Circulating Current Suppression for Modular Multi-level Converters with Direct Digital Control

Abstract

This paper presents circulating current suppression for modular multi-level converters (MMC) with a direct digital control. Circulating current with a second-order harmonic component is one of the pivotal issues in controlling MMC, since it will result in higher arm current and power loss in either upper or lower arm. A direct digital control is proposed to track inductor current command tightly regardless of wide filter inductance variation and to eliminate circulating current. In this paper, an MMC configuration is introduced first, and then a direct digital controller for circulating current suppression is derived in detail. Simulated results obtained from a 120 kW MMC have verified the analysis and discussion.