Matz 3

(1)

Moisture in Power Transformers

--

How

to

Es

ti

ma

te

and

Wh

at

to

Do?

Matz Ohlen, Megger Sweden

(2)

(3)

Content



_{The effects of water in a transformer}



_{How does water get into a transformer}



_{How can we measure moisture}



_{Interpretation and guidelines for moisture}



(4)

Water in a transformer

(5)

Transformer failure rates

(6)

Water is one of the worst enemies of a transformer...



_{A transformer with low moisture content is like a person in}

good condition

• A transformer can be used at high load without risk for catastrophic failure.

• A person can work hard without risk for heart attack



_{A wet transformer is like an overweight person in bad}

condition

• The transformer owner has to limit load to avoid bubbling (may lead to catastrophic failure)

• Moisture in insulation increases the rate of aging • The person can not run the marathon...



_{Water/moisture and (high) temperature will sooner or later kill}

(7)

Where is the water?



_{The insulation in a power transformer consists of oil}

impregnated cellulose and oil.

• 60 tons of oil with water content of 20 ppm) = 1.2 liter • 10 tons of cellulose with 3% water content = 300 liter



_{Almost all water is in the cellulose!}

20ppm (parts per million) in 60 tons of oil

3% water in 10 tons of cellulose

Typical values for a 300 MVA power transformer at 50 C

(8)

Water effects the transformer performance



_{Loading capability}

• Limits the loading capability due to decreased bubbling inception temperature



_{Dielectric strength}

• Decreases the dielectric strength of the oil and decreases PD inception voltage



_Aging

• High temperature and moisture will dramatically accelerate aging that lowers the mechanical strength of the cellulose insulation

(9)

High moisture limits the loading capability



_{Moisture determines the}

maximum loading/hot-spot

temperature for bubble

inception



_{Knowing moisture content}

and oil quality allows for

correct decision on

maximum loading

• Leave as-is • Dry-out/re-generate oil • Replace/Relocate • Scrap

G. K. Frimpong et al, “Estimation of Moisture in Cellulose and Oil Quality of Transformer Insulation using Dielectric Response Measurements”, Doble Client Conference,

Paper 8M, 2001.

(10)

Life of a transformer – Moisture and aging



_{During manufacturing, the cellulose insulation in the transformer is}

carefully dried out before it is impregnated with oil



_{The moisture content in the solid insulation of a new transformer is}

typically targeted to be < 0.5% by weight



_{As the transformer gets older, the moisture content will increase}

• Open-breathing transformers, typically around 0.2%/year

• Sealed conservator transformers, typically around 0.05%/year



_{In an old and/or severely deteriorated transformer, the moisture}

content can be > 4%



_{The aging process of the insulation is directly related to moisture}

(11)

0 5 10 15 20 25 30 35 40 45 50 80 90 100 110 120 130 140 Winding Hot Spot °C

A g i n g F a c t o r f o r K r a f t P a p e r ( I E E E ) IEEE Kraft 0.5% Water 1% 2% 3% 4% 5%

Moisture accelerates aging

Left: Lars E. Lundgaard, Walter Hansen, Dag Linhjell, Terence J. Painter, “Ageing of oil-impregnated paper in power

transformers”, IEEE PWRD, 2003

~2 years @ 3% ~12 years @ 1%

(12)

Where does the water come from?

 _{Leaking gaskets and faulty}

water traps may expose the inside of the transformer to moisture humid air

 _{Exposure to humid air}

during site

installation/commissioning

 _{Exposure to humid air}

during maintenance

 _{Normal aging of cellulose}

produces water

 _{Insufficient drying at}

manufacturing

 _{Typical moisture content in paper/pressboard:}

• New transformer: < 1% • Aged transformer: 2 - 4%

(13)

Interpretation of moisture content by various

standards and practices



_{< 1% - “As new”}



_{1-2% - “Dry”}



_{2-3% - “Moderately wet”}



_{3-4.5% - “Wet”}



_{> 4.5% - “Excessively wet”}

1) Original data in IEC60422, annex A, are presented as relative saturation in percentage. ”Dry”, ”Moderately wet”, ”Wet” and ”Extremely wet” are recalculated to percent moisture in cellulose.

2) Original data in IEC60422 for new transformers is presented as percent moisture in cellulose insulation.

3) Original data in CIGRE349 is by categories. “Good, “Fair”, “Probably wet” and “Wet” are relabeled to “Good/New”, “Dry” and “Wet”.

(14)

Moisture levels in practice...

1.0 %

☺

_{4.2 %}



2.6 %

?

Recommended approximate percent by weight of water in solid insulation (IEEE C57.106-2002)

 _{< 69 kV, 3% maximum}

 _{> 69 - < 230 kV, 2% maximum}

(15)

Measuring moisture in cellulose

insulation

(16)

Methods for moisture estimation in cellulose

insulation



_{Direct method}

• Take paper sample from transformer and measure moisture content using Karl Fisher titration



_{Indirect methods}

• Moisture in oil

– Absolute values – Relative saturation

• Power frequency tan delta/power factor measurements • Dielectric response measurements

oReturn Voltage Measurement (RVM) – DC method

oPolarization-Depolarization Current measurements (PDC) – DC method

(17)

Direct method - KFT on paper samples

– CIGRE brochure 414, 2010



_{Only possible during repair or}

tear-down



_{Moisture content in}

pressboard/paper samples is

pending where the sample was

located



_{Averaging many results is}

necessary to get a “true” value



_{Variations between different}

laboratories

Not practical as a standard

diagnostic method…

(18)

KFT measurements on paper samples

– Laboratory results

M. Koch, “Creating a Reliable Data Base for Moisture Evaluation of Power Transformers“,

Pax and KTH Workshop on Variable Frequency Diagnostics, Stockholm, 2007 0 0.5 1 1.5 2 2.5 3 3.5 4

SampleA SampleB SampleC

A B C D E F G

(19)

Absolute moisture in oil method

1. Oil sampling under service conditions

2. Measurement of water content by Karl Fischer titration

3. Deriving moisture content in paper via equilibrium diagrams

The procedure is easy to perform and very common but affected by

substantial potential errors:

 _{Sampling and transportation of samples}  _{Large variation in laboratory results}

 _{Diagrams only valid under equilibrium conditions (rarely happens during}

normal operation)

 _{Standard diagrams does not cover aged oil and/or cellulose that may have}

different solubility

(20)

Water in oil – Examples of laboratory analysis

– CIGRE Brochure 414, 2010

(21)

Water In Oil – Equilibrium charts (Oommen)

Very difficult to estimate water in paper from oil samples taken at low

temperatures!

Oil sample taken at 200_C

4.0% water = 6 ppm 1.0 % water = 3 ppm

(22)

Moisture saturation measurements (RS)

 _{Measure relative water saturation (“ppm/solubility”) in oil (%) instead of absolute}

moisture by weight (ppm)

 _{More accurate than oil sampling method since no oil handling is involved}  _{Moisture absorption capacity is less temperature dependent}

 _{Still requires equilibrium and charts are pending material...}

Moisture equilibrium chart for Kraft Paper at different temperatures

Water in oil solubility as a function of temperature Oil temperature, °C Water content in oil, ppm

0 22 10 36 20 55 30 83 40 121 50 173 60 242 70 331 80 446 90 592 100 772

(23)

Tan delta/power factor measurements

1. Measure tan delta/power factor at actual temperature 2. Convert data to reference temperature (20 C)

3. Compare with guidelines

Guidelines (examples):

 _{“Tan delta/power factor < 0.5% @ 20C is OK” (IEEE 62-1995)}

 _{“Tan delta/power factor < 1% @ 20C may be OK for a service aged}

transformer” (IEEE 62-1995)

 _{“Expect tan delta/power factor < 0.3% for a dry transformer” (Doble)}

The procedure is easy to perform and very common but is affected by errors and limitations

 _{Standard temperature correction tables are not accurate for the individual transformer}

 _{Moisture in paper has a low influence on tan delta/power factor at typical measurement}

temperatures

 _{Not possible to tell if an increased tan delta value is caused by high moisture in paper or}

(24)

0.10 1.00 10.00 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 New Service aged

 _{At tan delta = 1.0%, moisture can be around 4%}

 _{At tan delta = 0.5%, moisture can be around 3%}

 _{At tan delta = 0.3%, moisture can be 0.5 to 2%}

Tan delta (% @ 20C) vs moisture (%) for typical

core form new and service aged transformers

Moisture in cellulose, % Tan delta/PF, %

(25)

Tan delta vs moisture @ 20 C

Moisture in solid insulation

 _{Core type transformer}

 _{Oil conductivity 1 pS/m (Tan delta 0.02%)}

 _{Tan-delta @ 50 Hz reaches 0.5% at about 3-4%}

(26)

Dielectric Response measurements



_{DC methods – Time domain}

• Return Voltage Measurement – Voltage vs time

• Polarization-Depolarization Current measurement – Current vs time



_{AC method – Frequency domain}

• Dielectric Frequency Response measurements – Capacitance and dissipation factor vs frequency

(27)

Methods for DFR measurements

DC (Polarization-Depolarization

Current measurements)

 _Strength

 _{Shorter measurement time at very}

low frequencies

 _Weaknesses

 _{More sensitive to AC interference}

(microamps)

 _{More sensitive to DC interference}

(nanoamps)

 _{Limited frequency range (PDC}

only)

 _{Data conversion necessary}

(combined PDC/DFR only)

 _{Discharge before measurement}

may be needed

AC (Dielectric Frequency Response

measurements)

 _Strengths

 _{Less sensitive to AC interference}

(milliamps)

 _{Less sensitive to DC interference}

(microamps)

 _{Wide frequency range}

 _{No data conversion}

 _{No discharge necessary}

 _Weaknesses

 _{Longer measurement time for very}

low frequencies.

Multi-frequency test signal reduces measurement time with about 40%!

(28)

Comparing Dielectric Response measurements

– CIGRE D1-207, 2006

0 0.5 1 1.5 2 2.5 3 3.5 4 0 10 20 30 40 50 60 70 80 90 100 RVM, new oil RVM, old oil PDC, new oil PDC, old oil FDS, new oil FDS, old oil "True" KF titration

Moisture content in cellulose paper, %

(29)

Dielectric Frequency Response Measurements

–

Tan delta from mHz to kHz

V A Hi Lo Ground CHL C_L C_H

( )

ω ω I U Z =

( )

(

ε ε

)

ω ′′ ′ ⇒ and PF tand, , C Z

Measure at several frequencies

Use Ohms law:

(30)

Recommended frequency range for DFR

measurements on oil-paper insulation systems

�� 0-4.9 0.1 5-9.9 0.2 10-19.9 0.5 20-29.9 1 30-44.9 2 45-59.9 5 >60 10

(31)

What affects the frequency response?

- Temperature + Geometry Oil conductivity Geometry Moisture Moisture

(32)

FDS/DFR moisture assessment (AC)



_{Assign insulation temperature (winding/top-oil temperature)}



_{Measure tan delta from 1 kHz to 1 mHz (20-30 C)}



_{Align insulation model to measured data}



_Results:

• Moisture in solid insulation

• Conductivity/dissipation factor of the oil

• Power frequency tan delta/power factor and capacitance @ measurement temperature

• Power frequency tan delta/power factor @ 20 C reference temperature • Conductivity/dissipation factor of the oil @ 25 C reference temperature • Temperature dependence of power frequency tan delta/power factor

(33)

Transformer insulation (capacitor) X-Y modeling

Typical values for core form transformer: X = % barriers in the main duct (15-55%) Y= % spacers of the circumference (15-25%)

Cellulose – Blue Oil – Red

(34)

(35)

DFR - Investigating 0.5% tan delta values

Wet transformer (3.3%) with good oil

Dry transformer (0.5%) with aged oil (high conductivity/dissipation)

Service-aged transformer, 2% moisture and typical aged oil

(36)

Maintenance based on water in oil analysis…



_{Six transformers scheduled for oil regeneration}

and dehydration based on ppm water in oil data

�� % �� (�� ) � �� _�� _�� _�� _�� _�� _��

“ABB Advanced Diagnostic Testing Services Provide Detailed Results”, 2006

(37)

Maintenance based on DFR analysis…



_{Only one or maybe two transformer needed it!}

�� % �� (�� ) % �� (�� ) �� (��) � �� _�� _�� _�� _�� _�� _�� _�� _� _�� _��

(38)

37

(39)

Transformer drying – Methods/Examples



_{Two major techniques are used:}

• Drying the insulation by drying the oil – Field

• Drying the insulation with heat and vacuum – Field and factory



_{Drying the oil}

• Molecular sieves • Cellulose filters • Cold traps

• Combined oil regeneration and degassing



_{Drying the insulation}

• Vacuum and heat

• Pulsation drying through oil circulation • Hot oil spray drying

• Low frequency heating • Vapour phase drying

A. Gruber, ”Online Treatment of Transformers and Regeneration of Insulating Oil”, TechCon

(40)

Transformer drying – Comparing methods

P. Koestinger et al, ”Practical Experience With the Drying of Power Transformers In the Field, Applying the LFH Technology”, CIGRE A2-205,

2004

< 0.1 liter/day

> 10 liter/day

(41)

On-line oil regeneration and drying – Example

 _{25 MVA manufactured}

1972

 _{17 days of hot oil}

circulation with clay filtering (Fuller’s earth)

 _{PF down from 0.4 to 0.3%}  _{Moisture in cellulose not}

significantly reduced. 3% before drying and 2.7% after drying

 _{Degraded oil significantly}

improved. Conductivity before regeneration 12 pS/m and 1.6 pS/m after filtering Moisture 3% Oil conductivity 12 pS/m Moisture 2.7% Oil conductivity 1.6 pS/m

Poorvi Patel, B. Holmgren,”DFR - A Powerful Tool for Transformer Diagnostics”,TechCon AsiaPacific 2009

(42)

Summary and conclusions



_{Moisture is the worst enemy of the transformer!}

• Limits the loading capability • Accelerates the aging process • Decreases dielectric strength



_{The water/moisture in a transformer resides in the solid insulation, not}

in the oil



_{Dielectric Frequency Response Measurement is a great technique for}

moisture assessment as it measures:

• Moisture content in the cellulose insulation

• Conductivity/dissipation factor of the insulating oil

• Power frequency tan delta/power factor, accurately temperature corrected to 20 C reference temperature

