Transformer Winding Resistance Testing in Transformer Failure Analysis

When a transformer fails, it’s essential to conduct a thorough failure analysis to determine the root cause and prevent similar incidents in the future. Transformer Winding Resistance Testing (TWRT) plays a pivotal role in transformer failure analysis by helping to detect issues related to winding damage, insulation breakdown, and mechanical deformation. In this article, we’ll explore how TWRT is used in transformer failure analysis, what types of faults it can detect, and why it’s such an indispensable tool for diagnosing failures.

Why Transformer Winding Resistance Testing is Essential in Failure Analysis

1. Identifying the Cause of Transformer Failure

When a transformer fails, it’s often not immediately clear what caused the failure. TWRT provides valuable insights by measuring the resistance of the transformer windings, helping to detect internal issues like short circuits, winding deformation, or loose connections. This test allows engineers to pinpoint the exact location and nature of the fault, guiding the overall failure investigation.

2. Detecting Subtle Issues That May Have Led to Failure

In many cases, transformer failures occur gradually due to the slow degradation of internal components like insulation or windings. These issues may not be visible externally but can be detected through changes in winding resistance. TWRT is highly sensitive to even small changes in resistance, making it an effective tool for identifying the early signs of failure that might otherwise go unnoticed.

Example: In a transformer that failed after experiencing intermittent outages, TWRT was performed during the failure analysis. The test revealed a slight increase in resistance in one of the phases, which indicated a slow breakdown of the insulation. This subtle change had led to a short circuit that eventually caused the transformer to fail.

Key Faults Detected by Transformer Winding Resistance Testing in Failure Analysis

1. Winding Short Circuits

One of the most serious issues that can be detected by TWRT is a winding short circuit. When the insulation between winding turns breaks down, it can cause two or more turns to short-circuit. This drastically reduces the effective length of the winding, causing overheating, arcing, and eventual transformer failure. TWRT can detect these short circuits by identifying sudden drops in resistance.

How TWRT Detects It:

• Sudden Drop in Resistance: When a winding short circuit occurs, the resistance of the affected phase decreases significantly due to the shortened electrical path. TWRT can detect these abnormal drops in resistance, allowing engineers to identify the phase and location of the fault.

Example: After a large transformer at an industrial facility failed, TWRT was performed to investigate the root cause. The test revealed a drastic drop in resistance in the secondary winding, indicating that a short circuit between two turns had led to the failure. By locating the fault, the team was able to confirm that insulation breakdown was the cause.

2. Loose or Deteriorated Connections

Loose connections between windings and terminals can cause intermittent faults, arcing, and heat buildup, leading to eventual transformer failure. TWRT is particularly effective at detecting these issues by revealing abnormal increases in resistance due to poor or deteriorated connections.

How TWRT Detects It:

• High Resistance in a Phase: Loose or deteriorating connections increase resistance because of poor contact between the windings and the terminal leads. TWRT measures this increase, alerting engineers to the need for further investigation into the connections.

Example: In a case where a transformer experienced a sudden failure after a period of abnormal performance, TWRT revealed an increase in resistance in one of the phases. Upon further inspection, it was discovered that a connection between the winding and the terminal had loosened over time, leading to arcing and eventual failure.

3. Winding Deformation Due to Mechanical Stress

Transformers are exposed to mechanical stresses, especially during short-circuit events or heavy load conditions. These stresses can deform the windings, shifting them out of position and increasing resistance. Over time, this mechanical deformation can weaken the transformer’s structure, leading to insulation failure or even internal arcing.

How TWRT Detects It:

• Gradual Increase in Resistance: Winding deformation typically causes a gradual increase in resistance as the mechanical stresses affect the windings over time. By comparing TWRT results to baseline data, engineers can detect these subtle increases and investigate further.

Example: After a transformer failure following a short-circuit event, TWRT was performed as part of the failure analysis. The test showed a slight but consistent increase in resistance in one phase compared to baseline readings taken when the transformer was new. Further inspection revealed that the windings had been mechanically deformed by the short circuit, compromising the insulation.

4. Open Circuits in the Windings

Open circuits occur when the electrical continuity of the winding is broken, often due to a fault or mechanical failure. This results in a complete or partial failure of the transformer to carry current through the affected winding. TWRT can identify open circuits by showing an infinite or abnormally high resistance value.

How TWRT Detects It:

• Infinite or High Resistance: When an open circuit is present in the winding, the resistance measured by TWRT will be abnormally high or infinite, indicating that electrical continuity has been lost.

Example: A transformer at a power plant failed after experiencing several power fluctuations. TWRT showed that one phase had infinite resistance, signaling a complete break in the winding. This open circuit was caused by a severe fault that physically damaged the winding, leading to the failure.

5. Insulation Deterioration

Insulation deterioration is one of the leading causes of transformer failure. Over time, insulation breaks down due to heat, moisture, or aging, leading to higher winding resistance and, ultimately, short circuits or winding failure. TWRT can detect early signs of insulation degradation by identifying increases in resistance over time.

How TWRT Detects It:

• Gradual Increase in Resistance: As insulation breaks down, the winding resistance increases because of heat and moisture ingress. Regular TWRT can track these changes over time, signaling when insulation is approaching a critical failure point.

Example: During failure analysis of a transformer that had been in service for 25 years, TWRT showed a gradual rise in winding resistance over the previous decade. This resistance increase indicated that the insulation was deteriorating due to aging and moisture exposure. The analysis concluded that insulation failure had led to an internal short circuit, causing the transformer to fail.

Steps to Perform Transformer Winding Resistance Testing During Failure Analysis

1. Ensure the Transformer is De-Energized

Before performing TWRT, make sure the transformer is completely de-energized. This step is crucial for safety and to avoid inaccurate test results.

2. Set Up Proper Connections

Connect the Kelvin (4-wire) test leads to the transformer’s terminals. Clean the terminals to ensure a solid connection, and secure the leads tightly to avoid errors due to poor contact.

3. Measure Temperature and Apply Compensation

Measure the ambient and winding temperature to ensure that resistance values are corrected for temperature variations. Temperature compensation ensures that the resistance values are normalized to a standard reference temperature.

4. Perform the Test

Run the winding resistance test on all phases of the transformer. Record the resistance values and compare them to the baseline data (if available) or to industry standards to detect abnormalities.

5. Analyze and Compare Results

Compare the resistance values across different phases and between the current readings and previous tests. Look for any signs of significant resistance changes that could indicate issues like short circuits, loose connections, or winding deformation.

6. Investigate Further Based on Findings

If TWRT reveals unusual resistance readings, perform further inspections or tests to identify the specific fault. In some cases, disassembly of the transformer may be required to visually inspect the windings and connections.

Transformer Winding Resistance Testing is an invaluable tool in transformer failure analysis, helping engineers pinpoint the root cause of failures and prevent future incidents. From detecting winding short circuits and loose connections to identifying insulation degradation and winding deformation, TWRT provides critical insights that guide the overall failure investigation.

By performing TWRT during failure analysis, you can detect subtle issues that might otherwise go unnoticed, ensuring that transformers are repaired correctly and that similar failures are avoided in the future. In my years of experience, TWRT has consistently proven to be an indispensable part of diagnosing transformer failures, saving time, money, and preventing future breakdowns.