Distribution / Power Transformer
Transformer
The large oil- or dry-type transformers that step grid voltage down (e.g. 11 kV to 415 V) for homes and industry — the workhorse of every substation and the classic 'Transformers' chapter of an electrical machines course.
Schematic symbol
Transformer
How distribution / power transformer appears in a circuit diagram.
Types & variants
Key specs
kVA rating
kVA / MVAApparent power the transformer can deliver continuously. Distribution units are typically 25 kVA to 2500 kVA.
Voltage ratio
kV / VPrimary/secondary voltage, e.g. 11 kV / 433 V. Slightly above 415 V to allow for load regulation drop.
Vector group
—Winding connection and phase shift, e.g. Dyn11 = delta primary, star secondary, secondary lags by 330°. Must match for parallel operation.
Percentage impedance
%ZVoltage drop at full load as a percentage. Sets fault current level and load sharing between paralleled units (typically 4–6%).
Losses
WIron (core) loss is constant from energisation; copper (I²R) loss grows with load squared. Their ratio sets the maximum-efficiency load point.
Cooling class
—ONAN = oil natural air natural, ONAF adds fans, dry types use AN/AF. Determines rating at ambient temperature.
Markings
Nameplate lists kVA, voltage ratio, vector group, %Z, cooling class, oil volume/weight, tap positions and serial number. Bushings marked with H (HV) and X (LV) or by capital/lowercase letters (A,B,C / a,b,c).
Standard values
Indian distribution: 11 kV/433 V, Dyn11, at 25, 63, 100, 160, 250, 500, 990/1000, 1600, 2500 kVA. Off-circuit tap changer usually ±5% in 2.5% steps; larger units get on-load tap changers (OLTC).
How to choose
1) Sum the connected load in kVA and apply a diversity factor. 2) Size at 70–80% loading for growth and long life. 3) Match the vector group to the network (Dyn11 standard in India). 4) Check %Z against permissible fault level. 5) Pick oil-filled for outdoor pole/plinth mounting, dry-type for indoor or fire-sensitive sites. 6) Prefer BEE star-rated low-loss designs — losses run 24×7 for decades.
Pinout & package
HV bushings on one side, LV bushings on the other, tank with radiator fins, conservator with breather (silica gel), Buchholz relay between tank and conservator on oil units, and an earthing terminal on the tank. Pole-mounted, plinth-mounted, or package substation enclosures.
Example circuits
- 11 kV feeder → distribution transformer → 415 V LT panel of a building
- Generator step-up transformer at a power plant feeding the grid
- Two units in parallel sharing a growing industrial load (same ratio, vector group and %Z)
- Open-delta emergency operation after one phase of a bank fails
Common failures
Insulation ageing from overload heat (each 6–8 °C above rated roughly halves life), moisture ingress through a saturated breather, oil leaks, inter-turn shorts after lightning surges, tap-changer contact wear, and Buchholz trips from internal gassing.
How to test
Turns ratio test, winding resistance test, insulation resistance (megger HV–LV, HV–E, LV–E) with polarisation index, oil dielectric strength (BDV) test, and the classic open-circuit test (iron loss, from LV side) and short-circuit test (copper loss and %Z, from HV side) that every lab course runs.
Substitutes
Replace with the same voltage ratio and vector group; equal or higher kVA. For parallel operation the %Z must also match closely, or the units share load unevenly. A dry-type can replace oil in-doors if the kVA and impedance match.