POWER TRANSFORMERS
Dejan Susa on behalf of SINTEF Electric Power Technology Department
dejan.susa@sintef.no
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Transformer design
• Insulation
– Liquid, Air
– Solid
• Stresses
– Thermal
– Dielectric
– Mechanical
Electrical Breakdown
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Insulation Degradation: Remaining lifetime
1
1

EA
DPNOW DPEND
Re mainingLife 
 e RT
A
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Transformer Life Parameters
Load Current
Top-Oil Temp.
Hot-Spot Temp.
H2O
Ambient Temp.
DGA
Bottom-Oil Temp.
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Relative Position
Characteristic Temperatures
H x gr
Top Oil Tank
Hot-Spot
Top Oil-Winding
Cooler
HV winding
Average Winding
LV winding
Average Oil
Core
gr
Bottom OilWinding
Ambient
Temperature
rise
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IEC 60076-2: Temperature rises
Temperature rise limits
Requirements for
Top liquid
Average winding (by winding resistance variation):
– ON.. and OF.. cooling methods
– OD.. cooling method
K
60
65
70
78
Hot-spot winding
 Temperature limits are based on expected lifetime
Ambient temperatures
°C
monthly
yearly average
maximum
average
20
30
40
25
35
45
30
40
50
35
45
55
* Referred to the values given in Table 1.
Correction of
temperature
rise
K *
0
–5
–10
–15
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HEAT RUN TEST
 The heat run test, i.e. temperature rise test, is the type test carried out to verify the
guaranteed temperature rises for oil and windings
 It is also used to reveal the possible overheated locations inside and outside windings due
to high stray fields for the high powers
 In addition, as indicated above, the temperature rises obtained during a heat-run test are
used for estimating transformer loading capability by application of the relevant thermal
models
 During a heat run test the following temperatures could be directly measured:




Ambient temperature
Top-oil temperature
Bottom-oil temperature
Hot-spot temperature, hottest winding temperature (if fiber optic sensors are installed)
 There are two methods used for performing the heat run test:
 short circuit
 back-to-back
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Hot-Spot Temperature Direct Measurements
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= H • gr ·Ky • fhs-oil(t)
Top-oil rise over ambient
=(Top-oil rise)r •[(1+R·K2)/(1+R)]x • foil(t)
Ambient Temperature
Ambient Temperature
Hot-spot temperature
Hot-spot to top-oil gradient
Top-oil temp.
Transient state: Hot-spot and Top-oil Temperatures
fhs-oil(t)=f2(t)=k21[1-e-t/(k22w,r)]-(k21-1) [1-e-t/(o/k22)]
foil(t)=f1(t)=(1-e-t/(k11·o,r))
k21
f1(t)
f2(t)
oil
TIME
TIME
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Recommended thermal characteristics for exponential
equations
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Hot-Spot Temperature:400 MVA ONAF UNIT/120 kV WDG
IEC 76-7: New and Active Guide
100
Measured
90
Temperature, degC
80
70
60
50
IEC 354: OLD GUIDE
40
30
20
10
0
0
100
Time, min
200
300
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IEC 60076-7 versus IEC 354
oil
oil,ss
1.
2.
3.
4.
1
2
oil
Oil pocket
CT
Heating element
Matching Resistance
TIME
3,4
3
-IEC 354
-IEC 76-7
1
TIME
TIME
4
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Oil Viscosity Effect on Power Transformer Thermal
Performance: New Oils
-IEC 76-7
TIME
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Transformer Oils:
Mineral Oil- Natural Ester – Synthetic Ester
•
Viscosity-temperature
relationship is not strictly
Arrhenius type
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Transformer Oils
Ester
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Accelerated Ageing Testing
DP
0
1
1

DPEND DPSTART
k
t
t
lnk
1/T
t
Ageing rate of paper:
k  A e
EA

RT
1
1

EA
DPNOW DPEND
Re mainingLife 
 e RT
A
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A and E Parameters
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Moisture in Paper
•
Equilibrium Charts
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Pictures – Paper Sampler Rig
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Paper Sampler Rig - schematic
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Characteristic Ageing Parameters
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Application
1000
?
Year
2011
?
New
Condition scale
Now
End
200
DP scale
Age scale
 Load History
 Load Forecast
 Temperature
 Temperature
 A:Contamination
 Moisture, LMA
 Age
 EA:Process
 Material
DPNOW

 A e

E
 A
RT
1
t 
DPSTART



1
 A:Contamination Forecast
 Moisture, LMA
 EA:Process Forecast
 Material
1
1

EA
DPNOW DPEND
Re mainingLife 
 e RT
A
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Example:Eidsiva Anlegg AS; Gjøvik T1
•
•
•
•
•
Top-Oil Temperature=40 degC
Moisture in Paper=1.5% (Equilibrium Curves)
Production Year: 1986
DPSTART=1000
DPEND=200
DPNOW
E

 A
1
  A  e RT  t 
DPSTART




1
1
1

EA
DPNOW DPEND
Re mainingLife 
 e RT
A
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Example: Loss of Life (IEC 60076-7)

Relative Aging Rate due to Hot-Spot Temperature (V):
 Non-thermally Upgraded Paper
 Thermally Upgraded Paper
V  2 ( h 98) / 6
V e
(
15000
15000
)

110  273  h  273
 The relative ageing rate V = 1,0 corresponds to a temperature of 98 °C for non-thermally
upgraded paper and to 110 °C for thermally upgraded paper:
 Loss-of-Life (L)
 Over certain period: L=ΣVnxtn, where n(1,N)
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Loss of Life Example
Load Steps of the Transformer
Hot Spot Temperature Calculation
Load Profile
Hot-Spot Temperature Profile
2,50
Hot Spot Temp (Deg C)
Temperature, degC
250
Load Factor
Load
factor, K
2,00
1,50
1,00
0,50
200
150
100
50
0
0,00
4
8
12
16
20
0
24
4
8
12
16
20
24
Tim
e (hours)
Time,
hours
Tim e (hours)
Time,
hours
Life Calculation
Dry Loss
andofClean
IEC 76-7
100
90
Loss of Life (days)
Loss
of life, days
0
80
70
60
50
40
30
20
10
0
0
4
8
12
Time, hours
16
20
24
Tim e (hours)
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Monitoring Gasses
•
DGA
 Limited gas range monitor
•Hydrogen, H2 (100%)
•Ethylene, C2H4 (8 %)
•Acetylene, C2H2 (1.5)
•Carbon Monoxide, CO(18%)
 Complete gas range monitor
•Hydrogen, H2
•Methane, CH4
•Ethane, C2H6
•Ethylene*, C2H4
•Acetylene*, C2H2
•Carbon Monoxide, CO
•Carbon Dioxide, CO2
•Oxygen, O2
•Nitrogen, N2
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Typical gasses generated:
IN CASE OF TRACKING DISCHARGE IN WET INSULATION
Low energy electrical discharge: Hydrogen H2, Methane CH4
IN CASE OF CORE CIRCULATION CURRENTS
Circulation currents in the core: Ethylene C2H4, Methane CH4 and Hydrogen H2
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Typical gasses generated:
IN CASE OF OVERHEATED CONNECTION
Local hot-spots: CO and Ethylene C2H4, Methane CH¤
IN CASE OF WINDING OVERHEATING
Thermal degradation of insulating paper: CO
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