Transformers

A transformer is a static electrical machine that transfers energy between two or more circuits by electromagnetic induction, changing voltage and current levels in the process without changing frequency. It has no rotating parts: a magnetic core, a high-voltage winding, a low-voltage winding and an insulation system do all the work.

Alternating current in the primary winding sets up an alternating magnetic flux in the core. That flux links the secondary winding and induces a voltage in it. The ratio between the number of turns on each winding sets the ratio between the voltages, so a winding with fewer turns delivers a lower voltage at a proportionally higher current.

A power transformer generally sits in transmission and substation duty at higher capacity and voltage, and is designed to run near full load at high efficiency. A distribution transformer sits closer to the consumer, steps voltage down to utilisation levels, and spends its life on a load that varies through the day, so it is designed around good efficiency across a wide load range, not just at peak.

An oil-immersed transformer uses insulating oil for both insulation and heat transfer. A dry type transformer uses solid insulation and is cooled by air, so there is no transformer oil on site. Dry type units are often specified for indoor and occupied-building installations, while oil-immersed units remain the common choice outdoors and in industrial yards. Which one suits a project depends on the site, the load and the specification, not on one being better than the other.

Distribution transformers are grouped into energy-efficiency levels according to the losses the applicable standard permits at a given rating. A higher level means tighter permitted losses, which usually means more or better core and winding material, a higher purchase price, and lower losses over the life of the unit. Which level applies to a particular transformer depends on its rating, its design and the regulations in force at the time of supply.

Level 1 and Level 2 are energy-efficiency classifications for distribution transformers, defined by the maximum losses allowed at a given rating. Level 2 is the tighter of the two. Because the Indian efficiency framework is revised periodically, the level applicable to your transformer should be confirmed against the regulations current at the time of order rather than assumed from an older tender document.

Start from the actual maximum demand rather than the sum of every nameplate on site, then add realistic headroom for load you intend to connect later. Motor starting current, harmonic loading from drives and inverters, load unbalance, ambient temperature and duty cycle all affect the rating you need. An oversized transformer runs at poor loading and wastes no-load losses continuously. An undersized one runs hot and ages its insulation early.

Transformer losses fall into two groups. No-load losses, also called iron or core losses, occur whenever the transformer is energised regardless of load. Load losses, also called copper losses, rise with the square of the current drawn. Total losses at a given loading determine both the efficiency figure and the heat the cooling arrangement must remove.

A vector group describes how the windings are connected and the phase displacement between primary and secondary. In Dyn11, D is a delta-connected high-voltage winding, y is a star-connected low-voltage winding, n means its neutral is brought out, and 11 indicates the low-voltage side leads by 30 degrees. It matters because transformers intended to run in parallel must share a compatible vector group.

An off-circuit tap changer, OCTC, can only be moved with the transformer de-energised, so it is used to trim for a supply voltage that sits consistently high or low. An on-load tap changer, OLTC, changes taps while the transformer is carrying load, which is what you need when the incoming voltage moves through the day and the output has to be held steady without an interruption.

An isolation transformer separates a load from the supply galvanically, so there is no direct metallic path between the two circuits. It is specified where that separation itself is the requirement: breaking earth loops, containing fault current paths, or feeding equipment whose reference must float relative to the incoming supply, rather than where the aim is simply to change voltage.

Routine tests are performed on every unit, and type test certificates for the design are provided against the applicable standard for that product and rating. Where a project requires third-party inspection or witnessed testing, tell us at the enquiry stage so it is built into the schedule and the price rather than added afterwards.