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Bushing Health Assessment Techniques

  • Sep 9, 2025
  • 3 min read

Updated: 5 days ago

Bushing Health Assessment Techniques

Since the transformer bushings serve as the electrical and mechanical interface between transformer winding leads and transmission lines, their failure can result in outages for several months and may also result in fire and catasrophic failure; which further increases the transformer outages. Different health assessment techniques for bushings are described below.


1. Tan Delta Measurement (offline & online)


  • Tan Delta is the ratio of resistive current to capacitive current in the dielectric system.

  • Increased values of Tan Delta indicate insulation degradation e.g. moisture, contamination, or aging.

  • Mainly offline measurements are carried out at a fixed periodicity / or during shutdown for preventive maintenance. Online measurements are being used at few locations.

  • Tan Delta measurements at pre-commissioning stage are used as a reference & the limiting value during service is double of the pre-commissioning values.


2. Capacitance (C₁) Measurement (offline & online)


  • It is the capacitance between the air-end high-voltage terminal and the test tap, or the voltage tap of a capacitance-graded bushing. Increased C₁ indicates short / puncture between two or more condenser grading foils.

  • Mainly offline measurements are carried out at a fixed periodicity or during shutdown for preventive maintenance. Online measurements are being used at few locations.

  • C₁ Capacitance value is mentioned on the bushing's rating plate. However, the practice followed is to refer the capacitance value measured at pre-commissioning stage.

  • The permissible increase in C₁ during service is usually indicated by the bushing manufacturer. Alternately, reference should be made to CIGRE TB 755.


3. Dielectric Frequency Response (DFR) Analysis (offline)


  • DFR is an effective diagnostic method for evaluating the moisture content, oil conductivity, and health of the insulation in bushings. DFR involves measuring Tan Delta over a range of frequencies.


4. Dissolved Gas Analysis (DGA) (offline with oil samples drawn from OIP bushings & online)


  • DGA is a diagnostic tool and involves analysing the gases dissolved in the insulating oil in OIP bushings to detect and diagnose existing / potential faults. The key gases for bushing oil monitoring are hydrogen (H₂) and acetylene (C₂H₂) which are enough to detect early signs of faults.

Dissolved gas contents

Limiting values dissolved gas contents when

 bushing in service (ppm)

Hydrogen (H2)

< 140

Methane (CH4)

< 40

Acetylene (C2H2)

< 2

Ethylene (C2H4)

< 30

Ethane (C2H6)

< 70

Carbon Monoxide (CO)

< 1000

Carbon Dioxide (CO2)

< 3400

5. Moisture Content Analysis (offline with oil samples drawn from OIP bushings & online)


  • The presence of moisture compromises the insulating properties of the oil and thus is treated as an enemy of OIP Bushings. Karl Fischer titration is the most accurate method for determining moisture content in insulating mineral oil. When the bushing is in service, the limiting value of the oil sample for moisture content is < 10 ppm.


6. Infrared Thermography (online)


  • Abnormal hot spots or temperature gradients on the bushing surface can indicate internal problems or poor connections. An infrared (IR) scanner can monitor transformer bushings on a regular basis without making direct contact with them, however this is limited to visible components on the outside.


7. Visual Inspection (offline)


Visual inspection checks physical condition of bushing, preventing a possible damage in its early stage. It includes:


  • General inspection of different parts of bushings for surface conditions and damages.

  • In case of OIP bushings, oil level and its colour are to be observed.

  • Condition of test tap / voltage tap.

  • Earthing connection between the bushing flange and substation earth / earthing of transformer tank.

  • Verification of surface conditions of insulator sheds for pollution and presence of tracking / arcing marks.

  • Air-end HV terminal conditions for mechanical tightness and observe for colour changes due to overheating.


8. Partial Discharge (PD) Measurement (offline & online)


  • In offline PD testing, voltage is gradually increased to monitor PD inception, with the measurements recorded at specified levels after the one-minute withstand voltage and during voltage reduction.

  • Online partial discharge (PD) measurement can use either the conventional electrical method or the acoustic emission (AE) technique. AE detects ultrasonic waves from PD using piezoelectric sensors placed on the bushing surface and locates the source via triangulation by repositioning sensors in multiple tests.



 
 
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