Power Transformer Health Assessments

Power transformers are among the most expensive parts of the electrical network. Testing a power transformer is important in order to correctly assess the condition of the unit and to detect incipient faults at an early stage. The objective is to ensure safe operation and minimize the risk of outages. With a careful asset management program, it will be possible to extend the life of the power transformer while maintaining reliability.

Winding Resistance

 The winding resistance test of a transformer serves several crucial purposes:

  1. Quality Assurance and Type Testing:
    • During manufacturing, the winding resistance test ensures that the transformer’s windings have been correctly wound and that there are no short circuits or open circuits.
    • It verifies the quality of the transformer before it is put into service.
  2. Routine Maintenance and Field Testing:
    • Regularly testing the winding resistance helps monitor the health of the transformer during its operational life.
    • It detects issues such as loose connections, broken conductor strands, or high contact resistance in tap changers and bushings.
    • Deviations from expected resistance values can indicate problems that need attention.
  3. Assessment of Transformer Performance:
    • The winding resistance test provides information about the overall condition of the transformer.
    • Abnormal resistance values may indicate internal faults or degradation.
  4. Temperature Correction:
    • The winding resistance measurement is used to estimate the winding temperature.
    • Knowing the winding temperature is essential for assessing the transformer’s loading capacity and ensuring safe operation.

In summary, the winding resistance test is a fundamental diagnostic tool that helps maintain transformer reliability and prevent unexpected failures.

A winding resistance test for a transformer is essential for assessing the quality and condition of its windings, tap changer connections, and copper losses. Let’s dive into the details of how to perform this test:

  1. Purpose of the Winding Resistance Test:
    • Calculate the I²R lossesin the transformer.
    • Determine the winding temperature at the end of the temperature rise test.
    • Serve as a benchmark for assessing potential damages in the field.
  2. Measurement Procedure:
    • For star-connected windings, measure the resistance between the line and neutral terminals.
    • For star-connected autotransformers, measure the resistance of the HV side between HV and IV terminals, then between IV and neutral.
    • For delta-connected windings, measure the resistance between pairs of line terminals. Since individual winding resistance cannot be measured separately in delta connection, calculate the resistance per winding using the formula:
      • Resistance per winding = 1.5 × Measured value
  1. Temperature Considerations:
    • Transformer windings are typically immersed in insulation liquid and covered with paper insulation, making it impossible to directly measure the winding temperature during the test.
    • An approximation is used to calculate the winding temperature:
      • Temperature of winding = Average temperature of insulating oil
    • The average oil temperature should be taken 3 to 8 hours after de-energizing the transformer when the difference between top and bottom oil temperatures is less than 5°C.
  2. Measurement Methods:
    • Voltmeter-Ammeter Method:
      • Measure resistance using a simple voltmeter and ammeter.
      • Caution: Current should not exceed 15% of the rated current of the winding to avoid inaccuracies due to heating.
    • Current-Voltage Method:
      • Inject DC current into the winding and measure the corresponding voltage drop.
      • Apply Ohm’s law (R = V / I) to determine the resistance.

Remember to perform the winding resistance test at each tap. 

Demagnetization

When a transformer trips off-line, or after applying DC test signals in, for example, a winding resistance test, the transformer core remains magnetized. As this can cause issues for further tests, international standards and guides recommend demagnetization before doing excitation current and SFRA measurements. It is also recommended to perform demagnetization before the transformer is taken into service, thus avoiding unnecessary high in-rush currents.

Demagnetization of a power transformer is a crucial process to maintain its optimal performance and longevity. Over time, transformers can become magnetized due to various factors, such as electrical surges, improper grounding, or even natural phenomena like lightning strikes1. Let’s explore how demagnetization works and the methods used:

  1. Causes of Remanent Magnetism:
    • DC Testing: During winding resistance measurements (typically performed using DC current), the magnetic core becomes magnetized. After the test, the winding is discharged but remains magnetized.
    • Taking the Transformer Out of Service: When a transformer is taken out of service, the flux may remain in a non-zero state due to the phase difference between voltage and current.
    • Clearing High Fault Currents: Interruption of high short-circuit currents can also lead to magnetization.
  2. Effects of Remanent Magnetism:
    • Incorrect Relay Operation: Remanence can cause protective relays for overloads and internal faults to falsely operate and disconnect the transformer from the electrical system.
  3. Demagnetization Methods:

Remember that demagnetization helps prevent operational issues and ensures the reliable functioning of power transformers.

Procedure for demagnetizing a transformer:

  1. AC Voltage Demagnetization:
  2. Effects of Residual Magnetism:

Remember to follow safety protocols and manufacturer guidelines when demagnetizing transformers. Proper demagnetization ensures reliable transformer operation and accurate diagnostic results.

Transformer Turn Ratio (TTR) 

The turns ratio test for a power transformer is essential to ensure its optimal performance. Let’s delve into the details of this crucial test:

  1. Purpose: The turns ratio test determines the ratio of the high voltage winding to all other windings when the transformer is not under load. It is performed on all taps of every winding.
  2. Procedure:
    • Isolate the Equipment: Apply working grounds to all incoming and outgoing cables and disconnect them from the transformer bushing terminals. Ensure sufficient clearance between disconnected cables and switchgear terminals.
    • Connect Test Leads:
      • Connect the H designated three-phase test lead to the test set’s mating connection marked with an H.
      • Connect the X designated three-phase test lead to the test set’s mating connection marked with an X.
      • Connect the H1, H2, H3 designated test leads to the corresponding transformer terminals/bushings (H1, H2, H3). If an H0 terminal/bushing exists, connect the H0 test lead.
      • Similarly, connect the X1, X2, X3 designated test leads to the corresponding X1, X2, X3 transformer terminals/bushings. If an X0 terminal/bushing exists, connect the X0 test lead.
    • Measure Ratios: Perform turns ratio measurements for all tap positions.
    • Validate Ratios: Confirm that the measured ratios are within 5%of the calculated ratios.
  3. Importance: The turns ratio test assesses the condition of the transformer’s windings and core. It helps detect problems related to design, assembly, handling, overloading, fault conditions, or poor maintenance2.

Remember, accurate turns ratio ensures the transformer’s reliability and efficient operation!

Tap Changer Testing & Dynamic Resistance Measurement

The turns ratio test for a power transformer is essential to ensure its optimal performance. Let’s delve into the details of this crucial test:

  1. Purpose: The turns ratio test determines the ratio of the high voltage winding to all other windings when the transformer is not under load. It is performed on all taps of every winding.
  2. Procedure:
    • Isolate the Equipment: Apply working grounds to all incoming and outgoing cables and disconnect them from the transformer bushing terminals. Ensure sufficient clearance between disconnected cables and switchgear terminals.
    • Connect Test Leads:
      • Connect the H designated three-phase test lead to the test set’s mating connection marked with an H.
      • Connect the X designated three-phase test lead to the test set’s mating connection marked with an X.
      • Connect the H1, H2, H3 designated test leads to the corresponding transformer terminals/bushings (H1, H2, H3). If an H0 terminal/bushing exists, connect the H0 test lead.
      • Similarly, connect the X1, X2, X3 designated test leads to the corresponding X1, X2, X3 transformer terminals/bushings. If an X0 terminal/bushing exists, connect the X0 test lead.
    • Measure Ratios: Perform turns ratio measurements for all tap positions.
    • Validate Ratios: Confirm that the measured ratios are within 5%of the calculated ratios.
  3. Importance: The turns ratio test assesses the condition of the transformer’s windings and core. It helps detect problems related to design, assembly, handling, overloading, fault conditions, or poor maintenance2.

Remember, accurate turns ratio ensures the transformer’s reliability and efficient operation!

Tan Delta / Power Factor Measurements

Tan Delta test (or Dissipation Factor Test) on a transformer assesses the quality and condition of its insulation by measuring the dielectric loss, indicating how much energy is lost as heat, revealing issues like moisture, contamination, or aging that could lead to failure, with a higher value signaling poorer insulation. It works by measuring the phase angle difference between applied voltage and current, comparing it to the ideal 90-degree lag, and helps predict insulation health and transformer lifespan. 

Procedure for Transformer Tan Delta Test

Preparation & Safety:

Isolate the transformer from the power system (de-energize, open isolators).

Wear PPE and follow safety protocols.

Record ambient temperature & humidity.

Connections:

Short all terminals of the HV windings together and LV windings together with jumpers.

Connect the test set’s ground terminal to the transformer body/earth.

Connect HV lead of test set to the winding being tested, LV lead to the ‘tan delta’ (TD) tap of the bushing (after removing its earth connection).

Testing Modes (Common Modes):

UST (Unshielded Shielded Test): Measures insulation from winding to ground (with bushing’s TD tap).

GST (Guard or Grounded Shielded Test): Measures insulation between windings (HV to LV).

GSTg (Ground Shielded Guard Test): Measures insulation between winding and ground, often used for bushings.

Test Execution:

Apply a low-frequency test voltage (e.g., 10kV) using the test set.

Measure the tan delta value at different voltage levels (e.g., normal, 1.5x, 2x).

Data Analysis:

Correct the measured values to a standard temperature (e.g., 20°C) using correction factors.

Compare results with manufacturer standards or previous tests to identify insulation deterioration. 

it’s important for:

Early Fault Detection: Identifies insulation problems before they cause catastrophic failure.

Moisture Detection: Moisture significantly increases dielectric loss, making it a key indicator.

Aging Assessment: Reveals insulation degradation from normal aging.

Ensures Reliability: Helps maintain safe and reliable transformer operation, preventing costly outages.