The Ultimate 5-Step Guide to DC Winding Resistance Test

août 28, 2025

Abstract

The DC winding resistance test is a fundamental diagnostic procedure for assessing the integrity of power and distribution transformers. This examination measures the direct current resistance of the transformer's windings, providing critical insights into the condition of the internal conductors and connections. By applying a stable DC current and measuring the resulting voltage drop, technicians can calculate the resistance and compare it against manufacturer specifications, previous test results, and measurements from adjacent windings. Deviations from expected values can indicate a range of potential faults, including loose connections, broken conductor strands, high contact resistance in tap changers, or shorted turns. Proper execution and interpretation of this test are paramount for predictive maintenance programs, enabling the early detection of incipient faults, preventing catastrophic failures, and ensuring the long-term reliability of electrical power systems. The methodology requires careful temperature correction and an understanding of magnetic core saturation effects for accurate and repeatable results.

Key Takeaways

Always de-energize and ground the transformer before starting any test.
Use the four-wire (Kelvin) method for precise resistance measurements.
Allow readings to stabilize to counteract magnetic core inductance effects.
Perform a DC winding resistance test to detect winding and tap changer issues.
Correct all resistance readings to a common reference temperature for valid comparison.
Compare results between phases and against historical data for accurate analysis.
Document every test result meticulously for effective trend analysis.

Step 1: Understanding the Foundational Principles of Winding Resistance
Step 2: Assembling the Right Tools and Ensuring Safety
Step 3: Executing the DC Winding Resistance Test with Precision
Step 4: Interpreting the Data and Identifying Anomalies
Step 5: Advanced Diagnostics and Long-Term Management
Frequently Asked Questions
A Final Reflection on the Craft of Transformer Diagnostics
References

Step 1: Understanding the Foundational Principles of Winding Resistance

Before one can competently perform or interpret a DC winding resistance test, a deep appreciation for the underlying physical principles is necessary. This is not merely a procedural task of connecting leads and recording numbers; it is an interrogation of the transformer's most fundamental current-carrying paths. To understand the story the numbers tell, we must first learn the language in which it is written—the language of electrons moving through copper, of temperature's subtle influence, and of the hidden defects that disrupt this orderly flow. It is a dialogue with the physical heart of the transformer.

What is Winding Resistance and Why Does It Matter?

At its core, winding resistance is a deceptively simple concept, rooted in Ohm's Law. It is the measure of opposition to the flow of direct current (DC) through the transformer's windings. Imagine the winding as an immensely long, coiled copper wire. Just as a long, narrow pipe offers more resistance to water flow than a short, wide one, this long copper path presents a specific, measurable opposition to electrical current. The measurement itself involves applying a known DC voltage and measuring the resulting current, or more commonly, injecting a stable DC current and measuring the voltage drop across the winding. The resistance (R) is then a simple calculation: R = V/I (Voltage divided by Current), as described by Electrom Instruments (electrominst.com).

Why, though, do we concern ourselves with this specific property? The answer lies in its diagnostic sensitivity. The resistance of a winding is determined by the material of the conductor (almost always copper or aluminum), its length, and its cross-sectional area. These are design parameters, fixed at the time of manufacture. Therefore, any significant change in the measured resistance post-manufacture points to a physical change—and likely a degradation—within the winding structure. It is a direct health check on the transformer's circulatory system. A stable, correct resistance value suggests that the conductive pathways are intact and as the manufacturer intended. An incorrect value, however, is a symptom of an underlying pathology that demands investigation. It is one of the most powerful tools in our arsenal for peering inside the sealed tank of a transformer without physically dismantling it.

The Diagnostic Power: What Problems Can This Test Uncover?

The true value of the DC winding resistance test emerges when we consider the specific faults it can reveal. Its diagnostic reach is extensive, touching upon several critical aspects of a transformer's internal health. Think of it as a physician using a stethoscope to listen for irregularities in a patient's heartbeat.

First and foremost, it is exceptionally good at detecting problems with connections. This includes both internal connections, such as the crimped or brazed joints that connect winding sections, and external connections at the bushings and tap changer contacts. A loose or corroded connection introduces an extra, unwanted resistance into the circuit. This manifests as a higher-than-expected reading. Such a high-resistance joint can become a hot spot under load, leading to localized overheating, oil degradation, and eventually, a catastrophic failure.

Second, the test is sensitive to physical damage to the conductors themselves. If some of the parallel strands that make up a large winding conductor are broken, the overall cross-sectional area for current flow is reduced. This reduction forces the same amount of current through a smaller path, resulting in a higher measured resistance. This can happen due to mechanical stresses during shipping, installation, or from immense electromagnetic forces during a through-fault event.

Third, the DC winding resistance test is indispensable for assessing the health of tap changers, both on-load (OLTC) and de-energized (DETC). Each tap position represents a slightly different length of the winding, and therefore should have a predictable resistance value. By measuring the resistance at each tap position, one can verify the correct operation of the tap changer. A high contact resistance on a specific tap, a common problem in aging OLTCs, will show up as a clear anomaly in the readings. Open circuits within the tap changer assembly are also immediately obvious.

Finally, while less common, the test can sometimes provide an indication of severe short-circuited turns within a winding, particularly if a significant portion of the winding is shorted. This would cause a noticeable decrease in the overall resistance. However, for detecting a small number of shorted turns, other tests like sweep frequency response analysis (SFRA) or turns ratio testing are generally more sensitive. The power of the winding resistance measurement lies in its direct assessment of the conductor's continuity and quality.

A Matter of Temperature: The Correction Factor Explained

One of the most profound and often misunderstood aspects of this measurement is the influence of temperature. The electrical resistance of a conductor is not a static property; it is intrinsically linked to its temperature. For metals like copper and aluminum, resistance increases as temperature rises. Think of it in terms of atomic vibration. At higher temperatures, the atoms within the metal lattice vibrate more vigorously, creating more "obstacles" for the flowing electrons to navigate, thereby increasing resistance.

This relationship is not trivial; it is significant enough to render uncorrected measurements almost useless for comparative analysis. A winding resistance measurement taken on a cool morning at 15°C will be substantially different from a measurement of the same winding taken on a hot afternoon at 35°C, even if the winding's physical condition is identical. To make a meaningful comparison between a new reading and a factory test value (which was taken at a specific temperature) or a previous year's reading, we must translate them into a common language. We must correct them to a standard reference temperature.

This is done using a well-established formula:

R_adj = R_m * [(T_s + T_k) / (T_m + T_k)]

Where:

R_adj is the temperature-corrected resistance.
R_m is the measured resistance.
T_s is the standard reference temperature (e.g., 75°C or 85°C, depending on the standard).
T_m is the temperature of the winding when the measurement was taken.
T_k is a constant for the conductor material (234.5°C for copper, 225°C for aluminum).

This mathematical correction allows us to compare apples to apples. It removes the variable of temperature from the equation, so that any remaining difference between the corrected reading and the benchmark value can be attributed to a genuine change in the winding's physical condition. Accurate measurement of the winding temperature at the time of the test is therefore not an optional step; it is as fundamental as the resistance measurement itself. Typically, the top oil temperature is used as an approximation, after allowing the transformer to stabilize thermally. Without this correction, a technician might chase a "problem" that is merely a reflection of a hot day, or worse, miss a developing fault that is being masked by a cold one.

Step 2: Assembling the Right Tools and Ensuring Safety

Having grasped the theoretical underpinnings, we now move to the practical realm of preparation. The success and safety of a DC winding resistance test depend entirely on the groundwork laid before the first lead is ever connected. This involves a thoughtful selection of equipment and an unwavering commitment to a rigorous safety protocol. To proceed without this diligence is to risk not only inaccurate data but also severe personal injury and equipment damage. This stage sets the scene for a professional and effective diagnostic procedure.

Selecting Your Weapon: The Modern Winding Resistance Tester

The days of using simple multimeters or improvised bridge circuits for this task are long gone. The unique challenges of measuring very low resistances in a highly inductive environment demand specialized equipment. A modern, high-quality DC winding resistance tester is an investment in accuracy and efficiency. These instruments are specifically designed to overcome the hurdles presented by a large power transformer.

What should one look for in such an instrument? First, it must be capable of delivering a stable, ripple-free DC test current. The magnitude of this current is important; it needs to be high enough to produce a measurable voltage drop but not so high as to cause a significant temperature increase in the winding during the test (a general rule is to keep the test current below 15% of the winding's rated current). Instruments offering selectable test currents, perhaps from 1 A to 50 A or even higher for very large transformers, provide the necessary flexibility.

Second, the instrument must have a high-resolution voltage measurement circuit. We are often measuring resistances in the milliohm (mΩ) or even micro-ohm (μΩ) range. This requires a voltmeter that can accurately resolve very small potential differences.

Third, and perhaps most critically for efficiency, is the instrument's ability to handle the transformer's inductance. When DC current is first applied, the transformer's large iron core creates a powerful magnetic field, which induces a back-electromotive force (back-EMF) that opposes the current. This is why the reading takes time to stabilize. Modern testers employ advanced circuitry to saturate the core faster, significantly reducing this stabilization time. Many also feature simultaneous winding magnetization, allowing for the testing of multiple windings more quickly. When looking for a comprehensive suite of diagnostic tools, one might consider a range of testeurs de transformateurs that includes not just winding resistance meters but also insulation resistance testers and hipot testers, forming a complete health assessment package.

Table 1: Safety Checklist Before a DC Winding Resistance Test

Step	Action	Rationale
1	Verify De-energization	Confirm that the transformer is completely disconnected from all external power sources (both high and low voltage sides). Use a voltage detector.
2	Apply Safety Grounds	Connect working grounds to all incoming and outgoing terminals. This protects against accidental re-energization and induced static charges.
3	Review Lockout/Tagout (LOTO)	Ensure that all switches and disconnects are properly locked and tagged according to established site procedures.
4	Establish Exclusion Zone	Cordon off the test area to prevent unauthorized personnel from entering.
5	Inspect Test Equipment	Check test leads for damage (cracked insulation, frayed wires) and ensure the instrument itself is in good working order.
6	Wear Appropriate PPE	Use personal protective equipment including safety glasses, hard hat, and appropriate voltage-rated gloves if required by site policy.
7	Confirm Discharge Path	Understand that the tester will charge the winding. Know the instrument's discharge procedure and never disconnect leads until it signals safe.

The Safety Protocol: A Non-Negotiable Prerequisite

No amount of data is worth a human life. The environment around a power transformer, even when de-energized, holds potential dangers that must be respected. The most significant hazard during a DC winding resistance test is the stored magnetic energy. The transformer winding, when energized with DC current, becomes a powerful inductor. This stored energy must be safely dissipated before test leads can be disconnected.

The cardinal rule is to never, under any circumstances, disconnect the current-carrying leads while the test is in progress or before the instrument has completed its discharge cycle. Doing so would interrupt the current path through the inductor, causing the magnetic field to collapse rapidly. This rapid collapse induces a dangerously high voltage spike (V = L * di/dt), which can be thousands of volts. This voltage can cause a severe electric arc, posing an extreme risk of electrocution to the operator and potentially damaging the test instrument and the transformer's insulation.

Modern testing instruments are designed with safety in mind. They have internal discharge circuits that automatically and safely dissipate the stored energy after the test is complete. As noted by DV-Power (dv-power.com), these instruments typically have a visual indicator, such as a light or an on-screen message, that explicitly states when it is safe to remove the leads. Waiting for this "all-clear" signal is not optional; it is a critical safety step. The discharge time for large transformers can be several minutes, demanding patience from the test personnel. Rushing this process is a gamble with lethal stakes.

Beyond the hazard of stored energy, standard electrical safety practices are paramount. The transformer must be completely isolated from the power grid, and all terminals must be securely grounded before any work begins. This grounding protects against static charge build-up and the possibility of accidental re-energization from a remote location.

Preparing the Transformer for Testing

With the right equipment on hand and a firm grasp of the safety protocols, the final preparatory step is to ready the transformer itself. The goal is to create a stable and clean environment for the measurement.

First, the transformer should be in a thermally stable state. As discussed, temperature is a critical variable. If the transformer has recently been in service, it must be allowed to cool down until the winding temperature stabilizes. The oil temperature should be monitored, and testing should only commence after the temperature has remained constant for a period (e.g., an hour). This ensures that the temperature reading you take is a true representation of the winding temperature. Taking a measurement while the transformer is still rapidly cooling will lead to inconsistent and unreliable results.

Second, the connection points—the transformer bushings—must be clean and free of any dirt, grease, or oxidation. A poor connection between the test lead's clamp and the bushing terminal introduces an unwanted contact resistance. This extra resistance will be included in the measurement, leading to an erroneously high reading. The terminals should be wiped clean with a lint-free cloth and, if necessary, a suitable solvent. The contact surfaces of the test clamps should also be inspected and cleaned.

Finally, if the transformer has a de-energized tap changer (DETC), its position must be known and set to the desired tap for the test. It is crucial to ensure the DETC is properly seated on its contacts. It is good practice to cycle the tap changer through all its positions and back to the desired test position to wipe the contacts and ensure a good connection before starting the measurement. For transformers with on-load tap changers (OLTCs), the test will typically be performed on the principal tap and then on all other taps to verify the entire range of operation.

Step 3: Executing the DC Winding Resistance Test with Precision

With the foundational knowledge established and all preparations meticulously completed, we arrive at the heart of the process: the execution of the measurement itself. This is where theory meets practice, and where careful technique separates a meaningful diagnostic result from a misleading number. Precision, patience, and a methodical approach are the guiding virtues in this phase. The goal is to obtain a true and repeatable representation of the winding's resistance, free from the influence of procedural errors.

The Connection Conundrum: Correct Lead Placement

The single most important technical aspect of obtaining an accurate low-resistance measurement is the use of a four-wire connection, often called the Kelvin method. To understand why this is so critical, let's consider the alternative. If we were to use a simple two-wire method (like a standard multimeter), the instrument would measure the total resistance of the circuit, which includes not only the transformer winding but also the resistance of the test leads themselves and the contact resistance where the clips attach to the bushings. Since the winding resistance we are trying to measure is often extremely low (in the milliohm range), the resistance of the leads and contacts can be of a similar magnitude, introducing a very large error.

The Kelvin method elegantly solves this problem. It uses four separate leads: two for injecting the current and two for measuring the voltage.

Current Leads (I+, I-): These are the larger, heavier leads that carry the test current. They are connected to the transformer terminals, and the current flows from the tester, through the winding, and back.
Potential Leads (V+, V-): These are smaller leads that connect inside the current clamps, as close as possible to the transformer terminals. Their job is simply to measure the voltage drop directly across the winding.

Because the voltage measurement circuit in a modern tester has a very high internal impedance, almost no current flows through the potential leads. Since there is negligible current, the voltage drop across the potential leads themselves is virtually zero (V = I*R, and I ≈ 0). This means the instrument measures *only* the voltage drop across the transformer winding, completely excluding the resistance of the current leads and the contact resistance of the clamps. This technique is fundamental to achieving the accuracy required for a DC winding resistance test. Ensuring the potential clips make a clean, solid connection directly to the bushing stud, inside of where the main current clamps are attached, is a hallmark of a professional test setup.

The Test Current: How Much is Enough?

Selecting the appropriate test current is a balancing act. On one hand, a higher current is generally better. According to Ohm's Law (V = I*R), a higher current (I) will produce a larger voltage drop (V) for a given resistance (R). A larger voltage signal is easier for the instrument to measure accurately and is less susceptible to noise from stray electrical fields in the substation environment. A higher current also helps to "punch through" any minor oxidation or film on internal connections, providing a more representative reading of the true metallic contact.

On the other hand, a current that is too high can be detrimental. The flow of current through a resistance generates heat (P = I²R). If the test current is excessive, it can begin to heat the winding during the measurement process. As we have established, a change in temperature will cause a change in resistance, leading to a drifting, unstable reading and an inaccurate final result. The goal is to measure the resistance at a stable, known temperature, not to perform a temperature-run test.

A widely accepted guideline is to choose a test current that is less than 15% of the winding's rated AC current. For most transformers, a test current between 1 and 10 Amperes is sufficient. For very large generator step-up (GSU) transformers with extremely low winding resistances, currents of 50 A, 100 A, or even higher might be necessary to generate a measurable voltage drop. A good DC resistance tester will offer a range of selectable currents, allowing the technician to choose the optimal value for the specific transformer being tested. The key is to use a current that is high enough for a stable, strong signal but low enough to avoid heating effects.

Table 2: Common Connection Schemes for Three-Phase Transformers

Winding Config.	Test 1 (Phase A)	Test 2 (Phase B)	Test 3 (Phase C)	Typical Comparison
Delta (Δ)	H1 to H2	H2 to H3	H3 to H1	Compare A-B, B-C, C-A. A-B reading is Phase A winding in parallel with (Phase B + Phase C) windings.
Wye (Y) – Ungrounded	H1 to H2	H2 to H3	H1 to H3	Compare A-B, B-C. A-C is a check. A-B reading is Phase A + Phase B windings in series.
Wye (Y) – Grounded Neutral (H0)	H1 to H0	H2 to H0	H3 to H0	Compare A-N, B-N, C-N directly. Readings should be very close.

The Waiting Game: Achieving a Stable Reading

Perhaps the most common source of frustration for an inexperienced technician is the time it takes for the resistance reading to stabilize. After the current is applied, the reading on the meter will start low and slowly climb, only settling on a final value after a period that can range from a few seconds to many minutes. This is not a fault of the instrument; it is a direct consequence of the physics of the transformer core.

A transformer is a massive inductor. When a DC current is injected, it begins to build a magnetic field in the iron core. This changing magnetic field, in turn, induces a voltage (a back-EMF) that opposes the applied current. Initially, much of the energy from the test set is spent on building this magnetic field, and the current rises slowly. The instrument, calculating resistance based on the instantaneous current and voltage, shows a reading that changes as the current changes.

The reading will only become stable once the iron core is fully saturated with magnetic flux and the current has reached its steady-state DC value. At this point, the inductive effects cease, and the instrument is measuring pure DC resistance. The time this takes depends on the size of the transformer core and the winding's time constant (L/R). Large transformers with large cores have a very long time constant.

Patience is the only solution. The measurement must not be recorded until the instrument's reading is completely stable, meaning it is no longer changing significantly over a period of time (e.g., less than 1% change over 30 seconds). Modern testers help by using higher voltages to saturate the core more quickly and by employing sophisticated algorithms to predict the final value. Nevertheless, the technician must always wait for true stability before recording the result. Cutting this waiting period short is one of the most common sources of error in a DC winding resistance test.

Step-by-Step Procedure for a Three-Phase Transformer

Executing the test on a three-phase transformer requires a methodical sequence to ensure all windings are measured correctly and the results are comparable. The exact connection points depend on whether the winding is connected in a Delta (Δ) or Wye (Y) configuration.

Setup and Safety: Perform all safety checks as per the checklist. Connect the test set to a reliable power source. Connect the test leads to the instrument.
Connect for the First Measurement: Following the Kelvin principle, connect the current and potential leads to the first pair of bushings to be tested (e.g., H1 and H2 on a Wye-connected high-voltage winding). Ensure the potential leads are inside the current leads.
Set Test Parameters: Select the appropriate test current on the instrument.
Initiate the Test: Start the test. The instrument will apply the DC current.
Wait for Stability: Monitor the resistance reading. Wait patiently until the value is completely stable. This may take several minutes.
Record the Reading: Once stable, record the resistance value. Also, critically, record the oil temperature at the time of the measurement.
Discharge: Stop the test. The instrument will automatically begin its discharge cycle. DO NOT disconnect any leads.
Wait for Safe Signal: Wait until the instrument indicates that the discharge is complete and it is safe to handle the leads.
Reconfigure for Next Measurement: Safely disconnect and reconfigure the leads for the next measurement (e.g., H2 to H3).
Repeat: Repeat steps 4 through 9 for all required winding and tap changer positions. For a Wye winding, you would typically measure H1-H2, H2-H3, and H3-H1 (or H1-N, H2-N, H3-N if the neutral is available). For a Delta winding, you measure H1-H2, H2-H3, and H3-H1.

This systematic process ensures that each measurement is taken under the same conditions, providing a set of data that is reliable, repeatable, and ready for the crucial step of interpretation.

Step 4: Interpreting the Data and Identifying Anomalies

Obtaining a set of precise, stable resistance readings is only half the battle. The true skill in performing a DC winding resistance test lies in the interpretation of these numbers. A resistance value, in isolation, means very little. Its significance is unlocked only through comparison—comparison against a benchmark, against its siblings, and against its own past. This is the diagnostic phase, where the raw data is translated into actionable intelligence about the transformer's health. It is less a science of absolute numbers and more an art of recognizing meaningful patterns and deviations.

Establishing a Baseline: The Importance of Factory and Historical Data

The most powerful form of analysis is trend analysis over time. The single best benchmark for any winding's resistance is its own previous test history. Every transformer is unique, with slight variations in manufacturing. Therefore, the most reliable baseline is the data collected during factory acceptance testing (FAT). These initial resistance values, taken when the transformer was brand new and in perfect condition, are the "birth certificate" against which all future measurements should be judged.

When performing a test, the first step in interpretation is to correct the newly measured value to the same reference temperature as the factory data (e.g., 75°C). The corrected field measurement can then be directly compared to the factory value. A significant deviation, often considered to be more than 5%, is a red flag that warrants further investigation.

In the absence of factory data, the results from the commissioning test (the first test performed after installation but before energization) become the primary baseline. If neither is available, the first test you perform becomes the baseline for all future tests. This underscores the profound importance of meticulous record-keeping. As noted by industry experts, maintaining detailed records is a cornerstone of effective transformer diagnostics (powertransformerhealth.com). Without historical data, you are flying blind, able to spot only gross, immediate problems rather than subtle, long-term degradation. A robust asset management strategy, similar to those advocated by industrial service leaders like SKF, relies heavily on such historical trend data for predictive maintenance.

The Art of Comparison: Analyzing Phase-to-Phase Consistency

What if you have no historical data for a particular transformer? Are the measurements useless? Not at all. A three-phase transformer provides a wonderful built-in reference: its own phases. The three separate windings of a three-phase transformer are designed to be nearly identical. Therefore, their DC resistance values, when measured under the same conditions, should be very close to one another.

After completing the measurements for all three phases (e.g., H1-N, H2-N, H3-N for a Wye winding), the next step is to compare them. The expectation is that the values should be tightly clustered. A common rule of thumb is that the resistance values for the three phases should agree with each other to within 2-3%. Some standards may allow for up to a 5% difference, but a tighter agreement is more desirable.

Let's consider an example. Suppose for a Wye-connected winding, you measure the following (already corrected for temperature):

Phase A (H1-N): 12.4 mΩ
Phase B (H2-N): 12.5 mΩ
Phase C (H3-N): 12.3 mΩ

These values are all within approximately 1% of each other. This is an excellent result and provides high confidence that the windings are in good condition relative to each other.

Now, consider a different result:

Phase A (H1-N): 12.4 mΩ
Phase B (H2-N): 13.8 mΩ
Phase C (H3-N): 12.5 mΩ

Here, the resistance of Phase B is more than 10% higher than the other two phases. This is a significant anomaly. It immediately directs your attention to Phase B as having a potential problem, such as a poor internal connection, a damaged conductor, or a high-resistance contact in the tap changer for that phase. This comparative analysis is a powerful tool, even in the absence of historical benchmarks.

Red Flags: Recognizing Signs of Trouble

During interpretation, the analyst is essentially a detective looking for clues. Certain patterns in the data are classic "red flags" that point toward specific types of problems.

High and Stable Resistance: If one phase measures significantly higher than the others or its own historical values, but the reading is stable, this often points to a fixed issue in the current path. This could be a poorly made crimp connection, a partially failed brazed joint, or significant contact degradation in a de-energized tap changer. The problem is consistent and adds a fixed series resistance to the circuit.
Unstable or Fluctuating Resistance: A reading that will not stabilize and continues to wander or jump around, even after a long waiting period, is a serious warning sign. This often indicates a "make-and-break" connection. The test current may be causing slight thermal expansion or magnetic forces that are making a loose connection shift, changing its resistance moment by moment. This is a critical defect that can lead to severe arcing and failure under load.
Low Resistance: A resistance reading that is significantly lower than the baseline or the other phases is less common but can indicate shorted turns in the winding. A short circuit creates a parallel path for the current, effectively shortening the overall length of the conductor and thus lowering its total resistance.
Open Circuit: In the most extreme case, if the instrument is unable to pass any current and indicates an infinite or "open" resistance, it means there is a complete break in the winding or its associated connections. This could be a failed lead, a broken tap changer finger, or a burned-off conductor.

Recognizing these patterns allows the technician to move from simply collecting data to forming a diagnostic hypothesis about the nature and location of the potential fault.

The Role of Tap Changers in Resistance Measurement

Tap changers are a frequent source of problems in transformers, and the DC winding resistance test is one of the best ways to assess their condition. Both De-Energized Tap Changers (DETCs) and On-Load Tap Changers (OLTCs) are mechanical devices with moving contacts that can wear, corrode, and fail over time.

For a DETC, the resistance should be measured on the principal tap and compared to historical data. If there is a suspicion of a problem, it can be beneficial to measure the resistance on all tap positions. The resistance should change in a predictable, incremental way as you move from one end of the tap range to the other. A sudden jump or an anomalously high reading on one specific tap points directly to a problem with that tap's contact.

For an OLTC, the test is even more critical. The resistance should be measured on every single tap position. As the OLTC moves from one tap to the next, the resistance should change smoothly. Any significant spike or deviation on a particular tap indicates high contact resistance, which can cause severe overheating during operation. This is one of the primary methods for determining when an OLTC requires internal maintenance. The test verifies not only the static contacts but also the integrity of the diverter switches and transition resistors that are part of the complex OLTC mechanism. A thorough DC winding resistance test across all taps provides a comprehensive health report of this critical and vulnerable component.

Step 5: Advanced Diagnostics and Long-Term Management

Mastering the standard DC winding resistance test is the foundation of transformer conductor assessment. However, the field of diagnostics is ever-evolving. To achieve the highest level of asset management, we must look beyond the static test, integrate its findings into a holistic maintenance philosophy, and leverage technology for predictive insights. This final step is about transforming a single test result into a long-term strategy for reliability, moving from a reactive to a proactive and predictive mindset.

Beyond the Basics: Dynamic Winding Resistance Measurement

While the static DC resistance test is excellent for checking the condition of tap changer contacts when they are stationary, it does not provide information about their performance during the actual switching operation. For On-Load Tap Changers (OLTCs), the transition from one tap to another is a complex, rapid mechanical event involving diverter switches and transition resistors. Problems during this transition, such as open circuits or timing issues, can cause arcing and significant stress on the transformer.

This is where Dynamic Winding Resistance Measurement (DWRM), sometimes called Dynamic Resistance Measurement (DRM), comes into play. DWRM is an advanced application of the winding resistance test. In this test, a DC current is circulated through the winding, and the instrument records the current (or a voltage drop) as a high-speed graph while the OLTC is operated through its tap positions.

The resulting graph, or "signature," provides a wealth of information. A healthy OLTC will produce a clean and repeatable signature for each tap change. Deviations from this signature can reveal problems that a static test would miss. For example, a momentary dip in the current during the transition indicates an open circuit, a condition known as "make-before-break" failure, which can be catastrophic. The duration and shape of the transition ripple can also indicate the condition of the transition resistors. By comparing the dynamic signature of each tap change against the others and against previous tests, technicians can diagnose subtle mechanical and electrical problems within the OLTC mechanism before they lead to failure. It is a powerful proactive tool for OLTC maintenance.

Integrating Winding Resistance into a Comprehensive Maintenance Strategy

The DC winding resistance test, as powerful as it is, should not be viewed in isolation. It is one chapter in the comprehensive health story of a transformer. Its true diagnostic power is amplified when its results are correlated with findings from other electrical tests. A holistic maintenance strategy involves a suite of tests, each providing a different piece of the puzzle.

For instance, a high winding resistance reading might be corroborated by a high hot-spot temperature on a dissolved gas analysis (DGA) report, suggesting a problematic connection is overheating. An anomaly found during a winding resistance test might prompt a more detailed investigation using Sweep Frequency Response Analysis (SFRA) to check for winding deformation. Similarly, results from insulation resistance testers and power factor (tan delta) tests provide information about the health of the insulation system, which is distinct from but related to the conductor system. A problem in one system can often lead to problems in the other.

A comprehensive strategy also includes tests for other substation components. The health of the transformer is linked to the health of the entire system. This means periodic testing of circuit breakers and protective relays using specialized relay protection testers is essential to ensure the transformer is protected from damaging external events. Likewise, ensuring the integrity of the cables connecting to the transformer, perhaps using a cable fault locator, is part of a complete asset management plan. By integrating the findings from all these tests, a much clearer and more reliable picture of the overall system health emerges.

The Value of Trend Analysis and Predictive Maintenance

The ultimate goal of modern asset management is to move away from time-based maintenance (performing tasks on a fixed schedule) and reactive maintenance (fixing things after they break) towards predictive maintenance (intervening just before a failure is likely to occur). The DC winding resistance test is a cornerstone of this philosophy.

This is achieved through diligent trend analysis. By recording the corrected resistance values from every test over the transformer's life, a trend line can be established. A healthy winding should show a very flat trend, with only minor fluctuations. A gradual, consistent increase in resistance over several years might indicate slow degradation of a connection or general aging. A sudden, sharp increase from one test to the next is a clear alarm that a significant event has occurred—perhaps mechanical damage from a through-fault—that requires immediate attention.

This historical perspective allows engineers to predict the future. By extrapolating a negative trend, it may be possible to estimate when a winding's resistance will exceed acceptable limits, allowing for a planned outage and repair before an unplanned, catastrophic failure occurs. This predictive capability saves money, increases safety, and dramatically improves grid reliability. It transforms the test from a simple pass/fail check into a powerful tool for forecasting and risk management.

Documenting for the Future: Best Practices in Record-Keeping

None of this advanced analysis is possible without one final, crucial discipline: meticulous documentation. A test result without context is useless to the next person who comes along. A complete test report for a DC winding resistance test is the legacy you leave for the future health of that asset.

A best-practice report should include, at a minimum:

Asset Information: Transformer ID, serial number, location.
Test Conditions: Date and time of the test, weather conditions.
Winding Temperature: The measured oil temperature and the method used to determine it.
Test Equipment: The make, model, and serial number of the winding resistance tester used.
Raw Data: A clear table of all measured resistance values, for each phase, on each tap position, for both HV and LV windings.
Corrected Data: A corresponding table of all resistance values corrected to the standard reference temperature, clearly stating the reference temperature and formula used.
Analysis: A comparison of the results against factory data, previous tests, and between phases. Any deviations outside of acceptable limits (e.g., >2% between phases) should be highlighted.
Technician's Notes: Observations made during the test, such as difficulty stabilizing a reading, unusual sounds from the tap changer, or the condition of the bushings.
Recommendations: A clear conclusion about the condition of the windings and tap changer, with recommendations for any further investigation or maintenance actions.

This level of detailed documentation, as advocated by sources like Power Transformer Health (powertransformerhealth.com), ensures that each test builds upon the last, creating a rich historical database that is the true foundation of a successful long-term transformer management program.

Frequently Asked Questions

Why does the resistance reading take so long to stabilize? The slow stabilization is due to the transformer's high inductance. When DC current is applied, it must first build a large magnetic field in the iron core. This process induces a back-voltage that opposes the current, causing it to rise slowly. The resistance reading only becomes stable once the core is magnetically saturated and the current reaches its steady-state value. This can take several minutes for large transformers.

What is the Kelvin (four-wire) method? The Kelvin method is a technique for measuring low resistance accurately. It uses four wires: two to inject the test current and two separate wires to measure the voltage drop directly across the component being tested. This eliminates the resistance of the test leads and their contact points from the measurement, which is critical when measuring the very low milliohm values of transformer windings.

How often should a DC winding resistance test be performed? There is no single answer, as it depends on the age, criticality, and condition of the transformer. However, a common practice is to perform the test during factory acceptance, at commissioning, and then as part of a regular maintenance cycle every 3-5 years. It should also be performed after any significant event, such as a major through-fault, or if other tests like DGA suggest an internal problem.

What is an acceptable percentage difference between windings? For the three windings in a three-phase transformer, the resistance values (after temperature correction) should be very close. A difference of less than 2% between the highest and lowest reading is considered very good. Differences between 2% and 5% may be acceptable but warrant closer monitoring. A difference greater than 5% typically indicates a problem that requires investigation.

Can I use a standard multimeter for this test? No. A standard multimeter is not suitable for this test for two main reasons. First, it uses a two-wire method, which introduces significant error from lead and contact resistance. Second, it cannot supply the high, stable DC current needed to overcome the transformer's inductance and get a reliable reading. A specialized winding resistance ohmmeter is required.

Why is temperature correction so important? The resistance of copper changes significantly with temperature. Without correcting all measurements to a common reference temperature, it is impossible to make a valid comparison between a new test and historical data or factory specifications. An uncorrected reading could falsely indicate a problem on a hot day or mask a real problem on a cold day.

What does a high resistance reading indicate? A high resistance reading typically points to a problem in the current-carrying path. This could be a loose or corroded internal connection, a high-resistance contact in a tap changer, or several broken strands within the main winding conductor. It is a key indicator of a potential hot spot and future failure point.

A Final Reflection on the Craft of Transformer Diagnostics

The DC winding resistance test, in its essence, is a conversation with the transformer. It is an inquiry into the health of its most fundamental pathways. Through the careful application of current and the patient observation of voltage, we can discern the subtle whispers of degradation long before they become a roar of catastrophic failure. It is a procedure grounded in the elegant simplicity of Ohm's Law, yet its proper execution and interpretation demand a depth of understanding, a respect for safety, and a meticulous attention to detail that elevates it to a craft. In a world increasingly reliant on a stable supply of electric power, the skill to perform this test correctly is not just a technical competency; it is a vital contribution to the resilience and reliability of our shared infrastructure. It is a testament to the idea that by understanding the small things—the milliohms of resistance in a coil of wire—we can protect and preserve the largest and most critical of systems.

References

DV-Power. (2022). Why do we measure transformer winding resistance?. https://www.dv-power.com/why-do-we-measure-transformer-winding-resistance/

Electrom Instruments. (2023). Winding resistance measurement method. https://electrominst.com/test-technology/winding-resistance/

Power Transformer Health. (2025). Winding resistance test of a power transformer. https://powertransformerhealth.com/2025/06/01/winding-resistance-test-of-a-power-transformer/