Transformers & Power Distribution
Reading a transformer nameplate
What each field on a distribution transformer nameplate means, from rating and voltages to vector group, impedance, tappings and cooling, and why each matters on site.
The nameplate is the transformer's permanent identity. It records the ratings the unit was designed and tested for, and it is the first thing a maintenance engineer, an inspector or a replacement supplier will ask for. Reading it correctly avoids wrong assumptions about capacity, connection and protection.
The fields below are typical for a three-phase oil-immersed distribution transformer. The exact layout follows the standard the transformer was built to; in India that is commonly IS 2026 for power transformers and IS 1180 for distribution transformers.
Identity
- Manufacturer, serial number and year of manufacture: link the unit to its test records and drawings. Quote the serial number in every service request.
- Standard: the specification the transformer was designed and tested to.
Ratings
- Rated power (kVA): the continuous apparent power the transformer can deliver at rated voltage and frequency within its temperature-rise limits.
- Rated voltages: primary (high-voltage) and secondary (low-voltage) voltages at no load, for example 11,000 V and 433 V. The secondary is set slightly above the 415 V system voltage to allow for voltage drop under load.
- Rated currents: the full-load current on each side. These are the values against which cables and protection are checked.
- Frequency and phases: normally 50 Hz and three phase in India.
Connection
Vector group
The vector group describes how the windings are connected and the phase shift between primary and secondary. In Dyn11, a common distribution arrangement, the primary is delta (D), the secondary is star (y) with the neutral brought out (n), and the secondary lags the primary by 30° (the 11 o'clock position). Transformers that will run in parallel must have the same vector group.
Percentage impedance
Impedance, shown as a percentage, is the voltage needed to circulate full-load current with the secondary short-circuited, as a share of rated voltage. It sets the fault current the transformer can deliver and the voltage drop under load. Distribution transformers up to 2,500 kVA typically fall in the range of about 4.5–6.25%, increasing with rating. Units intended to share load in parallel should have closely matched impedance.
Tappings
Tappings allow the turns ratio to be adjusted to suit the actual supply voltage. A typical distribution transformer has an off-circuit tap changer with a range such as ±5% in steps of 2.5%. Off-circuit means the transformer must be de-energised before the tap position is changed. The nameplate lists the voltage for each tap position.
Cooling and temperature
- Cooling: a code such as ONAN (oil natural, air natural) describes the internal coolant and the external cooling medium and how each circulates.
- Temperature rise: the limits for oil and winding temperature above ambient that the design was tested to.
- Insulation level: the voltage withstand of the windings, sometimes shown as the basic insulation level.
Physical data
- Oil quantity and type: needed for top-up, filtration and containment design.
- Total mass and untanking mass: needed for lifting, transport and foundation design.
- Diagram: many nameplates include a connection diagram and the tap-position table.
| Parameter | Value | Condition |
|---|---|---|
| Rated power | 1,000 kVA | Continuous, at rated conditions |
| Rated voltage | 11,000 V / 433 V | No-load voltages |
| Vector group | Dyn11 | Common distribution arrangement |
| Impedance | About 5% | Typical band 4.5–6.25% up to 2,500 kVA |
| Tapping range | ±5% in 2.5% steps | Off-circuit |
| Cooling | ONAN | Oil natural, air natural |
As a quick check, the full-load secondary current is the rating in kVA × 1,000 ÷ (√3 × secondary voltage). For 1,000 kVA at 433 V that is about 1,333 A.
Nameplate and test report
The nameplate summarises the design. The routine test report for the individual unit records what was measured in the factory: winding resistance, turns ratio, impedance, no-load and load losses, and insulation tests. Keep the report with the site records and match it to the nameplate by serial number. When a transformer is relocated, repaired or replaced, both documents are needed.
Using the nameplate on site
- Check that the secondary voltage at the chosen tap suits the measured primary supply.
- Check cable and protection settings against the rated currents.
- Use the impedance to confirm downstream fault levels and breaker ratings.
- Confirm vector group and impedance before connecting two transformers in parallel.
- Record the serial number and ratings in the site's asset register.
A nameplate that is unreadable, painted over or missing should be replaced from the manufacturer's records. Never estimate ratings from the size of the tank.
Where to go next
- Electrical Power & Distribution Transformers
- Glossary: vector group, impedance and tap-changer terms.
- Discuss your requirement: share the nameplate data of an existing unit or the new specification.
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