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The present invention relates to a method for operating a converter system of a wind turbine, wherein the converter system includes converter modules capable of converting electric power produced by a generator to electric power applicable to a utility grid. The converter modules include generator i

The present invention relates to a method for operating a converter system of a wind turbine, wherein the converter system includes converter modules capable of converting electric power produced by a generator to electric power applicable to a utility grid. The converter modules include generator inverters and grid inverters. The method determines the enabling/disabling of the converter modules in response to a parameter related to the variable amount of electric power being produced by the generator. Advantages of the present invention are optimisation of power efficiency of the converter modules and improved reliability of the converter modules. Another advantage is the capability of fast enabling and disabling of the converter modules.

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The invention claimed is: 1. A method for operating a converter system of a wind turbine, said converter system comprising a plurality of converter modules capable of converting a variable amount of electric power produced by a generator to electric power applicable to a utility grid, wherein said

The invention claimed is: 1. A method for operating a converter system of a wind turbine, said converter system comprising a plurality of converter modules capable of converting a variable amount of electric power produced by a generator to electric power applicable to a utility grid, wherein said plurality of converter modules being configured in parallel, and wherein the method comprises the step of determining an enabling/disabling procedure of at least one of the converter modules in response to a parameter related to the variable amount of electric power being produced by the generator. 2. A method according to claim 1, wherein the parameter related to the variable amount of electric power being produced by the generator may be selected from the group comprising: temperatures of components of the converter module, a power reference (Sref) being the reference for the apparent power to the converter system, a power reference (Pref) being the reference for the active power to the converter system, a current reference (Iref) being a current reference to the converter system, a measured value (Smes) of the apparent power produced by the generator, a measured value (Pmes) of the active power produced by the generator, and a measured value (Imes) of the current produced by the generator. 3. A method according to claim 1, wherein a disabling of at least one of the converter modules may be a short term disabling, said short term disabling being characterised by the at least one converter module being in a ready-state allowing a fast enabling of the converter module being short term disabled, wherein a fast enabling is defined relative to the time required for operating contactors (Kgen, Kgrid, Kcharge). 4. A method according to claim 1, wherein a disabling of a least one of the converter modules may be a long term disabling, said long term disabling being characterised by the at least one converter module being in an off-state, only allowing a slow enabling the converter module being long term disabled, wherein a slow enabling is defined relative to the time required for fast enabling of converter modules. 5. A method according to claim 1, wherein the step of determining the enabling/disabling procedure of at least one of the converter modules is carried out so that the output power of each enabled converter modules is maximised in relation to the rated power of each of the converter modules, preferably carried out so that the output power of each enabled converter modules is at least 80% of the rated output power, more preferred at least 90% of the rated power. 6. A method according to claim 1, wherein the step of determining the enabling/disabling procedure of at least one of the converter modules is carried out so that the output power of a selected number of enabled converter modules is maximised in relation to the rated power of each of the converter modules, preferably carried out so that the output power of each enabled converter modules is at least 50% of the rated output power, more preferred at least 80% of the rated power. 7. A method according to claim 1, wherein the step of determining the enabling/disabling procedure of at least one of the converter modules is carried out so that the thermal cycling of each of the plurality of converter modules is minimised within a selected range of temperature when operating the converter modules, possibly carried out so that the temperature amplitude of thermal cycling of each enabled converter modules is minimised. 8. A method according to claim 1, wherein the step of determining the enabling/disabling procedure of at least one of the converter modules is carried out so that the thermal cycling of a selected number of the plurality of converter modules is minimised within a selected range of temperature when operating the converter modules and carried out so that the temperature amplitude of thermal cycling of the selected number of enabled converter modules is minimised. 9. A method according to claims 7, wherein the minimising of temperature amplitude of the thermal cycling is carried out relative to an interval of temperatures between 0 and 125 degrees Celsius, preferably relative to an interval of temperatures between 30 and 125 degrees Celsius, more preferably relative to an interval of temperatures between 60 and 125 degrees Celsius, or even more preferably relative to an interval of temperatures between 60 and 110 degrees Celsius. 10. A method according to claim 1, wherein the step of determining the enabling/disabling procedure of at least one of the converter modules is carried out so that the thermal cycling of each of the plurality of converter modules is minimised within a selected range of temperature when operating the converter modules, possibly carried out so that the frequency of the thermal cycling of each enabled converter modules is minimised, wherein the minimising of the frequency of the thermal cycling preferably is carried out relative to a maximum allowable frequency. 11. A method according to claim 1, wherein the step of determining the enabling/disabling procedure of at least one of the converter modules is carried out so that the thermal cycling of a selected number of the plurality of converter modules is minimised within a selected range of temperature when operating the converter modules and carried out so that the frequency of thermal cycling of the selected number of enabled converter modules is minimised, wherein the minimising of the frequency of the thermal cycling preferably is carried out relative to a maximum allowable frequency of thermal cycling. 12. A method according to claim 1, wherein the step of determining the enabling/disabling procedure of at least one of the converter modules is carried out so that the output power of a selected number of converter modules is maximised simultaneously with the thermal cycling of the selected number of converter modules is minimised, said maximising of output power and said minimising of thermal cycling being effected by calculating a single parameter and optimising said single parameter relative to a preset value, said single parameter being characteristic both of the output power relative to a rated power of a converter module and of the thermal cycling of a converter module. 13. A method according to claim 1, wherein the step of determining the enabling/disabling procedure of at least one of the converter modules is carried out so that the number of operational hours of each of the plurality of converter modules are equalised. 14. A method according to claim 1, wherein the step of determining the enabling/disabling procedure of at least one of the converter modules is carried out so that the number of operational hours of a selected number of the plurality of converter modules are equalised. 15. A method according to claim 1, wherein the step of determining the enabling/disabling procedure of at least one of the converter modules is carried out so that the harmonic voltages generated into the utility grid are minimised by phase shifting the Pulse-Width-Modulation (PWM) patterns of a plurality of converter modules. 16. A method according to claim 1, wherein a criteria for enabling/disabling at least one of the converter modules is determined from predetermined hysteresis parameters, wherein the hysteresis parameters are influenced by the temperature of components of the wind turbine. 17. A method according to claim 1, wherein a criteria for enabling/disabling at least one of the converter modules is determined from predetermined hysteresis parameters, wherein the hysteresis parameters are influenced by the ambient temperature. 18. A method according to claim 1, wherein said plurality of converter modules comprise generator inverters and grid inverters. 19. A method according to claim 18, wherein at least one of the converter modules are enabled by enabling both the generator inverter and the grid inverter. 20. A method according to claim 18, wherein at least one of the converter modules are disabled for short periods of time by disabling the generator inverter. 21. A method according to claim 18, wherein at least one of the converter modules are disabled for short periods of time by disabling the grid inverter. 22. A method according to claim 18, wherein at least one of the converter modules are disabled for short periods of time by disabling both the generator inverter and the grid inverter. 23. A method according to claim 1, wherein at least one of the converter modules are enabled by enabling the converter module according to a sequence of enabling procedures such as a sequence of enabling states according to the states of a state machine. 24. A method according to claim 1, wherein at least one of the converter modules are disabled for longer periods of time by disabling the converter module according to a sequence of disabling procedures such as a sequence of disabling states according to the states of a state machine.