Engineering · Power transformers

Power transformer specification checklist

Published 4 November 2026 by Jaybee Industries

Comparing offers is quicker, and fewer deviations come back, when the specification is complete. This checklist is written for EPC engineers and plant electrical teams in J&K and Uttarakhand buying power transformers in the 2.5–25 MVA, up to 66 kV range.

1. Rating and duty

State the continuous MVA rating and, for ONAN/ONAF units, the rating at each cooling stage. Describe the load: steady process load, solar export, or a mixed industrial intake. Note any overload requirement.

2. Voltages and connection

Give the HV and LV nominal voltages, the highest system voltage, the vector group (for example Dyn11 or YNd1) and neutral earthing arrangements. If a neutral point is needed on a delta winding, an earthing transformer may be part of the scope.

3. Impedance and fault level

Specify percentage impedance and its tolerance. It affects fault levels downstream, voltage regulation and parallel operation with existing transformers. If units will run in parallel, include the existing nameplate data.

4. Tap changer

Off-circuit (OCTC) or on-load (OLTC), the tapping range and step size, and whether you need an automatic voltage regulating relay and remote tap position indication.

5. Cooling and site conditions

ONAN or ONAF, the maximum and minimum ambient temperatures, and the site altitude. Valley sites in Kashmir and hill sites in Uttarakhand can sit above the 1,000 m reference, which affects cooling and external clearances. See cooling explained.

6. Losses

Either cap no-load and load losses, or give a loss capitalisation formula so offers can be compared on lifetime cost, not just price.

7. Tests and inspection

List routine tests, any type or special tests (short-circuit, impulse, temperature rise), and witness points. The main site's testing guide describes each test.

8. Fittings and accessories

Conservator and breather type, Buchholz, pressure relief, oil and winding temperature indicators, bushings or cable boxes, marshalling box, and the rollers or skid.

9. Transport and erection

Give the route to site, bridge and weight limits, the last reachable point for a trailer, and whether the unit should ship oil-filled or nitrogen-filled. These choices often decide the design for remote sites.

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