Control Strategy Of A Dual Active Bridge Converter For DC-bus Voltage Stability Control In DC Micro-grid

In order to solve the problems of the system power imbalance and DC-bus voltage fluctuation caused by the unstable output power of distributed power in DC micro-grid,more and more energy storage units are increasingly introduced into the construction of micro-grid.In this paper,the control strategy of stabilizing the DC-bus voltage in DC microgrid is studied for the energy storage unit composed of the lithium iron phosphate battery pack and the dual active bridge(DAB)converter.Firstly,the operating principle of the DAB converter and the operation characteristics of single phase-shift(SPS)control and extended phase-shift(EPS)control are introduced.Secondly,based on the state space average model of the DAB converter,the integrator of traditional PI control is omitted,the current feedforward control is introduced,and the DC-bus voltage stabilization control strategy based on the model predictive control(MPC)is studied.Aiming at the errors caused by the system line impedance,power switch voltage drop and dead time,as well as the error in the reasonable assumptions and simplification process of the MPC,fuzzy compensation control(FCC)is introduced to eliminate the error,thereby a compound control strategy based on model prediction and fuzzy compensation control is obtained.According to the small signal model of the DAB converter,the parameters of PI regulater in the current inner loop is designed.Considering the influence of dead time on the DAB converter,the critical condition to avoid voltage polarity reversal is derived.Thirdly,in view of the EPS control mode,the current stress optimization method of the EPS modulation during forward and reverse power transmission is studied according to the Lagrange multiplier method.In view of two cases of the maximum current stress in the mode of internal and external phase-shift ratio D1>D2,the image analysis method is introduced to find the optimal combination of the phase-shift ratio,and the current stress optimization result is combined with the compound control strategy under SPS modulation to optimize the current stress on the basis of maintaining the system power balance and stabilizing the DC-bus voltage The soft switching ranges of SPS modulation and EPS modulation are compared and analyzed It is pointed out that EPS modulation based on the current stress optimization can realize soft switching in the full power rangeFinally,a simulation circuit model is built to verify that EPS control based on the current stress optimization and SPS control based on MPC and FCC can both quickly stabilize the DC-bus voltage under various working conditions.Compared with the SPS modulation mode,the current stress under the EPS modulation mode is smaller and the transmission efficiency is higher.A prototype with low voltage and small power is built to verify two control methods The experimental results show that the energy storage unit composed of the DAB converter can suppress the DC-bus voltage fluctuation quickly under various operation conditions and switch between the charging mode and the discharging mode smoothly,the system has excellent dynamic performance,the effectiveness of the proposed control strategy and the correctness of the theoretical analysis are confirmed.