Grounding signal detected on the high-voltage cabinet during power outage

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Recently, a factory dealt with a fault in which a high-voltage cabinet lost power and generated a grounding signal. This post provides a detailed account of this fault phenomenon, in which a grounding alarm was triggered by induced electromotive force, as well as the process of fault diagnosis and resolution, in the hope of offering some insights to everyone. 【Faulty device】The grounding relay in a certain high-voltage distribution cabinet. 【Fault Phenomenon】During their shifts, the electricians on duty in the central control room repeatedly noticed intermittent grounding signals on the central signal screen. 【Diagnostic Analysis】Original URL: yunrun.com.cn/tech/1894.html ①After a thorough inspection by the electrical engineers at Changhui Instrument Manufacturing Co., Ltd., all operating circuits were found to be in normal condition, with no ground faults present. The grounding signal is generated by the grounding relay in the high-voltage switchgear of transformer No. 2, with a setting value of 30mA. ②However, at this time the 35kV high-voltage cabinet was under maintenance due to a power outage; the isolating switch and circuit breaker were both disconnected, and no faults had occurred in its primary or secondary circuits prior to the power outage. ③Testing the grounding circuit in the high-voltage cabinet revealed that there was indeed an intermittent grounding current, with a value of around 40 mA. ④Further inspection revealed two factors contributing to the ground current: first, the supply cable on the high-voltage side of transformer #2 was laid on the same support and at the same level in the cable trench as the other 4 high-voltage cables, with very short distances between them; all of these 4 high-voltage cables were in operation, which resulted in an induced electromotive force E in the cable of transformer #2 ； Secondly, after the shutdown of transformer No. 2, a set of grounding wires was installed at M1 located below the circuit breaker QF in the high-voltage cabinet, and another set was installed at M2 located at the front of the transformer. Thus, the induced electromotive force E forms a closed current circuit through these two sets of grounding wires and the ground, thereby generating an induced current i, as shown by the dashed lines in the figure below. When the value of the induced current exceeds 30 mA, the ground relay activates and sends a ground signal. http://yunrun.com.cn/upload/201804/02/201804021931505580.png 【Fault Resolution】After the maintenance of the high-voltage cabinet was completed, the grounding wires at M1 and M2 were removed, and the cabinet was put back into operation; the grounding signal no longer appeared. 【Experience Summary】When another charged conductor is present around a certain conductor, an induced electromotive force is generated in that conductor; if a closed circuit is formed, an induced current will flow.