[align=center]Research and Simulation on traction of urban train supplied by 24 pulse XIE Fang, FENG Xiao-yun, CHEN Shi-hao, Zhao xiaohao School of Electrical Engineering Southwest Jiaotong University, Chendu Sichuan 610031, China paper, the phase-shift principle of the 24-pulse rectifiers in urban's power supply is analyzed and modeled.considering SVPWM three-level inverter vector controlled traction system as the load, The running states are simulated in the two cases of supplying with ideally power supply and 24-pulse rectifiers.At the same time, harmonic of the AC side analyzed in different running states of the train as well as DC side voltage. Results shows that the 24-pulse supply has good performance,the combination simulation platform could effectively analyze the influence among AC, DC side and operation State of the train. KEY WORDS : 24-pulse vector control harmonic Abstract : This paper analyzes and models the phase-shifting principle of the 24-pulse power supply rectifier unit in urban rail transit. Using a vector control traction drive system powered by an SVPWM three-level inverter as the load, the operation of the motor is simulated under ideal power supply and 24-pulse power supply. Simultaneously, the harmonics of the power grid and the DC-side voltage are analyzed under various train operating conditions. Simulation results show that the 24-pulse power supply performs well, and the joint simulation platform can effectively analyze the mutual influence between the AC and DC sides and train operation. Keywords : 24-pulse vector control harmonics 1 Introduction The metro traction power supply system is a time-varying, AC/DC hybrid system. Its network structure is affected by the train's operating mode and operating time, making it a dynamic network. It is difficult to comprehensively consider the power supply system performance of each dynamic process and to grasp the characteristics of the entire power supply system. It includes three main parts: the AC network, the DC-side network, and the train. To establish a dynamic and real-time simulation software that can reflect the mutual influence of AC side circuit, DC side circuit, locomotive power, etc., it is necessary to conduct joint simulation of power supply system and train traction transmission system. However, power supply and train usually belong to two research directions. Many software programs for power supply system simulation treat the locomotive as an equivalent current source and simulate various working conditions such as locomotive coasting, acceleration, and regenerative braking through different current extraction modes [1]. It is difficult to comprehensively and in real-time reflect the characteristics of urban rail transit power supply system. This paper analyzes and models the 24-pulse rectifier circuit and urban rail train traction transmission system, establishes a joint simulation platform for 24-pulse DC traction power supply system, and simulates the mutual influence between AC and DC sides and train. It has certain reference significance for the development of power supply system simulation software. 2 Basic principle and modeling analysis of equivalent 24-pulse rectifier In order to improve the impact of high-order harmonics of rectifier device on power grid, communication and other equipment, the rectifier units in urban rail transit traction power supply system currently widely adopt equivalent 24-pulse rectifier circuit. The harmonic current generated by the 24-pulse rectifier unit is less than that of the 12-pulse rectifier. In particular, the 11th and 13th harmonics with the largest harmonic content in the 12-pulse rectifier can be reduced by more than 80%, which is currently a more ideal power supply method [2]. The equivalent 24-pulse rectifier unit consists of two 12-pulse rectifier units. The 12-pulse rectifier is composed of two 6-pulse 3-phase rectifier bridges connected in parallel. One of the 3-phase rectifier bridges is connected to the star winding of the secondary side of the rectifier transformer, and the other 3-phase rectifier bridge is connected to the delta winding of the secondary side of the rectifier transformer. Because the line voltage phases of the star winding and delta winding of the secondary side of each rectifier transformer are staggered by π/6, a 12-pulse rectifier circuit composed of two three-phase bridges connected in parallel can be obtained. When the windings of the rectifier transformers supplying the two 12-pulse rectifiers are connected in parallel on the high-voltage grid side using ±7.5° extended delta connection, the two rectifiers can be connected in parallel to form an equivalent 24-pulse rectifier [2]. [align=center]Fig.1 The diagram of phase-shift principle and vector[/align] Fig.1 shows the phase-shift principle and vector diagram of a phase-shifting transformer. The upper transformer group uses a left-extended connection on the high-voltage side, as shown in the left vector diagram. After adding the extended winding, the A-phase voltage shifts by +7.5º compared to before adding the extended winding. The lower transformer group uses a right-extended connection, as shown in the right vector diagram. Similarly, it can be seen that after adding the extended winding, the phase shift is -7.5º. Both transformer groups use a star-delta connection for their valve-side windings. The turns ratio of the delta-connected winding to the star-connected winding is [value missing]. This ensures that the line voltage amplitudes of the valve-side windings are equal, only differing in phase. The valve-side windings are connected to a 3-phase rectifier bridge in parallel, forming a 24-pulse circuit, with each pulse differing by a phase angle of 15º. Based on the principle of the phase-shifting transformer, the following simulation model was established using MATLAB/SIMULINK: [align=center] Fig. 2 Simulation model of 24 pulse rectifier circuit[/align] In Fig. 2, the three-phase power supply is selected to be AC ​​33KV, the same voltage level as the traction substation. The subsystem module is a 24-pulse rectifier transformer model. Through transformation and rectification, the DC1500V or DC750V voltage used for urban rail power supply can be obtained (the DC750V voltage system used in this paper is adopted). Under no-load conditions, the voltage waveform and spectrum analysis of the output terminal in one cycle are shown in Fig. 3. It can be seen that there are 24 wavefronts in one cycle, and the voltage pulsation is relatively small and relatively stable. The maximum harmonic order occurs at the 24th order, and the DC component is 781.5V. [align=center] Fig.3 The DC side output wave of 24-pulse rectifier [/align] Fig.4 shows the grid-side current waveform and harmonic analysis of the 24-pulse rectifier circuit after adding a resistive load. It can be seen from the figure that the grid-side current is close to a sine wave, with very small harmonics. The most noticeable harmonics are the 23rd and 25th harmonics, and the total distortion rate is only 1.71%. [align=center] Fig.4 The AC side current wave and spectrum of 24-pulse rectifier [/align] 3 Analysis and Simulation Model Establishment of Vector Control Strategy Based on SVPWM Three-Level Inverter Power Supply The AC drive system with field-oriented vector control can provide the optimal starting torque, enabling the train to start quickly and smoothly; the system has high speed accuracy and a wide adjustment range, ensuring stable operation of the train at various speed levels; it has ideal electrical braking function, enabling the train to brake reliably and stop accurately, while simultaneously feeding back electrical energy to the grid, making it very suitable for urban rail trains. The system control block diagram is shown in Figure 5. The traction drive system inverter adopts a vector control strategy based on SVPWM three-level voltage-type inverter (VSI) power supply. Compared with two-level VSI, the former has better output waveform, lower pulse frequency, lower device withstand voltage requirements, and lower output harmonic components. It has a certain effect on the operation of the motor and the power supply system. The main circuit current contains less pulsation component, which reduces the electromagnetic noise generated by the traction motor [3]. It has a certain effect on the improvement of AC side harmonics. [align=center] Fig.5 Rotor Field Oriented vector control frame diagram of asynchronism motor [/align] 4 Simulation results and analysis The simulation system of asynchronous motor vector control based on rotor field orientation is built using the Matlab/Simulink platform as shown in Figure 5. The space voltage vector modulation control three-level inverter is used to power the asynchronous motor. When powered by an ideal DC source, the system changes the speed setpoint from 40 rad/s to 80 rad/s in 0.2 seconds, decreases it to 50 rad/s in 0.5 seconds, and increases it back to 70 rad/s in 0.7 seconds. The system load increases abruptly from 20 N·m to 50 N·m in 0.8 seconds and decreases abruptly from 50 N·m to 20 N·m in 0.9 seconds. The simulation results are as follows: [align=center] Figure 6: Stator current wave when ideal DC source supplied Figure 7: Torque curve when ideal DC source supplied Figure 8: Velocity curve when ideal DC source supplied[/align] As can be seen from Figures 6, 7, and 8, the system can respond quickly when the speed setpoint changes, the stator current is stable with no speed fluctuation when the load changes, the torque dynamic response is fast, and the system control accuracy is high. Two equalizing capacitors were added to the output of the 24-pulse rectifier transformer to power the inverter. The 24-pulse power supply system and the vector-controlled three-level inverter main drive system were then jointly simulated. The simulation process was repeated, and the simulation results are as follows: [align=center] Fig. 9 Stator current wave when 24 pulse supplied Fig. 10 Torque curve when 24 pulse supplied Fig. 11 Velocity curve when 24 pulse supplied[/align] Comparing the simulation results under ideal DC power supply and 24-pulse power supply, it can be found that only the stator current and torque curves fluctuate slightly more, especially the torque pulsation, which is larger than under ideal DC power supply. However, the speed fluctuates almost nothing, and there are no obvious glitches during the speed and torque switching process. Furthermore, it does not affect the dynamic response speed of the system, indicating that the 24-pulse power supply performs well and does not cause significant adverse effects on train operation, meeting the requirements of urban rail power supply. Simultaneously, the simulation can also obtain the waveform of the current on the high-voltage power grid side, allowing for harmonic analysis of the current and examining the changes in harmonic content of the grid-side waveform under different operating conditions or load changes. The grid-side waveform and its harmonic analysis are shown in Figures 12, 13, 14, and 15: [align=center] Figure 12: AC side current waveform of 24-pulse rectifier Figure 13: 0.15s～0.17s AC side current waveform and spectrum Figure 14: 0.3s～0.32s AC side current waveform and spectrum Figure 15: 0.8s～0.82s AC side current waveform and spectrum Figure 16: DC side voltage waveform [/align] Simulation results show that at 0.15 seconds, the speed is stable at 40 rad/s, resulting in a good grid-side current waveform with no distortion and almost zero harmonics. However, at 0.3 seconds, the system's given speed has just increased to 80 rad/s, causing significant fluctuations in the grid current. The 23rd and 25th harmonics approach 10%, and the overall current distortion rate reaches 11.22%, although lower-order harmonics are relatively small. Similarly, at 0.8 seconds, the load changes, and the grid-side current begins a new round of fluctuations, with the waveform and harmonic characteristics similar to those at 0.3 seconds. This indicates that when the train's operating state changes, or even abruptly changes, the grid-side current is significantly affected, and harmonics increase accordingly, but it returns to a stable operating state within a relatively short time. Regarding the DC-side voltage, there are certain fluctuations under system acceleration, braking, and loading conditions, especially at 0.4 seconds when the train performs regenerative braking, causing a sudden increase in grid voltage and a voltage surge on the DC side. 4. Conclusion This paper establishes a joint simulation model of a 24-pulse power supply system and a vector-controlled train traction drive system. This model can evaluate the performance of the 24-pulse power supply and its impact on train operation, and accurately analyze the influence of train operation on grid-side harmonics and DC-side voltage. By treating the train traction drive system as the load for power supply, the harmonics of the grid-side current can be analyzed, providing a more realistic and referential analysis. Simulation results show that the 24-pulse power supply performs well, significantly reducing low-order harmonics, with a relatively small pulsation ratio, and has almost no significant impact on train operation performance. The joint simulation system can realistically analyze the influence of various train states on AC-side harmonics and DC-side voltage pulsation. References [1] Zhou Fulin, Li Qunzhan. Research on the development of simulation software for power supply system of urban rail transit. Urban Rail Transit Research. 2007, No. 5: 25-27. [2] Lin Huihan, Ling Wenjian. 24-phase axial double-split rectifier transformer. Transformer. 2002, No. 10: 9-10 [3] Wang Wei, Luo Longfu. Simulation of main drive system of three-level inverter for subway vehicle. Science, Technology and Engineering. 2007, No. 6: 1172-1176 [4] Zhang Chunjiang et al. Research on harmonic simulation of space vector PWM waveform. Journal of Yanshan University. 2000, No. 2: 141-144 Received date: Author's profile: Xie Fang (1980–), female, from Jinsha, Guizhou, is a master's student specializing in power electronics and electric drives. Feng Xiaoyun (1962–), female, from Xiayi, Henan, is a professor and doctoral supervisor specializing in power electronics and AC drives, automatic train control (ATC), and automatic train operation (ATO). Contact information: Mobile: 13982201059; E-mail: [email protected].