WO2022002332A1

WO2022002332A1 - Methods and control systems for voltage control of renewable energy generators

Info

Publication number: WO2022002332A1
Application number: PCT/DK2021/050214
Authority: WO
Inventors: Bo Yin
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2020-06-29
Filing date: 2021-06-28
Publication date: 2022-01-06
Anticipated expiration: 2022-12-29

Abstract

Aspects of the present invention relate to a method (100) for controlling a renewable energy generator (14). The method (100) comprises: receiving (102) a signal from a power plant controller (22) indicating a reactive power setpoint; determining (104) reactive power limits for the generator (14); generating (106) a control signal for the renewable energy generator (14), the control signal being based on the reactive power setpoint and limited based on the determined reactive power limits; and controlling (108) the generator (14) according to the control signal. Determining (104) the reactive power limits comprises: determining (110) a generator limit corresponding to the reactive power capability of the generator (14); determining (112) a terminal voltage of the generator (14); comparing (114) the determined terminal voltage to a voltage limit: determining (116) a voltage-based limit based on the comparison.

Description

METHODS AND CONTROL SYSTEMS FOR VOLTAGE CONTROL OF RENEWABLE ENERGY GENERATORS

TECHNICAL FIELD The present disclosure relates to a method and control systems for voltage control in renewable energy power plants.

BACKGROUND

Renewable power plants, such as wind power plants, are required to provide support for voltage levels of the power network to which they are connected. That is, power plants are controlled to meet requirements set out by power plant controllers and power network operators that specify how reactive power exchange should be controlled to regulate voltage. The main aim during this ‘voltage control’ is to maintain the voltage level of the power network within a voltage deadband, which is between approximately 0.9 p.u. and 1.1 p.u. voltage. Particular focus is placed on guiding the voltage levels towards a nominal or natural voltage level, typically 1 p.u. Maintaining voltage levels within the deadband and around the nominal voltage prevents deviations that result in the application of extraordinary measures such as under- or over-voltage ride-through protocols.

In order to support voltage recovery during drops, i.e. deviations below 1 p.u. within the dead-band, renewable energy generators of the renewable power plants are controlled to supply reactive power. Where voltage levels rise above the nominal level, reactive power is absorbed to reduce voltage levels. The supply and absorption of reactive power may be considered in terms of the way in which supply and absorption is achieved, and may accordingly be considered in terms of capacitive- and inductive-reactive power respectively.

When controlling generators to provide capacitive- or inductive-reactive power support, the local voltage level and the voltage level of the network are both altered. In some networks that have a so-called ‘weak grid interconnection’, small changes in reactive power exchange cause large variations in local voltage. Thus, capacitive-reactive power support may cause local voltage levels to rise out of the dead-band, which may lead to emergency measures to be implemented. Similarly, inductive-reactive power support may cause local voltage levels to drop out of the dead-band, with the result being an under-voltage ride-through being initiated.

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

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided a method for controlling a renewable energy generator of a renewable energy power plant. The method comprises: receiving a dispatch signal from a power plant controller indicating a reactive power set point; determining reactive power limits for the generator; generating a control signal for controlling the reactive power output of the renewable energy generator, the control signal being based on the reactive power set point and limited based on the determined reactive power limits; and controlling the renewable energy generator according to the control signal. Determining the reactive power limits comprises: determining a generator reactive power limit corresponding to the reactive power capability of the generator; determining a terminal voltage of the renewable energy generator; comparing the determined terminal voltage to a voltage limit: determining a voltage-based reactive power limit based on the comparison.

By implementing both a reactive power capability limit and a voltage-based reactive power limit, the generator can be controlled to prevent the reactive power from causing voltage limits to be exceeded or, in the event that the voltage limits are already exceeded, preventing further deviation. Accordingly, generators are controlled to more acutely and precisely avoid detrimental voltage deviations.

In the event the determined terminal voltage does not exceed the voltage limit, determining the voltage-based reactive power limit may comprise: determining an error value between the voltage limit and the determined terminal voltage; and determining the voltage-based reactive power limit based on the error value.

Determining the voltage-based reactive power limit based on the error value may comprise: determining a correction reactive power value based on the error value. Determining the correction reactive power value based on the error value may comprise passing the error value through a PI controller.

In the event the determined terminal voltage exceeds or is equal to the voltage limit, the method may comprise determining the voltage-based reactive power limit to reduce or prevent further exceedance of the voltage limit.

Generating the control signal may comprise: generating a main control signal portion for controlling the generator to generate reactive power at the reactive power set point received from the power plant controller; and generating an adjustment control signal portion for adjusting the main control signal portion, the adjustment control signal portion being generated based the reactive power limit; and adjusting the main control signal portion by the adjustment control signal portion.

In the event the terminal voltage exceeds or is equal to the voltage limit, the voltage- based reactive power limit may comprise a zero limit for the adjustment control signal portion.

In the event the terminal voltage does not exceed the voltage limit, determining the reactive power limit may comprise: comparing the voltage-based reactive power limit and the generator reactive power limit; and determining the reactive power limit as the reactive power limit closer to zero.

The method may comprise, in the event the terminal voltage does not exceed the voltage limit, comparing the reactive power limit with the received reactive power set point. The adjustment control signal portion may be limited based on the control signal limit. If the reactive power set point exceeds the reactive power limit, the control signal limit may comprise a zero signal.

The method may comprise determining the generator reactive power limit corresponding to the reactive power capability of the generator by referring to a P-Q data structure that specifies the reactive power limit for predetermined active power measurements. The P-Q data structure may be generated based on the measured terminal voltage. Determining the reactive power limit may comprise determining an upper reactive power limit and determining a lower reactive power limit.

The control signal may comprise a reactive current set point.

According to another aspect of the invention, there is provided a renewable energy generator controller configured to perform the method described above.

The renewable energy generator may comprise a wind turbine generator.

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

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 shows a schematic representation of a power network;

Figure 2 shows a block diagram of a voltage controller for a power plant controller;

Figure 3 shows a typical PQ chart for a wind turbine generator;

Figure 4 shows a block diagram of a voltage control unit for a wind turbine generator according to an embodiment of the invention;

Figure 5 shows a general method of operation for the voltage control unit of Figure 4 according to any embodiment of the invention; and Figure 6 shows a control system diagram of a voltage control unit according to an embodiment of the invention.

DETAILED DESCRIPTION

Generally, the invention described herein provides a method and a controller for implementing voltage control for a renewable energy generator. The method and controller ensure that operation of the generator, and particularly a converter of the generator, to provide reactive power support according to a received set point does not cause problematic voltage deviations and to mitigate existing deviations. This is achieved by implementing, at a local controller, limits on reactive power based on the physical capability of the generator and based on the voltage level. Advantageously, the generator controls the voltage level and reduces the occurrence of unwanted voltage deviations.

Figure 1 illustrates a typical architecture in which a wind power plant (WPP), which may also be referred to as a wind park or wind farm, is connected to a main grid as part of a wider power network. As will be understood by the skilled reader, a WPP comprises at least one wind turbine generator (WTG), and is also known as a wind park or a wind farm. A WTG is commonly referred to as a wind turbine. The examples shown are representative only and the skilled reader will appreciate that other specific architectures are possible, in relation to wind power plants, power plants for other renewable energy sources such as solar power plants, bio energy power plants, or ocean/wave/tidal energy plants, and to hybrid power plants having a combination of different types of renewable energy power plants. Thus, the invention also 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 to 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 network incorporating a WPP 12 and a power plant controller 22, referred to hereafter as PPC 22. 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 transmission network or main grid 16, as active power and/or current, for distribution. Individual generators may each be referred to in this description as a ‘unit’.

Although not illustrated in this Figure, the WPP 12 may also include compensation equipment, such as a static synchronous compensator (STATCOM) or another type of synchronous compensator, configured to provide reactive power or reactive current support as required. The WPP 12 may also include a battery energy storage system.

Each of the WTGs 14 is associated with a respective WTG controller 15. In some examples, a set of WTGs may share a single, semi-centralised WTG controller, such that there are fewer WTG controllers than WTGs. As would be understood by the skilled person, WTG controllers 15 can be considered to be computer systems capable of operating a WTG 14 in the manner prescribed herein, and may comprise multiple modules that control individual components of the WTG or just a single controller. The computer system of the WTG controller 15 may operate according to software downloaded via a communications network or programmed onto it from a computer- readable storage medium.

During normal operation of the WPP 12, the WTG controllers 15 operate to implement active and reactive current and/or power requests received from the PPC 22 to provide frequency and voltage support to the main grid 16. During extraordinary conditions, the WTG controllers 15 operate to fulfil predetermined network requirements, and also act to protect the WTGs 14 from any potentially harmful conditions.

The WPP 12 is connected to the main grid 16 (also called the main power network) by a connecting network 18. The WPP 12 and the main grid 16 are connected at a Point of Interconnection (Pol) 20, which is an interface between the WPP 12 and the main grid 16. The Pol 20 may also be referred to as the Point of Common Connection, which may be abbreviated to ‘PCC’ or ‘PoCC’.

The WTGs 14 are connected to one another locally by local grid 19, (also called the local power network or park grid). The function of the local grid is to channel power from each of the WTGs 14 to the connecting network 18 to the main grid 16. The Power Plant Controller (PPC) 22 is connected to the main grid 16 at a Point of Measurement (PoM) 24 and is connected to the WTG controllers 15. The role of the PPC 22 is to act as a command and control interface between the WPP 12 and the grid 16, and more specifically, between the WPP 12 and a grid operator, such as a transmission system operator (TSO) or a distribution system operator (DSO) 26. The PPC 22 is a suitable computer system for carrying out the controls and commands as described above and so incorporates a processing module 28, a connectivity module 30, a memory module 32 and a sensing module 34. The PPC 22 may also receive information regarding the grid 16 and/or the local buses, substations and networks from an energy management system (not shown). The WPP 12 is capable of altering its power or current output in reaction to commands received from the PPC 22.

As part of its operation, the PPC 22 generates and sends dispatch signals to the WTG controllers 15. The WTG controllers 15 control the WTGs according to set points contained within the dispatch signals.

During normal operation, the PPC 22 operates in one of a number of modes. One such mode is a voltage regulation, which may also be referred to as voltage control mode, in which the PPC 22 issues dispatch signals configured to cause the WTGs 14 to supply or absorb reactive power to regulate the voltage level of the power network. In particular, the PPC 22 supplies signals indicating reactive power set points to the WTGs 14 for maintaining voltage levels within a voltage range, specifically a dead-band, of between, approximately 0.9 per unit (p.u.) voltage and 1.1 p.u. voltage, as measured at the Pol 20 or PoM 24. In specific examples, the dead-band is between a lower voltage bound or limit of 0.87 p.u. and an upper voltage bound or limit of 1.13 p.u.

As would be understood by the skilled person, per-unit voltage is an expression of the voltage with respect to a base value which is used as a reference. 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 PPC 22 may issue a variety of different dispatch signals and set points to the WTG controllers 15 for implementation according to the mode in which the PPC 22 is operating. In the present embodiments, the PPC 22 is configured to issue dispatch signals to the WTG controllers 15 that indicate reactive power set points for the WTGs 14 to meet.

To regulate voltage, the WTG controllers 15 generate control signals indicating reactive current set points for controlling a line-side converter of the WTG 14. The reactive current set points are based on the reactive power set points dispatched by the PPC 22 to the WTG controller 15 and the present reactive power output of the WTG 14, as will be explained below. It will be appreciated that although the PPC 22 indicates a set point, the measured reactive power output of the WTG 14 may differ from this set point and so may require adjustment. Thus, the control signal from the WTG controller 15 is configured to alter the reactive power output based on an error between the set point and the output.

Before the voltage control of the WTG controller 15 is considered, the generation of reactive power set points by the PPC 22 will be described. To help illustrate this, Figure 2 shows a schematic representation of a PPC voltage control unit 40, contained within the processing module 28 of the PPC 22. The control unit 40 is configured to determine a reactive power reference value and to generate a plurality of reactive power set points that together achieve the reference value. It subsequently dispatches these set points to the WTG controllers 15.

As shown in Figure 2, the PPC voltage control unit 40 comprises a reactive power controller 42. The reactive power controller 42 receives a voltage reference, ‘U_ref’, and a measured voltage at the Pol 20, ‘IW. The reactive power controller 42 determines a deviation of the measured voltage from the voltage reference, and generates and outputs a reactive power reference value, ‘C . The reactive power reference value Q_ref is the reactive power output to be met by the WPP 12 at the Pol 20.

Where measured values are discussed in these examples, this is envisaged to encompass direct measurements and determinations by other means. The values described as measured may be determined or measured indirectly based on a proxy value.

The reactive power reference value is passed through a reactive power reference limiter 44. The limiter 44 applies reactive power limits to the reference value and outputs a limited reference value, ‘G ii_m’. In order to calculate the reference limits applied by the limiter 44, the PPC 22 receives reactive power capability values from each WTG 14. The capability values comprise a capacitive-reactive power capability, ‘Qavaiia_bie cap’, and an inductive-reactive power capability, ‘Q_{avaiiabie ind}’. These capability values correspond to the maximum possible reactive power that each WTG 14 can supply and absorb respectively. The capability values are indicative of the physical limits of the WTG 14 - i.e. they are the reactive power values that the WTG 14 is incapable of exceeding without causing damage to the systems. In some situations, the WTG 14 may be controlled to absorb or supply more reactive power but only for very short periods of time.

The limits for the reactive power reference limiter 44 are generated by summing the capacitive-reactive power capabilities of all WTGs 14 in the WPP 12 and the inductive- reactive power capabilities of all WTGs 14 in the WPP 12, to provide upper and lower limits for the reference value. Thus, the limits may be expressed by the formula:

The capacitive- and inductive-reactive power capabilities received from the WTGs 14 are typically generated based on a P-Q chart. P-Q charts, an example of which is shown in Figure 3, define the reactive power capability of the WTG 14 based on the active power output level of the WTG 14 and the voltage level. The P-Q chart of Figure 3 plots active power in megawatts, on the x-axis, against reactive power in megavolt-amperes reactive on the y-axis. In this example, a solid line forming a trapezoidal shape represents the power generation capability of the WTG 14 when the power converter is operating at nominal voltage (1 p.u.). Also shown is a line formed of short dashes for a terminal voltage at its lower voltage limit, 0.87 p.u., and a line formed of longer dashes for terminal voltages at the upper limit 1.13 p.u. The skilled reader will appreciate that this shape is typical for any P-Q chart for a power generator of a WTG. The lines shown on the P-Q chart therefore define the long-term power generating capability of the WTG 14. As will be appreciated, a P-Q chart may be generated for a plurality of voltage levels within the voltage dead-band.

Moving back to Figure 2, the limited reference value Q_ref n_m is passed to a reactive power dispatcher 46. The reactive power dispatcher 46 generates the individual reactive power set points, ‘Q_set_p n’ for dispatch to each WTG 14. The reactive power dispatcher 46 generates the reactive power set points by distributing the limited reference value between the WTGs 14.

In an example of this distribution, the limited reference value G n_m is evenly divided by the number of WTGs 14, i.e. Q_renim WTG = Qrenim/N, where N is the number of WTGs 14. For each WTG 14, the distributed value Q_refiimw_Tcis compared with the capability values, Qavaiiabie cap and Qavaiiabie ind for the respective WTG 14. If the WTG 14 cannot supply Q_ref nm WTG because that value of reactive power exceeds one of its capability values, then the set point for that WTG 14 is set as the capability value. Where this occurs, a difference between the distributed value and the relevant capability value is determined, and all these differences are summed to determine a total deficiency in reactive power for the WPP 12. This deficiency is re-distributed to WTGs 14 with excess capability (i.e. those for whom Qavaiiabie ind < Qref Nm WTG < Qavaiiabie cap) so that the set points for dispatching to the individual turbines are limited based on their capabilities and adjusted to provide additional reactive power where possible. Generally this redistribution is performed by distributing the deficiency based on the amount of extra reactive power capability a WTG can provide. In other embodiments, the distribution may be performed by dividing the limited reference value between the WTGs 14 equally or according to a different metric.

Figure 4 illustrates an example of WTG voltage control unit 60 for a WTG 14, which is housed within a WTG controller 15. The example in Figure 4 is a simplified block diagram of a voltage control unit for a WTG 14. Figure 6 shows a full control diagram of a voltage controller.

Beginning with Figure 4, the WTG voltage control unit 60 comprises a reactive power controller 62. The reactive power controller 62 receives the set point, Q_setp n, as part of the dispatch signal from the PPC voltage control unit 40. In addition, the controller 62 receives a locally-measured reactive power level, Qmeas, and a locally-measured voltage level, U_meas. By locally-measured it is meant that the values are measured at a terminal of the WTG 14. Inputs to blocks described in Figures 4 and 6 may be filtered to remove anomalous data before being utilised in the blocks.

Based on the set point Q_set _n received from the PPC voltage control unit 40, the measured reactive power level Qmeas, and the measured voltage level Umeas, the reactive power controller 62 generates a control signal for controlling the WTG 14 to meet the reactive power set point. Where the voltage control unit 60 is housed within a line-side converter, the control signal comprises a reactive current set point for controlling the line-side converter.

Importantly, in order to ensure that voltage limits and WTG capability limits are not exceeded, the control signal is generated by the reactive power controller 62 with respect to input from an upper reactive power limit unit 64 and a lower reactive power limit unit 66. These units 64, 66 determine reactive power limits based on the capability values and voltage levels and provide signals to the reactive power controller 62 to restrict the allowed range of reactive power that can be requested by the control signal.

Although upper and lower limits are discussed in these examples, it will be appreciated that only one of these limits may be applied in practice, and thus only one limit unit may be provided.

The limit units 64, 66 act to limit the control signal based on reactive power capability and on voltage level. The limit units 64, 66 implement limits according to the reactive power capability of the WTG 14 because the control signal generated by the controller 62 to meet the set point from the PPC 22 may exceed these capability limits. Although the reactive power set-point may have already been limited based on the capability values of the WTG 14 at the PPC voltage control unit 40, a measured reactive power value lower than the set-point may lead the reactive power controller 62 to generate a control signal requesting reactive power from the WTG 14 above its reactive power capabilities. Therefore, a further limit is implemented locally.

The limit units 64, 66 also limit the control signal based on voltage to prevent or mitigate exceedances of voltage limits. As can be seen, the limit units 64, 66 receive a measured voltage level I eas. In normal operation, i.e. operation where under- or over-voltage ride- through protocols are not operational, voltage levels both at the main grid and locally are maintained within the voltage dead-band. In the limit units 64, 66, the control signal is limited in one or more ways with respect to this voltage dead-band. The limit units 64, 66 may limit the control signal to prevent excursion of the voltage level outside of the voltage dead-band. If a measured voltage is outside the voltage dead-band range or equal to the limits, the limit units 64, 66 may limit the control signal to prevent further deviation or to return the voltage to the dead-band range. In either example, the limit units 64, 66 act to implement a further reactive power limit based on a comparison between the measured voltage and a voltage limit.

The limit units 64, 66, having generated at least the capability-based limit and, where required, the voltage-based limit, communicate a signal to the reactive power controller 62 to ensure adherence of the control signal output from the controller 62 to these limits.

To generalise, the WTG voltage control unit 60 operates according to a method 100, as shown in Figure 5. The method 100 comprises the steps of: receiving 102 a reactive power set point from a PPC; determining 104 at least one reactive power limit for the generator; generating 106 a control signal for controlling the reactive power output of the renewable energy generator based on the reactive power set point and limited by the reactive power limit; and controlling 108 the renewable energy generator based on the control signal. As described above, in relation to the limit units 64, 66, the determination step 104 of the at least one reactive power limit comprises the steps illustrated to the right of the method 100, including: determining 110 a first, generator- based reactive power limit corresponding to the reactive power capability of the generator; determining 112 a measured terminal voltage; comparing 114 the measured terminal voltage with a voltage limit and, in dependence on the comparison, determining 116 a second, voltage-based reactive power limit.

To further illustrate the implementation of this method 100 and the unit 60 in more detail, Figure 6 illustrates a control system diagram of a specific embodiment of the voltage control unit 60. Similar elements will be labelled with the same reference numerals to aid comparison between Figure 6 and Figure 4.

The voltage control unit 60 of Figure 6 is configured for controlling a line-side converter of a WTG 14 according to the set-point received from the PPC 22. Therefore, the output from the control unit 60 is a control signal indicating a reactive current set point, lo_controi, for the line-side converter (not shown).

The reactive current set point is determined from two constituent parts: a main reactive current value, l_QMam, derived from the reactive power set point Q_set _n alone, and a reactive current adjustment value, l_QAdjust, derived from the reactive power set point and the measured reactive power level Qmeas. The reactive current adjustment value generated to adjust the main reactive current value in order to raise or lower the measured reactive power value to the set point value. Therefore, if the measured reactive power value is equal to the set point value, the adjustment value is zero, as no further adjustment is required.

As will be familiar to the skilled person, reactive current is a function of reactive power and voltage. Accordingly, within the reactive power controller 62, reactive current levels are derived based on the measured and set point reactive power levels and the measured voltage levels input that are received as inputs by the reactive power controller 62. Particularly, the main reactive current level is derived based on the measured voltage level and the reactive power set point. The conversion from reactive power levels to reactive current levels is not depicted in this diagram for clarity.

To determine the reactive current adjustment value, an error value between the reactive power set point and the measured reactive power is calculated at a subtracting junction 68 within the reactive power controller 62. The error value is passed to a PI controller 70 to determine a reactive power adjustment value. From this adjustment value and the measured voltage level, the reactive current adjustment value is derived.

In the present embodiment, the upper limit unit 64 and the lower limit unit 66 generate reactive power limits, compare these limits with one another and then with the reactive power set point. The comparison of the limits with the set point is subsequently used to restrict the range of possible reactive current values. In other words, based on the reactive power limits generated in the limit units, the reactive current values are also limited.

In this embodiment, the adjustment value is limited by providing limits as inputs to the PI controller 70. By restricting the adjustment value to a particular range, the reactive power output requested based on the control signal can be prevented from exceeding the generator capacities and prevented from causing voltage deviations.

In other embodiments, the reactive power limits may be applied directly to the reactive current set point, or to the reactive power set point before calculation of the reactive current values. This depends upon how the limits, particularly the voltage limits, are intended to restrict the output of the system. In the embodiment of Figure 6, as a minimum, the intention is to: prevent the reactive power requested from the WTG 14 exceeding its capability; and, where the voltage level exceeds one of the voltage limits, i.e. is outside the allowable voltage range, to effectively ‘clamp’ the reactive power requested from the WTG 14 with respect to the set point. By clamping, it is meant that the set point is effectively set as a maximum reactive power level if the voltage exceeds the upper voltage limit, or as a minimum reactive power level if the voltage exceeds the lower voltage limit. In doing so, where voltage levels are outside one of the limits with respect to the voltage range, the clamping of the reactive power set point prevents an increase or decrease, as applicable, of the voltage level further from the dead-band range. As already explained, increases in reactive power supply lead to voltage increases, while increases in reactive power absorption lead to voltage decreases. Thus, by preventing further increase of the reactive power level beyond the set point value where the voltage is too high, or further decrease of the reactive power level beyond the set point value when the voltage level is too low, further, potentially more problematic deviations (i.e. those requiring over- or under-voltage ride through actions) can be avoided.

In order to achieve this, the upper and lower limit units 64, 66 are structured as shown in Figure 6. That is, that each unit 64, 66 receives the measured voltage level, I _eas, at block 72, and generates a voltage-based reactive power limit level as an output from block 72. Within block 72, a comparison is performed between the voltage limit and the measured voltage level to determine the voltage-based reactive power limit level. More specifically, the limit level is generated by finding an error between the limit and the measured voltage level, using a subtracting junction 74, and then utilising a PI controller 76 to generate the limit level from the error.

The output of the block 72 is a voltage-based reactive power limit, Qii_m u. The voltage- based reactive power limit is input to block 78, which performs an additional comparison of the voltage-based reactive power limit with the capability value, i.e. the generator- based, reactive power limit, Q_{avaiiabie cap} for upper limit unit 64 or Q_{avaiiabie ind} for the lower limit unit 66. The generator-based reactive power limit is determined based on the PQ chart as described above. In Figure 6, the comparison is performed by block 78 by comparing the voltage-based reactive limits with a reactive power range. This range is between 0 and the relevant capability value. The limiting blocks 78 compare the voltage-based reactive power limit with the range, and if the limit is within the range, the output is the voltage-based reactive power limit. If the limit is outside of the range, the output is the bound that the limit falls outside of. So, for example, in the upper limit block 64 if Qnmu > Q_{avaiiabie cap} the output of block 78 is Qavaiiabie cap, whereas if Qiim u < 0, the output of block 78 is 0.

The output of block 78 is passed to a further block 80, which compares the output from block 78 with the reactive power set-point Q_setp n. Based on this comparison, block 80 outputs a signal to adjust, and limit, the control signal of the reactive power controller 62. In other embodiments, the reactive power limit from block 78 may be output directly to the reactive power controller 62 and the comparison performed at the reactive power controller 62.

More specifically, in the comparator block 80, a comparison is performed at a subtracting junction 82 to find an error value between the output of block 78, nominally referred to as a reactive power limit, and the reactive power set-point. The error value is subsequently passed through a further limiter 84. The limits for the further limiter 84 are the reactive power limit from block 78 and 0.

The output of the further limiter 84 is passed to the reactive power controller 62 for use in limiting the reactive current adjustment value as described above. To permit limiting the adjustment value, the output of the limiter 84 is provided in the appropriate form, either by outputting a reactive power value which is subsequently converted to a reactive current limit based on voltage, or by converting the power levels to voltage at a point prior to the limiter 82. It will be appreciated that this may be carried out at any position prior to limiting the change value. It is envisaged that this will be performed between the subtracting junction 80 and limiter 82, with the limit value output from block 70 also being adjusted.

Although the elements are explained, the operation of the unit 60 are best demonstrated when considering particular scenarios. For the purposes of explaining the operation of the system of Figure 6, there are three scenarios to consider: a first scenario in which the voltage level is outside the allowable voltage range, and second and third scenarios in which the voltage level is within the voltage range. The scenarios will be explained in relation to the upper limit unit 64, but these explanations can be equally applied to the lower limit unit 66, which acts in concurrence with the upper limit unit 64.

Considering the first scenario, it will be assumed that the limit exceeded is the upper voltage limit. It will be appreciated that in the first scenario, the voltage level only exceeds the voltage limit of the upper limit unit 64 and thus the other of the limit units will be operating according to the second or third scenario.

In the first scenario, in the upper limit unit 64, the output from the subtracting junction 74 is negative as I eas > Umax. Therefore the correction value from the PI controller 76 will also be negative. Accordingly, when a negative value is limited within the limiting block 78, the negative value does not fall within the allowed range, and so the output of the limiting block 78 is the lower limit, which in this example is zero.

Zero is therefore the value passed to the comparator block 80. Regardless of the comparison at the subtracting junction 82, the output of the limiter 84 will also be zero, because the upper limit is zero and the lower limit is zero for this limiter 84. Thus, in the first scenario, the output of the upper limit unit 64 to the PI controller 70 in the reactive power controller 62 is zero.

In practice, this means that the adjustment value calculated at the PI controller cannot exceed zero, and thus cannot increase the reactive power set point further. Accordingly, the reactive power set point is clamped, and the WTG 14 is prevented from supplying reactive power that will cause further deviation of the voltage from the allowable range.

In other words, the voltage-based reactive power limit is calculated to prevent further exceedance of the voltage limit or to reduce the voltage level below the voltage-based reactive power limit. When the voltage level returns to within the allowable range, the zero-limit applied due to the exceedance is lifted, and the WTG 14 can adjust the set point in that direction again.

Although the limit unit 64 sends a zero through the system in the event of a voltage exceedance, it will be appreciated that this constitutes the application of a reactive power limit, albeit an indirect limit that prevents adjustment of the set point in that particular direction. Thus, in the first scenario, the zero signal can be considered to be a reactive power limit.

In both the second and third scenarios, where the voltage level is within the voltage range, the output of the subtracting junction 74 is positive, and therefore so is the output of the PI controller 76. In the second scenario, the voltage-based reactive power limit is outside the range, beyond the reactive power supply capability, so the output of the limiting block 78 is the reactive power capability value. In the third scenario, the output voltage-based reactive power limit is between zero and the reactive power capability value. Therefore, the output of the limiting block 78 is the voltage-based reactive power limit.

In the second scenario, as the reactive power set point has already been limited based on the reactive power capability value, it will not exceed the output of block 78 in this example. If the reactive power set point is at its limit, however, the subtracting junction 82 will yield a zero value, so the output of the limiter 84 will also be zero. On the other hand, if the reactive set point is less than its limit, the output of the subtracting junction 82 will be greater than zero, and so the output of the limiter 84 will also be equal to the error value or the reactive power capability limit. By using the error value or capability value in this scenario to limit the reactive current change value, the WTG 14 will be controlled according to its reactive power capability limit so that reactive power outside of the capability limits is not requested.

In general, therefore, where voltage is within a so-called ‘safe’ distance from its limits, the WTG 14 can be controlled to output reactive power according to its physical capability limits. Comparison of set point with the capability limits allows the limit unit 64 to determine a maximum change value based on the difference between the limit and the set point.

In the third scenario, comparison at the subtracting junction 82 may yield a zero value, a negative value, or a positive value depending upon the relative sizes of the output reactive power limit from block 78 and the set point. Therefore, in this scenario, the output of limiter 84 may be a variety of values, depending on the distance of the voltage level from the voltage limit and the parameters of the PI controller 76. In particular examples, if the voltage level is close to the limit, further increase or decrease (as appropriate for the level) may be prevented or restricted.

In this scenario, the WTG 14 is controlled according to a voltage-based limit, because, for example, the voltage level is close to a voltage limit. Thus, it is important to prevent increases in the reactive power level that would cause an exceedance of the voltage limits. The set point is therefore curtailed, according to its size relative to the determined voltage-based reactive power limit. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A method (100) for controlling a renewable energy generator (14) of a renewable energy power plant (12), the method (100) comprising: receiving (102) a dispatch signal from a power plant controller (22) indicating a reactive power set point; determining (104) reactive power limits for the renewable energy generator (14); generating (106) a control signal for controlling the reactive power output of the renewable energy generator (14), the control signal being based on the reactive power set point and limited based on the determined reactive power limits; and controlling (108) the renewable energy generator (14) according to the control signal, wherein determining (104) the reactive power limits comprises: determining (110) a generator reactive power limit corresponding to the reactive power capability of the renewable energy generator (14); determining (112) a terminal voltage of the renewable energy generator (14); comparing (114) the determined terminal voltage to a voltage limit: determining (116) a voltage-based reactive power limit based on the comparison.

2. The method (100) of claim 1, wherein, in the event the determined terminal voltage does not exceed the voltage limit, determining (116) the voltage-based reactive power limit comprises: determining an error value between the voltage limit and the determined terminal voltage; and determining the voltage-based reactive power limit based on the error value.

3. The method (100) of claim 2, wherein determining the voltage-based reactive power limit based on the error value comprises: determining a correction reactive power value based on the error value.

4. The method (100) of claim 3, wherein determining the correction reactive power value based on the error value comprises passing the error value through a PI controller.

5. The method (100) of any one of claims 1 to 4, wherein, in the event the determined terminal voltage exceeds or is equal to the voltage limit, determining (116) the voltage-based reactive power limit to reduce or prevent further exceedance of the voltage limit.

6. The method (100) of any one of claims 1 to 5, wherein generating the control signal comprises: generating a main control signal portion for controlling the generator (14) to generate reactive power at the reactive power set point received from the power plant controller; and generating an adjustment control signal portion for adjusting the main control signal portion, the adjustment control signal portion being generated based the reactive power limit; and adjusting the main control signal portion by the adjustment control signal portion.

7. The method (100) of claim 6, wherein in the event the terminal voltage exceeds or is equal to the voltage limit, the voltage-based reactive power limit comprises a zero limit for the adjustment control signal portion.

8. The method (100) of claim 6 or claim 7, wherein in the event the terminal voltage does not exceed the voltage limit, determining (116) the reactive power limit comprises: comparing the voltage-based reactive power limit and the generator reactive power limit; and determining the reactive power limit as the reactive power limit closer to zero.

9. The method (100) of claim 8, comprising, in the event the terminal voltage does not exceed the voltage limit, comparing the reactive power limit with the received reactive power set point, the adjustment control signal portion being limited based on the control signal limit.

10. The method (100) of claim 9, wherein if the reactive power set point exceeds the reactive power limit, the control signal limit comprises a zero signal.

11. The method (100) of any one of claims 1 to 10, comprising determining (110) the generator reactive power limit corresponding to the reactive power capability of the generator by referring to a P-Q data structure that specifies the reactive power limit for predetermined active power measurements.

12. The method (100) of any one of claims 1 to 11, wherein determining (104) the reactive power limit comprises determining an upper reactive power limit and determining a lower reactive power limit.

13. The method (100) of any one of claims 1 to 12, wherein the control signal comprises a reactive current set point.

14. A renewable energy generator controller (15) configured to perform the method (100) of any one of claims 1 to 13.

15. The method (100) of any one of claims 1 to 13 or the renewable energy generator controller (15) of claim 14, wherein the renewable energy generator (14) comprises a wind turbine generator.