Method and system for controlling a wind turbine generator in response to frequency deviation

Abstract

Aspects of the invention relate to methods (200, 300) for controlling the active power output of a wind turbine generator (14) in response to a frequency event on an electrical power network (16) to which the wind turbine generator (14) is connected, wherein the active power output of the wind turbine generator (14) is limited to be below an upper active power limit and / or above a lower active power limit. The methods (200, 300) comprise determining an active power reference value and distributing the active power reference value to a controller (15) of the wind turbine generator (14) for controlling the wind turbine generator (14). The methods operate to ensure that the active power reference value immediately after the frequency event is within the allowed active power range, thereby avoiding any mismatch between the reference value and the output which could cause delays in the process of returning to normal operation.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method and system for controlling a wind turbine generator in response to a frequency deviation. BACKGROUND

[0002] Regulators and operators of power networks expect connected power plants to adhere to a "grid code" and to provide specific services to the power network.

[0003] For example, some operators require power plants to provide support to the power network when the frequency of the power network deviates from a normal operating range, also known as the frequency deadband. A range of control strategies for wind power plants have been developed to provide support during frequency deviations. During these events, the power plant controller and the wind turbine controller implement frequency support by varying the active power output level to counteract the frequency deviation. In under-frequency events, where the frequency level deviates below the frequency deadband, the active power output level is increased to support the network. In over-frequency events, where the frequency level rises above the frequency deadband, the active power output level is decreased to provide support.

[0004] After the frequency deviation, the active power is ramped back to the normal level according to a limited ramp rate. However, in some cases, the active power output of the wind turbine is limited, typically based on available wind or another user-imposed limit. In contrast, the controller determines the active power output level based on a predefined curve, and can therefore issue an active power command that is outside the limit. This results in an effect known as "wind-up", in which the actual active power output level of the turbine is ramped back to the normal level after a delay due to the difference between the prescribed level from the controller and the limit level.

[0005] It is an object of the present invention to address one or more of the drawbacks associated with the prior art. SUMMARY

[0006] According to an aspect of the present application, there is provided a method for controlling an active power output of a wind turbine generator in response to a frequency event on an electrical 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 an active power reference value and assigning the active power reference value to a controller of the wind turbine generator for controlling the wind turbine generator. During the frequency event in which a frequency level of the electrical power network is outside a frequency deadband, the active power reference value is determined by: determining an active power value based on a measured frequency of the electrical power network; comparing the determined active power value to one of the upper active power limit or the lower active power limit; and setting the active power reference value equal to the limit value in case the determined active power value is outside the limit value, or setting the active power reference value equal to the determined active power value in case the determined active power value is not outside the limit value. After the frequency event in which the frequency level of the electrical power network is inside the frequency deadband, the active power reference value is determined to change from a final value during the frequency event to a base active power value according to a ramp rate limit.

[0007] The term reference value used herein refers to an active power setpoint. The above method ensures that throughout the entire deviation and immediately after the deviation, if the reference value assigned to the generator is not within an allowed range limited to be above or below a limit value, the reference value assigned to the generator will be at least equal to the limit value. Furthermore, the above method ensures that throughout the deviation, the output of the wind turbine generator and the reference value it receives are matched, so that when the deviation ends, a ramp can be started from a common value.

[0008] Determining the active power value based on the measured frequency of the electrical power network during the frequency event can comprise: determining an active power adjustment value corresponding to the measured frequency; and subtracting the active power adjustment value from an active power base value.

[0009] Determining the active power adjustment value can comprise comparing the measured frequency to a graph or lookup table indicating a correspondence between active power and frequency.

[0010] Setting the active power reference value equal to the limit value can comprise: determining an active power limit adjustment value as a difference between the active power base value and the active power limit value; and subtracting the active power limit adjustment value from the active power base value.

[0011] The active power base value can comprise a minimum of a nominal active power value and a reduced active power value.

[0012] If the frequency level is below the deadband during the frequency event, the relevant limit is an upper active power limit, which can be based on the available active power. If the frequency level is above the deadband during the frequency event, the relevant limit is a lower active power limit, which can be based on the user's preference.

[0013] According to another aspect of the application, there is provided a method for controlling an active power output of a wind turbine generator in response to a frequency event on an electrical 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 an active power reference value and assigning the active power reference value 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 electrical power network is outside a frequency deadband, the active power reference value is determined based on a measured frequency of the electrical power network. After the frequency event, in which the frequency level of the electrical power network is within the frequency deadband, the active power reference value is determined to change from a restart active power value to a base active power value according to a ramp rate limit, the restart active power value being determined based on a measured active power output of the wind turbine generator.

[0014] The above method utilises one point in time (i.e. the end of the event or deviation) to correct the reference value, thereby ensuring that the reference value is at or within the relevant limit when necessary. This also means that the reference value and the output will be at the same level when the frequency deviation ends, so that the ramp of the wind turbine generator starts directly without delay.

[0015] During the frequency deviation, the active power reference value can be determined by determining an active power adjustment value by comparing the measured frequency with a graph or look-up table indicating a correspondence between active power and frequency; and subtracting the active power adjustment value from the base active power value.

[0016] After the frequency deviation, determining the active power reference value can comprise determining a restart active power value. Determining the restart active power value can comprise determining a restart active power adjustment value by subtracting the measured active power output from the base active power value; and subtracting the restart active power adjustment value from the base active power value.

[0017] The base active power value can comprise the minimum of an available active power value and a reduced active power value.

[0018] If the frequency level is below the deadband during the frequency event, the relevant limit is an upper active power limit, which can be based on the available active power. If the frequency level is above the deadband during the frequency event, the relevant limit is a lower active power limit, which can be based on the user's preference.

[0019] Upon determining that the trigger condition is met and / or upon receiving a trigger generated in response to the trigger condition being met, the frequency deviation can be considered to have ended. The trigger condition can comprise at least one of: the frequency level being within a frequency deadband; the frequency level being equal to or exceeding a threshold frequency value; a difference between the measured active power value and the associated active power limit value being equal to or exceeding a threshold.

[0020] According to an aspect of the application, there is provided a power plant controller configured to perform one of the above-described methods.

[0021] According to an aspect of the application, there is provided a computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to perform one of the above-described methods.

[0022] According to an aspect of the application, there is provided a method for controlling an active power output of a wind turbine generator in response to a frequency event on an electrical power network to which the wind turbine generator is connected, wherein the active power output of the wind turbine generator is limited to an active power range between an active power upper limit and an active power lower limit. The method comprises determining an active power reference value and assigning the active power reference value to a controller of the wind turbine generator for controlling the wind turbine generator such that the active power reference value immediately after the frequency event is within the allowable active power range.

[0023] Within the scope of the present application, it is expressly intended that each aspect, embodiment and / or example given herein, and any combination of such, can be implemented independently of any other aspect, embodiment and / or example given herein and that all possible combinations of any two or more aspects, embodiments and / or examples given herein are to be expressly intended. In particular, with reference to the appended claims, each aspect of each claim can be implemented independently of any other aspect, embodiment and / or example given herein. Likewise, the application seeks to cover and includes any and all novel aspects, embodiments and / or examples of the various processes, systems and other implementations covered by this application, and falling within the scope of the appended claims. In addition, no limitation of the scope of the application shall be inferred from the description or drawings given herein, but any and all such limitations shall be reserved on the appended claims. BRIEF DESCRIPTION OF DRAWINGS

[0024] One or more embodiments of the application will now be described, by way of example only, with reference to the accompanying drawings in which:

[0025] Figure 1 A power network comprising a wind power plant and a power plant controller is shown;

[0026] Figure 2 A system diagram of a frequency controller of a power plant controller according to an embodiment of the application is shown; Figure 1 A system diagram of a frequency controller of a power plant controller according to an embodiment of the application is shown;

[0027] Figure 3 a pair of graphs showing the response of the frequency controller of Figure 2 to a frequency deviation;

[0028] Figure 4 a method of operation of the frequency controller of Figure 2 ;

[0029] Figure 5 a system diagram of the frequency controller of the power plant controller according to another embodiment of the application; Figure 1

[0030] Figure 6 a pair of graphs showing the response of the frequency controller of Figure 5 to a frequency deviation; and

[0031] Figure 7 a method of operation of the frequency controller of Figure 5 . DETAILED DESCRIPTION

[0032] In general, the present application relates to methods and systems for controlling a power plant controller and a wind turbine generator to ensure that a phenomenon known as "saturation" is avoided. Here, the term "saturation" refers to a mismatch between a set point and an output active power value of a wind turbine generator whose output is limited. The methods and systems described herein act to ensure that at the end of a frequency event, when the active power value can be ramped up or down to a normal level, the ramping of the output value does not have to wait for the set point value to reach a certain level. That is, the normal level is regained as quickly as possible while respecting the ramp rate limits imposed by the system on the change in active power. The benefit of this is that the generator can optimise its active power output, which it would not have been able to do before.

[0033] Figure 1 ​A typical architecture is shown in which a wind power plant (WPP, also known as a wind farm or wind park) is connected to a main grid that is 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 farm or wind park. A WTG is often referred to as a wind turbine. The example shown is representative only and the skilled reader will understand that other specific architectures are possible involving wind power plants, other renewable energy power plants (such as solar power plants, bio-energy power plants or ocean / wave / tidal energy power plants), and hybrid power plants with a combination of power plants formed from different types of renewable energy power plants. Thus, the present invention also relates to conventional renewable energy power plants and renewable energy power generators, rather than being specific to wind power plants and generators as in the figure. The components of the wind power plant and the power network are conventional and thus familiar to the skilled reader. It is expected that other known components can be included in addition to or as an alternative to those shown and described in Figure 1 Figure 1

[0034] Figure 1 A power network is shown comprising a WPP 12 and a power plant controller 22 (hereinafter referred to as PPC 22). The WPP 12 comprises a plurality of WTGs 14. Each of the plurality of WTGs 14 converts wind energy into electrical energy which is transmitted from the WPP 12 as active power and / or current to a main transmission network or main grid 16 for distribution. In this description, each individual power generator can be referred to as a “unit”.

[0035] Although not shown in this figure, the WPP 12 can also comprise a compensation device (such as a static synchronous compensator (STATCOM) or other type of synchronous compensator) configured to provide reactive power or reactive current support as required. The WPP 12 can also comprise a battery energy storage system.

[0036] Each of the WTGs 14 is associated with a respective WTG controller 15. In some examples, a group of WTGs can share a single semi-centralised WTG controller so that the number of WTG controllers is less than the number of WTGs. As will be understood by the skilled person, a WTG controller 15 can be considered to be a computer system capable of operating a WTG 14 in the manner specified herein and can comprise a plurality of modules controlling individual components of the WTG or just a single controller. The computer system of the WTG controller 15 can operate in accordance with software programmed onto it either downloaded through a communications network or from a computer readable storage medium.

[0037] ​​During normal operation of the WPP 12, the WTG controller 15 operates to fulfil active and reactive current and / or power requests received from the PPC 22 to provide frequency and voltage support to the main grid 16. During special conditions, the WTG controller 15 operates to meet predetermined network requirements and also to protect the WTG 14 from any potentially harmful conditions.

[0038] The WPP 12 is connected to the main grid 16 (also referred to as the main power network) by a connection network 18. The WPP 12 and the main grid 16 are connected at a point of interconnection (PoI) 20, which is an interface between the WPP 12 and the main grid 16. The PoI 20 can also be referred to as a point of common coupling, which can be abbreviated to “PCC” or “PoCC”.

[0039] A power plant controller (PPC) 22 is connected to the main grid 16 at a point of measurement (PoM) 24 and to the WTG controller 15. The role of the PPC 22 is to act as an interface for commands and control between the WPP 12 and the main grid 16, and more specifically, between the WPP 12 and a grid operator (such as a transmission system operator (TSO) or a distribution system operator (DSO)) 26. The PPC 22 is a suitable computer system for performing the above-mentioned control and commands and therefore contains a processing module 28, a connection module 30, a memory module 32 and a sensing module 34. The PPC 22 can also receive information about the main grid 16 and / or local buses, substations and networks from an energy management system (not shown). The WPP 12 is able to vary its power or current output in response to commands received from the PPC 22.

[0040] As part of its operation, the PPC 22 generates and sends allocation signals to the WTG controller 15. The WTG controller 15 controls the WTGs in accordance with the set points contained in the allocation signals.

[0041] The PPC 22 operates the WTGs 14 to provide frequency support to the main grid 16 when the frequency deviates from an acceptable frequency range (also referred to as a frequency deadband). To provide frequency support, the PPC 22 issues an allocation signal configured to cause the WTGs 14 to supply active power to provide frequency support to the power network. The signal is determined to control the active power output of the WTGs 14 so that the frequency level is supported in returning to the deadband. The deadband is typically a small region around the operating frequency of the power network, typically 50Hz, or in some examples 60Hz, as measured at the PoI 20 or PoM 24.

[0042] When the frequency level falls outside the deadband and thus below the deadband, the PPC 22 provides frequency support by assigning an increased active power setpoint to the WTG 14. When the frequency level rises above the deadband, the PPC 22 provides frequency support by assigning a decreased active power setpoint. These are respectively under-frequency events and over-frequency events.

[0043] In conventional PPCs, the active power setpoint is specified by an active power-frequency chart, which is in effect a suitable data structure containing associated values of frequency level and active power setpoint. Thus, using the active power-frequency chart, the PPC receives the measured frequency level and generates the corresponding active power setpoint, which is then assigned to the wind turbine controller. In some PPCs, the measured frequency level is used to determine an active power delta value (i.e. a change from a reference), and this is subtracted from a reference level to give the setpoint. In both cases, the setpoint is generated directly from the measured frequency level. Note that this functionality is conventional, and so further discussion will be limited.

[0044] As discussed herein, the reference active power level is used to indicate the active power level during normal operation. This can be, for example, the rated active power, or another active power set by the PPC according to various control aspects. In some cases, the reference active power level can correspond to a reduced active power level.

[0045] In this conventional case, the PPC assigns the setpoint without regard to limits on the active power output of the WTG. For example, upon an under-frequency event, the active power output of the WTG above the reference level is limited by the maximum active power value corresponding to the wind-based available active power output. Similarly, in an over-frequency event, the active power output below the reference level is typically limited by grid codes or by a minimum output level generated or determined by the user. In some examples, the active power output can also be limited by physical constraints placed on the WTG.

[0046] The PPC can therefore demand a setpoint for active power that is more or less than the corresponding maximum or minimum limit of the WTG. If so, the WTG will output active power at the limit, not the setpoint, but the setpoint will remain at a value outside the limit. Once the frequency event is over and the frequency level returns to the deadband, the PPC ramps the active power setpoint back to the reference level. If the PPC has demanded a setpoint outside the limit but the WTG outputs at the limit, there will be a delay between the time the PPC starts ramping the setpoint and the time the setpoint reaches the limit (and between the time the WTG can start ramping its output). This delay results in under- or over-generation of active power, which is unnecessary or undesirable and can result in the power plant being penalized by the transmission system operator, and is therefore best avoided.

[0047] This phenomenon of mismatch between the setpoint assigned by the PPC and the WTG output, and the delay that occurs in regaining the reference active power output after a frequency event caused by this mismatch, can be referred to as PPC saturation. It should be noted that the limit causing the mismatch is typically a variable limit, such as available active power.

[0048] As described below, Figures 2 to 4 and Figures 5 to 7 are provided as two embodiments of the present invention that counteract and substantially eliminate the delay associated with PPC saturation. In Figure 2 and Figure 5 the control schemes or algorithms of the embodiments operate to achieve the same result but by different means. The result achieved is that the active power setpoint generated by the controllers ramps from the limit level back to the reference level after a frequency event, without delay in the WTG output ramping back to the reference as well. In essence, therefore, the embodiments both explicitly specify the manner in which the setpoint (herein referred to as the active power setpoint or active power reference) can be determined to ensure that its value immediately after a frequency event is equal to or within the limit set by available power (upper limit) and user preference or WTG constraints (lower limit).

[0049] Figure 2 A frequency control scheme, frequency control algorithm or frequency controller 100 is shown, which forms part of the processing module 28 of the PPC 22. Figure 3 and Figure 4 diagrams illustrating example scenarios of the frequency controller 100 using Figure 2 and Figure 2 the method of operation of the frequency controller 100.

[0050] InFigure 2 In particular, the frequency controller 100 receives various active power levels and measured frequencies and outputs a value for the active power setpoint. The frequency controller 100 includes an active power-frequency correspondence table 102, which can be referred to as a P-f table 102. Initially, the frequency controller 100 receives a measured frequency f meas at the P-f table 102. The P-f table 102 determines an active power adjustment or change value ΔP as an output. The determined active power adjustment value is input to an adaptive limiter 104. The P-f table 102 can include a lookup table of measured frequencies to output active power changes ΔP, or can include a lookup table of frequency changes (i.e., frequency error values f nom -f meas ) relative to a nominal value f nom to output active power changes ΔP.

[0051] The adaptive limiter 104 receives as inputs values such as one or more power limits, referred to herein as P ava and P lim , and a reference active power value P base . In the case where a reduced active power value P curtail is also provided to the PPC 22, this value is passed through a ramp rate limiter, referred to herein as RRL2 or reference numeral 106, to give a ramped reduced active power P curtail_ramp , and this value is also provided as an input to the adaptive limiter 104.

[0052] The adaptive limiter 104 performs a comparison to identify whether the setpoint based on the adjustment value ΔP from the P-f table 102 would be outside the relevant limit. In other words, the adaptive limiter 104 compares the proposed setpoint value to the limit and determines whether the proposed setpoint is above the upper active power limit (i.e., the available active power P ava ), or below the lower active power limit (i.e., the customer limit P lim ). The output of the adaptive limiter is a new adjustment value ΔP'. The value of ΔP' is based on the outcome of the comparison. If the proposed setpoint does fall outside the relevant limit, then ΔP' is determined such that the resulting setpoint is at least equal to the limit. This is achieved by setting the value of ΔP' to be equal to the limit minus the reference value. If the proposed setpoint does not fall outside the relevant limit, then the value of ΔP' is the adjustment value ΔP.

[0053] In the adaptive limiter 104, the proposed setpoint (which is compared to the limit) is determined by subtracting the adjustment value ΔP received from the P-f table 102 from the minimum of the ramped reduced active power value and the reference active power value.

[0054] Thus, in the present embodiment, the determination made by the adaptive limiter 104 can be classified by a pair of equations. During an over-frequency event, the determination is as follows:

[0055]

[0056] During an under-frequency event, the determination is as follows:

[0057]

[0058] The output of the adaptive limiter (i.e. the new adjustment value ΔΡ') is passed through a ramp rate limiter 108 to ensure that the change does not exceed a predetermined ramp rate before being input to two difference junctions 110 and 112. The difference junctions 110 and 112 subtract the adjustment value ΔΡ' output from the limiter from the reduced ramped active power value and the reference active power value respectively. The minimum of these two differences is determined at 114 to determine the setpoint value P ref . The reference value is passed through a final hard limiter 116 to provide a final setpoint value P refFreq . This final setpoint value is assigned to the WTG 14.

[0059] To illustrate the effect that the adaptive limiter 104 of the frequency controller 100 has on the assigned setpoint value, Figure 3 a graph is shown indicating two over-frequency events. Initially, the active power setpoint is equal to the reference level. When the measured frequency deviates above the frequency deadband, a first over-frequency event occurs at time t 1a .

[0060] In response, the frequency controller 100 calculates an adjustment value ΔΡ based on the P-f table 102 relative to the reference level. Since this adjustment value does not result in a proposed setpoint below the limit value P lim , the output of the limiter 104 (i.e. the new adjustment value ΔΡ') is the same as the adjustment value ΔΡ. Once the frequency event ends at t 1b , the active power setpoint ramps back to the reference level according to the ramp rate limit.

[0061] A second over-frequency event occurs at time t 2a . The deviation is greater than the first event, so the response of the controller is also greater. As can be seen from Figure 3 , the ΔΡ value generated from the P-f table 102 based on the measured frequency will result in a setpoint value below the limit, i.e. min(P curtail_ramp , P base ) - ΔΡ ≤ P lim , as per the equation above. Thus, the adaptive limiter 104 recalculates the value of ΔΡ' in this case so that the setpoint will be equal to the limit value P lim .

[0062] Thus, once the frequency event ends at time t 2b , both the active power setpoint and the active power output of the WTG ramp back to the reference value P lim together from the limit. This ramp is denoted in Figure 3 as P ref_anti-winddup and can be compared and contrasted with what happens in a conventional arrangement (shown by the dotted line denoted P ref_original ). In a sense, therefore, the adaptive limiter 104 acts to keep the setpoint at the reference limit if the required set value falls below the reference limit P lim .

[0063] This action of the frequency controller 100 can be shown as a method 200, as shown in Figure 4 . The method 200 shows the determination of a conventional active power setpoint; it will be appreciated that the setpoint is allocated by the PPC 22 to the WTGs 14 and their controllers 15 in an appropriate manner.

[0064] As shown in Figure 4 , at a first step 202, an active power value is determined based on the measured frequency. In the context of Figure 2 , the determined active power value is the proposed setpoint (i.e. the adjustment from the reference value, where the adjustment is determined based on the measured frequency). The active power value in this step is used for comparison with the relevant active power limit (which will be discussed in the next step), and so the active power value can be determined in any manner, whether it is based wholly or partially on the frequency, a lookup table, a graph, a formula or by other suitable means.

[0065] At a next step 204, the value is compared with the relevant active power upper or lower limit. As discussed in the context of Figure 2 and Figure 3 , this is to determine whether the proposed setpoint value is outside the limit.

[0066] At step 206, if the value is outside the limit, the active power setpoint is set to the limit. At step 208, if the value is not outside the limit, the active power setpoint is set to the determined value. Step 206 corresponds to the situation discussed in the context of the over-frequency event at time t 2a in Figure 3 , while step 208 corresponds to the over-frequency event at time t 1a .

[0067] When it is determined at step 210 that the frequency event has ended, the method ends with step 212 by changing the active power setpoint from the final value during the frequency event to the reference active power value. In other words, the active power setpoint, and the active power output of the WTGs, is ramped back to normal levels according to the ramp rate limit.

[0068] While the above method is described as being performed during a frequency deviation, it will be appreciated that determining the setpoint from the measured frequency can be performed at any time, and the checks performed during the method and ramping are only applied at the end of the frequency event.

[0069] In Figure 5 In a second embodiment of the frequency controller shown, the frequency controller 120 incorporates a ramp rate initialization module 122 in place of the adaptive limiter 104. The other features of the controller 120 are the same as those of the frequency controller 100 shown in Figure 2 so have been identified with the same reference numerals.

[0070] The role of the ramp rate initialization module 122 is to produce a setpoint equal to or greater than the limit immediately after the end of a frequency event, so that the setpoint starts ramping from this value, rather than a lower value.

[0071] To do this, the ramp rate initialization module 122 receives the adjustment value ΔP determined from the P-f table 102 and the reduced power value P curtail_ramp and the reference power value P base . The ramp rate initialization module 122 also receives the measured active power output value P meas of the WTGs.

[0072] During a frequency deviation, the ramp rate initialization module 122 outputs a value ΔP' which is equal to the value of ΔP received from the P-f table 102. At the end of the frequency deviation, typically in response to a trigger indicating the end of the frequency deviation, the ramp rate initialization module 122 determines a restart or re-initialization value ΔP' which is equal to the difference between the minimum of the reference or reduced active power values and the measured active power value. This can be expressed as ΔP' = ΔP before the trigger, and immediately after the trigger ΔP ′ = min(P curtail_ramp , P base ) - P measOnce this restart value has been determined and assigned, the ramp rate initialization module 122 returns to setting ΔΡ' = ΔΡ. However, since the frequency excursion has ended and the frequency level has returned to the deadband, the ΔΡ value determined from the P-f table 102 and the resulting ΔΡ' value will be minimal, possibly zero. This change in ΔΡ' will be moderated by the ramp rate limiter 108 so that the change in ΔΡ' will not result in a sudden jump in the value of the setpoint. Thus, the active power value will ramp back to the reference value slowly.

[0073] As noted above, the end of a frequency excursion is typically marked by a trigger. This means that when the trigger criteria are met, a trigger is communicated to the frequency controller. The trigger criteria typically indicate that the frequency excursion has ended. The trigger criteria can include the frequency level returning to the deadband, which can be determined based on a change in the ΔΡ value from the P-f table 102 from a substantial ΔΡ value to a negligible ΔΡ value. In other embodiments, the trigger criteria can be met when it is determined that the frequency level is above or below a particular threshold, when it is determined that the rate of change of the frequency reaches a particular threshold, or when the difference between the measured active power level and the active power limit level is at a threshold. The difference between the measured active power and the limit active power being at a threshold can indicate that the measured frequency is still outside the deadband, but the power value will no longer be outside its corresponding limit, so saturation will not occur.

[0074] This process is illustrated in Figure 6 , which Figure 6 is a graph showing two overfrequency events of similar size and time to the events of Figure 3 . Thus, the same time points t 1a to t 1b and t 2a to t 2b are used for the frequency events.

[0075] The first overfrequency excursion is between time t 1a and t 1b . At tl a, the frequency rises above the deadband. Thus, according to the P-f table 102, a ΔΡ value is calculated and passed through the limiter without change to produce the setpoint. During the excursion, the ΔΡ value passed from the P-f table 102 is unchanging.

[0076] At time t 1b , the overfrequency excursion ends as the frequency level returns to the deadband. The ramp rate initialization module 122 receives a trigger indicating this. In response, the ramp rate initialization module 122 generates a restart ΔΡ' value, as described above. In this case, since the active power output and the setpoint did not exceed the limit during the excursion, the setpoint produced using the restart adjustment value is the same as the setpoint during the excursion, so the ramp back to the reference starts from the same setpoint.

[0077] In contrast, in the second deviation, from time t 2a Initially, the frequency deviates to a larger value. Therefore, during the deviation period, the setpoint generated based on the ΔP value is below the limit. However, it is understandable that the WTG output will be at the limit during this period.

[0078] At the end of the deviation, at time t 2b When the frequency returns to the dead zone and in response to receiving this trigger, the ramp rate initialization module 122 determines the restart adjustment value so that the subsequent setpoint is equal to the measured active power output. As already mentioned, the measured active power output is at the limit, so the setpoint is set to the limit immediately after the deviation ends, and ramping begins from that level according to the ramp rate limit.

[0079] Similarly, as Figure 3 As shown, the dotted line indicates the case without an adaptive limiter, in which the slant change is subject to PPC saturation.

[0080] Figure 7 Controls are shown Figure 5 and Figure 6 The typical method 300 of the embodiment. Method 300 and... Figure 4 Similar to method 200, this demonstrates how to generate an active power reference value. Understandably, PPC 22, once determined, appropriately assigns the active power reference value to WTG 14 and its controller 15.

[0081] like Figure 7 As shown, in the first step 302 of the method, a reference value for active power is determined based on the measured frequency of the power network. This step can be performed at any time during controller operation, rather than immediately after the frequency event, as will be discussed below, or it can be performed during the measured frequency, depending on the implementation of the system.

[0082] In the next step 304 of the method, a check is performed to see if a trigger has been received, indicating that the deviation from the trigger standard frequency has ended. If no trigger is received, the method returns to step 302. If a trigger has been received, the restart active power value is determined in step 306. Subsequently, in step 308, the active power value is transitioned from the restart value to the reference active power value.

[0083] The effect of both methods 200, 300 described above is to ensure that the setpoint and the output active power values match at the end of the frequency deviation, in general, so that there is no delay in ramping the active power to the reference level. Each method achieves this effect in a different way - the first method is based on a comparison between a specific limit and a determined value, the second method is based on a point in time at which the frequency deviation is considered to have ended. Although the embodiments differ in detail, it will be appreciated that the same technical effect is achieved.

[0084] It will be appreciated that various modifications and changes can be made to the present application without departing from the fair scope of the application.

Claims

1. A method for controlling the active power output of a wind turbine generator (14) in response to a frequency event on an electric power network to which the wind turbine generator (14) is connected, wherein the active power output of the wind turbine generator (14) is limited to below an active power upper limit and / or above an active power lower limit, the method comprising: A controller (15) for determining an active power reference value and assigning the active power reference value to the controller of the wind turbine generator (14) is used to control the wind turbine generator (14). Wherein, during the frequency event in which the frequency level of the power network is outside the frequency dead zone, the active power reference value is determined by the following steps: The active power value is determined based on the measured frequency of the power network; The determined active power value is compared with one of the upper and lower limits of active power; and If the determined active power value is outside the limit, the active power reference value is set to be equal to the limit; or if the determined active power value is not outside the limit, the active power reference value is set to the determined active power value. Wherein, after the frequency event in which the frequency level of the power network is within the frequency dead zone, the active power reference value is determined to change from the final value during the frequency event to the reference active power value according to the slack rate limit.

2. The method according to claim 1, wherein, Determining the active power value based on the measured frequency of the power network during the frequency event includes: Determine the active power adjustment value corresponding to the measured frequency; and Subtract the active power adjustment value from the reference active power value.

3. The method according to claim 2, wherein, Determining the active power adjustment value includes comparing the measured frequency with a graph or lookup table (102) indicating the correspondence between active power and frequency.

4. The method according to any one of claims 1 to 3, wherein, Setting the active power reference value to be equal to the limit value includes: Determine the active power limit adjustment value as the difference between the reference active power value and the active power limit; and Subtract the active power limit adjustment value from the reference active power value.

5. The method according to any one of claims 1 to 3, wherein, The reference active power value includes the minimum of the nominal active power value and the reduced active power value.

6. The method according to any one of claims 1 to 3, wherein, If the frequency level is below the dead zone during the frequency event, the relevant limit is the active power upper limit, which is based on available active power; and if the frequency level is above the dead zone during the frequency event, the relevant limit is the active power lower limit, which is based on user preferences.

7. A method for controlling the active power output of a wind turbine generator (14) in response to a frequency event on an electric power network to which the wind turbine generator (14) is connected, wherein the active power output of the wind turbine generator (14) is limited to below an active power upper limit and / or above an active power lower limit, the method comprising: A reference value for active power is determined and assigned to the controller of the wind turbine generator for controlling the wind turbine generator. Wherein, during the frequency event in which the frequency level of the power network is outside the frequency dead zone, the active power reference value is determined based on the measured frequency of the power network; and Wherein, after the frequency event in which the frequency level of the power network is within the frequency dead zone, the active power reference value is determined as a baseline active power value that changes from a restart active power value according to a ramp rate limit, the restart active power value being determined based on the measured active power output of the wind turbine generator (14).

8. The method according to claim 7, wherein, The active power reference value is determined during the frequency deviation period by the following steps: The active power adjustment value is determined by comparing the measured frequency with a graph or lookup table indicating the correspondence between active power and frequency; and Subtract the active power adjustment value from the reference active power value.

9. The method according to claim 7 or 8, wherein, Determining the active power reference value after the frequency deviation includes: The restart active power value is determined by the following steps: The restart active power adjustment value is determined by subtracting the measured active power output from the reference active power value; and Subtract the restart active power adjustment value from the baseline active power value.

10. The method according to claim 7 or 8, wherein, The reference active power value includes the minimum of the available active power value and the reduced active power value.

11. The method according to claim 7 or 8, wherein, When the frequency level is below the dead zone during the frequency event, the relevant limit is the active power upper limit, which is based on available active power; and when the frequency level is above the dead zone during the frequency event, the relevant limit is the active power lower limit, which is based on user preferences.

12. The method according to claim 7 or 8, wherein, When a triggering condition is determined to be met, the frequency deviation is considered to have ended, and the triggering condition includes at least one of the following: the frequency level is within the frequency dead zone; The frequency level is equal to or exceeds the threshold frequency value; the difference between the measured active power value and the relevant limit of the active power is equal to or exceeds the threshold.

13. A power plant controller (22) configured to perform the method according to any one of claims 1 to 12.

14. A computer-readable storage medium comprising instructions that, when executed by a computer, cause the computer to perform the method according to any one of claims 1 to 12.

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