1. Analysis of faulty grounding of DC system The DC system has a wide distribution range, many exposed parts, many cables and long, and the environment is relatively harsh. It is easily corroded by dust and moisture, which makes the insulation performance of some components of the DC system lower, even insulated. Destruction causes DC grounding. The reasons for analyzing DC grounding are as follows:
1.1 The secondary circuit insulation material is unqualified, the insulation performance is low, or it is in disrepair and severely aged. Or there are some injuries, such as abrasions, bruises, crushes, sprains or burns caused by overcurrent.
1.2 The secondary circuit and equipment are seriously polluted and damp, and the junction box is filled with water and steam, which makes the DC-to-ground insulation seriously degrade.
1.3 Small animal entry or small metal parts falling on the component cause DC ground fault, such as small animals such as mice and cockroaches entering the live loop; some components are wired, unused screws, washers, etc., falling on the live loop .
2. Hazard of DC system grounding fault In DC grounding fault, the two-point grounding is harmful, which may cause serious consequences. If a two-point ground fault occurs in the DC system, it may constitute a short circuit to the ground, causing relay protection, signal, automatic device malfunction or refusal, or DC fuse blown, so that the protection and automatic devices and control circuits lose power. In the complex protection circuit, the two poles are grounded at the same pole. It is also possible to short-circuit some relays and not to trip, causing a step-by-step trip.
2.1 Two-point grounding may cause the circuit breaker to trip erroneously. When the DC grounding occurs at two points, the current relay contact is short-circuited, and the trip coil TQ is energized, and the TQ contact is closed and tripped.
2.2 Two-point grounding may cause the circuit breaker to refuse to move the circuit breaker and may cause a refusal.
2.3 Two-point grounding causes the fuse to blow to the grounding point, which may cause the fuse to blow. When the protection action is performed, not only the circuit breaker refuses to jump, but also the fuse is blown, and there is a possibility that the relay contact is burned out. The DC system ground fault is not only bad for the equipment, but also poses a threat to the safety of the entire power system. Therefore, the procedure stipulates that the DC grounding should reach zero voltage or zero voltage (below 70V), and the other pole to ground voltage rises (higher than 70V) or rises to the busbar full power. Everything on the network works and chooses to find the grounding point to prevent grounding at two points.
3. DC Grounding Fault Handling When the DC system is found to be grounded, the grounding polarity should be determined by an insulation monitoring device. When the voltage of the positive pole to ground is greater than 70V, the negative pole is grounded; when the voltage of the negative pole to ground is greater than 70V, the positive pole is grounded; and the situation is reported to the department and the leader of the Ministry of Health, and the relevant personnel of the thermal, relay and other personnel are contacted in time to carry out the grounding point. Find.
3.1 Method for finding DC grounding point
3.1.1 When there is work on the secondary circuit, or when there is equipment maintenance test, it should be stopped immediately. Pull on its power and check if the grounding is removed.
3.1.2 Ask if there is any motor start and stop operation, if any, first check if the circuit is grounded;
3.1.3 In the case of rainy weather, power outages may be made to outdoor systems or branch roads with harsh environmental conditions such as steam and water leakage;
3.1.4 If there is a temporary power supply, this power should be stopped first.
3.1.5 Divide the DC system first, separate the loop from the parallel loop, and then cut each feeder one by one.
3.1.6 For the less important DC load and the branch that cannot be transferred, use the method of “instantaneous power failure†to check whether the loop in the branch has a ground fault.
3.1.7 For important DC loads, use the transfer load method to check the shunt with or without a ground fault.
3.1.8 Find the DC system ground fault, which is carried out by two or more people. One of them operates (the cut-off time is 1-2 seconds), and one person monitors and monitors the meter indication and signal changes.
3.1.9 When selecting DC grounding by means of instantaneous power failure, the following sequence should be followed:
3.1.9.1 Pull unimportant loads first, such as coal, chemical combustion, dust removal, etc., after pulling important loads;
3.1.9.2 First pull and close the power supply, pull back operation, signal power supply;
3.1.9.3 Take the DC insurance of the insulation monitoring circuit, flash circuit and voltmeter circuit;
3.1.10 When the grounding of a loop power supply is removed, the grounding branch shall be determined, and the branching section shall be re-tested, narrowed and the grounding range determined.
3.1.11 When the above-mentioned pull-in branch method is invalid, the method of switching the rectifying device and the battery pack should be adopted, and the method of switching the DC bus should be searched to check whether the rectifier device, the battery pack and the DC bus are grounded;
4. Precautions when searching for ground faults:
4.1 When the DC power supply is stopped instantaneously, it should be approved by the chief engineer. The time should not exceed 3 seconds. The action should be quick to prevent the loss of the protection power supply or the reclosing power supply for too long.
4.2 In order to prevent misjudgment and observe whether the grounding phenomenon disappears, it should be judged comprehensively from the indications of signals, light plates and insulation monitoring meters.
4.3 Prevent artificial short circuit or grounding at another point, resulting in false tripping.
4.4 According to the actual drawings, prevent the terminal wire ends from being removed, and prevent the left or wrong connection when the wiring is restored; the wire ends should be recorded and marked.
4.5 When using the instrument inspection, the internal resistance of the meter should be no less than 2000 ohms/volt.
4.6 If the fault is found, it must be carried out by two or more people to prevent personal electric shock and to perform safety monitoring.
4.7 Prevent protection from malfunction. If necessary, remove the protection from possible misoperation before the power supply is turned off.
4.8 Pull the heat control power cabinet power supply to obtain the consent of the thermal protection class.
4.9 Pull all protective power and operation (control) power supply, excitation regulator power supply to obtain the consent of the electrical protection class;
4.10 pull #7, 8 disk control power switch to obtain the consent of the thermal and electrical protection class;
5 Conclusion DC grounding will not cause great harm to equipment and unit operation in a short time, but once a two-point grounding short circuit occurs, it will threaten the safety of the equipment and the unit, cause the equipment to burn out, the unit will be disassembled, and the power grid will be threatened. Security. Therefore, DC grounding should be organized in time to find out and eliminate it as soon as possible.
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