Distribution Transformers and Substation Equipment
This domain covers the 11/0.433 kV distribution transformer and the equipment around it: HV and LV switching, protection, instrument transformers, bushings, lightning arresters, earthing and auxiliaries. The exam tests nameplate interpretation, current calculations and the purpose of each item in a distribution substation.
Core rules
- A distribution transformer is rated in kVA, not kW, because its heating depends on voltage and current, not on load power factor alone.
- Common vector group for distribution transformers is Dyn11; the neutral is brought out and earthed. The LV neutral earthing is essential for consumer safety.
- HV protection may be a fuse, circuit breaker or relay-controlled switchgear depending on transformer rating. LV protection is by MCCB or ACB with overload and short-circuit releases.
- Oil-filled transformers use a conservator, breather with silica gel, Buchholz relay, oil temperature indicator and winding temperature indicator. Dry-type transformers do not use oil or Buchholz protection.
- Instrument transformers: CTs step current down for metering and protection; PTs/VTs step voltage down. Lightning arresters divert surge energy to earth.
Worked example
A 630 kVA, 11 kV/433 V transformer supplies a load of 500 kW at 0.85 power factor. Find the load kVA and the LV current.
Load kVA = kW / pf = 500 / 0.85 = 588 kVA. This is below the 630 kVA rating. LV current = 588000 / (√3 × 433) = 588000 / 750 ≈ 784 A. The transformer is not overloaded in kVA, but the LV current must be checked against the LV breaker, busbar and cable ratings. The HV current would be 588000 / (√3 × 11000) ≈ 30.9 A.
Traps that cost marks
- Comparing kW directly with kVA without dividing by power factor.
- Using 415 V instead of 433 V for transformer secondary current.
- Expecting a Buchholz relay on a dry-type transformer.
- Ignoring ambient temperature and cooling conditions when assessing loading.
- Confusing CTs and PTs; a CT secondary must never be open-circuited while energised.