Distribution Transformers and Substation Equipment
Domain 5 - Distribution Transformers and Substation Equipment. The exam tests transformer ratings, connections, protection and the equipment found in a 33/11 kV or pole-mounted substation.
Transformer facts
- Standard DTs are 11/0.433 kV, rated 25, 63, 100, 160, 250, 400 or 630 kVA, ONAN cooled with mineral oil.
- Vector group Dyn11: delta HV, star LV with neutral brought out and 11 o'clock displacement; the neutral is earthed, giving 240 V phase-to-neutral.
- Accessories: conservator, silica-gel breather, oil level gauge, thermometer, explosion vent, Buchholz relay on larger units, off-circuit tap changer usually ±5% in 2.5% steps, and the rating plate.
- Tests: ratio, vector group, winding resistance, insulation resistance with a 500 V or 1 kV megger, oil BDV (at least 30 kV for 11 kV class), and temperature rise.
- Protection: HV DO fuse, AB switch for isolation, lightning arrester on the HV side, LV breaker, and two separate earths for body and neutral.
Worked example
For a 250 kVA, 11/0.433 kV transformer: HV current = 250 000/(√3 × 11 000) = 13.1 A; LV current = 250 000/(√3 × 433) = 333 A. The HV DO fuse must sit above 13.1 A and above the magnetising inrush, so utilities choose from a standard table, often 1.5–2 times full-load current, rather than at exactly 13.1 A.
Traps
- Sizing the DO fuse at full-load current, so it blows on every energisation.
- Omitting the neutral earth, leaving the LV system floating.
- Confusing the conservator with the breather, or describing the Buchholz relay as an overcurrent device.
- Recording a tap position without noting it on the commissioning sheet.