Control Strategy for Modular Multilevel Converters with Redundant Sub-Modules Using Energy Reallocat

Abstract

By analyzing the topology and mathematical model of modular multilevel converters (MMCs), a control scheme based on the energy reallocation is proposed to control the MMC with redundant sub-modules. This control scheme is not only appropriate for the MMC in normal state but also applicable to the MMC in sub-modules fault state. Considering the circulating currents characteristics of the MMC in sub-module fault state, a phase energy controller using proportional integral resonance (PIR) control is presented, and the circulating currents can be suppressed. As the power loss of the MMCs is closely related to switching frequency of the IGBTs, a new modulation strategy based on arm current feedback is proposed in this paper, and this strategy considers not only the switching frequency’s effect but also the collector current’s effect on the switching losses.

Existing System

Nearest Level Control (NLC) Voltage Balancing Method.

Proposed System

In this paper, the proposed approaches include the following features: The control scheme based on energy reallocation can control the MMC even when the MMC is in SM fault conditions or the AC power system has an unbalanced voltage sag, meanwhile, the SM capacitor voltages will be kept at the rated value. The differential currents controller based on PIR controller can suppress the circulating currents when the MMC is in SM fault conditions. A new modulation strategy based on arm current feedback is proposed, and this strategy considers not only the switching frequency’s effect, but also the collector current’s effect on the switching losses.

Architecture

(a) Topology structure of MMC (b) Structure of sub-module