Discussion on DC system grounding faults in substations and power plants The DC systems of substations and power plants provide power for control, protection, signal and automatic devices. The safe and continuous operation of DC systems is of great importance to ensure power supply. Since the DC system is an ungrounded system that is floating, if two points are grounded, it may cause the above device to malfunction or refuse, causing a major accident. Therefore, when a grounding occurs, the grounding point should be accurately and quickly detected while ensuring the normal power supply of the DC system, and the grounding fault should be eliminated, thereby avoiding the harm caused by the two-point grounding.
Under normal circumstances, both positive and negative ground are insulated. When a grounding occurs in the system, the DC system is normally not affected. However, when two or more points are grounded, the positive and negative terminals will be short-circuited. The switch and protection circuit are malfunctioning and refusing. At the same time, the failure of the DC system may cause more troubles. Therefore, it is very important to use good instruments and accurate judgment methods when grounding occurs.
1. The type of grounding is summarized from the composition of the actual DC system grounding: positive grounding and negative grounding; direct grounding; metallic grounding (also called complete grounding) and indirect grounding, non-metallic grounding (also called non- Fully grounded,). In addition, there are single point grounding, multi-point grounding, loop grounding, insulation reduction and AC half-grounding. Since the direct grounding conditions vary from place to place, many new names have been created, which are generally these names.
2. DC system grounding generation When the insulation level between the positive or negative pole of the DC system and the earth drops to a certain setting or lower than a certain value, it is collectively referred to as DC system grounding; when the positive insulation level is lower than a certain The specified value is called positive grounding; when the negative insulation level is lower than a certain value, it is called negative grounding.
3. Why does the DC system grounding DC system be a long-term live system with uninterrupted work, many branches, and wide load, which will cause cable aging, terminal aging, and component damage due to environmental changes, climate change, pollution, high temperature, etc. Problems such as the device itself cause a drop in insulation levels. In general, the longer the commissioning time, the higher the probability of grounding, especially if the power plant is grounded more frequently than the substation.
4. What are the hazards caused by the DC system grounding? 1) Harm of the positive pole grounding Because the circuit breaker trip coil is connected to the negative pole power supply, when the positive pole of the system is grounded, the positive pole passes through the earth to form a loop. Grounding can cause tripping of the circuit breaker.
2) Harm of the negative grounding The grounding of the negative pole may cause the circuit breaker to refuse to act, and the grounding may cause the circuit breaker to refuse to move.
Earlier we said that the DC system grounding is generated and dangerous. From the DC grounding hazard, we can see that whether it is the grounding of the positive pole or the grounding of the negative pole, as long as there is a grounding, it is required to form a new grounding loop to the ground. Quickly eliminate, otherwise there will be a failure or even an accident if two or more points are grounded. From the current situation and experience in the field, Top Electric believes that there are roughly the following methods.
1. Pulling method Once the DC grounding circuit is disconnected from the DC system, the positive and negative poles of the DC bus will be balanced against the ground voltage. Therefore, people usually lose power from the DC grounding circuit and determine whether the DC grounding point occurs in the circuit. This is called the “pull method”. The DC system is an uninterruptible power supply. Based on its particularity, people cannot arbitrarily power off. In recent years, with the extensive use of computers, the microcomputer protection also does not allow people to disconnect the DC power at will. In the field troubleshooting, abnormal closed loops often occur, using dual power supply circuits, and the substation has no signal difference between the DC load signal loop, control loop and protection loop during the construction, expansion, and repair process. It makes the difficulty of finding DC ground faults more difficult. The "pull method" often causes accidents such as a control loop or a protection loop trip.
2, automatic grounding inspection instrument to find the loop There are many DC grounding line selection devices on the market. Generally designed and produced by the three principles of “signal injection method”, “Hall sensor monitoring method” and “magnetic saturation monitoring method”, the general situation is to install a through-current current transformer on each branch circuit of DC, each The signal sensed by the transformer is analyzed and judged by the DC grounding line selection device, and the branch circuit of the DC ground is determined. The sensor number installed on the branch circuit and the grounding detector display part of the corresponding number of the circuit. The advantage is that it can monitor the grounding status of each branch circuit in real time, and when checking the grounding loop, such as “automatic path-by-channel test method”, if the instrument measurement is highly accurate, it is an impossible automation. equipment. However, due to its low measurement accuracy, high false alarm rate, poor anti-interference ability, and inconsistencies in various scenes, some problems have arisen in use.
3, portable instrument search and positioning method Using a portable DC ground fault finder, find DC grounding is a good method, as an auxiliary tester of the pull loop method, the elimination of ground fault is a good helper in terms of time and safety. Its characteristics are artificial pull method, there is no need to disconnect the DC loop power supply, and the mobile acquisition transformer is measured on each distribution loop. If there is a ground loop, it will alarm.
4, manual troubleshooting method Although the DC grounding of the substation is complex, whether it is conventional protection or microcomputer protection, the troubleshooting method is consistent. The method of using the pull path to find the segmentation process is to first signal and illuminate the part, and then to operate the part; first, the principle of the outdoor part and the back room. The troubleshooting experience based on the site is as follows:
1) First determine whether the positive pole is grounded or the negative pole is grounded. Measure the positive and negative poles to ground voltage, and effectively distinguish whether the positive pole is grounded or the negative pole is grounded.
2) The distinction between the two busbars will not widen the searched ground, and determine which segment of DC grounding occurs.
3) If there is a DC grounding line selection device that cannot be accurately determined and there is a false alarm, please exit the running DC grounding detector.
4) If the secondary circuit in the station is under construction or has an inspection test, stop it immediately, pull off its working power supply and see if the signal is eliminated.
5) Using the segmented and part-part pull method, the operating power must be powered by the battery, and first stop important circuits, such as signal loops and lighting circuits.
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn

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