On-line generation of gating signals for current source converter topologies

PWM rectifiers and current source inverters (CSI´s) are usually gated using optimized patterns stored in EPROMs. Patterns are therefore mostly of fixed modulation index, since variable modulation index requires storing patterns for a finite numbr of indices. Furthermore, the modulation index can only be changed once per cycle, thus reducing the dynamic response of the system. This paper proposes a technique of generating gating patterns online for CSI topologies based on carrier PWM techniques (PWM rectifiers or inverters). The proposed technique is designed and implemented for the standard three-phase six-switch configuration. It delivers the appropriate gating signals for all switching patterns and modulation indices including overmodulation. It is therefore possible to extend the concept of duality in the power circuit topology between VSI´s and CSI´s to carrier PWM modulation techniques. In the proposed approach, the requirements imposed on gating signals are satisfied by the appropriate combinations of single phase switching patterns and the introduction of the required complementary and overlap pulses. Implementation in a mixed analog/digital circuit is described and experimental results obtained on a 2 kVA voltage-controlled current source inverter unit are given.