FIELD OF THE INVENTION

The invention relates to a method for controlling a wind turbine, particularly to a method for controlling a wind turbine during power ramping.

BACKGROUND OF THE INVENTION

According to grid code requirements, wind turbines may be required to produce power according to a frequency-power curve. The frequency-power curve specifies if the power produced by the wind turbine or a wind turbine park should be reduced or increased in order to support a stable grid frequency. In order avoid undesired structural loads on the wind turbine, the wind turbine may limit the rate of change in produced power as required in response to a request to change the output power by a certain amount as dictated by the frequency- power curve.

In order to meet grid code requirements to change the output power within a certain response time there is a need to improve control systems to handle such grid code requirements. SUMMARY OF THE INVENTION

It is an object of the invention to improve the control of a wind turbine in relation to handling grid code requirements, particularly to improve the wind turbine's capability to handle requirements to change the output power within a certain response time in order to be able support grid frequency stability.

In a first aspect of the invention there is provided a method for controlling a wind turbine to change a power reference dependent on a grid frequency, the method comprises

- obtaining the grid frequency,

- comparing the grid frequency with a frequency threshold, and

- setting a power ramp rate for the power reference for a power production of the wind turbine dependent on the comparison of the grid frequency with the frequency threshold.

By setting the power ramp rate dependent on the comparison of grid frequency with the frequency threshold, the allowable rate of change of the power produced according to the power reference can be adaptively changed dependent on the deviation of the grid frequency from the nominal grid frequency. In this way, the output power from the wind turbine can be used to reduce deviations of the grid frequency from the nominal grid frequency within a short response time by allowing fast power ramp rates for grid frequencies above the frequency threshold.

According to an embodiment, the method comprises adjusting the power reference as a function of the grid frequency. By adjusting the power reference as a function of grid frequency, e.g. by determining a power request for power production as a function of the grid frequency the wind turbine may autonomously adjust the output power to support stabilisation of the grid frequency.

According to an embodiment, the comparison of the grid frequency with the frequency threshold comprises determining if the grid frequency is within a predetermined frequency range. For example, it could be determined if the grid frequency is within one of two or more frequency ranges. Accordingly, different frequency ranges could be defined for different degrees of deviations of the grid frequency from the nominal frequency to provide different actions for frequency stabilisation dependent on the severity of the grid deviation.

According to an embodiment, comparing the grid frequency with the frequency threshold comprises determining if the grid frequency is within a first or a second frequency range. For example, the second frequency range may comprise frequencies above or below the frequency threshold where the frequency threshold is respectively larger or smaller than a nominal frequency. The first frequency range may comprise frequencies between the frequency threshold being smaller than the nominal frequency and the frequency threshold being larger than the frequency threshold.

Setting of the power ramp rate may comprise selecting among different power ramp rates the power ramp rate for the power ramp limiter. Thus, the power ramp rate may be selected dependent on whether the output frequency is within the first or the second frequency range. According to an embodiment, the different power ramp rates comprises first and second power ramp rates, where the second power ramp rate allows a higher rate of change than the first power ramp rate. For example, the first power ramp rate may be selected if the grid frequency is within a first frequency range and the second power ramp rate may be selected if the grid frequency is within a second frequency range.

According to an embodiment, setting the power ramp rate may comprise setting the power ramp rate for a primary power rate limiter arranged for limiting the rate of change of the power reference. Accordingly, a controller of the wind turbine may comprise a primary power rate limiter for the purpose of implementing the power ramp rate function.

According to an embodiment, the method comprises setting a further power ramp rate for the power reference dependent on the comparison of the grid frequency with the frequency threshold. The further power ramp rate may be selected from different power ramp rates. Advantageously, the further ramp rate allows setting rate of change limitation of the power reference dependent on other changes of the power reference, e.g. dependent on a request to change the power reference to an externally provided power request.

For example, the further power ramp rate may be for limiting a rate of change of the power reference caused by changing (e.g. due to an external request) between the power reference determined (by the primary power rate limiter) according to the power ramp rate and an externally provided power reference.

According to an embodiment, the different power ramp rates for the further power ramp rate comprises third and forth power ramp rates, where the fourth power ramp rate allows a higher rate of change than the third power ramp rate.

Advantageously, the further power ramp rates may be set to other values than the first and second power ramp rates to provide different ramp rate limitations to other requests for changing the power reference.

For example, the forth power ramp rate may allow a rate of change higher than or equal to the second power ramp rate and the third power ramp rate may allow a rate of change higher than or equal to the first power ramp rate. In this way, the rate of change of the power reference determined by primary power rate limiter will not be further limited by the further power ramp rates.

Setting the further power ramp rate may be comprises setting the power ramp rate for a secondary power rate limiter arranged for limiting the rate of change of the power reference. Accordingly, a controller of the wind turbine may comprise a secondary power rate limiter for the purpose of implementing the power ramp rate limitation of the further power ramp rate. The secondary power rate limiter may be arranged to select the potentially rate limited power reference from the primary power rate limiter or an externally provided power reference for the power reference and to rate limit a change between the potentially rate limited power reference and the externally provided power reference.

A second aspect of the invention relates to a controller for controlling a wind turbine to change a power reference dependent on a grid frequency, where the controller comprises

- a power rate limiter arranged to limit a rate of change of the power reference for a power production of the wind turbine, and

- a ramp rate selector arranged to set a power ramp rate for the power rate limiter dependent a comparison of the grid frequency with a frequency threshold.

The controller may additionally comprise the second power rate limiter, in which case the power rate limiter is the primary power rate limiter. The ramp rate selector may be configured to set power ramp rate both for the primary and secondary power rate limiters.

A third aspect of the invention relates to a wind turbine or a wind turbine part comprising a control system according to the second aspect.

A fourth aspect of the invention relates to a wind park controller for controlling at least one wind turbine of a wind park and for changing a power reference for the wind park dependent on a grid frequency, the wind park controller comprises - a power rate limiter arranged to limit a rate of change of the power reference for the power production of the wind park,

- a ramp rate selector arranged to set a power ramp rate for the power rate limiter dependent a comparison of the grid frequency with a frequency threshold.

In general, the various aspects and embodiments of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 shows a wind turbine,

Fig. 2 shows a control system for controlling a wind turbine to change a power reference,

Fig. 3 shows the allowable rate of change of the power reference, and

Fig. 4 shows different power ramp rates for different frequency ranges.

DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a wind turbine 100 (WTG) comprising a tower 101 and a rotor 102 with at least one rotor blade 103, such as three blades. The rotor is connected to a nacelle 104 which is mounted on top of the tower 101 and being adapted to drive a generator situated inside the nacelle. The rotor 102 is rotatable by action of the wind. The wind induced rotational energy of the rotor blades 103 is transferred via a shaft to the generator. Thus, the wind turbine 100 is capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades and, subsequently, into electric power by means of the generator. The generator may include a power converter for converting the generator AC power into a DC power and a power inverter for converting the DC power into an AC power to be injected into a utility grid. The generator is controllable to produce a power corresponding to a power reference. The output power may be adjusted according to the power reference (Pref in Fig. 2) by adjusting the pitch of the rotor blades 103 or by controlling the power converter to adjust the power production. Accordingly, the power reference is used for controlling the amount of wind power to be extracted by the wind turbine.

Fig. 2 shows a controller 200 for controlling a wind turbine to change a power reference Pref dependent on a grid frequency f. The power reference Pref is supplied to a power control system 205 of the wind turbine which is responsible for controlling the wind turbine, e.g. by means of pitch control, to produce power according to the supplied power reference Pref. For example, the power control system 205 may be a rotor speed controller which adjusts the pitch of the blades 103 to extract power from the wind according to the supplied power reference Pref.

The grid frequency is the voltage oscillation frequency of the power grid or utility network with which the power output of the wind turbine is connected. The power output of the wind turbine may be the output of the power inverter. The grid frequency may be measured by a grid meter or other suitable measurement device at the power output of the wind turbine, at a point of common connection between one or more wind turbines and the power grid or other locations.

The controller 200 comprises a power rate limiter 202 arranged to limit a rate of change of the power reference Pref. The power rate limiter receives a power request Preq, i.e. a requested power value, from the frequency- power component 201. For example, the frequency- power component 201 may determine the power request Preq according to a frequency-power function, e.g. a function in the form of a curve or look-up table which provides the power request as a function of frequency. For example, as illustrated, the frequency power function may set a factor K between 0 and 1 for scaling a power input Pi (not shown) so that Preq = K x Pi. For frequencies between the nominal frequency, here 50 Hz, and a frequency threshold fT the factor is one, whereas for frequencies above fT, the factor decreases as a function of the frequency to reduce power request Preq. The power input Pi may be provided from a source such as a grid operator, a central power plant controller or from other external source as an external power reference Pext. Alternatively, the power input Pi may be provided from an internal sources of the wind turbine 100 such as a look-up table with different available power references.

Although not shown, the frequency-power component 201 may in the same way determine a power request for frequencies below the nominal frequency, e.g. by scaling the power input Pi with factors greater than one if the frequency f is less than a lower frequency threshold fT. Accordingly, the frequency-power component 201 may be configured to determine the power request by comparing the frequency f with upper and lower frequency thresholds, fTu and fTI, respectively.

The frequency- power component 201 or the frequency power function may be set according to grid code requirements so that in case of frequency deviations of the grid frequency from the nominal grid frequency, the power produced by the wind turbine 100 can be adjusted to support a reduction of the frequency deviation.

The frequency-power component 201 may be comprised by the wind turbine 100, e.g. by the control system 200, or may be located externally to the wind turbine 100. Accordingly, if the frequency-power component 201 is part of the wind turbine 100, the modification of the power request may be performed

autonomously by the a control system of the wind turbine 100.

Due to the frequency-power component 201 the power reference Pref may be adjusted as a function of the grid frequency.

The control system 200 further comprises a ramp rate selector 204 arranged to set a power ramp rate for the power rate limiter 202 dependent a comparison of the grid frequency f with the frequency threshold fT. Accordingly, the control system 200 is capable of setting a power ramp rate for the power production of the wind turbine 100 dependent on the grid frequency and frequency threshold FT.

Changing the power reference Pref instantaneously according to the instantaneous changes of the power request Preq would cause significant loads which could damage the wind turbine. Accordingly, an instantaneous change of the power request Preq needs to be fulfilled by a gradual change of the power reference Pref. The power rate limiter 202 sets a maximum rate of change limit and therefore determines the ramp rate limited power request Pramp as a function of time according to an allowable rate of change. For example, the rate limited power request Pramp may be determined according to Fig. 3 which shows the allowable change APramp, or change APref of the power reference Pref, as a function of time, where the allowable change is given by the rate limit curve 301. The slope of the rate limit curve 301 corresponds to the ramp rate set by the power rate limiter 202.

Accordingly, by means of the control system 200 the power reference Pref and, therefore, the produced power can be changed from a first level to a second level where the rate of the change is limited to a maximum rate according to the power ramp rate.

Fig. 4 shows an example of setting the power ramp rate PR dependent on grid frequency and the frequency threshold. Curve 401 illustrates different power ramp rates RR1 and RR2 for the power rate limiter 202 as a function of frequency f. In Fig. 4 the frequency fN refers to the nominal frequency, e.g. 50 Hz.

By comparing the grid frequency with the frequency threshold fT such as the upper frequency threshold fTu or the lower frequency threshold fTI it is possible to determine if the grid frequency is above or below one of the frequency thresholds, i.e. if the grid frequency is within a predetermined frequency range. For example, one or more predetermined frequency ranges including a range with frequencies up to fTI, a range from fTI to fTu, a range from fTI to fN, a range from fN to fTu, and a range with frequencies above fTu could be defined. Accordingly, if the grid frequency is within a first range from fN to fTu or fTI to fTu, the power ramp rate could be set to a first ramp rate RR1. If the grid frequency is within a second range of frequencies above fTu, the power ramp rate could be set to a second ramp rate RR2. The second range could also include frequencies below fTI, or a third ramp rate could be associated with a third range of frequencies below fTI. For example, the upper frequency threshold fTu could be 50.2 Hz when the nominal frequency fN is 50 Hz. Similarly, the lower frequency threshold fTI could be 49.8 Hz.

In general the ramp rate selector 204 may be arranged to set the power ramp rate RR by selecting from different available power ramp rates a particular power ramp rate dependent on the grid frequency. In order to be able to provide a fast change of the output power in order to support stabilisation of the grid frequency within a short response time, the second power ramp rate RR2 could be set to a high value in order to achieve a fast change of produced power from one level to another level. For example, the second ramp rate RR2 could be within a range from 0.05 to 0.2 PU/s which, depending on the nominal power production of the wind turbine, could range from 100 kW/s to 2 MW/s.

When the grid frequency is close to the nominal frequency fN, e.g. below the upper frequency threshold fTu, there is no requirement for fast changes of the output power. Accordingly, the first power ramp rate RRl could be set to a low value. For example, the first power ramp rate RRl could be within a range from 0.05 to 0.2 PU/min which, depending on the nominal power production of the wind turbine, could range from 100 kW/min to 2 MW/min. In general the second power ramp rate RR2 allows a higher rate of change than the first power ramp rate RRl, when the second power ramp rate RR2 is associated with grid frequencies above the upper frequency threshold fTu or below the lower frequency threshold fTI (i.e. a second frequency range) and when the first power ramp rate RRl is associated with grid frequencies between the upper and lower threshold frequencies (i.e. a first frequency range).

Referring again to Fig. 2, the controller 200 may comprise both the power rate limiter 202 referred to above and a secondary power rate limiter 203. In this case the power rate limiter 202 is referred to as the primary power rate limiter 202. The secondary power rate limiter 203 is arranged for limiting the rate of change of the power reference Pref similarly to the primary power rate limiter, but not for rate limitation of changes of the power request Rreq. Instead the secondary power rate limiter 203 is arranged to select the potentially rate limited power reference Pramp from the primary power rate limiter 202 or an externally provided power reference Pext for the power reference Pref and to rate limit a possible change between the power reference Pramp and the externally provided power reference Pext. The purpose of the secondary power rate limiter 203 is to select the power request Preq from the frequency-power component 201 or an externally provided power reference Pext. For example, the wind turbine may be requested to change the output power independent of grid frequency. For example, the wind turbine may be requested to reduce the output power even if the grid frequency is within the first frequency range, i.e. from fTI to fTu.

Again, changing the power reference Pref instantaneously according to the instantaneous change between the potentially rate limited power reference Pref and the external power reference Pext of the power request Preq would cause significant loads which could damage the wind turbine. Accordingly, an

instantaneous change between the Pramp and Pext needs to be fulfilled by a gradual change of the power reference Pref.

The secondary power rate limiter 203 operates in the same way as the primary power rate limiter 202 in response to changes of the provided input power value intended to be used for the power reference Pref.

Accordingly, the secondary power rate limiter 203 is arranged to set a further power ramp rate RR for the power reference Pref. Similarly to the primary power rate limiter 202, the power ramp rate for the secondary power rate limiter 203 is set dependent on the comparison of the grid frequency with the frequency threshold and may involve selection of the power ramp rate from a plurality of different power ramp rates dependent on the grid frequency and the frequency threshold or frequency ranges, i.e. by determining if the grid frequency is within a first or a second frequency range. The one or more power ramp rates RR for the secondary power rate limiter 203, e.g. the power ramp rates RR, e.g. the ramp rates associated with the first and second frequency ranges, may be equal to the ramp rates RR for the primary power rate limiter 202 for the same threshold frequencies or frequency ranges. Alternatively, the one or more power ramp rates RR for the secondary power rate limiter 203 may be different from those of the primary power ramp limiter 202 for the corresponding frequency thresholds or frequency ranges. For example, for a given frequency range, the ramp rate RR for the secondary power rate limiter 203 may be larger than the corresponding power ramp rates RR of the primary power rate limiter in order to ensure that the ramp rate of the secondary ramp rate limiter 203 does not further limit the rate of change of the power reference Pref.

Referring again to Fig. 4, curve 402 illustrates different power ramp rates RR3 and RR4 for the secondary power rate limiter 203. As shown, for grid frequencies within a range from fTI to fTu, the third ramp rate RR3 is greater than the corresponding first ramp rate RR1, but could also be equal to the first ramp rate RR1. Similarly, for frequencies within the second range of frequencies above fTu or below fTI, the fourth ramp rate RR4 is greater than the corresponding second ramp rate RR3, but they could also be equal.

Accordingly, if the grid frequency is within a first range from fN to fTu or fTI to fTu, the power ramp rate for the secondary power rate limiter 203 could be set to a third ramp rate RR3. If the grid frequency is within a second range of

frequencies above fTu, the power ramp rate of the secondary power rate limiter 203 could be set to a fourth ramp rate RR2. The second frequency range could also include frequencies below fTI, or another ramp rate could be associated with a third range of frequencies below fTI for the secondary power rate limiter 203.

It is noted that the fourth ramp rate RR4 for the second range of frequencies above fTu could be different from another ramp rate RR4' (not shown) of the secondary power rate limiter 203 for a third range of frequencies below fTI.

Additionally or alternatively, the second ramp rate RR2 for the second range of frequencies above fTu could be different from another ramp rate RR2' (not shown) of the primary power rate limiter 202. As shown in Fig. 4, the power ramp rates RR for frequencies above or below the frequency thresholds fTu, fTI, allows a higher rate of change than the power ramp rates RR for frequencies between the frequency thresholds fTu, fTI. The functions of the controller 200 for controlling a wind turbine may used in an alternative configuration of a wind park controller for controlling one or more wind turbines. In this case the ramp rate limiters 202, 203 are configured in the same way to limit the rate of change of a power reference Pref for controlling the amount of wind power to be extracted by the one or more wind turbines of the wind park. In this case the power reference for the wind part is not supplied to the power control system 205 of a wind turbine, but may be divided among the producing wind turbines of the park. For example, a fraction of the power reference Pref for the park may be supplied as an externally provided power reference Pext to a control system of a wind turbine.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is to be interpreted in the light of the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.