Optimized pole and zero placement with state observer for LCL-type grid-connected inverter

Due to the attractive harmonic elimination capability, LCL filter is widely adopted in the grid-connected converters. Combined with complete state feedback control, PI current controller of the converters offers almost full controllability, resulting in super grid current waveform. By proposing an optimal pole and zero placement strategy for the PI state-space controller in this paper, the dominant poles and non-dominant poles are carefully specified to ensure system stability even in worst cases of low LCL resonant frequency and weak grid. With the proposed method, the high-order system behaves like a typical second-order system. The obtained maximum bandwidth varies with the filter size and the control burden is moderate. The computer aid design (CAD) based selection of control parameters is also provided with simplicity and adaptability. Moreover, only grid current is required as the state variable with the developed reduced-order state observer. Experimental results are provided to verify the feasibility and validity of the proposed method.