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Western Mining Electric Association San Antonio TX Transformer Loading & Short Circuit Considerations NOVEMBER 16, 2012
© SPX Transformer Solutions, Inc.
Layer vs. Disk Windings Discussion
PRESENTED BY
David L. Harris, PE
Customer Technical Executive
SPX Transformer Solutions, Inc.
Office: 262-521-0166
Cell: 262-617-3039
Dave has a BS Electrical Engineering from Clarkson University, Potsdam, New York, and an MS
Engineering Management from Milwaukee School of Engineering. He has been in the transformer
industry for 43 years in design, development, manufacturing, testing, marketing, sales and
management of transformers and load tap changers. Currently, he holds the position of Customer
Technical Executive for SPX Transformer Solutions. Dave is a Life Member of the IEEE and is
active in the Electric Power Industry as a past chair of several Working Groups and
Subcommittees for the IEEE Substations Committee and IEEE Transformers Committee. Dave is
an individual member of CIGRE.
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2010
GLOBAL INFRASTRUCTURE X PROCESS EQUIPMENT X DIAGNOSTIC TOOLS
Impedance
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2010
IEEE Standard Impedance
IEEE Std C57.12.10-2010
High Voltage BIL Without LTC With LTC
≤ 110 5.5 --
150 6.5 7.0
200 7.0 7.5
250 7.5 8.0
350 8.0 8.5
450 8.5 9.0
550 9.0 9.5
650 9.5 10.0
750 10.0 10.5
Table 3 Percent impedance at self-cooled (ONAN) rating
The percent impedance voltage at the self-cooled rating as measured on the
rated voltage connection shall be as listed in Table 3 if the user does not
specify another value.
For cases not covered in Table 3, the percent impedance voltage value shall
be agreed between user and manufacturer, and the user should perform a
system study to determine the proper value of impedance.
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2010
IEEE Std C57.12.10-2010
IEEE Standard Impedance AutoTransformers
For autotransformers, the percent impedance voltage
shall be as specified by the user, or it should be the
lower of the value from Table 3 and the value obtained
according to the following equation:
Autotransformer impedance voltage = (Value from
Table 3) × (Autotransformer co-ratio) × 1.5 where
Autotransformer co-ratio = (High-Voltage – Low-
Voltage)/(High-Voltage)
This impedance voltage is the autotransformer
impedance and not the equivalent autotransformer
impedance.
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2010
Impedance: Directly Proportional - Frequency & MVA
Inversely Proportional - Volts/Turn
c a
b h
h
cba*turnsIX% 3
1
3
1
2
cestansisReWindingIR%
22 IR%IX%IZ%
Reactance
Resistance
Impedance
Core
Inner
Wdg
Outer
Wdg
Directly Proportional Frequency & MVA
Inversely Proportional Volts/Turn (Excitation)
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2010
Impedance Consequences
Low Impedance High Impedance
Increased Secondary Fault Currents
Reduced Secondary Fault Currents
Improved Voltage Regulation Potential Voltage Regulation Issues
Higher Short Circuit Withstand Forces
Lower Short Circuit Withstand Forces
Higher Interrupting Capacity for Secondary Equipment
Lower Interrupting Capacity for Secondary Equipment
Low Leakage Flux, Stray Losses, Winding Losses, Core Losses
Higher Leakage Flux, Stray Losses, Winding Losses, Core
Losses
Smaller footprint, Lighter, Reduced Cooling
Equipment Requirements
Larger footprint, Heavier, Increased Cooling Equipment
Requirements
NOTE: Both high and low impedance
increase costs
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2010
Impedance Effect on Voltage Regulation
18/24/30 MVA Transformer
Load Losses = 60 kW @ 18 MVA; Z = 8.0 @ 18 MVA base
= 166.67 kW @ 30 MVA, Z = 13.33 @ 30 MVA base
18 MVA 30 MVA
Power Factor % Regulation Power Factor % Regulation
1.0 0.64 1.0 1.43
0.9 4.02 0.9 6.95
0.8 5.24 0.8 8.92
18/24/30 MVA Transformer
Load Losses = 65kW @ 18 MVA; Z = 10.0 @ 18 MVA base
= 185.56 kW @ 30 MVA, Z = 16.67 @ 30 MVA base
18 MVA 30 MVA
Power Factor % Regulation Power Factor % Regulation
1.0 0.86 1.0 1.99
0.9 5.05 0.9 8.82
0.8 6.57 0.8 11.25 (> 10% LTC)
9
Nominal Voltage Case Study Example The transmission system is a nominal 138 kV transmission line, and the customer
distribution system is regulated 13.2 kV using LTCs for voltage regulation. Assuming the
HV DETCs are set on C tap and the transformer is rated 15/20/25(28) MVA 55ºC/65ºC rise.
Transformer
HV – 138 kV Nominal
Transformer (Alternate)
HV – 134 kV Nominal
HV DETC TAPS: 144.9, 141.45, 138.0, 135.55,
131.1 kV
HV DETC TAPS: 140.7, 137.35, 134.0, 130.65,
127.3 kV
LV: 13.8/7.967 kV GRDY
Rated current @ 28 MVA = 1171 amps
LV: 13.2/7.621 kV GRDY
Rated current @ 28 MVA = 1225 amps
When HV @ C tap (138 kV) and transmission
line operates @ 138 kV; LV @ Neutral tap = 13.8
kV; @ 7L = 13.2 kV
LTC efectively 13.2kV + 23 (14.4%)/- 9 (5.9%) @
steps = 0.0653%(86.25 kV)/step
When HV @ C tap (134 kV) and transmission line
operates @ 138 kV (1.03 per unit over excitation);
LV @ Neutral tap = 13.6 kV; @ 5L = 13.2 kV
LTC effectively 13.2 kV + 21( 13.1%) / - 12(9.25%)
@ steps = 0.0644%(82.5 kV)/step
Transformer Ratio HV C tap – LV N = 17.32 Transformer Ratio HV C tap – LV N = 17.58
Impedance @ 100 % excitation C-N = 8.0% Impedance @ 103% excitation C-N = 7.54%
Impedances and LTC steps are different are different: paralleling issues.
Ratios are different: paralleling issues
138 kV HV Transformer should be specified with full capacity LTC to 13.2 kV to
avoid capacity reduction
134 kV HV Transformer operates as standard @ 1.03% over excitation,
increases sound level, reduces impedance
Largest Shell Transformer Made in
the U.S. Completed and Shipped
• Efacec Power Transformers Inc. has successfully shipped the largest shell transformer made in the U.S. in more than 20 years. The electrical transformer has the unique feature that it can be delivered in four pieces, overcoming significant transportation restrictions.
• The 700-MVA 230-kV GSU transformer is the first of its type ever made in the U.S. It was completely designed, manufactured and tested at the firm’s Rincon, GA facility. A large electric utility bought the first unit made in the U.S., which was shipped last month.
• The huge shell transformer uses “disassociated phase technology," which allows it to be built in four pieces – three units and a fourth piece that fits on top – and assembled at its destination. The technology, perfected at Efacec’s Portugal plant, makes shipping such large transformers feasible.
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2010
Voltage Regulation
IEEE C57.12.80
3.495 voltage regulation of a constant-voltage transformer:
The change in output (secondary) voltage that occurs
when the load (at a specified power factor) is reduced from
rated value to zero, with the primary impressed terminal
voltage maintained constant.
14
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2010
Voltage Regulation IEEE C57.12.90 - 2010
15
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011
Agenda
Thermal Design Considerations
Industry Guides
Limiting Parameters for Overloading
Theoretical Life
Functional Life
User’s Practice – Worldwide Survey
16
Let’s make this interactive!
If you have questions during the
presentation, please stop me to ask
them.
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011
Thermal Design Considerations
To Control Vary the Design Parameter
Average oil Amount of coolers (rads,
temperature over fans, pumps) and unit’s
ambient thermal time constant
Winding Gradient Current densities in winding
conductors and cooling
surface within winding
Hot Spot Gradient Current densities in winding
conductors and stray loss
distribution
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011
Hot-spot temperature calculation – C57.12.00-2010, 5.11.1.1
a) Direct measurement during a thermal test in accordance with IEEE Std C57.12.90 A sufficient
number of direct reading sensors should be used at expected locations of the maximum temperature
rise as indicated by prior testing or loss and heat transfer calculations.
b) Direct measurement on an exact duplicate transformer design per a).
c) Calculations of the temperatures throughout each active winding and all leads. The calculation
method shall be based on fundamental loss and heat transfer principles and substantiated by tests on
production or prototype transformers or windings.
The maximum (hottest-spot) winding temperature rise above ambient temperature shall be included in the
test report with the other temperature rise data. A note shall indicate which of the above methods was used
to determine the value.
Electrical Design Process
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
The circulation of oil
through the coils into the
tank and cooling is the
process that removes
heat from the windings
to the surrounding
environment.
Non-directed oil flow is
where the oil is allowed
to flow freely through the ducts and other oil channels in the windings
If the free flow of oil is directed along specific pathways in the
windings, it is known as directed oil flow
Circulation of the Oil
19
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
coils
radiators
T bot Q c T bot
Q r
T r,bot
Q n
Q s
Q c T
c,top
T top
Q r
T top
T varies linearly here
tank
Assumed oil temperature distribution inside tank. The oil flows, Q , as
well as the flow weighted temperatures are also indicated.
.
Typical Thermal Performance Calculation Variables
Electrical Design Process
20
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS 21
Natural Circulation of the Oil
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011
22
Winding
Thermal Design Considerations
Oil is free to
find its own
path from the
bottom of the
winding to the
top of the
winding.
Strategic
washers are
placed in the
winding to
direct the oil
flow.
Non Directed Oil Flow___ ____ Directed Oil Flow___
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
coils
radiators
T bot Q c T bot
Q r
T r,bot
Q n
Q s
Q c T
c,top
T top
Q r
T top
T varies linearly here
tank
Assumed oil temperature distribution inside tank. The oil flows, Q , as
well as the flow weighted temperatures are also indicated.
.
Typical Thermal Performance Calculation Variables
Electrical Design Process
23
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Hot-Spot Temperature Calculation – C57.12.00-2010, 5.11.1.1
a) Direct measurement during a thermal test in accordance with IEEE Std C57.12.90 A sufficient
number of direct reading sensors should be used at expected locations of the maximum temperature
rise as indicated by prior testing or loss and heat transfer calculations.
b) Direct measurement on an exact duplicate transformer design per a).
c) Calculations of the temperatures throughout each active winding and all leads. The calculation
method shall be based on fundamental loss and heat transfer principles and substantiated by tests on
production or prototype transformers or windings.
The maximum (hottest-spot) winding temperature rise above ambient temperature shall be included in the
test report with the other temperature rise data. A note shall indicate which of the above methods was used
to determine the value.
24
Electrical Design Process (cont.)
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
28 kV, 175.5 kV 28 kV, 195.5 kV 28 kV, 255.5 kV 28 kV, 235.5 kV
Electrical Design Process (cont.)
25
Hot-Spot Temperature Calculation – Example
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Hot-Spot Temperature ( C Rise over ambient)
@ 175.5 kV @ 195.5 kV @ 255.5 kV @ 235.5 kV Hottest
Inner Winding 84.1 83.9 88.4 86.1 88.4
Winding 2 77.0 76.9 79.8 78.4 79.8
Winding 3 80.9 77.0 71.2 72.7 80.9
Winding 4 65.7 65.7 70.0 69.8 70.0
Outer Winding 65.4 65.6 69.5 66.2 69.5
Hot-Spot Temperature Calculation – Results
The inner winding and the Winding 3 had to be redesigned to lower the
hot-spot temperature rise below 80 C.
26
Electrical Design Process (cont.)
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
The two transformer oil circulation systems in common use:
Natural circulation, or thermosiphon, which relies on the viscosity change of the oil from temperature variation to produce oil flow (explain thermal head)
Forced circulation, which is the use of pumps as the means to create oil flow
Oil Circulation Systems
27
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011
External Cooling
Radiators are the most common means used to increase the amount
of exposed oil surface area to the surrounding air in order to increase
the efficiency of heat exchange rate.
If dictated by loading or space requirements, higher efficiency heat
exchangers that employ pumps or water coolers can be utilized, with a
significant increase in cost.
Fans are a relatively inexpensive means to increase the rate of heat
dissipation from the radiators by increasing the volume of air moving
over the radiator surface.
Noise generated by the cooling fans varies with the blade design and
the operational speed of the cooling fans, and often becomes a limiting
factor in transformer loading and cooling.
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Finite Element Analysis (FEA) is
performed on every new design
to generate electromagnetic field
plots
Analysis of loss distribution is
used to calculate the gradient
between the hottest-spot rise and
oil rise
Hottest-Spot Calculations
29
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011
Oil to Winding Temperature Gradient
The gradient is defined as the difference between the average oil
temperature in the tank and the average conductor temperature
in the windings and is directly related to the amount of conductor
surface exposed to the surrounding oil in the windings.
Resistance and eddy losses per unit area are calculated and
entered into the following formula to determine the gradient.
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 31
Industry Practice on Transformer Loading
Industry Guides
C57.91-1981
Distribution Tr.
C57.92-1981
Power Tr.
ANSI/IEEE C57.91-1995 Guide for Loading
Transformers *
C57.115-1991
Power Tr.>100MVA
* This guide is currently being revised by the IEEE
Transformers Committee
IEC Publication 60076-7 is a 2005 update of IEC Pub 60354-1991
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 32
Industry Practice on Transformer Loading
Industry Guides
• Main differences between IEEE and IEC standards and guides
are limits and calculation methods for Hot-spot temperature
• IEC limits are lower (160 °C) than the IEEE limit of 180 °C
• Previous IEC guide utilized the H factor of 1.1(for Distribution
Tr.) to 1.3 (for Power Tr.)
• New IEC revision states this H factor ranges 1.0-2.1
o The factor H should be defined either by direct
measurement or by a calculation procedure based on
fundamental loss and heat transfer principles, and
substantiated by direct measurements on production or
prototype transformers or windings.
o General practice still is to use the default values of H
• IEEE requires more accurate calculation and verification
(C57.12.00)
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 33
Industry Practice on Transformer Loading
An example of following two industry guides (IEEE v.s.
IEC)
• Typical Power Transformer will have a “gradient” value of 20 C
• Following the IEEE guide, manufacturers can usually estimate
the hot-spot temperatures with 5 C accuracy
• Following the old IEC practice, HST is 26 C (20 X 1.3)
• CIGRE reported that the Hot-Spot factor ranges from 0.9 to 2.1,
verified by fiber optic sensors
o This translates to HST could be 18 to 42 C over the oil
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 34
Industry Practice on Transformer Loading
Limiting Parameters for Overloading
• Under old guides;
o Top oil temperature 110 C
o Winding Hot-spot temperature 180 C
o Maximum short-time loading 2 times Maximum Nameplate rating
• Under 1995 guide; (for distribution transformers)
o Top oil temperature 120 C
o Winding Hot-spot temperature 200 C
o Short-time loading (1/2 hours or less) 3 times Maximum Nameplate rating
• Under 1995 guide; (for power transformers)
o Top oil temperature 110 C
o Winding Hot-spot temperature 180 C
o Maximum loading 2 times Maximum Nameplate rating
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 35
Under IEC 60076-7, 2005;
Types of Loading Distribution
Transformers
Medium Power
Transformers
Large Power
Transformers
Normal cyclic loading
Current (p.u.) 1.5 1.5 1.3
Hot-spot temperature and metallic
parts in contact with insulating
material ( C)
120
120
120
Top-oil temperature ( C) 105 105 105
Long-time emergency cyclic loading
Current (p.u.) 1.8 1.5 1.3
Hot-spot temperature and metallic
parts in contact with insulating
material ( C)
140
140
140
Top-oil temperature ( C) 115 115 115
Short-time emergency cyclic loading
Current (p.u.) 2.0 1.8 1.5
Hot-spot temperature and metallic
parts in contact with insulating
material ( C)
N/A
160
160
Top-oil temperature ( C) N/A 115 115
Industry Practice on Transformer Loading
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Transformer Thermal Loading Specification
36
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
604.27273
15000exp
HSTLife
Where, Life = Life in hours at temperature HST
HST = Hot Spot Temperature in C
Theoretical Life
37
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Rule of Thumb: For every 6 to 8 degrees the hot spot
temperature is reduced, the theoretical life of the
transformer insulation doubles.
38
Theoretical Life (cont.)
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Basis
Normal Insulation Life
Hours Years
50% retained tensile strength of insulation (former IEEE Std C57.92-1981 criterion)
65,000 7.42
25% retained tensile strength of insulation 135,000 15.41
200 retained degree of polymerization in insulation 150,000 17.12
Interpretation of distribution transformer functional life test data (former IEEE Std C57.91-1981 criterion)
180,000 20.55
"Normal insulation life" of a
well-dried, oxygen-free,
65 C average winding
temperature rise insulation
system at the reference
temperature of 110 C.
Theoretical Life (cont.)
39
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Insulation Life Testing Process
40
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Aging of insulation materials is
caused by a number of factors • Moisture
• Oxygen
• Temperature
• Time
Proper application of oil
preservation systems and
maintenance can minimize
the moisture and oxygen
content
Proper loading can minimize
the hot-spot temperature
Oil Preservation Systems
41
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 42
MPEG Video
Industry Practice on Transformer Loading
Functional Life
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Defined as “conditions which the transformer does not function as intended”
Overloading can cause “bubbles” in the oil that cause dielectric failures (hot-spot temperatures)
Overloading can cause tank pressure build-up that cause gasket leaks and PRD operation (average oil temps)
Other loading related issues: • Current carrying components’ ratings
• CT saturation
• Lead heating
• Leakage flux
• Overheating
Functional Life (cont.)
43
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Sources of bubbles
• Gasses dissolved in oil
• Gasses generated from
decomposition of insulation
• Water vapor from paper insulation
in windings
Sudden release of gas/vapor
as bubbles is possible under
overloading
Functional Life – Bubbles
44
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Next series of slides were
presented at the 2001
IEEE T&D Conference in
Atlanta by T.V. Oommen*
Based on two EPRI
reports:
• EL-6761 (March 1990)
• EL-7291 (March 1992)
Functional Life – Bubbles (cont.)
* Permission to use them granted by the author.
45
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 46
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 47
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COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 49
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 50
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COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 53
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 54
Industry Practice on Transformer Loading
Functional Life (Bubbles)
• Revised Loading Guide C57.91 will contain the equation for
Bubble evolution temperature.
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 55
Industry Practice on Transformer Loading
Functional Life (Bubbles)
• Summary from these reports on Bubbles
o Bubble Generation from Overload is mostly due to water
vapor released from paper insulation
o Gas blanked units and conservator units show little
difference in bubble evolution at low moisture levels
o Increasing gas saturation in oil lowers bubble evolution
temperature only at high moisture levels
o Accepting 140 °C as Hot-spot temperature limit appears
to be valid for moisture content above 1.5%
• CIGRE did survey in 1995 and found similar practice (140 °C
limit)
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 56
Industry Practice on Transformer Loading
User’s Practice – Worldwide Survey
(According to the CIGRE Working Group 12.09)
• Why should the Hot-spot temperature be limited?
o 60% of the users Free Gas Bubbles
o 20% of the users Thermal Decomposition
o 20% of the users Both
• How high this temperature can be?
o Depends on many factors including moisture content, gas
saturation level, static pressure, etc
o Generally below 140 °C
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 57
Industry Practice on Transformer Loading
Conclusion and Recommendations
Monitoring, Maintenance, Proper implementation of the Operating strategy, and taking appropriate actions in time protect your investment and provide maximum return on that investment
Specifications, Design, Execution of these at OEM, and Verification testing assures built-in robustness
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Conductor Tilting Force – Critical
58
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Winding Radial Forces
59
Maximum Radial Stress (psi):
Failure Modes:
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS
Winding Beam Bending Forces
60
COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.
TRANSFORMERS | SERVICE | TRAINING | COMPONENTS
TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS 61
Typically a problem
for “Layer” winding
Can happen to “disk”
or “helical” windings
Extent of damage to
paper insulation will
determine how soon
a total unit failure will
happen
Winding Spiral Tightening Forces
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011
Wild Life Outages www.wildlifeoutages.com
62
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011
Wild Life Outages www.wildlifeoutages.com
63
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011
Wild Life Outages www.wildlifeoutages.com
64
COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011
Thank you
January 9, 2013 65