Research On Modulation Strategies And Control Method For Railway Traction System With PMSM

The permanent magnet synchronous motor?PMSM?is attracting growing attention in traction systems for railway vehicles due to its advantages,such as high efficiency,high power density,low noise and flexible structure.In this paper,the PMSM fed by a high power three-phase two-level inverter is taken as the research subject.And the research is focused on the optimized synchronous modulation technologies of high power traction inverter with low switching frequency and control strategies of PMSM.For the three-phase two-level inverter,after briefly introducing the relationship between asynchronous space voltage vector pulse width modulation?SVPWM?and carried-based pulse width modulation?PWM?,rules for arranging the optimal synchronous space voltage vector sequences are derived.And the output phase voltage waveforms generated by these vector sequences satisfy periodic symmetry,half wave symmetry,quarter wave symmetry and 3-phase symmetry,and there are no pulses of opposite polarity in the line-line voltage waveforms,and there is no simultaneous switching of more than one phase.Then a generalized method is proposed to construct the optimal synchronous space voltage vector sequences.Using this method,the vector sequences with pulse ratio below 11 are constructed,and the mathematic relationship between the duration angles of vectors in the sequences and the optimal switching angles in the corresponding phase voltage waveforms is analyzed.Thereafter,the duration angles of vectors in the sequences are selected as variables,and the current harmonic distortion minimization SVPWM?CHM-SVPWM?is proposed.And the optimization of this problem is executed through three steps.It is testified that,compared with phase-voltage-oriented current harmonic distortion minimization PWM?CHMPWM?,CHM-SVPWM can achieve the global optimal solution at any given initial values of iteration.And the optimized results are validated by the simulations and experiments.For the PMSM fed by a three-phase two-level inverter,the mathematic modes and vector control strategies are simply introduced,and the expression of stator voltage vectors with the optimized synchronous modulation employed is given.And then the expression of the PMSM stator current generated by the optimized synchronous modulation is derived.Thereafter,the advantages and disadvantages of existing two current harmonic evaluation indexes,weighted total harmonic distortion?WTHD?and no-load current harmonic evaluation index for salient pole motor?_i0,are analyzed.With consideration of the salient pole characteristic and load angle of PMSM,a harmonic current evaluation index,?_i,is proposed.After introducing an optimized synchronous PWM method based on?_i0,named CHMPWM-ESM because this method was firstly proposed for electrically excited synchronous motor,the current harmonic performance of CHMPWM and CHMPWM-ESM are compared by using WTHD??_i00 and?_i as evaluation index respectively,and the comparison results are different when the different evaluation indexes are used.Then the experiment shows that the comparison results of PMSM total current harmonic distortion?THD?produced by the two methods,CHMPWM and CHMPWM-ESM,are coincide with the comparison results of current harmonic performance based on?_i.WTHD and?_i00 cannot describe the effects of the salient pole characteristic and the changing load angle on PMSM current harmonics,and?_i can be used to evaluate the salient PMSM current harmonic performance correctly and effectively.Based on the PMSM stator current expression,the stator current harmonic spectrums of CHMPWM and CHMPWM-ESM are analyzed when pulse ratio is 11.And the experimental results verified the analysis.Corresponding to CHMPWM-ESM,optimized synchronous SVPWM based on minimum?_i0,named CHMSVM-ESM,is constructed and the optimized results are given.Thereafter,considering the salient pole characteristic and the load angle of PMSM,an optimized synchronous SVPWM with minimum?_i?CHMSVM-PM?is proposed,and the effects of the saliency ratio on optimization process are analyzed.For a PMSM whose saliency ratio is 2.5,the optimized results of CHMSVM-PM are presented and the difference in optimized solutions with changing load angle is shown.Then using?_i as evaluation index,the current harmonic performance of CHMSVM-PM,CHMSVM-ESM and CHM-SVPWM are compared.And the comparison results show that CHMSVM-PM is superior to the other two methods in the overall range and the two methods are just special cases of CHMSVM-PM in some operation situation.The experimental results validated the correctness of the comparison results using?_i as evaluation index and the advantage of CHMSVM-PM.Based on the PMSM stator current expression using optimized synchronous modulation,the analytical expression of torque harmonic is derived.Through comparing the calculation results and the simulation results,the correctness of deduction is verified.And it builds the theoretical foundation for analyzing and evaluating the torque harmonic with optimized synchronous modulation and developing the new synchronous PWM based on optimized torque performance.The asynchronous modulation used in low speed range and the optimized synchronous modulation used in medium or high speed range can be combined as a hybrid modulation strategy.When the hybrid modulation is employed as part of the vector control system,the dynamic response is relatively poor in the situation where optimized synchronous modulation is in service.To solve this problem,the fast dynamic response of direct torque control?DTC?method is considered,and a DTC method which is suitable for the whole operation range of PMSM is developed to improve the dynamic response of the control system.For PMSM operating in the low speed range,a DTC method with fixed switching frequency and asynchronous SVPWM is simply introduced.Then a stator flux observer based on the equivalent flux is proposed,and the effectiveness of the flux observer under various motor parameter deviations is verified through simulations.By analyzing torque equation of PMSM,a deadbeat DTC with asynchronous SVPWM is presented and the high dynamic performance of this method is testified by simulation results and experimental results.For PMSM operating in the medium or high speed range,the features of stator flux trajectories generated by various optimized synchronous PWM modes are analyzed,and the synchronous modulation strategies are implemented with corresponding stator flux trajectories.Thereafter,by combining the steady-state performance of optimized synchronous PWM and dynamic performance of DTC,a DTC method,including flux weakening control,based on optimized stator flux trajectories is proposed,and the simulations and experimental results validate the high dynamic performance of this method.