Research On Transient Control Strategies Of DAB DC-DC Converter

With the development of the power system towards the energy Internet,the DC distribution network will be extremely widely used,and has become a research focus at home and abroad.And the DC transformer is a key device for energy transmission and conversion.The dual active bridge(DAB)DC-DC converter is considered to be the best choice for constructing a DC transformer because of its advantages of electrical isolation,wide voltage conversion range,high power density,and bidirectional flow of energy.When the DAB converter is operating in a steady state,there is no DC bias in the inductor current;however,this condition is no longer satisfied during the transient state.The DAB converter may cause problems such as current surge,transformer saturation,and unreliable operation during the transient process.In the DAB converter,AC and DC variables exist at the same time,making modeling and controller design more difficult.Mostly model and design control strategies were based on steady-state characteristics,and could not meet the control requirements of transient processes.Therefore,this thesis takes the transient control of DAB converter as the research object,analyzes its transient characteristics,and puts forward transient control strategies,and verifies its effectiveness and feasibility through simulations and experiments.The specific research contents are as follows:First,the steady state characteristics of the DAB converter under single phase shift modulation and double phase shift modulation are analyzed,and the switching state and operation mode of the converter are elaborated in detail.At the same time,the shortcomings of the steady-state control scheme during the transient process are analyzed in combination with simulations,which provides theoretical support for the subsequent research on transient characteristics and transient control.Second,in order to better describe the transient characteristics of the converter,two modeling methods for transient models are studied;the generalized averaging model and the inductor voltage direct modeling method.Based on the generalized averaging model,the complex Fourier decomposition of the inductor current is performed,and the state space equation is established for the low-frequency component.Based on the inductor voltage direct modeling method,from the perspective of Laplace transform and Fourier series expansion,the transient expression of inductor current is directly obtained.Through two transient models,the transient characteristics of the inductor current and active power are analyzed,and the causes and expressions of the inductor current DC bias are demonstrated.And a simulation platform is built to verify the accuracy of the two models and transient characteristics.Third,the converter works under the voltage closed loop,and the transient process will change the phase shift.Therefore,the transient waveform and current surge characteristics are analyzed after the step of phase shift.For the DAB converter with double phase shift modulation,a transient process optimization strategy based on predictive control is proposed.By introducing the transition phase shift,the DC bias is eliminated in one switching cycle,which effectively reduces the current impact.On this basis,a double loop compensation control method is proposed which the inner loop is inductor current and the outer loop is output voltage.In addition,the effect of the double loop compensation control method is verified through simulations.While quickly stabilizing the output voltage,it can greatly reduce the peak current of the inductor current and accelerate the transient response processFinally,a 2 kW DAB converter prototype is designed.The steady state control,predictive control and double loop compensation control are experimentally verified and compared.The experimental results show that,by comparing different sets of PI parameters,it is difficult to achieve the goals of rapid output voltage stability and small transient current overshoot for the steady state controller.Comparing with the non-applied transient control,the predictive control can eliminate the transient bias in one switching cycle,and effectively suppressing the occurrence of magnetic saturation.On this basis,the double loop compensation control can quickly stabilize the output voltage,effectively reduce the transient current shock and improve the reliability of converter operation.Based on the DAB converter with dual phase shift modulation,this thesis studies its transient control strategies and verifies their accuracy.In the end,a transient process with no current shock and fast transient response is realized.Also,the output voltage is regulated quickly and accurately and a DAB converter with high performance and high reliability is realized.It is of important meaning for the application of DC transformers in the DC distribution network.