1. Introduction
The no-load current test – also called the single-phase excitation current test – is used to measure the transformer’s excitation current under no-load conditions.
The measured current is an indicator of:
- Problems in the magnetic circuit, such as an unintended air-gap between the yoke and the core limb caused by incorrect core core bolts.
- Short-circuited loops (shorted turns) in the transformer windings.
2. Test Method and Practical Notes
In this test, the no-load (excitation) current of the transformer is measured using single-phase excitation.
Key points are:
- Residual flux in the core
After the transformer has been de-energized, or after a DC winding resistance test, residual magnetic flux remains in the core. Therefore, before performing the no-load current test, this residual flux should be removed as far as possible. - Applied voltage and minimum current level
- A 220 V single-phase AC voltage is applied to the high-voltage winding of the transformer.
- The no-load current is measured on the same side.
- If the measured current is less than 10 mA, it is strongly recommended to increase the test voltage so that the measured current exceeds 10 mA. This improves measurement accuracy.
- Tap positions
It is required that this test be performed at least on the first tap, the rated (nominal) tap, and the last tap. - Quantities to be measured
It is recommended to measure, in addition to the no-load current:- the active part of the no-load loss, and
- the reactive part of the no-load loss.
- Use of the results as a reference
Because the applied test voltage is usually not equal to the rated voltage of the transformer, the measured current is not the true rated no-load current; it is only a certain percentage of it.
For this reason, the test should be carried out at the time of commissioning, and the results should be kept as reference data for comparison with future tests.
If commissioning data are not available, the results of the first available test under known conditions shall be used as the reference for later comparisons. The test connections are shown in Fig 1.
Figure 1: Excitation current test with ALLINA T1
3. Acceptance Criteria for Test Results
The no-load current test is considered satisfactory when all of the following conditions are fulfilled:
- Phase-to-phase current differences (three-phase transformers)
- The difference between the no-load currents of the two outer limbs (phases A and C in a star-connected winding) is normally less than 10 %.
- For the middle phase, a difference of up to 30 % compared with the two outer phases is acceptable.
- In five-limb transformers, the current pattern will be different, but similar limits apply.
If the measured current is below 10 mA, instrument errors may make the 10 % and 30 % criteria unreliable. In this situation it is recommended to:
a) Plot the variation of no-load current versus tap position and check whether the transformer behavior appears physically reasonable.
b) Compare the no-load currents of similar transformers. If the differences are of the same order of magnitude, the condition may be accepted; a large discrepancy between similar units requires further investigation.
- Single-phase transformers
For single-phase transformers, the difference between the no-load currents of comparable units is normally less than 10 %. - Comparison with previous tests of the same transformer
The difference between the present test results and previous results on the same transformer, under identical conditions (same test voltage), should be less than 10 %.
4. Additional Checks When Criteria Are Not Met
If any of the above conditions is not satisfied, the following steps are recommended:
- Repeat the test after removing residual flux
First, ensure that the residual flux in the core has been eliminated as far as possible. Then repeat the no-load current test. - If discrepancies persist
If, even after demagnetization, the criteria are still not met, additional investigations based on other tests are necessary. A useful approach is:- Measure the no-load current by applying a single-phase voltage with higher magnitude.
- As an example, reference IEEE C157.152 recommends performing this test with voltage up to 10 kV with tan delta test kit.
Furthermore, carrying out a magnetic balance test and comparing its results with previous records can also help to identify the nature and location of the problem.
5. Cases Requiring Measurement from the Delta Side
In the following situations it is recommended to perform the no-load current measurement from the delta-connected side of the transformer:
- The no-load current flowing from the star-connected high-voltage side is so small that it cannot be measured accurately; therefore, it is necessary to perform the test from the delta-connected low-voltage winding.
- The transformer vector group is Dyn.
- There is a possibility of a short-circuited turns in the delta-connected secondary winding.
6. Typical No-Load Current Patterns in Healthy Transformers
When the no-load current test is carried out on a healthy transformer, one of the following patterns is usually observed.
6.1 Pattern 1 – Outer phases > middle phase
The no-load currents of the two outer phases are larger than that of the middle phase. This occurs in:
- Three-phase, three-limb core-type power transformers.
- Five-limb core-type or shell-type power transformers with a delta-connected secondary winding.
6.2 Pattern 2 – Outer phases < middle phase
The no-load currents of the two outer phases are smaller than that of the middle phase. This occurs in:
- Three-limb core-type power transformers when the test procedure is not correctly followed.
- Cases where the neutral of the high-voltage (Y-connected) side is not accessible.
- Transformers with a delta connection whose third corner is grounded (guarded).
- Four-limb core-type power transformers.
6.3 Pattern 3 – All three phase currents equal
The no-load currents of all three phases are approximately equal. This is typical for:
- Five-limb core-type or shell-type power transformers without a delta-connected secondary winding.
These characteristic patterns can be very helpful in judging whether an observed imbalance is normal for a given core design or indicates a possible defect.
7. Conclusions
The no-load current test is a simple but powerful diagnostic tool for assessing transformer condition. By:
- Observing the practical precautions (residual flux removal, sufficient current level, multiple tap positions),
- Applying clear acceptance criteria (phase-to-phase differences and comparison with reference data), and
- Recognizing typical current patterns for different core configurations,
operators can detect many potential problems in the magnetic circuit and windings at an early stage. The results obtained at commissioning or during the first documented test should always be preserved as reference fingerprints, against which all later no-load current tests of the same transformer are compared.
