Transformer Bushing Test Items

Transformer Bushing Inspection Items

Transformer bushings are critical components that connect the internal windings to external circuits, providing both insulation and mechanical support. Their performance directly impacts the safe and stable operation of the transformer. Inspection items for bushings primarily fall into the following categories:

I. Routine Inspections (Typically performed during outage or live-line patrols)

Visual Inspection:

Check the porcelain/ceramic housing (or composite insulator) surface for cracks, damage, contamination, electrical tracking, glaze shedding, or flashover marks.

Check metal flanges, connecting hardware, and grading rings for corrosion, deformation, or damage.

Check sealing points (e.g., porcelain-to-flange interface) for signs of oil seepage (oil-impregnated bushings) or gas leakage (SF6 bushings).

Check the top terminal connector ("General's Cap") is tight and shows no signs of overheating or discoloration.

Check the oil level gauge (oil-impregnated bushings) is legible and the oil level is within the normal range (considering ambient temperature effects).

Check pressure gauges (oil-filled or gas bushings) for normal readings.

Check the measurement terminal ("tap" or "test tap") is intact and reliably grounded.

Oil Level / Pressure Check: For oil-impregnated or gas-filled bushings, regularly observe and record oil level or gas pressure to ensure it remains within the manufacturer's specified normal range.

Infrared Thermography (Live-line Detection): Periodically perform infrared scanning on all bushing parts (especially the top terminal, flange, housing surface, and measurement terminal ground lead) to detect abnormal hot spots (indicating poor contact, excessive dielectric loss, internal faults, etc.).

 

II. Preventive Tests (Typically performed during outage maintenance)

These are key tests to assess the insulation condition of the bushing:

Insulation Resistance Measurement:

Main Insulation: Measure insulation resistance between the high-voltage conductor and the measurement terminal (or ground). Reflects overall moisture ingress and contamination of the main insulation.

Measurement Terminal Insulation: Measure insulation resistance between the measurement terminal and ground. Poor insulation here severely affects the accuracy of dielectric loss tests and is itself a potential failure point. Requirements are very high (typically > 1000 MΩ).

Dielectric Dissipation Factor (Tan Delta) and Capacitance Measurement:

Core Test: Measure the dielectric dissipation factor (tan δ) and capacitance (C) of the bushing's main insulation. This is one of the most sensitive and important methods for diagnosing insulation aging, moisture ingress, delamination, or local defects.

Measurement Method: Typically performed using a high-voltage bridge (e.g., Schering Bridge) or automatic tan delta tester, at rated voltage or specified test voltage. Measure between the high-voltage conductor and the measurement terminal (with measurement terminal grounded).

Key Considerations: Compare results against factory values, historical data, and similar bushings. Both the absolute value of tan δ and its yearly trend (rate of increase) are critical. Capacitance changes (compared to factory or previous measurements) exceeding a certain limit (e.g., ±5%) may indicate internal structural changes (e.g., breakdown or open circuit of capacitive layers).

AC Withstand Voltage Test (Power Frequency):

Verifies the ability of the main insulation to withstand a specified power frequency test voltage, assessing its insulation strength.

This is a destructive test, usually performed during commissioning acceptance, after major repairs, or when severe defects are suspected. Must be performed strictly according to regulations. Test voltage is significantly higher than operating voltage.

Dissolved Gas Analysis (DGA) of Oil:

For oil-impregnated bushings. Take a sample of the internal insulating oil for chromatographic analysis (DGA).

Detects dissolved gases like Hydrogen (H₂), Methane (CH₄), Ethane (C₂H₆), Ethylene (C₂H₄), Acetylene (C₂H₂), Carbon Monoxide (CO), and Carbon Dioxide (CO₂).

Uses characteristic gas concentrations and ratios to diagnose internal latent faults such as partial discharge, overheating, or arcing (e.g., capacitive layer discharge, contact overheating).

Oil Performance Tests:

For oil-impregnated bushings. Measure oil breakdown voltage, dielectric dissipation factor, moisture content, acidity, interfacial tension, etc.

Assess the electrical performance and aging state of the insulating oil itself. Oil degradation directly affects bushing insulation performance.

Moisture Content Test:

Oil-Impregnated: Measure trace moisture content in the oil (ppm).

SF6 Gas: Measure moisture content in the SF6 gas (dew point or ppm).

Moisture is a major enemy of insulation; excessive levels significantly reduce dielectric strength and accelerate aging.

SF6 Gas Leak Check (for SF6 Bushings): Use a leak detector to check bushing seal integrity, ensuring no SF6 gas leakage.

 

III. Special Testing and Diagnostics (Performed as needed)

Partial Discharge (PD) Measurement:

Offline Measurement: Measure PD magnitude (pC) and patterns during AC withstand tests or extended voltage application using a PD detector.

Online Monitoring: Use high-frequency current transformers (HFCT) or ultra-high frequency (UHF) sensors installed on the measurement terminal or ground lead to monitor PD signals during operation.

PD is a key indicator of insulation deterioration, effectively detecting internal voids, contaminants, floating potentials, etc.

X-ray or Computed Tomography (CT) Scan: Used when severe internal mechanical deformation, displacement, or foreign objects are suspected and difficult to diagnose via conventional tests, providing internal structural images.

Ultrasonic Testing (UT): Primarily used to detect hidden cracks or defects within porcelain housings (if the structure allows).

Vibration and Noise Monitoring: Monitor bushings during operation for abnormal vibration or noise, which may indicate internal looseness or mechanical issues.

Measurement Terminal Current / Online Monitoring: For bushings equipped with online monitoring systems, monitor the current on the measurement terminal ground lead (related to dielectric loss) or PD signals in real-time or periodically.