Research On Control Strategy For Power Electronic Transformer In Traction Drive Systems

With the development of power semiconductors,power electronic transformers(PET)are able to replace the line-frequency transformers in traction drive systems for electric locomotive or electric multiple units.Cascade H-bridge(CHB)connects to the grid as the front end of PET,while dual-active bridge(DAB)DC-DC converters achieve galvanic isolation and provide stable DC power supplies for traction inverters as the back end of PET.This dissertation studies the control strategies of CHB and DAB,making PET adapt to the complex transmission working conditions and realize high performance in full operating range.The discretization of the adaptive filter for grid synchronization is studied in this dissertation.An optimized hierarchical discretization method is proposed to guarantee the accuracy and efficiency of grid synchronization under high normalized frequency.After hierarchizing the adaptive filter,the accuracy and stability based on discretizing different hierarchies and using various discretization methods are studied.It is the integrators in adaptive filter that are proposed to be discretized by the pre-warping bilinear transform method,realizing good accuracy and low coefficients sensitivity.To further increase the computational efficiency,a state variable in the adaptive filter is re-selected,while the Taylor series is introduced to calculate accurate coefficients.A design method of multi-vector-proportional-integrator resonant controller in CHB is proposed to achieve zero steady-state error and fast transient response of the fundamental current,and suppress harmonic currents.Considering the large system delay in CHB current loop,phase compensation is required.However,the phase compensation also leads to a non-minimum phase system,resulting in coefficients coupling.Hence,a tuning method is proposed to solve this problem.The frequency response is analyzed using the Nichols chart.Tuning of gains is transformed into tuning of minimum gain margin and multiple phase crossover frequencies for decoupling.The settling time and overshoot optimization is considered to design the optimized coefficients and improve the transient response.A phase-shift modulation strategy of DAB with DC bias current reduction at full operating range is proposed to avoid transformer saturation and even power devices breakdown caused by the DC bias current in transient process.The strategy is achieved by staggering the load time of primary bridge and secondary bridge switching signals.The bias of voltage-second values in primary and secondary sides are counteracted,and the DC bias current is greatly reduced without extra transient periods.To achieve the phase-shift modulation in full operating range,the phase-shift range limitation with threelevel modulation is proposed in case of switching signal missing.A phase-shift control strategy of DAB is proposed to achieve accurate power control,avoiding the PET bus voltage fluctuation generated by the power deviation from the deadtime effect at low power flow.The duty cycle ratio between the primary side and secondary side is determined by keeping voltage-second balance.The transformer current returns to zero in half a period,and the backflow power causing by dead-time is reduced.The growth rate of duty cycles followed by power increase is further restricted to expand the well-compensated range of dead-time.Finally,the effect of dead-time is eliminated,the practical power is equals to the ideal power,and the fluctuation of bus voltage is suppressed.A prototype of PET with traction control unit is designed in this dissertation.Experimental results in full operating range show that CHB achieves low current harmonics and rapid transient response,DAB achieves stable output and bidirectional power flow.These PET control strategies suit the requirement of traction drive systems.