Abstract: Urban rail transit vehicles generally use DC power supply, and the auxiliary power supply system on the vehicle powers the auxiliary equipment. This paper provides a simple analysis and introduction to the circuit structure, form, and auxiliary power supply system of auxiliary inverters for urban rail transit vehicles, and points out the application and development of auxiliary power supply systems for urban rail transit vehicles. Keywords: Auxiliary inverter, PWM modulation, isolation transformer, chopper control power supply. In urban rail transit vehicles, DC voltage (generally 1500VDC and 750VDC) is usually obtained from the power grid and converted to 380VAC output by the auxiliary inverter (also called static inverter) to power the auxiliary equipment on the train. Urban rail transit vehicles generally use two types of vehicles. For the two types of vehicles, the inverters operate differently: Type A vehicles are trailer cars, and their inverters supply one path to the train lighting and fan motors; the other path outputs 110VDC control power and also charges the batteries; Type B/C vehicles are motor cars, and their inverters output 380VAC to power the train's air conditioning units and air compressors respectively. The following is an analysis and introduction of the auxiliary circuit system of urban rail transit vehicles. 1. Auxiliary Inverter Circuit Structure There are two common forms of auxiliary inverter circuits in urban rail transit vehicles: one uses direct inversion (DC-AC), as shown in Figure 1; the other uses a chopping (boost/buck chopping) followed by inversion (DC-DC-AC), as shown in Figure 2. Siemens uses the DC-DC-AC form, such as in the Shanghai Metro Lines 1 and 2 and Guangzhou Metro Line 1 vehicles; Bombardier uses the DC-AC form, applied in vehicles produced in Changchun. In the DC-DC-AC boost/buck chopping method, the boost chopping system is used in applications with a DC 750V power grid voltage; the buck chopping system is used in applications with a DC 1500V power grid voltage. The purpose of using boost/buck chopping is to stabilize the inverter's input voltage, ensuring a stable output voltage when the load changes or the voltage fluctuates. Currently, the switching frequencies of switching devices such as GTOs and IGBTs are sufficient to achieve stable inverter output and full-load operation using PWM modulation within the grid voltage fluctuation range. Therefore, vehicles currently in production often adopt direct inverter methods. 2. Auxiliary Inverter Forms Currently, there are two forms of auxiliary inverters used in urban rail transit vehicles in China: one is a single inverter, and the other is two inverters connected in series. For example, Shanghai Metro Line 1 DC trains use a single inverter, while Shanghai Metro Line 2 trains use two inverters connected in series. 2.1 Single Inverter Form For auxiliary inverters with a grid voltage of 1500V and a capacity of approximately 200kVA, 3300V/400A IGBT components are generally used. This form has a simple and reliable structure, and the inverter uses PWM modulation to keep the harmonic content of the output voltage within the limit value, making it the most commonly used form at present. 2.2 There are two schemes for the series connection of two inverters: one is to connect the outputs of the two inverters to an isolation transformer, and then output the circuit or magnetic circuit of the isolation transformer through superposition, followed by filtering. The advantage of this scheme is that the inverters can use low-voltage IGBT elements; the other is to control the phase difference of the output voltages of the two inverters. After they pass through the transformer's circuit or magnetic circuit superposition, the harmonics of the transformer's output voltage are reduced, thus reducing the requirements for the output filter, i.e., reducing the size and weight of the filter. It should be noted that this circuit is more complex, especially the transformer. The secondary winding of the transformer using circuit superposition is more complex. The magnetic circuit design of the transformer using magnetic circuit superposition is more complex. On the other hand, this circuit emerged when the level of IGBT elements was not very high in the early days. Therefore, this form is basically no longer used. 2.3 Auxiliary power supply system Taking Shanghai Metro Line 1 as an example, its static inverter principle block diagram is shown in Figure 3. The DC 1500V power supply is filtered by an LC filter and then chopped and regulated to 770V by a GTO chopper. After passing through an intermediate filter, it is fed into a six-pulse GTO inverter, whose output becomes AC 380V after passing through an isolation transformer. A tap is also located on the secondary side of the isolation transformer, whose output AC voltage is rectified to provide DC 110V power. The core of the control unit is a microprocessor, comprising four functional packages: Power Supply Package (P-PAC) – provides control pulses for the power supply, chopper, and inverter; Communication Package (C-PAC) – transmits various signals from the inverter and the train, and stores actual values ​​of process parameters; Interface Package (I-PAC) – determines required parameter values ​​and monitors inverter voltage, current, temperature, delay time, and operating process; Fast Protection and Control Package (F-PAC) – controls the inverter's operation, stores analog values ​​of actual process parameters, and implements fast inverter protection. 3. Application and Development On Wuhan Light Rail Line 1, the auxiliary power system uses IGBT modules (1700V/1200A) to form static inverters, outputting stable three-phase AC380V, DC110V, and DC24V power to supply the air compressors, air conditioners, lighting, and electric heaters on the train, and to charge the batteries. Each train is equipped with two sets of auxiliary power inverters with a capacity of 140KVA. On Shanghai Metro Line 2, the auxiliary power system uses static inverters composed of IGBT modules (3300V/1200A), outputting AC380V power. Each car has one auxiliary inverter with a capacity of 90KVA. The inverter in car A supplies power to half of the train's lighting and fan motors, and also provides DC110V power. The inverters in cars B and C each supply power to half of the train's air conditioning units. Most metro cars use a two-motor-one-trailer (3 cars) configuration to form a unit, with two units constituting one train. Each car is equipped with a static inverter, and each unit shares a single DC 110V control power supply. The auxiliary inverter capacity of each car is 75-80KVA, and the DC 110V control power supply is approximately 25KW. Metro cars manufactured by ALSTON in France have been modified to have two static auxiliary inverters per unit, each with a capacity of 120KVA, and each including a DC 110V control power supply with a power of 12KW. Recently, foreign-made metro cars have adopted centralized control, with each unit in a 6-car formation equipped with only one static auxiliary inverter, with a capacity of approximately 250KVA, and one DC 110V control power supply, approximately 25KW. Currently, most metro and light rail auxiliary systems worldwide use Insulated Gate Bipolar Transistor (IGBT) (or IPM) modules. For personal safety, the low-voltage system and control power supply are isolated from the high-voltage system by a transformer in terms of potential. At present, both domestic and foreign countries adopt the scheme of DC-DC conversion and high-frequency transformer isolation. From the perspective of redundancy and axle load balance, the distributed power supply scheme is often selected. References [1] Xu An, Tao Shenggui, Pang Qianlin. Urban Rail Transit Electric Traction [M]. Beijing: China Railway Publishing House, 1998. [2] Gao Shuang. Subway Vehicle Structure and Maintenance Management [M]. Beijing: China Railway Publishing House, 2003.