Field of the invention The present invention relates to a control system for controlling a plurality of electrical high-power components in a power converter of an electrical power generating system, such as a variable speed wind turbine.

Background of the invention

The power converter is an essential and expensive component in electrical power generating systems. Without the power converter, the power generating system is not able to produce power, which can be supplied to the utility grid. Therefore, one way of optimising the performance of a power generating system, such as a variable speed wind turbine, is to be able to control the power converter more efficiently.

Brief description of the invention

It is an object of the present invention to provide a more efficient control system for power converters of electrical power generating systems, such as variable speed wind turbines.

The present invention relates to a power converter control system for controlling a plurality of electrical high-power components in a power converter of an electrical power generating system, such as a wind turbine, said electrical high-power components being arranged and controlled to perform active current sharing so that, for a given phase, a number of the electrical high-power components together supply the load current of that phase, wherein the power converter control system is arranged to detect if one or more of the electrical high-power components will no longer be able to deliver a requested load current of one or more phases, and control the overall performance of the electrical power generating system so that at least a minimum of output from the electrical power generating system is maintained.

A situation, in which it could be relevant to secure a minimum output from the electrical power generating system, is when an electrical high power component is damaged instantly and therefore only is able to carry less or no load current. Instead of shutting down the electrical power generating system, the control system is able to keep the electrical power generating system in operation by performing active power sharing. This would include loading the healthy electrical high power components more while loading the damaged ones less.

Typically, the active current sharing is used to distribute load between healthy electrical high power components when one or more electrical high power components fail (partly or completely) and the maximum output from the electrical power generating system from the actual wind speed is less than or equal to the capacity of the healthy electrical high power components. In this way, at least a minimum of production of energy from the electrical power generating system is maintained. The active current sharing between the electrical high-power components may be controlled by individual electrical high-power component controllers or from a more centrally placed controller, such as from the control system or from a higher-level controller. According to an advantageous embodiment of the invention, the expression "electrical high-power component" may refer to electronic power components, which together are capable of handling current, for instance by converting an AC input to an AC output or to a DC output. Such electronic components may also be referred to together as a part of a power converter or as a power stack. For the sake of convenience, the latter expression will be used throughout this document without limiting the scope of protection only to power stacks. Preferably, the electrical power generation system is a wind turbine, but it could also be an electrical power generation system producing power based on energy from the sun or from waves or it could be a hydroelectric plant, etc. Throughout this document, the electrical power generation system will be referred to as a wind turbine without limiting the scope of protection only to wind turbines.

When referring to current sharing and control of a power converter or the power stacks of a power converter, it is important to distinguish between active current sharing and passive current sharing as known in the art. The latter is often a simple oversizing of the mechanical construction of the power stacks, which ensures that the power stacks enable the power converter to carry more load than normally necessary. This, of course, leads to a more expensive construction. Alternatively, if not oversized (sufficiently), the wind turbine is simply shut down.

In some embodiments, as described below, the control system of the present invention is advantageous in that it is able to monitor the load current of the individual power stacks during operation of the power converter. Furthermore, in some embodiments, in case of malfunction of one or more power stacks of the power converter, the control system is able to provide other controllers of the wind turbine, e.g. the wind turbine controller, with information regarding the malfunction. In case the malfunctioned power stack(s) reduces the capacity of the power converter, the control system may initiate a derating of the overall energy production of the wind turbine for ensuring at least a minimum production of energy. Thereby it is possible to continue producing energy, which is very advantageous compared to shutting down the production of energy. Being able to keep the wind turbine in operation is very advantageous to the owner of the energy produced by the wind turbine in order to get as fast return of investment as possible. Furthermore, it is very advantageous that service of the malfunctioned power stack(s) can be planned, i.e. a service visit at the wind turbine can wait until planned service or until other components of the wind turbine requires service. This is especially true if the wind turbine is located offshore. In an embodiment of the invention, the power converter control system is controlling the overall performance of the electrical power generating system according to a given set of rules.

In case something is happening, e.g., with one or more of the electrical high power components, which reduces the capability of such electrical high power component(s) to carry a load, the power converter control system facilitates control based on a given set of rules. The given set or rules may then determine the load distribution in the power converter, derate components, reduce nominal output, etc. The rules may be predetermined, i.e. they may be developed to act on a given fault situation, or they may facilitate control based on measurements of the actual occurring fault.

Typically, one or more rules from the set of rules is used to distribute load between healthy electrical high power components when one or more electrical high power components fail (partly or completely) and the maximum output from the electrical power generating system from the actual wind speed is equal to or higher than the capacity of the healthy electrical high power components. In this situation, a derating of the overall production is an option to ensure at least a minimum of production from the electrical power generating system. In an embodiment of the invention, the power converter control system is arranged to be able to send a request to a central controller of the electrical power generation system for reducing the nominal output from the electrical power generation system.

The control system of the present invention is able to communicate with the wind turbine controller which is very advantageous in that the control system then is able to provide the wind turbine controller with information related to the status of the load of the power converter and/or the individual power stacks. This information may be used by the wind turbine controller to decide the control strategy in relation to maximum production, derating of the overall energy production of the wind turbine, etc.

This decision may include economic considerations, i.e. weighing the current energy price with a prediction on how long the power converter or the individual power stacks is able to be in operation at different load levels.

Furthermore, beside the derating considerations, the wind turbine controller may, in worst case, use the information to completely shut down the production of energy.

In an embodiment of the invention, the power converter control system is arranged to be able to send a request to a central controller of the electrical power generation system for derating the performance of one or more of the electrical high-power components.

If it is detected that an electrical high power component is not able to be loaded fully, the part of the load current handled by that particular electrical high power component may be reduced or the electrical high power component may be completely shut down. In an embodiment of the invention, the power converter control system further is arranged to be able to detect if a given electrical high-power component will no longer be able to deliver a requested load current without an increased risk of critical failure, and either adjust an operational parameter for the performance of that particular electrical high-power component or disconnect the electrical high-power component in order to avoid such critical failures without taking the power converter out of operation.

In an embodiment of the invention, the power converter control system is further arranged to detect if, within a certain period of time, a given electrical high-power component will no longer be able to deliver a requested load current without an increased risk of critical failure.

In an embodiment of the invention, the control procedures based on the given set of rules include initiation of a derating of the overall production of the electrical power generation system

In an embodiment of the invention, the control procedures based on the given set of rules include initiation of an offline test of the electrical power generation system.

In an embodiment of the invention, the control procedures based on the given set of rules include initiation of a stop of production of the electrical power generation system.

The control system of the present invention is advantageous in that it is able to control the load current of the individual power stacks of the power converter. During normal operation, the power stacks of the power converter are able to share the total load current of the power converter among each other and the control system is able to monitor the load of the individual power stack.

In the case of detecting a malfunction or other abnormalities of one particular power stack in the power converter, the control system is able to control the operation of the power converter, e.g. at least partly take over control of how the load current is to be shared among the remaining power stacks, preferably based on predetermined rules. The predetermined rules may be different depending on the type of malfunction, which or how many power stacks are malfunctioned, etc. The control system may control the operation of the power converter based on operational parameters. Such operational parameters may be used in the control of the individual power stacks to determine the load current of the individual power stacks, i.e. by controlling operational parameters for all power stacks in the power converter, the control system is able to determine the current sharing between the power stacks of the power converter.

By being able to control the distribution of the load current among the power stacks, the control system is able to keep the wind turbine in operation in spite of the before- mentioned malfunction situation.

In an embodiment of the invention, the control of the active current sharing includes at least one individual operational parameter for the performances of the individual electrical high-power components. As previously mentioned, the control of the active current sharing between power stacks may be provided by individual power stack controllers, the control system or higher-level controllers. In any case, the set of rules defining how much of the current of a given phase, the individual power stack is to handle, may be accompanied with operational parameters.

Preferably, the operational parameters are individual operational parameters for the control of the individual power stack, i.e. the operational parameters supplied to the individual power stacks may differ from power stack to power stack. By means of the operational parameters, the control system is capable of taking over control of the individual power stacks completely or to partly control the individual power stacks. The latter situation may include deciding a frame of the operation for the individual power stack, so that the overall control of all the power stacks comply with an overall control strategy for the power converter and/or the overall control strategy of the wind turbine.

In an embodiment of the invention, the at least one individual operational parameter is defined by the power converter control system either as absolute values or as percentages of the total load current to be delivered by the one or more phases, to which the electrical high-power component belongs.

As previously mentioned, the control system may at least partly take over control of the individual power stacks or at least supply the individual power stack controllers with new operational parameters. Preferably, this could be done by adjusting the operational parameters dedicated to a specific power stack to reflect a desired percentage of the load current of a phase, which that specific power stack is to handle.

In an embodiment of the invention, in normal operation of the power converter, the at least one individual operational parameter is defined taking into consideration lifetime considerations for the electrical high-power components, so that less performance is requested by electrical high-power components with less expected remaining lifetime.

As previously mentioned, the control system may at least partly take over control of the individual power stacks. Preferably, this could be done by adjusting the operational parameters dedicated to a specific power stack to reflect lifetime considerations of the individual power stack to be controlled.

In an embodiment of the invention, in normal operation of the power converter, the at least one individual operational parameter is defined taking into consideration the electrical high-power component position in a cooling system so that less performance is requested by electrical high-power components cooled by warmer cooling medium.

As previously mentioned, the control system may at least partly take over control of the individual power stacks. This could preferably be done by adjusting the operational parameters dedicated to a specific power stack to reflect the position of that power stack in the cooling system. If the cooling system is based on cooling liquid, the performance of the power stack cooled by the warmest cooling liquid may be reduced.

In an embodiment of the invention, the individual operational parameter is found by testing the electrical high-power component.

The testing may result in a characteristic of the electrical high-power component. This characteristic may describe the electrical properties of the electrical high-power component. Such electrical properties may, e.g., be the resistance and thereby indirectly the electrical losses and/or the maximum load of the individual electrical high power component. This may be used in the active current sharing in order to spread the load current between different electrical high-power components and thereby obtain a more equal wear of the individual electrical high-power components.

The test may be made before the electrical high-power component is taken into production, i.e. during production of the component or when it is mounted in a power converter.

In an embodiment of the invention, during fault conditions, in which one or more electrical high-power components are derated or disconnected and the others deliver higher load currents than in normal operation, the control system is arranged to recalculate the individual operational parameters of the electrical high-power components in operation according to the given set of rules describing the actual situation.

Operational parameters may, for instance, be current sharing factors, absolute current loads for an electrical high-power component, etc.

As previously mentioned, the control system may at least partly take over control of the individual power stacks. This could preferably be the case if a power stack fails and thereby is not able to operate. In this case, the load current for the one or more phases with the failed power stack(s) are divided between the remaining power stacks supplying the current for that one or more phases, respectively. This may require that the overall production is derated, but still it is possible to operate the power converter, which is very advantageous. In an embodiment of the invention, the recalculation is done taking into consideration experience from previous similar situations, for instance in the form of well-functioning configurations.

When deciding how the load current from the failed power stack is to be divided between the remaining power stacks, it is very advantageous to be able to include experiences of previous situations, in which a power stack has failed. This may lead to faster decisions, which are advantageous due to the very short time a power stack can handle critical overload before failing. In an embodiment of the invention, the recalculation is based on a rule based control system.

The use of a rule based system, such as Fuzzy logic, is advantageous in that it makes it possible to predetermine the control strategy according to different input scenarios, i.e. what should happen if the capacity of one power stack is reduced with 50%, two power stacks with 50%, one is completely broken, etc. Preferably, the rule based system used may be based on a control theory chosen from the list comprising: Neural networks, Bayesian probability, Fuzzy logic, Machine learning and Evolutionary computation or genetic algorithms.

In an embodiment of the invention, the given set of rules is used by the rule based control system to control the electrical power generation system in order to ensure a minimum of production.

In an embodiment of the invention, the electrical power generation system is a wind turbine.

Figures A few exemplary embodiments of the invention will be described in more detail in the following with reference to the figures, of which fig. 1 illustrates a wind turbine according to an embodiment of the invention, fig. 2 illustrates a control system for a wind turbine according to an

embodiment of the invention, fig. 3 illustrates a power converter according to an embodiment of the

invention and fig. 4 illustrates a control system of a power converter according to an

embodiment of the invention. Detailed description of the invention

Fig. 1 illustrates an electrical power generating system in form of a variable speed wind turbine 1 according to an embodiment of the invention. The wind turbine 1 comprises a tower 2, a nacelle 3, a hub 4 and two or more blades 5. The blades 5 of the wind turbine 1 are rotatably mounted on the hub 4, together with which they are referred to as the rotor. The rotation of a blade 5 along its longitudinal axial is referred to as pitch. The wind turbine 1 is controlled by a control system comprising a wind turbine controller 6, sub controllers 7 for controlling different parts of the wind turbine 1 and communication lines 8.

The wind turbine 1 further comprises a generator 9 and a power converter 10. The generator 9 transforms rotational energy from the rotor into electrical energy and the power converter 10 "shapes" the electrical energy from the generator 9 into a form, which complies with utility grid demands. The electrical energy is transported from the generator 9 to the converter 10 and further to the utility grid 12 via high voltage cables 11.

Fig. 2 illustrates a control system according to an embodiment of the invention. The wind turbine 1 is controlled by a main controller 6, often referred to as the wind turbine controller 6. The wind turbine controller 6 communicates via communication lines 8 with a plurality of sub controllers 7. The sub controllers 7 control different parts of the wind turbine 1, respectively, such as for example the pitch of the blades 5 and the operation of the power converter 10. Hence, the wind turbine controller 6 may receive information about, e.g. wind speed from a sub controller 7 or from a sensor device (not illustrated) and information of capacity, e.g. of the power converter 10, and based hereon, the wind turbine controller 6 may describe a pitch angle for the blades 5, which relates to the production of a given amount of electrical energy. Based in inputs from different parts of the wind turbine 1, the wind turbine controller 6 will adjust the electrical energy production during production. The power converter 10 comprises electrical high-power components 13 for shaping the current from the generator 9 to at least partly comply with utility grid demands. It should be mentioned that other (not illustrated) components, such as, e.g. a transformer, may be needed to completely comply with utility grid demands.

The electrical high-power components 13 may be referred to as power stacks 13. Preferably, each power stack 13A, 13B, 13C, 13n converts three phases.

According to a preferred embodiment of the invention, each of the power stacks is controlled by a power stack controller 14A, 14B, 14C, 14n.

If the wind turbine 1 is part of a wind power park, the wind turbine controller 6 may communicate via communication lines 8 with a wind power park controller 15. Further, the wind turbine controller 6 may communicate via communication lines 8 via a public data communication network 16, such as the Internet, with the owner 17 of the wind turbine 1, with utility grid operators 18, etc. In the same way, the wind power park controller 15 may communicate with its surroundings.

Furthermore, it should be mentioned that the control system may also communicate with meteorological stations, transformers and other sub stations (not shown). The communication via the communication lines 8 may be in the form of wired or wireless data communication.

Fig. 3 illustrates a power converter 10 according to an embodiment of the invention. High voltage cables 11 provide the power converter 10 with current from the generator 9, which is positioned within the nacelle 3. The power stacks 13 A, 13B, 13C, ... , 13n shape the current and delivers the shaped current to the utility grid 12 via other high voltage cables 11.

Fig. 3 illustrates that each of the power stacks 13 A, 13B, 13C, ... , 13n carries load from three phases. It should be mentioned that in other configurations, a given power stack 13 A, 13B, 13C, ... , 13n may carry the load from only one single phase. This could be the case if, e.g., the generator 9 comprises a plurality of individual sets of windings. Thus, in another embodiment, if the generator 9 comprises a plurality of windings, the current of one set of windings may be handled by one power stack 13 A, the current from a second set of windings may be handled by a second power stack 13B, etc. This embodiment is not illustrated in fig. 3.

According to the illustrated embodiment, the wind turbine controller 6 controls the power converter 10 via a sub controller 7, i.e. a power converter controller 7 A. The power converter controller 7A controls the individual power stacks 13A, 13B, 13C, 13n via individual power stack controllers 14A, 14B, 14C, 14n. During operation, this control includes providing reference values to the individual power stack controllers 14A, 14B, 14C, ... , 14n, based on which the load current of each of the respective power stacks 13 A, 13B, 13C, 13n can be determined. Furthermore, the power converter controller 7A performs the overall control of the power converter 10, e.g. start up, time of connecting to the grid, etc.

The power converter controller 7A may comprise an algorithm for processing input from the wind turbine controller 6 and from the power stack controllers 14 A, 14B, 14C, ... , 14n in order to provide reference values, which are used by the individual power stack controllers 14A, 14B, 14C, ... , 14n to control the respective power stacks 13 A, 13B, 13C, ... , 13n. Furthermore, the algorithm may supply the wind turbine controller 6 with information on the status, max capacity, etc., of the power converter 10. During normal operation, the algorithm of the power converter controller 7A may control the power converter 10 according to a normal operation mode or according to a first set of rules. If anything deviates from the normal operation scenario, the algorithm may start to control the power converter 10 according to another operation mode depending on the deviation or according to one or more of a plurality of different sets of rules. These different sets of rules may be stored on a data storage (not illustrated), which may be part of the power converter controller 7A and may be referred to as second sets of rules.

This way of building up the control of the power converter 10, i.e. by equipping each power stack 13 A, 13B, 13C, ... , 13n with a respective power stack controller 14A, 14B, 14C, ... , 14n for controlling the individual switches of the power stacks 13 A, 13B, 13C, ... , 13n enables for an autonomous and very fast adaption to changes in the individual power stack 13 A, 13B, 13C, ... , 13n or elsewhere in the system, for instance due to failure of a power stack 13 A, 13B, 13C, ... , 13n.

This is explained in further details in relation to fig. 4, which illustrates power stacks 13A, 13B, 13C, ... , 13η, power stack controllers 14A, 14B, 14C, ... , 14η, a power converter controller 7A, a wind turbine controller 6 and one additional sub controller in form of a pitch controller 7B.

All communication between the elements illustrated in fig. 4 takes place via communication lines 8, which may be wired, e.g. in form of optical fibres or copper cables, or use any known wireless communication protocol, such as Bluetooth. During operation of the power converter 10, the power stack controllers 14 A, 14B, 14C, ... , 14n exchange operational data. This could be done by circulating a telegram with information of operational parameters related to the individual power stacks 13 A, 13B, 13C, ... , 13n. In this way, each of the power stack controllers 14A, 14B, 14C, ... , 14n always knows the operational status of all the power stacks 13 A, 13B, 13C, 13n.

Preferably, the telegram comprises information on current, voltage and a set of rules from the power converter controller 7A, based on which the load currents of the individual power stacks 13 A, 13B, 13C, ... , 13n are to be controlled. The set of rules includes operational parameters for the individual power stacks 13 A, 13B, 13C,

13n, such as load share of maximum capacity of the power stack 13 A, 13B, 13C, ... , 13n, load share in percentage of the demand to the power converter 10, voltage, current, etc.

Hence, during normal operation of the power converter 10, a mutual understanding of the operational status of the power stacks 13 A, 13B, 13C, ... , 13n is achieved among the power stack controllers 14A, 14B, 14C, 14n.

Based on a first set of the rules from the power converter controller 7A, the power stack controllers 14A, 14B, 14C, ... , 14n know the load current to be handled and due to the mutual knowledge of operational status of the power stacks 13 A, 13B,

13C, ... , 13n, the power stack controllers 14A, 14B, 14C, ... , 14n are able to perform a mutual distribution of the load current ordered from the power converter controller 7A. Typically, the first set of rules for normal operation prescribes that the load current is shared equally between the power stacks 13 A, 13B, 13C, ... , 13n, i.e. if the power converter 10 comprises four power stacks 13A, 13B, 13C, 13D, each of these four power stacks 13A, 13B, 13C, 13D handles 25% of the load current. This is advantageous because then the individual power stacks 13 A, 13B, 13C, 13n are evenly worn and, thereby, service or replacement of the power stacks 13 A, 13B, 13C, ... , 13n can be expected more or less at the same time.

Alternatively, the first set of rules may prescribe that the power stack 13A, 13B, 13C, ... , 13n, e.g. with the least estimated life time remaining is to handle the least load current.

It should be mentioned that the use of different set of rules (referred to as first set, second set, third set, etc.) may depend on whether the wind turbine 1 is producing energy at the rated/nominal level, i.e. if a 2 MW wind turbine is actually producing 2 MW. If the wind turbine 1 is not producing at the rated level and a temperature of a certain power stack is increasing, a set of rules may simply demand the other power stacks to take over some of the load current from the warm power stack and thereby continue the production below the rated level.

If the wind turbine 1 is producing at the rated level and a power stack temperature increases, a set of rules may be used, which decreases the overall power production from the wind turbine 1. Alternative, a set of rules may prescribe to continue the production at the rated level, which will then cause either operating the warm power stack ignoring the increased temperature or loading the remaining power stacks above their optimal load level. The latter mentioned set of rules may be used for a certain period of time, for instance if the price of energy is high at that time.

In case the temperature continues to increase when the power converter is controlled according to a second set of rules, a third set of rules may be applied, based on which the power converter is then to be controlled. The third set of rules may command the warm power stack to not take any share of the load current and command the rest of the power stacks to take as much load current as possible while keeping the temperature under, e.g., 60 degrees Celsius. This may result in the wind turbine having to limit the production, for instance from 2 MW to 1.5 MW. In the same way, if a power stack for one reason or another suddenly disconnects or breaks down without any pre-warning or symptoms, the converter controller may act based on, e.g., a fourth set of rules. Such fourth set of rules may include calculating the remaining capacity of the converter and sending this result to the wind turbine controller in order for the wind turbine controller to be able to control the wind turbine according to the result of these calculations.

Minor deviations from normal operation could be that the temperature of, e.g., the power stack 13 A slowly increases from a predefined normal temperature of 55 °C to 65 °C. According to an embodiment of the invention, the control system of the power converter 10 could handle such minor deviations by acting based on a second set of rules to manage this increase of temperature without shutting down the wind turbine 1, thus ensuring that a minimum of output from the electrical power generating system is maintained as will be explained below.

The power stacks 13 A, 13B, 13C, ... , 13n may be equipped with temperature sensors (not illustrated) and the temperature of power stack 13 A may therefore be

communicated to the power stack controller 7A, e.g. via communication line 8A. When the increase is registered by the power stack controller 7A, a second set of rules prescribing a new way of sharing the load current is communicated to the power stack controllers 14A, 14B, 14C, 14n. This second set of rules prescribes or recalculates the operational parameters determining how the load current handled by the power stack 13 A should be decreased and the load current handled by the rest of the power stacks 13B, 13C, ... , 13n should be increased accordingly. In this way, the power converter controller 7A reduces the load on power stack 13 A until the temperature is back at a normal level.

The power converter controller 7A may communicate the increase in temperature of power stack 13 A, the result of decreasing the load of power stack 13 A, etc., to the wind turbine controller 6 for later use as part of the normal data log procedure. As indicated above, the power stacks 13 A, 13B, 13C, ... , 13n are monitored in that temperatures, currents, voltages, etc., are measured and preferably communicated to the power converter controller 7A via communication lines 8 or 8 A, 8B, 8C, ... , 8n.

If, within a period of time, a tendency is monitored and found to be continuing, and if it is estimated that this tendency is most likely to result in overload and/or a critical failure of a power stack 13 A, 13B, 13C, ... , 13n, the reference values of that specific power stack 13 A, 13B, 13C, ... , 13n may be adjusted according to the set of rules in order to avoid shut down of the power converter 10 and, thereby, of the entire wind turbine 1. The period of time, through which a tendency is monitored, depends on the character of the physical quantity being monitored and on the change in the monitored values. Hence, the period of time, through which an increase in a temperature is monitored before taking action and adjusting (also referred to as recalculating) an operational parameter of a power stack 13 A, 13B, 13C, ... , 13n, may be longer than the period of time, through which the increase of a current is monitored before adjusting an operational parameter of the power stack 13 A, 13B, 13C, 13n.

As can be understood from the above, both the start and the end of the period of time, through which a tendency is to be monitored, may be hard to define. What is easier to define is the time to react on a tendency, which could lead to critical failure. A power stack 13 A, 13B, 13C, ... , 13n according to the invention may be capable of handling an overcurrent corresponding to up to 110% of the rated current before an overcurrent shutdown occurs. Hence, if the monitoring of an increase of current in a power stack a power stack 13A, 13B, 13C, ... , 13n shows that a power stack 13A, 13B, 13C, ... , 13n is overloaded with, e.g., 108% of the rated current, then the control system has the time it takes before that power stack a power stack 13A, 13B, 13C, 13n is overloaded with 110% and thereby shuts down, to take corrective control actions.

As previously mentioned, if a power stack 13 A, 13B, 13C, 13n shuts down, the remaining power stacks 13 A, 13B, 13C, ... , 13n instantly take over the load until the wind turbine controller 6 derates the overall production (reduces the nominal output), e.g. by adjusting the pitch. The time passing from a critical failure is measured in the power stack 13 A, 13B, 13C, ... , 13n to this information is provided to the wind turbine controller 6 is typically 1 or 2 milliseconds. Then, the wind turbine controller 6 may request the pitch controller 7B to change the pitch of the blades 5, and before the blades 5 are in the new positions, a few seconds may have gone. A typical example could be that the change of the pitch starts less than 20 milliseconds from the communication between wind turbine controller 6 and pitch controller 7B and that the physical pitch angle is changed by between 5 degrees and 10 degrees per second.

Hence, when a power stack 13 A, 13B, 13C, ... , 13n shuts down, the remaining power stacks 13 A, 13B, 13C, ... , 13n are to handle the entire load current in a couple of seconds. This means that preferably, during normal operation, each of the power stacks 13 A, 13B, 13C, ... , 13n is controlled so that it has a buffer capacity in order to be able to perform this kind of power converter control. Thereby, it may be ensured that the power generating system keeps producing at least a minimum of energy instead of shutting down.

As can be understood from the above, the reaction time of the wind turbine 1 defines the time passing from when a deviation is measured to when corrective actions have taken place.

If the temperature of a given power stack 13 A continues to increase despite the implementation of the above-described second set of rules, the power stack 13 A may have a safety mechanism, which disconnects the power stack 13 A completely if the temperature reaches, e.g., 100°C. Such a completely disconnection of a power stack 13 A is considered a critical failure, and such a critical failure may lead to further critical failures in one or more of the rest or the power stacks 13B, 13C, ... , 13n.

Furthermore, component failures, sensor failures or loose electric connections may also be characterised as critical failures or events, which cause the rule based control to take over. When such a critical failure occurs, one or more of the controllers controlling the wind turbine 1 may detect that the electrical high-power components, such as the power stacks (including the one failed), will no longer be able to deliver the requested load current of one or more phases. Control based on one or more given set of rules may then be activated, thereby facilitating a continued production of energy. It should be mentioned that failures or critical failures as described above could also occur instantly. For instance, a power stack may simply break down due to electrical or mechanical wear, which cannot be measured or foreseen before the breakdown. In this situation, the controllers controlling the wind turbine will also switch to control based on a given set of rules as fast as possible.

According to the invention, such critical failures may be handled according to the third set of rules, so that the wind turbine 1 can continue to produce electrical power.

The control system illustrated in fig. 4 can be divided into three sub control systems, of which the first consists of the power stack controllers 14A, 14B, 14C, ... , 14n, the second consists of the power converter controller 7A, and the third consists of the wind turbine controller 6.

In case of critical failure, such as, e.g. a shut-down of a power stack 13 A, the first sub control system, i.e. the power stack controllers 14A, 14B, 14C, 14n, reacts instantly (within a few micro seconds) and independently of the power converter controller 7A. Hence, during the first microseconds after the shut-down, the remaining power stacks 13B, 13C, ... , 13n share the load current from power stack 13A, whereby these power stacks 13B, 13C, 13n become overloaded. Such overload will lead to critical failure if not dealt with within a few seconds.

Instead of shutting down the production of electrical power, the power converter controller 7 A communicates new values to the power stack controllers 14 A, 14B, 14C, ... , 14n and to the wind turbine controller 6 according to the third set of rules. New reference values (also referred to as operational parameters) for the current sharing are communicated to the power stack controllers 14A, 14B, 14C, 14n, and the reduced amount of power, which can be handled by the power converter 10, is communicated to the wind turbine controller 6. The new reference values are calculated based on a set of rules and facilitate that the power generating system keeps producing at least a minimum of energy.

The wind turbine controller 6 derates the production of electrical power, e.g. by pitching the blades 5 out of the wind to reduce rotation of the generator 9. The power stack controllers 14A, 14B, 14C, ... , 14n distribute the load current according to the received reference values according to the third set of rules. In its most extreme version, such derating could constitute a complete shut-down of the power converter of the wind turbine 1, but this is not preferred.

The distribution of load current among the power stacks 13A, 13B, 13C, ... , 13n may be determined taking into consideration the locations of the respective power stack 13 A, 13B, 13C, ... , 13n in the cooling circuit, the ages of the respective power stacks 13 A, 13B, 13C, 13n, individual power stack performance parameters, etc.

Three sets of rules, based on which the wind turbine 1 and, thereby, also the power converter 10 can be controlled have been described above. It should be mentioned that it is possible to create a plurality of sets of rules taking a plurality of different errors and conditions into consideration. Using such sets of rules, the power converter controller 7A can control the power stacks 13 A, 13B, 13C, ... , 13n and indirectly the wind turbine 1 by communicating the load capacity of the power converter 10 to the wind turbine controller 6.

Preferably, the plurality of different sets of rules consists of predetermined sets of rules, which the wind turbine controller or power converter controller may use to control the performance of the wind turbine 1 or parts of the wind turbine. Hence, the sets of rules which are predetermined may define control strategies, which define how, e.g., the power converter is to be controlled in specific situations, for instance in case of power stack failure, cooling system failure, cosmic radiation, sensor errors or loose connections, etc. Control according to the set of rules may implement derating of the overall production, distribution of load, production stop or initiation of offline tests, etc.

The offline tests may for instance be used to control if the broken power stack is electrical intact (i.e. if the switches are not damaged but one or more sensors may be broken) and thereby, if the power converter is operational, to enable the wind turbine 1 to produce energy without the damaged power stack.

As mentioned, different sets of rules may be created before the errors occur but they could also be created continuously when or after errors occur, thus making the control system adaptive. Such adaptive control systems are advantageous in that new scenarios, measurements, operational values, etc. can be used to optimise existing sets of rules or create new sets of rules. Controlling according to a set of rules (and preferably predetermined rules) facilitates that if something happens in the wind turbine to be controlled, the control system is able to find a way on its own to keep the production of the wind turbine up and running. Such a control system is preferably at least partly rule based. This should be understood so that when controlling the wind turbine 1 in the normal mode, this control may be done based on non-rule based control principle.. If something happens, which is critical or could lead to critical failure of parts of the wind turbine, such as, e.g. the power converter, the rule based control principles may be used to determine how the wind turbine should be controlled to avoid such critical failure.

Figuratively, it could be said that the normal wind turbine control is wrapped in a rule based control system, which knows how to control the wind turbine depending on the risk of occurrence of different critical failures. Of course, it could be said that the normal operation mode is a first set of rules under the control of this rule based control system. A plurality of different rule based systems could be used, of which some are: Neural networks, Bayesian probability, Fuzzy logic, Machine learning and Evolutionary computation or genetic algorithms.

Besides being a stack-wise current sharing, the above-described current sharing may also be used to control the current load of each phase within the individual power stacks 13 A, 13B, 13C, ... , 13n. This is advantageous in that symmetrical loads of the individual power stacks 13A, 13B, 13C, 13n may be obtained.

From the above, it can be seen that the control system of the present invention comprises a normal operation mode, which is "wrapped up" in a non-normal operation mode comprising predefined rules for handling errors or failures, thereby ensuring continued production rather than shut-down of the wind turbine 1.

As described above, during normal operation of the wind turbine and thereby also of the power converter, the control is based on a first set of rules and/or a normal control system. The normal control may include control of current sharing between the power stacks of the power converter.

The goal for the current sharing control is to control the load of the individual power stacks in a way so that the end of the lifetime of the power stacks of a power converter ideally is reached at the same time. One input to such current sharing control is individual power stack performance parameters. By individual power stack performance parameters is understood that, due to the manufacturing process, quality of components, etc, not two power stacks are identical from "birth". Hence, before using a power stack in the power converter, each power stack is tested to find at least the internal resistance, which is relevant for the heat generation, which in turn is relevant for end of lifetime estimations in the long run. The tests may also find other individual power stack performance parameters, such as the temperature of coolant at the location of the power stack in the power converter, the time delay in communication path of the power stack, etc.

Some of the test may be performed during production of the power stack, others when the power stack is implemented in a power converter.

It should be mentioned that some of the individual power stack performance parameters may change during the lifetime of the power stack and therefore additional tests or adjustments of these parameters in the current sharing control is expected.

Such adjustments may be made based on feedback from the power stacks during operation. Such feedback may be used to verify the test results, optimise the current sharing control, quality assurance, etc.

Furthermore, in faulty situations where the wind turbine and/or the power converter is controlled by a rule based control system, the above described current sharing control may also be taken into account when deciding the load distribution between power stacks when critical faults occurs.

It should be understood that the term "derate" is used to describe both reduction of the nominal output of the wind turbine and reduction of the load of a power stack, for instance when the power stack temperature is increasing, in which case the load of that power stack is derated. The term "reducing nominal output" is used to describe reducing the output from the wind turbine, for instance when the wind turbine is producing at its nominal output (when a 2 MW wind turbine is actually producing 2 MW). List of reference numbers

1. Wind turbine

2. Tower

3. Nacelle

4. Hub

5. Blade

6. Wind turbine controller

7. Sub controller

8. Communication line

9. Generator

10. Power converter

11. High voltage cable

12. Utility grid

13. Power stack

14. Power stack controller

15. Wind power park controller

16. Public data communication network

17. Owner of wind turbine

18. Utility grid operator