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Bushing Testing for Transformers

Bushings - Purpose

• Allows an energized conductor to penetrate a ground plane

• Provide insulation for an energized conductor where it passes through the grounded wall of an apparatus tank or chamber.

• Support/Terminals for other energized working parts of the transformer

Bushings - Types

• Capacitor/Condenser Type - Complex insulation system employing a

mechanism to grade voltage across the insulation system.

• Oil-Impregnated Paper Insulation • Resin Bounded Paper Insulation

• Non-Condenser Type - Simple insulation system no voltage grading,

typically employed on low voltage or gas filled bushings

• Solid

• Alternate Layers of Solid and Liquid Insulation • Gas-Filled

Difference Between Power Factor Test and Potential Taps

• Test Tap • Potential Tap - Bushing Rated to <= 69 - Bushing Rated > 69

kV kV - Test Voltage: 500 V - Test Voltage: 2 kV - Grounded in Service - In Service: Floating or

Grounded - Used for testing purposes only - In addition to test

purposes, can also be used to monitor line voltage.

Bushing Tests

• Ungrounded-Specimen Test •

Center Conductor to Tap, C1 •

Tap Insulation Test

• Tap to Flange,

C2 • Hot Collar Test

• Externally Applied Collar to Center

Conductor • Overall

• Center Conductor to Flange

Investigative Bushing Tests

• Inverse Ungrounded-Specimen

Test • Tap to Center Conductor, C1 •

Do Not Exceed Tap Voltage Rating •

Tip-up Test

• Repeat C1 Test at 2 and 10 kV or 2 and 5 kV if Less Than 10kV

• Alternate C2 Test • GST Ground Test

• Tap to Center Conductor (C1 + C2) • Supplemental Hot Collar Tests

Bushing Test Set-Up

•Short-Circuit All Windings on Transformer

•Clean Bushings to Minimize Surface

Leakage •Ground Opposite Winding

•Remove Test Tap Cover From Bushing Under Test

Only •Performed C1, Main Core Insulation Test in UST

Mode •Perform C2, Tap Test in GST-Guard Mode

•Replace Test Tap Cover

Transformer Bushings

• Prior to Bushing Tests, Ensure

• High Side Winding Shorted

• Low Side Winding Shorted Including Neutral

• WHY???

Higher Than Normal Tan δ Values May Be Obtained

• NOTE Remove Only One Tap Cover at a time….

C1 Test Result Analysis Tan δ

• Modern Condenser Type Bushings •Generally of the Order of 0.5% •Temperature Correction to 20 Degrees C

• Deteriorated Bushings •Generally Between 0.5% to 1.0%

• Investigate Bushings •Above 1.0%

EXCEPTIONS…

C2 TESTS

• Why Perform C2 Tests ?????

C2 TESTS

•Internal Flashover Around the Main Core is a Real and Serious Threat to all Sealed Capacitance Graded Bushings

•The C2 Test has Been Shown, in Some Cases, to be a More Apparent Indicator of Internal Fluid Contamination Than the C1 Test

Hot-Collar Test - When To Perform

• Oil Filled Bushings Not Equipped With Taps and Overall Test Cannot be Performed: - Single Hot-Collar Test on Small Bushings 15 kV

and Below - Several Single Hot-Collar Tests for Bushings

Rated Above 15 kV

• Check Oil Level on all Liquid-Filled Bushings Without Liquid-Level Gauges or Sight Glass • Check Bushing With Liquid-Level Gauges Whenever the Gauge is Suspect • As Supplementary Test When Overall, UST,

Tap Tests Indicate Possible Problem

Hot Collar Test Result Analysis

Hot Collar Test Results

•Increased Watt-Losses •Contamination of the Insulation • Maximum Acceptable Limit, 0.1 W

•Decreased Current (Amperes) •Voids •Low Liquid or Compound Level

Field Technique Problems

• Drying Tap Area With Heat • May Result in Values Affected by Temperature

• Cleaning Porcelain Surface With Alcohol-Based or Evaporative Cleaner • May Cool Surface When Cleaner

Evaporates, Resulting in Further Condensation

• Leads Used to Short-Circuit Bushings • To Avoid Stray Capacitive Currents to Ground, Leads Should Not Sag Near Tank Surface

• THE NUT BEHIND THE WHEEL THEORY

Case Study

• Three bushings 132 kV - GOB 550 • Values are given below

Insulation A B C

C1 2,53 1,00 0,37

C2 0,61 0,54 4,49

•Red indicates areas of concern •Perform tip-up test to find problem

Things to look out for!

• Increase in tanδ • Perform tip-up on suspect bushings • Ensure connects are correct and retest • Be conscious of the applied voltage to C2 • Replace test tap cover • Ensure bushings are clean before testing

End of Bushing

Thank you !

Any Questions ?