The Suppression of Voltage Overshoot and Oscillation During the Fast Recovery Process From Load Short-Circuit Fault for Three-Phase Stand-Alone Inverter

Abstract

When a three-phase stand-alone inverter feeds several loads simultaneously, to ensure high reliability and continuity of power supply for the large load network, the inverter needs to operate continuously, even during faulty conditions. Moreover, the inverter should provide large enough current by switching from the voltage control mode (VCM) to the current control mode (CCM) to trigger the circuit breaker connecting the short-circuit fault branch to recover the power supply. Therefore, when a load short-circuit fault occurs, the inverter will switch from VCM to CCM. In CCM, the short-circuit currents can be controlled by the current-limiting strategy. Once the fault is cleared, to quickly supply the remaining loads, the output voltages should be restored to the rated voltage as soon as possible. However, the fast recovery process from the load short-circuit fault may cause significant voltage overshoot and oscillation. The voltage overshoot may lead to load damage, and the voltage oscillation may extend fault recovery time. To effectively solve the above problems, the voltage overshoot introduced by large fault current and postfault load impedance after fault clearance is first analyzed in this article, and then the suppression strategy based on the parallel virtual impedance is proposed. Second, the voltage oscillation introduced by the initial phase difference between the voltage references and the output voltages when the inverter switches from CCM to VCM is analyzed, and then the effective suppression strategy which can achieve seamless mode switching after fault clearance is proposed. The experimental results have been carried to verify the effectiveness of the proposed strategies.