1. Introduction
Transformer bushings provide the critical interface that allows high-voltage conductors to pass safely through grounded transformer tanks. As one of the most stressed insulation components, a bushing’s condition directly affects transformer reliability. Aging, thermal stress, contamination, and moisture ingress can all degrade the insulation system, leading to partial discharge or dielectric failure.
The tan delta test—also known as the dissipation factor of power factor test—is a widely accepted diagnostic method to assess bushing insulation integrity. By measuring dielectric losses and capacitance, the test enables maintenance engineers to detect early-stage defects, trend insulation deterioration, and make informed decisions about cleaning, drying, or replacement.
Monitoring tan delta and capacitance over time ensures that insulation problems are identified before they evolve into catastrophic transformer failures.
2. Testing Principle
The tan delta test measures the power loss within the insulation when an AC test voltage is applied. For condenser bushings, which consist of multiple concentric insulation layers forming two main capacitors (C1 and C2), this test is performed separately on each section.
- C1 represents the main insulation between the central conductor and the test tap.
- C2 represents the insulation between the test tap and ground.
During testing, an AC voltage is applied between the central conductor and ground. The test equipment measures both the capacitive current and the resistive (loss) component. The ratio of these two components defines the dissipation factor (tan δ).
Measurements can be taken in various configurations depending on the instrument and bushing design, typically using grounded specimen test (GST) or ungrounded specimen test (UST) modes. For C1 testing, the high-voltage lead is connected to the conductor, the flange is grounded, and the test tap is appropriately connected or guarded to isolate leakage paths. For C2 testing, the measurement is performed between the test tap and ground with the central conductor grounded. The wiring diagram of these tests with CAPTAN 12 for spare and mounted bushings are explained in our previous article (High Voltage Bushings Dissipation Factor (Tan Delta or Power Factor) Measurement).
It is important that all connections are clean, dry, and secure. Only one reference ground point should be used to prevent measurement interference. When environmental noise or electromagnetic interference is present, results are often averaged from tests at 45 Hz and 55 Hz to improve accuracy.
Temperature has a significant influence on tan delta readings, as dielectric losses increase with temperature. Therefore, the measured value must be corrected to a standard reference temperature of 20°C using the relation:
tan δ₍₂₀₎ = β × tan δ₍meas₎
where β is the temperature correction factor depending on the bushing type and test temperature. For example:
- For OIP (Oil Impregnated Paper) bushings, β ≈ 0.87 at 0°C, 0.94 at 10°C, 1.04 at 30°C, and 1.05 at 50°C.
- For RIP (Resin Impregnated Paper) bushings, β ≈ 0.91 at 0°C, 0.97 at 10°C, 1.10 at 30°C, and 1.06 at 50°C.
- For RBP (Resin Bonded Paper) bushings, β ≈ 0.62 at 0°C, 0.83 at 10°C, 1.18 at 30°C, and 1.25 at 40°C.
- For RIS (Resin Impregnated Synthetic) bushings, β ≈ 0.66 at 0°C, 0.83 at 10°C, 1.25 at 30°C, and 1.39 at 40°C.
The corrected 20°C value allows direct comparison of results taken under different environmental conditions.
Hot-Collar Test:
The hot-collar test is a supplementary method for locating localized insulation defects on porcelain bushings. A metal collar is placed around one shed at a time, and losses are measured between the collar and the central conductor.
- Normal losses are less than 100 mW.
- If losses are higher, the collar is moved down one shed, and the test is repeated until the fault region is identified.
This technique is particularly useful for detecting cracks, pinholes, or localized moisture ingress.
3. Result Evaluation
When evaluating test results, the corrected tan delta at 20°C is compared to three benchmarks: factory nameplate data, previous test records, and similar bushings in service.
Dissipation Factor (tan δ):
For modern condenser bushings, the typical dissipation factor at 20°C is approximately 0.5%. Acceptable ranges for new and old (more than 15 years) bushings are as follows:
- RIP and OIP bushings: about 0.3–0.4% typical for a new one, with a maximum acceptable value of 0.7%.
- RBP bushings: about 0.5–0.6% typical for a new one, with a maximum acceptable value of 1.5%.
According to If the measured value exceeds twice the nameplate value or the specified limit, the bushing should be inspected for contamination or insulation deterioration. Occasionally, negative dissipation factors accompanied by small decreases in capacitance may occur; these typically indicate unusual external leakage, internal carbon tracking, or a poor connection at the test tap.
Capacitance (C1 and C2):
Capacitance should remain within ±5% to ±10% of the nameplate value, depending on the number of condenser layers.
- An increase in capacitance suggests that one or more condenser layers may be short-circuited.
- A decrease in capacitance may indicate a floating ground sleeve or a poor test-tap contact.
For C2, the dissipation factor is typically around 1% and is not temperature corrected. Any significant deviation or increasing trend indicates deterioration of the tap insulation or contamination.
Trending Over Time:
It is essential to monitor changes in tan delta and capacitance over the life of the bushing. A gradual increase in tan delta indicates insulation aging or moisture ingress. For OIP and RIP bushings, the yearly increase should remain below approximately 0.6% to 0.8%. If growth exceeds this limit, or if the total value surpasses 1.1% for OIP or 0.9% for RIP, additional diagnostic tests should be scheduled.
A sharp rise in tan delta or a simultaneous increase in capacitance suggests internal breakdown or shorted layers, which may require immediate action. Where the factory reference is unavailable, results should be compared with similar bushings in the same transformer or sister units under comparable operating conditions.
4. Typical Fault Indicators
Several characteristic patterns in tan delta and capacitance results indicate specific bushing faults:
- Surface Contamination or Moisture: A uniform increase in dissipation factor during overall tests, often reversible after cleaning or drying the porcelain.
- Internal Moisture in OIP Bushings: Tan delta increases significantly with temperature and tends to rise over successive test periods even if capacitance remains stable.
- Shorted Capacitor Layers: Noticeable increase in capacitance beyond the upper tolerance limit, often accompanied by a higher dissipation factor.
- Floating Ground Sleeve or Poor Test-Tap Connection: Capacitance decreases below the expected range, sometimes with unstable or negative tan delta readings.
- Partial Discharge or Corona: Rapid growth in tan delta with increasing test voltage and possible ultrasonic activity.
- Carbon Tracking or Localized Defects: Abnormally high losses confined to specific porcelain sheds, confirmed by the hot-collar method.
5. Maintenance Insights
Testing Recommendations:
- Perform tan delta and capacitance measurements at commissioning, after major system faults, and periodically based on service conditions—typically every three to four years for OIP bushings.
- Always use clean, dry, and well-shielded test connections. Ensure only one grounding point to avoid circulating currents.
- Avoid testing under rain or high humidity. If unavoidable, dry the bushing surface and correct results to 20°C.
- Record ambient temperature, oil or bushing top temperature, and humidity for every measurement.
- When electromagnetic interference is suspected, perform dual-frequency testing (45 Hz and 55 Hz) and average the results.
Trending and Action Thresholds:
Establish a baseline using factory data or the first reliable field test. Subsequent tests should be trended against this reference.
- A tan delta increase of less than 2 times the prior value indicates normal aging.
- A rise between 2 and 3 times the previous value suggests accelerated aging; repeat testing should be scheduled sooner.
- An increase greater than 3 times or an absolute tan delta above the acceptable limit (0.7% for OIP/RIP or 1.5% for RBP) requires immediate investigation and possible replacement.
For capacitance, a change less than 5% from the reference is acceptable. A deviation between 5% and 10% warrants further analysis, while a variation greater than 10% typically indicates an internal defect or shorted layers and necessitates bushing replacement.
6. Example Test by CAPTAN12
7. Conclusion
Tan delta testing remains one of the most effective predictive maintenance tools for assessing the health of transformer bushings. By combining dissipation factor and capacitance measurements, and by applying temperature correction and trend analysis, utilities can accurately monitor insulation degradation and prevent unplanned outages.
Consistent testing, adherence to correct procedures, and disciplined trending allow early identification of issues such as moisture ingress, partial discharge, or layer shorting. Implementing clear acceptance criteria and action thresholds ensures timely maintenance decisions and contributes directly to improved transformer reliability and operational safety.
