Methods and systems for controlling a renewable energy power plant with power-to-x converters

Abstract

According to an aspect of the invention, there is provided a power plant controller for a renewable energy power plant comprising one or more renewable energy generators and one or more Power-to-X units. Each Power-to-X unit is configured to convert electric power to X from the one or more renewable energy generators and / or a power network to which the renewable energy power plant is connected. The power plant controller is configured to execute machine readable instructions to: receive a signal indicative of available active power for supply to the one or more Power-to-X units from the one or more renewable energy generators; compare the indicated available active power to a base power usage level for the one or more Power-to-X units; and determine and dispatch set points for reducing the electrical power usage of the one or more Power-to-X units to the base power usage level when the indicated available active power is less than the base power usage level. The electric power shortfall from the one or more renewable energy generators is provided to the one or more Power-to-X units from the connected power network.

Description

[0001] METHODS AND SYSTEMS FOR CONTROLLING A RENEWABLE ENERGY POWER PLANT WITH POWER-TO-X CONVERTERS

[0002] Technical field

[0003] The present disclosure relates to methods and systems for controlling a renewable energy power plant that is connected to a power network and comprises one or more power-to-X units.

[0004] Background to the invention

[0005] Power plant arrangements are increasingly adopting power-to-X (P2X) units for electric power conversion to X. The P2X units are capable of converting surplus electric power generated by the plant into other useful forms, for example for energy storage and reconversion. One such example is an electrolysis system, which may be included for the production of hydrogen using electrical power produced by generators of the power plant.

[0006] In this context, the electrolysis system uses the energy from the electric power supply to decompose an input of water into respective flows of hydrogen and oxygen. The hydrogen is typically the desired product that can be used as feedstock in various ways, while the oxygen may be regarded as a byproduct. When the hydrogen is produced using electrical power produced by renewable energy sources, the resulting hydrogen may be regarded as ‘green hydrogen’.

[0007] However, energy availability from renewable energy sources is prone to fluctuation and can change rapidly, e.g. due to wind-gusting and clouding. This can lead to inconsistent Power-to- X conversion, frequent shutdowns I startups of the P2X units, and ultimately a loss of production and revenue.

[0008] It is against this background that the invention has been devised.

[0009] Summary of the invention

[0010] According to an aspect of the invention, there is provided a power plant controller for a renewable energy power plant comprising one or more renewable energy generators and one or more Power-to-X units. Each Power-to-X unit is configured to convert electric power to X from the one or more renewable energy generators and / or a power network to which the renewable energy power plant is connected. The power plant controller is configured to execute machine readable instructions to: receive a signal indicative of available active power for supply to the one or more Power-to-X units from the one or more renewable energy generators; compare the indicated available active power to a base power usage level for the one or more Power-to-X units; and determine and dispatch set points for reducing the electrical power usage of the one or more Power-to-X units to the base power usage level when the indicated available active power is less than the base power usage level. The electric power shortfall from the one or more renewable energy generators is provided to the one or more Power-to-X units from the connected power network.

[0011] In this manner, the power plant controller is able to control the Power-to-X unit(s) to remain at the base power usage level, even if there is a transient shortfall in the available active power supply, providing more predictable performance of the Power-to-X unit(s), facilitating enhanced X production control.

[0012] In an example, the base power usage level may correspond to a minimum power usage for maintaining the one or more Power-to-X units in a production state, converting electric power to X at a minimum production rate. In this manner, the power plant controller is able to control the Power-to-X unit(s) to secure a minimum rate of X production, corresponding to the base power usage level, even if there is a transient shortfall in the available active power supply. This facilitates X production agreements and enhanced commercialisation of the X production.

[0013] In an example, the base power usage level may correspond to a minimum power usage for maintaining the one or more Power-to-X units in a production-ready state according to which the one or more Power-to-X units are configured to convert electric power to X, and switch to the production state, upon receiving set points greater than the base power usage level. In this manner, the Power-to-X units is secured in an active state, ready to convert electric power to X without delay, when the power availability increases.

[0014] Optionally, the power plant controller may be further configured to determine and dispatch set points at a reference active power level when the indicated available active power is greater than or equal to the reference active power level. For example, the reference power level may be a rated power usage of the one or more Power-to-X units.

[0015] Optionally, the power plant controller may be further configured to determine and dispatch set points for reducing the amount of electrical power converted to X from the reference active power level when the indicated available active power is less than the reference active power level, the set points being determined using a prescribed ramp rate limit.

[0016] In an example, the set points determined for reducing the amount of electrical power converted to X are greater than or equal to the indicated available active power. The electric power shortfall from the one or more renewable energy generators may, for example, be provided to the one or more Power-to-X units from the connected power network when the determined set points exceed the indicated available active power.

[0017] Optionally, the power plant controller is further configured to stop determining set points and dispatch a command for controlling the one or more Power-to-X units to switch to a standby state in dependence on detecting a state change condition, the state change condition being associated with a magnitude, or an accumulation, of the electric power shortfall.

[0018] In an example, the state change condition may be detected in dependence on one or more of the following: the indicated available active power being less than a lower threshold power level, the lower threshold power level being less than the base power usage level; the indicated available active power being less than the lower threshold power level for a threshold period; and / or the indicated available active power being less than the base power usage level for a threshold period.

[0019] According to another aspect of the invention, there is provided a renewable energy power plant connected to a power network. The renewable energy power plant comprises: one or more renewable energy generators; one or more Power-to-X units, each Power-to-X unit being configured to convert electric power to X; and a power plant controller as described in a previous aspect of the invention.

[0020] Optionally, the one or more Power-to-X units comprise a power-to-gas unit configured to convert electric power from the power plant to gas. For example, the power-to-gas unit may be configured to generate hydrogen gas by electrolysis using the electric power supply.

[0021] In an example, the one or more renewable energy generators may comprise: a wind turbine generator; and / or a photovoltaic generator.

[0022] According to yet another aspect of the invention, there is provided a method of operating a renewable energy power plant comprising one or more renewable energy generators and one or more Power-to-X units. Each Power-to-X unit is configured to convert electric power to X from the one or more renewable energy generators, and / or a power network to which the renewable energy power plant is connected. The method comprises: obtaining a signal indicative of available active power for supply to the one or more Power-to-X units from the one or more renewable energy generators; and comparing the indicated available active power to a base power usage level for the one or more Power-to-X units; and determining and dispatching set points for reducing the electrical power usage of the one or more Power-to-X units to the base power usage level when the indicated available active power is less than the base power usage level. The electric power shortfall from the one or more renewable energy generators is provided to the one or more Power-to-X units from the connected power network.

[0023] According to a further aspect of the invention, there is provided a computer program or a computer-readable medium comprising instructions which, when the program or the instructions is / are executed by a computer, cause the computer to carry out a method as described in a previous aspect of the invention.

[0024] 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.

[0025] Brief description of the drawings

[0026] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0027] Figure 1 schematically shows a renewable energy power plant, including a power plant controller, connected to a power network;

[0028] Figure 2 schematically shows an exemplary controller, which may form part of the Power Plant Controller, shown in Figure 1 ; Figure 3 shows an exemplary method of operating the renewable energy power plant of Figure 1 in accordance with an embodiment of the invention;

[0029] Figure 4 shows exemplary sub-steps, in accordance with an embodiment of the invention, of the method shown in Figure 3;

[0030] Figures 5 and 6 are graphs showing an available power supply and a power usage or demand of one or more P2X units of the power plant, shown in Figure 1 , when operated according to the method shown in Figure 3;

[0031] Figure 7 shows another exemplary method of operating the renewable energy power plant of Figure 1 in accordance with an embodiment of the invention;

[0032] Figure 8 shows exemplary sub-steps, in accordance with an embodiment of the invention, of the method shown in Figure 7;

[0033] Figure 9 is a graph showing an available power supply and a power demand or usage of one or more P2X units of the power plant, shown in Figure 1 , when operated according to the method shown in Figure 7.

[0034] Figure 10 shows a further exemplary method of operating the renewable energy power plant of Figure 1 in accordance with an embodiment of the invention; and

[0035] Figures 11 and 12 are graphs showing a power output supply to the connected power network, shown in Figure 1 , a power supply generated by one or more renewable energy generators of the power plant, shown in Figure 1 , and a power usage or demand of one or more P2X units of the power plant, shown in Figure 1 , when operated according to the method shown in Figure 10.

[0036] Detailed description of embodiments of the invention

[0037] In general terms, embodiments of the invention relate to renewable energy power plant arrangements incorporating one or more power-to-X units for converting electrical power into X and one or more renewable energy generators, such as a wind turbine, generating electrical power for supply to the Power-to-X unit(s) and / or a connected power network. The power-to- X unit(s) may be configured for power-to-heat conversion, power-to-chemical conversion, and / or power-to-fuel conversion, such as hydrogen gas or another fluid medium that can be used as a chemical feedstock or for energy storage.

[0038] Within this context, the available power supply from the renewable energy generators is susceptible to rapid fluctuations due to the intermittent nature of such energy sources. Embodiments of the invention therefore relate to methods and systems for controlling the P2X units accordingly to mitigate downtime and balance the demands of X production against other factors, such as grid stability and market prices. These factors influence the trade-offs between X production and electricity export to the power network.

[0039] 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 (WTGs). A WTG is commonly referred to as a ‘wind turbine’. The example shown is representative only and the skilled reader will appreciate that other architectures are possible. The WPP also includes one or more Power- to-X units for converting electrical power into X, e.g. for the purposes of electricity conversion, energy storage, and / or reconversion pathways. In this context, the power-to-X unit(s) may be configured for power-to-heat conversion, power-to-chemical conversion, and / or power-to-fuel conversion, such as the production of hydrogen gas or another fluid medium that can be used as a chemical feedstock or for energy storage. In other examples, the power plant may include alternative or additional 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 generators. 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 depicted in the Figures.

[0040] 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] Considered in more detail, Figure 1 shows a power system 10 incorporating a WPP 12. In this example, the WPP 12 includes a plurality of WTGs 14, one or more Power-to-X (P2X) units 20, and a power plant controller (PPC) 22. Each of the WTGs 14 converts wind energy into electrical energy, which is transferred, as active power and / or current, to the P2X unit(s) 20 for conversion to X, and / or to a main power network or ‘main grid’ 16 (connected to the WPP 12) for distribution. The P2X unit(s) 20 are configured to convert electric power to X and, for this purpose, the P2X unit(s) 20 may be configured to use electric power supplies from the WTGs 14, the main grid 16, and / or other sources (not shown in Figure 1). Hence, while this disclosure is primarily concerned with the use of power generated by the WTGs 14 for power- to-X conversion, the P2X unit(s) 20 may receive a portion of their input power from the main grid 16, or other non-renewable energy sources, at least in certain conditions, as shall be described in more detail.

[0042] In this example, each of the WTGs 14 is associated with a respective local WTG controller 15 and each of the P2X units 20 is associated with a respective local P2X controller 17. In other implementations, individual sets of the WTGs 14 and / or P2X units 20 may share semicentralised controllers though, such that there are fewer WTG controllers 15 and / or P2X controllers 17 than respective numbers of WTGs 14 and / or P2X unit(s) 20. As will be understood by the skilled person, the WTG controllers 15 and P2X controllers 17 can each be considered to be local computer systems capable of operating a WTG 14 or a P2X unit 20 respectively in the manner prescribed herein. The WTG controllers 15 and P2X controllers 17 may each comprise multiple modules that control individual components of the WTG 14 / P2X unit 20 or just a single controller. The computer system of each WTG controller 15 I P2X controller 17 may operate according to software downloaded via a communications network or programmed onto it from a computer-readable storage medium.

[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) 21 , which is an interface between the WPP 12 and the main grid 16. The Pol 21 may also be referred to as the Point of Common Coupling, which may be abbreviated to ‘PCC’ or ‘PoCC’.

[0044] The Pol 21 allows for the supply of electrical power from the WPP 12 to the main grid 16, and also provides for the supply of electrical power from the main grid 16 to the WPP 12, for example supplying power to the P2X unit(s) 20 when there is insufficient power supply from the WTGs 14.

[0045] The 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 indicative of the power supply to / from the main grid 16 and / or a frequency level of the main grid 16. The PPC 22 is also connected to each of the WTG controllers 15 and P2X controllers 17 and the role of the PPC 22 is to act as a command and control interface between the WPP 12 and the grid 16. In particular, the PPC 22 is configured to act as a command and control interface between the WPP 12 and a grid operator 26, such as a transmission system operator (TSO) or a distribution system operator (DSO).

[0046] The PPC 22 is a suitable computer system for carrying out the controls and commands as described herein. The PPC 22 may therefore 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.

[0047] 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, the P2X unit(s) 20, and / or the WPP 12. 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 or the P2X controllers 17, 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.

[0048] During normal operation, the PPC 22 generates and sends 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. The set points may be determined to provide frequency and voltage support to the main grid 16, and / or to provide an electrical power supply to the P2X unit(s) 20. The WTG controllers 15 control the WTGs 14 according to the set points contained within the dispatch signals. Various methods are known in the art for determining such set points for controlling the operations of the WTGs 14, which are not described in detail here to avoid obscuring the invention.

[0049] The PPC 22 is further configured to generate and send dispatch signals to the P2X controllers 17. The dispatched signals may contain commands for setting an activation state of the P2X unit(s) 20, and / or set points (e.g. current and / or power set points) for controlling the power-to- X conversion of the P2X unit(s) 20. For example, the P2X unit(s) 20 may be operable in one of a plurality of activation states, including both production and non-production states, and the dispatched signals may contain commands for controlling the P2X unit(s) 20 to switch from one activation state to another. For example, such commands may be executed to switch between activation states by starting- up or shutting-down one or more auxiliary systems or subsystems of the P2X unit(s) 20.

[0050] In the production state, the P2X controllers 17 may control the P2X unit(s) 20 according to the set points contained within the dispatched signals, operating the P2X unit(s) 20 to deliver a corresponding rate of X production. The PPC 22 may determine such set points based, at least in part, on an available active power supply from the WTGs 14, for example, as shall be described in more detail.

[0051] Bi-directional control networks may therefore be arranged between the PPC 22 and the WTG controllers 15, and between the PPC 22 and the P2X controllers 17. In each case, the bidirectional control networks enable two-way communication. For example, an uplink direction (i.e. the direction from the central PPC 22 to the local WTG controllers 15 / P2X controllers 17) may be used to send commands and reference values, e.g., for voltage, current, and / or power, from the PPC 22 to the local WTG controllers 15 and P2X controllers 17. In contrast, a downlink direction may be used by the WTG controllers 15 and P2X controllers 17 to return information about the current operational state of the respective WTG 14 or P2X unit 20 to the central PPC 22, for example regarding the amount of active power currently being produced or consumed. 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).

[0052] It is noted at this point that the system view of Figure 1 is schematic in form and so does not represent a complete practical system, which may include other components such as compensation equipment (e.g. a static synchronous compensator (STATCOM) or another type of synchronous compensator) configured to provide reactive power or reactive current support as required, as well as power inductors, chokes, filters, isolation switches, power dissipation choppers, breakers, and so on. However, such system components are within the purview of the skilled person and so are not discussed in detail in this disclosure.

[0053] The P2X unit(s) 20 are configured to receive electrical power from the WTGs 14, and / or the main grid 16, via a supply cable 38 extending from the connecting network 18. The P2X unit(s) 20 are shown to be separate from the WTGs 14 in this example, although it is also possible for the P2X unit(s) 20 to be integrated with the WTGs 14 and embodiments of the invention are also applicable to such arrangements. In addition, although the P2X unit(s) 20 are illustrated as part of the WPP 12 in this example, in other examples a power plant arrangement may include P2X unit(s) 20 that are physically separate from the power plant, but electrically connected to the power plant for receiving electrical power produced by the electrical generators.

[0054] The electrical power received from the connecting network 18 is supplied in alternating current (AC) form. In Figure 1 , the electrical power input is shown to be supplied directly to the P2X unit(s) 20, via the connecting network 18. Although not shown it shall be appreciated that the P2X unit(s) 20 may include front-end units that process the AC power received from the connecting network 18 into a direct current (DC) form that is suitable for components of the P2X unit 20. For example, the front-end units may include a transformer configured to provide a voltage step-up and an AC-DC converter configured to convert the input power to DC.

[0055] The P2X unit(s) 20 are identical in this example and are independently operable so that the level of electrical power that is supplied to each P2X unit 20 can be adjusted individually, to allocate the total power received from the connecting network 18 between the P2X unit(s) 20 as desired. The P2X unit(s) 20 are also electrically and physically distinct from one another, and can be replaced separately and individually, for example if a P2X unit 20 becomes degraded or otherwise reaches the end of its operating life. However, this is not intended to be particularly limiting on the scope of the present disclosure, which relates to overall control of the activation states and power demands of the P2X unit(s) 20.

[0056] For some embodiments, “X” with regard to the P2X unit(s) 20 may comprise or consist of one or more fuels. By way of example, the P2X unit(s) 20 may include a power-to-gas (P2G) unit configured to convert electric power to gas. In particular, the power-to-gas unit may be configured to use electric power to produce, a gas or gas mixture comprising or consisting of one or more of the group of: hydrogen; oxygen; and methane. For example, the power-to-gas unit may be configured to convert electric power to a gas mixture comprising hydrogen and oxygen. To give an example, the P2X unit(s) 20 may include an electrolysis system fed with an input water stream by an appropriate water source. That water source may supply fresh water, for example from storage tanks or from a pipe. Alternatively, in the case of a system based offshore, a de-saliniser may be used to remove salts from seawater and supply fresh water to the electrolysis system. Such a de-saliniser is a known system that would be understood by the skilled person and so is not described here. The electrolysis system provides a hydrogen output stream into a fluid line, through which the hydrogen stream can be conveyed to a user of the generated hydrogen. For example, the reference to the user may be a direct supply to a distribution network, for example, or it may be a suitable storage capacity such as a set of tanks. The hydrogen user may also include a suitable compressor / dryer system to compress the hydrogen to a suitable pressure level (e.g., approximately 35 bar, or even 700 bar or higher) before storage and / or transportation. Although the P2X unit 20 produces hydrogen gas in this embodiment, in other embodiments an P2X unit(s) 20 may produce other fluids, such as methane for example.

[0057] Moreover, in principle, any suitable type of electrolysis system may be used, the specification of which would be within the understanding of a skilled person. For example, the electrolysis system may comprise one or more of the group of: an alkaline electrolyser; an unpressurized alkaline electrolyser; a pressurized alkaline electrolyser; a proton exchange membrane electrolyser; an unpressurized proton exchange membrane electrolyser; a pressurized proton exchange membrane electrolyser; a polymer electrolyte membrane electrolyser; an unpressurized polymer electrolyte membrane electrolyser; a pressurized polymer electrolyte membrane electrolyser; and a solid oxide electrolyser (SOEC). Similarly, it is to be understood that the P2X unit(s) 20 may include further units, components and / or other devices, such as pumps, vents, storage tanks and / or separators, required for a Power-to-X unit.

[0058] As noted previously, the P2X unit(s) 20 are each operable in both production and nonproduction states, where the non-production states may include a production-ready state and a standby state, amongst others.

[0059] In the production state, the one or more P2X unit(s) 20 are configured to actively convert electric power to X according to set points dispatched from the PPC 22. For example, the power conversion of the P2X unit(s) 20 may be varied according to the set points between a rated power of the P2X unit(s) 20 and a baseline or base power usage level, corresponding to a minimum rate of X production. For context, an electrolysis module may have a power rating of between 10 to 20 MW, for example, although this may vary and the P2X unit may have a baseline power usage level of approximately 15% of the rated power output. The base power usage level may correspond to a minimum rate at which the electrochemical reactions are sustainable over extended periods and / or a minimum activation energy for the X production.

[0060] In the production-ready state, the P2X unit(s) 20 do not actively convert electric power to X but electrical power is consumed by the P2X unit(s) to prepare for X production upon receiving a set point from the PPC 22. For example, electrical power may be used to power one or more auxiliary systems or sub-systems of the P2X unit(s) 20 that are necessary for operating the P2X unit(s) 20 to produce X. For example, in an electrolysis system such auxiliary systems may include systems for a flow loop; a lye circulation for electrolysis stacks; gas circulation for electrolysis stacks; gas / liquid pressure balancing for electrolysis stacks; pressure equalization of cells within the electrolysis stacks; pH and KOH (potassium hydroxide) balancing for the electrolysis stacks; and any other equipment that may be used in the production of the raw hydrogen and oxygen gas. The P2X unit(s) 20 may therefore be associated with a baseline power usage level for maintaining the P2X unit(s) in the production-ready state, which may correspond to a power demand of the auxiliary system(s) of the P2X unit(s) 20.

[0061] In the standby state, the P2X unit(s) 20, and their auxiliary I sub-systems are shut down, requiring startup before the P2X unit(s) 20 can convert electric power to X. Accordingly, in the standby state, the power demand of the P2X unit(s) 20 may be reduced to zero or a negligible amount.

[0062] The following methods and systems of the present invention are concerned with mitigating downtime of the P2X unit(s) 20, reducing activations and deactivations, and controlling power- to-X conversion, whilst balancing the demands of X production against other factors, such as energy availability, grid stability and market prices.

[0063] For this purpose, Figure 2 illustrates a control scheme, algorithm, or “controller” 100, which may form part of the processing module 28 of the PPC 22 for controlling the P2X unit(s) 20.

[0064] In an example, the controller 100 may be configured to determine set points for controlling the power-to-X conversion of the P2X unit(s) 20 in the production state. For this purpose, the controller 100 may be configured to receive or obtain a signal indicative of available active power for supply to the P2X unit(s) 20 from the WTGs 14 and determine set points for the P2X unit(s) 20 based thereon. The active power set points are dispatched to the P2X unit(s) 20 to control, in real-time, a target value for the active power conversion to X.

[0065] Various methods are known in the art for determining such active power set points and it shall be appreciated that the following examples are not intended to be limiting on the scope of the invention.

[0066] For example, the controller 100 may be configured to determine set points at a reference active power level, such as a rated power of the P2X unit(s) 20, when the indicated available active power is greater than or equal to the reference active power level, and reduce the electrical power conversion to X towards the available active power level when the indicated available active power is less than the reference power level. For this purpose, the controller 100 may include one or more predetermined ramp rates and / or ramp rate limits for determining the active power set points based on the indicated available active power from the WTGs 14. For example, the control module 100 may store a look-up table comprising pre-determined ramp rates for respective differences between the current active power level of the P2X unit(s) 20 and the indicated active power supply from the WTGs 14. Such ramp rates may be used when the indicated available active power is determined to reduce at a greater rate than the one or more ramp rate limits. Here it shall be appreciated that the set points dispatched to the P2X unit(s) 20 may cause the P2X unit(s) 20 to draw power from the main grid 16 to make-up the electric power shortfall where the determined set points exceed the indicated available active power supply from the WTGs 14.

[0067] In examples, the controller 100 may be configured to control reduction of the power-to-X conversion of the P2X unit(s) 20 to a base power usage level for maintaining the P2X unit(s) 20 in a power-production state, and determine the set points at the base power usage level when the indicated available active power is less than or equal to the base power usage level. Again, it shall be appreciated that the dispatched set points may cause the P2X unit(s) 20 to draw power from the main grid 16 to make up for the electric power shortfall in this condition. In this manner, the PPG 22 may be configured to control the WPP 12 to provide an agreed X production service, and draw power from the main grid 16 to maintain the minimum agreed rate of X production when the available active power supply from the WTGs 14 is temporarily insufficient. For example, the operators of the WPP 12 may have agreed to produce X at a prescribed minimum rate for a service period of 12 hours.

[0068] However, excessive power draw from the main grid 16 is not desirable and, in examples, the controller 100 may therefore be further configured to stop determining set points and, instead, determine a command for controlling the one or more P2X unit(s) 20 to switch from the production state to a non-production state in dependence on the power usage from the main grid 16. For example, the controller 100 may be configured to detect a state change condition associated with a magnitude, or an accumulation, of the electric power shortfall.

[0069] In other examples, the controller 100 may additionally or alternatively be configured to detect state change conditions based on the available active power supply, and control the activation states of the P2X unit(s) 20 accordingly. Again, for this purpose, the controller 100 may be configured to receive or obtain a signal indicative of available active power for supply to the P2X unit(s) 20 from the WTGs 14 and detect a state change condition in dependence on a comparison of the indicated available active power to one or more power thresholds associated with the current activation state. The thresholds may be configured to provide a tolerance for mitigating unnecessary activations / deactivations of the P2X unit(s) 20 when the available power supply deviates from a reference or base line power usage level associated with each activation state. Additionally, the controller 100 may be configured to determine an elapsed time period while the available active power remains outside the threshold and compare the elapsed time to a threshold period for detecting a state change condition. In this manner, the threshold period provides a further tolerance for mitigating unnecessary activations / deactivations of the P2X unit(s) 20, when the available active power supply fluctuates and temporarily moves outside such thresholds.

[0070] Each activation state of the P2X unit(s) 20 may therefore be associated with a respective active power range with upper and lower power thresholds for detecting state change conditions, In particular, the controller 100 may be configured to detect: (i) a low-power state change condition when the indicated available active power is less than a lower threshold of the active power range for a threshold period; and / or (ii) a high-power state change condition when the available active power is greater than an upper threshold of the active power range for a threshold period.

[0071] If a low-power state change condition is detected, the PPC 22 may dispatch a control command for controlling the P2X unit(s) 20 to switch to an activation state associated with a lower active power demand than the current activation state, e.g. switching from a production state to a production-ready state. Meanwhile, if a high-power state change conditions is detected, the PPC 22 may dispatch a control command for controlling the P2X unit(s) 20 to switch to an activation state associated with a higher active power demand than the current activation state, e.g. switching from a standby state to a production-ready state.

[0072] In this manner, the controller 100 may control switching between activations states of the P2X unit(s) 20 based on the indicated available active power supply and use the elapsed time outside the power thresholds for controlling such switching.

[0073] In further examples, the controller 100 may additionally or alternatively be configured to monitor an active power supply output to the main grid 16 from the WPP 12 in order to detect one or more shut-down conditions of the P2X unit(s) 20. In particular, the controller 100 may be configured to receive or obtain a signal indicative of the active power supply output from the WTGs 14 to the main grid 16, e.g. at the PoM 24, and compare the power supply to one or more active power thresholds. For example, such power thresholds may include an upper power threshold and / or a lower power threshold. The controller 100 may use the upper power threshold for triggering a soft shut-down of the P2X unit(s) 20, while the lower power threshold may be used for triggering a hard shut-down of the P2X unit(s) 20. In particular, if the signal indicates that the power supply to the main grid 16 is less than or equal to the upper power threshold, the controller 100 may be configured to determine a soft shut-down command for controlling the P2X unit(s) 20 to gradually reduce the electric power-to-X conversion, for example at a prescribed ramp rate, before switching to a non-producing state, for example upon reaching a base power level. Additionally or alternatively, the controller 100 may be configured to determine a hard shut-down command if the signal indicates that the power supply to the main grid 16 is less than or equal to the lower power threshold, where the hard shut-down command controls the P2X unit(s) 20 units to switch directly to the non-production state, stopping the electric power-to-X conversion (without a ramp down). It shall be appreciated that the hard shut-down command may be dispatched following a soft-shut down command if the power supply to the to the main grid 16 continues to fall despite the reduction of the electric power-to-X conversion. In this manner, the lower power threshold may be configured to prevent or limit power usage from the main grid 16, and may correspond to a zero power supply to the main grid 16 or a negative power level (e.g. based on a reference allowable power usage from the main grid 16). The upper power threshold may be based on a reference power usage level of the P2X unit(s) 20 and include a power margin that is based on, or derived from, historic data indicative of fluctuations or rates of decline of the active power supply from the WTGs 14 to the grid 16.

[0074] In this manner, the controller 100 may control the shutdown of the P2X unit(s) 20 to maintain an electricity export to the grid 16 when the available power from the WTGs 14 temporarily reduces, for example in low wind speed conditions.

[0075] Methods of controlling the P2X units 20 to mitigate downtime and balance the demands of X production against other factors, such as grid stability and market prices, shall now be described with additional reference to Figures 3 to 12.

[0076] It shall be appreciated that the following methods are intended to be complementary to one another. Although described as separate methods, the skilled person shall appreciate that each of the following methods may be used alone or in conjunction with one or more of the other methods for controlling the P2X unit(s) 20. For example, the methods may be implemented in respective modes of operation as may be selectively activated or deactivated by a plant operator. Figure 3 shows an example method 300 of controlling the WPP 12 in accordance with an embodiment of the invention. In particular, the method 300 may be executed to control the activations and deactivations of the P2X unit(s) 20 during operation of the WPP 12.

[0077] In step 302, the PPC 22 obtains a signal indicative of an available active power level, Pavaii_PTx, for supply to the P2X unit(s) 20 from the WTGs 14. That is, the PPC 22 obtains a signal indicative of the amount of active power that is generated by the WTGs 14 and available for supply to the P2X unit(s) 20, for example accounting for system losses between the WTGs 14 and the P2X unit(s) 20 and other allocations of the generated power. For example, the PPC 22 may receive one or more signals indicative of an active power level of the WTGs 14 and a power demand of the main grid 16 and determine an available active power level, Pavaii_PTx, for supply to the P2X unit(s) 20 based thereon (e.g. based on a difference therebetween). The active power level of the WTGs 14 may be received as a signal from one or more sensors at the output of the WTGs 14, for example. Similarly, the power demand of the main grid 16 may be received as a signal output from the PoM 24, for example.

[0078] In step 304, the PPC 22 analyses the indicated available active power level, Pavaii_PTx, to detect any state change condition(s) of the P2X unit(s) 20. That is, the PPC 22 determines whether the activation state(s) of the P2X unit(s) 20 should be changed in view of the indicated available active power level, Pavaii_PTx. In this context, a shortage of available active power may prompt a deactivation or shut-down operation of the P2X unit(s) 20, while an excess of available active power may prompt an activation or start-up operation of the P2X unit(s) 20. For this purpose, the PPC 22 compares the indicated available active power level, Pavaii_PTx, to an active power range associated with the current activation state of the P2X unit(s) 20. Here, it shall be appreciated that the active power range used for this purpose acts as a tolerance for transient fluctuations of the available power supply to the P2X unit(s) 20, allowing the P2X unit(s) 20 to maintain their current state of operation. Moreover, the PPC 22 is configured to allow the indicated available active power level, Pavaii_PTx, to temporarily fall outside of the tolerance, i.e. the relevant active power range, without prompting a state change of the P2X unit(s) 20. In particular, the PPC 22 determines the time, Ts, that has elapsed while the indicated available active power level, Pavaii_PTx, remains outside of the active power range, and detects a state change condition of the P2X unit(s) 20 if a threshold period is exceeded. In this manner, the PPC 22 mitigates unnecessary downtime of the P2X unit(s) 20 and limits grid power usage by applying hysteresis thresholding. It shall be appreciated that the hysteresis thresholding used for detecting state change conditions of the P2X unit(s) 20 may take various suitable forms within the scope of the invention though. By way of example, Figure 4 shows exemplary sub-steps 306 to 322 for detecting the state change condition(s) of the P2X unit(s) 20, in accordance with an embodiment of the invention.

[0079] In sub-step 306, the PPC 22 compares the indicated available active power level, Pavaii_PTx, to the lower power threshold associated with the current activation state of the P2X unit(s) 20. The lower power threshold may be based on a reference power usage level of the P2X unit(s) 20 in the respective activation state, for example with a tolerance threshold, e.g. 5% of the reference power usage level.

[0080] If the indicated available active power level, Pavaii_PTx, is less than or equal to the lower power threshold, the PPC 22 determines the time, Ts, elapsed in that condition, in sub-step 308 (i.e. the PPC 22 determines how long the indicated available active power level, Pavaii_PTx, has been less than or equal to the lower power threshold). The PPC 22 compares the elapsed time, Ts, to a threshold period, Tthreshoidi, for detecting a state change condition, in sub-step 310.

[0081] If the elapsed period, Ts, is less than the threshold period, Tthreshoidi , the PPC 22 returns to checking the indicated available active power level, Pavaii_PTx, against the lower power threshold, Tthreshoidi , in sub-step 306.

[0082] However, if it is determined, in sub-step 310, that the elapsed period, Ts, is greater than or equal to the threshold period, Tthreshoidi , the PPC 22 detects a first state change condition of the P2X unit(s) 20, in sub-step 312.

[0083] The first state change condition corresponds to a trigger for reducing the power usage of the P2X unit(s) 20 by switching to an activation state that is associated with a lower power usage. For example, detecting the first state change condition may act as a trigger for switching the P2X unit(s) 20 from a production state to a production-ready state, or from a production-ready state to a shutdown state. In response to detecting the first state change condition, the PPC 20 may therefore output a command for shutting down one or more auxiliary systems or subsystems of the P2X unit(s) 20, as shall be described in more detail.

[0084] First though, returning to sub-step 306, if the PPC 22 determined that the indicated available active power level, Pavaii_PTx, is greater than the lower power threshold, the PPC 22 proceeds to make a further comparison to the upper power threshold associated with the current activation state of the P2X unit(s) 20, in sub-step 316. Thereafter, the method 300 may proceed substantially as described previously for detecting a second state change condition of the P2X unit(s) 20. In particular, if the indicated available active power level, Pavaii_PTx, is greater than or equal to the upper power threshold, the PPC 22 determines the time, Ts, elapsed in that over-power condition, in sub-step 318, and compares the elapsed time, Ts, to a threshold period, Tthreshoid2, in sub-step 320. If the elapsed period, Ts, is less than the threshold period, Tthreshoid2, the PPC 22 returns to checking the indicated available active power level, Pavaii_PTx, against the upper power threshold again, in sub-step 316. However, if it is determined, in sub-step 320, that the elapsed period, Ts, is greater than or equal to the threshold period, Tthreshoid2, the PPC 22 detects the second state change condition of the P2X unit(s) 20, in sub-step 322.

[0085] The second state change condition corresponds to a trigger for increasing the power usage of the P2X unit(s) 20 by switching to an activation state that is associated with a higher power usage. For example, in response to detecting the second state change condition, the PPC 20 may output a command for switching the P2X unit(s) 20 from a shutdown state to a productionready state, or from a production-ready state to a production state. Detecting the second state change condition may therefore produce a trigger for activating or starting up one or more auxiliary systems or sub-systems of the P2X unit(s) 20, as shall be described in more detail.

[0086] Finally, if the PPC 22 determines, in sub-step 316, that the indicated available active power level, Pavaii_PTx, is less than the upper power threshold, and therefore falls within an allowed active power range, the PPC 22 determines that the current activation state is suitable, in substep 314. The PPC 22 therefore operates the P2X unit(s) 20 accordingly, proceeding to check the power availability in the next iteration cycle.

[0087] Returning to Figure 3, if the PPC 22 detects a state change condition of the P2X unit(s) 20 in step 304, the PPC 22 proceeds to determine and dispatch a command to change the activation state of the P2X unit(s) 20 accordingly, in step 324. For example, when the first state change condition is detected, in step 304, the PPC 22 dispatches a command signal for reducing the power usage of the P2X unit(s) 20 by switching the P2X unit(s) to a lower-power activation state. The commands dispatched from the PPC 22 may therefore be configured to shut-down one or more auxiliary system or sub-systems of the P2X unit(s) 20 when the PPC detects the first state change condition, switching the P2X unit(s) 20 from the production state to a nonproduction state and / or from a production ready-state to a shutdown state.

[0088] In contrast, when the second state change condition is detected, in step 304, the PPC 22 dispatches a command signal for increasing the power usage of the P2X unit(s) 20 by switching the P2X unit(s) to a higher-power activation state. For example, the P2X unit(s) 20 may be controlled to switch from a non-production state to a production state and / or from a shutdown state to a production-ready state. The commands dispatched from the PPC 22 may therefore be configured to start or activate one or more auxiliary- or sub- systems of the P2X unit(s) 20 when the PPC detects the second state change condition. In each case, the command signals may be dispatched by the PPC 22 to the P2X controllers 17 for operating the P2X unit(s) 20 accordingly.

[0089] By way of example, Figures 5 and 6 show first and second graphs illustrating the control of the P2X unit(s) 20 according to the method 300 in response to variations of the available active power supply from the WTGs 14, when operating in a particular activation state. For the purposes of this example, the P2X unit(s) 22 may be considered to be operating in a production-ready state initially, however this is not intended to be limiting on the scope of the invention.

[0090] Each graph shows a first signal 502 indicative of the available active power level, Pavaii_PTx, for supply to the P2X unit(s) 20 from the WTGs 14; a second signal 504 corresponding to a power usage level of the P2X unit(s) 20, PCOn_PTx; a first line 506 corresponding to an upper power threshold, Pupper, of the current activation state of the P2X unit(s) 20; and a second line 508 corresponding to a lower power threshold, Piower, of the current activation state of the P2X unit(s) 20.

[0091] In Figure 5, the available active power level, Pavaii_PTx, is initially shown to be greater than the power usage level, Pcon_pix, of the P2X unit(s) 20 and between the upper and lower power thresholds Pupper, Piower. However, the available active power level, Pavaii_PTx, fluctuates, e.g. due to a drop in wind speed, and reduces to less than the power usage level, PCOn_PTx, of the P2X unit(s) 20, at the point T1. At this time, the power usage level, PCOn_PTx, of the P2X unit(s) 20 therefore exceeds the available active power level, Pavaii_PTx, of the WTGs 14 and the P2X unit(s) 20 therefore consume power from other sources, such as the main grid 16. The available active power level, Pavaii_PTx, continues to fall and eventually reduces below the lower power threshold, P|Ower, at the point T2. In accordance with the method 300, the PPC 22 therefore determines that the available active power level, Pavaii_PTx, is less than the lower power threshold, P|Ower, in sub-step 306, and determines the time elapsed, Ts, in this condition, in sub-step 308. However, the available active power level, Pavaii_PTx, increases again shortly after and returns to the active power range (between the upper and lower power thresholds, Pupper, Piower) at the point T3. Accordingly, the elapsed time period, Ts, does not exceed the time threshold, Tthreshoidi , and the PPC 22 does not detect a state change condition. As a result, shutdown is avoided and the PPC 22 controls the P2X unit(s) 20 to maintain the productionready state, shown by the substantially constant power usage level Pcon_PTx. The available active power level, Pavaii_PTx, pf the WTGs 14 continues to increase to the power usage level, Pcon_PTx, of the P2X unit(s) 20 at the point T4 and, thereafter, the WTGs 14 supply all of the power to maintain the P2X unit(s) 20 in the production-ready state as the available active power level, Pavaii_PTx, stabilises once more after the point T5. In this manner, the PPC 22 controls the P2X unit(s) 20 to maintain the current activation state and ride-through transient fluctuations of the active power available from the WTGs 14, temporarily drawing power from other sources to avoiding unnecessary shutdowns and downtime of the P2X unit(s) 20.

[0092] In contrast, in the example shown in Figure 6, the available active power level, Pavaii_PTx, falls below the lower power threshold, Piower, at the point T2, but remains less than the lower power threshold, P|Ower, for a prolonged period. The PPC 22 therefore determines, in sub-step 310, that the elapsed period, Ts, has exceeded the threshold period, Tthreshoidi, at the point T6. The PPC 22 therefore detects the first state change condition, in sub-step 312, and dispatches a command, in step 324, causing the P2X unit(s) 20 to switch to a lower-power activation state, reducing the power usage by shutting down the auxiliary system(s) to enter the standby state. In Figure 6, this is shown by the reduction in the power usage level, PCOn_PTx, from the point T6 onwards. In this case, the P2X unit(s) 20 are shut down by the PPC 22 and therefore stop consuming electrical power at the point T7. As shown in Figure 6, the available active power level, Pavaii_PTx, increases after the point T6 and the method 300 may proceed to determine whether there is a sustained increase of the available active power level, Pavaii_PTx, exceeding an upper power threshold of the standby state that would prompting a startup of the P2X unit(s). However, this is not evident in the present example. Instead, the available active power level , Pavaii_PTx, below the power usage level, PCOn_PTx, of the P2X unit(s) 20 in the productionready state as shown up to the point T6.

[0093] In this manner, the method 300 uses the power thresholds and time thresholds to mitigate unnecessary activations and deactivations of the P2X unit(s) 20. It is anticipated that the method 300 will therefore lead to reduced downtime of the P2X unit(s) 20, increased revenues, and reduced power usage from the main grid 16.

[0094] Figure 7 shows an example method 700 of controlling the P2X unit(s) 20 in accordance with another embodiment of the invention. In particular, Figure 7 relates to a method 700 of controlling the Power-to-X conversion of the P2X unit(s) 20 in the production state, for example to provide a minimum rate of X production. The method 700 is particularly applicable to situations where the plant operator may have agreed an X-production service, e.g. at a respective minimum rate, for a predetermined period.

[0095] In step 702, the PPC 22 obtains a signal indicative of an available active power level, Pavaii_PTx, for supply to the P2X unit(s) 20 from the WTGs 14, substantially as described previously in step 302 of the method 300.

[0096] In step 704, the PPC 22 determines and dispatches active power set points for controlling the P2X unit(s) 20 based on the indicated available active power level, Pavaii_pix, the dispatched active power set points being greater than or equal to base power usage level corresponding to a minimum rate of X production. The base power usage level may therefore correspond to a minimum rate of X production pre-agreed by the plant operator, or the base power usage level may correspond to a minimum power level for maintaining the P2X unit(s) 20 in the production state, for example where the power-to-X conversion is associated with a minimum activation energy below which the P2X unit(s) 20 are unable to produce X. In each case, the base power usage level of the P2X unit(s) 20 may be associated with a corresponding minimum rate of X production. The dispatched active power set points therefore cause the P2X unit(s) 20 to use corresponding amounts of electric power for X production, where that power may vary between a maximum rated power of the P2X unit(s) 20 and the base power usage level of the P2X unit(s) 20.

[0097] In this mode of operation, it shall be appreciated that the base power usage level is prioritised over other factors, and the PPC 22 may therefore dispatch active power set points, in step 704, for controlling the P2X unit(s) 20 to convert electric power to X, even when there is a shortage of available active power supply from the WTGs 14. Accordingly, if the functionality is enabled, the PPC 22 will control the P2X unit(s) 20 to use an electric power supply from the main grid 16 to provide the electric power shortfall from the WTGs 14 and maintain the base power usage level of the P2X unit(s) 20. The WPP 12 can therefore be operated to satisfy minimum rates of X production, which may be agreed in advance, despite temporary drops in the renewable energy production.

[0098] It shall be appreciated that the PPC 22 may use one or more methods for determining the active power set points, in step 704, to dispatch to the P2X unit(s) 20 based on the indicated available active power level, Pavaii_PTx- By way of example, Figure 8 shows example sub-steps 706 to 714, in accordance with an embodiment of the invention, for controlling the P2X unit(s) 20 based on the indicated available active power level, Pavaii_PTx.

[0099] In sub-step 706, the PPC 22 compares the indicated available active power level, Pavaii_PTx, to a reference power level of the P2X unit(s) 20 in the production state. For example, the reference power level may be a rated power of the P2X unit(s) 20.

[0100] If the PPC 22 determines that the indicated available active power level, Pavaii_PTx, is greater than or equal to the reference power level, the PPC 22 is configured to determine and dispatch set points equal to that reference power level, in sub-step 708.

[0101] Accordingly, when the PPC 22 determines that the available active power level, Pavaii_PTx, for supply to the P2X unit(s) 20 is greater than or equal to the rated power of the P2X unit(s) 20, the PPC 22 dispatches set points for controlling the P2X unit(s) 20 to produce X at the rated power level. The method may then check the indicated available active power level, Pavaii_PTx, in sub-step 706, during a subsequent iteration.

[0102] If the PPC 22 determines, in sub-step 706, that the indicated available active power level, Pavaii_PTx, is less than the reference power level, the PPC 22 compares the indicated available active power level, Pavaii_PTx, to a baseline power usage level, Pbase_PTx, of the P2X unit(s) 20 in sub-step 710. For example, the baseline power usage level, Pbase_PTx may be a power level of the P2X unit(s) 20 corresponding to an agreed or pre-programmed minimum rate of production of X.

[0103] If the PPC 22 determines that the indicated available active power level, Pavaii_PTx, is less than the reference power level but greater than the baseline power usage level, Pbase_PTx, the PPC 22 is configured to determine and dispatch set points based on the indicated available active power level, Pavaii_PTx, in sub-step 712. However, power increases and decreases of the P2X unit(s) may be subject to one or more limits, and the PPC 22 may therefore access one or more predetermined ramp rates and / or ramp rate limits for determining the active power set points based on the indicated available active power level, Pavaii_PTx- The ramp rate may therefore limit the rate of increase or decrease of the active power set points and the power usage of the P2X unit(s) 20 may therefore reduce at a slower rate than the indicated available active power level, Pavaii_PTx. This may result in a power shortage of the power supplied form the WTGs 14. It shall be appreciated that the P2X unit(s) 20 may therefore draw power form the main grid 16 to satisfy the dispatched set points. The method 700 may then check the indicated available active power level, Pavaii_PTx, in sub-step 710, during a subsequent iteration.

[0104] If the PPC 22 determines that the indicated available active power level, Pavaii_PTx, is less than the baseline power usage level, Pbase_PTX, the PPC 22 is configured to determine and dispatch set points based on the baseline power usage level, Pbase_PTX, in sub-step 714. That is, the PPC 22 may determine and dispatch set points for reducing the power usage of the P2X unit(s) 20 to the baseline power usage level, Pbase_PTX, and maintaining the P2X unit(s) 20 at the baseline power usage level, Pbase_PTX, thereafter while the indicated available active power level, Pavaii_PTx, remains less than the baseline power usage level, Pbase_PTX. Here, again, it shall be appreciated that the power shortage may therefore be supplied to the P2X unit(s) 20 from the main grid 16 in order to satisfy the dispatched set points and maintain a minimum rate of production of X.

[0105] By way of example, Figure 9 shows a graph illustrating the example method 700 of controlling the P2X unit(s) 20 in the production state to provide a minimum rate of X production. The graph includes a first signal 902 corresponding to the available active power level, Pavaii_PTx, for supply to the P2X unit(s) 20 from the WTGs 14; a second signal 904 corresponding to the setpoints, Pset_PTx, dispatched from the PPC 22 to the P2X unit(s) 20; a third signal 906 corresponding to reference power level or rated power level, Prated_PTx, of the P2X unit(s) 20; and a fourth signal 508 corresponding to a base power usage level, Pbase_PTX, of the P2X unit(s) 20.

[0106] The available active power level, Pavaii_PTx, is initially greater than the rated power, Prated_PTx, of the P2X unit(s) 20, up to the point T1. In step 704, the PPC therefore determines and dispatches set points, Pset_PTx, at the rated power level, Prated_PTx, during this period. However, after the point T1 , the available active power level, Pavaii_PTx, falls below the rated power, Prated_PTx, of the P2X unit(s) 20. The PPC 22 therefore determines set points for reducing the power usage of the P2X unit(s) 20 to the available active power level, Pavaii_PTx, in sub-step 712, and may use a prescribed ramp rate for this purpose. However, as shown in Figure 9, the set points, Pset_PTx, determined by the PPC 22 reduced at a slower rate than the available active power level, Pavaii_PTx, of the WTGs 14. Between the points T1 and T2, the set points, PSet_PTx, therefore exceed the available active power level, Pavaii_PTx, for supply to the P2X unit(s) 20 and the P2X unit(s) 20 are therefore configured to draw the power shortage from the main grid 16. At the point T2, the available active power level, Pavaii_PTx, falls below the base power usage level, Pbase_PTX, and the PPC 22 therefore determines and dispatches set points based on the baseline power usage level, Pbase_PTX, in sub-step 714. As show in Figure 9, the dispatched set point therefore continue to reduce at the first ramp rate, reaching the base power usage level, Pbase_PTX, at the point T3. Thereafter the PPC 22 determines and dispatches set points to maintain the P2X unit(s) 20 at the base power usage level, Pbase_PTX. Again, during this period, the power usage of the P2X unit(s) 20 exceeds the available active power supply from the WTGs 14 and the P2X unit(s) 20 therefore use power supplied from the main grid 16 to maintain the minimum rate of X production.

[0107] In this manner, the PPC 22 may control the WPP 12 to provide an agreed X production service and draw power from the main grid 16 to maintain the minimum agreed rate of X production, even when there is insufficient active power supply from the WTGs 14. This can have economic advantages for the plant operator and contractual advantages between off-taker and plant operator as fixed delivery is secured.

[0108] In examples, the power usage of the P2X unit(s) 20 may be subject to further limitations though. For example, the PPC 22 may be configured to switch the P2X unit(s) 20 to a nonproduction state if the PPC 22 determines that the indicated available active power level, Pavaii_PTx, falls below a lower threshold for the production state (e.g. for a threshold period, substantially as described previously), or the amount of energy consumed from the main grid 16 exceeds a threshold amount. That is, having determined and dispatched set points for reducing and / or maintaining the power usage of the P2X unit(s) 20 at the base power usage level, Pbase_PTX, in sub-step 714, the PPC 22 may be further configured to determine whether the indicated available active power level, Pavaii_PTx, falls below a lower threshold, e.g. for a threshold period, or otherwise determine whether the amount of energy consumed from the main grid 16 exceeds a threshold amount. If the PPC 22 determines that either such condition is satisfied the PPC 22 may determine and dispatch a command signal for switching the P2X unit(s) 20 to a non-production state, such as a production-ready state or a standby state.

[0109] Figure 10 shows an exemplary method 1000 of controlling the P2X unit(s) 20 in accordance with another embodiment of the invention. In particular, Figure 10 relates to a method 1000 of controlling the P2X unit(s) 20 for safe shut-down from the production state.

[0110] In contrast to the previous method, the safe-shut down commands may be controlled in dependence on the active power supply output to the main grid 16 from the WTGs 14, instead of the available active power for supply from the WTGs 14 to the P2X unit(s) 20. Accordingly, in step 1002, the PPC 22 obtains a signal indicative of the active power supply output to the main grid 16 from the WTGs 14. For example, the PPC 22 may receive an active power output level, Pmeas_ppc, measured at the PoM 24, which corresponds to the active power output to the main grid 16 from the WTGs 14.

[0111] In step 1004, the PPC 22 compares the active power output level, Pmeas_ppc, to a soft shutdown threshold. The soft shutdown threshold may be based on a reference power level of the P2X unit(s) 20, such as a base power usage level of the P2X unit(s) 20, and further include a margin (relative to the base power usage level), e.g. for preserving a minimum power export from the WTGs 14 to the main grid 16.

[0112] If the active power output level, Pmeas_ppc, is less than or equal to the soft shut-down threshold, the PPC 22 proceeds to determine and dispatch a soft-shutdown command to the P2X unit(s) 20, in step 1006. The dispatched soft-shutdown command causes the P2X unit(s) 20 to reduce the electric power conversion to X at a prescribed ramp rate and switch to a non-production state upon reaching a minimum power level, such as the base power usage level. Accordingly, once the P2X unit(s) 20 reach the minimum power level, the soft-shut-down command further causes the P2X unit(s) to stop converting the electric power to X and switch to a nonproduction state.

[0113] In this manner, the power demand of the P2X unit(s) 20 gradually reduces to the minimum power level and, once the minimum power level is reached, the P2X unit(s) 20 are shutdown to impart a sudden step reduction of the power demand, which boosts the active power available for supply from the WTGs 14 to the main grid 16.

[0114] Once the soft-shutdown command has been determined and dispatched to the P2X unit(s) 20, in step 1006, the available active power supply from the WTGs 14 may continue to fall, further reducing the available power for supply to the WTGs 14 and possibly leading to a situation where the WPP 12 must draw power from the main grid 16 to satisfy its power demands.

[0115] To mitigate this risk, the PPC 22 continues monitoring the active power output level, Pmeas_ppc, after dispatching the soft shut-down command and compares the active power output level, Pmeas_ppc, to a hard shut-down threshold, in step 1008. The hard shutdown threshold may correspond to a minimum power export from the WTGs 14 to the main grid 16 or a zero I negligible power level at the PoM 24, for example. If the PPC 22 determines that the active power output level, Pmeas_ppc, is less than or equal to the hard shutdown threshold, the PPC 22 proceeds to determine and dispatch a hard shutdown command to the P2X unit(s) 20, in step 1010. The dispatched hard shutdown command causes the P2X unit(s) 20 to switch directly to the non-production state. That is, the hard shutdown command causes the P2X unit(s) 20 to immediately stop converting electrical power to X and to switch to the non-production state. For example, the hard shutdown command may cause the P2X unit(s) 20 to tigger shutdown or deactivation one or more of their auxiliary systems or subsystems. In this manner, the dispatched hard shutdown command produces a sudden reduction or step change of the electric power demand from the P2X unit(s) 20, resulting in a sudden boost of active power available for supply from the WTGs 14 to the main grid 16. In this manner, the PPC 22 may control the P2X unit(s) 20 to avoid a negative power supply to the main grid 16, i.e. a power draw from the main grid 16.

[0116] By way of example, Figures 11 and 12 show first and second graphs illustrating the control of the P2X unit(s) 20 according to the method 1000. The first graph, shown in Figure 11 , includes a first signal 1102 indicative of the active power output level, Pmeas_ppc, supplied to the main grid 16 from the WTGs 14. The first graph also include a first line 1104 corresponding to the soft shutdown threshold, PSOft_shutdown; and a second line 1106 corresponding to the hard shutdown threshold, Phard_shutdown. The second graph, shown in Figure 12, includes a second signal 1202 indicative of the active power supply, P active_WTG, generated by the WTGs 14, and a third signal 1204 corresponding to a power usage level, Pcon_PTx, of the P2X unit(s) 20. The second graph also includes a line 1206 corresponding to the soft shutdown threshold, P soft_shutdown-

[0117] As shown in Figures 11 and 12, the active power supply, P active_WTG, generated by the WTGs 14 is initially large enough to provide an active power output level, Pmeas_ppc, supplied to the main grid 16 that exceeds the soft shutdown threshold, with a surplus power being used for X production as shown by the power usage level, Pcon_PTx, of the P2X unit(s) 20. At the point T 1 , the active power supply, P active_WTG, generated by the WTGs 14 begins to reduce, e.g. due to a drop in wind speed, but the P2X unit(s) 20 continue to produce X at the same rate. According between the points T1 and T2, the power usage level, Pcon_PTx, of the P2X unit(s) 20 remains substantially constant but the active power output level, Pmeas_ppc, supplied to the main grid 16 decreases, reflecting the drop of the active power supply, Pactive_wrG, generated by the WTGs 14. At the point T2, the active power output level, Pmeas_ppc, supplied to the main grid 16 reaches the soft shutdown threshold, PSOft_shutdown. The PPC 22 therefore proceeds to determine and dispatch a soft-shutdown command to the P2X unit(s) 20, in step 1006. Accordingly, between the points T2 and T3, the power usage level, PCOn_PTx, of the P2X unit(s) 20 is shown to reduce towards zero at a prescribed ramp rate. During this period, the active power supply, Pactive_wrG, generated by the WTGs 14 continues to reduce at a constant rate, however that power is redistributed between the P2X unit(s) 20 and the main grid 16 (being transferred away from the P2X unit(s) 20 and towards the main grid 16. The active power output level, Pmeas_ppc, supplied to the main grid 16 is therefore shown to reduce at a slower rate between the points T2 and T3. The soft shutdown command may therefore cause the P2X unit(s) 20 to gradually reduce their power demand towards zero and enter a nonproduction state. However, in this example, the reduction of the power usage by the P2X unit(s) is too slow to compensate for the loss of power generated by the WTGs 14 and, at the point T3, the active power output level, Pmeas_ppc, supplied to the main grid 16 reaches the hard shutdown threshold, Phard_shutdown. In other words, the soft shutdown is insufficient and the PPG 22 therefore proceeds to determine and dispatch a hard-shutdown command to the P2X unit(s) 20, in step 1010. Accordingly, at the point T3, the P2X unit(s) 20 enters the nonproduction state and the power usage level, PCOn_PTx, drops to zero. This produces a corresponding step-increase of the active power output level, Pmeas_ppc, supplied to the main grid 16 at the point T3. Thereafter, the active power supply, Pactive_wrG, generated by the WTGs 14 continues to reduce to zero, and at the point T4, the graphs show a zero power supply and output to the main grid 16.

[0118] In this manner, the method 1000 controls the P2X unit(s) 20 so as to avoid a power draw from the main grid 16, which may be expensive or contrary to an agreed manner of operation.

[0119] It is expected that the present invention will therefore provide for enhanced integration of P2X units in renewable energy power plants, providing further opportunities for energy storage and conversion and increased revenues from the power plant.

[0120] 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.

Claims

1. CLAIMS1. A power plant controller for a renewable energy power plant comprising one or more renewable energy generators and one or more Power-to-X units, each Power-to-X unit being configured to convert electric power to X from the one or more renewable energy generators and / or a power network to which the renewable energy power plant is connected, the power plant controller being configured to execute machine readable instructions to: receive a signal indicative of available active power for supply to the one or more Power-to-X units from the one or more renewable energy generators; compare the indicated available active power to a base power usage level for the one or more Power-to-X units; and determine and dispatch set points for reducing the electrical power usage of the one or more Power-to-X units to the base power usage level when the indicated available active power is less than the base power usage level, the electric power shortfall from the one or more renewable energy generators being provided to the one or more Power-to-X units from the connected power network.

2. A power plant controller according to claim 1 , wherein the base power usage level corresponds to a minimum power usage for maintaining the one or more Power-to-X units in a production state, converting electric power to X at a minimum production rate.

3. A power plant controller according to claim 1 , where the base power usage level corresponds to a minimum power usage for maintaining the one or more Power-to-X units in a production-ready state according to which the one or more Power-to-X units are configured to convert electric power to X, and switch to the production state, upon receiving set points greater than the base power usage level.

4. A power plant controller according to any preceding claim, wherein the power plant controller is further configured to determine and dispatch set points at a reference active power level when the indicated available active power is greater than or equal to the reference active power level.

5. A power plant controller according to claim 4, wherein the reference power level is a rated power usage of the one or more Power-to-X units.

6. A power plant controller according to claim 4 or claim 5, wherein the power plant controller is further configured to determine and dispatch set points for reducing the amountof electrical power converted to X from the reference active power level when the indicated available active power is less than the reference active power level, the set points being determined using a prescribed ramp rate limit.

7. A power plant controller according to claim 6, wherein the set points determined for reducing the amount of electrical power converted to X are greater than or equal to the indicated available active power, and wherein the electric power shortfall from the one or more renewable energy generators is provided to the one or more Power-to-X units from the connected power network when the determined set points exceed the indicated available active power.

8. A power plant controller according to any preceding claim, wherein the power plant controller is further configured to stop determining set points and dispatch a command for controlling the one or more Power-to-X units to switch to a standby state in dependence on detecting a state change condition, the state change condition being associated with a magnitude, or an accumulation, of the electric power shortfall.

9. A power plant controller according to claim 8, wherein the state change condition is detected in dependence on one or more of the following: the indicated available active power being less than a lower threshold power level, the lower threshold power level being less than the base power usage level; the indicated available active power being less than the lower threshold power level for a threshold period; and / or the indicated available active power being less than the base power usage level for a threshold period.

10. A renewable energy power plant connected to a power network, the renewable energy power plant comprising: one or more renewable energy generators; one or more Power-to-X units, each Power-to-X unit being configured to convert electric power to X; and a power plant controller according to any preceding claim.

11. A renewable energy power plant according to claim 10, wherein the one or more Power-to-X units comprise a power-to-gas unit configured to convert electric power from the power plant to gas.

12. A renewable energy power plant according to claim 10 or claim 11 , wherein the one or more renewable energy generators comprise: a wind turbine generator; and / or a photovoltaic generator.

13. A method of operating a renewable energy power plant comprising one or more renewable energy generators and one or more Power-to-X units, each Power-to-X unit being configured to convert electric power to X from the one or more renewable energy generators, and / or a power network to which the renewable energy power plant is connected, the method comprising: obtaining a signal indicative of available active power for supply to the one or more Power-to-X units from the one or more renewable energy generators; and comparing the indicated available active power to a base power usage level for the one or more Power-to-X units; and determining and dispatching set points for reducing the electrical power usage of the one or more Power-to-X units to the base power usage level when the indicated available active power is less than the base power usage level, the electric power shortfall from the one or more renewable energy generators being provided to the one or more Power-to-X units from the connected power network.

14. A computer program or a computer-readable medium comprising instructions which, when the program or the instructions is / are executed by a computer, cause the computer to carry out a method according to claim 13.

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