Patent Description:
Regulators and operators of power networks expect connected power plants to adhere to a 'grid code' and to provide particular services to the power network.

For example, some operators require power plants to support the power network when the frequency of the power network deviates from the normal operational range, also referred to as a frequency dead band. A range of control strategies have been developed for wind power plants to provide support during frequency deviations. During these events, power plant controllers and wind turbine controllers implement frequency support by changing active power output levels to counteract the frequency deviation. In under-frequency events, where the frequency level deviates below the frequency dead band, active power output levels are increased to support the network. In over-frequency events, where the frequency level rises above the frequency dead band, active power output levels are decreased to provide support.

After the frequency deviation, active power is increased or decreased back to normal levels according to a limited ramp rate. However, in some situations, active power output of a wind turbine is limited, based generally on the available wind or another user-implemented limit. In contrast, controllers determine the active power output levels based on predefined curves, and so may issue active power commands that are outside the limits. This causes an effect known as wind-up where the actual active power output level of the turbine ramps back to normal levels after a delay due to the difference between the stipulated level from the controller and the limit level.

Among relevant prior art are <CIT>, <CIT>, <CIT> and <CIT>.

According to an aspect of the present invention there is provided a method for controlling active power output of a wind turbine generator in response to a frequency event on a power network to which the wind turbine generator is connected, and wherein the active power output of the wind turbine generator is limited to be below an upper active power limit and/or above a lower active power limit. The method comprises determining and dispatching active power references to a controller of the wind turbine generator for controlling the wind turbine generator. During the frequency event, in which the frequency level of the power network is outside a frequency deadband, the active power references are determined by: determining an active power value based on a measured frequency of the power network; comparing the determined active power value with one of the upper active power limit or the lower active power limit; and if the determined active power value is outside the limit the active power reference is set equal to the limit, or if the determined active power value is not outside the limit the active power reference is set as the determined active power value. After the frequency event, in which the frequency level of the power network is within the frequency deadband, the active power references are determined to change from the final value during the frequency event to a baseline active power value according to a ramp rate limit.

The term reference is used herein to mean an active power set point. The above method ensures that, throughout the deviation and immediately after the deviation, the reference dispatched to the generator will be at least equal to the limit, if not within the allowable range defined above or below the limit. Furthermore, it ensures that there is a match between the output of the wind turbine generator and the reference it receives all through the deviation, so that when the deviation ends, the ramping can begin from a common value.

During the frequency event, determining the active power value based on the measured frequency of the power network may comprise: determining an active power adjustment value that corresponds to the measured frequency; and subtracting the active power adjustment value from an active power baseline value.

Determining the active power adjustment value may comprise comparing the measured frequency with a chart or look-up table indicating a correspondence between active power and frequency.

Setting the active power reference to be equal to the limit may comprise: determining an active power limit adjustment value as the difference between an active power baseline value and the active power limit; and subtracting the active power limit adjustment value from the active power baseline value.

The active power baseline value may comprise a minimum value of a nominal active power value and a curtailed active power value.

If during the frequency event the frequency level is below the deadband, the relevant limit is the upper active power limit, and the upper active power limit may be based on the available active power. If during the frequency event the frequency level is above the deadband, the relevant limit is the lower active power limit and the lower active power limit may be based on a user preference.

According to another aspect of the invention, there is provided a method for controlling active power output of a wind turbine generator in response to a frequency event on a power network to which the wind turbine generator is connected, wherein the active power output of the wind turbine generator is limited to be below an upper active power limit and/or above a lower active power limit. The method comprises: determining and dispatching active power references to a controller of the wind turbine generator for controlling the wind turbine generator. During the frequency event, in which the frequency level of the power network is outside a frequency deadband, the active power references are determined based on a measured frequency of the power network. After the frequency event, in which the frequency level of the power network is within the frequency deadband, the active power references are determined to change from a restart active power value to a baseline active power value according to a ramp rate limit, the restart active power value being equal to a measured active power output of the wind turbine generator.

The above method makes use of a point in time, the end of the event or deviation, to correct the value of the reference, thereby ensuring that the reference is at or within the relevant limit when it is necessary for it to be. This also means that the reference and output will be at the same level when the frequency deviation ends so that ramping begins for the wind turbine generator straight away and with no delay.

During the frequency deviation the active power reference value may be determined by: determining an active power adjustment value by comparing the measured frequency with a chart or look-up table indicating a correspondence between active power and frequency; and subtracting the active power adjustment value from a baseline active power value.

After the frequency deviation determining the active power references may comprise determining the restart active power value. Determining the restart active power value may comprise: determining a restart active power adjustment value by subtracting the measured active power output from a baseline active power value; and subtracting the restart active power adjustment value from the baseline active power value.

The active power baseline value may comprise a minimum of an available active power value and a curtailed active power value.

If during the frequency event the frequency level is below the deadband, the relevant limit is the upper active power limit and the upper active power limit may be based on the available active power. If during the frequency event the frequency level is above the deadband, the relevant limit is the lower active power limit and the lower active power limit may be based on a user preference.

The frequency deviation may be deemed to have ended upon determination that a trigger condition is met and/or upon receipt of a trigger generated in response to a trigger condition being met. The trigger condition may comprise at least one of: the frequency level being within the frequency deadband; the frequency level is equal to or exceeds a threshold frequency value; a difference between the measured active power value and the relevant active power limit is equal to or exceeds a threshold value.

According to an aspect of the invention, there is provided a power plant controller configured to perform one of the methods described above.

According to an aspect of the invention, there is provided computer-readable storage medium comprising instructions that, when executed by a computer, cause the computer to perform one of the methods described above.

According to an aspect of the invention, there is provided a method for controlling active power output of a wind turbine generator in response to a frequency event on a power network to which the wind turbine generator is connected, and wherein the active power output of the wind turbine generator is limited to an active power range between an upper active power limit and a lower active power limit. The method comprises determining and dispatching active power references to a controller of the wind turbine generator for controlling the wind turbine generator such that the value of the active power reference immediately after the frequency event is within the allowable active power range.

That is, all embodiments and/or features of any embodiment can be combined in any way within the scope of the appended claims.

Generally, the present application relates to methods and systems of controlling a power plant controller and a wind turbine generator to ensure that the phenomenon known as "wind-up" is avoided. Here, the term "wind-up" refers to the mismatch between set point and output active power values of an output-limited wind turbine generator. The methods and systems described herein act to ensure that, at the end of a frequency event, when the active power value is able to ramp back up or down to normal levels, the ramping of the output value does not have to wait for the set point value to reach a certain level. That is, that the normal levels are regained as soon as possible whilst adhering to the ramp rate limit imposed by the system on active power changes. The benefit of this is that the generator can optimise its active power output where it may have previously been experiencing a delay.

<FIG> illustrates a typical architecture in which a wind power plant (WPP), which may also be referred to as a wind park or wind farm, is connected to a main grid as part of a wider power network. As will be understood by the skilled reader, a WPP comprises at least one wind turbine generator (WTG), and is also known as a wind park or a wind farm. A WTG is commonly referred to as a wind turbine. The examples shown are representative only and the skilled reader will appreciate that other specific architectures are possible, in relation to wind power plants, power plants for other renewable energy sources such as solar power plants, bio energy power plants, or ocean/wave/tidal energy plants, and to hybrid power plants having a combination of different types of renewable energy power plants. Thus, the invention also relates to renewable energy power plants and renewable energy generators in general, rather than being specific to wind power plants and generators as in the Figures. The components of the wind power plant and power network are conventional and as such would be familiar to the skilled reader. It is expected that other known components may be incorporated in addition to or as alternatives to the components shown and described in <FIG>. Such changes would be within the capabilities of the skilled person.

<FIG> shows a power network <NUM> incorporating a WPP <NUM> and a power plant controller <NUM>, referred to hereafter as PPC <NUM>. The WPP <NUM> includes a plurality of WTGs <NUM>. Each of the plurality of WTGs <NUM> converts wind energy into electrical energy, which is transferred from the WPP <NUM> to a main transmission network or main grid <NUM>, as active power and/or current, for distribution. Individual generators may each be referred to in this description as a 'unit'.

Although not illustrated in this Figure, the WPP <NUM> may also include compensation equipment, such as a static synchronous compensator (STATCOM) or another type of synchronous compensator, configured to provide reactive power or reactive current support as required. The WPP <NUM> may also include a battery energy storage system.

Each of the WTGs <NUM> is associated with a respective WTG controller <NUM>. In some examples, a set of WTGs may share a single, semi-centralised WTG controller, such that there are fewer WTG controllers than WTGs. As would be understood by the skilled person, WTG controllers <NUM> can be considered to be computer systems capable of operating a WTG <NUM> in the manner prescribed herein, and may comprise multiple modules that control individual components of the WTG or just a single controller. The computer system of the WTG controller <NUM> may operate according to software downloaded via a communications network or programmed onto it from a computer-readable storage medium.

During normal operation of the WPP <NUM>, the WTG controllers <NUM> operate to implement active and reactive current and/or power requests received from the PPC <NUM> to provide frequency and voltage support to the main grid <NUM>. During extraordinary conditions, the WTG controllers <NUM> operate to fulfil predetermined network requirements, and also act to protect the WTGs <NUM> from any potentially harmful conditions.

The WPP <NUM> is connected to the main grid <NUM> (also called the main power network) by a connecting network <NUM>. The WPP <NUM> and the main grid <NUM> are connected at a Point of Interconnection (Pol) <NUM>, which is an interface between the WPP <NUM> and the main grid <NUM>. The Pol <NUM> may also be referred to as the Point of Common Connection, which may be abbreviated to 'PCC' or 'PoCC'.

The Power Plant Controller (PPC) <NUM> is connected to the main grid <NUM> at a Point of Measurement (PoM) <NUM> and is connected to the WTG controllers <NUM>. The role of the PPC <NUM> is to act as a command and control interface between the WPP <NUM> and the grid <NUM>, and more specifically, between the WPP <NUM> and a grid operator <NUM>, such as a transmission system operator (TSO) or a distribution system operator (DSO). The PPC <NUM> is a suitable computer system for carrying out the controls and commands as described above and so incorporates a processing module <NUM>, a connectivity module <NUM>, a memory module <NUM> and a sensing module <NUM>. The PPC <NUM> may also receive information regarding the grid <NUM> and/or the local buses, substations and networks from an energy management system (not shown). The WPP <NUM> is capable of altering its power or current output in reaction to commands received from the PPC <NUM>.

As part of its operation, the PPC <NUM> generates and sends dispatch signals to the WTG controllers <NUM>. The WTG controllers <NUM> control the WTGs according to set points contained within the dispatch signals.

The PPC <NUM> acts to operate the WTGs <NUM> to provide frequency support to the grid <NUM> where frequency deviates from an acceptable frequency range, also known as a frequency deadband. To provide frequency support, the PPC <NUM> issues dispatch signals configured to cause the WTGs <NUM> to supply active power to provide frequency support to the power network. The signals are determined to control the active power output of the WTGs <NUM> so that frequency levels are supported in returning to the deadband. The deadband is generally a small region around the operating frequency of the power network, typically <NUM>, or in some examples, <NUM>, as measured at the Pol <NUM> or PoM <NUM>.

When frequency level drops outside of the deadband and is therefore below the deadband, the PPC <NUM> provides frequency support by dispatching increased active power set points to the WTGs <NUM>. When frequency levels rise above deadband, the PPC <NUM> provides frequency support by dispatching decreased active power set points. These are under- and over-frequency events respectively.

In conventional PPCs, the active power set point is dictated by an active power-frequency chart, which is in effect a suitable data structure containing associated values of frequency levels against active power set points. As such, using the active power-frequency chat, the PPC receives a measured frequency level and generates a corresponding active power set point, which it then dispatches to the wind turbine controllers. In some PPCs, the measured frequency level is used to determine an active power delta value, i.e. a change from baseline, and this is subtracted from the baseline level to give the set point. In either circumstance, a set point is generated directly from the measured frequency level. Note that this functionality is conventional and so further discussion will be limited.

A baseline active power level, as discussed herein, is used to indicate an active power level during normal operation. For example, this may be the rated active power, or another active power that was set by the PPC according to various control aspects. In some cases, the baseline active power level may correspond to a curtailed active power level.

In this conventional situation, the PPC dispatches set points with no regard to limitations on the active power output of the WTG. For example, where an underfrequency event occurs, the active power output of the WTG above the baseline level is limited by a maximum active power value corresponding to the available active power output based on the wind. Similarly, in overfrequency events, the active power output below the baseline level is often limited by the grid code or by a user-generated or user-determined minimum output level. In some examples, the active power output may also be limited by physical constraints placed on the WTG.

The PPC may therefore demand more or less active power via its set point than the respective maximum or minimum limit of the WTG. If it does, the WTG will output active power at the limit, not the set point, but the set point will remain at the value outside the limit. Once the frequency event ends and the frequency level returns to the deadband, the PPC ramps the active power set point back to the baseline level. If the PPC has demanded a set point outside the limit but the WTG is outputting at the limit, there will be a delay between when the PPC begins to ramp the set point and when the set point meets the limit and hence when the WTG can begin to ramp its output. This delay causes under- or over-generation of active power where it is not necessary or desirable, and may cause the plant to be penalised by the transmission system operator, and therefore is best avoided.

This phenomenon of the mismatch between PPC dispatched set point and WTG output and the resulting delay in regaining the baseline active power output after the frequency event caused by this mismatch may be referred to as PPC wind-up. It should be noted that the limits that cause the mismatch are typically variable limits, such as available active power.

<FIG> and <FIG>, as described below, are provided as two embodiments of the invention that counteract and substantially eliminate the delays associated with PPC wind-up. The control schemes or algorithms of the embodiments illustrated in <FIG> and <FIG> operate to achieve the same outcome but do so by different means. The outcome achieved is that after the frequency event, the active power set point generated by these controllers ramps from the limit level back to the baseline level so that there is no delay in the output of the WTGs ramping back to the baseline either. Essentially, therefore, the embodiments both specify ways in which the set point (which may be referred to an active power set point or an active power reference herein) can be determined to ensure that its value immediately after the frequency event is equal to or within the limits set by the available power (upper limit) and user preference or WTG constraint (lower limit).

<FIG> illustrates a frequency control scheme, algorithm, or "controller" <NUM>, which forms part of the processing module <NUM> of the PPC <NUM>. <FIG> and <FIG> show a chart indicating an example scenario using the controller <NUM> of <FIG> and a method of operation of the controller <NUM> of <FIG>.

In <FIG>, the frequency controller <NUM> receives various active power levels and measured frequency and outputs a value for the active power set point. The controller <NUM> comprises an active power-frequency correspondence table <NUM>, which may be referred to as a P-f table <NUM>. Initially, the controller <NUM> receives a measured frequency, fmeas, at the P-f table <NUM>. The P-f table <NUM> determines an active power adjustment or change value, ΔP, as an output. The determined active power adjustment value is input to an adaptive limiter <NUM>. The P-f table <NUM> may comprise a look-up table of measured frequency vs change in output active power ΔP, or may comprise a look-up table of change in frequency from a nominal value fnom (i.e. a frequency error value of fmeas-fnom) vs change in output active power ΔP.

The adaptive limiter <NUM> receives values as inputs such as one or more power limits, here referred to as Pava and Plim, and the baseline active power value Pbase. Where a curtailed active power value, Pcurtail, is also provided to the PPC <NUM>, this value is passed through a ramp rate limiter, which is referred to as RRL2 or by reference numeral <NUM> here, to give a ramped curtailed active power, Pcurtail_ramp, and this value is also provided as an input to the adaptive limiter <NUM>.

The adaptive limiter <NUM> performs a comparison to identify whether the set point based on the adjustment value ΔP from the P-f table <NUM> would be outside the relevant limit. In other words, the adaptive limiter <NUM> compares a proposed set point value with the limits and determines if the proposed set point is above an upper active power limit, i.e. available active power Pava, or is below a lower active power limit, i.e. a user limit Plim. The output of the adaptive limiter is a new adjustment value ΔP'. The value of ΔP' is based on the comparison. If the proposed set point does fall outside the relevant limit, then ΔP' is determined so that the resulting set point is at least equal to the limit. This is achieved by setting the value of ΔP' to be equal to the limit value subtracted from a baseline value. If the proposed set point does not fall outside the relevant limit then the value of ΔP' is the adjustment value ΔP.

In the adaptive limiter <NUM>, the proposed set point, which is compared with the limit, is determined by subtracting the adjustment value ΔP received from the P-f table <NUM> from the from the minimum of the ramped curtailed active power value and the baseline active power value.

Thus, the determination made by the adaptive limiter <NUM> in this embodiment can be classified by a pair of equations. During an overfrequency event, the determination is follows: <MAT>.

During an underfrequency event, the determination is as follows: <MAT>.

The output of the adaptive limiter, the new adjustment value ΔP', is passed through a ramp rate limiter <NUM> to ensure that the change does not exceed a predetermined ramp rate, before being input to two difference junctions <NUM> and <NUM>. The difference junctions <NUM>, <NUM> subtract the adjustment value output from the limiter ΔP' from the curtailed ramped active power value and the baseline active power value respectively. A minimum of these two differences is determined at <NUM> to determine a set point value, Pref. The reference value is passed through a final hard limiter <NUM> to provide a final set point value PrefFreq. This value is dispatched to the WTGs <NUM>.

To illustrate the effect the adaptive limiter <NUM> of the frequency controller <NUM> has on the dispatched set point value, <FIG> illustrates a chart indicating two overfrequency events. Initially, the active power set point is equal to the baseline level. A first overfrequency event occurs at time t1a, when the measured frequency deviates above the frequency deadband.

In response, the frequency controller <NUM> calculates an adjustment value ΔP from the baseline level based on the P-f table <NUM>. As the adjustment value does not cause the proposed set point to be below the limit Plim, then the output of the limiter <NUM>, namely the new adjustment value ΔP', is the same as the adjustment value ΔP. Once the frequency event ends at t1b the active power set point ramps back to the baseline level according to the ramp rate limit.

At time t2a a second overfrequency event occurs. The deviation is greater than the first event so the response by the controller is greater. As can be seen in <FIG>, the ΔP value generated from the P-f table <NUM> based on the measured frequency would cause the set point value to be below the limit, i.e. min(Pcurtail_ramp, Pbase) - ΔP ≤ Plim, as in the equation above. Thus, the adaptive limiter <NUM> in this circumstance recalculates the value of ΔP' so that the set point will be equal to the limit value Plim.

Therefore, once the frequency event ends at time t2b, the active power set point and the active power output of the WTG both ramp up together from the limit back to the baseline value Plim. This ramp is labelled Pref_anti-windup in <FIG>, and can be compared and contrasted with what would happen in conventional arrangements, which is illustrated by the dotted and dashed line labelled Pref_original. In a sense, therefore, the adaptive limiter <NUM> acts to clamp the set point to the baseline limit value Plim if the request setpoint falls below that limit value.

This action by the frequency controller <NUM> can be illustrated as a method <NUM> as shown in <FIG>. The method <NUM> illustrates the determination of active power set points in general; it will be appreciated that the set points are dispatched to the WTGs <NUM> and their controllers <NUM> by the PPC <NUM> in a suitable manner.

As shown in <FIG>, at a first step <NUM>, an active power value is determined based on measured frequency. When considered in relation to <FIG>, the determined active power value is the proposed set point, i.e. the adjustment value subtracted from the baseline value, where the adjustment value has been determined based on the measured frequency. The active power value in this step is for comparison with the relevant active power limit, as will be discussed in the next step, and so can be determined in any way, whether it is based fully or partially on the frequency, a look-up table, a chart, on an equation, or by other suitable means.

At the next step <NUM>, the value is compared with the relevant upper or lower active power limit. As discussed in relation to <FIG> and <FIG>, this is to determine whether the proposed set point value is outside the limit.

At step <NUM>, if the value is outside the limit, the active power set point is set as the limit value. At step <NUM>, if the value is not outside the limit, the active power set point is set as the determined value. Step <NUM> corresponds to the situation discussed in relation to the overfrequency event at time t2a in <FIG>, while step <NUM> corresponds to the overfrequency event at time t1a.

When it is determined, at step <NUM> that the frequency event has finished, the method concludes with step <NUM> by changing the active power set point from the final value during the frequency event to the baseline active power value. In other words, the active power set point, and, therefore, the active power output of the WTG, is ramped back to normal levels according to the ramp rate limit.

While the above method is described as being performed during a frequency deviation, it will be appreciated that the determination of set points according to measured frequency may be performed at all times, and the checks performed during the method and ramping are applicable only when a frequency event is ending.

In the second embodiment of the frequency controller, shown by <FIG>, the frequency controller <NUM> incorporates a ramp rate initialization module <NUM> in place of the adaptive limiter <NUM>. The other features of the controller <NUM> are the same as in the controller <NUM> of <FIG>, so have been labelled with the same reference numerals.

The ramp rate initialization module <NUM> acts to generate a set point equal to or greater than the limit value immediately after the end of the frequency event so that the set point ramps from that value rather than a lower value.

To do so, the module <NUM> receives the adjustment value ΔP, determined from the P-f table <NUM> and the curtailed power value and baseline power value Pcurtail_ramp and Pbase. The module <NUM> also receives a value of the measured active power output value of the WTG, Pmeas.

During the frequency deviation, the module <NUM> outputs a ΔP' value equal to the ΔP value received from the P-f table <NUM>. At the end of the frequency deviation, typically in response to a trigger indicating the end of the frequency deviation, the module <NUM> determines a restart or reinitialization value, ΔP', that is equal to the difference between the minimum of the baseline or curtailed active power value and the measured active power value. This can be represented as before the trigger, ΔP' = ΔP, while immediately after the trigger ΔP' = min(Pcurtail_ramp, Pbase) - Pmeas. Once this restart value has been determined and dispatched, the module <NUM> returns to setting ΔP' = ΔP. However, as the frequency deviation will have ended and frequency levels returned to the deadband, the ΔP value determined from the P-f table <NUM> and consequently the value ΔP' will be minimal, possibly zero. This change in ΔP' will be mitigated by the ramp rate limiter <NUM> so that the change in ΔP' does not cause a sudden jump in the value of the set point. Accordingly, the active power value will slowly ramp back to the baseline value.

It is noted above that the end of the frequency deviation is typically marked using a trigger. By this it is meant that a trigger is communicated to the frequency controller when a trigger criteria is met. The trigger criteria typically indicates that the frequency deviation is ended. The trigger criteria may comprise the return of the frequency level to the deadband, and this may be determined based on a change in the ΔP value from the P-f table <NUM> from a substantial ΔP value to a negligible ΔP value. In other embodiments, the trigger criteria may be met when it is determined that the frequency level is above or below a particular threshold value, when it is determined that a rate of change of frequency is meets a particular threshold, or when a difference between the measured active power level and an active power limit level is at a threshold value. The difference between measured and limit active power being at a threshold may indicate that the measured frequency is still outside the deadband, but that the power value will not longer be outside its respective limit, so no wind-up will occur.

This process is illustrated in <FIG>, which is a chart illustrating two overfrequency events that are similar in size and timing to those of <FIG>. Accordingly, the same time points t1a to t1b and t2a to t2b are used for the frequency events.

The first overfrequency deviation is between times t1a and t1b. At t1a, the frequency rises above the deadband. Accordingly, as per the P-f table <NUM>, a ΔP value is calculated and passed through the limiter without change to generate a set point. The ΔP value passed from the P-f table <NUM> is unchanged during the deviation.

At time t1b the overfrequency deviation ends as the frequency level returns to the deadband. A trigger is received by the adaptive limiter <NUM> indicating this. In response, the module <NUM> generates the restart ΔP' value, as described above. In this case, as the active power output and set point during the deviation has not exceeded the limit, the set point generated using the restart adjustment value is the same as the set point during the deviation, and therefore the ramping back to the baseline is from the same set point.

In contrast, in the second deviation, beginning at time t2a, the frequency deviates to a greater value. Accordingly, during the deviation, the set point generated based on the ΔP value is below the limit. It will be appreciated, however, that the output of the WTG will be at the limit during this period.

At the end of the deviation, at time t2b, the frequency returns to the dead band and in response to receiving this trigger, the module <NUM> determines the restart adjustment value so that the set point immediately after is equal to the measured active power output. The measured active power output is at the limit, as already mentioned, so the set point immediately after the deviation ends is set to the limit and the ramping is begin from this level according to the ramp rate limit.

Again, as shown in <FIG>, a dotted and dashed line indicates the situation without the adaptive limiter where the ramping is subject to the wind-up of the PPC.

<FIG> illustrates a general method <NUM> governing the embodiment of <FIG> and <FIG>. The method <NUM>, as with the method <NUM> of <FIG>, demonstrates how active power references are generated. It will be appreciated that the PPC <NUM> dispatches the active power references to the WTGs <NUM> and their controllers <NUM> appropriately after determination.

As shown in <FIG>, at a first step <NUM> of the method active power references are determined based on a measured frequency of the power network. This step may be performed at all times during the operation of the controller other than immediately after the frequency event, as will be discussed below, or may be performed during a measured frequency depending upon the implementation of the system.

At the next step <NUM> of the method, a check is performed to see if a trigger has been received, indicating that the frequency deviation has ended according to the trigger criteria. If the trigger has not been received, the method returns to step <NUM>. If it has been received, a restart active power value is determined at step <NUM>. Subsequently, at step <NUM>, the active power value is ramped from the restart value to the baseline active power value.

The two methods <NUM>, <NUM>, described above both act to generally ensure that the set point and output active power values are matched at the end of a frequency deviation, so there is no delay in ramping the active power back to baseline levels. Each achieves this effect in a different way - the first method being based on the specific limit and comparison between determined values, the second based on a point in time at which the frequency deviation is deemed to have ended. While the implementations differ in the details, it will be appreciate that the same technical effect is achieved.

Claim 1:
A method (<NUM>) for controlling active power output of a wind turbine generator (<NUM>) in response to a frequency event on a power network (<NUM>) to which the wind turbine generator (<NUM>) is connected, wherein the active power output of the wind turbine generator (<NUM>) is limited to be below an upper active power limit and/or above a lower active power limit, the method (<NUM>) comprising:
determining and dispatching active power references to a controller (<NUM>) of the wind turbine generator (<NUM>) for controlling the wind turbine generator (<NUM>),
wherein, during the frequency event, in which the frequency level of the power network (<NUM>) is outside a frequency deadband, the active power references are determined by:
determining (<NUM>) an active power value based on a measured frequency of the power network (<NUM>);
comparing (<NUM>) the determined active power value with one of the upper active power limit or the lower active power limit; and
if the determined active power value is outside the limit the active power reference is set equal to the limit (<NUM>), or if the determined active power value is not outside the limit the active power reference is set as the determined active power value (<NUM>);
wherein, after the frequency event, in which the frequency level of the power network (<NUM>) is within the frequency deadband, the active power references are determined to change (<NUM>) from the final value during the frequency event to a baseline active power value according to a ramp rate limit.