Brief Analysis of Auxiliary Power System for Urban Rail Transit Vehicles

Abstract: Urban rail transit vehicles generally use DC power supply, and the auxiliary power supply system on the vehicle is used to power the vehicle auxiliary equipment. This paper briefly analyzes and introduces the circuit structure, form and auxiliary power supply system of the auxiliary inverter of urban rail transit vehicles, and points out the application and development of the auxiliary power supply system for urban rail transit vehicles.
Keywords: auxiliary inverter PWM modulation isolation transformer chopper control power supply
In urban rail transit vehicles, DC voltage (usually 1500VDC and 750VDC) is usually obtained from the power grid, and the auxiliary inverter (also called static inverter) is used to convert and output 380VAC 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 working form of the inverter is different: Type A vehicles are trailers, and their inverters supply train lighting and fan motors in one way; the other outputs 110VDC control power and charges the battery at the same time; Type B/C vehicles are motor vehicles, and their inverters output 380VAC to power the train's air conditioning unit and air compressor 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 adopts direct inversion (DC-AC), as shown in Figure 1; the other adopts first chopping (boost/buck chopping) and then inversion (DC-DC-AC), as shown in Figure 2. Siemens uses the DC-DC-AC form, such as Shanghai Line 1 and Line 2 and Guangzhou Line 1 subway vehicles; Bombardier uses the DC-AC form, which is used in vehicles produced in Changchun.




Among them, in the DC-DC-AC mode of boost/buck chopping, the boost chopping system is used in the occasion of DC750V power supply network voltage; the buck chopping system is used in the occasion of DCl500V network voltage. The purpose of using boost/buck chopping is to stabilize the input voltage of the inverter and ensure that the chopper has a stable output voltage when the load changes or the voltage fluctuates.
At present, the switching frequency of switching devices represented by GTO and IGBT is sufficient to meet the requirements of stable inverter output and full-load operation within the range of grid voltage fluctuations using PWM modulation. Therefore, vehicles currently produced often use direct inverters.
2 Auxiliary inverter form
At present, there are two forms of auxiliary inverters used in urban rail transit vehicles in China: one is a single inverter form, and the other is a two-inverter series form. For example, Shanghai Line 1 DC subway vehicles use a single inverter form, and Shanghai Line 2 subway vehicles use two inverters in series.
2.1 Single inverter form
For auxiliary inverters with a grid voltage of 1500V and a capacity of about 200kVA, 3300V/400A IGBT components are generally used. This form has a simple structure and is reliable. The inverter uses PWM modulation to keep the harmonic content of the output voltage within the limit value. It is currently a commonly used form.
2.2 Two inverters in series
There are two solutions: one is to output two inverters to the isolation transformer, through the circuit superposition of the isolation transformer, or the magnetic circuit superposition, and then output after filtering. The advantage of this solution is that the inverter can use low-voltage IGBT components; the other is to control the phase difference of the output voltage of the two inverters. When they pass through the circuit superposition or magnetic circuit superposition of the transformer, the harmonics of the transformer output voltage are reduced, so that the requirements for the output filter can be reduced, that is, the volume and mass of the filter can be reduced.
It should be pointed out that this circuit is relatively complex, especially for transformers. The secondary winding of transformers with circuit superposition is relatively complex. The magnetic circuit design of transformers with magnetic circuit superposition is relatively complex. On the other hand, this circuit was produced when the level of IGBT components was not very high in the early days. Therefore, this form is basically no longer used.
2.3 Auxiliary power supply system
Take Shanghai Metro Line 1 as an example, the principle block diagram of its static inverter is shown in Figure 3. The DC1500V power supply is chopped and regulated to 770V by the GTO chopper after passing through the LC filter, and then sent to the six-pulse GTO inverter through the intermediate filter. Its output becomes AC380V after passing through the isolation transformer. There is also a set of taps on the secondary side of the isolation transformer, and its output AC voltage is rectified to provide DC110V power supply.

The core of the control unit is a microprocessor, which includes four function packages:
power function package (P-PAC) - provides control power supply and pulses for chopping and inverter.
Communication function package (C-PAC) - transmits various signals on the inverter and train, and stores the actual values ​​of process parameters.
Interface function package (I-PAC) - determines the required values ​​of parameters, monitors the voltage, current, temperature, delay time and working process of the inverter.
Fast protection and control function package (F-PAC) - controls the working process of the inverter, stores the analog value of the actual value of the process parameters, and realizes fast protection of the inverter.
3 Application and development
On Wuhan Light Rail Line 1, the auxiliary power system uses IGBT modules (1700V/1200A) to form a static inverter, which outputs stable three-phase AC380V power, DC110V and DC24V power for the air compressor, air conditioner, lighting and electric heater on the train, and charges the battery. Each train is equipped with two sets of auxiliary power inverters with a capacity of 140KVA.
The auxiliary power system of Shanghai Metro Line 2 uses a static inverter composed of IGBT modules (3300V/1200A) to output AC380V power. Each car of the train is equipped with an auxiliary inverter with a capacity of 90KVA. The inverter of car A supplies power to half of the lighting and fan motors of the train, and also provides DC110V power. The inverters on cars B and C supply power to half of the air conditioning units of the train respectively.
Most subway vehicles are composed of two motor vehicles and one trailer vehicle (3 vehicles), and two units form a train. Each vehicle is equipped with a static inverter, and each unit shares a DC110V control power supply. The capacity of the auxiliary inverter of each vehicle is 75-80KVA, and the power of the DC110V control power supply is about 25KW. The subway vehicles produced by ALSTON in France are changed to 2 static auxiliary inverters in one unit, each with a capacity of 120KVA, each with a DC110V control power supply, and a power of 12KW. Recently, subway vehicles produced abroad adopt centralized control. In the 6-car formation, each unit is equipped with only one static auxiliary inverter with a capacity of about 250 KVA and a DC110V control power supply of about 25KW.
At present, most of the subway and light rail auxiliary systems in the world are composed of 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 transformers. At present, both domestic and foreign countries use DC-DC conversion and high frequency transformer isolation. From the perspective of redundancy and axle load balance, a distributed power supply solution is often used.
References
[1] Xu An, Tao Shenggui, Pang Qianlin. Electric traction of urban rail transit [M]. Beijing: China Railway Publishing House, 1998.
[2] Gao Shuang. Metro vehicle structure and maintenance management [M]. Beijing: China Railway Publishing House, 2003.