Analysis and Reduction of Zero-Sequence LC-Resonant Current in DPWM-Modulated 3 LCL-Type Grid-Connected Inverter With Neutral Line

Discontinuous pulsewidth modulation (DPWM) is often adopted in the three-phase (3 phi) grid-connected inverter (GCI) for enhancing the efficiency. However, DPWM will increase common voltage at the neutral point of the filter capacitors and may deteriorate the safe operation of the inverter. To reduce the common voltage, some applications connect the neutral point of the filter capacitors and midpoint of dc voltage. Nevertheless, there may be zero-sequence LC-resonant current in the filter inductor, leading to a larger current ripple and further increasing the power loss. This letter demonstrates the inherent mechanism how DPWM generates the zero-sequence resonance current. On the basis, this letter unveils the straightforward ideas for problem solving, regardless of the DPWM method, and then proposes an easy-implementing solution to reduce it from the perspective of control method, which is different from the state-of-the-art works by modifying modulation method, yet can reserve the efficiency improvement brought from DPWM. Finally, experimental results based on a 3.6-KVA 3 phi GCI prototype are provided to verify the analysis and effectiveness of the proposed method.