Capacitor voltage balancing techniques for a multi-pulse pulse rectifier with active injection

Two techniques are presented to balance the filter capacitor voltages of a multi-pulse rectifier with active injection operating on a variable frequency supply. The active injector consists of a low-rated, bidirectional switch that modifies the behaviour of a 12-pulse rectifier at either low or high-frequency, resulting in 24-pulse or multi-level PWM operation respectively. The operation of the rectifier and the injector, the practical issues that unbalance the capacitor voltages and the methods to correct this imbalance are explained together with experimental results obtained with a 4 kW prototype.