Patent Description:
Harmonic oscillations in power grids, i.e. oscillations at multiples of the grid frequency, are a well-known problem. When connecting a wind turbine generator to a power grid, it is therefore desirable to know how the wind turbine generator may contribute and react to such oscillations.

There may thus be a need for a simple and efficient way of obtaining useful information on the harmonic behavior of a wind turbine generator.

Document <CIT> discloses a method for determining a source parameter value and an impedance parameter value for an electrical model of a wind turbine generator according to the preamble of claim <NUM>.

According to a first aspect of the invention there is provided a method of determining a source parameter value and an impedance parameter value for an electrical model of a wind turbine generator, wherein the electrical model is representative of a selected harmonic occurring during operation of the wind turbine generator. The method comprises (a) obtaining a set of measured voltage values and a set of measured current values corresponding to the selected harmonic, (b) calculating an initial source parameter value and an initial impedance parameter value based on the set of measured voltage values and the set of measured current values, (c) calculating a respective fitting index for each of a plurality of candidate combinations of respective candidate source parameter values and candidate impedance parameter values, wherein each candidate source parameter value is obtained as a multiple of the initial source parameter value divided by a first predetermined number and each candidate impedance parameter value is obtained as a multiple of the initial impedance parameter value divided by a second predetermined number, (d) selecting a subset of the candidate combinations for which the corresponding fitting index is maximal, and (e) determining the source parameter value and the impedance parameter value based on the selected subset of candidate combinations.

This aspect of the invention is based on the idea that for a selected harmonic (such as <NUM>*f, <NUM>*f, <NUM>*f, <NUM>*f, <NUM>*f and so on, where f denotes the grid frequency), a set of model parameter values (source parameter value and impedance parameter value) is determined, which describes the electrical behavior of the wind turbine generator with regard to the selected harmonic. This is done by obtaining measured voltage and current values corresponding to the selected harmonic (e.g. by applying DFT (Discrete Fourier Transform) or FFT (Fast Fourier Transform) to raw measurement data). Based on these sets of measurement data, initial parameter values are calculated and then a fitting index is calculated for each of a plurality of candidate combinations of candidate source parameter values and candidate impedance values. The fitting index is indicative of how well a given candidate combination (and the corresponding electrical model) fits the measured reality. The candidate source parameter values are obtained as respective multiples of the initial source parameter values divided by a first predetermined number and the candidate impedance parameter values are obtained as respective multiples of the initial impedance parameter value divided by a second predetermined number. In other words, a matrix containing a fitting index for each candidate combination is formed. Then, the subset of candidate combinations having the maximal fitting index is identified and the (final) source parameter value and the (final) impedance parameter value are determined based on this subset. Thus, the candidate combinations with the maximum fitting index are used to determine the desired parameter values (source parameter value and impedance parameter value).

According to an embodiment of the invention, the set of measured voltage values and the set of measured current values are obtained during operation of the wind turbine generator while it is connected to a power grid.

In other words, the measurements are obtained during normal operation of the wind turbine generator while it is connected to the power grid. In order for the measurements to be representative of a wide variety of situations, the measurements should be collected over a substantive period of time, such as during a week or more of continuous operation.

The measurements are preferably obtained at a coupling point between the wind turbine generator and the power grid or at a point in the vicinity of this coupling point.

According to a further embodiment of the invention, the set of measured voltage values and the set of measured current values are obtained during operation of the wind turbine generator while it is connected to a power grid simulator.

The grid simulator is capable of electrically behaving like a power grid in a wide range of situations. This can in particular be utilized to obtain a wider variation in the collected data in a significantly shorter period of time as compared to the above embodiment which relies on a true power grid.

According to a further embodiment of the invention, the power grid simulator is operated to generate a plurality of harmonics at a coupling point between the wind turbine generator and the power grid simulator.

Thereby, it can be assured that sufficient measurement data representative of each harmonic of interest is actually available.

According to a further embodiment of the invention, the electrical model is a Thevenin equivalent circuit, the source parameter value is a Thevenin generator voltage, and the impedance parameter value is a Thevenin series impedance.

In other words, the electrical model consists of an ideal (alternating) voltage generator coupled in series with an impedance. The frequency of the voltage generator is a multiple of the grid frequency (e.g. <NUM>, <NUM>, <NUM>, etc.).

According to a further embodiment of the invention, the initial source parameter value is calculated as a median value of the set of measured voltage values, and the initial impedance parameter value is calculated as the sum of all measured voltage values in the set of measured voltage values divided by the sum of all measured current values in the set of measured current values.

According to a further embodiment of the invention, the electrical model is a Norton equivalent circuit, the source parameter value is a Norton generator current, and the impedance parameter value is a Norton parallel impedance.

In other words, the electrical model consists of an ideal (alternating) current generator coupled in parallel with an impedance. The frequency of the voltage generator is a multiple of the grid frequency (e.g. <NUM>, <NUM>, <NUM>, etc.).

According to a further embodiment of the invention, the fitting index is indicative of how well the corresponding candidate combination fits the obtained sets of measured voltage values and measured current values.

The fitting index may preferably be a number between <NUM> and <NUM>, where a fitting index equal to <NUM> indicates no or a very bad fit while a fitting index equal to <NUM> indicates a perfect or very good fit.

According to a further embodiment of the invention, determining the source parameter value comprises (a) determining a minimum source parameter value and a maximum source parameter value in the selected subset of candidate combinations, and (b) determining the source parameter value as a mean value of the minimum source parameter value and the maximum source parameter value.

According to a further embodiment of the invention, determining the impedance parameter value comprises (a) determining a minimum impedance parameter value and a maximum impedance parameter value in the selected subset of candidate combinations, and (b) determining the impedance parameter value as a mean value of the minimum impedance parameter value and the maximum impedance parameter value.

According to a second aspect of the invention there is provided a system for determining a source parameter value and an impedance parameter value for an electrical model of a wind turbine generator, wherein the electrical model is representative of a selected harmonic occurring during operation of the wind turbine generator. The system comprises (a) a measurement unit configured to obtain a set of measured voltage values and a set of measured current values corresponding to the selected harmonic, and (b) a processing unit configured to (b1) calculate an initial source parameter value and an initial impedance parameter value based on the set of measured voltage values and the set of measured current values, (b2) calculate a respective fitting index for each of a plurality of candidate combinations of respective candidate source parameter values and candidate impedance parameter values, wherein each candidate source parameter value is obtained as a multiple of the initial source parameter value divided by a first predetermined number and each candidate impedance parameter value is obtained as a multiple of the initial impedance parameter value divided by a second predetermined number, (b3) select a subset of the candidate combinations for which the corresponding fitting index is maximal, and (b4) determine the source parameter value and the impedance parameter value based on the selected subset of candidate combinations.

This aspect of the invention is essentially based on the same idea as the first aspect described above.

According to a third aspect of the invention, there is provided a computer program comprising computer executable instructions which, when executed by a computer processor, are adapted to cause the processor to perform the method according to the first aspect or any of the embodiments thereof.

It is noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to method type claims whereas other embodiments have been described with reference to apparatus type claims.

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiments to be described hereinafter and are explained with reference to the examples of embodiments. The invention will be described in more detail hereinafter with reference to examples of embodiments.

The illustration in the drawing is schematic. It is noted that in different figures, similar or identical elements are provided with the same reference numerals or with reference numerals which differ only within the first digit.

<FIG> shows an electrical model <NUM> utilized in embodiments of the present invention. The electrical model <NUM> is a Thevenin equivalent circuit of a wind turbine generator and comprises a voltage generator in series with an impedance Zv,WEA. The voltage generator provides an alternating voltage with magnitude Uv,WEA and frequency corresponding to a selected v-th harmonic of a grid frequency (e.g. v*<NUM>, v = <NUM>, <NUM>, <NUM>,. The wind turbine generator represented by the model <NUM> is coupled to a power grid <NUM> or a power grid simulator <NUM>. At the point of interconnection between the wind turbine generator represented by the model <NUM> and the power grid <NUM> or power grid simulator <NUM>, a voltage Uv,NAP and current Iv,NAP corresponding to the v-th harmonic are measured, e.g. by applying FFT or DFT to corresponding raw measurement signals.

<FIG> shows another electrical model <NUM> utilized in embodiments of the present invention. The electrical model <NUM> is a Norton equivalent circuit of a wind turbine generator and comprises a current generator in parallel with an impedance Zv,WEA. The current generator provides an alternating current with magnitude Iv,WEA and frequency corresponding to a selected v-th harmonic of a grid frequency (e.g. v*<NUM>, v = <NUM>, <NUM>, <NUM>,. Like in <FIG>, the wind turbine generator represented by the model <NUM> is coupled to a power grid <NUM> or a power grid simulator <NUM>. At the point of interconnection between the wind turbine generator represented by the model <NUM> and the power grid <NUM> or power grid simulator <NUM>, a voltage Uv,NAP and current Iv,NAP corresponding to the v-th harmonic are measured, e.g. by applying FFT or DFT to corresponding raw measurement signals.

<FIG> shows a flowchart of a method <NUM> according to an embodiment of the present invention, more specifically a method <NUM> of determining a source parameter value Uv,WEA and an impedance parameter value Zv,WEA for an electrical model <NUM> (see <FIG>) of a wind turbine generator, wherein the electrical model is representative of a selected harmonic occurring during operation of the wind turbine generator. In other words, the method aims at determining the source parameter value Uv,WEA and impedance parameter value Zv,WEA for a given v-th harmonic.

The method <NUM> begins at <NUM> and continues at <NUM> with obtaining a set of measured voltage values Uv,NAP(k) and a set of measured current values Iv,NAP(k) corresponding to the selected harmonic. The index k has integer values between <NUM> and Ntot. The measured values may be obtained over a longer period of time while the wind turbine generator is connected to a power grid <NUM> or during a (possibly) shorter period of time while the wind turbine generator is connected a power grid simulator <NUM>. This will be discussed in more detail below in conjunction with <FIG>. In any event, the obtained measurement values represent a variety of voltages and currents associated with the selected v-th harmonic of the grid frequency.

Then, an initial source parameter value Uv,WEA,<NUM> and an initial impedance parameter value Zv,WEA,<NUM> based on the set of measured voltage values and the set of measured current values are calculated at <NUM>. Here, a first index i is set to one, i.e. i = <NUM>. The initial values are starting points for the algorithm and may in particular be calculated as follows: <MAT> <MAT>.

At <NUM>, a candidate source parameter value Uth is calculated as <MAT> and a second index j is set to one, i.e. j = <NUM>.

At <NUM>, a candidate impedance value Zth is calculated as <MAT>.

Then, at <NUM> a fitting index r is calculated for the combination of candidate values Uth and Zth. The fitting index is preferably a value between <NUM> (zero) and <NUM> (one), where a larger value indicates that the combination of candidate values fits the measurements better while a smaller value indicates that the combination of candidate values fits the measurements worse. The calculation of the fitting index r will be discussed in more detail below in conjunction with <FIG>.

At <NUM>, the calculated fitting index r is stored in a matrix rmat at a position corresponding to row i (i=<NUM>) and column j (j=<NUM>), and then j is incremented by one (j = j + <NUM>) at <NUM>.

At <NUM>, it is determined whether j > <NUM> and as long as this is not the case, the method returns to <NUM> and repeats the inner loop, i.e. steps <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> for each incremented value of j and the corresponding new candidate impedance parameter value Zth calculated at <NUM>.

When it is determined at <NUM> that j > <NUM> (i.e. j = <NUM>), the method continues to <NUM> where the first index i is incremented by one (i = i + <NUM>).

Then, at <NUM>, it is determined whether i > <NUM> and as long as this is not the case, the method returns back to <NUM> and repeats the inner loop (i.e. steps <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>) for the corresponding new candidate source parameter value Uth calculated at <NUM>.

When it is determined at <NUM> that i > <NUM> (i.e. i = <NUM>), the method continues to <NUM>. At this stage, a <NUM> x <NUM> matrix rmat has been established by calculating a respective fitting index r for each of a plurality of candidate combinations of respective candidate source parameter values Uth and candidate impedance parameter values Zth, wherein each candidate source parameter value is obtained as a multiple of the initial source parameter value divided by a first predetermined number (i.e. Uth = Uv,WEA,<NUM>·i/<NUM>) and each candidate impedance parameter value is obtained as a multiple of the initial source parameter value divided by a second predetermined number (i.e. Zth = Zv,WEA,<NUM>·j/<NUM>). Here, both the first and second predetermined number is equal to <NUM>.

At <NUM>, optimum values Uv,WEA,opt and Zv,WEA,opt calculated by selecting a subset of the candidate combinations for which the corresponding fitting index r is maximal, and determining the source parameter value Uv,WEA,opt and the impedance parameter value Zv,WEA,opt based on the selected subset of candidate combinations. More specifically, this is done by applying the algorithm 360A shown in <FIG> and the algorithm 360B shown in <FIG> to the matrix rmat as will be discussed in more detail further below.

Having obtained the optimum parameter values Uv,WEA,opt and Zv,WEA,opt, the method <NUM> continues to <NUM> where the optimum parameter values are output before the method ends at <NUM>.

<FIG> shows a flowchart of a part of the method <NUM> shown in <FIG>. More specifically, <FIG> shows the calculating <NUM> of the fitting index r for a given candidate combination of Uth and Zth. The calculating process <NUM> begins at <NUM> where the measurement index k is set to one (k = <NUM>) and the value rsum is set to zero (rsum = <NUM>). Then, at <NUM>, it is determined whether the k-th voltage measurement value Uv,NAP(k) is less than the candidate voltage value Uth, i.e. whether Uv,NAP(k)<Uth. If that is the case, the process continues to <NUM>, otherwise it continues to <NUM>.

At <NUM>, it is determined whether <MAT>.

If this is the case, the process continues to <NUM> where rsum is incremented by one (i.e. rsum = rsum + <NUM>) and then to <NUM> where k is incremented by one (i.e. k = k + <NUM>). If not, the process goes directly to <NUM> without incrementing rsum.

Similarly at <NUM>, it is determined whether <MAT>.

Again, if this is the case, the process continues to <NUM> where rsum is incremented by one (i.e. rsum = rsum + <NUM>) and then to <NUM> where k is incremented by one (i.e. k = k + <NUM>). If not, the process goes directly to <NUM> without incrementing rsum.

Thereafter, it is determined at <NUM> whether all measurement values have been used, i.e. whether k > Ntot. If this is not case, the process returns to <NUM> and repeats the steps <NUM>, <NUM>/<NUM>, <NUM>, <NUM>, and <NUM>. Otherwise, it proceeds to <NUM> and calculates the fitting index r as r = rsum/Ntot before the method <NUM> continues at <NUM> as discussed above in conjunction with <FIG>.

<FIG> shows a flowchart of another part of the method <NUM> shown in <FIG>. More specifically, <FIG> shows the process 360A of calculating the optimum source parameter value Uv,WEA,opt. The process begins at <NUM> where the index k is set to one (k = <NUM>) and the value rmax is set to zero (rmax = <NUM>). Then, at <NUM>, the optimum fitting index r for the k-th row (i.e. in this case the first row) in the matrix rmat is found as <MAT>.

Then, at <NUM>, it is determined whether ropt(k) is larger than rmax, i.e. whether ropt(k) > rmax. If this is the case, the process continues to <NUM> where Umin is set equal to Uv,WEA,<NUM>*k/<NUM> and rmax is set equal to ropt(k) before the process continues to <NUM>. Otherwise, the process continues to <NUM> where it is determined whether ropt(k) is equal to rmax. If this is the case, the process continues to <NUM> where Umax is set equal to Uv,WEA,<NUM>*k/<NUM>. If not, <NUM> is skipped and the process continues at <NUM> by incrementing k by one (i.e. k = k + <NUM>).

Then, at <NUM>, it is determined whether k exceeds <NUM>, i.e. whether k > <NUM>. If not, the process returns to <NUM> and repeats the steps <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> as described above. Otherwise, the process continues to <NUM> where the optimum source parameter values Uv,WEA,opt is determined as the middle value of Umax and Umin, i.e. as <MAT> before the method <NUM> continues at <NUM> as discussed above in conjunction with <FIG>.

<FIG> shows a flowchart of yet another part of the method <NUM> shown in <FIG>. More specifically, <FIG> shows the process 360B of calculating the optimum impedance parameter value Zv,WEA,opt. The process begins at <NUM> where the index k is set to one (k = <NUM>) and the value rmax is set to zero (rmax = <NUM>). Then, at <NUM>, the optimum fitting index r for the k-th column (i.e. in this case the first column) in the matrix rmat is found as <MAT>.

Then, at <NUM>, it is determined whether ropt(k) is larger than rmax, i.e. whether ropt(k) > rmax. If this is the case, the process continues to <NUM> where Zmin is set equal to Zv,WEA,<NUM>*k/<NUM> and rmax is set equal to ropt(k) before the process continues to <NUM>. Otherwise, the process continues to <NUM> where it is determined whether ropt(k) is equal to rmax. If this is the case, the process continues to <NUM> where Zmax is set equal to Zv,WEA,<NUM>*k/<NUM>. If not, <NUM> is skipped and the process continues at <NUM> by incrementing k by one (i.e. k = k + <NUM>).

Then, at <NUM>, it is determined whether k exceeds <NUM>, i.e. whether k > <NUM>. If not, the process returns to <NUM> and repeats the steps <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> as described above. Otherwise, the process continues to <NUM> where the optimum impedance parameter value Zv,WEA,opt is determined as the middle value of Zmax and Zmin, i.e. as <MAT> before the method <NUM> continues at <NUM> as discussed above in conjunction with <FIG>.

The processes 360A and 360B are preferably performed in parallel.

As can be seen from the above discussion of <FIG> and <FIG>, a subset of candidate combinations (i.e. combinations of candidate source parameter values and impedance parameter values) for which the corresponding fitting index is maximal is selected by going through the matrix rmat. Thereafter, the optimum source parameter value and the optimum impedance parameter value are determining based on the selected subset, in the present embodiment as the mean value of the maximum and minimum values in the subset.

An important aspect of the present invention is that the corresponding method relies on measured voltage and current values corresponding to the selected harmonic. Thus, the quality of the determined parameter values evidently depends on the quality of the measurement data which can generally be obtained in one of two ways: either during normal operation of the wind turbine generator while it is connected to the power grid or during operation of the wind turbine generator while it is connected to a power grid simulator. In the first case, measurements must be made during an extensive period of time in order to obtain data corresponding to a wide variety of operating conditions and power grid events. In the second case, the diversity of the measurement data may be assured by corresponding programming of the power grid simulator such that the desired conditions and events are sufficiently represented. This takes much less time and generally leads to a more precise determination of the model parameters as shown in the following.

<FIG> shows a plot of measured harmonic voltages Uv and currents Iv in accordance with an embodiment of the present invention where the wind turbine generator is connected to a power grid simulator <NUM> (see <FIG>). As can be seen, the measurement values <NUM> fit very well into a rectangle <NUM>. Accordingly, there is a clear distinction between combinations of harmonic current and voltage that fit and do not fit the model (represented by the rectangle <NUM>). This illustrates the point already made above, namely that a very precise model (source parameter value and impedance parameter value) can be obtained in this case.

As a contrast, <FIG> shows a plot of measured harmonic voltages Uv and currents Iv in accordance with another embodiment of the present invention where the wind turbine is connected to a power grid <NUM> (see <FIG>) during normal operation. In this case, the obtained measurement values <NUM> cover a much smaller area in the graph and both rectangles <NUM> and <NUM> appear to surround them quite well. Thus, in this case it is more difficult to obtain a precise model (source parameter value and impedance parameter value) than in the case discussed above in conjunction with <FIG>.

Claim 1:
A method of determining a source parameter value (Uv,WEA, Iv,WEA) and an impedance parameter value (Zv,WEA) for an electrical model (<NUM>, <NUM>) of a wind turbine generator, wherein the electrical model is representative of a selected harmonic occurring during operation of the wind turbine generator, characterised in that the method comprising
obtaining (<NUM>) a set of measured voltage values (Uv,NAP) and a set of measured current values (Iv,NAP) corresponding to the selected harmonic,
calculating (<NUM>) an initial source parameter value (Uv,WEA,<NUM>) and an initial impedance parameter value (Zv,WEA,<NUM>) based on the set of measured voltage values and the set of measured current values,
calculating (<NUM>) a respective fitting index (r) for each of a plurality of candidate combinations of respective candidate source parameter values and candidate impedance parameter values, wherein each candidate source parameter value is obtained as a multiple of the initial source parameter value divided by a first predetermined number and each candidate impedance parameter value is obtained as a multiple of the initial impedance parameter value divided by a second predetermined number,
selecting a subset of the candidate combinations for which the corresponding fitting index is maximal, and
determining (<NUM>) the source parameter value (Uv,WEA,opt) and the impedance parameter value (Zv,WEA,opt) based on the selected subset of candidate combinations.