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Moisture Estimation in

Peter Fong

e ec ron cs orp.

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Content

1. The Effects of Water in Transformer

2. How Does Water Get In?

.

4. What can be Done about it?

OMICRON 9 November 2009 Page 2

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The Effects of Water in Transformer

Generates BubblesPremature Failure

r ggers art a sc arge of Good Transformers

Accelerates the A in of InsulationShorten Life Expectancy

r

ans ormer

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Effect 1: Bubbling200

Effect:Bubbling inception 180perature[C]

Kraft Paper New Oil

TU Paper New Oil

Aged Kraft Aged Oil

decrease with higher

moisture content 140

160Tem

120Risks: Ince tion of Partial Dischar e

80

100

1,0 2,0 3,0 4,0 5,0 6,0

Lower Dielectric Strength Short-Circuit of Windings

OMICRON Page 4

Moisture in paper [%]

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Effect 2: Dielectric Strength

Effect: PD Inception Voltage

=

rea own o age

content Dielectric strength of Oildecreases Tem erature

w ,

Risks:

a ure o rans ormerInsulation

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Effect 3: Aging

Effect:High temperature and100

1000

ctance/

a

Dry1%

dramatically lower the

mechanical strength of

10Lifee

xp

2%

3%

paper nsu a on

Risks:50 70 90 110 130

0,1

4%

ower t e expecte e otransformer Run transformer at lower rating

Temperature / C

OMICRON Page 6

L. E. Lundgaard, Aging of oil-impregnated paper in power transformers,IEEE Transactions on Power Delivery, Jan. 2004

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Paper Insulation

Network transformersGenerator stepup units

of chains of glucose rings As the paper ages, theserings breaks up and formswater molecules Water also accelerates the

breaking up of these rings

New: ~ 1200 DPEnd of Life: ~ 200 DP

[Neumann, Micafil-Symposium,Stuttgart, 2004] +H2O-H2O

Condensation Hydrolysis+H2O-H2O

Condensation Hydrolysis

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The Aging Process

When a transformer is manufactured in the factory,the paper insulated windings are subjected to

At this stage, the transformer has a moisture contentof < 0.5% b wei ht in a er and 6 m in oil

As the transformer ages, the moisture content willincrease progressively

In a severely deteriorated system, the moisturecontent could reach > 4%

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Where Does Water Come From?

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I. Grundlagen: Feuchtigkeit in l-Papier

u Normal aging of paperproduces water

ea s cou expose

insulation to atmosphericmoisture

Exposure to atmosphericmoisture duringmaintenance

Failure to dry out theinsulation during

Increase of water

0,1-0,2% per year

Water content in the paper/Pressboard: New: 0.4 to 0.8 % Aged: 3 to 5 %

Page: 10 November 09

[Cigr WG12.18 Life Management ofTransformers, 1999]

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ere s e a erMost of the water is contained in the cellulose!->>> ore mpor an o measure e wa er

content in paper than oil

100000 kg

13000 kg

Water content at 60 C:40 ppm

Water content at 60 C:4 %

Mass of the water, desolved in theoil:

Mass of the water contained in thepaper:

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Insulation?

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Moisture Estimation

There is no practical direct way of measuringmoisture levels in transformer insulation, therefore awet condition may go unnoticed for a long time

This has led to indirect method of moisture estimation

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Direct Method

Take paper sample from transformer and test

Limited use since possible only during repair or

tear-down

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Indirect Methods

Measure properties of the insulation which

1. Moisture-in-Oil Measurements (Chemical

Method)2. Moisture Saturation (Relative Humidity)

3. Dielectric Spectroscopy (Electrical Method)

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Moisture-In-Oil Method

Karl Fischer Titration on oil sample Use E uilibrium Curves to correlate moisture in oil to

moisture content in paper

Easy to Perform Need to wait for equilibrium (days ~ weeks)

Uncertainty in Estimates

Not accurate for aged oil

-

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Equilibrium Curves

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Round Robin Test at Oil Samples

340,5 ppm]

50

60

noil[

ppm]

Uni StuttgartBC

54,8160

180

average[

40Moist

urei

E

GF

44,3

39,8 39,7

35,3120

140

eviation

fro

30

,

19 8

80

100

10

20

5 86,7

16,2

11,2

15,2

8,99,512,212,1

7,5

40

Sample A Sample B Sample C0

,3,5

,

0US B C D E F G

December 2007

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Moisture Saturation Method

instead of ppm as in the lab Moisture probes inside the transformer measures

changes in capacitance of water molecules

Moisture diffused into the probe and changes its,determined

More accurate than oil sampling method since no

handling is involved Instrusive

equ res qu r um

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Dielectric Spectroscopy

Electrical Method

Method of Choice due to: on- es ruc ve