Abstract：The AC-DC-AC electric locomotive with four-quadrant pulse width modulation (PWM) control is gradually becoming the main type. Compared with traditional AC-DC electric locomotives, it alleviates reactive power and low-order harmonic problems of electrified railways. However, the increase in traction power exacerbates the negative sequence problem of the power grid. Therefore, referring to the structure of the rotary power flow controller (RPFC) and static var generator (SVG), a new negative-sequence compensation topology based on a single-phase dual rotary phase shifting transformer (S-DRPST) is proposed to suppress the unbalanced currents caused by traction loads on the grid side. Firstly, S-DRPST is mainly composed of two single-phase rotary phase shifting transformers, which change the phase angle between the stator and rotor voltages by driving the S-RPST to rotate through a servo motor. The secondary side combines the two stator voltage vectors to form the output voltage of S-DRPST in series with the line, and changing the amplitude and phase of the output voltage can effectively control the line current of S-DRPST. Then, the S-DRPST negative sequence compensation strategy is studied in the application scenario of V/x traction power supply system. Considering the traction and braking states of the electric train load, the amplitude and phase of the output voltage of two S-DRPSTs are calculated based on the power of the supply arm, and then the set value of the phase shift angle is obtained. At the same time, the phase shift angle control of a single S-DRPST is designed using flexible coupling before the compensation equipment is put into operation. After the equipment is put into operation, the coordinated variable-speed control for phase shift angle is adopted to suppress the output voltage oscillations. Finally, a simulation model of the high-speed railway power supply system is established to verify the proposed S-DRPST topology and control strategy. The results show that the S-DRPST in the closed-loop scenario can flexibly reduce the impact current on the traction network, decreasing the impact coefficient from 1.26 to 0.99. Considering the traction and braking states of the power supply arm throughout the operation, the unbalanced voltages under different operating conditions are controlled to be below 0.1%. In scenarios where the traction load changes rapidly and continuously, the S-DRPST can respond promptly to compensate for negative-sequence currents, keeping the voltage imbalance within 2% in less than 100 ms. The conclusions are as follows. (1) S-DRPST can adjust the phase shift angle in the V/x traction power supply system to change the total current of the supply arm and achieve a symmetrical current on the grid side. (2) In the phase shift angle control of S-DRPST, flexible coupling reduces the impact current caused by the compensation equipment, and the coordinated variable-speed control suppresses output voltage oscillations during the adjustment process. (3) S-DRPST controls the grid-side current to be three-phase symmetrical by considering the traction and braking states of the supply arm. (4) S-DRPST can respond promptly to rapid and continuous power fluctuations of high-speed railway loads and provide corresponding compensation. The proposed S-DRPST is an electromagnetic device with obvious advantages in unit capacity cost, which is suitable for existing traction power supply systems and addresses the negative sequence problem in high-speed railways.
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2024, Vol. 39 
