Independent Control of Three-Phase DC Voltage Balancing for Cascaded H-Bridge Rectifier With Unbalanced Load and Grid

As the challenge of unbalanced load and grid condition in power electronic transformer (PET) application system, the dc voltage balancing control is essential for cascaded H-bridge (CHB) rectifier. This article proposes a novel hybrid dc voltage balancing control strategy for three-phase, four-wire CHB rectifiers. To achieve interphase voltage equalization, d-q current control is applied to each phase. Without additional negative or zero sequence voltages injected, there is no grid current distortion introduced to the system. As the active and reactive power of each phase is controlled independently, interphase dc voltage balance can be maintained even in phase loss conditions. For the inner-phase unbalance problem, a modulation method combining real-time voltage level calculation with voltage sorting is adopted. Finally, experimental results with different unbalanced load and grid conditions verify the effectiveness of the proposed method based on a three-phase, four-wire, seven-level CHB rectifier platform; moreover, the dynamic performance is also improved with the proposed hybrid control strategy. The rectifier still can run for 12 s under phase loss conditions, which comply with the low voltage ride through (LVRT) grid codes defined in standard GB/T 36995-2018. When the grid returns to normal, the rectifier only needs 1.5 s to resume the three-phase operation.