Modeling and Resonance Damping for the CM Loop of the MLCL-Filtered Grid-Connected Inverter With Variable Inductors Under the Weak Grid Condition

The modified LCL (MLCL) is commonly applied in photovoltaic (PV) string inverters to suppress the leakage current and enhance the electromagnetic interference (EMI) performance of the grid current. However, the potential risk of resonance appears in the common mode (CM) loop, which was caused by the MLCL filter. In this article, the CM model of the inverter is first built considering the inductance saturation. It is found that the variable inductor brings phase margin decreasing problem and additional resonance points in the CM loop. To suppress the CM resonance (CMR), novel passive and active damping strategies are explored in this article. The improved passive damping (IPD) strategy is realized by introducing a paralleled RC unit, which greatly reduces the damping resistor losses without affecting the damping performance. Besides, a novel active damping (NAD) strategy is proposed to compensate the time delay and to raise the phase margin. Moreover, considering the gradient mutation issue brought by the discontinuous pulsewidth modulation (DPWM) scheme, an antisaturation PWM (ASPWM) scheme is applied to eliminate the oscillation. Finally, experiments are conducted on an 80-kW PV string inverter. After applying the proposed CMR damping strategy with the ASPWM scheme, CMR is fully eliminated in the whole power range despite the grid condition.