Leakage Current in Power Distribution System of Subway Stations

Urban rail transit equipment is complex, large in number, and widely distributed, and there are fire hazards in long-term continuous operation. Among subway fires at home and abroad, fires caused by electrical reasons account for the largest proportion, reaching 37%, among which the leakage current of the power supply line is an important factor causing electrical fires. Fire departments and industry experts often require subway stations to set up electrical fire monitoring systems, but in the subway monitoring and prevention measures, the monitoring of leakage current is not perfect. The existing leakage current monitoring system has false alarms, which makes the leakage protection of the distribution system frequently trip. To this end, finding the cause of frequent false alarms and taking relevant technical measures to limit leakage current are important tasks for subway safety risk management.

Our company studies the leakage current of power distribution equipment and lines in urban rail transit subway stations, and proposes targeted measures to limit leakage current, solve the problem of frequent tripping of leakage protection of distribution system and frequent alarm of electrical fire monitoring system, and create conditions for the implementation of safe and reliable smart subway distribution system.

Analysis of the cause of leakage current in subway stations

The electro mechanical equipment and lighting power loads of subway stations are divided into 3 levels according to their different uses and importance. The primary load is an important load in the subway station. From the two bus sections of the substation, one power supply is drawn to the equipment (group). The two power supplies are automatically switched at the end of the line. The switching method is mainly through contactors, STS static switches, PC-level automatic train monitoring systems (ATS), CB-level ATS, etc. The secondary load feeds a single-circuit power line from the secondary load bus of the step-down substation or the environmental control room to the power box of the equipment. The tertiary load draws a single-circuit power line from the tertiary load bus section to the equipment.

By comparing and analyzing the on-site data, the power supply method of each load and the power distribution method inside the equipment, it can be found that there are three reasons for the leakage current in the subway station power distribution system.

1) The neutral line of the dual power switching circuit is connected in parallel.

2) 1.2 TN-S system has multiple grounding points.

3) Construction wiring errors are used for operation.

Generation of residual current

Residual current is commonly known as leakage current. In fact, any electrical equipment and any power supply line has more or less residual current. Residual current is generally the normal leakage current of the live conductor to the ground, so it is also called leakage residual current. The leakage residual current is directly related to the electrical construction process, construction level, insulation layer medium and thickness, line laying method, power supply line form of electrical equipment, the length of time the electrical equipment and lines are used, the load category and quantity, the surrounding geographical climate environment (temperature, humidity, air pressure, etc.), power supply voltage, current, frequency and power pollution degree and other parameters, as well as the nature of building use. Although there is no need to have a very accurate leakage residual current value in actual work, as an important parameter in electrical design, there should also be a more correct estimated reference value. The residual current to the ground generated by the equipment under normal use in normal environment should be called normal leakage residual current.

Measures to Limit Leakage Current

Through the above analysis, the fundamental reason affecting the leakage current of the subway station distribution system is that the current flowing out of the power supply side does not return according to the established circuit, which is specifically manifested in the parallel connection of the neutral line of the dual power switching circuit, multi-point grounding in the TN-S system, and construction and installation wiring problems. Therefore, in view of the above reasons, the power distribution system of urban rail transit stations can adopt the following measures to limit and reduce the leakage current of the power distribution system of subway stations.

1) For the single busbar disconnection wiring method, the bus tie switch should use a four-pole circuit breaker.

2) The neutral line (N line) should not be shared in the TN-S system feeder circuit.

3) The phase line and neutral line of the same circuit should be laid in parallel.

4) After construction and installation, the power distribution system should be tested for insulation and leakage current before power is turned on to ensure that the system, line, and electrical equipment are in good condition.

Acrel ASJ series residual current relay can meet the protection of the above-mentioned leakage conditions. It can be used in conjunction with the remote control trip switch to cut off the power supply in time, prevent indirect contact, and limit the leakage current. It can also be directly used as a signal relay to monitor power equipment. It is particularly suitable for the safety protection of power distribution systems in subway stations, schools, commercial buildings, factory workshops, trade markets, industrial and mining enterprises, national key fire units, intelligent buildings and communities, subways, petrochemicals, telecommunications.

Type	Function	Installation	Appearance
ASJ10-LD1C	AC-type residual current measurement; Alarm prompt of current crossing line; Rated residual operating current can be measured; Limit non-driving time can be set; Two sets of relay output; With local and remote "test" and "reset" functions.	Guide (DIN35mm)
ASJ10-LD1A	A-type residual current measurement; Current percentage light column display; Rated residual operating current can be measured; Limit non-driving time can be set; Two sets of relay output (settable) [1]; With local and remote "test" and "reset" functions.
ASJ10L-LD1A [2]	A-type residual current measurement; Rated residual operating current can be measured; Limit non-driving time can be set; Two sets of relay output (settable); Transformer breakage alarm can be set; Pre-alarm value can be set; Return value can be set. With local and remote "test" and "reset" functions. LCD display, 25 event records.
ASJ20-LD1C	AC-type residual current measurement; Alarm prompt of current crossing line; Rated residual operating current can be measured; Limit non-driving time can be set; Two sets of relay output; With local and remote "test" and "reset" functions.	Panel (48*48)
ASJ20-LD1A	A-type residual current measurement; Current percentage light column display; Rated residual operating current can be measured; Limit non-driving time can be set; Two sets of relay output (settable)note 1; With local and remote "test" and "reset" functions.	Panel (48*48)