A two-stage combined control strategy for parallel operation of inverter

Conventional control strategies for parallel operation of inverters are mainly classified into two categories. One is based on interconnection line which transfers the active and reactive power of each inverter. However, once a fault occurs at the communication channel, the whole system must be shut down. The other prevalent approach is using droop control theory, which needs no communication mechanism but has a power sharing problem. In this paper, a two-stage combined control scheme is proposed to overcome the shortcomings of traditional methods. The classical interconnection-based controller is applied to share the active power accurately and reactive power is controlled by means of Q-ω droop controller in the first stage. In the second stage, all inverters switch to P-E droop control mode with introducing an adaptive virtual impedance. This method can achieve accurate active and reactive power sharing as well as high reliability. Experimental results are provided to verify the feasibility of the proposed method.