Similar Transformer: When No Short-Circuit Test Is Needed

Why the concept exists

Testing a power transformer for short circuit is not always possible. Large transformers sometimes exceed the capacity of test stations. That is why IEC 60076-5 allows, if purchaser and manufacturer agree before the order, dynamic withstand to be demonstrated by calculation and design and manufacturing considerations (clause 4.2.1). For more on the short-circuit test procedure for a transformer: transformer short-circuit test.

One way to do that is to compare the transformer with another one that has already been tested successfully, called the reference transformer. That comparison is valid only if the transformer being assessed is similar to the reference. Annex B, which is informative, defines when it is.

The nine criteria of Annex B

A transformer is similar to a reference transformer if they share these characteristics:

Criteria 6 and 7 are numerical. The others are about matching the design.

How the power criterion is applied

Short-circuit apparent power is the rated power divided by the per-unit short-circuit impedance.

Take a 100 MVA reference transformer with 12% impedance. Its short-circuit apparent power is 100 / 0.12 = 833 MVA. The similarity range goes from 30% to 130% of that value, that is, from 250 to 1,083 MVA. More on the thermal short-circuit withstand over 2 s in thermal short-circuit withstand of 2 s.

• A 60 MVA transformer with 11% has 545 MVA. It is inside the range.
• A 150 MVA transformer with 10% has 1,500 MVA. It is outside.

Meeting this is not enough. The other eight criteria must be met too.

When the reference is accepted

Annex A, clause A.3.3.2.1, adds conditions for a transformer to serve as a reference:

• It is similar to the one being assessed.
• It was designed with basically the same calculation methods and the same mechanical strength criteria.
• It was manufactured with basically the same practices, quality assurance and quality control instructions.
• The validity range of the short-circuit design rules covers the characteristics of both.
• It passed the short-circuit test successfully.

In addition, the test report, the main electromagnetic design data, the calculations and mechanical criteria used, and the manufacturing and quality practices are examined.

If the manufacturer does not have a single suitable reference, clause A.2 allows the transformer to be compared with several tested ones, provided each Annex B characteristic matches that of at least one of them. The standard gives an example: in a three-phase autotransformer with a reduced-capacity delta tertiary, the series and common windings are compared with an autotransformer without tertiary, and the tertiary with a three-phase transformer that has a tested reduced-capacity tertiary.

Comparing forces and stresses

If the reference is acceptable, clause A.3.3.2.2 requires first comparing the winding structures, the main insulation and the clamping system. It must be possible to conclude that the assessed transformer is, in principle, as robust as the reference in its basic mechanical structure.

Then the forces and stresses of each are compared, in Tables A.1 or A.2. The transformer is considered suitable if no figure exceeds 1.2 times that calculated for the reference. There are exceptions in core-type transformers, where the limit drops to 1.1 times: the average compressive stress in disc, helical and single-layer windings, the equivalent compressive stress in multi-layer ones, and the thrust on the low-voltage lead-outs. On the forces a short circuit produces in the windings, read short-circuit forces in transformer windings.

Who declares similarity

The Annex B criteria do not declare that a given transformer is similar to another. That conclusion comes from the design review with the manufacturer’s complete documentation. The information the manufacturer provides remains its intellectual property and must be kept confidential (clause A.3.4).

Category and test

Annex A applies mostly to categories II and III. For category I, which is normally bought in batches, the standard prefers testing one or two units from the batch, because it considers that the quickest and cheapest way to verify conformity. It also admits a design review. The categories are defined by rated power: up to 2,500 kVA, from 2,501 to 100,000 kVA and above 100,000 kVA. Further detail on the short-circuit withstand categories and the design current: short-circuit withstand: categories and design current.

What to ask for as purchaser

1. Decide the demonstration method before the order and put it in writing.
2. If a design review is accepted, ask for the reference test report and the comparison in Tables A.1 or A.2.
3. Ask for the written statement of a positive result. It does not release the manufacturer from its obligations on the transformer’s withstand capability.

We can help with the specification and the tests

At Tiza Group we carry out short-circuit studies that give the short-circuit power and fault current at your transformer’s point of connection, data you must hand to the manufacturer, remotely and worldwide. We also measure short-circuit impedance and FRA on site, to compare them with the factory reference, in Colombia, or elsewhere on request. Contact us with the single-line diagram and the nameplate.

Related guides

• Short-circuit study
• Factory tests of transformers
• Transformer short-circuit test: procedure
• Short-circuit forces in transformer windings
• Transformer Short-Circuit Test: Procedure
• Thermal Short-Circuit Withstand: 2 s and Temperature
• Short-Circuit Forces in Transformer Windings
• Short-Circuit Withstand: Categories and Design Current

References

• IEC 60076-5, Power transformers – Part 5: Ability to withstand short circuit, clause 4.2.1 and Annexes A and B (informative).

Frequently asked questions