Common Faults of UPS in Rail Transit Communication Signal Power Supply System and Improvement Scheme of Selection(1)

Abstract: At present, the failure rate of ups in the rail transit communication signal power supply system is high, especially in the small capacity section below 20KVA. The reason is that there are serious defects in the selection and system scheme of UPS, which not only does not save the cost for users, but also buries potential safety hazards in the operation. In view of these practical problems, this paper puts forward the selection and improvement plan.

1. Introduction of communication signal system of rail transit and composition of communication signal power supply system

A. Introduction of rail transit communication system

The task of rail transit communication system is to establish an audio-visual link network, improve the level of modern management and transmit various information such as voice, data, image and text. The system is mainly composed of transmission system, official telephone system, special telephone system, wireless communication system, broadcasting system, clock system, video monitoring system, passenger information system, power supply and grounding system, communication integrated network management system and other subsystems.

B. Introduction of rail transit signal system

Urban rail transit signal system is the key system equipment to ensure the safety of train operation, realize the modernization of train operation, and improve the transportation efficiency.

The signal system of urban rail transit is usually controlled by automatic train control system (ATC). ATC system includes three subsystems:

1. Automatic train monitoring system, referred to as ATS, 2. Automatic train protection subsystem, referred to as ATP, 3. Automatic train operation system, referred to as ATO.

The three subsystems form a closed-loop system through information exchange network, which realizes the combination of ground control and on-board control, and the combination of local control and central control, and forms a train automatic control system based on safety equipment, which integrates the functions of train operation command, operation adjustment and train driving automation.

C. At present, due to the cost, most of the rail transit communication signal power systems adopt commercial UPS single machine, and the composition diagram is as follows:

Figure 1 current rail transit communication power system

Figure 2 current rail transit signal power system

From Figure 1 and Figure 2, it can be analyzed that in the rail transit communication signal power supply system, UPS actually affects the safe operation of rail transit, and causes the operators and manufacturers many problems:

1. In terms of system composition, two lines of municipal power are input to the voltage regulator (the voltage regulator itself has bypass) through ATS mutual input, and the voltage regulator is output to UPS power supply, and then the UPS is output to AC distribution cabinet, that is to say, the uninterrupted operation of rail transit communication power supply system is completed by UPS, so the importance of UPS can be seen. Unfortunately, because 1kva-20kva uses a commercial UPS single machine system, the static bypass of this kind of UPS is not separated from the input of rectifier. In actual operation, there is no bypass power supply after the battery discharge of UPS is completed due to the trip of the input switch Ki of UPS itself, resulting in the system load power failure, which seriously affects the safety and reliability of rail transit operation.

There are many reasons for the trip of UPS input switch Ki, including: 

1) The switch itself has quality problems. Some switches with poor quality, once used for a long time, will be tired and trip from time to time. 

2)There is a short circuit on the secondary side of the switch. For example: UPS rectifier fault, UPS input terminal short circuit phenomenon, this kind of trip often occurs in actual operation. 

3)Trip due to overload. That is, all possible causes of overcurrent. Such as harmonic, surge, voltage sag, starting current, virtual connection and so on.

2. In the aspect of UPS selection of the system, because the commercial UPS is adopted, in the actual operation, not only will the rectifier failure occur frequently due to the low immunity designed by the ups itself, but also because the overload of the inverter can not be automatically restored to the power supply of the inverter after the overload of the inverter trips by-pass, and the inverter needs to be manually started again, which brings serious defects to the safety of rail transit operation.

From figures 1 and 2, we can see that: Due to the overload of UPS (for example, when the communication signal power supply is output to the load, the isolation transformer is often used to isolate the output. If the transformer has the starting excitation current, the inverter will be overloaded). When the UPS is converted to the bypass operation, if the overload is removed and the inverter cannot be restored automatically, then any power failure of the city will cause the power failure of the load. Because the switching time of ATS is more than 50ms at least.

Therefore, the ups host shall be the industrial UPS exclusive to rail transit, It shall have: 

I. high level immunity, which shall be applied to severe electrical environment (immunity includes: 1. Radiation sensitivity test, 2. Power frequency magnetic field radiation sensitivity test, 3. Conduction sensitivity of RF field induction, 4. Immunity of electric fast transient pulse group, 5. Surge immunity, 6. Immunity of voltage drop and interruption, 7. Induction / contact of power line, 8 Electrostatic discharge immunity);

II. The rectifier and the static bypass input Ki and KP;

III. after the inverter trips the bypass due to overload, the overload release can automatically restore the inverter power supply.

2. Selection and improvement of industrial UPS for rail transit communication signal power system.

Figure 3 improved rail transit communication power system

Figure 4 improved rail transit signal power system

It can be seen from Fig. 3 and Fig. 4 that in the industrial UPS system of rail transit communication signal power supply, the rectifier and static bypass of ups have two lines of municipal power input Ki and KP, which eliminates the risk of only input switch KI. The power supply of the voltage regulator only supplies power to the bypass, because the main input of the industrial UPS, that is, the input of the rectifier, does not need the voltage regulator to protect, and the response time of the voltage regulator is usually about 1 second, which is suitable for long-term high-voltage or low-voltage adjustment. If the voltage of the bypass standby power supply is often unstable, it can be adjusted by the voltage regulator. After all, when the ups trips the bypass, the bypass standby power supply is directly supplied load.

In addition, the problem of industrial UPS should be emphasized here. In a word, the reliability of industrial UPS is higher than that of commercial ups.

Author of this paper is He Chun, Chief Engineer of AVIC-TECH 

About AVIC-Tech

AVIC-Tech (Xiamen)Electric Power Technology Co.,Ltd is a high-tech enterprise specializing in R&D, manufacture, sales and service of high-end power supply. Our flagship products are industrial UPS (1kVA-800kVA), dynamic voltage restorer (10kVA-2000kVA), and lithium battery system, which are widely used in nuclear power, electric power, petroleum and petrochemical, rail transportation, industrial manufacturing and other fields.

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