Off-grid microgrid

Abstract

A method of operating an off-grid microgrid (8), the microgrid (8) comprising at least one wind turbine (12), a conversion device (18) configured to produce fluid fuel from electrical power generated by the at least one wind turbine (12), and an energy storage device (16) The 5 method comprises operating the energy storage device (16) in an isochronous mode to absorb active power imbalances in the microgrid (10).

Description

[0001] OFF-GRID MICROGRID

[0002] Technical field

[0003] The invention relates to off-grid microgrids, in particular microgrids including wind turbine generators.

[0004] Background to the invention

[0005] In some instances, a group of wind turbine generators (WTGs) forming a wind power plant, or ‘wind park’, may be ‘off-grid’, in that there is no electrical connection between the power plant and an external power grid. This situation may be temporary, as for the case of a wind power plant that is configured to disconnect selectively from a power grid to operate in an island mode as a contingency measure. Alternatively, a power plant may be permanently off-grid and thus never electrically connected to a power grid. For example, a power plant may be installed in a remote location to exploit available wind resources, where it is impractical establish an electrical connection to a power grid.

[0006] Wind power plants that are off-grid, either temporarily or permanently, may be equipped with means to store the electrical energy that is generated by the WTGs, enabling power production to continue when a grid connection is disrupted, for example. Energy storage may take the form of an electrical storage device such as a battery electric storage system (BESS) that is incorporated into the power plant, for example. Another option is to provide means to convert the generated power into a fluid fuel, or chemical fuel, such as hydrogen, for example using an electrolyser, thereby producing ‘green hydrogen’. This approach may be particularly useful for a remotely-located power plant, in that it may be easier to transport hydrogen from the power plant to where it is needed than to transmit the generated electrical power directly to a power grid.

[0007] In a wind power plant, the WTGs may connect to energy storage devices and / or one or more loads through a plant bus to form a microgrid.

[0008] When a microgrid is connected to a large external network, the frequency on the plant bus is defined by the external network, and active power injection may be controlled based on the frequency and exchange of active power with the external grid. When disconnected from the external network and operating in an island mode so that the microgrid is ‘off-grid’, or if the microgrid is permanently off-grid, the frequency is determined by any imbalance between active power production and consumption.

[0009] It is against this background that the invention has been devised.

[0010] Summary of the invention

[0011] An aspect of the invention provides a method of operating an off-grid microgrid. The microgrid comprises at least one wind turbine, a conversion device configured to produce fluid fuel from electrical power generated by the at least one wind turbine, and an energy storage device. The method comprises operating the energy storage device in an isochronous mode to absorb active power imbalances in the microgrid.

[0012] By absorbing active power imbalances, the energy storage device can be utilised to maintain a stable grid frequency in the absence of a connection to an external grid, against variations in the power produced by the at least one wind turbine and the power consumed by the conversion device.

[0013] Operating the energy storage device in an isochronous mode may entail operating the energy storage device to inject and absorb active power to and from the microgrid at a frequency corresponding to the frequency of the microgrid, for example a frequency on a bus of the microgrid, thereby supporting the grid frequency.

[0014] The energy storage device may be operated as a grid-forming device. The energy storage device may be operated as a virtual synchronous machine. The microgrid frequency may be stabilised by operating the energy storage device to compensate for a difference between the power produced by the at least one wind turbine and the power consumed by the conversion device. The at least one wind turbine may be configured and / or operated as grid-following.

[0015] The method may comprise operating the energy storage device to provide load balancing for the conversion device.

[0016] The method may comprise measuring electrical power flowing into or from the energy storage device, and controlling delivery of power generated by the at least one wind turbine to the conversion device and to the energy storage device in accordance with the measured power into or from the energy storage device. The method may comprise varying power generation and / or power delivery to the conversion device to control a state of charge of the energy storage device.

[0017] The method may comprise operating the microgrid so that a power factor is substantially equal to one at a point of connection to the energy storage device, so that real power and apparent power substantially balance. The method may comprise operating the microgrid in a loadfollowing mode with respect to the conversion device. The method may comprise controlling power generation and / or power delivery within the microgrid so that an average power delivered to the energy storage device tends toward zero. These measures may help to allow the capacity of the energy storage device to be relatively low.

[0018] The energy storage device may be operated to compensate for a difference in respective ramp rates of the at least one wind turbine and the conversion device.

[0019] The energy storage device may comprise at least one of: a battery, such as a battery electric storage system; a supercapacitor; and a fuel cell, which may act in combination with a source of fluid fuel to provide energy storage.

[0020] The conversion device may comprise an electrolyser. The fluid fuel produced by the conversion device may be hydrogen. The microgrid may comprise multiple conversion devices, which may be of different types and / or have differing ramp rates.

[0021] The microgrid may comprise a connection to a remote power grid, in which case the microgrid may be off-grid when the connection to the power grid is disrupted. Alternatively, the microgrid may be permanently off-grid. In other words, the microgrid may be off-grid either temporarily or permanently. References in this specification to an ‘off-grid microgrid’ therefore include microgrids that are off-grid only for a portion of their operating time and are connected to an external grid at other times.

[0022] The method may comprise operating the energy storage device to balance reactive power in the microgrid. The energy storage device may be operated to stabilise voltage in the microgrid.

[0023] The invention also extends to a control system configured to perform the above method.

[0024] Another aspect of the invention provides a controller or control system for an off-grid microgrid. The microgrid comprises at least one wind turbine, a conversion device configured to produce fluid fuel from electrical power generated by the at least one wind turbine, and an energy storage device. The controller or control system is configured to operate the energy storage device in an isochronous mode to absorb active power imbalances in the microgrid. The controller may comprise an input configured to receive one or more signals indicative of active power imbalances in the microgrid. The controller may comprise a processor configured to process the received signals and to generate one or more control signals for controlling operation of the energy storage device. The controller may comprise an output configured to output the control signals.

[0025] The invention also extends to a microgrid comprising the controller or control system of the above aspects.

[0026] Another aspect of the invention provides an off-grid microgrid, comprising: at least one wind turbine; a conversion device configured to produce fluid fuel from electrical power generated by the at least one wind turbine; an energy storage device; and a control system configured to operate the energy storage device in an isochronous mode to absorb active power imbalances in the microgrid. The microgrid may comprise a bus to which the at least one wind turbine, the conversion device and the energy storage device are connected.

[0027] The microgrid may comprise a sensor configured to produce signals indicative of electrical power flowing into or from the energy storage device, in which case the control system may be configured to control delivery of power generated by the at least one wind turbine to the conversion device and to the energy storage device in accordance with signals received from the sensor.

[0028] In some embodiments, a capacity of the energy storage device does not exceed 20% of a total power production capability of the microgrid.

[0029] It will be appreciated that preferred and / or optional features of each aspect of the invention may be incorporated alone or in appropriate combination in the other aspects of the invention also.

[0030] Brief description of the drawings

[0031] So that it may be more fully understood, the invention will now be described, by way of example only, with reference to the remaining drawings, in which like features are assigned like reference numbers, and in which: Figure 1 shows a wind power plant defining a microgrid according to an embodiment of the invention;

[0032] Figure 2 shows schematically an architecture of a controller of the wind power plant of Figure 1 ; and

[0033] Figure 3 is a diagram representing control logic used by the controller of Figure 2.

[0034] Detailed description of embodiments of the invention

[0035] In general terms, embodiments of the invention relate to wind power plants (WPPs) defining off-grid microgrids that incorporate ‘P2X’ (power-to-X) capability, namely means to convert generated electrical power into a useful substance such as hydrogen, alongside one or more WTGs.

[0036] The microgrid may be off-grid temporarily, for example when a connection between the microgrid and an external power grid is disrupted so that the WPP operates in an island mode. Alternatively, a microgrid may be permanently off-grid, for example if the microgrid is in a remote location where establishing an electrical connection to a power grid is impractical. In either case, the microgrid is configured for stable operation and maximised production while off-grid.

[0037] Maintaining stability in an off-grid microgrid having P2X capability presents unique challenges. For example, electrolysers and other conversion devices that may be used to convert electrical power produced by the WTGs into hydrogen or another fluid fuel typically have an inherent inertia, in that an electrolyser has a defined ramp rate for moving between different levels of production. This ramp rate is typically significantly different to the ramp rate of the WTGs, creating an imbalance between the produced power and the power that the electrolyser is capable of utilising in certain operating scenarios. For example, the WTGs may respond more quickly than the electrolyser to a fast change in the energy available from the wind, leading to a transient power surplus or deficit while the electrolyser ramps between production levels. Such a power imbalance may destabilise the microgrid, in that the voltage and / or frequency may drift from the prescribed levels. If a ramp rate limit in the positive direction is implemented in the wind turbines, energy may be spilled. Other scenarios in which the respective ramp rates of the WTGs and the electrolyser may have an impact include at startup and energisation of the WPP, or following a fault. Other challenges presented by the inclusion of electrolysers, or other conversion devices, include a requirement for current-controlled operation as is typical for conventional electrolysers, which may complicate fault management. The presence of other uncontrolled auxiliary loads in the microgrid may also influence the power balance. More generally, connecting WTGs to a microgrid and starting production of varying power can be complicated in an off-grid environment.

[0038] To address these challenges, embodiments of the invention incorporate one or more energy storage devices, such as a BESS, into the microgrid, which can be used as a buffer between the WTGs and the electrolyser or other conversion device. The energy storage device can provide load balancing by acting dynamically to absorb and / or compensate for any transient difference between the power output by the WTGs and the present production capability of the electrolyser.

[0039] For example, in some embodiments this is achieved by operating the energy storage device in an isochronous mode to act as an isochronous frequency controller, so that the energy storage device absorbs and injects power into the microgrid at a fixed frequency corresponding to the operating frequency of the microgrid, thereby supporting a stable grid frequency. This may involve operating the device as a virtual synchronous machine (VSM), to act as a grid-forming device that balances active power, and optionally also reactive power, in the microgrid. Specifically, the energy storage device may be used as a slackbus, or ‘swingbus’, for the microgrid. In this way, the microgrid can be operated in a manner that maintains a stable frequency and voltage while maximising hydrogen production relative to the potential available energy from wind, avoiding a need to curtail power production. In other embodiments, the energy storage device may not be operated as a VSM and an alternative fast-response implementation may be used instead.

[0040] Operating the energy storage device in an isochronous mode as a grid-forming device facilitates the use of grid-following WTGs in the microgrid, by providing for a stable frequency and voltage in the microgrid. The ability to use grid-following WTGs, in turn, may be beneficial as the maturity of this technology minimises obstacles to designing and installing the microgrid.

[0041] In practice, operating an energy storage device in the manner described above may involve placing a sensor such as a power meter on a line through which the energy storage device is charged and discharged, to provide signals indicative of active and / or reactive power flowing into and out from the device. Charging and discharging of the energy storage device can then be managed, in part, based on those signals.

[0042] While the energy storage device could be relatively large, for example of a capacity matching the production capability of the microgrid, in some embodiments a control strategy is implemented that minimises demands placed on the energy storage device used as an isochronous frequency controller, allowing the capacity of the energy storage device to be minimised and thereby reducing the overall cost of installing and maintaining the microgrid. For example, the capacity of the energy storage device may be 20% or less of the total installed power production capability of the WPP. This may be achieved by implementing control that minimises active power exchange with the energy storage device and that controls its state of charge by dispatching a load, for example an electrolyser, and production, for example a wind power plant, in a manner that optimises utilisation of the production resource. The control strategy may involve operating the microgrid in a load-following mode with respect to the electrolyser, applying plant-level control to force, indirectly, the average power delivered to the energy storage device to zero in steady-state, and implementing plant control to maintain a power factor of approximately one, so that real power substantially matches apparent power, at a point of connection to the energy storage device.

[0043] Another measure that may be implemented for stability in the microgrid is to control a state- of-charge (SoC) of the energy storage device according to plant-level requirements. For example, this may be implemented by temporarily increasing power production and / or curtailing power consumption by the electrolyser relative to the levels entailed by other operational objectives, to enable the energy storage device to be charged or discharged as necessary to reach a target SoC. Altering the SoC of the energy storage device to a desired level may temporarily override the requirement to hold the average power delivered to the device at zero, and therefore allow a net flow of power into or out from the device. Once the desired SoC is reached, the control scheme can resume controlling charging and discharging the device in a manner that averages power to the device to zero.

[0044] By accommodating a delay between producing electrical power and converting that power into hydrogen or another substance, embodiments of the invention provide load-shifting that may allow for the microgrid to incorporate a group of WTGs having a combined power production capacity exceeding the capacity of one or more conversion devices of the microgrid, thereby further exploiting the available wind resources and enhancing overall production of hydrogen or other substance. An example of an off-grid microgrid 8 as described above is shown in Figure 1 , although it should be appreciated that this example is representative only and embodiments of the invention may be implemented in, or applicable to, other power plant and power network architectures.

[0045] More specifically, Figure 1 shows a microgrid 8 including a WPP 10 that includes a group of WTGs 12, also referred to as ‘wind turbines’, and a power plant controller (PPG) 14. Although a single WTG 12 is represented in Figure 1 for simplicity, in practice the group may include several WTGs 12. The microgrid 8 further includes an energy storage device in the form of a battery electric storage system (BESS) 16, and a primary load in the form of a hydrogen generation system 18. The WTGs 12, the BESS 16 and the hydrogen generation system 18 each connect to a common node defining a plant bus 20 of the microgrid 8.

[0046] The microgrid 8 also includes a connection line 22 through which the plant bus 20 is connectable, via a point of common coupling 24, to an external power grid 26. The connection line 22 includes a switch 28 that enables the plant bus 20 to be disconnected from the power grid 26 selectively. Opening the switch 28 disconnects the plant bus 20 from the power grid 26 and so places the microgrid 8 and its WPP 10 in an ‘off-grid’ state, in which the microgrid 8 is operated as an independent entity in an island mode. It is reiterated that other embodiments of the invention provide microgrids that are permanently off-grid, in that there is no connection to an external power grid at all.

[0047] The microgrid 8 further includes a reactive power compensation device 30, and one or more auxiliary loads and supplies represented generally at 32.

[0048] The reactive power compensation device 30 comprises one or more capacitors, inductors and resistors that can compensate for any imbalance in reactive power on the plant bus 20 as necessary, as is conventional. Accordingly, the reactive power compensation device 30 acts as a filter for the microgrid 10, providing a combination of reactive power balancing and harmonic filtration for the hydrogen generation system 18.

[0049] The auxiliary loads and supplies 32 may include, for example, an auxiliary plant supply through which electrical power may be delivered to the plant bus 20 to provide for an auxiliary demand within the microgrid 8, for example from the power grid 28, black start supplies such as a diesel generator and / or a biogas generator, and a methanol synthesis system. These examples are non-limiting and various other sources and loads may be included in a microgrid. The WTGs 12 are typical horizontal-axis wind turbines (HAWTs) in this example. Such WTGs are well known in the art and so shall not be described in detail here. In overview, each WTG 12 comprises a rotor 34 including a set of blades that turn around a generally horizontal axis in response to wind, to drive an electrical generator 36 of the WTG 12 and thereby convert wind energy into electrical power. The power output from the generator 36 is typically three- phase, and is conditioned by a two-stage AC-AC converter 38 and a step-up transformer 40 to the voltage required by the plant bus 20, which in this example is 33kV at 50Hz but may be different in other examples, for example 60Hz. From there, when the switch 28 of the connection line 22 is closed, at least some of the produced power is transferred to the power grid 26, as active power and / or current, for distribution.

[0050] The power produced by the WTGs 12 and delivered to the plant bus 20 can also be used to drive the loads connected to the plant bus 20, in particular the hydrogen generation system 18. Power can also be selectively stored in the BESS 16, by charging the BESS 16 from the plant bus 20. Conversely, the BESS 16 can be discharged selectively to deliver power to the plant bus 20 on demand.

[0051] The BESS 16 includes an AC-DC converter 42 and a battery stack 44, in this case of lithium- ion batteries. The BESS converter 42 is configured to process power received from the plant bus 20 into the form required by the battery stack 44 when charging, which in this example is 690V DC. The BESS converter 42 is further configured to process power discharged from the battery stack 44 into the form required by the plant bus 20 when discharging.

[0052] The hydrogen generation system 18 includes an electrolysis system and a rectifier 46, which can be a thyristor rectifier or an IGBT converter for example, that is configured to process power received from the plant bus 20 into the form required by the electrolysis system. The rectifier 46 is a line commutated device, hence the reactive power compensation device 30 is included to provide reactive power balancing and harmonic filtration. The hydrogen generation system 18 includes a transformer 47 that provides a required step-down in voltage from the plant bus 20.

[0053] The electrolysis system comprises an electrolysis cell stack, or ‘electrolyser’ 48, that consumes electrical power to produce hydrogen. The electrical power consumed by the electrolyser 48 is supplied by the rectifier 46, which operates in a current or power control mode. The electrolyser 48 is fed with an input water stream from an appropriate water source. That water source may supply fresh water, for example from storage tanks or from a pipe. Alternatively, a de-saliniser may be used to remove salts from seawater and thereby supply fresh water to the electrolyser 48.

[0054] The electrolyser 48 provides a hydrogen output stream into a fluid line 50, through which the hydrogen stream can be conveyed to a user 51 of the generated hydrogen. The user 51 may represent a distribution network, for example, or it may be a suitable storage capacity such as a set of tanks. The hydrogen generation system 18 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. It is noted that although the electrolyser 48 produces hydrogen in this embodiment, in other embodiments a WPP electrolyser may produce another fluid, such as methane for example.

[0055] In principle, any suitable type of electrolyser 48 may be used, the specification of which would be within the understanding of a skilled person. By way of example, the electrolyser 48 may be an alkaline electrolyser, a polymer-electrolyte membrane (PEM) electrolyser, or a solid- oxide electrolyser (SOEC), or a combination of these options.

[0056] It follows from the above that the microgrid 8 defines a ‘P2X’ system, in that it includes means for converting electrical power into a useful substance, in this case hydrogen and optionally methanol. Providing P2X capability enables the WPP 10 to continue to generate power while in an off-grid state, thereby increasing the overall production of the WPP 10.

[0057] P2X capability also enables the microgrid 8 to be permanently off-grid in other embodiments, in that the electrical power that is produced can all be converted to hydrogen, for example, which can then be taken to where the power is needed. For example, hydrogen may be conveyed through a fluid line or, if held in storage tanks, may be collected periodically.

[0058] In this embodiment, the PPC 14 is configured to control all functions of the WPP 10, including the BESS 16, the hydrogen generation system 18, the plant bus 20 and the auxiliary loads and supplies 32, and so acts as a control system for the overall microgrid defined by the WPP 10. In this respect, Figure 1 shows in dashed lines communication channels between the PPC 14 and the various parts of the microgrid 8.

[0059] In other embodiments, a separate microgrid controller may be provided to control the overall operation of the microgrid 8, while the PPC 14 controls operation of the WTGs 12 and energy storage device(s) only, and acts in response to demands received from the microgrid controller. Various other control arrangements are possible, including more distributed arrangements involving dedicated controllers for each part of the system.

[0060] Although not shown in Figure 1 , each of the WTGs 12 may be associated with a respective WTG controller, in which case each WTG controller operates its respective WTG 12 to implement active and reactive current, and / or power, requests received from the PPG 14. In other examples, a set of WTGs 12 may share a single, semi-centralised WTG controller, such that there are fewer WTG controllers than WTGs. It may also be possible for the PPG 14 to control operation of the WTGs 12 directly.

[0061] Similarly, Figure 1 shows that the hydrogen generation system 18 includes a P2X controller 49, which operates the rectifier 46 and the electrolyser 48 in response to commands received from the PPG 14. Specifically, the P2X controller 49 receives a reference value, Pref_P2x, for the power of the electrolyser 48, which corresponds to the rate at which hydrogen is produced, and applies current control to the rectifier 46 to control operation of the electrolyser 48.

[0062] As would be understood by the skilled person, the PPG 14, the P2X controller 49 and the WTG controllers may be regarded as computer systems capable of operating the WPP 10 and the microgrid 8 in the manner prescribed herein. Each may comprise multiple modules that control individual components of the microgrid 8, or just a single controller. The computer system may operate according to software downloaded via a communications network or programmed onto it from a computer-readable storage medium.

[0063] The PPG 14 is configured to act to ensure the stability of the microgrid 8 while in an off-grid state, in a manner that maximises production of power and the conversion of that power into hydrogen. In this embodiment, as noted above this involves adopting a control strategy that utilises the BESS 16 in an isochronous mode, specifically to operate as a virtual synchronous machine to act as a grid-forming device, in that the frequency on the plant bus 20 is held substantially constant by the governor control applied to the BESS 16, which therefore helps to control frequency by providing a virtual inertia response to transient changes. Meanwhile, automatic voltage regulator control is implemented to provide instantaneous responses to transient changes in the plant bus voltage.

[0064] In practice, operating the BESS 16 in an isochronous mode may entail operating the BESS converter 42 to inject and absorb active power to and from the plant bus 20 at a constant frequency corresponding to the plant bus frequency, which is 50Hz in this example, thereby supporting the frequency on the plant bus 20. Conversely, the provision of a steady grid frequency enables the WTGs 12 to be configured as grid-following, which may ease configuration of the microgrid 8 as the technology for gridfollowing WTGs is relatively mature.

[0065] More specifically, the BESS 16 is used as a slackbus for the microgrid 8, so that the BESS 16 acts to balance the active and reactive power in the plant bus 20. In this respect, in practical terms the BESS 16 is charged / discharged to compensate for transient active power imbalances at the plant bus 20 when the output from the WTGs 12 exceeds the capability of the electrolyser 48, for example due to a difference in respective ramp rates, and conversely is discharged to inject active and / or reactive power to the plant bus 20 to compensate for any shortfall in the power delivered by the WTGs 12 at other times, to balance transient active power and reactive power in the plant bus 20. In this way, the BESS 16 is operated to mitigate transient deviations in the microgrid power, thereby providing a stiff voltage and frequency in the plant bus 20 against varying wind conditions.

[0066] The use of the BESS 16 as a slackbus is implemented in a manner that minimises demands on the BESS 16 and thus enables the capacity of the BESS 16 to be minimised, in turn reducing the cost of the system, both in terms of procurement and maintenance. The specific control strategies used to achieve this are discussed below.

[0067] To implement this control, in this embodiment the PPG 14 receives an input from a sensor in the form of a power meter 52 that is connected to an input / output of the BESS 16, to provide signals indicative of the active and reactive power flowing into or out from the BESS 16. These signals are used as an input to the control of the microgrid 8.

[0068] Considering the architecture of the PPC 14 in more detail, in this example, the PPC 14 includes a dispatch module, or ‘dispatcher’, that is configured to generate and send dispatch signals to the WTG controllers and the P2X controller 49. For example, the WTG controllers may control their respective WTGs 12 according to commands or set points contained within the dispatch signals. More generally, as Figure 2 shows the PPC 16 may incorporate a processing module 54, a connectivity module 56, a memory module 58 and a sensing module 60. During normal operation, the connectivity module 56, the memory module 58, and / or the sensing module 60 are configured to provide the processing module 54 with one or more measurement signals comprising information that is indicative of various parameters for controlling the microgrid 8. For example, such measurement signals may comprise information that is indicative of: power levels, current levels and / or voltage levels of the individual WTGs 12, and / or the plant bus 20; and / or measurements indicative of the available power at the WTGs 12, such as a wind speed and / or a wind direction at each of the WTGs 12. The measurement signals also include the signals received from the power meter 52.

[0069] In general terms, the sensing module 60 may, for example, receive such measurement signals directly from one or more connected sensors (e.g. at the plant bus 20 and / or the WTGs 12), including the power meter 52, and communicate the information to the processing module 54. Alternatively, or additionally, the information may be determined by one or more systems that are connected to the connectivity module 56, such as the WTG controllers, and the information may be communicated, in turn, through the connectivity module 56 to the processing module 54. In each case, the determined information may also be stored permanently, or temporarily, in the memory module 58, from which it may be recalled, on demand, by the processing module 54. The PPG 14 may also receive information regarding the power grid 26 and / or local buses, substations and networks from an energy management system (not shown).

[0070] More specifically, and as Figure 2 shows, in this example the PPG 14 receives or derives at least the following inputs:

[0071] • AC power input to the transformer 47 of the hydrogen generation system 18, Pp2x_ac, taken at the position indicated in Figure 1 ;

[0072] • DC power output from the rectifier 46, Pp2x_dc, taken at the position indicated in Figure 1 ;

[0073] • active power on the plant bus 20, PBEss_meaS, as measured by the power meter 52;

[0074] • reactive power on the plant bus 20, QBEss_meaS, as measured by the power meter 52;

[0075] • estimated available power from wind, Pavaii_wrG, which is estimated by the PPC 14 based on wind readings;

[0076] • a power reference for the BESS 16 for controlling SoC, Pbess_soc-

[0077] The PPC 14 is configured to act so that the active power delivered to the BESS 16 averages to zero in steady-state. It is noted, however, that the active power delivered to the BESS 16 can be non-zero and either positive or negative over short periods to support control of the frequency on the plant bus 20 and / or to allow the SoC of the BESS 16 to be adjusted.

[0078] Using the above inputs, the PPC 14 generates at least the following outputs: • active power reference values for the WTGs 12, Pref w G;

[0079] • reactive power reference values for the WTGs 12, Qref_wrG;

[0080] • the reference value for the P2X controller, Pref_P2x; and

[0081] • a power reference value for the measured power at the power meter to achieve a target SoC for the BESS 16, Pref_soc.

[0082] The value of Pref_soc is adjusted for the purposes of maintaining the SoC of the BESS 16 at a defined level. This value acts as a control trim signal that acts to curtail the electrolyser load and / or to increase the power output by the WTGs 12 transiently, to adjust the BESS SoC and thereby provide flexibility in the use of the BESS 16.

[0083] The PPC 14 is configured to determine values for these outputs that have the effect of operating a control scheme, or algorithm, for maximising power production while maintaining microgrid stability when the microgrid is in an off-grid state.

[0084] In general terms, and as noted above, in this embodiment the plant frequency, namely the frequency on the plant bus 20, is regulated by operating the BESS 16 in an isochronous mode to balance a difference between the generated power and the power consumed by the loads. In principle, if the BESS 16 operates at a fixed frequency, the power flow will automatically reach a level where it compensates for any power imbalances in the microgrid 8. The generated power corresponds to the combined output of the WTGs 12, and may be measured at an output from the AC-AC converter 38 of each WTG 12 for example. The power consumed by the loads corresponds to the consumption of the electrolyser 48, as indicated by the value of Pp2x_ac, together with the consumption of the auxiliary loads 32 and thermal losses in the collector network. Differences in the generated power and the consumed power may arise when the output of the WTGs 12 changes following fluctuations in wind conditions, for example, or when the demand of any of the loads varies. The BESS 16 is responsive to any such changes to provide rapid compensation and thereby minimise any impact on the microgrid frequency and / or voltage.

[0085] By filling any transient gaps between generation and load to regulate frequency and voltage on the plant bus 20, the BESS 16 acts as a grid-forming device and as a buffer by effectively adjusting the ramp rate of the WTGs 12 to match that of the electrolyser 48.

[0086] To maximise the proportion of the energy produced by the WTGs 12 that is ultimately converted into hydrogen, a general control regime is applied that involves operating the BESS 16 to provide time-shifting, or load-shifting, so that power output by the WTGs 12 that cannot immediately be utilised by the electrolyser can be stored and then used later. In this way, the BESS 16 is utilised to avoid curtailing power production. Disregarding the effect of ramp rate limits, this regime may be summarised as follows:

[0087] • if Pp2X_max *• Pavaii_ TG - Paux+ioss, then the BESS 16 is charged;

[0088] • if Pp2x_max > Pavaii_wrG - Paux+ioss, then the BESS 16 is discharged; and

[0089] • if Pp2x_max = Pavaii_wrG - Paux+ioss, then the BESS 16 SoC is maintained.

[0090] In the above, Pp2x_max refers to the maximum load on the electrolyser 48, and PaUx+ioss refers to the total power consumed by the auxiliary loads 32 combined with losses in the WPP 10. This charging regime can be adapted to take into account a target SoC for the BESS 16, by deviating from the above to charge or discharge the BESS 16 as may be necessary to reach that target SoC. It follows that varying the target SoC of the BESS 16 provides a means for manipulating power flow within the microgrid 8.

[0091] This regime may also allow the total installed capacity of the WPP 10 to exceed the capacity of the electrolyser 48, thereby further exploiting the available wind resources. A further benefit is that using the BESS 16 in this way creates flexibility for the electrolyser load to be manipulated according to its operating state and / or ambient conditions, for example to reduce the power consumed by the electrolyser 48 when desirable.

[0092] Within this broad control regime, some specific measures are implemented to enable the capacity of the BESS 16 to be minimised. For example, despite its grid-forming role, the capacity of the BESS 16 may be no more than 20% of the total installed capacity of the WPP 10. These measures include manipulating the plant control to force the average power delivered to the BESS 16 to zero as noted above, by ensuring that the BESS 16 is charged and discharged to approximately equal extents in the medium term, whilst retaining the freedom to charge or discharge the BESS 16 as needed to support the frequency on the plant bus 20. Another measure that is implemented is to make the WPP 10 load-following so that the electrolyser load follows the power production, as set out in more detail below. In addition, as noted above the PPG 14 acts to provide a power factor of approximately one at the point of coupling to the BESS 16.

[0093] Figure 3 shows an example of part of the control logic that is applied by the PPG 14 to implement load-following control. This portion of the control logic primarily acts to determine values for Pref_wrG and Pref_P2x so that the load of the electrolyser 48 is determined to follow the production of the WTGs 12 whilst accounting for plant losses and the consumption of the auxiliary loads 32.

[0094] In practical terms, the electrolyser load is controlled with reference to the power controller of the rectifier 46, for example by controlling the firing angle in the case of a thyristor rectifier.

[0095] It follows from Figure 3 that the PPG 14 controls the measured power, PBEss_meas to the setpoint, Pref_soc. The setpoint can come from a state of charge controller 62 within a BESS power controller 64, which can be embedded in the BESS 16, in the PPG 14 or in another device. The controller can for example be a PI D controller.

[0096] The output of the BESS power controller 64, plus the available power from the WPP 10, is sent as a setpoint, Pref_P2x, to the electrolyser rectifier 46.

[0097] The setpoint to the WTGs 12, Pref_wrG, is the available power from the WTGs 12 minus the difference between the setpoint for the electrolyser rectifier 46 and the actual power delivered by the electrolyser 48, Pp2x_ac. In steady state, when the available power of the WTGs 12 does not exceed the maximum power of the electrolyser 48, the WTGs 12 will produce the available power, and the electrolyser 48 will adjust the consumption to balance the power of the microgrid 8. The WTGs 12 will reduce the power when the electrolyser 48 reaches a limit.

[0098] In other embodiments the microgrid 8 may include multiple electrolysers.

[0099] The control scheme also encompasses operating modes for specific scenarios. For example, at energisation of the WPP 10 the BESS 16 is used to establish the microgrid. This involves first ramping the voltage on the plant bus 20 and energising the various transformers and cables, synchronising the WTGs 12 and then commencing power delivery by discharging power as the electrolyser 48 and the auxiliary loads 32 are connected, to meet their demands before the WTGs 12 have reached full production. In this scenario, the PPG 14 determines a value for Pref_wrG that matches the instantaneous power discharged from the BESS 16, which corresponds to the value for PBEss_meas as measured by the power meter 52, which by convention is negative when the BESS 16 is discharging power. Accordingly, this may be expressed as Pref_wrG = - PBEss_meaS- The BESS 16 therefore acts to compensate for any initial shortfall in power while the WTGs 12 ramp up and thus aids with establishing the microgrid at start-up, until the WTGs 12 reach steady-state production. The skilled person will appreciate that modifications may be made to the specific embodiments described above without departing from the inventive concept as defined by the claims.

[0100] For example, although in the above example an energy storage device in the form of a BESS is used, other types of energy storage devices may be used in other embodiments, such as supercapacitors or fuel cells for example. It is also possible for a microgrid to incorporate multiple energy storage devices, which may optionally be of different types. If multiple storage devices are used collectively to stabilise the microgrid, internal power balancing between the devices may be implemented.

[0101] Also, although in the above example the BESS is used to balance both active and reactive power in the microgrid, in other embodiments the BESS may be operated to prioritise balancing of active power to support the microgrid frequency, while reactive power is supplied by the WTGs to support the microgrid voltage, for example.

Claims

Claims1. A method of operating an off-grid microgrid (8), the microgrid (8) comprising at least one wind turbine (12), a conversion device (18) configured to produce fluid fuel from electrical power generated by the at least one wind turbine (12), and an energy storage device (16), the method comprising operating the energy storage device (16) in an isochronous mode to absorb active power imbalances in the microgrid (10).

2. The method of claim 1 , comprising operating the energy storage device (16) to provide load balancing for the conversion device (18).

3. The method of any preceding claim, comprising measuring electrical power flowing into or from the energy storage device (16), and controlling delivery of power generated by the at least one wind turbine (12) to the conversion device (18) and to the energy storage device (16) in accordance with the measured power into or from the energy storage device (16).

4. The method of any preceding claim, comprising operating the energy storage device (16) as a grid-forming device.

5. The method of any preceding claim, comprising varying power generation and / or power delivery to the conversion device (18) to control a state of charge of the energy storage device (16).

6. The method of any preceding claim, comprising operating the microgrid (8) so that a power factor is substantially equal to one at a point of connection to the energy storage device (16).

7. The method of any preceding claim, comprising operating the energy storage device (16) to compensate for a difference in respective ramp rates of the at least one wind turbine (12) and the conversion device (18).

8. The method of any preceding claim, comprising stabilising frequency in the microgrid (8) by operating the energy storage device (16) to compensate for a difference between the power produced by the at least one wind turbine (12) and the power consumed by the conversion device (18).

9. The method of any preceding claim, comprising operating the energy storage device (16) as a virtual synchronous machine.

10. A control system (14, 49) configured to perform the method of any preceding claim.

11. An off-grid microgrid (8), comprising: at least one wind turbine (12); a conversion device (18) configured to produce fluid fuel from electrical power generated by the at least one wind turbine (12); an energy storage device (16); and a control system (14, 49) configured to operate the energy storage device (16) in an isochronous mode to absorb active power imbalances in the microgrid (8).

12. The microgrid (8) of claim 11 , comprising a sensor (52) configured to produce signals indicative of electrical power flowing into or from the energy storage device (16).

13. The microgrid (8) of claim 12, wherein the control system (14, 49) is configured to control delivery of power generated by the at least one wind turbine (12) to the conversion device (18) and to the energy storage device (16) in accordance with signals received from the sensor (52).

14. The microgrid (8) of any of claims 11 to 13, wherein a capacity of the energy storage device (16) does not exceed 20% of a total power production capability of the microgrid (8).

15. The microgrid of any of claims 11 to 14, comprising a bus (20) to which the at least one wind turbine (12), the conversion device (18) and the energy storage device (16) are connected.

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