Improvement of PGU Simulation Models based onFRT Test Rig with adjustable Voltage

Vector and Short-Circuit PowerRainer Klosse, Karsten Kuch, Jorg Jahn, Julius Gerdes

WindGuard Certification GmbHOldenburger Str. 65

26316 Varel, GermanyEmail: r.klosse@windguard.de

Abstract—This paper shows influences on the behaviourof power generating units due to variations in the gridimpedance. In order to investigate these effects, an alternativetransformer based test equipment has been developed thatallows for simultaneous voltage vector magnitude and anglejumps. A complex transformation ratio is defined to proveelectrical equivalence of the test systems in combination withadditional impedances. It is assumed, that simulation modelscan be improved by including additional measurements intothe validation process which are not part of the prototypemeasurements yet.

I. INTRODUCTION

Riding through voltage dips without tripping is one of themost crucial requirements for distributed generators (DG)such as wind energy converters (WEC). In order to getmeaningful results out of fault-ride-through (FRT) testing,the test equipment has to reproduce real FRT conditions asadequate as possible. Otherwise, successfully riding throughthe tests would not necessarily result in successful ride-through events in ”real” fault situations and system stabilitymay be harmed.

Testing – especially when it comes to field tests – isalways a compromise, because the test equipment has to betransportable (thus robust), affordable and should have a lowimpact on the upstream network. The first test equipment thatfulfilled these requirements was based on inductive voltagedivision and is still state of the art nowadays. This kind oftest equipment will be referred to as standard test equipmentin this paper.

In the last years in Germany a couple of grid situationsoccurred, which were followed by a subsequent andsignificant loss of DG. This led to more attention regardingthe conditions during FRT events. As a consequence FRTrequirements in the German grid codes [3] were specifiedmore precisely.

This paper deals with some effects that have beenobserved during several years of measurements – mainlythe influence of short-circuit power and phase angle jumps.These effects influence the test results of low-voltage ridethrough testing but depend on the site specific grid situation.Simulation models that validate these measurement data maynot show realistic values in deviating grid situations. Thesevereness of these possible deviations should be consideredin future discussions.

II. EXPERIENCES GAINED FROM SEVERAL YEARS OFMEASUREMENTS

FRT testing are standard procedures within the windindustry since many years. Nowadays these measurement areapplied to different power sources such as photovoltaics orcombined heat and power systems. The following sectionswill show examples for deviating behaviour of powergenerating units (PGU) connected to the grid, based on bothinverters or electrical machines. Firstly the conditions duringgrid faults will be extended by two more variables, that is,the short-circuit power and the phase angle of the voltagevector.

A. Effects of sudden changes of the grid impedance

The effect of sudden changes of the grid impedanceZgrid (which for simplification reasons is already included inZseries) does not just occur while utilising the standard testequipment. “Real” grid faults such as short-circuits, split upthe grid impedance as well, resulting in a sudden change ofthe magnitude of the remaining Zgrid between the point ofconnection (POC) of the DG and the fault location. Sincethe R/X-ratio differs between the voltage levels this “split”of Zgrid will also result in a varied R/X-ratio.

So two effects (in addition to the reduced voltage) occurduring grid faults for the downstream connected DG:

1) The different magnitude of Zgrid will result in a changeof short-circuit power.

2) The difference in the R/X-ratio will force the voltagevector of the remaining voltage to a angle jump. Thejumps sometimes have a magnitude of more than 30◦.

The FRT-stability of a network is highly dependent on thecapability of the power generating units (PGU) to handlethese uncommon, however, sensitive grid situations. Botheffects are currently not part of the standard testing of DGand, thus, not part of the EUT’s simulation model.

B. Influence of short-circuit power variations

Regarding inverters, the output filter typically producesa short current spike on fault-entry. Figure 1 shows asymmetric low-voltage ride through on an inverter basedgenerator including a current spike of appr. 10 times thecurrent’s nominal value.

With exception of the short-circuit power, the generator ismeasured under the same measurement conditions as shown

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Figure 1. Inverter based generator during LVRT test at a connection pointwith high short-circuit power. Note the current spike of 10 · Ir.

in Figure 2. The different short-circuit power completelyavoids the current spike. While in Figure 1 the ratio of theshort-circuit power to the generator’s apparent power is 3 inFigure 2 this ratio is 33. So both measurements fulfill therequirements of FGW TR3. [1]

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Figure 2. Inverter-based generator during LVRT test at a connection pointwith low short-circuit power

Different behaviour cannot only be observed on fault-entry but on voltage recovery as well. Figure 3 shows thebehaviour of an inverter-based generator on voltage recovery,while the available short-circuit power is high. The inverterimmediately starts to ramp up current and, thus, power again.

Figure 3. Inverter based generators during voltage recovery, high SSC

Figure 4 shows the same inverters on voltage recoveryto the same kind of voltage drop. The available short-circuitpower is significantly lower than in the first case. In contrastto Figure 3, in Figure 4 the inverters’ current control is notworking properly and a power ramp up cannot be identified.

Figure 4. Inverter based generators during voltage recovery, low SSC

The standard test equipment (see section III for details)requires a large series impedance in order to limit theshort-circuit current ISC when switch S2, see Figure 7, isclosed. The available short-circuit power at the terminals ofthe Equipment Under Test (EUT) is significantly reduced.Additionally to the jump in the short-circuit power SSC,another effect occurs, that is described in the followingsection.

C. Influence of vector angle jumps

As described in the previous section, the series impedanceof standard test equipment causes a significant reduction inSSC. On the other hand, this impedance influences the R/Xratio at the terminals of the EUT. So the series impedancedoes not just influence the magnitude of the voltage dropacross the grid impedance (from the EUT’s point of view).In addition, it influences the phase angle of the voltage dropresulting in a vector angle jump in the terminal voltage.

Figure 5 shows a LVRT test sequence using standardtest equipment on an electrical machine. In the second plotof Figure 5 it can be seen that the active power in thepositive sequence Ppos shows oscillations every time whenthe switching state of S1 or S2 changes. It should be notedthat the largest magnitude of this oscillation occurs when thebypassing switch S2 is closed again.

Applying different vector angle jumps leads to differentoutput currents. In Figure 6 the current response of anelectrical machine is shown during a 3-phase LVRT test.In the upper plot (77 % residual voltage) no vector anglejump is applied. In the middle and lower plots (both 70 %residual voltage) a vector angle jump of appr. 19◦ laggingand leading is applied. On the one hand, the maximumamplitude of the current response is lower when no vectorangle jump is applied. On the other hand the times when themaximum/minimum values of the damped oscillations occuris different and the maximum values are higher.

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Figure 5. Test sequence voltage drop to 60 % residual voltage includingopening and closing of by-passing switch S1. Upper plot: Positive sequencevoltage. Middle plot: Real, reactive and apparent power. Lower plot: Angledrift in relation to fixed 50,0 Hz.

Figure 6. Current response of an electrical machine during a 3-phasevoltage dip. Upper plot: without additional vector angle jump, 77 % residualvoltage. Middle: vector jump 19◦ lagging, 70 % residual voltage. Lower:vector jump 19◦ leading, 70 % residual voltage.

III. STANDARD TEST EQUIPMENT

The standard test equipment for testing low-voltage-ride-through events is based on the principle of inductive voltagedivision. In IEC 61400-21 [2], regarding field-testing ofwind turbines, it is described with the explicit indicationthat it is an example. But especially regarding field testingwind turbines this principle is still state of the art. Thistest rig consists of two inductances, Zseries and Z fault. Zseriesis combined with a bypassing switch S1 while Z fault iscompleted with a series switch S2, see Figure 7.

Figure 7. Conventional test equipment

During normal operation (NOP) S1 is closed, S2 is openand the equipment under test (EUT) is not confronted withdeviating conditions. The test procedure starts by openingS1 and closing S2 subsequently. When S2 is closed, thefault event (FEV) is started. Zseries will limit the short-circuitcurrent that will flow from the grid while S2 is closed. Theratio between Zseries and Z fault will determine the residualvoltage for the EUT.

IV. INTERMEDIATE CONCLUSION

The effects described in the previous section imply thatthere are more effects than the depth of the voltage dip thatshould be considered during LVRT measurements.

During 3-phase voltage drops the following effects can beobserved in theory:

• The voltage dip during the fault will cause on fault-entry and fault clearance a jump in the voltage vector.This jumps affect both magnitude and angle.

• The vector angle jump will be leading or laggingdependent from the jump of the R/X-ratio betweennormal and fault operation.

• The resulting grid impedance during the fault is not onlydepending on the parallel connection of Zseries + Zgridwith Z fault. It will depend on the impedance betweenthe junction to Z fault and the terminals of the EUT. Thisimpedance includes e. g. transformer and cabling.

• EUTs tend to be tested on (too) weak grids so thatcapacitor discharging effect are disregarded.

• If the ratio of the nominal power of the test equipmentand the EUT is high the stability of the EUT will notbe tested adequately.

During unsymmetrical (e. g. 2-phase) voltage dips, inaddition, the following effect occurs:

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• The vector group of the transformer(-s) causefundamental changes on the components of the voltagevector at the terminals of the EUT.

It has been shown so far that there are more influencingparameters during grid faults. The standard test equipmentis not able to give proper consideration to all of the abovementioned effects. However, these effects have a stronginfluence on measurement results and, thus, the design ofsimulation models of the EUT. So it should be questionedif at least some effects could be taken into account usingalternative test equipment.

In the next sections, on the one hand, will be shown that itis possible to configure the transformer based test equipmentequally to the standard test equipment or simplified gridmodels. On the other hand it will be shown that thetransformer based test equipment is capable of additionalfeatures.

Two switching states will be considered (no-loadcondition):

1) Pre- and post-fault conditions, (normal operation,index NOP):Standard test equipment: Switches S1 and S2 are open,see Figure 7Transformer based equipment: Initial transformationratio, see Figure 13

2) Fault event (index FEV):Standard test equipment: Switch S1 is open while S2

is closed, see Figure 10Transformer based equipment: Altered transformationratio, see Figure 14

V. TRANSFORMER BASED TEST EQUIPMENT

Transformer based test set-ups are not widely spread butsome test laboratories use this kind of equipment since manyyears.

Mainly the transformer based test equipment consistsof a transformer which offers as many taps as necessaryon the secondary winding. By switching between the tapsthe transformation ratio is changed and voltage jumps areapplied to the EUT, see Figure 8.

Figure 8. Schematic diagram of transformer based test equipment [4]

A. Advantages of autotransformer based test equipment

One of the main advantages in comparison tothe conventional test equipment are reduced systemperturbations to the upstream network since it is not loadedwith short-circuit current during the voltage drops [4]. Theavailable short-circuit power SSC can be applied to the EUTand varied by additional series impedances.

Besides low-voltage ride through events, high-voltage ridethrough events can be easily achieved. So requirements thathave been defined in up-to-date grid codes can be fulfilled.

Since occurring currents and, hence, losses are lowtransformer based equipment is more efficient. So LVRT, orHVRT events, respectively, can be applied for longer timesthan usual. The maximum ride through times of EUTs canbe easily determined.

Utilising additional winding sections a large variety oftransformations can be realised. Tests can be carried outon low-voltage level since vector diagram that are result ofthe vector group of the EUT’s transformer can be directlymodeled by the test setup

B. Additional options of transformer based test equipment

When modifying equations 7 and 8 one can see, thattransformer based test equipment is able to produce testsituations that cannot be achieved in real (and operating)grids. But by choosing adequate values, worst-case scenarioscan be applied to the EUT.

VI. COMPARISON BETWEEN STANDARD ANDTRANSFORMER BASED TEST EQUIPMENT

A. Thevenin equivalent of standard test equipment

Figure 9. Standard test equipment during normal operation (NOP),switching state 1

Figure 10. Standard test equipment during fault condition (FEV), switchingstate 2

Both states can be simplified into their Theveninequivalent circuits with a resulting voltage source and aseries impedance.

UEUT,NOP = U 0,NOP!= U 0 (1)

ZNOP = Zseries + Zunit (2)

Impedances Zseries and Zunit are without effect on theresult of equation 1 due to no-load condition of Figure 9.

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UEUT,FEV = U 0Zseries

Zseries + Z fault(3)

ZFEV =Zseries · Z fault

Zseries + Z fault+ Zunit (4)

Figure 11. Thevenin equivalent circuit during normal operation, switchingstate 1

Figure 12. Thevenin equivalent circuit during fault condition, switchingstate 2

During no-load condition the test rigs are configuredin a way that a certain ratio V between both voltages –representing the depth of the voltage dip – is reached.

V =UEUT,FEV

UEUT,NOP=

Z fault

Zseries + Z fault(5)

It should be kept in mind that the voltage vector jumps incase that the R/X-ratio of Z fault is different to the R/X-ratioof Zseries and, thus, this ratio represents a complex value.

B. Thevenin equivalent for transformer based test equipment

Since the standard test equipment is considered as stateof the art every alternative test equipment has to prove thattest conditions are at least equal to the standard equipmentor even closer to real grid situations.

Following the approach of section VI-A, two switchingstates of the transformer based test equipment areconsidered. Figures 13 and 14 present these two switchingstates using the example of an autotransformer with tertiarywinding.

The ratio V is defined by the no-load voltages of the twoswitching states.

V =U 0,FEV

U 0,NOP(6)

Figure 13. Autotransformer during normal operation (NOP), switchingstate 1

Figure 14. Autotransformer during fault condition (FEV), switching state 2

The impedances Zseries,T and Zunit,T have to be chosen insuch way that they are equal to the values of the Theveninequivalents represented by equations 7 and 8.

Zseries,T =ZNOP − ZFEV

1− V 2 (7)

Zunit,T = ZNOP − Zseries,T (8)

VII. CONCLUSION & OUTLOOK

From the shown examples it can be seen that withincreasing series impedances, the tendency towards anoscillation or instability of the power generating unitincreases. These effects can be triggered by voltage vectorjumps. In electrical grids, vector angle jumps related tovoltage dips are rather the rule than the exception. Highshort-circuit powers in the other hand, stimulate, amongstothers, discharge effects that may result in significant currentspikes. Using conventional test equipment, short-circuitpower variation test can only barely be performed, as itreduces available short-circuit power due to required seriesimpedances. Models of power generating units are utilisedto verify, inexpensively, the unit on reliable behaviour, forgrid situations not yet tested. The question remains whetheror not a simulation model is sufficiently validated, if certainbasic functionalities had never been tested by measurement.

The presented measurement data is the result ofmeasurements utilising an autotransformer basedtest systems with nominal power of 11 kW. Theseautotransformer-based test systems offer multiple additionaloptions to perform tests that are not part of standardprototype testing yet. The electrical equivalence to standardtest equipment has been proven. In the next steps, test haveto be carried out on PGUs with higher rated power.

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ACKNOWLEDGEMENT

The authors would like to thank Project ManagementJulich and the German Federal Ministry of EconomicsAffairs and Energy as well as the Federal EnvironmentAgency for funding (project number 0325721). Furthermore,the authors would like to thank the project participantsSMA Solar Technology AG, GE Wind Energy GmbH andEWE Netz GmbH for their support. Special thanks go tothe Institute for Electrical Drives, Power Electronics andDevices (IALB) of the University of Bremen and WRDGmbH, System Innovation for offering their laboratories forvalidation measurements.

REFERENCES

[1] Fordergesellschaft Windenergie und andere Erneuerbaren Energien,FGW e.V., Technical Guidelines for Power Generating Units, Part3: “Determining the Electrical Properties of Power Generating Unitsconnected to Medium-, High- and Extra-High-Voltage Grids”, TR3,Rev. 23, 01.05.2013.

[2] IEC 61400-21:2008, Wind turbines - Part 21: Measurement andassessment of power quality characteristics of grid connected windturbines, 2nd ed., 2008.

[3] Verband der Elektrotechnik, Elektronik, Informationstechnik e. V.(VDE), VDE-AR-N 4120, Technische Bedingungen fur den Anschlussund Betrieb von Kundenanlagen an das Hochspanungsnetz (TABHochspannung), Berlin, Germany: VDE-Verlag, 2015.

[4] J. Jahn, F. Santjer, R. Klosse Fault-Ride-Trough tests based on low-voltage measurements and their feedback to the medium voltageequipment, DEWEK, Bremen, Germany, 2010.

[5] R. Klosse High-Voltage-Ride-Through Test System based onTransformer Switching, The 12th International Workshop onLarge-Scale Integration of Wind Power into Power Systems as well ason Transmission Networks for Offshore Wind Power Plants, London,England, 2013.

[6] R. Klosse, F. Santjer Fault Ride Through Test based on TransformerSwitching, The 8th International Workshop on Large-Scale Integrationof Wind Power into Power Systems as well as on TransmissionNetworks for Offshore Wind Power Plants, Bremen, Germany, 2009.

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