Quasi-Selective Harmonic Elimination (Q-SHE) Modulation-Based DC Current Balancing Method for Parallel Current Source Converters

Abstract

Parallel operation of current source converters (CSCs) is an attractive approach for medium-voltage high-power conversion applications. In order to achieve proper dc rail current balancing of parallel CSCs while maintaining a good control of ac line current quality at low switching frequency (typically at a few hundred hertz), an improved quasi-selective harmonic elimination (Q-SHE) modulation method with online delay angle and bypass band adjustment is proposed. At first, each CSC positive rail dc current is regulated by tuning the delay angle of the corresponding line reference current with respect to the grid voltage. At the same time, the negative rail dc currents balancing errors are compensated by injecting small bypass pulses into the conventional SHE pattern. By the coordinated control of the aforementioned methods, an accurate dc rail currents balancing and an enhanced line current quality can be achieved at both the steady and the transient states.