Transformer cores can retain residual magnetism after various electrical tests such as winding resistance measurement, DC injection tests, or even after sudden disconnection from the power supply. This residual flux trapped in the core material can cause problems during re-energization. Inrush currents can spike to dangerously high levels, protection relays may trip unnecessarily, and the transformer may produce unusual noise or vibration upon re-energization.
Demagnetization of the transformer core is a necessary step that engineers and technicians must perform after any test that introduces DC current through the windings.
In this technical guide, we will discuss everything you need to know about transformer core demagnetization after testing, including its working principle, causes of residual magnetism, demagnetization methods, equipment required, step-by-step procedures, relevant industry standards, and practical field tips. Practical examples are included throughout to help you apply these concepts in real-world scenarios confidently.
1. What is Residual Magnetism in a Transformer Core?
Residual magnetism, also called remanence, is the magnetic flux that remains in the transformer core after the magnetizing force (current) has been removed. Transformer cores are made from grain-oriented silicon steel, which is designed to carry magnetic flux efficiently. However, this same property means the core material tends to “remember” its last magnetic state.
Think of it this way. If you take a steel nail and rub it with a magnet several times in one direction, the nail itself becomes a small magnet. The transformer core behaves similarly. After a DC current flows through the winding during testing, the core retains some level of magnetization even after the current stops flowing.
The amount of residual flux depends on the type of core material, the magnitude of the DC current applied, the duration of DC application, and how the current was removed. Abrupt disconnection of DC current tends to leave higher residual flux compared to a gradual reduction.
According to the B-H curve (hysteresis loop) of the core material, the residual flux density (Br) is the value of flux density remaining at zero magnetizing force. For grain-oriented silicon steel, this value can be as high as 60 to 80 percent of the saturation flux density. That is a large amount of trapped magnetism.
2. Why Does Testing Cause Residual Magnetism?
Several common transformer tests introduce DC current into the windings. Each of these tests has the ability to magnetize the core to varying degrees. Let us look at the most common ones.
2.1 Winding Resistance Measurement
This is one of the most frequently performed tests on power transformers. A DC current source is connected across the winding, and the voltage drop is measured to calculate resistance using Ohm’s law. The DC current flowing through the winding creates a magnetomotive force (MMF) that magnetizes the core. After the measurement is complete, if the DC source is simply switched off, the core retains residual flux.
2.2 DC Insulation Resistance Testing (Megger Testing)
During insulation resistance testing, a high DC voltage is applied between windings or between winding and ground. Although the current is small, it can still contribute to core magnetization over time, especially in large power transformers with highly permeable core materials.
2.3 Transformer Turns Ratio (TTR) Testing
Some TTR test sets apply a voltage to one winding and measure the induced voltage on the other. Depending on the instrument design, a DC component may be introduced. Older analog TTR test sets are more likely to leave residual magnetism in the core.
2.4 CT Saturation Testing and Other Diagnostic Tests
Current transformer saturation testing and sweep frequency response analysis (SFRA) can also affect the magnetic state of the core. Any test procedure that passes direct current through the winding creates a risk of core magnetization.
2.5 Sudden Disconnection from AC Supply
Even during normal operation, if a transformer is suddenly disconnected from the AC power supply, the core flux freezes at whatever instantaneous value it had at the moment of disconnection. This frozen flux becomes the residual flux. The value can range from zero (if disconnection happened at the exact moment of zero flux) to near-peak flux density.
3. Why Is Demagnetization Necessary?
Residual magnetism in the transformer core is not just a theoretical concern. It has real operational consequences that can affect equipment performance and power system protection.
3.1 Excessive Inrush Current
The most well-known consequence is the generation of high inrush currents during re-energization. Inrush current occurs because the transformer core saturates during the first few cycles after energization. If residual flux is already present in the core, the total flux (residual plus the new flux from the applied AC voltage) can push the core deep into saturation. This results in inrush currents that can reach 8 to 15 times the rated full-load current.
For example, consider a 50 MVA power transformer with a rated full-load current of 1200 A on the HV side. If residual flux is present, the inrush current upon energization could reach 10,000 A or more. This level of current can cause mechanical stress on the windings and may trigger differential protection relays to operate incorrectly.
3.2 False Relay Operations
Overcurrent relays, differential relays, and restricted earth fault relays can misinterpret inrush current as a fault condition. Modern numerical relays have second harmonic blocking features to distinguish between inrush and fault currents. However, high residual flux can alter the harmonic content of the inrush waveform and defeat these blocking algorithms.
3.3 Inaccurate Test Results
Residual magnetism can also affect the accuracy of subsequent diagnostic tests. For instance, if you perform an excitation current test immediately after a winding resistance test without demagnetizing, the results will be skewed due to the existing magnetic bias in the core.
3.4 Mechanical Stress and Noise
The asymmetric flux created by residual magnetism causes uneven forces on the core laminations and windings. This results in increased vibration and audible noise during energization. Over time, repeated high inrush events can loosen winding clamping structures.
4. Methods of Transformer Core Demagnetization
There are several methods used in the field and in laboratories to demagnetize transformer cores. Each method has its own advantages and limitations.
Method 1: AC Voltage Application with Gradual Reduction (Variac Method)
This is the most traditional and widely understood method. An AC voltage is applied to one of the transformer windings using a variable AC source (variac or autotransformer). The voltage is initially set high enough to drive the core into saturation and then gradually reduced to zero over several minutes.
Step-by-Step Procedure:
- Connect a variac to the low-voltage winding of the transformer.
- Start with zero voltage and slowly increase the AC voltage until the core reaches saturation. You can identify saturation by monitoring the excitation current. A sharp increase in current indicates the core is saturating.
- Once saturation is confirmed, begin reducing the voltage very slowly.
- Continue reducing the voltage in small increments over a period of 3 to 5 minutes.
- Reduce the voltage all the way to zero without any sudden jumps.
- Disconnect the variac.
This method is effective and easy to understand. However, it requires a sufficiently rated variac and may not be practical for very large power transformers because the voltage required to saturate the core from the LV side can still be substantial.
Example: For a 132/33 kV, 50 MVA transformer, applying voltage to the 33 kV winding requires a variac capable of supplying several thousand volts. In practice, you may apply voltage to the tertiary winding (if available) or use a lower voltage winding tap.
Method 2: DC Current with Polarity Reversal and Decreasing Magnitude
This method mimics AC demagnetization using a DC source. A DC current is applied to the winding, then the polarity is reversed and the magnitude is slightly reduced. This process is repeated multiple times with decreasing current levels until the current reaches zero.
Step-by-Step Procedure:
- Connect a DC source to one of the transformer windings.
- Apply a DC current large enough to saturate the core.
- Reverse the polarity and reduce the current magnitude by about 10 to 20 percent.
- Reverse the polarity again and reduce the current further.
- Continue this process for 20 to 30 reversals.
- End with zero current.
This method is more practical in the field because DC sources are readily available (the same equipment used for winding resistance testing can often be used). However, it is time-consuming and requires careful manual control of current levels and polarity reversals.
Method 3: Using Dedicated Demagnetization Equipment
Several test equipment manufacturers now offer dedicated transformer demagnetization devices. These instruments automate the demagnetization process by generating a controlled, decreasing oscillating current or voltage waveform. They monitor the core flux in real-time and confirm when the core has been fully demagnetized.
Popular manufacturers of such equipment include Omicron, Megger, Doble, and DV Power. For example, the Omicron CPC 100 has a built-in demagnetization function that can be used after winding resistance testing. The device automatically generates the required waveform and indicates when the process is complete.
These instruments save time, reduce the risk of operator error, and provide documented proof that demagnetization was performed. For utilities and large industrial facilities, investing in such equipment is a wise decision for long-term transformer maintenance programs.
Method 4: Energization at Reduced Voltage
In some cases, the transformer can be demagnetized by energizing it at a reduced AC voltage (around 10 to 20 percent of rated voltage) and then gradually increasing to full voltage. The initial low-voltage application creates alternating flux in the core that gradually erases the residual magnetism without generating excessive inrush current.
This method is less controlled than the variac method and may not fully demagnetize the core in all cases. It is sometimes used as a practical compromise in field situations where other equipment is not available.
Method 5: Multiple Short-Circuit Energizations
This is an older technique where the transformer is energized with one winding short-circuited. The short-circuit current generates alternating flux that opposes the residual magnetism. The transformer is energized and de-energized several times in succession. Each cycle reduces the residual flux.
This method is not recommended for routine use because it subjects the transformer to repeated short-circuit stresses. It should only be considered as a last resort.
5. How to Verify That the Core Has Been Demagnetized
After performing demagnetization, it is good practice to verify that the process was successful. There are several ways to check.
5.1 Excitation Current Test
Perform an excitation current measurement on all three phases. If the core is properly demagnetized, the excitation current values should be balanced and consistent with previous baseline measurements. An unbalanced reading or abnormally high reading on one phase indicates residual magnetism may still be present.
5.2 Flux Measurement
Some advanced test equipment can measure the flux in the core directly using a fluxmeter or by integrating the voltage waveform across the winding. A near-zero DC flux component confirms successful demagnetization.
5.3 Comparison with Previous Test Records
Compare current test results with factory test reports or previous field test data. If the demagnetization was successful, the excitation current and other magnetic parameters should match the historical values within acceptable tolerance.
6. Practical Tips for Field Engineers
Here are some practical recommendations for transformer demagnetization that experienced field engineers follow.
- Always demagnetize after winding resistance testing. This is the number one rule. Winding resistance testing is performed on almost every transformer outage, and it injects DC current into the core. Make demagnetization a standard part of your test procedure.
- Demagnetize before performing excitation current tests. If you run excitation current tests on a magnetized core, the results will be misleading and may trigger unnecessary alarm.
- Use the correct winding. Apply the demagnetizing voltage or current to the winding with the lowest voltage rating. This requires less voltage from your equipment and is safer.
- Take your time. Rushing the demagnetization process by reducing the voltage too quickly will leave residual flux. A gradual reduction over 3 to 5 minutes is recommended for large power transformers.
- Document the process. Record that demagnetization was performed in your test report. Include the method used, the winding to which the signal was applied, and the duration of the process. This documentation is valuable for asset management and future reference.
7. Example: Complete Test and Demagnetization Sequence
Let us walk through a practical example of a complete testing and demagnetization sequence for a 66/11 kV, 20 MVA power transformer during a routine maintenance outage.
Step 1: Perform visual inspection and oil sampling.
Step 2: Perform insulation resistance testing using a 5 kV Megger on all windings.
Step 3: Perform winding resistance testing using a micro-ohmmeter with 10 A DC test current on all tap positions. After completing each phase measurement, reduce the DC current gradually before disconnecting.
Step 4: Immediately after winding resistance testing, perform demagnetization. Connect a variac to the 11 kV LV winding. Increase the AC voltage until the excitation current shows saturation (approximately 3 to 5 percent of rated current). Then gradually reduce the voltage to zero over 4 minutes.
Step 5: Verify demagnetization by performing an excitation current test. Compare results with factory test report values. Confirm balanced readings across all three phases.
Step 6: Proceed with turns ratio testing, sweep frequency response analysis, and other remaining tests.
Step 7: Document all test results, including the demagnetization step, in the final test report.
This sequence follows the recommendations in IEEE Std C57.152-2013 and ANSI/NETA MTS standards.
8. Relevant Industry Standards
8.1 IEEE Std C57.152-2013
This IEEE standard covers diagnostic testing of power transformers and includes recommendations for demagnetization after DC-based tests. Section references within this standard describe the need for demagnetization after winding resistance testing and provide general guidance on acceptable methods.
8.2 IEC 60076-1
The IEC standard for power transformers includes general requirements for testing and mentions the need to consider residual magnetism effects during routine and type tests.
8.3 ANSI/NETA MTS
The ANSI/NETA Maintenance Testing Specifications (MTS) standard, published by the InterNational Electrical Testing Association, provides maintenance testing procedures for electrical power equipment. It references the need for demagnetization after DC testing of transformers and includes demagnetization as a step in the recommended test sequence for power transformers.
8.5 ANSI C57.12.90
This standard covers test code procedures for liquid-immersed distribution, power, and regulating transformers. It addresses winding resistance measurements and the associated demagnetization requirements. Per this standard, demagnetization should be performed after any test that could leave residual magnetism in the core.
8.6 CIGRE Technical Brochures
CIGRE (International Council on Large Electric Systems) has published several technical brochures on transformer testing and diagnostics that include detailed discussions on demagnetization methods, measurement of residual flux, and the impact on transformer energization.
9. Impact on Power System Protection
Residual magnetism has a direct impact on transformer protection schemes. Differential protection relays (ANSI code 87T) are particularly sensitive to inrush currents caused by residual flux.
Modern numerical differential relays use second harmonic restraint or blocking to distinguish between magnetizing inrush and internal fault currents. The second harmonic component in normal inrush current is typically between 15 and 25 percent of the fundamental component. However, high residual flux can cause the inrush waveform to have a different harmonic signature, sometimes with a lower second harmonic ratio. This can cause the relay to interpret the inrush as a fault and trip the transformer.
Overcurrent relays (ANSI code 51) set on the transformer feeders can also operate during high inrush events if the current exceeds their pickup settings. Ground fault relays (ANSI code 51N/51G) may operate if the inrush current has an asymmetric zero-sequence component.
Proper demagnetization before re-energization eliminates these protection coordination issues and prevents unnecessary outages. For power system engineers working on relay coordination studies and transformer energization planning, accounting for residual flux conditions is an important part of the analysis.
10. Conclusion
Demagnetization of the transformer core after testing is a straightforward but often neglected procedure. Residual magnetism left in the core after DC-based tests like winding resistance measurement can cause excessive inrush currents, false relay operations, and inaccurate diagnostic test results. The process of demagnetization involves applying a decreasing alternating magnetic field to the core until the residual flux is reduced to near zero.
Make demagnetization a standard step in your transformer testing procedure. It takes only a few minutes and can prevent hours of troubleshooting protection relay trips and operational disruptions during transformer re-energization.
11. Frequently Asked Questions (FAQs)
The duration depends on the method used and the size of the transformer. For the variac method, the gradual voltage reduction should take 3 to 5 minutes for medium-sized power transformers (10 to 100 MVA). Automated demagnetization instruments can complete the process in 2 to 10 minutes. The DC polarity reversal method may take 15 to 30 minutes depending on the number of reversal steps.
Residual magnetism itself does not directly damage the transformer. However, the high inrush currents it causes during re-energization can create mechanical stresses on the windings and may lead to cumulative damage over many energization cycles.
Yes. Any test that injects DC current into the transformer winding can magnetize the core. Demagnetization should be performed after winding resistance testing, DC insulation tests, and any other test that applies DC to the windings.
Energizing at full voltage will reduce the residual flux over several AC cycles. However, the initial inrush current can be very high and may trip protection relays.
For the variac method, you need a variable AC source (autotransformer) with sufficient voltage and current rating. For the DC reversal method, you need a DC power supply with polarity reversal capability. Dedicated demagnetization instruments from manufacturers like Omicron, Megger, and DV Power provide automated solutions.
Three-phase simultaneous energization does not eliminate the inrush current problem caused by residual flux. Controlled switching (point-on-wave switching) can reduce inrush current, but it requires knowledge of the residual flux magnitude and polarity in each core limb.
The most practical way to check is by performing an excitation current test. If the three-phase excitation currents are unbalanced or higher than expected, the core likely has residual magnetism.
Yes. Distribution transformers can also retain residual magnetism after testing. The same principles apply, although the smaller size makes demagnetization easier.