Paralleled Current Sharing And DC Bias Suppression For Dual Active Bridge DC/DC Converter

With the high-speed development of modern electric locomotive,The traction power system with power electronic traction transformer(PETT)as the key component satisfies the demands of high-speed locomotive for small volume,light weight and high efficiency.Dual-active-bridge(DAB)DC/DC converter is one of the core element of PETT.To study the dynamic performance and parallel efficiency optimization of DAB,this paper proposed DC bias suppression strategy and parallel current sharing control strategy.Firstly,this paper introduces main circuit topologies and modulation techniques of DAB converter.The working principle of DAB under the modulation of the single-phase-shift(SPS)and the triple-phase-shift(TPS)is discussed in detail.The large-signal model and small-signal model of DAB converter are established.Based on this mathematical model,the power transmission characteristic,soft-switching range and power backflow phenomenon are analyzed.Secondly,this paper proposes a DC bias suppression strategy based on TPS modulation which aimed at the problem of the DAB magnetic bias.Multi-frequency component approximation method is used to analyze all working modes of TPS modulation.The proportional relationship between the DC bias current and the duty cycle of the active bridges is derived.The transformer DC bias current detection circuit is designed,and the output voltage duty ratio of the active bridges is accurately controlled in real time according to the magnitude of the bias current to ensure the volt-second balance of the high-frequency transformer windings.Thus the magnetic saturation problem due to DC bias is effectively eliminated.Thirdly,the transfer function of a single DAB converter is solved by small disturbance equation.According to the function,the equivalent model of parallel output DAB is analyzed.For the power imbalance caused by the input voltage fluctuation and the mismatching of electrical parameters,this paper proposes a parallel current sharing control strategy.The proposed strategy reconstructs the voltage transfer ratio to control DAB tracking current signal.In this strategy,each DAB converter share one voltage outer loop to realize the closed-loop control of the output voltage,and each DAB has its own independent current inner loop to track the unified reference current.Compared with the traditional current sharing strategy,the proposed strategy does not need to add high frequency current sampling circuit,simplifies the hardware circuit design and reduces manufacture cost.Finally,the software and hardware design process of parallel DAB converter experimental platform is introduced in detail.Including the main circuit element parameter design,drive and test circuit design and control system design.With TMS320F28335 as the core control system,a 12kW system consists of three DAB converters connected in parallel with independent 750V input and 750V parallel output were built.The effectiveness of the proposed control strategy is verified by experiments.