Current source converter on-line pattern generator switching frequency minimization

Abstract

On-line PWM pattern generators for current source converters offer a number of control advantages over off-line optimized patterns. However, when implemented using the principles applying to voltage source inverter PWM pattern generators, the switching frequency is equal to: (a) the carrier frequency in standard carrier-based implementations and to (b) 2/3 the cycle frequency in space vector implementations. This paper shows that this frequency can be reduced to 1/2 of the respective frequencies. Two pattern generators are investigated: (a) a carrier based technique, namely the modified dead-band technique; (b) a space vector based technique, where the advantage is taken of the extra zero state available in current source converters. It is shown that a significant reduction of AC line current distortion is obtained with the modified dead-band technique for modulation indices greater than 0.4. Furthermore, with the two proposed pattern generators, there is no penalty on the distortion of the DC bus voltage. The principles of operation of the proposed schemes are explained. DSP implementation algorithms of the space vector scheme are given. Experimental results on a 5 kVA current rectifier and a 5 kVA current source inverter confirm the feasibility of the proposed pattern generators, and illustrate the input and output frequency spectra obtained. >