Common-Mode Resonance Damping and DC Voltage Balancing Strategy for LCCL-Filtered Three-Level Photovoltaic Grid-Tied Inverters

The leakage current caused by common-mode (CM) voltage is a critical issue in transformerless three-level photovoltaic (PV) inverters, which can increase the output current distortion, bring extra power losses, aggravate the electromagnetic interference, and even cause degradation of the panels and safety issues. The LCCL filter was proven to be effective to mitigate the leakage current, whose capacitor is split into two parts, and the common point of the smaller one is connected to the neutral-point (NP) of the DC-link. However, the LCCL-filtered three-level inverter suffers from the CM resonance of the neutral current and the NP voltage imbalance. Therefore, this paper proposes a CM resonance damping and NP voltage balancing strategy for the transformerless LCCL-filtered three-level PV inverter. A novel dual-cascade-loop with a dc voltage-difference outer-loop and a neutral-current inner-loop is proposed to control the CM voltage with a $\alpha \beta \gamma $ -frame space vector pulse width modulation (SVPWM) for damping the CM resonance and balancing the NP voltage. At last, the effectiveness of the proposed strategy is experimentally validated through a 10 kW transformerless LCCL-filtered T-type three-level PV inverter.