The zero-sequence current harmonics are too high
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In the newly installed equipment, it has been recently found that when the load on the outgoing cabinet exceeds around 300A, zero-sequence current levels become excessive. By checking the waveform records in the monitoring backend, it was found that the peak value of the zero-sequence current reached 1 KA. And the protection device has no records. The zero-sequence CT was replaced, and the protection device was tested, but the issue persisted. By checking the harmonic content analysis, it was found that the harmonic content of the zero-sequence current is 200% or even higher. Multiple checks did not solve the problem. During the subsequent commissioning of the equipment, it was found in another switchgear room that single-unit equipment exhibited a high level of zero-sequence harmonics during startup. The same issue of equipment replacement still exists. Changing the shielding of the three-core power cable to ground at one end, and changing the cable of the zero-sequence CT to a shielded cable, had no effect. I hope experts on the forum can help answer my questions.
This post was last edited by chb1534 on 2018-7-9 at 16:13. It is likely caused by the fact that the third harmonic levels of the electrical appliances in question do not meet the required standards; it is recommended to turn on each appliance individually and check the zero-sequence current to determine which appliance is causing the problem
This is a problem that affects most of the outgoing circuit breakers in a substation; it occurs whenever the load exceeds 300A. The motors in another substation also exhibit this phenomenon whenever the load exceeds 200A. It was initially suspected that it was caused by the arc-suppression grounding coil; after removing that coil, the problem still persists. This phenomenon occurs not in a single device but in this substation, and it is also related to the load. So I’m puzzled.
How is the zero sequence sampled? How is the zero-sequence transformer connected? Is there a photo of the nameplate on the zero-sequence transformer?
The zero-sequence CT is of the split-type like LXK’s; there is no issue with the cable shielding grounding, as it has been checked. We have also tried CTs from other manufacturers for the zero-sequence circuit, and the same problems occurred. There are really no photos of the nameplate, so I’ll go take some pictures there tomorrow. Is the CT model not suitable?