Voltage And Power Balance Control Strategy For Cascaded Power Electronic Transformer

Power Electronic Transformer (Power electronic transformer, PET), also known as Solid-State Transformers, with advantages of high power density、better controllability and so on, is widely concerned in some occasions such as future smart grid, new energy power and railway traction. In this paper, the cascaded power electronic transformers are taken as research objects. The research works include theoretical analysis, computer simulations, and experimental verifications.Firstly, this paper introduces the control strategy for Cascaded H-Bridges Rectifier (CHBR). At the same time, based on single-phase PWM rectifier, the operating principle and the control strategies of single-phase PWM are analyzed, at the same time, the design process of current loop, voltage loop and phase-locked loop are detailed introduced, and designing methods of these controllers are presented. Then the modulation schemes suitable for CHBR are analyzed and compared. To overcome the problem of the different switching frequency and loss, the modulation scheme by swapping the pulses is proposed.Secondly, this paper analyzes the factors affecting the unbalanced voltage of DC capacitor in the cascade H-bridge PWM rectifier, The mechanism of the happened voltage unbalanced is presented. Based on the analysis, Two kinds of control strategies are proposed:duty-cycle compensated balance control strategy and voltage sorting balance control strategy.Thirdly, based on the deep analysis of power balance of DC-DC converter parallel output, three kinds of power balance control strategies are proposed, including based on power feedback control strategy、current sensorless power balance strategy and voltage following control strategy.The power feedback control strategy has the advantage of fast responsibility and the disadvantage of high cost. However the number of device is reduced and the dynamic performance is better with the current sensorless power balance strategy. Compared with two other control strategies, the voltage following control strategy is much easier to implement.Finally, the three cells Cascaded Power Electronic Transformer for experiment is designed and set up. The experimental results show the validity and feasibility of control strategies which are put forward in this paper.