The DC Circulation Current Control Strategy Based on Decoupled Control Scheme in the Multi-module Paralleled Inverter System

This paper proposes a novel Control strategy based on decoupled control scheme to eliminate DC circulation current in the Multi-module paralleled uninterrupted power supply(UPS) system.On the basis of the original paralleled system for load sharing by active and reactive power control,a separate control circuit is added in each paralleled inverter module to eliminate the DC component of output voltage.The voltage between the middle point of inverter half bridge and the middle point of the DC capacitors is measured to accurately detect DC voltage offset of inverter.The difference between the feedback DC voltage and the reference DC component is regulated by proportional-integral controller to obtain the compensating DC voltage component.Therefore the DC voltage component of each inverter output voltage can be regulated to zero by adding the feedback DC voltage component to inverter output voltage reference.This method is based on the idea that DC voltage offset of each inverter is decoupled.In this way,the DC circulation current of paralleled UPS system Can be eliminated by regulating each parallel inverter module' DC voltage component to zero.Because each module only regulates its own DC voltage component,the DC circulation current control is decoupled.The experiment in three paralleled 15 kVA three-phase UPS system proves feasibility and reliability of the control strategy.