Wind turbine converter control

A full-scale converter system for converting active power produced by a generator of a variable speed wind turbine to be fed into an electricity grid, the converter system comprising:

• a converter control system,
• a series-connection of parallel-connected generator-side inverters and a series-connection of parallel-connected grid-side inverters, the series-connections of the generator- and grid-side inverters are connected back to back on their DC sides to form a common DC link,
• wherein generator-side inverters and grid-side inverters at a same voltage level of the series-connection form at least first and second converter-strings,
• wherein the generator-side and grid-side inverters form a generator-side and grid-side voltage-center-point, respectively, at a voltage level between the inverters connected in series, wherein the voltage-center-points are coupled together by a center-line conductor, wherein the converter control system is programmed to cause the converter system to perform conversion operation with a de-rated maximum active power-output comprising:
• in response to at least one of (i) the grid-side inverter and (ii) the generator-side inverter of the first converter-string being disabled, disabling active power production of at least one of (i) the grid-side inverter and (ii) the generator-side inverter of the second converter-string, or correspondingly reducing active power production of the second converter-string, thereby preventing a compensation current along the center-line conductor.