Space-Vector-Modulated Hybrid Bidirectional Current Source Converter

Current source converters (CSCs) present the desirable characteristics of inherent load and converter short-circuits protection. Six-switch (fully controllable) CSCs are used in applications requiring the flexibility and performance, which is lacking in more affordable phase-controlled SCR-based CSCs. Hybrid CSC (HCSCs) with both SCRs and force-commutated switches usually offer a compromise solution regarding cost and performance. This paper discusses the use of space vector modulation (SVM) techniques to enhance the performance of a bidirectional three-SCR four-switch HCSC. The main challenge is to generate the gating signals online, so that the SCRs are safely commutated with variable power factor, reduced switching losses and harmonic distortion, and increased gain. This requires the use of appropriate sequences of states with minimum states on times. The commutation issues of SCRs in an HCSC are discussed in details, and two SVM strategies are proposed to implement HCSCs with only active and with active and natural commutation. They result in HCSCs with features comparable to those of the conventional six-switch CSC. Analytical equations describing the limitations of the two techniques are derived. A sample case study discussing the power losses on individual switches is presented. Experimental results obtained in a laboratory prototype are provided to verify the theoretical analysis and demonstrate the feasibility of the proposed techniques.