A method for the controlling a wind turbine generator

Abstract

A method (300) for controlling a wind turbine generator (104) connected to an electric power grid (102), wherein the wind turbine generator (104) comprises an electric generator (126) and a power converter arrangement (132) for electric power conversion, wherein the method (300) comprises: in response to an indication of a fault condition affecting the wind turbine generator (104), comparing (304) one or more determined electric currents at one or more first locations (162) to one or more determined electric currents at one or more second locations (164), the first and second locations (162, 164) being spaced apart from one another and located between the electric generator (126) and the electric power grid (102); and based on the comparison of the determined electric currents at the first and second locations (162, 164) and if the determined electric current at the first location (162) is different from the determined electric current at the second location (164), terminating (306) an electric power conversion of the power converter arrangement (132).

Description

[0001] A METHOD FOR THE CONTROLLING A WIND TURBINE GENERATOR

[0002] Technical field

[0003] Aspects of the present invention relate to a method for controlling a wind turbine generator connected to an electric power grid.

[0004] Background

[0005] In general, an electric power grid based on synchronous generators and alternating voltage, for example referred to as a utility grid, may have defined parameters, for example a defined alternating voltage frequency, such as 50 Hz or 60 Hz. The stability of the electric power grid parameters, including the load angle of synchronous generators, is dependent on a variety of variables including the balance between generated electric power and consumed electric power in the electric power grid. In general, any imbalance between generated electric power and consumed electric power results in changes in the grid frequency of the electric power grid. In general, it is important to have a stable grid frequency in the electric power grid.

[0006] In general, a grid code may be specified for an electric power grid, for example by the electric power grid operator, wherein the grid code defines parameters a power plant connected to the electric power grid has to meet, such as a power plant including one or more wind turbine generators, for example to provide sufficient frequency support to the electric power grid, to provide sufficient voltage support to the electric power grid, or to provide sufficient power oscillation damping support to the electric power grid.

[0007] In general, a wind turbine generator, or a power plant including one or more wind turbine generators or other renewable electric power generating units, includes one or more power converters for electric power conversion.

[0008] Summary

[0009] The inventors of the present invention have found drawbacks in conventional solutions for wind turbine generators, or power plants including wind turbine generators, to provide support to the electric power grid. For example, some conventional solutions do not provide a sufficiently appropriate voltage support to the electric power grid.

[0010] An object of the invention is to provide a solution which mitigates or solves the drawbacks and problems of conventional solutions.

[0011] The above and further objects are solved by the subject matter of the independent claims. Further advantageous embodiments of the invention can be found in the dependent claims.

[0012] According to a first aspect of the invention, the above mentioned and other objects are achieved with a method for controlling a wind turbine generator connected to an electric power grid, wherein the wind turbine generator comprises an electric generator and a power converter arrangement for electric power conversion, wherein the electric generator is connected to the electric power grid via the power converter arrangement, wherein the method comprises: in response to an indication of a fault condition affecting the wind turbine generator, comparing one or more determined electric currents at one or more first locations to one or more determined electric currents at one or more second locations, the first and second locations being spaced apart from one another and located between the electric generator and the electric power grid; and based on the comparison of the determined electric currents at the first and second locations and if the determined electric current at the first location is different from the determined electric current at the second location, terminating an electric power conversion of the power converter arrangement .

[0013] It is to be understood that the second location is different from the first location. For some embodiments, the first and second locations may be spaced apart from one another by one or more electrical or electronic components, said one or more electrical or electronic components being different from a sole electrical cable or a sole straight electrical conductor. For some embodiments, it may be defined that the electric potential of the first location is different form the electric potential of the second location. For some embodiments, it may be defined that there is an electric potential difference between the first and second locations.

[0014] The IEC standard IEC60909-0:2016 specifies a method for calculating a short circuit current and defines how to perform short circuit calculations and which contributors to include in the calculations. IEC60909-0:2016 states that full-scale converters, FSC, within wind power plants must be regarded as contributors to the short circuit current and thereby be included in the calculations. In IEC60909-0:2016, it is stated that the level of contribution must be defined by the power converter manufacturer.

[0015] Grid codes for wind turbine generators vary among countries. A requirement of some grid codes is the wind turbine generator’s ability to manoeuvre through fault events, during which the wind turbine generator must stay connected to the electric power grid and help to stabilize the electric power grid. This behaviour, or requirement, is known as “fault ride through”, FRT. For example, an FRT can be a fault ride through for an over voltage event, OVRT, or a fault ride through for a low voltage event, LVRT, alternatively, under-voltage ride through, LIVRT.

[0016] According to some grid codes, the power converter performance of the wind turbine generator must comply with the grid codes regarding FRT including low voltage ride through. According to some grid codes, the response to an FRT is that the power converter must support the voltage level of the electric power grid by injecting reactive power or current to the electric power grid. The intension of FRT grid codes is to ensure that the wind turbine generator’s performance helps to support and stabilize the electric power grid by providing reactive power to clear the fault. In general, according to most grid codes, the FRT compliance only relates to faults on the grid side of wind turbine generator. In general, according to most grid codes, faults inside the wind turbine generator do not fall under the FRT requirements.

[0017] In general, according to conventional control implementation, a fault condition, such as a short circuit, occurring inside the power converter of the wind turbine generator will result in a voltage drop which triggers the power converter’s FRT behaviour in the same manner as if the fault condition would occur outside, or is external to, the wind turbine generator. Thus, according to conventional control implementation, the result is that both the electric power grid and the power converter will contribute to the fault level internally in the power converter, or in the wind turbine generator. Conventionally, whatever the prospective (or maximum available) grid fault level is, the power converter’s contribution will be added to the grid short circuit level.

[0018] The inventors of the present invention have found that, based on a comparison of electric currents determined at the two different locations between the electric generator and the electric power grid, it can be determined if the fault condition is occurring inside the wind turbine generator or outside the wind turbine generator. If the fault condition is occurring inside the wind turbine generator, i.e., when the electric currents at the two different locations are different from one another, the electric power conversion of the power converter arrangement can be terminated, whereby the power converter arrangement’s contribution to a fault condition inside the wind turbine generator is avoided. Thus, by way of embodiments of the method according to the first aspect, it is possible to the distinguish between external faults external to the wind turbine generator and internal faults internal to the wind turbine generator, and / or internal to the power converter arrangement.

[0019] Thus, an advantage of the method according to the first aspect is that, upon a fault condition affecting the wind turbine generator, the fault type can be determined, or detected, and the power converter-generated current or power contribution can be controlled dependent on the location of the fault. An advantage of the method according to the first aspect is that it is prevented that the power converter arrangement feeds current into a fault inside the wind turbine generator, and thus, the power converter arrangement does not make a fault inside the wind turbine generator worse.

[0020] An advantage of the method according to the first aspect is that the contribution of the power converter arrangement of the wind turbine generator to a fault condition, such as a short circuit current, more specifically, a fault condition internal to the wind turbine generator, can be defined as zero, for example with regard to the IEC standard IEC60909-0:2016.

[0021] An advantage of the method according to the first aspect, besides the removal of the power converter’s contribution to a fault condition or event, such as a short circuit fault (more specifically, a fault condition internal to the wind turbine generator), so as to lower the short circuit specification for the power converter (for example, such that the components with lower rating for producing the power converter may be used), is that it be will be ensured that the power converter does not contribute to a fault in, or within, the power converter, irrespective if the fault impedance is high or low.

[0022] An advantage of the method according to the first aspect is an improved control of a wind turbine generator connected to an electric power grid. An advantage of the method according to the first aspect is an improved fault ride through performance of a wind turbine generator for the support and / or stabilization of the electric power grid upon fault events, for example an improved OVRT or LVRT. An advantage of the method according to the first aspect is an improved and / or more efficient / appropriate support, such as an improved and more efficient / appropriate voltage support, to the electric power grid by the wind turbine generator in relation to conventional solutions. An advantage of the method according to the first aspect is an improved and / or more efficient wind turbine generator in relation to conventional wind turbine generators.

[0023] An advantage of the method according to the first aspect is that the short circuit, SC, level for the power converter arrangement of the wind turbine generator is limited or reduced, i.e., the short circuit rating of components of the power converter of the wind turbine generator can be reduced, which means that lower rating components (i.e., components with lower rating), which are less expensive than higher rating components, can be used for the power converter. An advantage of the method according to the first aspect is that the design of the power converter can be simplified. Thus, an advantage of the method according to the first aspect is an improved fault ride through performance of the wind turbine generator without increased costs, or to lower costs, in relation to conventional solutions. When the determined electric current at the first location is different from the determined electric current at the second location, it is to be understood that the electric current at the first location and the electric current at the second location are unequal, or not equal, for example, that the electric current at the first location and the electric current at the second location differs by a sufficient value, or the difference is above a sufficient value, for example taking into account the rating of the components of the wind turbine generator, the rating of the electric power grid and / or the operation / power level of the wind turbine generator and / or of the electric power grid.

[0024] Further advantages of embodiments of the method according to the first aspect are disclosed in the detailed description hereinbelow.

[0025] For some embodiments, the fault condition may, for example, comprise a fault current, a short circuit (SC) fault, or a short circuit (SC) current.

[0026] According to an advantageous embodiment of the method according to the first aspect, the method further comprises: if the determined electric current at the first location is different from the determined electric current at the second location, determining that the fault condition is occurring in the wind turbine generator; and if it is determined that the fault condition is occurring in the wind turbine generator, terminating an electric power conversion of the power converter arrangement.

[0027] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0028] According to a further advantageous embodiment of the method according to the first aspect, the method further comprises: based on the comparison of the determined electric currents at the first and second locations and if the determined electric current at the first location is equal to the determined electric current at the second location, controlling the power converter arrangement according to a fault condition response control scheme to provide support to the electric power grid.

[0029] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0030] For some embodiments, the fault condition response control scheme may correspond to FRT, for example OVRT and / or LVRT, for example according to one or more grid codes.

[0031] When the determined electric current at the first location is equal to the determined electric current at the second location, it is to be understood that the electric current at the first location and the electric current at the second location are substantially equal, for example, that the electric current at the first location and the electric current at the second location are sufficiently close to one another, for example are within the same limited range, for example taking into account the rating of the components of the wind turbine generator, the rating of the electric power grid and / or the operation / power level of the wind turbine generator and / or of the electric power grid.

[0032] According to another advantageous embodiment of the method according to the first aspect, the method further comprises: if the determined electric current at the first location is equal to the determined electric current at the second location, determining that the fault condition is occurring outside the wind turbine generator; and if it is determined that the fault condition is occurring outside the wind turbine generator, controlling the power converter arrangement according to a fault condition response control scheme to provide support to the electric power grid.

[0033] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0034] According to yet another advantageous embodiment of the method according to the first aspect, wherein the fault condition response control scheme comprises a low voltage response control scheme for providing voltage support to the electric power grid. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved LVRT. An advantage of this embodiment is a further improved voltage support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0035] According to still another advantageous embodiment of the method according to the first aspect, the low voltage response control scheme comprises the control of an injection of reactive power to the electric power grid for providing voltage support to the electric power grid. An advantage of this embodiment is a further improved LVRT.

[0036] According to an advantageous embodiment of the method according to the first aspect, wherein the method further comprises: determining an electric current at the first location; and determining an electric current at the second location.

[0037] According to a further advantageous embodiment of the method according to the first aspect, wherein the power converter arrangement comprises one or more power converters, wherein the method comprises: if the determined electric current at the first location is different from the determined electric current at the second location, terminating the electric power conversion of one or more of the one or more power converters.

[0038] According to another advantageous embodiment of the method according to the first aspect, the power converter arrangement comprises a machine-side converter and a line-side converter, wherein the machine-side converter is connected to the electric power grid via the line-side converter while the line-side converter is connected to the electric generator via the machine-side converter, wherein the method comprises: if the determined electric current at the first location is different from the determined electric current at the second location, terminating the electric power conversion of the line-side converter.

[0039] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0040] According to yet another advantageous embodiment of the method according to the first aspect, the power converter arrangement comprises a machine-side converter and a line-side converter, wherein the machine-side converter is connected to the electric power grid via the line-side converter while the line-side converter is connected to the electric generator via the machine-side converter, wherein the method comprises: if the determined electric current at the first location is different from the determined electric current at the second location, stopping a control signal to the lineside converter.

[0041] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0042] According to still another advantageous embodiment of the method according to the first aspect, the method comprises: if the determined electric current at the first location is equal to the determined electric current at the second location, controlling the line-side converter according to a fault condition response control scheme to provide support to the electric power grid. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0043] According to an advantageous embodiment of the method according to the first aspect, the power converter arrangement comprises a machine-side converter and a line-side converter, wherein the machine-side converter is connected to the electric power grid via the line-side converter while the line-side converter is connected to the electric generator via the machine-side converter, and wherein the first and second locations are located between the line-side converter and the electric power grid.

[0044] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0045] According to a further advantageous embodiment of the method according to the first aspect, the power converter arrangement comprises a machine-side converter and a line-side converter, wherein the machine-side converter is connected to the electric power grid via the line-side converter while the line-side converter is connected to the electric generator via the machine-side converter, wherein the power converter arrangement comprises a line-side inductor, wherein the line-side converter is connected to the electric power grid via the line-side inductor, wherein the first location is located between the line-side inductor and the electric generator, and wherein the second location is located between the line-side inductor and the electric power grid.

[0046] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0047] According to another advantageous embodiment of the method according to the first aspect, the first location is located between the line-side converter and the line-side inductor. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0048] According to still another advantageous embodiment of the method according to the first aspect, the wind turbine generator comprises a circuit breaking apparatus comprising one or more circuit breakers, wherein the power converter arrangement is connected to the electric power grid via the circuit breaking apparatus, wherein the first location is located between the circuit breaking apparatus and the electric generator, and wherein the second location is located between the circuit breaking apparatus and the electric power grid.

[0049] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator. According to yet another advantageous embodiment of the method according to the first aspect, the second location is located between the power converter arrangement and the electric power grid. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0050] According to an advantageous embodiment of the method according to the first aspect, the first location is located between the electric generator and the second location.

[0051] According to a further advantageous embodiment of the method according to the first aspect, the electric generator is connected to the electric power grid via a transformer, wherein the first and second locations are located between the electric generator and the transformer.

[0052] According to another advantageous embodiment of the method according to the first aspect, the electric generator is connected to the transformer via an EMI / EMC filter arrangement comprising one or more EMI / EMC filters, wherein the first location is located between the electric generator and the EMI / EMC filter arrangement, and wherein the second location is located between the EMI / EMC filter arrangement and the transformer.

[0053] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0054] According to still another advantageous embodiment of the method according to the first aspect, the power converter arrangement comprises a harmonic filter arrangement comprising one or more harmonic filters and one or more capacitors, wherein the electric generator is connected to the electric power grid via the harmonic filter arrangement, wherein the first location is located between the electric generator and the harmonic filter arrangement of the power converter arrangement, and wherein the second location is located between the electric power grid and the harmonic filter arrangement of the power converter arrangement.

[0055] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0056] According to yet another advantageous embodiment of the method according to the first aspect, the power converter arrangement comprises a machine-side converter and a line-side converter, wherein the machine-side converter is connected to the electric power grid via the line-side converter while the line-side converter is connected to the electric generator via the machine-side converter, wherein the line-side converter is connected to the electric power grid via the harmonic filter arrangement, and wherein the first location is located between the line-side converter and the harmonic filter arrangement of the power converter arrangement.

[0057] An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0058] According to an advantageous embodiment of the method according to the first aspect, the method comprises: in the comparison of the determined electric currents at the first and second locations, including one or more determined electric currents at one or more third locations in the harmonic filter arrangement. An advantage of this embodiment is a further improved comparison of the electric currents at the first and second locations and / or a further improved determination whether the electric currents at the first and second locations are different from or equal to one another. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator. It is to be understood that the third location is different from the first and second locations. For some embodiments, it may be defined that the third location is spaced apart from the first and second locations.

[0059] According to a further advantageous embodiment of the method according to the first aspect, the method further comprises: determining one or more electric currents at one or more third locations in the harmonic filter arrangement; and comparing the determined electric currents at the first and second locations while considering the determined electric current at the third location.

[0060] An advantage of this embodiment is a further improved comparison of the electric currents at the first and second locations and / or a further improved determination whether the electric currents at the first and second locations are different from or equal to one another. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0061] According to another advantageous embodiment of the method according to the first aspect, the electric current at the third location comprises a filter current of the harmonic filter arrangement. An advantage of this embodiment is a further improved comparison of the electric currents at the first and second locations and / or a further improved determination whether the electric currents at the first and second locations are different from or equal to one another. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0062] According to yet another advantageous embodiment of the method according to the first aspect, the method comprises: in the comparison of the determined electric currents at the first and second locations, including a filter capacitance of the harmonic filter arrangement and one or more determined line voltages at one or more fourth locations between the electric power grid and the harmonic filter arrangement of the power converter arrangement. An advantage of this embodiment is a further improved comparison of the electric currents at the first and second locations and / or a further improved determination whether the electric currents at the first and second locations are different from or equal to one another. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator. It is to be understood that the fourth location is different from the first and second locations. For some embodiments, it may be defined that the fourth location is spaced apart from the first and second locations.

[0063] According to an advantageous embodiment of the method according to the first aspect, wherein the method further comprises: determining one or more line voltages at one or more fourth locations between the electric power grid and the harmonic filter arrangement of the power converter arrangement, and comparing the determined electric currents at the first and second locations while considering the determined line voltage at the fourth location and a filter capacitance of the harmonic filter arrangement. An advantage of this embodiment is a further improved comparison of the electric currents at the first and second locations and / or a further improved determination whether the electric currents at the first and second locations are different from or equal to one another. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0064] According to a further advantageous embodiment of the method according to the first aspect, the method comprises: in the comparison of the determined electric currents at the first and second locations, including one or more estimated filter currents of the harmonic filter arrangement estimated based on a filter capacitance of the harmonic filter arrangement and on one or more determined line voltages at one or more fourth locations between the electric power grid and the harmonic filter arrangement of the power converter arrangement.

[0065] An advantage of this embodiment is a further improved comparison of the electric currents at the first and second locations and / or a further improved determination whether the electric currents at the first and second locations are different from or equal to one another. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0066] According to another advantageous embodiment of the method according to the first aspect, the method further comprises: determining one or more line voltages at one or more fourth locations between the electric power grid and the harmonic filter arrangement of the power converter arrangement, estimating one or more filter currents of the harmonic filter arrangement based on the one or more determined line voltages at the one or more fourth locations and on a filter capacitance of the harmonic filter arrangement, and comparing the determined electric currents at the first and second locations while considering the estimated one or more filter currents of the harmonic filter arrangement.

[0067] An advantage of this embodiment is an improved comparison of the electric currents at the first and second locations and / or a further improved determination whether the electric currents at the first and second locations are different from or equal to one another. An advantage of this embodiment is a further improved control of the wind turbine generator. An advantage of this embodiment is a further improved fault ride through of the wind turbine generator, for example a further improved OVRT or LVRT. An advantage of this embodiment is a further improved support to the electric power grid. An advantage of this embodiment is a further improved wind turbine generator.

[0068] According to yet another advantageous embodiment of the method according to the first aspect, the method further comprises: detecting a fault condition affecting the wind turbine generator.

[0069] According to a second aspect of the invention, the above mentioned and other objects are achieved with a computer program or a computer-readable medium comprising instructions which, when the program or the instructions is / are executed by a computer, cause the computer to carry out the method according to any one of the embodiments disclosed above. Advantages of the computer program or the computer-readable medium according to the second aspect correspond to advantages of the method according to the first aspect and its embodiments mentioned above or below.

[0070] According to an aspect of the invention, the above-mentioned computer program or the computer-readable medium is configured to implement the method and its embodiments described herein. According to a third aspect of the invention, the above mentioned and other objects are achieved with a control arrangement for controlling a wind turbine generator connected to an electric power grid, wherein the wind turbine generator comprises an electric generator and a power converter arrangement for electric power conversion, wherein the electric generator is connected to the electric power grid via the power converter arrangement, and wherein the control arrangement is configured to: in response to an indication of a fault condition affecting the wind turbine generator, compare one or more determined electric currents at one or more first locations to one or more determined electric currents at one or more second locations, the first and second locations being spaced apart from one another and located between the electric generator and the electric power grid; and based on the comparison of the determined electric currents at the first and second locations and if the determined electric current at the first location is different from the determined electric current at the second location, terminate an electric power conversion of the power converter arrangement.

[0071] Advantages of the control arrangement according to the third aspect correspond to advantages of the method according to the first aspect and its embodiments mentioned above or below.

[0072] It is to be appreciated that all the embodiments described for the method aspects of the invention are applicable also to the control arrangement aspects of the invention. Thus, all embodiments described for the method aspects of the invention may be performed by the control arrangement, which may include one or more controllers, control units, or control devices. As mentioned above, the embodiments of the control arrangement have advantages corresponding to advantages mentioned above for the method and its embodiments.

[0073] According to a fourth aspect of the invention, the above mentioned and other objects are achieved with a wind turbine generator comprising an electric generator, and a power converter arrangement for electric power conversion, wherein the electric generator is connectable to an electric power grid via the power converter arrangement, and wherein the wind turbine generator comprises a control arrangement according any one of the embodiments disclosed above or below.

[0074] Advantages of the wind turbine generator according to the fourth aspect correspond to advantages of the method according to the first aspect and its embodiments mentioned above or below.

[0075] According to a fifth aspect of the invention, the above mentioned and other objects are achieved with a power plant for providing electric power to an electric power grid, wherein the power plant comprises one or more wind turbine generators, and wherein the power plant comprises one or more of the group of:

[0076] • a control arrangement according any one of the embodiments disclosed above or below; and

[0077] • a wind turbine generator according any one of the embodiments disclosed above or below.

[0078] An advantage of the power plant according to the fifth aspect is an improved control of a power plant connected to an electric power grid. An advantage of the power plant according to the fifth aspect is an improved fault ride through performance of a power plant for the support and / or stabilization of the electric power grid upon fault events, for example an improved OVRT or LVRT. An advantage of the power plant according to the fifth aspect is an improved and / or more efficient support, such as an improved and more efficient voltage support, to the electric power grid by the power plant in relation to conventional solutions. An advantage of the power plant according to the fifth aspect is an improved and / or more efficient power plant in relation to conventional power plants. Otherwise, advantages of the power plant according to the fifth aspect correspond to advantages of the method according to the first aspect and its embodiments mentioned above or below.

[0079] The above-mentioned features and embodiments of the method, the computer program, the computer-readable medium, the control arrangement, the wind turbine generator and the power plant, respectively, may be combined in various possible ways providing further advantageous embodiments.

[0080] Further advantageous embodiments of the method, the computer program, the computer-readable medium, the control arrangement, the wind turbine generator and the power plant according to the present invention and further advantages with the embodiments of the present invention emerge from the detailed description of embodiments.

[0081] Brief Description of the Drawings

[0082] Embodiments of the invention will now be illustrated, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, where similar references are used for similar parts, in which:

[0083] Figure 1 is a schematic diagram illustrating an embodiment of the power plant according to the fifth aspect of the invention including one or more embodiments of the wind turbine generator according to the fourth aspect of the invention;

[0084] Figure 2 is a schematic diagram illustrating aspects of embodiments of the wind turbine generator according to the fourth aspect of the invention, which may be included in the power plant of figure 1 ;

[0085] Figure 3 is a schematic diagram illustrating aspects of embodiments of the wind turbine generator of figure 2;

[0086] Figure 4 is a schematic flow chart illustrating aspects of embodiments of the method according to the first aspect of the invention;

[0087] Figure 5 is another schematic flow chart illustrating further aspects of embodiments of the method according to the first aspect of the invention; Figure 6 is a schematic diagram illustrating further aspects of embodiments of the method according to the first aspect of the invention;

[0088] Figure 7 is a schematic diagram illustrating further aspects of embodiments of the method according to the first aspect of the invention;

[0089] Figure 8 is a schematic diagram illustrating further aspects of embodiments of the method according to the first aspect of the invention;

[0090] Figure 9 is a schematic diagram illustrating further aspects of embodiments of the method according to the first aspect of the invention; and

[0091] Figure 10 is a schematic diagram illustrating an embodiment of the control arrangement according to the third aspect of the invention, in which a method according to any one of the herein described embodiments may be implemented.

[0092] Detailed Description

[0093] With reference to figure 1 , a power plant 100 for providing electric power, or electrical energy, to an electric power grid 102 according to the fifth aspect of the invention is schematically illustrated. The power plant 100 includes one or more wind turbine generators 104 and / or one or more electric power generating units 103, which may be referred to as renewable electric power generating units 103. One or more of the one or more electric power generating units 103 may include one or more of the group of: a wind turbine generator, WTG, 104; an electric battery energy storage system 112; a photo-voltaic panel 108; and a fuel cell 110. The electric battery energy storage system 112 may include one or more electric battery units. The power plant 100 may include one or more additional power sources or power generators, such as one or more additional electric battery units. Thus, the power plant 100 may include one or more wind turbine generators 104, for example two, three or more wind turbine generators 104. Aspects of embodiments of the wind turbine generator 104 according to the fourth aspect are disclosed in further detail hereinbelow in connection with figure 2.

[0094] With reference to figure 1 , the wind turbine generator 104, the electric battery energy storage system 112, the photo-voltaic panel 108, or the fuel cell 110 may be described as a power source of the power plant 100 or as a power generator of the power plant 100. For some embodiments, the power plant 100 may be referred to as a hybrid power plant. The power plant 100 may be connected, or connectable, more specifically electrically connected / connectable, to the electric power grid 102 via a point of common coupling, PCC, 114. The electric power grid 102 may be referred to as a utility grid, an electrical grid, or an electric power network. For example, the power plant 100 may be located offshore or on land. The power plant 100 may include a control arrangement 116 for controlling the power plant 100. The control arrangement 116 may comprise, or be referred to as, a power plant controller, PPC.

[0095] With reference to figure 2, aspects of embodiments of the wind turbine generator, WTG, 104 according to the fourth aspect are schematically illustrated. For some embodiments, the wind turbine generator 104 may comprise a rotor 118 including one or more blades 120, or rotor blades 120, for example two or more blades 120, such as three blades 120, or more. The wind turbine generator 104 may comprise a tower 122 and a nacelle 124 mounted to the top of the tower 122. The rotor 118 may be connected, such as rotatably connected or mounted, to the nacelle 124. The wind turbine generator 104 comprises an electric generator 126. The rotor 118 may be connected to the electric generator 126. The rotor 118 is configured to drive the electric generator 126. The electric generator 126 may be a permanent magnet, PM, generator, or any other type of electric generator. The nacelle 124 may house the electric generator 126.

[0096] With reference to figure 2, the rotor 118 is rotatable by action of the wind. The wind- induced rotational energy of the blades 120 and rotor 118 may be transferred via a coupling 128, for exampling including one or more shafts 130, to the electric generator 126. Thus, the wind turbine generator 104 may be described to be configured to convert kinetic energy of the wind to mechanical energy, or rotational energy, by way of the blades 120 and, subsequently, to electric power by way of the electric generator 126.

[0097] With reference to figure 2, the wind turbine generator 104 comprises a power converter arrangement 132 for electric power conversion. The electric generator 126 may be connected, such as electrically connected, to the power converter arrangement 132. The electric generator 126 may be connected, or connectable, more specifically electrically connectable / connected, to the electric power grid 102 via the power converter arrangement 132. For some embodiments, the power converter arrangement 132, and / or the wind turbine generator 104, may comprise one or more power converters 133a, 133b, for example, connected, more specifically electrically connected, to the electric generator 126. The wind turbine generator 104 and / or the electric generator 126 may be connected, more specifically electrically connected, to the electric power grid 102 via said one or more power converters 133a, 133b, and / or via the power converter arrangement 132. The one or more power converters 133a, 133b may comprise a first power converter 133a for converting AC power from the electric generator 126 to DC power. The one or more power converters 133a, 133b may comprise a second power converter 133b for converting DC power from the first power converter 133a to AC power to be provided to the electric power grid 102. The nacelle 124 may house the one or more power converters 133a, 133b, or the one or more power converters 133a, 133b may be located elsewhere, such in the tower 122 or outside the wind turbine generator 104. Accordingly, the nacelle 124 may house the power converter arrangement 132, or the power converter arrangement 132 may be located elsewhere.

[0098] With reference to figure 2, alternatively, it may be defined that the power converter arrangement 132 includes a machine-side converter, MSC, 133a and a line-side converter, LSC, 133b. The machine-side converter 133a is connected, or connectable, more specifically electrically connectable / connected, to the electric power grid 102 via the line-side converter 133b while the line-side converter 133b is connected, or connectable, more specifically electrically connectable / connected, to the electric generator 126 via the machine-side converter 133a.

[0099] With reference to figure 2, for some embodiments, it may be defined that the power converter arrangement 132 comprises a DC link 135 connecting the machine-side converter 133a to the line-side converter 133b, more specifically, connecting the DC sides of the machine-side converter 133a and of line-side converter 133b to one another. With reference to figure 2, alternatively, it may be defined that the DC link 135 connects the first and second power converters 133a, 133b to one another. For some embodiments, the DC link 135 may comprise a DC link chopper. With reference to figure 2, the wind turbine generator 104 comprises a control arrangement 134, or controller 136, for controlling the wind turbine generator 104. The control arrangement 134 of the wind turbine generator 104 may comprise the controller 136, which may be referred to as a wind turbine generator controller 136. The control arrangement 134 of the wind turbine generator 104 may be configured to communicate with and / or be connected to, or be part of, the control arrangement 116 of the power plant 100 (see figure 1 ) comprising one or more wind turbine generators 104.

[0100] With reference to figure 2, for some embodiments, the wind turbine generator 104 may be referred to as a variable-speed wind turbine generator. It is to be understood that the wind turbine generator 104 may include further unites, components and / or devices, such as sensors, required for a wind turbine generator 104. For example, the wind turbine generator 104 may be located offshore or on land.

[0101] With reference to figure 3, further aspects of embodiments of the wind turbine generator 104 according to the fourth aspect are schematically illustrated. For some embodiments, the line-side converter 133b may include, or be built up by, one or more power converter stacks 138a, 138b, 138c, 138d. In a corresponding way, the machineside converter 133a may be include, or be built up by, one or more power converter stacks, however, not shown in any figure of the drawings. The line-side converter 133b and / or the power converter stacks 138a, 138b, 138c, 138d may include circuits and semiconductor devices, such as transistors, thyristors and diodes, according to various conventional designs. For some embodiments, it may be defined that the line-side converter 133b of the power converter arrangement 132 is configured to convert DC power to AC power to be provided to the electric power grid 102, for example, AC power with various voltages and / or various frequencies.

[0102] With reference to figure 3, for some embodiments, the power converter arrangement 132 may include one or more line-side inductors 140a, 140b, 140c, 140d, for example, one line-side inductor 140a, 140b, 140c, 140d for each power converter stack 138a, 138b, 138c, 138d of the line-side converter 133b. For some embodiments, the inductor may be referred to as a reactor, or coil. When the line-side inductor 140a, 140b, 140c, 140d is included, the line-side converter 133b is connected, or connectable, more specifically electrically connectable / connected, to the electric power grid 102 via the line-side inductor 140a-d.

[0103] With reference to figure 3, for some embodiments, the power converter arrangement 132 may include a harmonic filter arrangement 142 comprising one or more harmonic filters 144 and one or more capacitors 146. When the harmonic filter arrangement 142 is included, the electric generator 126 is connected, or connectable, more specifically electrically connectable / connected, to the electric power grid 102 via the harmonic filter arrangement 142.

[0104] With reference to figure 3, for some embodiments, the wind turbine generator 104 may include a circuit breaking apparatus 148 comprising one or more circuit breakers 150a, 150b. When the circuit breaking apparatus 148 is included, the power converter arrangement 132 is connected, or connectable, more specifically electrically connectable / connected, to the electric power grid 102 via the circuit breaking apparatus 148.

[0105] With reference to figure 3, for some embodiments, the electric generator 126, and / or the wind turbine generator, WTG, 104, may be connected, or connectable, more specifically electrically connectable / connected, to the electric power grid 102 via a transformer 152, for example, so as to change the AC voltage level, such as step-up, or increase, the voltage level before input of the electric power to the electric power grid 102. For some embodiments, the grid side of the transformer 152 may be referred to as the primary side, and the WTG side of the transformer 152 may be referred to as the secondary side. For other embodiments, the grid side of the transformer 152 may be referred to as the high voltage side, and the WTG side of the transformer 152 may be referred to as the low voltage side. For some embodiments, the transformer 152 may be referred to as a grid transformer, i.e. , a transformer located between the electric power grid 102 and any one of the group of: the wind turbine generator 104; the power converter arrangement 132; and the line-side converter 133b. With reference to figures 2 and 3, for some embodiments, the electric generator 126 may be connected, or connectable, more specifically electrically connectable / connected, to the transformer 152 via an EMI / EMC filter arrangement 154 comprising one or more EMI / EMC filters 156.

[0106] With reference to figure 3, for illustrative purposes, an example internal fault location 158 of a fault condition occurring in the wind turbine generator 104, or of an internal fault condition, or of an internal fault, is indicated as being located between the harmonic filter arrangement 142 and the circuit breaking apparatus 148. Further, for illustrative purposes, an example external fault location 160 of a fault condition occurring outside the wind turbine generator 104, or of an external fault condition, or of an external fault, is indicated as being located on the gride side of the transformer 152. For example, each one of the internal and external fault conditions may comprise a short circuit or a short circuit fault. However, it is to be understood that the internal fault location 158 and the external fault location 160 may be located elsewhere within the wind turbine generator 104 and outside the wind turbine generator 104, respectively, i.e. , that the respective fault condition may occur at other locations than what is illustrated in figure 3.

[0107] With reference to figures 4 to 6, aspects of embodiments of the method 300 for controlling a wind turbine generator 104 connected to an electric power grid 102 according to the first aspect of the invention are schematically illustrated. The wind turbine generator 104 includes an electric generator 126 and a power converter arrangement 132 for electric power conversion, wherein the electric generator 126 is connected to the electric power grid 102 via the power converter arrangement 132, for example, as illustrated above.

[0108] With reference to figure 4, embodiments of the method 300 include the steps of: in response to an indication (or detection) of a fault condition affecting (or impacting) the wind turbine generator 104, such as affecting the control of the wind turbine generator 104,

[0109] • comparing 304 one or more determined electric currents at one or more first locations 162 to (or with) one or more determined electric currents at one or more second locations 164, wherein the first and second locations 162, 164 are spaced apart from one another and located between the electric generator 126 and the electric power grid 102; and

[0110] • based on the comparison of the determined electric currents at the first and second locations 162, 164 and if the determined electric current at the first location 162 is different from the determined electric current at the second location 164, terminating 306 (or interrupting) an electric power conversion of the power converter arrangement 132.

[0111] It is to be understood that the first location 162 is different from the second location 164. For some embodiments, the first and second locations 162, 164 may be spaced apart from one another (or separated, or placed with a distance to one another) by one or more electrical or electronic components (such as an inductor 140a), wherein said one or more electrical or electronic components is / are different from a sole electrical cable or a sole straight electrical conductor. For some embodiments, it may be defined that the electric potential of the first location 162 is different form the electric potential of the second location 164. For some embodiments, it may be defined that there is an electric potential difference between the first and second locations 162, 164. Said fault condition may, for example, be a short circuit fault or short circuit fault condition

[0112] When the determined electric current at the first location 162 is different from the determined electric current at the second location 164, it is to be understood that the electric current at the first location 162 and the electric current at the second location 164 are unequal, or not equal, for example, that the electric current at the first location 162 and the electric current at the second location 164 differs by a sufficient value, or the difference is above a sufficient value, for example taking into account the rating of the components of the wind turbine generator 104, the rating of the electric power grid 102 and / or the operation / power level of the wind turbine generator 104 and / or of the electric power grid 102. For example, for some embodiments, by different is meant different enough, such at least a 1 % difference, such as least a 3 % difference, for example at least a 5 % difference, such as at least a 10 % difference. With reference to figure 4, for some embodiments, the step of terminating 306 the electric power conversion of the power converter arrangement 132 may comprise stopping 306a (see figure 6) one or more control signals, such as one or more Pulse- Width Modulation (PWM) signals, to the power converter arrangement 132, for example to one or more of the line-side converter 133b and machine-side converter 133a of the power converter arrangement 132, such as to the power converter stacks 138a-d of one or more of the line-side converter 133b and machine-side converter 133a. For example, when the control signal to only the line-side converter 133b is stopped, the machine-side converter 133a may still be operating, and then it is advantageously to dissipate electric energy / power from the DC link chopper of the DC link 135 of the power converter arrangement 132. For some embodiments, the step of terminating 306 the electric power conversion of the power converter arrangement 132 may comprise, or may be followed by, the step of opening 306b (see figure 6) one or more of the one or more circuit breakers 150a, 150b of the circuit breaking apparatus 148.

[0113] With reference to figure 5, for some embodiments, the method 300 may the additional steps of:

[0114] • if the determined electric current at the first location 162 is different from the determined electric current at the second location 164, determining 305 that the fault condition is occurring in 158 the wind turbine generator 104; and

[0115] • if it is determined that the fault condition is occurring in 158 the wind turbine generator 104, terminating 306 an electric power conversion of the power converter arrangement 132.

[0116] With reference to figure 6, which schematically illustrates several different embodiments, for some embodiments, the method 300 may comprise one or more of the steps of: based on the comparison of the determined electric currents at the first and second locations and if the determined electric current at the first location is equal to the determined electric current at the second location, controlling 308 the power converter arrangement 132 according to a fault condition response control scheme to provide support to the electric power grid 102;

[0117] • if the determined electric current at the first location is equal to the determined electric current at the second location, determining that the fault condition is occurring outside 160 the wind turbine generator 104, for example, in the electric power grid 102, on the grid side of the point of common coupling 114, or on the grid side of the transformer 152, or elsewhere; and

[0118] • if it is determined that the fault condition is occurring outside 160 the wind turbine generator 104, controlling 308 the power converter arrangement 132 according to a fault condition response control scheme to provide support to the electric power grid 102.

[0119] When the determined electric current at the first location 162 is equal to the determined electric current at the second location 164, it is to be understood that the electric current at the first location 162 and the electric current at the second location 164 are substantially equal, for example, a tolerance range may be taken into account. For example, that the electric current at the first location 162 and the electric current at the second location 164 are sufficiently close to one another, for example are within the same limited range, for example taking into account the rating of the components of the wind turbine generator 104, the rating of the electric power grid 102 and / or the operation / power level of the wind turbine generator 104 and / or of the electric power grid 102.

[0120] For some embodiments, the fault condition response control scheme may comprise, or correspond to, an FRT, for example an OVRT and / or an LVRT, for example according to one or more grid codes. For some embodiments, the fault condition response control scheme may comprise a low voltage response control scheme for providing voltage support to the electric power grid 102. With reference to figure 6, for some embodiments, the low voltage response control scheme may comprise the control 308a of an injection of reactive power to the electric power grid 102 for providing voltage support to the electric power grid 102. With reference to figure 6, which schematically illustrates several different embodiments, for some embodiments, the method 300 may comprise one or more of the steps of:

[0121] • detecting 301 a fault condition affecting the wind turbine generator 104;

[0122] • determining 302 an electric current at the first location 162. For example, the electric current at the first location 162 may be determined by being measured, for example by way of an electric current measuring unit 170, which may comprise one or more sensors, such as electric current sensors, for example electric current transformers. However, the electric current at the first location 162 may be determined in other conventional ways and / or by way of other conventional equipment known to the person skilled in the art;

[0123] • determining 303 an electric current at the second location 164. For example, the electric current at the second location 164 may be determined by being measured, for example by way of an electric current measuring unit 172, which may comprise one or more sensors, such as electric current sensors, for example electric current transformers. However, the electric current at the second location 164 may be determined in other conventional ways and / or by way of other conventional equipment known to the person skilled in the art;

[0124] • if the determined electric current at the first location 162 is different from the determined electric current at the second location 164, terminating 306 the electric power conversion of one or more of the one or more power converters 133a, 133b of the power converter arrangement 132;

[0125] • if the determined electric current at the first location 162 is different from the determined electric current at the second location 164, terminating 306 the electric power conversion of the line-side converter 133b;

[0126] • if the determined electric current at the first location 162 is different from the determined electric current at the second location 164, stopping 306a a control signal to the line-side converter 133b. For example, when the control signal to the line-side converter 133b is stopped, the machine-side converter 133a may still be operating, and then, electric energy / power from the DC link chopper of the DC link 135 connecting the machine-side converter 133a to the line-side converter 133b (more specifically, connecting the DC sides of the machine-side converter 133a and line-side converter 133b to one another) is advantageously dissipated; and

[0127] • if the determined electric current at the first location 162 is equal to the determined electric current at the second location 164, controlling 308 the lineside converter 133b according to a fault condition response control scheme to provide support to the electric power grid 102.

[0128] With reference to figure 6, for some embodiments, the method 300 may comprise the step of:

[0129] • in the comparison of the determined electric currents at the first and second locations 162, 164, including 309 one or more determined electric currents at one or more third locations 166 (see figure 3) in the harmonic filter arrangement 142.

[0130] It is to be understood that the third location 166 is different from the first and second locations 162, 164. For some embodiments, it may be defined that the third location 166 is spaced apart from the first and second locations 162, 164. For some embodiments, it may be defined that the electric potential of the third location 166 is different form the electric potential of the first and of the second location 162, 164. For some embodiments, it may be defined that there is an electric potential difference between the third location 166 and any one of the first and second locations 162, 164.

[0131] With reference to figure 6, for some embodiments, the method 300 may comprise the steps of:

[0132] • determining 309a one or more electric currents at one or more third locations 166 (see figure 3) in the harmonic filter arrangement 142. For example, the electric current at the third location 166 may be determined by being measured, for example by way of an electric current measuring unit 174, which may comprise one or more sensors, such as electric current sensors. However, the electric current at the third location 166 may be determined in other conventional ways and / or by way of other conventional equipment known to the person skilled in the art; and • comparing 304 the determined electric currents at the first and second locations 162, 164 while considering the determined electric current at the third location 166. For some embodiments, it may be defined that the electric current at the third location 166 comprises a filter current of the harmonic filter arrangement.

[0133] By way of one or more of the above-mentioned steps 309 and 309a, a further improved comparison of the electric currents at the first and second locations 162, 164 and / or a further improved determination whether the electric currents at the first and second locations 162, 164 are different from or equal to one another is / are provided. Because of the harmonic filter arrangement 142, a comparison of the electric currents at the first and second locations 162, 164 may, under some circumstances, have some current offsets. This is because of the harmonic filter capacitor’s 146 di / dt response to fast voltage transients. By including a harmonic filter current in the comparison of the electric currents at the first and second locations 162, 164, the determination whether the electric currents at the first and second locations 162, 164 are different from or equal to one another and whether the fault condition is internal or external to the wind turbine generator is further improved. The harmonic filter current may be determined by the above-mentioned step 309a, for example, by way of one or more electric current sensors.

[0134] With reference to figure 6, for some embodiments, the method 300 may comprise the step of:

[0135] • in the comparison of the determined electric currents at the first and second locations 162, 164, including 310 a filter capacitance of the harmonic filter arrangement 142 and one or more determined line voltages at one or more fourth locations 168 (see figure 3) between the electric power grid 102 and the harmonic filter arrangement 142 of the power converter arrangement 132.

[0136] It is to be understood that the fourth location 168 is different from the first and second locations 162, 164. For some embodiments, it may be defined that the electric potential of the fourth location 168 is different form the electric potential of the first and of the second location 162, 164. For some embodiments, it may be defined that there is an electric potential difference between the fourth location 168 and any one of the first and second locations 162, 164. When applicable, it is to be understood that the fourth location 168 is also different from the third location 166. For some embodiments, it may be defined that the fourth location 168 is spaced apart from the first and second locations 162, 164. When applicable, for some embodiments, it may be defined that the fourth location 168 is also spaced apart from the third location 166.

[0137] With reference to figure 6, for some embodiments, the method 300 may comprise the steps of:

[0138] • determining 310a one or more line voltages at one or more fourth locations 168 between the electric power grid 102 and the harmonic filter arrangement 142 of the power converter arrangement 132. For example, the line voltage at the fourth location 168 may be determined by being measured, for example by way of a voltage measuring unit 176, which may comprise one or more sensors, such as voltage sensors. However, the line voltage at the fourth location 168 may be determined in other conventional ways and / or by way of other conventional equipment known to the person skilled in the art; and

[0139] • comparing 304 the determined electric currents at the first and second locations 162, 164 while considering the determined line voltage at the fourth location 168 and a filter capacitance of the harmonic filter arrangement 142.

[0140] With reference to figure 6, for some embodiments, the method 300 may comprise the step of:

[0141] • in the comparison of the determined electric currents at the first and second locations 162, 164, including 310b one or more estimated filter currents of the harmonic filter arrangement 142 estimated based on a filter capacitance of the harmonic filter arrangement 142 and based on one or more determined line voltages at one or more fourth locations 168 between the electric power grid 102 and the harmonic filter arrangement 142 of the power converter arrangement 132. With reference to figure 6, for some embodiments, the method 300 may comprise the steps of:

[0142] • determining 310a one or more line voltages at one or more fourth locations 168 between the electric power grid 102 and the harmonic filter arrangement 142 of the power converter arrangement 132;

[0143] • estimating 310c one or more filter currents of the harmonic filter arrangement 142 based on the one or more determined line voltages at the one or more fourth locations 168 and on a filter capacitance of the harmonic filter arrangement 142; and

[0144] • comparing 304 the determined electric currents at the first and second locations 162, 164 while considering the estimated one or more filter currents of the harmonic filter arrangement 142.

[0145] As disclosed above, because of the harmonic filter arrangement 142, a comparison of the electric currents at the first and second locations 162, 164 may, under some circumstances, have some current offsets. The above-mentioned steps 310 and 310a- c provide advantageous alternatives to the above-mentioned steps 309 and 309a in order to include the harmonic filter current in the comparison of the electric currents at the first and second locations 162, 164. For example, an online estimator may be provided, which is configured to estimate the filter current of the harmonic filter arrangement 142, for example based on the measured line voltage (such as at the fourth location 168) and a known filter capacitance of the harmonic filter arrangement 142. The estimated electric current flowing into the harmonic filter arrangement can then be subtracted, and a better current loss can be estimated for the comparison of the determined electric currents at the first and second locations 162, 164. Under some circumstances, a challenge for the online estimator may be that the filter capacitance of the harmonic filter arrangement 142 may be reduced over time. However, this effect can be mitigated by having the power converter arrangement 132, during startup of the WTG 104 and of the power converter arrangement 132 (the capacitors are discharged before startup), estimating the actual capacitance of the harmonic filter arrangement 142, or simply having enough margin for the comparison of the electric currents at the first and second locations 162, 164. In order to further illustrate aspects of the above-mentioned steps 310 and 310a-c, reference is made to figures 8 and 9. The diagram 500 of figure 8 schematically illustrates a three-phase zero pu voltage fault ride through (FRT) entry with a filter estimator included, which calculates the total current leak per phase. The current spike shown is because of the transients into the harmonic filter, since the filter estimator model operates only at lower frequency. In order to increase detection performance, it is advantageous to have limits divided into two sections. One section for low impedance faults with a high current trip threshold for a short period (>0.5-2 ms) and another section for high impedance faults with a low current trip threshold for a longer period (>5-20 ms). The diagram 600 of figure 8 schematically illustrates a three-phase 0.2 pu voltage fault ride through (FRT) entry, showing the transient behaviour when the filter estimator is detecting on the difference of power converter stack currents (ILSC), such as at the first locations 162 in figure 3, versus line currents (IL), such as at the second location 162 in figure 3.

[0146] Unless disclosed otherwise, it should be noted that the method steps illustrated in figures 4 to 6 and described herein do not necessarily have to be executed in the order illustrated in figures 4 to 6. The steps may essentially be executed in any suitable order. Further, one or more steps may be added without departing from the scope of the appended claims. One or more steps may be excluded without departing from the scope of the appended claims. Method steps of the different embodiments of figures 6 and 7 may be combined in various ways so as to form additional embodiments.

[0147] With reference to figure 3, with regard to the first and second locations 162, 164, for some embodiments, the first and second locations 162, 164 may be located between the line-side converter 133b and the electric power grid 102. For some embodiments, the first location 162 may be located between the line-side inductor 140a-d and the electric generator 126 while the second location 164 may be located between the lineside inductor 140a-d and the electric power grid 102. For some embodiments, the first location 162 may be located between the line-side converter 133b and the line-side inductor 140a-d. For some embodiments, the first location 162 may be located between the circuit breaking apparatus 148 and the electric generator 126 while the second location 164 may be located between the circuit breaking apparatus 148 and the electric power grid 102. For some embodiments, the second location 164 may be located between the power converter arrangement 132 and the electric power grid 102. For some embodiments, the first location 162 may be located between the electric generator 126 and the second location 162.

[0148] With reference to figure 3, with regard to the first and second locations 162, 164, for some embodiments, the first and second locations 162, 164 may be located between the electric generator 126 and the transformer 152. For some embodiments, the first location 162 may be located between the electric generator 126 and the EMI / EMC filter arrangement 154 while the second location 164 may be located between the EMI / EMC filter arrangement 154 and the transformer 152. For some embodiments, the first location 162 may be located between the electric generator 126 and the harmonic filter arrangement 142 of the power converter arrangement 132 while the second location 164 may be located between the electric power grid 102 and the harmonic filter arrangement 142 of the power converter arrangement 132. For some embodiments, the first location 162 may be located between the line-side converter 133b and the harmonic filter arrangement 142 of the power converter arrangement 132.

[0149] It is to be understood that the method 300 and the wind turbine generator 104 are applicable to three-phase applications or three-phase electric power, although this is not explicitly shown in the figures for illustrative purposes. For example, figure 3 is a single line, or single phase, diagram, which only illustrates one of two or three phases.

[0150] With reference to figure 7, further aspects of the method 300 according to the first aspect are disclosed and explained. Figure 7 shows a schematic diagram 400 related to general short circuit maximum bolted faults, such as between two phases, which also may be referred to as low impedance faults. Two aims reached by embodiments of the method 300 according to the first aspect are as follows:

[0151] • Avoiding that the power converter arrangement’s 132 contribution exceeds the maximum grid fault level before the sine function reaches its amplitude level in the first % period of the sine function (see “d” in see figure 7) in case the fault is inside the power converter arrangement 132 or inside the wind turbine generator 104, but still complying with fault ride through (FRT) requirements specified by the grid code if the fault is outside the wind turbine generator 104; and

[0152] • Detecting short circuit faults with high impedance, such as arc faults (the “opposite” to bolted faults or low impedance faults), internal to the power converter arrangement 132 or to the wind turbine generator 104 so as to avoid the power converter arrangement’s 132 contribution to the fault.

[0153] By way of embodiments of the method 300 according to the first aspect, an advantageous and efficient way to distinguish between faults internal to the power converter arrangement 132, or to the wind turbine generator 104, and faults external to the power converter arrangement 132, or to the wind turbine generator 104, is provided, while suppressing the power converter arrangement’s 132 electric current contribution to an internal fault. Further, by way of embodiments of the method 300 according to the first aspect, it is also ensured that a high impedance short circuit fault is detected and stopped before it evolves into a severe low impedance fault.

[0154] In the diagram or graph 400 of figure 7, which is related to general short circuit maximum bolted faults, a, b, c, and V indicate the following: a. is the maximum grid contribution amplitude. The short circuit level must not increase by the addition of the power converter contribution; b. is an example of the short circuit amplitude with the power converter contribution magnitude added to the amplitude; c. is an example of an area or region where the power converter contribution is acceptable, since the sum is still within the maximum grid contribution; and d. is an example of an available time period for power converter control (or WTG control) to determine whether the fault is internal to the wind turbine generator 104 or external to the wind turbine generator 104 in order to proceed with fault ride through (FRT) according to grid code requirements or to terminate, or interrupt, the electric power conversion of the power converter.

[0155] In general, with reference to figure 3, a low impedance fault internal to the WTG 104 or to the power converter (i.e. , bolted short circuit faults) causes the voltage to collapse, i.e. , go to substantially zero or close to zero, in the short circuit location, such as at 158. Conventionally, the power converter stack 138a-d will feed electric current to the fault 158, which will cause the electric current to rise instantaneously because of the low voltage at the short circuit location 158. Conventionally, a conventional autonomous power converter stack protection system will be activated and terminate electric power conversion, for example at a threshold current of 2 x pu (per-unit). Therefore, in general, the power converter contribution to an internal low impedance fault will therefore be stopped within microseconds. This will ensure that the power converter contribution to the internal fault will be stopped before the AC grid current amplitude reaches maximum, a, in the first % period of the sine function (wave), as indicated in figure 7. Hereby, a reduction of a short circuit maximum fault is attained.

[0156] With reference to figure 3, in general, the manner in which embodiments of the method 300 avoid contribution to internal faults may be described as follows:

[0157] • For faults outside the WTG 104 (such as fault ride through (FRT) events), for example at the external fault location 160, substantially the same electric current (currents with substantially the same ampere) will be measured / sensed at first location 162 (for example, by way of one or more power stack sensors) and at the second location 164 (for example, by way of a line-side AC sensor), since the power converter arrangement 132 contributes to the fault outside of and on the grid-side of the WTG 104;

[0158] • For faults internal the WTG 104, for example at the internal fault location 158, different electric currents (currents of different ampere) will be measured / sensed at first location 162 (for example, by way of one or more power stack sensors) and at the second location 164 (for example, by way of a line-side AC sensor), since at the first location 162, the stack sensor senses the power converter contribution to the internal fault while at the second location 164, the line-side AC sensor senses the electric power grid’s contribution to the internal fault; and

[0159] • By comparing 304 the determined electric current at the first location 162 to the determined electric current at the second location 164, the fault event / condition can be determined, such as if the fault is internal or external to the WTG 104. If the electric current sensors do not measure / sense the same electric current (currents with substantially the same ampere), the fault can be considered internal to the WTG 104, upon which electric power conversion (or PWM) is immediately terminated / interrupted so as to stop contribution to internal faults.

[0160] With reference to figures 2 and 10, aspects of embodiments of the control arrangement 134 for controlling a wind turbine generator 104 connected to an electric power grid 102 according to the third aspect of the invention are schematically illustrated, wherein the wind turbine generator 104 comprises an electric generator 126 and a power converter arrangement 132 for electric power conversion, and wherein the electric generator 126 is connected to the electric power grid 102 via the power converter arrangement 132. Embodiments of the control arrangement 134 are configured to: in response to an indication of a fault condition affecting the wind turbine generator 104, compare 304 one or more determined electric currents at one or more first locations 162 to one or more determined electric currents at one or more second locations 164, the first and second locations 162, 164 being spaced apart from one another and located between the electric generator 126 and the electric power grid 102; and based on the comparison of the determined electric currents at the first and second locations 162, 164 and if the determ ined electric current at the first location 162 is different from the determined electric current at the second location 164, terminate 306 an electric power conversion of the power converter arrangement 132.

[0161] With reference to figure 2, some embodiments of the control arrangement 134 may include a detection unit 134a for detecting 301 a fault condition affecting the wind turbine generator 104 in order to perform step 301 in figure 6. The detection unit 134a may be configured to be connected to and / or to communicate with a sensor or detector for measuring or detecting a signal or fault current / voltage associated with the fault condition, in the wind turbine generator 104, in the power converter arrangement 132, in the point of common coupling, PCC, 114, in the electric power grid 102, or elsewhere. With reference to figure 2, some embodiments of the control arrangement 134 may include a first determination unit 134b for determining an electric current and / or a line voltage in order to perform steps 302, 303, 309a and 310c in figure 6. For some embodiments, the control arrangement 134 may include a comparison unit 134c for comparing one or more determined electric currents at one or more first locations 162 to one or more determined electric currents at one or more second locations 164 in order to perform step 304 in figures 4 to 6. For some embodiments, the control arrangement 134 may include a second determination unit 134d for determining whether the fault condition is occurring in 158 or outside 160 the wind turbine generator 104 in order to perform steps 305 and 307 in figures 5 and 6. For some embodiments, the control arrangement 134 may include a termination unit 134e for terminating the electric power conversion in order to perform step 306 in figures 4 to 6.

[0162] With reference to figure 2, for some embodiments, the control arrangement 134 may include a controlling unit 134f for controlling the power converter arrangement 132 according to a fault condition response control scheme in order to perform steps 308 and 308a in figure 6. For some embodiments, the control arrangement 134 may include an inclusion unit 134g for performing steps 309, 310 and 310b in figure 6. For some embodiments, the control arrangement 134 may include an estimation unit 134h for estimating one or more filter currents of the harmonic filter arrangement 134 in order to perform step 310c in figure 6.

[0163] With reference to figure 2, for some embodiments, the control arrangement 134 is configured to directly or indirectly communicate, for example via signal lines (or cables or wires) or wirelessly, with one or more of the group of: the power converter arrangement 132; the wind turbine generator 104; the power plant 100; the electric power grid 102; the measuring units 170, 172, 174, 176; sensors; and other devices or systems of the wind turbine generator 104, of the power plant 100, or of the electric power grid 102.

[0164] Figure 10 shows in schematic representation an embodiment of the control arrangement 134 according to the third aspect of the invention, which may include a control unit 700, which may correspond to or may include one or more of the above- mentioned units 134a-h of the control arrangement 134. The control unit 700 may comprise a computing unit 701 , which can be constituted by essentially any suitable type of processor or microcomputer, for example a circuit for digital signal processing (Digital Signal Processor, DSP), or a circuit having a predetermined specific function (Application Specific Integrated Circuit, ASIC). The computing unit 701 is connected to a memory unit 702 arranged in the control unit 700. The memory unit 702 provides the computing unit 701 with, for example, the stored program code and / or the stored data which the computing unit 701 requires to be able to perform computations. The computing unit 701 is also arranged to store partial or final results of computations in the memory unit 702.

[0165] With reference to figure 9, in addition, the control unit 700 may be provided with devices 711 , 712, 713, 714 for receiving and transmitting input and output signals. These input and output signals may contain waveforms, impulses, or other attributes which, by means of the devices 711 , 713 for the reception of input signals, can be detected as information and can be converted into signals which can be processed by the computing unit 701 . These signals are then made available to the computing unit 701 . The devices 712, 714 for the transmission of output signals are arranged to convert signals received from the computing unit 701 in order to create output signals by, for example, modulating the signals, which, for example, can be transmitted to other parts and / or systems of, or associated with, the wind turbine generator 104 (see figures 1 and 2), the electric power grid 102 (see figures 1 and 3) and / or the power plant 100 (see figure 1 ). Each of the connections to the devices for receiving and transmitting input and output signals can be constituted by one or more of a cable; a data bus; and a wireless connection.

[0166] Here and in this document, units are often described as being provided for performing steps of the method according to embodiments of the invention. This also includes that the units are designed to and / or configured to perform these method steps.

[0167] With reference to figure 2, the units 134a-h of the control arrangement 134 are in figure 2 illustrated as separate units. These sperate units may, however, be logically separated but physically implemented in the same unit, or can be both logically and physically arranged together. The units 134a-h may for example correspond to groups of instructions, which can be in the form of programming code, that are input into, and are utilized by a processor / computing unit 701 (see figure 10) when the units are active and / or are utilized for performing its method step.

[0168] With reference to figures 2 and 10, the control arrangement 134, which may include one or more control units 700, for example one or more devices, controllers or control devices, according to embodiments of the present invention may be arranged to perform all of the method steps mentioned above, in the claims, and in connection with the herein described embodiments. The control arrangement 134 is associated with the above-described advantages for each respective embodiment of the method.

[0169] With reference to figure 10, according to the second aspect of the invention, a computer program 703 or a computer-readable medium is provided, comprising instructions which, when the program or the instructions is / are executed by a computer, cause the computer to carry out the method according to any one of the embodiments disclosed above.

[0170] The person skilled in the art will appreciate that the herein described embodiments of the method according to the first aspect may be implemented in a computer program 703 (see figure 10), which, when it is executed in a computer, instructs the computer to execute the method. The computer program is usually constituted by a computer program product 703 stored on a non-transitory / non-volatile digital storage medium, in which the computer program is incorporated in the computer-readable medium of the computer program product. The computer-readable medium comprises a suitable memory, such as, for example: ROM (Read-Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically Erasable PROM), a hard disk unit, etc.

[0171] With reference to figures 1 to 3, according to the fourth aspect of the invention, a wind turbine generator 104 is provided, wherein the wind turbine generator 104 includes an electric generator 126 and a power converter arrangement 132 for electric power conversion. The electric generator 126 is connectable, or connected, more specifically electrically connected / connectable, to an electric power grid 102 via the power converter arrangement 132. The wind turbine generator 104 includes a control arrangement 134 according to any one of the embodiments disclosed above. Further embodiments of the wind turbine generator 104 are disclosed above.

[0172] With reference to figure 1 , according to the fifth aspect of the invention, a power plant 100 is provided. The power plant 100 is configured to provide electric power to an electric power grid 102. The power plant 100 comprises one or more wind turbine generators 104 and one or more of the group of: • a control arrangement 134 according to any one of the embodiments disclosed above; and

[0173] • a wind turbine generator 104 according to any one of the embodiments disclosed above.

[0174] Further embodiments of the power plant 100 are disclosed above.

[0175] The present invention is not limited to the above-described embodiments. Instead, the present invention relates to, and encompasses all different embodiments being included within the scope of the independent claims.

Claims

Claims1 . A method (300) for controlling a wind turbine generator (104) connected to an electric power grid (102), wherein the wind turbine generator (104) comprises an electric generator (126) and a power converter arrangement (132) for electric power conversion, wherein the electric generator (126) is connected to the electric power grid (102) via the power converter arrangement (132), wherein the method (300) comprises: in response to an indication of a fault condition affecting the wind turbine generator (104), comparing (304) one or more determined electric currents at one or more first locations (162) to one or more determined electric currents at one or more second locations (164), the first and second locations (162, 164) being spaced apart from one another and located between the electric generator (126) and the electric power grid (102); and based on the comparison of the determined electric currents at the first and second locations (162, 164) and if the determined electric current at the first location (162) is different from the determined electric current at the second location (164), terminating (306) an electric power conversion of the power converter arrangement (132).

2. A method (300) according to claim 1 , wherein the method (300) further comprises: if the determined electric current at the first location (162) is different from the determined electric current at the second location (164), determining (305) that the fault condition is occurring in the wind turbine generator (104); and if it is determined that the fault condition is occurring in the wind turbine generator (104), terminating (306) an electric power conversion of the power converter arrangement (132).

3. A method (300) according to claim 1 or 2, wherein the method (300) further comprises:based on the comparison of the determined electric currents at the first and second locations (162, 164) and if the determined electric current at the first location (162) is equal to the determined electric current at the second location (164), controlling (308) the power converter arrangement (132) according to a fault condition response control scheme to provide support to the electric power grid (102).

4. A method (300) according to claim 3, wherein the method (300) further comprises: if the determined electric current at the first location (162) is equal to the determined electric current at the second location (164), determining (307) that the fault condition is occurring outside the wind turbine generator (104); and if it is determined that the fault condition is occurring outside the wind turbine generator (104), controlling (308) the power converter arrangement (132) according to a fault condition response control scheme to provide support to the electric power grid (102).

5. A method (300) according to any one of the claims 1 to 4, wherein the power converter arrangement (132) comprises a machine-side converter (133a) and a lineside converter (133b), wherein the machine-side converter (133a) is connected to the electric power grid (102) via the line-side converter (133b) while the line-side converter (133b) is connected to the electric generator (126) via the machine-side converter (133a), wherein the power converter arrangement (132) comprises one or more lineside inductors (140a-d), wherein the line-side converter (133b) is connected to the electric power grid (102) via the line-side inductor (140a-d), wherein the first location (162) is located between the line-side inductor (140a- d) and the electric generator (126), and wherein the second location (164) is located between the line-side inductor (140a-d) and the electric power grid (102).

6. A method (300) according to claim 5, wherein the first location (162) is located between the line-side converter (133b) and the line-side inductor (140a-d).

7. A method (300) according to any one of the claims 1 to 6, wherein the wind turbine generator (104) comprises a circuit breaking apparatus (148) comprising one or more circuit breakers (150a, 150b), wherein the power converter arrangement (132) is connected to the electric power grid (102) via the circuit breaking apparatus (148), wherein the first location (162) is located between the circuit breaking apparatus (148) and the electric generator (126), and wherein the second location (164) is located between the circuit breaking apparatus (148) and the electric power grid (102).

8. A method (300) according to any one of the claims 1 to 7, wherein the second location (164) is located between the power converter arrangement (132) and the electric power grid (102).

9. A method (300) according to any one of the claims 1 to 8, wherein the power converter arrangement (132) comprises a harmonic filter arrangement (142) comprising one or more harmonic filters (144) and one or more capacitors (146), wherein the electric generator (126) is connected to the electric power grid(102) via the harmonic filter arrangement (142), wherein the first location (162) is located between the electric generator (126) and the harmonic filter arrangement (142) of the power converter arrangement (132), and wherein the second location (164) is located between the electric power grid (102) and the harmonic filter arrangement (142) of the power converter arrangement (132).

10. A method (300) according to claim 9, wherein the power converter arrangement (132) comprises a machine-side converter (133a) and a line-side converter (133b), wherein the machine-side converter (133a) is connected to the electric power grid (102) via the line-side converter (133b) while the line-side converter (133b) is connected to the electric generator (126) via the machine-side converter (133a),wherein the line-side converter (133b) is connected to the electric power grid (102) via the harmonic filter arrangement (142), and wherein the first location (162) is located between the line-side converter (133b) and the harmonic filter arrangement (142) of the power converter arrangement (132).

11. A method (300) according to claim 9 or 10, wherein the method (300) comprises: in the comparison of the determined electric currents at the first and second locations (162, 164), including (309) one or more determined electric currents at one or more third locations (166) in the harmonic filter arrangement (142).

12. A method (300) according to any one of the claims 9 to 11 , wherein the method (300) comprises: in the comparison of the determined electric currents at the first and second locations (162, 164), including (310) a filter capacitance of the harmonic filter arrangement (142) and one or more determined line voltages at one or more fourth locations (168) between the electric power grid (102) and the harmonic filter arrangement (142) of the power converter arrangement (132).

13. A computer program (703) or a computer-readable medium comprising instructions which, when the program or the instructions is / are executed by a computer, cause the computer to carry out the method (300) according to any one of the claims 1 to 12.

14. A control arrangement (134) for controlling a wind turbine generator (104) connected to an electric power grid (102), wherein the wind turbine generator (104) comprises an electric generator (126) and a power converter arrangement (132) for electric power conversion, wherein the electric generator (126) is connected to the electric power grid (102) via the power converter arrangement (132), and wherein the control arrangement (134) is configured to:in response to an indication of a fault condition affecting the wind turbine generator (104), compare (304) one or more determined electric currents at one or more first locations (162) to one or more determined electric currents at one or more second locations (164), the first and second locations (162, 164) being spaced apart from one another and located between the electric generator (126) and the electric power grid (102); and based on the comparison of the determined electric currents at the first and second locations (162, 164) and if the determined electric current at the first location (162) is different from the determined electric current at the second location (164), terminate (306) an electric power conversion of the power converter arrangement (132).

15. A wind turbine generator (104) comprising an electric generator (126), and a power converter arrangement (132) for electric power conversion, wherein the electric generator (126) is connectable to an electric power grid (102) via the power converter arrangement (132), and wherein the wind turbine generator (104) comprises a control arrangement (134) according to claim 14.

16. A power plant (100) for providing electric power to an electric power grid (102), wherein the power plant (100) comprises one or more wind turbine generators (104), and wherein the power plant (100) comprises one or more of the group of:• a control arrangement (134) according to claim 14; and• a wind turbine generator (104) according to claim 15.

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