An Integrated Common-Mode Fast-Balancing Mechanism for Three-Phase Three-Level Converter With LCL Filter

A three-phase three-level converter with an LCL filter has been used in the industry because of its low switching harmonics. However, it inevitably suffers from dc neutral-point imbalance, which may unnecessarily stress semiconductors and deteriorate power quality. The imbalance will, nevertheless, naturally rebalance, but it usually takes a long time. A common rectification is, thus, to add an active or a passive booster to speed up the balancing. An active booster requires sensitive sensors and control modification, which will increase the system cost and complexity. A passive booster is, therefore, preferred owing to its exemption of monitoring and control efforts. The existing three-phase passive booster, however, contains several extra separate LC components, which are normally not functioning during the balanced conditions. They should, hence, be reduced. One approach is to dual use the existing LCL (or other high-order) filter for both harmonic filtering and dc-link balancing. Topologically, it requires only the addition of one extra capacitor along the return common-mode path. Using newly formulated criterions related to sizing, harmonic, and modulation selections, the balancing performance of the resulting capacitive balancer has been evaluated comprehensively. Finally, an experimental prototype is built for testing the balancing mechanism under different operating conditions.