Post Time: 2024-09-07 16:33:59
Transformer grounding requirements are exactly the same as system requirements, so its grounding method, grounding resistance and other calculations and selection are also the same as system requirements.
“Y-Y” and ‘Y-Δ’ wiring: Normally, the neutral point of the ‘Y’ side is grounded.
“Δ-Δ” wiring: according to the requirements of the system, grounding can be carried out on one side by means of a grounding transformer or an appropriate voltage transformer; under safe operating conditions, grounding can also be carried out at one point of the ‘Δ’ wiring.
Normally supplied by 3, 6 or 10 kV systems with ungrounded neutrals through step-down transformers. If the low-voltage side of the transformer is a neutral ungrounded system, there have been accidents in which high voltage has escaped to the low-voltage circuit resulting in insulation breakdown of low-voltage equipment. In order to prevent this situation, should be used in the neutral point not grounded low voltage system neutral point or phase line through the breakdown fuse grounding. Specific measures include:
Transformer with “Y” connection: Connect the breakdown fuse to the transformer neutral.
Transformer with “Δ” wiring: connect the blow-through fuse to one of the phase lines.
Irrespective of whether the neutral point is directly earthed or earthed by means of a blow-through fuse, the earthing resistance must not exceed 4 Ω. The earthing requirements for high voltage must also be met.
That is, three-winding transformer, when the low-voltage winding open-circuit operation, due to electrostatic induction may damage the insulation of the low-voltage winding, the low-voltage winding of a phase out of the line temporarily grounded or grounded by the lightning arrester.
♦ Lightning arrester installation: lightning arrester should be installed in the drop open fuse and transformer, as close as possible to the transformer installation, the closer the better.
♦ Grounding: lightning arrester grounding line should be connected with the transformer low-voltage winding neutral and transformer metal shell together, using the “trinity” grounding method. This method ensures that when the arrester discharge, the transformer main insulation voltage is only the residual voltage of the arrester, and the voltage on the grounding device will not act on the main insulation of the transformer, thus reducing the risk of insulation breakdown.
♦Low-voltage protection: For 3~10kV distribution transformers, low-voltage surge arresters are installed on the low-voltage outgoing lines to protect the transformer’s low-voltage windings and to prevent damage to the insulation of the high-voltage windings when lightning waves are switched from the low-voltage windings to the high-voltage windings.
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