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Thermal Performance of Power Transformers
Tutorial of Cigre Working Group A2.24Convener: Jan Declerc, Belgium
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Aim of the Tutorial
Thermal Performance of Power Transformers
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
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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
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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 ↓
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Introduction (cont’d)
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
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Introduction (cont’d)
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
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Introduction (cont’d)
Load P
Current I
Losses RI2
Temperature
Lifetime
Tins 98 C
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Thermal Performance of Power Transformers
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
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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 life Life 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 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
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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 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
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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
)
InsuldurKraft
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) 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!)
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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 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
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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
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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 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
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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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Thermal Performance of Power Transformers
Scope Fundamentals of thermal ageing Ratings of new transformers Practical 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
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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
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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
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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)
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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
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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
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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
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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:
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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
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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
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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
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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
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• 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
Direct measurement of hot-spot temperature with fibre optic sensor
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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
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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
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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
]
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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
Direct measurement of hot-spot temperature with fibre optic sensor
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Limitations from components other than winding
Bushings Leads and connections Tap changers Magnetic screen Circulating current in the core
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Overloading Bushings
Risks associated with overloading: Pressure build-up Gasket seals Tan delta increase Dielectric performances Stray magnetic flux
( IEEE C57.19.100 )
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Leads and connections
Lead cross section Extra insulation Restricted cooling Defective connections
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Tap changers
Excessive contact temperature
Increase of tap-changer contact resistance
Shorter contact life
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Circulating current in the core induced by winding leads
Concern :Overheating of lamination or arcing in core joints
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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
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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
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Thermal Performance of Power Transformers
Scope Fundamentals of thermal ageing Ratings of new transformers Practical applications for in service transformers
WG A2.24 Thermal Performance of Power Transformers
TF in co-operation with Dr. Viktor Sokolov
Contents of Tutorial
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Ageing DP and moisture profilesPost mortem analysis
140C
110C90C
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End of life can come under 20 yearsAging profile of 700 MVA, 420 kV, 14 years
Hot spot area
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Critical ageing comprises only limited hot spot area
700 MVA transformer ,23 years
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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
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Temperatures in transformer cell
Transformer : 60/90 MVA, 12 radiators and fansCell : Front inlet opening and 4 extraction fans
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Temperatures in transformer cell
Transformer and air temperatures on-line 22 type T thermocouples connected to Tempscan and stored
every minute on PC
Oil information
Air information
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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
Temperatures in transformer cell
Air temperatures around transformer, transformer fans on
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Temperatures in transformer cell
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
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Temperatures in transformer cell
0 60 90 Load (MVA)
Ttopoil
T air cell
Tamb
Assumption : transformer fans stay on
T (C)
+8
+49
+15
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Temperatures in transformer cell
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
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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
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*>179.4°F
*<29.3°F
40.0
60.0
80.0
100.0
120.0
140.0
160.0
Transformer overheating case
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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
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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.
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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
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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
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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
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Insulation system
Homogenous insulation system
All conventional insulation transformers60/65/78
High temperature transformer80/120/145
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Insulation system
Hybrid or mixed insulation system60/95/115 60/65/95
3
2
3
4
2
4
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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
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Thermal Performance of Power Transformers
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
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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
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For more information: CIGRE Brochure 393
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