Selecting a distribution transformer

What this guide covers

A transformer enquiry is useful when the electrical duty, the installation and the documentation requirement are stated together. This guide walks through those decisions in the order they are usually resolved. It supports the preparation of a specification; the rating and construction are confirmed only after an engineering review of the actual project.

Rating and load

Start from the maximum demand the transformer will carry and the growth the project expects over its working life. A transformer chosen only for today's load may run close to its limit after an extension; one chosen far above the load runs lightly and carries no-load losses throughout. State both figures, and the basis on which they were estimated.

Describe the load character as well as its size. Large motor starts, welding sets, rectifier loads and variable-speed drives produce voltage dips or harmonic currents that can affect losses and temperature rise. If harmonic loads form a substantial part of the demand, say so; the design may need to account for them.

Invitrona's electrical power and distribution transformers are typically 25–2,500 kVA, three phase and oil immersed, with natural oil and natural air cooling (ONAN).

Voltage class and vector group

State the primary system voltage, typically the 11 kV class, with the 22 kV and 33 kV classes considered by assessment, and the required secondary, typically 433 V for low-voltage distribution. The vector group sets the phase relationship between primary and secondary and the way the neutral is made available. Distribution transformers commonly use a delta primary and a star secondary with the neutral brought out, often written Dyn11; if the project requires another group, for example to parallel with an existing transformer, state it with the details of the existing unit.

Tapping arrangement

Taps let the secondary voltage be adjusted to the actual primary voltage. Off-circuit taps are changed with the transformer de-energised and suit supplies that are reasonably steady. An on-load tap changer adjusts while the transformer carries load and suits supplies that vary through the day, at a higher cost and with more maintenance. A common off-circuit range for distribution transformers is ±5% in 2.5% steps; the range needed should follow from the measured or declared primary variation.

Losses and efficiency level

No-load and load losses decide running cost over the transformer's life and are often fixed by the specification. Where a utility or project specification cites a standard, such as IS 1180 for distribution transformers or IS 2026 for power transformers, the enquiry should name it and the efficiency level required, so the quotation can state how the offer relates to it.

Installation and interfaces

Record whether the transformer will stand indoors or outdoors, on what kind of plinth, and how it will be reached for delivery and lifting. State the termination on each side, such as cable boxes, bushings or a busduct flange, and the cable sizes where known. List the fittings the project expects, such as temperature indicators, a gas-and-oil-actuated relay, pressure relief and marshalling, and say who supplies the protection relays and switchgear that the fittings connect to.

Testing and documents

State which tests the specification calls for, whether any are to be witnessed, and which drawings and documents must be approved before manufacture. Approval steps take time and are planned into the programme; listing them in the enquiry avoids a change of scope after the order.

Next step

Use the transformer enquiry checklist to collect the inputs, then summarise them in the enquiry form and note which documents are available, such as the specification, single-line diagram and site layout. Documents are requested when the review begins. The review states the proposed rating, construction and assumptions, and lists any departure from the specification so it can be accepted or resolved before order.