thermal performance of power transformers -...

Thermal Performance of Power Transformers

Tutorial of Cigre Working Group A2.24Convener: Jan Declerc, Belgium

2222Thermal Performance of Power Transformers – Tutorial od Cigre WG A2 24

2

Aim of the Tutorial


Facilitate and develop the exchange of knowledge and information, in all countries

Add value to the knowledge and information by synthesizing state-of-the-art and world practices

Set up bridges between Manufacturers, Utilities, Laboratories, Research Centres, Universities, ....

Identify the research avenues that appear most promising

This tutorial will be further developed by Cigre A2


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Introduction

Transformers are critical components in T&D systemsOptimal investment, optimal load, life time expectancy

BUTLoad of transformers ~ Power ~ U.I ~ RI2~ heatWinding with highest temperature alias HOT SPOTLife of insulation depends on temperature riseMontsinger relation + 6 K lifetime/2

So let’s cool ONAN, ONAF, OFAF, ODAF


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Introduction (cont’d)

Damage mechanisms in transformers:Thermal ageing of paper

Ageing at hot spot determines life expectancyInfluences mechanical strength, not dielectric propertiesHot spot is mostly not mechanically most stressed region=> lower mechanical strength acceptable at those places

Bubble generation due to high temperaturesLeads to free gas in oilLowers dielectrical strength

Copper sulphide deposition on paperIncreases electrical conductivity of oil impregnated paperRisk on dielectrical strength reductionDepends on oil composition

Static electrificationCaused by to high oil flowIncreases with temperature ↓


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Transformer rating is based on thermal capacityElectrical Insulation System EIS is a critical factor for transformer loading capabilityEIS contains important information for reliability and condition

Until now design test baytop oil temperature x xaverage winding gradient x xhot spot factor (1.1 … 1.3) xIEC 60076-2, ANSI C57.12


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Trend increasing average load of transformersincreasing use in off-design conditions (overload)using hot spot as important performance indicatorconsulting and troubleshootingReliability and condition assesment lack of mastering of material performance

oil chemical characteristicstreatment for paper upgrading

tendency to extend life expectancy of generation systemneed to evaluate transformer condition: replace or reinforce


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Load P

Current I

Losses RI2

Temperature

Lifetime

Tins 98 C


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ScopeFundamentals of thermal ageingRatings of new transformersPractical applications for in service transformers

WG A2.24 Thermal Performance of Power Transformers

Contents of Tutorial


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Fundamentals in Thermal Ageing

Provide information about thermal ageing of the cellulose insulation to help utilities to better manage their transformers.

Role of chemical environment on paper ageing.

Solubility of ageing markers in oil and paper.

Thermal aspects.

Diagnostics and condition assessment

Condition management and maintenance

TF D1.01.10 - Paper Ageing - Convener: Lars LundgaardSupply information for WG A2.24 ”Thermal performance” concerning mineral oil impregnated cellulose insulation


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Mechanical strength and DP

Classical view (IEC 60354):Mechanical strength of cellulose determines lifeLife duration = e-p*T

Influence of condition of insulation not appreciated


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Mechanical strength and DP

Classical view (IEC 60354):Mechanical strength of cellulose determines lifeLife duration = e-p*T

Influence of condition of insulation not appreciated

Modern approach:Mechanical strength determined by length of cellulose chains in fibresDegree of polymerisation (DP) of cellulose molecules describes ageing condition

Tensile strength and DP relates:

1250 1000 750 500 250 0DP-value

0

40

80

120

Tens

ile in

dex

[Nm

/g]

TIME


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Ageing: Arrhenius model

For one ageing process:E describes temperature dependence (activation energy) For hydrolysis, the ageing rate

doubles every 7oC

A factor describes influence of ”contamination condition”Can increase ageing tenfold,

equivalent to 20-30oC

teAScissionsChain TRE

⋅⋅= +⋅−

)( 273

Ageing rate

0.0024 0.0025 0.0026 0.0027 0.0028 0.0029 0.003

1/Tabs [K-1]

-20

-18

-16

-14

-12

ln (a

gein

g ra

te)

Wet paperDry paper

130oC

110oC

90oC

70oC

A

E


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Kraft paper

Oxidation may increase ageing 2-3 timesHydrolysis (Acid catalyzed) may increase ageing 10-15 times when water increases to 3 %

Normal ageing is governed mainly by oxidation and hydrolysis:

1/Tln

(re

acti

on r

ate)

O2H2O

Increasing temperature

At thermal defects, withtemperatures exceeding normal conditions, pyrolysis becomes active

Activation energies of oxidation, hydrolysis and pyrolysis are different

The total ageing is the sum of these processes


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Thermally upgraded kraft paper

Paper may be upgraded in many different ways

cyanoethylether, dicyandiamind, melamine, urea.

Ageing mechanism of upgraded paper is less known than for kraft paper

Ageing rate is lower than for kraft paper; (1/3 for some types)Less sensitive to hydrolysis

0.0025 0.0026 0.0027 0.0028 0.0029

1/Tabs [K-1]

-20

-18

-16

-14

-12

LN (r

eact

ion

rate

)

Insuldur

Kraft

With water added


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Water is an ageing accelerator

50 70 90 110 130

Temperature [oC]

0.1

1

10

100

1000

Life

exp

ecta

ncy

[yea

rs] Dry paper

1 %

1,5 %

2 %

3 %

4 %

But: Equally important as the water are the low molecular acids produced by ageing of the cellulose (and maybe by some oils?)Hottest areas = most dry areas

=> Transformer can still live long with higher average moisture content

This not fully investigated yet

Water and high temperature may give very short life for a transformer:

Water is produced by ageing


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Sensitivity of paper to oxygen in oil

0

0.5

1

1.5

2

2.5

3

Seal Type Membrane Free breathingOil preservation system

Age

ing

acce

lera

tion

fact

or


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СO2 CO

ACIDSH2O

Levoglucosane

Depolymerization

Hydrolysis Pyrolysis

acidsО2

waterCOCO2

Oil oxidationCelluloseoxidation

Temperature oxygen

Furans

Dehydration

H2O

Cigre WG 12.18


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Thermal Life is a function oftemperature, water and by-products

][36524

11

_ 27313350

yearseA

DPDPLifeExpected TStartEnd +⋅

∗•

−=

Hot spot temperature

Water & acids


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Diagnostics (direct and indirect)

Estimated from knowledge of load temperatures, contamination of insulation, and materials performance.

Indirectly decided by chemical matters produced from ageing:Furanes (production depending on paper type)WaterCO and CO2Low Molecular AcidsSludge

Sampling of paper from transformerMechanical strength cannot be measured, only DP-valueHow representative is a sample for hotspot conditions?=> Impossible to get paper from hot spot area and/or oldest area

(Hot spot area not always oldest area!)


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Furanic compound analysis

Furanes are produced by kraft paper, but to a less degree by upgraded paper (Insuldur)

Furanes, (and also low molecular acids) behave like water, are mainly located in the cellulose, and their concentration in the oil samples varies with temperatureFuranes degrade with timeCorrelation to paper condition is complex

0.01 0.1 1 10

# Chain scissions

1E-005

0.0001

0.001

0.01

0.1

1

10

2 FA

L/ g

ram

cel

lulo

se [m

g] Dry

Oxygenated

1 % water added

3 % water added

0.01 0.1 1 10

# Chain scissions

1E-005

0.0001

0.001

0.01

0.1

1

10

2 FA

L/ g

ram

cel

lulo

se [m

g]

Kraft Insuldur


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End of life criteria

In order to evaluate the economic consequences of accelerated ageing, a “normal life duration” must be definedOld IEC 354 gave no indication IEC 57 92 1981 indicated 65 000 hours at 100ºC hot spot yearly averageIEC 60076-7 indicates 150 000 hoursIEEE indicate, for thermally upgraded paper operating continuously at 110°C,

50% retained tensile strength : 65 000 hours25% retained tensile strength : 135 000 hours200 retained degree of polymerisation : 150 000 hours

Thermal Life:Time to critical decomposition DP<200 (Mechanical life of paper) only 10 to 15 % of failuresDielectric Life: Time span to critical reduction of dielectric safety Mechanical life : critical mechanical weakness and deformation of windings


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End of life : examples of deposit


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End of life : examples of deposit


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Bubble generation hazard

Residual moisture in winding insulation can lead to generation of gas bubbles at high temperatureThis is the dominant concern in the selection of a limiting hot spot temperature for safe operationPhysical determinant factors for bubble generation have been identified in laboratory:

Moisture content in insulationHydrostatic pressureDuration of the high temperature

Real life determinant factors:Too high rate of temperature rise

Caused by high load cold startMoisture does not get enough time to migrate

Mostly linked to overload conditions


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Advices

Remember, to improve knowledge of ageing mechanism and validate ageing model:

Use improved ageing model based on DP

Laboratory ageing experiments do only mimic reality.

Keep track of thermal and condition history of units

Take post mortem analysis of scrapped units in a systematic way to learn

Link with design of transformer -> Part 2


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ScopeFundamentals of thermal ageingRatings of new transformersPractical applications for in service transformers

Contents of Tutorial


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Thermal design of transformer

Critical parameters:•Stray magnetic field•Leakage flux control•Loss density (in conductor)•Oil flow pattern and pressure drop singularities•Insulation coverage•Eddy loss density in metallic parts•Unpredicted hot spot


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Recommended limits for winding temperature rise

Averagewinding

temperaturerise

Hot-spottemperature

rise

IEC ON, OF cooling 65 K 78 KIEC OD cooling 70 K 78 KIEEE Thermally

upgraded paper65 K 80 K

IEEE Normal kraft paper 55 K 65 K

thermal performance of power transformers -...

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