System and method for neutral point balancing for back-to-back voltage source converters

A three-level back-to-back voltage source power conversion assembly, comprising:

• a first power converter comprising a first set of switching devices;
• a second power converter coupled to the first power converter via a direct current (DC) link;
• a converter controller communicatively coupled to the first and second power converters, the converter controller configured to implement a balancing algorithm for the power conversion assembly, the balancing algorithm comprising:
• in response to receiving an indication of at least one of a direct current (DC) or an alternative current (AC) unbalance occurring in upper or lower capacitor voltages of the DC link, determining a deviation of a midpoint voltage of the DC link as a function of a total voltage of the DC link;
• calculating a voltage compensation needed for pulse-width modulation signals of the power conversion assembly based on the deviation in the midpoint voltage;
• and
• coordinating common mode voltage injection from each of the first and second power converters independently at a neutral point of the voltage source power conversion assembly based on the voltage compensation, and applying adaptive controller gains to each of the first and second power converters based on one or more operating parameters of the voltage source power conversion assembly, thereby minimizing the at least one of the DC unbalance or the AC unbalance at any given operating condition.

A wind turbine power system, comprising:

• a generator comprising a rotor and stator;
• a three-level back-to-back voltage source power conversion assembly electrically coupled to the generator, the power conversion assembly comprising:
• a rotor-side converter comprising a first set of switching devices;
• a line-side converter coupled to the rotor-side converter via a direct current (DC) link, the line-side converter comprising a second set of switching devices;
• a converter controller communicatively coupled to the rotor-side and line-side converters, the converter controller configured to implement a balancing algorithm for the power conversion assembly, the balancing algorithm comprising:
• in response to receiving an indication of at least one of a direct current (DC) or an alternative current (AC) unbalance occurring in voltage of the DC link, determining a deviation of a midpoint voltage of the DC link as a function of a total voltage of the DC link;
• calculating a voltage compensation needed for pulse-width modulation signals of the power conversion assembly based on the deviation in the midpoint voltage;
• and
• coordinating common mode voltage injection from each of the first and second power converters independently at a neutral point of the voltage source power conversion assembly based on the voltage compensation, and applying adaptive controller gains to each of the first and second power converters based on one or more operating parameters of the voltage source power conversion assembly, thereby minimizing the at least one of the DC unbalance or the AC unbalance at any given operating condition.