Methods and systems for controlling a renewable energy power plant

Abstract

According to an aspect of the invention, there is provided a method of controlling a renewable energy power plant comprising a plurality of wind turbine generators, each wind turbine generator having a plurality of controllable systems that must satisfy respective production-ready conditions for that wind turbine generator to enter a production-ready state, the method comprising: for each wind turbine generator: receiving one or more sensor signals indicative of a current condition of each controllable system of that wind turbine generator; estimating, for each controllable system, a delay for that controllable system to satisfy the respective production-ready condition based on the current condition indicated for that controllable system; and estimating a start-up period for that wind turbine generator to enter the production-ready state based on the estimated delays for the controllable systems; and controlling a startup of at least one of the plurality of wind turbine generators based on the estimated startup periods.

Description

[0001] METHODS AND SYSTEMS FOR CONTROLLING A RENEWABLE ENERGY POWER PLANT

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to methods and systems for controlling a renewable energy power plant comprising a plurality of wind turbine generators.

[0004] BACKGROUND

[0005] Wind turbines or wind turbine generators are used to capture energy in the wind as it flows past them, and to generate electrical power from the captured energy, e.g. to be supplied to an electrical grid. Often, multiple wind turbines are located in relatively close proximity to one another in a geographical area, where such a group of wind turbines may be referred to collectively as forming a wind park or wind farm. A renewable energy power plant may comprise one or more such wind parks or wind farms.

[0006] During operation of the power plant, the wind turbines may be switched between various states of operation, including both power production and non-production states. For this purpose, each wind turbine includes a plurality of controllable systems, such as a set of auxiliary systems, that are controllable to maintain or alter the operational state of the wind turbine, as required. For example, the blade pitch systems may be operated to feather the turbine blades in a standby state and to adjust the pitch of the blades into the wind when switching to a production-ready state.

[0007] However, when an urgent need to switch operational states arises, for example in response to a change in the power demand, a variable delay is observed as the wind turbine switches to the production-ready state. The variable delay (which may be referred to as the start-up period) impairs the ability to accurately control the power production of the power plant.

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

[0009] SUMMARY OF THE INVENTION

[0010] According to an aspect of the invention, there is provided a method of controlling a renewable energy power plant comprising a plurality of wind turbine generators. Each wind turbine generator comprises a plurality of controllable systems that must satisfy respective production-ready conditions for that wind turbine generator to enter a production-ready state. The method comprises: for each wind turbine generator: receiving one or more sensor signals indicative of a current condition of each controllable system of that wind turbine generator; estimating, for each controllable system, a delay for that controllable system to satisfy the respective production-ready condition based on the current condition indicated for that controllable system; and estimating a start-up period for that wind turbine generator to enter the production-ready state based on the estimated delays for the controllable systems; and controlling a startup of at least one of the plurality of wind turbine generators based on the estimated startup periods.

[0011] In this manner, sensor signals are used to monitor the current condition of each controllable system and to estimate respective delays for each controllable system to satisfy respective conditions for power production. The respective delays are then assessed to give a best estimate of the start-up period for each wind turbine generator. The estimated startup periods can be used in a variety of ways to assess the impact of individual wind turbine generators on the production capability of the entire power plant, and to control startup of the wind turbine generators accordingly. It is envisaged that presently disclosed embodiments will therefore provide enhanced control of wind turbine generators and renewable energy power plants.

[0012] In an example, estimating the start-up period based on the estimated delays may comprise determining a maximum delay of the estimated delays and estimating the startup period as being equal to the maximum delay. Advantageously, this provides a relatively simple method for estimating the start-up period.

[0013] Optionally, a first controllable system of the plurality of controllable systems may be dependent on another controllable system of the plurality of controllable systems such that the production-ready condition of the first controllable system can only be satisfied once the production-ready condition of the other controllable system is satisfied. Advantageously, the method therefore accounts for the fact that the controllable systems may be complex and interrelated. Optionally, a first controllable system of the plurality of controllable systems may be dependent on one or more other controllable systems of the plurality of controllable systems such that the production-ready condition of the first controllable system can only be satisfied once the production-ready conditions of the one or more other controllable systems are satisfied. In an example, the method may comprise estimating the delay for the first controllable system to satisfy the respective production-ready condition based on the indicated current conditions of each of the first controllable system and the other controllable system.

[0014] Optionally, estimating the respective delay for the first controllable system to satisfy the respective production-ready condition may comprise: estimating a partial delay for the production-ready condition of the first controllable system to be satisfied based on the current condition indicated for the first controllable system; and combining the estimated delay for the production-ready condition of the other controllable system and the estimated partial delay for the first controllable system. Advantageously, the delay for the first controllable system may be estimated even if the first controllable system is not yet making progress towards satisfying its respective production-ready condition.

[0015] One or more of the controllable systems may be independently controllable systems. For each independently controllable system, the delay may be estimated independently of the respective production-ready conditions of any other controllable systems being satisfied. Advantageously, if the controllable system is independently controllable, then the delay for that controllable system to satisfy its production-ready condition can be estimated more simply.

[0016] In an example, the controllable systems may comprise: a yaw system; a pitch system; a rotor system; a heating system and / or a lubrication system. Advantageously, the controllable systems may comprise systems or subsystems that are necessary for the wind turbine generator to be able to produce electrical power. Optionally, the current conditions indicated for the controllable systems may comprise one or more of the following: a yaw rate; a yaw angle; a pitch rate; a pitch angle; a hydraulic pressure; a rotor speed; a rotor acceleration; and a lubrication fluid temperature; of that wind turbine generator. The current conditions may therefore be indicated by current values for one or more parameters of the controllable systems that influence or determine the ability of the wind turbine generator to produce power. In an example, the respective productionready conditions of the controllable systems may comprise one or more of the following: a target yaw angle; a target pitch angle; a target hydraulic pressure; a target rotor speed; and / or a target fluid temperature. The production-ready conditions may therefore relate to target values for respective parameters of the controllable systems that determine the ability of the wind turbine generator to produce power.

[0017] In an example, the first controllable system may take the form of the rotor system and the other controllable system may take the form of the yaw system or the pitch system. Optionally, the method may comprise estimating the delay for the rotor system to satisfy the respective production-ready condition based on: a yaw rate and yaw angle, and / or a pitch rate and a pitch angle, of each blade; a hydraulic pressure of each hydraulic pitch actuator; and / or a rotor speed and a rotor acceleration. Estimating the delay for the rotor system to satisfy the respective production-ready condition may comprise estimating a partial delay for the production-ready condition of the rotor system to be satisfied based on the rotor speed and the rotor acceleration; and combining the estimated partial delay for the rotor system with a maximum delay of the estimated delays for the yaw system and / or the pitch system.

[0018] In an example, the method may comprise: for each wind turbine generator: obtaining a target start-up period for that wind turbine generator; comparing the target start-up period to the estimated start-up period for that wind turbine generator; and controlling at least one of the controllable systems of that wind turbine generator to reduce a difference between the target start-up period and the estimated start-up period for that wind turbine generator based on the comparison.

[0019] Optionally, the method may comprise controlling the controllable system corresponding to the maximum delay of the estimated delays to reduce the difference between the target start-up period and the estimated start-up period for that wind turbine generator. The start-up period of the wind turbine generator may therefore be dependent on the maximum delay, and therefore the respective controllable system may be controlled to reduce the difference between the target start-up period and the estimated start-up period in a simple manner.

[0020] In an example, the method may comprise estimating an overall start-up period for the plurality of wind turbine generators to each enter the production-ready state based on the estimated start-up periods for each wind turbine generator. Advantageously, a period for the power plant to reach a state where all of the wind turbine generators are producing power may be estimated with increased accuracy.

[0021] Optionally, the method may comprise: for each wind turbine generator: obtaining a ramp rate for the active power output of each wind turbine generator; and forecasting an active power level for that wind turbine generator based on the obtained ramp rate and the estimated start-up period for that wind turbine generator. Advantageously, by using the estimated start-up period and the ramp rate, a forecast of an active power level of the wind turbine generator may be made more accurate. In an example, the method may comprise estimating a period for each wind turbine generator to reach a threshold active power level based on the respective forecast. Advantageously, a period for the power plant to reach a state where all of the wind turbine generators are producing power a threshold active power level may be estimated with increased accuracy.

[0022] Optionally, the method may comprise controlling a start-up of the renewable energy power plant based on the estimated period for each wind turbine generator to reach the threshold active power level and a target production time. Advantageously, the renewable energy power plant can be controlled to more accurately reach a desired active power level at a target time according to demand or available production conditions (e.g. weather).

[0023] In an example, the method may comprise forecasting an active power level for the plurality of wind turbine generators based on the forecast active power level for each wind turbine generator.

[0024] Optionally, the method may comprise estimating a period for the plurality of wind turbine generators to collectively reach a further threshold active power level based on the forecast for the plurality of wind turbines. Advantageously, by using the estimated startup periods, it can be more accurately estimated when the renewable energy power plant will reach a certain threshold active power level, for example a desired active power level according to demand or available production conditions (e.g. weather).

[0025] According to a further aspect of the invention, there is provided controller for a renewable energy power plant comprising a plurality of wind turbine generators, the controller being configured to execute machine readable instructions to perform any method described herein. The controller may, for example, take the form of a wind turbine generator controller or a power plant controller.

[0026] 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. 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.

[0027] BRIEF DESCRIPTION OF THE DRAWINGS

[0028] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0029] Figure 1 schematically shows a renewable energy power plant that includes a power plant controller;

[0030] Figure 2 shows a schematic illustration of an exemplary wind turbine in accordance with an embodiment of the invention;

[0031] Figure 3 shows an exemplary method of operating the renewable energy power plant of Figure 1 in accordance with an embodiment of the invention;

[0032] Figure 4 shows a graph illustrating example delays, following a startup request, for a plurality of controllable systems of the wind turbine, shown in Figure 2, in accordance with an embodiment of the invention;

[0033] Figure 5 shows a further exemplary method of operating the renewable energy power plant of Figure 1 in accordance with an embodiment of the invention; and

[0034] Figure 6 shows a graph illustrating active power levels, following a startup request, of the plurality of wind turbines of an exemplary renewable energy power plant, in accordance with an embodiment of the invention. DETAILED DESCRIPTION

[0035] Embodiments of the present invention relate to methods and systems for controlling a renewable energy power plant including a plurality of wind turbine generators. In particular, the methods and systems are provided for controlling startup of at least one of the wind turbine generators based on their estimated start up periods.

[0036] Here it shall be appreciated that the wind turbines may be operated in various states, including both production and non-production states, during operation of the power plant, and production delays are observed when a startup request is issued requiring a change of the operational state. The production delay corresponds to the time taken for various controllable systems of the wind turbine, such as a yaw system and / or a pitch system of the wind turbine, to enter a production-ready condition. For example, the nonproduction states may include a standby state and a production-ready state (i.e. a state wherein the wind turbine generator is ready to produce power, but is not yet producing power), amongst others. In the standby state, the controllable systems may be operated to minimise power consumption and / or to protect the wind turbine from damage. For example, the blades of the wind turbine may therefore be feathered out of the wind and the yaw system may direct the nacelle out of the prevailing wind direction. In order to enter a production-ready state, all of the individual controllable systems must satisfy respective production-ready conditions. For example, the yaw system must align the nacelle with the prevailing wind direction and the pitch system must pitch the turbine blades into the wind. Accordingly, when a startup request is issued, a production delay is observed while the controllable systems adjust the wind turbine to a production-ready state.

[0037] The production delay will vary depending on the state or condition of the controllable systems, when the startup request is received, and the variable delay affects the ability to accurately control the power output from the power plant. The methods and systems of the present invention are therefore configured to continuously or repeatedly estimate the time required for each wind turbine generator to switch from its current state to a production-ready state. Herein, this period shall be referred to as the ‘production-delay- time (PDT)’ or the ‘start-up period’ for that wind turbine generator.

[0038] For this purpose, sensors on each wind turbine provide signals indicative of the current condition of each controllable system, and one or more schemes, rules, or algorithms are used to estimate a delay for that controllable system to satisfy a respective production-ready condition based on the indicated condition. The estimated delays are then assessed to estimate an overall start-up period for each wind turbine generator, for example by identifying the controllable system that incurs the longest delay, and one or more startup commands are issued to the wind turbines on that basis.

[0039] In examples, the estimated start-up periods for the individual wind turbine generators may also be used to estimate the impact of each start-up period on the production capability of the entire power plant, and therefore more accurately forecast the production capability of the plant for enhanced startup control.

[0040] In this manner, it is envisaged that the presently disclosed embodiments will provide enhanced control of a plurality of wind turbine generators and the power production therefrom. The power plant may therefore be controlled to provide enhanced responsiveness to varying grid demands, taking proactive steps to ready the wind turbines for forecast changes in power demand.

[0041] Embodiments of the invention will now be discussed below with reference to Figures 1 to 6.

[0042] Figure 1 illustrates a typical architecture in which a renewable energy power plant 12 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 specific architectures are possible. In other examples, the power plant may include other renewable energy sources such as a solar power plant, a bio energy power plant, an ocean / wave / tidal energy plant, or a hybrid power plant having a combination of different types of renewable energy power plants. Thus, the invention relates to renewable energy power plants and renewable energy generators in general, rather than being specific to wind power plants and generators as in the Figures. The components of the wind power plant and power network are conventional and as such would be familiar to the skilled reader. It is expected that other known components may be incorporated in addition, or as alternatives, to the components shown and described in Figure 1. Such changes would be within the capabilities of the skilled person. Figure 1 shows a power system 10 comprising a renewable energy power plant 12. In this example, the WPP 12 includes a plurality of WTGs 14. Each of the plurality of WTGs 14 converts wind energy into electrical energy, which is transferred from the WPP 12 to a main power network, or ‘main grid’ 16, as active power and / or current, for distribution. It will be appreciated that while three wind turbines 14 are schematically shown in Figure 1 , any suitable number of wind turbines may be present in the WPP 12 in different examples.

[0043] Although not illustrated in Figure 1 , the WPP 12 also includes compensation equipment, such as a static synchronous compensator (STATCOM) or another type of synchronous compensator, configured to provide reactive power or reactive current support as required.

[0044] The WPP 12 also includes a connecting network 18 for connecting the WPP 12 to the main grid 16. Although not shown in Figure 1 , the WTGs 14 may be connected together in the WPP 12 by respective feeder lines I systems that connect to one or more substations or collection points of the WPP 12. Such substations or collection points may therefore from part of the connecting network 18, where the power outputs from the WTGs 14 are combined, and the connecting network 18 may further include one or more transmission lines connecting the substations I distribution points to the main grid 16. The power generated by each WTG 14 may therefore be transmitted, via the respective feeder system, to the one of the substation or distribution point(s) and, in turn, to the main grid 16, via the transmission line(s).

[0045] In this example, the WPP 12 and the main grid 16 are connected at a Point of Interconnection (Pol) 20, which is an interface between the WPP 12 (or the transmission line(s) thereof) and the main grid 16. The Pol 20 may also be referred to as the Point of Common Coupling, which may be abbreviated to ‘PCC’ or ‘PoCC’.

[0046] The WPP 12 further includes a power plant controller 22, referred to hereafter as PPC 22, for centralised control of the WTGs 14, and each of the WTGs 14 is associated with a respective local WTG controller 15. As will be understood by the skilled person, the WTG controllers 15 can be considered to be local control systems capable of operating a WTG 14 in the manner prescribed herein, and may comprise multiple control modules that control individual components of the WTG or just a single controller with multiple sub-modules. The computer system of the WTG controllers 15 may operate according to software downloaded via a communications network or programmed onto it from a computer-readable storage medium.

[0047] Although each WTG 14 is associated with a respective local WTG controller 15 in this example, such an arrangement is not intended to be limiting on the scope of the invention. In other implementations, a set of WTGs may share a single, semi-centralised WTG controller, for example, such that there are fewer WTG controllers than WTGs.

[0048] In any case, a bi-directional control network may be arranged between the PPG 22 and the WTG controllers 15 enabling two-way communication. For example, the uplink direction (i.e. the direction from the central PPG 22 to the local WTG controllers 15) is used to send reference values, e.g., for voltage, active power and / or reactive power, and start-up requests from the PPG 22 to the local WTG controllers 15. In turn, the WTG controllers 15 use the reference values to operate various controllable systems of the respective WTGs 14, and thereby control the WTGs 14 to satisfy the respective reference values. The downlink direction may be used by the WTGs 15 to return information about the current operational state of the respective WTGs 14 to the central PPG 22. For example, the returned information may include information about the current operational states or conditions of the controllable systems of the WTGs 14, along with information relating to the amount of reactive power currently produced and / or a local voltage level. The returned information may therefore include signals from one or more sensors that are indicative of the condition of each controllable system of the WTGs 14. Such signals may, for example indicate a pitch angle, pitch rate, yaw angle, yaw rate, rotor speed, and / or rotor acceleration of the WTGs 14, amongst others parameters, as shall be described in more detail. It shall be appreciated that 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).

[0049] The role of the PPC 22 is to provide centralised control of the WTGs 14 and to act as a command and control interface between the WPP 12 and the grid 16 (more specifically, a grid operator 26). For example, the grid operator 26 may be a transmission system operator (TSO) or a distribution system operator (DSO).

[0050] The PPC 22 is therefore configured to generate and send control signals to the WTG controllers 15. The control signals effectively control the operational states of the WTGs 14 and contain active and reactive current, and / or power, set points determined by the PPC 22 to provide frequency and voltage support to the main grid 16 based on measurements of the power characteristics of the WTGs 14, the WPP 12 and / or the main grid 16.

[0051] In turn, the WTG controllers 15 control the WTGs 14 according to the set points contained within the dispatch signals and switch the WTGs 14 between operation states accordingly, for example to reduce power consumption and / or protect the WTGs 14 when the power demand allows. In this manner, the WPP 12 is capable of altering its power or current output in reaction to set points received from the PPG 22.

[0052] The PPG 22 is a suitable computer system for carrying out the controls and commands as described herein and so may incorporate a processing module 28, a connectivity module 30, a memory module 32 and a sensing module 34, amongst others, as shown in Figure 1.

[0053] The WTGs 14 shall now be considered in more detail with additional reference to Figure 2, which schematically illustrates an exemplary embodiment of one of the WTGs 14.

[0054] The WTG 14 is shown to include a tower 42 supporting a nacelle 44 to which a rotor 46 is mounted. The rotor 46 is operatively coupled to a generator housed inside the nacelle 44. The rotor 46 features a plurality of wind turbine blades 48 that extend radially from a hub 50. In this example, the rotor 46 comprises three blades 48, although other configurations of the WTG 14, including any suitable number of blades and rotors are also possible. In addition to the generator, the nacelle 44 houses other components required for converting wind energy into electrical energy and various components needed to operate, control, and optimise the performance of the WTG 14.

[0055] The WTG 14 is also schematically shown to include a plurality of controllable systems 54 for controlling the operational state of the WTG 14 and a sensor system 56 for monitoring each controllable system 54. That is, the sensor system 56 may include one or more sensors for measuring parameters indicative of a current condition of each controllable system 54. The one or more sensors 56 may therefore be configured to output sensor signals indicative of the current condition of each controllable system 54 for use in estimating a delay to power production. It shall be appreciated that the controllable systems 54 may typically be situated in or around the nacelle 44 of the WTG 14, despite being schematically illustrated at the tower 42 in Figure 2. In embodiments, the controllable systems 54 may also be situated, partly or wholly, in any part of the tower 42 or elsewhere on the WTG 14 though, and the schematic illustrations are provided only as an example. Similarly, the sensors may be located in, or on, any suitable part of the wind turbine generator 14 for measuring a respective parameter indictive of the current conditions of the controllable systems 54.

[0056] The plurality of controllable systems 54 may relate to those sub-systems or auxiliary systems of WTG 14 that are controlled to selectively configure the WTG in one of a plurality of power production and non-production states according to set points or control signals issued from the PPG 22. For example, the controllable systems 54 may include a set of auxiliary systems that must satisfy respective production-ready conditions in order to switch the WTG 14 from a standby state to a production ready state. Accordingly, the sensor system 56 may include one or more sensors for monitoring respective parameters indicative of the condition of each controllable system 54, enabling calculation of the deviation of such parameters from respective target values for power production. The target values for satisfying the respective production-ready conditions, i.e. for enabling power production, may be pre-programmed and / or determined for the current operating conditions, for example according to one or more methods that are known in the art that may involve the use of one or more look-up tables.

[0057] To give an example, the controllable systems 54 of the WTG 14 may include a rotor system, a pitch system, a yaw system, and / or a heating system, as shall be described in more detail below.

[0058] The rotor system is configured to control the rotation of the rotor 46 and, for this purpose, may control a converter that controls a generator torque acting against the rotor rotation; and / or the pitch of the blades 48, via the pitch system; amongst other components. However, the WTG 14 is unable to generate power below a threshold rotor speed or target speed and, in the standby state, the rotor 46 may be stationary, and inactive, or otherwise rotating at a speed less than the target speed. Accordingly, when switching from the standby state to the production-ready state of the rotor system, there is a delay for the rotor 46 to reach the target speed and satisfy a corresponding production-ready condition of the rotor system. The sensor system may therefore include one or more sensors for measuring a speed of the rotor 46, and / or an acceleration of the rotor 46, amongst other parameters, thereby indicating the current condition of the rotor system for assessing the progress towards the production-ready condition.

[0059] The pitch system controls a pitch of one or more of the wind turbine blades 48, and may, for example, include one or more hydraulic actuators arranged to adjust the blade pitch angle (also known as blade pitch). That is, for each rotor blade 48 the pitch system may comprise a hydraulic cylinder for adjusting a pitch angle of the blade 48, and a pitch piston movable in the hydraulic cylinder for controlling the position of each blade 18 independently. The pitch system may be at least partly located in the rotor hub 50 of the WTG 14 and may be capable of pitching each blade 48 to any position in the range of, for instance, -90 degrees to +90 degrees. In the standby state, the pitch system may control the pitch of one or more of the wind turbine blades 48 to minimise the rotor torque generated by passing wind, for example to protect the WTG 14 from damage. For this purpose, the pitch system may be configured to lock the wind turbine blades 48 and / or to adjust the pitch angle of the blades 48 parallel to the wind. In contrast, for power production, the pitch system may control the blades 48 to respective target pitch angles, for optimum power generation. The target pitch angles may be determined based on operating conditions of the wind turbine, e.g. wind conditions in the vicinity of the WTG 14. Accordingly, when switching from the standby state to the production ready-state, the pitch system is configured to adjust the blade pitch towards the target pitch angle in order to satisfy a production-ready condition. The pitch system is typically configured to adjust blade pitch at a certain rate of change, or pitch rate. That is, the pitch system adjusts the blade pitch to the target pitch angle at a prescribed pitch rate. Typically, the pitch rate is a constant rate and is predetermined in a design phase of a wind turbine. In general, such a constant rate is set at a level that aims to ensure there is always sufficient hydraulic pressure available in the pitch system to adjust the blades to the target pitch angle. The sensor system 56 may therefore include one or more sensors for measuring a pitch angle of each blade 48, a pitch rate of each blade 48, and / or a hydraulic pressure of each hydraulic actuator. In this manner, the sensor system 56 may indicate the current condition of the pitch system for assessing the progress towards the production-ready condition. As will be understood, the pitch angle, pitch rate, and hydraulic pressure, may each be determined based on measurements taken by one of the sensors. Such a sensor may be conveniently arranged at the hydraulic actuator, for example where the pitch angle may be determined by measuring the actual position of the actuator, and pitch rate may be determined by measuring the rate of movement of the actuator.

[0060] The yaw system is configured to control a yaw offset, also referred to as yaw angle, yaw error or yaw misalignment of the WTG 14. The yaw offset is a difference between the incoming wind direction at the WTG 14 and a direction in which the nacelle 44 or rotor 46 of the wind turbine generator 14 is oriented. The yaw system may be arranged between the tower 42 and nacelle 44 and allows for rotational motion of the nacelle 44 (and attached components, including the rotor 46 and the blades 48) relative to the tower. In the standby state, the yaw system may control the yaw angle to direct the rotor 46 out of the wind. For example, the yaw system may control the yaw angle according to the wind direction to minimise the rotor torque generated by passing wind in the standby state. In contrast, for power production, the yaw system may control the yaw angle according to a target yaw angle for the WTG 14 (e.g. 0 degrees, i.e. facing the incoming wind to maximise energy capture). The yaw angle may therefore output a control signal to control a yaw drive mechanism of the yaw system to rotate the nacelle 44 relative to the tower 42 via the yaw bearing in accordance with the target yaw angle. Accordingly, when switching from the standby state to the production ready-state, the yaw system is configured to adjust the yaw angle towards the target yaw angle in order satisfy a production-ready condition. The sensor system 56 may therefore include one or more sensors for measuring a yaw angle of the nacelle 44 (relative to the tower 42 or the incoming wind angle), and / or a yaw rate of the nacelle 44 (relative to the tower 42 or the incoming wind angle). In this manner, the sensor system 56 may indicate the current condition of the yaw system for assessing the progress towards the productionready condition.

[0061] The heating system is configured to provide heating to the WTG 14, for example to facilitate optimum power production. For example, the heating system may be controlled in co-operation with a lubrication system, such as an oil system, to provide heating and lubrication to moving parts of the WTG 14 for effective power production. In a standby state, the heating system may be inactive and cooled to ambient temperature. However, when switching from the standby state to the production ready-state, the heating system may be configured to heat one or more parts, or subsystems, of the WTG 14 to a target temperature in order satisfy a production-ready condition. For example, the heating system may be configured to heat a fluid of a lubrication system to a target temperature for effective lubrication, e.g. to reduce the fluid viscosity for effective lubrication and reduction of frictional forces. The sensor system 56 may therefore include one or more temperature sensors for measuring a temperature of the heating system, or any heated components of the WTG 14, and thereby indicating the current condition of the heating system for assessing the progress towards the production-ready condition.

[0062] It will be understood that this example set of controllable systems 54 is not intended to be limiting on the scope of the invention though, and the WTGs 14 may include alternative or additional controllable systems 54 in other examples. For example, other controllable systems 54 may include a lubrication system, amongst other controllable systems required for power-production.

[0063] It shall also be appreciated that one or more of the controllable systems 54 may be dependent on other controllable system(s) 54 such that a first one of the controllable systems 54 may be unable, or otherwise not allowed, to make progress towards its respective production-ready condition until a second one of the controllable systems 54 has satisfied its respective production-ready condition. For example, the rotor system may be partly or wholly dependent on the pitch system and the yaw system, such that the rotor speed will not be increased to the target rotor speed until the pitch system has also adjusted the blades 48 to the target pitch angle and the yaw system has adjusted the yaw to the target yaw angle.

[0064] In any case, each of the controllable systems 54 must satisfy a respective production — ready condition in order for the respective WTG 14 to enter the production- ready state and start generating power.

[0065] In Figure 2, the controllable systems 54 are shown to be connected to and controlled by the WTG controller 15, which may be placed inside the nacelle 44, in the tower 42, or distributed at a number of locations inside (or externally to) the WTG 14 and communicatively connected to one another. The WTG controller 15 may therefore be considered to act as the command and communication interface between the controllable systems 54 and the PPG 22. For example, the WTG controller 15 may therefore be configured to receive the sensor signals from the sensor system 56 and communicate the signals to the PPG 22 that determines the set points and start-up requests.

[0066] However, when a startup request is issued to one of the WTGs 14, it shall be appreciated that a variable production delay is observed depending on the current conditions of the controllable systems 54. For example, the production delay may vary in dependence on the difference between the target rotor speed for power generation and the actual rotor speed of the WTG 14 when the startup request is received. The variable delay affects the ability to accurately control the power output from each WTG 14 and, in turn, the power output of the WPP 12.

[0067] To mitigate this issue, the PPG 22 may further include one or more controller(s) 100 for estimating the time required for each WTG 14 to switch from its current state to the production-ready state, i.e. for estimating the PDT or start-up period for each WTG 14. For example, the controller(s) 100 may be configured to estimate the startup period for each WTG 14 continuously, or periodically, e.g. at a prescribed frequency. The controller(s) 100 may be incorporated into the processing module 28 of the PPG 22, as shown in Figure 1 for example, however this arrangement is not intended to be limiting on the scope of the invention.

[0068] The controller(s) 100 are configured to receive sensor signal(s) from the sensor systems 56 of each WTG 14, indicating the current condition of each controllable system 54. The time taken for the controllable systems 54 to satisfy their respective production-ready conditions will vary according to their indicated conditions when the startup request is received, and the WTG 14 cannot enter the production-ready state until each controllable system 54 has satisfied the respective production ready condition. The controller(s) 100 are therefore further configured to estimate a delay for each controllable system 54 to satisfy the respective production ready condition based on the received sensor signals.

[0069] It shall be appreciated that the controller(s) 100 may include one or more schemes, rules, or algorithms for estimating the time taken for each controllable system 54 to transition from the current condition, indicated by the sensor signal(s), to the productionready condition. For example, the schemes, rules, or algorithms may be configured to compare sensed and target values for a respective parameter of each controllable system 54 and use one or more rate of change functions for estimating the time taken for the controllable system 54 to satisfy the target value. To give an example, the controller(s) 100 may receive a sensor signal indicative of the rotor speed of one WTG 14, determine a difference between the indicated rotor speed and a target rotor speed for power production, and estimate the time taken for the rotor of that WTG 14 to reach the target rotor speed using a function for the rate of change of the rotor speed. Alternatively or additionally, the schemes, rules, or algorithms may access one or more look-up tables or graphs that describe the rate of change, or time taken, for each controllable system 54 to reach the respective target value based on the sensed value.

[0070] Moreover, the controller(s) 100 may include suitable logic for recognising the dependency of the controllable systems 54, such as a first one of the controllable systems 54 being dependent on a second one of the controllable systems 54. The logic may be programmed into the schemes, rules, or algorithms, such that the controller 100 estimates the delay for the first controllable system additionally based on the estimated delay for the second controllable system. That is, the controller(s) 100 may be configured to estimate a partial delay for the first controllable system, substantially as described above, based on the difference between sensed and target values for generating power, and estimate the overall delay for the first controllable system by further adding the estimated delay for the second controllable system. In this manner, the controllers(s) 100 may be configured to estimate the delay for the first controllable system based on the indicated current conditions of both the first and second controllable systems. Moreover, where one of the controllable systems 54 is dependent on a set of controllable systems 54, the controller(s) 100 may be configured to estimate the overall delay for the dependent controllable system 54 based on the maximum delay from the set of controllable systems 54. For example, the controller(s) 100 may be configured to estimate the delay for the rotor system to satisfy the production-ready condition by adding a maximum one of the estimated delays for: (i) the pitch system, and (ii) the yaw system, to the partial delay for the rotor system.

[0071] The controller(s) 100 are further configured to estimate the overall PDT or start-up period for each WTG 14 based on the estimated delays for each controllable system 54 of that WTG 14. Here, it shall be appreciated that the WTG 14 can only enter the productionready state, and start producing power once each of the controllable systems 54 satisfy their respective production-ready conditions. The controller(s) 100 may therefore include one or more further schemes, rules, or algorithms, for this purpose. For example, the controller(s) 100 may be configured to determine a maximum delay of the estimated delays and estimate the start-up period as equal to the maximum delay. In other examples, other functions may be used for estimating the overall startup period based on the estimated delays of the individual controllable systems 54.

[0072] The PPG 22 is configured to control startup of one or more of the WTGs 14 based on the estimated startup periods. The PPG 22 may include various control strategies for analysing the estimated startup periods and determining one or more WTGs 14 to control. It shall be appreciated that the PPG 22 may send the start-up requests to one or more of the WTGs 14, via the respective WTG controllers 15, for example along with active or reactive power set points.

[0073] Following startup, the WTG 14 enters a power-production state and the WTG controller 15 controls the WTG 14 to ramp up the active power output towards a target active power output, provided by active power set points issued from the PPG 22. During this period, the active power output of the WTG 14 increases at a predetermined ramp rate or in accordance with a predetermined ramp rate limit. The active power output of the WTG 14 therefore gradually increases towards the target active power output, and a further delay is therefore observed while the WTG 14 approaches the active power set points issued from the PPG 22.

[0074] In examples, in order to further enhance the control of the WTGs 14, the controller(s) 100 may therefore be further configured to estimate a period for each WTG 14 to reach a threshold active power level, which may correspond to a rated power level of the WTG 14 or an active power level corresponding to the active power set points. In particular, the controller(s) 100 may include one or more schemes, rules, or algorithms, for forecasting the active power output of each WTG 14 or otherwise estimating a time taken for each WTG to reach the threshold power level. For this purpose, the schemes, rules, or algorithms may use a prescribed ramp rate or ramp rate limit for each WTG 14. For example, the PPG 22 may store a look-up table comprising pre-determined ramp rates for respective differences between the current active power level of the wind turbine generator 14 and a target active power output of the wind turbine generator 14. The controller(s) 100 may be configured to obtain one of the predetermined ramp rates and / or ramp rate limits from the look-up table on this basis, and use the ramp rate I ramp rate limit to estimate the active power output of each WTG 14. In this manner, the controller(s) 100 may determine a time-varying forecast of the active power output from each WTG 14.

[0075] It shall be appreciated that, although the controller(s) 100 have been described above as being implemented at the PPG 22, the controller(s) 100 may be implemented at the WTG controllers 15 or a separate, standalone, system in other examples.

[0076] A method of controlling a renewable energy power plant in accordance with embodiments of the invention shall now be discussed with additional reference to Figures 3 and 4.

[0077] Figure 3 schematically illustrates a method 300 of controlling a renewable energy power plant, such as the WPP 12, comprising a plurality of WTGs in accordance with an embodiment of the invention. While the method 300 is described as being performed by the PPG 22 in the following, it shall be appreciated that this is not intended to be limiting on the scope of the invention and any individual steps or sub-steps of the method may instead be performed by the WTG controllers 15, for example. The method 300 involves steps 301 to 303 for estimating the startup period for each WTG 14. In the following description, the method steps 301 to 303 are therefore described for estimating the startup period of one of the WTGs 14, referred to as ‘the WTG 14’, though it shall be appreciated that the steps 301 to 303 are repeated for each of the WTGs 14 in an identical manner.

[0078] In step 301 , the method 300 involves receiving one or more sensor signals indicative of a current condition of each controllable system 54 of the WTG 14. The sensor signals may be determined by the sensor system 56 and received at the PPG 22, for example via the WTG controller 15. Here it shall be appreciated that the sensor signals may be determined by the sensor system 56, substantially as described in relation to Figure 2, and the sensor signals may therefore include measured parameters indicative of the current condition of each controllable system 54.

[0079] To continue the previous example, the received sensor signals may therefore include sensor signals indicative of the yaw rate and / or yaw angle of the nacelle 44, the pitch rate and / or pitch angle of each blade 48, the hydraulic pressure of each hydraulic pitch actuator, the rotor speed and / or rotor acceleration of the rotor 46, and / or the temperature(s) sensed by the temperature sensor(s).

[0080] In step 302, the method 300 involves estimating a delay for each controllable system 54 of the WTG 14 to satisfy a respective production-ready condition based on the current condition indicated for that controllable system 54.

[0081] For this purpose, the PPG 22 may be configured to estimate the delay for each controllable system 54 to satisfy the respective production-ready condition using the schemes, rules, or algorithms, of the controller(s) 100. In particular, the PPG 22 may be configured to determine the difference between the target values for satisfying the respective production-ready conditions of each controllable system 54 and the respective values indicated by the received sensor signals. For each controllable system 54, the determined difference may then be used to estimate the delay using a rate of change function and / or by accessing a look-up table or graph defining the rate of change I time taken according to the determined difference.

[0082] It will be understood that the specifics of estimating the delay for one of the controllable systems 54 to satisfy the respective production-ready condition will be dependent on which controllable system 54 the delay is being estimated for. For example, for the pitch system, the PPC 22 may be configured to estimate the delay for the pitch angle of each blade 48 to reach the respective target pitch angle. For this purpose, the PPC 22 may compare the pitch angle measurements received from the sensor system to the respective target pitch angles and estimate the delay for each blade 48 using a respective pitch rate function to calculate how long it will take the blade 48 to move from the current pitch angle to the target pitch angle.

[0083] Figure 4 is a graph 400 illustrating example delays for a plurality of controllable systems 54. Specifically, to continue the previous example, the graph 400 shows example delay times in seconds for the heating system, the yaw system, the pitch system and the rotor system.

[0084] Each of the respective delays begin at 0 seconds, i.e. the current time, indicated by in the graph 400. This point represents the start time for the delay if a start-up request were to be received for the WTG 14. When the start-up is requested, each of the controllable systems 54 may be in a standby state, a production-ready state, or another non-production-ready state. In this example, each of the controllable systems 54 is configured in a standby state when the startup request is received.

[0085] Each dashed line in Figure 4 represents the time during which that controllable system 54 is making progress towards the respective production-ready condition. The time at which the delay finishes, i.e. the time at which the controllable system 54 satisfies the production ready condition, is shown as a in the graph 400.

[0086] However, as noted previously, some of the controllable systems 54 may be dependent on other controllable system(s) 54 and unable to make progress towards the respective production-ready condition until the other controllable system(s) 54 have satisfied their respective production-ready condition(s). In relation to such dependent controllable systems 54, the PPC 22 includes suitable logic for recognising the dependency of the controllable systems 54 and estimates the delay for the dependent controllable system 54 additionally based on the estimated delay(s) for those other controllable system(s) upon which it depends.

[0087] In Figure 4, the dependency is illustrated by the absence of a dashed line, which indicates that progress is not being made towards meeting the production-ready condition or entering the production-ready state. For example, as shown in the graph 400, the rotor system does not start making progress towards satisfying the respective production ready condition until the pitch system and the yaw system have each satisfied their respective production ready conditions. In this case, this is because the rotor system is dependent on the pitch system and the yaw system. That is, the rotor speed does not increase to a speed suitable for generating power (i.e. the target rotor speed) until the blades 48 are pitched at the target pitch angle, and the nacelle 44 is yawed to the target yaw angle.

[0088] The estimated delays therefore include time during which the controllable systems 54 are making progress towards satisfying the respective production-ready conditions, and also include time when no progress is being made towards satisfying the respective production-ready conditions (the latter being indicated by the absence of the dashed line).

[0089] For example, as shown in the graph 400 of Figure 4, the PPC 22 may be configured to estimate the delay for the rotor system to satisfy the respective production-ready condition based on the maximum delay of the yaw system and the pitch system, in addition to the partial delay for the rotor system to subsequently reach the target rotor speed. The PPC 22 may therefore estimate the rotor system delay based on the sensor signals that indicate the yaw rate and the yaw angle of the nacelle 44, the pitch rate and the pitch angle of each blade 48, and the rotor speed and the rotor acceleration of the rotor 46. Advantageously, estimating the delay for the rotor system may take into account the complexity of the controllable systems, and the interplay between the rotor system, the yaw system, and the pitch system.

[0090] Meanwhile other controllable systems 54 may be independently controllable. For example, as shown in the graph 400 of Figure 4, the yaw system, the pitch system, and the heating system are independently controllable systems. The PPC 22 is configured to estimate the delay for these controllable systems 54 independently of the respective production-ready conditions of any other controllable systems 54 being satisfied.

[0091] Returning to Figure 3, in step 303, the method 300 involves estimating a start-up period for the WTG 14 to enter the production-ready state based on the estimated delays for the controllable systems 54. For example, the PPC 22 may estimate the delay for each controllable system 54 to satisfy the respective production-ready condition, in step 302, and apply one or more rules, functions, or algorithms for estimating an overall startup period for the WTG 14 based thereon. For example, the PPC 22 may be configured to determine a maximum delay of the estimated delays and estimate the start-up period as being equal to the maximum delay. That is to say, the start-up period may be estimated as being equal to the longest delay of the controllable systems 54 to satisfy their respective productionready condition. Advantageously, estimating the start-up period may therefore be dependent on the maximum delay, and the estimation is therefore simplified.

[0092] For example, Figure 4 further illustrates the estimated start-up period for the WTG 14 (labelled as “Total for WTG”). Here, the dashed line in Figure 4 represents the time during which the WTG 14 is making progress towards entering the production-ready state, and entering the production-ready state is shown as a in the graph 400. As can be seen from the graph 400, the start-up period for the wind turbine generator 14 in this example is equal to the maximum delay of the controllable systems 54, which corresponds to the delay of the rotor system determined in step 302.

[0093] Returning to Figure 3, once the start-up period has been estimated for each WTG 14, according to steps 301 to 303, the method 300 further includes controlling a start-up of at least one of the WTGs 14 based on the estimated start-up periods, in step 304.

[0094] Here it shall be appreciated that, having estimated the start-up period for each WTG 14, such estimates may be used in a variety of control strategies to improve the control of the power output from the WTGs 14 and the WPP 12. For example, the estimated startup periods may be compared to a threshold or target start-up period and the PPC 22 may issue a start-up command to respective WTGs 14 based on the comparison to ensure that the WPP 12 is able to satisfy the target start-up period.

[0095] The target start-up period may, for example, depend on conditions such as a date, time, power demand of the main grid 16, and / or wind conditions, for example. For example, the target start-up period may be determined based on a target power production over a particular time period. The target power production may be determined based on, for example, a power demand (i.e. and amount of power desired by a power plant operator or grid operator) or a weather forecast. Therefore, if the demand is high, or the weather forecast is good for power production, then a relatively short target start-up period may be chosen. If demand is lower, or the weather forecast is bad for power production, then a relatively long target start-up period may be chosen (i.e. relatively longer than the short target start-up period).

[0096] In this manner, the method 300 monitors the controllable systems 54 of each WTG 14 to estimate the responsiveness of each WTG 14 to a startup demand, and controls the startup of the WTGs 14 according for enhanced control of the power output.

[0097] Figure 5 shows a further exemplary method 500 of controlling a renewable energy power plant, such as the WPP 12, comprising a plurality of WTGs, in accordance with an embodiment of the invention. Again, while the method 500 is described as being performed by the PPG 22 in the following, it shall be appreciated that this is not intended to be limiting on the scope of the invention and any individual steps or sub-steps of the method may instead be performed by the WTG controllers 15, for example.

[0098] The method 500 initially proceeds by estimating the startup period of each of the WTGs 14 according to steps 301 , 302, and 303 of the previous method 300. Thereafter, the method 500 proceeds to estimate a period for each WTG 14 to reach a threshold active power level. For example, the PPG 22 may receive, obtain, or otherwise determine a target active power level for each WTG 14 and estimate the time taken for each WTG 14 to reach that target active power level. The target active power level may, for example, be determined based on forecast power demands on weather conditions.

[0099] In step 501 , the method 500 therefore involves obtaining a ramp rate for the active power output of each WTG 14. For example, the PPG 22 may be preprogrammed to associate each WTG 14 with a respective ramp rate or ramp rate limit, or otherwise access a lookup table comprising pre-determined ramp rates or ramp rate limits for respective values of the current and / or target active power levels of the WTG 14 (or the difference between the current and target active power levels).

[0100] In step 502, the method involves forecasting the active power level for each WTG 14 based on the obtained ramp rate and the start-up period estimated for that WTG 14, in step 303. That is, the PPG 22 may estimate the active power level of each WTG 14 after a prescribed period has elapsed from a startup request, or otherwise estimate the active power level of each WTG 14 as a time-varying forecast, using the estimated startup period and the obtained ramp rate. Although not shown, in examples the method 500 may also include the exemplary substeps of forecasting the combined active power level for the plurality of WTGs 14 based on the forecast active power level for each WTG 14, and / or estimating a period for the WTGs 14 to collectively reach a further threshold active power level (i.e. a period for the combined active power level to reach a respective threshold). Here it shall be appreciated that the combined active power level for the plurality of WTGs 14 may be determined by summing together the time-varying forecasts determined in step 502.

[0101] By way of example, Figure 6 is a graph 600 illustrating the forecast active power level for a plurality of WTGs 14. In this example, the WPP 12 includes five WTGs 14 and the graph 600 shows an estimated active power production for each WTG 14 (in kW) over time (in seconds). Additionally, the graph 600 shows an estimated total active power production for the WPP 12 (in kW) over time (in seconds), which is shown to combine the estimated power production for the five WTGs 14. The estimated power production for each WTG 14 is labelled as “WTGi P_Production”, where i is an integer between 1 and 5. The estimated total power production for the WPP 12, i.e. the cumulative estimated power production of the five WTGs 14, is labelled as “P_Production Total”. The graph 600 also shows a target active power production for the WPP 12, which is labelled as “P_Target”.

[0102] As can be seen from the graph 600, the WTGs 14 enter into their respective productionready states (i.e. begin actively producing power) at different times. Furthermore, each of the WTGs 14 has a ramp period, during which the active power output of the WTG 14 is increasing, before reaching a peak predicted active power output. These two factors create a non-linear increase of the WPP 12 power production, as varied numbers of the WTG 14 start-up at different times, and may ramp up at different rates.

[0103] In this manner, the method 500 is able to forecast or estimate when the WPP 12 will reach a threshold active power level, following a startup request, enabling enhanced control of the power output from the WPP 12.

[0104] The method 500 then performs the step 304 of controlling a start-up of at least one of the plurality of wind turbine generators 14 based on the estimated start-up periods.

[0105] Here it shall be appreciated that the step 304 of the method 500 may further include the exemplary sub-step of controlling a start-up of one or more of the WTGs 14 based on the estimated period for each WTG 14, or the WPP 12, to reach the threshold active power level and a target production time. Advantageously, the WPP 12 can therefore be controlled to more accurately reach a desired active power level at a target time according to demand or available production conditions (e.g. weather).

[0106] 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.

[0107] In other examples, the methods may additionally, or alternatively, include a step of comparing the start-up periods estimated, in step 303, for each WTG 14 to respective target start-up periods, and controlling one or more WTGs 14 to reduce the estimated startup periods in dependence thereon. For example, where the estimated startup period exceeds the target startup period for one of the WTGs 14, the PPG 22 may identify the maximum delay of the estimated delays for the WTG 14, and control the respective controllable system 54 to reduce the difference between its current condition and the production-ready condition. In turn, this will reduce the estimated startup period, and the step may be iteratively repeated until the estimated startup period for the WTG 14 is less than or equal to the target startup period.

[0108] Alternatively or additionally, if the target start-up period is shorter than the estimated start-up period, then the PPG 22 may be configured to increase the rate at which the controllable system 54 makes progress towards satisfying the respective productionready condition. For example, the pitch rate is a constant rate in some operation modes and is predetermined in a design phase of the wind turbine generator 14. However, this pitch rate is adjustable, and may be increased as long as there is sufficient hydraulic pressure in the hydraulic actuator for that blade 48. Therefore, the pitch system, controlled by the PPG 22, can increase the pitch rate and therefore reach the target pitch angle sooner than estimated, reducing the delay and, in turn, the estimated startup period for the WTG 14.

[0109] Likewise, if the target start-up period is longer than the estimated start-up period, then the PPG 22 may be configured to decrease the rate at which the controllable system 54 makes progress towards satisfying the respective production-ready condition.

Claims

CLAIMS1. A method of controlling a renewable energy power plant comprising a plurality of wind turbine generators, each wind turbine generator having a plurality of controllable systems that must satisfy respective production-ready conditions for that wind turbine generator to enter a production-ready state, the method comprising: for each wind turbine generator: receiving one or more sensor signals indicative of a current condition of each controllable system of that wind turbine generator; estimating, for each controllable system, a delay for that controllable system to satisfy the respective production-ready condition based on the current condition indicated for that controllable system; and estimating a start-up period for that wind turbine generator to enter the production-ready state based on the estimated delays for the controllable systems; and controlling a startup of at least one of the plurality of wind turbine generators based on the estimated startup periods.

2. A method according to claim 1 , wherein estimating the start-up period based on the estimated delays comprises determining a maximum delay of the estimated delays and estimating the start-up period as being equal to the maximum delay.

3. A method according to claim 1 or 2, wherein a first controllable system of the plurality of controllable systems is dependent on another controllable system of the plurality of controllable systems such that the production-ready condition of the first controllable system can only be satisfied once the production-ready condition of the other controllable system is satisfied.

4. A method according to claim 3, comprising estimating the delay for the first controllable system to satisfy the respective production-ready condition based on the indicated current conditions of each of the first controllable system and the other controllable system.

5. A method according to any preceding claim, wherein one or more of the controllable systems are independently controllable systems; and wherein, for each independently controllable system, the delay is estimated independently of the respective production-ready conditions of any other controllable systems being satisfied.

6. A method according to any preceding claim, wherein the controllable systems comprise: a yaw system; one or more pitch systems; a rotor system; a heating system and / or a lubrication system.

7. A method according to claim 6, wherein the current conditions indicated for the controllable systems comprise one or more of the following: a yaw rate; a yaw angle; a pitch rate; a pitch angle; a hydraulic pressure; a rotor speed; a rotor acceleration; and a lubrication fluid temperature; of that wind turbine generator.

8. A method according to claim 6 or 7, when dependent on claim 3 or 4, wherein the first controllable system takes the form of the rotor system and the other controllable system takes the form of the yaw system or the pitch system.

9. A method according to any preceding claim, further comprising: for each wind turbine generator: obtaining a target start-up period for that wind turbine generator; comparing the target start-up period to the estimated start-up period for that wind turbine generator; and controlling at least one of the controllable systems of that wind turbine generator to reduce a difference between the target start-up period and the estimated start-up period for that wind turbine generator based on the comparison.

10. A method according to any preceding claim, wherein the method further comprises: estimating an overall start-up period for the plurality of wind turbine generators to each enter the production-ready state based on the estimated start-up periods for each wind turbine generator.

11. A method according to any preceding claim, further comprising: for each wind turbine generator: obtaining a ramp rate for the active power output of each wind turbine generator; andforecasting an active power level for that wind turbine generator based on the obtained ramp rate and the estimated start-up period for that wind turbine generator.

12. A method according to claim 11 , further comprising: estimating a period for each wind turbine generator to reach a threshold active power level based on the respective forecast.

13. A method according to claim 11 , further comprising: forecasting an active power level for the plurality of wind turbine generators based on the forecast active power level for each wind turbine generator.

14. A method according to claim 13, further comprising: estimating a period for the plurality of wind turbine generators to collectively reach a further threshold active power level based on the forecast for the plurality of wind turbines.

15. A controller for a renewable energy power plant comprising a plurality of wind turbine generators, the controller being configured to execute machine readable instructions to perform the method of any of claims 1-14.

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