An Improved Beatless Control Method of AC Drives for Railway Traction Converters

ABSTRACT:  

The traction converter consists of a single phase AC-DC rectifier and a three-phase DC-AC inverter. Due to special structural characteristics of single phase rectifier, a fluctuating voltage component with the frequency twice of the grid’s, exists in DC-link voltage. Fed by fluctuating DC-link voltage, a beat phenomenon occurs in traction motor, and harmonic components appear in both stator current and electromagnetic torque, especially when motor operates near the ripple frequency. In this paper, the mechanism and influence of fluctuating voltage are analyzed in detail. Based on modeling analysis of motor and switching function of inverter, a frequency compensation factor is derived in vector control of induction motor. Then an improved frequency compensation control method is proposed to suppress beat phenomenon without LC resonant circuit. Finally the modified scheme is verified by simulation and experiment.

KEYWORDS:

1.      Fluctuating DC voltage

2.      Beat phenomenon

3.      Vector control

4.      Beatless control

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fig. 1. FOC with frequency compensation for Induction Motor

 EXPECTED SIMULATION RESULTS:

Fig. 2. Waves of stator current and electromagnetic torque of traction Motor

Fig. 3. FFT of stator current and electromagnetic torque before adding frequency compensation method

Fig. 4. FFT of stator current and electromagnetic torque after adding traditional frequency compensation method

Fig. 5. FFT of stator current and electromagnetic torque after adding improved frequency compensation method

 CONCLUSION:

In high-power traction converters, without LC filter circuit paralleled in DC-link, a fluctuating voltage twice of the grid frequency contains in DC-link voltage. This paper aims at adopting software control method to suppress beat phenomenon in traction motor caused byDC ripple voltage. According to theoretical analysis, DC ripple voltage is influenced by output power of motor, DC-link capacitor and power factor. Then, influences of fluctuating voltage are analyzed in detailed from aspect of switching function and motor model. Based on above analysis, combining with rotor field oriented control of traction motor, the frequency of switching function is modified to suppress beat phenomenon. An improved frequency compensation control method is proposed. Simulation model is built to verify the proposed scheme. Finally, the proposed control method is verified by drag experiment on a dynamometer test platform.

REFERENCES:

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[3] K Nakata, T Nakamachi , K Nakamura, “A beatless control of inverter-induction motor system driven by a rippled DC power source,” Electrical Engineering in Japan, Vol.109, No.5, pp.122-131,1989.

[4] Z Salam, C.J. Goodman, “Compensation of fluctuating DC link voltage for traction inverter driver,” Power Electronics and Variable Speed Drives, 1996. Sixth International Conference on (Conf. Publ. No. 429), pp. 390-395, 1996.

[5] S. Kouro, P. Lezana, M. Angulo and J. Rodriguez, "Multicarrier PWM With DC-Link Ripple Feedforward Compensation for Multilevel Inverters," IEEE Transactions on Power Electronics, vol. 23, no. 1, pp. 52- 59, Jan. 2008.