in the HPPP eliciting a response to the new frequency signal. This tool can only injector mimic grid frequency signals.
6.4. Adaptation of the test profiles for frequency injectionTo conduct the testing the profiles described in section 6.1 needed to be converted into xmlcode. There is also a limitation that the frequency injection tool will only update the frequencyin the HPPP every second. This had implications for especially the extreme frequency profiletests in section 6.1.3. At a rate of frequency change of 3 Hz/s the frequency injected to theHPPP would be 47 Hz for an extreme non credible frequency event would exceed theextreme frequency excursion tolerance limit as set out in the frequency operating standard[R 9]. This does not impact on the accuracy or the validity of the test.
This is also the limit that is required by the Hornsdale Stage 2 Generator performancestandard. In the minds of Siemens Gamesa it was prudent to stay within the frequencyoperating standard and accept that the profile injected would have a frequency gradientslightly less than 4 Hz/s.
Figure 9 shows the RoCoF withstand characteristic for the SWT 3.2 DD 113 wind turbine.
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Figure 9 - Hornsdale 2 rate of change of frequency withstand capability
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7. Modelling methodologyThe goal of the simulations is to replicate the frequency injection tests conducted onHornsdale 2 wind farm. To do this a following mathematical representations are required:
· grid as an aggregated representation with a varying power frequency is required;· an aggregated model of the wind farm turbines with all the associated control
algorithms;· an aggregated model of the wind turbine step up transformers;· an aggregated model of the wind farm collector network; and· a model of the wind farm main transformer.
All the equivalence values are calculated in accordance with [R 10].
Power system modelling is inherently time consuming therefore the following elements thatplay no role in the frequency response have not been included in the modelling effort:
· harmonic filters;· Hornsdale Stage 1; and· Hornsdale Stage 3;
Although the adjacent wind farms share the same connection point they will not respond tothe changes occurring at Hornsdale Stage 2.
To replicate the changes in frequency the voltage source modelled at the Connection Point iscontrolled via an input signal. Sometimes referred to as a “play back” signal. These modellingresults were provided as a part of the registration process to verify that the engineeringcapability tests and were undertaken prior to the Market Trial.
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7.1. Set up in Power Factory
Figure 10 - Power Factory Single Line Diagram representation of Hornsdale Stage 2
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7.2. Injection of frequency Profiles in to Power FactoryThe control of the frequency in the grid model is done via a DigSilent Simulation Language(DSL) model frame shown in Figure 11.
Figure 11 - Voltage source control model frame
A closer look at the intermediate block in Figure 13 shows the implementation of the controlover the voltage source from an external file.
The voltage source can be used to control the active power, reactive power or the voltageangle at a busbar. To do this, it is necessary to define a load flow controller model and selectit as a voltage magnitude controller and/or a voltage angle controller.
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Figure 12 - Voltage source controlled by external load flow controller [R 11]
The output signal of the load flow controller is automatically connected to the voltage setpoint signal, uctrl, or to the voltage angle set point signal, phictrl. These two internal inputsignals are used to define the positive sequence voltage set point:
( ) ( )( )1 cos sinset nomU uctrl U phictrl dphiu j phictrl dphiu= × × + + × + (1.1)
The zero- and negative sequence voltage set points are similarly derived. This description isfrom [R 11].
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Figure 13 - block definition of the input signals
Below is an example of the input file:
The input file can only contain text and spaces, the number 2 refers to the number of inputcolumns, the first column is time, the second column is voltage in per unit and the thirdcolumn is per unit frequency.
25 1.04 16 1.04 1.00257 1.04 1.0058 1.04 1.00759 1.04 1.0139 1.04 1.0169 1.04 1.0199 1.04 1.01129 1.04 1.01159 1.04 1.01189 1.04 1.01
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219 1.04 1.01249 1.04 1.01279 1.04 1.01309 1.04 1.01339 1.04 1.01369 1.04 1.01399 1.04 1.01429 1.04 1.01459 1.04 1.01489 1.04 1.01519 1.04 1.01549 1.04 1.01579 1.04 1.01609 1.04 1.01639 1.04 1.01669 1.04 1.01699 1.04 1.01700 1.04 1.0075701 1.04 1.005702 1.04 1.005703 1.04 1.0025704 1.04 1734 1.04 1
8. Analysis
8.1. Comparison of Simulation and Test results
8.1.1. Differences between the field test setup and actual operation of the windfarm
To conduct the FCAS trial of the contingency services the only difference between thenormal operation and the trial set up is that the frequency injection tool “highjacks” thefrequency input to the wind farm controller the HPPP [R 8]. The park pilot continues torespond to the other grid variables independent of the frequency.
With respect to the regulation services there are no differences between the trial and normaloperation except the introduction of the AGC signal into the wind farm SCADA as describedin section 5.3.
8.1.2. Issues impacting the ability for Hornsdale Stage to provide Fast raiseFCAS
Description of the issue:
To assess the ability of Hornsdale 2 wind farm to provide contingency services AEMOrequire that the impact of a combined frequency event and short circuit event be assessed,
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and the projected FCAS amounts are then assessed on this basis. A simulated example ofthis event is shown in Figure 14. The critical active power response is shown in Figure 15.
Figure 14 - FCAS assessment combined frequency and short circuit event
The active power response (Figure 15) shows that after the combined frequency and shortcircuit event the active power from the wind farm reduces from 74 MW to 47 MW beforerecovering and beginning to provide frequency support. The response that is obtained fromthe wind farm without the short circuit event is shown in Figure 17.
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Figure 15 - FCAS active power response
Impact on the provision of FCAS:
The impact of the active power reducing is that the Hornsdale Stage 2 wind farmdemonstrates no contribution to FCAS for the six second fast raise service (R6). For theavoidance of doubt: the wind farm can provide fast raise services in the absence of a voltageevent.
Explanation for this response:
The first obligation for any wind farm is to stay connected and the obligation to do this is setout in detail in the Hornsdale Generator Performance Standards [R 12] particularly S5.2.5.4and S5.2.5.5. The second obligation is to support the network voltages during faults and sofar as possible restore the surrounding network to a stable operating state.
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The fundamental constraint is the amount of energy available from the wind to achieve thesegoals this control optimisation is shown in Figure 18. There is also a requirement for stableoperation.
I d&
I q
Figure 16 - Overview of wind turbine control relationships
The control system design of the Siemens Gamesa therefore has a control block to managethese constraints, it is referred to as the “Active Current Controller”.
Description of the Active Current Controller
The network bridge active current controller operates to control the active power exported tothe network. The power demand is determined either by the Wind Turbine Control (WTC)power controller or the fault ride through power limiter controller.
In normal operation, the network bridge active current controller controls the quadrature axis(Q-axis) network current based on the power demand, and the system voltage magnitude,during network faults, a new power limit, is calculated by the power limit controller and thisreduced power reference is used to determine the Q-axis current reference.
In normal operation the power limit is greater than the power demand and the fault ridethrough is inactive therefore the Q-axis network current is calculated by dividing the powerdemand by the system voltage.
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During a voltage dip the fault ride through (FRT) sequence is initiated. The maximum valueof Q-Axis current is then determined by the selection of the appropriate overload limit. Thesevalues represent the maximum allowable converter Q-axis current, and equivalent activepower during a fault.
Furthermore, during a fault the turbines will ignore signals from the HPPP and optimise thewind turbine controls to support network voltage and stay connected to the network.
Figure 17 - FCAS response without combined short circuit event
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Generator
MVA
Pmax
MVA
NormalOperation
MVA
Qmax
Grid Fault
Qmax
Pmax
Figure 18 - MVA optimisation
8.1.3. Post Fault Reactive power controlPost fault reactive power control depends on the voltage control droop set in the HPPP andthe voltage control set-point. The HPPP has a two levels of control:
· Responding to its droop setting depending on the relative position of the voltagecontrol set point and the measured grid voltage.
· In turn the voltage control sends new voltage control set-points to each turbine, andthe turbine level voltage control uses a PI controller to fix the voltage set-point at theturbine level.
It should also be noted that these controls are deliberately much slower than the FRTcontrols, and respond over some seconds.
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8.1.4. Zero order hold and the injection toolAlthough the WPS frequency injection tool (see section 6.3) works well it has somelimitations for doing this type of testing. The issues and their resolution and or explanationare described below. The main cause is an artefact of the discrete signal and limitationsupdating the frequency signal.
8.1.5. Frequency Injection Tool implementation
Figure 19 – Frequency Injection Tool Implementation
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Figure 20 - Frequency Injection Tool Active
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8.1.6. Operation
Figure 21 - HPPP (Wind Farm Controller) Response to injection tool
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8.1.7. Internet and Software Communication DelayThe injection tool communicates with the target HPPP via an internet data link, this creates ashort time delay between the HPPP receiving, de-coding and acting upon the signal. At thispoint the amount of delay in the communication between them is unclear.
8.1.8. Poling and wind farm delaysThe HPPP frequency controller takes some time to respond and calculate new set-points andis modelled as Td_input (communication time delay) with a value of 0.125 seconds. Thisvalue has been developed from wind farm testing. The wind farm also takes some time tosend new set-points to each turbine in a wind farm. This is modelled as Td_Psp_Park and ismodelled as 0.2 seconds this is mostly due to the time that is taken to pole each turbine inthe wind farm. There are also some measurement delays due to the required signal filteringand calculation of the frequency from voltage zero crossing measurement.
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Figure 22 - Time delays modelled in power factory
Digital to analogue conversion and zero order hold
The accuracy that a waveform can be replicated depends on the rate at which the waveformis sampled. There is an example below in figures Figure 23 and Figure 24.
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Figure 23 - effect of zero order hold (1 second sample rate)
0 1 2 3 4 5 6 7 8 9 10-10
-8
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0
2
4
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8
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Figure 24 – Effect of zero order hold (0.2 second sample rate)
Unexpected delay in the wind farm response
An example of what the HPPP receives from the frequency injection tool is shown inFigure 25. This accounts for the offset response from the wind farm.
Stepped response from the test results
An example of what the HPPP receives from the frequency injection tool is shown inFigure 25. This accounts for the stepped response from the wind farm. If the input file for thevoltage source model in Power Factory is made to be more representative of the injectionsignal it received (overcoming the linearization automatically done by the program) during thefield testing the results in Figure 26 are obtained.
0 1 2 3 4 5 6 7 8 9 10-10
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0
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Figure 25 - Example of a 1 second sample of a standard frequency ramp from Figure 4
0 1 2 3 4 5 6 7 8 9 10
49.4
49.5
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Figure 26 - Stepping response from power factory
8.2. N-1 AnalysisThe analysis of the response to a frequency event in combination with a short circuit eventshows a disappointing outcome, when assessed against the FCAS assessment tool the windfarm does not provide any fast raise service in this circumstance. The reasons for this areoutlined in section 8.1.2. As the simulation model is an accurate representation of the controlsystems and strategies in the actual turbine, Siemens Gamesa has no reason to question theoutcome of the simulations.
However this model response has never been validated.
8.3. Speed of the FCAS response from HWFCurrently the maximum rate of response from the wind farm is 21.5 MW/s set as anachievable value in the HPPP.
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8.4. Performance characteristics of HWF in providing FCAS inSouth Australia, specifically in instances when SA islands fromthe NEM
To the knowledge of Siemens Gamesa no events that islanded the South Australian regionfrom the NEM have been the result of a combination of frequency and voltage events,Siemens Gamesa would therefore propose that the combination of such events be excludedfrom the assessment of the capability to provide FCAS. Siemens Gamesa expects that thewind farm would perform as tested and as modelled. It is expected that the greatercompetition and service provision from this technology would be of assistance in anycircumstance.
As demonstrated in Figure 9 the wind turbines can remain connected in a wide range offrequency conditions. The Siemens Gamesa technology is able to meet the requirements ofthe minimum and automatic standards.
8.5. Learnings and observations such as how the definitions ofthe services might be varied to better utilise wind generatorcapabilities in frequency control
The main observation from a technical perspective is whether the standard droop control isadequate to provide those services. There are a range of functionalities and modes that havenot been implemented in this trial. The HPPP is highly configurable and could be shaped toprovide support where there are gaps in the FCAS landscape.
There is also a “spinning reserve” characteristic that maintains an offset between theavailable active power and the amount of enabled frequency support.
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OffsetΔ
P
Figure 27 - Spinning Reserve
Some investigation could also be done to investigate complementary technologies, forexample the use of wind generation with synthesised inertia in combination with hydrogeneration enabled for FCAS.
An example of the configuration options available from the Danish Grid Code [R 15] is shownin Figure 28. The Danish Grid Code requires this characteristic in be enabled in wind farms.It is parameterised in Table 4.
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Table 4 – DK grid parameters
Parameter NameFDKUnderFrequencyRatio
FDKUnderFrequencyDeviationFDKUnderFrequencyLimit
FDKOverFrequencyDeviationFDKOverFrequencyRatioFDKOverFrequencyLimit
FTargetFrequency
Figure 28 - parameterisation of the Danish Grid Code frequency response
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8.6. Learnings as to whether FFR capabilities (beyond the currentMASS specifications) might or might not be delivered by WTGs.
Siemens Gamesa believes there is a role for wind farms providing FFR. Converter connectedgeneration is capable of providing very rapid changes in power output and an FFRcharacteristic could easily be defined and implemented.
As discussed in section 8.1.2 wind farms would have to be given some derogation forproviding this kind of service should there be concurrent short circuit events.
9. Further work required beyond the trial to allow Hornsdale 2 tooffer FCAS Services on a permanent basisThe original operational concept for Hornsdale Stage 2 did not consider participating in theFCAS markets. To continue operating in the FCAS markets some upgrades to thecommunication and control systems are required for practical reasons. These reasons are:
· flexibility, the wind farm needs to be able to select the mode it operates in;· agility, the market operator needs to be able to take advantage of changing market
conditions; and· reliability, operator at this point does not have the ability to set the wind farm to the
correct modes seamlessly.
As described in section 5.2 some of these changes have been made, however the missinglink is for the bidding control room to have access to the frequency modes that are available.
10. ConclusionSiemens Gamesa has worked through the FCAS trial and detailed each of the challengesthat were faced and attempted to answer each of the questions that were raised. SiemensGamesa also believe the FCAS trial demonstrated the ability of converter connectedgeneration to participate in the FCAS markets with few limitations.
The most complex issue to resolve is the participation of wind generation in the fast raiseFCAS market. Some optimisation may be possible to improve the performance of the windfarms in this regard. It may also require a relaxation by AEMO of the requirement for the windgeneration to provide R6 service where there is a concurrent voltage and frequency events.
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Siemens Gamesa has also looked into the impacts on the wind turbines themselves todetermine if there is any extra wear and tear while providing FCAS or if any extra service andmaintenance is required.
To date the data back from locally and internationally is inconclusive and anecdotal. Siemenswill continue to into the maintenance requirements but at the time of writing has nothing toreport.
