Circulating resonant current suppression for current-controlled inverters based on output impedance shaping

Current-controlled LCL-filtered inverters are usually paralleled together for interconnecting the renewable energy resources into the grid. When the inverter output impedance is not well designed, the harmonic circulating currents out of the control bandwidth may be easily aroused. To reveal this resonant phenomenon and to enhance the stability of the parallel system, this paper builds the parallel system model with the consideration of the asynchronous carrier wave. Aiming at optimizing the inverter output impedance around the resonant frequency, a simple control method, which cascades the classical active damping loop with a proportional resonant controller with phase compensation, is proposed for the parallel inverter system. Each inverter is carefully designed with a minimum phase behavior by tuning the control parameters. Both the high modulus and the passivity of the output impedance are guaranteed to mitigate the circulating resonant currents. The effectiveness of the proposed control method is experimentally validated by a three-parallel inverter system.