1. Doble Power Factor Bushing Test
By: Cody Dickson
&
Chris Chappell

2. Doble M4000 Insulation Analysis Test Set

3. Most failures of electrical equipment are the result of a breakdown in the insulation system.
Power factor testing is an effective way to detect damaged electrical equipment insulation.
Why Power factor?

4. Is the Power Factor Test AC or DC?
The power factor test is an AC test that can put out up to 10KV and 300mA for > 4 Minutes.
Is the Power Factor Test AC or DC?

5. Construction/Purpose of a Bushing
The bushing in its entirety is to allow an energized conductor to penetrate a grounded plane (the transformer)
Can be gas, oil, or dry type bushings.
Gas filled and Bulk bushings have no active components.
Condenser bushing on the other hand do have an active component.
Construction/Purpose of a Bushing

8. The purpose of the power factor test on bushings
Bushings are a critical part of the electrical system that transforms and switches ac voltages ranging from a few hundred volts to several thousand volts. Bushings not only handle high electrical stress, they could be subjected to mechanical stresses, affiliated with connectors and bus support, as well. Although a bushing may be thought of as somewhat of a simple device, its deterioration could have severe consequences.
The purpose of the power factor test on bushings
The Power Factor Test is the ratio of the power dissipated in the insulation, in Watts, to the product of the effective voltage and current in volt amperes, when tested under a sinusoidal voltage and prescribed conditions.

9. PF testing is ran every few years on transformers and the bushings are usually included when these tests are ran.
PF is another tool used to track and trend the condition of the test apparatus.
When to the run the test

10. Nameplate information
Bushing nameplates provide a range of information used to analyze the results of the test.
Also the manufactures factory tests should be compared to the nameplate ratings and a field test should be ran upon installation to determine that they match the factory test results.(This is especially true for the “T” type bushings used on GSU).
Nameplate information

11. Bushing Power Factor Tests
C1- The C1 test (UST), tests the main core insulation; the center conductor to the test tap.
C2- The C2 test (GST), tests the Tap insulation; the test tap to the flange.
Hot Collar- The Hot Collar test, tests the external collar to the center conductor.
Overall- The overall test, tests the center conductor to the flange.(This is usually used on spare bushings).

12. Hot Collar Power Factor Test
For bushings not equipped with either a test tap or a voltage tap, the only field measurement which can be performed is the hot collar test.
Uses the conductive hot collar band which fits closely to the porcelain surface directly under the first skirt
Center conductor of bushing is grounded.
The test provides a measurement of the losses in the section directly beneath the collar and is especially effective in detecting conditions such as voids in compound filled bushings or moisture penetration.
Watts-loss values less than 100mw - bushing acceptable
Watts-loss values of 100mw or more - bushing unacceptable (contamination)
Hot Collar Power Factor Test

13. Procedure: How to Run Test for Bushings in Transformer
Clarify what test voltage to run on the bushings you are going to test.
Connect High Voltage Lead to C1 and low voltage lead to C2; for C1 power factor test.
C1 Main Core Insulation Test Use 10KV if bushing rating is greater than 15KV
C2 Tap Insulation Test 69KV and less-500V; 69KV and up- 2KV; IEEE Standard
Once all bushings have been tested, you will then switch your leads on the C1 and C2 taps and repeat this for every bushing.
Procedure: How to Run Test for Bushings in Transformer

16. If you do not thoroughly clean the porcelain on the bushing
If you do not thoroughly clean the porcelain on the bushing. (Clean using alcohol, soap and water, Windex with ammonia, and a soft cloth)
If there is high humidity, or moisture in your C2 test tap. (To get rid of moisture in the test tap use a heat gun to dry out the tap being sure not to over heat the area, (this as well could skew results))
Do not test on rainy days.
Higher than normal temperatures may skew results. (Test set corrects for temperature assuming the correct temperature is entered.)
Low oil. (Check your sight glass)
Problems May Arise

17. This tap is connected inside the bushing to a determined position on the core of the bushing.
When running a test on the C2 test tap it is a test of the degradation of the insulation inside the bushing.
This test is also a lot better test to determine the level of fluid contamination inside the bushing.
Not all bushing have this C2 tap, this may result in you having to perform a hot collar test on the particular bushing.
C2 Tap

18. As your run the power factor tests on the bushings you may begin to notice slight rises in the things such as capacitance and insulation values.
Once you get over a certain amount either of these values you must really start looking harder into what are some of the reasons that would cause things like that to happen.
After you have checked connections, cleaned bushings, and you have had some one to peer check behind you to make sure that you have not overlooked some miniature detail causing your skewed results, you must then begin to think about what some of the reasons would be in side the bushing itself as to why it would be going bad.
Results

19. C1
Most companies allow for a + or – 5% power factor on bushing, but this still may vary from company to company and between different manufacturers.
Between nameplate pf and up to twice nameplate pf - bushing acceptable
Between twice nameplate pf and up to 3 times nameplate pf - monitor bushing closely
Above 3 times nameplate pf - replace bushing
Nameplate capacitance +/-5% - bushing acceptable
Nameplate capacitance +/-5% to +/-10% - monitor bushing closely
Nameplate capacitance +/-10% or greater - replace bushing
Results Continued

20. C2
pf compare to prior test on same bushing or similar test on similar bushings
C2 pf generally around 1%
Nameplate capacitance +/-5% - bushing acceptable
Nameplate capacitance +/- 5% to +/-10% - monitor bushing closely
Nameplate capacitance +/-10% or greater - replace bushing
Results Continued

21. Questions???