Table 1 of IEC 60076-1 sets the tolerances on guaranteed values: total losses +10 %, component losses +15 % without exceeding the total-loss tolerance, no-load voltage ratio the lesser of ±0.5 % or ±1/10 of the impedance, impedance ±7.5 % or ±10 % on the principal tapping depending on its value, and no-load current +30 %. A transformer complies with the standard if the quantities stay within those tolerances.
Short answer
A transformer does not leave the factory with the exact values the manufacturer guaranteed. Clause 9 of IEC 60076-1 recognizes this and gives a table of tolerances. With it you decide whether a value measured in a test is acceptable. More on the correction of losses to the reference temperature in transformer losses and correction to 75 °C.
Why there are tolerances
The standard gives the reason. In large transformers, with several windings and relatively low rated voltages, it is not always possible to obtain turns ratios that match the specified voltage ratio exactly. Other quantities are not known precisely at the time of the offer. Others carry manufacturing and measurement uncertainty. That is why tolerances are accepted on certain guaranteed values. On uncertainty when measuring transformer losses: transformer loss measurement and uncertainty.
Two reading rules:
- If the table gives no tolerance in one direction, there is no restriction in that direction.
- A transformer is considered compliant when the quantities subject to tolerance do not go outside the limits of table 1.
Table 1
| Quantity | Tolerance |
|---|---|
| Total losses | +10 % of the total losses |
| Component losses | +15 % of each component loss, without exceeding the tolerance on total losses |
| No-load voltage ratio, principal tapping, first pair of windings | The lesser of: ±0.5 % of the declared ratio, or ±1/10 of the actual percentage impedance on the principal tapping |
| Ratio on other tappings of the same pair, and on other pairs | To be agreed, but not lower than the lesser of the two values above |
| Short-circuit impedance, two separate windings or first pair, principal tapping | ±7.5 % if the impedance is 10 % or more. ±10 % if it is below 10 % |
| Impedance, same case, any other tapping of the pair | ±10 % if it is 10 % or more. ±15 % if it is below 10 % |
| Impedance, auto-connected pair or second pair of a multi-winding transformer, principal tapping | ±10 % |
| Impedance, same case, any other tapping | ±15 % of the value declared for that tapping |
| Impedance of additional pairs | To be agreed, but 15 % or more |
| No-load current | +30 % of the declared value |
“Declared value” means the value declared by the manufacturer.
Three notes to the table
- In transformers with more than two windings, the loss tolerances apply to each pair of windings, unless the guarantee says they refer to a given loading condition.
- In certain autotransformers and in regulating transformers, the impedance is small and justifies a wider tolerance. Transformers with large tapping ranges, especially asymmetric ones, may also need special treatment. On the other hand, if the transformer is to be combined with existing units, it may be justified to agree narrower impedance tolerances. Special tolerances must be flagged in the offer and agreed between manufacturer and purchaser.
- The “declared value” is the one declared by the manufacturer.
Worked examples
The numbers are illustrative. They show how the table is applied.
Losses. The manufacturer guarantees 20 kW no-load and 80 kW load loss. Guaranteed total losses are 100 kW. The maximum allowed for the total is 110 kW. Each component loss may exceed its value by up to 15 %, that is, 23 kW no-load and 92 kW load loss. But both maxima cannot occur at the same time, because the sum (115 kW) would exceed the 110 kW of the total.
Impedance. A transformer with 12.5 % declared on the principal tapping has a tolerance of ±7.5 %. The acceptable range is 11.56 % to 13.44 %. Another with 8 % declared has a tolerance of ±10 %. The range is 7.2 % to 8.8 %.
Ratio. With an actual impedance of 8 % on the principal tapping, 1/10 is 0.8 %. The lesser of 0.5 % and 0.8 % is 0.5 %. With an actual impedance of 4 %, 1/10 is 0.4 %. The lesser of 0.5 % and 0.4 % is 0.4 %. The ratio rule becomes stricter when the impedance is below 5 %.
No-load current. With 0.3 % declared, the maximum is 0.39 %. There is no lower limit.
How they are used in practice
- In the offer. Special tolerances must be flagged at the time of the offer and agreed between manufacturer and purchaser (note 2 of the table).
- In the factory test. Each measured value is compared with the guaranteed one and its tolerance. See transformer factory tests.
- At site acceptance. The ratio is verified with the transformer turns ratio test. If it falls outside tolerance, see turns ratio out of tolerance.
- For impedance. The short-circuit impedance (Z) test measures the value that is compared with the table.
- At no load. The no-load excitation test measures the no-load current and losses.
For impedance, clause 5.5 adds that, if the enquiry gives a range between upper and lower limits, the measured values may differ from the guaranteed ones within the tolerances, but may not go outside those limits, which are given without tolerance.
What not to do
- Do not apply the ±0.5 % tolerance to every ratio. On the principal tapping, take the lesser of ±0.5 % and 1/10 of the impedance.
- Do not add up the component loss tolerances beyond the total.
- Do not use ±7.5 % for an impedance below 10 %. In that case the tolerance on the principal tapping is ±10 %.
- Do not reject or accept a lot without reading the contract. The table is the rule of the standard. Special tolerances are agreed by the parties.
When to call us
At Tiza Group we measure ratio, impedance, losses and no-load current and compare them with the guarantee and its tolerance, at the factory and on site. See our service of power transformer testing and commissioning in Colombia, or elsewhere on request. If you need to set the tolerances in the specification, see engineering design and studies. Write to us and we will send you a quote.
Related guides
- Turns ratio out of tolerance
- Short-circuit impedance (Z) test on power transformers
- No-load excitation test
- Transformer factory tests
- How to quote power transformer tests
- Transformer Loss Measurement and Uncertainty
- Transformer Losses and Correction to 75 °C
References
- IEC 60076-1, Power transformers - Part 1: General, clause 9, table 1, and clause 5.5. Clause numbering of edition 2.1 (1993, with amendment 1 of 1999).
Frequently asked questions
What is the tolerance on total losses?
+10 % of the guaranteed total losses. There is no lower limit: where a tolerance is missing in one direction, the standard says there is no restriction in that direction.
What tolerance does short-circuit impedance have?
On the principal tapping of a two-winding transformer, ±7.5 % of the declared value if the impedance is 10 % or more, and ±10 % if it is below 10 %. On other tappings of the pair, ±10 % and ±15 % respectively. For autotransformers and for the second pair of a multi-winding transformer: ±10 % on the principal tapping and ±15 % on the others.
When is a transformer considered compliant?
When the quantities subject to tolerance are not outside the tolerances of table 1 (IEC 60076-1, clause 9).