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Oil-immersed grounding transformer
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The grounding transformer is connected to the three-phase busbar of the system and, according to its internally designed Z-connection configuration, taps the system neutral point for use by the arc-suppression coil.
In a power supply system, if the transformer windings are connected in a Y configuration with a neutral point tapped, there is no need to use a grounding transformer. However, if the transformer windings are connected in a Δ configuration without a neutral point tapped, when selecting an arc-suppression grounding device, it is essential to use a grounding transformer to bring out the neutral point.
The grounding transformer employs a Z-connected transformer. Due to the Z-connection on the high-voltage side of the transformer, each phase winding consists of two segments, each located on a different core column belonging to separate phases. The two segments of the coil are connected in opposite polarities. As a result, the zero-sequence magnetic fluxes generated by the two phase windings cancel each other out, leading to a very low zero-sequence impedance and extremely low no-load losses. When using a conventional transformer equipped with an arc-suppression coil, the capacity of the arc-suppression coil should not exceed 20% of the transformer's rated capacity.
In addition to being equipped with an arc-suppression coil, a grounding transformer can also be connected to a secondary load, thereby replacing the station service transformer. Specifically, for a ZN,yn11 connection, when the grounding transformer is loaded with a secondary load, its primary capacity should equal the sum of the arc-suppression coil’s capacity and the secondary load’s capacity. When the grounding transformer is operated without a secondary load, its capacity equals that of the arc-suppression coil alone.
Grounding transformers can be configured in oil-immersed, dry-type, or integrated designs that combine an oil-immersed arc-suppression coil with a grounding transformer, depending on practical application requirements.
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