Research On DC Solid State Transformer Based On Push-pull Resonance

With the development of high-efficiency distributed power supply systems and the demand for diversified DC loads,the DC power supply system has attracted more attention due to its high reliability,low power loss,no frequency instability,and low cost of distributed energy access.In the DC distribution network,the DC conversion link is essential to realize the energy transfer of different voltage levels.Since AC-type magnetic coupling energy transfer cannot be realized in DC distribution networks,DC solid-state transformer based on power electronic devices have become its core devices.In view of the low-voltage and high-current applications of medium and low-voltage DC distribution networks and the functional requirements of electrical isolation and bidirectional energy transmission,this paper proposes a DC solid-state transformer based on push-pull resonance and cascade half-bridge structure.And the operating characteristics,control Strategy and fault protection schemes of it were studied.In this paper,the working modes and circuit characteristics of the push-pull resonant circuit and the cascade half-bridge circuit,the basic components of the DC solid-state transformer,are first described.The effect of the switching frequency of the push-pull resonant circuit in the forward and reverse modes on the operating efficiency is analyzed.By establishing a mathematical model of the cascade half-bridge circuit,the relationship between the output voltage on the high voltage side and the duty cycle,current ripple,and output inductance is analyzed.Secondly,the operating characteristics and control methods of the DC solid-state transformer based on push-pull resonance are analyzed.The push-pull resonance cascade half-bridge sub-module adopts the IPOS(Input Parallel Output Series)structure to expand the system capacity to adapt to the low-voltage DC distribution network,and the multi-mode voltage/current control method ensures the flexible flow of energy in the solid-state transformer.In order to reduce the current ripple on the low voltage side,phase shift control is adopted between the sub-modules,and the influence of the number of sub-modules and output capacitance(inductance)on the current ripple is explored to provide a basis for circuit design.Thirdly,the fault protection scheme for DC solid-state transformers under extreme working conditions is studied.Aiming at the voltage fluctuations in the DC distribution network,the loop current feedforward scheme to deal with low-voltage side fluctuations and the loop virtual capacitance scheme to deal with high-voltage side fluctuations are studied to improve the system power supply reliability.In order to avoid cascading damage in the sub-module fault state,the sub-module fault removal scheme based on the logic block of the drive signal is given,and the phase shift control between the modules can automatically adjust according to the fault state.And the simulation model of the proposed DC solid-state transformer was built and verified by simulation.Finally,a solid-state transformer experimental platform based on push-pull resonance cascade half-bridge structure that contains two sub-modules is built,and verifies the operating conditions and control schemes of each mode.The experimental results verify the effectiveness and feasibility of the circuit topology and control scheme proposed in this paper.