SVPWM Strategy For Traction Inverter In Entire Modulation Range

The traction inverter is the main component of the AC-DC-AC type electric traction drive system,and its modulation strategy is one of the key techniques of electric locomotive and electric multiple unit(EMU).The two-level traction inverter is studied in the dissertation,and theoretical study and practical experiment of the SVPWM strategies for three-phase six-switch inverter and fault-tolerant three-phase four-switch inverter(TPFSI)in entire modulation range are conducted.The research results have certain theoretical significance and practical value for improving the output performance,enhancing the driving capability and increasing the reliability of the traction inverter.Firstly,the topology and working principle of two-level inverter are briefly summarized,and the basic principle of SVPWM is analyzed.Limited by the switching frequency of the traction inverter,there is a deviation between the actual output voltage and its reference value of the traction inverter with five kinds of existing synchronized SVPWM strategies under low pulse number.An online correction strategy of reference modulaiton index for the linear modulation region is proposed,which can be employed by traction inverter to output the more accurate voltage.In addition,based on the regular sampling rule,the equivalent carrier-based PWM(CBPWM)strategies of the synchronized SVPWM strategies are derived,which simplify the implementation and reduce the real-time computing burden of the digital controller.Secondly,in order to improve utilization of DC voltage,extend the output voltage range and enhance the drive capability of traction inverter,the SVPWM strategy in overmodulation region is studied.Considering the existing problems of the conventional overmodulation algorithm and the limitation on the number of sampling points in a fundamental period,the synchronized SVPWM strategy in the overmodulation region using superposition principle is presented.Meanwhile,the correction strategy of reference modulation index is extended to the overmodulation region to realize the accurate output voltage in entire modulation range.Besides,the equivalent CBPWM modulation functions of the synchronized SVPWM strategies in the overmodulation region are derived,which are suitable for the engineering implementation.Thirdly,for the purpose of improving the reliability of the traction inverter,the topology and operation mechanism of the fault-tolerant TPFSI are described,and the SVPWM strategy in the linear modulation region is designed.Meanwhile,focusing on the limited maximum output voltage of TPFSI and the drawbacks of the conventional overmodulation algorithm,a novel SVPWM strategy based on virtual voltage vector for TPFSI in the overmodulation region is proposed.Consequently,the equivalent CBPWM modulation functions of the SVPWM strategies in entrie modulation range are derived,and the variation characteristics of modulation functions are summarized.Finally,based on the 5kVA experimental platform of induction motor drive system,the practical experiment research for the designed modulation strategies of three-phase six-switch inverter and fault-tolerant TPFSI in entire modulation range is carried out.The experimental results verify the feasibility and effectiveness of the proposed SVPWM strategies,which have certain guiding significance for the engineering application of traction inverter.