10kV distribution networks, due to long lines, complex paths, and many users, often experience line trips or single-phase grounding caused by external damage, improper user equipment maintenance, or operational errors. How to quickly determine the nature of faults, locate and isolate fault points, rapidly reduce the scope of power outages, and shorten outage duration is a long-term research topic for power supply enterprises. Jiangsu Suqian Power Supply Company has conducted in-depth research and made beneficial explorations in solving line faults.
Type Determination
When a line fault occurs, the first step is to determine the fault type. Generally, there are several situations: if one phase voltage decreases but not to zero, and the other two phases increase and exceed the line voltage, it can be judged as resonance overvoltage; if one phase voltage decreases to zero or not to zero, and the other two phases increase but do not exceed the line voltage, it can be judged as a single-phase grounding fault; if the three-phase voltages rise in sequence alternately, and the three-phase voltages show low-frequency oscillation within different ranges, it can be judged as resonance overvoltage; if one phase voltage drops to zero and the other two phases remain unchanged, it can be judged as a blown fuse in one phase of the voltage transformer.
Among these, the occurrence of single-phase grounding should be based on the action of signal devices (significant imbalance in three-phase voltages) and the action of the arc suppression coil. At this time, duty personnel at power plants and substations should immediately report to the duty dispatcher the four aspects: phase-to-ground voltages, neutral point displacement voltage, arc suppression coil current, and equipment abnormalities.
Workflow
When it is determined that there is single-phase grounding in the system, but after inspection of the relevant substations no grounding fault point is found, the faulty equipment should be identified in the following order:
First, if there are two sets of transformers and their 10kV busbars are operated in parallel through a bus tie switch, the bus tie switch can be opened first to check which busbar system has grounding.
Second, trial trip the line switches in the prescribed order. The duty dispatcher can, based on feedback from relevant users, trip the line switches in sequence; for lines with self-generation, the generator should be disconnected before trial tripping; for users with dual power supplies, no power transfer is performed, and the line switch is directly tripped (note that the other line must not be overloaded).
The principle for determining the tripping sequence is: first, no-load lines and capacitors; second, lines without important users, especially those with long and numerous branches; third, lines with fewer important users; finally, lines with more important users. In addition, lines under first-level power protection must not be trial tripped.
Third, after determining the grounded line, immediately notify the relevant units to inspect the line.
Fourth, if only the first-level power protection line remains untripped and the grounding line has not been determined, immediately notify the relevant units to organize personnel to inspect the first-level power protection line. While ensuring power supply, the line's partial sections and branch switches can be trial tripped to determine the grounding point. If the grounding phenomenon disappears after tripping the end section or branch switch of the line, that switch will not be re-energized until the grounding point is eliminated or isolated. If no fault point is found on the line, all other outgoing line switches on the busbar where the line is located should be opened to determine whether the grounding point is on that line. If it is on that line, continue searching.
Once a line is found to have grounding, if it does not threaten personal or equipment safety, it can continue to operate with grounding until the specified time. After the grounding fault point is found, the relevant units should be notified to organize efforts to eliminate the grounding fault point as soon as possible, and the faulty line should be removed within the specified time.
