Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 76

Having established the grid transformation to be γ/Δ for the primary distribution, the 13.8 kV delta system becomes ungrounded. A stable and detectable grounding system is required for this project, to insure the neutral point will not be floating in case of a ground fault. This can be accomplished by grounding/station service transformers. These will be 300 kVA, 13.8 kV to 480 V, Zg/γ with a 100 A grounding resistor on the primary. The secondary neutral is grounded directly to ground without a resistor to render a four‐wire system, suitable for the LV 277/480 V services.

The grounding/station service transformers are connected to the 13.8 kV buses with fused links.

The MV 13.8 to 4.16 kV transformers are Δ/γ. The LV neutral is resistance grounded to limit the fault current to about 100 A. The selected fault current is low enough to limit the damage caused by ground faults to the MV equipment and high enough to be used for selective relay protection and most importantly to limit the insulation overstressing due to spiking arcing ground faults caused by the system capacitances.