TECHNICAL FIELD

[0001] The present invention relates generally to a method for monitoring structural health of a filter comprising a capacitor in a wind turbine, a system for monitoring structural health of a filter comprising a capacitor in a wind turbine, and a wind turbine.

BACKGROUND

[0002] With a recent trend of using green power, there is an increase in a demand for wind turbine power. Thus, a wind turbine may need to generate more power to meet the increase in the demand of the wind turbine power. As such, more filters, for example grid harmonic filters, may be required in the wind turbine to comply with grid requirements. With more filters in the wind turbine, it may be more difficult to perform a timely maintenance of the filters. Failures of the filters for example the grid harmonic filter may lead to a failure in grid compliance since higher harmonic current and voltage will be fed to the grid causing a higher total harmonic distortion (THD). Thus, the IEC standard THD requirement of about <5 may not be complied. Further, the higher harmonics can also lead to increased power losses and thus increased heating on the converter system components like the generator, transformers and other capacitors which could lead to damages of these components.

[0003] Thus, it is desirable to have a method and a system for monitoring structural health of a filter in a wind turbine. SUMMARY

[0004] According to one embodiment of the present invention, a method for monitoring structural health of a filter comprising a capacitor in a wind turbine is provided. The method includes measuring a capacitor harmonic current and a capacitor harmonic voltage; determining an impedance value based on the measured capacitor harmonic current and the measured capacitor harmonic voltage; comparing the determined impedance value with a reference impedance value; and determining the structural health of the filter based on the difference between the determined impedance value and the reference impedance value. The capacitor harmonic current may be measured at an AC side of an inverter and after a grid choke. The capacitor harmonic voltage may also be measured at an AC side of an inverter and after a grid choke. The structural health of the filter in the wind turbine may be monitored online, i.e. during operation of the wind turbine. Thus, an early warning for maintenance of the filter may be provided. Failures of the filter which may lead to a failure in grid compliance can be avoided. Damages to the converter system components like the generator, transformers and other capacitors can also be avoided.

[0005] According to one embodiment of the present invention, an amplitude of the capacitor harmonic current and an amplitude of the capacitor harmonic voltage are measured at a carrier frequency and at least one subsequent harmonic of the carrier frequency. [0006] According to one embodiment of the present invention, different types of filters may be used in a wind turbine. Some examples of the filters include a harmonic filter, a resonant filter, a differential filter and a smoothing filter.

[0007] According to one embodiment of the present invention, different kinds of capacitors may be used in the filters. For example, the capacitor may be a film capacitor. The capacitor may be an AC film capacitor or a DC film capacitor.

[0008] According to one embodiment of the present invention, determining the structural health of the filter includes determining one or more of a life state of the capacitor and a remaining lifetime of the capacitor.

[0009] According to one embodiment of the present invention, the life state of the capacitor and the remaining lifetime of the capacitor are further determined based on different kinds of factors. For example, the factors may include a reference equivalent series resistance of the filter and a temperature of the filter. The factors may also include a common mode total harmonic distortion in the filter.

[0010] According to one embodiment of the present invention, a system for monitoring structural health of a filter comprising a capacitor in a wind turbine is provided. The system includes a current determining unit configured to measure a capacitor harmonic current; a voltage determining unit configured to measure a capacitor harmonic voltage; a processing unit coupled to the current determining unit and the voltage determining unit, wherein the processing unit is configured to determine an impedance value based on the measured capacitor harmonic current received from the current determining unit and the measured capacitor harmonic voltage received from the voltage determining unit, compare the determined impedance value with a reference impedance value, and determine the structural health of the filter based on the difference between the determined impedance value and the reference impedance value. The current determining unit may measure the capacitor harmonic current at an AC side of the inverter and after a grid choke. The voltage determining unit may measure the capacitor harmonic voltage at an AC side of the inverter and after a grid choke. The structural health of the filter in the wind turbine may be monitored online, i.e. during operation of the wind turbine. Thus, an early warning for maintenance of the filter may be provided. Failures of the filters which may lead to a failure in grid compliance can be avoided. Damages to the converter system components like the generator, transformers and other capacitors can also be avoided.

[0011] According to one embodiment of the present invention, the current determining unit is configured to measure an amplitude of the capacitor harmonic current at a carrier frequency and at least one subsequent harmonic of the carrier frequency.

[0012] According to one embodiment of the present invention, the current determining unit includes at least one current sensor configured to detect the capacitor harmonic current; at least one band pass filter, wherein each band pass filter is coupled to a corresponding current sensor and the processing unit, and wherein each band pass filter is configured to eliminate high order frequencies of the capacitor harmonic current and to transmit a signal indicative of the amplitude of the capacitor harmonic current measured at the carrier frequency and the at least one subsequent harmonic of the carrier frequency to the processing unit. [0013] According to one embodiment of the present invention, the voltage determining unit is configured to measure an amplitude of the capacitor harmonic voltage at the carrier frequency and the at least one subsequent harmonic of the carrier frequency.

[0014] According to one embodiment of the present invention, the voltage determining unit includes at least one resistor-capacitor (RC) tuned circuit; at least one isolation transformer, wherein each isolation transformer is coupled to a corresponding RC tuned circuit; at least one band pass filter, wherein each band pass filter is coupled to a corresponding isolation transformer and the processing unit, and wherein each band pass filter is configured to eliminate high order frequencies of the capacitor harmonic voltage and to transmit a signal indicative of the amplitude of the capacitor harmonic voltage measured at the carrier frequency and the at least one subsequent harmonic of the carrier frequency to the processing unit.

[0015] According to one embodiment of the present invention, different types of filters may be used in a wind turbine. Some examples of the filters include a harmonic filter, a resonant filter, a differential filter and a smoothing filter.

[0016] According to one embodiment of the present invention, different kinds of capacitors may be used in the filters. For example, the capacitor may be a film capacitor.

The capacitor may be an AC film capacitor or a DC film capacitor.

[0017] According to one embodiment of the present invention, the processing unit is configured to determine one or more of a life state of the capacitor and a remaining lifetime of the capacitor.

[0018] According to one embodiment of the present invention, the processing unit is configured to further determine the life state of the capacitor and the remaining lifetime of the capacitor based on different kinds of factors. For example, the factors may include a reference equivalent series resistance of the filter and a temperature of the filter. The factors may also include a common mode total harmonic distortion in the filter.

[0019] According to one embodiment of the present invention, a wind turbine is provided. The wind turbine includes at least one filter having a capacitor, and a system as described above coupled to the at least one filter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which:

[0021] Figure 1 illustrates a common setup of a conventional wind turbine.

[0022] Figure 2 shows a flow chart diagram of a method for monitoring a structural health of a filter in a wind turbine according to an embodiment of the present invention.

[0023] Figure 3 shows a schematic drawing of a system for monitoring a structural health of a filter in a wind turbine according to an embodiment of the present invention.

[0024] Figure 4 shows a schematic drawing of a system for monitoring structural health of a filter having a capacitor in a wind turbine according to an embodiment of the present invention. [0025] Figure 5 shows a schematic drawing of a processing unit of a system for monitoring structural health of a filter having a capacitor in a wind turbine according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0026] Figure 1 illustrates a common setup of a conventional wind turbine 100. The wind turbine 100 is mounted on a base 102. The wind turbine 100 includes a tower 104 having a number of tower sections. A wind turbine nacelle 106 is placed on top of the tower 104. The wind turbine rotor includes a hub 108 and at least one rotor blade 110, e.g. three rotor blades 110. The rotor blades 110 are connected to the hub 108 which in turn is connected to the nacelle 106 through a low speed shaft which extends out of the front of the nacelle 106. . The low speed shaft typically drives a generator (not shown) for producing electrical power. The electrical power generated is thereafter usually conditioned by a converter system (not shown), comprising a power converter, prior to delivery from the wind turbine.

[0027] The wind turbine 100 may include one or more filters. Different types of filters may be used in the wind turbine. The filters may include, but are not limited to, harmonic filters, resonant filters, differential filters and smoothing filters.

[0028] In one embodiment, the filters include a capacitor. For example, a smoothing filter located at the DC link may include a single capacitor or a bank of capacitors. The capacitor may be a film capacitor. Aging, high overload and poor quality of capacitors in the filters may result in failure in the filters. There may also be a reduction of capacitance of the filters which may cause an increase in total harmonic distortion in the filters.

[0029] Thus, structural health of the filter(s) may be monitored so that an early warning or alert for maintenance may be provided. Failures of the filters which may lead to a failure in grid compliance can be avoided. Damages to the converter system components like the generator, transformers and other capacitors can also be avoided.

[0030] Figure 2 shows a flow chart 200 of a method for monitoring structural health of a filter comprising a capacitor in a wind turbine. At 202, a capacitor harmonic current and a capacitor harmonic voltage are measured. The capacitor harmonic current may be measured at an AC side of an inverter and after a grid choke. The capacitor harmonic voltage may also be measured at an AC side of an inverter and after a grid choke. At 204, an impedance value is determined based on the measured capacitor harmonic current and the measured capacitor harmonic voltage. At 206, the determined impedance value is compared with a reference impedance value. In one embodiment, the reference impedance value is predetermined and relates to a healthy state of the filter. In another embodiment, the reference impedance value is obtained from supplier data recorded during production testing. At 208, the structural health of the filter is determined based on the difference between the determined impedance value and the reference impedance value.

[0031] Figure 3 shows a schematic drawing of an embodiment of a system 300 for monitoring a structural health of a filter comprising a capacitor in a wind turbine. The system 300 includes a current determining unit 302, a voltage determining unit 304 and a processing unit 306. The current determining unit 302 and the voltage determining unit 304 are coupled to the processing unit 306.

[0032] The current determining unit 302 measures a capacitor harmonic current. In one embodiment, the current determining unit 302 may measure the capacitor harmonic current at an AC side of an inverter and after a grid choke. The current determining unit 302 measures an amplitude of the capacitor harmonic current at a carrier frequency and at least one subsequent harmonic of the carrier frequency. The voltage determining unit 304 measures a capacitor harmonic voltage. In one embodiment, the voltage determining unit 304 may measure the capacitor harmonic voltage at an AC side of an inverter and after a grid choke. The voltage determining unit 304 measures an amplitude of the capacitor harmonic voltage at the carrier frequency and the at least one subsequent harmonic of the carrier frequency.

[0033] The carrier frequency may be the carrier frequency or switching frequency of a power converter of the wind turbine 100. The carrier frequency may also be the fundamental 50Hz or 60Hz frequency. The carrier frequency may be a fundamental frequency or a 1 ^st harmonic. Each harmonic of the carrier frequency is a multiple of the carrier frequency. For example, a 2 ^nd harmonic has a frequency of (2 x carrier frequency), a 3 rd harmonic has a frequency of (3 x carrier frequency), and so on.

[0034] The processing unit 306 determines an impedance value based on the measured capacitor harmonic current received from the current determining unit 302 and the measured capacitor harmonic voltage received from the voltage determining unit 304. The processing unit 306 compares the determined impedance value with a reference impedance value, and determines the structural health of the filter based on the difference between the determined impedance value and the reference impedance value. The reference impedance value is predetermined and relates to a healthy state of the filter. In another embodiment, the reference impedance value is obtained from supplier data recorded during production testing.

[0035] In one embodiment, the system 300 can include a monitoring interface 308 coupled to the processing unit 306. The monitoring interface 308 receives signals indicative of the structural health of the filter and monitors results of the structural health of the filter.

[0036] In another embodiment, the structural health of the filter, which may refer to a state of the capacitor, is fed back to the main controller of the wind turbine or to a wind turbine monitoring system, allowing for further actions to be taken. For example, an alarm is raised once the structural health of the filter, comprising the impedance of a capacitor, deviates from certain predetermined margins. The alarm may trigger maintenance to check on the capacitors in the filters respectively.

[0037] Figure 4 shows a schematic drawing of one embodiment of the system 300. The system 300 is coupled between a power converter 402 of a wind turbine and a transformer 404 of a wind turbine. The transformer 404 is coupled to a power grid (not shown). A grid choke 406 is coupled between the power converter 402 and the system 300. As shown in Figure 4, the power converter 402 may be an inverter.

[0038] In one embodiment, the wind turbine may include a plurality of filters. As an illustration, two harmonic filters 408 and a resonant filter 410 are shown in Figure 4. The harmonic filters 408 and the resonant filter 410 each comprise at least one capacitor. The capacitors may be film capacitors (e.g. AC or DC film capacitors). The current determining unit 302 and the voltage determining unit 304 are coupled to the two harmonic filters 408 and the resonant filter 410.

[0039] The current determining unit 302 has a plurality of current sensors 412. Each current sensor 412 may be located at an AC side of the power converter 402 (e.g. inverter) and after the grid choke 406. Each current sensor 412 is coupled to a corresponding filter 408, 410, in particular to the capacitor in the corresponding filter 408, 410. Each current sensor 412 detects a capacitor harmonic current of the corresponding filter 408, 410. The capacitor harmonic current may be measured at a carrier frequency of the inverter 402 and at least one subsequent harmonic of the carrier frequency. The capacitor harmonic current may be measured at an AC side of the power converter 402 (e.g. inverter) and after the grid choke 406.

[0040] By measuring the capacitor harmonic current at an AC side of the power converter 402 (e.g. inverter) and after the grid choke 406, each current sensor 412 is connected in series with a capacitor (e.g. AC harmonic capacitor) of the corresponding filter 408, 410. Each current sensor 412 directly measures harmonic current passing through the capacitor of the corresponding filter 408, 410. Therefore, lower rating current sensors can be used for the current sensors 412 since the direct harmonic signal passing through the capacitors include mainly switching and harmonic frequencies and there may not be a necessity to separate the low frequency 50Hz output current from the harmonic currents.

[0041] Further, the capacitor harmonic current measured by each sensor 412 may include three phase currents la, lb and Ic. Based on the three phase currents la, lb and Ic, the unbalanced state of the harmonic currents can be determined. Thus, failure of any leg of the three phase AC harmonic filter 408 or resonant filter 410 can be determined.

[0042] DC link capacitor failure can also be determined by analyzing the data from the harmonic current sensed on the AC harmonic filter 408 and from the input and output current and/or voltage sensed on the power converter . The wind turbine converter system comprises a conventional AC/DC/AC power converter (not shown in the drawings) which comprises a AC/DC rectifier coupled to a DC/ AC inverter through a DC link. The power converter output corresponds to the DC/ AC inverter output, and the output is coupled to the AC harmonic filter. The power converter usually comprises input and output current and/or voltage sensing, but does not necessarily comprise DC link capacitor current sensing. Using the data from input and output current sensing of the conventional power converter together with the harmonic current sensed on the AC harmonic filter 408, it can be determined if the DC link capacitor is degraded or damaged.

[0043] The current determining unit 302 further comprises a plurality of band pass filters 414. Each band pass filter 414 is coupled to a corresponding current sensor 412 and the processing unit 306. In an embodiment, the processing unit 306 is the power converter controller. Each band pass filter 414 eliminates high order frequencies of the capacitor harmonic current. Each band pass filter 414 then transmits a signal indicative of the amplitude of the capacitor harmonic current measured at the carrier frequency and the at least one subsequent harmonic of the carrier frequency to the processing unit 306. In one embodiment, each band pass filter 414 may eliminate frequencies which are above 50 ^th harmonic of the capacitor harmonic current. Thus, each band pass filter 414 transmits a signal indicative of the amplitude of the capacitor harmonic current measured at the carrier frequency and the respective amplitudes of the capacitor harmonic current measured at frequencies of 2 ^nd to 50 ^th harmonic of the carrier frequency.

[0044] The voltage determining unit 304 comprises a plurality of resistor-capacitor (RC) tuned circuits 416 (e.g. two RC circuits 416). One RC circuit 416 is coupled to the two harmonic filters 408 and the other RC circuit 416 is coupled to the resonant filter 410. The voltage determining unit 304 further comprises voltage sensors which detects the filter harmonic voltage of the corresponding filter(s) 408, 410 and provides the capacitor harmonic voltage measurements to the corresponding RC circuit 416. The capacitor harmonic voltage may be measured at the carrier frequency of the inverter and at least one subsequent harmonic of the carrier frequency. The capacitor harmonic voltage is passed to the RC tuned circuit 416 to allow only switching harmonics of a lower voltage level. The voltage sensed is usually on the level of about 600-1000V and should be lower around 0-5V to be fed to a typical controller circuit. If the sensed voltage is passed to a conventional step down transformer to lower the voltage, the resolution of the sensed voltage is reduced due to inherent high inductance of the conventional step down transformer. Thus, the RC tuned circuit 416 provides a higher resolution of the sensed voltage since the RC filter is designed to allow only the carrier frequency and harmonics of interest. An isolation transformer of lower inductance is added for isolation purposes and will not affect the resolution of the sensed voltage since a lower inductance can be selected.

[0045] The voltage determining unit 304 also comprises a plurality of isolation transformers 418 (e.g. two isolation transformers 418). Each isolation transformer 418 is coupled to a corresponding RC tuned circuit 416. The voltage determining unit 304 further comprises a plurality of band pass filters 420 (e.g. two band pass filters 420). Each band pass filter 420 is coupled to a corresponding isolation transformer 418 and the processing unit 306. Each isolation transformer 418 transmits a signal indicative of the capacitor harmonic voltage to the corresponding band pass filter 420. As explained above, using a conventional step down transformer would introduce additional inductance. Thus, using an isolation transformer 418 may prevent unintended coupling effect to the voltage determining unit 314 due to introduction of additional inductance of a conventional step down transformer. The isolation transformer 418 also provides isolation from high voltage circuit to low voltage circuit and serves as a safety measure. Isolation is provided by means of the air gap insulation and tape insulation, leading to a low leakage current between the primary and secondary windings, within the isolation transformer 418. The isolation transformer 418 may also provide isolation of noise at certain frequencies from interfering with the quality of the measured signal. The RC tuned circuit 416 and the isolation transformer 418 are selected with a consideration that the resolution of measurement readings are not affected when provided to the processing unit 306.

[0046] Each band pass filter 420 eliminates high order frequencies of the capacitor harmonic voltage. Each band pass filter 420 then transmits a signal indicative of the amplitude of the capacitor harmonic voltage measured at the carrier frequency and the at least one subsequent harmonic of the carrier frequency to the processing unit 306. In one embodiment, each band pass filter 420 may eliminate frequencies which are above 50 ^th harmonic of the capacitor harmonic voltage. Thus, each band pass filter 420 may transmit a signal indicative of the amplitude of the capacitor harmonic voltage measured at the carrier frequency and the respective amplitudes of the capacitor harmonic voltage measured at frequencies of 2 ^nd to 50 ^th harmonic of the carrier frequency.

[0047] The signal indicative of the amplitude of the capacitor harmonic current measured at the carrier frequency and the at least one subsequent harmonic of the carrier frequency, and the signal indicative of the amplitude of the capacitor harmonic voltage measured at the carrier frequency and the at least one subsequent harmonic of the carrier frequency may be represented by the following Fourier series equation:

(S _N f)(x) = ^ +∑[a„ cos(nx) + b„ sm(nx)], N≥0

„=l [0048] The processing unit 306 may determine the impedance value of each filter 408, 410 based on the corresponding signal indicative of the amplitude of the capacitor harmonic current measured at the carrier frequency and the at least one subsequent harmonic of the carrier frequency received from the current determining unit 302 and the corresponding signal indicative of the amplitude of the capacitor harmonic voltage measured at the carrier frequency and the at least one subsequent harmonic of the carrier frequency received from the voltage determining unit 304. In one embodiment, the determined impedance value of each filter 408, 410 may be based on a harmonic frequency set ranging from 1 ^st to 50 ^th harmonic of the inverter carrier frequency. The processing unit 306 compares the determined impedance value of each filter 408, 410 with a reference impedance value, and determines the structural health of each filter 408, 410 based on the difference between the determined impedance value and the reference impedance value. The reference impedance value is predetermined and relates to a healthy state of the filter 408, 410. In another embodiment, the reference impedance value is obtained from supplier data recorded during production testing. The reference impedance value of the harmonic filters 408 and the reference impedance value of the resonant filter 410 may be the same or different.

[0049] The monitoring interface 308 receives signals indicative of the structural health of the filter 408, 410 from the processing unit 306 and monitors results of the structural health of the filter 408, 410.

[0050] In an embodiment, the processing unit 306 determines a life state of the capacitor and/or a remaining lifetime of the capacitor in each filter 408, 410. The life state of the capacitor and/or the remaining lifetime of the capacitor in each filter 408, 410 may be determined based on the difference between the determined impedance value and the reference impedance value. If a degraded capacitor is detected, the monitoring interface 308 receives an alert that the capacitor is degraded.

[0051] Capacitance and equivalent series resistance of each filter 408, 410 may be derived from the determined impedance value of each filter 408, 410. Thus, the determined impedance value of each filter 408, 410 may be further analyzed based on a reference equivalent series resistance of the corresponding filter 408, 410 and a temperature of the corresponding filter 408, 410 to predict the remaining lifetime of the capacitors and/or the life state of the capacitors in each filter 408, 410. The reference equivalent series resistance of each filter 408, 410 and the temperature of each filter 408, 410 may be correction and tolerance factors to provide more accurate results. The remaining lifetime of the capacitors in each filter 408, 410 may also be displayed on the display unit. In the event that only one leg of a capacitor is degraded, a common mode total harmonic distortion (THD) may be used for analysis to provide a more accurate prediction of the life state of the capacitor.

[0052] In one embodiment, the system 300 may include surge protection 422 to prevent damage to the processing unit 306 during abnormal conditions.

[0053] Figure 5 shows a schematic drawing of the processing unit 306 of the system 300. The processing unit 306 includes an analog-to-digital (A/D) conversion block 502. The A/D conversion block 502 may receive a signal indicative of a capacitor harmonic current from the current determining unit 302 and a signal indicative of a capacitor harmonic voltage from the voltage determining unit 304. The capacitor harmonic current may include three phase currents la, lb, Ic. The capacitor harmonic voltage may include three phase voltages Va, Vb, Vc. The A/D conversion block 502 may convert the signal indicative of the capacitor harmonic current and the signal indicative of the capacitor harmonic voltage into digital signals respectively.

[0054] The processing unit 306 includes a digital filter block 504 which is coupled to the A/D conversion block 502. The digital filter block 504 may receive the digital signal indicative of the capacitor harmonic current and the digital signal indicative of the capacitor harmonic voltage from the A/D conversion block 502 and may digitally filter the received digital signals. The processing unit 306 includes a Fourier transform block 506 which is coupled to the digital filter block 504. The Fourier transform block 506 may receive the digitally filtered signal indicative of the capacitor harmonic current and the digitally filtered signal indicative of the capacitor harmonic voltage from the digital filter block 504 and may perform a Fourier transform of the digitally filtered signals. [0055] The processing unit 306 includes a determining block 508 which is coupled to the Fourier transform block 506. The determining block 508 may receive the signal indicative of the capacitor harmonic current and the signal indicative of the capacitor harmonic voltage from the Fourier transform block 506 and may determine a resultant spectrum of the capacitor harmonic current (la, lb, Ic) and the capacitor harmonic voltage (Va, Vb, Vc). The processing unit 306 includes a first calculating block 510 which is coupled to the determining block 508. The first calculating block 510 may determine a capacitance value of the capacitor (e.g. three phase capacitor) and harmonic levels based on the resultant spectrum of the capacitor harmonic current (la, lb, Ic) and the capacitor harmonic voltage (Va, Vb, Vc).

[0056] The processing unit 306 includes a first comparison block 512 which is coupled to the first calculating block 510. The first comparison block 512 may compare the determined capacitance value with the reference capacitance value. In one embodiment, the reference capacitance value may be the same for each phase. In another embodiment, the reference capacitance value may be different for each phase. The first comparison block 512 may determine if the determined capacitance value is smaller than the reference capacitance value. If the determined capacitance value is smaller than the reference capacitance value, it may be determined that the harmonic filter 408 and/or the resonant filter 410 are degraded or damaged.

[0057] In addition, the processing unit 306 includes a second calculating block 514 which is coupled to the digital filter block 504. The second calculating block 514 may receive the digitally filtered signal indicative of the capacitor harmonic current from the digital filter block 504 and may perform a current averaging of each phase of the capacitor harmonic current (la, lb, Ic). The second calculating block 514 may then sum up the average values of each phase of the capacitor harmonic current (la, lb, Ic) to obtain a summation value Iabc. The processing unit 306 includes a second comparison block 516 which is coupled to the second calculating block 514. The second comparison block 516 may compare the summation value Iabc with a reference maximum unbalanced current value and may determine if the summation value Iabc is greater than the reference maximum unbalanced current value. If the summation value Iabc is greater than the reference maximum unbalanced current value, there may be an unbalanced current fault occurring due to e.g. one leg of a three phase grid capacitor being degraded or damaged.

[0058] The processing unit 306 includes a first notification block 518 which is coupled to the second comparison block 516. If the summation value Iabc is greater than the reference maximum unbalanced current value, the first notification block 518 may send an unbalanced current fault notification to a circuit breaker protection block 520 located in an external system 522 to e.g. activate the circuit breaker(s) to turn off the power converter.

[0059] In one embodiment, the first comparison block 512 may be coupled to the circuit breaker protection block 520 so that the circuit breaker(s) may be activated if the determined capacitance value is smaller than the reference capacitance value.

[0060] The processing unit 306 includes a data storage block 524 which is coupled to the determining block 508, the first comparison block 512 and the second comparison block 516. The resultant spectrum of the capacitor harmonic current (la, lb, Ic) and the capacitor harmonic voltage (Va, Vb, Vc) determined by the determining block 508, the comparison results of the determined capacitance value and the reference capacitance value determined by the first comparison block 512, and the comparison results of the summation value Iabc and the reference maximum unbalanced current value determined by the second comparison block 516 may be stored in the data storage block 524.

[0061] The processing unit 306 also includes a second notification block 526 which is coupled to the data storage block 524. Grid capacitance fault notifications from the second notification block 526 may also be stored in the data storage block 524.

[0062] The data storage block 524 may be coupled to a diagnostic/protection block 528 located in the external system 522. The data storage block 524 may transmit the information stored in the data storage block 524 to the diagnostic/protection block 528. The diagnostic/protection block 528 may perform appropriate analysis of the faults (e.g. fault in the harmonic filter and/or the resonant filter, unbalanced current fault, grid capacitance fault) occurring in the wind turbine based on the information received the data storage block 524 and carry appropriate protection measures for the wind turbine.

[0063] The above described method and system use harmonic current and harmonic voltage to determine a structural health of the filter(s) in the wind turbine online, i.e. during operation of the wind turbine. The above described method and system may also determine life state of the capacitors in the filter(s) and the remaining lifetime of the capacitors in the filter(s). The above described method and system are economical and provide an early warning for maintenance of the filter(s). Failures of the filters which may lead to a failure in grid compliance can be avoided. Damages to the converter system components like the generator, transformers and other capacitors can also be avoided. [0064] While embodiments of the invention have been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.