Transformer Testing

Power transformers should undergo testing at various stages throughout their life cycle to ensure their performance, reliability, and safety.

Here are the key stages when power transformers should be tested:

Factory Testing
Power transformers are tested at the factory before they are shipped to the customer. These tests verify the transformer's compliance with design specifications, performance characteristics, and quality standards.

Acceptance Testing (New Equipment)
Upon delivery and installation, before being put into service, power transformers should undergo acceptance testing. This ensures they're ready for operation and helps spot any manufacturing defects or transportation-related issues.

Low voltage Transformer
Pad mounted Transformer

Routine Maintenance Testing
Regular maintenance testing is performed at predetermined intervals, such as 3-5 years, to assess the condition of the transformer and identify any developing issues. Routine maintenance testing helps detect early signs of degradation, insulation breakdown, or other abnormalities that could impact the transformer's performance.

Periodic Diagnostic Testing
Periodic diagnostic testing delves deeper into the transformer's condition, surpassing routine maintenance. It encompasses detailed insulation testing, partial discharge analysis, sweep frequency response analysis (SFRA), and dissolved gas analysis (DGA). These tests offer invaluable insights into the transformer's internal health, aiding in the anticipation and prevention of potential failures or malfunctions.

After Repair or Overhaul
After any significant repair, refurbishment, or overhaul of a power transformer, it is essential to conduct post-repair testing. These tests ensure that the transformer has been restored to its proper functioning and meets the required performance standards.

Transformer Testing, Low Voltage (< 600V)

NETA Test Procedure Categories

1. Dry Type Transformers

Rating:
≤ 1000 KVA

Voltage Level:
Low voltage (<600V)

Common Locations:
Typically found in commercial, industrial, and residential facilities.

Example: :
A transformer used in a small business facility to manage operational loads within safety limits.

Test Procedures, Low Voltage (< 600V)

Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition.
  3. Verify that resilient mounts are free and that any shipping brackets have been removed.
  4. Verify that connections are as specified.
    5. transformer anchorage
  5. Compare equipment nameplate data with drawings and specifications.
  6. Inspect , alignment, and .
    transformer anchorage
    transformer grounding
Visual and Mechanical Inspection list:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition.
  3. Inspect anchorage, alignment, and grounding.
  4. Verify that resilient mounts are free and that any shipping brackets have been removed.
  5. Verify the unit is clean.
  6. Verify that control and alarm settings on temperature indicators are as specified.(optional)
  7. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform survey in accordance with Section 9.(optional)
    8. Dry type transformer Infrared image
    Transformer Infrared Thermography Image
  8. Perform specific inspections and mechanical tests as recommended by the manufacturer.
  9. Verify that as-left tap connections are as specified.
  10. Verify the presence of surge arresters.
Visual and Mechanical Inspection list:
  1. Perform insulation-resistance tests winding-to-winding and each winding-to-ground.
  2. Apply test voltage to each winding
    Record a one minute and ten minute value for each test.
    Applied test voltage and results should be in accordance with
Neta Table 100.1
NETA Table 100.5 shows recommended minimum test result values
  1. Primary Winding to Secondary Winding
    (Phases ABC, All phases are shorted internally)
  2. Primary Winding to Ground Bus
    (Phases ABC, All phases are shorted internally)
  3. Secondary Winding to Ground Bus
    (Phases ABC, All phases are shorted internally)
  1. Calculate Polarization Index.
    Polarization index is an extension of the insulation resistance test.
    Divide the ten-minute value by the one-minute value.

\( \Large PI= \huge \frac{10\; minutes}{1\; minutes} \)

Polarization Index Value < 1
Indicates potential insulation deterioration and warrants further investigation.

Procedure:
  1. Acceptance Testing: Perform turns-ratio tests at all tap positions
    Maintenance Testing: Perform turns-ratio on as found tap position.
  2. Turns-ratio test results shall not deviate by more than one-half percent( 0.05% ) from either the adjacent coils or the calculated ratio.
Transformer Turns Ratio

This is the ratio of the number of turns in the primary winding (N1) to the number of turns in the secondary winding (N2). The voltage times the amperage on the primary winding is equal to the voltage times the amperage on the secondary winding.

\( \Large \text {turns ratio = } \)

\( \Large \frac{N_{1}}{N_{2}} =\frac{V1}{V2}=\frac{I2}{I1} \)

  1. \(\large {N_{1} :}\)\(\text{ pri. turns}\)
  2. \(\large {N_{2} :}\)\(\text{ sec. turns}\)
  1. \(\large {V_{1} :}\)\(\text{ pri. volts}\)
  2. \(\large {V_{2} :}\)\(\text{ sec. volts}\)
  1. \(\large {I_{2} :}\)\(\text{pri. current}\)
  2. \(\large {I_{1} :}\)\(\text{sec. current}\)
Test Description

The Transformer Turns Ratio (TTR) test is conducted to determine the turns ratio of a transformer. This test measures the number of turns in the primary winding compared to the secondary winding. The accepted values for the TTR test should not exceed a 0.5% deviation from the calculated values.

  • Verifies the transformer's input and output voltage ratio.
  • Confirms nameplate ratio, polarity, and vectors.
  • Identifies possible winding deficiencies, such as open-circuits and short-circuits of turn-to-turn sensitivity.
Test Result Values

ANSI/IEEE C57.12.00-2006, Section 9.1 The standard deviation between test results and calculated values should be within 0.5% of nameplate markings, with rated voltage applied to one winding.

TTR Wiring
Insulation resistance (IR) test:

The IR test is of value for future comparative purposes and also for determining the suitability of the transformer of energizing or application. The IR test must be successfully completed for factory warranty to be valid. The IR test must be conducted prior to energizing.

Description:
The IR test is crucial for future comparative analysis and determining the transformer's readiness for energization. A transformer insulation test is a diagnostic test performed on power transformers to assess the condition and integrity of the insulation system. The insulation system in a transformer consists of various components, such as solid insulation, oil insulation, and insulation between windings, which are crucial for maintaining electrical isolation and preventing short circuits.

Purpose:
The primary purpose of a transformer insulation test is to measure the insulation resistance or insulation resistance to ground. This test helps evaluate the insulation's ability to withstand electrical stress and identifies any potential insulation weaknesses or deterioration. It is typically conducted using a high-voltage DC test source and a megohmmeter, which measures resistance in the range of megaohms (MΩ).

Polarization index (PI) test:
This is an extension of the Insulation Resistance (IR) test. It involves taking two IR measurements: the first reading at 1 minute and the second at 10 minutes. The ratio of the 10-minute reading to the 1-minute reading is then calculated to determine the PI dielectric absorption value. The PI should be determined for both winding-to-winding and winding-to-ground.

\( \Large PI= \huge \frac{10\; min}{1\; min} \)

Polarization Index Value < 1
Indicates potential insulation deterioration and warrants further investigation.

Transformer Testing, High Voltage ( > 600V)

NETA Test Procedure Categories

1. Dry Type Transformers > 500 KVA

Voltage Level:
Low voltage (<600V) or Medium Voltage (>600V)

Common Locations:
Frequently used in commercial, industrial, and residential facilities that require higher power capacity.

Example: :
A transformer in a large manufacturing plant that requires high power for heavy industrial loads.

2. Liquid Filled Transformers

Voltage Level:
Medium Voltage (>600V)

Common types:
Station transformers

Common Locations:
Used in power generation stations, large commercial and industrial facilities, and extensive residential complexes.

Example: :
A transformer at a power station that steps down voltage for distribution to various consumers.

Station Transformer

Test Procedures, High Voltage Voltage Voltage (>600V)

New Equipment (Acceptance)
Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition.
  3. Inspect anchorage, alignment, and grounding.
  4. Verify that resilient mounts are free and that any shipping brackets have been removed.
  5. Verify the unit is clean.
  6. Verify that control and alarm settings on temperature indicators are as specified. (optional)
  7. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform in accordance with Section 9.(optional)
    8. Dry type transformer Infrared image
    Transformer Infrared Thermography Image
  8. Perform specific inspections and mechanical tests as recommended by the manufacturer.
  9. Verify that as-left tap connections are as specified.
  10. Verify the presence of surge arresters.
Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition.
  3. Inspect impact recorder prior to unloading.
  4. Test dew point of tank gases.(optional)
  5. Inspect anchorage, alignment, and grounding.
  6. Verify the presence of
    7. transformer pcb labeling
    transformer non-pcb labeling
  7. Verify removal of any shipping bracing after placement.
  8. Verify the bushings are clean.
  9. Verify that alarm, control, and trip settings on are as specified.
    10. transformer pcb labeling
    Temperature Contact settings for alarms or trip signals
  10. Verify operation of alarm, control, and trip circuits from .   ,   ,   , and  
    11. transformer pcb labeling
    Transformer temperature (left) and oil level indicator (right).
    Transformer pressure relief device
    Transformer pressure relief device.
    Transformer gas relay-Buchholz relay
    Gas Accumulator (Bulchholz Relay).
    Transformer gas relay-Buchholz relay
    Fault Pressure Relay
    margin: auto;
    Fault Pressure Relay under test
  11. Verify that cooling fans and pumps operate correctly and have appropriate overcurrent protection.
  12. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform survey in accordance with Section 9.(optional)
    13. transformer Infrared image
    Transformer Infrared Thermography Image
  13. Verify correct liquid level in tanks and bushings.
  14. Verify valves are in the correct operating position.
  15. Perform inspections and mechanical tests as recommended by the manufacturer.
  16. Verify that positive pressure is maintained on gas-blanketed transformers.
  17. Test load tap-changer
  18. Verify presence of transformer
    transformer pcb labeling
    margin: auto;
  19. Verify de-energized tap-changer position is left as specified.
Procedure:
  1. Acceptance Testing: Perform turns-ratio tests at all tap positions
    Maintenance Testing: Perform turns-ratio on as found tap position.
  2. Turns-ratio test results shall not deviate by more than one-half percent( 0.05% ) from either the adjacent coils or the calculated ratio.
Visual and Mechanical Inspection list:
  1. Perform insulation-resistance tests winding-to-winding and each winding-to-ground.
  2. Apply test voltage to each winding
    Record a one minute and ten minute value for each test.
    Applied test voltage and results should be in accordance with
Neta Table 100.1
NETA Table 100.5 shows recommended minimum test result values
  1. Primary Winding to Secondary Winding
    (Phases ABC, All phases are shorted internally)
  2. Primary Winding to Ground Bus
    (Phases ABC, All phases are shorted internally)
  3. Secondary Winding to Ground Bus
    (Phases ABC, All phases are shorted internally)
  1. Calculate Polarization Index.
    Polarization index is an extension of the insulation resistance test.
    Divide the ten-minute value by the one-minute value.

\( \Large PI= \huge \frac{10\; minutes}{1\; minutes} \)

Polarization Index Value < 1
Indicates potential insulation deterioration and warrants further investigation.

Station Transformer
Doble M4000 test set
Doble M4000 Test Procedure
  1. Transformer is de-energized and completely isolated from the power source.
  2. Transformer housing is properly grounded.
  3. All bushing of HV and LV winding, including the neutral are shorted to make them into an equivalent HV and LV bushings. Neutrals must be ungrounded.
  4. Transformers equipped with load-tap-changers should be set to some position off neutral, and this position should be noted on the test data sheet.
  5. Assemble test set in accordance with the instruction book.
  6. Check the ground and the HV connections and safety switches
  7. Connect specimen for the desired test. Raise test voltage to desired level.
  8. Adjust the meter reading to full scale. The test set is ready to us
  9. Measure total charging current as the product of the meter reading and a multiplier within the proper range.
  10. Specimen loss is measured by turning the milliwatts-adjust which cancels the capacitive reactance. When the capacitive reactance is cancelled the meter will be at minimum reading. This meter reading and the proper multiplier provides the milliwatt for the dielectric loss in the insulation.
  11. The capacitance of the specimen is automatically obtained as a product of capacitance multiplier and capacitance dial reading at minimum milliwatts balance.
  12. The product of the meter reading and the meter multiplier is the milliwatts loss of the specimen.
  13. The power supply polarity is reversed to cancel the effect of electrostatic interference. The average of the two milliwatts results is calculated as the specimen milliwatts loss.
  14. The test set has shielding to minimize the effect of electrostatic interference, as well as electromagnetic interference. For cases where high interference is encountered, a special interference cancellation circuit is available with the 10 kV test sets from Doble Engineering. The more advanced design of the Doble M4000 test set does not require interference cancellation circuit.
Transformer Turns Ratio

This is the ratio of the number of turns in the primary winding (N1) to the number of turns in the secondary winding (N2). The voltage times the amperage on the primary winding is equal to the voltage times the amperage on the secondary winding.

\( \Large \text {turns ratio = } \)

\( \Large \frac{N_{1}}{N_{2}} =\frac{V1}{V2}=\frac{I2}{I1} \)

  1. \(\large {N_{1} :}\)\(\text{ pri. turns}\)
  2. \(\large {N_{2} :}\)\(\text{ sec. turns}\)
  1. \(\large {V_{1} :}\)\(\text{ pri. volts}\)
  2. \(\large {V_{2} :}\)\(\text{ sec. volts}\)
  1. \(\large {I_{2} :}\)\(\text{pri. current}\)
  2. \(\large {I_{1} :}\)\(\text{sec. current}\)
Test Description

The Transformer Turns Ratio (TTR) test is conducted to determine the turns ratio of a transformer. This test measures the number of turns in the primary winding compared to the secondary winding. The accepted values for the TTR test should not exceed a 0.5% deviation from the calculated values.

  • Verifies the transformer's input and output voltage ratio.
  • Confirms nameplate ratio, polarity, and vectors.
  • Identifies possible winding deficiencies, such as open-circuits and short-circuits of turn-to-turn sensitivity.
Test Result Values

ANSI/IEEE C57.12.00-2006, Section 9.1 The standard deviation between test results and calculated values should be within 0.5% of nameplate markings, with rated voltage applied to one winding.

TTR Wiring
Insulation resistance (IR) test:

The IR test is of value for future comparative purposes and also for determining the suitability of the transformer of energizing or application. The IR test must be successfully completed for factory warranty to be valid. The IR test must be conducted prior to energizing.

Description:
The IR test is crucial for future comparative analysis and determining the transformer's readiness for energization. A transformer insulation test is a diagnostic test performed on power transformers to assess the condition and integrity of the insulation system. The insulation system in a transformer consists of various components, such as solid insulation, oil insulation, and insulation between windings, which are crucial for maintaining electrical isolation and preventing short circuits.

Purpose:
The primary purpose of a transformer insulation test is to measure the insulation resistance or insulation resistance to ground. This test helps evaluate the insulation's ability to withstand electrical stress and identifies any potential insulation weaknesses or deterioration. It is typically conducted using a high-voltage DC test source and a megohmmeter, which measures resistance in the range of megaohms (MΩ).

Polarization index (PI) test:
This is an extension of the Insulation Resistance (IR) test. It involves taking two IR measurements: the first reading at 1 minute and the second at 10 minutes. The ratio of the 10-minute reading to the 1-minute reading is then calculated to determine the PI dielectric absorption value. The PI should be determined for both winding-to-winding and winding-to-ground.

\( \Large PI= \huge \frac{10\; min}{1\; min} \)

Polarization Index Value < 1
Indicates potential insulation deterioration and warrants further investigation. in

Winding Resistance

Transformer winding resistance testing is a common diagnostic test conducted on power transformers. It involves measuring the resistance of the transformer's windings to evaluate their condition and identify any potential issues. Here's an overview of transformer winding resistance testing:

Purpose

The primary purpose of winding resistance testing is to assess the integrity of the transformer's windings and connections. By measuring the resistance, it helps detect abnormalities such as loose connections, high resistance joints, or damaged windings that could affect the transformer's performance.

inspection
Procedure

Transformer winding resistance testing typically involves the following steps:

  1. De-energization: The transformer must be de-energized and isolated from the power source before conducting winding resistance testing. Safety precautions should be followed, and appropriate lockout/tagout procedures should be implemented.
  2. Test Equipment: Specialized test equipment, such as a low-resistance ohmmeter, is used for winding resistance testing. This equipment provides accurate measurements of the winding resistance.
  3. Measurement Setup: The test leads are connected to the terminals of each winding, including the high-voltage (HV) winding, low-voltage (LV) winding, and any other auxiliary windings. Care should be taken to ensure proper connections and to account for any tap changer connections.
  4. Measurement: The resistance measurement is taken by applying a known test current to the winding and measuring the resulting voltage drop across the winding.
Interpretation of Results:

The measured winding resistance values are compared to the expected or baseline values. Deviations from the expected values may indicate various issues:

  1. Abnormal Resistance: Higher-than-expected resistance readings could indicate loose connections, high resistance joints, or increased winding resistance due to deterioration, corrosion, or damage.
  2. Significant Variation: Significant differences in resistance values between different phases or windings may suggest imbalances or anomalies that require further investigation.
  3. Consistency Check: The measured resistance values are compared with the transformer manufacturer's specifications or previous test results for consistency.
Power Factor and Dissipation Testing

Power Factor Testing and Dissipation Factor Testing are both methods used to assess the condition of insulating materials in electrical equipment, such as transformers, cables, and capacitors. These tests measure the dielectric losses within the insulation system to determine its health and detect potential issues such as moisture, contamination, or aging.

Power Factor Testing and Dissipation Factor Testing are both methods used to assess the condition of insulating materials in electrical equipment, such as transformers, cables, and capacitors. These tests measure the dielectric losses within the insulation system to determine its health and detect potential issues such as moisture, contamination, or aging.

Power Factor

Definitions: The power factor is the ratio of the real power (active power) dissipated in the insulation to the apparent power (total power) applied to the insulation.

Purpose: Power factor testing is used to measure the insulation's dielectric losses at a specific frequency (usually the operating frequency, such as 60 Hz or 50 Hz). It helps in identifying the presence of moisture, contamination, and overall insulation condition.

Dissipation Factor (DF) Testing:

Purpose: Dissipation factor testing provides a more direct measure of the dielectric losses in the insulation material. It is particularly useful in identifying degradation in the insulating material itself, such as contamination, moisture ingress, or chemical breakdown.

Definitions: The dissipation factor, also known as the loss tangent or tan δ, is the tangent of the phase angle between the resistive (real power) and reactive (stored energy) components of the current in the insulation.

Transformer Inspection and Maintenance Checklist
General Inspection Items Frequency
Load current Hourly or use recording meters
Voltage Hourly or use recording meters
Liquid level Hourly or use recording meters
Temperature Hourly or use recording meters
Protective devices Yearly
Protective Alarms Monthly
Ground Connections Every 6 months
Tap Changer Every 6 months
Lightning Arresters Every 6 months
Pressure-relief Devices Every 6 months
Breather Monthly
Auxiliary Equipment Annually
External inspection Every 6 months
Internal inspection 5 to 10 years
Insulating Liquids Frequency
Dielectric strength Annually
Color Annually
Neutralization number Annually
Interfacial tension Annually
Power Factor Test Annually
Moisture content Annually
Gas-analysis test Annually
Solid insulation (winding) Inspections Frequency
Insulation Resistance Annually
DC winding resistance Annually
Power Factor Annually
Frequency Response Analysis Annually
Interfacial tension Annually
*Hi-pot (AC or DC) Five years or more
Induced voltage Five years or more
Polarization recovery voltage Annually

Maintenance

Transformer maintenance schedules should be determined according to the critical or noncritical nature of the transformer and the load that is connected to it.

Proper maintenance includes regular inspections and repairs, maintaining and testing the transformer's liquid, and checking and testing the winding insulation. You should also follow any additional maintenance recommendations from the transformer's manufacturer.

A guide for power transformer maintenance and testing, including how often you should do these tasks, is shown in the tables below.

Routine inspection and repair involve visually checking the transformer's operating conditions and making any needed repairs. How often you do this depends on how critical the transformer is and the environment it's in.

Transformer Inspection and Maintenance Checklist
General Inspection Items Frequency
Load current Hourly or use recording meters
Voltage Hourly or use recording meters
Liquid level Hourly or use recording meters
Temperature Hourly or use recording meters
Protective devices Yearly
Protective Alarms Monthly
Ground Connections Every 6 months
Tap Changer Every 6 months
Lightning Arresters Every 6 months
Pressure-relief Devices Every 6 months
Breather Monthly
Auxiliary Equipment Annually
External inspection Every 6 months
Internal inspection 5 to 10 years
Insulating Liquids Frequency
Dielectric strength Annually
Color Annually
Neutralization number Annually
Interfacial tension Annually
Power Factor Test Annually
Moisture content Annually
Gas-analysis test Annually
Solid insulation (winding) Inspections Frequency
Insulation Resistance Annually
DC winding resistance Annually
Power Factor Annually
Frequency Response Analysis Annually
Interfacial tension Annually
*Hi-pot (AC or DC) Five years or more
Induced voltage Five years or more
Polarization recovery voltage Annually
Common Transformer Failures

Overview

Transformers are crucial components in the electrical power distribution and transmission networks, but they are prone to various types of failures due to their complex nature and operational stress. Understanding common transformer failures and the purpose and description of relevant diagnostic tests is essential for maintaining their reliability and extending their service life. Here are some common transformer failures and the corresponding tests used to diagnose and potentially prevent these issues:

Mechanical Damage

Description:
Mechanical damage can occur from mishandling, installation errors, or environmental factors such as earthquakes.

Mechanical factors can cause significant damage to transformer windings, potentially rupturing the solid insulation and leading to electrical failure. Damage to transformer windings may occur due to electromechanical forces or during shipping. Additional reasons for transformer failures include:

Liquid Leakage
Leakage can occur through screw joints, around gaskets, welds, casting, pressure-relief device, and so on. The main causes are improper assembly of mechanical parts, improper fi lters, poor joints, improper finishing of surfaces

Gas Leakage
In gas-sealed transformers, additional problems can be the loss of gas, oxygen content above 5%, or gas regulator malfunctions. These problems are caused by gas leaks above the oil, leaky valve seats, insufficient gas space, and/or insufficient flushing of gas space with nitrogen.

Broken Ground Connections
The transformer tank should always be solidly grounded to eliminate electric shock.

Diagnostic Tests:
Visual inspections and mechanical integrity tests.

Example:
After a minor earthquake, visual inspections revealed structural damages to the transformer tank, necessitating detailed assessments and repairs.

Insulation Breakdown

Description:
Insulation breakdown occurs when the electrical insulation material within the transformer can no longer withstand the electrical stress, leading to a short circuit or arcing. This can be due to aging, overheating, mechanical damage, or contamination, often resulting in catastrophic failure and potential fire hazards.

Solid Insulation:
Solid insulation, comprised of cellulose-based materials like pressboard and paper, is utilized between transformer windings to ensure electrical isolation. Cellulose is made up of long chains of glucose rings, which degrade over time, resulting in shorter chains. The condition of the paper insulation is assessed by its degree of polymerization (DP), which represents the average number of glucose rings per chain. Fresh paper typically has a DP value between 1200 and 1400, whereas a DP value below 200 indicates significantly weakened mechanical strength. This reduction in strength can compromise the paper's ability to withstand mechanical stresses such as those from short circuits.

This type of solid insulation is particularly vulnerable to mechanical damage, which can occur from transformer movement or forces generated during short circuits. Additionally, faults in the insulation material might arise from the formation of copper sulfate (CuSO4) or from hot spots, which can develop due to insufficient oil levels or transformer overloading.

Diagnostic Tests:
Dielectric breakdown voltage test and insulation resistance test.

Example:
A utility company regularly performs insulation resistance testing on transformers before the summer peak loads to prevent unexpected failures due to increased demand.

Winding Failures

Causes:
This is an electrical fault in the transformer winding insulation where it can involve phase-to-ground, phase-to-phase, three-phase and/or ground, or turn-to-turn type short-circuit. The causes for this type of failure may be due to a short-circuit fault, lightning, overload or overcurrent condition, or transformer liquid containing moisture and contaminants.

  1. External faults Overheating
  2. short circuits between turns,
  3. Open winding Deterioration
  4. Phase-to-phase faults Mechanical failures
  5. Moisture
  6. Terminal Board Failures Turn-to turn failures Surges
  7. Improper blocking of turns Grounds

Diagnostic Tests:
Transformer Turns Ratio (TTR) test and Sweep Frequency Response Analysis (SFRA).

Overheating

Description:
Overtemperature can be caused by an overcurrent, overvoltage, insuffi cient cooling, low liquid level, sludge in the transformer liquid, high ambient, or short-circuited core. In dry-type transformers, this condition can be due to clogged ducts.

Diagnostic Tests:
Thermographic inspections and oil analysis for dissolved gases.

Thermographic cameras used during routine maintenance can identified hot spots on the transformer casing, indicating potential overheating issues.

Bushing Failure

Causes:
Bushing failure can be caused by fl ashover due to dirt accumulation and/or lightning strikes.

  1. Aging of the insulation
  2. Cracking
  3. Contamination
  4. Flashover due to animals Flashover due to surges Moisture
  5. Low oil or Fluid

Diagnostic Tests:
Power factor or dissipation factor testing and capacitance measurements.

Example:
Dissipation factor testing conducted during a routine inspection detected deteriorating insulation health in several bushings, prompting their replacement.

Core Faults

Causes:
This condition is due to the failure of core laminations, core, bolts, clamps, and so on.

  1. Core insulation failures
  2. Shorted laminations
  3. Loose clamps, bolts, and wedges
  4. Broken ground strap
  5. Ground Issues (increased noise, vibration, and eddy current losses.)
  6. High Exciting Current: high exciting currents are due to short-circuited core and/or open core joints.

Diagnostic Tests:
Core ground test and SFRA.

Example:
Unusual noise and vibrations from a transformer were traced back to core lamination faults using core ground testing.

Oil Contamination

Description:
Contamination of transformer oil by moisture, particles, or gases can lead to a loss of insulating properties.

Transformer Oil:
Transformer oil, a highly refined mineral crude oil product, plays a crucial role in providing insulation between windings and cooling within transformers. This oil is composed of various hydrocarbons, including paraffin, naphthalene, and aromatic oils.

Cooling oil failures typically arise from two primary issues: malfunctioning oil circulation systems or inadequate heat transfer to the secondary cooling circuit. These problems can lead to increased oil viscosity and excessively high temperatures in the secondary cooling circuit.

Additionally, the combination of moisture and oxygen with heat significantly contributes to oil contamination, which can produce conducting particles. As a result, the internal temperature of the transformer may rise, compromising the oil's insulating properties and potentially leading to a short circuit.

Diagnostic Tests:
Dissolved Gas Analysis (DGA) and moisture content tests.

Example:
DGA revealed high levels of acetylene and hydrogen gases, indicating internal arcing due to contaminated oil in a high-voltage transformer.

External Short Circuits

Description:
External short circuits due to faults in connected equipment result in excessive current flow, damaging windings and insulation.

Diagnostic Tests:
Protective relay testing and system analysis.

Example:
System analysis after a fault event on a connected power line helped in quickly isolating and addressing an external short circuit affecting a transformer.

Tap Changer Failures

Causes:

  1. Failures in on-load tap changers (OLTC) due to wear
  2. Loose connections Leads (open)
  3. Links
  4. oil contamination
  5. improper voltage regulation
  6. Moisture
  7. Insufficient insulation Tracking Short-circuits

Diagnostic Tests:
Dynamic resistance measurements and OLTC analyzer tests.

Example:
Routine dynamic resistance measurements identified irregular patterns in an OLTC, suggesting worn contacts that were replaced to avoid voltage regulation issues.

Oil Sample Testing

Overview

Purpose

Transformer oil serves critical functions in power transformers, acting as both an insulating material and a cooling medium. Testing transformer oil is essential to ensure its effectiveness in these roles, as it helps evaluate physical and chemical properties that are crucial for the transformer's performance and safety. Regular testing can detect contaminants and chemical changes in the oil, which may indicate issues like insulation breakdown, overheating, or other operational faults within the transformer.

Recommended Testing Schedule

  1. Initial and Commissioning Tests:

    2. Oil should be tested when the transformer is first installed to establish a baseline for future comparisons.

  2. Routine Monitoring:

    3. Annually Standard practice for transformers under typical operating conditions.

    Bi-annually or Quarterly: Recommended if previous tests show deteriorating oil conditions or the transformer is in a harsh operating environment.

  3. Following a Fault Events:

    4. Testing is crucial after incidents such as electrical faults or lightning strikes, as these can alter the oil's properties.

  4. Special Considerations:

    5. For critical transformers or those with a history of issues, testing might be required more frequently, such as every 3 to 6 months, to monitor conditions closely.

Acid Neutralization Number (ANN)

Purpose

The primary purpose of the ANN test is to monitor the degradation of transformer oil over time. As the oil ages, it can become contaminated with oxidation products, which increase its acidity. This rise in acid content can lead to the corrosion of metal parts and degradation of the paper insulation within the transformer, potentially leading to operational failures. Therefore, the test helps in preventive maintenance planning by indicating when the oil needs to be treated or replaced to maintain the transformer's reliability and efficiency.

Description

During the ANN test, a known quantity of transformer oil is titrated with a base (usually potassium hydroxide) to determine the amount of acid in the oil. The results are expressed in milligrams of KOH needed to neutralize the acids in one gram of oil, indicated as mg KOH/g. A higher Acid Neutralization Number suggests a greater presence of acidic compounds, indicating more severe oil degradation. Regular testing and monitoring allow for timely corrective actions, such as oil purification or replacement, to avoid costly repairs and downtime due to transformer damage.

Industry Standards

ASTM D974-Measures the acidity of petroleum products, including transformer oils, through color-indicator titration.

ANSI/IEEE C57.106 -Provides guidelines for the acceptance and maintenance of insulating oil in electrical equipment, advising on testing and maintenance practices to ensure equipment integrity.

IEC 62021-1-International standard specifically tailored for determining the acidity in insulating oils.

Test Result Standards:

New Oil: Should typically have an ANN of less than 0.1 mg KOH/g, indicating minimal acidity.

In-Service Oil: An ANN value of up to 0.2 mg KOH/g is generally acceptable; values beyond this suggest potential oil degradation.

Action Level: ANN values reaching or exceeding 0.5 mg KOH/g indicate significant degradation, necessitating immediate corrective actions such as oil treatment or replacement.

Color Test

Purpose

The main purpose of the Color Test is to assess the color of transformer oil as an indicator of its condition and suitability for continued use. This test is important because the color of transformer oil can change due to aging, oxidation, contamination, or the presence of by-products from thermal and electrical stresses. A darker color can indicate degradation or contamination, which may affect the oil's insulating and cooling properties.

Description

The Color Test is typically conducted by visually comparing the color of the oil sample to a standardized color chart or by using automated colorimeters that provide a precise measurement of color in terms of hue, saturation, and brightness. The test is simple: a sample of the transformer oil is placed in a clear container or special cuvette, and its color is then evaluated against standard color scales like the ASTM Color Scale.

Industry Standards

ASTM D1500- This standard describes the method for measuring the color of petroleum products by comparing them to a color scale under controlled lighting. The color is reported as a numerical value from 0.5 (very light) to 8.0 (very dark), providing a quantifiable measure of the oil's color.

Test Result Standards:

New Oil: New transformer oil typically has a very light color, usually around 0.5 to 1.0 on the ASTM color scale, indicating minimal impurities and good refining.

In-Service Oil: As the oil is used within a transformer, it can darken due to aging, oxidation, and the effects of heat and electrical stresses. Oils with a color rating up to 2.0 are generally considered acceptable, but any noticeable change from the new oil should be investigated.

Action Level: Transformer oils showing a color rating above 2.5 might indicate severe degradation or contamination, requiring further testing and potentially oil treatment or replacement. A dark color, especially nearing or exceeding 3.0, suggests urgent attention may be needed to prevent equipment damage or failure.

Dielectric Breakdown Voltage

Purpose

The main purpose of the Dielectric Breakdown Voltage test is to measure the ability of transformer oil to withstand electrical stress without failure. This test is crucial because transformer oil serves as both an insulator and a coolant. Over time, the oil can become contaminated with water, particles, and other impurities that significantly reduce its insulating properties. The Dielectric Breakdown Voltage test helps in assessing the quality and purity of the oil, thereby ensuring that it can effectively perform its insulating function.

Description

The Dielectric Breakdown Voltage test involves applying an AC voltage at a controlled rate to a sample of transformer oil between two electrodes. The voltage is increased until the oil fails electrically and allows current to pass through by arcing between the electrodes. The voltage at which this breakdown occurs is recorded as the dielectric breakdown voltage of the oil. The test is typically conducted under standard conditions to ensure consistency, with multiple tests performed on the same sample to establish a reliable breakdown voltage value.

Industry Standards

ASTM D877- A standard test method that uses a specific electrode configuration and test procedure to measure the dielectric breakdown voltage of insulating liquids.

ASTM D1816- Provides a more sensitive measurement using a smaller electrode gap and is generally used for oils where higher levels of impurity are suspected.

IEC 60156- An international standard that outlines procedures similar to ASTM but is widely adopted globally and may include specific variations tailored to regional testing practices.

Test Result Standards:

New Oil: Typically, new transformer oil should exhibit a dielectric breakdown voltage of at least 30 kV. This indicates good insulating properties suitable for effective transformer operation.

In-Service Oil: The acceptable minimum values can vary, but generally, a dielectric breakdown voltage below 25 kV may suggest that the oil is no longer effective as an insulating medium and requires further treatment or replacement.

Action Level: If the dielectric breakdown voltage falls below 20 kV, it is generally considered critical, and immediate actions such as oil filtration, degassing, or replacement are recommended to restore its insulating properties.

Dissolved Gas Analysis (DGA)

Purpose

The primary purpose of Dissolved Gas Analysis is to identify the presence and concentrations of specific gases dissolved in transformer oil that are produced by the decomposition of the oil and solid insulation under high temperature or electrical stress. By analyzing these gases, DGA can provide early warning signs of conditions such as overheating, arcing, and partial discharges within a transformer. This allows for preventative maintenance and can help avoid catastrophic failures and costly unplanned outages.

Description

During DGA, a sample of transformer oil is extracted and analyzed using gas chromatography or similar methods to quantify the presence of key gases such as hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H2), carbon monoxide (CO), and carbon dioxide (CO2). Each of these gases can indicate different types of faults or degradation processes within the transformer. For example, hydrogen and methane can indicate low-energy discharges or early-stage overheating, while acetylene is a marker for high-energy arcing.

Industry Standards

ASTM D3612- This standard provides various methods for performing dissolved gas analysis in electrical insulating oils by gas chromatography.

IEC 60567- An international standard that outlines the procedures for oil sampling and handling, gas extraction, and interpretation of dissolved gas analysis results in transformers.

Test Result Standards:

Normal Operation: Small amounts of gases such as nitrogen and oxygen are typically present due to the solubility of air in the oil. Low concentrations of fault gases can also be normal, depending on the transformer design and operating conditions.

Fault Conditions: Specific gas ratios and concentrations can indicate different types of fault conditions:

  • Partial Discharge: Characterized by elevated hydrogen and methane.
  • Thermal Faults: Identified by rising levels of methane, ethane, and ethylene, with specific temperature thresholds indicated by the ratios of these gases.
  • Arcing: Marked by the presence of acetylene and, in severe cases, also by significant amounts of hydrogen.

Action Levels: DGA results are often interpreted using the Duval Triangle or other diagnostic tools that help classify the type of fault and its severity based on the ratios of the dissolved gases. Actions can range from continued monitoring (for minor anomalies) to immediate operational changes or shutdowns (for severe or critical gas levels).

Interfacial Tension

Purpose

The primary purpose of the IFT test is to evaluate the condition of transformer oil by detecting the presence of oxidation by-products and other polar contaminants. A high interfacial tension indicates that the oil is in good condition with minimal contamination, while a lower interfacial tension suggests that the oil contains more polar compounds and may be nearing the end of its useful life.

Description

The IFT test is performed by measuring the force required to break the interface between the transformer oil and a water layer under controlled conditions. This is usually done using a tensiometer, which measures the force in dynes per centimeter (dyn/cm). The test involves placing a drop of transformer oil on the surface of distilled water in a special apparatus and gradually increasing the surface area until the oil-water interface breaks.

Industry Standards

ASTM D971- This standard describes the method for measuring the interfacial tension of oil against water by the ring method. It is commonly used to determine the presence of polar compounds which are typically oxidative degradation products.

IEC 62961- This international standard provides guidelines for the measurement of the interfacial tension of insulating oils, particularly focusing on the assessment of changes in interfacial tension as an indicator of oil degradation.

Test Result Standards:

New Oil: Typically, new transformer oil will have an interfacial tension of about 40 to 45 dynes/cm, indicating that the oil is relatively free of polar contaminants and suitable for use in electrical equipment.

In-Service Oil: As the transformer oil ages, its interfacial tension usually decreases. An IFT value above 30 dynes/cm is generally considered acceptable for in-service transformer oils. Regular monitoring helps track the degradation over time.

Action Level: If the IFT value falls below 25 dynes/cm, it suggests significant contamination and degradation. Oil with an IFT below 20 dynes/cm is generally considered unsuitable for continued use without treatment, and immediate action may be required to prevent damage to the transformer.

Power Factor (Dissipation Factor)

Purpose

The primary purpose of the Power Factor or Dissipation Factor test is to measure the dielectric losses in transformer oil, which indicate how much electrical energy is lost in the form of heat. This test provides insight into the condition of the oil, particularly its purity and degree of contamination by moisture, oxidation products, and other impurities that can affect its performance as an insulating medium.

Description

The test involves measuring the power factor (cos φ) or dissipation factor (tan δ) of the transformer oil at a specified temperature (usually 25°C or 100°C). These factors are measures of the dielectric loss angle in the insulating material and are calculated by applying an AC electric field to the oil in a test cell and measuring the resulting current. The power factor is the cosine of the phase angle between the applied voltage and the total current, while the dissipation factor is the tangent of this phase angle. Low values indicate good insulating properties, whereas higher values indicate greater contamination and reduced insulating effectiveness.

Industry Standards

ASTM D924- This standard describes the method for determining the dielectric constant and dissipation factor of electrical insulating liquids such as transformer oil. It provides detailed procedures for test cell setup, temperature control, and accurate measurement of these electrical properties.

IEC 60247- This international standard outlines methods for measuring the relative permittivity, dielectric dissipation factor, and DC resistivity of insulating liquids. It covers a range of test conditions and provides guidance on interpreting test results.

Test Result Standards:

New Oil: Typically, new transformer oil should have a power factor or dissipation factor of less than 0.5% at 25°C, indicating high insulating quality and minimal conductive or polar contaminants.

In-Service Oil: For in-service transformer oil, the power factor/dissipation factor should generally remain below 1.0% at 25°C. Values within this range suggest that the oil still maintains good insulating properties.

Action Level: If the power factor/dissipation factor exceeds 1.0% at 25°C, it may indicate significant contamination or degradation of the oil, necessitating further investigation, oil treatment, or replacement. Values significantly higher than this threshold can impair the transformer's performance and increase the risk of failure.

Specific Gravity

Purpose

The primary purpose of the Specific Gravity test is to determine the density of transformer oil, which affects how the oil behaves under temperature changes and its ability to effectively cool and insulate the transformer. Specific gravity also helps in identifying potential contamination with foreign substances that could alter the oil’s density and insulating properties.

Description

The Specific Gravity test measures the ratio of the density of transformer oil to the density of water at a specified temperature (usually 15°C or 25°C). This is done using a hydrometer, pycnometer, or density meter. The oil sample is placed in a test container, and its density is measured and compared to that of water. The result is a dimensionless number that indicates whether the oil is heavier or lighter than water.

Industry Standards

ASTM D1298- This standard outlines the method for determining the density, relative density (specific gravity), or API gravity of crude petroleum, petroleum products, or mixtures of petroleum and non-petroleum products.

Test Result Standards:

New Oil: Typically, the specific gravity of new transformer oil ranges from about 0.875 to 0.895 at 15°C. These values indicate that the oil is lighter than water and has the appropriate physical properties for use in transformers.

In-Service Oil: The specific gravity of in-service transformer oil should not deviate significantly from the initial values. Stable specific gravity readings indicate that the oil maintains its original properties and effectiveness as a cooling and insulating medium.

Action Level: Significant changes in specific gravity may indicate contamination (e.g., water ingress, which increases specific gravity) or degradation (e.g., by-products from oxidation, which might alter density). If specific gravity deviates markedly from normal ranges, further investigation or oil treatment may be required to restore its properties or prevent equipment damage.

Moisture

Purpose

The primary purpose of the Moisture Content test is to measure the water content in transformer oil to ensure it remains within safe limits. Water in transformer oil reduces its dielectric strength, promotes the formation of acids, and accelerates the aging of both the oil and the paper insulation. Monitoring moisture levels helps in maintaining the effectiveness of the oil as an insulating medium and in preventing transformer failures.

Description

The Moisture Content test, also known as the Karl Fischer titration, is a quantitative chemical analysis method used to determine the exact amount of water in transformer oil. The test involves mixing a sample of the oil with a reagent that reacts specifically with water. The amount of reagent consumed is measured and directly correlates to the amount of water in the sample. The results are typically expressed in parts per million (ppm).

Industry Standards

ASTM D1533- This standard describes the test method for determining water in insulating liquids by Coulometric Karl Fischer titration. It is highly precise and suitable for measuring even low levels of moisture in transformer oil.

IEC 60814- This international standard outlines the method for determining water content in insulating liquids by automatic coulometric Karl Fischer titration. It is widely used for routine moisture content testing in transformer oils.

Test Result Standards:

New Oil: Typically, new transformer oil should contain less than 35 ppm of water to ensure optimal insulating properties and prevent any immediate risk to the transformer’s operation.

In-Service Oil: For in-service transformer oil, moisture content should ideally remain under 50 ppm. Consistent monitoring helps in detecting any rise in moisture levels that could signify leaks, condensation, or other issues affecting the transformer.

Action Level: Moisture levels exceeding 50 ppm in transformer oil are generally considered actionable, and levels above 100 ppm are deemed critical, requiring immediate attention. At these levels, the risk of insulation failure increases significantly, and measures such as oil dehydration or replacement might be necessary to restore the oil’s performance and protect the transformer.

NETA Test Procedure

NETA ATS-2017

7.2.1.1 Transformers, Dry Type, Air-Cooled, Low-Voltage, Small

NOTE: This category consists of power transformers with windings rated 600 volts or less and sizes equal to or
less than 167 kVA single-phase or 500 kVA three-phase.
A. Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition.
  3. Inspect anchorage, alignment, and grounding.
  4. Verify that resilient mounts are free and that any shipping brackets have been removed.
  5. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  6. Verify that as-left tap connections are as specified.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable..
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data or in the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index.
  3. Perform turns-ratio tests at all tap positions. (optional)
  4. Verify correct secondary voltage phase-to-phase and phase-to-neutral after energization and prior to loading.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of the thermographic survey shall be in accordance with Section 9.(optional)
  4. Tap connections are left as found unless otherwise specified.
D. Test Values – Electrical
  1. Compare bolted electrical connection resistances to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Minimum insulation-resistance values of transformer insulation shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall not be less than 1.0.
  3. Turns-ratio test results shall not deviate by more than one-half percent from either the adjacent coils or the calculated ratio.
  4. Phase-to-phase and phase-to-neutral secondary voltages shall be in agreement with nameplate data.

NETA ATS-2017

7.2.1.2 Transformers, Dry Type, Air-Cooled, Large

NOTE: This category consists of power transformers with windings rated higher than 600 volts and low-voltage transformers larger than 167 kVA single-phase or 500 kVA three-phase.
A. Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition.
  3. Inspect anchorage, alignment, and grounding.
  4. Verify that resilient mounts are free and that any shipping brackets have been removed..
  5. Verify the unit is clean.
  6. Verify that control and alarm settings on temperature indicators are as specified.(optional)
  7. Verify that cooling fans operate and that fan motors have correct overcurrent protection.
  8. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  9. Perform specific inspections and mechanical tests as recommended by the manufacturer.
  10. Verify that as-left tap connections are as specified.
  11. Verify the presence of surge arresters.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index.
  3. Perform power-factor or dissipation-factor tests on all windings in accordance with the test equipment manufacturer’s published data.
  4. Perform a power-factor or dissipation-factor tip-up test on windings greater than 2.5 kV.(optional)
  5. Perform turns-ratio tests at all tap positions.
  6. Perform an excitation-current test on each phase.(optional)
  7. Measure the resistance of each winding at each tap connection.(optional)
  8. Measure core insulation resistance at 500 volts dc if the core is insulated and the core ground strap is removable.
  9. Perform an applied voltage test on all high- and low-voltage windings-to-ground. See ANSI/IEEE C57.12.91, Sections 10.2 and 10.9.(optional)
  10. Verify correct secondary voltage, phase-to-phase and phase-to-neutral, after energization and prior to loading.
  11. Test surge arresters in accordance with Section 7.19.
C. Test Values – Visual and Mechanical
  1. Control and alarm settings on temperature indicators shall operate within manufacturer’s recommendations for specified settings.
  2. Cooling fans shall operate.
  3. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  4. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  5. Results of the thermographic survey shall be in accordance with Section 9.(optional)
  6. Tap connections shall be left as found unless otherwise specified.
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Minimum insulation-resistance values of transformer insulation shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall not be less than 1.0.
  3. The following values are typical for insulation power factor tests:
    1. CHL Power transformers: 2.0 percent or less
    2. CHL Distribution transformers: 5.0 percent or less
    3. CH and CL power-factor or dissipation-factor values will vary due to support insulators and bus work utilized on dry transformers. Consult transformer manufacturer’s or test equipment manufacturer’s data for additional information.
  4. Power-factor or dissipation-factor tip-up exceeding 1.0 percent shall be investigated.
  5. Turns-ratio test results shall not deviate more than one-half percent from either the adjacent coils or the calculated ratio.
  6. The typical excitation current test data pattern for a three-legged core transformer is two similar current readings and one lower current reading.
  7. Temperature-corrected winding-resistance values shall compare within one percent of previously-obtained results.
  8. Core insulation-resistance values shall not be less than one megohm at 500 volts dc.
  9. AC dielectric withstand test voltage shall not exceed 75 percent of factory test voltage for one minute duration. DC dielectric withstand test voltage shall not exceed 100 percent of the ac rms test voltage specified in ANSI C57.12.91, Section 10.2 for one minute duration. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand test, the test specimen is considered to have passed the test.
  10. Phase-to-phase and phase-to-neutral secondary voltages shall be in agreement with nameplate data.
  11. Test results for surge arresters shall be in accordance with Section 7.19.

NETA ATS-2017

7.2.2 Transformers, Liquid-Filled

A. Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition.
  3. Inspect impact recorder prior to unloading.
  4. Test dew point of tank gases (optional)
  5. Inspect anchorage, alignment, and grounding.
  6. Verify the presence of PCB content labeling.
  7. Verify removal of any shipping bracing after placement.
  8. Verify the bushings are clean.
  9. Verify that alarm, control, and trip settings on temperature and level indicators are as specified.
  10. Verify operation of alarm, control, and trip circuits from temperature and level indicators, pressure relief device, gas accumulator, and fault pressure relay.
  11. Verify that cooling fans and pumps operate correctly and have appropriate overcurrent protection.
  12. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  13. Verify correct liquid level in tanks and bushings.
  14. Verify valves are in the correct operating position.
  15. Verify that positive pressure is maintained on gas-blanketed transformers.
  16. Perform inspections and mechanical tests as recommended by the manufacturer.
  17. Test load tap-changer
  18. Verify presence of transformer surge arresters.
  19. Verify de-energized tap-changer position is left as specified.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter if applicable.
  2. "Perform insulation-resistance tests, winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index."
  3. Perform turns-ratio tests at all tap positions.
  4. Perform insulation power-factor or dissipation-factor tests on all windings in accordance with test equipment manufacturer’s published data.
  5. "Perform power-factor or dissipation-factor tests on each bushing equipped with a power-factor/ capacitance tap. In the absence of a power-factor/ capacitance tap, perform hot-collar tests. These tests shall be in accordance with the test equipment manufacturer’s published data."
  6. Perform excitation-current tests in accordance with test equipment manufacturer’s published data.
  7. "Measure the resistance of each high-voltage winding in each de-energized tap-changer position. Measure the resistance of each low-voltage winding in each de-energized tap-changer position."
  8. Perform leakage reactance three-phase equivalent and per phase tests. (optional)
  9. If core ground strap is accessible, remove and measure core insulation (optional) resistance at 500 volts dc.
  10. Measure the percentage of oxygen in the gas blanket. (optional)
  11. Remove a sample of insulating liquid in accordance with ASTM D 923. Sample shall be tested for the following.
    1. Dielectric breakdown voltage: ASTM D 877 and/or ASTM D 1816
    2. Acid neutralization number: ANSI/ASTM D 974
    3. Specific gravity: ANSI/ASTM D 1298 (optional)
    4. Interfacial tension: ANSI/ASTM D 971
    5. Color: ANSI/ASTM D 1500
    6. Visual Condition: ASTM D 1524
    7. Water in insulating liquids: ASTM D 1533.
    8. Power factor or dissipation factor in accordance with ASTM D 924. (optional)
  12. Remove a sample of insulating liquid in accordance with ASTM D923 and perform dissolved-gas analysis (DGA) in accordance with ANSI/IEEE C57.104 or ASTM D3612.
  13. Test instrument transformers.
  14. Test surge arresters, if present.
  15. Test transformer neutral grounding impedance device, if present.
  16. Verify operation of cubicle or air terminal compartment space heaters.
C. Test Values – Visual and Mechanical
  1. Alarm, control, and trip circuits from temperature and level indicators as well as pressure relief device and fault pressure relay shall operate within manufacturer’s recommendations for their specified settings.
  2. Cooling fans and pumps shall operate.
  3. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  4. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  5. Results of the thermographic survey shall be in accordance with Section 9.(optional)
  6. Liquid levels in the transformer tanks and bushings shall be within indicated tolerances.
  7. Positive pressure shall be indicated on pressure gauge for gas-blanketed transformers.
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Minimum insulation-resistance values of transformer insulation shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall not be less than 1.0.
  3. Turns-ratio test results shall not deviate by more than one-half percent from either the adjacent coils or the calculated ratio.
  4. Maximum winding insulation power-factor/dissipation-factor values of liquid-filled transformers shall be in accordance with the manufacturer’s published data. In the absence of manufacturer’s published data use Table 100.3. Distribution transformer power factor results shall compare to previously obtained results.
  5. Investigate bushing power-factor values that vary from nameplate values by more than 150 percent. Investigate bushing capacitance values that vary from nameplate values by more than five percent. Investigate bushing hot-collar test values that exceed 0.1 Watts.
  6. Typical excitation-current test data pattern for a three-legged core transformer is two similar current readings and one lower current reading.
  7. Sweep frequency response analysis test results should compare to factory and previous test results.
  8. Consult the manufacturer if winding-resistance test values vary by more than two percent from factory test values or between adjacent phases.
  9. Investigate leakage reactance per phase test results that deviate from the average of the three readings by more than 3%. The three-phase equivalent test results serve as a benchmark for future tests.
  10. Core insulation values shall be compared to the factory test value but not less than one mega-ohm at 500 volts dc.
  11. Investigate the presence of oxygen in the nitrogen gas blanket.
  12. Insulating liquid values shall be in accordance with Table 100.4.
  13. Evaluate results of dissolved-gas analysis in accordance with ANSI/IEEE Standard C57.104.
  14. Results of electrical tests on instrument transformers shall be in accordance with Section 7.10.
  15. Results of surge arrester tests shall be in accordance with Section 7.19.
  16. Compare grounding impedance device values to manufacturer’s published data.
  17. Heaters shall be operational.

NETA MTS-2019

7.2.1.1 Transformers, Dry Type, Air-Cooled, Low-Voltage, Small

NOTE: This category consists of power transformers with windings rated 600 volts or less and sizes equal to or less than 167 kVA single-phase or 500 kVA three-phase.
A. Visual and Mechanical Inspection:
  1. Inspect physical and mechanical condition.
  2. Inspect anchorage, alignment, and grounding.
  3. Prior to cleaning the unit, perform as-found tests, if required.
  4. VClean the unit.
  5. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  6. Perform as-left tests..
  7. Verify that as-left tap connections are as specified.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data or in the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index.
  3. Perform turns-ratio tests at all tap positions. (optional)
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value. (7.2.1.1.A.5.1)
  2. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of the thermographic survey shall be in accordance with Section 9.(optional)
  4. Tap connections are left as found unless otherwise specified.
D. Test Values – Electrical
  1. Compare bolted electrical connection resistances to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Minimum insulation-resistance values of transformer insulation shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall not be less than 1.0.
  3. Turns-ratio test results shall not deviate by more than one-half percent from either the adjacent coils or the calculated ratio.

NETA MTS-2019.

7.2.1.2 Transformers, Dry Type, Air-Cooled, Large

NOTE: This category consists of power transformers with windings rated higher than 600 volts and low-voltage transformers larger than 167 kVA single-phase or 500 kVA three-phase.
A. Visual and Mechanical Inspection:
  1. Inspect physical and mechanical condition.
  2. Inspect anchorage, alignment, and grounding.
  3. Prior to cleaning the unit, perform as-found tests, if required.
  4. Clean the unit.
  5. Verify that control and alarm settings on temperature indicators are as specified. (optional)
  6. Verify that cooling fans operate correctly.
  7. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  8. Perform specific inspections and mechanical tests as recommended by the manufacturer.
  9. Perform as-left tests.
  10. Verify that as-left tap connections are as specified.
  11. Verify the presence of surge arresters.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index.
  3. Perform insulation power-factor or dissipation-factor tests on all windings in accordance with the test equipment manufacturer’s published data.
  4. Perform a power-factor or dissipation-factor tip-up test on windings rated greater than 2.5 kV. (optional)
  5. Perform turns-ratio tests at the designated tap position.
  6. Perform an excitation-current test on each phase. (optional)
  7. Measure the resistance of each winding at the designated tap position.(optional)
  8. Measure core insulation resistance at 500 volts dc if the core is insulated and if the core ground strap is removable.
  9. Perform an applied voltage test on all high- and low-voltage windings-to-ground. See IEEE C57.12.91-2001, Sections 103.(optional)
  10. Verify correct secondary voltage phase-to-phase and phase-to-neutral after energization and prior to loading.
  11. Test surge arresters in accordance with Section 7.19.
  12. Perform online partial-discharge survey on winding rated higher than 600 volts. (optional)
C. Test Values – Visual and Mechanical
  1. Control and alarm settings on temperature indicators shall operate within manufacturer’s recommendations for specified settings.
  2. Cooling fans shall operate.
  3. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  4. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  5. Results of the thermographic survey shall be in accordance with Section 9.(optional)
  6. Tap connections shall be left as found unless otherwise specified.
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Minimum insulation-resistance values of transformer insulation shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall not be less than 1.0.
  3. The following values are typical for insulation power factor tests:
    1. CHL Power transformers: 2.0 percent or less
    2. CHL Distribution transformers: 5.0 percent or less
    3. CH and CL power-factor or dissipation-factor values will vary due to support insulators and bus work utilized on dry transformers. Consult transformer manufacturer’s or test equipment manufacturer’s data for additional information.
  4. Power-factor or dissipation-factor tip-up exceeding 1.0 percent shall be investigated.
  5. Turns-ratio test results shall not deviate more than one-half percent( 0.05% ) from either the adjacent coils or the calculated ratio.
  6. The typical excitation current test data pattern for a three-legged core transformer is two similar current readings and one lower current reading.
  7. Temperature-corrected winding-resistance values shall compare within one percent of previously-obtained results.
  8. Core insulation-resistance values shall not be less than one megohm at 500 volts dc.
  9. AC dielectric withstand test voltage shall not exceed 65 percent of factory test voltage for one minute duration. DC dielectric withstand test voltage shall not exceed 100 percent of the ac rms test voltage specified in IEEE C57.12.91, Section 10.2 for one minute duration. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand voltage test, the test specimen is considered to have passed the test.
  10. Phase-to-phase and phase-to-neutral secondary voltages shall be in agreement with nameplate data.
  11. Test results for surge arresters shall be in accordance with Section 7.19.
  12. Results of online partial-discharge survey should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, refer to Table 100.23.

NETA MTS-2019

7.2.1.2 Transformers, Dry Type, Air-Cooled, Large

A. Visual and Mechanical Inspection:
  1. Inspect physical and mechanical condition..
  2. Inspect anchorage, alignment, and grounding.
  3. Prior to cleaning the unit, perform as-found tests, if required.
  4. Clean bushings and control cabinets.
  5. Verify operation of alarm, control, and trip circuits from temperature and level indicators, pressure-relief device, gas accumulator, and fault-pressure relay.
  6. Verify that cooling fans and/or pumps operate correctly.
  7. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  8. Verify correct liquid level in tanks and bushings.
  9. Verify that positive pressure is maintained on gas-blanketed transformers.
  10. Perform inspections and mechanical tests as recommended by the manufacturer.
  11. Test load tap-changer.
  12. Verify the presence of transformer surge arresters.
  13. Perform as-left tests.
  14. Verify de-energized tap-changer position is left as specified.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter in accordance with Section 7.2.2.A.8.1.
  2. Perform insulation-resistance tests, winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index.
  3. Perform turns-ratio tests at the designated tap position.
  4. Perform insulation power-factor or dissipation-factor tests on all windings in accordance with test equipment manufacturer’s published data.
  5. Perform power-factor or dissipation-factor tests on each bushing equipped with a powerfactor/ capacitance tap. In the absence of a power-factor/capacitance tap, perform hot-collar tests. These tests shall be in accordance with the test equipment manufacturer’s published data.
  6. Perform excitation-current tests in accordance with the test equipment manufacturer’s published data.
  7. Perform sweep frequency response analysis tests.(optional)
  8. Measure the resistance of each winding at the designated tap position.
  9. If the core ground strap is accessible, remove and measure the core insulation resistance at 500 volts dc. (optional)
  10. Measure the percentage of oxygen in the gas blanket. (optional)
  11. Remove a sample of insulating liquid in accordance with ASTM D923. The sample shall be tested for the following.
    1. Dielectric breakdown voltage: ASTM D 877 and/or ASTM D 1816
    2. Acid neutralization number: ANSI/ASTM D 974
    3. Specific gravity: ANSI/ASTM D 1298 (optional)
    4. Interfacial tension: ANSI/ASTM D 971
    5. Color: ANSI/ASTM D 1500
    6. Visual Condition: ASTM D 1524
    7. Water in insulating liquids: ASTM D 1533.
    8. Measure power factor or dissipation factor in accordance with ASTM D924. (optional)
  12. Remove a sample of insulating liquid in accordance with ASTM D3613 and perform dissolved-gas analysis (DGA) in accordance with IEEE C57.104 or ASTM D3612.
  13. Test the instrument transformers.
  14. Test the surge arresters.
  15. Test the transformer neutral grounding impedance devices.
C. Test Values – Visual and Mechanical
  1. Alarm, control, and trip circuits from temperature and level indicators as well as pressure relief device and fault pressure relay should operate within manufacturer’s recommendations for their specified settings.
  2. Cooling fans and/or pumps should operate.
  3. "Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value."
  4. Bolt-torque levels should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  5. Results of the thermographic survey shall be in accordance with Section 9.(optional)
  6. Liquid levels in the transformer tanks and bushings should be within indicated tolerances.
  7. Positive pressure should be indicated on pressure gauge for gas-blanketed transformers.
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Minimum insulation-resistance values of transformer insulation should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall be compared to previously obtained results and should not be less than 1.0.
  3. Turns-ratio test results should not deviate by more than one-half percent from either the adjacent coils or the calculated ratio.
  4. Maximum power-factor/dissipation-factor values of liquid-filled transformers corrected to 20° C should be in accordance with the transformer manufacturer’s published data. Representative values are indicated in Table 100.3.
  5. Investigate bushing power-factor values that vary from nameplate values by more than 50 percent. Investigate bushing capacitance values that vary from nameplate values by more than five percent. Investigate bushing hot-collar test values that exceed 0.1 Watts.
  6. Typical excitation-current test data pattern for a three-legged core transformer is two similar current readings and one lower current reading.
  7. Sweep frequency response analysis test results should compare to factory and previous test results.
  8. Temperature corrected winding-resistance values should compare within two percent of previously obtained results.
  9. Core insulation values should be comparable to previously obtained results but not less than one megohm at 500 volts dc.
  10. Investigate the presence of oxygen in the nitrogen gas blanket.
  11. Insulating liquid values should be in accordance with Table 100.4.
  12. Evaluate results of dissolved-gas analysis in accordance with IEEE C57.104.
  13. Results of electrical tests on instrument transformers shall be in accordance with Section 7.10.
  14. Results of surge arrester tests shall be in accordance with Section 7.19.
  15. Compare grounding impedance device values to previously obtained results. In the absence of previously obtained values, compare obtained values to manufacturer’s published data.
NETA ATS / MTS
TABLE 100.5
Neta Table 100.5
NETA ATS / MTS
Neta Table 100.5
Neta Table 100.5
NETA Test Procedure

NETA ATS-2017

7.10.1 Instrument Transformers, Current Transformers

A. Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition.
  3. Verify correct connection of transformers with system requirements.
  4. Verify that adequate clearances exist between primary and secondary circuit wiring.
  5. Verify the unit is clean.
  6. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  7. Verify that all required grounding and shorting connections provide contact.
  8. Verify appropriate lubrication on moving current-carrying parts and on moving and sliding surfaces.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable.
  2. Perform insulation-resistance test of each current transformer and its secondary wiring with respect to ground at 1000 volts dc for one minute. For units with solid-state components that cannot tolerate the applied voltage, follow manufacturer’s recommendations.
  3. Perform a polarity test of each current transformer in accordance with ANSI/IEEE C57.13.1.
  4. Perform a ratio-verification test using the voltage or current method in accordance with ANSI/IEEE C57.13.1.
  5. Perform an excitation test on transformers used for relaying applications in accordance with ANSI/IEEE C57.13.1.
  6. Measure current circuit burdens at transformer terminals in accordance with ANSI/IEEE C57.13.1.
  7. When applicable, perform insulation-resistance tests on the primary winding with the secondary grounded. Test voltages shall be in accordance with Table 100.5.
  8. Perform dielectric withstand tests on the primary winding with the secondary grounded. Test voltages shall be in accordance with Table 100.9. (optional)
  9. Perform power-factor or dissipation-factor tests in accordance with manufacturer's published data.
  10. Verify that current transformer secondary circuits are grounded and have only one grounding point in accordance with ANSI/IEEE C57.13.3. That grounding point should be located as specified by the engineer in the project drawings.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of thermographic survey shall be in accordance with Section 9.(optional)
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Insulation-resistance values of instrument transformers shall not be less than values shown in Table 100.5.
  3. Polarity results shall agree with transformer markings.
  4. Ratio errors shall be in accordance with IEEE C57.13.
  5. Excitation results shall match the curve supplied by the manufacturer or be in accordance with IEEE C57.13.1.
  6. Measured burdens shall be compared to instrument transformer ratings.
  7. Insulation-resistance values of instrument transformers shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5.
  8. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand test, the primary winding is considered to have passed the test.
  9. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.
  10. Test results shall indicate that the circuits have only one grounding point.

NETA ATS-2017

7.10.2 Instrument Transformers, Voltage Transformers

A. Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition.
  3. Verify correct connection of transformers with system requirements.
  4. Verify that adequate clearances exist between primary and secondary circuit wiring.
  5. Verify the unit is clean.
  6. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  7. Verify that all required grounding and shorting connections provide contact.
  8. Verify appropriate lubrication on moving current-carrying parts and on moving and sliding surfaces.
  9. Perform as-left tests.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Test voltages shall be applied for one minute in accordance with Table 100.5. For units with solidstate components that cannot tolerate the applied voltage, follow manufacturer’s recommendations.
  3. Perform a polarity test on each transformer to verify the polarity marks or H1- X1 relationship as applicable.
  4. Perform a turns-ratio test on all tap positions.
  5. Measure voltage circuit burdens at transformer terminals.
  6. Perform a dielectric withstand test on the primary windings with the secondary windings.(optional) connected to ground. The dielectric voltage shall be in accordance with Table 100.9. The test voltage shall be applied for one minute.
  7. Perform power-factor or dissipation-factor tests in accordance with manufacturer's published data.
  8. Verify that voltage transformer secondary circuits are grounded and have only one grounding point in accordance with ANSI/IEEE C57.13.3. The grounding point should be located as specified by the engineer in the project drawings.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of the thermographic survey in accordance with Section 9. (optional)
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Insulation-resistance values of instrument transformers shall not be less than values shown in Table 100.5.
  3. Polarity results shall agree with transformer markings.
  4. Ratio errors shall be in accordance with IEEE C57.13.
  5. Measured burdens shall be compared to instrument transformer ratings.
  6. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand test, the primary winding is considered to have passed the test.
  7. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.
  8. Test results shall indicate that the circuits have only one grounding point.

NETA ATS-2017

7.10.3 Instrument Transformers, Coupling-Capacitor Voltage Transformers

A. Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition.
  3. Verify correct connection of transformers with system requirements.
  4. Verify that adequate clearances exist between primary and secondary circuit wiring.
  5. Verify the unit is clean.
  6. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  7. Verify that all required grounding and shorting connections provide contact.
  8. Verify appropriate lubrication on moving current-carrying parts and on moving and sliding surfaces.
  9. Perform as-left tests.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Test voltages shall be applied for one minute in accordance with Table 100.5. For units with solidstate components that cannot tolerate the applied voltage, follow manufacturer’s recommendations.
  3. Perform a polarity test on each transformer to verify the polarity marking. See ANSI/IEEE C93.1 for standard polarity marking.
  4. Perform a turns-ratio test on all tap positions.
  5. Measure voltage circuit burdens at transformer terminals.
  6. Perform a dielectric withstand test on the primary windings with the secondary windings.(optional) connected to ground. The dielectric voltage shall be in accordance with Table 100.9. The test voltage shall be applied for one minute.
  7. Perform power-factor or dissipation-factor tests in accordance with manufacturer's published data.
  8. Verify that voltage transformer secondary circuits are grounded and have only one grounding point in accordance with ANSI/IEEE C57.13.3. The grounding point should be located as specified by the engineer in the project drawings.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of the thermographic survey shall be in accordance with Section 9. (optional)
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Insulation-resistance values of instrument transformers shall not be less than values shown in Table 100.5.
  3. Polarity results shall agree with transformer markings.
  4. Ratio errors shall be in accordance with IEEE C57.13.
  5. Measured burdens shall be compared to instrument transformer ratings.
  6. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand test, the primary winding is considered to have passed the test.
  7. Capacitance of capacitor sections of coupling-capacitor voltage transformers shall be in accordance with manufacturer’s published data.
  8. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.
  9. Test results shall indicate that the circuits have only one grounding point.

NETA MTS-2019

7.10.1 Instrument Transformers, Current Transformers

A. Visual and Mechanical Inspection:
  1. Inspect physical and mechanical condition.
  2. Prior to cleaning the unit, perform as-found tests, if required.
  3. Clean the unit.
  4. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  5. Verify that all required grounding and shorting connections provide contact.
  6. Verify appropriate lubrication on moving current-carrying parts and on moving and sliding surfaces.
  7. Perform as-left tests.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter.
  2. Perform insulation-resistance test of each current transformer and its secondary wiring with respect to ground at 1000 volts dc for one minute. For units with solid-state components that cannot tolerate the applied voltage, follow manufacturer’s recommendations.
  3. Perform a polarity test of each current transformer in accordance with ANSI/IEEE C57.13.1. (optional)
  4. Perform a ratio-verification test using the voltage or current method in accordance with ANSI/IEEE C57.13.1. (optional)
  5. Perform an excitation test on transformers used for relaying applications in accordance with ANSI/IEEE C57.13.1. (optional)
  6. Measure current circuit burdens at transformer terminals in accordance with ANSI/IEEE C57.13.1. (optional)
  7. Perform insulation-resistance tests on the primary winding with the secondary grounded. Test voltages shall be in accordance with Table 100.5.
  8. Perform dielectric withstand tests on the primary winding with the secondary grounded. Test voltages shall be in accordance with Table 100.9.
  9. Perform power-factor or dissipation-factor tests in accordance with manufacturer's published data. (optional)
  10. Verify that current circuits are grounded and have only one grounding point in accordance with IEEE C57.13.3.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of the thermographic survey shall be in accordance with Section 9. (optional)
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Insulation-resistance values of instrument transformers shall not be less than values shown in Table 100.5.
  3. Polarity results shall agree with transformer markings.
  4. Ratio errors should not be greater than values shown in IEEE C57.13.
  5. Excitation results shall match the curve supplied by the manufacturer or be in accordance with IEEE C57.13.1.
  6. Compare measured burdens to instrument transformer ratings.
  7. Insulation-resistance values of instrument transformers shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5.
  8. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand test, the primary winding is considered to have passed the test.
  9. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.
  10. Test results shall indicate that the circuits have only one grounding point.

NETA MTS-2019

7.10.2 Instrument Transformers, Voltage Transformers

A. Visual and Mechanical Inspection:
  1. Inspect physical and mechanical condition.
  2. "Prior to cleaning the unit, perform as-found tests, if required."
  3. Clean the unit.
  4. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  5. Verify that all required grounding and connections provide contact.
  6. Verify correct operation of transformer withdrawal mechanism and grounding operation.
  7. Verify correct primary and secondary fuse sizes for voltage transformers.
  8. Use appropriate lubrication on moving current-carrying parts and on moving and sliding surfaces.
  9. Perform as-left tests.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Test voltages shall be applied for one minute in accordance with Table 100.5. For units with solidstate components that cannot tolerate the applied voltage, follow manufacturer’s recommendations.
  3. Perform a polarity test on each transformer to verify the polarity marks or H1- X1 relationship as applicable. (optional)
  4. Perform a turns-ratio test on as-found tap position.
  5. Measure voltage circuit burdens at transformer terminals. (optional)
  6. Perform a dielectric withstand test on the primary windings with the secondary windings connected to ground. The dielectric voltage shall be in accordance with Table 100.9. The test voltage shall be applied for one minute. (optional)
  7. Perform power-factor or dissipation-factor tests in accordance with manufacturer's published data.
  8. Verify that potential circuits are grounded and have only one grounding point in accordance with IEEE C57.13.3.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of the thermographic survey shall be in accordance with Section 9. (optional)
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Insulation-resistance values of instrument transformers shall not be less than values shown in Table 100.5.
  3. Polarity results shall agree with transformer markings.
  4. In accordance with IEEE C57.13.8.1.1 the ratio error should be as follows:
    1. Revenue metering applications: equal to or less than ±0.1 percent for ratio and ±0.9 mrad (three minutes) for phase angle.
    2. Other applications: equal to or less than +1.2 percent for ratio and ±17.5 mrad (one degree) for phase angle.
  5. Compare measured burdens to instrument transformer ratings.
  6. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand voltage test, the primary windings are considered to have passed the test.
  7. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.
  8. Test results should indicate that the circuits have only one grounding point.

NETA MTS-2019

7.10.3 Instrument Transformers, Coupling-Capacitor Voltage Transformers

A. Visual and Mechanical Inspection:
  1. Inspect physical and mechanical condition.
  2. Prior to cleaning the unit, perform as-found tests, if required.
  3. Clean the unit..
  4. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.(optional)
  5. Verify correct operation of transformer withdrawal mechanism and grounding operation.
  6. Verify correct primary and secondary fuse sizes for voltage transformers.
  7. Use appropriate lubrication on moving current-carrying parts and on moving and sliding surfaces.
  8. Perform as-left tests.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Test voltages shall be applied for one minute in accordance with Table 100.5. For units with solidstate components that cannot tolerate the applied voltage, follow manufacturer’s recommendations.
  3. Perform a polarity test on each transformer, as applicable, to verify the polarity marks or H1-X1 relationship. See IEEE C93.1 for standard polarity marking. (optional)
  4. Perform a ratio test on the as-found tap position.
  5. Measure voltage circuit burdens at transformer terminals. (optional)
  6. Perform a dielectric withstand test on the primary windings with the secondary windings connected to ground. The dielectric voltage shall be in accordance with Table 100.9. The test voltage shall be applied for one minute. (optional)
  7. Measure capacitance of capacitor sections.
  8. Measure insulation power factor or dissipation factor in accordance with test equipment manufacturer’s published data
  9. Verify that potential circuits are grounded and have only one grounding point in accordance with IEEE C57.13.3.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of the thermographic survey shall be in accordance with Section 9. (optional)
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt-torque levels should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Polarity results shall agree with transformer markings.
  4. In accordance with IEEE C57.13; 8.1.1 the ratio error should be as follows:
    1. Revenue metering applications: equal to or less than ±0.1 percent for ratio and ±0.9 mrad (three minutes) for phase angle.
    2. Other applications: equal to or less than +1.2 percent for ratio and ±17.5 mrad (one degree) for phase angle.
  5. Compare measured burdens to instrument transformer ratings.
  6. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand voltage test, the test specimen is considered to have passed the test.
  7. Capacitance of capacitor sections of coupling-capacitance voltage transformers should be in accordance with manufacturer’s published data.
  8. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.
  9. Test results should indicate that the circuits have only one grounding point.
NETA ATS / MTS
TABLE 100.5
Neta Table 100.5
NETA ATS / MTS
Neta Table 100.5
Neta Table 100.5