Review, Understanding, and Interpretation of Industry Field Test Limits on Transformer

Presented By:
Charles Sweetser
OMNICRON Electronics Corp, USA
TechCon 2021

I. Abstract
II. Introduction
III. Electrical Tests and Limits

• a. Overall Power Factor and Capacitance
• b. Bushing Power Factor and Capacitance
• c. Exciting Current
• d. Turns Ratio
• e. Leakage Reactance
• f. DC Winding Resistance
• g. Sweep Frequency Response Analysis (SFRA)

IV. Case Studies
V. Conclusion
VI. References

Abstract

The electric power industry is always looking for the best approach to better determine and continually track the condition of power transformers. It is important to understand the value of effectively analyzing field test results. Often, industries field limits are applied in the analysis process without much thought.

Through careful selection, hierarchal value, and appropriate times of use, today transformer diagnostics generally consists of a comprehensive suite of basic or standard electrical field tests including:

• Power Factor (Overall and Bushings)
• Exciting Current
• Turns/Voltage Ratio
• Leakage Reactance
• DC Winding Resistance
• Sweep Frequency Response Analysis (SFRA)

These specific diagnostic tests have been selected as the primary focus for this presentation and discussion.

This paper focuses on the interpretation of industry field test limits as applied to power transformers as part of the standard condition assessment protocol. The audience will be provided with an understanding, interpretation, and analysis of test limits, supported by specially selected case studies.

Introduction

The primary goal when performing diagnostic tests on power transformers is to ensure safe operation and accomplish life extension. Understanding the condition of the power transformer is essential. Maintenance personnel must manage moisture, heat, and oxygen, while attempting to protect the power transformer from dielectric, thermal, and mechanical failure modes. This paper is going to focus on a subset of electrical diagnostic tests. We will focus on the test result outcome. We will review and provide understanding of both analysis techniques and use of existing industry limits. We introduce and focus on the following tests:

1. Overall Power Factor and Capacitance
2. Bushing Power Factor and Capacitance
3. Exciting Current Test
4. TTR – Transformer Turns Ratio
5. Leakage Reactance (3-Phase Equivalent and Per Phase)
6. DC Winding Resistance
7. Sweep Frequency Response Analysis (SFRA)

The analysis/limits recommendations found in this paper are based on the contents of:

• IEEE C57.152-2013, “IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors”.
• ANSI/NETA MTS-2019, “Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems”.
• IEEE C57.149-2012, “IEEE Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers”.

Transformer Electrical Tests and Limits

1. Overall Power Factor and Capacitance

The overall power factor measurement is used to assess the integrity of the insulation system within a transformer. The unit-less value of power factor represents efficiency. With respect to insulation, we expected the insulation system to be efficient with respect to power loss. Several contributing factors may affect the efficiency of the insulation. The insulation system may become compromised due to one or more of the following contributing factors:

• Natural aging and deterioration
• Overheating
• Moisture ingress
• Localized defects (such as partial discharge, voids, cracks, and partial or full short-circuits)

When the insulation system of a transformer becomes compromised, the insulation becomes mechanically and/or dielectrically weaker, which may lead to an undesired failure mode.

For discussion purposes, we will consider a two-winding transformer; delta-wye (Dyn1). When studying the two-winding transformer, there are three insulation components that can be isolated and tested when the overall power factor is performed, which includes,

1. CH: High-voltage winding-to-ground insulation, including the high-voltage bushing insulation
2. CL: Low-voltage winding-to-ground insulation, including the low-voltage bushing insulation
3. CHL: High-voltage to low-voltage (inter-winding) insulation, which does not include the bushing insulation

Special attention is given to analyzing the CHL insulation. This inter-winding insulation component consists of major insulation between windings. This measurement performed in the UST mode is exempt from external influences, such as the bushing insulation and external surfaces.

Test Preparations:

When performing overall power factor and capacitance measurements, the following test preparations are recommended:

1. Ensure that the transformer tank and core are solidly grounded, also connect both the test instrument and power source ground to this point. We will refer to this point as the “GROUND” node.
2. Ensure that all bushing surfaces are clean and dry.
3. Completely isolate the transformer terminals; remove external connections and buswork from H1, H2, H3, X1, X2, X3 and X0.
4. Bond/short the H1, H2, and H3, making sure that they are isolated. We will refer to this point as the “HV” node.
5. Bond/short the X1, X2, X3, and X0 making sure that they are isolated. We will refer to this point as the “LV” node.
6. Document tap-positions, temperatures, humidity, fluid levels, and pressures.

Test Procedure:

When performing overall power factor capacitance measurements, the following test procedures are recommended:

The test voltages will be limited and should not exceed the line-to-ground rating of the insulation system. Often, a 10 kV maximum is applied; due to the limits of portable test equipment. When convenient, Variable Frequency Power Factor Tests will be performed on CH, CL, and CHL insulation components, along with Power Factor Tip-Up measurements.

Before each measurement, ensure that the cable is in the clear.

Shown below, in Table 1, is a typical test plan for overall power factor in capacitance measurements:

Review of Industry Limits:

The following shall be expected regarding power factor measurements transformers:

IEEE C57.152 [1]

• PF < 0.5% at 20 °C for “new” liquid filled power transformers rated under 230kV
• PF < 0.4% at 20 °C for “new” liquid filled power transformers rated over 230kV
• PF < 1.0% at 20 °C for “service aged” liquid filled power transformers
• PFs between 0.5% and 1.0% at 20 °C warrant additional testing and investigation

NETA MTS [2]

• PF < 1.0% for liquid filled power transformers
• PF < 2.0% for liquid filled distribution transformers
• PF < 2.0% for dry-type power transformers (CHL insulation)
• PF < 5.0% for dry-type distribution transformers (CHL insulation)
• PF Tip-Up for dry-type insulation should be < 1.0%

Note: Measured values should also be compared to the manufacturer’s published data.

Additional Analysis Recommendations:

• Analyze the Variable Frequency Power Factor Tests. Focus on the interwinding insulation. If tested at or near 20 C, the slope of the Variable Frequency Power Factor curve should be positive at 60 Hz. This does not guarantee but provides strong evidence of dry cellulose/mineral oil insultation.
• Confirm the presence of Tip-up. The limit for dry-type insulation is stated above. Liquid filled units should not produce Tip-Up.
• Inter-winding measurements on complex insulation system, such as 3 winding transformers, often result in low charging current. This indicates a possible shield or a design that prohibits the direct measurement of a specific inter-winding insulation component. IEEE C57.152 states [1]: “A complex insulating system consists of three or more terminals insulated from each other. PF calculations should not be used to determine the integrity of insulation if the measured current is less than 0.3 mA. At low measured currents, PF calculations are susceptible to large swings, which could be misleading. Therefore, in those cases, the test results should be evaluated based on current and loss readings.” However, the watts loss reading can only be used to compare to future readings. There are no recommended limits for such “Watts Loss” measurements.

2. Bushing Power Factor and Capacitance

A bushing power factor measurement is used to assess the integrity of the insulation system within a bushing. Availability of a test tap or a potential tap will allow testing of the main insulation, C1, and the tap insulation, C2. If neither are available, a hot collar test will be performed. The nameplate ratings of the bushings will determine applicable test voltages.

Note: Only remove one bushing tap adapter cap at a time. Immediately return each bushing tap cap after every test.

Test Preparations:

When performing bushing power factor measurements, the following test preparations are recommended:

1. Ensure that the transformer tank, bushing flanges, and core are solidly grounded, also include both the test instrument and power source ground to this point. We will refer to this point as the “GROUND” node.
2. Identify the bushing type and characteristics, such as tap type (potential tap or test tap). It is also important to identify whether the bushing insulation system is oil impregnated paper or resin impregnated paper.
3. Identify the bushing’s line to ground rating. This will help in selecting the appropriate test voltage.
4. Ensure that all bushing surfaces and tap areas are clean and dry.
5. Completely isolate the transformer terminals; remove external connections and buswork from H1, H2, H3, X1, X2, X3 and X0.
6. Bond/short the H1, H2, H3, and H0, making sure that they are isolated. We will refer to this point as the “HV” node.
7. Bond/short the X1, X2, X3, and X0 making sure that they are isolated. We will refer to this point as the “LV” node.
8. Prepare and obtain any necessary bushing tap adapters and hot collar straps.
9. Document temperatures, humidity, and bushing fluid levels/color.

Test Procedure:

When performing bushing power factor and capacitance and hot collar measurements, the following test procedures are recommended:

The test voltages will be limited and not exceed the line-to-ground rating of the insulation system. Often, a 10 kV maximum is applied; due to the limits of portable test equipment. When convenient, Variable Frequency Power Factor Tests can be performed on C1 insulation components, along with Power Factor Tip-Up measurements.

Before each measurement, ensure that the cable is in the clear, especially for the C2 measurement.

Shown below, in Table 2, is a typical test plan for overall power factor and capacitance measurements:

Notes:

• Bushings shall remain shorted, similar to the overall power factor test. Failure to short the bushing terminals, may result in compromised measurements.
• Hot Collar tests are optional; they will not be performed if test taps or potential taps are available.
• Test taps and potential taps can be identified, based on the bushing rating, as follows:
  • ➢ Test Taps <= 350 kV BIL
  • ➢ Potential Taps > 350 kV BIL
• C2 tests must be performed carefully, ensuring that the “hook” is in the clear, completely.

Review of Industry Limits:

The following shall be expected regarding power factor measurements:

IEEE C57.152 [1]

• The bushing results should compare well with the nameplate data or initial reading. Power Factor values should not exceed 1.5X to 2.0X nameplate data or initial reading. Any reading that exceeds 3.0X should be removed from service.
• Bushing capacitance should not exceed +/- 5% of nameplate data.
• Increasing/trending PF should warrant concern.
• The hot collar results are analyzed from watts loss. The HC test should not exceed 100 mW loss.

NETA MTS [2]

• Bushing PF results should not vary from nameplate by more than 50%.
• Bushing capacitance should not exceed +/- 5% of nameplate data.
• The HC test should not exceed 100 mW loss.

Additional Analysis Recommendations:

• C1 and C2 values should compare well amongst similar bushings.
• Analyze the Variable Frequency Power Factor Tests. Focus on the C1 main insulation component. If tested at or near 20 °C, the slope of the Variable Frequency Power Factor curve should be positive at 60 Hz. This does not guarantee but provides strong evidence of healthy OIP insultation.
• Confirm the presence of Tip-up. OIP bushing should not produce Tip-Up.
• Bushing with shorter lower side flanges may not produce the same PF results once installed. Experience has shown that undesired coupling can affect both C1 and C2 at times.

3. Exciting Current

The exciting current measurement is performed by applying an AC (60Hz) Voltage (typically at 10kV) across a primary winding of the transformer, while the secondary and other windings are open circuited. Both current and watts loss are measured and recorded. The exciting current test is a single-phase test, and therefore, a series of three measurements are required to measure the exciting current of each phase. They should be repeated on each tap position. These patterns can then be compared and analyzed.

The exciting current test is used to detect the following transformer failure modes,

• Compromised/shorted insulation (turn-to-turn, inter-winding, and/or winding-to-ground insulation)
• Core and core ground defects, including magnetization
• Poor connections and/or open circuits

The analysis of the exciting current measurement is unique, because it does not typically involve applying industry limits or even a comparison to a factory or baseline value. Instead, the analysis of the exciting current measurement involves phase-to-phase or LTC pattern validation and recognition.

Test Preparation:

1. Ensure that the transformer tank and core are solidly grounded, also include both the test instrument and power source ground to this point. We will refer to this point as the “GROUND” node.
2. Completely isolate the transformer terminals; remove external connections and buswork from H1, H2, H3, X1, X2, X3, and X0.
3. Isolate X1, X2, X3, and X0 making sure that they are not connected together. All three of these points must remain in the clear. If present, X0 should be grounded.
4. Document temperatures, humidity, and DETC/OLTC tap positions.

Test Procedure:

Three single-phase tests will be performed. Depending on the rating and burden of the open circuit losses, up to 10 kV will be applied. It is recommended to perform the test in the as found DETC position, while testing each position on the OLTC. Shown below, in Table 3, is a typical test plan for exciting currents on one OLTC tap position.

If the required test current exceeds 200 mA, the test voltage may have to be reduced. The test instrument will automatically stop the test if the current limit has been exceeded.

Review of Industry Limits:

The analysis of the exciting current measurement is unique, because it does not typically involve applying industry limits or even a comparison to a factory or baseline value. Instead, the analysis of the exciting current measurement involves phase-to-phase or LTC pattern validation and recognition. The typical excitation current test data pattern for a transformer is two similar current readings (on the windings of the outer phases of the core) and one lower current reading (on winding on the center phase of the core).

However, other patterns can surface in addition to H-L-H:

1. High – Low – High (HLH) Pattern (most common)
   • Expected for a 3-legged core type transformer
   • Expected for a 5-legged core (or shell) type transformer with a Delta connected secondary winding
2. Low – High – Low (LHL) Pattern
   • Will be obtained on a 3-legged core type transformer if the traditional test protocols are not followed
   • Neutral on high side Wye-configured transformer is inaccessible
   • Forget to ground 3rd terminal on a Delta-connected transformer
3. All 3 Similar Pattern
   • Expected for a 5-legged core (or shell) type transformer with a non-delta secondary winding

Magnetization can and will affect the results.

4. TTR – Transformer Turns Ratio

The transformer turns-ratio (TTR) test is a functional check of the transformer, used to assess if it is properly transforming voltage, according to the nameplate value. If the TTR test does not “pass”, then the transformer is usually not returned to service until the source of the issue has been identified and resolved.

The TTR measurement is used to detect the following transformer failure modes:

• Compromised Insulation (turn-to-turn, inter-winding, and/or winding-to-ground insulation)
• Core Defects
• Severe Discontinuities, Poor Connections, and/or Open-Circuits
• Severe Mechanical Failures (e.g. winding movement or deformation)

Test Preparation:

1. Ensure that the transformer tank and core are solidly grounded, also include both the test instrument and power source ground to this point. We will refer to this point as the “GROUND” node.
2. Completely isolate the transformer terminals; remove external connections and buswork from H1, H2, H3, X1, X2, X3, and X0.
3. Isolate H1, H2, and H3, making sure that they are not connected together.
4. Solidly ground X0.
5. Isolate X1, X2, and X3, making sure that they are not connected together.
6. Document temperatures, humidity, and DETC/OLTC tap positions.

Test Procedure:

We are assuming that the vector group is a “Dyn1”. Anything from a few volts to several hundred volts can be applied as long as the L-G rating or test instrument ratings are not exceeded.

It is recommended to perform the test in the as found DETC position, while testing each position on the OLTC.

Shown below, in Table 4, is a typical test plan for Turns Ratio on one OLTC tap position.

Review of Industry Limits:

IEEE C57.152 [1]

Within 0.5% of the specified nameplate voltage for all windings and winding taps.

NETA MTS [2]

Turns-ratio test results shall not deviate more than one-half of one percent from either the adjacent coils (phases) or from the calculated winding ratio.

Additional Analysis Recommendations:

• Would like to see all phases compare within +/- 0.1%, Not required, but adds confidence to the results.
• Some tap positions, often near the extremes, may slightly exceed the +/- 0.5% limit. This may not be a problem. It is recommended to consult the manufacturer in this scenario.

5. Leakage Reactance

The field leakage reactance test is an AC (60Hz) short-circuit impedance test, which is performed to detect mechanical winding movement and/or deformation within a power transformer. There are two methods for performing leakage reactance tests, as follows:

1. Three Phase (3-Phase) Equivalent Test
2. Per-Phase Test

Test Preparation:

1. Ensure that the transformer tank and core are solidly grounded, also include both the test instrument and power source ground to this point. We will refer to this point as the “GROUND” node.
2. Completely isolate the transformer terminals; remove external connections and buswork from H1, H2, H3, X1, X2, X3, and X0.
3. Isolate H1, H2, and H3, making sure that they are not connected together.
4. Document temperatures and humidity.
5. Supply #4 solid bare copper conductor and C Clamps/Vice Grips/Channel Nuts.
6. Solidly short X1, X2, and X3, do NOT include X0; ground X0.
7. Identify impedance, base power, and base voltage from nameplate.
8. Verify that the DETC and OLTC are in the nominal rated tap position. If not, three-phase equivalent measurement will not be comparable to nameplate.

Test Procedure:

Six tests are to be performed; 3 (3 Phase Equivalent) and 3 (Per-Phase). A four-wire Kelvin connection will be applied. An AC test current should be injected to establish a 30 -100 VAC drop across the primary winding. Table 5 and Table 6, shown below, provided the connections for both the 3 Phase Equivalent tests and Per-Phase tests, respectively.

Review of Industry Limits:

IEEE C57.152 [1]

• The purpose of the 3-Phase equivalent test is to produce a test result to compare to the factory short-circuit impedance percentage value (Z% nameplate), which can be found on the transformer nameplate. A deviation greater than ±3% of the reported value should be investigated.
• If one or more of the Per-Phase measurements is dissimilar from the others, a mechanical failure may exist within the transformer, which should then trigger further investigation. We recommend that the measured impedance (Ω) values of the three Per-Phase measurements compare to within ±3% of the average of the three (Ω) values.

6. DC Winding Resistance

The DC Winding Resistance test is used routinely in the field to validate and assess the continuity of the current carrying path between terminals of a power transformer winding. The DC Winding Resistance test is looking for a change in the continuity or real losses of this circuit, generally indicated by high or unstable resistance measurements. The DC Winding resistance test is used to identify problems such as loose lead connections, broken winding strands, or poor contact integrity in tap changers.

Understanding the expected resistance values is important for setting up and performing a DC Winding Resistance measurement. It is recommended to compare phase measurements, review previous results, or consult the factory test report for determining the expected results. Typical transformer winding resistances generally range from a few milli-Ohms (mΩ) to several Ohms (Ω).

It is recommended to compare phase measurements, review previous results, or consult the factory test report for determining the expected results.

Performing the DC Winding Resistance test quickly and accurately is often challenging. The challenge is due to the fact that the transformer core must be saturated to remove the reactive component of the test circuit before the resistance can be isolated and measured. Testing low resistance windings is often problematic because in order to achieve an adequate terminal voltage, the injected test current must be relatively large, and saturation may take a long time.

Test Preparation:

1. Ensure that the transformer tank and core are solidly grounded, also include both the test instrument and power source ground to this point. We will refer to this point as the “GROUND” node.
2. Completely isolate the transformer terminals. Remove external connections and buswork from H1, H2, H3, X1, X2, X3, and X0; verify that all surfaces are clean and dry.
3. Isolate H1, H2, and H3, making sure that they are not connected together.
4. Isolate X1, X2, and X3, making sure that they are not connected together.
5. Solidly ground X0.
6. Document temperatures, humidity, and bushing fluid levels.

Safety:

• Strictly follow all local safety policies and procedures
• Potential high voltage is present when applying the DC output to test objects with a high inductance
• As long as energy is flowing in the measurement circuit, NEVER connect or disconnect test objects and/or cables.
• Always swap leads at bushing terminals and never at test equipment.
• Use separate clamps for current and voltage connections on both sides of the test object to avoid hazards in case one clamp falls off during the test.

Test Procedure:

Six tests will be performed; 3 on the HV windings and 3 on the LV windings. These tests are shown in Table 7 and Table 8. The optimal current injection level is unknown until the actual test is performed. The tables below recommend current injection ranges. It is recommended that the user start with a default of 1 A DC and 10 A DC, for the HV and LV sides, respectively. The actual injected current will be determined once a preliminary measurement is performed.

Review of Industry Limits:

IEEE C57.152 [1]

The test results will be compared one phase to another. In the phase-comparison it is expected that the resistance measurements compare to within ±2%, however, ±5% is allowable.

NETA MTS [2]

Temperature corrected values should compare with 2% of a previous measurement.

Additional Analysis Recommendations:

• Special consideration will be given to measurements below 10 mΩ; small variation can cause large “%” differences.
• Temperature correction is not required; however, it is recommended.

7. Sweep Frequency Response Analysis

Sweep Frequency Response Analysis (SFRA) is a diagnostic tool used to assess the mechanical and electrical integrity of power transformers. The SFRA test consists of measuring the transfer function (Vout/Vin) of a power transformer winding over a wide sweep of frequencies from 20 Hz to 2 MHZ. The equivalent circuit of a transformer winding includes the coil resistance and inductance as well as capacitances between the turns and the other windings, and between the winding, the tank wall, and the core. Winding movement and/or deformation will cause changes in these passive RLC elements, thus changing the frequency response of the transformer winding. Deviations in the SFRA Measurements can be used to identify the following mechanical failure modes:

• Radial Deformation (faults)
• Axial Deformation (faults)
• Bulk Winding Movement (transportation)

It can also identify electrical problems such as:

• Broken or Loose Connections
• Shorted Turns (Compromised Insulation)

Test Preparation:

1. Ensure that the transformer tank and core are solidly grounded, also include both the test instrument and power source ground to this point. We will refer to this point as the “GROUND” node.
2. Completely isolate the transformer terminals; remove external connections, such as cables, from H1, H2, H3, X1, X2, X3, and X0.

Test Procedure:

Based on the IEEE C57.149 guide, 9 tests are recommended for the Dyn1 configuration. The 9 tests are shown in Table 9:

Review of Industry Limits:

The test results are to be analyzed in accordance with IEEE C57.149. [3]

Focus on key characteristics:

• Radial and Axial Deformation (Post Fault)
• Bulk Movement (Transportation Damage)

Case Studies

To better demonstrate the use of industry limits, this paper will present all test results in a visual format instead of just displaying the data as numbers in a table. A service aged 34.5 kV transformer will be used for this case study, and the following test results will be presented and compared to the known industry limits.

• Overall Power Factor
• Bushing Power Factor and Capacitance
• Exciting Current
• Turns Ratio
• DC Winding Resistance

Overall Power Factor:

In this example we expect the PF measurements to be < 1.0 % at 20 °C. All measurements are well below this threshold shown in Figure 1. They are assigned a PASS.

Other Factors to Consider:

• There is no initial/previous data
• Test were performed at 18°C
• The bushings have not been compensated/removed from PF measurements

Bushing C1 Power Factor and Capacitance:

Power Factor values should not exceed 1.5X to 2.0X nameplate data or initial reading. Any reading that exceeds 3.0X should be removed from service. Bushing capacitance should not exceed +/- 5% of nameplate data. All 3 bushings PASS the capacitance change limit, however, all 3 FAIL the 1.5X nameplate recommendations. These comparisons to nameplate are shown in Figure 2. This warrants an investigation.

Other Factors to Consider:

• There is no initial/previous data
• Test were performed at 18°C
• These are GE type U bushings

Bushing C2 Power Factor and Capacitance:

Nameplate data was not avialible, either was initial/previous data. An analysis cannot be performed against industry limits. However, the values are low and somewhat consistent. These C2 measurements, shown in Figure 3, would not be used to further condemn the bushings even considering the poor C1 results.

Exciting Current:

The Exciting Current produced the expected High-Low-High pattern. The results also match the expected pattern of a reactor type tap changer; the bridging positions are higher than the non-bridging positions. The results in Figure 4 are given a PASS.

TTR:

The results shown in Figure 5 are clearly within 0.5% of the specified nameplate voltage for all windings, winding taps, and adjacent windings. These results are easily assigned a PASS and require no other considerations.

HV DC Winding Resistance:

The DC Winding Resistance measurements compare to within ±2%. A PASS is assigned.

It should be noted that initial/previous data was not avialible, so only the 2% rule was applied, no other considerations were applied.

LV DC Winding Resistance:

The DC Winding Resistance measurements compare to within ±2%. A PASS is assigned.

It should be noted that initial/previous data was not avialible, so only the 2% rule was applied, no other considerations were applied.

Conclusion

• Knowing how to use and apply documented industry limits plays a key role in the analysis and decision-making process.
• NETA and IEEE standards and guides provide comprehensive information regarding test plans test procedures test preparations, and analysis of the results.
• As materials and technologies advance, it is more important to stay educated on the content that is available regarding industry limits.

References

[1] IEEE C57.152-2013, “IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors”.

[2] ANSI/NETA MTS-2019, “Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems”.

[3] IEEE C57.149-2012, “IEEE Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers”.