Methods and systems for improving control of a renewable energy power plant

Abstract

According to an aspect of the invention there is provided a control system for a renewable energy generator of a renewable energy power plant comprising a plurality of renewable energy generators. The control system comprises one or more controllers configured to execute machine readable instructions to: determine a first power reference value for the renewable energy generator using a droop control technique, the first power reference value being determined based on a frequency signal indicative of a frequency level of the renewable energy generator; receive a second power reference value from a power plant controller associated with the plurality of renewable energy generators; and control a power level of the renewable energy generator based, at least in part, on the first and second power reference values.

Description

[0001] METHODS AND SYSTEMS FOR IMPROVING CONTROL OF A RENEWABLE ENERGY POWER PLANT

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to methods and systems for improving control of a renewable energy power plant connected to a power network.

[0004] BACKGROUND

[0005] 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 measured frequency of the power network deviates from the normal operational range or a permissible range, also referred to as a frequency contingency deadband.

[0006] In order to support the power network, the individual energy generators of a power plant are typically controlled by a central power plant controller during normal modes of operation (i.e. in the absence of a fault). For example, the power plant controller may provide reference values (i.e. target values or "set points") for one or more electricproduction parameters, such as the active power, the reactive power, and / or the voltage, to be produced by the individual renewable energy generators of the power plant. These reference values or setpoints may be dispatched to local controllers, associated with respective ones of the renewable energy generators, that operate to implement the set points received from the power plant controller. In this manner, the power plant controller provides effective steady-state control of the power plant in order to support the frequency of the power network.

[0007] However, when transient conditions are experienced in the grid (such as an abrupt frequency drift) the data communication between the central plant controller and the individual generator controllers may be too slow for the central control system to cope with the response-time requirements. The drift may therefore be insufficiently mitigated, leading to a grid fault and instability.

[0008] It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION

[0009] According to an aspect of the present invention, there is provided a control system for a renewable energy generator of a renewable energy power plant comprising a plurality of renewable energy generators. The control system comprises one or more controllers configured to execute machine readable instructions to: determine a first power reference value for the renewable energy generator using a droop control technique, the first power reference value being determined based on a frequency signal indicative of a frequency level of the renewable energy generator; receive a second power reference value from a power plant controller associated with the plurality of renewable energy generators; and control a power level of the renewable energy generator based, at least in part, on the first and second power reference values. The second power reference value may be determined based on a frequency signal indicative of a frequency level of the power network, for example.

[0010] For example, the control system may be configured to control an active power level of the renewable energy generator based, at least in part, on the first and second power reference values, where such power reference values take the form of active power references.

[0011] It shall be appreciated that the control system may take the form of a single controller with multiple modules controlling the renewable energy generator, or a system comprising multiple controllers that perform the same function.

[0012] In each case, the control system determines the first power reference value for a respective one of the plurality of renewable energy generators based on a local frequency level of that generator. In this context, the frequency level of the renewable energy generator is intended to refer to the frequency of the current and / or voltage as measured at an output of the renewable energy generator, for example at respective terminals of the renewable energy generator. The control system further receives the second power reference value from the central power plant controller, which may determine such power reference values for each of the plurality of renewable energy generators. Advantageously, the control system then combines the first and second power reference values to control the renewable energy generator. In doing so, the control system effectively combines a centralized and a decentralized approach to controlling the active power output of the renewable energy generator.

[0013] The second power reference value incorporates the centralised control, which is well- suited to steady state conditions, while the first power reference value incorporates local control at the generator for adaptation to transient conditions. In particular, the first power reference value can be updated much more frequently than the second power reference value (which is delayed by transmission from the power plant controller to the local controller), allowing for more frequent adjustment of the control signal and providing a faster response time (as often required for mitigating drift in transient grid conditions).

[0014] Embodiments of the invention will therefore provide for improved control during transient grid conditions, as well as increasing the control reliability in the event of a loss, or compromise, of communication between the central power plant controller and the local control system of the renewable energy generator.

[0015] The second power reference value may, for example, be determined based on a frequency signal indicative of a frequency level of a power network to which the power plant is connected. By way of example, the frequency signal may be indicative of a frequency level at a point of interconnection of the power plant to the power network. For example, the frequency signal may be based on measurements obtained at a point of measurement, such as a power meter, suitably arranged to monitor the power supply to the power network and / or the frequency level of the power network. If the power plant is temporarily disconnected from the power network, for example in an island mode of operation, the second power reference value may be determined based on a predetermined frequency level, such as a frequency level of the power network preceding the disconnection from the power network. The first power reference value is therefore repeatedly updated by the control system while the power plant is disconnected, adjusting the control signal with a suitably fast response time for adapting to transient conditions. In an example, the control system may be configured to control the power level of the renewable energy generator by determining an output power value for the renewable energy based, at least in part, on the first and second power reference values. For example, the first and second power reference values may be summed together.

[0016] Optionally, the control system is configured to control the power level of the renewable energy generator by comparing the determined output power value to a power feedback value, obtained by the control system, indicative of the power output from the renewable energy generator. In this manner, the control system is able to operate in a feedback mode of operation for enhanced control of the renewable energy generator.

[0017] The control system may, for example, be configured to determine the first power reference value at a first frequency; and receive the second power reference value at a second frequency. The second frequency may be less than the first frequency, for example. The first power reference value is therefore updated more frequently to adjust the control signal accordingly to adapt to transient conditions.

[0018] The control system may be configured to determine the first power reference value for the renewable energy generator using a droop reference frequency, for example. The droop reference frequency may be based on one or more of the following: a predetermined reference frequency; and / or the frequency signal indicative of the frequency level of the renewable energy generator.

[0019] Optionally, the control system is configured to select the droop reference frequency from respective inputs of the predetermined reference frequency and the frequency signal based on a control input.

[0020] In an example, the control system may be configured to apply a low pass filter to the frequency signal indicative of the frequency level of the renewable energy generator; and compare the low pass filtered signal to the droop reference frequency.

[0021] Optionally, the control system may be further configured to determine the first power reference value based on one or more active power limits associated with the renewable energy generator. In this manner, the first power reference value is limited upside and downside to permissible levels of active power.

[0022] Optionally, the control system may be further configured to: determine an available power level of the renewable energy generator; and determine at least one of the one or more active power limits based on the determined available power level.

[0023] In an example, the control system may be further configured to determine the first power reference value based on one or more rate of change limits associated with the power output of the renewable energy generator.

[0024] Optionally, the control system may be configured to determine at least one of the one or more rate of change limits based on a maximum permissible rate of change of active power of the renewable energy generator.

[0025] In an example, the control system may be configured to determine the first power reference value using a droop gain. The droop gain may, for example, be determined based on the frequency signal indicative of the frequency level of the renewable energy generator. In this manner, the droop gain is tunable according to the frequency ranges and functional across the frequency range.

[0026] The one or more controllers may, for example, be configured to determine the droop gain based on a curve relating respective values of the droop gain and the indicated frequency level of the renewable energy generator. The droop may therefore incorporate an (adjustable) frequency dead band . Optionally, the control system may be configured to select the curve relating respective values of the droop gain and the indicated frequency level from amongst a plurality of curves relating respective values of the droop gain and the indicated frequency level.

[0027] Optionally, the first power reference value is output to the power plant controller for determining the second power reference value. For example, the first power reference value may be output to the power plant controller for determining the second power reference value in a subsequent iteration. According to another aspect of the invention, there is provided a power plant control system for a renewable energy power plant comprising a plurality of renewable energy generators. The power plant control system comprises one or more control systems, as described in a previous aspect of the invention, for controlling respective ones of the plurality of renewable energy generators. In an example, the power plant control system may further comprise a power plant controller configured to determine and dispatch the second power reference value to the or each control system.

[0028] According to yet another aspect of the invention, there is also provided a method of operating a renewable energy power plant comprising a renewable energy generator. The method comprises: determining a first power reference value for the renewable energy generator, the first power reference value being determined by a local controller associated with the renewable energy generator using a droop control technique, the first power reference value being determined based on a frequency signal indicative of a frequency level of the renewable energy generator; determining a second power reference value for the renewable energy generator, the second power reference value being determined by a power plant controller; dispatching the second power reference value from the power plant controller to the local controller; receiving the second power reference value at the local controller; and controlling, via the local controller, a power level of the renewable energy generator based, at least in part, on the first and second power reference values.

[0029] Within the scope of this invention it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0031] Figure 1 shows schematically a power network connected to a renewable energy power plant that includes a power plant controller;

[0032] Figure 2 shows an exemplary control structure of the power plant of Figure 1.

[0033] Figure 3 shows exemplary sub-modules of the control structure shown in Figure 2; and

[0034] Figure 4 shows an exemplary method of operating the renewable energy power plant of Figure 1 in accordance with an embodiment of the invention.

[0035] DETAILED DESCRIPTION

[0036] Embodiments of the present invention relates to methods and systems for improving control of a renewable energy power plant, particularly in relation to abrupt frequency drifts.

[0037] This is achieved by way of coordinating a centralized and a decentralized approach to controlling the active power output of the renewable energy generators, which supports the frequency of a connected power network. In particular, as in conventional systems, a central power plant controller is configured to determine reference values (i.e. target values or “set points”) for one or more electric-production parameters, such as the active power, the reactive power, and / or the voltage, to be produced by the individual renewable energy generators of the power plant. The reference values or setpoints are dispatched to local controllers, associated with respective ones of the renewable energy generators, that operate the renewable energy generators to implement the set points received from the power plant controller. In this manner, the power plant controller provides centralised control of the renewable energy generators, producing effective steady-state control that supports the frequency and voltage of a connected power network. Advantageously though, in order to deal with transient grid fluctuations (such as abrupt frequency drift), the local controllers of the renewable energy generators are further configured to determine another power reference value using a droop control technique. For example, the droop control technique may determine a first power reference value based on a frequency level of the associated renewable energy generator (e.g. a frequency level determined from measurements at terminals of the generator), and the local controller may receive a second power reference value, dispatched from the power plant controller. The local controllers are configured to control a power level of the renewable energy generator based on the first and second power reference values. For example, the first and second power reference values may be combined to determine a control signal for controlling one or more electric-power production parameters of the renewable energy generator. The control signal may, for example, be determined with comparison to feedback measurements of the power output from the renewable energy generator.

[0038] The power plant controller therefore dispatches the second power reference value to the local controllers to provide centralised control, which is well-suited to steady state conditions, while each local controller further determines the first power reference value, providing decentralised control for adaptation to transient grid conditions. In particular, by determining the first power reference value locally, transmission delays are mitigated, and the first power reference value can be updated more frequently than the second power reference value to adjust the control signal with a fast response time. Such a rapid response time is often required for mitigating drift in transient grid conditions.

[0039] In this manner, it is envisaged that embodiments of the invention will provide improved control during transient grid conditions, as well as increasing the control reliability in the event of a loss, or compromise, of communication between the central power plant controller and the local controllers of the renewable energy generators. For example, the first power reference value can be updated repeatedly while the power plant is disconnected, adjusting the control signal with a fast response time to adapt to transient conditions.

[0040] Figure 1 illustrates a typical architecture in which a renewable energy power plant is connected to a main grid or wider power network. In the example shown in Figure 1 , the renewable energy power plant is a wind power plant (WPP). As will be understood by the skilled reader, a WPP comprises a plurality of wind turbine generators (WTG)s. A WTG is commonly referred to as a ‘wind turbine’. The example shown is representative only and the skilled reader will appreciate that other specific architectures are possible. In other examples, the power plant may include other renewable energy sources such as a solar power plant, a bio energy power plant, an ocean / wave / tidal energy plant, or a hybrid power plant having a combination of different types of renewable energy power plants. Thus, the invention 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, or as alternatives, to the components shown and described in Figure 1. Such changes would be within the capabilities of the skilled person.

[0041] Figure 1 shows a power system 10 incorporating a WPP 12. In this example, the WPP 12 includes a plurality of WTGs 14. Each of the plurality of WTGs 14 converts wind energy into electrical energy, which is transferred from the WPP 12 to a main power network, or ‘main grid’ 16, as active power and / or current, for distribution.

[0042] Although not illustrated in Figure 1 , the WPP 12 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.

[0043] The WPP 12 also includes a connecting network 18 for connecting the WPP 12 to the main grid 16. In this example, the WPP 12 and the main grid 16 are connected at a Point of Interconnection (Pol) 20, which is an interface between the WPP 12 and the main grid 16. The Pol 20 may also be referred to as the Point of Common Coupling, which may be abbreviated to ‘PCC’ or ‘PoCC’.

[0044] The WPP 12 further includes a power plant controller 22, referred to hereafter as PPC 22, for centralised control of the WTGs 14 and, in this example, each of the WTGs 14 is associated with a respective local WTG controller 15. The WTG controllers 15 act as local controllers of the renewable energy generators in this example. As will be understood by the skilled person, the WTG controllers 15 can be considered to be local control systems capable of operating a WTG 14 in the manner prescribed herein, and may comprise multiple controller modules that control individual components of the WTG or just a single controller with multiple sub-modules (as shall be described in the following examples). The computer system of the WTG controllers 15 may operate according to software downloaded via a communications network or programmed onto it from a computer-readable storage medium.

[0045] A bi-directional control network may be arranged between the PPG 22 and the WTG controllers 15 enabling two-way communication. For example, the uplink direction (i.e. the direction from the central PPG 22 to the local WTG controllers 15) is used to send reference values, e.g., for voltage and / or reactive power, from the PPG 22 to the local WTG controllers 15. The downlink direction may be used by the WTGs 15 to return information about their current operational state, e.g. about the amount of active power currently produced and / or a locally determined power reference, to the central PPG 22. Such a control network may, for example, be implemented as a bus system, i.e. a CAN bus (ISO 11898) or an Ethernet bus (IEEE 802.3).

[0046] The role of the PPC 22 is to provide centralised control of the WTGs 14 and to act as a command and control interface between the WPP 12 and the grid 16 (more specifically, a grid operator 26). For example, the grid operator 26 may be a transmission system operator (TSO) or a distribution system operator (DSO).

[0047] The PPC 22 is configured to generate and send dispatch signals to the WTG controllers 15. The dispatched signals contain active and reactive current, and / or power, set points determined by the PPC 22 to provide frequency and voltage support to the main grid 16 based on measurements of the power supply from the WPP 12 to the main grid 16 and / or a frequency level of the main grid 16.

[0048] In turn, the WTG controllers 15 control the WTGs 14 according to the set points contained within the dispatch signals and, in this manner, the WPP 12 is capable of altering its power or current output in reaction to set points received from the PPC 22. For this purpose, the Power Plant Controller (PPC) 22 is connected to the main grid 16 at a Point of Measurement (PoM) 24, such as a power meter. For example, the PPC 22 may be configured to receive one or more measurement signals from the PoM 24 comprising measurements of the power supply from the WPP 12 to the main grid 16 and / or a frequency level of the main grid 16.

[0049] The PPC 22 is a suitable computer system for carrying out the controls and commands as described herein and so may incorporate a processing module 28, a connectivity module 30, a memory module 32 and a sensing module 34, amongst others, as shown in Figure 1.

[0050] The connectivity module 30, the memory module 32, and / or the sensing module 34 are configured to provide the processing module 28 with information that is indicative of a frequency level of the main grid 16, as well as power levels, current levels and / or voltage levels of the WTGs 14 and / or the WPP 12.

[0051] For example, the sensing module 34 may receive such information directly from one or more connected sensors or power meters (e.g. at the PoM 24) and communicate the information to the processing module 28. Alternatively, or additionally, the information may be determined by one or more systems that are connected to the connectivity module 30, such as the WTG controllers 15, and the information may be communicated, in turn, through the connectivity module 30 to the processing module 28. In each case, the determined information may be stored permanently, or temporarily, in the memory module 32, from which it may be recalled, on demand, by the processing module 28. The PPC 22 may also receive information regarding the grid 16 and / or local buses, substations and networks from an energy management system (not shown).

[0052] In contrast to conventional arrangements, the local WTG controllers 15 are additionally configured to determine further power references or set points for controlling the respective WTGs 14 using a droop control technique. For example, each WTG controller 15 may be configured to determine further power references for controlling a respective WTG 14 based on the frequency level of the power output from that WTG 14. In this manner, the local WTG controllers 15 are therefore configured to control a power level of the respective WTG(s) 14 by determining a control signal based on a combination of power reference values received from the PPG 22 and determined locally by the WTG controller 15. It shall be appreciated that the locally determined power reference values may be updated at a greater frequency than the power reference values determined by the PPG, for example due to the relatively slow data communication between the PPG and the WTG controllers 15. Accordingly, the power reference values determined by the WTG controllers 15 are much more responsive to transient grid conditions, providing effective control of the WTGs 14. Moreover, in the absence of a power reference value from the PPG 22, e.g. following a loss of communication, the WTG controllers 15 are able to provide decentralised control of the respective WTG(s) 14 improving the stability of the WPP 12. It shall be appreciated that the control arrangement may also provide effective control during a fault of the main grid 16, where the central control of the PPG 22 may be replaced by autonomous control carried out by the local WTG controllers 15.

[0053] The PPG 22 and the local WTG controllers 15 may therefore each be arranged to work in a feedback mode in which they compare reference values, e.g. from reference inputs, with measured values, e.g. from measurement inputs, and produce a control signals or references based on the difference between the two input values.

[0054] An exemplary control structure in accordance with an embodiment of the invention shall now be described in more detail with additional reference to Figure 2.

[0055] As shown in Figure 2, the overall controller structure of the WPP 12 comprises the central PPG 22 and the local WTG controllers 15. For simplicity, a single WTG controller 15 is shown in Figure 2, but it shall be appreciated that the PPG 22 connects to each local WTG controller 15 in a substantially identical manner.

[0056] The central PPG 22 and the local WTG controllers 15 each include both reactive-power and active-power controllers, however the following description is limited to aspects of active power control in order to avoid obscuring the invention, and so only active power controllers are shown in Figure 2. In this example, the PPC 22 is principally configured to regulate the frequency of a connected power network using frequency feedback measurements and is shown to receive, amongst other inputs, a frequency signal, f_WPP, indicative of a frequency level of the main grid 16 (e.g. a frequency level at the Pol 20). For example, the PPC 22 may receive a series of time-varying measurements indicative of one or more power characteristics at the PoM 24, which may be used to derive a frequency signal in accordance with one or more known methods. Such measurements may include voltage and / or current measurements, for example.

[0057] The frequency signal, f_WPP, is provided to an active-power controller 37 (i.e., P controller 37) of the PPC 22 and may be used in one or more active power control loops thereof to regulate the grid frequency.

[0058] The P controller 37 may therefore receive various signals which are relevant for active power control, for example including the frequency signal, f_WPP, and a feedback signal, Pmeas_WPP, indicative of the active power output from the WPP 12 to the grid 16. In examples, the P controller 37 may also receive feedback signals, Pmeas_WTG, indicative of the active power output from each of the WTGs 14, as shown in Figure 2.

[0059] Although not shown in Figure 2, it shall be appreciated that the P controller 37 may also receive signals from each of the local WTG controllers 15 indicative of the available active power, i.e. that amount of active power that could currently be produced at maximum by the individual WTGs 14. For example, a function of the local WTG controllers 15 referred to as "Pavail" may also determine the amount of active power that could be produced at maximum. For example, each WTG controller 15 may measure and / or determine the currently prevailing wind speed and calculate, for example based on a power curve of the WTG 14, what amount of active power could be produced under the prevailing wind condition. It produces the Pavail signal, which may be fed back to the PPC 22.

[0060] In some embodiments, the P controller 37 may also receive external control inputs, e.g. a curtailment signal from the grid provider by which the grid provider can prescribe to what extent active power production of the WPP 12 shall be curtailed. The curtailment prescription may be expressed in absolute terms (e.g. in MW), or in relative terms, e.g. as a percentage of the plant's nominal active power or the active power currently producible by the WPP 12, that is the sum of the Pavail.

[0061] As shown in Figure 2, the P controller 37 comprises a frequency (f) and active-power (P) controller 42, in this example, and a P set-point dispatcher 43. The f-and-P controller 42 produces an internal overall reference value, Pref_WPP, for the active power to be produced by the WPP 12, based on the inputs to the P controller 37 (e.g. the frequency signal and the active power output from the WPP 12 to the grid 16). For example, when the frequency signal is above a frequency threshold, the f-and-P controller 42 may generate a reduced value of, Pref_WPP.

[0062] If the WPP 12 operates in a curtailed mode, the f-and-P controller 42 may also generate an increased value of Pref_WPP when the frequency signal is below a frequency threshold, in order to contribute to frequency control of the grid. In embodiments with an external curtailment-functionality, the external curtailment signal also influences the evaluation of Pref_WPP. For example, if the signal demands that the active power produced shall be x % of the active power currently producible by the WPP 12, Pref_WPP will be set to x % of the sum of all Pavail.

[0063] The P set point dispatcher 43 splits the overall Pref_WPP (determined by the f-and-P controller 42) into individual active power set points or reference values, Pref2_WTG, for the local WTG controllers 15, for example using the Pavail information. The splitting does not have to be uniform, for example WTGs 14 of the first row subjected to higher load could be curtailed more than those with lower load. The individual power reference values, Pref2_WTG, are dispatched to the respective WTG controllers 15.

[0064] The local WTG controllers 15 comprise a local P controller 45. The local P controllers 45 receive the respective Pref2_WTG value, dispatched from the PPG 22, as an input. In a conventional system, the local WTG controllers 15 would have controlled the WTGs 14 according to such inputs, for example operating in a feedback mode of operation.

[0065] Advantageously, the local P controllers 45 further include a droop controller 46 in the control structure. The droop controller 46 receives a signal, f_WTG, indicative of the frequency level of the power output from the respective WTG 14 and a droop reference frequency, F_droop, as inputs and determines another reference value, Pref1_WTG, for the active power to be produced by the respective WTG 14. The droop reference frequency, F_droop, may be a nominal frequency level for the WTG 14 or the droop reference frequency, F_droop, may be based on the signal, f_WTG.

[0066] The droop controller 46 includes one or more schemes, rules, or algorithms for droop control. That is, the droop controller 46 implements one or more droop control techniques for determining the active power reference value, Pref1_WTG, based on the droop reference frequency input, F_droop and frequency level, f_WTG, of the power output from the WTG 14. For example, when the frequency level, f_WTG, is above a frequency threshold, the f-and-P controller 42 may generate a reduced value of, Pref2_WTG. If the WPP 12 operates in a curtailed mode, the f-and-P controller 42 may also generate an increased value of Pref1_WTG when the frequency signal, f_WTG, is below a frequency threshold, in order to contribute to frequency control of the grid. The active power reference value, Pref1_WTG, determined by the droop controller 46 may be further limited according to one or more active power limits, which may be determined based on an available power level of the WTG 14 for example, and / or according to one or more rate of change limits associated with the power output of the WTG 14, such as a maximum permissible rate of change of active power of the WTG 14, as shall be described in more detail.

[0067] The P controllers 45 each determine a control signal for controlling the respective WTG 14 based on the power reference value, Pref2_WTG, received from the PPG 22 and the power reference value, Pref1_WTG, determined by the droop controller 46. In the following description, the power reference value, Pref1_WTG, determined by the droop controller 46 may therefore be referred to as a ‘first power reference value’ provided to the P controller 45 and the power reference value, Pref2_WTG, received from the PPG 22 may, in turn, be referred to as a ‘second power reference value’ provided to the P controller 45. Another input to the P controller 45 is a signal indicative of the active power measured at the output of the respective WTG 14, referred to as "Pmeas_WTG" in Figure 2. P controller 45 therefore combines the first and second power reference values, Pref1_WTG and Pref2_WTG, to the measured active power output Pmeas_WTG and produces a control signal, C, based on the difference between the input values. The determined control signals, C, control the WTGs 14 to produce a corresponding amount of active power.

[0068] Exemplary details of the control structure shall now be discussed with additional reference to Figure 3.

[0069] In the example shown in Figure 3, the f-and-P controller 42 of the PPG 22 includes first and second control modules in this example. In particular, the f-and-P controller 42 includes a frequency droop controller 100 and an active power loop 102 for determining the active power reference, Pref_WPP, for the WPP 12. However, the PPG 22 is not particularly limited in this respect and, in other examples, the f-and-P controller 42 may include additional or alternative modules for determining the active power reference value, Pref_WPP.

[0070] In this example, the frequency signal, f_WPP, indicative of the frequency level of the main grid 16 (e.g. as determined at the Pol 20), is provided as an input to the frequency droop controller 100. The frequency droop controller 100 includes one or more droop control techniques for determining a droop power reference, Pctrl_PoC_ref, which is provided as an input to the active power loop 102. For this purpose, the frequency droop controller 100 may further receive a reference frequency for comparison to the frequency signal. For example, the reference frequency may correspond to a nominal frequency of the main grid 16 (typically 50 Hz or 60 Hz).

[0071] The active power loop 102 may further receive inputs of feedback measurements of the power output from the PPG 22 to the WPP 12, Pmeas_WPP, as measured at the PoM 24, and, optionally, the power output from each of the WTGs 14, Pmeas_WTG. In addition, the active power loop 102 may receive the locally determined reference values for each of the WTGs, Pref1_WTG, and / or active power reference values, Pref, from other P-related control aspects, which may be provided in addition to or as an alternative to the droop power reference, Pctrl_PoC_ref, (e.g. when operating in an island mode of operation).

[0072] The active power loop 102 includes one or more schemes, rules, or algorithms for combining the various active power reference values and comparing the power to the active power feedback measurements. On this basis, the active power loop 102 determines the active power reference, Pref_WPP, for the WPP 12, which is provided to the P set-point dispatcher 43. As discussed previously, although not shown, the P set-point dispatcher 43 may receive Pavail signals from the WTG controllers 15 and determine the respective second power reference values, Pref2_WTG, for each of the WTG controllers 15 based on such inputs.

[0073] Now turning to the WTG controller 15, the droop controller 46 is shown to determine the first power reference value, Pref1_WTG, for the WTG 14 based on a droop reference frequency, F_droop, and a signal, f_WTG, indicative of the frequency level of the respective WTG 14.

[0074] The droop reference frequency, F_droop, may be determined by one or more parameters controllable by a grid or plant operator. In particular, in the example shown in Figure 3, the droop controller 46 may receive the measured frequency level, f_WTG, of the WTG 14, and determine the droop reference frequency, F_droop, as either a constant setpoint I nominal frequency, fSet_WTG, or a low-pass filtered value of the measured frequency, f_WTG, according to a configurable parameter “UseFiltRef”. In this manner, an operator, such as the power plant operator, may selectively configure the droop reference frequency, F_droop, by setting the configurable parameter, UseFiltRef, to use the constant setpoint, fSet_WTG, (e.g. with UseFiltRef = 1), or a low-pass filtered value of the measured frequency, f_WTG, (e.g. with UseFiltRef = 0), or a combination thereof.

[0075] In parallel, the droop controller 46 separately applies a low pass filter, at the block 103, to the indicated frequency level, f_WTG, of the WTG 14 and compares the droop frequency, F_droop, to the output low-pass filtered frequency level. The error is then provided to a droop gain block 104 of the droop controller 46.

[0076] A droop gain value of the droop gain block 104 may, itself, be determined by a control input or the droop gain value may be configured to vary with, and be determined based, on the droop reference frequency, F_droop, or the indicated frequency level, f_WTG, of the WTG 14. For example, the droop controller 46 may include one or more control modules (not shown) configured to determine the droop gain based on a curve relating respective values of the droop gain and the droop reference frequency, F_droop, or the WTG frequency level, f_WTG. For example, the curve may be selected from a plurality of such curves based on a control input from a grid or plant operator, for example. In this manner, the droop gain is tuneable according to the operating frequency range.

[0077] The output of the droop gain block 104 is a power reference value for the WTG 14, which is passed to a power limit block 108 and a rate limiter block 110, as shown in Figure 3, before the first power reference value, Pref1_WTG, is output to the P controller 45.

[0078] The power limit block 108 applies upper and lower active power limits to the power reference value, where the WTG controller 15 may determine the upper and / or lower limits based on the feedback of the available power level of the WTG 14, Pavail. The rate limiter block 110 may further apply one or more rate of change limits to the power reference value. Again, the WTG controller 15 may determine the rate of change limits based on a maximum permissible rate of change of active power of the WTG 14, as may be prescribed for a particular type of WTG for example. In this manner, the WTG 14 can adjust its active power production during transient and semi-steady-state conditions if a frequency deviation occurs.

[0079] The output of the power limit block 108 is the first active power reference value, Pref1_WTG, which is provided to the P controller 45 of the WTG controller 15. The P controller 45 of the WTG controller 15 is configured to determine the control signal, C, for the WTG 14 by combining the first and second active power reference values, Pref1_WTG and Pref2_WTG, received from the droop controller 46 and the PPG 22, and comparing the combined reference value to the feedback active power measurement, Pmeas_WTG, from the WTG 14. For example, the P controller 45 of the WTG controller 15 may be configured to combine the first and second active power reference values, Pref1_WTG and Pref2_WTG, received from the droop controller 46 and the PPG 22 by summing the reference values together, and the P controller 45 may be calibrated accordingly. The output control signal, C, may take the form of an active power reference for the WTG 14 to follow by adjusting one or more control parameters of the WTG 14. That is, the power output of the WTG 14 may be controlled according to the control signal, C, to minimise the error between the combined power reference values and the power measurement feedback. For this purpose, the WTG 14 and / or the WTG controller 15 may therefore include one or more additional control modules or instructions for transforming the control signal, C, into one or more corresponding commands for controlling respective parameters of the WTG 14, such as pitch, yaw, etc., and / or its converter.

[0080] It shall also be appreciated that the locally determined first power reference values, Pref1_WTG, may be updated at a greater frequency than the second power reference values, Pref2_WTG, received from the PPG 22, for example due to the relatively slow data communication between the PPG 22 and the WTG controllers 15. Accordingly, the first power reference values, Pref1_WTG, determined by the WTG controllers 15 are much more responsive to transient conditions, providing effective control of the WTGs 14.

[0081] A method 400 of operating the WPP 12 in accordance with an embodiment of the invention shall now be described in more detail with additional reference to Figure 4.

[0082] In step 402, the PPG 22 determines a power reference value, Pref_WPP, for the WPP 12. The method used to determine the WPP power reference value, Pref_WPP, is not intended to be particularly limiting on the scope of the invention, and may be based on the frequency level of the main grid 16 and / or using other active power reference values, such as, Pref, determined by other P-related control aspects of the PPG 22.

[0083] To give an example, the PPG 22 may obtain a frequency signal indicative of the frequency level, f_WPP, of the main grid 16, in step 402. For example, the PPG 22 may receive or otherwise obtain a series of measurements, such as current and / or voltage measurements at the PoM 24, and derive a frequency signal, f_WPP, based thereon.

[0084] The PPG 22 may then determine a power reference value, Pref_WPP, for the WPP 12 based on the frequency signal. For example, the PPG 22 may determine the droop power reference, Pctrl_PoC_ref, using the droop controller 100 of the PPG 102 based on the frequency signal, f_WPP. The determined droop power reference, Pctrl_PoC_ref, is subsequently used to determine the power reference value, Pref_WPP, for the WPP 12, using the active power loop 102. In particular, the droop power reference, Pctrl_PoC_ref, may be compared against measured power levels of the WPP 12, Pmeas_WPP, and the WTGs 14, Pmeas_WTG, as provided from the WTG controllers 15. The active power loop 102 may also factor in other power reference values, such as the first power reference values, Pref1_WTG, determined in a previous iteration, and / or the power reference values, Pref, from other P-related control aspects, as shall be described in more detail.

[0085] In step 406, the P set-point dispatcher 43 receives the power reference value, Pref_WPP, for the WPP 12 and determines and dispatches individual second power reference values, Pref2_WTG, or set points to the WTG controllers 15. For example, the active power reference value, Pref_WPP, for the WPP 12 is passed to the P set-point dispatcher 43, which receives feedback of the available power, Pavail, of the WTGs 14 (and the active power output of the WTGs 14). Based on such inputs the P set-point dispatcher 43 uses one or more rules, schemes, or algorithms to determine individual active power reference values, Pref2_WTG, for each of the WTGs 14, which are then dispatched in dispatch signals to the associated WTG controllers 15.

[0086] In parallel, in step 408, each WTG controller 15 receives a frequency signal, f_WTG, indicative of the frequency level of the power output of the respective WTG 14 and determines another power reference value, Pref1_WTG, for controlling the WTG 14 using the droop controller 46. By way of example, the droop controller 46 of the WTG controller 15 may receive the frequency signal, f_WTG, apply a low pass filter to the frequency signal and compare the frequency signal to the droop frequency, F_droop, which correspond to a nominal frequency or a low pass-filtered value of the frequency signal, f_WTG. The droop controller 46 determines a frequency error, which is provided to the droop gain block 104. The droop gain block 104 applies the droop gain to the error signal and produces a corresponding power reference. The power reference signal is further limited by the upper and lower active power limits of the limit block 106 and a rate of change of the power reference signal is limited by the rate limiter block 108. The power reference value, Pref1_WTG, is output from the rate limiter block and provided to the P controller 45 of the WTG controller 15.

[0087] In step 410, the WTG controller 15 receives the dispatched active power reference value, Pref2_WTG, from the PPG 22. In step 412, the WTG controller 15 combines the active power reference values, Pref1_WTG and Pref2_WTG, received from the PPG 22 and determined by the droop controller 46 and determines a control signal, C, for controlling the respective WTG 14. For example, the P controller of the WTG controller may combine the first and second active power reference values, Pref1_WTG and Pref2_WTG, by summing them together and comparing the product to a feedback signal, Pmeas_WTG, of the measured power output from the respective WTG 14, using one or more active power control loops to determine the active power control signal. As the locally determined power reference values, Pref1_WTG, may be updated at a greater frequency than the power reference values, Pref2_WTG, received from the PPG 22, the control signal may be updated at a first frequency according to the changes in the power reference values, Pref1_WTG, determined by the WTG controllers 15, and updated at a second frequency (i.e. a lower frequency) according to the changes in the power reference values, Pref2_WTG, received from the PPG 22.

[0088] In step 414, the determined control signal, C, is output from the WTG controller 15 to the respective WTG 14 or otherwise used to control one or more parameters of the WTG 14 accordingly to reduce the error between the combined active power reference values and the active power output of the WTG 14.

[0089] In this manner, the local WTG controllers 15 rapidly update the first power reference value, Pref1_WTG, and, in turn, update the control signal, C, such that the WTG 14 is controlled to respond rapidly to frequency deviations, while the slower communication of power reference values, Pref2_WTG, from the PPG 22, provide effective steady state control of the WTGs 14.

[0090] Moreover, it shall be appreciated that if there is a loss of communication, such that the WTG controller 15 fails to receive the second active power reference value, Pref2_WTG, from the PPG 22, in step 410, the WTG controller 15 continues to determine the local active power reference value, Pref1_WTG, based on the droop control function. The WTG controller 15 therefore determines a suitable control signal, C, for continued operation of the WTG 14, improving the stability of the WTG 14 operation and the WPP 12 in general. It is expected that the present invention will therefore improve grid stability and strength, and thereby contributing to greater power generating capabilities of the WPP 12.

[0091] It will be appreciated that various changes and modifications can be made to the examples described above without departing from the scope of the present invention.

[0092] For example, while the above-described examples focused on the operation of the WPP 12 whilst the WPP 12 is connected to the main grid 16, providing frequency support, embodiments of the present invention also encompass operation of the WPP 12 when the WPP 12 is temporarily disconnected from the main grid 16, for example in a shortterm island condition.

[0093] In particular, while the WPP 12 is connected to the main grid 16, the PPC 22 may determine a power reference value, Pref_WPP, for the WPP 12 based on the frequency level, f_WPP, of the main grid, substantially as described in step 402. However, if the WPP 12 is disconnected from the main grid 16, for example following operation of a circuit breaker of the WPP 12, the WPP 12 may be configured to enter an island mode of operation. During the island mode of operation, the WPP 12 may be configured to operate substantially as described in the method 400, with the exception that the PPC 22 is no longer able to determine the power reference value, Pref_WPP, for the WPP 12 based on the frequency level, f_WPP, of the main grid 16. Instead, in the island mode of operation, the PPC 22 may be configured to determine the power reference value, Pref_WPP, for the WPP 12 based on the active power reference value(s), Pref, from one of the other P-related control aspects or based on a predetermined frequency level, such as a frequency level of the main grid 16 that preceded the disconnection. For example, a substantially constant frequency level may be used, upon entering the island mode of operation, to determine the power reference value, Pref_WPP, for the WPP 12, in step 402, until the grid connection is restored.

[0094] In this manner, the WPP 12 is operable in a variety of conditions, able to maintain operation when temporarily disconnected from the main grid 16 and provide frequency and / or voltage support when the WPP 12 is reconnected to the main grid 16.

Claims

CLAIMS1 . A control system for a renewable energy generator of a renewable energy power plant comprising a plurality of renewable energy generators, the control system comprising one or more controllers configured to execute machine readable instructions to: determine a first power reference value for the renewable energy generator using a droop control technique, the first power reference value being determined based on a frequency signal indicative of a frequency level of the renewable energy generator; receive a second power reference value from a power plant controller associated with the plurality of renewable energy generators; and control a power level of the renewable energy generator based, at least in part, on the first and second power reference values.

2. A control system according to claim 1 , wherein the control system is configured to control the power level of the renewable energy generator by determining an output power value for the renewable energy based, at least in part, on the first and second power reference values.

3. A control system according to claim 2, wherein the control system is configured to control the power level of the renewable energy generator by comparing the determined output power value to a power feedback value, obtained by the control system, indicative of the power output from the renewable energy generator.

4. A control system according to any preceding claim, wherein the control system is configured to determine the first power reference value at a first frequency; and receive the second power reference value at a second frequency, the second frequency being less than the first frequency.

5. A control system according to any preceding claim, wherein the control system is configured to determine the first power reference value for the renewable energygenerator using a droop reference frequency, the droop reference frequency being based on one or more of the following: a predetermined reference frequency; and / or the frequency signal indicative of the frequency level of the renewable energy generator.

6. A control system according to claim 5, wherein the control system is configured to select the droop reference frequency from respective inputs of the predetermined reference frequency and the frequency signal based on a control input.

7. A control system according to claim 5 or claim 6, wherein the control system is configured to apply a low pass filter to the frequency signal indicative of the frequency level of the renewable energy generator; and compare the low pass filtered signal to the droop reference frequency.

8. A control system according to any preceding claim, wherein the control system is further configured to determine the first power reference value based on one or more active power limits associated with the renewable energy generator.

9. A control system according to claim 8, wherein the control system is further configured to: determine an available power level of the renewable energy generator; and determine at least one of the one or more active power limits based on the determined available power level.

10. A control system according to claim 8 or claim 9, wherein the control system is further configured to determine the first power reference value based on one or more rate of change limits associated with the power output of the renewable energy generator.

11. A control system according to claim 10, wherein the control system is configured to determine at least one of the one or more rate of change limits based on a maximum permissible rate of change of active power of the renewable energy generator.

12. A control system according to any preceding claim, wherein the control system is configured to determine the first power reference value using a droop gain, the droop gain being determined based on the frequency signal indicative of the frequency level of the renewable energy generator.

13. A control system according to 12, wherein the one or more controllers are configured to determine the droop gain based on a curve relating respective values of the droop gain and the indicated frequency level of the renewable energy generator, optionally, wherein the control system is configured to select the curve relating respective values of the droop gain and the indicated frequency level from amongst a plurality of curves relating respective values of the droop gain and the indicated frequency level.

14. A control system according to any preceding claim, wherein the first power reference value is output to the power plant controller for determining the second power reference value.

15. A method of operating a renewable energy power plant comprising a renewable energy generator, the method comprising: determining a first power reference value for the renewable energy generator, the first power reference value being determined by a local controller associated with the renewable energy generator using a droop control technique, the first power reference value being determined based on a frequency signal indicative of a frequency level of the renewable energy generator; determining a second power reference value for the renewable energy generator, the second power reference value being determined by a power plant controller; dispatching the second power reference value from the power plant controller to the local controller; receiving the second power reference value at the local controller; andcontrolling, via the local controller, a power level of the renewable energy generator based, at least in part, on the first and second power reference values.

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