Transformer bushings are the high-voltage entry points into the grounded tank of a power transformer. A single bushing failure can result in transformer damage, unplanned outages, and expensive repairs. The capacitance and tan delta test also called the dissipation factor test or power factor test is one of the most trusted non-destructive diagnostic methods for evaluating bushing insulation condition during commissioning and routine maintenance.
This test measures two parameters for each bushing: capacitance (C) and dissipation factor (tan δ). A healthy bushing shows stable capacitance and low tan delta values throughout its service life. Any deviation from the factory nameplate values acts as an early warning sign of moisture ingress, insulation aging, contamination, or condenser layer failure. Performing this test correctly and interpreting the results accurately can prevent expensive transformer failures.
In this technical guide, we will discuss everything you need to know about the capacitance and tan delta test of transformer bushings, including applicable IEEE and IEC standards, test equipment requirements, test modes (UST, GST, GSTg), step-by-step field test procedures, acceptance criteria for OIP and RIP bushings, temperature correction methods, and common field testing mistakes to avoid. Practical examples are included throughout to help you apply these concepts in real-world scenarios confidently.
1. Types of Transformer Bushings
Two major types of condenser bushings are used in modern power transformers. They are Oil-Impregnated Paper (OIP) and Resin-Impregnated Paper (RIP). The type of bushing directly affects the expected tan delta values and acceptance limits. So it is important to understand the differences before running any diagnostic test.
1.1 Oil-Impregnated Paper (OIP) Bushings
OIP bushings have been the most widely used type for decades. In an OIP bushing, capacitance grading is achieved by wrapping Kraft paper multiple times around the conductor core and placing conductive foil inserts at specific intervals during wrapping. Afterwards, this main insulation system is impregnated with mineral insulating oil.
OIP bushings require periodic oil-level monitoring. They are susceptible to oil leakage, moisture ingress through faulty gaskets, and thermal degradation of the paper-oil system over time. The thermal class rating for OIP bushings is Class-A (up to 105°C) as per IEC 60085.
1.2 Resin-Impregnated Paper (RIP) Bushings
RIP bushings offer several advantages over OIP designs. They are maintenance-free with no oil monitoring requirements. There is no risk of oil leakage. They exhibit lower partial discharge levels (less than 2 pC). New RIP bushings from reputable manufacturers show tan delta values of 0.35% or less, well below the IEEE C57.19.01 factory acceptance limit of 0.85%. They also have a higher thermal class rating of Class-E (up to 120°C) compared to Class-A (up to 105°C) for OIP. 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.
The solid epoxy-resin construction makes RIP bushings more suitable for harsh environmental conditions and seismic zones. However, RIP bushings have higher initial costs compared to OIP designs. This cost difference is an important consideration for power transformer procurement and lifecycle budgeting.
2. What Are C1 and C2 Capacitances in a Condenser Bushing?
In condenser-type bushings, two principal capacitances are defined and measured: C1 (main capacitance) and C2 (tap capacitance). These two capacitances form the basis of the entire diagnostic test. Let us look at each one in detail.
2.1 C1 (Main Capacitance)
C1 capacitance is 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.
Think of C1 as a stack of many small capacitors connected in series. Each capacitor is made up of a conductive foil layer and a paper insulation layer between them. The combined capacitance of this entire stack is C1.
The C1 capacitance value is specified on the bushing nameplate and serves as a diagnostic baseline. In a healthy bushing, C1 should remain relatively constant over time.
2.2 C2 (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 with a test tap configuration, the C2 insulation is shorted to ground through the test tap cover, meaning it experiences no voltage stress under operating conditions. However, if the bushing has a voltage tap (potential tap) instead of a test tap, the tap is connected to a voltage divider or monitoring device and is not shorted to ground.
Particularly when a gasket fails on a bushing, water accumulates in the tap compartment and attacks the main insulation core from the outermost layers first. A C2 test primarily includes this most susceptible insulation and provides notification that moisture ingress is a problem before a C1 test does. This is why measuring C2 is just as important as measuring C1. In many real-world cases, C2 degradation is the first warning sign. Regular transformer bushing testing that includes both C1 and C2 measurements gives engineers the best chance of detecting problems early.
3. Test Equipment and Setup
The right test equipment and proper setup are the foundation of accurate capacitance and tan delta measurements. Here is what you need.
3.1 Test Equipment
A fully automatic capacitance and tan delta test kit (also called a power factor test set or dissipation factor test bridge) rated at 10 kV or 12 kV is the standard instrument for this test. Popular test sets used in the industry include the Megger Delta series, OMICRON CPC series, and Doble M series. These instruments measure capacitance (in pF) and dissipation factor (tan δ in %) simultaneously at the applied test voltage and frequency (50 Hz or 60 Hz).
The test set should have a tan delta measurement resolution of at least 0.01% and must have a current calibration certificate. Using an uncalibrated instrument is one of the most common sources of measurement error in the field.
The test kit should include:
- High-voltage (HV) output lead
- Low-voltage (LV) measurement lead
- Guard lead
- Ground cable
- Test tap adapters compatible with the bushing tap size
3.2 Pre-Test Setup Checklist
Before starting the test, complete the following preparation steps:
- Isolate the transformer from all external connections (bus bars, cables, surge arresters, CTs, PTs).
- Verify that the transformer is completely de-energized and properly grounded using a grounding set.
- Check weather conditions before proceeding. Do not perform capacitance and tan delta tests during rain, fog, or when the relative humidity exceeds 75%. Surface moisture on porcelain insulators creates leakage paths that produce falsely elevated tan delta readings. IEEE C57.152 recommends postponing testing under these conditions.
- Record the ambient temperature and transformer oil temperature. You will need this data later for temperature correction.
- Clean all exposed bushing insulator surfaces and the test tap area to remove dust, moisture, and contamination. Surface contamination creates leakage paths that produce falsely high tan delta values.
- Allow sufficient time after de-energization for residual charges to dissipate and for temperatures to stabilize before beginning measurements.
- Inspect all test cables and leads. Make sure they are dry, undamaged, and free from surface contamination.
- Connect the test set ground cable firmly to the transformer tank ground.
- Verify that the ground connection has good continuity.
3.3 Test Taps and Voltage Taps
Bushings may have either a test tap or a voltage tap (potential tap). A test tap is used only for offline diagnostic measurements and remains grounded during normal operation. A voltage tap is designed for continuous use and can be connected to online monitoring instruments during service. Voltage taps have a built-in capacitance divider, which means the C2 value measured through a voltage tap will differ from a test tap configuration. The interpretation and acceptance limits may also differ between the two.
Always confirm which tap type your bushing has before starting the test. The bushing nameplate or manufacturer documentation will specify this. The test procedure and acceptance limits differ between the two configurations, so using the wrong reference values will lead to incorrect conclusions.
4. Test Modes: UST, GST, and GSTg
Three measurement modes are available on a standard capacitance and tan delta test kit. Each mode is designed for a specific test configuration.
4.1 UST (Ungrounded Specimen Test)
UST mode is the standard method for measuring C1 capacitance and tan δ of the main bushing insulation. In this mode, the specimen under test is electrically isolated from ground and has two accessible terminals. The HV lead from the test kit connects to the bushing conductor. The LV measurement lead connects to the test tap. The guard circuit eliminates any stray capacitance to ground, so only the insulation between the conductor and the outermost grading foil (C1) is measured.
C1 measurement is performed in UST mode at 10 kV test voltage.
4.2 GST (Grounded Specimen Test)
GST mode is used for measuring the total insulation to ground. It does not have a guarded return path. This mode is commonly applied for testing transformer windings, reactor windings, or bushings without accessible test taps. In GST mode, the HV terminal connects to the conductor being tested, and the grounded tank forms the return path.
4.3 GSTg (Grounded Specimen Test with Guard)
GSTg mode is a refined version of GST that separates total values into component parts. This mode is commonly used to measure C2 capacitance, the insulation between the test tap and the grounded mounting flange. However, the exact mode selection for C2 measurement can vary depending on the test set manufacturer and bushing configuration. Always consult your specific instrument’s operating manual and the bushing manufacturer’s test procedure to confirm the correct mode for C2 measurement.
C2 measurement is performed in GSTg mode at lower test voltages from 500 V to 2000 V, or as specified by the bushing manufacturer.
Never exceed the manufacturer’s rated test voltage for the tap. Applying excessive voltage to the C2 section can permanently damage the insulation.
4.4 Quick Reference Table
| Test | Mode | Test Voltage | What It Measures |
|---|---|---|---|
| C1 (main insulation) | UST | 10 kV | Capacitance and tan δ between conductor and test tap |
| C2 (tap insulation) | GSTg | 500 V – 2 kV | Capacitance and tan δ between test tap and ground flange |
| Total insulation (no tap access) | GST | 10 kV | Overall capacitance including all ground paths |
5. Step-by-Step Field Test Procedure
Here is the complete step-by-step procedure for performing the capacitance and tan delta test on transformer bushings during commissioning or periodic maintenance.
Step 1: Shorting and Grounding Bushing Groups
Short all the bushings of the same voltage level together. For example, short all three HV bushings (R, Y, B) together using jumper cables. Do the same for LV bushings and neutral bushings if applicable.
Ground the bushing groups that are NOT being tested. Only the group under test should remain ungrounded and connected to the test set. All groups of untested bushings must be grounded to the transformer tank. This grounding serves two purposes: it protects personnel from induced voltages, and it eliminates floating capacitances from ungrounded windings that would couple into the measurement circuit and produce erroneous readings.
Example: On a 132/33 kV transformer, if you are testing the HV bushings first, short R-Y-B HV bushings together and connect them to the HV lead. Ground the LV (a-b-c) bushing group and the neutral bushing to the tank.
Step 2: C1 Measurement (UST Mode)
Remove the test tap cover of the first bushing under test. Connect the HV lead of the test kit to the shorted HV bushing group. Connect the LV measurement lead to the test tap of the specific bushing being measured. Set the test mode to UST on the instrument. Apply 10 kV test voltage. Record the C1 capacitance (pF) and tan δ (%) values displayed by the instrument.
Note that in this configuration, the HV lead energizes all three shorted bushings simultaneously. However, the UST mode with the LV measurement lead connected to only one specific test tap isolates and measures only that individual bushing’s C1 capacitance.
Now move only the LV measurement lead to the test tap of the next bushing in the same group. Repeat the measurement. Continue until all bushings in that voltage group are tested.
Example: For HV R-phase bushing, connect the LV lead to the R-phase test tap. Record C1 = 520 pF, tan δ = 0.28%. Move the LV lead to the Y-phase test tap. Record C1 = 518 pF, tan δ = 0.30%. Move to the B-phase test tap. Record C1 = 521 pF, tan δ = 0.29%. All three readings are close to each other and close to the nameplate values — this indicates healthy insulation.
Step 3: C2 Measurement (GSTg Mode)
After completing all C1 tests in the group, proceed to C2 measurements. Disconnect the HV lead from the bushing group. Connect the HV lead of the test kit to the test tap of the bushing under test. Set the test mode to GSTg. Apply the recommended test voltage (500 V to 2 kV — check manufacturer specifications for the maximum safe tap voltage). Record the C2 capacitance (pF) and tan δ (%) values. Compare C2 values with the nameplate data.
C2 values are more sensitive to external factors than C1, especially for bushings rated below 115 kV. Stray capacitances from surrounding structures, porcelain surface contamination, air gaps, and oil conditions can influence C2 readings. Deviations of up to ±50% from nameplate values may be acceptable for test tap configurations.
For bushings rated 115 kV and above, C2 depends primarily on the internal paper insulation and is more stable. Large deviations in C2 for these higher-rated bushings should be taken seriously.
Step 4: Testing Lower Voltage Bushings
After completing the HV bushing tests, move to the LV or tertiary bushing group. Ground the HV bushings (already shorted together). Connect the HV lead of the test kit to the shorted LV bushing group. Connect the LV measurement lead to the test tap of the LV bushing under test. Follow the same UST procedure for C1 and GSTg procedure for C2 as described above.
Step 5: Safety and Cleanup
After testing each bushing at high voltage (10–12 kV), ground the test terminals immediately before touching any connections. The capacitance in the bushing can hold a residual charge that is dangerous.
Reinstall the test tap cover or earthing strip on every bushing after testing. Verify proper grounding of each tap by performing a continuity test. An ungrounded test tap can develop dangerous voltage levels during transformer operation and must never be left floating.
Remove all test leads and jumper cables. Restore the transformer connections to their original configuration. Document all test results with the date, temperature, and instrument serial number.
6. Acceptance Criteria for Test Results
Interpreting the test results accurately is just as important as performing the test itself. Here are the acceptance limits based on IEEE and IEC standards for both OIP and RIP bushings.
6.1 C1 Tan Delta Limits
According to IEEE C57.19.01-2000, the tan delta limit is 0.5% for OIP (Oil-Impregnated Paper) bushings and 0.85% for RIP (Resin-Impregnated Paper) bushings at the reference temperature of 20°C. In practice, new bushings from reputable manufacturers show values between 0.2% and 0.4%.
For field evaluation of in-service bushings, IEEE C57.19.100 provides the following guidelines for the C1 dissipation factor:
- Tan δ between nameplate value and up to 2× nameplate value → Bushing is acceptable
- Tan δ between 2× and 3× nameplate value → Monitor the bushing closely; schedule follow-up tests
- Tan δ above 3× nameplate value → Replace the bushing
| Condition | OIP Bushing Tan δ (C1) | RIP Bushing Tan δ (C1) |
|---|---|---|
| New / Factory (at 20°C) | ≤ 0.5% | ≤ 0.85% |
| Good field value | 0.2% – 0.4% | ≤ 0.35% |
| Requires close monitoring | 0.5% – 1.0% | 0.5% – 0.85% |
| Requires investigation / replacement | > 1.0% | > 0.85% |
6.2 C1 Capacitance Limits
The measured C1 capacitance should not increase more than 3% above the nameplate value. An increase of more than 3% indicates partial puncture or short-circuiting of condenser layers inside the bushing. This happens because a short-circuited layer reduces the number of series capacitors in the condenser stack, which raises the total capacitance. This is a dangerous condition that requires immediate bushing replacement.
A change in capacitance exceeding 5% is a clear cause to remove the bushing from service and investigate its suitability for continued operation.
A decrease in C1 capacitance compared to factory values may indicate physical damage during transportation or installation. Such a bushing should not be energized without further investigation.
| Capacitance Change from Nameplate | Interpretation | Recommended Action |
|---|---|---|
| Within ±1% | Normal | No action needed |
| +3% to +5% | Possible condenser layer failure | Investigate; compare with historical data |
| > +5% | Probable layer short-circuiting | Remove from service immediately |
| Decrease from nameplate | Possible transport or installation damage | Do not energize; investigate further |
6.3 C2 Tan Delta Limits
The tan δ of the test tap to flange insulation (C2) generally varies between 0.4% and 3%. C2 values are expected to be higher than C1 values because the C2 section is more exposed to external environmental factors. Always compare C2 readings with nameplate values and historical records from previous tests.
A sudden increase in C2 tan delta even if C1 appears normal is a strong early indicator of moisture ingress into the bushing. Moisture enters through failed gaskets and attacks the outermost insulation layers (C2 section) first before reaching the main insulation (C1 section). This makes C2 testing an extremely valuable early warning tool.
7. Applicable Industry Standards for Bushing Testing
Every capacitance and tan delta test must be performed and evaluated against recognized industry standards. Here are the primary standards that govern this test.
IEEE C57.19.01-2017 defines the performance characteristics and dimensions for power transformer and reactor bushings with BIL ratings of 150 kV and above. This standard specifies the factory acceptance limits for C1 power factor and capacitance values that serve as the baseline for all field comparisons.
IEC 60137 covers insulated bushings for alternating voltages above 1000 V and is the primary reference in regions following the IEC framework. This standard also defines dissipation factor limits at the reference temperature of 20°C.
IEEE C57.152 is the IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors. This standard provides the field testing guidelines, including test procedures, modes, and interpretation of results for bushing diagnostics.
IEEE C57.19.100 is the IEEE Guide for Application of Power Apparatus Bushings. It offers guidance on applying and evaluating bushings in service conditions.
IEEE C57.19.00 covers general requirements and the test code for outdoor apparatus bushings.
In North America, the three main bushing standards used are CAN CSA C88.1, IEEE C57.19.00/C57.19.01, and IEC 60137. CIGRE Technical Brochure TB 755 and TB 445 are also used internationally as supplementary references for bushing reliability and tan delta categorization.
Here is a quick reference table:
| Standard | Scope |
|---|---|
| IEEE C57.19.01-2017 | Performance characteristics for power transformer bushings (BIL ≥ 150 kV) |
| IEEE C57.19.00 | General requirements and test code for outdoor apparatus bushings |
| IEEE C57.19.100 | Guide for application of power apparatus bushings |
| IEEE C57.152 | Guide for diagnostic field testing of fluid-filled power transformers |
| IEEE C57.12.90 | Standard test code for liquid-immersed transformers |
| IEC 60137 | Insulated bushings for alternating voltages above 1000 V |
| CIGRE TB 755 | Bushing reliability guidelines |
| CIGRE TB 445 | Tan delta / power factor categorization |
8. 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.
8.1 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.
8.2 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:
- 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.
- Perform automated sweep: Modern test sets automatically step through the frequency range, measuring tan delta and capacitance at each frequency point.
- Plot response curves: The test results are typically displayed as tan delta versus frequency curves for visual analysis.
8.3 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.
8.4 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.
9. Conclusion
The capacitance and tan delta test of transformer bushings is one of the most valuable non-destructive diagnostic tools available to electrical engineers and commissioning professionals. This test helps detect insulation degradation, moisture ingress, contamination, and condenser layer failures long before they can cause major damage to the power transformer. Measuring both C1 and C2 capacitance along with their corresponding tan delta values provides a complete picture of the bushing’s internal health.
Always compare your field measurements against the bushing nameplate data and applicable standards such as IEEE C57.19.01, IEEE C57.152, and IEC 60137.