FIELD OF THE INVENTION

The invention relates to the field of wind turbines. In particular, the invention relates to a method for testing a wind turbine, a control system for a wind turbine and to a wind turbine system.

BACKGROUND OF THE INVENTION

In the past, wind turbines were designed to be decoupled automatically from the utility grid, which they were connected to, in the case of a huge grid disturbance. This loss of power generation potential may have a direct impact to the stability of the utility grid and may lead to cascaded effects and earning losses.

For today's state of the art multi megawatt wind turbines it is required that they are able to stay connected during such huge grid disturbances. These grid disturbances may comprise undervoltage, overvoltage, low frequency and high frequency disturbances. The capability to stay connected during undervoltage and overvoltage is usually called LVRT (Low Voltage Ride Through) resp. HVRT (High Voltage Ride Through) capability.

Different nations and regional grid network operators have different requirements for power generating units, which requirements are called grid code requirements. But not only the grid code requirements itself may be different. Every grid operator itself may define several tests which have to be passed to prove that the wind turbine is able to fulfill the grid code requirements. For example, these tests may vary by the amplitude of the grid voltage disturbance, the duration of the disturbance, whether the fault is a single phase fault or affects two or three phases, and/or priority requirements for active and reactive power.

But also when the specified test for different grid codes may vary, they usually all have one common condition. They have to be performed with the complete wind turbine under realistic conditions. The wind turbine to be tested not only comprises the mechanical parts like a pitch system, but also a power converter, an auxiliary power supply, a main transformer, and control parts such as an upper turbine controller.

The test of a complete wind turbine may be necessary to not only certify the power converter's behavior, which usually has the biggest influence to the grid fault behavior, but also to test all other components of the wind turbine, which have to be able to ride through to the same tests without tripping.

Because the whole wind turbine may have to be tested for such specified tests, it is usually not possible just to test the behavior of the power converter in a test lab with special grid infrastructure. A special test setup usually has to be installed between the wind turbine and its injection point to the power grid. This test setup may contain huge inductors and/or resistors which may be switched in between the grid voltage phases and/or between single phases and ground. Big additional inductors, called grid weakening inductances, in between test setup and power grid are usually used to minimize the feedback of the test setup to the grid, however, this can never be completely avoided. So, test setups like this may always lead to visible flicker, which may lead to trips of other power generating units and/or may disturb electrical equipment connected to the power grid. Furthermore, big grid weakening inductances may lead to huge short circuit currents heating up the inductances, which may complicate to run a test with a duration longer than a few seconds.

A test setup like described above is often installed in one or several shipping containers, because it is voluminous and it must be capsuled for safety reasons. These containers are then usually installed close to the wind turbine for the time of the test. The installation of such a test set for an offshore wind turbine may be complicated, because one or several shipping containers may have to be mounted directly on a wind turbine tower in the middle of the sea.

On the other hand, running the defined test cases directly with an offshore wind turbine may be the most efficient way, because the wind conditions offshore usually are much better than they are onshore. This may result in less time needed to run through all the defined tests, which may be several dozen necessary certifications tests.

EP 2 620 780 A1 , EP 2 461 026 A1 and EP 2 461 027 A1 relate to simulation devices for LVRT (low voltage ride through), which are coupled between a utility grid and a wind turbine.

DE 10 201 1 007434 A1 and DE 10 201 1 002842 A1 relate to software based simulation for LVRT.

US 2014 0015555 A1 describes a hardware in the loop simulator for grid faults. The control hardware and software of a wind turbine is integrated, but the utility grid itself is simulated by software.

EP 2 236 821 A1 relates to a system for island operation of wind turbines, in which one wind turbine is used as a power supply for other wind turbines.

DESCRIPTION OF THE INVENTION

It is an objective of the invention to provide a fast, exact and cost-effective test method for tests that a wind turbine fulfils required grid code requirements. This objective is achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

A first aspect of the invention relates to a method for testing a wind turbine. A wind turbine in general may be a device adapted for converting wind energy into electrical energy. A wind turbine may comprise a rotor with a vertical or horizontal axis, a generator for converting the mechanical energy from the rotor into electrical energy and a converter for converting the power directly produced by the generator into a power that may be supplied to an electrical grid (or at least to a transformer connected to the electrical grid).

According to an embodiment of the invention, the method comprises: supplying an island grid, at least one wind turbine to be tested is connected to, with at least one grid emulating wind turbine, the grid emulating wind turbine generating electrical energy based on wind energy and the grid emulating wind turbine being adapted for maintaining a nominal voltage and nominal frequency in the island grid, the nominal voltage and nominal frequency being the voltage of a utility grid, which is disconnected from the island grid during testing of the wind turbine to be tested; controlling the at least one grid emulating wind turbine, such that a test scenario with a different voltage different from the nominal voltage and/or a different frequency different from the nominal frequency is produced in the island grid to simulate a utility grid with a specific fault scenario; controlling the at least one grid emulating wind turbine, such that surplus electrical energy supplied to the island grid by the at least one wind turbine to be tested is dissipated via a resistive load from the island grid to maintain the different voltage and/or different frequency.

With the method, at least two wind turbines may be coupled in such a way that at least one of the wind turbines (the grid emulating turbine) supplies an island grid, to which at least one other wind turbine is also connected, and at least a voltage amplitude and/or a frequency of this island grid may be controlled in such a way that one or more test scenarios, which for example may be required for grid certification, are reproduced, wherein the energy management for the island grid is done by the grid emulating turbine. The grid emulating wind turbine is controlled such that surplus electrical energy produced by the wind turbine to be tested and/or the grid emulating wind turbine is dissipated by a resistor, which may be directly connected to the island grid or may be a part of the grid emulating wind turbine.

The nominal voltage (i.e. nominal voltage amplitude) and/or nominal frequency may be the voltage and/or frequency of a utility grid, the wind turbines usually supply with electrical energy, when they are not performing a test. A utility grid may be a large scale electrical network interconnecting the wind turbines with remote loads. During the test, the island grid, which may be a grid or a part of a grid interconnecting the wind turbines with the utility grid, may be disconnected from t e utility grid and the grid simulating wind turbine may simulate a utility grid with specific fault scenarios. These fault scenarios may be standardized with specific test scenarios that may be provided by international or national standards and/or grid network operators.

The grid simulating wind turbine may be seen as voltage source and/or may be operated in a so-called island mode. Because only two or more wind turbines are coupled to the island grid, the grid simulating wind turbine is free to change grid voltage (amplitude) and frequency according to the defined test scenarios. The wind turbine to be tested then reacts to these impressed test cases as it would react if the "real" utility grid would be disturbed accordingly.

With the method, the energy management in the island grid may also be performed by the grid simulating wind turbine. Usually, during a test scenario (and possible before and after) not only the grid simulating wind turbine will supply electrical energy to the island grid but also the wind turbine to be tested. In such a case, mainly to maintain the correct voltage (i.e. the correct nominal and/or correct test scenario voltage), the grid simulating wind turbine may have to reduce the energy supplied by itself into the island grid and/or even may have to reverse its internal power flow.

It has to be noted that several test scenarios dictated by the above mentioned standards or grid network operators require that the wind turbine to be tested run at least a couple of seconds before the test with the same amount of load for which the test is for. The thereby necessary island grid energy management will then be handled by the grid simulating wind turbine.

With the method, the test scenarios (such as grid code certification tests) may be performed without any additional test equipment. With a passive load and/or resistor added to the island grid, some or all of the surplus electrical energy may be dissipated. Additionally or alternatively, the grid emulating wind turbine may dissipate electrical energy internally with a resistor that, for example, is connected to the DC link. Also, the grid simulating wind turbine may mechanically dissipate electrical energy by adding torque to its generator instead of applying it. The surplus energy from the island grid may be then converted into rotational energy.

According to an embodiment of the invention, a resistor for dissipating electrical energy may be directly connected to the island grid. The at least one grid emulating wind turbine may be controlled such that the different voltage and/or different frequency of the test scenario is maintained, while surplus electrical energy from the island grid is dissipated by the resistor. The resistor may be seen as a passive and/or dump load and/or may continuously dissipating energy from the island grid. In this case, the grid simulating turbine may introduce so much electrical energy into the island grid that in sum with the electrical energy from the wind turbine to be tested, it is compensated by the dissipated electrical energy.

According to an embodiment of the invention, a resistor for dissipating electrical energy is connected to a DC link of the grid emulating wind turbine. For example, the resistor may be part of a voltage limiter of the wind turbine which is connected to the DC link.

It may be possible that, when the wind turbine to be tested introduces additional electrical energy into the island grid, the grid simulating turbine reverses its internal energy flow. For example, a converter of the at least one grid emulating wind turbine may be controlled such that electrical energy from the island grid flows to the DC link. The converter may comprise a rectifier for generating a DC current for a DC link from an AC current generated by the generator of the wind turbine and an inverter for converting the DC current from the DC link into a further AC current to be supplied to the island grid. During reverse energy flow, the rectifier may be operated as an inverter and the inverter may be operated as a rectifier. According to an embodiment of the invention, the at least one grid emulating wind turbine is controlled such that surplus electrical energy from the island grid is dissipated by mechanical components of the at least one grid emulating wind turbine. Also in this case, the wind turbine may reverse its energy flow. The converter may be controlled such that electrical energy from the island grid flows to the mechanical components. During reverse energy flow, the rectifier of the converter may be operated as an inverter and the inverter may be operated as a rectifier.

According to an embodiment of the invention, an electrical generator of the at least one grid emulating wind turbine is operated as electrical motor and rotational energy generated in the electrical motor is dissipated by braking the generator with a mechanical brake system and/or blade bitch control system. Also a speed up of the generator is possible. The fact, that such a test takes a least only a couple of seconds, a too massive speed up may be prohibited by a large moment of the inertia of the one or more grid emulating wind turbines. There are several possibilities, how the surplus electrical energy from the island grid may be dissipated with the wind turbine itself. The grid simulating wind turbine may act as a load on the island grid. For example, the rotor interconnected with the generator may be braked such that it does not accelerate due to the generator operating in motor mode. Also, the pitch of the rotor blades may be changed such that the rotor is braked with the aid of wind energy.

According to an embodiment of the invention, at least two grid emulating wind turbines, which are connected to the island grid, are controlled to produce the test scenario and to maintain the different voltage and/or different frequency from the test scenario in the island grid.The method is not limited to only two interconnected wind turbines. Several wind turbines may be coupled to operate as grid simulating wind turbines and/or to produce the needed voltages and/or frequencies in the island grid, while managing the energy in the island grid.

According to an embodiment of the invention, the method further comprises: during a test scenario, controlling the at least one wind turbine to be tested, such that the at least one wind turbine is operating in a normal mode, in which the wind turbine supplies a utility grid with the nominal voltage and the nominal frequency. It has to be understood that the wind turbine to be tested should be operated as it was connected to the utility grid (for example indirectly via the island grid) and/or it should be operated in a normal mode, in which it tries also to produce electrical energy during the test scenarios.

According to an embodiment of the invention, at least two wind turbines, which are connected to the island grid, are tested simultaneously during a test scenario. It is also possible to run more than one test wind turbine on this island grid. In this case, also the grid disturbance behavior of several interconnected wind turbines may be tested and certified. According to an embodiment of the invention, during a test scenario, the at least one grid emulating wind turbine is controlled such that for a predefined test scenario time, the grid emulating wind turbine maintains a test scenario voltage different from the nominal voltage and/or a test scenario frequency different from the nominal frequency in the island grid. Usually, during a test scenario, the nominal voltage and/or nominal frequency is altered to a predefined pattern dictated by a test standard and/or a grid network operator. For example, the voltage in the island grid may be lowered and/or increased suddenly and/or continuously.

The test scenario voltage may comprise an undervoltage, being at least 10% lower than the nominal voltage. The test scenario voltage may comprise an overvoltage, being at least 10% higher than the nominal voltage. The test scenario frequency may be at least 1 % lower than the nominal frequency and/or the test scenario voltage may be at least 1% higher than the nominal frequency.

According to an embodiment of the invention, during the test scenario, at least one fault inductance is connected to the island grid. For example, test equipment (which may be standardized and/or may be assembled in a test container) may be connected to the island grid and/or between the wind turbine to be tested and the island grid. For example, the test equipment may comprise a fault inductance, for grounding at least one phase of the island grid. The fault inductance may be interconnected via a switch with the respective phases, which switch may be controlled by a control system performing the test scenarios. For example, an LVRT test container may be required by a certification institute to keep the certification setup of different wind turbine producers comparable. With one or more fault inductances, which are added to the island grid, the tests scenarios required for grid certification may be reproduced.

According to an embodiment of the invention, a control system of the grid emulating wind turbine, which is adapted for controlling a converter of the grid emulating wind turbine for converting an electric power from a wind powered electrical generator into a power to be supplied to the island grid, controls the grid emulating wind turbine for producing the test scenario. For example, the control system may be adapted for controlling the grid emulating wind turbine in a normal mode, in which the grid emulating wind turbine is adapted for supplying a utility grid with electrical energy.

In other words, the control system for the wind turbine, which may control a converter of the wind turbine during a normal mode, in which the wind turbine is supplying a utility grid with electrical energy, also may be adapted for operating in an island mode, in which the test scenarios may be generated in the island grid.

Further aspects of the invention relate to a control system for a wind turbine and a wind turbine system adapted for performing the method as described in the above and in the following. It has to be understood that features of the method as described in the above and in the following may be features of the control system and/or the wind turbine system as described in the above and in the following, and vice versa.

According to an embodiment of the invention, the wind turbine system comprises at least two wind turbines, an island grid interconnecting the at least two wind turbines, wherein at least one of the wind turbines comprises a control system adapted for performing the method of one of the preceding claims. The wind turbine adapted for performing the method may be used as grid simulation turbine, which generates the test scenarios in the island grid and which performs the energy management in the grid. The other wind turbine may be the wind turbine to be tested that runs in an operation mode as it would be connected to the utility grid.

It has to be understood that the wind turbines may be equally designed and/or may have equally designed controllers and that during a first test, the first turbine may be the grid simulating turbine and the second turbine is tested, and during a second test, the second turbine may be the grid simulating turbine and the first turbine is tested.

For example, the wind turbine system may be an (offshore or onshore) wind park with a plurality of wind turbines that are interconnected with a wind park grid, which is connected with the utility grid. During the testing, the wind park grid may be disconnected from the utility grid and one or more of t e wind turbines may be used as grid simulating wind turbines, whereas one or more of the remaining wind turbines may be tested.

According to an embodiment of the invention, the island grid is connectable to a utility grid. For example, the wind turbine system may comprise a switch for disconnecting the utility grid from the island grid.

According to an embodiment of the invention, the island grid comprises a fault inductance, via which at least one phase of the island grid is groundable. For example, for specific tests, in which an earth fault may be simulated, the island grid may be temporally interconnected with test equipment such as a fault inductance.

According to an embodiment of the invention, the island grid comprises a resistor for dissipating energy from the island grid. Such a resistive load may already be present in a wind park, for dissipating energy in the case, the wind park is disconnected from a utility grid, but the wind turbines and their control systems should stay operating, while supplying themselves with energy.

It has to be understood that the term "wind turbine" in the context of the present disclosure not only may relate to mechanical parts but also to electrical power conversion devices and/or to the corresponding control system.

For example, a wind turbine may comprise one or more of the following components: a mechanical turbine, a blade pitch control system for setting a pitch for blades of the mechanical turbine, a generator for generating electrical power when driven by the mechanical turbine, a mechanical brake system for braking the mechanical turbine and/or the generator, a converter for converting the electrical power from the generator into an electrical power to be supplied to an electrical grid, a controller for controlling the converter, an auxiliary power supply for supplying the controller, the auxiliary power supply being connected to the island grid, and/or a transformer for transforming a power from the converter into a power to be supplied to the electrical grid. The converter furthermore may comprise an inverter, a DC link and a rectifier. The DC link may comprise a voltage limiter with a resistor.

During a test based on the method as described above and in the following (for example a test, whether the wind turbine conforms to grid codes or not), all these components may be tested onsite as a complete assembly. No simulations for specific components have to be included in the test.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

Fig. 1 schematically shows a wind turbine system according to an embodiment of the invention.

Fig. 2 shows a flow diagram for a method for testing a wind turbine according to an embodiment of the invention.

Fig. 3 schematically shows a wind turbine system according to a further embodiment of the invention.

The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 1 shows a wind turbine system 10 with two wind turbines 12a, 12b that are interconnected with an island grid 14.

Each wind turbine 12a, 12b comprises a rotor/mechanical turbine 16 that is connected to a generator 18 for generating electrical power when driven by the rotor 16. The mechanical parts of the wind turbine 12a, 12b furthermore comprise a blade pitch control system 20 for setting a pitch for blades of the rotor 16 and/or a mechanical brake system 22 for braking the rotor 16 and/or the generator 18.

For example, the generator 18 may generate a three-phase electrical power of variable voltage amplitude and/or frequency. This AC current is supplied to a converter 24, which comprises a rectifier 26, a DC link 28 and an inverter 30. The rectified current from the generator 18 is stored in the DC link and is then inverted into a further AC current, to be supplied to the island grid 14 via a transformer 32.

The wind turbine 12a, 12b comprises a control system 34, for example with controllers 34a, 34b for controlling the inverter 30 and the rectifier 26, respectively. The whole wind turbine system 34 is supplied from an auxiliary power supply 36 retrieving its electrical energy via a further transformer 38 from the island grid 14.

It has to be noted that the wind turbines 12a, 12b may be equally designed and/or may comprise equally designed control systems 34.

As will be described in detail below, the first wind turbine may be controlled as a grid simulating wind turbine 12a, which will then supply the island grid 14 with electrical energy and will control the energy management of the island grid 14 during a test of the test wind turbine 12b. In general, the wind turbine 12a will generate test scenarios with a predefined, specified varying voltage amplitude and/or varying frequency in the island grid 14 and will maintain the specified voltage amplitude and/or frequency although the wind turbine 12b introduces further electrical energy during the test scenarios. This surplus electrical energy may be dissipated via a resistor/resistive load 40 directly connected to the island grid 14. It has to be noted that the island grid 14 may be a one-phase or multi-phase grid and/or that the resistive load 40 may comprise one or more resistors interconnecting the phases of the island grid 14 with the ground.

Alternatively or additionally, the surplus electrical energy may be dissipated by a resistor/resistive load 42 connected to the DC link 28, which, for example, may be a part of a voltage limiter connected to the DC link 28.

Alternatively or additionally, the surplus electrical energy may be dissipated mechanically by the mechanical brake system 22 and/or with the rotor 16, which blades are set by the blade pitch control system 20 in such a way that the generator 18 may be braked with the rotor 16.

Furthermore, Fig. 1 shows that due to test reasons, a test equipment 44 may be connected to the island grid 14. The test equipment 44 may comprise fault inductors 46 and/or switches 48 for connecting the fault inductors 46 to one or more phases of the island grid 14. For example, when a fault inductor 46 connects one phase of the island grid to ground, an earth fault may be simulated.

The control systems 34 of the wind turbine and/or the test equipment may be controlled by a superordinated controller 50, which, for example, may control the wind turbine 12a to switch into grid simulation mode and to generate a specified test scenario. The controller 50 also may control the test equipment 44 to simulate the specific faults as demanded by the specified test scenario. The controller 50 also may control the wind turbine 12b to switch into a normal operation mode, in which the wind turbine 12a operates as connected to a utility grid and its behavior in this mode with respect to the test scenarios can be tested.

Fig. 2 shows a flow diagram for describing a test method that may be performed by the wind turbine system 10 of Fig. 1 .

It has to be noted that the test method described in the above and in the following may be performed with the controllers/control systems 34, 34a, 34b, 50 and/or may be implemented in software. I.e. the controllers/control systems 34, 34a, 34b, 50 may comprise processors, which run computer programs that are adapted to perform the test method. However, it is not excluded that the test method is at least partially implemented in hardware and/or that specific steps of the method are performed by a person conducting the test. In step S10, the wind turbine 12a is switched into a grid simulating mode, for example by the controller 50, and the island grid 14 is supplied by the grid emulating wind turbine 12a with electrical energy. The grid emulating wind turbine 12a generates this electrical energy based on wind energy. Furthermore, a nominal voltage and nominal frequency in the island grid 14 is maintained by the grid emulating wind turbine 12a. In other words, the grid emulating wind turbine 12a simulates a utility grid. It has to be noted that the auxiliary supply 38 of the grid emulating wind turbine 12a is also coupled to the island grid 14 and is therefore supplied by the grid emulating wind turbine 12a.

In step S12, the grid emulating wind turbine 12a is controlled, such that a test scenario with a different voltage and/or a different frequency is produced in the island grid 14. For example, the controller 50 may command the control system 34 of the wind turbine 12a to start a specific scenario. Alternatively, the control system 34 may perform a specified set of test scenario autonomously.

During a test scenario, the converter 24 and in particular the inverter 30 of the grid emulating wind turbine 12a is controlled such that a different voltage amplitude (such that an under- or overvoltage and/or under- or overfrequency) with respect to the nominal voltage amplitude resp. frequency is generated. Since the converter 24 of the grid simulating turbine 12a may be seen as the main voltage source of the island grid 14, the converter 24 may generate any voltage amplitude and frequency independent from a utility grid. In such a way, the converter 24 of the grid emulating wind turbine 12a may be controlled in such a way, that the test scenarios required for grid certification can be reproduced.

In step S14, the wind turbine 12b is switched into a normal mode, for example by the controller 50, when it is not already in the normal mode. In particular, during a test scenario, the wind turbine 12b to be tested should be controlled as if it was connected to a utility grid. In normal mode, a wind turbine is controlled in such a way that it is adapted for supplying a utility grid with the nominal voltage and the nominal frequency.

It may be possible that during a further test, the wind turbine 12b acts as grid emulating turbine and/or the wind turbine 12a is tested, i.e. acts as test wind turbine.

In step S16, the grid emulating wind turbine 12a is controlled, such that electrical energy supplied to the island grid 14 by the at least one wind turbine 12b to be tested is dissipated via one or more resistive loads 40, 42 from the island grid 14 to maintain the different voltage and/or different frequency. Since the wind turbine 12b to be tested introduces further electrical energy into the island grid 14, the grid emulating wind turbine 12a has to reduce its supply of electrical energy into the island grid or even has to absorb the surplus electrical energy. In general, the converter 24 of the wind turbine 12a may be controlled such that the test voltage amplitude and test frequency may be kept at their specified values. Surplus electrical energy may be dissipated by t e resistive load 40 directly connected to the island grid 14 and/or the internal resistive load 42 of the grid emulating wind turbine 12a.

Additionally, surplus electrical energy from the island grid 14 may be dissipated by mechanical components 16, 22 of the at least one grid emulating wind turbine 12a. In this case, the electrical generator 18 of the grid emulating wind turbine 12a may be operated as electrical motor and rotational energy generated in this electrical motor may be dissipated by braking the generator 18 with the mechanical brake system 22 and/or blade bitch control system 20.

When electrical energy from the island grid has to be dissipated by the grid emulating wind turbine 12a itself, it may be necessary, that the energy flow in the converter 24 or at least in the inverter 30 is reversed. The converter 24 of the grid emulating wind turbine 12a may be controlled such that electrical energy from the island grid 14 flows in direction to the internal resistive load 42 and/or towards the mechanical components 16, 22. In this case, the inverter 30 may be operated as rectifier and/or the rectifier 26 may be operated as inverter.

During the test, a test equipment 44 may support the test scenarios. For example, during a test scenario, at least one fault inductance 46 may be connected to the island grid 14, for example by closing the switch 48 via the control of the controller 50. In such a way, one or more phases of the island grid 14 may be grounded via the fault inductance 46, for example to simulate an earth fault.

Furthermore, for example, if a certification institute has objections against the mentioned test setup and claims that the grid faults may have to be impressed by fault inductances 46 to keep certification process for different wind turbines uniform and comparable, it has to be noted that also with additional test equipment, at least grid weakening inductances and a circuit feedback to the grid may be avoided with the test method.

Fig. 3 shows a further wind park system 10, which, for example, may be an offshore or onshore wind park. A plurality of wind turbines 12a, 12b, 12c, 12d may be connected to the island grid 14, which in this case may be a wind park distribution grid, which may be connected to a large scale utility grid 52, for example via a transformer 54 and a switch 56.

The superordinated controller 50 may be part of a wind park control system. The controller 50 may disconnect the grid 14 from the utility grid 52, for example by opening the switch 56, and thus may transform the grid 14 into an island grid.

Furthermore, during one test scenario, more than one wind turbine may be the grid emulating wind turbines (for example 12a and 12c). This may stabilize the voltage amplitude and/or the frequency in the island grid 14 during a test scenario. Furthermore, more than one wind turbine 12a, 12c may be used for dissipating electrical energy. For example, t e internal resistive loads 42 of two wind turbines 12a, 12c may be used for dissipating the surplus electrical energy in the island grid 14.

Also, more than wind turbine may be the wind turbines to be tested (for example 12b and 12d). In such a way, the collective behavior of several wind turbines 12b, 12d connected to one grid 14 may be tested.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SYMBOLS

10 wind turbine system

12a, 12b, 12c, 12d wind turbine

14 island grid

16 rotor

18 generator

20 blade pitch control system

22 mechanical brake system

24 converter

26 rectifier

28 DC link

30 inverter

32 transformer

34 control system

34a rectifier controller

34b inverter controller

36 auxiliary power supply

38 auxiliary transformer

40 resistor/resistive load

42 resistor/resistive load

44 test equipment

46 fault inductor

48 switch

50 superordinated controller

52 utility grid

54 park transformer

56 switch