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Title:
METHOD OF CONTROLLING A WIND TURBINE GENERATOR
Document Type and Number:
WIPO Patent Application WO/2019/120405
Kind Code:
A1
Abstract:
METHOD OF CONTROLLING A WIND TURBINE GENERATOR A method of controlling a wind turbine generator (1) comprising a power converter (22) and at least one circuit breaker (25, 41). The method comprises determining a condition of the circuit breaker (25, 41) and modifying one or more operational parameters of the power converter (22) in accordance with the condition of the circuit breaker (25, 41). In particular, the invention proposes that the converter (22) is derated if one or more of the circuit breakers (25, 41) is close to violating a protection characteristic, thereby avoiding tripping of the circuit breaker (25, 41) and the associated lost production. [Figure 3]

Inventors:
ANDERSEN, Gert Karmisholt (Grumstrupvej 10, 8732 Hovedgård, 8732, DK)
LUND, Torsten (Herslev Bygade 14, 7000 Fredericia, 7000, DK)
DOAN, Duy Duc (Havkærparken 17, 8381 Tilst, 8381, DK)
RABI, Morteza (Helga Pedersens Gade 5, 8000 Århus C, 8000, DK)
DYRLUND, Poul Møhl (Elmehaven 1A, 8520 Lystrup, 8520, DK)
Application Number:
DK2018/050328
Publication Date:
June 27, 2019
Filing Date:
December 06, 2018
Export Citation:
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Assignee:
VESTAS WIND SYSTEMS A/S (Hedeager 42, 8200 Aarhus N, 8200, DK)
International Classes:
H02H5/04; F03D7/02
Foreign References:
US20100327599A12010-12-30
US20170145989A12017-05-25
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Claims:
Claims

1. A method of controlling a wind turbine generator (1 ) comprising a power converter (22) and at least one circuit breaker (25, 41 ), the method comprising determining whether the circuit breaker (25, 41 ) is approaching violation of a protection characteristic and modifying one or more operational parameters of the power converter (22) in accordance with the condition of the circuit breaker (25, 41 ).

2. The method of claim 1 , comprising modifying operational parameters of the power converter (22) to prevent violation of the protection characteristic of the circuit breaker (25, 41 ).

3. The method of claim 1 or claim 2, wherein the protection characteristic defines a function of electrical current with respect to time.

4. The method of claim 1 or claim 2, wherein the protection characteristic defines a function of temperature with respect to time.

5. The method of any preceding claim, wherein determining the condition of the circuit breaker (25, 41 ) comprises monitoring at least one operating parameter of the circuit breaker.

6. The method of any preceding claim, wherein determining the condition of the circuit breaker (25, 41 ) comprises estimating the value of one or more of the at least one operating parameters of the circuit breaker (25, 41 ) based on a model.

7. The method of claim 5 or claim 6, wherein the at least one operating parameter comprises one or more of: an electric current conducted through the circuit breaker (25, 41 ); and a temperature of the circuit breaker (25, 41 ).

8. The method of claim 7 when dependent on claim 1 , comprising monitoring the electric current over a time period to determine whether the circuit breaker (25, 41 ) is approaching violation of a protection characteristic.

9. The method of any preceding claim, wherein modifying operational parameters of the power converter (22) comprises derating the power converter (22).

10. The method of any preceding claim, wherein modifying operational parameters of the power converter (22) comprises updating an active power limit and/or a reactive power limit for the converter (22). 1 1 . The method of claim 10, wherein modifying operational parameters of the power converter (22) comprises updating a P-Q chart relating to the power converter (22).

12. The method of any preceding claim, comprising modifying one or more operational parameters of a generator (20) of the wind turbine generator (1 ) in accordance with the condition of the circuit breaker (25, 41 ).

13. A control system (27, 36) for a wind turbine generator (1 ), the wind turbine generator (1 ) comprising a power converter (22) and at least one circuit breaker (25, 41 ), the control system comprising:

an input configured to receive one or more signals indicative of the circuit breaker

(25, 41 ) is approaching violation of a protection characteristic;

a processing module configured to determine the condition of the circuit breaker (25, 41 ) based on the one or more signals, and to generate a control signal arranged to modify one or more operational parameters of the power converter (22) in accordance with the condition of the circuit breaker (25, 41 ); and

an output configured to output the control signal.

14. A wind power plant (12, 80) comprising the control system of claim 13.

Description:
METHOD OF CONTROLLING A WIND TURBINE GENERATOR

Technical field

The invention relates to a method of controlling a wind turbine generator comprising at least one circuit breaker.

Background to the invention

A wind power plant converts energy contained in wind into electrical power, which is typically delivered to a power grid. The changeable nature of wind entails an electrical power output of varying characteristics, and so circuit breakers are provided at various positions within the power plant to protect both the internal components of the power plant and the grid. For example, a stator of a generator of the wind power plant may include circuit breakers, and in most architectures circuit breakers are provided at a grid-side of a power converter that modifies the electrical power produced by the generator into a suitable form for transferring to the grid.

Such circuit breakers are configured to open to create a break in an electrical circuit, or‘trip’, when a certain electrical condition defining a protection characteristic arises. The protection characteristic is a function that defines the conditions under which the circuit breaker opens if violated, and is typically non-linear. For example, the protection characteristic may be defined by the square of an electrical current passing through the circuit breaker over a given time period. For example, a circuit breaker may open after exposure to a current of 1000 amperes over a period of five seconds, or after conducting 400 amperes for 50 seconds. Alternatively, the protection characteristic may define a function of the temperature of the circuit breaker with respect to time.

A circuit breaker trip forces a sudden shut-down of the power plant, which can induce stress on other elements of the plant. For example, since the generator will typically be operating at high load when a circuit breaker trips, the elements of the generator may be subjected to a sudden and significant change in torque in a double-fed induction generator (DFIG) architecture due to the sudden loss of excitation current. Over successive trip events, such stresses can exacerbate wear and fatigue.

Although some circuit breakers can be closed remotely after opening, many wind power plant systems opt for circuit breakers that must be closed manually once they have tripped. This creates significant down-time on the wind power plant until an operator can visit to reset the circuit breaker. The associated lost production can be particularly significant for power plants located offshore or in otherwise remote locations that are not readily accessible at short notice.

In some arrangements, a supervision application monitors operation of the circuit breakers and can intervene to trigger a soft trip of the circuit breaker when required by shutting the power plant down temporarily, for example over a period of 10 minutes, and thereby remove the load from the circuit breaker. Such operations are referred to as disconnections, and can be reversed remotely by the supervision application and controlled to minimise stress to the power plant components, thereby avoiding some of the problems associated with lengthy and sudden shut-downs. However, a disconnection nonetheless interrupts operation of the power plant and so increases the lost production.

A consequence of advances in wind turbine generator technology is that higher electrical power is typically generated for given wind conditions. Meanwhile, the circuit breakers used within the power plants have not changed, with efforts typically being directed towards optimising usage of power plant components instead of upgrading them. Accordingly, the frequency of circuit breaker trip or disconnection events is rising, in turn negatively impacting lost production.

Although it may be possible to replace the presently used circuit breakers with larger models that can cope with the electrical power that they are expected to be exposed to, suitable circuit breakers available at present are configured in discrete sizes and may be twice the size of those currently used, with a corresponding relative cost increase. So, this is not an attractive approach to minimising lost production in view of the limited space available within the systems of a wind power plant and a general objective to minimise the supported mass at the top of a wind turbine generator support tower. Moreover, the level of protection that must be provided for the grid is substantially constant, and so a larger circuit breaker may not be suitable in any event.

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

Summary of the invention

An aspect of the invention provides a method of controlling a wind turbine generator comprising a power converter and at least one circuit breaker. The method comprises determining whether the circuit breaker is approaching violation of a protection characteristicand modifying one or more operational parameters of the power converter in accordance with the condition of the circuit breaker.

Determining the condition of the circuit breaker may comprise determining whether the circuit breaker is approaching violation of a protection characteristic, in which case the method may comprise modifying operational parameters of the power converter to prevent violation of the protection characteristic of the circuit breaker. In this way, circuit breaker trip or disconnection events, and the associated lost production from the wind turbine generator, can be avoided.

The protection characteristic may define a function of electrical current with respect to time or a function of temperature with respect to time, for example, or a combination of the two.

Determining the condition of the circuit breaker optionally comprises monitoring at least one operating parameter of the circuit breaker, such as an electrical current conducted by the circuit breaker or another indication of its status, for example a signal output by the circuit breaker to indicate its status. Alternatively, or in addition, determining the condition of the circuit breaker may comprise estimating the value of one or more of the at least one operating parameters of the circuit breaker based on a model. In either case, the at least one operating parameter may comprise one or more of: an electric current conducted through the circuit breaker; and a temperature of the circuit breaker. Such methods may comprise monitoring the electric current over a time period to determine whether the circuit breaker is approaching violation of a protection characteristic.

In some embodiments, modifying operational parameters of the power converter comprises derating the power converter. This can enable a short-term sacrifice in terms of power output in return for a long-term net benefit to power production by avoiding a shut-down of the wind turbine generator, for example due to a circuit breaker trip.

Modifying operational parameters of the power converter may comprise updating an active power limit and/or a reactive power limit for the converter, for example by updating a P-Q chart relating to the power converter. This may involve derating the converter, for example to avoid violating a protection characteristic of the circuit breaker. Alternatively, this may entail increasing the active power limit and/or reactive power limit to overload the converter in the short-term when the condition of the circuit breaker indicates that this is possible without risking a trip or disconnection event. The method may comprise modifying one or more operational parameters of a generator of the wind turbine generator in accordance with the condition of the circuit breaker, providing greater flexibility in control of the wind turbine generator in response to the circuit breaker condition. For example, operation of the generator may be derated in accordance with the circuit breaker condition in a similar way to the power converter.

Another aspect of the invention provides a control system for a wind turbine generator, the wind turbine generator comprising a power converter and at least one circuit breaker. The control system comprises: an input configured to receive one or more signals indicative of a condition of the circuit breaker; a processing module configured to determine the condition of the circuit breaker based on the one or more signals, and to generate a control signal arranged to modify one or more operational parameters of the power converter in accordance with the condition of the circuit breaker; and an output configured to output the control signal.

The invention also extends to a wind power plant comprising the control system of the above aspect.

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

Brief description of the drawings

So that it may be more fully understood, the invention will now be described, by way of example only, with reference to the following drawings, in which:

Figure 1 is a schematic diagram of a wind turbine generator that is suitable for use with embodiments of the invention;

Figure 2 is a schematic diagram of an architecture of a full-scale converter based wind power plant that is suitable for use with embodiments of the invention;

Figure 3 is a block diagram of a converter controller of the converter of Figure 2;

Figure 4 is a block diagram representation of a thermal capability manager of the converter controller of Figure 3; Figure 5 is a representation of a typical P-Q chart used by the converter controller of Figure 3; and

Figure 6 is a schematic block diagram of an architecture of a DFIG arrangement that is suitable for use with embodiments of the invention.

Detailed description of embodiments of the invention

In general terms, embodiments of the invention provide methods and control systems for controlling various aspects of wind power plant operation to avoid circuit breaker trips or disconnections, by derating the power plant before such events occur. Although derating the power plant reduces its short-term output, by avoiding the need to shut the plant down and subsequently reactivate the plant, the approach provides a long-term net benefit to power plant performance.

This approach also avoids the stress and associated wear and fatigue that can be induced in components of the wind power plant in a sudden shut down caused by a circuit breaker trip.

In each embodiment, characteristics of one or more circuit breakers are monitored directly or modelled with reference to other operating parameters, to determine whether the circuit breaker is approaching a protection characteristic and therefore at risk of tripping or disconnecting. If so, operation of a generator and/or a power converter of the wind power plant is derated to mitigate the risk of a trip or disconnection event.

A specific implementation of such an approach is outlined below with reference to Figures 1 to 5, which demonstrates how the inventive concept can be incorporated into an existing control architecture that is already configured to derate operation of a wind power plant in view of other considerations. It should be appreciated that this implementation is described by way of example only. The skilled reader will appreciate that there are various ways in which the risk of a circuit breaker trip or disconnection event occurring can be evaluated, and in which derating operation of the power plant to avoid such events can be managed.

Moreover, although embodiments of the invention are described in relation to a particular type of wind power plant having a full-scale architecture, in practice the invention is applicable to any type of wind power plant or wind turbine system, such as DFIG arrangements. Accordingly, to provide context for the invention, Figure 1 shows an individual wind turbine generator 1 of a kind that may be controlled according to embodiments of the invention. The wind turbine generator 1 shown is a three-bladed upwind horizontal-axis wind turbine (HAWT), which is the most common type of turbine in use.

The wind turbine generator 1 comprises a turbine rotor 2 having three blades 3, the rotor 2 being supported at the front of a nacelle 4 in the usual way. It is noted that although three blades is common, different numbers of blades may be used in alternative embodiments. The nacelle 4 is in turn mounted at the top of a support tower 5, which is secured to a foundation (not shown) that is embedded in the ground.

The nacelle 4 contains a generator (not shown in Figure 1 ) that is driven by the rotor 2 to produce electrical energy. Thus, the wind turbine generator 1 is able to generate electrical power from a flow of wind passing through the swept area of the rotor 2 causing the rotation of the blades 3.

With reference now to Figure 2, an example of a wind power plant 12 to which methods according to embodiments of the invention may be applied is shown. The example shown in Figure 2 is based on a full-scale converter architecture, although as noted above embodiments of the invention may be used with other types of converter and in general terms the invention is suitable for use with all topologies.

Moreover, the components of the wind power plant 12 are conventional and as such familiar to the skilled reader, and so will only be described in overview.

The wind power plant 12 shown in Figure 2 includes a single wind turbine generator 1 such as that shown in Figure 1 , but in practice further wind turbine generators may be included.

As already noted, the wind turbine generator 1 comprises an electrical generator 20 that is driven by the rotor 2 to produce electrical power. The electrical generator 20 includes a central armature 21 that is driven by the rotor 2 to rotate within a stator 23. The stator 23 contains one or more sets of three-phase windings (not shown) in which electrical current is induced in response to varying magnetic flux created by rotation of the armature 21 , under the control of a turbine controller 27.

Generator circuit breakers 25 are included on each output phase of the stator 23, to protect elements of the wind power plant 12 beyond the generator 1 . The wind power plant 12 also includes a low voltage link 14 defined by a bundle of low voltage lines 16 terminating at a coupling transformer 18, which acts as a terminal that connects the wind turbine generator 1 to a grid transmission line that in turn connects to a power grid. Electrical power produced by the wind turbine generator 1 is delivered to the grid through the coupling transformer 18.

The electrical generator 20 of a full-scale architecture typically produces multiphase electrical power. In this embodiment, the power produced in the electrical generator 20 is three-phase AC, but is not in a form suitable for delivery to the grid, in particular because it is typically not at the correct frequency or phase angle. Accordingly, the wind turbine generator 1 includes a power converter 22 and a filter 24 disposed between the electrical generator 20 and the coupling transformer 18, to process the electrical generator 20 output into a suitable waveform having the same frequency as the grid and the appropriate phase angle.

The power converter 22 provides AC to AC conversion by feeding electrical current through an AC-DC converter 26 followed by a DC-AC converter 28 in series. The AC-DC converter 26 is connected to the DC-AC converter 28 by a conventional DC link 30, which includes a switched resistor 32 to act as a dump load to enable excess energy to be discharged, and a capacitor 34 providing smoothing for the DC output.

Any suitable power converter 22 may be used. In this embodiment, the AC-DC and DC-AC parts of the power converter 22 are defined by respective bridges of switching devices (not shown), for example in the configuration of a conventional two level back-to-back converter. Suitable switching devices for this purpose include integrated gate bipolar transistors (IGBTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs). The switching devices are typically operated using pulse-width modulated drive signals.

The smoothed DC output of the AC-DC converter 26 is received as a DC input by the DC-AC converter 28 and creates a three-phase AC output for delivery to the coupling transformer 18.

The AC output of the power converter 22 is carried by the three power lines 16 that together define the low voltage link 14, each line 16 carrying a respective phase. The low voltage link 14 includes the filter 24, which in this embodiment comprises a respective inductor 38 with a respective shunted filter capacitor 40 for each of the three power lines 16, to provide low-pass filtering for removing switching harmonics from the AC waveform. The three power lines 16 also each include a respective grid-side circuit breaker 41 for managing faults within the wind power plant 12 and to protect the grid from exposure to excessive electrical current.

As noted above, the low voltage link 14 terminates at the coupling transformer 18 which provides a required step-up in voltage. A high voltage output from the coupling transformer 18 defines a wind turbine generator terminal 42, which acts as a point of common coupling for the wind power plant 12.

The low voltage link 14 also includes three branches, one for each phase, that define auxiliary power lines 44 that divert some of the power that is output from the filter 24 for powering auxiliary systems of the wind power plant 12.

As noted above, in a full-scale architecture the DC-AC converter 28 is configured to provide a level of control over the characteristics of the AC power produced, for example to increase the relative reactive power in dependence on grid demand. Noting that the magnitude, angle and frequency of the output is dictated by grid requirements, and that the voltage is set at a constant level in accordance with the specifications of the low voltage link 14, in practice only the current of the AC output is controlled, and a converter controller 36 is provided for this purpose. The converter controller 36 and the turbine controller 27 together form part of an overall control system that controls operation of the wind power plant 12. The converter controller 36 is described in more detail later with reference to Figure 3.

The control system acts based on sample data obtained by a sampling system that probes the wind turbine generator 1 at various stages to sample electrical signals that are indicative of current or voltage, for example. In particular, as is typical for a full-scale architecture, the sampling system gathers raw data relating to the current and voltage of the outputs from the stator 23 of the generator 20, and from the power converter 22 on the grid side. This raw data is processed into sample data, which is then passed to the converter controller 36, for example. The converter controller 36 uses the sample data to determine operating parameters for the wind turbine generator 1 . For example, the duty cycle of the control signals for generator-side IGBTs of the power converter 22 may be determined, at least in part, based on the instantaneous properties of the generated power supplied by the electrical generator 20.

The control system also includes a supervision application that monitors the condition of various components of the wind power plant 12, including the circuit breakers 25, 41 , and is configured to take action in some circumstances. For example, as noted earlier the supervision application may intervene when a circuit breaker trip appears imminent by disconnecting the circuit breaker 25, 41 in question.

As Figure 3 shows, the converter controller 36 of this embodiment comprises an active power controller 46, a reactive power controller 48, a thermal capability manager 49, and a software block defining a power management module 50. The skilled reader will appreciate that in practice the converter controller 36 may include various other control modules, but for the purposes of the present disclosure only those that relate to power control are referred to.

The active power controller 46 and the reactive power controller 48 operate in tandem to interface to current controllers (not shown), which issue drive signals to the switching devices of the power converter 22 to control the active and reactive components of its AC output based on signals received from the active and reactive power controllers 46, 48. The active power controller 46 is configured to receive an active power reference from the power management module 50, and the reactive power manager 48 is configured to receive a reactive power reference from the power management module 50.

The thermal capability manager 49 includes modelling and evaluation resources that enable the thermal performance of various components of the wind power plant 12 to be determined based on the sample data provided by the sampling system, which enables optimised usage of those components. This may avoid the need to take direct measurements of temperature for each component, which is often impractical; although thermal capacity evaluation can also be based on direct measurements where available, which the thermal capability manager 49 is arranged to take into account in its modelling.

In this embodiment, alongside its main role of thermal modelling, the thermal capability manager 49 is adapted to monitor the sample data that it already has access to - which includes indications of the electrical currents passing through the generator circuit breakers 25 and the grid-side circuit breakers 41 - to determine whether any of the circuit breakers 25, 41 are close to violating respective defined protection characteristics and therefore trigger a trip or disconnection by the supervision application.

The thermal capability manager 49 is also configured to model the temperature of each circuit breaker 25, 41 . As the protection characteristic for each circuit breaker 25, 41 is dependent on temperature, the thermal capability manager 49 takes the temperature of the circuit breakers 25, 41 into account when assessing their condition. Indeed, as noted above in some arrangements the protection characteristic is defined in terms of the temperature of the circuit breaker 25, 41 instead of, or in combination with, the electrical current that it conducts. In such embodiments, modelling of the temperature of the circuit breaker 25, 41 enables the thermal capability manager 49 to determine whether the protection characteristic is close to being violated.

By monitoring the condition of each circuit breaker 25, 41 , the thermal capability manager 49 is also able to estimate the additional capacity available that can be used to load the generator 20 at any given time without risking a trip or disconnection.

Figure 4 shows the architecture of the thermal capability manager 49 in schematic form. The functionality provided by the thermal capability manager 49 extends beyond the scope of the present disclosure, and so shall only be described in overview to avoid obscuring the invention with unnecessary detail.

The thermal capability manager 49 includes a series of modules that each provides a dedicated function, which process sample data received from the sampling system in a sequence of stages for the purpose of generating derate references that are passed to the power management module 52. The derate references limit the output of the generator 20 and the converter 22 in accordance with the estimated temperatures of components of the wind power plant 12 and the condition of the circuit breakers 25, 41 .

Specifically, the thermal capability manager 49 comprises four modules: an operating point perturber 60; a component model 62; a converter capability evaluator 64; and a derate controller 66. These modules may be embodied as software blocks, for example, or alternatively as dedicated hardware components.

The operating point perturber 60 receives input data including the present active and reactive power references, and sample data indicating operating parameters such as voltage and current at various stages of the wind power plant 12, as well as ambient temperature. Based on this information, the operating point perturber 60 calculates active and reactive power levels above nominal power to evaluate the capacity of the circuit breakers 25, 41 relative to the present conditions. This enables the thermal capability manager 49 to assess how close the circuit breakers 25, 41 are to violating their respective protection characteristics. The results of this analysis are forwarded to the component model 62. The component model 62 determines information relating to the electrical current passing through each circuit breaker 25, 41 , which can then be used to evaluate whether any of the circuit breakers 25, 41 are approaching violation of a protection characteristic and therefore close to tripping, for example, taking also the sensitivity of the system to perturbations into account. The component model 62 is also adapted to estimate the effect of temperature in eroding a trip margin, namely a proximity to violation of a protection characteristic, for each circuit breaker 25, 41 .

Although only one component model 62 is shown in Figure 4 for simplicity, in practice a respective component model 62 is included for each component that is to be monitored. Other component models 62 may operate to use mathematical models to simulate the operating parameters of a given component, to estimate the power dissipated in that component. This can in turn be used to estimate the temperature of the component. For example, the component model 62 may utilise Foster models to simulate component behaviour and the response of the system to perturbations, although any other suitable model may be used.

The outputs from the, or each, component model 62 are passed to the converter capability evaluator 64, which collates estimates of the condition of a range of components and analyses the overall state of the converter 22 to evaluate its present operating capability, including its sensitivity to perturbations.

The converter capability evaluator 64 outputs an indication of the present capability of the converter 22 to the derate controller 66, which generates respective derate factors for active power and reactive power accordingly. The derate factors are passed to the power management module 50, which uses them to update active and reactive power references that are used to control operation of the converter 22 and the generator 20.

Accordingly, between them the operating point perturber 60, the component model 62 and the converter capability evaluator 64 enable the thermal capability manager 49 to plan for and schedule short-term overloading of the circuit breakers 25, 41 , without having to derate the system. Where this is not possible, for example if one of the circuit breakers 25, 41 is operating too close to violating its protection characteristic, the derate controller 66 is utilised to derate the converter 22 and in turn the generator 20, and thereby avoid a trip from occurring.

Returning to Figure 3, the power management module 50 provides a suite of functions that enable the processing and optimisation of power references that arise within the wind power plant 12, and those received from external sources such as a transmission system operator responsible for the grid, a power plant controller responsible for multiple wind turbine generators within a single wind power plant, or the turbine controller 27, for example.

The power management module 50 is further arranged to modify the power references in accordance with internally-derived degrade factors, which take the derate factors received from the thermal capability manager 49 into account.

Like the thermal capability manager 49, the power management module 50 is modularised, in that it comprises a set of discrete modules that each provide a specific function. In this embodiment, those modules are implemented as individual software blocks within a common processing unit, but in other arrangements dedicated hardware modules could be used.

The modularised arrangement enhances integration with the converter controller 36, in particular because it enables individual functions to be developed and upgraded without impacting other functions. Moreover, a clearly defined hierarchy between the different functions can be created, thus improving interaction between the functions and therefore improving the efficiency of the converter controller 36.

More specifically, in this embodiment the power management module 50 includes a power reference manager 52, a power capability manager 54 and a degrade mode manger 56. These modules are ordered according to a hierarchy in which the degrade mode manager 56 provides inputs to the power capability manager 54, which in turn provides inputs to the power reference manager 52, which then transmits an active power reference and a reactive power reference to the active power controller 46 and the reactive power controller 48 respectively.

The degrade mode manager 56 is arranged to degrade, or derate, the power generating capability of the generator 20 based on instantaneous operating parameters as well as the derate factors output by the thermal capability manager 49. By taking the latter into account, the generating capability of the wind power plant 12 is degraded if approaching violation of a protection characteristic is detected or expected in one of the circuit breakers 25, 41 , thereby avoiding a circuit breaker trip and so preventing interruptions in operation of the wind power plant 12.

The degrade mode manager 56 therefore relates to the level of power that the wind turbine generator 1 is able to produce at a fundamental level, in view of either safety considerations or physical constraints. To this end, the degrade mode manager 56 calculates degrade factors of between 0 and 1 that are applied globally throughout the system. In a simple example, if the degrade mode manager 56 determines that the generator 20 is only capable of outputting half of its normal capacity in terms of active power due to elevated coolant temperature, the degrade mode manager 56 calculates a degrade factor of 0.5 for active power. It should be appreciated that the degrade factors can be implemented in any suitable form, for example as absolute or relative values.

In this respect, it is noted that as circuit breaker trip or disconnection events should be avoided, in this embodiment the derate factors that are calculated to avoid such events define a maximum capability of the converter 22 and, in turn, the generator 20. Any other factors that might prompt further degradation, such as those mentioned below, are then applied within the parameters established by the derate factors. So, for example, if the derate factors are set at 0.8, the degrade mode manager will output a degrade factor in the range 0 to 0.8.

The degrade factors calculated by the degrade mode manager 56 are output to the power capability manager 54, which uses the degrade factors to update a P-Q chart that defines the ratio of active power to reactive power that the wind turbine generator 1 is able to produce, as well as absolute magnitudes for each type of power. An example of a P-Q chart 70 that may be used by the converter controller 36 is shown in Figure 5, which plots active power in kilowatts, on the x-axis, against reactive power in kilovolt-amperes reactive, on the y-axis.

A solid line 72 forming a trapezoidal shape represents the capability of the generator 20 when operating at full capacity. The skilled reader will appreciate that this shape is typical for any P- Q chart for a generator 20 of a wind turbine generator. Within the solid line 72, a dashed line 74 forming a smaller trapezium represents a degraded capability for the generator 20.

It is noted that in the example shown in Figure 5, both active and reactive power are degraded in the degraded capability represented by the dashed line 74, and each by equal amounts. However, in other operating modes only one of these may be degraded, or different weighting may be applied to each type of power. For example, if active power is prioritised over reactive power, reactive power is degraded to a greater extent than active power, and optionally only reactive power is degraded. Correspondingly, if reactive power is prioritised, active power is degraded to a greater extent than reactive power.

The lines shown on the P-Q chart 70 therefore define the long-term power generating capability of the wind turbine generator 1 . The power capability manager 54 updates the P-Q chart 70 according to the degrade factors generated by the degrade mode manager 56, if those factors fall below 1 . The power capability manager 54 then generates active and reactive power limits by checking the updated P-Q chart 70 against a prioritisation of active power against reactive power, which is defined by an operating mode of the wind turbine generator 1 as indicated by the power plant controller or the turbine controller.

For example, if reactive power is prioritised, but a reactive power reference supplied by the power plant controller or the turbine controller 27 cannot be met within the limits of the updated P-Q chart 70, the power capability manager 54 adjusts the active and reactive power limits accordingly by degrading the active power limit further to enable the reactive power demand to be met.

In turn, once the power capability manager 54 has updated the P-Q chart 70 in accordance with the degrade factor supplied by the degrade mode manager 56, and generated active and reactive power limits in accordance with the prioritisation between the two types of power, those power limits are communicated back to the power plant controller or turbine controller 27 as a request for power reduction. The power plant controller and turbine controller 27 can then take the request into account when generating the next set of power references, thereby providing a feedback loop for this element of the control. In this way, the changes defined by the derate factors based on circuit breaker 25, 41 condition are propagated throughout the wind power plant 12.

The updated P-Q chart 70 is transmitted to the power reference manager 52, which also receives several power references from various sources. In this respect, the power reference manager 52 includes an input (not shown) that is configured to receive the various power references. The power reference manager 52 further includes a processor 58 that is arranged to analyse the input power references to determine output active and reactive power references, and an output (not shown) configured to transmit those references to the power converter 22, as shall be described.

The references received at the input of the power reference manager 52 include the active and reactive power references received from the power plant controller or turbine controller, along with various internal active power references that together define an auxiliary demand.

The power reference manager 52 also prioritises reactive power over active power - or vice- versa - in the short-term, according to the same prioritisation applied by the power capability manager 54. Typically, the power plant controller or turbine controller 27 issues an active power reference indicating the level of real power that the wind power plant 12 must deliver, along with a reactive power reference. As an alternative to a reactive power reference, or in addition to one, the power plant controller or turbine controller 27 may supply a power factor (or‘CosPhi’) reference, that defines the ratio of real power to the total power dissipated in the system, or ‘apparent power’, in which case the power reference manager 52 is responsible for determining a reactive power reference based on the active power reference and the power factor reference. As the skilled person would understand, the reactive power reference can be derived from these inputs using basic geometric and trigonometric relations. The power reference manager 52 may have the option either to calculate the reactive power reference from the power factor and active power references, or to use the reactive power reference supplied by the power plant controller or turbine controller 27.

The power reference manager 52 compares the power references that it receives with the present capability of the wind turbine generator 1 as indicated by the P-Q chart 70 received from the power capability manager 54, and determines whether the demands to which those references relate can all be met whilst simultaneously supplying adequate reactive power.

If the demands can be met, the power reference manager 52 simply generates active and reactive power references that represent the respective totals of the different active and reactive power references that it receives. If demand cannot be met within the constraints of the P-Q chart 70, the power reference manager 52 prioritises the references that it receives according to a pre-determined regime. The converter controller 36 may also communicate the inability to meet demand to the turbine controller 27 or the power plant controller, in case derating is required.

By creating the active and reactive power references based on the various demands arising throughout the system, the power reference manager 52 avoids operating the wind turbine generator 1 at its operational limits - as indicated by the power capability manager 54 - at all times. This in turn increases operational efficiency and reduces the risk of circuit breaker trip or disconnection events.

As already noted, embodiments of invention are also applicable to other types of wind turbine system, including DFIG topologies having a doubly fed induction generator with a rotor- connected converter. Although the skilled person will be familiar with such arrangements, for completeness Figure 6 shows in overview an example of a wind power plant 80 having such an architecture.

The wind power plant 80 of Figure 6 has a generator 82 comprising a set of rotor windings that are driven by the rotor 2, and a set of stator windings. To enable the generator 82 to produce electrical power when the rotor windings rotate, an excitation current is fed to the rotor windings by a power converter 84.

The output of the generator 82 is connected to a three-way coupling transformer 86 that provides electrical connection to a point of common coupling (not shown) to a grid, and to the power converter 84. In turn, the power converter 84 is connected to the rotor windings of the generator 82, thereby defining a feedback loop. Thus, once power generation commences, the power converter 84 can use the output of the generator 82 to produce the excitation current that is delivered to the rotor windings.

In this arrangement, as for the full-scale architecture of Figure 2, circuit breakers are used to protect the grid from excessive electrical power. In the Figure 6 configuration, generator circuit breakers 25 are positioned between the stator of the generator 82 and the coupling transformer 86, optionally on the stator itself. Grid-side circuit breakers 41 are located between the power converter 84 and the coupling transformer 86.

The above described principles of monitoring the condition of each circuit breaker 25, 41 and modifying operation of the wind power plant accordingly apply equally to the arrangement of Figure 6 as for that of Figure 2. Moreover, the architecture of the elements of the converter controller 36 that oversee this process are also the same for the wind power plant 80 shown in Figure 6 as for the full-scale architecture of Figure 2.

In summary, the present invention makes use of the functionality of the thermal capability manager 49 to monitor the condition of circuit breakers 25, 41 within the wind power plant 12. This in turn is used to update derate factors that are transmitted to the power management module 52, which are incorporated into degrade factors that define limits on the capability of the converter 22 as defined by the P-Q chart 70. This eventually feeds through to the final active and reactive power references that are output by the power reference manager 52 and transferred to the active power controller 46 and reactive power controller 48 accordingly. This in turn feeds back to the turbine controller 27 and thus effects limitation of the generator output as required. In this way, the new approach for avoiding circuit breaker trips by limiting the power output by the generator 20 and processed by the converter 22 is integrated into an existing architecture that already includes provisions for derating the system in view of other considerations. The skilled person will appreciate that modifications may be made to the specific embodiments described above without departing from the inventive concept as defined by the claims.

In particular, while it is convenient to incorporate derating for protecting circuit breakers into the existing control architecture, there is no requirement to do so and many other implementations are possible. For example, the electrical current passing through a circuit breaker could be modelled based on sample data collected elsewhere in the system, instead of being monitored directly. In addition, any signal output by a circuit breaker that is indicative of its status may be taken into account. Similarly, the condition of a circuit breaker may be assessed by a dedicated hardware or software module, which may then output a derate factor based solely on the circuit breaker condition, which can be passed to the power management module or equivalent control structure to influence the active and reactive power references that are used to operate the wind power plant.