Research On Dual-Active-Bridge DC-DC Converter And Its Series Voltage Equalization Control Technology

Dual active full bridge DC-DC converter is one of the most widely used bidirectional DC-DC converters in DC microgrid and other occasions,because of its advantages of bidirectional energy flow,electrical isolation and easy soft switching.In this paper,dual-active full-bridge DC-DC converter is taken as the research object,and DAB inductive current DC bias elimination method and DAB output series voltage equalization control technology under phase shift control are studied.The main contents and innovations are as follows:(1)This paper firstly introduces the circuit topological structure of DAB,and analyzes the phase-shifting control principle of DAB with half-bridge midpoint potential as the entry point.Then,the research status of DAB phase shifting control,DAB inductive DC bias current suppression method and voltage balancer in bipolar DC distribution network are summarized.(2)DC current bias is one of the main reasons leading to magnetic saturation of INDUCtors and transformers of DAB.In addition,DAB operation characteristics will be affected in its attenuation process.Aiming at the problem that DAB auxiliary inductors have DC bias under normal phase-shifting control,this paper analyzes the relationship between DAB inductive voltage-second product and half-bridge potential voltage-second product.Based on introducing the concept of half-bridge reference potential,a phase-shifting control method of DAB inductive current without DC bias is proposed.Theoretical analysis and Matlab simulation results show that the phase shifting method can effectively eliminate the DC bias of inductance current when DAB starts and shift ratio changes.(3)In bipolar DC power distribution system,high power and high performance voltage balancer can effectively increase distribution capacity and reduce distribution loss.DAB has the advantages of large transmission power and high operation efficiency.Aiming at the voltage balancer formed by two DAB outputs in series structure,this paper proposes a dual-loop control strategy of series voltage outer loop and equal voltage inner loop.In this strategy,the series voltage is controlled to a given value through the outer loop,and the two output voltages are equal through the inner loop.The proposed control strategy only needs one voltage sensor to achieve the balance of two DAB output voltages,reducing the cost of the device.Matlab simulation results show that the proposed control strategy can effectively keep the output voltage equal when the load of two DAB is unbalanced.(4)In order to verify the proposed DAB inductive current phase-shifting method without DC bias and DAB output tandem double-loop voltage equalization control strategy,two DAB series and parallel experimental platforms are designed and built.The hardware design of the experimental platform includes main circuit and control circuit design,and the software design includes digital filter and control program design.Finally,DAB stand-alone,output series,current stress contrast and other experiments are carried out on the experimental platform.The experimental results prove the effectiveness of the proposed control method and strategy.