Diagnostic Measurements and Condition Assessment of High ...€¦ · Diagnostic Measurements and Condition Assessment of High Voltage Bushings CEPED Conference on Electrical Power

Diagnostic Measurements and Condition

Assessment of High Voltage Bushings

CEPED Conference on Electrical Power Equipment Diagnostics

September 2-4, 2015, Kuala Lumpur, Malaysia

Dr. Michael Krüger, OMICRON

Cigre A2.37 Transformer Failure Statistics22.000 grid transformers with 150.000 sercice years

Tenbohlen et. al.: „ DEVELOPMENT AND RESULTS OF A WORLDWIDE TRANSFORMER RELIABILITY SURVEY” CIGRE SC A2 COLLOQUIUM 2015, Shanghai

Causes for transformer breakdowns

Bushings = 17,16 %

PAGE 2

Cigre A2.37 Transformer Failure Statistics

Tenbohlen et. al.: „ DEVELOPMENT AND RESULTS OF A WORLDWIDE TRANSFORMER RELIABILITY SURVEY” CIGRE SC A2 COLLOQUIUM 2015, Shanghai

External effects of bushing failures

Fire 30%

Explosion, Burst

10%

PAGE 3

Transformer Oil-Oil Bushings

PAGE 5

Transformer Oil-Gas Bushings

Source: HSP Germany

PAGE 6

LV Bushings of GSU Transformers

Technical Data:

Ur = 36 kV

Ir = 17 kA

PAGE 7

Outdoor GIS-Air Bushings

Source: HSP Germany

400kV GIS Substation, Vorarlberger Illwerke, Buers Austria

PAGE 8

Wall Bushings

PAGE 9

Wall Bushing, HSP Germany

Generator Bushings

PAGE 10

Insulation Systems of High Voltage Bushings

RBP Resin Bonded Paper

RIP Resin Impregnated Paper

OIP

Oil Impregnated Paper

PAGE 11

RBP RIPOIP

1913 to 1974 since 1950 since 1960

Recommended measures Time schedule

Visual check – leakage and mechanical damage In case of maintenance work

Measurement of capacitance and

dielectric dissipation factor

- after installation as reference for later measurements

(fingerprint)

- 10 years after commissioning

- dependend on results after each 5 years

(uncritical values) or shorter

Insulation Systems of High Voltage Bushings

Source: B. Heil (HSP Troisdorf, Germany), „Diagnose und Bewertung von Durchführungen“, OMICRON AWT Germany 2010

PAGE 12

Share of Products

Source: MICAFIL

PAGE 13

Resin Impregnated Synthetics (RIS)

Source: MICAFIL

PAGE 14

Today up to 170kV

• Craped paper is replaced by a

open synthetic textile

• Textile can be impregnated with a

filled resin with high viscosity

(filler = Aluminiumoxide or

Siliciumoxide)

• Filled resin systems are proven to

be reliable for use in HV

applications for several 10 years

• Improved thermal conductivity

and mechanical strength

• No long drying period needed

• Reduced manufacturing time and

costs

• Available up to 170kV today

Resin Impregnated Synthetics (RIS)

Source: MICAFIL

PAGE 15

High Voltage AC Test in the Factory Lower Part in a Oil Filled Vessel

Source: MICAFIL

PAGE 16

Capacitive Bushings (1)

Emax= high

A

Emax=

smaller

A

without

capacitive

layers

with

capacitive

layersEarthing Cap

PAGE 17


PAGE 18

Aluminium foils with equal areas

Aluminium foil

Paper

Aluminium foil Conductor


PAGE 19

Current at the Measurement Tap

50𝐻𝑧 → 𝐼𝐶 = 𝑈 𝜔 𝐶 = 10 …100𝑚𝐴

𝐵𝐼𝐿 1.2μ𝑠 → 𝐼𝐶 = 𝐶 𝑑𝑈/𝑑𝑡 = 200𝐴

Capacitive Bushing of a 123kV CT

PAGE 20

Measuring and Voltage Tap

PAGE 21

Source: Passoni & Villa

Bushing Measuring Tap

PAGE 22

Measuring Taps

MICAFIL

ABB

PAGE 23

Measuring Taps

HSP

PAGE 24

Measuring Taps

GE

PAGE 25

Arcing at the Measurement Tap

PAGE 26

Source: Norbert Koch „Diagnoseverfahren an Hochspannungsdurchführungen aus Herstellersicht“, Diagnoseverfahren an Schaltanlagen und Transformatoren,

HdT Essen 2013

Partly Burned Measurement Tap of a

245kV-RIP Busing

PAGE 27

Source: Hubert Goebel GmbH

Partly Burned Measurement Tap of a

400kV-OIP Busing

PAGE 28


Diagnostic

Methods

PAGE 29

Type of Fault

Moisture

Ageing

Contact problems and high impedance

faults between layers

Shorted grading layers

Leakage

Partial discharges

Diagnostic Technique

OIP 5 1 5 5 5 5

RBP 5 1 5 5 5 5

RIP 5 1 5 5 5 5

OIP 4 5 4 3 4 5

RBP 4 5 4 3 4 5

RIP 4 5 4 3 4 5

OIP 5 4* 1 3 4 4

RBP 5 4* 1 3 4 4

RIP 5 4* 1 3 4 4

OIP 3 5 3 1** 2 2

RBP 3 5 3 1** 2 2

RIP 3 5 3 1** 2 2

OIP 3 5 1 1 1 1

RBP 3 5 1 1 1 1

RIP 3 5 1 1 1 1

OIP 1 5 3 1 5 4

RBP 1 5 3 1 5 4

RIP 1 5 3 1 5 4

Dissolved gas analysis 1 5 1 1 4 5

Moisture in oil 5 5 4 5 3 1

DF/PF 4 5 4 3 2 2

Conductivity of oil (IEC 61620) 4 5 4 3 2 2

Particles in oil 5 5 5 5 2 5

Analysis of Furanic components 5 5 5 5 1 5

Test for corrosive Sulfur 5 5 5 5 2 5

1=very good, 2=good 3=fair 4=poor 5=not applicable

*big leakages can be detected with hot collar test **DF/PF from 2-12kV

OIP

Ge

ne

ral

Ele

ctr

ica

l

Ba

sic

Dielectric Response

with FDS / PDC

Partial discharge measurementEle

ctr

ica

l

Ad

va

nce

dO

il

Capacitance

DF/PF

Visual Inspection

Thermography

Infrared Thermography

PAGE 30

Source: ABB Brochure „Bushing diagnostics and conditioning“, 2750 515-142 en

C-Tan δ Progression of a Bushing

C Tan-Delta Progression of a 220 kV RBP (Hard Paper) Bushing

Year of Manufacturimg 1961, horizontal mounted, oil filled

-5

0

5

10

15

20

25

30

75 77 79 81 83 85 87 89

Date of Measurement

Tan

Delt

a x

10

-3

Ch

an

ge o

f C

ap

acit

an

ce %

Tan Delta Change of Capacitance

Source: RWE

PAGE 31

Source: Volker Seitz: „Vorbeugende Instandhaltung an Leistungstransformatoren“, OMICRON Anwendertagung 2003, Friedrichshafen

Bushing Fault

•Source: RWE

PAGE 32

Source: Volker Seitz: „Vorbeugende Instandhaltung an Leistungstransformatoren“, OMICRON Anwendertagung 2003, Friedrichshafen

Fault Mechanisms and Diagnosis

• Partial breakdowns

– Measurement of

capacitance

– TanDelta measurement

– PD measurement

• Voids, cracks

– Measurement of

capacitance (RBP)

– PD measurement

• Contact problems on measurement taps

– Tan Delta voltage sweep (tip-up test)

• Ageing, moisture

– Dielectric response measurements

– TanDelta

Voltage

[kV]

No. of

layers

%

change

123 14 7.1

245 30 3.3

420 40 2.5

550 55 1.8

PAGE 33

Definitions of Dielectric Losses

Dielectric losses are caused by

• Conductive losses

• Polarization losses

• Partial discharges

•

0.00E+00

5.00E-04

1.00E-03

1.50E-03

2.00E-03

2.50E-03

0 50 100 150 200 250 300 350 400

Parallel

Parallel

Parallel circuit Serial circuit

RPCP

RS

CStot

R

SS

C

R

U

U

CRU

U

cos:PF

tan:DF

||

||cos:PF

1

||

||tan:DF

tot

RP

PPCP

RP

I

I

CRI

I

0.00E+00

5.00E-04

1.00E-03

1.50E-03

2.00E-03

2.50E-03

0 50 100 150 200 250 300 350 400

Serial

Serial

0.00E+00

5.00E-04

1.00E-03

1.50E-03

2.00E-03

2.50E-03

0 50 100 150 200 250 300 350 400

Serial

Parallel

Sum

PAGE 34

Standards

Type RIP OIP RBP

Main insulation Resin

impregnated

paper

Oil

impregnated

paper

Resin bonded

paper

DF / tan delta (20°C,

IEC60137)

< 0.7 % < 0.7 % < 1.5 %

PF cos phi (20°C,

IEEE C57.19.01)

< 0.85 % < 0.5 % < 2 %

Typical new values 0.3-0.4 % 0.2-0.4 % 0.5-0.6 %

PD (IEC60137) at Um

1.5 Um /3

1.05 Um /3

< 10 pC

< 5 pC

< 5 pC

< 10 pC

< 5 pC

< 5 pC < 300 pC

PAGE 35

Cigre WG A2.34 Brochure 445 Guide for Transformer Maintenance

www.e-cigre.org

PAGE 36

Frequency

http://www.e-cigre.org/

• Not free of cavities – Partial Discharges possible

also at rated voltage

• Higher dielectric losses can feed to thermal

instability

• RPB has cavities and cracks in the

paper which are normally filled

with the surrounding oil

• Increase of capacitance

• After a longer storage period this oil is running out.

The PD level is increasing and the capacitance is

getting smaller

RBP - Bushings

PAGE 37

RBP – Bushings Cracks

in the Insulation

PAGE 38

Measurement on 220kV RBP Bushings (1971)

PAGE 39

TanDelta 15-400Hz

PAGE 40

RBP Bushing Oil-Filled Cracks Oil Ingress by Capillare Effect

PAGE 41

RBP Bushing 123kV (1963)

PF (f) A, B, C

0.0%

0.2%

0.4%

0.6%

0.8%

1.0%

1.2%

0.0

Hz

50.0

Hz

100.0

Hz

150.0

Hz

200.0

Hz

250.0

Hz

300.0

Hz

350.0

Hz

400.0

Hz

450.0

Hz

A

Influence of

Moisture

PAGE 42

Micafil UTXF 24 (Drysomic) RBP Bushings

A,B,N humid

after wrong

storage

C dried

PAGE 43

Micafil UTXF 24 RBP Bushings A,B,N never used, wrongly stored, C dried

•Messung bei 20°CDF (f) A, B, C, N

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

6.0%

0.0

Hz

50.0

Hz

100.0

Hz

150.0

Hz

200.0

Hz

250.0

Hz

300.0

Hz

350.0

Hz

400.0

Hz

450.0

Hz

A

B

C

N

dried

humid

PAGE 44

Micafil UTXF 24 FDS Messung

dried

humid

PAGE 45

RBP Bushing 123kV

?

PAGE 46

RBP Bushing 123kV

PF (V) A, B, C

0.5%

0.6%

0.7%

0.8%

0.9%

1.0%

1.1%

0.0

V

20

00

.0V

40

00

.0V

60

00

.0V

80

00

.0V

10

00

0.0

V

12

00

0.0

V

14

00

0.0

V

A

B

C

PAGE 47

RBP Bushing 123kV

PF (f) A, B, C

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

0.0

Hz

50

.0H

z

10

0.0

Hz

15

0.0

Hz

20

0.0

Hz

25

0.0

Hz

30

0.0

Hz

35

0.0

Hz

40

0.0

Hz

450.0

Hz

A

B

C

PAGE 48

RBP Bushing 123kV

Bushing has to be

exchanged

defective contact on the

measuring tap or

defective connection between the

innermost layer and the HV

conductor or

partial breakdown between layers

PAGE 49

Measurement Tap O.K.

PAGE 50

Innermost Layer was not Properly

Connected to the HV Conductor.

PAGE 51

123kV RBP Bushing

PAGE 52

Dissipation Factor Measurement

0,0%

1,0%

2,0%

3,0%

4,0%

5,0%

6,0%

7,0%

8,0%0

,0H

z

50,0

Hz

100

,0H

z

150

,0H

z

200

,0H

z

250

,0H

z

300

,0H

z

350

,0H

z

400

,0H

z

450

,0H

z

DF (f) A, B, C

A

B

C

PAGE 53

Measurement of the Removed Bushing

PAGE 54

Measurement of the Removed Bushing

0,0%

2,0%

4,0%

6,0%

8,0%

10,0%

12,0%

14,0%

16,0%0

,0H

z

50,0

Hz

100

,0H

z

150

,0H

z

200

,0H

z

250

,0H

z

300

,0H

z

350

,0H

z

400

,0H

z

450

,0H

z

DF (f)

C1

PAGE 55

Aktivteil

PAGE 56

1

1,5

2

2,5

3

3,5

2 kV 4 kV 6 kV 8 kV 10 kV 12 kV

Ta

n δ

[%]

Faulty Contact on a Head Connection 245kV RBP Bushing

PAGE 57


Faulty Contact on a Head Connection 245kV RBP Bushing

PAGE 58


OIP Bushings

• Paper of the OIP bushings ages particularly at high

temperatures

• Through aging the dielectric losses will increase

-> this increases the power factor

• Temperature dependent aging decomposes the Paper

and produces additional water

-> this accellerates the aging

PAGE 59

OIP Bushings Winding

Machine

PAGE 60

Source: Trench Brochure „OIP Transformer Outdoor Bushings“

Assessment of 50/60Hz C-Tanδ Results (OIP)

PAGE 61

Source: Proposal from OMICRON, GÖBEL and HSP for the Cigre Working Group A2.43 „Reliability of HV Bushings“

Tan δ Dependency on the Temperature

PAGE 62

Source: ABB: „Bushing Diagnostics and Conditioning“ Brochure 2750515-142 en, Ludvika 2000

C-Tan δ Dependency on the Temperature

PAGE 63

Source: HSP Manual

Limits für Measurement Results

a) Capacity:

Voltage Level / Change of Capacitance

≥245 kV 2.3 %

≥362 kV 1.7 %

≥420 kV 1.5 %

≥550 kV 1.3 %

>550 kV 0.8 %

b) tan delta

Normal values are between 0.2 % and 0.4 %

The Temperature Influence can be neglected

between 20°C bis 70°C.

Values between 0.4 % and 0.5 %: → Contact HSP

Values > 0.55 % can be an indicator for an internal

problem and should be investigated by a DGA

Breakdown in a OIP Bushing

Source: B. Heil, „Diagnose und Bewertung von Durchführungen”, OMICRON AWT Dresden 2010

PAGE 64

OIP Bushing Fault

PAGE 65


OIP Bushing Fault

PAGE 66


OIP Bushing Fault

PAGE 67


OIP Bushing Breakdown

at the Sharp Edge of the Foil

PAGE 69


OIP Bushing

PAGE 70


33kV OIP Bushings

New bushings

Removed

bushings

C-Tan-Delta Meas.

PAGE 71

Tan Delta (T) at 50Hz (OIP DF)

0.10

1.00

10.00

20 30 40 50 60 70 80 90°C

[%]0.1-0.3% Water in paper

0.4-0.6% Water in paper

1% Water in paper

Source: ABB, Bushing diagnostics and conditioning

PAGE 72

Tan Delta (f) at 30°C (33kV OIP DF)

Tan Delta (f) A, B, C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0

50

10

0

15

0

20

0

25

0

30

0

35

0

40

0

45

0Hz

%

A

B

C

A Removed

B Removed

C Removed

PAGE 73

123kV OIP Bushings TanDelta = f(U)

PF (V) A, B, C

0.4%

0.5%

0.6%

0.7%

0.8%

0.9%

1.0%

1.1%

1.2%

1.3%0

.0V

20

00

.0V

40

00

.0V

60

00

.0V

80

00

.0V

10

00

0.0

V

12000.0

V

14

00

0.0

V

A

B

C

PAGE 74

123kV OIP Bushings TanDelta = f(f)

PAGE 75

123kV OIP DF Corroded Measuring Tap

PAGE 76

Ageing of RIP Bushings

• Partial breakdowns between capacitive layers are rather

seldom

• Decrease of the power factor with increasing test voltage can

be an indicator for partial breakdowns

• Also defective connections of the measurement layer to the test

tap or of the innermost layer to the high voltage conductor may

be the reason for a decrease of the power factor with

increasing test voltage

• Increase of the capacitance after a partial breakdown between

two layers:

• Cnew = Cold x n / (n-1)

n= number of layers approx. 4-7 kV per layer

PAGE 77

RIP Bushings

PAGE 78

RIP Bushing

PAGE 79

Partial Breakdown on a RIP Bushing

PAGE 80

Source: B. Heil, „Diagnose und Bewertung von Durchführungen”, OMICRON AWT Germany 2010

Closed Grading FoilsBetter design for Very Fast Transients (VFT)

PAGE 81

Breakdown in Oil Causes a Flash-Over in the Earth Lead due to Fast Transient Earth Currents

PAGE 82

Breakdown in Oil Causes a Flash-Over in the Earth Lead due to Fast Transient Earth Currents

PAGE 83

Measurement on a 420kV Bushing

PAGE 84

Measurement of the Capacitance and the Dielectric

Dissipation Factor on C1

PAGE 85



PAGE 86

UST

GST



PAGE 87

0,0%

1,0%

2,0%

3,0%

4,0%

5,0%

6,0%

0,0

V

2000,0

V

4000,0

V

6000,0

V

8000,0

V

10000,0

V

12000,0

V

14000,0

V

DF (V)

C…

0,0%

1,0%

2,0%

3,0%

4,0%

5,0%

6,0%

7,0%

0,0

Hz

50,0

Hz

100,0

Hz

150,0

Hz

200,0

Hz

250,0

Hz

300,0

Hz

350,0

Hz

400,0

Hz

450,0

Hz

DF (f)

C…

Insulation Resistance Measuring Tap against Flange

PAGE 88

782 Ohm

Partial Discharge Measurement

PAGE 89

Inception Voltage below 20kV

PAGE 90

Partial Discharges > 2nC at 45 kV

PAGE 91

PD Location

PAGE 92

Measuring Tap

PAGE 93

Measuring Tap

PAGE 94

Cutted Bushing

PAGE 95

Burned Contact Spring

•PAGE 96

50Hz: I = U C 37 mA

VFT: I = C du/dt > 100A

Relative Tan δ Dependency on the Temperature

PAGE 98

Source: ABB: „Bushing Diagnostics and Conditioning“ Brochure 2750515-142 en, Ludvika 2000

C-Tan δDependency on the Temperature

PAGE 99

Source: HSP Operation Instruction BAL EKTG03e

220kV RIP Bushing, Stored Outside

PAGE 100

220kV RIP Bushing, Stored Outside

0.25 %

0.35 %

0.45 %

0.55 %

0.65 %

0.75 %

0.85 %

0.0 Hz 100.0 Hz 200.0 Hz 300.0 Hz 400.0 Hz 500.0 Hz

Ta

n D

elt

a

after removing from TR

3 months after removing

6 months afetr removing

Minimum

Minimum

Minimum

PAGE 101

PAGE 102

Influence of Humidity –for 123kV Bushings with Silicone Coated Composite Insulators

Mounted Horizontally

PAGE 103

DF (f)

0.2%

0.3%

0.4%

0.5%

0.6%

0.7%

0.8%

0.9%

1.0%

0.0Hz 50.0Hz 100.0Hz 150.0Hz 200.0Hz 250.0Hz 300.0Hz 350.0Hz 400.0Hz 450.0Hz

T31 2U2(f)

T31 2V2(f)

T31 2W2(f)

T32 2U2(f)

T32 2V2(f)

T32 2W2(f)

T33 2U2(f)

T33 2V2(f)

T33 2W2(f)

Influence of Humidity –for 123kV Bushings with Silicone Coated Composite Insulators


PAGE 104

Influence of Humidity for 123kV Bushings with Silicone Coated Composite Insulators


DF (f)

0.2%

0.3%

0.4%

0.5%

0.6%

0.7%

0.8%

0.9%

1.0%

0.0Hz 50.0Hz 100.0Hz 150.0Hz 200.0Hz 250.0Hz 300.0Hz 350.0Hz 400.0Hz 450.0Hz

A

B

C

DF (f) A, B, C

0.0%

0.1%

0.2%

0.3%

0.4%

0.5%

0.6%

0.0

Hz

50.0

Hz

100.0

Hz

150.0

Hz

200.0

Hz

250.0

Hz

300.0

Hz

350.0

Hz

400.0

Hz

450.0

Hz

A

B

C

Bushings with

moss and outer

humidity

Bushings cleaned,

measured at dry weather

PAGE 105

Measurement with GuardLeakage current is bypassing the meter

Meas. tap

Guard

Capacitive

layer

Meas.

layer

Leakage

current

PAGE 106

Measurement With High Humidity With and

Without Guard

PAGE 107

Measurement With High Humidity With and

Without Guard

without

Guard

with Guard

• The oil side of RBP and RIP bushings

doesn't need a housing

• Cellulose near to the surface can absorb

water, if bushings are not stored properly

• Incoming water, also from the ambient air

reduces the dielectric strength – this causes

an increase of the dielectric dissipation factor

Moisture in RBP and RIP Bushings

PAGE 108

Water in RBP and RIP Bushings

RIP wet RIP dry

Drying of RBP

and RIP bushings

is limited

PAGE 109

Measurement of the Dielectric Response

with FDS und PDC

PAGE 110

PDC FDS

Combination of PDC

und FDS reduces

measurement time

Combined FDS-PDC Measurement

on a RIP Bushing

PAGE 111

RBPRIP

OIP

FDS Results on RIP, RBP and OIP Bushings

PAGE 112

25°C

60°C

FDS/PDC on a RIP Bushing at Different

Temperatures

50Hz tanδ

PAGE 113

Source: G. Kopp, „Measurement of the dielectric response on HV bushings “

dry surface

wet surface

FDS/PDC with Dry and Wet Surface

50Hz tanδ

equal

PAGE 114

Source: G. Kopp, „ Measurement of the dielectric response on HV bushings “

Drying of a 145kV RBP Bushing

TanDelta Measurement

PAGE 115

Drying of a 145kV RBP Bushing

TanDelta (V)

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

0 2 4 6 8 10 12 14

kV

TanDelta (f)

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

3.0%

3.5%

0 50 100 150 200 250 300 350 400 450

Hz

1. Measurement before drying

2. Measurement after drying

1. Measurement before drying

2. Measurement after drying

Drying:

1200h at 90°C / 200°F

PAGE 116

High Voltage Test with PD Measurement

PAGE 117

PD Measurement - Phase Resolved

Pattern @ 157kV

PAGE 118

Separation of PD Sources in 3CFRD

2MHz

500kHz

8MHz

500

kHz

2

MHz

8

MHz

A(500kHz)>A(2MHz)>A(8Mhz)3CFRD

•timeframe

1 µs

500 k

Hz

2 M

Hz

8 M

Hz

3CFRD = 3 Center Frequency Relation Diagram

PAGE 119

PD Measurement Cluster 1

500 kHz

2.8 MHz

8 MHz

50

0 k

Hz

2.8

MH

z

8 M

Hz

PAGE 120


500 kHz

2.8 MHz

8 MHz

50

0 k

Hz

2.8

MH

z

8 M

Hz

PAGE 121


500 kHz

2.8 MHz

8 MHz

50

0 k

Hz

2.8

MH

z

8 M

Hz

PAGE 122

Assessment and Interpretation

Indication RBP OIP RIP

increase of

capacitance

oil in cracks

or partial breakdownspartial breakdowns partial breakdowns

high dissipation

factor

partial breakdowns;

insulator surface wet or

dirty (clean the

insulator);

ageing of the inner

insulation;

water in the inner

insulation;

partial breakdowns;


dirty (clean the

insulator);

ageing of the inner

insulation;

water in the inner

insulation;

partial breakdowns;


dirty (clean the

insulator);

ageing of the inner

insulation;

water in the inner

insulation;

dissipation factor is

decreasing with

increasing voltage

bad potential

connections;

partial breakdowns

bad potential

connections;

partial breakdowns

bad potential

connections;

partial breakdowns

dissipation factor is

strongly increasing

with

increasing

temperature

high moisture in the

insulation;

high degree of ageing


insulation;



insulation;


partial discharges normal, if constant

Discharges produce

gasses;

Errosion of the

cellulose;

production of x-wax

partial breakdowns;

cracks or voids after

electrical or mechanical

stress;

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Documents

Diagnostic Measurements and Condition Assessment of High ...€¦ · Diagnostic Measurements and Condition Assessment of High Voltage Bushings CEPED Conference on Electrical Power