Circulating current´s characteristics analysis and the control strategy of parallel system based on double close-loop controlled VSI

Real and reactive power circulation is produced because of the differences of amplitude and phase of each module´s output voltage in parallel inverter system. The expression of equivalent output impedance is acquired through the close-loop transfer functions of double close-loop controlled inverter, then the characteristics of circulating current of parallel system are well researched from the point of view of equivalent output impedance. Theoretic analysis and experimental results prove that the magnitude of circulating current is proportional to the voltage difference and to the reciprocal of equivalent output impedance´s modulus, the ratio of reactive and real power circulation is determined by the tangent of output impedance´s angle. Finally a new control strategy for this kind of parallel inverter system is proposed and verified by simulation and experimental results.