Capacitance and Tan Delta Test of Transformer Bushings: Step By Step Test Procedures

The health of transformer bushings is critical for ensuring the safe and reliable operation of power transformers in electrical substations. Among the various diagnostic tests performed during transformer testing and commissioning, the Capacitance and Tan Delta Test stands as one of the most effective non-destructive methods for evaluating the insulation condition of bushings. In this blog post we will explore the principles, procedures, and significance of capacitance and tan delta testing for transformer bushings, providing essential knowledge for electrical engineers and technicians involved in transformer commissioning and maintenance.

What is a Transformer Bushing?

A transformer bushing is a specialized insulating device that allows electrical conductors to pass safely through the grounded metal tank of a transformer while maintaining electrical isolation. Bushings serve as the critical interface between the high-voltage conductors and the transformer’s internal windings, providing both mechanical support and electrical insulation.

For high-voltage applications, condenser-type bushings (also called capacitance-graded bushings) are universally employed due to their superior ability to control electric field distribution along the length of the insulation.

The condenser bushing incorporates multiple layers of conductive foils alternating with insulating material (typically oil-impregnated paper or resin-impregnated paper) to create a capacitive voltage divider. This graded design ensures uniform voltage distribution along the bushing length, reducing electric field stress and enabling more compact dimensions compared to non-graded designs. The importance of bushing health cannot be overstated, as bushing failures account for approximately 14-20% of all major transformer failures, making them the third leading cause of transformer outages after tap changers and windings.

Types of Transformer Bushings

Transformer bushings are primarily classified based on their insulating material composition:

Oil-Impregnated Paper (OIP) Bushings

OIP bushings utilize cellulose paper layers impregnated with transformer-grade mineral oil as the dielectric medium. The condenser core is wound from kraft paper, treated, and impregnated with insulating oil, which remains in liquid form throughout the bushing’s operational life. The core is housed within a porcelain insulator, with the space between the core and porcelain filled with the same insulating oil. OIP bushings are the traditional technology and remain widely used, particularly in existing transformer installations. They typically exhibit tan delta values of 0.45% or lower and partial discharge levels below 5 pC.

However, OIP bushings have inherent vulnerabilities. They are susceptible to moisture ingress through worn seals, oil leakage due to gasket degradation, and explosive failures when subjected to lightning strikes or internal faults. The liquid oil content makes them vulnerable to environmental contamination and requires periodic monitoring of oil level and quality.

Resin-Impregnated Paper (RIP) Bushings

RIP bushings represent an advanced technology where the paper condenser core is impregnated with epoxy resin under vacuum conditions, forming a solid dielectric structure after curing. This eliminates the need for liquid oil in the condenser core, though the bushing may still be installed in an oil-filled transformer. RIP bushings can be housed in either porcelain or silicone rubber composite insulators, with the intervening space filled with polyurethane foam, gel, or other solid insulating materials.

RIP bushings offer several advantages over OIP designs: they are maintenance-free with no oil monitoring requirements, have no risk of oil leakage, exhibit lower partial discharge levels (typically less than 2 pC), demonstrate superior tan delta values (0.35% or less), possess higher insulation class ratings (Class-E up to 120°C versus Class-A up to 105°C for OIP), and show improved seismic and mechanical strength. The solid construction provides better performance in harsh environmental conditions and reduces the risk of catastrophic failures. However, RIP bushings typically have higher initial costs compared to OIP designs.

Capacitance in Bushings

In condenser-type bushings, two principal capacitances are defined and measured: C1 (main capacitance) and C2 (tap capacitance).

C1 Capacitance (Main Capacitance)

C1 capacitance represents the total capacitance of the main insulation system, measured between the high-voltage center conductor and the test tap electrode. This capacitance is formed by all the capacitive layers from the central current-carrying conductor to the outermost grading foil, which is connected to the test tap. For bushings rated 115 kV and above, C1 values are highly predictable and stable since they depend solely on the internal paper insulation and condenser design, which are strictly controlled during manufacturing.

The C1 capacitance value is typically specified on the bushing nameplate and serves as a critical diagnostic parameter. In a healthy bushing, C1 should remain relatively constant over time. An increase in C1 capacitance of more than 3% from the factory value indicates partial breakdown or short-circuiting of condenser layers within the bushing core—a serious condition requiring immediate bushing replacement.

Conversely, a decrease in C1 capacitance compared to factory values may indicate physical damage during transportation or installation, suggesting the bushing should not be put into service. C1 capacitance measurement is performed using the Ungrounded Specimen Test (UST) mode at typically 10 kV test voltage.

C2 Capacitance (Tap Capacitance)

C2 capacitance is the capacitance between the test tap electrode and the grounded mounting flange of the bushing. This capacitance comprises a small section of the condenser core insulation between the outermost grading foil (connected to the test tap) and the grounded flange. During normal transformer operation, the C2 insulation is typically shorted to ground through the test tap cover, meaning it experiences no voltage stress under operating conditions.

For bushings equipped with voltage taps (as opposed to test taps), the C2 section acts as part of a capacitive voltage divider when connected to potential devices for voltage measurement or monitoring purposes. The C2 capacitance is more sensitive to external factors than C1, particularly for bushings rated below 115 kV, where it may be affected by stray capacitances, contamination on porcelain surfaces, air gaps, and surrounding oil conditions. For higher voltage bushings (115 kV and above), C2 is primarily dependent on the internal paper insulation and is therefore more stable and predictable.

C2 measurement is performed using the Grounded Specimen Test with Guard (GSTg) mode at lower test voltages, typically 500-2000 V or 1.0 kV as specified. Due to the influence of stray capacitances, C2 values may show deviations of up to ±50% from nameplate values, particularly for test tap configurations.

Tan Delta (Dissipation Factor) in Bushings

Fundamental Principle

The tan delta test, also known as the dissipation factor test or power factor test, is a diagnostic measurement that quantifies the dielectric losses in insulation materials.

In an ideal, pure insulator with perfect dielectric properties, the material behaves as a perfect capacitor. When an AC voltage is applied, the resulting current is purely capacitive and leads the voltage by exactly 90 degrees. Under these ideal conditions, there is no resistive component to the current, meaning zero energy is dissipated as heat.

However, in real-world insulation systems, the material is never perfectly pure. Aging, moisture ingress, contamination, and inherent material impurities introduce conductive paths through the insulation. These impurities cause a resistive component of current to flow through the insulator. This resistive current is in phase with the applied voltage, while the capacitive current remains 90 degrees ahead of the voltage.

The phase angle between the applied voltage and the total resulting current becomes less than 90 degrees due to this resistive component. The loss angle \((\delta)\) is defined as the complementary angle, and \(\tan\delta\) is simply the tangent of this loss angle. Mathematically, tan δ equals the ratio of the resistive current \((I_R)\) to the capacitive current \((I_C)\):

\( \tan \delta = \frac{I_R}{I_C} \)

For a healthy insulator with minimal impurities, tan δ is very low because the resistive current is negligible compared to the capacitive current. As insulation degrades due to moisture, aging, or contamination, the resistive component increases, resulting in higher tan δ values.

Capacitance and Tan δ Test Modes

Three primary measurement modes are employed for bushing testing, each suited to specific test configurations:

Ungrounded Specimen Test (UST) Mode

The UST mode is used when the specimen is electrically isolated from ground and has two accessible terminals for measurement. This configuration is ideal for transformer bushings equipped with test taps or voltage taps, current transformers with accessible test taps, capacitor voltage transformers, and circuit breaker voltage grading capacitors.

In UST mode, high voltage is applied to the bushing conductor while the test tap is connected to the low-voltage terminal of the test set. The mounting flange is grounded. This configuration effectively eliminates the influence of stray capacitance losses to ground and reduces interference from nearby energized equipment. UST mode is the standard method for measuring C1 capacitance and tan δ of the main bushing insulation.

Grounded Specimen Test (GST) Mode

The GST mode is employed when the specimen does not have two isolated terminals for measurement, meaning the specimen is inherently grounded or cannot be separated from ground. This mode is commonly used for testing transformer windings, reactor windings, current transformers without test taps, and overall bushing insulation when the test tap is not accessible.

In GST mode, the high-voltage terminal is connected to the conductor being tested, while the grounded components form the return path. This mode measures total insulation including all ground capacitances.

Grounded Specimen Test with Guard (GSTg) Mode

The GSTg mode is a refined version of GST that separates total values into component parts for more detailed analysis. This mode is frequently used to measure C2 capacitance of bushings, where the test tap is energized, and losses from the tap to the grounded flange are measured.

GSTg mode uses a guard terminal to exclude certain capacitances from the measurement, allowing isolation of specific insulation sections. For C2 testing, the high-voltage conductor is connected to the guard terminal to eliminate its influence, the test tap receives the high-voltage test signal, and the low-voltage terminal connects to ground. This mode is typically performed at lower voltages (500-2000 V) to avoid over-stressing the relatively small C2 insulation section.

The GSTg mode is also used in conjunction with UST mode to confirm and validate test readings, particularly when troubleshooting suspect bushing measurements.

Test Equipment

Accurate capacitance and tan delta testing requires specialized high-voltage test equipment. The standard instrumentation includes a 10 kV or 12 kV fully automatic capacitance and tan delta test kit, which provides accurate measurement and excellent repeatability of test results. These test sets are often referred to as “Doble testers” in the industry, named after the Doble Engineering Company that pioneered this testing methodology and manufactures widely-used test equipment (such as the M4000 series).

Modern test equipment incorporates advanced features including digital signal processing, interference suppression capabilities to handle electromagnetic noise, variable frequency testing (typically 15-400 Hz range), high-precision measurement circuits (with capacitance accuracy of 0.3% and tan delta accuracy of 1×10⁻⁴), integrated high-voltage sources, touchscreen interfaces for ease of operation, and automated test sequencing with reporting capabilities.

Testing Procedure for Transformer Bushings

Pre-Test Preparations

Before commencing capacitance and tan delta testing, several critical safety and procedural steps must be observed:

• Personnel qualification: Testing must be carried out only by experienced and certified personnel with appropriate training in high-voltage test procedures.
• De-energization: Absolutely ensure that the transformer and all associated equipment are completely de-energized and isolated from all power sources. Never connect test equipment to energized apparatus under any circumstances.
• Grounding: The ground cable must be connected first before any other connections and removed last after testing is complete. After completing high-voltage testing (10-12 kV), all test terminals must be grounded before being touched by personnel.
• Test lead inspection: There should be no joints or splices in testing cables. High-voltage leads must be double-shielded screened cables. Verify that shields do not have internal shorting, as this would prevent GST/GSTg mode testing. Check cable integrity using a 100V insulation tester.
• Bushing surface preparation: Bushings should be cleaned before testing to remove contamination from porcelain surfaces. Surface contamination and moisture can cause inaccurate results.
• Environmental conditions: Avoid testing during unfavorable weather conditions, especially high relative humidity, heavy rain, or fog, as these can affect surface leakage on porcelain bushings and compromise measurement accuracy.
• Test tap access: Only remove the test tap cover from the bushing currently under test. All other bushings in the same voltage group should remain with their test taps covered and grounded to prevent dangerous voltages from developing at open test taps.
• Connection hardware: Use proper adapters provided by the test set or bushing manufacturer to ensure secure, well-insulated connections to the test tap. Improper connections can affect results and pose safety hazards.
• Documentation review: Have available the bushing nameplate data, factory test reports, and previous field test results for comparison purposes.

Step-by-Step Testing Procedure for Bushings

The following procedure describes the standard method for testing transformer bushings on a three-phase auto-transformer, though the principles apply to any transformer configuration:

Initial Setup

For a three-phase auto-transformer with high-voltage (HV), intermediate-voltage (IV), and low-voltage (LV) bushings:

1. Short together all HV and IV bushings: Connect all 400 kV, 220 kV, 132kV and Neutral bushings together (isolated from earth) using appropriate bus bars or heavy-duty cables. Label this group “HV”.
2. Short all LV bushings and ground them: Connect all 33 kV bushings together and connect this group solidly to the transformer tank/earth. Label this “LV Grounded”.
3. Organize test leads: Prepare the high-voltage (HV) cable with crocodile clip, the low-voltage (LV) screened cable for test tap connection, the guard cable (if performing GSTg tests), and grounding cables.

Measurement of C1 Capacitance and Tan Delta (UST Mode)

This is the primary test for bushing main insulation:

1. Connect HV lead: Attach the crocodile clip of the HV cable from the test set to the top terminal of the shorted HV/IV bushing group.
2. Access test tap: For 245 kV OIP bushings, unscrew the test tap cover and insert the connection pin into the central test tap stud. For 420 kV OIP bushings, remove the earthing strip from the flange by unscrewing the holding screw.
3. Connect LV lead to test tap: Connect the LV screened cable from the test set to the test tap of the bushing under test. Ensure a secure, well-insulated connection.
4. Ground the flange: Connect the flange body solidly to ground/earth.
5. Configure test set: Set the instrument to UST mode. Set test voltage to 10 kV for C1 measurement (or as specified by manufacturer).
6. Perform measurement: Execute the automated test sequence. The instrument will apply the test voltage, measure capacitance and tan delta, and display results.
7. Record results: Document the measured C1 capacitance (in pF), tan delta (in percentage), test temperature, and date. Compare with nameplate values and previous test results.
8. Repeat for each bushing: Change only the LV lead connection to the test tap of each bushing to be tested while keeping the HV and ground connections as established. Test all bushings in the HV/IV group.

Measurement of C2 Capacitance and Tan Delta (GSTg Mode)

This secondary test evaluates the tap-to-flange insulation:

1. Reconfigure connections: Connect the HV lead from the test set to the test tap of the bushing under test (use additional crocodile clip if needed).
2. Connect conductor to guard: Connect the high-voltage bushing conductor to the Guard terminal of the test set. This excludes the C1 section from the measurement.
3. Connect LV lead to ground: Connect the LV terminal of the test kit to ground/earth.
4. Configure test set: Set the instrument to GSTg mode. Set test voltage to 1.0 kV for C2 measurement (or as specified).
5. Perform measurement: Execute the test sequence to measure C2 capacitance and tan delta.
6. Record results: Document C2 values and compare with nameplate data.

Testing 33 kV (LV) Bushings

For lower voltage bushings in the same transformer:

1. Reverse grounding: Ground the HV/IV bushings (already shorted together).
2. Connect test leads: Apply the HV lead of the test kit to the shorted 33 kV bushing group, connect the LV lead to the test tap of the bushing under test.
3. Perform UST measurement: Test in UST mode following the same procedure as for HV bushings.

Post-Test Procedures

After completing all measurements:

1. Ground test terminals: Immediately after testing each bushing at high voltage (10-12 kV), ground the test terminals before touching them.
2. Re-install test tap covers and grounds: It is critical to ensure that test tap points are properly grounded after measurements. Reinstall the test tap cover or earthing strip. Verify proper grounding by performing a continuity test.
3. Remove test equipment: Disconnect all test leads in reverse order of connection—remove HV leads first, then LV leads, and finally ground leads last.
4. Documentation: Complete all test documentation including measured values, nameplate comparison, temperature at time of testing, weather conditions, any observations, and recommendations based on results.

Frequency Sweep Tan Delta Testing

In addition to standard power frequency (50/60 Hz) tan delta measurement, frequency sweep testing provides valuable additional diagnostic information about bushing condition, particularly for detecting moisture and early-stage degradation.

Principle and Advantages

Frequency sweep testing involves measuring tan delta and capacitance at multiple frequencies across a range, typically from 15-17 Hz up to 400 Hz. Different dielectric phenomena shows at different frequencies:

• Low frequency response (15-50 Hz): The measurement of dissipation factor at low frequencies enables detection of moisture with very high sensitivity. Moisture effects are most pronounced at lower frequencies due to ionic conduction mechanisms.
• Power frequency (50/60 Hz): Standard diagnostic frequency for routine testing and comparison with historical data.
• Higher frequencies (100-400 Hz): Help characterize the overall insulation condition and detect certain types of degradation.

By examining the tan delta response across multiple frequencies, engineers can better distinguish between different failure mechanisms and assess insulation condition more comprehensively than single-frequency testing alone.

Frequency Sweep Procedure

Frequency sweep measurements should be carried out for all condenser bushings, particularly those rated above 245 kV. The measurement procedure is similar to standard testing, but the test set automatically varies the frequency while maintaining constant voltage:

1. Select standard frequencies: To enable consistent analysis and comparison, use standardized frequency points. A typical frequency sequence includes: 17, 25, 34, 43, 51, 68, 85, 102, 119, 136, 187, 255, 323, and 391 Hz.
2. Perform automated sweep: Modern test sets automatically step through the frequency range, measuring tan delta and capacitance at each frequency point.
3. Plot response curves: The test results are typically displayed as tan delta versus frequency curves for visual analysis.

Interpretation of Frequency Sweep Results

The shape of the tan delta frequency response curve provides diagnostic information:

Healthy OIP bushings: Show relatively flat tan delta response across the frequency range, with values remaining fairly constant or showing only slight variation. The tan delta at all frequencies should remain below 0.5%.

Healthy RIP bushings: Exhibit a rising tan delta trend with increasing frequency, which is normal behavior for resin-impregnated insulation. However, the absolute values remain low, and the pattern should be consistent with factory baseline data.

Moisture-contaminated bushings: Display significantly elevated tan delta at low frequencies (15-50 Hz) compared to the 50/60 Hz value. An increase in tan delta at 17 Hz of more than 0.1% compared to the 51 Hz value indicates moisture ingress. The tan delta “tips down” at low frequencies when moisture is present.

Aged or deteriorated bushings: Show elevated tan delta across the entire frequency range, with values exceeding nameplate specifications.

Important note: Sometimes disturbances may appear in the tan delta response near power frequency (50/60 Hz) when unfavorable weather conditions exist, especially high relative humidity. These are typically surface effects. The porcelain bushing surface should be cleaned and dried to minimize surface leakage effects before concluding that internal insulation problems exist.

Recommended Frequency Sweep Limits for OIP Bushings

For new OIP bushings, indicative limits at key frequencies are:

• 17 Hz: tan delta ≤ 0.5% maximum, or not more than 0.1% increase from 51 Hz value
• 50/51 Hz: tan delta ≤ 0.4% maximum
• 391 Hz: tan delta ≤ 0.5% maximum, or not more than 0.1% increase from 51 Hz value

Any bushing exceeding these limits or showing significant deviation from factory baseline curves should be investigated further and potentially replaced.

Acceptable Limits and Diagnostic Criteria

Tan Delta Limits

For all types of bushings, the tan delta at 50/60 Hz should be less than 0.5% after temperature correction to 20°C. This limit is specified in both IEC 60137 and IEEE C57.19.01 standards.

However, the limiting value specified in standards represents the maximum acceptable for new equipment. More stringent criteria apply for diagnostic evaluation of bushings in service:

Comparison with nameplate data: The measured tan delta should not exceed twice the nameplate or factory test value. Values between 2-3 times nameplate warrant close monitoring, while values exceeding 3 times nameplate indicate the bushing should be replaced.

Variation from baseline: The tan delta should not increase by more than 0.1% from previous measurements or factory values when corrected to the same temperature. Increases exceeding this threshold indicate progressive deterioration.

Absolute limits by bushing type:

• OIP bushings: Typically exhibit tan delta values of 0.45% or lower when new. Values of 0.5-1.0% may be acceptable for older bushings but require investigation. Values exceeding 1.0% at 20°C should be investigated for potential replacement.
• RIP bushings: Should demonstrate tan delta values of 0.35% or less. The solid epoxy insulation typically yields lower loss factors than oil-paper systems.

Capacitance Limits

C1 Capacitance:

• Should not vary by more than ±3% from the nameplate or factory value
• An increase exceeding 3% indicates partial puncture of condenser layers and requires immediate bushing replacement
• A decrease compared to factory values suggests transport damage and the bushing should not be installed

C2 Capacitance:

• Shows greater natural variation than C1 due to stray capacitance effects
• Deviations up to ±50% from nameplate values may be acceptable for test tap configurations due to external influences
• For potential tap bushings (IEEE designs), variations should be within ±10% as stray effects are minimized
• A 5-10% increase or decrease in C2 from baseline measurements warrants investigation

Common Causes of Bushing Failures

Moisture Ingress and Insulation Deterioration

Moisture contamination is one of the most serious threats to bushing insulation integrity. Water can enter bushings through degraded seals and gaskets (particularly after temperature cycling), loose seals on the upper end cover, damaged or improperly installed mounting flanges, porcelain cracks or chips, improper storage before installation (bushings stored outdoors or in humid environments), and prolonged storage without proper sealing.

Even small amounts of moisture dramatically increase tan delta values, particularly at low frequencies, while simultaneously reducing dielectric strength and increasing the risk of partial discharge and flashover. The oil-paper insulation in OIP bushings is especially vulnerable, as moisture dissolves in the oil and migrates into the paper insulation.

Thermal Overload and Overheating

Bushings are subjected to continuous electrical and mechanical stress, and excessive heat accelerates insulation degradation. Overheating can result from transformer overloading beyond rated capacity, poor electrical contacts at bushing terminals creating resistive heating, inadequate cooling system performance, high ambient temperatures combined with solar heating on outdoor bushings, and internal partial discharge activity generating localized hot spots.

Elevated temperatures cause accelerated aging of insulation materials. The insulation class rating (Class A for OIP at 105°C, Class E for RIP at 120°C) represents the maximum continuous operating temperature. Exceeding these temperatures, even periodically, significantly shortens bushing life.

Partial Discharge Activity

Partial discharges (PD) are localized electrical breakdowns in insulation voids, defects, or regions of high electrical stress that do not completely bridge the insulation. PD activity generates several damaging effects: chemical decomposition of insulation materials producing acidic byproducts and gases, physical erosion creating carbonized tracking paths, progressive enlargement of voids and defects, and localized heating accelerating deterioration.

PD typically originates from manufacturing defects (voids, contamination in the condenser core), excessive mechanical stress from short circuits or seismic events, thermal aging reducing insulation integrity, moisture creating regions of locally reduced dielectric strength, and overvoltage transients from lightning or switching events.

Once initiated, PD is a progressive failure mode. Discharge channels become carbonized, creating conductive paths. Eventually, enough condenser foils short circuit that the remaining layers cannot withstand the voltage stress, leading to catastrophic flashover and bushing explosion.

Mechanical Damage and Stress

Physical damage to bushings can compromise insulation integrity:

• Transportation and handling damage: Impact, vibration, or improper lifting can crack porcelain insulators, damage internal condenser cores, or loosen internal connections
• Seismic events: Earthquakes impose severe mechanical stress. RIP bushings generally demonstrate superior seismic performance compared to OIP bushings
• Through-fault forces: Short circuit currents create enormous electromagnetic forces, potentially causing mechanical displacement of bushing components or loosening of terminal connections
• Vandalism and external impact: External damage to porcelain or mounting flanges

Electrical Transients and Overvoltages

Bushings are exposed to various transient overvoltage conditions that can degrade insulation or cause immediate failure:

• Lightning strikes: Generate very fast transients with rise times in the nanosecond range and amplitudes potentially exceeding 2.0 per unit (twice nominal voltage)
• Switching transients: Result from circuit breaker operations, reactor switching, or capacitor bank switching, with durations in the millisecond range
• Resonance conditions: Can produce sustained overvoltages at harmonic frequencies

While bushings are designed with adequate Basic Insulation Level (BIL) ratings to withstand specified transient magnitudes, repeated exposure, particularly combined with aged insulation, progressively damages the dielectric structure. OIP bushings are more vulnerable to explosive failure from transient-induced breakdowns compared to RIP designs.

Long-Term Aging

All bushings experience gradual deterioration over their service life:

• Expected lifetime: Approximately 25-30 years under normal operating conditions, versus 50+ years for transformers, meaning most transformers require bushing replacement during their service life
• Aging mechanisms: Electrical stress causing molecular chain scission in polymers, thermal cycling producing mechanical stress and seal degradation, chemical reactions producing acidic byproducts, and UV exposure degrading external polymer materials

Conclusion

The Capacitance and Tan Delta Test represents the most reliable and widely-adopted non-destructive diagnostic method for assessing transformer bushing insulation condition. This test provides quantitative measurements that directly correlate with insulation quality, offering superior sensitivity for detecting moisture ingress, aging, contamination, and incipient failures compared to traditional insulation resistance testing.

As transformer bushings continue to be a leading cause of transformer failures (representing 14-20% of major failures), the implementation of rigorous testing protocols using capacitance and tan delta measurements, combined with proactive maintenance based on test results, significantly enhances transformer reliability, prevents costly outages, and optimizes asset management in electrical power systems.