Research On DC Transformer Control Strategy Based On Series Resonant Dual-active Full-bridge DC-DC Converter

With the development of distributed energy and DC distribution network,the DC transformer has gradually become a research hotspot.In this thesis,the working principle,control strategies of the input-series-output-parallel(ISOP)DC transformer and the control strategies of the power unit series resonant dual active bridge DC-DC converter(SRDAB)were studied.1.The operation principle of SRDAB with single phase shift control was studied and the fundamental wave mathematical model was also established.Based on these work,the soft switching characteristics and power transmission characteristics of the SRDAB were analyzed and verified in Simulink.By comparing typical SRDAB soft-start control strategies,an improved soft-start strategy was proposed in this thesis,and a simulation model was built in Simulink to verify the superiority of this strategy.2.We established the time-domain mathematical model of SRDAB by considering the internal equivalent resistance of the circuit as well as all the harmonic components of resonant current and square wave voltages at both ends of the high-frequency isolated transformer.Then,the current balance control strategy at switching point was proposed,which can prevent the loss of soft switching caused by excessive equivalent internal resistance of the circuit.At the same time,the phase shift-variable frequency control strategy(PS-VF)was further studied.By adjusting the frequency and phase shift angle,the sum of the absolute values of the turn-off current do not increase along with the transmission power,and can maintain the optimal solution.Finally,the correctness of the mathematical model of SRDAB and the feasibility of the two control strategies were verified in Simulink.3.We first designed the output voltage stabilization control system by using the small signal analysis method.Then the input voltage equalization control strategy of the DC transformer was analyzed.Above this,the independent control between the output voltage stabilization and the input voltage equalization was achieved through decoupling control.Finally,the effectiveness of the control strategy was verified by simulation.4.According to the actual engineering requirements,we completed the design of the DC transformer parameters and the construction of the SRDAB experimental platform.This experimental platform was used to verify the current balance control strategy at switching point and phase shift-variable frequency control strategy.The laboratory’s DC transformer prototype was used to verify the input voltage equalization control strategy.These experimental results were compared with theoretical results and simulation results to verify the correctness and feasibility of the above control strategies.As the key equipment of DC distribution network and AC-DC mixed distribution network,the DC transformer studied here can not only realize conversion between different levels of DC voltage,but also meet the grid connection of distributed energy.This research work has great academic research value and engineering application value.