Patent Description:
In order to maximise their generating potential, it is becoming increasingly common for power producers to combine power storage systems and different types of renewable energy generator to form hybrid power plants. For example, a hybrid power plant might typically comprise wind turbine generators, photovoltaic generators, and battery storage systems. To interact with a power network, or power grid, the different types of generator within a hybrid power plant are operated as one, with a single point of interconnection (Pol) connecting the generators to the grid and with the plant as a whole being expected to comply with requirements of the grid specified by a grid operator.

A hybrid power plant controller (HPPC) is incorporated to ensure compliance with the grid requirements by controlling the generators and storage in accordance with instructions received from the grid operator or according to pre-programmed operation. The HPPC monitors electrical parameters of the grid and power plant, and acts to operate the power plant's reactive and active power outputs to achieve steady state control of voltage and frequency respectively. Conventional HPPCs, upon receipt of instructions from the grid operator, generate reference levels for active and/or reactive power to be met by the generators or storage systems in line with pre-set control methods.

However, while HPPCs are able to distribute reference levels to generators, the response differs between different types of generator. In particular, the rates of change of active and/or reactive power output of the generators are fixed at predetermined levels that are well within the capabilities of the generators. Consequently, it is likely that the full capability of each type of generator is under-utilized, and may possibly lead to non-compliance with grid requirements in some situations.

In extreme cases, the fixed response has the potential to cause problematic oscillations of grid voltage or to cause other instabilities of the grid.

Therefore, there exists a need for a control method for a hybrid power plant that utilizes the full capabilities of the available generators while ensuring compliance with grid requirements and stability of the grid. It is an aim of the present invention to address this need. Documents <CIT> and <CIT> are prior art examples.

<FIG> illustrates a typical architecture in which a hybrid power plant (HPP) is connected to a main transmission grid as part of a wider power network and comprises three sub-plants: a battery storage system, photovoltaic generators, and wind turbine generators. The same example set-up is used in <FIG>. As will be understood by the skilled reader, a HPP is a power plant that is capable of generating electrical energy from at least two different renewable energy sources. Thus, the examples shown in the figures are representative only and the skilled reader will appreciate that other specific architectures of HPPs are possible. For example, it is possible that more than three sub-plants may be incorporated into a HPP, or the HPP may comprise two sub-plants only.

Furthermore, it will be understood by the skilled reader that a sub-plant forming the HPP may be formed by a single generator. Therefore, as a sub-plant may comprise a single generator and a hybrid power plant requires two or more sub-plants, a hybrid power plant may be defined as a power plant incorporating at least two renewable energy generators, in which the power generated by the power plant is generated from at least two different sources of renewable energy.

The skilled reader will appreciate that methods, systems and techniques also described below may be applicable to many different configurations of power network. Moreover, the components of the hybrid power plant and power network are conventional and as such would be familiar to the skilled reader. It is expected that other known components may be incorporated in addition to or as alternatives to the components shown and described in <FIG> or <FIG>. Such changes would be within the capabilities of the skilled person.

Considering the figures in more detail, <FIG> shows a power network <NUM> incorporating a HPP <NUM>. The HPP <NUM> includes three sub-plants <NUM>: a solar power sub-plant <NUM>, a wind power sub-plant <NUM>, and a battery storage sub-plant <NUM>. The solar sub-plant <NUM> comprises a plurality of photovoltaic (PV) generators <NUM>, more commonly called PV cells, configured to convert solar energy into electrical energy. The wind sub-plant <NUM> comprises a plurality of wind turbine generators (WTGs) <NUM> configured to convert wind energy into electrical energy. The battery sub-plant <NUM> comprises at a plurality of electrochemical battery units <NUM>, lithium-ion storage units for example, operable to store and release electrical energy as required. Single WTGs <NUM>, PV cells <NUM>, or battery units <NUM> would also be possible in each of these sub-plants <NUM>, <NUM>, <NUM>. The electrical energy generated or released by each sub-plant <NUM>, <NUM>, <NUM> is transferred to a main transmission network or main grid <NUM>, as active current, for distribution.

For simplicity, battery units <NUM>, WTGs <NUM>, and PV cells <NUM> will be referred to collectively as 'generators' <NUM> hereafter, unless reference to an individual one of these is required, in which case it will be indicated which is being referred to. Using this definition, each sub-plant <NUM> may be considered to incorporate at least one generator <NUM>, each generator <NUM> generating power from a renewable energy source or from storage.

Each of the generators <NUM> within the sub-plants <NUM> of <FIG> is associated with a respective generator controller <NUM>. In some embodiments, a sub-set of generators <NUM>, <NUM>, <NUM>, such as those within the wind power sub-plant <NUM>, may share a single, semi-centralised controller, such that there are fewer generator controllers than generators. As would be apparent to the skilled person, generator controllers <NUM> can be considered to be computer systems capable of operating a PV cell <NUM>, WTG <NUM> and/or battery unit <NUM> in the manner prescribed herein, and may comprise multiple modules that control individual components of each generator <NUM>.

During normal operation of the HPP <NUM>, the generator controllers <NUM> operate to implement active and reactive current requests received from a hybrid power plant controller (HPPC) <NUM> at their respective generator(s) <NUM>. During extraordinary conditions, the generator controllers <NUM> operate to fulfil predetermined network requirements, and also act to protect the generators <NUM> from any potentially harmful conditions.

Within each sub-plant <NUM>, each of the generators <NUM> is connected to a local grid (not shown) that links the generators <NUM>. Each of the sub-plants <NUM> is, in turn, suitably connected to a collector bus <NUM> via a respective feeder line <NUM>. The collector bus <NUM> may be at a voltage level that is suitable for relatively short distance power transmission, for example in the region of <NUM> kV to <NUM> kV, most usually between <NUM> kV and <NUM> kV.

The collector bus <NUM> is connected to a medium voltage bus <NUM>, which in turn is connected to a main step-up transformer <NUM>. The collector bus <NUM>, medium voltage bus <NUM> and main step-up transformer <NUM> are connected by transmission lines <NUM>, <NUM>. The main transformer <NUM> is in turn connected to the main grid <NUM> at a Point of Interconnection (Pol) <NUM> by another transmission line <NUM>. The Pol <NUM> is an interface between the HPP <NUM> and the main grid <NUM> and comprises a Pol bus <NUM> whose nominal voltage level is higher than that of the collector and medium voltage buses <NUM>, <NUM>.

While the collector and medium voltage buses <NUM>, <NUM> may be required to span distances up to around <NUM>, the main grid <NUM> and Pol bus <NUM> may be an international, national, or regional grid such as the National Grid of Great Britain, for example, and therefore may be required to span distances of up to around <NUM> or more. Accordingly, the voltage level of the main grid <NUM> and the Pol bus <NUM> may be much higher than the voltage level of the collector and the medium voltage buses <NUM>, <NUM> for better transmission efficiency. As such, the main transmission grid <NUM> may comprise a plurality of substations and additional buses operating at different voltages as well as further transformers to increase the voltage for improved transfer of power. The transmission grid <NUM> shown in <FIG> includes at least one substation <NUM> and an associated feeder bus <NUM>, connected to the Pol bus <NUM> by a transmission line <NUM>.

The connecting lines such as the transmission and feeder lines <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may each include a protection system to protect individual components from damage during or following extreme conditions. For example, it is envisaged that at least an appropriate circuit breaker will be included in each line.

Hereinafter, it should be assumed that references to components being connected or connections between components comprise suitable feeder or transmission lines as described above unless it is otherwise indicated.

A Hybrid Power Plant Controller (HPPC) <NUM> is connected to the power network <NUM> at a Point of Measurement (PoM) <NUM> and is also connected directly to each of the sub-plants <NUM> of the HPP <NUM>. The role of the HPPC <NUM> is to act as a command and control interface between the HPP <NUM> and the grid <NUM>, and more specifically, between the sub-plants <NUM> and a grid operator or transmission system operator (TSO) <NUM>. The HPPC <NUM> is a suitable computer system for carrying out the controls and commands as described above and so incorporates a processor <NUM>, a connectivity module <NUM>, a memory module <NUM>, and a sensing module <NUM>. The HPPC <NUM> may also receive information regarding the grid <NUM> and/or the local buses <NUM>, <NUM>, <NUM>, <NUM>, substations <NUM> and networks from an energy management system (not shown).

The HPPC <NUM> is connected to the transmission line <NUM> between the main transformer <NUM> and the Pol bus <NUM> at the PoM <NUM> to allow monitoring and regulation of the output of the HPP <NUM> and to interpret the power demands correctly. The HPPC <NUM> is also connected to the medium voltage bus <NUM> to measure a variety of parameters that are representative of the state of the grid <NUM> and HPP <NUM>, and that can be used to improve the outputs of the HPP <NUM> to best meet the requirements of the TSO <NUM> or as set out in a set of grid-specific requirements.

The HPPC <NUM> is equipped to measure a variety of parameters including a representative power output that will be supplied to the main grid <NUM> at the Pol <NUM> by the HPP <NUM>. As the PoM <NUM> is not at the Pol <NUM>, the measured parameters are only representative as losses in the lines between the PoM <NUM> and Pol <NUM>, and between the PoM <NUM> and the HPPC <NUM>, may have an effect on the measurements. Suitable compensation may take place to account for the losses to ensure that the measurements are accurate.

The HPPC <NUM> measures parameters of the power output such as reactive and active power exchange between the HPP <NUM> and the main grid <NUM>, and the voltage level of the main grid <NUM>. The HPPC <NUM> compares the measured parameters against specific grid requirements and, in a suitable manner, communicates control commands to specific components of the HPP <NUM> accordingly. The sub-plants <NUM> of the HPP <NUM> are capable of altering their power output in reaction to commands received from the HPPC <NUM>. It will be noted that <FIG> is a schematic view, so the way in which the control commands are transferred is not depicted explicitly. However, it will be appreciated that suitable cabling may be provided to interconnect the HPPC <NUM> and the sub-plants <NUM>, generators <NUM> or generator controllers <NUM>. The interconnections may be direct or 'point to point' connections, or may be part of a local area network (LAN) operated under a suitable protocol (CAN-bus or Ethernet for example). Also, it should be appreciated that rather than using cabling, the control commands may be transmitted wirelessly over a suitable wireless network, for example operating under WiFi™ or ZigBee™ standards (IEEE802. <NUM> and <NUM>. <NUM> respectively).

As discussed above, the HPPC <NUM> manages the HPP <NUM> according to a set of grid requirements that are specific to the main grid <NUM>. Specific circumstances require different modes of operation. <FIG> illustrates the flow of information between components in <FIG> required to implement such modes of operation.

<FIG> illustrate embodiments of the network <NUM> that relate to active power generation, while <FIG> relates to a configuration for the reactive power control the within a HPPC according to an alternative embodiment. It will be appreciated that the invention relates to both active and reactive power generation and, therefore, aspects in the discussion below in relation to <FIG> are equally applicable to reactive power. Where specific differences apply between the application of the invention in respect of reactive power, these differences will be indicated accordingly. Otherwise, it should be assumed that where active power is referred to, the principles, methods and/or systems may be implemented in a similar manner for reactive power generation.

<FIG> shows the flow of information, such as measured parameters and operational commands, between the TSO <NUM>, PoM <NUM>, HPPC <NUM>, and sub-plants <NUM>, <NUM>, <NUM> of the HPP <NUM>. Modules <NUM>, <NUM> of the processor <NUM> of the HPPC <NUM> are also shown schematically in <FIG>. The structure and operation of the modules of the processor <NUM> shown in <FIG> will be discussed later in more detail in relation to <FIG>.

Initially focussing on <FIG>, the HPPC <NUM> receives a plurality of inputs relevant to the control of active power from the TSO <NUM>, the PoM <NUM>, and from each sub-plant <NUM>. In particular, the HPPC <NUM> receives an active power reference level, Pref, from the TSO <NUM>, a measured active power level, Pmeas, from the PoM <NUM>, and an available active power capacity of the generators <NUM> of each sub plant <NUM>, Pavailable, from the sub-plants <NUM>. In some embodiments, a reference level may alternatively be determined by the HPPC <NUM> by reference to predetermined requirements rather than being received from a TSO <NUM>. More generally, the HPPC <NUM> can be considered to determine a reference level, which may also be considered to be a target power output for the plant.

In addition, frequency requirements and measurements may also be received by the HPPC <NUM> to permit the HPPC <NUM> to control active power output according to frequency requirements as well, although these are not depicted here.

The HPPC's processor <NUM> comprises an active power calculation module <NUM> in communication with active power ramp rate determination modules <NUM>. The HPPC processor <NUM> will include a plurality of ramp rate determination modules <NUM>, each being specific to a sub-plant <NUM> of the HPP <NUM>. The HPPC processor <NUM> will also include a reactive power ramp rate determination module for each sub-plant <NUM> and a reactive power calculation module, whose operations are similar to the active power modules <NUM>, <NUM>. The reactive power modules are not shown in <FIG>.

Using the active power reference level, Pref, received by the HPPC <NUM> from the TSO <NUM>, the measured active power level, Pmeas, and the fed-back available active power levels from each of the sub-plants <NUM>, Pavailable, the HPPC <NUM> generates active power set points, Pset, for each sub-plant <NUM> at its active power calculation module <NUM>. While the reference level is intended to be a target power output for the HPP <NUM> to inject into the main grid <NUM>, the set points are considered to be target or destination power output levels for the sub-plants <NUM>, or for individual generators <NUM> to meet. The HPPC <NUM> varies the set points to demand active power from the sub-plants <NUM> according to their available capacity.

The HPPC <NUM> also determines a bespoke ramp rate for each sub-plant <NUM> using the ramp rate determination modules <NUM>, provided a trigger condition is enabled. Enablement of the trigger will be discussed later. The ramp rates are each labelled dP/dt in <FIG>. Each of the determined ramp rates and active power set points are then communicated to the relevant sub-plant <NUM> using the HPPC's connectivity module <NUM>, and the sub-plants <NUM> consequently operate, or are operated, to meet both the specified ramp rate and to change their active power output, Pout, towards their specified set point accordingly.

So, the HPPC <NUM> receives active power reference, or target, levels from the TSO <NUM>, active power measurements from the PoM <NUM>, and available active power capacity from each of the battery sub-plant <NUM>, the PV sub-plant <NUM>, and the WTG sub-plant <NUM>. Using these parameters, and possibly others including frequency measurements and set points, the HPPC <NUM> generates active power set points and ramp rates for each of the sub-plants <NUM>. To the WTG sub-plant <NUM>, an active power set point, Pset(WTG), and a ramp rate dP/dt(WTG) are communicated. To the PV sub-plant <NUM>, an active power set point, Pset(PV), and a ramp rate dP/dt(PV) are communicated. To the battery sub-plant <NUM>, an active power set point, Pset(battery), and a ramp rate dP/dt(battery) are communicated. Similarly, reactive power set points and ramp rates specific to each sub-plant <NUM> are communicated as required.

In turn, the WTG sub-plant <NUM> feeds its available active power, Pavailable(WTG), back to the HPPC <NUM>, and operates to output an active power level, Pout(WTG), which changes according to the received ramp rate, with the intention of converging upon the active power set point that was also communicated to it. Similarly, the PV sub-plant <NUM> and battery sub-plant <NUM> feed their available active power, Pavailable(PV)/Pavailable(battery) back to the HPPC <NUM>, and operate to output active power levels, Pout(PV)/Pout(battery), which change according to the received ramp rate, with the intention of converging upon the respective active power set point target levels that were also communicated to them.

The operation of the HPP <NUM> described above is in contrast to conventional, known operation of a hybrid power plant, in which a hybrid power plant controller receives an active power reference level from a TSO, as well as measured active power levels, and calculates a difference between the measured and reference levels. From the received available active power levels, an active power set point is calculated for each sub-plant to meet, and these set points are communicated to the sub-plants, which change their active power output levels according to a preset, non-variable ramp rate, such that their output level changes from the initial level to a destination level within a predictable time period.

Thus, the conventional power output from sub-plants of HPPs results in an under-utilisation of the available resources, and often a slower response than could actually be achieved. Conversely, where very slow responses are required, the conventional approach may result in an output that ramps too quickly. The bespoke ramp rates generated according to the present invention are suitable for the type of generator <NUM> in each sub-plant <NUM>, taking into account the operational characteristics that dictate how a generator <NUM> is able to change its output. These ramp rates are also able to take into account operational characteristics of the power network <NUM> and main grid <NUM> more generally so as to allow full compliance. Thus, implementing a variable and adaptive mix of ramp rates that depend upon the capability of the generator <NUM>, and on the requirements for the mode of operation of the HPP <NUM>, achieves a more efficient, suitable, and compliant HPP <NUM>.

Bespoke ramp rates are generated for each sub-plant <NUM> because of and based on the different attributes of the generators <NUM>, and particularly because of the different types of generator <NUM>, and because of differences in how the generators <NUM> extract power from their respective sources. For example, because a WTG <NUM> has many more moving parts than a PV cell <NUM> or battery unit <NUM>, relying upon a torque generated by wind energy, the speed with which a WTG <NUM> can increase or decrease its output power levels is typically slower than the speeds that are achievable using PV cells <NUM> or battery units <NUM>. The ramp rate capabilities of WTGs <NUM> may be affected by mechanical speed and/or drive train constraints, while the capability of a PV cell <NUM> may be dependent upon the available surface area exposed to solar radiation. Battery units may be reliant on their state of charge. Other differences between WTGs <NUM>, PV cells <NUM> and battery units <NUM>, or other renewable energy sources if available, may dictate the rate of change of power levels.

Considering the modules <NUM>, <NUM> of the HPPC <NUM> in slightly more detail, <FIG> schematically illustrates the active power calculation module <NUM> and one of the active power ramp rate determination modules <NUM>, specifically the module for a wind-powered sub-plant. The operational principles for the modules are similar for reactive power generation, and so the modules shown in <FIG> may also be used for reactive power adaptive ramp rate implementation, with appropriate changes such as including a voltage controller rather than a frequency controller. An alternative embodiment having a reactive power calculation module and reactive power ramp rate determination module is described later in relation to <FIG>.

In <FIG>, the active power calculation module <NUM>, which is considered to be known and so is only described briefly here, comprises a frequency controller <NUM> that receives frequency parameters such as set points and measurements, and distributes information to each of an active power set point selector <NUM>, a ΔP calculator <NUM>, and an operational mode selector <NUM>. The frequency controller <NUM> may distribute a difference in frequency between the measured frequency of the grid <NUM> and the frequency set point, indicating that a change in active power output from the HPP <NUM> is required to support the frequency of the grid <NUM>. Also included in this module <NUM> are a ramp rate limiter <NUM>, and a signal conditioning unit <NUM>.

Based on the output from the frequency controller <NUM>, the operational mode selector <NUM> provides input to the frequency controller <NUM> and ΔP calculator <NUM> to influence the required set points.

As discussed in relation to <FIG>, the active power calculation module <NUM> also receives inputs as active power parameters, and these are received at its active power set point selector <NUM>. The parameters are communicated to the ΔP calculator <NUM>, which calculates a difference between the required and measured active power levels, and communicates this back to the set point selector <NUM>, and to the ramp rate limiter <NUM>.

The set point selector <NUM> then generates set points for each sub-plant <NUM> based upon the calculated difference, the frequency controller's output and the received active power parameters. These are communicated to the signal conditioning unit <NUM> for dispatching to the sub-plants <NUM> along with a selected ramp rate.

In conventional power plants, where no ramp rate determination module <NUM> is present, the ramp rate limiter would act only to limit the ramp rate if particular operational criteria are exceeded.

In the present invention, the ramp rate limiter <NUM> receives a further input in addition to the difference between the reference and the measured active power level. This further input is received from the ramp rate determination module <NUM>, in the form of a selected, bespoke ramp rate or a command to use a preset ramp rate. Selected, bespoke ramp rates are selected based upon a variety of criteria, including attributes of the generator(s) <NUM> of each sub-plant <NUM>. Whether the bespoke or preset ramp rate is used is based on a trigger condition as will be described later.

Once the ramp rate limiter <NUM> has received inputs from the calculator <NUM> and ramp rate module <NUM>, it distributes either the bespoke ramp rate or the command to use the preset to a signal conditioning unit <NUM>. The ramp rate and set points received by the signal conditioning unit <NUM> are then dispatched to the individual sub-plants <NUM>, as shown in <FIG>. The signal conditioning unit <NUM> performs the function of generating signals that are in the correct format for implementation by the sub-plants <NUM>.

In particular, if a bespoke ramp rate is received, the limiter <NUM> is effectively bypassed or disabled such that the bespoke ramp rate passes directly through to the signal conditioning unit <NUM> before being output. On receiving a command that the preset ramp rate is to be used, the ramp rate limiter <NUM> may act to implement a limitation on the preset ramp rate if necessary.

The ramp rate for each sub-plant <NUM> is determined by its ramp rate determination module <NUM>, which is also shown in <FIG>. The ramp rate determination module <NUM> includes a ramp rate selector <NUM>, a ramp rate dispatcher <NUM>, a ramp rate calculator <NUM> and a trigger <NUM>. The ramp rate selector <NUM> distributes a ramp rate to the ramp rate limiter <NUM> as required, where the ramp rate that is distributed is dependent upon whether the trigger <NUM> is in an active state or not.

To put this another way, if a monitored criteria or condition is met at the trigger <NUM>, the ramp rate selector <NUM> is able to enable the use of an adaptive, determined ramp rate, provided to it by the ramp rate dispatcher <NUM>, and as calculated by the ramp rate calculator <NUM>. The 'enablement state' of the trigger <NUM> is effectively an instruction to utilise the calculated ramp rates from the ramp rate calculator <NUM>. On the other hand, if the trigger <NUM> adopts an 'inhibit state', such that the criteria or condition for adaptive ramp rate selection is not met, or if an inhibit condition is identified, the selector <NUM> effectively ignores the calculated adaptive ramp rate, and instead distributes a command to utilise the sub-plant's preset ramp rate, effectively performing an override function. The ramp rate selector <NUM> may alternatively distribute the preset ramp rate itself.

Thus, there are at least two choices of ramp rate available to the selector <NUM>: a determined ramp rate and a preset ramp rate. The determined ramp rate is determined by the calculator <NUM> based on at least an attribute of the generators <NUM>, and is adaptive such that it may be altered based on monitored parameters. The preset ramp rate is a value that is effectively hard-wired into the generator <NUM> or controller <NUM> prior to installation, or that, at the very least, is not determined in real-time according to measured parameters or attributes of the generators <NUM> as the determined rate is.

The ramp rate dispatcher <NUM> and calculator <NUM> operate to generate adaptive ramp rates and provide these to the selector <NUM> periodically. It will be appreciated that ramp rates may be provided only on request by the calculator <NUM> and dispatcher <NUM>, or that the calculation may be influenced by the operation of the trigger <NUM>.

The ramp rate calculator <NUM> utilises monitored attributes and calculates ramp rates accordingly. In one embodiment, the ramp rate calculator <NUM> may utilise a plurality of threshold values to identify a set of ramp rates that apply to the attribute of generator type, communicating the set of ramp rates to the dispatcher <NUM> and/or selector <NUM>, or even just a single ramp rate. Therefore, the ramp rate calculator <NUM> may utilise a look-up table, or a model of the power network <NUM> to determine appropriate ramp rates, or may use a predetermined logic to determine an operating mode, according to which the ramp rates are set.

As used herein, the 'type' of generator is defined by the source of the energy that is converted by generator into electrical energy. Types of generator may include wind turbine generators, battery units, or photovoltaic cells.

An example look-up table is shown below, in which three threshold values for rate of change of frequency are used with ramp rates being set for each type of generator <NUM>:.

It will be appreciated that each of the ramp rates provided in the above table are provided in per-unit format. As would be understood by the skilled person, per-unit active power is an expression of the active power with respect to a base value which is used as a reference. Similarly, per-unit reactive power, or per-unit voltage is an expression of the reactive power/voltage with respect to a reference base value. Using a per-unit system allows for normalization of values across transformers and other components that may change the value by an order of magnitude.

The function of the ramp rate dispatcher <NUM> is to communicate the calculated ramp rate to the selector <NUM>. The effective gain of the ramp rate dispatcher <NUM> is <NUM>.

Returning to <FIG>, the trigger's enablement state is entered if a criterion is met. One criterion for active power adaptive ramp rate selection to be triggered is a threshold value being reached for a short-circuit ratio, which may indicate that there is a weak grid interconnection between the HPP <NUM> and the main grid <NUM>. In weak grid interconnections, fast ramp rates are discouraged as they are likely to result in dangerous voltage oscillations.

A SCR for the network <NUM> can typically be determined for the network at the medium voltage bus <NUM>. The SCR can be calculated in real-time by measuring the voltage level change for a given reactive power change at the medium voltage bus <NUM> and the SCR is given as the ratio of this reactive power change to the voltage level change. These values are typically sampled over a short sampling window. The SCR may also be measured at other busses <NUM>, <NUM> downstream of the medium voltage bus <NUM>.

In some embodiments, the threshold SCR value for enabling selection of an adaptive ramp rate is <NUM>. In other embodiments, the threshold value is less than <NUM>. For example, the threshold may be a value between <NUM> and <NUM>. In exceptional circumstances, the threshold value may be lower than <NUM>.

An alternative trigger criterion may be the rate of change of measured frequency of the main grid meeting a predetermined threshold. High rate of change of frequency may require fast changes in active power, while low rates of change of frequency require slower changes in active power. A plurality of threshold values may be used to implement different ramp rates as necessary.

If the enablement criteria are not met, then the ramp rate selector <NUM> is inhibited from sending the adaptive ramp rates, instead sending a command to use predetermined rates. There may also be some inhibit conditions, which act to maintain the trigger <NUM> in the inhibit state even if the enablement criteria are met. In other words, inhibit conditions take precedence over trigger conditions. For example, for a wind sub-plant <NUM>, operation of WTGs <NUM> above a threshold speed may be used as an inhibit condition, or other mechanical constraints, while PV cells <NUM> of PV sub-plants <NUM> may be limited by thermal constraints and battery units <NUM> in battery sub-plants <NUM> by state of charge and/or thermal constraints. Other operational parameters may be used.

The process for distributing ramp rates and set points to the sub-plants is shown as a flow chart in <FIG>, and will now be discussed, whilst also referring back to <FIG> and <FIG>.

In the method <NUM>, the new Pmeas, Pavailable, and Pref values are received <NUM> by the HPPC <NUM>. The Pref values are received from either the TSO <NUM> or from an internal memory <NUM> that includes the grid requirements that the HPP <NUM> is required to meet. The difference between the Pref and Pmeas is determined <NUM> by the ΔP calculator <NUM> of the HPPC's active power calculation module <NUM>.

Based on the determined difference, and the Pavailable values for each sub-plant <NUM>, the HPPC generates <NUM> new Pset values for each of the sub-plants <NUM> to meet. The Pset values for each sub-plant <NUM> will together result in output to the grid <NUM> that meets the required, target level specified as Pref in the initial step <NUM> of the method <NUM>.

At the next step <NUM>, a query is performed, to determine whether the trigger condition is fulfilled. The fulfilment of the trigger condition is here intended to mean that the criteria for the enablement state to be entered is met, and that there are no inhibit conditions present that restrict the enablement of the ramp rate selector <NUM> to select a ramp rate provided by the ramp rate calculator <NUM>.

If the condition is not fulfilled, i.e. the enablement criterion is not met, or an inhibit condition is present, then the ramp rate selector <NUM> does not utilise the calculated adaptive ramp rate, instead selecting <NUM> that a predetermined ramp rate should be used and dispatching <NUM> this command to the signal conditioning unit <NUM> and, consequently, the sub-plants <NUM>. The generated active power set point for each sub-plant <NUM>, Pset, is also dispatched <NUM> to the sub-plants <NUM> along with this command.

If the condition is fulfilled, such that the trigger enters its enablement state, an adaptive ramp rate mode can be considered to have been entered <NUM>. The ramp rates for each sub-plant <NUM> are calculated <NUM> by the ramp rate calculator <NUM>, and the correct or appropriate ramp rate is selected <NUM> by the ramp rate selector <NUM>, i.e. the ramp rate calculated by the calculator <NUM>. This chosen ramp rate and the Pset values are then distributed <NUM> to the appropriate sub-plants, via the limiter <NUM> and signal conditioning unit <NUM>.

The trigger <NUM> acts to monitor the parameters by which it inhibits or enables operation of the selector <NUM>, either continuously or periodically, so that a new ramp rate can be distributed <NUM> when necessary.

While it is expected when the ramp rates and set points are initially dispatched that the target output for the sub-plants <NUM> will be reached, it is entirely possible that new grid conditions or new reference levels may require an alteration of the ramp rates or set points. In these circumstances, the ramp rate calculator <NUM> may generate new ramp rates to be selected by the selector.

<FIG> schematically illustrates an alternative embodiment for reactive power modules <NUM>, <NUM> of the processor <NUM>, these modules <NUM>, <NUM> being responsible for reactive power control. As shown in this figure, the processor <NUM> comprises a reactive power calculation module <NUM> and a reactive power ramp rate determination module <NUM> for each sub-plant <NUM>, only one of which is shown.

In this embodiment, the reactive power calculation module <NUM> comprises a reactive power generation controller <NUM>, capable of generating target set point outputs based upon received measurements, available generation capacity, and reference levels received from a TSO <NUM>. The set points, and general operation of the generation controller <NUM> may also be based upon an operational mode selected by the operational mode selector <NUM>.

The generation controller <NUM> communicates the relevant generated set points to the signal conditioning unit <NUM> for dispatching to the signal conditioning unit <NUM>. The reactive power generation controller <NUM> also communicates with a compensation equipment controller <NUM> to identify how much compensation equipment is required, and to prepare the equipment to compensate reactive power accordingly. The compensation equipment also dispatches set points and commands to the signal conditioning unit <NUM> for dispatch.

Moreover, the processor <NUM> also comprises reactive power ramp rate determination modules <NUM> for each sub-plant <NUM>. Each of these ramp rate determination modules <NUM> has a similar structure and make-up to the active power ramp rate determination modules <NUM> described above, comprising a trigger <NUM>, a ramp rate selector <NUM>, a ramp rate dispatcher <NUM>, and a ramp rate calculator <NUM>. The reactive power trigger's enablement criteria may be similar or the same as those used for the active power trigger.

The reactive power ramp rate determination module <NUM> may also incorporate a ramp rate limiter (not shown).

The selected ramp rates or the commands to use predetermined rates are communicated by the determination module to the signal conditioning unit <NUM>, and these are dispatched along with the set points to the individual generators <NUM> or sub-plants <NUM> accordingly.

The generators <NUM> of the sub-plant <NUM> are then operated to change their power output according to the ramp rate provided or the predetermined rates so as to achieve the targeted generator output indicated by the set points.

Claim 1:
A method of controlling a hybrid power plant (<NUM>) connected to a power network (<NUM>), the hybrid power plant having a first renewable energy generator (<NUM>) and a second renewable energy generator (<NUM>), the first renewable energy generator being configured to generate power using a different source of renewable energy to the second renewable energy generator, the method comprising:
determining (<NUM>-<NUM>) a target plant power output for the hybrid power plant;
determining (<NUM>) a target generator power output (Pset(WTG)) for each of the first and second generators based on the target plant power output;
the method being characterized by further comprising:
determining (<NUM>) a first ramp rate (dP/dt(WTG)) for a first renewable energy generator based, at least in part, on an attribute of the first renewable energy generator; and determining a first preset ramp rate for the first renewable energy generator that is different from the first ramp rate;
determining (<NUM>) a second ramp rate for the second renewable energy generator based, at least in part, on an attribute of the second renewable energy generator; and determining a second preset ramp rate for the second renewable energy generator that is different from the second ramp rate;
monitoring (<NUM>) at least one trigger condition, comprising comparing a short-circuit ratio of the power network with a predetermined threshold, or comparing rate of change of frequency level of the power network, wherein the trigger condition is fulfilled if the short-circuit ratio is below the predetermined threshold or if the rate of change of frequency exceeds the predetermined threshold,
operating the first renewable energy generator to change its power output to the power network to achieve the target generator power output by communicating (<NUM>,<NUM>) to the first renewable energy generator the respective target generator power output and i) the first ramp rate if the trigger condition is fulfilled, and ii) the first preset ramp rate if the trigger condition is not fulfilled; and
operating the second renewable energy generator to change its power output to the power network to achieve the target generator power output by communicating to the second renewable energy generator the respective target generator power output and i) the second ramp rate if the trigger condition is fulfilled and ii) the second preset ramp rate if the trigger condition is not fulfilled.