Abstract: This paper analyzes the circuit structure schemes of auxiliary inverter power supplies for subway vehicles using direct inverter, chopper step-down inverter, and double chopper step-down inverter methods. Based on the technical conditions of the imported auxiliary inverter power supply for the Beijing "Fuxing Line 8" subway vehicle, this paper introduces the development scheme and main technical characteristics of a domestically produced DC750V auxiliary inverter power supply for subway vehicles. Experimental waveforms of the scheme are presented. 1 Introduction In recent years, the auxiliary power supplies for subway vehicles imported by cities such as Shanghai, Guangzhou, and Beijing in China have all adopted static auxiliary inverter power supplies. Guangzhou Metro and Shanghai Metro Line 2 use IGBT auxiliary inverter power supplies; Beijing's "Fuxing Line 8" uses a GTO heat pipe radiator self-cooling auxiliary inverter power supply. Therefore, developing and manufacturing domestically produced static auxiliary inverter power supplies for subway vehicles is an inevitable trend in the development of urban rail transit in China. The comparison between static auxiliary inverter power supply and traditional electric generator set power supply is as follows: (1) Static auxiliary inverter power supply is directly powered from the third rail of the subway train. After DC/DC chopper conversion, it provides a stable input voltage to the three-phase inverter. Through VVVF frequency conversion voltage regulation control, the inverter outputs three-phase AC voltage to supply power to the load. For multi-output power supplies, the circuit adopts transformer isolation. The advantages of this auxiliary inverter power supply are good output voltage quality factor, high power efficiency, and safe and reliable operation. (2) Traditional subway auxiliary power supply usually adopts the power supply scheme of rotary electric generator set. The motor is powered from the DC750V third rail, and the generator outputs three-phase AC voltage to supply power to the load. For some electrical equipment with DC110V and DC24V, power is still required through three-phase transformer and rectifier. This power supply method has a large unit size, small output capacity, low efficiency, and the power supply is easily affected by the operating conditions of the DC generator set. The output voltage fluctuates greatly and the reliability is poor. 2 Comparison of Auxiliary Power Supply System Schemes for Metro Vehicles The following section analyzes and compares several commonly used auxiliary inverter power supply circuit structures for DC750V metro vehicles. 2.1.1 Direct Inverter Method Figure 1 shows the simplest basic circuit structure of an auxiliary inverter power supply for metro vehicles. High-power GTOs, IGBTs, or IPMs are typically used as switching components. The auxiliary inverter power supply directly draws current from the third power rail. The inverter is controlled by a constant V/f ratio, outputting a three-phase pulse-width modulated voltage to supply power to the load. This circuit is characterized by its simple structure, few components, and convenient control. However, its disadvantages include the inverter's output voltage being easily affected by fluctuations in the mains input voltage, lack of input-output isolation, poor output voltage quality factor, high harmonic content, and low load efficiency. [align=center] Figure 1 Schematic diagram of direct inverter auxiliary power supply circuit[/align] 2.1.2 Chopper-based Step-down Inverter Method Figure 2 shows the auxiliary power supply circuit structure of the chopper-based step-down and inverter method. This circuit mainly consists of a single-tube DC/DC chopper, a two-point inverter, a three-phase filter, an isolation transformer, and a rectifier circuit. After the inverter output is filtered by the three phases, it outputs a stable sinusoidal three-phase AC voltage, which serves as the power supply for driving three-phase AC loads such as air conditioners and fans. At the same time, after the three-phase AC voltage is rectified by the transformer, multiple DC outputs of the power supply can be realized. Its characteristics are as follows: (1) The output voltage of the three-phase inverter is not affected by the fluctuation of the input grid voltage. The closed-loop control of the DC/DC chopper can keep the input voltage of the inverter constant. (2) Each auxiliary inverter power supply chopper only needs one high-power high-voltage IGBT element, and the inverter can use a lower voltage IGBT element. (3) Since the input voltage of the inverter is constant, for inverters that only require CVCF control, only a certain number of ladder outputs are needed to ensure that the inverter outputs a stable pulse width modulation voltage with a harmonic content of less than 5%. (4) The choppers are distributed on the power supply of each vehicle, and the unit structure is uniform. For the power grid, although the switching frequency of each power supply chopper is the same, the chopper phase difference between them is random, and the chopper can also achieve multi-phase multi-chopping function. (5) The use of isolation transformers realizes the electrical isolation between the power grid input and output load. [align=center] Figure 2 Circuit structure diagram of chopper step-down inverter[/align] 2.1.3 The auxiliary power supply circuit of the double chopper step-down inverter method is basically the same as that of the single-tube direct DC/DC chopper step-down inverter method. The double choppers replace the DC/DC single-tube choppers, and the switching components can be GTO, IGBT or IPM. The circuit structure diagram is shown in Figure 3. Its characteristics are as follows. (1) By using a dual chopper, when the control phases of the upper and lower choppers are staggered by 180°, the switching frequency of the chopper can be doubled accordingly. This can greatly reduce the size and weight of the filter device, reduce the voltage ripple of the DC link in the inverter, and improve the anti-interference capability of the auxiliary inverter power supply. (2) The closed-loop control of the dual chopper plays a role in voltage stabilization and transformation, thus improving the output efficiency of the inverter. (3) Compared with a single-tube chopper, the dual DC/DC chopper has twice as many switching components and chopper accessories, but the withstand voltage of the tube can be reduced by half, improving the usage margin of components and the safety and reliability of the equipment. (4) The transformer isolation between the DC power supply network and the load, as well as the correspondingly designed filter, can ensure that the harmonics of the three-phase AC voltage output by the inverter are minimized, and can reduce the impact of overcharge voltage on the load, thus improving the service life of the load. [align=center]Figure 3 Circuit Structure Schematic of Double-Chopper Buck Inverter[/align] 2.1.4 Buck-Buck Chopper Inverter Method Figure 4 shows the circuit structure schematic of a subway auxiliary power supply with buck-buck chopper and inverter. The front-stage chopper consists of a smoothing reactor, two switching transistors, diodes, and an energy storage reactor. The buck-buck chopper is essentially equivalent to a two-phase DC/DC converter. The control system adopts PWM control. The two switching transistors alternately turn on and off, and the pulse width is appropriately controlled according to the output voltage to obtain a constant DC output voltage opposite to the input voltage. The rear-stage inverter output consists of a two-point three-phase inverter and a three-phase filter. The switching components of the chopper and inverter can be GTOs, IGBTs, IPMs, etc. The characteristics of this circuit are: fluctuations in the mains voltage do not affect the constant stability of the chopper output voltage. When the mains voltage is higher than the chopper output voltage, the chopper operates in buck chopping control mode; when the mains voltage is lower than the chopper output voltage, the chopper operates in boost chopping control mode. The alternating on and off of the two switching transistors increases the chopper switching frequency, reduces the size and capacity of the energy storage reactor and the voltage stress of the switching devices, and reduces the output voltage ripple. [align=center] Figure 4 Circuit structure diagram of buck-boost chopper inverter[/align] 3 Development and research of subway auxiliary inverter power supply As early as the late 1980s, the Locomotive and Rolling Stock Research Institute of the China Academy of Railway Sciences began to use advanced converter control technology and new high-power GTO and IGBT components to develop on-board power supply products. They successively developed high-power GTO choppers, two-phase bridge IGBT choppers, and on-board IGBT inverters for driving high-power linear motors and subway vehicles. In 1999, an air conditioning inverter power supply for the DC600V power supply system of passenger cars was developed, and it passed performance tests at the Sifang Rolling Stock Research Institute of the Ministry of Railways in June of that year. In September, it was installed and put into operation on the K79/80 train at the Wuchang Rolling Stock Depot. In 2000, an air conditioning inverter power supply for diesel and electric locomotives was developed and tested. This product has been installed and tested at the Nanchang Diesel Locomotive Depot and the Shaowu Electric Locomotive Depot. In 2002, in response to the technical requirements of imported auxiliary inverter power supplies for the Beijing "Fuxing-Bazhong" subway trains, the Locomotive and Rolling Stock Research Institute of the China Academy of Railway Sciences developed a domestically produced DC750V auxiliary power supply unit for subway trains, which passed type testing at the Locomotive and Rolling Stock Inspection Station of the Product Quality Supervision and Inspection Center of the Ministry of Railways. The developed DC750V subway auxiliary power supply has a total capacity of 40 kVA, and its main loads are lighting, ventilation fans, driver's cab air conditioning units, and DC110V and DC24V control power supplies for the vehicles. Considering the reliability of the power supply and the random multiplicity of multiple power supplies on the vehicle, the main power supply circuit adopts a single-tube chopper step-down inverter circuit, high-power IGBT switching elements, and heat pipe heat dissipation. The control adopts chopper and inverter dual closed-loop pulse width modulation control technology to ensure good stability of the three-phase AC output voltage and low harmonic content. Its main technical parameters are shown in Table 1. Table 1 Main technical parameters of subway auxiliary power supply device This subway auxiliary power supply has the following characteristics. (1) The auxiliary power supply chopper adopts chopper closed-loop control mode to ensure that the voltage of the intermediate DC link of the inverter power supply is stable when the input voltage changes. (2) The switching frequency of the output inverter is set to 214 kHz, and the harmonic suppression method is adopted to effectively suppress the harmonic content of the output voltage and current and the impact on the output high-frequency isolation transformer, thereby improving the power factor of the inverter and the utilization efficiency of the load. (3) A three-phase filter device and an isolation transformer are adopted to realize electrical isolation between input and output, AC load and DC output power supply. (4) The frequency conversion starting method is adopted, which reduces the starting current impact of the electrical load and helps to extend the service life of the load equipment. (5) The control system uses the MC80C196 sixteen-bit microcontroller as the main control unit, which has functions such as control implementation, protection, self-diagnosis, self-recovery, fault storage, LED indicator and Chinese character display, data transmission, and command reception. (6) The control system is equipped with fault protection functions such as short circuit, overvoltage, undervoltage, overcurrent, overheating, and grounding. It automatically resumes operation after the protection signal disappears, which improves the safety and reliability of the subway auxiliary inverter power supply. (7) The main control unit uses a box-type plug-in structure, which is convenient for maintenance, repair and equipment replacement. In order to adapt to the characteristics of large impact and large vibration during locomotive operation, the chassis adopts a metal frame structure, which has high mechanical strength and good electromagnetic shielding effect. The rated load test waveform of DC750V subway auxiliary power supply is shown in Figures 5 to 8. [align=center] Figure 5 Steady-state waveforms of input and output voltage[/align] [align=center] Figure 6 Output voltage and current waveforms Figure 7 Start-up, steady-state, and shutdown processes of intermediate stage voltage[/align] 4 Conclusions (1) Replacing traditional DC generator sets with static auxiliary inverter power supplies is an inevitable trend in the development of urban rail transit such as subways and light rail. (2) The selection of static auxiliary inverter power supply schemes should be combined with the development of domestic power electronics technology, the level of component usage, and the development direction of auxiliary inverter power supplies for subway electric vehicles abroad, to develop and manufacture auxiliary inverter power supply systems suitable for urban rail transit subways and light rail vehicles in China. (3) The successful development of subway static auxiliary inverter power supplies signifies that we have the capability to develop and produce domestically produced subway auxiliary power supplies. [align=center] Figure 8 Output voltage, current start-up, steady state, shutdown process[/align] References [1] Kikuchi Takahiro. New type of inverter for Japanese railway vehicles[J]. Foreign Railway Vehicles, 2000, 37(5): 23-26. [2] Application manual for third generation IGBT and intelligent power module[M]. Mitsubishi Electric, 1996. [3] SIV instruction manual[Z]. Toyo Electric Manufacturing Co., Ltd., 1998. [4] Wu Zhong, Li Hong, Zuo Peng, et al. Harmonic analysis and suppression strategy of natural sampling SPWM inverter power supply[J]. Power Grid Technology, 2001, 24(4): 1-5. [5] Wu Zhong, Li Hong, Zuo Peng, et al. Nonlinear output feedback control of DC/DC boost converter[J]. Chinese Journal of Railway Science, 2000, 21(3): 21