Control Strategy For Cascaded Power Electronic Transformer

Power electronic transformer (PET) is combined with power electronic devices and high-frequency transformer. This kind of power electronic converter could achieve but not limit to traditional transformer functions. As a novel power electronic device, PET is expected in occasions such as future smart grid, railway traction, new energy power generation, etc. In this paper, individual and coupling core high-frequency transformer-isolation cascaded power electronic transformers are taken as research objects. The research works include theoretical analyses, computer simulations, and experimental verifications.Firstly, mathematical models of LC resonant and non-resonant DC-DC converter are established, power transmission expression are derived. The effect of voltage phase angle difference, switching angular frequency, resonance angular frequency on the power transmission characteristic is systematic analyzed. Based on the analysis of the power transmission characteristic, a power control strategy is research and proposed. The correctness of the proposed theory and control strategy is verified through simulation and measured results.Secondly, based on the deep analysis of the voltage imbalance of DC capacitor in the cascade H-bridge PWM rectifier, an optimized DC capacitor voltage balancing control strategy is proposed. Simulation and experimental results show that the proposed control method could reduce the switching frequency of the switching device through decrease the switching states of H-bridge.Thirdly, based on the deep analysis of power balance of DC-DC converter parallel output in the individual high-frequency transformer, a power balance control based on power feedback of DC-DC converter parallel output is proposed. Simulation and measured results show that the proposed power balance control strategy could avoid the power imbalance due to the parameter variation.Finally, the mathematical model of multi-winding transformer is established and the power transmission expression is derived. Based on the deep analysis of the power transmission characteristic, a power balance control strategy utilizing power feedback+power forward is proposed. Simulation and measured results show that the proposed power balance control strategy could still achieve power balance even if the parameter variations.