The cable fault testerunder Wuhan UHV can help many power workers conduct various power tests more conveniently.
To determine the occurrence of a single-phase grounding fault, the following methods and steps can be used, which integrate relevant information and descriptions from the reference article:
1、 Observe voltage changes
Voltage drop of faulty phase: When a single-phase grounding fault occurs, the voltage of the faulty phase will drop to near zero. For example, if phase A is completely grounded, the voltage of phase A will drop to zero.
Non fault phase voltage rise: At the same time, the voltage of the non fault phase will rise to the line voltage. This is because in a low current grounding system, when one phase is grounded, the neutral point potential will shift, causing the non fault phase voltage to rise.
2、 Utilize the voltage at the triangular opening of the voltage transformer
Voltage at the open triangle of the voltage transformer: When a single-phase grounding fault occurs, a specific voltage value will appear at the open triangle of the voltage transformer. For example, for a fully grounded fault, there may be three times the original phase voltage (such as 100V) at that location, triggering the action of the voltage relay and issuing a grounding signal.
3、 Check the fuse and wire breakage situation
Fuse melting: If a phase (such as phase A) of the voltage transformer's high-voltage side experiences a fuse melting, the voltage of the faulty phase will decrease but the indication will not be zero, while the voltage of the non faulty phase will still be the phase voltage. A voltage value of around 35V will appear at the opening triangle of the transformer, and the relay will be activated to send out a grounding signal.
Disconnected situation: When the secondary fuse of one phase of the bus voltage transformer is blown, the voltage of one phase is zero, and the voltage of the other two phases is normal. In this case, the central signal alarm bell will ring and display the light sign "Voltage Transformer Disconnected".
4、 Consider other possible factors
Series resonance: When there are components with capacitive and inductive parameters in the system, it may cause ferromagnetic resonance, causing the relay to operate and emit a grounding signal. This is usually due to mismatched parameter combinations.
False grounding of no-load busbar: During no-load operation of the busbar, three-phase voltage imbalance may occur and a grounding signal may be issued. But after sending a line, the grounding phenomenon usually disappears on its own.
Insulation monitoring instrument problem: When the neutral point of the insulation monitoring instrument is disconnected, even if the power grid is single-phase grounded, the three-phase voltage may still be normal, but the grounding signal has already been sent out.
5、 Processing strategy
Record and report: Once a possible single-phase grounding fault is discovered, the on duty personnel should immediately record and report it to the on duty dispatch and relevant responsible personnel.
Detailed inspection: Firstly, check for any obvious signs of faults in the electrical equipment inside the station, and then gradually search for the grounding of the circuit.
Split the power grid: Divide the power grid into several parts that are not directly connected electrically to determine the single-phase grounding area.
Trial pull and search: Use a one pull and one close method to conduct a trial pull and search for the fault point. When the grounding phenomenon of a circuit breaker on a certain line disappears, it can be judged as a faulty line.
By following the above steps and methods, it is possible to accurately determine the occurrence of single-phase grounding faults and take corresponding measures.
