Control of three-phase grid-connected LCL-filtered inverters with adaptability to non-ideal grid

Non-ideal grid conditions include the voltage imbalance, distortion and frequency variation of three-phase grid voltage. In addition to positive-sequence components, three-phase grid contains negative-sequence, zero-sequence and harmonic components under non-ideal grid conditions. First, an oscillating signal of twice the grid frequency on the d-axis was generated by the negative sequence, which caused an error in the phase-locked loop unfortunately. Adopting some digital filters can eliminate the error, but the adaptation to the frequency variation cannot be achieved. Thus, the variable sampling period phase-locked loop (VSP_PLL) is needed. Second, the negative-sequence voltage and voltage harmonics usually caused asymmetrical and poor grid current quality. Third, additional sensors or complex algorithms have to be used to damp the resonance of LCL filter. To solve these issues, this paper discusses a symmetrical control strategy which consists of the VSP_PLL with Goertzel filter, grid-voltage feedforward and inverter-side current control with proper design of LCL parameters. With such control, a three-phase inverter can achieve symmetrical three-phase grid current with good quality even under grid distortion and imbalance. At last, experimental results obtained from a grid-connected LCL-filtered inverter verify the effectiveness of the proposed strategy.