Power transformer

01

Compare no-load and load losses, not just price

EU Regulation 548/2014 sets maximum losses for power transformers from 1 kVA used in 50 Hz networks or industry, with exclusions such as instrument, rectifier, furnace, offshore and traction transformers. For three-phase liquid-immersed transformers with one winding up to 24 kV and the other up to 1.1 kV, Tier 2 (from 1 July 2021) limits a 1,000 kVA unit to 7,600 W load loss and 693 W no-load loss; a dry-type 1,000 kVA unit is limited to 9,000 W and 1,395 W. [1]

The Regulation defines medium power transformers as above 1.1 kV up to 36 kV and from 5 kVA to below 40 MVA, and large power transformers as above 36 kV or from 40 MVA; for medium (where applicable) and large units the Peak Efficiency Index must be declared. Documentation must state rated power, load and no-load losses, cooling power at no load and the weight of main components, and these must also appear on the rating plate. [1]

As an editorial calculation example only: a 1,000 kVA liquid-immersed unit at the Tier 2 no-load limit loses 693 W for every hour it is energised, about 6,070 kWh a year. Assuming load loss varies with the square of load, at an average 50% load the 7,600 W full-load loss becomes about 1,900 W, or about 16,600 kWh a year. Weigh both against your load profile and energy price.

Give the supplier your expected load profile and energy price, and ask for guaranteed no-load and load losses with test tolerances. A lower purchase price can bring higher running cost; compare total losses over your load profile.

02

Choose the cooling class for the site

Reclamation's transformer maintenance manual names heat as a destroyer of transformers and requires the cooling class to appear on the nameplate; where fans are added to an oil-immersed self-cooled unit, they start automatically at a preset temperature. [2]

The manual warns that winding temperature indicators only simulate the hottest spot from top-oil temperature and a heater fed by a CT, are at best accurate at rated load, and should not be relied on for accuracy, though they can switch cooling and alarms. [2]

State ambient temperature, altitude, indoor or outdoor installation, ventilation and any harmonic loads, and ask for the rating at each cooling stage.

03

Protect insulation life from heat and moisture

Reclamation states that operating only 10 °C above rating cuts transformer life by 50%, and that each doubling of moisture in a transformer halves insulation life; the life of the transformer is the life of its paper. It advises planning a dry-out at 2% moisture by dry weight in the paper and never exceeding 2.5% or 30% oil saturation. [2]

For dissolved gas analysis (DGA) the manual says to sample after one month of service and once a year, to compare each result with earlier ones to see trends, and that DGA is unreliable on a new unit or within one to two weeks after oil processing. [2]

It also reports that about 90% of preventable bushing failures come from moisture entering through leaky gaskets, cracks or seals. Ask the supplier how the oil is preserved (conservator, sealed tank or nitrogen) and which sampling valves and gauges are fitted. [2]

04

S11, S13, amorphous-core and buried designs in the reviewed ranges

XBRELE, for example, lists amorphous-alloy transformers and 11–35 kV cast-resin dry-type units; Bokong lists S11 and S13 oil-immersed distribution transformers; Changan lists S-M oil-immersed and S-M.D buried power transformers. Treat loss and efficiency claims as figures to confirm in routine test reports. [3] [4] [5]

05

Core, windings, bushings and oil preservation: where losses and failures start

Core and windings tie straight to the EU loss limits above (core material to no-load loss, conductor to load loss); the remaining rows follow the points Reclamation's manual asks maintainers to watch.

06

Rating-plate data: kVA, voltages, losses, cooling and weight

Rated power and highest voltage decide which EU loss table applies, so state those first; the two loss rows carry the Tier 2 limits for a 1,000 kVA liquid-immersed unit as a reference point. [1]

07

Send the supply, load and site data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Supply: [primary and secondary voltage; frequency; vector group; tapping]
Load: [rated kVA; load profile; harmonics; energy price]
Losses: [required EU tier or other rule; guaranteed no-load and load losses]
Type: [liquid-immersed or dry-type; core type]
Cooling: [cooling class; ambient; altitude; indoor or outdoor]
Protection: [oil preservation; gauges; alarms; sampling valves]
Tests: [routine tests; loss measurement; temperature-rise test]
Documents: [rating plate data; technical documentation; test reports]

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