Research On Control System Of H-bridge Cascaded Power Electronic Transformer

As a new power transformer, power electronic transformer(PET) can overcome some of the inherent shortcomings of the traditional transformers, make voltages and currents of its primary side and secondary side flexibly controlled, and own the potential to solve many new problems of modern power system by the introduction of power electronic conversion technology on the basis of traditional power transformers.The topology of cascaded H-bridge PET, operating principle and advantages are introduced in this article, especially the related control strategies of power electronic transformers are studied. First of all, detailed analysis of topology structure, operating principle and switch modes for the single-phase PWM rectifier, isolated bidirectional DC-DC converters with atransformer, and three-phase inverter are presented. Then voltage and current double closed-loop decoupling control based on D-Q transformation of single-phase PWM rectifier is proposed by the construction of virtual axis, which makes the reference current tracked quickly and accurately by current, and the same input voltage and current in phase can also be realised. After that, the phase-shift control of bidirectional isolated DC-DC is proposed with the achievement of bidirectional controllable power flow, then the maximum power transfer points are also analysed. Finally, voltage and current double closed-loop decoupling control of three-phase inverter based on the LC filter is proposed, which realises a good dynamic performance and low distortion rate of the output voltage.In addition, cascaded H-bridge DC-link voltage balance control is proposed for unbalancd DC-link voltage, which is led by hardware differences among H-bridges, realising balanced voltages among capacitors by adding correction on each H-bridge. Dual active bridge(DAB) power balance control is also proposed for unbalancd power flow, which is led by hardware differences among DABs, realising power flow balancement by adding correction on phase angle. Fluctuations in DC-link voltage and output voltage distortion may be caused by load fluctuations of PET. For this problem, PET feed-forward control is proposed based on the conservation power between input and output, which improves the response capability of input and isolation stage to load fluctuations and stability of PET by the feed-forward of load power to input and isolation stage. Results of simulation verify the feasibility and effectiveness of the proposed control strategy.Based on DSP and FPGA development platform, this article introduces the hardware and software design of PET. First, the overall control structure and the parameter design method of input, isolation and output stage are presented. Then hardware circuits are also introduced, including phase voltage reference circuit, sample conditioning circuit, fault protection circuit and grid resistors. Finally, the joint DSP-FPGA control structure and module design in DSP and FPGA program is introduced.