Research On Control Strategy Of Cascaded Power Electronic Transformer

Power electronic transformers(PET)have broad application prospects in the fields of renewable energy power generation grid-connected systems,power routers,traction transformers,and battery energy storage systems.PET generally consists of an AC/DC converter and a high-frequency isolation DC/DC converter.PET based on cascaded H-bridge(CHB)and dual active bridge(DAB)converters is the most widely studied topology.Aiming at the control strategy of PET from start-up to normal operation,this dissertation has carried out a series of research work:Aiming at the contradictory problem of increasing the start-up speed of PET and reducing the impulse current during the start-up process,a soft start-up strategy based on energy feedback was proposed.Different from the traditional hard start resulting in excessive impulse current or the traditional ramp start resulting in slow speed and difficult slope design: on the one hand,this strategy always makes the DAB work in the peak current control mode to speed up the start-up speed;on the other hand,this strategy is directly based on the current and power model of each converter in PET,which can calculate and control the peak current in real time.Therefore,the proposed strategy can maximize the power transmission capacity of DAB while ensuring the system security.It not only avoids the complicated design of starting process control parameters,but also takes into account the factors of starting safety and starting rapidity.Aiming at the problem that high and low voltage DC bus voltages are prone to large fluctuations when the dynamic response characteristics of the closed-loop control of CHB and DAB are quite different,a coordinated control strategy based on the internal energy flow of PET is proposed.Unlike the traditional strategy of CHB and DAB respectively controlling the high and low voltage DC side voltages,which causes the two parts of control to be coupled to each other.From the perspective of energy flow rather than voltage,this strategy divides the control functions of CHB and DAB into two parts: energy "total" control and energy "transfer" control,and realizes the decoupling of CHB from DAB control activities.This strategy quantitatively allocates the fluctuation of the total energy storage of the PET DC bus to the high and low voltage DC bus,so as to minimize the square sum of the relative deviation of the energy of all the DC bus.By introducing model predictive control at the DAB,the PET’s two-stage fourth-order system is simplified to a single-stage second-order system,which greatly reduces the complexity of system control parameter design.This strategy effectively improves the dynamic response performance of the DC voltage,and is robust against inconsistent circuit parameters.Finally,a low-power experimental platform was built to verify the correctness and effectiveness of the proposed control method.