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This post was last edited by liwei618 on 2020-2-22 at 22:06. Handling of single-phase grounding faults in microcomputer protection devices: Fault earthing, also known as grounding fault, refers to an accidental connection between a conductor and the ground. When the impedance of the connection is small enough to be ignored, such a connection is called \"fully grounded\". Fault grounding can be divided into three situations: using the overcurrent protection installed in the distribution lines as ground fault protection as well ； Utilizing zero-sequence current for ground fault protection ； Ground fault protection is achieved using residual current. Characteristics of a ground fault: (1) When one phase (such as phase A) is partially grounded, that is, through a high resistance or an arc, the voltage of the faulty phase decreases while the voltage of the non-faulty phases increases. These voltages are higher than the phase voltage but do not reach the line voltage. When the voltage at the open delta terminal of the voltage transformer reaches the set value, the voltage relay activates and sends a ground fault signal. (2) If a complete ground fault occurs in phase A, the voltage of the faulty phase drops to zero, while the voltage of the non-faulty phases rises to the line voltage. At this point, a voltage of 100 V appears at the open delta terminal of the voltage transformer, causing the voltage relay protection to activate and send out a ground fault signal. (3) A open circuit occurs in one phase (Phase A) on the high-voltage side of the voltage transformer, or its fuse blows. In this case, the indication for the faulty phase is not zero. This is because, in the secondary circuit, the voltmeter for this phase forms a series circuit with the transformer coils and the voltmeters for the other two phases; as a result, a relatively small voltage reading is displayed. However, this value does not represent the actual voltage of that phase. For the non-faulty phases, the voltage remains at the phase voltage level. A voltage of around 35V appears at the open delta terminal of the transformer, which activates the relay and sends a ground fault signal. (4) Due to the presence of components with capacitive and inductive parameters in the system, especially ferromagnetic inductance components with cores, ferromagnetic resonance can occur when the parameter combinations are mismatched, causing the relay to activate and send a ground signal. (5) False grounding phenomenon of the unloaded busbar. When the bus is operating without a load, three-phase voltage imbalance may also occur, along with a ground fault signal being generated. But once a circuit is connected, the grounding phenomenon disappears on its own. Handling of single-phase ground fault (1) Steps for dealing with a ground fault: ① After a single-phase ground fault occurs, the on-duty personnel should immediately activate the alarm system, make records, and promptly report to the on-duty dispatcher and relevant responsible persons. They should then locate the source of the ground fault according to the instructions given by the dispatcher, though the specific method of investigation is left to the discretion of the on-site personnel. ②Thoroughly inspect the electrical equipment in the facility for any obvious signs of malfunction; if the fault point cannot be identified, then proceed to check for ground faults in the wiring. ③Operate the busbars in segments, and run the transformers operating in parallel separately, in order to identify the single-phase grounded area. ④Then disconnect the bus reactive power compensation capacitor circuit breaker as well as the unloaded line. For multi-power supply line protection, load transfer and changes in the power supply method should be employed to locate the ground fault. ⑤A pull-and-close test is carried out to locate the fault point; when the grounding issue disappears after pulling the circuit breaker of a certain line, it can be determined that this line is the faulty one. The incident is immediately reported to the on-duty dispatcher for further handling, and additional inspections are conducted on the circuit breaker, disconnect switch, bushing, and other equipment associated with that faulty line. (2) Requirements for handling ground faults: ① When searching for and dealing with single-phase ground faults, safety measures must be taken to ensure personal safety. When the equipment is grounded, one must not approach the fault location within 4 meters indoors, or within 8 meters outdoors. Personnel entering these areas must wear insulating boots and gloves, as well as use specialized tools. ②To reduce the scope and negative impact of power outages, when searching for single-phase ground faults, lines that are long, have many branches, have experienced numerous faults in the past, have light loads, and serve less critical purposes should be checked first; after that, lines that are short, have heavy loads, few branches, and serve important functions should be examined. Users with dual power supplies can switch to the alternate power source before attempting to pull it, while dedicated lines require prior notification. If someone reports signs of a fault on a certain line, it is possible to try pulling that line first. ③If the high-voltage fuse of the voltage transformer blows, it must not be replaced with a regular fuse. Fuses made of porcelain tubes filled with quartz sand, with a rated current of 0.5A, must be used; such fuses possess excellent arc-quenching capabilities and a high current-breaking capacity, and they serve to limit short-circuit currents.