Discussion on Triangular-Carrier Frequencies of a Modular Multilevel Cascade Converter Based on Double-Star Chopper Cells (MMCC-DSCC) with Phase-Shifted PWM

This paper presents a theoretical discussion on triangular-carrier frequencies of a modular multilevel cascade converter based on double-star chopper cells (MMCC-DSCC, hereafter referred to as the DSCC) with phase-shifted PWM. The DSCC consists of multiple cascaded chopper cells and a center-tapped inductor per leg. Low voltage steps bring significant reductions in harmonic voltage and current to BTB (back-to-back) or HVDC (high-voltage direct-current) systems using two DSCCs. When the DSCC adopts the 6-kV GCTs (gate-commutated thyristors) that are commercially available in the market, the switching frequency should be less than 200Hz to improve conversion efficiency. Theoretical analysis developed in this paper reveals that the most suitable triangular-carrier frequency is 125Hz or 175Hz. Simulated waveforms obtained from a three-phase 66-kV, 200-MW DSCC are included to verify this analysis. The analytical and simulated results agree well with each other.