diagnosis of electrical performance and aging behavior of ... · power factor test or tan delta...

Diagnosis of Electrical Performance

and Aging Behavior of

Transformer Dielectrics

CIGRE Thailand

29th November 2013, Bangkok

Supatra A. Bhumiwat

Independent HV Diagnostics Consultant

www.kea-consultant.com

2

Agenda

• The Nature of Electrical Insulation or Dielectrics

• Classification of problems in transformer dielectrics

• Identification of aging in transformer dielectrics by

oil analysis

• Identification of aging in transformer dielectrics by

electrical tests

3

The Nature of Electrical Insulation

Electrical Insulation is sometimes called

“Dielectrics”

due to its 2 basic electrical properties:

• Ability to persist in electrostatic field for a long time

(High Resistance or Low Conduction)

• Ability to be polarized

(Polarization)

“Conduction” and “Polarization” Phenomena

occur in every dielectric material, more or less.

4

Polarization

“Every kind of insulating materials consists, at an

atomic level, of negative and positive charges

balancing each other…..

As soon as a material is exposed to an electric field,

the positive and negative charges become oriented

thus forming different kinds of dipoles….”

From Walter S. Zaengl; “Dielectric spectroscopy in time and frequency domains for HV power

equipment, Part I: Theoretical Considerations”, IEEE Electrical Insulation Magazine,

vol. 19 no. 5, pp. 5-19, September/October 2003.

5

Polarization

Arrangement of charges during “Polarisation”

6

Polarization

Recovery Voltage happens to the phenomenon of “Polarisation”

But not to the phenomenon of “Conduction”

7

Equivalent Circuit of Electrical Insulation

C R Rpi + Cpi Rpn +Cpn

8

Polarization Conduction

1 The charges linked with

definite molecules of matter are

brought into motion. These

charges cannot leave the

confines of a given molecule.

1 The free charges (carriers) can

move through the entire thickness

of a dielectric from one electrode

to the other.

2 “Polarization” takes place in all

the molecules of a dielectric

and causes chemical change or

deterioration

2 “Conduction” is often determined

by the presence of impurities or

contaminants. It is not attributed

to its basic substance

3 When the voltage applied to a

dielectric is discontinued, the

displaced charges may tend to

return to their initial positions

3 The return (or recovery) voltage

never happens to the phenomenon

of electrical conduction.

4 The absorption current due to

polarization decays to zero

slowly under a direct voltage.

4 The conduction current exists and

keeps constant so long as a direct

voltage is applied to a dielectric.

From B. Tareev “Physics of Dielectric Materials”, MIR Publishers, Moscow, 1975

9

Each problem in a dielectric is produced by

the mechanism of

either “Conduction” or “Polarization” or both.

An ability to diagnose and identify

one from the other

allows problem in a dielectric

to be solved correctly.

10

Classification of problems

in Transformer Insulation

CLASSIFICATION OF PROBLEMS IN TRANSFORMER INSULATION

Polarization Conduction

Moisture in paper / pressboard Free water in oil

Oxidation by-products (acid & non-acid type) Surface humidity

Polar molecules in oil

(polar aromatics, polar compounds, etc)

Surface contaminants

Sludges at oil-paper interface Tracking

Thermal aging by-products Carbon dust

By products from Partial discharges Metal particles in oil

Arcing by-products Debris from fault

Corrosive products in oil Copper sulphide

11

O2+ Unstable Hydrocarbon

(in Oil)

+ Catalyst(.Moisture. Copper. Iron )

+Accelerators( . Heat. Vibration. Surges. Stresses )

= Oxidationby

product ( .peroxide. alcohols. ketones. acid )

Deterioration of Oil by Oxidation

Sludges are the accumulation of Oxidation-by-products

Sludges in the main ducts block cooling system

or prevent heat transfer.

12

Identification of Aging in Transformer dielectrics

by Oil analysis

• Fault (DGA)

• Normal Aging of Oil

- Contaminants which cause Conduction

Moisture, Particles, Metal-in-oil,

Dielectric Breakdown Voltage

- Deterioration products, acid or non-acid type

which cause Polarization

Conductivity & DDF by IEC61620 (since previous decade),

Acid, IFT, Inhibitors, etc.

**** Tan delta & DC Resistivity (IEC 60247)

or Power factor test of oil (ASTM-D924 detects

the combination of Conduction and Polarization****

• Normal Aging of Paper (Furans, Methanol & Ethanol )

13

Performance of Oil

based on IEC61620

14

Oil Test Results showing the Independency of

Moisture-in-Oil & Oil-Conductivity (IEC61620)

ID Oil Condition Oil temperature (oC) Moisture-in-oil (ppm) Conductivity at 20

oC (pS/m) Acidity

when sample is taken (Karl-Fischer method) (oil test: IEC61620) (mgKOH/g

Tx-001 A 37.6 35 7.06 0.086

C 40.0 37 1.30 0.029

B + C 17.0 23 0.47 0.013

Tx-002 A 40.7 36 4.81 0.082

C 39.7 37 1.54 0.019

B + C 17.0 25 0.50 0.012

Tx-003 A 16.0 21 10.06 0.082

B 10.0 10 7.88 0.080

B + C 11.0 5 0.83 0.007

Tx-004 A 14.0 14 18.6 0.129

D 9.0 7 19.5 0.128

E 5 5 0.858 -

Note: A: Before treatment, B: after vacuum dehydration, C; after clay treat, D after on-line drying, E: after oil replacement

Oil Conductivity / DDF based on the new IEC61620 allows the identification

of Polarisation phenomena in the insulation.

So we can solve oil problem correctly.

15

Moisture-in-oil VS

Conductivity based on IEC61620

(Data from 1,207 oil samples)

Moisture VS Conductivity at 20oC


1.E+00

1.E+01

1.E+02

1.E-02 1.E-01 1.E+00 1.E+01 1.E+02Conductivity (pS/m)

Mo

istu

re (

pp

m)

16

DDF (IEC60247) VS



DDF (60247) at 90oC VS Conductivity (61620) at 20

oC

(Data from 1,066 oil samples

1.E-03

1.E-02

1.E-01

1.E+00

1.E-01 1.E+00 1.E+01 1.E+02 1.E+03

s at 20oC (pS/m)

DD

F (

60

24

7)

at 9

0oC

17

Acidity (Neutralization Number) VS



Acidity VS Conductivity at 20oC


1.E-03

1.E-02

1.E-01

1.E+00

1.E-02 1.E-01 1.E+00 1.E+01 1.E+02

Conductivity (pS/m)

Aci

dit

y

18

Oil analysis results of

New and Young transformers

Oil from Rating of Age Water content s at 20oC Acidity IFT at 25

oC DDF

transformer transformer in service (ppm) (PS//m) (mg KOH/g) (dynes/cm) at 90oC

no. (MVA) ASTM-D1533 IEC-61620 ASTM-D974 ASTM-D971 IEC60247

[35 max] [0.03 max] [40 min]

NT-001 22 5 months 3 5.03 0.004 39.00 -

NT-002 13 0 3 3.299 0.003 39.7 -

10 1 month 9 3.644 0.005 37.6 -

NT-003 13 3 months 3 3.914 0.005 37.6 -

NT-004 25 2 years 7 5.50 0.01 35.4 0.0177

NT-005 25 4 years 6 16.39 0.01 46.4 0.0483

NT-006 20 0 5 0.03 0.003 39.4 -

NT-007 20 1 month 3 0.03 0.001 41.0 -

NT-008 20 1 year 7 0.12 0.001 35.7 -

NT-009 20 1 year 3 0.10 0.001 39.0 -

19

Classification of oil according to its

Conductivity (Test method of new IEC61620)

“New” unused oil (Laboratory quality) < 0.05 pS/m

“New” unused oil (Industrial quality) 0.05..0.10 pS/m

Light used oil in good condition 0.1….1.0 pS/m

Middle used oil in acceptable condition 1.0….5.0 pS/m

Heavy used oil in bad condition > 5.0 pS/m

20

Identification of Aging in

Transformer dielectrics by Electrical Test

It is very important that test voltage for in-service transformer

(or any in-service power equipment) is non-destructive because

insulating materials are service-aged more or less and we do not

want any diagnosis to further deteriorate or shorten its life.

Since many years problems or aging caused by “Polarization”

and “Conduction” are diagnosed in combination through

Insulation Resistance Measurement (with Polarization Index).

Power factor test or Tan delta test also does not identify

“Conduction” and “Polarization” in Dielectrics.

Since the last decade, Dielectric Response Analysis by

Polarization Depolarization Current (PDC) analysis identifies

“Conduction” and “Polarization” in Dielectrics

(not only for transformers).

21

Currents during

Insulation Resistance Measurement

The steady-state Conduction Current is caused by Conduction phenomena.

The time-dependant Absorption Current is caused by Polarization phenomena.

So Insulation Resistance measurement detects both Conduction and Polarization

22

Principle of Test arrangement

for PDC measurement

Current measurement: from 1 pA

HV equipment can be tested at low voltage (e.g. 50V)

23

Insulation between windings

a New Power Transformer

Measurement results of Polarisation Depolarisation Currents

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1 10 100 1,000 10,000

Time (s)

Cu

rren

t (A

)

Test Voltage 100V

Polarisation Current

Depolarisation Current

24

Influence of Oil conductivity

1E+0 1E+1 1E+2 1E+3

Time (s)

1E-9

1E-8

1E-7

Cu

rren

t (A

)ipol

idepol

new 3-phase power transformer

Uc = 1'000 V

Tc = 5'000 s

q ~ 15 °C

pressboard moisture content = 1.0%.

measured

soil = 0.1 pS/m

soil = 0.4 pS/m

soil = 1.6 pS/m

25

Influence of Moisture in Pressboard

1E+0 1E+1 1E+2 1E+3

Time (s)

1E-10

1E-9

1E-8

1E-7

Cu

rren

t (A

)ipol

idepol

new 3-phase power transformer

Uc = 1'000 V

Tc = 5'000 s

q ~ 15 °C

soil = 0.4 pS/m

.

measured

m. c.: 0.2%

m. c.: 1.0%

m. c.: 2.5%

26


Power Transformers


27

Insulation between windings3f, 20 MVA, 33/11 kV Transformer

PDC Test on Transformer without oil

(Teat Voltage 100 V)


28

Some PDC Measurement results

1.E-11

1.E-10

1.E-09

1.E-08

1 10 100 1,000 10 ,000

Time (s)

Cu

rren

t (A

)

T4

T5(5.7)

PDC at 50V, 20oC

(no oil)

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1 10 100 1,000 10 ,000

Time (s)

Cu

rren

t (A

)

I pol.

I depol.

PDC at 50V, 30oC

(no oil)

(5.9)1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1 10 100 1,000 10 ,000

Time (s)

Cu

rren

t (A

)

I pol.

I depol.

PDC at 50V, 24oC

(no oil)

(5.8)

1.E-09

1.E-08

1.E-07

1 10 100 1,000 10 ,000

Time (s)

Cu

rren

t (A

)

(5.2)I pol.

I depol.

PDC at 500V, 5oC

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1 10 100 1,000 10 ,000

Time (s)

Cu

rren

t (A

)

(5.1)I pol.

I depol.

PDC at 100V, 18oC

1.E-10

1.E-09

1.E-08

1 10 100 1,000 10 ,000

Time (s)

Cu

rren

t (A

)

I pol.

I depol.

PDC at 100V, 25oC

(5.3)

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1 10 100 1,000 10 ,000

Time (s)

Cu

rren

t (A

)

I pol.

I depol.

(5.4)

PDC at 100V, 15oC

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1 10 100 1,000 10 ,000

Time (s)

Cu

rren

t (A

)

I pol.

I depol.

(5.5)

PDC at 100V, 23oC

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1 10 100 1,000 10 ,000

Time (s)

Cu

rren

t (A

)

I pol.

I depol.

PDC at 100V, 26oC

(5.6)

From: S. Bhumiwat, S. Lowe, P. Nething, J. Perera, P. Wickramasuriya, P. Kuansatit,

“Performance of oil and paper in transformers based on IEC61620 and dielectric

response techniques.” IEEE EI Magazine, vol. 26 no. 3, May/June 2010, pp. 16-23.

(can be downloaded from www.kea-consultant.com)

29

PDC Measurement Resultsof a Transformer which has

Overheating in the Solid Insulation between windings

From a case study in “Identification of overheating in transformer solid insulation

by Polarization Depolarization Current Analysis” by Supatra A. Bhumiwat,

in 2013 Electrical Insulation Conference, Ottawa, Canada, 2-5 June 2013


30

Depolarization Index (dP.I.) from PDC(to determine overheating of solid insulation)

From a case study in “Identification of overheating in transformer solid insulation

by Polarization Depolarization Current Analysis” by Supatra A. Bhumiwat,

in 2013 Electrical Insulation Conference, Ottawa, Canada, 2-5 June 2013


31

Evaluation of

PDC Measurement Results

• Oil Conductivity (only oil-paper transformer)

• Moisture in paper (any oil-paper equipment)

• Insulation Resistance and Polarisation Index

• Frequency scan of Capacitance

• Frequency scan of DDF (tan d)

• Recovery Voltage Polarisation Spectrum

32

Evaluation of

Insulation Resistance between windings and P.I.

Polarisation Current

1.E+10

1.E+11

1.E+12

1 10 100 1,000 10,000

Time (s)

Resi

sta

nce (

W)

33


Power Transformers

R 1 min. 72.1 GW

R 10 min. 99.8 GW

PI 1.38

tan d at 1 Hz 0.09%

Moisture in pressboard 1.0%

Oil conductivity at 20oC 0.32 pS/m

R 1 min. 63.5 GW

R 10 min. 78.6 GW

PI 1.24

tan d at 1 Hz 0.09%

Moisture in pressboard 1.5%

Oil conductivity at 20oC 0.12 pS/m

Polarisation Index is not a key to judge the wetness of Transformer insulation.

34

PDC Evaluation of

Capacitance and DDF

35

Capacitance and DDF

1E-4 1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4

Frequency (Hz)

1E-3

1E-2

1E-1

1E+0

tan

d

tand

.

unaged (m.c.: 4.0%)

unaged (m.c.: 2.5%)

2mm pressboard, q = 20 °C

unaged (m.c.: 1.0%)

unaged (m.c.: 0.2%)

1E-4 1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4

Frequency (Hz)

200

300

400

C' (p

F)

C'

.

unaged (m.c.: 4.0%)

unaged (m.c.: 2.5%)

2mm pressboard, q = 20 °C

unaged (m.c.: 1.0%)

unaged (m.c.: 0.2%)

C & tan delta is more sensitive at lower frequencies than at 50 Hz for the diagnosis

of aging in dielectrics. Tan delta tells you good / bad but does not tell you why.

Deterioration in insulating materials increase Capacitance at low frequencies.

Since previous decade (at the same time of PDC), there has been a diagnostic tool

which determines moisture in paper/pressboard based on frequency domain.

36

PDC Evaluation results of DDF and C ratio

1.E-02

1.E-01

1.E+00

1.E+01

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

Frequency (Hz)D

DF

T6 at 26oC

(6.2)1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

Frequency (Hz)

DD

F

T4 at 15oC

T5 at 23oC

(6.1)

1.E+00

1.E+01

1.E+02

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

Frequency (Hz)

C r

atio

T6

(6.5)1.E+00

1.E+01

1.E+02

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

Frequency (Hz)

C r

atio

T4

T5

(6.4)1.E+00

1.E+01

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

Frequency (Hz)

C r

atio T4 (no oil)

T5 (no oil)

(6.6)

1.E-02

1.E-01

1.E+00

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

Frequency (Hz)

DD

F

(6.3)

T4 (no oil)

T5 (no oil)

From: S. Bhumiwat, S. Lowe, P. Nething, J. Perera, P. Wickramasuriya, P. Kuansatit,

“Performance of oil and paper in transformers based on IEC61620 and dielectric

response techniques.” IEEE EI Magazine, vol. 26 no. 3, May/June 2010, pp. 16-23.


37

Conclusion

1. Every problem in a dielectric is produced by

mechanism of “Conduction” or “Polarization” or

both. Aging behavior of transformer dielectric can

be classified by its nature.

2. It is important to choose diagnostic tool which

can identify problem or identify the aging type of

the dielectric, in order to solve the problem

correctly so insulation life can be prolonged.

3. The use of non-destructive technique or low

voltage methods ensures that the trouble or fault

is not modified or worsen by the tests.

38

THANK YOU

Publications can be downloaded from:

www.kea-consultant.com

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