Complete List of Methods for Finding Ground Faults in Single-phase Motors (2)

3. Change the neutral grounding method to find

The distribution system adopts neutral point ungrounded or grounded through arc suppression coils, which has both advantages and disadvantages. In response to the "drawbacks" of difficult fault finding and the resulting personal and property safety issues, users themselves are also further considering and have come up with two main solutions:

(1) Change the neutral point to a mode of small resistance+circuit breaker or medium resistance+high-voltage contactor. The circuit breaker or high-voltage contactor is usually in the open position, and only when the zero sequence voltage of the system is detected to rise, it will delay closing and become grounded with a small or medium resistance for a short time. Then, the fault can be detected through the detection method of grounding with a small resistance. In addition, due to the flexible control of the on/off of the neutral point resistor, it is possible to control the on/off of the resistor at a certain time sequence after the action of the arc suppression coil, in order to activate the protection device or allow the ground fault indicator to recognize the signal and indicate the path of the ground current.

(2) Change the neutral point to ground through a small resistor. After the renovation, the use of the zero sequence two-stage protection function and short circuit fault indicator of the outlet circuit breaker can basically solve about 70% of the grounding fault detection problem, but there are still about 30% of medium resistance and high resistance grounding faults that are difficult to find, and there may be problems with the protection coordination with the line fuse. A good solution for this system is to use digital fault indicators to send measurement data of line zero sequence current (cable), line total current (overhead), and ground insulation voltage (overhead) indicators to the dispatch system through wireless communication network. After comprehensive analysis of real-time and historical information of the substation, the location of the grounding point can be determined.

4. Manual search

If there is no grounding line selection device installed in the substation, and there is no grounding fault indicator or short-circuit grounding two in one fault indicator installed on the line, and there is no good grounding fault detector, then the foolish method of manual search will have to be adopted. The search steps are as follows:

(1) By manually (or dispatching, the same below) sequentially pulling the gate, one can know which outgoing line of the substation is grounded and which phase is grounded through dispatching.

(2) Next, there are two methods to find the fault point: one is to segment the line step by step, or to open the frequently faulty line and use a 2.5kV megger to measure the ground to ground insulation. The section with low insulation resistance is the fault section, in order to narrow the search range (of course, safety protection measures such as hanging grounding wires must be taken on the outgoing side of the substation); The second is to segment the line as much as possible, and then conduct trial power transmission step by step, interacting and cooperating with the dispatch. When there is a zero sequence voltage alarm, this section is considered a faulty section.

The manual search method is very troublesome to operate. If the line is long, there are many branches, and there are few switch segments, it is difficult to operate. In addition, the weather and weather are not good, and it is even more difficult to handle. It is recommended to use some technological means with low equipment investment to cooperate with manual search, which can achieve twice the result with half the effort, providing power supply reliability and social benefits, as well as creating economic benefits.

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