High Voltage Bushing

Richmond Community College

Types of Bushings

The primary function of a bushing is to provide an insulated

entrance for an energized conductor into an apparatus tank or

chamber.

• Condenser type:

Oil-Impregnated paper insulation

Resin bounded paper insulation

• Non-condenser type:

Solid

Alternate layers of solid and liquid insulation

Gas-filled

Bushing

• For outdoor bushings, the primary insulation is contained in a weather-proof housing, usually porcelain.

• The space between the primary insulation and the weather shed is generally filled with an insulating oil or compound (also used are plastic and foam).

• Some of the solid homogeneous types may use oil to fill the space between the conductor and the inner wall of the weather-shed.

• Bushings also may use gas, such as SF6 as an insulating medium between the center conductor and outer weather-shed.

• Bushings may be further classified generally as being equipped, or not equipped, with a potential tap or power-factor test tap or electrode.

• NOTE “Potential” taps are sometimes also referred to as “capacitance” or “voltage” taps.

Bushing Components

C11

C12

C13

C14

C1

C2

Anatomy of a

Condenser Bushing

Condenser Bushing Construction

Bushing Core Designs

Bushing Test Tap Example

Bushing Potential Tap Example

Tap Adapters

Westinghouse Type O+ Bushings

Bushing Test Adapter

Westinghouse Type S, OS, GOS Bushing

Bushing Test Adapter

OS

Westinghouse Type S, OS, GOS Bushing

Bushing Test Adapter

Bushing Test Adapter

Bushing Troubles

• Operating records show that about 90 percent of all preventable bushing failures are caused by moisture entering the bushing through leaky gaskets or other openings.

• High-voltage bushings, if allowed to deteriorate, may explode with considerable violence and cause extensive damages to adjacent equipment.

• Flashovers may be caused by deposits of dirt on the bushings, particularly in areas where there are contaminants such as salts or conducting dusts in the air. These deposits should be removed by periodic cleaning.

Bushing Tests

• Ungrounded-Specimen Test (center conductor to tap, C1). • Tap Insulation Test (tap to flange, C2) • Collar Test (externally applied collar to center conductor) • Overall (center conductor to flange) • Inverse Ungrounded-Specimen Test (tap to center conductor, C1. Do not exceed tap voltage rating!) • Tip-up Test (repeat C1 at 2 and 10 kV or 2 and L-G kV if less than 10)

Bushing Troubles TROUBLE POSSIBLE RESULTS METHODS OF DETECTION

Cracked porcelain Moisture enters. Oil and/or gas Leaks. Filler leaks out.

Visual inspection. Power factor test. Hot collar test

Deterioration of cemented joints

Moisture enters. Oil and/or gas Leaks. Filler leaks out.

Visual inspection. Power factor test. Hot collar test

Gasket leaks Moisture enters. Oil and/or gas Leaks. Filler leaks out.

Visual inspection. Power factor test. Hot collar test . Hot-wire test for moisture. Insulation resistance.

Moisture in insulation Moisture enters.

Power factor test. Hot collar test .

Bushing Troubles TROUBLE POSSIBLE RESULTS METHODS OF DETECTION

Solder seal leak Moisture enters. Filler leaks out.

Visual inspection. Power factor test. Hot collar test . Hot-wire test for moisture. Leak detector

Broken connection between ground sleeve and flange

Moisture enters. Oil and/or gas Leaks. Filler leaks out.

Visual inspection. Power factor test. Hot collar test

Gasket leaks Sparking in apparatus Tank or within bushing. Discolored oil.

Power factor test.

Voids in compound Internal corona. Power factor Tip up test. Hot collar test .

Bushing Troubles TROUBLE POSSIBLE RESULTS METHODS OF DETECTION

Oil migration Filler contamination Visual inspection. Power factor test. Hot collar test .

No Oil Oil leaks out Moisture enters.

Visual inspection. Power factor test. Hot collar test.

Displaced grading shield. Internal sparking discolors oil.

Hot collar test .

Electrical flashover Cracked or broken porcelain. Complete failure.

Visual inspection. Hot collar test .

Lightning Cracked or broken porcelain. Complete failure.

Visual inspection. Test lightning arrester

Bushing Troubles

TROUBLE POSSIBLE RESULTS METHODS OF DETECTION

corona Internal breakdown. Radio interference. Treeing along surface of paper or internal surfaces.

Power factor test Hot collar test. Hot-wire test. RRIV

Short-circuited condenser sections

Increased capacitance. Reduced voltage at capacitance tap terminal. Adds internal stress to insulation.

Power factor test. Voltage test at capacitance tap. Capacitance test.

Darkened oil Radio interference, Poor test results.

Power factor test. Hot collar test.

Preparing for

Tests

Bushing Cleaning

Recommendations: • Collinite Wax • Clean, dry cloth • Soap & Water !! • Windex with Ammonia • Apply heat (lamp) to fully dry all surfaces

Alcohol NOT recommended

Spare Bushings

Do not test in a wooden crate (capacitive coupling) Support on a metal stand if possible If hanging from a sling, the sling’s cleanliness may affect the test, and it

should be kept away from energized points Connect ground lead directly to bushing flange, and ground both test set

and the specimen to adequate ground Clean upper and lower surfaces before testing

Bushing C1 (conductor to tap)

Insulation Test

Dielectric Circuit: Main-

Insulation/C1 Test

Dielectric Circuit: C2 Insulation

Bushing Tap-Insulation (C2) Test

Test Result Analysis

Bushing Power Factor •C1 power factor for modern condenser type bushings are typically near 0.5% after correction to 20 degrees C. •C2 should be <1.0% PF for condenser and <2.0% for non-paper/oil filled bushings. • C2 insulation greater than a 1.0% power factor is questionable and warrants further investigation.

Evaluating Bushing Test Results

Good: (G) • Power Factor matches nameplate. • Hot-Collar test shows no abnormal losses. • Capacitance measurements are normal. • Visual inspection shows that there are no cracks or

oil/compound leaks in the cylindrical portion of the porcelain weather casting.

Evaluating Bushing Test Results

Deteriorated: (D) • Overall GST and/or UST power factors are approximately

twice the nameplate values for a new bushing, and/or the Hot-Collar test values are deteriorated.

• If a visual inspection shows that compound is leaking from the bushing in the vicinity of a gasket, this fact should be recorded on the test data sheet. A defective gasket will eventually permit moisture to enter.

Evaluating Bushing Test Results Ct.

Investigate: (I) • If power factor is significantly different than nameplate value

and overall GST or UST value is greater than 1.0 %PF. • If Hot-Collar test values are abnormal. • If the capacitance is abnormal.

Evaluating Bushing Test Results Ct.

Bad: (B) • If overall GST or UST power factor is showing a trend away from the

initial benchmark. For bushings with initial UST values of .5% or less, the absolute UST power factor should not be allowed to exceed 1.5% before being removed from service

• If Hot-Collar is abnormal • If capacitance is abnormal (5-15%) increase indicates short circuited

layers • If the cylindrical portion of the porcelain weather castings are cracked.

Variation Of Power Factor with

Temperature

• Electrical characteristics of all insulating materials vary with temperature.

• In order to compare results of periodic tests on the same apparatus while at different temperatures, it is necessary that the manner in which the results vary with temperature be known.

• The results then can be converted to a common temperature base.

Temperature Factor Correction

Temperature-Correction is used in the following manner: 1. Calculate the specimen (e.g., bushing) power factor.

2. Determine the test-specimen temperature.

3. Obtain the appropriate correction factor from the Temperature-Correction Table corresponding to the specimen temperature.

4. Multiply (1) and (3)

Example Ohio Brass Company bushing Class GK, 115 kV

(1) Calculated power factor = 0.42%

(2) Ambient temperature = 30°C

(3) Multiplier from the Temperature-Correction Table at

30°C = 1.11

(4) Corrected to 20°C power factor = 0.42% x 1.11 = 0.47%