Single-phase-to-ground fault definition:
In a three-phase power system, insulation damage occurs only between one-phase conductors and ground.
In a broad sense, single-phase-to-ground fault includes single-phase-to-ground short-circuit. Single-phase-to-ground short-circuit is a special phenomenon in single-phase ground fault, which is only present in the transformer neutral point grounding system.
In a narrow sense, single-phase-to-ground fault and single-phase-to-ground short-circuit are two concepts. One is a malfunction, the other is an accident.
Power system failures and accidents are narrowly distinguished.
Single-phase grounding short circuit means that the live wire (any phase of electricity) is directly connected to the ground wire without passing through the load. In the 380/220 power supply system or other grounding systems, since the neutral wire of the transformer is grounded, and the grounding resistance is very small , the live wire (any phase of electricity) is directly connected to the ground wire without going through the load, and a large current will be generated in an instant, burning the wire, power distribution equipment or tripping, etc., so a single-phase grounding short circuit is a serious accident, which is a power The system tries to avoid it as much as possible.
In the power system, there are many places where the ungrounded system is used for power supply. For example, in special places such as severe dust, flammable and explosive, all 6-10 kV and 35 kV power supply systems in our country are neutral. For a power supply system that is not grounded or grounded through a large resistance, after the single-phase grounding of such a system, since there is no loop or no loop that generates a large current, there is no ground current or no large ground current, so it does not affect the normal operation of the system.
Although it does not affect the normal operation of the system, the single-phase grounding is a kind of fault. In order to prevent the grounding from expanding to two-phase grounding or further damage to the grounding equipment, the regulations stipulate that after a single-phase grounding fault occurs, it must be checked within 2 hours. If the cause cannot be found within 2 hours or cannot be dealt with, the power supply can only be supplied after the power outage is dealt with.
Encyclopedia of single-phase grounding fault finding methods
For small current grounding systems, how to quickly find single-phase grounding faults, I will introduce some simple and feasible methods to you.
1. Manual search method
If there is no grounding line selection device installed in the substation, no grounding fault indicator or short-circuit grounding two-in-one fault indicator is installed on the line, and there is no good grounding fault detector, then the stupid method of manual search has to be used. Find the steps as follows:
(1) By manually (or dispatching, the same below) opening the gates in turn, you can know which outlet line of the substation is grounded, and which phase is grounded through dispatching.
(2) Next, there are two ways to find the fault point: one is to segment the line step by step, or pull apart the line that is often faulty, and use a 2.5kV shaker to measure the grounding relative to ground insulation, and the section with low insulation resistance It is the fault segment, so as to narrow the search range (of course, safety protection measures such as hanging ground wires must be taken on the outgoing side of the substation); the second is to segment the line as much as possible, and then test the power transmission step by step, and cooperate with the dispatching interaction , when there is a zero sequence voltage alarm, this section is the fault section.
The manual search method is very troublesome to operate. If the line is long, there are many branches, and there are few switch sections, it will be difficult to operate. In addition, the weather and weather are not good, it will be even more difficult to handle. It is recommended to use some scientific and technological means with less equipment investment to cooperate with manual search, which can achieve twice the result with half the effort, which not only provides power supply reliability and social benefits, but also creates economic benefits.
2. Use grounding line selection device and fault indicator to find
The substation is generally equipped with a grounding line selection device. Although it is sometimes inaccurate, it can provide a technical reference for manual switching. Then install some ground fault indicators (or short-to-ground two-in-one fault indicators) on the line to indicate the ground fault path. The more reliable ground fault detection method is to use the signal source method, and the more sensitive ground fault detection method is to use the first half-wave method or the DC transient analysis method. It is recommended to use a combination of two ground fault indicators to find ground faults. The signal source method is the main method, supplemented by the first half-wave method or the DC transient analysis method.
3. Use feeder automation methods to find
If the user has money, feeder automation methods are recommended to find ground faults. This method utilizes the logic function of intelligent switches (electric load switch, sectionalizer, circuit breaker, recloser + FTU) to replace the traditional manual search method, and can automatically realize fault isolation, recovery and transfer of power supply. Suppose a dual power supply hand-in-hand line is divided into 6 sections, that is, there are 5 intelligent switches (with three-phase five-pole signal PT or capacitive PT on both sides), the middle one is connected, and the ground fault point is connected to the first switch. Take the second smart switch as an example, the specific implementation steps are as follows:
(1) The substation will turn off the grounding outlet, and the intelligent switches of each subsection on the line will automatically open.
(2) The substation is closed for power transmission, and the zero-sequence voltage of the substation does not alarm, and the power transmission of this section is restored successfully; the first intelligent switch FTU detects the line voltage, but does not detect the zero-sequence voltage, and automatically after a delay for a period of time When the switch is closed, the zero-sequence voltage is detected because it is closed to the ground fault, and the switch is immediately opened and "blocked"; the second intelligent switch also detects the zero-sequence voltage, the switch does not move and "blocks", cancels the "delay of electricity" "Auto close" function.
(3) The intermediate tie switch detects the loss of power on one side, and automatically closes after a delay for a period of time. Because the fault point is not in this section and no zero-sequence voltage is detected, the section is successfully transferred to power supply.
(4) After the tie switch is powered on, the second intelligent switch detects the line voltage, but does not detect the zero-sequence voltage, but there is a "blocking" in front, so the "power-on delay automatic closing" function is canceled, and the switch is kept at quantile.
(5) So far, the ground fault point section has been separated, and other non-fault sections have been restored or transferred power supply.
4. Change the neutral grounding method to find
The power distribution system adopts the neutral point ungrounded or grounded through the arc suppression coil, which has advantages and disadvantages. In view of the "disadvantage" of difficulty in finding faults and some personal and property safety problems caused by them, users themselves are also doing further thinking. There are two main solutions:
(1) Change the neutral point to ground via a small resistance. After the transformation, the zero-sequence two-stage protection function and short-circuit fault indicator of the outlet circuit breaker can basically solve about 70% of the ground fault finding problems, but there are still about 30% of the medium-resistance and high-resistance grounding faults. Find, there may also be a problem with the protection of the line fuse. For this kind of system, a better solution is to use digital fault indicators to transmit the measurement data of indicators such as line zero-sequence current (cable), line total current (overhead), and ground insulation voltage (overhead) through wireless communication. The network is sent to the dispatching system, and the location of the grounding point can be judged by comprehensively analyzing the real-time and historical information of the substation.
(2) The neutral point is changed to the mode of small resistance + circuit breaker or medium resistance + high voltage contactor. The circuit breaker or high-voltage contactor is usually in the sub-position. Only when it is detected that the zero-sequence voltage of the system is raised, the closing is delayed, and it becomes a small resistance or a medium resistance grounding in a short time, and then the detection method in the small resistance grounding method is used. to find the fault. In addition, since the on-off of the neutral point resistor can be flexibly controlled, the on-off of the resistor can be controlled with a certain closing and opening sequence after the arc suppression coil is activated, so as to make the protection device operate or let the ground fault indicator identify the signal and indicate the ground current path.
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