Voltage Balance Control Strategy for a Cascaded H-Bridge Converter-based Power Electronic Transformer

The three-phase asymmetry phenomenon commonly exists in medium voltage distribution networks. In this paper, a cascaded H-bridge converter-based power electronic transformer with separated DC bus is applied. A new method with negative-sequence voltage injection is proposed to control cluster voltage balance of DC capacitors when the input voltages and output loads are asymmetric. The relationship between the active power of each phase and the negative-sequence voltage is established and the negative-sequence voltage is calculated in the dq frame to avoid the complicated trigonometric calculation. The calculation results do not contain the grid currents, therefore, only a single inner current loop is adopted and the positive and negative sequences of grid currents are not necessary to calculate. Besides, individual voltage balancing control is realized by trimming the modulation wave in the output stage. The effectiveness of the proposed control strategy is verified by simulation results.