Controlling a wind farm

Abstract

Provided is a method of controlling a wind turbine group having a plurality of wind turbines. Each wind turbine generates electrical power as wind turbine output power for feeding into an electrical supply network. The group feeds a group power output into the network at a grid connection point, and the group power output is substantially formed as the sum of all the turbine power outputs of the group. A maximum group power output is specified for limiting the group power output, a control value for compliance with the maximum group power output is transferred to each wind turbine in the group in order to limit the output power of the respective wind turbine to a maximum value defined by the control value, and a control relationship is determined between potential control values and potential group power outputs using the control relationships and depending on the maximum group power output.

Claims

1. A method of controlling a wind turbine group having a plurality of wind turbines, comprising: forming a group power output as a sum of wind turbine power outputs of the wind turbine group, wherein each wind turbine of the wind turbine group respectively generates electrical power as a respective wind turbine power output for feeding into an electrical supply network; feeding, by the wind turbine group, the group power output into the electrical supply network at a grid connection point; specifying a maximum group power output for limiting the group power output; to comply with the maximum group power output, sending a control value to each wind turbine of the wind turbine group for limiting the respective wind turbine power output to a maximum value defined by the control value; and determining a control relationship between a plurality of potential control values and a respective plurality of potential group power outputs to specify the control value, wherein the control value is determined using the control relationship and the control value is determined depending on the maximum group power output; and storing the control relationship as a table of value tuples, wherein: each value tuple includes a potential control value and a potential group power output, or the control relationship includes a threshold control value which is representative of an associated control value at which an associated group power output reaches a current maximum value of the associated group power output, and the threshold control value is composed of the associated control value and a reserve value, or the control value is specified as a relative value in relation to a nominal power.

2. The method according to claim 1, wherein, to generate the control relationship successively for the plurality of potential control values, from a predefinable first potential control value to a predefinable last potential control value, calculating a respective potential group power output for a respective potential control value, wherein: the potential group power output for the respective potential control value is calculated as a sum of power outputs that can be generated by each of the wind turbines of the wind turbine group, and a power output that can be generated by a wind turbine is calculated depending on a nominally limited power output which is calculated using a nominal power of the wind turbine and the potential control value, and an available power output denoting an electrical power output which can currently be generated by the wind turbine.

3. The method according to claim 2, wherein the nominally limited power output is a product of the nominal power of the wind turbine and the potential control value.

4. The method according to claim 2, wherein a smaller of the nominally limited power output and the available power output is used as the power output that can be generated.

5. The method according to claim 1, wherein: the control relationship is updated within predefinable time intervals, wherein, in particular.

6. The method according to claim 5, wherein the predefinable time intervals are less than one minute.

7. The method according to claim 1, comprising: determining a reference relationship, wherein: a relationship between potential group power outputs and potential control values is determined as the reference relationship based on the control relationship to obtain a control value from the maximum group power output using the reference relationship.

8. The method according to claim 7, wherein the relationship is determined for some of the potential control values by using every n-th potential control value from the potential control values of the control relationship, wherein n is an integer from 2 to 100 in order to: find, in a first assignment step, a quick first relationship between a specified group power output and a potential control value assigned in a first approximation; and determine, in a second assignment step based on the potential control value assigned in said first approximation and on the control relationship, a more precisely assigned potential control value and to use the control value.

9. The method according to claim 8, wherein an interpolation between two potential control values of the control relationship is performed in the second assignment step.

10. The method according to claim 1, comprising: determining, by a central group computer for the wind turbine group, the control relationship; receiving, by the central group computer, operating values on a regular basis from the plurality of wind turbines of the wind turbine group; and determining, by the central group computer, the control value.

11. The method according to claim 1, wherein: the control value is composed of a fixed control term and a variable control term, wherein: the fixed control term is determined using the control relationship and as a function of the maximum group power output, the variable control term is determined as a function of a setpoint/actual value comparison between the maximum group power output; and the group power output fed to a grid at the grid connection point, and the variable control term is limited to a maximum of one tenth of the maximum value of the control value.

12. The method according to claim 11, wherein: setpoint/actual comparison of values are taken into account as a control error, and the variable control term includes an integral term of the control error, wherein: the integral term obtained from the control error is determined by integration using an integration factor.

13. The method according to claim 12, wherein: the integral term is limited to a fraction of a maximum value of the control value, and the fraction is in a range between 5% and 20% of the maximum value of the control value, or the variable control term, after a change in the specified maximum group power output by a value greater than a specified change limit, is determined or supplied with a time delay, or at least completely determined or supplied with a time delay initially, wherein the time delay ranges between 1 and 20 seconds.

14. The method according to claim 12, wherein the integration factor depends on an amplitude of the control error, wherein the integration factor is positively correlated with a magnitude of the control error.

15. The method according to claim 11, wherein the variable control term is determined with an aid of an integrator and an output value of the integrator is reset when there is a change in the maximum group power output.

16. A method of controlling a wind farm having a plurality of wind turbine groups, comprising: controlling each wind turbine group of the plurality of wind turbine groups using the method according to claim 1; specifying a maximum farm output power to limit a farm power output fed to a grid at the grid connection point; subdividing the maximum farm output power into a plurality of maximum power output fractions in accordance with a distribution rule; sending, to each wind turbine group of the plurality of the wind turbine groups, one of the plurality of maximum power output fractions as the maximum group power output; and controlling each wind turbine group of the plurality of the wind turbine groups depending on the maximum group power output.

17. The method according to claim 16, wherein the distribution rule splits the maximum farm output power into the plurality of maximum power output fractions, depending on: a nominal group power output of each wind turbine group of the plurality of wind turbine groups, a status of each wind turbine group, or an operational condition of each wind turbine group of the plurality of wind turbine groups.

18. The method according to claim 1, wherein the sum of wind turbine power outputs of the wind turbine group excludes power losses in a wind farm.

19. A wind turbine group having a plurality of wind turbines, comprising: a group computer including a group controller configured to control the wind turbine group, wherein each of the plurality of wind turbines generates electrical power as wind turbine power output for feeding into an electrical supply network, wherein the wind turbine group feeds a group power output into the electrical supply network at a grid connection point, wherein the group power output is formed as a sum of a plurality of wind turbine power outputs of the wind turbine group, and the group controller is configured such that: a maximum group power output is specified for limiting the group power output; a control value for compliance with the maximum group power output is sent to each wind turbine of the wind turbine group to limit the respective wind turbine power output of the respective wind turbine to a maximum value defined by the control value; a control relationship is determined as a relationship between potential control values and potential group power outputs, in order to specify the control value, and the control value is determined using the control relationship, and depending on the maximum group power output; and the control relationship is stored as a table of value tuples, wherein: each value tuple includes a potential control value and a potential group power output, or the control relationship includes a threshold control value which is representative of an associated control value at which an associated group power output reaches a current maximum value of the associated group power output, and the threshold control value is composed of the associated control value and a reserve value, or the control value is specified as a relative value in relation to a nominal power.

20. A wind farm, comprising: a plurality of wind turbine groups including the wind turbine group according to claim 19; the grid connection point; and a central wind farm computer.

21. The wind turbine group according to claim 19, wherein the sum of the plurality of wind turbine power outputs of the wind turbine group excludes power losses in a wind farm.

22. A method of controlling a wind turbine group having a plurality of wind turbines, comprising: forming a group power output as a sum of wind turbine power outputs of the wind turbine group, wherein each wind turbine of the wind turbine group respectively generates electrical power as a respective wind turbine power output for feeding into an electrical supply network; feeding, by the wind turbine group, the group power output into the electrical supply network at a grid connection point; specifying a maximum group power output for limiting the group power output; to comply with the maximum group power output, sending a control value to each wind turbine of the wind turbine group for limiting the respective wind turbine power output to a maximum value defined by the control value; determining a control relationship between a plurality of potential control values and a respective plurality of potential group power outputs to specify the control value, wherein the control value is determined using the control relationship and the control value is determined depending on the maximum group power output; and to determine the control relationship successively for the plurality of potential control values, from a predefinable first potential control value to a predefinable last potential control value, calculating a respective potential group power output for a respective potential control value, wherein: the potential group power output for the respective potential control value is calculated as a sum of power outputs that are capable of being generated by each of the wind turbines of the wind turbine group, and a power output capable of being generated by a wind turbine is calculated depending on a nominally limited power output which is calculated using a nominal power of the wind turbine and the potential control value, and an available power output denoting an electrical power output which can currently be generated by the wind turbine.

23. The method according to claim 22, wherein the sum of wind turbine power outputs of the wind turbine group excludes power losses in a wind farm.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Exemplary embodiments of the invention shall now be described in greater detail with reference to the accompanying Figures, in which

(2) FIG. 1 shows a perspective view of a wind turbine.

(3) FIG. 2 shows a schematic view of a wind farm.

(4) FIG. 3 shows a schematic view of a wind farm comprising three wind turbine groups.

(5) FIG. 4 shows a diagram illustrating the creation of a control relationship.

(6) FIG. 5 illustrates a control structure for controlling a wind turbine group.

DETAILED DESCRIPTION

(7) FIG. 1 shows a wind turbine 100 comprising a tower 102 and a nacelle 104. A rotor 106 having three rotor blades 108 and a spinner 110 is arranged on nacelle 104. Rotor 106 is made to rotate by the wind and drives a generator in nacelle 104 as a result.

(8) Wind turbine 100 has an electric generator 101, which is indicated in nacelle 104. Electric power can be generated by means of generator 101. An infeed unit 105, which can be in the form of an inverter, is provided for feeding electric power into the grid. This can be used to produce a three-phase infeed current and/or a three-phase infeed voltage having an amplitude, a frequency and a phase, for feeding into the grid at a grid connection point PCC. That can be done directly, or also collectively with other wind turbines in a wind farm. A system controller 103 is provided to control wind turbine 100 and also infeed unit 105. System controller 103 can also receive externally specified values, in particular from a central wind farm computer.

(9) FIG. 2 shows a wind farm 112 comprising, in this example, three wind turbines 100, which may be identical or different. The three wind turbines 100 are thus representative of basically any number of wind turbines in a wind farm 112. Wind turbines 100 deliver their power, namely the generated current, via an electrical wind farm network 114. The respective current or power outputs respectively generated by the individual wind turbines 100 are added up, and a transformer 116 is usually provided that transforms the voltage in the wind farm in order to feed it into supply network 120 at infeed point 118, which is also referred to generally as the point of common coupling (PCC). FIG. 2 is only a simplified view of a wind farm 112. Wind farm network 114 may be designed differently, for example by a transformer also being provided at the output of each wind turbine 100, to mention just one other embodiment.

(10) Wind farm 112 also has a central wind farm computer 122, which can also be referred to synonymously as the central wind farm controller. This can be connected to wind turbines 100 via data lines 124, or wirelessly, so as to exchange data with the wind turbines via those connections, in particular to receive measured values from wind turbines 100 and to transmit control values to wind turbines 100.

(11) FIG. 2 thus shows a wind farm 112 having three wind turbines 100 in the example shown here. These three wind turbines 100 can form a wind turbine group. Wind farm 112 can thus comprise a total of one wind turbine group. Each of these wind turbines 100 can therefore generate an electrical power output as its wind turbine output power, and these wind turbine power capacities are added up in wind farm network 114 to form a group power output, and this group power output can be fed into the electrical supply network 120 at grid connection point 118.

(12) In order to specify and comply with a maximum for this group power output, such a maximum group power output can initially be given to the central wind farm computer 122 externally, for example by a grid operator. The central wind farm computer 122 can then determine a control value on the basis of a control relationship and transmit it to wind turbines 100 via data lines 124 or, alternatively, wirelessly. To that end, a control relationship is determined in the central wind farm computer 122 and is then used to determine the control value. The control value is thus determined in wind farm computer 122 using the control relationship that is also determined there, and depending on the specified maximum group power output, and is transmitted to wind turbines 100.

(13) FIG. 3 shows a wind farm 312 comprising three wind turbine groups 301, 302 and 303 in this example. Each wind turbine group 301, 302 and 303 has a plurality of wind turbines 100. According to FIG. 3, each wind turbine group contains three wind turbines, but this should be understood as an example only, and each wind turbine group can have any other number of wind turbines, and the number of wind turbines can also differ between the wind turbine groups. There may also be more or fewer wind turbine groups. Wind turbines 100 can be the same as wind turbine 100 in FIG. 1 or like a wind turbine 100 in FIG. 2, so reference sign 100 was chosen in FIG. 3 also, for the sake of simplicity. However, wind turbines 100 may also differ from each other, not only within a wind turbine group, but also from one wind turbine group to another.

(14) Each wind turbine 100 generates a wind turbine output power P.sub.A. The respective wind turbine power outputs P.sub.A may differ, of course, and an individual index has been refrained from here for the sake of clarity. The sum of wind turbine power outputs P.sub.A of a wind turbine group 301, 302 and 303 forms the respective group power output P.sub.G1, P.sub.G2 and P.sub.G3, respectively. These group power outputs P.sub.G1, P.sub.G2 and P.sub.G3 are likewise added up in wind farm network 314, namely to form the wind farm power output P.sub.WP. This wind farm power output P.sub.WP is then fed into the electrical supply network 320 at grid connection point PCC. In the illustrative view shown in FIG. 3, any power losses are neglected. The wind farm power output P.sub.WP fed into electrical supply network 320 at grid connection point PCC is therefore the sum of group power outputs P.sub.G1, P.sub.G2 and P.sub.G3.

(15) In order to limit the wind farm power output P.sub.WP at grid connection point PCC, also marked with reference sign 318, a limiting signal specifying a maximum farm output power P.sub.PCCm can be specified externally. This maximum farm output power P.sub.PCCm can be entered for this purpose in a distribution module 330. Distribution module 330 can split this maximum farm output power P.sub.PCCm into individual maximum power output fractions, namely maximum group power outputs P.sub.Gm1, P.sub.Gm2 and P.sub.Gm3. These individual maximum group power outputs are transferred to corresponding group modules 331, 332 and 333 assigned to the respective wind turbine group.

(16) Depending on the respectively specified maximum group power output P.sub.Gm1, P.sub.Gm2 and P.sub.Gm3, each of these group modules 331, 332 and 333 then determines a control value P.sub.C1, P.sub.C2 and P.sub.C3. These control values are then transmitted to the respective wind turbines in the relevant wind turbine group, namely via data lines 324, which are all marked with the same reference sign for the sake of simplicity. These data lines 324 can also be used to obtain information on the individual wind turbines, in particular status information, operating values and measured values.

(17) The structure of distribution module 330, together with group modules 331, 332 and 333, is for illustration purposes, and these modules can also be combined in a process control computer/controller, for example. Whether separate or in combination, they can in any case, and preferably together, be part of the central wind farm computer 322, which is therefore marked as a broken-line rectangle containing the group modules and the distribution module.

(18) FIG. 4 shows an illustrative diagram that is meant to explain the control relationship and how it is determined. FIG. 4 is based on a simplified example of a wind turbine group comprising three wind turbines each having a nominal power rating of 2 MW. This results in a nominal power of 6 MW for the group power output, i.e., a nominal group power output of 6 MW.

(19) The control value P.sub.C is shown as a percentage on the x-axis of the diagram. The control values of 0-100% are potential control values in this respect, because they can be selected as the control value in each case. The power output P is entered on the y-axis in MW. This power output may relate to the group power output or also to the power output of a single wind turbine, depending on which value or curve is shown. The curve for the potential group power output P.sub.GP basically represents the result of the control relationship to be determined. This curve for the potential group power output P.sub.GP is calculated as follows.

(20) For each potential control value P.sub.C, an associated value of the potential group power output P.sub.GP is calculated as follows. 101 or 100 control values can be used, for example, namely values from 0% or 1% to 100%, in 1% increments.

(21) For each potential control value, the potential group power output is calculated as the sum of the power outputs that can theoretically be generated by each of the wind turbines in the wind turbine group given the respective control value. The power that can theoretically be generated by a wind turbine is either a nominally limited power output P.sub.NL or an available power output P.sub.AV. The smaller of the nominally limited power output P.sub.NL and the available power output P.sub.AV is used in each case as the power output that can theoretically be generated. The nominally limited power P.sub.NL is the product of the respective control value P.sub.C and the nominal power output of the respective wind turbine. The nominally limited power output P.sub.NL in the diagram in FIG. 4 thus forms a diagonal from the origin to its nominal power output P.sub.AN at a control value of 100%. This diagonal is drawn as a broken line in FIG. 4, the broken line being partly covered by a solid line that will be discussed further below. As the three wind turbines examined here by way of example have the same nominal output power, namely P.sub.NL=2 MW, there is also one diagonal only.

(22) The available power output P.sub.AV is the power that the wind turbine could generate, on the basis of the prevailing wind conditions, if it is not being subjected to limitation. Such limitation could also result from an earlier or current value of a specified maximum group power output for limiting the group power output. If the wind turbine is not operated with a limitation, the currently available power is the power that is actually being generated at that moment. Instead of calculating the available power, it is possible to that extent to take the actually generated power as the available power.

(23) In the illustrative example in FIG. 4, it is assumed that the three wind turbines have different available power outputs. This may be due, for example, to the fact that the wind turbines receive different strengths of wind depending on their position in the wind farm, for example because one of the wind turbines is in a more exposed position than another of the wind turbines. For that reason, accordingly, three different power outputs are also drawn into FIG. 4, namely as P.sub.AV1, P.sub.AV2 and P.sub.AV3. The value of the respective available power does not depend on the control value P.sub.C and for that reason these three available power outputs P.sub.AV1, P.sub.AV2 and P.sub.AV3 are drawn as horizontal dot-dash lines, unless they are covered by a solid line.

(24) The power that can theoretically be generated is thus selected for each individual wind turbine at each control value P.sub.C as the smaller value between the nominally limited power output P.sub.NL and the available power output P.sub.AV1, P.sub.AV2 and P.sub.AV3. This results in the three solid lines for the respective power P.sub.T1, P.sub.T2 and P.sub.T3 that can theoretically be generated. These lines initially coincide with the nominally limited power P.sub.NL, and then the respective value of the available power P.sub.AV1, P.sub.AV2 or P.sub.AV3.

(25) The potential group power output P.sub.GP is then the sum of these three individual power outputs that can theoretically be generated. It is calculated as the sum of these three power outputs that can theoretically be generated for each control value P.sub.C.

(26) Depending on the potential control value P.sub.C, the result can therefore be represented by the characteristic curve of the potential group power output P.sub.GP. For practical implementation purposes, the values of this characteristic curve of the potential group power output P.sub.GP can be stored in a table along with the respective potential control value P.sub.C as a pair of values, and this table can then be regarded as a control relationship.

(27) In order to now determine an associated control value P.sub.C as a function of a specified maximum group power output, this specified maximum group power output can be looked up on the characteristic curve for the potential group power output P.sub.GP shown in FIG. 4, or in a corresponding table, and the associated potential control value P.sub.C can be read off and used as control value P.sub.C. For example, if a value of 2.5 MW is specified as the maximum group power output for the example illustrated in FIG. 4, then this value is found on the characteristic curve for the potential group power output P.sub.GP at a control value P.sub.C of 40%. This illustrative example is shown as marked in FIG. 4 as the maximum group power output P.sub.Gm.

(28) It can be read from the curve of the potential group power output P.sub.GP that it no longer increases once a potential control value P.sub.C is reached. The associated potential control value P.sub.C is referred to as the threshold control value P.sub.CL and is stored additionally as information. Complying with this threshold control value P.sub.CL prevents excessively large control values P.sub.C being used, which could no longer be reached at all due to the horizontal part of the curve for the group power output P.sub.GP. The threshold control value P.sub.CL is preferably chosen slightly higher than the first value of the control value after which the group power output P.sub.GP no longer increases. By this means, it is possible to ensure that the power to be fed into the grid is not inadvertently limited to an unnecessarily strong extent when there are slight changes in the power situation.

(29) To actually control the wind turbines in the case of a specified maximum group power output, an appropriate control value can thus be determined in the manner described with reference to FIG. 4. However, the control value can be supplemented by a variable term, in particular by an integral term. In this case, the control value may be composed of a fixed term and a variable term. The fixed term is obtained from FIG. 4 in the manner described. Purely for the sake of simplicity, FIG. 4 and the descriptions above assumed the control value P.sub.C and does not specify that the fixed term of the control value is in fact determined in accordance with FIG. 4. In the case where no variable term is used, the control value as a whole and the fixed term of the control value can be identical.

(30) The variable term may result from a control arrangement, in particular from a setpoint/actual value comparison in which the specified maximum group power output is compared with the actually generated group power output. A control error resulting from the difference between said setpoint and actual value is preferably entered via a controller, in particular via an integrator or a controller that includes an integrator.

(31) FIG. 5 illustrates a control structure 500 (controller) for controlling a wind turbine group 502. By means of control structure 500, wind turbine group 502 is to be controlled in such a way that it maintains a specified maximum group power output P.sub.Gm. This specified maximum group power output P.sub.Gm thus forms an input variable or an input value for this control structure. This specified maximum group power output P.sub.Gm is entered into control block 504. Control block 504 contains a control relationship. This control relationship assigns a control value to a maximum group power output that has been entered. To that end, control block 504 can basically have a reverse relationship to the diagram in FIG. 4 or a reverse assignment between the potential group power output and the potential control value. The input variable of the relationship stored in control block 504 is not the control value, therefore, but the group power output. This is indicated by the symbol in control block 504.

(32) This means there is a clear assignment between the group power output that has been entered and the control value that results. The resultant control value is fixed, therefore, and is thus referred to as C.sub.F in the control structure 500 of FIG. 5. Control block 504 thus outputs a fixed control term C.sub.F of a control value C. The fixed control term C.sub.F is outputted as a percentage value. In summing element 506, the fixed control term C.sub.F is added to a variable control term C.sub.V, described below, to form control value C. The variable control term C.sub.V and also control value C are likewise provided in the form of percentage values.

(33) The control value C is then passed on to the individual wind turbines 100. The individual wind turbines 100 respond by adjusting their power generation accordingly, and they generate a wind turbine output power P.sub.A1, P.sub.A2 and P.sub.A3, respectively. These power outputs may be equal, but for practical reasons alone may differ at least slightly. They are combined and together, i.e., in total, form the group power output P.sub.G. This group power output P.sub.G is also fed into an electrical supply network, of course, whereby control structure 500 in FIG. 5 does not play a crucial role in that regard, so it is not shown.

(34) To determine the variable control term C.sub.V in the difference element 508, the generated group power output P.sub.G is subtracted from the specified maximum group power output P.sub.Gm to form a control error e. This control error e is initially routed via a gain block 510 with variable gain k (e, t). The result is then fed to an integrator 512. The variable gain factor k (e, t) can thus be understood as an integral gain factor, or as a gain factor of integrator 512. Its inverse can be understood as an integration time constant. However, in order to highlight the manifold importance of this variable gain factor k (e, t), this gain block 510 is drawn as a separate element.

(35) The variable gain factor k (e, t) therefore depends on control error e and also on time. Not only is control error e multiplied by a factor k, therefore, but such a factor depends also and additionally on control error e. To that extent, one can also speak of a nonlinear gain, because it depends on the amplitude of its input signal. It is proposed, in particular, that the gain is selected as being all the greater, the greater the magnitude of the control error. The variable gain factor k (e, t) can also be referred to synonymously as an integration factor.

(36) The variable gain factor k (e, t) is also dependent on time, and this dependency on time is provided, in particular, so that the gain factor k (e, t) initially takes a very small value after a jump in the control error, which value can also be zero, or less than 10% of an otherwise smallest value of the variable gain factor k (e, t). In this case, this small value, or even the value zero, is provided for a short period of time, in particular for 10 seconds or less. This is based on the idea that a jump in the control error e can only be caused by a jump in the specified maximum group power output P.sub.Gm, because for physical reasons it is not possible for the actually generated wind farm power output P.sub.G to jump, apart from a safety device being triggered, which would lead, however, to an error anyhow and to an error signal.

(37) A jump in the control error e thus means that a new value is specified for the maximum group power output. In that case, integrator 512 should not perform any upward integration initially. Integrator 512 is preferably reset, in addition, after such a jump in control error e. This is indicated by a reset line 514. This is meant to indicate that gain block 510, which has detected such a jump in control error e, also passes this information to integrator 512, so that the latter can be reset.

(38) In any case, the variable gain factor k (e, t) is so small shortly after the jump in the control error, i.e., a short time after a change in the specified maximum group power output, that integrator 512 is effectively inactive. In that period, ideally, the value zero or at least a very small value is also the result for the variable control term C.sub.V.

(39) However, determination of the fixed control term C.sub.F is not inactive, but said fixed control term C.sub.F is recalculated immediately after the change in the maximum group power output, instead, in accordance with the stored control relationship. Immediately after the change in the specified maximum group power output P.sub.Gm, an assigned fixed control term C.sub.F is determined, and that should already be a good value for controlling wind turbine group 502. The total control value C, therefore, is initially equal, when the variable control term C.sub.V is zero or negligible, to the fixed control term C.sub.F.

(40) The wind turbines 100 in wind turbine group 502 then have time to adjust their wind turbine output power P.sub.A1, P.sub.A2 and P.sub.A3 to the new total control value C. In other words, only control branch 516 is meant to be initially active at the beginning of a changed maximum group power output. This prevents integrator 512 from performing an upward integration too quickly, in response to such an abrupt change in the specified maximum group power output, to a large value that must then be reduced again. In other words, control branch 518 should be as inactive as possible at the beginning of a changed maximum group power output. The result is that the specified maximum group power output is substantially implemented by the control branch and thus by a controller in the first place, and that the control branch, in particular the integral controller, regulates only minor deviations, in particular that a steady-state accuracy is achieved which, of course, can hardly be achieved by a controller.

(41) In this sense, integrator 512 generates an output signal, once the initial period has elapsed, for regulating any residual deviation and for achieving steady-state accuracy. In order to also ensure that the integral term or control branch 518 is not too dominant in implementing a specified maximum group power output, a limiting element 520 is also provided. This limiting element 520 thus limits the output of integrator 512. A value of 10% is stated there by way of illustration, and this is meant to mean that the variable control term C.sub.V is 10% at a maximum. However, this is an example, and it is preferable even that the variable control term C.sub.V take a maximum value of only 5%.

(42) Limiting element 520 is also to be understood symbolically in this respect, in that not only is the output of integrator 512 to be limited, but the integrator 512 itself is then to cease performing any upward integration. For that purpose, a limiting feedback loop 522 is symbolically provided to illustrate that this limitation of the output variable is actually intended to limit integrator 512.

(43) Aspects shall now be described with reference to further examples.

(44) The proposed method comprises two components:

(45) 1. A control component that determines a fixed control term of the control value by means of an auxiliary table, and

(46) 2. a slow integral-action controller that still performs the fine adjustment using a setpoint/actual value comparison and determines a variable control term of the control value.

(47) It is proposed that a central wind farm controller sends all the wind turbines in the wind farm one and the same control value for the active power to be set. This control value is distributed as a broadcast to all the wind turbines. It is referred to as a control value, but due to the power output of the wind turbines being dependent on the wind, it is really a power limitation value (Pmax). This is because the wind turbine can limit its power output, but cannot increase it arbitrarily. The maximum power output depends primarily on the wind, and then on other limitations such as the apparent power to be complied with, the temperatures to be complied with, the noise levels to be complied with, technical availability, etc.

(48) However, it is also possible that the wind turbines in a wind farm are subdivided into a plurality of wind turbine groups, which can also be referred to as cluster control. The concept there is that the wind park be virtually subdivided into groups, and that each group receives its own specified value, namely its own maximum group value, which also constitutes a Pmax value.

(49) The idea of controlling by means of a control relationship that can be stored in a table, which can be called an auxiliary table, works for both types of control—not only when there is one control value for all, but also in the case of cluster control.

(50) Further information: The control value sent to the wind turbines is a setpoint percentage value and relates in each case to the nominal power of the respective wind turbine.

(51) The auxiliary table for the fixed control term can be created in the following manner.

(52) The central wind farm controller communicates constantly with each wind turbine. Control values are sent and current operating data are received. Obtaining a complete map of the relevant data from the wind farm takes up to one second.

(53) Three variables are relevant for this procedure:

(54) 1. The availability of the wind turbine indicates whether the turbine is in order and ready to feed power to the grid. This also includes whether communication with the central wind farm controller is in order, etc.

(55) 2. The current wind turbine output power refers to the active power currently fed-in by the wind turbine.

(56) 3. The available power from the wind turbine indicates the available active power of the wind turbine that the wind turbine could generate and feed into the grid if it did not have to comply with certain limitations.

(57) The wind turbine supplies the following four signals: The power output that can currently be generated from the prevailing wind. A technically available power output that takes into account current technical conditions, for example the fact the technically available active power can decrease when a converter is deactivated. A power output that can still be generated and fed into the grid, depending on extreme environmental conditions such as a storm, and which can also be referred to, therefore, as power output dependent on force majeure. An externally specified maximum power output, which can be referred to as the maximum available power after taking an externally specified limitation or control into account. This also includes a limitation or control caused by specification of a maximum group power output.

(58) A nominal power of each wind turbine can preferably be transmitted as well, although it suffices to transmit seldom, for example only once a day.

(59) The available power that is relevant for the proposed method is the minimum value resulting from the power that can currently be generated from the prevailing wind, the technically available power and the power dependent on force majeure. The externally specified maximum power does not limit the value used as the available power, because the control value of the proposed control method is itself included here and is distorted as a result.

(60) A table, namely the control relationship, is continuously produced in the background, independently of the power control being performed. This table contains two items of information in each case: A virtual control value (ranging from 0 to 100%), which can also be referred to synonymously as the potential control value, and a theoretical active power output of all the wind turbines and matching the control value, which can be referred to as the potential group power output.

(61) The procedure for this is as follows:

(62) In one pass, a potential control value, for example 10%, is specified. The theoretical power output is then calculated for all the wind turbines. To do that, a check is performed to determine whether the wind turbine is available. If it is, then the potential control value is multiplied by the nominal power of the wind turbine to obtain an absolute value for the output power. This output power is compared with the available active power, which can also be referred to simply as the available power. The smaller power value is taken as the theoretical power. This calculation is performed and added up for all the wind turbines in the wind farm or wind turbine group. In the end, for a potential control value of 10%, the table indicates a potential group power output of XXX kW as the sum of the theoretical active power outputs of all the wind turbines in the wind farm, for example 1870 kW. On the next pass, the virtual control value or potential control value is increased, for example to 11%, and the calculated is performed once again. The sum of all the theoretical power outputs, i.e., the potential group power output, could then be 2050 kW, for example. This calculation is performed for all the virtual control values or potential control values ranging from 0% to 100%.

(63) In the background, a table is created in which a value for the potential group power output is obtained for all the virtual control values or potential control values.

(64) After a complete run, an additional table for a simplified search is filled. This table can then be referred to as a reference table, has two entries in each case, and indicates the starting index for the first table in 10% increments, in relation to the installed capacity of the wind turbine group, in particular, which is the sum of the nominal power ratings of all the wind turbines in the wind farm. This is proposed for a simplified search so that it is not necessary to search through the entire first Table 1 while the controller is calculating. If the power output is to be limited due to the wind or other conditions, the maximum setpoint power output for a wind turbine, normed relative to its nominal power, is stored in the end as a threshold control value, preferably plus a reserve. This can be done in such a way that, from then on, the table is filled to the last entry with the threshold control value.

(65) The result is a function that receives as input a setpoint power or maximum power, namely the maximum group power output, in particular as an absolute value, and that returns an ideal control value as its output.

(66) In this function, the setpoint power is normed to the installed power, and the starting index for Table 1 is determined with the aid of Table 2.

(67) A search algorithm that determines the best possible control value then begins in Table 1.

(68) Starting with the starting index, each value for the sum of the theoretical power values, i.e., each value for the potential group power output, is looked at and compared with the setpoint power output. Another condition is that the value of the potential group power output must increase in comparison with the previous index, since otherwise the final value would have been reached. So if the power value increases relative to the previous index, and the power value is still lower than the setpoint power, then the index is increased. If one of the conditions is not met, the process stops. The auxiliary variables contain the index, the value of the potential group power output, i.e., the sum of the theoretical power values for the index, but where the power value is less than the setpoint power, and the potential group value, where the value is greater than the setpoint power. The ideal control value is then linearly interpolated between these values.

(69) This control value is returned as the result of the function.

(70) To illustrate this, the tables and a sample calculation (all idealized) are provided below:

(71) The installed power capacity Pinst is calculated as the sum of the nominal power ratings of all the wind turbines in the wind farm or wind turbine group.

(72) As an example, a wind farm containing five wind turbines each having a nominal power of 2000 kW, and five wind turbines each having a nominal power of 3000 kW, is assumed.

(73) The installed capacity Pinst is calculated as 5*2000 kW+5*3000 kW=25000 kW.

(74) The maximum available power, normed to the nominal power of a wind turbine, here in the example to one with a nominal power rating of 3000 kW, is 1740 kW.Math.1740 kW*100%/3000 kW+5% reserve=63%. This, therefore, is the threshold control value.

(75) With a control value of 63%, none of the wind turbines would operate with a limit, because the total available power of the wind turbines is below the 63% figure.

Example of Table 1 (the Index Corresponds to the Potential Control Value

(76) TABLE-US-00001 Sum of the theoretical power outputs (potential group Index Control value power output) 0  0% 0 KW 1  1% 250 kW 2  2% 500 kW . . . . . . . . . 50 50% 12500 kW 51 51% 12800 kW 52 52% 13050 kW 53 53% 13300 kW 54 54% 13500 kW 55 55% 13700 kW 56 56% 13880 kW 57 57% 13950 kW 58 58% 14000 kW 59 59% 14000 kW . . . . . . . . . 100 100%  14000 kW

(77) Detailed description of how to determine the theoretical power using the example of a virtual control value or potential control value of 2%.

(78) Wind turbines can also be referred to synonymously as WTGs (wind turbine generators). It is assumed that a control value of 2% is sent to WTG 1. The WTG has a nominal power of 2000 kW*2%=40 kW. The available power is more than 40 kW, so 40 kW is taken as the theoretical power. WEAs 1 to 5 are identical. WTG 6 has a nominal power rating of 3000 kW. 3000 kW*2%=60 kW. In this case also, the available power is more than 60 kW. The same applies to the remaining turbines 7 to 10. This results in a total power output of 5 WTGs à 40 kW+5 WEAs à60 kW=500 kW total theoretical power, i.e., potential group power output. This value is entered in the table. The same calculation is then performed with a virtual or potential control value of 3%, and so on up to 100%.

(79) When this is finished, Table 1 provides, for each potential control value, a sum total of all the theoretical wind turbine power outputs as a potential group power output. From the control perspective, however, this table is needed exactly the other way round. The controller would like to have the control value for a particular power output. This is the purpose of Table 2: In 10% increments, Table 2 provides a suitable entry point for Table 1, so that it is not necessary to search through the entire table. For a potential group power output, the table returns the starting index equal to the initial control value.

(80) Note: From index 6 onwards, the maximum control value is found. This is the value that was obtained above on the basis of the maximum available output power of a WTG in the wind farm. If this control value is sent, all the wind turbines feed their maximum available power into the grid. More than that is not possible.

(81) TABLE-US-00002 TABLE 2 Starting P.sub.SP in % index from Index of Pinst Table 1 0  0% 0 1 10% 10 2 20% 20 3 30% 30 4 40% 40 5 50% 50 6 60% 63 7 70% 63 8 80% 63 9 90% 63 10 100%  63

(82) Specimen calculation of the control value for a power output of, for example, 13550 kW:

(83) Specified maximum group power output, or P_setpoint_%=13550 kW*100%/25000 kW=54.2%. Index 5

(84) Calculation: P_setpoint_%/10 and rounded down 54.2%/10 and rounded down=5

(85) Table 2 Index 5 returns a starting index of 50 for Table 1.

(86) Table 1, starting index of 50 returns a potential group power output of 12500 kW.

(87) The index is incremented until the setpoint power is reached, i.e., the specified maximum group power output of 13550 kW.

(88) TABLE-US-00003 54 54% 13500 kW (P_temp1) 55 55% 13700 kW (P_temp2)

(89) Index 54 is less than the setpoint power, and index 55 is more. A linear interpolation between the two values is performed and a control value is determined with 0.1% accuracy:

(90) Control value=(Index_P_temp1)+(Psetpoint-P_temp1)*10I (P_temp2-P_temp1)

(91) Control value=54%+(13550 kW-13500 kW) I (13700-13500)

(92) Control value=54%+0.25.fwdarw.round up and to 0.1% accuracy=54.3%

(93) The function then returns an ideal control value of 54.3%.

(94) When the wind is evenly distributed within the wind farm, Table 1 is relatively linear up to the maximum available power. See the example. At extreme locations, however, for example two WTGs in a valley and one WTG on a mountain, the distribution is very different, so the proposed method offers a good solution.

(95) With regard to the description of the controller:

(96) As described in detail at the outset, the controller works with two components: A control component that uses the table, and a slow integral-action controller for fine adjustment.

(97) The control component, i.e., the fixed control term, is determined first. This is done by calculating the power to be fed into the grid.

(98) The power output that the wind farm is to feed into the grid, which is calculated from the setpoint power value at the point of common coupling, which can also be called the grid connection point, for example as specified by a grid operator, and from the power loss in the wind farm. The power loss in the wind farm may be caused, for example, by losses in cables, transformers, by own power consumption, or even by a small factory. The power loss could also be negative. For example, if other as yet unknown producers of power are integrated into the wind farm, or there is one wind turbine in the wind farm that does not have data bus communication with the central wind farm controller, but nevertheless feeds in a specified power output of 40%, for example.

(99) The power loss is calculated in the central wind farm controller as follows:

(100) P_losses=sum of the current WTG power from the wind farm (sum of P_WTG_current)−current power output (Pcurrent) at the grid connection point.

(101) This can be explained with reference to the example above: The wind farm operates without limitations and feeds a maximum output power of 14000 kW into the grid. The central wind farm controller measures an output power of 13450 kW at the grid connection point.

(102) P_losses=(14000 kW)−13450 kW=550 kW. The losses are somewhat larger, perhaps because there are also a high-voltage transformer and some stations installed in the wind farm.

(103) The controller is given a setpoint value of 13000 kW from the energy utility company.

(104) In order to reach the 13000 kW at the grid connection point, all the wind turbines together would have to feed in a power output of 13000 kW (specified by the energy utility company)+550 kW (wind farm losses). The ideal setpoint value for a total wind turbine power output of 13550 kW is therefore requested from the table. The function above then returns the ideal control value of 54.3%, which thus forms a fixed control term of the control value.

(105) It should be noted that any change in the active power or reactive power of the wind turbines may also change the power losses in the wind farm. The effect of losses could theoretically be taken into account by changing the power output, and an even better control value would then be obtained in the event of large changes in setpoint values. However, it was realized that this aspect does not need to be taken into consideration. It was realized, in particular, that this error is quickly eliminated due to the power losses in the wind farm being continuously calculated.

(106) The controller add an integral term, namely a variable control term, to this ideal control value, i.e., to the fixed control term.

(107) The integral term is calculated as follows:

(108) A power difference is formed and then normed to the nominal wind farm power output ((setpoint power−actual power) I Prated). If there is a control difference greater than 6%, an integral term of 1.0 is used, which term can be referred to synonymously as an integration factor. If there is a control difference greater than 3%, an integral term of 0.3 is used, and if less than 3% an integral term of only 0.1 is used.

(109) The integral term is limited to +−10% (=+−10 pu). The integral term is only meant to apply control within a range of +−10%, in order to compensate for any inaccuracies.

(110) Furthermore, in the event of a large jump in the setpoint value of 8%, the integral term is set to only 0.0001. This is intended to give the wind turbine time to respond to the ideal control value. The wind turbines are given four seconds to do so. A control difference of 8% is also set if there is a downward jump in the setpoint value, and is re-initialized to zero if the integral term has run to its positive limit.

(111) Resetting an output value of the integrator is also proposed for this controller. This takes effect as soon as the actual power output is 0.6% more than the setpoint value and the reported maximum actual power (in pu) from a WTG (P_Max_WTG_pu) is more than the total control value (ideal control value from the table+integral term). The integral term is then initialized to P_Max_WTG-Pu−ideal control value. Here too, the integral term is limited to +−10%.

(112) The total control value, i.e., the control value, is composed of the ideal control value, i.e., the fixed control term, plus the integral term, i.e., the variable control term, and is sent to the wind farm.

(113) The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.