Protection of stray current in the hottest Urban R

2022-09-23
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Protection of stray current in urban rail transit Abstract: stray current does great harm to the environment and infrastructure around the power supply system. The principle of "prevention first, Drainage second, combination of prevention and drainage, and strengthening monitoring" should be adopted for the protection of stray current. Combined with years of on-site construction experience, the protection of stray current in urban rail transit is discussed from the aspects of stray current protection methods, technical principles and stray current monitoring

key words: rail transit, stray current, drainage cabinet, one-way conduction device

1 protection principle of stray current

in the rail transit DC traction power supply system, as long as the running rail is used as the return conductor, the generation of stray current is inevitable. In order to reduce the harm of stray current, we should try to reduce the stray current flow. Therefore, it is necessary to take effective measures to prevent stray current and control the stray current flow within the allowable range. The protection works of stray current basically adopt the principle of "prevention first, Drainage second, combination of prevention and drainage, and strengthening monitoring"

(1) prevention first

control all possible stray current leakage ways to reduce stray current entering the main structure of the rail transit system, equipment and structural reinforcement of relevant facilities along the line. During the specific implementation, due to the many disciplines involved, all disciplines and types of work must be closely coordinated, especially in the construction design stage, comprehensive prevention and control measures should be considered to minimize the electrical connection between the DC system and other buildings. The measures that can be taken include: the DC power supply equipment in the traction substation and the section is insulated from the structural reinforcement and the main body of the structure during installation; During the construction of the running track, the installation method of insulation with the track bed shall be adopted; Metal pipelines introduced from the outside into the rail transit or from the rail transit shall be introduced and led out after insulation treatment; Set up electrical connection of structural reinforcement inside the rail transit line, connect all structural reinforcement and grounding points together, and discharge the leaked stray current back to the DC system

(2) set the collection system of stray current with drainage as the auxiliary

. This collection system is the return channel of the first small resistance encountered after the stray current leaks from the return rail, which can limit the stray current inside the system as far as possible to prevent the stray current from leaking outside the system

2 stray current drainage system in different sections

in the specific implementation, corresponding drainage measures should be taken in different sections

2.1 stray current drainage system in elevated section

Shanghai Rail Transit elevated line generally adopts the structure of cast-in-situ concrete simply supported box girder, which is installed between the box girder and the pier through plate type rubber bearings to achieve the effect of insulation installation. In the stray current protection system, the surface reinforcement inside the cast-in-situ concrete simply supported box girder is welded with the reinforcement of the main structure. As a channel for collection and drainage, it is the first line of defense for stray current protection. Use the copper bar to lead out the structural surface as the drainage copper terminal, and use 1 × 150mm2 cable connects the drainage copper terminals of each cast-in-place concrete simply supported box girder, and then discharges the stray current scattered in the section to the DC power supply system through the drainage cabinet installed in the substation, playing a protective role of stray current gathering 400 + industry elite flow. At the same time, the copper terminal can also be used as the monitoring point of the stray current monitoring system

2.2 stray current drainage system in shield tunnel sections

shield tunnel sections are connected by circular segments one by one through bolts, and each shield segment has structural reinforcement. The segments installed in the tunnel are prefabricated. According to the requirements of stray current discipline, the structural reinforcement in each segment is shaped and welded together, so that the structural reinforcement in the segment is electrically connected, and the electrical connection points are well led out through steel washers. During the assembly of tunnel segments, the structural reinforcement of each tunnel segment is electrically connected through bolts and nuts to form an equipotential body. Each segment and each ring segment are connected into one by bolts in the ring and longitudinal directions to form a Faraday cage, which can electrically shield the subway to prevent the leakage of stray current of the subway and reduce the pollution to the underground environment

2.3 protection of stray current of monolithic track bed

when monolithic track bed is used in underground section tunnel, because the monolithic track bed is located under the running track and is closest to the track, it is easiest to directly collect the stray current leaked by the track. Therefore, in the shield tunnel, using the electrical connection of the internal structural reinforcement of the monolithic track bed to establish the collection of main stray current can most fundamentally solve the protection problem of stray current. However, the design of the monolithic track bed needs to consider the impact of natural conditions such as earthquakes. This exhibition will gather more than 10 world apex graphene teams and key enterprises in Changzhou, Jiangsu Province. Set a settlement joint at a certain position, and lead out the ballast bed reinforcement at the settlement joint to connect the copper terminal with 1 × 150mm2 cable electrically connects the track bed structural reinforcement on both sides of the settlement joint, and then discharges the stray current back to the DC system through the drainage cabinet of the traction substation, playing the role of stray current protection. At the same time, the copper terminal provides a direct monitoring point for stray current monitoring

when the monolithic track bed is used in elevated sections, due to the use of cast-in-place concrete simply supported box girder, the stray current is basically controlled. Therefore, the structural reinforcement inside the monolithic track bed is no longer electrically connected for stray current protection, but only serves as the main reinforcement of the structure

2.4 requirements of other relevant disciplines

the installation of fasteners such as contact, cable support and drainage pipes of water supply and drainage disciplines shall be fixed with expansion bolts. During the installation and construction in the tunnel and elevated section, the main reinforcement of segment and box girder structure must be avoided. When reserving bolts, water swelling sealing washers should be set, which is conducive to waterproof, and will not contact with the embedded parts of metal bolts, isolating the transmission path of stray current

3 application of drainage cabinet and one-way conduction device

3.1 working principle of drainage cabinet

drainage cabinet installed in traction substation plays an important role in drainage system. The drainage cabinet can effectively prevent the stray current from damaging the overhead cast-in-situ concrete simply supported box girder. It adopts 18 carbon fiber composite fan blades developed by SaiFeng company and ceramic matrix composite turbine parts, tunnel structural reinforcement, integral track bed structural reinforcement and metal equipment along the line. At the same time, it can prevent the stray current from leaking to the outside of rail transit, It is an effective method to protect the underground public environment of rail transit

the stray current drainage diode branch between the negative busbar of the negative cabinet and the stray current protection collection is the main working circuit of the drainage cabinet. Each diode branch is composed of diodes, fuses, shunts and rheostats in series, and each circuit is connected in parallel with an RC circuit to suppress overvoltage. The schematic diagram of the drainage cabinet is shown in Figure 1

the stray current collection busbar and the traction substation are measured by force measuring sensor, amplifier and data processing system. There is a current measuring device between the negative busbar to measure the stray current. Generally, the rated range is 600A. The measuring device includes an ammeter, overcurrent relay and a time relay. The ammeter indicates the magnitude of stray current, the adjustment range of overcurrent relay is DC 50 ~ 250A, and the adjustment range of time relay is 5 ~ 25s. When the measured stray current exceeds the design range, start the time relay, send an alarm signal to the signal lamp on the panel of the current drainage cabinet, and send it to the control center, so as to check whether the DC power supply system has faults or other related professional accident hazards

3.2 application of one-way conduction device

in rail transit parking lots and train maintenance garages, there are many reasons for stray current leakage due to complex operating environment and human factors. In addition, due to the operation of the electrical system, the track is equipped with an insulating joint at this position. Therefore, the adoption of one-way conduction device not only solves the problem of electrical connection of insulating joints, but also solves the problem of stray current protection

the unidirectional conduction device is composed of three circuits in parallel, including diode circuit, disconnector circuit and discharge gap device (see Figure 2), and the steel rails at both ends of the insulating joint are connected with cables. During normal operation, the disconnector is open and the diode circuit works, so that the rail current is conducted in one direction and cut off in the other direction, effectively preventing the increase of the stray current of the whole rail transit due to the poor insulation level of some rails. In case of emergency braking of the train at the parking lot and train inspection shed, regeneration mode shall be adopted first. If there is no other locomotive to absorb the generated regenerative current, when the rail voltage at both ends of the insulating joint rises to a certain value (1000V), the discharge gap will break down and discharge (see Figure 3), there is current flowing in the circuit r-m1-m2-cp-p, and the rail potential will be reduced, which can avoid personal injury accidents. When the current value of circuit r-m1-m2-cp-p reaches 200A, the bypass switch is closed due to the joint action of M1 and M2 coils, and the r-m1-k-p circuit is connected. When the current flowing in M1 is reduced to less than 50a, M1 electromagnetic force is not enough to keep the bypass switch closed, and the bypass switch is opened. In case of diode circuit failure, the isolating switch circuit can be used to directly connect the track insulation joint to ensure the normal operation of the locomotive

4 set up stray current monitoring system

a complete stray current monitoring system is designed to monitor the magnitude of stray current, which can provide data basis for operation maintenance and prevention of stray current

the potential analysis method in electrochemistry is the principle of stray current measurement at this stage. The reference electrode with constant potential and providing measurement potential standard is selected as the basic potential to measure the potential of structural reinforcement, drainage copper bar, rail, etc. to measure the stray current in rail transit

the stray current monitoring system is composed of reference electrode, track potential test terminal, current drainage test terminal, main structure reinforcement test terminal, potential measurement box and stray current comprehensive test device (see Figure 4). At present, in the stray current monitoring system of Shanghai rail transit, dqu-350 series cu-cuso4 electrode is mostly used as the reference electrode, and the data is collected by the data acquisition instrument during the measurement. It receives the signal of potential measurement from the potential measurement box, records and saves it. It can be connected with the computer to monitor and record synchronously. If it finds any abnormality after analyzing the data, it will send an alarm signal. This method is convenient for mastering the situation of stray current and dealing with it in time to protect the track construction project

through the combination of the above prevention, drainage, monitoring and other methods, it can basically prevent the harm of stray current and protect the rail transit and the nearby underground public environment

references

1 Wang Yuanyuan Research and Discussion on stray current "source processing" method Urban rail transit research, 2001 (1): 42

2 Li Wei Monitoring and prevention of subway stray current Urban rail transit research, 2003 (4): 48 (end)

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