A Phase-Shifted-PWM D-STATCOM Using a Modular Multilevel Cascade Converter (SSBC)—Part I: Modeling, Analysis, and Design of Current Control

This paper presents and discusses a phase-shifted-PWM distribution static synchronous compensator (D-STATCOM) using the modular multilevel cascade converter based on single-star bridge cells (MMCC-SSBC). A three-phase 140-V 10-kVA SSBC-based STATCOM with a cascade count N=6 is designed, constructed, and tested for verifying the performance and functionality of the medium-voltage D-STATCOM in both steady and transient states. The 13-level line-to-neutral (25-level line-to-line) voltage at the ac side yields an almost sinusoidal line current with a total harmonic distortion value of 1.7%. An analytical model for the phase-shifted PWM is proposed to design the current control gains. Two phase-shifted PWM methods, which are named as one-cell and all-cells update methods, are theoretically and experimentally compared. The all-cells update method can reduce the inherent time delay, thus resulting in a faster and more stable system response than the one-cell update method.