Fault Ride-through Capability Test Unit for Wind Turbines

Transcription

1 WIND ENERGY Wind Energ. 2008; 11:3 12 Published online 7 November 2007 in Wiley Interscience ( Research Article Fault Ride-through Capability Test Unit for Wind Turbines Juan Carlos Ausin* and Daniel Navarro Gevers, Research and Development Department, Gamesa I&T, Ciudad de la Innovacion 9 11, Sarriguren (Navarra), Spain Björn Andresen, Research and Development Department, Gamesa Wind Engineering ApS, Vejlsøvej 51, DK-8600 Silkeborg, Denmark Key words: fault ride-through capability (FRT); voltage dip; field test; grid code certification; wind turbine modelling; simulation and validation The GAMESA voltage ride-through capability test unit was designed as a tool for voltage dip studies for different wind turbine (WT) configurations and to investigate specific grid code requirements. By generating a voltage dip at the WT terminals, the WT manufacturer is able to check that the equipment works according to the design specifications, fulfils the grid code requirements and can validate the simulation models. This paper presents a description of the unit, the methodology used in the field tests and the results of the study. Copyright 2007 John Wiley & Sons, Ltd. Received 20 November 2006; Revised 23 August 2007; Accepted 3 September 2007 Introduction During the last few years, there has been a special interest on the grid integration of wind turbines (WTs). 1 This has lead to a re-definition of the grid connection conditions by the system operator (SO) in different countries. 2,3 The behaviour of WTs during voltage dips has been one of the main subjects, topic of many research activities and the main concern of most grid operators during these years. Modern grid codes include fault ride-through capability (FRT) requirements similar to the curve plotted in Figure 1. This curve is obtained from a list of requirements from a set of international grid codes including E.ON Netz (Germany), Red Eléctrica de España (REE-Spain) and Federal Energy Regulatory Commission (FERC-EEUU). The curve covers the critical voltage drop where disconnection from the grid is not allowed. In order to help the WTs to ride through these grid events, manufacturers have been forced to introduce new hardware components and new control strategies in the WT. 5 On the other hand, simulation studies may be useful for evaluating the WT control strategy and the impact on the network. Frequently, simulations are also used by the SOs to evaluate the performance of the wind farm/turbine under different fault conditions. It is clear that the control strategies and the network stability simulations may only be trusted if the WT models are validated against real measurements. For the validation of the generator + converter system, normally, test benches are used since the test can be performed and repeated under well-defined conditions. However, these tests do not take some of the real components in the WT into account. These components have a very important influence in the WT response during and after the fault. Test benches frequently do not include, e.g. the inertia of blades and gearbox, the behaviour of the pitch regulation, the electrical back-up system (UPS), the mechanical oscillations in the drive train or the protection settings in the switchgear and in the control system. On the contrary, on-field tests include all the relevant variables, and therefore, they reproduce the real behaviour of the whole WT. * Correspondence to: J. C. Ausin, Technical Responsible, Ibaia Energia, C/JM Iturrioz 26-Beasain, (Guipuzcoa), Spain jcausin@ibaiaenergia.com Copyright 2007 John Wiley & Sons, Ltd.

2 4 J. C. Ausin, D. Navarro Gevers and B. Andresen Figure 1. International voltage ride-through requirements International Standards and Grid Codes for Voltage Dips On-site Testing In order to have some well-defined test and measurement conditions, the IEC describes in the new draft version 6 some test requirements for the voltage dip testing (Table I). The voltage profile to be tested is defined for off-line conditions, i.e. when the turbine under test is disconnected from the grid, and therefore, does not contribute to modify the voltage shape. The IEC also includes the calculation methodology for the positive sequence of the fundamental components of voltage and current. By using the results of these calculations in the proposed equations, active and reactive power values are estimated. 6 For other specific requirements or more detailed assessment of the simulation models, other tests and measurements can optionally be carried out. 7 The type of testing included in all these engineering recommendations can be used as a type test/approval for the WT. However, grid code requirements are usually requested at the point of connection where the full wind farm exports power to the grid. In order to perform an approval for a whole wind farm, real tests are not feasible, and therefore, it would be necessary to perform simulation studies. 8 In these simulations, all internal Table I. Test requirements for the voltage dip testing according IEC Case Magnitude of voltage phase to phase (fraction of voltage before the dip) Magnitude of voltage (positive sequence) Duration (s) VD1 symmetrical three-phase 0,90 +/ 0,05 0,90 0,5 +/ 0,05 voltage dip VD2 symmetrical three-phase 0,50 +/ 0,05 0,50 0,5 +/ 0,05 voltage dip VD3 symmetrical three-phase 0,20 +/ 0,05 0,20 0,2 +/ 0,05 voltage dip VD4 two-phase voltage dip 0,90 +/ 0,05 0,95 0,5 +/ 0,05 VD5 two-phase voltage dip 0,50 +/ 0,05 0,75 0,5 +/ 0,05 VD6 two-phase voltage dip 0,20 +/ 0,05 0,60 0,2 +/ 0,05 Shape

3 Fault Ride-through Capability Test Unit 5 grid characteristics (cable, transformer, short circuit level, etc.) contribute to change the voltage shape seen at the WT terminals. The final voltage dip shape is therefore site-specific. Moreover, the different grid codes have different additional requirements for the voltage ride-through capability (FRT). Some of them require riding through symmetrical faults only while others include requirements for unsymmetrical faults also. Some of them require measuring the voltage at the point of connection while others require the voltage to be measured at the generator terminals (see Figure 1). In order to be able to fulfil most of the existing grid codes, international regulations or engineering recommendations, it is very useful to have a configurable and flexible tool that is able to test all kind of voltage dip with little or none hardware modification. This is the main objective of GAMESA RTC equipment. Description of the Test/RTC Test Unit Container The GAMESA RTC test unit (Figure 2) was designed to provoke a voltage dip at the WT terminals, while the network conditions remain almost unchanged for the rest of the connected turbines and network users. In order to follow most of the grid codes and obtain realistic results, the test unit was designed to reproduce all kind of symmetrical faults. The test unit can also be used to study the behaviour of the WT in the event of an inverse voltage sequence caused by asymmetrical voltage dips as required in some of the grid codes, and the measurement and assessment standards. 6,7 The voltage dip test is performed at the LV side of the WT transformer (690 V). This allows the use of the RTC for tests at different sites and in countries with different medium voltage level. In order to see the balancing effects of the Dy 20 kv/690 V turbine transformer, an additional Dy 690 V/690 V transformer is included in the RTC unit. The short circuit is produced by closing the switchgear that connects to ground the fault impedance. This generates a current to the ground that produces a voltage drop proportional to the impedances rate value. The fault impedance (magnitude and angle) is designed to be as realistic as possible in order to reproduce the effect of a fault produced on the transmission system. The grid side impedance is designed in order to limit the short circuit power at the fault point, to approximately K-times the generator nominal power. The K factor is a security factor to ensure that the short circuit does not affect the rest of the wind farm turbines and network users while keeping the turbine operating in a sustainable grid scenario. As long as the grid is strong enough to accommodate the wind farm, the value of the grid impedance can be considered independent of the network characteristics, and therefore independent of the country and site. Figure 2. Pictures from the RTC test unit container

4 6 J. C. Ausin, D. Navarro Gevers and B. Andresen There are other possible configurations to generate the voltage dip. For example, it can be generated at the medium instead of at the low voltage side, and it can be produced by means of resistances or combination of resistances and reactances with similar results. The proposed RTC configuration was considered the most cost-efficient and flexible solution. Flexibility of the RTC Unit One of the main features of the RTC is the flexibility of the design with the objective of reproducing any kind of voltage dip curve and failure type with few software changes. The bypass function allows operating the turbine when no tests are being made, without dismounting the container (see Figure 3). The RTC unit is able to perform a wide set of voltage drop values and durations for any of the phases: it can reproduce three-phase, two-phase and single-phase faults. 9 The change from one voltage profile to another can be done by easy changes to the proper switches at the command panel (see Figure A1). The switches can be selected according to different requirements from the utilities, the measurements institutes or the customers. Figure 3. Single-line diagram of the short circuit test Test Results The RTC test unit has been used for testing the GAMESA prototypes in La Plana wind farm, located in the north of Spain. Several symmetrical and unsymmetrical voltage dips have been tested, and the response of the WT has been registered in detail. The flexibility of the RTC unit allows programming the value of the voltage dip change in steps of around 20%. The duration of the event can be programmed in the range from 0,05 up to 3 s, which depends on the size of the connected turbine and the fault type (two or three phase). Furthermore, the voltage recovery shape can be changed to reproduce a slow response time. Table II includes the possible deep and time limits used for the G52 (WT of 850 kw) and G80 (WT of 2 MW).

5 Fault Ride-through Capability Test Unit 7 Table II. Maximum allowed short circuit time (s), versus voltage drop deepness Drop deep (phase to ground) G52 Two-phase short circuit (s) Single- or three-phase short circuit (s) G80 Two-phase short circuit (s) Single- or three-phase short circuit (s) 80% (320 V) 1,1 1,0 0,6 0,5 60% (240 V) 3,3 3,0 1,1 1,0 40% (160 V) 3,3 3,0 1,1 1,0 20% (80 V) 2,5 2,0 1,1 1,0 15% (60 V) 2,5 2,0 1,1 1,0 The performance of the RTC and the response of the WT are detailed in the following plots. In Figure 4, the response of the WT under a three-phase voltage drop can be seen. The RTC was programmed to produce a drop up to 0,35 pu for 500 ms. The voltage waveform, the RMS value, the calculated active and reactive power are shown. All variables were plotted in per-unit values and were measured at the fault point. After about 100 ms, a slight voltage recovery was observed due to the injection of reactive current as required by the E.ON and REE grid codes. The RTC test unit was installed at the site close to the tower base. The unit was placed in a 40 ft length container. All necessary test equipment, UPS systems, cabling, measurement connections, etc., were placed inside the container. This configuration makes it easy to transport the equipment to different test sites. The electrical configuration is shown in the single-line diagram in Figure 3. Figure 5 shows a three-phase voltage drop and the recovery in three intermediate steps in approximately 1,2 s. The measured active and reactive powers are also shown. A quick and equilibrated response of the RTC can be seen in the voltage plot. Figure 6 shows the response of the WT under a two-phase voltage dip. After the event, the WT remained stable and recovered the initial conditions in a very short time. Riding through asymmetrical voltage dips is a present requirement in some grid codes, and therefore, this is an essential feature of the RTC. The results obtained by the RTC were used to demonstrate and certificate the RTC of GAMESA WT and to validate the models that are sent to the SOs for their transient stability analysis. Simulation Models Validation The stability studies intend to demonstrate that the stability of the network, when subjected to short circuits located at different locations, is not affected by the connection of the wind farm. SOs require, as a part of the wind farm interconnection agreements, to provide a validated mathematical model to represent the behaviour of the wind farm in network stability studies. Of course, these studies will reflect the real behaviour of the wind farm only if the simulation models are validated against real measurements. Currently, there is no standard model validation procedures accepted by SO, and it is frequently the WT manufacturers who establish their own validation procedure and guarantee that the model behaves as the real WT during a short circuit. In order to validate the models, a similar characteristic as the on-field test scenario must be simulated. The network can be easily simulated by means of the Thevening equivalent by using short circuit power and impedance values at the point of connection. The RTC is easily modelled by introducing the impedances of the voltage divider and the equivalent transformer. The behaviour of the WT could be slightly different depending on the working point. Doubly fed induction machines commonly use two different control areas: one for the optimal rotor speed/optimal power production at low wind speed values, and the other, for nominal rotor speed conditions for high wind speed values. In

6 8 J. C. Ausin, D. Navarro Gevers and B. Andresen Figure 4. Three-phase voltage drop: voltage, active and reactive power order to assure the robustness of the simulation models, the validation procedures should at minimum include tests in the two control areas, i.e. for high and low wind speed values. Moreover, the response to voltage dip depends mainly on the magnitude of voltage step change rather that on the voltage dip duration. This requires that simulation models are validated against a set of different voltage dip values from the maximal (depending on the grid code) to a minimum of about 20% deep close to the turbine steady-state operational limits. Although not all possible conditions can be simulated, by covering high and low wind speed values, and large and small voltage dip, a wide range of operational states is represented. Moreover, transient stability studies are normally performed close to wind farm full power output as this represents the worst-case scenario for the grid stability. The RTC test results are used for the validation of GAMESA G80 and G52 Power System Simulator for Electrical System Planning (PSS/E) simulation models. These models are very simplified and exclusively include the necessary loops to represent the WT behavior in a transient stability study. 10,11 Simulations are performed for high and low wind speed conditions, and for large and small voltage variations.

7 Fault Ride-through Capability Test Unit 9 Figure 5. Three-phase drop with intermediate recovery steps: voltage wave form, voltage RMS, active power and reactive power Figure 7 shows a comparison of the on-site test results and the PSS/E G8X model simulation output when the RTC unit provokes a voltage drop to 0,25 pu during 500 ms at full power output. The RMS voltage, and the active and reactive power are plotted. There is a good match between the two plots that indicates that the use of the RTC is a valuable tool for validating models for grid stability studies. Conclusions This paper presents the main features of the GAMESA RTC unit. The RTC has proven to be a useful tool for certifying the behaviour of the WT during voltage dips and for validating the simulation models. The main RTC features can be summarized as:

8 10 J. C. Ausin, D. Navarro Gevers and B. Andresen Figure 6. Double-phase 50% voltage drop for G80: voltage wave form, voltage RMS, active power and reactive power The unit is transportable, independent of the grid characteristics, which makes it valuable for use in different sites and countries. It is a flexible unit able to reproduce the voltage drop profiles required by most SO to fulfil the FRTC requirement. The RTC allows reproduction of symmetrical and unsymmetrical voltage drops. Figures 4 6 show that the WT can ride-through symmetrical and unsymmetrical faults in accordance with most grid codes all over the world. The RTC is a valuable tool for obtaining measurements for validating simulation models. By using the RTC test results, transient stability models can be validated for SO stability studies. Acknowledgements This paper was made possible through the valuable work of Lorenz Feddersen from FeCon GmbH together with GAMESA in the development of the RTC unit. We thank the GAMESA people involved in all the testing campaign.

9 Fault Ride-through Capability Test Unit 11 Figure 7. Comparison of RTC results (left) and PSS/E simulation results (right) for a G80 machine for a large voltage disturbance Appendix Figure A1 shows the command panel of the RTC unit. Figure A1. Voltage dip shape confi guration and panel control settings References 1. Ausin JC. The impact of a large penetration of intermittent sources on the power system operation and planning. PhD submitted to The University of Manchester (UMIST), E.ON Netz GmbH. Netzanschlussregeln für Hoch und Höchstspannung, Bayreuth 1, August Bolik S, Andresen B, Birk J, Nielsen J. Vestas handles grid code requirements. Advanced control strategies for wind turbines, EWEC Madrid, Requisitos de respuesta frente a huecos de tensión de las instalaciones eólicas REE, PO.12.3, REE draft document, January Navarro Gevers D. Beitrag zur Regelung einer doppeltgespeisten Asynchronmaschine für Windkraftanlagen ohne Lagegeber. PhD submitted to T.U.Ilmenau, 2004.

10 12 J. C. Ausin, D. Navarro Gevers and B. Andresen 6. IEC Ed 2.0. Wind turbine generator systems Part 21: measurement and assessment of power quality characteristics of grid connected turbines. Committee draft (CDV), February Technische Richtlinien für Windkraftanlagen, Teil 4-Bestimmung der Elektrischen Eigenschaften Fördergesellschaft für Windenergie e.v (FGW), 2. Revision 20, January Procedimiento de verificación, validación y certificación de los requisitos del PO 12.3 sobre la respuesta de las instalaciones eólicas ante huecos de tensión. AEE draft document, January RTC Test Unit, Manual V07. FeCon GmbH. Flensburg, Ekanayake JB, Holdsworth L, Jenkins N. Comparison of 5th and 3rd Order Machine Models for Doubly Fed Induction Generator (DFIG) Wind Turbines. Elsevier Power Systems Research 2003, 67(3): Erlich I. Modelling of Wind Turbines Equipped with Doubly-Fed Induction Machines for Power System Stability Studies, PSCE 2006, IEEE.