An excitation current test measures the current a transformer winding draws when a low voltage is applied, and it is used as a screening check on the magnetic circuit: the core, the air gaps, the grounding of core components and, in some cases, the winding itself.
The value of the test is comparative. A single reading means little; the pattern across phases, tap positions and previous records is what reveals a problem. Excitation current findings are evidence for further investigation, not a complete diagnosis on their own.
What the Excitation Current Test Measures
The test applies a low AC voltage to one winding, usually at a reduced level well below the rated voltage, and measures the current that flows. That current is dominated by the magnetizing behaviour of the core: the flux path, the number of turns effectively in the circuit, the core material condition and the presence of any circulating path that changes the magnetic balance. Because the reading is sensitive to the magnetic circuit, it can reveal conditions that a purely electrical ratio check does not show clearly.
The measurement is normally made phase by phase, and the current is expressed in amperes or as a percentage of the rated full-load current. Repeating the measurement at more than one tap position, where a tap changer is fitted, adds information because the effective turns and flux distribution change with tap position. The exact voltage used is set by the instrument and the test procedure; what matters for interpretation is that the same voltage and connection arrangement are used every time so that readings remain comparable.
How the Test Is Configured Across Phases and Taps
There is no single connection scheme that suits every transformer, because the vector group, the neutral arrangement and the tap-changer position all change the flux path. A common approach is to excite one phase at a time and record the current in each phase under identical conditions, then compare the three readings. Some testers automate the sequence and display the three currents together, which makes a phase-to-phase imbalance immediately visible.
When a tap changer is fitted, record the tap position with every reading. The excitation current normally changes with tap position because the effective ratio changes; the useful comparison is between phases at the same tap, and between the present reading and the baseline recorded at the same tap in earlier tests. Mixing readings from different tap positions without recording them is one of the most common reasons excitation data becomes impossible to interpret later.
What Normal and Abnormal Patterns Can Indicate
In a healthy three-phase transformer, the excitation currents across phases are usually close to each other, although small differences can exist because of core geometry and build asymmetries. A clearly higher current in one phase, or a pattern that changes between tap positions, is a flag that the magnetic circuit is not behaving as it did at baseline. Possible contributors include problems in the core such as shorted laminations or poor core grounding, an unintended circulating path, and in some cases winding-related conditions that change the effective turns balance.
The important discipline is to treat excitation current as a screening indicator. A pattern change says that something is different; it does not, by itself, identify which component is at fault. The finding should be combined with the transformer history, the results of related tests such as turns ratio and winding resistance, and the manufacturer’s guidance before a conclusion is drawn.
Phase Imbalance and the HLH/LHL Pattern Question
Three-phase transformers can show characteristic patterns where one phase reads high or low relative to the others. Practitioners sometimes describe patterns such as “high-low-high” or “low-high-low” across phases, and these patterns have been associated with particular core or winding conditions in specific transformer designs. The value of pattern language is that it draws attention to the phase order of the anomaly, which is often more informative than the absolute magnitude of any single reading.
However, pattern labels are not universal laws. Whether a given pattern is abnormal depends on the vector group, the core construction, the test connection and the tap position. Before using a pattern label in a report, confirm that it comes from a source that matches your transformer type and test method, and describe what you actually observed rather than relying on the label alone.
The table below organises common excitation current patterns as screening evidence only. It is not a fault dictionary: each row ends in a next step, not a verdict.
| Observed pattern | What it may relate to (evidence only) | Next step before any verdict |
|---|---|---|
| One phase clearly higher than the other two at the same tap | A magnetic-circuit change in that phase path, core grounding or a circulating path; residual magnetism is also possible | Demagnetise and repeat; compare with the same-tap baseline; confirm identical connections |
| Pattern changes between tap positions | Tap-changer path or effective turns balance changing with position | Re-test at adjacent taps; add turns ratio and winding resistance checks |
| Uniform rise across all phases after DC testing | Residual magnetism or a common condition rather than a single defect | Demagnetise and repeat before deeper diagnosis |
| A characteristic phase order such as high-low-high or low-high-low | Design-dependent; may relate to specific core or winding conditions in that transformer type | Confirm the source matches the transformer type and method; never treat the label as a verdict |
Separating Test Conditions from Core Condition
Before an abnormal pattern is assigned to the transformer, the test conditions must be ruled out. Residual magnetism in the core, left by a recent DC test such as a winding resistance measurement, can change the excitation behaviour and produce readings that are not representative. Connections that are not identical between phases, contact resistance at the terminals, and a test voltage that was not the same on every phase can all create imbalance that has nothing to do with the transformer.
If residual magnetism is suspected, demagnetize the core and repeat the measurement. If the pattern persists after demagnetization and after the connections are checked, the evidence is more credible. Recording the conditions with every reading, including the tap position, test voltage and any recent DC testing, is what makes this separation possible months later when the data is reviewed.
When to Escalate to Other Diagnostic Tests
An excitation current anomaly is a reason to widen the investigation, not to stop at the anomaly itself. The next step depends on the transformer and the pattern. Turns ratio testing can confirm whether the effective turns balance is correct, and its interpretation is covered in How to Interpret Transformer Turns Ratio Test Results. Winding resistance testing can separate connection and winding-resistance effects from core effects. In some cases, a frequency-response or SFRA measurement adds information about mechanical and magnetic circuit changes, and post-transport and post-fault testing explains when those deeper checks are justified.
Escalation decisions should also use the transformer’s operating history and any other recent findings. A small phase difference that has been stable for years is different from a new imbalance appearing after a through-fault or after transport. Keep the evidence chain in the report: what was measured, under what conditions, compared with which baseline, and what further test is recommended as a result.
Repeatability is the first test of any excitation finding. Before treating a pattern as real, measure the suspect phase twice without changing anything. If the two readings do not agree within the instrument’s expected stability, the problem is more likely in the connection or the conditions than in the transformer. If they do agree, compare the reading with the same phase at the same tap position in the previous test record. A worked example makes the logic concrete: a three-phase unit whose excitation currents were 2.1%, 2.2% and 2.3% of rated at baseline, and now reads 2.2%, 4.8% and 2.3%, shows a single-phase anomaly that deserves a turns ratio check and a careful look at the core and tap-changer history. The same unit reading 4.0%, 4.2% and 4.1% across all phases after a recent DC test is more likely to be affected by residual magnetism or a common condition, and demagnetization should be tried before deeper tests are ordered.
Frequently Asked Questions
What causes high excitation current?
High excitation current can be caused by conditions in the magnetic circuit, such as shorted laminations, poor core grounding, unintended circulating paths, or in some cases winding-related changes that affect the effective turns balance. It can also be caused by test conditions such as residual magnetism or inconsistent connections, so those must be ruled out before the transformer itself is blamed.
What is a magnetic balance test?
Magnetic balance testing is a related low-voltage method that checks whether flux is distributed evenly across the phases of a three-phase core. Excitation current testing is one way to observe that balance; the reading is compared across phases under identical conditions. Like excitation current, magnetic balance is a screening check whose findings should be confirmed with other diagnostic tests.
Can residual magnetism affect the reading?
Yes. Residual flux left in the core by a recent DC test can change the excitation behaviour and produce readings that are not representative of the normal condition. Demagnetize the core and repeat the measurement before interpreting an anomaly, and record any recent DC testing in the test record.
For the equipment used in this and related maintenance checks, see the transformer maintenance test equipment range. To place the test inside the full lifecycle programme, read What Tests Are Required for a Power Transformer.