Failure Rate Characteristics Typical Electrical Equipment

Time

Failure Rate Infant

MortalityWear-out RegionRandom

Failures

Useful Life

Wear Out Region

!Time!Material quality!Material workmanship!Family of components show wear at the

same rate

Random Failure Region

!Serious material defects!Serious workmanship defects!Anomalies causing shorter useful life

expectancy!External effects (contamination,

lightning, etc.)

Bushings

!Hermetically sealed!Described as maintenance free by manufacturer!Manufacturer in the past has recommended not

to disturb seal!Not possible to inspect primary components!Small volume of oil!Condition assessment critical

Bushings continued

!European statistics show 80% of bushing failures occur between 12-20 years (mid life of our transformer population)

!Failure followed by catastrophic event such as tank rupture, fire etc.

!Bushing failures initiate 30% of transformer failures

!Very similar device to an instrument transformer

Serviced Aged Bushings

!Pressure has been lost!Substantial number have leaks

Common Bushing Failure Mechanisms

!Moisture ingress due to leaking or deteriorated gaskets

!Partial discharge due to internal deficiencies!Overheating of connections due to the use of

dissimilar metals!Improper storage of spare bushings!Use of conductive ink instead of foils

Partial Discharge

!Inadequate impregnation of oil into paper (X wax build up on paper)

!Deteriorated oil!Porcelain contaminated with semi-

conducting sediment!Discharge at foil ends (X wax present)

Small Oil Volume Equipment

!Instrument Transformers and Bushings!Common failure modes!Partial discharge is a common symptom

(known as predominant failure cause in CT’s)!Hydrogen is a product and an excellent

indicator of partial discharge (Balteau experience)

Condition Assessment Tools

!Infrared Scanning!Power Factor Testing!DGA !External Inspection (not applicable for internal

problems although high oil level or inflated bellows in CTs can indicate internal issues)

Weaknesses Infrared Scan

"Difficult to pick up internal problems below flange.

"Load and weather condition effect readings.

"Bright sun inhibits effectiveness.

.

Weaknesses Power Factor Testing

"External conditions such as humidity, temperature and contamination effect readings.

"Due to the bushing being mounted in the transformer, an overall reading is impossible to obtain.

"Certain types of internal problems will be indicated by negative readings, which can sometimes make analysis confusing.

Weaknesses Power Factor Testing

"Different bushing types and manufacturers have varying levels of acceptance.

"C2 readings can be influenced by insulation not within the bushing cavity.

" Partial discharge in it’s early stages is difficult to pick up.

Weaknesses DGA

"Limited amount of oil."Secondary failure mode can be

introduced during sampling."Not a lot of data to compare readings.

Strengths Infrared

"In most cases results are very definitive and can pin point problem areas above flange.

"Non-intrusive unit in service while assessment is being made.

Strengths Power Factor Testing

" Most companies have a large database of previous readings, making trending easy.

"With proper understanding of the theory of the UST readings, analysis can be done.

"Can detect both carbon and moisture as failure mechanisms.

Strengths DGA

" Test is performed under repeatable conditions (laboratory).

" Sampling is quite simple as long as precautions are taken.

"Done in conjunction with PPM of H2O all types of failure mechanisms are identified (partial discharge, carbon tracking, water ingress).

Summary of Observations

" Instrument transformers and bushings have a lot in common

"As in instrument transformers partial discharge in bushings seems to be a symptom of several failure causes (hydrogen and X wax ever present)

"The failure of each is a concern due to the collateral damage they cause

Altalink Case Studies Bushing DGA Testing

Dissolved Gas Values for Small Oil Volume Equipment From IEC 60599

2501030300Instrument Transformers

2703040140Bushings

C2H2

C2H6

C2H4

CH4H2

Dissolved-Gas Analysis COBType GK-40 Bushing SER. # 25610

542Acetylene (C2H2)2,291Ethylene (C2H4)2,185Ethane (C2H6)6,523Methane (CH4)49,495Hydrogen (H2)

Concentration (ppm in oil)

Component Gas

Dissolved-Gas Analysis COBType GK-40 Bushing SER. # 25600

Not detectableEthylene (C2H4)traceAcetylene (C2H2)

29Ethane (C2H6)Not detectableMethane (CH4)

Hydrogen (H2)

Component Gas

6.5

Concentration (ppm in oil)

Dissolved-Gas Analysis COBType GK-40 Bushing SER. # 15940

<0.1Acetylene (C2H2)1.8Ethylene (C2H4)71Ethane (C2H6)146Methane (CH4)

1705Hydrogen (H2)

Concentration (ppm in oil)

Component Gas

Dissolved-Gas Analysis COBType GK-40 Bushing SER. # 19147

0.2Acetylene (C2H2)11Ethylene (C2H4)409Ethane (C2H6)

1256Methane (CH4)19,131Hydrogen (H2)

Concentration (ppm in oil)

Component Gas

Field Test History for COBType GK-40 Bushing SER. # 25610

198919851979Nameplate

0.1360.5990.8570.575Power Factor %

C1

1.560.4530.721Power Factor %

C2

Field Test History for COBType GK-40 Bushing SER. # 25600

199119891979Nameplate

0.2610.3090.5930.30Power Factor %

C1

.2480..2460.45Power Factor %

C2

Field Test History for COBType GK-40 Bushing SER. # 15940

shop19931989Nameplate

.401.230.440.31Power Factor %

C1

1.560.4530.721Power Factor %

C2

Field Test History for COBType GK-40 Bushing SER. # 19147

shop19931989Name Plate

.771.140.310.29Power Factor %

C1

.270.270.25Power Factor %

C2

Graphical Representation #25610

00.20.40.60.8

11.21.41.6

1979 1985 1989 1989Years

Pow

er F

acto

r

PF C1PF C2

Overall Test for COBType GK-40 Bushing SER. # 25610

Capacitance (pF)

%Power Factor

kV

458.88.8010Overall Test (GS-Ground)

Dissolved-Gas Analysis CGE500 KV Reactor Bushings

000Acetylene (C2H2)

020Ethylene (C2H4)

313Ethane (C2H6)

1274Methane (CH4)

252031Hydrogen (H2)

Dissolved-Gas Analysis CGE240 KV Bushing SER. # 252816

0Acetylene (C2H2)

0Ethane (C2H6)3Ethylene (C2H4)

6Methane (CH4)Hydrogen (H2)

Component Gas

30

Concentration (ppm in oil)

Dissolved-Gas Analysis CGE138 KV Bushing SER. # 232854

0Acetylene (C2H2)

15Ethane (C2H6)3Ethylene (C2H4)

54Methane (CH4)Hydrogen (H2)

Component Gas

230

Concentration (ppm in oil)

The Questions We Now Face Are:

!Do I perform DGA analysis on all my bushings?

!If not what criteria do I use to choose which ones to test?

The Questions We Now Face Are:

!Due to the small amount of oil what is my sampling rate?

!What are the critical gases and at what level should I be concerned?

!When do I remove suspect bushings from service?

Other Utilities

!National Grid in the United Kingdom has 4000 bushing DGA results

!BC Hydro has initiated a DGA gas sampling program for their 500 KV bushings