Evaluation of the High Voltage Bushing Tan Delta Test Results

1.Introduction

Transformer bushings provide a critical interface in power networks. They allow high-voltage conductors to pass safely through grounded transformer tanks. However, bushings experience severe electrical and thermal stress during operation. Aging, thermal stress, contamination, and moisture ingress can degrade the insulation system over time. Consequently, this degradation leads to partial discharge or complete dielectric failure.

The tan delta test—also known as the dissipation factor or power factor test—is an essential diagnostic tool. Maintenance engineers use this test to evaluate bushing insulation integrity. Specifically, the test measures dielectric losses and capacitance. As a result, engineers can detect early-stage defects and trend insulation deterioration effectively. Furthermore, these insights help teams make timely decisions regarding cleaning, drying, or replacement. Regular monitoring ensures that operators identify problems before catastrophic transformer failures occur.

2.Testing Principle

The tan delta test measures power loss within the insulation under an applied AC test voltage. Condenser bushings contain multiple concentric insulation layers that form two main capacitors: C1 and C2. Therefore, technicians test each section separately.

• C1: Represents the main insulation between the central conductor and the test tap.
• C2: Represents the insulation between the test tap and the ground flange.

(High Voltage Bushings Dissipation Factor (Tan Delta or Power Factor) Measurement).

During the test, the instrument applies an AC voltage between the central conductor and ground. It simultaneously measures the capacitive current and the resistive loss component. The ratio of these two currents defines the dissipation factor (tan δ).

Technicians configure the instrument in either Grounded Specimen Test (GST) or Ungrounded Specimen Test (UST) mode. For C1 testing, the engineer connects the high-voltage lead to the conductor and grounds the flange. Then, they guard or connect the test tap properly to isolate surface leakage paths. For C2 testing, the operator measures the loss between the test tap and ground while grounding the central conductor.

Ensure Clean Connections

Technicians must always keep test connections clean, dry, and secure. In addition, you should use only one reference ground point to eliminate ground loops. When strong electromagnetic interference occurs in the field, dual-frequency testing (45 Hz and 55 Hz) helps improve measurement accuracy.

Ambient and insulation temperature directly affects tan delta readings. Dielectric losses increase significantly as the temperature rises. Therefore, engineers must convert measured values to a standard reference temperature of 20°C:

Here, β represents the temperature correction factor based on the bushing type and test temperature:

• OIP (Oil Impregnated Paper): β ≈ 0.87 at 0°C, 0.94 at 10°C, 1.04 at 30°C, and 1.05 at 50°C.
• RIP (Resin Impregnated Paper): β ≈ 0.91 at 0°C, 0.97 at 10°C, 1.10 at 30°C, and 1.06 at 50°C.
• RBP (Resin Bonded Paper): β ≈ 0.62 at 0°C, 0.83 at 10°C, 1.18 at 30°C, and 1.25 at 40°C.
• RIS (Resin Impregnated Synthetic): β ≈ 0.66 at 0°C, 0.83 at 10°C, 1.25 at 30°C, and 1.39 at 40°C.

Consequently, normalized 20°C values enable direct comparisons between tests conducted across different seasons.

The Hot-Collar Test

The hot-collar test serves as a supplementary method for locating localized defects in porcelain housings. The operator wraps a conductive collar around one shed at a time. Then, the test set measures losses between the collar and the central conductor.

Normal losses remain below 100 mW. However, if losses exceed this threshold, the operator moves the collar down one shed. They repeat the measurement until they isolate the exact fault region. Consequently, this technique helps detect localized cracks, internal voids, or moisture ingress.

3.Result Evaluation

Engineers evaluate test data by comparing the normalized 20°C tan delta value against three key benchmarks: nameplate data, historical test logs, and sister units in service.

Dissipation Factor (tan δ)

Modern condenser bushings typically exhibit a dissipation factor around 0.5% at 20°C. Acceptable limits vary depending on insulation technology:

• RIP and OIP bushings: Typically 0.3%–0.4% when new, with a maximum acceptable threshold of 0.7%.
• RBP bushings: Typically 0.5%–0.6% when new, with a maximum acceptable limit of 1.5%.

If the measured tan delta exceeds twice the initial baseline or exceeds these limits, inspect the bushing for moisture ingress or oil degradation. Occasionally, negative dissipation factors occur alongside minor capacitance drops. These anomalies usually indicate external surface leakage, internal carbon tracking, or an improper test-tap connection.

Capacitance (C1 and C2)

Capacitance must remain within ±5% to ±10% of the factory nameplate value.

• Capacitance Increase: Indicates that breakdown or puncture has short-circuited one or more condenser layers.
• Capacitance Decrease: Indicates a floating ground sleeve or poor contact at the test tap.

For C2, the dissipation factor typically stays near 1.0% and requires no temperature correction. However, any significant upward trend warns of tap insulation deterioration or internal moisture.

Historical Trending

Asset managers should systematically track changes in tan delta and capacitance over the bushing lifecycle. A gradual rise in tan delta reflects normal dielectric aging.

Conversely, a sharp jump in tan delta—or a sudden shift in capacitance—indicates an active fault. Such shifts require immediate operational intervention. When factory nameplate values are missing, compare the data with adjacent phases or identical sister units operating under similar conditions.

4.Typical Fault Indicators

Distinct measurement patterns help maintenance teams identify specific bushing failure modes:

• Surface Contamination or Moisture: Causes a uniform increase in dissipation factor across overall tests. Cleaning and drying the porcelain sheds quickly restores normal values.
• Internal Moisture in OIP Bushings: Causes tan delta to increase steeply with temperature. The loss factor continues to rise across successive testing cycles even when capacitance remains constant.
• Shorted Capacitor Layers: Produces a notable capacitance increase above the upper tolerance limit, often accompanied by higher dielectric loss.
• Floating Ground Sleeve or Loose Tap: Lowers measured capacitance below tolerance and frequently produces erratic or negative tan delta readings.
• Partial Discharge or Corona: Drives a rapid rise in tan delta as test voltage increases, often generating detectable ultrasonic signals.
• Carbon Tracking: Generates abnormally high losses localized to specific porcelain sheds during hot-collar testing.

5.Maintenance Insights & Action Thresholds

Practical Testing Recommendations

• Schedule Regular Intervals: Measure tan delta and capacitance at commissioning, after system fault currents, and every 3 to 4 years during routine maintenance.
• Ensure Clean Terminations: Always clean and shield measurement leads.
• Control Grounding: Maintain a single solid ground connection to prevent ground loops.
• Account for Weather: Avoid testing during heavy rain or extreme humidity. If testing is necessary, clean and dry the housing thoroughly.
• Log Environmental Conditions: Always record ambient temperature, oil temperature, and relative humidity.

Action Thresholds

• Normal Aging: A tan delta increase below 2× the initial baseline indicates acceptable aging.
• Accelerated Aging: A tan delta rise between 2× and 3× the baseline suggests accelerated deterioration. Schedule a follow-up test sooner.
• Critical Fault / Replacement: A tan delta increase exceeding 3× the baseline—or exceeding 0.7% for OIP/RIP and 1.5% for RBP—demands immediate investigation and potential replacement.
• Capacitance Limits: Capacitance shifts below 5% are acceptable. Deviations between 5% and 10% warrant closer monitoring. Any shift beyond 10% confirms shorted layers and requires immediate replacement.

6.Example Test by CAPTAN12

7.Conclusion

Tan delta testing provides an indispensable predictive maintenance method for high-voltage transformer bushings. By evaluating dissipation factor and capacitance together—and applying proper temperature corrections—maintenance teams can monitor insulation health accurately.

Furthermore, consistent test procedures and disciplined trend analysis help operators identify moisture ingress and layer degradation before outages occur. Applying clear action thresholds ensures timely maintenance decisions and maximizes transformer reliability and safety.