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 andinformation, in all countries

Add value to the knowledge and information bysynthesizing 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


Introduction

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

BUT Load of transformers ~ Power ~ U.I ~ RI2~ heat Winding with highest temperature alias HOT SPOT Life of insulation depends on temperature rise

Montsinger relation + 6 K lifetime/2

So let’s cool ONAN, ONAF, OFAF, ODAF


Introduction (cont’d)

Damage mechanisms in transformers: Thermal ageing of paper

Ageing at hot spot determines life expectancy Influences mechanical strength, not dielectric properties Hot spot is mostly not mechanically most stressed region

=> lower mechanical strength acceptable at those places

Bubble generation due to high temperatures Leads to free gas in oil Lowers dielectrical strength

Copper sulphide deposition on paper Increases electrical conductivity of oil impregnated paper Risk on dielectrical strength reduction Depends on oil composition

Static electrification Caused by to high oil flow Increases with temperature ↓



Transformer rating is based on thermal capacity Electrical Insulation System EIS is a critical factor for transformer

loading capability EIS contains important information for reliability and condition

Until now design test bay top oil temperature x x average winding gradient x x hot spot factor (1.1 … 1.3) x IEC 60076-2, ANSI C57.12



Trend increasing average load of transformers increasing use in off-design conditions (overload) using hot spot as important performance indicator consulting and troubleshooting Reliability and condition assesment lack of mastering of material performance

oil chemical characteristics treatment for paper upgrading

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



Load P

Current I

Losses RI2

Temperature

Lifetime

Tins 98 C



Scope Fundamentals of thermal ageing Ratings of new transformers Practical applications for in service transformers

WG A2.24 Thermal Performance of Power Transformers

Contents of Tutorial


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


Mechanical strength and DP

Classical view (IEC 60354): Mechanical strength of

cellulose determines life Life duration = e-p*T

Influence of condition of insulation not appreciated


Mechanical strength and DP

Classical view (IEC 60354): Mechanical strength of

cellulose determines life Life duration = e-p*T

Influence of condition of insulation not appreciated

Modern approach: Mechanical strength

determined by length of cellulose chains in fibres

Degree 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


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


Kraft paper

Oxidation may increase ageing 2-3 times

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

Normal ageing is governed mainly by oxidation and hydrolysis:

1/T

ln (

reac

tion

rat

e)

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


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

)

InsuldurKraft

With water added


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


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


СO2 CO

ACIDSH2O

Levoglucosane

Depolymerization

Hydrolysis Pyrolysis

acidsО2

waterCOCO2

Oil oxidationCelluloseoxidation

Temperature oxygen

Furans

Dehydration

H2O

Cigre WG 12.18


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

][36524

11

_ 27313350

yearseA

DPDPLifeExpected TStartEnd

Hot spot temperature

Water & acids


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) Water CO and CO2 Low Molecular Acids Sludge

Sampling of paper from transformer Mechanical strength cannot be measured, only DP-value How 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!)


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 temperature

Furanes degrade with time Correlation 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


End of life criteria

In order to evaluate the economic consequences of accelerated ageing, a “normal life duration” must be defined

Old IEC 354 gave no indication IEC 57 92 1981 indicated 65 000 hours at 100ºC hot spot yearly average IEC 60076-7 indicates 150 000 hours IEEE indicate, for thermally upgraded paper operating continuously at 110C,

50% retained tensile strength : 65 000 hours 25% retained tensile strength : 135 000 hours 200 retained degree of polymerisation : 150 000 hours

Thermal Life:Time to critical decomposition DP<200 (Mechanical life of paper) only 10 to 15 % of failures

Dielectric Life: Time span to critical reduction of dielectric safety Mechanical life : critical mechanical weakness and deformation of windings


End of life : examples of deposit

23232323Thermal Performance of Power Transformers – Tutorial od Cigre WG A2.24

23

End of life : examples of deposit


Bubble generation hazard

Residual moisture in winding insulation can lead to generation of gas bubbles at high temperature

This is the dominant concern in the selection of a limiting hot spot temperature for safe operation

Physical determinant factors for bubble generation have been identified in laboratory: Moisture content in insulation Hydrostatic pressure Duration of the high temperature

Real life determinant factors: Too high rate of temperature rise

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

Mostly linked to overload conditions


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


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


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


Average oiltemperature

Bottom oiltemperature

Top-oiltemperature

Average windingtemperature

Hot-spottemperature

g

Hg

Top of winding

Bottom of winding

Temperature rise29

Determination of hot spot temperature from test results or detailed computation


IEC Hot Spot Factor (H)

Analytical determination of hot-spot factor (H) has been attempted without success.

Experimental determination of hot-spot factor on a group of 34 different transformers show: Wide dispersion of result (0.5 to 2) 65% of result evenly spread between 1 and 1.5 No correlation with size or type of cooling

Thot-spot = Ttop-oil + (Rated hot-spot rise )2y


Determination of winding hot-spot temperature at rated load

Direct measurement with fibre optic sensor Calculated values from manufacturer model Analytical determination from test results Default values ( Loading guide maximum value)


Thermal model – state of the art

Global modeling approach and assumptions

1 2

4

5

8

6

3

7

(1) core;(2) disc type winding; (3) layer type winding; (4) tank; (5) radiator; (6) top oil volume; (7) bottom oil volume; (8) oil pump

• 3D effects are neglected• modelled 2D geometry and components:

• Geometry is parameterised• Every transformer is represented by a similar channel network


Thermal model – state of the art

Q2()Qi()Qn()

Rmt()Rm2()Rmi()Rmn()

Un

Rm1()

Q1(,omg)

Ui U2 U1

M

Qk

n,pni,pi

2,p20,pt

0,p0

Hp

1,p1


Thermal model – state of the artOil flow model

• Oil flow in the radial oil channels of a disc-type winding can be neglected

• Oil flow in the transformer is modelled by a parallel axial channel network :

continuity: 0zu

momentum:

(1)

(2)

rur

rrzpg

tu


In most cases oil flow is buoyancy driven : gdz))t,z(T(

PqzTvc

zTk

tTc v

2

2

Numerical solution- time discretisation: explicit Euler - spatial discretisation:

convection term: upwinddiffusion: central difference

- non-linear terms: time-lagging

Thermal model – state of the artTemperature distribution in windings

Similar equations for the radiator and core channels

Solve 1D- transient convection diffusion equation for each oil channel:


Thermal model – state of the artTemperature of other components

transfer heat

radiation and convectiongeneration

heat internal

tTcV i

viii

)TT(AhPt

TcV oilcorecoilcorewcore

vcorecorecore

• for example: core mass

A lumped system approach is used despite Bi>1

Numerical solution:- time discretisation: explicit Euler - non-linear terms: time-lagging


Thermal model – state of the artExample

Output : velocities, oil temperatures, wall temperatures and conductor temperatures

High Voltage Winding Conductor temperature profile 32 discs of 68 discs, 5 layers

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32S1

S2

S3

S4

S594-9890-9486-9082-8678-8274-7870-7466-7062-66


Thermal model - Remarks

Detail of thermal model ↑ Calculation time ↑=> Every model is a balance between: Available time and computing power Necessary detail and information

Current models could still be improved Radial flow could occur in disk winding Non steady state effects can have certain influence Neglected details in geometry can have certain influence …

Available computing power increases steadily=> Model complexity can follow at same pace


Direct measurement of hot-spot temperature with fibre optic sensor

Fibre optic sensor are available from many manufacturer

They have to be installed at time of transformer manufacturing

Usually there will be 10 to 20 sensor installed at various location in the winding, in oil duct or on the core


• Comparison with data from fibre optic sensors during factory temperature rise tests of 1 transformer :

factors influencing interpretation of fibre optic measurements:- sensor is encapsulated in spacer material - sensor is in contact only with conductor wall- uncertainty of positioning- accuracy: 3°C



Transformer 1: 125 MVA 149 kW ONAN / 220 kW ONAF

0

10

20

30

40

50

60

70

0 20000 40000 60000 80000

time [s]

tem

pera

ture

[°C

] TradiTraduTtop-expTradi-expTradu-exp


Transformer 2: 68MVA 144 kW ONAN / 403 kW ONAF

20

30

40

50

60

70

80

0 20000 40000 60000 80000

time [s]

tem

pera

ture

[°C

]

TradiTraduTradi-expTradu-expTtop-exp

Radiator inlet and outlet temperatures


HV conductor temperature 100% height (transformer 2)

HV internal conductor modelHV conductor wall model

HV conductor exp

0 20000 40000 60000 80000time [s]

20

30

40

50

60

70

80

90

100

tem

pera

ture

[°C

]


60 MVA Oil Natural Air Forced 150% overload

0

20

40

60

80

100

120

140

160

0 5000 10000 15000 20000 25000 30000

Time (sec)

Tem

p (C

)

HV W topHV W top oilHV W midHV W mid oilLV W top



Limitations from components other than winding

Bushings Leads and connections Tap changers Magnetic screen Circulating current in the core


Overloading Bushings

Risks associated with overloading: Pressure build-up Gasket seals Tan delta increase Dielectric performances Stray magnetic flux

( IEEE C57.19.100 )


Leads and connections

Lead cross section Extra insulation Restricted cooling Defective connections


Tap changers

Excessive contact temperature

Increase of tap-changer contact resistance

Shorter contact life


Circulating current in the core induced by winding leads

Concern :Overheating of lamination or arcing in core joints


Testing under overload conditions

Thermal test recommended for thermally stressed transformers

IEC and IEEE loading guide recommend that overload requirements be specified at time of purchase

Some utilities request overload tests on new transformers with DGA control

Some utilities perform overload test in the field to confirm overload capacity on some class of transformers

Accurate hot-spot temperature indicator is essential


GSU transformers

Remains in steady state regime Maximal load during entire lifetime IEEE: steady-state operation

=> life expectancy of 22years at 100ºC=> life expectancy of 7 years at 110°C

GSU transformer application is a good way to test the original design of a manufacturer.

GSU are larger units => particular transport restrictions





TF in co-operation with Dr. Viktor Sokolov



Ageing DP and moisture profilesPost mortem analysis

140C

110C90C


End of life can come under 20 yearsAging profile of 700 MVA, 420 kV, 14 years

Hot spot area


Critical ageing comprises only limited hot spot area

700 MVA transformer ,23 years


Temperature and Ageing profile of 80 MVA, 88 kV, 32 years

HV LV

Hot Spot 65 K 74 K

Average 57 K 62 K

Bottom 50 K 55 K


Temperatures in transformer cell

Transformer : 60/90 MVA, 12 radiators and fansCell : Front inlet opening and 4 extraction fans



Transformer and air temperatures on-line 22 type T thermocouples connected to Tempscan and stored

every minute on PC

Oil information

Air information


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

S12

S13

S14

S15

S16

IBT1-ONAF at 60MVA Temperature profile at 0.5 m

47-50

44-47

41-44

38-41

35-38


Air temperatures around transformer, transformer fans on



RADIATOR TEMPERATURES IBT-2radiator front and back

0

10

20

30

40

50

60

70

9:48

:27

10:0

5:44

10:2

3:01

10:4

0:18

10:5

7:35

11:1

4:52

11:3

2:09

11:4

9:26

12:0

6:43

12:2

4:00

12:4

0:31

12:5

7:48

13:1

5:05

13:3

2:22

13:4

9:39

14:0

6:56

14:2

4:13

14:4

1:30

14:5

8:47

15:1

6:04

15:3

3:21

15:4

9:44

16:0

7:01

16:2

4:18

16:4

1:35

16:5

8:52

17:1

6:09

17:3

3:26

17:5

0:01

18:0

7:18

18:2

4:35

18:4

1:52

18:5

9:09

TIME

TEM

PERA

TURE front bottom

front topback bottomback top

ONAN ONAF transformer fans on ONAF transformer fans and additional fans on



0 60 90 Load (MVA)

Ttopoil

T air cell

Tamb

Assumption : transformer fans stay on

T (C)

+8

+49

+15



Cells and shelter can obstruct transformer cooling

Check complete heating system of transformer + shelter

Provide extra fans and air ducts if necessary

Check for other restrictions on heat dissipation and air flow


Transformer overheating

Transformer Voltage - 34.5kV/13.2kV with LTC Rating - 12/16/20 MVA Oil preservation system - sealed tank with nitrogen blanket

Location High Tech/Manufacturing Loads Sunshine Altitude is 6,200 Feet Open site with no restrictions for wind or heat dissipation


*>179.4°F

*<29.3°F

40.0

60.0

80.0

100.0

120.0

140.0

160.0

Transformer overheating case


Transformer overheating case On site measurements

Industrial Park #2

-10

0

10

20

30

40

50

60

70

80

90

28/09/9911:55

28/09/9916:43

28/09/9921:31

29/09/992:19

29/09/997:07

29/09/9911:55

29/09/9916:43

29/09/9921:31

30/09/992:19

30/09/997:07

30/09/9911:55

Time

Tem

p (C

)

Tamb1

Tamb sw room

Tair 5

Tair 4

Tair 3

Tbot 5

Ttop 5

Tbot 4

Ttop 4

Tbot 3

Ttop 3

Cover

right

Back OTI

Left


Transformer overheating caseConclusions

Stronger fans at right location ! Reconsider the overload philosophy due to the following

system changes Due to growing high tech and manufacturing loads,

historical winter peak is slowly changing to summer. Due to the same reasons as above, some of transformer

peak loads are occurring during the hottest part of the day instead of in the late afternoon and early evening.

Reference all MVA ratings to 7.000 feet instead of 3,300 feet otherwise 5 % derating.

Require a heat run for all new transformers. Require the installation of a Transformer Temperature

Monitor. In addition to specifying the ANSI/IEC Overload Guide,

develop a specific transformer overload specification.


References and further studies

Cigre technical brochure no 96 : thermal aspect of transformers IEC 60076-7 : Loading guide for oil-immersed power transformers IEC 60076-14 : Application guide of high temperature materials IEEE C57.91 : Guides for Loading Mineral-Oil_Immersed

Transformers EPRI EL-5384 : Bubble formation in transformers

New developments at Cigre D1-01 Aging profile data acceptance considering temperature profile, oil

parameters, estimation of aging rateExperience of scrapped units

Verification of suggested diagnostic approach in-field New methods based on oil analysis


International Standards

IEC 60076 Part 14: Guide for the design and application of liquid-immersed power transformers using high-temperature insulation materials

NEEDS CAREFULL ATTENTION AND EXPERIENCE!!!!

Impact on all components (tank, bushings) Check compatibility and use of materials Only thermal test differs Impact on losses at reference temperature Different alarm settings


New Electric Insulation systems Solid and liquid (fire safe and bio degradable)

Solid insulationDissipation Factor

(%) Material Thermal

class IEC

Standard Reference

Relative permittivity at

25°C

At 25°C At 100°C

Moisture Absorption

(%)

Density (g/cm3)

Cellulose 105 60554-3 3,3 – 4,1 0,4 1,0 7,0 0,97 – 1,2

Cellulose 105 60641-3 2,9 – 4,6 0,4 1,0 7,0 0,8 – 1,35

Polyester Glass1) 130 - 200 60893-3 4,8 1,3 – 7,0 N/A 0,2 – 1,1 1,8 – 2,0

Polyester Glass1) 130 - 220 61212-3 N/A N/A N/A 0,16 – 0,28 1,8 – 2,0 S

Polyimide 220 60674-3 3,4 0,2 0,2 1,0 – 1,8 1,33 – 1,42

Aramid 220 60819-3 1,6 – 3,2 0,5 0,5 5,0 0,72 – 1,10

Aramid 220 61629-1 2,6 – 3,5 0,5 0,5 5,0 0,70 – 1,15

NOTE 1 All data has been taken from measurements in air

NOTE 2 Relative permittivity and dissipation factor data are referenced to 50/60 Hz

NOTE 3 Moisture data is based on air having a relative humidity of 50%

1) Typically only used in lower voltage applications due to possible air entrapment during the manufacturing process


Insulation system

Homogenous insulation system

All conventional insulation transformers60/65/78

High temperature transformer80/120/145


Insulation system

Hybrid or mixed insulation system60/95/115 60/65/95

3

2

3

4

2

4


Applications

Traction transformers High Speed Train transformers

Convertor Transformers In hot spot regions due to extra losses

Refurbishment USA practice for upgrading old units Rethink cooling capacity

Mobile substations with compact power transformers


Conclusions

Transformers are believe to have inherently some margin of overload capacity

Concerns: Insulation Ageing Bubbling Limitations from components other than winding

Method need to be developed for: Analytical assessment of older transformer overloading

capability Overloading test procedure in the field

Cost / benefit analysis of overloadingEconomic value of insulation loss of life can be assessed if a “normal life duration” is agreed

Additional monitoring can guide user in ageing


For more information: CIGRE Brochure 393

75


THANK YOU

thermal performance of power transformers -...

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