Improved Square Wave Injection Strategy for the Voltage Balance Control of Cascaded H-Bridge Rectifier

The modular cascaded H-bridge (CHB) adopted as the active front-end rectifier in the power electronic traction transformer (PETT) tends to suffer from dc voltage imbalance issues when the load powers of each module are severely unbalanced. The existing square wave injection strategy (SWIS) can improve the capacitor voltage balancing performance, but the maximum amount of the injected square wave in the high-power modules is only 1 - M (M is the average modulation index). In order to extend the load imbalance operation range of the CHB, the improved SWIS (ISWIS) proposed in this article not only increases the square wave content in the high-power module to 1 (the modulation index is increased to 1.27) but also reduces the phase between the ac voltage and ac current of the high-power module to 0 when the load imbalance becomes severer. These two important changes allow the high-power H-bridge (HB) modules to transmit the maximum active power. Moreover, the overmodulation problem caused by the superposition of the opposite compensation signal in the low-power modules is analyzed, and an effective solution is then proposed to avoid the current distortion issue. Both simulation and experimental results prove the correctness and validity of the proposed strategy.