Power Balance Control Strategy For Input-series Output-parallel Dual Active Bridge Converter

The dual active bridge(DAB)has the advantages of high voltage utilization,high power density,and bidirectional power transfer capability.In the application of medium and high voltage-low voltage DC power grid,the input-series output-parallel(ISOP)DAB improves the voltage of the series side and realizes a large voltage conversion ratio,but there are problems such as power imbalance between modules.In this paper,the following researches are carried out on the module difference and power balance control for ISOP-DAB which consist of three-level neutral point clamed(3L-NPC)half-bridge:First,the ISOP-DAB system structure and sub-module topology are analyzed.In this paper,the working principle,power and soft switching characteristics of DAB under 3L-NPC half-bridge balance are studied,as well as the optimization of inductor current RMS under triple phase shift(TPS)modulation is also studied.In this paper,when the DAB is under the unbalanced conditions,and under the action of some specific switching modes during the softstart process,the balance principle of port voltage and inductor current in both primary and secondary sides is also analyzed.Secondly,this paper analyzes the relationship of voltage,current and power between ISOP-DAB modules,then discusses many factors that affect the consistency of modules.The self-balancing ability of the system under disturbance is analyzed,and the difference of control stability between series side and parallel side under bidirectional power is also analyzed.A power balance control strategy based on TPS modulation is proposed,which does not require sampling voltage or current of module,and simplifies the sampling circuit and control design.The strategy can also be used as a backup solution for other multi-module balance strategies when the sensor fails.Then,a method to estimate the parameter difference of multi-module is proposed.The core idea of this method is to inject forced components as disturbance and observe the change of feature that can reflect the module difference.Combined with the power balance control strategy,the estimation method can be summarized and promoted to the application of N modules.This paper takes three modules as an example to simulate and verify the effectiveness of the method in MATLAB/Simulink.The constraints of control parameter used to avoid overpower are studied in this paper,and the variation trend of the power boundary margin with respect to the number of modules and module power are also studied.Finally,a two-module ISOP-DAB converter experiment platform is designed,which uses FPGA controller as the core,to realize the 220 V to 24 V voltage conversion.The feasibility of the module difference estimation method is verified under four disturbances of different magnitudes and positive and negative values.Experimental results show that,in the output voltage type of 20/24/28 V and the power range from 10 W to 435 W,the system can significantly improve the voltage balance of the series side under the power balance control,and the efficiency is basically higher than 90%.