Reduced distortion and increased availability in wind turbines through load dependent operation of parallel inverter units with variable DC link voltage

A new control concept for inverter systems in wind turbines is proposed. Thereby only as many parallel connected inverter units as needed are activated to feed in the momentary generated power into the grid. Each inverter unit is equipped with an individual controller without additional communication between the inverter units. The load dependent activation and deactivation of inverter units is controlled by the use of a variable DC link voltage. The proposed control strategy reduces the total harmonic distortion in part load operation significantly and adds additional redundancy in case of failures in any of the inverter units to the whole inverter system. Arising zero sequence currents between the inverter units are effectively reduced by controlling the switching frequencies of the inverter units. The new control strategy is studied for three parallel connected grid sided inverter units in simulations as well as on a 20.7 kVA laboratory test setup.