1. Introduction
Among practical diagnostic methods in transformer maintenance, the magnetic balance test plays a key role in evaluating core integrity and detecting winding abnormalities. Even a single short-circuited turn can significantly disturb the magnetic flux path, alter induced voltages, and make this method highly sensitive to internal faults.
2. Purpose of the Magnetic Balance Test
2.1 Evaluation of Magnetic Flux Behavior
Magnetic balance test verifies whether magnetic flux forms correctly and flows properly through the transformer core. In a healthy transformer, the relationship between applied voltage and induced voltages follows a predictable pattern that the magnetic circuit geometry defines.
2.2 Detection of Abnormal Conditions
If a shorted turn exists, it forms a low-impedance loop, generates circulating current, and distorts the local magnetic field. As a result, induced voltages deviate from expected values. Therefore, engineers can detect faults that insulation-based diagnostic tests may not easily reveal.
2.3 Comparative Diagnostic Approach
In practice, engineers typically use magnetic balance test as a comparative method rather than an absolute one. They evaluate the results against previous measurements or compare the three phases. Consequently, this approach improves reliability when identifying deviations from normal transformer behavior.
3. Basic Test Principle
3.1 Voltage Injection and Measurement Concept
The method requires the technician to apply a low AC voltage to one phase winding and measure the induced voltages in the remaining windings. The resulting voltage pattern then reflects the magnetic balance within the transformer core.
3.2 Expected Behavior in a Three-Leg Core
When the technician applies voltage to the middle leg, nearly equal voltages appear in the side legs. However, when the technician applies voltage to a side leg, the middle leg shows a higher induced voltage than the opposite side. This difference reflects the geometry of the magnetic path.
3.3 Voltage Relationship Criterion
In all configurations, the sum of the induced voltages must equal the applied voltage. Therefore, any significant deviation from this relationship may indicate abnormal magnetic paths, core problems, or winding faults such as shorted turns.
Fig. 1: Transformer magnetic balance test circuit using the multi-function substation tester (ALLINA T1).
4. Recommended Test Arrangement
4.1 Selection of Test Side and Conditions
Technicians typically perform magnetic balance test on the side with the highest number of turns because this choice improves sensitivity and increases the reliability of comparisons. In addition, they should keep the tap position consistent to obtain comparable results.
4.2 Step-by-Step Test Procedure
First, select a phase and apply a low AC voltage, typically 230 V. Next, measure the induced voltages in the other windings. Then, repeat the process for all three phases (A, B, and C) to obtain a complete set of comparative data.
4.3 Result Evaluation Method
Engineers should compare the measured voltages with expected phase relationships and, whenever possible, with historical test records. If the results remain consistent across phases, the transformer likely operates normally. On the other hand, deviations may suggest abnormalities in the magnetic circuit or windings.
5. Detection of Shorted Turns
5.1 Effect of Short-Circuited Turns
A shorted turn acts as a closed loop with very low impedance. Because of this, large circulating currents flow through the loop. These currents oppose the normal magnetic flux and disrupt the expected distribution pattern within the transformer core.
5.2 Impact on Induced Voltages
When a faulty winding participates in the magnetic circuit, its induced voltage decreases significantly while another phase remains energized. In severe cases, such as a direct short, the induced voltage may drop close to zero.
5.3 Diagnostic Indicators
Engineers identify abnormal results by comparing measured voltages with expected values, previous test data, or corresponding phase measurements. If the deviations become significant, the probability of a winding fault becomes high.
6. Additional Verification Method
6.1 Cross-Phase Testing Approach
For further confirmation, the technician applies AC voltage to one winding while measuring current and induced voltage in another winding. Then, the technician repeats this procedure across phases to check consistency.
6.2 Improved Diagnostic Confidence
If a shorted turn exists, abnormal current and reduced induced voltage consistently appear whenever the affected winding participates in the magnetic circuit. As a result, this pattern strengthens confidence in the diagnosis of internal faults.
6.3 Use in Complex Cases
This method becomes especially useful when the initial results remain inconclusive or when transformer design makes direct interpretation more difficult. In such cases, cross-comparison provides additional insight into the magnetic behavior of the system.
Fig. 2: Example test results obtained using the multi-function substation tester (ALLINA T1).
7. Important Practical Notes
7.1 Full Three-Phase Testing Requirement
Engineers should always test all three phases. Testing only one phase may hide asymmetries, whereas sequential testing of phases A, B, and C provides a complete understanding of magnetic balance behavior.
7.2 Controlled Low-Voltage Application
Technicians usually apply a low AC voltage, typically around 230 V, to maintain safe and controlled testing conditions. This voltage level provides enough input for comparative analysis without adding excessive stress to the transformer.
7.3 Influence of Residual Magnetism
Engineers should remove residual core magnetism from previous DC tests before running magnetic balance test. Otherwise, the remaining flux can distort magnetic behavior and complicate result interpretation.
Fig. 3: Multi-function substation tester (ALLINA T1) in operation.
8. Safety and Testing Precautions
8.1 General Safety Considerations
Although the applied voltage remains relatively low, the transformer stays energized during the test. Therefore, technicians must ensure proper isolation, grounding, and secure connections before applying voltage.
8.2 Test Setup Verification
Engineers should verify transformer isolation, document the tap position, and confirm that the test device has the correct connection and rating. In addition, they must consider the induced voltages in non-energized windings during setup.
8.3 Handling Induced Voltages
Because induced voltages appear in other windings, personnel must treat all terminals as energized. Accordingly, the team must take proper precautions to protect personnel and prevent unintended contact during the test.
9. Conclusion
The magnetic balance test offers a practical and sensitive way to evaluate transformer core behavior and detect shorted turns. By applying low AC voltage and analyzing induced voltages, engineers gain valuable insight into magnetic performance. Moreover, when they perform the test systematically, they support accurate diagnosis and reliable transformer condition assessment.
