Ground and Stray Current

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summary: When a large DC current flows along a track laid on the ground, the DC current will not only flow in the track, but also leak from the track to the earth, flow on various metal objects in the earth, and then return to the power system. This part of the leaked current is called stray current, which is also called stray current in subway engineering. keywords: Grounded Rail Impedance Stray Current When large DC current flows along the track laid on the ground, the DC current will not only flow in the track, but also leak from the track to the earth, flow on various metal objects in the earth, and then return to the power system. This part of the leaked current is called stray current, which is also called stray current in subway engineering. (1) Characteristics of stray current An important project has an underground oil pipeline. Shortly after it was put into use, a perforation and oil leakage occurred. After a survey of the perforation and oil leakage, it was found that there was a small DC electric railway on the ground with a total length of about 2500m, of which about 1000m was parallel to the oil pipeline. The distance between the two was very close, generally only a few dozen meters, and in some places only 25m apart. 　　In order to find out whether the oil pipeline leakage is caused by stray current, measure the potential of the oil pipeline to the ground (note: The outside of the oil pipeline has been insulated and wrapped with glass ribbon and asphalt), and the following results were obtained from the measurement: 　　1. The tube-ground potential is unstable. Several points in the corrosion zone were measured and it was found that the tube-ground potential at each point fluctuated with time. Within 10 to 15 minutes, the maximum fluctuation amplitude could reach several volts, as shown in Figure 32.: At G measuring point, the tube-to-ground potential fluctuates between -1. 0V ~ -3. 5V, and at E measuring point, the potential fluctuates between 1. 5V ~ -1. 0V. 　　Whether the tube-to-ground potential is stable is an important indicator of whether there is stray current in the ground. The above measurement results show that the tube-to-ground potential is unstable, which means there is stray current in the ground at the detection point. 　　2． The pipe-to-ground potential seriously deviates from the normal value. This project tested the pipe-to-ground potential at 10 points. The results are shown in Table 17. 　　 According to the measurement results, the natural potential in the area is about -0. 6 ~ -0. 7V. The middle value -0. 65V is taken as the normal value of the natural potential in the area. Therefore, 10 maximum measured values ​​deviate from the normal value are obtained. Among them, the measured value at A is relatively close to the normal value, and the other points all deviate significantly from the normal value. The maximum negative deviation reaches 3.15V, and the maximum positive deviation reaches 2. 45V. 　　When there is a serious deviation from the normal value of potential, it indicates that this area is invaded by quite strong underground stray current. 　　3． Abnormal soil potential gradient Under normal natural conditions, the potential gradient of soil is generally very small, and the measurement error is at the same level as the conventional method. The soil potential gradient measurement results of the two points with the largest deviation from the normal value in this project are shown in Table 18. According to the classification standard, when the soil potential gradient is less than 0. 5mV/m, it is an area with weak interference from stray currents; when it is between 0.5 and 5mV/m, it is an area with medium-strength interference; when it is greater than 5mV/m, it is an area with strong interference. Point G is in a strong interference area, and point E is in a moderate interference area. 　　According to the principle of electric corrosion, stray current flows into the part of the pipe, that is, the pipe-ground potential is negatively biased to the cathode area. ； The part where stray current flows out of the pipe, that is, the pipe-ground potential is positively biased to the anode area, which is where corrosion occurs. 　　It can be seen from Table 17 ； The section from measuring point B to measuring point F is the anode area, that is, the corrosion area. ； Near point E, the tube-ground potential has the largest positive bias and the corrosion is the most severe. (2) Methods to Reduce Stray Current Currently, subways generally adopt the following methods to reduce stray current. 　　1． Reduce rail impedance. The rails that subway trains run on also serve as traction trains for the return flow of people. Therefore, the smaller the rail impedance, the smaller the stray current flowing out from the rails. An effective way to reduce rail impedance is to use long rails. The longer the rails, the fewer rail joints and the smaller the rail impedance. In addition to connecting the rail joints with fishplate bolts, two more than 120mm2 insulated copper cables are used to connect the two rails. 　　2． The use of point supports for running rails to reduce the contact surface between the rails and the ground is also one of the methods to reduce stray currents. For this reason, the running rails use point supports, that is, concrete soft sleepers are used as supports. 　　3． Insulation between the rail and the ground The better the insulation between the rail and the ground, the smaller the stray current. Therefore, insulation is adopted between the rail and the concrete soft sleeper, between the fastening bolts and the concrete soft sleeper, and between the fasteners and the concrete soft sleeper. It is required that the leakage resistance of each kilometer of track to the stray current collection network is greater than 10Ω. 　　4． Set up a stray current collection network and use DC power supply for Shanghai subway electric vehicles. The rated voltage is 1500V and the rated current is up to 3000A. Although insulation measures are taken between the running rails that also serve as return currents and the ground, long rails are used, and copper cables are welded at the rail joints, the rails themselves have resistance. When current flows through the rails, a potential difference is generated in the resistance. Since the insulation resistance of the rails to the ground cannot be infinite, the potential difference will produce stray currents. That is, a small part of the current of large rails will flow out of the track. This stray current is regarded as a "miscurrent" in the subway. When the stray flow enters the structural steel bars of the subway tunnel and the metal pipes, supports, bridges, etc. that are poorly insulated from the tunnel, these metal equipment will be subject to electrical corrosion in the presence of electrolyte. To this end, a stray current collection network is installed in the track bed under the subway concrete soft sleeper. 　　 The stray current collection network consists of upper and lower rows of longitudinal steel bars. Each row of steel bars is 5 Φ12mm steel bars. Every 50m, the 5 longitudinal steel bars are welded into a whole with a Φ25mm or more transverse steel bar. At the same time, two Φ20mm steel bars are used to weld the upper and lower 2 transverse connecting steel bars into one body, as shown in Figure 33. In addition to collecting stray currents, the five steel bars in the upper row also play a role in fixing the concrete soft pillow. Holes are pre-pierced on the concrete soft pillow, and the steel bars are penetrated during construction. The lower row of steel bars is fixed in the concrete track bed. 　　 The stray current collection network and the structural steel bars of the tunnel should be insulated and cannot be connected. 　　The stray current collection network is equipped with lead-out terminals at both ends of each traction substation to measure and collect stray currents. (3) Methods to prevent stray currents The above measures can reduce stray currents * * Reduced, but there is still a small amount of stray current that inevitably flows from the concrete track bed to the metal conductors in the tunnel structure. If no measures are taken, this part of the stray current will cause corrosion of the metal conductors. Methods to prevent stray current are introduced below according to each type of equipment. 1. Cable bridge The Shanghai subway project uses metal cable bridge. It is required that the insulation resistance between each bracket of the bridge and the steel bars of the tunnel structure is not less than 10kΩ. For this reason, insulation expansion bolts are used when fixing the bracket, and a multimeter is used when measuring insulation. If it is an insulation resistance meter (megger), it is difficult to read because the value of 10kΩ is relatively small. When the bridge is integrated and the insulation resistance between the bridge and the tunnel's main structural steel is measured, a "false short circuit" will appear. However, there is often a potential difference between the measured bridge and the tunnel's main structural steel. The reason for this potential difference is that: 　　(1) The earth is not an absolute equipotential body. In power engineering, the earth is always regarded as zero potential, but this is not actually the case. As shown in Figure 34, after the neutral point of the transformer is grounded, measure the phase-to-ground voltage from the same point on the earth. If the three phase-to-ground voltages are equal, then measure the L1-to-ground voltage from point A, the L2-to-ground voltage at point B, and the L3-to-ground voltage at point C. At this time, the three-phase voltage values ​​are often unequal. Because the potential of points A, B, and C of the earth is usually different. The potential of each point of the earth is the composite result of natural electric field and artificial electric field. Therefore, the earth is not an absolute equipotential body. 　　(2) Concrete is not a complete insulator. Although the bridge bracket is fixed with nylon expansion bolts for insulation, there is still contact between the bridge bracket and the concrete. The longer the bridge length, the more contact surfaces between the bridge and concrete, and the smaller the insulation resistance between the two. 　　(3) Potential exists between copper and iron. The Shanghai Metro uses corrosion-resistant copper grounding electrodes as working grounding. A subway station only has one grounding point. The grounding device and the tunnel main structural steel bars are insulated from each other. Therefore, a battery effect is generated between the copper grounding electrode and the tunnel steel bars, and a potential difference appears between the bridge and the main steel bars connected to the copper grounding electrode. 　　2． Power and lighting piping Shanghai Metro's power and lighting piping are all made of flame-retardant PVC pipes to avoid electrical corrosion of the protective tubes by stray currents. 　　3． Station water supply and drainage pipes (1) All water supply and drainage pipes entering the station should be added with a 2m long insulating pipe for insulation isolation before entering the station. The insulating pipe is located outside the station, 150mm away from the main structure. 　　 (2) The water supply and drainage pipes leaving the subway area should be added with a 2m-long UPVC plastic insulating pipe before they can be led out to the ground. The insulating ribs should be located in a dry and easy to inspect and repair location. 　　 (3) Install a section of insulated pipe from the water storage system where the water pipe is pressed to electrically insulate the water pipe system from the water pump-motor unit. 　　 (4) The water supply and drainage of the interregional tunnels must be electrically connected, and both ends of the water pipes should be connected to the ground electrode at stations with substations. 　　 (5) Use UPVC plastic insulated pipes for water supply and drainage pipes that pass through the track bed. 　　4． Station environmental control system (l) Electrical devices installed on metal dampers should be installed using the insulation method. 　　 (2) The motor-wind turbine unit installed on the same metal base should be insulated by a rubber lightning arrester during installation. A short section of insulated air duct should be added to the metal air supply and exhaust ducts leading from the fan to electrically insulate the air duct system from the motor-fan unit. 　　5. The main structure of subway cut-and-cover stations, rectangular tunnels and circular tunnels (1) The steel bars of subway cut-and-cover stations and the main structure of rectangular tunnels should be electrically connected. 　　 (2) The steel bars in each ring segment of the subway circular tunnel are required to form an electrical connection loop in the circumferential and longitudinal directions. 　　6． Other cable armors are only allowed to be grounded at the outer end. ； PE wires must not be grounded repeatedly ； AC and DC high and low voltage switch cabinets, transformers, power lighting distribution boxes, etc. in subway stations and substations are all installed insulated from the steel bars of the main structure, and insulated grounding wires are separately led from the grounding bar for grounding protection.