A distribution transformer is selected by its rated power, which is the continuous load it carries without exceeding temperature limits; by its voltages and taps; by its impedance, which sets the fault current; and by site conditions. Losses and their tolerances are agreed in the contract.
Short answer
Selecting a distribution transformer means fixing a few data points and agreeing with the manufacturer how they are guaranteed. The data are the power, the voltages and taps, the impedance, the vector group, the insulation type and the site. To learn what it is and how it is built, see distribution transformer. The substation types where it is installed are described in Distribution substations: types and components; the short-circuit data it must withstand, in Short-circuit current as a substation design input. More on what the rated power of a transformer means in transformer rated power.
IEC 60076-7 treats as distribution transformers those up to 2,500 kVA three-phase or 833 kVA single-phase. In North American practice the equivalent specifications are IEEE C57.12.00 (liquid-immersed) and IEEE C57.12.01 (dry-type), with IEEE C57.91 as the loading guide.
1. Rated power
According to IEC 60076-1, rated power refers to continuous duty and is the reference value for guarantees and for load loss and temperature rise tests. If several values are assigned for different cooling modes, the rated power is the highest.
The standard recommends taking values from the R10 series in transformers up to 10 MVA: 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1,000 kVA and so on.
It is calculated from the expected maximum load and its growth over the life of the equipment. IEC 60076-7 warns that loading above the nameplate or at an ambient temperature above the design value brings risk and accelerated aging. For that reason it is not advisable to size the unit counting on overload; see loading a transformer above nameplate.
2. Voltages and taps
The primary voltage (that of the medium-voltage network), the secondary voltage and the tap range to compensate for network variation are fixed. The standard recognizes that the voltage at the secondary terminals differs from the rated value because of the internal drop, and that this must be taken into account when specifying rated voltage and taps. It also requires the transformer to withstand without damage an overfluxing of up to 5 % above the rated voltage/frequency ratio.
3. Short-circuit impedance
It sets two opposing things: the voltage drop under load and the fault current on the secondary. A higher impedance limits the fault but increases the drop. The tolerance is ±7.5 % if the value is 10 % or greater and ±10 % if it is lower, on the principal tap. Before buying, compare it with the short-circuit study. See also short-circuit withstand. With the chosen impedance you can estimate the short-circuit current on the secondary using our free short-circuit current tool.
4. Losses and tolerances
No-load and load losses are guaranteed in the contract. IEC 60076-1 allows a tolerance of +10 % on total losses and +15 % on each component, without exceeding the total tolerance. The no-load current may exceed the declared value by up to 30 %. The no-load voltage ratio has a tolerance of ±0.5 % of the declared value, or one tenth of the percentage impedance, whichever is smaller.
When comparing offers, compare the guaranteed losses as well as the price.
5. Vector group and insulation type
The vector group is written with letters and a clock number (for example Dyn11) and must match that of the other transformers operating in parallel and the low-voltage grounding system. Insulation can be oil or dry-type, depending on the site. See pole-mounted substation, pad-mounted transformer and dry-type transformer maintenance.
6. Site conditions
IEC 60076-1 defines normal service with altitude up to 1,000 m, air between -25 °C and +40 °C, a practically sinusoidal and symmetrical voltage, and an environment with low pollution and no significant seismic risk. If the site departs from these, it must be stated in the request for quotation. See normal service conditions of a transformer.
7. Protection and switching
Protection on the medium-voltage side can be a fuse or a breaker, depending on the rating and the importance of the load. See transformer protection with fuse or breaker. The grounding of these substations is detailed in Grounding of distribution substations, which should be adapted to the local code and the utility’s requirements.
Questions to settle before buying
- Is the load constant or intermittent? A load with short peaks and long periods below the rated power ages the insulation less than a continuous load at the maximum value. IEC 60076-7 measures that aging by the winding hot-spot temperature: the relative aging rate is 1 when the hot spot is at 98 °C with non-thermally-upgraded paper and at 110 °C with thermally upgraded paper.
- Will there be loads with harmonics? IEC 60076-1 uses as a conventional rule a voltage distortion not exceeding 5 % in total or 1 % in even harmonics. Keep in mind also that harmonic current increases load losses and heating. If the plant has drives or rectifiers, say so in the request for quotation.
- Will it operate in parallel with another transformer? Then the vector group, the voltage ratio and a compatible impedance must match. Ask the manufacturer for the declared impedance of each unit and compare them.
- Is it inside a building or outdoors? It defines whether it is dry-type or oil-immersed and what fire protection is required.
Which documents to ask for
Ask for the routine test report with the measured losses, the no-load current, the impedance and the voltage ratio at each tap, and compare with the guaranteed values and their tolerances. The same values serve as a reference at commissioning and in later maintenance, because they show whether the equipment has changed over the years. See transformer maintenance.
What to include in the request for quotation
| Item | Note |
|---|---|
| Rated power | Load and growth |
| Voltages and tap range | Medium-voltage network and load |
| Impedance | Consistent with the short-circuit level |
| Vector group | Paralleling and grounding |
| Insulation and cooling | Oil or dry-type |
| Guaranteed losses | With the contract tolerances |
| Site conditions | Altitude, temperature, pollution |
How we help
We size and specify your transformer in the engineering design and studies service and verify it at commissioning, with ratio, winding resistance and insulation tests, in Colombia or elsewhere on request.
Related guides
- Distribution transformer
- Power transformer selection and specification
- Loading a transformer above nameplate
- Short-circuit withstand
- Distribution substations: types and components
- Short-circuit current as a substation design input
- Grounding of distribution substations
- Transformer Rated Power: What It Means (IEC 60076-1)
References
- IEC 60076-1, power transformers: general (1993 edition with 1999 amendment); clauses 1.2, 4.1, 4.3, 4.4 and the tolerance table.
- IEC 60076-7, loading guide for oil-immersed power transformers; clauses 3 and 5.
Frequently asked questions
What rated power should I choose?
The expected maximum load plus growth over the service life of the equipment. The rated power is what the transformer carries continuously within the temperature limits (IEC 60076-1). Loading it above that accelerates insulation aging (IEC 60076-7), so overloads should not be relied on as a rule.
What is the tolerance on a transformer's impedance?
IEC 60076-1 sets ±7.5 % of the declared value on the principal tap if the impedance is 10 % or more, and ±10 % if it is lower, for a two-winding transformer. The no-load current may exceed the declared value by up to 30 %.
What counts as a distribution transformer under IEC?
IEC 60076-7 defines it as a power transformer of up to 2,500 kVA three-phase or 833 kVA single-phase.