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Theses and Dissertations
1-1-1980
Overloading power transformers utilizing projected
daily ambient temperatures.
Daniel Chaply
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OVERLOADING POWER TRANSFORMERS UTILIZING
PROJECTED DAILY AMBIENT TEMPERATURES
By
Daniel Chaply
A Thesis
Presented to the Graduate Faculty
of Lehigh University
in Candidacy for the Degree of
Master of Science
in
Electrical Engineering
Lehigh University
1980
ProQuest Number: EP76094
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Certificate of Approval
This thesis is accepted and approved in partial fulfillment
of the requirements for the degree of Master of Science in
Electrical Engineering.
(Date)
Professor in Charge
Chairman of Department
XI
Acknowledgements
The author expresses his appreciation to the Pennsylvania
Power and Light Company for providing the opportunity to
complete this thesis through its Education Assistance
Program and to Mr. J. K. Redmon for his direction and
guidance as advisor to this thesis.
111
Table of Contents
Page
Abstract
1
Chapter 1
Introduction
4
Chapter 2
Background
7
Chapter 3
Insulation Life
10
Chapter 4
Environmental Factors and Effects
15
Chapter 5
Overload Methodologies
22
Chapter 6
Ambient Temperatures and Load Profiles
40
Chapter 7
Results and Discussions
46
Chapter 8
Dielectric Strength of Conductors in
Contact With Hot Spots
90
Chapter 9.
Summary
128
Chapter 10
Conclusions
138
141
Bibliography
Appendix A
Listing of Transformers Used in
This Study
148
Appendix B
Climatological Data
149
Appendix C
Oil Temperature Change Equation
150
151
Vita
IV
List of Tables
Page
Table 4-1
Wind Velocity Effects on Top
Oil Temperature
19
Table 7-1
Per Cent Overloads Using the Day/Night
Model For 2 Hour Emergency Period
59
Table 7-2
Per Cent Overloads Using the Day/Night
Model For 6 Hour Emergency Period
60
Table 7-3
Per Cent Overloads Using the Day/Night
Model For 10 Hour Emergency Period
61
Table 7-4
Per Cent Overloads Using the Varying
Ambient Model For 2 Hour Emergency
Period
62
Table 7-5
Per Cent Overloads Using the Varying
Ambient Model For 6 Hour Emergency
Period
63
Table 7-6
Per Cent Overloads Using the Varying
Ambient Model For 10 Hour Emergency
Period
64
Table 7-7
Per Cent Overloads Using the PJM Method
65
Table 7-8
Tabulation of Temperatures for 10 MVA
Transformer 2 Hour Emergency, Hot Spot
Limit of 180°C
78
Table 7-9
Tabulation of Temperatures for 10 MVA
Transformer 6 Hour Emergency, Hot Spot
Limit of 180°C
79
Table 7-10
Tabulation of Temperatures for 10 MVA
Transformer 10 Hour Emergency, Hot Spot
Limit of 180°C
80
Table 7-11
Tabulation of Temperatures for 15/20/25
MVA Transformer 2 Hour Emergency, Hot
Spot Limit of 180°C
81
Page
Table 7-12
Tabulation of Temperatures for 15/20/25
MVA Transformer 6 Hour Emergency, Hot
Spot Limit of 180°C
82
Table 7-13
Tabulation of Temperatures for 15/20/25
MVA Transformer 10 Hour Emergency, Hot
Spot Limit of 180°C
83
Table 7-14
Tabulation of Temperatures for 150 MVA
Transformer 2 Hour Emergency, Hot Spot
Limit of 180°C
84
Table 7-15
Tabulation of Temperatures for 150 MVA
Transformer 6 Hour Emergency, Hot Spot
Limit of 180°C
85
Table 7-16
Tabulation of Temperatures for 150 MVA
Transformer 10 Hour Emergency, Hot Spot
Limit of 180°C
86
Table 7-17
Tabulation of Temperatures for 300 MVA
Transformer 2 Hour Emergency, Hot Spot
Limit of 180°C
87
Table 7-18
Tabulation of Temperatures for 300 MVA
Transformer 6 Hour Emergency, Hot Spot
Limit of 180°C
88
Table 7-19
Tabulation of Temperatures for 300 MVA
Transformer 10 Hour Emergency, Hot Spot
Limit of 180°C
89
Table 8-1
Per Cent Overloads with the Day/Night
Model 2 Hour Emergency, 150°C Hot Spot
Limit
97
Table 8-2
Per Cent Overloads with the Day/Night
Model 6 Hour Emergency, 150°C Hot Spot
Limit
98
Table 8-3
Per Cent Overloads with the Day/Night
Model 10 Hour Emergency, 150°C Hot Spot
Limit
99
VI
Page
Table 8-4
Per Cent Overloads with the Varying
Ambient Model 2 Hour Emergency, ,150°C
Hot Spot Limit
100
Table 8-5
Per Cent Overloads with the Varying
Ambient Model 6 Hour Emergency, 150°C
Hot Spot Limit
101
Table 8-6
Per Cent Overloads with the Varying
Ambient Model 10 Hour Emergency, 150°C
Hot Spot Limit
102
Table 8-7
Per Cent Overloads Using the.PJM Method
with Hot Spot Limit at 150°C
103
Table 8-8
Tabulation of Temperatures for 10 MVA
Transformer 2 Hour Emergency, Hot Spot
Limit of 150°C
116
Table 8-9
Tabulation of Temperatures for 10 MVA
Transformer 6 Hour Emergency, Hot Spot
Limit of 150°C
117
Table 8-10
Tabulation of Temperatures for 10 MVA
Transformer 10 Hour Emergency, Hot Spot
Limit of 150°C
118
Table 8-11
Tabulation of Temperatures for 15/20/25
MVA Transformer 2 Hour Emergency, Hot
Spot Limit of 150°C
119
Table 8-12
Tabulation of Temperatures for 15/20/25
MVA Transformer 6 Hour Emergency, Hot
Spot Limit of 150°C
120
Table 8-13
Tabulation of Temperatures for 15/20/25
MVA Transformer 10 Hour Emergency, Hot
Spot Limit of 150°C
121
Table 8-14
Tabulation of Temperatures for 150 MVA
Transformer 2 Hour Emergency, Hot Spot .
Limit of 150°C
122
Table 8-15
Tabulation of Temperatures for 150 MVA
Transformer 6 Hour Emergency, Hot Spot
Limit of 150°C
123
vn
Page
Table 8-16
Tabulation of Temperatures for 150 MVA
Transformer 10 Hour Emergency, Hot Spot
Limit of 150°C
124
Table 8-17
Tabulation of Temperatures for 300 MVA
Transformer 2 Hour Emergency, Hot Spot
Limit of 150°C
125
Table 8-18
Tabulation of Temperatures for 300 MVA
Transformer 6 Hour Emergency, Hot Spot
Limit of 150°C
126
Table 8-19
Tabulation of Temperatures for 300 MVA
Transformer 10 Hour Emergency, Hot Spot
Limit of 150°C
127
Vlll
List of Figures
Page
Figure 5-1
Insulation Life - Loss Curves
26
Figure 6-1
Idealized Daily Ambient Temperature
Curve
43
Figure 7-1
Overloads for 10 MVA Transformer Two
Hour Emergency, 180°C Hot Spot Limit
66
Figure 7-2
Overloads for 15/20/25 MVA Transformer
Two Hour Emergency, 180°C Hot Spot Limit
67
Figure 7-3
Overloads for 150 MVA Transformer Two
Hour Emergency, 180°C Hot Spot Limit
68
Figure 7-4
Overloads for 300 MVA Transformer Two
Hour Emergency, 180°C Hot Spot Limit
69
Figure 7-5
Overloads for 10 MVA Transformer Six
Hour Emergency, 180°C Hot Spot Limit
70
Figure 7-6
Overloads for 15/20/25 MVA Transformer
Six Hour Emergency, 180°C Hot Spot Limit
71
Figure 7-7
Overloads for 150 MVA Transformer Six
Hour Emergency, 180°C Hot Spot Limit
72
Figure 7-8
Overloads for 300 MVA Transformer Six
Hour Emergency, 180°C Hot Spot Limit
73
Figure 7-9
Overloads for 10 MVA Transformer Ten
Hour Emergency, 180°C Hot Spot Limit
74
Figure 7-10
Overloads for 15/20/25 MVA Transformer
Ten Hour Emergency, 180°C Hot Spot Limit
75
Figure 7-11
Overloads for 150 MVA Transformer Ten
Hour Emergency, 180°C Hot Spot Limit
76
Figure 7-12
Overloads for 300 MVA Transformer Ten
Hour Emergency, 180°C Hot Spot Limit
77
Figure 8-1
Overloads for 10 MVA Transformer Two
Hour Emergency, Hot Spot Limit of 150°C
IX
104
Page
Figure 8-2
Overloads for 15/20/25 MVA Transformer
Two Hour Emergency, Hot Spot Limit
of 150°C
105
Figure 8-3
Overloads for 150 MVA Transformer Two
Hour Emergency, Hot Spot Limit of 150°C
106
Figure 8-4
Overloads for 300 MVA Transformer Two
Hour Emergency, Hot Spot Limit of 150°C
107
Figure 8-5
Overloads for 10 MVA Transformer Six
Hour Emergency, Hot Spot Limit of 150°C
108
Figure 8-6
Overloads for 15/20/25 MVA Transformer
Six Hour Emergency, Hot Spot Limit
of 150°C
109
Figure 8-7
Overloads for 150 MVA Transformer Six
Hour Emergency, Hot Spot Limit of 150°C
110
Figure 8-8
Overloads for 300 MVA Transformer Six
Hour Emergency, Hot Spot Limit of 150°C
111
Figure 8-9
Overloads for 10 MVA Transformer Ten
Hour Emergency, Hot Spot Limit of 150°C
112
Figure 8-10
Overloads for 15/20/25 MVA Transformer
Ten Hour Emergency, Hot Spot Limit
of 150°C
113
Figure 8-11
Overloads for 150 MVA Transformer Ten
Hour Emergency, Hot Spot Limit of 150°C
114
Figure 8-12
Overloads for 300 MVA Transformer Ten
Hour Emergency, Hot Spot Limit of 150°C
115
Abstract
In an era where financial, environmental, and public
constraints are placed on additional facilities needed by
utilities, a greater demand is placed on the industry to
maximize the capability of all the equipment that is used to
transfer power from generation sites to usage areas.
It has
been known for many years that power transformers can be
loaded such that the output can be greater than the unit's
nameplate rating.
The factor that must be determined by
each user is the amount of overload that can be tolerated
without subjecting the transformer to undue deterioration
and possible premature failure.
Various methodologies for overloading have been developed
over the years.
All have taken into account the temperature
of the air that surrounds the transformer.
Approximation of
this ambient temperature have been made so that overloads
could be readily computed.
This thesis examined the various factors that determine
the allowable output of medium and large power units installed
in outdoor locations.
These factors included transformer
design characteristics, paper insulation life, and external
environmental effects.
Use of projected daily ambient
temperatures instead of a presently used two temperature
approximation (one temperature for summer and another for
winter) was investigated.
Short time emergency situations
were specifically addressed.
Two methods for incorporating
a changing daily ambient were devised, and the results
obtained with these methods were compared to that obtained
with the present method.
The comparison was made for a
series of short time emergency situations that were assumed
to occur randomly and with equal probability throughout the
year.
Analysis of the results indicated that, in most of the
cases, the use of an assumed constant ambient temperature
did not handicap the overall potential overload capability
of a unit.
There were certain periods of the year that the
overload capability was greater when calculated with the
projected ambient temperature rather than with one of the
two constant values.
However, when comparisons were made
for the year as a whole, the advantage gained by using
projected ambient temperatures was overcome.
It was concluded that, in the general case, the use of
one constant ambient value for summer and another for winter
in the short time emergency calculations would yield results
that adequately project overload capabilities.
Based on internal temperature comparisons, it was
determined that additional overload capability can be obtained
by performing a separate computation for the month of April.
The remaining months of the year would still remain on the
two temperature basis.
Calculations were made to determine if the relative
performance of the methods would change when the operation
of the transformer was within the constraints of a lower hot
spot temperature.
The results indicated that the relative
overload capability would remain unchanged.
CHAPTER 1
INTRODUCTION
Much of the equipment needed to direct the flow of
electrical power on a utility's transmission and distribution
system is rated to carry a given'amount of ultimate load
without exceeding some predetermined constraints.
constraints are usually thermal in nature.
These
Since this
equipment does have losses associated with it, a portion of
these losses are in the form heat dissipation.
When the
amount of heat generated is such that material strengths and
characteristics become affected, then the thermal constraint
is reached.
This thermal constraint will not only be influ-
enced by the design parameters of the piece of equipment,
but also by the system loading profiles and the external
environmental conditions.
The last two factors generally
vary from hour to hour and consequently will have a pronounced effect as to whether or not that thermal constraint
is exceeded.
For power transformers, the thermal loading capability
is based on the concept that the maximum or hot spot temperature, when applied for a given amount of time, will age
paper insulation at a certain rate.
Hence, one can see that
both time and temperature influence the maximum loading that
is allowed.
To obtain the maximum capability of any piece
of electrical equipment, including transformers, requires
real time monitoring of the equipment's thermal and electrical
characteristics, as well as the environmental conditions
external to that piece of equipment.
In the past, the
installation of monitoring equipment was rare, either because
installation was economically unjustifiable or techniques
for such monitoring did not exist.
Consequently, various
methodologies for overloading power transformers have been
- A 8»21
created.
'
However, with the increasing burden on utilities to
keep costs down and still furnish service for customer's
needs with previous reliability, equipment rating methods
must be scrutinized to insure that components are operated
at maximum capability without sacrifice of reliability.
In
this study, the effect of environmental factors on transformer
overload capability was analyzed.
Two methods were devised
to incorporate those factors that were determined to have
the greater effect on that capability.
Results obtained
from these calculations were compared with those overloads
obtained with present methods.
Analysis was made so as to
determine what benefit inclusion of such climatological data
could have on increasing the present overloading limits on
transformers.
To determine what effect such calculations had on
transformers typically used in substations, a selection of
medium and large power transformers was picked to cover a
range of 10 MVA to 300 MVA (55°C rating).
Data from four
units, of four different MVA sizes and kV ratings, manufactured by four different companies, were used in the calculations.
This study was limited to oil-immersed medium and
large power transformers that are installed in outdoor
locations.
These units are of recent vintage and all use
the thermally upgraded 65°C rise insulation system.
CHAPTER 2
BACKGROUND
On May 1, 1885, K. Zipernowsky, M. Beri, and 0. T.
Blathy displayed the first closed magnetic circuit transformer
at the Hungarian National Exhibition in Budapest.
18
The
following year, George Westinghouse used this new apparatus
and, in Great Barrington, Massachusetts, developed the first
commercial lighting system.
During the same year, oil was
introduced as an insulating and cooling media.
Such were
the beginnings of the electric power distribution system as
we know it today.
Electric power requirements have skyrocketed.
In 1882, Edison's Pearl Street Station had a capacity of 30
kW.
In 1980, the total capacity of the U.S. electric power
system is over 600 million kW.
Nameplate ratings originally were based on standards
that assumed continuous full loading at the maximum ambient
temperature, usually 40°C, and a hot spot temperature of
approximately 105°C.
Intensive research into insulation
life led to the concept of thermal aging processes.
Since
most units experience variable loadings, short time overloads
were allowed to use up the surplus insulation life that
resulted from cyclic operations at a lower rate than normal
loading.
These short time overloads would be compensated by
loadings at lower levels during the remaining time of the
load cycle.
Later, standards, '
were developed to allow an
increase in loss of life allowances for emergency overloads.
24
Various formula and calculation methods for computing insulation degradation and permissible overloads have been introduced.
The method described in the report
"Determination
of Power Transformer Overloads" is one of the latest of
these methods.
Present day American National Standards Institute
(ANSI) - National Electrical Manufacturers Association
(NEMA) - Institute of Electrical and Electronics Engineers
(IEEE) guides define the power transformer rated output by
the power that the secondary winding can continuously deliver
while operating in an ambient of•30°C continuously.
The
maximum hot spot temperature of 180°C must not be exceeded,
and normal thermal life must be expected.
That maximum hot
spot temperature limit applies for transformers whose average
winding temperature rise by resistance does not exceed 65°C.
The vast majority of transformers that are constructed for
use in the United States have this upgraded thermal insulation.
However, it should be noted that the vast majority of transformers do not operate in an ambient of 30°C continuously
nor do these transformers operate at full load continuously.
CHAPTER 3
INSULATION LIFE
To gain a very precise knowledge of the expected life
of a transformer, functional life tests should be performed.
These tests measure the capability of all of the materials
in the context of a system.
At the present time, no func-
tional life testing of medium and large power transformers
has proven to be practical and economically justifiable.
Full scale life testing is economically unjustifiable so
that life tests of a model is necessary.
The model, as a
whole, must apply the same stresses, in the proper sequence,
as the unit itself.
The model must also use the same material,
and, in the overall sense, accurately simulate the configuration of the system itself.
The stresses that are involved
are thermal, dielectric, and mechanical in nature.
The life
of a transformer can become a function of any of these
stresses.
19
The limits must be properly defined to adequately
correlate with actual experiences.
Hence, one is required to rely on material life tests
to make projections of expected life for the unit itself.
10
Cellulosic insulation material (paper and press board)
has been commonly used in transformers to separate the
current carrying parts.
With high temperature tests, the
degradation of the insulation has been shown to be a gradual
and cumulative process.
With time, the material exhibits
dryness and charring, increased brittleness, and loss of
mechanical strength rather than dielectric strength.
The
dielectric strength retains a relatively high constant value
until, after a certain point, it decreases rapidly.
The
degradation was also found to be quicker when the insulation
3
was oil impregnated.
Once the insulation had deteriorated
so badly that 1) adequate dielectric strength cannot be
maintained against impulse voltages and 2) adequate mechanical
strength cannot prevent conductor separation when stressed
by short circuits, then the insulation, and consequently the
unit itself, is subjected to a very high probability of
failure.
Different tests can be performed to determine the
mechanical strength.
These include folding, tearing,
stretching, bursting, and tensile tests.
yields the longest insulation life.
The tensile test
It is a simple test to
perform, and reproducible results are obtainable.
11
Loading
guides have, in one form or another, used the tensile strength
criteria as the means of defining the length of functional
service life of transformers.
Results of early experiments
13
showed that deterioration
occurred at some definite rate at all times, but that for
approximately every 8°C increase in temperature, the rate of
aging doubled.
This aging phenomena was also found to be
cumulative and progressive.
The relationship defining this
deterioration of the insulation in oil is based on a chemical
reaction theory.
Equation 3-1, shown below, defines this
4 21
relationship, known as the Arrhenius Reaction Rate. '
Log1Q L = A + y
(eqn. 3-1)
where:
L = minimum life expectancy in hours
A = constant = -13,391 for 65°C rise insulation
= -14,133 for 55°C rise insulation
B = constant = 6972.15
T = Absolute temperature in °K
Equation 3-1 states that the logarithm of time that it
takes for the insulation strength to reach a limit is linearly
proportional to the reciprocal of temperature.
12
It should be
noted that the factor L is the minimum life expectancy.
When the tensile strength of the insulation drops to 50% of
the initial strength, then it is assumed that the insulation's
strength will not withstand thermal and mechanical forces.
Consequently the transformer's end-of-life is assumed to be
reached.
After the concept of a transformer and the idea of
parallel operation of transformers was introduced in 1885,
improvements in design continued so that by 1913, 55°C
average winding rise transformers were available.
In 1958,
Westinghouse Electric Corporation introduced the Insuldur
system to improve thermal endurance.
Initially it was amine
stabilizer that was added to the transformer oil.
But, by
1960, the stabilizer was actually added to the cellulose.
By 1964, improvements allowed the insulation impregnated
with Insuldur to withstand higher temperatures than was
possible with the regular kraft paper.
The composition of cellulose is a chain of glucose
(CfiH, _(),.) molecules with three hydroxyl (OH) radicals associated with each glucose unit.
When the cellulose is soaked
in oil and heated, hydrogen ions join with the hydroxyl
radicals and form Water.
The water remains in the insulation
13
and, by hydrolysis, acts to deteriorate the cellulose.
The
Insuldur acts as a stabilizer and retards the ability of the
hydroxyl radical to dissociate.
With the use of Insuldur and its greater thermal capability came an increase in the average winding rise from
55°C to 65°C.
With this higher temperature came an increase
of approximately 12% in the continuous load rating of the
transformer.
If a hot spot temperature of 110°C (for 65°C average
winding rise at rated power) is assumed, then, by using
equation 3-1, page 12, a theoretical minimum transformer
life of 65,020 hours can be expected, or approximately 1\
years of operation.
This figure is based on the assumption
that the unit is operating continuously at either 110PC
maximum hot spot temperature for a unit with a 65°C average
winding rise or at 95°C hot spot temperature for a unit with
a 55°C average winding rise.
In practice, the typical
transformer on a utility system is not continuously loaded
at full load in a 30°C ambient.
Actual installations undergo
varying duty cycles and changing temperatures.
Hence, based
5
on operating experience, unit lives between 30-50 years are
expected.
14
CHAPTER 4
ENVIRONMENTAL FACTORS AND EFFECTS
Although the modern power transformer is very efficient
(ranging from approximately 99 to over 99.5% efficient),
there is a small percentage quantity of energy that is lost
in the transformation.
Most of the loss is in the form of
2
heat, originating either from the transformer winding's I R
losses, eddy losses, core losses, or stray losses in the
metallic structure of the unit.
This heat must be removed
or thermal runaway will occur and the unit will fail prematurely.
Three means of heat transfer exist in a transformer:
convection, conduction, and radiation.
The heat from the
core and coils is transferred to the oil principally by convection and thermosiphon circulation.
25
The heat is then
transferred to the tank and radiators by conduction.
Finally,
radiation and conduction transfer the heat to the outside
air.
Convection sets the air molecules in motion, creating
air currents which bring in new air molecules to the radiating
surface.
Of the three modes that account for heat dissipation
in a transformer, convection is by far the most important accounting for approximately 75% of the heat dissipation.
The heat transfer can be significantly improved by forced
15
13
movement of air, as would be accomplished by wind or with
fans.
The forced air movement breaks up and decreases the
air film next to the radiating surface.
This decreases the
thickness through which heat must flow and thus allows for
quicker removal of heat.
Hence, one can see that the ultimate capability of the
transformer is not only dependent on the designed parameters
of the core and coils, but also on the external environmental
factors.
The factors that were considered in this study
included ambient air temperature, wind, and solar radiation.
The scope of the study was limited to outdoor installations
only.
Wind
Wind is defined as moving air.
The velocity and duration
of wind can determine the heat loss rate from a piece of
electrical equipment.
However, since wind is difficult to
predict, variable, omnidirectional, and nonuniformly distributed in height,
its use in the computation of overloads
for general study purposes is not easy to incorporate.
In
actual installations, studies would be required to accurately
profile wind characteristics at a particular location.
16
The
resulting wind rises would indicate not only speed, but
direction and probability of occurrence.
Monitoring facilities
would also be required to give real time indication of wind
conditions at the time that the piece of equipment would be
loaded above nameplate.
Two points need to be considered when analyzing the
value of considering the effects of wind on a transformer's
ratings.
1.
The vast majority of large MVA transformers are cooled
using heat exchangers and fans.
These fans force air
directly through the cooling coils of the heat exchanger.
The effect of any additional movement of air that could
be contributed by the wind would probably be negligible.
2.
For small transformers that are not cooled with heat
exchangers, i.e. such as OA, OA/FA, or OA/FOA types,
the cooling surfaces are radiator banks.
Substation
facilities that contain these smaller transformers tend'
to be straightforward installations that are located to
serve a relatively small area.
Consequently, the
number of such facilities is quite large.
An intelli-
gence hookup with some central operator's office usually
17
does not exist.
The costs involved with the installation
of monitoring facilities that would indicate wind speed
and direction can become a significant percentage of
the total cost and would probably prove to be economically
unjustifiable when compared with the benefits derived.
However, where the loadings on a transformer are such
that real time wind monitoring equipment is desirable and
justifiable, the additional heat transferred can significantly
lower internal temperatures especially for OA units.
Heat
can be dissipated "at a rate almost linearly with the air
velocity that is moving past a radiating surface.
This
holds true for velocities up to approximately 95 kilometers
per hour.
32
A rough approximation of the dissipation magni-
tude can be made by realizing that, in ducts, 0.00155 watts/sq.
cm./0.3 km/minute/ degree centrigrade above average air
temperature can be dissipated.
Experiments
2
9
have been performed to determine the effect of
wind on the top oil temperature of a transformer.
Listed in
Table 4-1, page 19, are the results that were obtained with
a 30 MVA transformer.
18
TABLE 4-1
Wind Velocity Effects on Top Oil Temperature
Top Oil Rise in % of
Rise in Still Air
Wind Velocity
(kilometers per hour)
0.00
100
5.86
80
17.17
50
23.44
30
35.16
25
It must be noted that the effect of the wind will not
be the same for all transformers.
In addition, oil tempera-
ture decreases may not hold when the direction of the wind
changes, since most medium and large power transformers are
rectangular in shape but do not have the radiating surfaces
arranged in a symmetrical fashion around the transformer.
In light of the above, the effects of wind on a power transformer's
ratings were not incorporated into the calculations,
Solar
Of the 1,350.9 watts/square meter of solar heater
radiation on a plane surface that occurs outside the earth's
19
atmosphere, the peak amount that eventually reaches the
ground in the Pennsylvania-New Jersey-Maryland area varies
from 870 watts/square meter in July to 964.5 watts/square
meter in March.
14
The attenuation and variation is due to
absorption and scattering by water vapor, clouds, haze,
dusk, and the angle of incidents between direct rays and the
exposed surface.
On a clear day, the maximum solar intensity
that can be expected at sea level is approxmately 940 watts/
square meter.
14
The effect that such solar intensity can
have on increasing the top oil temperature of a transformer
can be calculated by the equation in Appendix C, page 150.
This temperature calculation takes into account the low
temperature total emissivities as well as the coefficient of
absorption of solar radiation.
The absorption coefficient
ranges from about 0.07 for highly polished silver to 0.75 for
gray paint and 0.97 for black matte.
The emissivity coeffi-
cient can be as low as 0.02 for highly polished silver to
0.95 for gray paint as well as lamp black.
However, it has been found that the temperature of the
oil of a transformer is practically independent of color.
Tests
14
23 2
'
have indicated that for units (5 kVA) loaded
continuously, the maximum difference in the top oil tempera-
20
ture of units painted with various colors (including white
and black) was approximately 2°C. For units with little
loading and for those with no loads, the top oil temperature
difference between the different colored units increased.
But, it was less than a 4-5°C difference.
These tests were conducted continuously during a three
day period and night cooling results were also recorded.
The top oil temperature between the various colored tanks
was much closer, with 1°C and less differences prominent.
Based on these results, the effects of solar irradiation on
transformer tanks have been neglected in the overload calculations.
Consequently, the overload calculations have been done
with ambient temperatures being the most influential environmental factor.
21
CHAPTER 5
OVERLOAD METHODOLOGIES
Although a transformer will wear out, it will wear out
from thermal deterioration and the resultant lowering of
electrical dielectric strengths and short circuit capabilities
rather than from frictional degradation of moving parts.
Since the thermal deterioration is dependent on a summation
of the various temperatures of the transformer, the ambient
temperature that is chosen becomes an important factor in
determining the allowable insulation deterioration.
Various
methods for calculating overloads have been introduced over
the years.
Each model the transformer and its environment
by including certain assumptions and simplifications so that
calculations could be done without too much difficulty.
In
1969, the Transmission and Substation Design Subcommittee of
the Pennsylvania-New Jersey-Maryland Interconnection (PJM)
published a report
entitled "Determination of Power Transformer
Ratings" and thereby setforth a standard procedure for
establishing overload thermal ratings for old and new power
transformers.
The method/procedures contained in that
report work within the framework of the ANSI and NEMA
22
standards that have been established for power transformers.
The calculations in this study used that report as the
preliminary basis for overload calculations.
The largest loading that a transformer can be expected
to carry is proportional to the hottest spot conductor
temperature that exists in the unit.
This hot spot tempera-
ture is calculated by adding the ambient temperature, top
oil rise over ambient, and the hottest spot rise over top
oil, as shown in equation 5-1 below.
6
HS
= T
a
+ 0
o
+ 9
^»-
g
where:
6WC, = Hot spot temperature (°C)
T
= Ambient temperature (°C)
6
= Top oil rise over ambient temperature (°C)
8
= Hot spot rise over top oil temperature (°C)
O
Predicting the overload capability of the transformer
for some future time period requires a need to approximate
the ambient temperature for that time period.
According to
the NEMA Standards, the average daily temperature for the
month involved should be used for loads where normal life
expectancy is expected.
Where short terra overloads are
23
5 X
" )
involved, the average of the maximum daily temperatures for
the month involved should be used.
The report recommended that, from the viewpoint of
ambient temperatures, the year be divided into two parts.
The months April through October were defined as "summer"
and, for emergency overload periods of less than 24 hours,
an ambient of 35°C was assumed.
For periods greater than 24
hours, the ambient was assumed to be 30°C.
The months of
November through March were defined as "winter", and, for
all overload calculations, 10°C was assumed.
The daily load cycle has been assumed to be two step,
with peak loading occurring for 14 hours at one per unit and
0.7 per unit off-peak loading for 10 hours.
Short time
emergencies would occur in the final hours of the peak
period so as to include all of the insulation life deterioration that occurs because of the emergency.
The NEMA Guide is the basis for the insulation life
curve.
The equation 5-2, page 25, is based on the Arrhenius
Chemical Reaction Rate Theory.
In the report, the 55°C
insulation loss of life curve was chosen to be parallel to
24
v
the 65°C curve, such that proportional life expectancies
between the two insulation types could be obtained.
% Loss of Life/Hour = lOOe
(K
" 6K + T
hs
a
+ 273)
(eqn
*
5 2)
"
where:
0,
= Hot spot rise over ambient temperature °C
T
= Ambient Temperature °C
K
= 32.543 for 55°C rise insulation
30.834 for 65°C rise insulation
These insulation life curves are shown in Figure 5-1,
page 26.
Certain limits were imposed so that additional
overloading on the transformer would be prevented.
For the
65°C rise transformer, these limits included a maximum top
oil temperature of 110°C, a maximum hot spot temperature of
180°C, for short time emergencies (less than 24 hours) a
maximum loss of life of 1.5% per day, and a maximum overloading
of 200%.
These suggested limits were based on a concensus
of the PJM member companies.
Each member utility may modify
these limits to best suit their operating conditions, experience,
and philosophy.
In the case of the Pennsylvania Power &
Light Company, Allentown, Pennsylvania, the allowable top
oil temperature was set at 10°C higher (120°C), the short
25
FIGURE 5-1
INSULATION LIFE-LOSS CURVES
1.0 ■-
RISE INSULATION
0.1 ■-
C RISE INSULATION
P<
o.oi •-
s
160
140
120
HOT SPOT TEMPERATURE
26
time emergency loss of life was increased to 5% per day, and
a maximum overload was set at 2.25 per unit.
In this thesis, the computations involved the following
scenario.
Only emergency periods were considered, and, for
evaluation and comparison purposes, these were assumed to be
2, 6, and 10 hours long.
The need to overload a transformer
was assumed to occur randomly and unexpectedly.
For compar-
ison purposes, it was assumed that in any given day throughout
the year, there was an equal probability that a condition
may exist which would require the transformer to surpass its
rated nameplate rating.
Most actual operating situations
are such that transformers experience a higher probability
of having an overload only during certain periods of the
year.
But, there are also instances, such as transfer of
power between utilities, that a need for short time overloads
may indeed occur at any time throughout the year rather than
being more prevalent during only a certain part of the year.
To simplify the calculations, every day of a given
month has been assumed to be identical.
ambient temperature models were required.
Thus, monthly .
The values for
the temperature obtained from these models have been used in
the calculations.
An overload computation was necessary for
27
each month and for each of the three emergency periods.
Data from four different transformers (shown in Appendix A,
page 148) was used in the computations.
Although the PJM
Report addressed both the 55 and 65°C rise insulations, only
the results obtained with transformers with the 65°C insulation were studied here.
New transformers with this thermally
upgraded insulation are used almost exclusively by every
utility in the United States.
Units by four different
manufacturers were chosen, with sizes ranging from 10 MVA to
300 MVA.
The limits used in the overload calculations were
the same as used by the Pennsylvania Power & Light Company,
i.e. a hot spot limit of 180°C, top oil temperature limit of
120°C, loss of life for the short time 2, 6, and 10 hour
emergencies of 5% per day, and a maximum rating of 2.25 per
unit.
The rating method developed in the PJM Report calculates
the allowable overloads by determining the winding hot spot
temperature for a given loading.
To make this calculation,
it is necessary to use two groups of data.
The first set
includes information on the transformer's thermal and physical/
electrical properties.
These include such factors as no-load
i
f
losses, load losses, weight of the core and coils, weight of
the tank and fittings, gallons of oil, average conductor
28
rise over ambient at rated load (from a factory test report),
top oil rise over ambient at rated load (again from a factory
test report), hot spot over top oil temperature values
(based on a manufacturer's design), and rated insulation
temperature rise (55°C or 65°C).■
The second set of information that is necessary concerns
the external factors that influence the environment that the
transformer operates in.
The load cycle is of most importance.
Although 24 different hourly values of loading (with peak
load defined as one per unit) can be placed into the program,
it is much more convenient to use a two step load curve.
The NEMA Standard Publication #TR98 derives the means of
converting the continuously varying daily load cycle into
the two step cycle.
Again, the peak is defined as one per
unit and the off-peak is a function thereof.
As previously
stated, the cycle that was agreed on was one per unit peak
for 14 hours and 0.7 per unit off-peak for 10 hours.
Finally, it should be noted that the computation technique
developed in the report is done in two parts.
Long time
overload calculations are done for periods of one day and
greater.
Short time emergency calculations are performed
for periods of between 1 through 24 hours.
29
For long time
ratings, the entire load cycle is overloaded with an incrementally higher per unit loading until one of the limits is
reached.
However, even at the higher loading, the relation-
ship between the peak and the off-peak values remains as in
the normal case (i.e. 1 to 0.7 per unit).
For the short time emergency calculations, the load
cycle is used as before.
However, the day that the emergency
occurs, the emergency period is automatically assigned to
the final hours of the peak period.
Thus, a computation of
the overload for a 2 hour emergency period would assume that
it had occurred starting during the 22nd hour.
It is this
two hour period that would be loaded with a higher per unit
loading until a limiting constraint is reached.
Normal load
cycles are assumed for the day before and after the day the
emergency occurs.
In this manner, the entire loss of life
attributable to the emergency is included.
Aside from a load cycle, the factor that causes significant variation in allowable overload is the ambient temperature.
A quick calculation showed that for a 10/11.2 MVA transformer,
the temperature change from -15°C to 45°C yielded overload
capability ranging from 223% to 89% respectively on a 10 MVA
basis.
As stated above, the report assumed that the ambient
30
temperature was a constant value throughout the load cycle.
If, instead of using a constant value, an ambient temperature
value that more closely approximated the actual value at the
time that the emergency occurs could possibly yield greater
allowable overloads without decreasing the expected life of
the unit.
One proposal that would get away from using only one
temperature value involves using a two step ambient temperature model.
The temperature values would be adjusted at
sunrise and sunset.
At sunrise, the projected high tempera-
ture of the day would be used as the temperature value in
the computation of the overloads for the sunrise through
sunset period.
For the remaining hours of the 24 hour day,
the temperature recorded at sunset would be used.
In this
study, this model was referred to as the Day/Night Model.
Additional development of it was done in Chapter 6, page 40.
In the development of this study, it was determined
that adequate information was available to allow the use of
the continuous temperature profiles to formulate a more
realistic model of the transformer's operation.
This model
was referred to as the Varying Ambient Model and it, also,
was developed more completely in Chapter 6, page 40.
31
The computation of the short time emergency overloads
with the ambients as described in the PJM Report (referred
to as the PJM Model), the Day/Night Model, and the Varying
Ambient Model followed the same sequence.
Using the values
of the load cycle as well as the ambient temperature, the
top oil temperature was calculated for each hour of the 24
hour period.
For the hours prior to the emergency, the
normal loading was used.
During the time of the emergency,
an assumed peak loading was used.
The top oil rise over
ambient at the end of the 24 hour period was calculated.
The assumed peak loading was increased, and an hour by hour
determination of the oil temperature was again made.
When,
at the end of the two successive 24 hour periods, the top
oil rise over ambient was within a specified tolerance, then
the calculated loss of life was checked against the allowed
limit.
Should the calculated loss exceed the limit, the
loading was decreased until the calculated value was under
the limit.
The calculated hot spot temperature was then
checked to determine if that limit had been surpassed.
If
needed, the loading was again decreased to bring it below
the limit.
The top oil temperature was finally checked and
the limiting check and adjustment was made here as well.
Once the loading was such that the above limits are not
exceeded, then the allowable loading above nameplate had
32
been determined.
The temperature determination equation
used in the calculations are described as follows.
The thermal time constant for rated load with an initial
temperature rise of 0°C is defined as T .
The time constant
is the length of time that would be required for the temperature of the oil to change from the initial value to the
ultimate value if the initial rate of change were continued
until the ultimate temperature was reached.
Since transfor-
mers not rated OA or OA/FA are designed such that the
exponential power of temperature rise versus loss equals
unity (i.e. n = 1), then 63% of the temperature change
occurs in the length of time equal to the time constant.
This occurs regardless of the initial and ultimate temperatures.
The equation for the time constant is:
T r = JP*
r
(eqn. 5-3)
fl
where:
T
= Rated time constant of the top oil rise (hours)
H
= Thermal capacity (watt hours/degree)
T
= Top oil rise over ambient at rated load (°C)
Pf, - Totaliwatts loss at rated kVA
33
The thermal capacity H for non-self cooled transformers
can be calculated by:
H = 0.06 WCC + 0.06 WT + 1.93 GO (0.06 WCC + 0.06 WT
+ 1.93 GO)
(eqn. 5-4)
where:
WCC = Weight of the core and coils (lbs)
WT
= Weight of the tank and fittings (lbs)
GO
= gallons of oil
For units that are self cooled (n = 0.8), equation
5-3, page 33, holds only for full rating starting cold.
In
these units, the time constant becomes a function of both
the initial and final temperature rises.
Hence, this time
constant can be computed using the equation 5-5, below.
ou T
0
T = T
"e ou
T 0_
1
n
01
To
e Ol.'
1
n
(eqn. 5-5)
_T o
where:
ou
Ultimate top oil rise over ambient temperature (°C)
0 01. = Initial top oil rise over ambient temperature (°C)
T
= Top oil rise over ambient at rated load (°C)
n
= Exponential power of temperature rise versus loss
0.8 for self cooled transformers
1.0 for FOA, 0A/F0A, and 0A/FA/FA transformers
34
The thermal capacity H is calculated by changing the
coefficients of the tank weight and gallons of oil from 0.06
to 0.04 and 1.93 to 1.33, respectively, in equation 5-4,
page 34.
A resistance correction factor C
for any average
winding temperature is:
_ 234.5 + AWT
r " 234.5 + D
,
r
C
_ ,,
(eqn
-
5_6)
where:
D
= Design average winding temperature of 75°C for
55°C transformers and 85°C for 65°C units (°C).
AWT = Average Winding Temperature (°C).
The resistance correction factor compensates for the
variation in load loss as the winding temperature changes.
Therefore, the resistance correction factor, C, modified to
take into effect of winding and stray resistance, is as
i
shown below.
CL
C = CJ
r
££
LL
+
X
I
C
where:
J2L = Rated copper loss (watts)
LL = Rated load loss (watts)
35
LL
(eqn. 5-7)
The steady state top oil rise for constant load can be
calculated using equation 5-8, below.
8
ou = To NfTT1] "
<«*•• 5"«
where:
8
ou
= Ultimate top oil rise over ambient temperature
(oc)
T
= Top oil rise over ambient at rated load, as
determined by factory tests (°C)
C
= Resistance correction factor for ultimate
winding temperature
K
= Per unit load
R
= Ratio of load to no-load losses at rated load
on the temperature test tap.
An hour by hour tabulation of the transient top oil
rise is next made using equation 5-9, below.
6
O
= (6
OU
- 6 .)(1 - e"
Ol
/T
) + 6 .
01
(eqn. 5-9)
^
where:
9
= Top oil rise over ambient temperature at the
end of time t
T
= Thermal time constant for the temperature rise
interval 6 . to 6
oi
ou
36
These calculations are done for whole hour periods,
with the previous hour's results impacting the results of
the next hour's calculations.
Hour-by-hour computations are then made of the average
winding temperature.
The average winding temperature rise
over ambient is first calculated, as shown below.
6
av=0o+ (Tav - To)(CK2)n
(eon. 5-10)
where:
0
av
T
= Average winding temperature rise over ambient
for a load K (°C)
= Average winding temperature rise over ambient,
as determined by factory test at rated load
(°C)
The average winding temperature can then be determined
by adding the above result to the ambient temperature, T
as
shown in equation 5-11, below:
AWT = T
a
+ 6
av
(eqn. 5-11)
^
Using this calculated value for the average winding
temperature, the resistance correction factor can now be
37
recalculated and this new value used to correct the top oil
rises and the winding temperature rise.
With these recalculated values, the transient hot spot
gradient'can be calculated.
This rise, 0,
, is the hot spot
rise over ambient temperature for an amount of time (t) for
a load K, and is the sum of the top oil rise and the hot
spot rise over top oil per equation 5-12, below.
8 hs' = 6 o +6 g
v(eqn.
^
5-12)'
where:
0,
= Hot spot rise over ambient temperature (°C)
0
= T
8
T
8
(CK2)n
8
= Hot spot rise over top oil temperature at
per unit load K (°C)
= Hot spot rise over top oil temperature at
rated load on the test tap, as per the design
of the transformer (°C)
Finally, the hottest spot temperature, 0trq> °f the
system can now be computed by equation 5-13, below.
6
HS
38
= 6
hs
+ T
a
(e< n
* '
5 13)
"
The loss of life of the insulation for the computed hot
spot can be determined using equation 5-2, page 25.
With
all 3 temperature-related limiting factors now available, a
check is made to determine if the limits have been exceeded
on the top oil temperature, hot spot temperature, and the
loss of life.
If, for the given loading, none of the limits
are above their limit, then the per unit loading is increased
and the above computations are redone.
When the per unit loading is such that an additional
incremental increase will produce a violation in a limiting
constraint, then the proper loading has been attained.
This
per unit loading is then translated into a MVA reading and a
percentage reading.
39
CHAPTER 6
AMBIENT TEMPERATURES AND LOAD PROFILES
The Development of an Ambient Temperature Model
To determine the various ambient temperatures for use
in subsequent calculations, it was necessary to develop a
representation for the temperature variation throughout a
given day.
The heating of the air during the daytime will
be directly dependent upon the rate that the sun can warm
the earth's surface and, by convection, heat the surroundings.
This can be represented by a sinusoidal function.
Nighttime
cooling is exponential, but can adequately be represented as
a sinusoidal function as well.
9
A harmonic function that represents the changing temperatures throughout a day in a given month can be expressed by
40
an equation of the following form:
AMB(t) = M + D sin
|^ (t - (P - 6))
(eqn. 6-1)
where:
AMB(t) = Ambient temperature during any given
time t (°C)
M
= Mean temperature (°C)
T
= Period duration, expressed as 24:00 hours
D
= Temperature deviation from the mean (°C)
t
= Time of day (hour on the basis of 00:00
to 24:00)
P
= Time that peak temperature occurs (hour
expressed on the basis of 00:00 to 24:00)
In this study, it was assumed that all days in a given
month are identical.
To make the overload computations, the
idealized ambient temperature values was derived using
equation 6-1, above, and the data on ambient temperature
averages.
These averages are obtained from the US Department
of Commerce National Weather Service's tabulation of averages
for a 30 year period.
various locations.
These averages are tabulated for
In particular, the data from the Allentown-
Bethlehem-Easton Airport location was used in the study.
Appendix B, page 149, average maximums, minimums, and mean
temperatures for each month are shown.
41
In
Studies
have indicated that the peak in daily ambient
temperature occurs about one hour after the peak in the
intensity of solar radiation.
Since solar radiation peaks
occur around noon, the high ambient temperature of the day
can be expected to occur about 1 PM.
Substituting P equals
13:00 hours (1 PM on a 24 hour clock basis) in equation 6-1
results in a continuous profile of ambient temperatures for
a typical day of the month in question.
displayed in Figure 6-1, page 43.
This is graphically
To determine the ambient
temperature data required to make the Day/Night calculations,
the average time that sunset and sunrise occurs for a given
month was required.
These times were obtained from an
almanac, and the respective temperatures profiles were
determined using equation 6-1, page 41.
Shown in Appendix
B, page 149,are the average peak and sunset temperatures, as
well as the times chosen for sunrise and sunset, that were
used for subsequent overload calculations.
For the calculations with the Varying Ambient method,
the temperature values were obtained directly from equation
6-1, page 41, on an hour-by-hour basis.
42
o
O
W
^-1
VO
O
H
a
n
w
Q
M
43
Correlating Load Curves With Changing Ambient Temperature:
The typical daily load cycle on a transformer is usually
a curve that is continuously oscillating; varying from a
minimum point to a maximum point back to a minimum point
within a 24 hour period.
As detailed in the NEMA Standards
such a curve can be reduced to a two-step rectangular function.
This function would consist of a peak and an off-peak period.
The values for the periods used here were the same as used
in the PJM report "Determination of Power Transformer Rating",
i.e. 14 hours at 1 pu peak and 10 hours at 0.7 pu off-peak.
The load cycle was assumed to remain constant each day
throughout t!he year.
For the Day/Night methodology, the
peak period was arbitrarily chosen to start at 5 a.m. and
continue for 14 hours through 7 p.m.
The sunrise and sunset
times for a given month, as previously determined, were then
located relative to the load cycle.
Peak temperatures were
allocated to the daytime hours between sunrise and sunset,
and sunset temperatures were allocated to the remaining
hours.
The emergency periods were assumed to occur during
the final hours of peak period.
Thus, the two hour emergency
was assumed to begin at 5 p.m. and continue until 7 p.m.
For the Varying Ambient calculations, the correlation
of the ambient temperature model with the daily load cycle
44
model required additional areas of consideration.
shown
20
It can be
that the change in the top oil temperature will lag
a given ambient temperature change.
In several hours, the
oil temperature's change will asymptotically approach the
ambient temperature change.
Since both the ambient and oil
temperatures determine the final internal temperatures of
the transformer and, consequently, the total permissible
overload, there was a need-to determine at what point the
peak ambient temperature and the load cycle produced the
most conservative overloads.
Calculations were made and the
resulting sensitivity models indicated that placing the peak
ambient temperature in the second last hour of the peak
period resulted in the lowest permissible overloads.
Again, it was arbitrarily assumed that the peak period
would begin at 5 a.m. and continue for 14 hours until 7 p.m.
Since the ambient temperature peak must occur in the second
last hour of the peak cycle, the overload calculations with
the Varying Ambient models positioned the peak daily temperature at 6 PM.
The hourly values of the ambient temperature
for each month were determined using equation 6-1, page 41,
and P equal to 18:00.
These values were then used in the
calculations to determine final allowable overloads.
45
CHAPTER 7
RESULTS AND DISCUSSIONS
In this chapter the results obtained with the methodologies previously described are analyzed and compared with
each other.
Because of the large amount of data that was
obtained from these calculations, it was necessary to organize
it in the form of tables and charts to better convey the
various trends of the limits and overloads that will be
considered.
As previously mentioned, each day of the month was
assumed to have an identical temperature and temperature
variation.
These variations were as defined by equation
6-1, page 41, and the temperature values were as listed in
Appendix B, page 149.
In addition, the load cycle configur-
ation was assumed to be rectangular and identical each day
throughout the year.
To effectively compare the three methods, the results
with the Day/Night and the Varying Ambient methods were
compared with the PJM method.
To extend the scope of this
comparison, a selection of new vintage medium and large
46
power transformers, ranging in size from 10/11.2 MVA to
300/336 MVA (on a 55/65°C rating basis) was chosen, as
stated in the introduction.
A hypothetical emergency scenario
was devised so that overloads can be calculated.
It was
assumed that, for example, one two-hour emergency would
occuri.in each and every month for a year's time period.
The
permissible overloads that resulted with the Day/Night and
the Varying Ambient methods were tabulated and compared with
those obtained with the PJM method.
Such calculations were
repeated for the 6 and 10 hour emergency periods.
If, for
example, the permissible overload with the Day/Night method
was 5% over that allowed with the PJM method, then a +5 is
assigned for that month.
A negative value, on the other
hand, indicates that a permissible overload using the new
method was less than what was obtained with present methods.
These values are shown after the slash (/) in Tables 7-1
through 7-6, pages 59 through 64.
The positive and negative
values for each month were then summed for the entire year,
and the resulting net positive/negative value was part of
the overall analysis and comparison.
Using these net values,
one can quickly determine which method was allowing a greater
net overload and for what emergency overload periods.
The
method that allows for less emergency overloading than that
obtained by the present means was also quickly pointed out.
47
The per cent overloads over nameplate were also shown
in the tables mentioned above.
preceding the slash.
These values were shown
These values were plotted and shown in
Figures 7-1 through 7-12, pages 66 through 77.
Each chart
represents one emergency period (either 2, 6, or 10 hour)
for each transformer.
Hence, one can get a quick represen-
tation of the magnitude and time of the deviation of the
overloads with the new methods as compared with those obtained
with the existing method.
One can also get an idea of how
the two methods compared with one another.
Finally, since most of the time the two limiting factors
that prevent additional transformer overloading were the oil
temperature and the hot spot temperature, a group of tables
were made to show these two temperatures as a function of
the different months of the year.
Again, only one transformer
size and one overload period was listed per table and the
temperatures from all three methods were listed in that
table.
An asterisk (*) before the temperature indicates
that the allowable limit has been reached and further overloading was prevented.
48
The following summarizes the grouping of these tables
and charts:
A)
Tables 7-1 through 7-7 inclusive (Pages 59 through
65) list the allowable overloads as determined by
the various methods.
B)
Figures 7-1 through 7-12 inclusive (Pages 66
through 77) graphically display the overloads from
the various methods for a given time per transformer.
C)
Tables 7-8 through 7-19 (Pages 78 through 89)
compare the limiting criteria preventing additional
overloads for the different methods.
The discussion of the results was broken down into two
parts.
The first covered the results from the time (2, 6,
or 10 hour) standpoint, while the second part reviewed the
results from an individual transformer basis.
First, the
analysis was made on the short time emergencies.
The Two Hour Emergency Case:
In this short time emergency case, the overload
calculations with the Day/Night method exceeded the
49
values calculated with the present method, with a
difference ranging from 11-24%.
The largest gains
occurred in the month of April, September and October.
Smaller but still pronounced gains occurred in February
and March.
Less overload than allowed with the present
method was found during the summer months of June, July
and August.
In most cases, the factor that limited additional
loading throughout the year was the hot spot temperature.
The exception was the small 10 MVA transformer for
which the oil temperature during the summer months
reached its allowable limit.
With the Varying Ambient method, the allowable
overloads that resulted usually proved to be less than
what would be allowed with the present methods.
Overall,
one would, for the year, have additional (depending on
the MVA size) capability of 27-58% over that allowed
with the present method.
The only months in which the
Varying Ambient method allowed more loading were April
and October.
The hot spot temperature prevented additional
loading throughout the year for the large MVA transfor50
mers and, during the spring, fall and winter months,
for the medium size transformers.
Oil temperature
limits were reached during the summer months thereby
preventing additional loading on the latter units.
The Six Hour Emergency Case:
In this situation, the Day/Night method yielded
results which, on the balance, proved to be out performed
by the present method by as much as 15%.
The majority
of the loss occured during the winter and summer months.
Very similar results were noted in May and September,
but, during April and October, the Day/Night method
allowed greater overloads.
The limiting factor for all except the small 10
MVA transformer was the 180°C hot spot temperature.
For the 10 MVA transformer, additional overloading was
stopped because the oil temperature limit was reached.
Investigating the 6 hour emergency case with the
Varying Ambient method showed the results to be very
similar to the 2 hour case!
With the present method,
the allowable overloads exceeded by anywhere from
51
23.7% to 38% what was calculated with the Varying
Ambient method.
Except for April, May and October, the
allowable loading above nameplate was less throughout
the remainder of the year.
Again, the hot spot temper-
ature limited loadings for the two larger transformers,
t
while a combination of oil/hot spot and just oil temperature limits were noted for the two smaller units.
The Ten Hour Emergency Case:
The 10 hour case gave the most pessimistic results
for the Day/Night method.
The present method allowed a
transformer to exceed nameplate ratings by 10-38% for
the 12 month period.
As with the above cases, the
results with the Day/Night method exceeded the present
method only in April and October while in the remaining
months of the year, the results were noticeably the
opposite.
The limiting constraints for this case varied
widely.
For the small 10 MVA unit, oil temperature
limited overloads during all of the months.
A combin-
ation of oil temperature (for the summer months) and
hot spot temperatures (for the remainder of the year)
52
prevented additional loading on the 15/20/25 MVA unit.
On the large 150 MVA unit, neither the oil nor hot spot
temperatures were the limiting constraint.
Rather, the
5% loss of life limit prevented more loading.
For the
300 MVA unit, overloads during all of the months were
limited by hot spot temperature.
Results similar to the above were obtained with
the Varying Ambient temperature model.
For the 12
month period, the present method allowed up to 33% more
loading above nameplate than what the calculations with
the Varying Ambient method allowed.
The April, May and
October months were again the only months that the
present method did not allow more MVA output than what
was shown with the Varying Ambient method.
The limiting constraints for this method for all
except the 150 MVA transformer proved to be the same as
with the Day/Night method.
For the 150 MVA unit, the
hot spot temperature instead of the loss of life limit
was reached.
It should be pointed out that the values mentioned
above are not that extremely out of line.
53
A 12% difference
in favor of one method for a 12 month period yields an
average difference of 1% per month in that method's favor.
Although the magnitude of that percentage is important, it
is felt that it is of greater value to determine which
method is actually working out for the better during different
times of the year.
By analyzing the results on an individual transformer
size basis, one can determine how the limiting constraints
changed with the different methods as well as with the
length of time of the overload, as represented by the emergency period.
For the 10 MVA unit, all three methods showed, from
winter to summer, increasingly higher oil temperatures.
The
oil temperature activated the overload cutoff in the summer
for the short 2 hour emergencies.
But, in the longer 6 and
10 hour emergency periods, the oil temperature limit was
reached for the Day/Night and the Varying Ambient methods
throughout the year.
With the PJM method, the limiting
constraints changed from oil temperature in the summer to
the hot spot limit for the winter.
It should be noted that
the temperatures obtained for both oil and hot spot at the
allowable overload were very close with all three methods.
54
Many times, there was a difference of less than 5°C in any
given month when the oil or hot spot temperatures were
compared between methods.
In the case of the 15/20/25 MVA unit, the hot spot
temperature remained very close to the 180°C limit.
In most
instances, it was this temperature that limited additional
overloads.
The oil temperature remained relatively cool in
the winter, but got increasingly warmer in the summer.
Occasionally, it got hot enough in the summer to reach its
limit, thereby cutting off additional overloading.
Again,
the difference in temperatures obtained for the oil and hot
spot with these three methods, for any given month, was very
close.
Whereas the two smaller transformers above have both
the oil and hot spot temperature limits reached, the limiting
constraint for the two larger units, except' for two instances,
was the hot spot limit only.
The oil temperature, although
varying between the cooler and warmer ambients, remained
relatively cool as compared with the smaller units above.
As mentioned previously, in two time periods of the
year, the overload capability, as calculated with the PJM
55
method, gave results that were less than what was calculated
with the other two methods.
and September/October.
These two periods were April/May
All of these months lie within the
"summer" range as defined in the PJM methodology.
When looking at the value of the oil temperature as
computed in the PJM method and comparing it with that obtained
from the other two methods, acceptable differences were
shown for the months of May, September and October.
the values obtained for April are noteworthy.
However,
The oil
temperature for April in the PJM method was closer to that
obtained with the Day/Night and the Varying Ambient method
for the warmer month of May.
This implies that the assumed
ambient of 29°C used in the PJM program may be unduly high,
and therefore unnecessarily overstate the temperature increases
inside the transformer.
However in most instances, to include April as a "winter"
month and use the 4°C ambient may prove to be too liberal.
Since the average temperature for April is approximately
10 °C (with a deviation of +/-11°C), consideration should be
given to making a separate overload computation for that
month with an assumed ambient in the 10-20°C range.
56
The
resultant greater transformer overloads will then decrease
the large differential that was seen when results were
compared between the three methods.
It was of interest to note how the results vary as the
length of time of the emergency changes.
For all four
transformers, the calculations with the Day/Night method
yielded overloads that were very favorable, as compared with
the PJM method, for the short time periods, i.e., two hours.
As the time periods began to get longer, the overloads went
in favor of the PJM method and became increasingly so with
longer times.
This was irrespective of the transformer MVA
size:
On the other hand, a different trend was detected with
the Varying Ambient method.
For short time periods, the PJM
method was very much ahead in total overload, as per the
scenario.
However, as the length of time of the emergency
continued, the advantage began to decrease.
Although the
advantage never switched to the Varying Ambient method (at
least as far as these calculations could determine), the
fact that this trend existed is more significant.
First it
should be recalled that, in developing the Varying Ambient
method, it was decided to position the peak temperature at a
57
location opposite the load cycle that would produce the most
conservative results, i.e. in the 23rd hour of the peak of
the load cycle.
The two hour emergency would begin at
exactly that instant.
However, with increasing longer
emergency periods, the start of these periods, while still
occurring during the peak portion of the load cycle, was
also occurring at a time period where the ambient temperature
was significantly lower than the peak temperature.
Hence
the transformer began to bear the emergency overload during
a period when the ambient was still cool, as compared to the
peak temperature.
With the Day/Night method, the peak temperature was
either in existance or proceeded all overloads.
With longer
emergency periods, the effect of the higher projected peak
ambient temperature became increasingly more important.
Consequently, the allowable overload decreased.
seen for all transformer sizes in the study.
58
This was
TABLE 7-1
PER CENT OVERLOADS USING THE DAY/NIGHT MODEL
FOR 2 HOUR EMERGENCY PERIOD
Size of Transformer
10 MVA
Month
15/20/25 MVA
150 MVA
300 MVA
January
197/0
219/1
167/0
178/0
February
200/3
221/3
169/2
180/2
March
195/-2
220/2
167/0
178/0
April
195/15
217/13
164/9
176/11
May
181/1
205/1
156/1
167/1
June
176/-4
202/-2
154/-1
164/-2
July
172/-8
200/-4
152/-3
163/-3
August
174/-6
201/-3
153/-2
164/-2
September
187/7
212/8
160/5
172/6"
October
190/10
213/9
162/7
173/7
November
193/-4
215/-3
164/-3
175/-3
December
196/-1
217/-1
166/-1
177/-1
Net overload gain (+)/loss (-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
+11%
+24%
+13%
+16%
The net overload gain or loss was determined by summing the
monthly per cent differentials when results from the above
method are compared to those of the PJM method. This
difference is shown after the slash (/) above.
59
TABLE 7-2
PER CENT OVERLOADS USING THE DAY/NIGHT MODEL
FOR 6 HOUR EMERGENCY PERIOD
Month
10 MVA
Size of Transformer
150 MVA
15/20/25 MVA
300 MVA
January
182/-2
201/0
159/-2
166/0
February
182/-2
202/1
159/-2
167/1
March
175/-9
197/-4
156/-5
162/-4
April
169/10
193/7
153/5
159/5
May
161/2
187/1
149/1
155/1
June
156/-3
184/-2
146.1/-1.9
152/-2
July
153/-6
182/-4
145/-3
151/-3
August
154/-5
183/-3
146/-2
151/-3
September
160/1
188/2
148.2/0.2
155/1
October
170/11
194/8
153.4/5.4
160/6
November
175/-9
197/-4
156/-5
163/-3
December
181/-3
200/-1
158.2/-2.8
165/-1
Net overload gain (+)/loss (-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
-15%
+1% .
-12.1%
-2%
60
TABLE 7-3
PER CENT OVERLOADS USING THE DAY/NIGHT MODEL
FOR 10 HOUR EMERGENCY PERIOD
Size of Transformer
10 MVA
Month
15/20/25 MVA
150 MVA
300 MVA
January
176/-5
193/-2
155.8/-2.2
161/-2
February
176/-5
193/-2
155.8/-2.2
161/-2
March
171/-10
189/-6
153.2/-4.8
157.2/-5.8
April
165/8
185/6
150.3/5.3
154.2/4.2
May
159/2
180/1
146.8/1.8
151/1
June
153/-4
176/-3
144/-1
148/-2
July
151/-6
173/-6
142.5/-2.5
147/-3
August
152/-5
175/-4
143.4/-1.6
147.1/-2.9
September
156/-1
180/1
145.8/0.8
150/0
October
163/6
185/6
149.4/4.4
154/4
November
169/-12
189/-6
152.2/-5.8
157/-6
December
175/-6
192/-3
155/-3
160/-3
Net overload gain (+)/loss (-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
+38%
-18%
-10.8%
-17.5%
61
TABLE 7-4
PER CENT OVERLOADS USING THE VARYING AMBIENT MODEL
FOR 2 HOUR EMERGENCY PERIOD
10 MVA
Month
Size of Transformer
150 MVA
15/20/25 MVA
300 MVA
January
191/-6
212/-6
164/-3
173/-5
February
191/-6
212/-6
163/-4
173/-5
March
188/-9
210/-8
161/-6
171/-7
April
185/5
207/3
158.1/3.1
169/3
May
180/0
203/-1
156/1
166/0
June
174/-6
200/-4
153/-2
163/-3
July
170/-10
199/-5
152/-3
162/-4
August
172/-8
200/-4
152.1/-2.9
163/-3
September
176/-4
202/-2
154/-1
165/-1
October
183/3
205/1
157/2
168/2
November
187/-10
208/-10
160/-7
170/-8
December
190/-7
211/-7
163/-4
173/-5
Net overload gain (+)/loss (-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
-58%
-49%
-26.8%
-36%
62
TABLE 7-5
PER CENT OVERLOADS USING THE VARYING AMBIENT MODEL
FOR 6 HOUR EMERGENCY PERIOD
10 MVA
Month
Size of Transformer
150 MVA
15/20/25 MVA
300 MVA
January
178/-6
197/-4
157.1/-3.9
163/-3
February
178/-6
196/-5
157/-4
163/-3
March
173/-11
194/-7
155/-6
160.1/5.9
. 168/9
190/4
152/4
158/4
May
162/3
187/1
149/1
155/1
June
156/-3
183/-3
146.2/-1.8
152/-2
July
153/-6
182/-4
145/-3
150/-4
August
155/-4
183/-3
146/-2
151/-3
185/-1
148/0
153/-1
April
September
,159/0
i
October
165/6
189/3
151/3
156.1/2.1
November ,
177/-13
192/-9
154/-7
159/-7
December
177/-7
196/-5
157/-4
162/-4
Net overload gain (+)/loss (-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
-38%
-34%
-23.7%
-25.8%
63
TABLE 7-6
PER CENT OVERLOADS USING THE VARYING AMBIENT MODEL
FOR 10 HOUR EMERGENCY PERIOD
Month
10 MVA
Size of Transformer
15/20/25 MVA
150 MVA
300 MVA
January
176/-5
191/-4
156/-2
160/-3
February
176/-5
191/-4
156/-2
159/-4
March
171/-10
188/-7
154/-4
. 157/-6
April
166/9
184/5
151/6
154/4
May
160/3
181/2
148/3
151/1
June
154/-3
177/-2
145/0
July
151/-6
174/-5
144/-1
147/-3
August
153/-4
176/-3
145/0
148/-2
September
157/0
179/0
147/2
150/0
October ,
163/6
183/4
150/5
153/3
November
169/-12
186/-9
152.3/-5.7
156/-7
December
175/-6
190/-5
155.2/-2.8
159/-4
' 148.1/-1.9
Net Overload gain (+)/loss (-) for. the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
-33%
-28%
-1.5%
-22.9%
64
TABLE 7-7
PER CENT OVERLOADS USING THE PJM METHOD
Size of Transformer:
10 MVA
Emergency Period
Summer
Winter
2 hours
6 hours
10 hours
180
159
157
197
184
181
Size of Transformer:
15/20/25 MVA
Emergency Period
Summer
Winter
2 hours
6 hours
10 hours
204
186
179
218
201
195
Size of Transformer:
150 MVA
Emergency Period
Summer
Winter
2 hours
6 hours
10 hours
155
148
145
167
161
158
Size of Transformer:
300 MVA
Emergency Period
Summer
Winter
2 hours
6 hours
10 hours
166
154
150
178
166
163
65
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TABLE 7-8
TABULATION OF TEMPERATURES FOR 10 MVA TRANSFORMER?
2 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Month
Day/Night
Hot Spot
Oil
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
PJM
Hot Spot
°C
Jan
117.0
*178.5
119.7
*178.3
117.1
*178.6
Feb
116.4
*179.5
120.2
*178.8
117.1
*178.6
Mar
117.2
*178.7
121.3
*178.5
117.1
*178.6
Apr
118.9
*179.5
123.6
*179.4
*124.7
178.1
May
123.9
*177.8
*124.4
177.8
*124.7
178.1
Jun
*124.4
175.9
*124.3
174.9
*124.7
178.1
Jul
*123.8
173.4
*123.8
172.5
*124.7
178.1
Aug
*123.9
174.5
*123.9
173.5
*124.7
178.1
Sep
121.5
*178.2
*123.8
175.3
*124.7
178.1
Oct
120.1
*178.3
123.9
*178.7
*124.7
178.1
Nov
119.0
*178.6
122.6
*179.3
117.1
*178.6
Dec
118.0
*179.0
120.2
*178.4
117.1
*178.6
indicates Limiting Constraint
78
TABLE 7-9
TABULATION OF TEMPERATURES FOR 10 MVA TRANSFORMER
6 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Month
Day/Night
Oil
Hot Spol
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
PJM
Hot Spot
°C
Jan
*123.9
178.2
*124.1
176.5
123.4
*178.7
Feb
*124.5
178.9
*124.8
177.2
123.4
*178.7
Mar
*124.6
175.6
*124.0
174.0
123.4
*178.7.
Apr
*124.4
172.6
*124.8
172.6
*124.1
167.1
May
*123.9
168.5
*124.6
169.7
*124.1
167.1
Jun
*i*24.5
166.9
*124.2
168.6
*124.1
167.1
Jul
*124.4
165.5
*124.3
165.6
*124.1
167.1
Aug
*123.8
165.3
*124.5
166.5
*124.1
167.1
Sep
*124.2
168.4
*124.6
168.3
*124.1
167.1
Oct
*124.8
173.5
*124.0
170.4
*124.1
167.1
Nov
*124.2
175.2
*124.5
173.6
123.4
*178.7
Dec
*124.5
178.3
*124.7
176.6
123.4
*178.7
^Indicates Limiting Constraint
79
TABLE 7-10
TABULATION OF TEMPERATURES FOR 10 MVA TRANSFORMER
10 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Month
Day/Night
Oil
Hot Spoi
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
PJM
Hot Spot
°C
Jan
*124.0
175.4
*124.1
175.6
123.0
*177.0
Feb
*124.7
176.2
*124.8
176.3
123.0
*177.0
Mar.
*124.3
173.5
*124.0
173.1
123.0
*177.0
Apr
*124.1
170.4
*124.9
171.7
*124.4
167.3
May
*124.3
168.0
*124.7
168.8
*124.4
167.3
Jun
*123.8
164.8
*124.2
165.8
*124.4
167.3
Jul
*124.8
165.0
*124.3
164.5
*124.4
167.3
Aug
*124.1
164.7
*124.6
165.7
*124.4
167.3
Sep
*123.9
166.2
*124.6
167.5
*124.4
167.3
Oct
*123.9
169.4
*124.1
169.6
*124.4
167.3
Nov
*124.3
172.5
*124.5
172.7
123.0
*177.0
Dec
*124.7
175.7
*124.7
175.7
123.0
*177.0
^Indicates Limiting Constraint
80
TABLE 7-11
TABULATION OF TEMPERATURES FOR 15/20/25 MVA TRANSFORMER
2 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Month
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
PJM
Oil
°C
Hot Spot
°C
Jan
96.5
*179.2
119.7
*178.3
117.1
*178.6
Feb
94.5
*178.6
120.2
*178.8
117.1
*178.6
Mar
95.9
*179.3
121.3
*178.5
117.1
*178.6
Apr
97.4
*178.5
123.6
*179.4
*124.7
178.1
May
106.3
*178.9
*124.4
177.8
*124.7
178.1
Jun
108.3
/*178.8
*124.3
174.9
*124.7
178.1
Jul
109.1
*178.3
*123.8
172.5
*124.7
178.1
Aug
108.6
*178.4
*123.9
173.5
*124.7
178.1
Sep
101.8
*179.3
*123.8
175.3
*124.7
178.1
Oct
100.2
*178.3
123.9
*178.7
*124.7
178.1
Nov
98.6
*178.3
122.6
*179.3
117.1
*178.6
Dec
97.1
*178.3
120.2
*178.4
117.1
*178.6
'Indicates Limiting Constraint
81
TABLE 7-12
TABULATION OF TEMPERATURES FOR 15/20/25 MVA TRANSFORMER
6 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Month
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying; Ambient
Oil
Hot Spot
°C
°C
Oil
°C
PJM
Hot Spot
°C
Jan
109.8
*179.6
112.1
*179.2
109.1
*178.9
Feb
108.0
*178.5
111.9
*178.3
109.1
*178.9
Mar
112.3
*179.4
114.1
*179.2
109.1
*178.9
Apr
114.2
*178.6
116.0
*178.4
119.3
*179.3
May
118.4
*179.0
118.6
*179.1
119.3
*179.3
Jun
120.6
*179.3
120.2
*178.2
119.3
*179.3
Jul
121.5
*178.9
121.9
*179.3
119.3
*179.3
Aug
120.9
*179.0
121.2
*179.3
119.3
*179.3
Sep
117.6
*178.7
119.5
*178.8
119.3
*179.3
Oct
113.7
*178.7
117.2
*179.0
119.3
*179.3
Nov
112.2
*179.3
114.7
*178.5
109.1
*178.9
Dec
110.8
*180.0
112.7
*179.1
109.1
*178.9
^Indicates Limiting Constraint
82
TABLE 7-13
TABULATION OF TEMPERATURES FOR 15/20/25 MVA TRANSFORMER
10 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Jan
113.9
*178.3
115.7
*178.8
113.6
*179.3
Feb
114.5
*179.0
116,3
*179.5
113.6
*179.3
Mar
117.5
*179.4
117.8
*179.0
113.6
*179.3
Apr
119.4
*178.7
119.7
*178.4
123.5
*179.1
May
122.4
*178.6
122.6
*179.4
123.5
*179.1
Jun
*123.9
177.6
124.1
*178.5
123.5
*179.1
Jul
*123.8
175.7
*124.2
176.7
123.5
*179.1
Aug
*124.2
177.2
*124.4
178.1
123.5
*179.1
Sep
123.1
*179.2
123.5
*179.1
123.5
*179.1
Oct
119.2
*178.3
121.1
*179.1
123.5
*179.1
Nov
117.4
*17?.2
118.3
*178.2
113.6
*179.3
Dec
114.5
*178.3
116.2
*178.5
113.6
*179.3
Month
PJM
Oil
°C
Hot Spot
°C
c
^Indicates Limiting Constraint
83
TABLE 7-14
TABULATION OF TEMPERATURES FOR 150 MVA TRANSFORMER
2 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Month
Day/:Night
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
PJM
Hot Spot
°C
Oil
°C
Jan
77.5
*178.5
82.3
*180.0
77.3
*178.2
Feb
76.1
*179.5
82.1
*178.5
77.3
*178.2
Mar
77.9
*179.0
84.4
*178.6
77.3
*178.2
Apr
80.9
*178.5
87.4
*178.3
91.0
*178.6
May
90.0
*178.6
90.8
*179.6
91.0
*178.6
Jun
93.1
*179.7
93.4
*178.8
91.0
*178.6
Jul
94.2
*178.5
95.2
*179.6
91.0
*178.6
Aug
93.5
*179.0
93.9
*178.4
91.0
*178.6
Sep
86.0
*179.1
92.0
*178.4
91.0
*178.6
Oct
83.4
*178.7
88.6
*178.3
91.0
*178.6
Nov
81.0
*178.6
85.9
*179.0
77.3
*178.2
Dec
78.7
*178.5
83.0
*179.5
77.3
*178.2
^Indicates Limiting Constraint
84
TABLE 7-15
TABULATION OF TEMPERATURES FOR 150 MVA TRANSFORMER
6 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Month
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying; Ambient
Oil
Hot Spot
°C
°C
PJM
Oil
°C
Hot Spot
°C
Jan
86.5
*178.4
88.4
*178.2
85.5
*179.7
Feb
87.2
*179.2
89.8
*178.7
85.5
*179.7
Mar
91.1
*179.8
91.5
*179.0
85.5
*179.7
Apr
94.0
*179.5
94.4
*178.7
98.8
*178.9
May
97.7
*178.8
97.3
*178.4
98.8
*178.9
Jun
100.2
*178.3
100.0
*178.3
98.8
*178.9
Jul
102.0
*179.1
101.9
*178.9
98.8
*178.9
Aug
101.4
*179.5
101.2
*179.2
98.8
*178.9
Sep
98.1
*178.3
99.3
*179.4
98.8
*178.9
Oct
93.8
*179.2
95.7
*179.0
98.8
*178.9
Nov
91.1
*179.8
93.0
*179.5
85.5
*179.7
Dec
87.4
*178.4
90.0
*179.8
85.5
*179.7
'Indicates Limiting Constraint
85
TABLE 7-16
TABULATION OF TEMPERATURES FOR 150 MVA TRANSFORMER
10 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Month
Day/Night (1)
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
PJM (1)
Hot Spot
°C
Jan
89.2
177.5
89.8
*178.3
85.7
177.1
Feb
89.9
178.2
90.5
*179.1
85.7
177.1
Mar
92.3
177.8
93.0
*179.5
85.7
177.1
Apr
95.3
177.7
95.9
*179.2
99.3
176.8
May
98.2
176.9
98.8
*178.9
99.3
176.8
Jun
100.9
176.7
101.4
*178.3
99.3
176.8
Jul
102.4
176.7
103.4
*179.5
99.3
176.8
Aug
101.6
176.8
102.7
*179.8
99.3
176.8
Sep
99.7
177.4
100.8
*i80.0
99.3
176.8
Oct
96.2
177.7
97.3
*179.5
99.3
176.8
Nov
93.2
177.6
93.7
*178.3
85.7
177.1
Dec
90.1
177.4
90.5
*178.2
85.7
177.1
■^Indicates Limiting Constraint
(1)
The limiting contraint for all months was the 5% Loss
of Life Limit.
86
TABLE 7-17
TABULATION OF TEMPERATURES FOR 300 MVA TRANSFORMER
2 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Month
Day/Night
Hot Spot
Oil
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
PJM
Oil
°C
Hot Spot
°C
Jan
90.2
*178.9
94.2
*178.2
90.8
*179.7
Feb
88.5
*179.2
94.6
*178.7
90.8
*1,79.7
Mar
89.6
*178.3
96.5
*178.6
90.8
*179.7
Apr
91.4
*178.3
99.2
*179.6
101.5
*179.1
May
100.7
. *179.3
101.5
*179.1
101.5
*179.1
Jun
102.6
*178.4
103.5
*178.4
101.5
*179.1
Jul
103.9
*178.9
105.1
*179.2
101.5
*179.1
Aug
103.5
*179.3
104.5
*179.5
101.5
*179.1
Sep
96.4
*179.6
102.9
*179.6
101.5
*179.1
Oct
94.3
*178.3
100.2
*179.7
101.5
*179.1
Nov
92.7
*178.6
97.6
*178.9
90.8
*179.7
Dec
91.1
*178.9
95.4
*179.4
. 90.8
*179.7
*Indicates Limiting Constraint
87
TABLE 7-18
TABULATION OF TEMPERATURES FOR 300 MVA TRANSFORMER
6 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Day/Night
Oil
Hot Spot
°C
°C.
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
Jan
102.0
*179.6
104.2
*179.1
100.8
*178.4
Feb
101.1
*179.6
104.8
*179.7
100.8
*178.4
Mar
104.6
*178.6
105.9
*178.2
100.8
*178.4
Apr
106.9
*178.3
109.2
*179.8
112.5
*179.7
May
111.6
*17.9..6."
111.5
*179.5
112.5
*179.7
Jun
113.5
*179.0
113.5
*179.0
112.5
*179.7
Jul
115.1
*179.8
114.5
*178.2
112.5
*179.7
Aug
113.7
*178.3
113.9
*178.5
112.5
*179.7
Sep
110.8
*178.7
112.2
*178.5
112.5
*179.7
Oct
106.1
*178.3
109.4
*178.3
112.5
*179.7
Nov
]05.0
*180.0
107.1
*178.5
100.8
*178.4
Dec
102.9
*179.6
104.6
*178.7
100.8
*178.4
Month
^Indicates Limiting Constraint
88
u
PJM
Hot Spot
°C
TABLE 7-19
TABULATION OF TEMPERATURES FOR 300 MVA TRANSFORMER
10 HOUR EMERGENCY, HOT SPOT LIMIT OF 180°C
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
Jan
106.0
*179.2
107.5
*179.8
104.8
*179.8
Feb
106.7
*179.9
106.9
*178.2
104.8
*179.8
Mar
108.6
*178.4
109.1^
*178.7
104.8
*179.8
Apr
111.0
*178.3
111.5
*178.6
101.5
*179.1
May
114.8 . *179.5
113.9
*178.5
101.5
*179.1
Jun
116.7
*178.9
116.1
*178.3
101.5
*179.1
Jul
118.5
*179.9
118.0
*179.3
101.5
*179.1
Aug
117.0
*178.4
117.4
*179.6
101.5
*179.1
Sep
114.9
*178.7
115.7
*179.6
101.5
*179.1
Oct
111.1
*178.2
112.7
*179.0
101.5
*179.1
Nov
108.9
*178.6
110.5
*179.3
104.8
*179.8
Dec
106.5
*178.8
107.9
*179.3
104.8
*179.8
Month
*Indicates Limiting Constraint
89
PJM
Hot Spot
°C
CHAPTER 8
r
DIELECTRIC STRENGTH OF CONDUCTORS
IN CONTACT WITH HOT SPOTS
Test results from recent investigations
17 22
'
have
uncovered a concern about the generation of visible gas
bubbles from the thermal degeneration of cellulose insulation
material that is in contact with hot metal.
These tests
have shown that the gas generated can reduce the dielectric
strength of the insulating oil and therefore possibly lead
to premature electrical failure of the transformer.
When
the transformer oil is heated, gases, such as hydrogen,
methane, acetylene, ethylene, and ethane are generated.
27 33
'
Evidence that these gases are generated can be seen at
temperature/ of 350°C and above.
However, the investigations
found that when cellulose (winding insulation) was immersed
in transformer oil and heated, carbon dioxide, water, and
carbon monoxide were detected at significantly lower temperatures.
These tests also indicated that the dielectric
strength decreased 5% for every 25°C increase in conductor
temperature for temperatures up to 15D°C.
Above 150°C, the
loss of dielectric strength became significantly greater in one case dropping from approximately 22 kV to 13 kV for a
hot spot temperature increase from 150 to 175°C.
90
The implication from those results is that the probability
of experiencing a dielectric failure becomes increasingly
greater as the temperature of the conductor is increased.
Undoubtedly additional research will be performed to determine
what effect this thermal deterioration of cellulose material
at overload temperatures will have on the life of transformers.
Should this research point the way toward greater
concern with this decreasing dielectric strength, one may
find it to one's advantage, on certain transformers where a
very high degree of reliability is justifiable, to accept a
more conservative philosophy on overloading and lower the
hot spot limit to 150°C.
As was seen in the previous calculations, the vast
majority of the transformer overloads were limited by the
180°C limit imposed on the hot spot temperature.
A decrease
of this limit by 30°C will obviously decrease overload
capability.
However, what needs to be determined is 1) what
a decrease in the allowable hot spot temperature will do to
the overload capability when calculated by either the Day/Night
or the Varying Ambient method and 2) if there is any advantage
in using any one method in this particular case.
91
Consequently previously performed calculations were
redone incorporating the lower hot spot limit of 150°C.
Again, to cover the range of medium and large power transformers, the selection of the four units was used as before.
Results and discussions are presented as was done in Chapter
7, page 46.
All assumptions made previously, including load
cycling, ambient temperature, etc. were used here as well.
The following summarizes the tables and charts that are
used in this discussion:
A.
Tables 8-1 through 8-7 inclusive (Pages 97 through 103)
list the allowable overloads as determined by the
various methods.
B.
Figures 8-8 through 8-12 inclusive (Pages 104 through
115) graphically display the overloads from the various
transformers for a given time per transformer.
C.
Tables 8-8 through 8-19 inclusive (Pages 116 through
127) compare the limiting criteria preventing additional
overloads for the various methods.
The discussion of the results were again broken down
into two parts.
The first covered the results from the time
92
standpoint, while the second reviewed the results on a
transformer size basis.
For all transformer sizes, big gains with the Day/Night
method were found in April, September and October.
Overall,
for the year as a whole, the Day/Night method resulted in a
1-20% more overload capacity than would have been expected
with the PJM method.
As anticipated, winter months showed
token differences in favor of the Day/Night method, while
the summer months showed a similar difference in favor of
the PJM method.
The vast majority of the difference was
found in the spring and fall periods of the year.
All
limits to additional overloads were as a result of the 150°C
hot spot limit being reached.
The oil temperature, cooler
in the winter than in the summer, varied within a small band
width and never approached its limit.
For the smaller of
the four units, the oil varied from about 90-ll0°C while for
the larger units the oil remained significantly cooler,
usually varying in the 60-90°C range.
The results obtained with the Varying Ambient method
showed that a more significant gain in overload capacity can
i
be realized by using the PJM method.
For the two smaller
units the difference was about 55% in favor of the PJM
93
method, while for the two larger units, the difference was
approximately 37%.
Again, there were two months - namely
April and October - where these differences were reversed by
a small percentage.
The 150°C limit was reached for all cases, thereby
preventing additional overloads.
The temperature variations
for the oil and hot spot were very similar to that obtained
with the Day/Night method and comments made above to that
effect hold here as well.
For the six hour case, very close correlation of results
was seen when the overloads obtained with the Day/Night and
the Varying Ambient methods were compared.
However, both
methods were outperformed by the PJM method, with the Day/Night
results lower by 11-26% and the Varying Ambient results
lower by about 27-37%.
The decreases in the summer months
were expected, but the consistent decreases in the fall and
winter months were not.
As before, the only positive months
were April and October.
However, the difference between the
PJM method and either of the two new methods was relatively
small.
Allowing a longer time period for the emergency to
persist, the temperature of the oil was higher than that
94
which was calculated in the two hour case.
At no time, with
any of the methods, did the temperature come close to reaching
its prescribed limit.
the hot spot limit.
All overload cutoffs were because of
As with the two hour case, the oil
temperatures for two smaller units were higher than for the
two larger units.
For the Day/Night method, the pattern setforth in the
six hour case was continued in the ten hour emergency case.
The overload capability was less here than in the six hour
case for all transformers, with a range of 20.8-37% in favor
of the PJM method.
Again, the three months of April, May
and October reversed the.trend.
However, the remaining nine
months were so negative so as to overwhelmingly cancel out
these gains.
As in the six hour case, the Day/Night and the
Varying Ambient methods yielded results that are very close.
Oil temperatures in this case were higher than in the
six hour case.
However, in all instances, the hot spot
limit was reached before the oil temperature was even close
to its limit.
The oil temperature for the larger units
again remained cooler than for the smaller units.
95
In the discussions on the trend of the overload capabilities (with the hot spot limit set at 180°C) it was noted
that the two new methods exhibited opposite performance.
The same was also found to occur when the hot spot limit was
set at 150°C.
With the Day/Night method, overloads were
more favorable for short time periods while with the Varying
Ambient method, overloads were changing such that, while
still indicating less allowable loading than was obtained
with the PJM method, nonetheless showed decreasing differences
as the time period increased.
The same reasons apply here
as before.
In addition, the repetition of the large margin of
difference during April and October was the same as was
shown for the 180°C case.
Similar comments apply here as
well.
96
TABLE 8-1
PER CENT OVERLOADS WITH THE DAY/NIGHT MODEL
2 HOUR EMERGENCY, 150°C HOT SPOT LIMIT
Month
10 MVA
Transformer Size
150 MVA
15/20/25 MVA
300 MVA
January
173/0
190/0
151.1/-0.9
158/1
February
175/2
193/3
153/1
160/3
March
172/-1
187/-3
148/-4
155/-2
April
168/14
183/10
145/7
152/8
May
156/2
174/1
139/1
145/1
June
152/-1
171/-2
136/-2
142/-2
July
150/-4
170/-3
135/-3
141/-3
August
151/-3
170/-3
135/-3
141/-3
September
162/8
178/5
141/3
148/4
October
165/11
184/11
146/8
153/9
November
168/-5
186/-4
148/-4
155/-2
December
171/-2
189/-1
150/-2
157/0
Net overload gain(+)/loss(-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
+20%
. +14%
+1.1%
+14%
The net overload gain or loss was determined by summing the
monthly per cent differentials when results from the above
method are compared to those of the PJM method. This
difference is shown after the slash (/) above.
97
TABLE 8-2
PER CENT OVERLOADS WITH THE DAY/NIGHT MODEL
6 HOUR EMERGENCY, 150°C HOT SPOT LIMIT
Transformer Size
150 MVA
15/20/25 MVA
Month
10 MVA
January
162/-3
179/-1
145/-3
February
161/-4
179/-1
145/-3
March
157/-8
174/-6
142/-6
146/-5
April
153/7
170/7
138.3/4.3
142.1/5.1
May
147/1
164/1
135/1
138/1
June
143/-3
160/-3
132/-2
135/-2
July
141/-5
158/-5
130/-4
131/-6
August
142/-4
160/-3
131/-3
134/-3
September
146/0
164/1
134/0
138/1
October
152/6
170/7
138/4
. 143/6
November
156/-9
174/-6
141/-7
146/-5
December
161/-4
178/-2
144/-4
149/-2
.
300 MVA
150/-1
149.1/-1.9
Net overload gain(+)/loss(-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
-26%
-11%
-18.7%
-12.8%
98
TABLE 8-3
PER CENT OVERLOADS WITH THE DAY/NIGHT MODEL
10 HOUR EMERGENCY, 150°C HOT SPOT LIMIT
Transformer Size
150 MVA
15/20/25 MVA
Month
10 MVA
300 MVA
January
159/-5
174/-3
143/-4
February
159/-5
173/-4
143/-4
146/-3
March
155/-9
169/-8
141/-6
143/-6
April
151/6
165/6
137.2/4.2
140/5
May
146/1
160/1
134/1
136/1
June
142/-3
157/-2
131/-2
133/-2
July
140/-5
158/-4
129.1/-3.9
131/-4
August
141/-4
156/-3
130/-3
132/-3
September
144/-3
159/0
132.3/-0.7
135/0
October
149/4
165/6
136.1/3.1
139/4
November
154/-10
169/-8
December
158/-6
173/-4
146.2/-2.8
139.2/-7.JB 142/-7
143/-4
146/-3
Net overload gain(+)/loss(-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
-37%
-23%
-27.1%
-20.8%
99
TABLE 8-4
PER CENT OVERLOADS WITH THE VARYING AMBIENT MODEL
2 HOUR EMERGENCY, 150°C HOT SPOT LIMIT
Transformer Size
150 MVA
15/20/25 MVA
Month
10 MVA
300 MVA
January
166/-7
183/-7
147/-5
,*- 153/-4
February
166/-7
183/-7
147/-5
152/-5
March
163/-10
180/-10
145/-7
150/-7
April
159/4
176/3
142/4
147/3
May
154/0
172/-1
138.1/0.1
144/0
June
150/-4
170/-3
135/-3
141/-3
July
148/-6
167/-6
134/-4
140/-4
August
150/-4
168/-5
135/-3
140/-4
September
152/-2
171/-2
137/-1
142/-2
October
157/3
175/2
140/2
146/2
November
161/-12
178/-12
143/-9
149/-8
December
165/-8
182/-8
146.1/-5.9
152/-5
Net overload gain(+)/loss(-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
-53%
-56%
-36.8%
-37%
100
TABLE 8-5
PER CENT OVERLOADS WITH THE VARYING AMBIENT MODEL
6 HOUR EMERGENCY, 150°C HOT SPOT LIMIT
Transformer Size
150 MVA
15/20/25 MVA
Month
10 MVA
January
160/-5
176/-4
144/-4
February
160/-5
175/-5
144/-4
147/-4
March
156/-9
172/-8
141.2/-6.8
145/-6
April
152/6
168/5
138/4
141.1/4.1
May
148/2
164/1
135/1
138/1
June
143/-3
160/-3
132/-2
135/-2
July
141/-5
158/-5
130/-4
133/-4
August
142/-4
159/-4
131/-3
134/-3
September
145/-1
162/-1
133.1/-0.9
137/0
October
150/4
166/3
137/3
140/3
November
155/-10
170/-10
140/-8
143/-8
December
159/-6
174/-6
143.1/-4.9
147/-4
300 MVA
147.1/-3.9
Net overload gain(+)/loss(-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
-36%
-37%
-29.7%
-26.8%
101
TABLE 8-6
PER CENT OVERLOADS WITH THE VARYING AMBIENT MODEL
10 HOUR EMERGENCY, 150°C HOT SPOT LIMIT
Transformer Size
Month
10 MVA
150 MVA
300 MVA
January
159/-5
173/-4
143.3/-3.7
146/-3
February
159/-5
172/-5
143/-4
145/-4
March
156/-8
169/-8
141/-6
143/-6
April
151/6
165/6
138/5
140/5
May
147/2
161/2
134.1/1.1
136/1
June
142/-3
157/-2
131/-2
133/-2
July
140/-5
155/-4
130/-3
August
141/-4
156/-3
September
144/-1
159/-0
133/0
October
149/4
163/4
136.1/3.1
138.1/3.1
November
154/-10
167/-10
139.2/-7.8
142/-7
December
158/-6
171/-6
143/-4
145/-2
15/20/25 MVA
131.2/-3.8
130.2/-2.8 132.1/-2.9
135/0
Net overload gain(+)/loss(-) for the 12 months:
10 MVA:
15/20/25 MVA:
150 MVA:
300 MVA:
-35%
-30%
-24.1%
-21.6%
102
TABLE 8-7
PER CENT OVERLOADS USING THE PJM METHOD
WITH HOT SPOT LIMIT AT 150°C
Size of Transformer:
10 MVA
Emergency Period
Summer
Winter
2 hours
6 hours
10 hours
15*%
146%
145%
173%
165%
164%
Size of Transformer:
15/20/25 MVA
Emergency Period
Summer
Winter
2 hours
6 hours
10 hours
173%
163%
159%
190%
180%
177%
Size of Transformer:
150 MVA
Emergency Period
Summer
Winter
2 hours
6 hours
10 hours
138%
134%
133%
152%
148%
147%
Size of Transformer:
300 MVA
Emergency Period
Summer
Winter
2 hours
6 hours
10 hours
144%
137%
135%
157%
151%
149%
103
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TABLE 8-8
TABULATION OF TEMPERATURES FOR 10 MVA TRANSFORMER
2 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Month
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying; Ambient
Oil
Hot Spot
°C
°C
Oil
°C
PJM
Hot Spot
°C
Jan
99.7
*149.2
102.3
*148.7
99.8
*149.4
Feb
98.2
*148.6
102.7
*149•2
99.8
*149.4
Mar
99.7
*148.8
104.2
*149.3
99.8
*149.4
Apr
101.5
*148.9
106.1
*149.5
107.7
*148.9
May
107.4
*149.5 .
107.4
*148.6
107.7
*148.9
Jun
108.9
*149.3
109.1
*148.6
107.7
*148.9
Jul
109.9
*149.4
110.1
*148.8
107.7
*148.9
Aug
109.3
*149.3
110.0
*149.6
107.7
*148.9
Sep
104.3
*149.0
108.4
*148.8
107.7
*148.9
Oct
102.8
*148.8
106.6
*149.1 ,
107.7
*148.9
Nov
101.4
*148.7
104.9
*149.1
99.8
*149.4
Dec
100.1
*148.7
102.9
*148.9
99.8
*149.4
^Indicates Limiting Constraint
116
TABLE 8-9
TABULATION OF TEMPERATURES FOR 10 MVA TRANSFORMER
6 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
;
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
Jan
104.7
*149.4
105.4
*148.9
102.7
*148.7
Feb
104.5
*148.7
105.7
*149.0
102.7
*148.7
Mar
106.7
*149.2
106.5
*148.5
102.7
*148.7
Apr
108.9
*149.7
108.7
*i49.1
111.6
*149.5
May
110.4
*148.7
110.9
*149.7
111.6
*149.5
Jun
112.2
*148.8
111.9
*148.6
111.6
*149.5
Jul
113.2
*149.1
113.1
*149.0
111.6
*149.5
Aug
112.5
*148.8
112.3
*i48.6
111.6
*149.5
Sep
111.3
*149.2
111.2
*148.7
111.6
*149.5
Oct
108.6
*149.0
109.2
*148.8
111.6
*149.5
Nov
106.7
*148.8
108.1
*149.8
102.7
*148.7
Dec
105.4
*149.7
105.8
*149.1
102.7
*148.7
Month
^Indicates Limiting Constraint
117
PJM
Hot Spot
°C
TABLE 8-10
TABULATION OF TEMPERATURES FOR 10 MVA TRANSFORMER
10 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
Jan
105.4
*148.7
105.4
*148.7
103.1
*148.7
Feb
106.0
*149.4
106.0
*149.4
103.1
*148.7
Mar
107.0
*148.7
107.8
*149.9
103.1
*148.7
Apr
109.3
*149.3
109.1
*149.0
112.2
*149.7
May
110.9
*148.8
111.4
*149.7
112.2
*149.7
Jun
112.8
*149.1
112.4
*148.6
112.2
*149.7
Jul
113.9
*149.4
113.5
*149.0
112.2
*149.7
Aug
113.2
*149.1
112.8
*148.6
112.2
*149.7
Sep
111.8
*148.9
111.6
*i48.7
112.2
*149.7
Oct
109.5
*148.7
109.6
*148.7
112.2
*149.7
Nov
108.4
*149.7
108.5
*149.7
103.1
*148.7
Dec
106.1
*149.1
106.0
*149.0
103.1
*148.7
Month
^Indicates Limiting Constraint
118
PJM
Hot Spot
°C
TABLE 8-11
TABULATION OF TEMPERATURES FOR 15/20/25 MVA TRANSFORMER
2 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Month
Day/Night
Oil
Hot Spoi
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
PJM
Oil
°C
Hot Spot
°C
Jan
86.9
*148.9
91.1
*148.7
87.9
*149.9
Feb
85.1
*149.1
91.4
*149.0
87.9
*149.9
Mar
89.1
*149.2
93.4
*149.1
87.9
*149.9
Apr
91.2
*148.8
95.4
*148.8
97.3
*148.9
May
96.7
*148.8
97.6
*148.5
97.3
*148.9
Jun
98.8
*149.2
100.1
*149.9
97.3
*148.9
Jul
100.0
*149.8
100.9
*149.0
97.3
*148.9
Aug
99.2
*149.2
100.2
*148.9
97.3
*148.9
Sep
94.4
*148.9
98.9
*149.3
97.3
*i48.9
Oct
90.7
*148.9
96.5
*149.2
97.3
*148.9
Nov
89.1
*148.6
94.2
*148.7
87.9
*149.9
Dec
87.9
*149.3
92.1
*149.1
87.9
*149.9
^Indicates Limiting Constraint
119
TABLE 8-12
TABULATION OF TEMPERATURES FOR 15/20/25 MVA TRANSFORMER
6 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Month
Day/Night
. Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
PJM
Hot Spot
°C
Jan
93.9
*149.4
96.3
*149.6
93.5
*149.2
Feb
94.5
*149.6
96.2
*148.9
93.5
*149.2
Mar
97.4
*149.6
98.4
*149.2
93.5
*149.2
Apr
99.7
*149.5
100.6
*149.3
103.7
*149.5
May
102.7
*149.1
102.8
*149.2
103.7
*149.5
Jun
104.5
*148.7
104.8
*149.0
103.7
*149.5
Jul
105.6
*148.7
106.0
*149.1
103.7
*149.5
Aug
105.5
*149.7
105.2
*148.9
133.7
*149.5
Sep
102.9
*149.3
104.0
*149.3
103.7
*149.5
Oct
98.9
*148.6
101.3
*148.7
103.7
*149.5
Nov
96.8
*148.9
99.2
*149.0
93.5
*149.2
Dec
94.6
*149.1
96.8
*149.0
93.5
*149.2
^Indicates Limiting Constraint
120
TABLE 8-13
TABULATION OF TEMPERATURES FOR 15/20/25 MVA TRANSFORMER
10 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Month
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
PJM
Hot Spot
°C
Jan
97.3
*149.4
97.9
*149.5
95.8
*149.7
Feb
97.1
*i48.6
97.7
*148.6
95.8
*149.7
Mar
99.5
*148.7
100.0
*149.2
95.8
*149.7
Apr
101.8
*148.7
102.4
*149.3
105.7
*149.4
May
104.6
*148.7
104.7
*149.4
105.7
*149.4
Jun
107.3
*149.8
106.7
*149.3
105.7
*149.4
Jul
108.4
*149.8
107.9
*149.4
105.7
*149.4
Aug
107.7
*i49.7
107.1
*149.1
105.7
*149.4
Sep
105.2
*148.8
105.9
*149.5
105.7
*149.4
Oct
102.5
*149.4
103.0
*148.8
105.7
*149.4
Nov
100.3
*149.5
,100.8
*148.8
95.8
*149.7
Dec
98.0
*149.6
98.3
*148.7
95.8
*149.7
^Indicates Limiting Constraint
121
TABLE 8-14
TABULATION OF TEMPERATURES FOR 150 MVA TRANSFORMER
2 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Month
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying; Ambient
Oil
Hot Spot
°C
°C
PJM
Oil
°C
Hot Spot
°C
Jan
67.8
*148.5
72.1
*148.7
68.1
*149.8
Feb
66.1
*148.8
72.6
*149.2
68.1
*149.8
Mar
71.2
*148.8
75.1
*149.7
68.1
*149.8
Apr
74.3
*148.9
78.1
*149.9
81.6
*149.4
May
80.4
*149.2
80.8
*148.6
81.6
*149.4
Jun
83.2
*149.1
83.6
*148.5
81.6
*149.4
Jul
84.9
*150.0
85.4
*149.5
81.6
*149.4
Aug
83.7
*148.6
84.6
*149.6
81.6
*149.4
Sep
78.9
*149.7
82.6
*149.4
81.6
*149.4
Oct
73.9
*149.6
79.0 . *148.7
81.6
*149.4
Nov
71.4
*149.0
76.0
*148.6
68.1
*149.8
Dec
69.0
*148.6
72.9
*148.6
68.1
*149.8
■^Indicates Limiting Constraint
122
TABLE 8-15
TABULATION OF TEMPERATURES FOR 150 MVA TRANSFORMER
6 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Month
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
PJM
Hot Spot
°C
Jan
74.3
*148.9
75.2
*148.8
71.9
*149.5
Feb
74.9
*149.6
75.8
*149.5
71.9
*149.5
Mar
78.0
*149.6
77.8
*148.6
71.9
*149.5
Apr
80.6
*148.7
80.9
*148.6
85.7
*149.8
May
84.5
*149.5
84.1
*149.1
85.7
*149.8
Jun
87.3
*149.5
87.0
*149.3
85.7
*149.8
Jul
88.5
*148.8
88.3
*148.7
85.7
*149.8
Aug
87.7
*149.0
87.5
*148.7
85.7
*149.8
Sep
85.6
*149.7
85.4
*148.6
85.7
*149.8
Oct
81.4
*149.2
82.3
*149.2
85.7
*149.8
Nov
78.3
*149.0
79.3
*149.0
71.9
*149.5
Dec
75.1
*148.7
76.0
*148.7
71.9
*149.5
indicates Limiting Constraint
123
TABLE 8-16
TABULATION OF TEMPERATURES FOR 150 MVA TRANSFORMER
10 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Month
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
PJW[
Oil
°C
Hot Spot
°C
Jan
75.7
*148.6
75.7
*148.6
72.3
*148.9
Feb
76.1
*148.7
76.1
*148.7
72.3
*148.9
Mar
79.1
*149.9
78.8
*149.5
72.3
*148.9
Apr
81.6
*148.6
82.1
*149-5
86.3
*149.4
May
85.0
*149.1
84.5
*148,6
86.3
*149.4
Jun i
87.9
*149.2
87.4
*148.6
86.3
*149.4
Jul
89.1
*148.7
89.5
*149.9
86.3
*149.4
Aug
88.3
*148.6
88.0
*148.5
86.3
*149.4
Sep
86.2
*148.. 6
86.6
*149.7
86.3
*149.4
Oct
82.7
*148.6
82.7
*148.7
86.3
*149.4
Nov
79.7
*148.6
79.7
*148.6
72.3
*148.9
Dec
77.2
*149.8
77.1
*149.8
72.3
*148.9
*Indicates Limiting Constraint
c
124
'
TABLE 8-17
TABULATION OF TEMPERATURES FOR 300 MVA TRANSFORMER
2 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Method
Month
Day/Night
Oil
Hot Spot
°C
°C
Varying; Ambient
Oil
Hot Spot
°C
°c
PJW[
Oil
°C
Hot Spot
°C
Jan
80.5
*149.3
84.9
*149.5
80.8
*148.7
Feb
78.7
*149.2
84.9
*148.6
80.8
*148.7
Mar
82.8
*149.1
86.8
*149.0
80.8
*148.7
Apr
85.2
*149.0
89.3
*149.1
91.7
*149.2
May
90.9
*149.1
91.8
*149.2
91.7
*149.2
Jun
93.1
*149.0
94.0
*149.1
91.7
*149.2
Jul
94.4
*149.6
95.5
*149.9
91.7
*149.2
Aug
93.5
*148.6
94.4
*148.8
91.7
*149.2
Sep
88.8
*149.4
92.8
*148.7
91.7
*149.2
Oct
85.0
*149.7
90.3
*149.3
91.7
*149.2
Nov
83.3
*149.5
88.0
*149.4
80.8
*148.7
Dec
81.5
*149.5
85.6
*149.4
80.8
*148.7
^Indicates Limiting Constraint
125
TABLE 8-18
TABULATION OF TEMPERATURES FOR 300 MVA TRANSFORMER
6 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Month
Oil
°C
Day/Night
Hot Spoi
°'C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
Oil
°C
PJM
Hot Spot
°C
Jan
87.2
*149.4
88.8
*148.6
85.9
*148.8
Feb
87.2
*148.6
89.3
*149.0
85.9
*148.8
Mar
90.7
*149.7
91.5
*149.8
85.9
*148.8
Apr
92.6
*148.6
93.5
*148.7
97.2
*L49.3
May
96.1
*149.0
96.1
*148.9
97.2
*149.3
Jun
98.4
*149.1
98.4
*149.1
97.2
*149.3
Jul
99.4
*148.6
99.4
*148.6
97.2
*149.3
Aug
98.7
*148.6
98.8
*i48.7
97.2
*149.3
Sep
96.9
*149.8
97.8
*150.0
97.2
*149.3
Oct
93.0
*149.7
94.6
*149.0
97.2
*149.3
Nov
90.5
*149.'5
92.1
*148.7
85.9
*148.8
Dec
87.8
*149.2
90.2
*150.0
85.9
*148.8
"Indicates Limiting Constraint
126
TABLE 8-19
TABULATION OF TEMPERATURES FOR 300 MVA TRANSFORMER
10 HOUR EMERGENCY, HOT SPOT LIMIT OF 150°C
Month
Day/Night
Oil
Hot Spot
°C
°C
Method
Varying Ambient
Oil
Hot Spot
°C
°C
PJM
Oil
°C
Hot Spot
°C
Jan
89.5
*148.6
90.6
*149.6
87.4
*148.7
Feb
89.9
*148.9
90.2
*148.5
87.4
*148.7
Mar
92.4
*149.0
92.5
*149.2
87.4
*148.7
Apr
95.2
*149.6
95.4
*149.8
99.0
*149.7
May
97.9
*149.3
97.1
*148.5
99.0
*149.7
Jun
100.3
*149.5
99.6
*148.8
99.0
*149.7
Jul
101.1
*148.9
100.7
*148.6
99.0
*149.7
Aug
100.5
*149.0
100.0
*148.5
99.0
*149.7
Sep
98.8
*149.5
99.1
*149.7
99.0
*149.7
Oct
95.3
*148.9
95.8
*148.7
99.0
*149.7
Nov
92.7
*148.5
94.1
*150.0
87.4
*148.7
Dec
90.9
*149.9
90.2
*150.0
87.4
*148.7
^Indicates Limiting Constraint
127
CHAPTER.9
SUMMARY
After reviewing the results of the computations from
the three methods, one can see that, in certain instances, a
particular method may give an increased overload capability
over that which can be presently obtained.
However, as can
be seen quite vividly, these gains are offset by lower than
presently used overloads during other periods.
These periods
can be length-of-time periods or time-of-year periods.
In
the Day/Night method, the length of time that the emergency
period persisted determined whether the method gave an
overall net increase or decrease in the final allowable
overload.
For short periods of time, the total allowable
overload for one year of the portrayed scenario was greater
than with the existing PJM method.
Increasingly longer
periods of time reversed the result so that overloads became
increasingly in favor of.the PJM method.
When the results
of the calculations during the various months of the year
are reviewed, it is seen that there are certain times of the
year when use of the Day/Night method results in a greater
allowable overload.
128
It was shown that, with the Varying Ambient method, set
so as to follow a conservative policy and yield safe results,
the overloads that were computed for all three emergency
periods proved to be lower than what would be allowed with
the PJM method.
Although the difference was quite large for
the short time periods, the difference began to decrease as
longer emergency periods were used.
Although the Varying Ambient method gave the lowest
overloads of the three methods, it must be remembered that
the peak ambient temperature was adjusted within the load
cycle so as to give the most conservative results.
A calcu-
lation was made to determine what change would occur if the
peak ambient temperature would be placed more in the middle
of the 1 per unit part of the load cycle.
Calculations were
made for both the 150°C and 180°C hot spot limited cases.
The change in overload capability for the 15/20/25 MVA unit
during the month of March can be seen on page 130.
The
calculations assume that the peak temperature occurs in the
sixteenth hour of the load cycle.
Hence, it can be seen
that such a change in the positioning of the peak ambient
temperature within the load cycle can substantially modify
the overloads that are allowed.
129
Hot Spot Limit Set at 180°C
Emergency
Period
Hours
Overload
%
Change
%
2
223
13
218
6
206
12
201
10
Ove:trload From
PJM Method %
7.8
195.8
195
Hot Spot Limit Set at 150°C
Emergency
Period
Hours
Overload
%
Change
%
2
199
19
190
6
186
14
180
10
178
9
177
Overload From
PJM Method %
The time of year that an emergency occurred produced
different results as far as overload capability.
The.PJM
method showed that, the lower ambient of the "winter" months,
as per the definition of winter in the methodology, gave
greater overloads than the "summer" months.
The other two
methods used in this text also gave overloads that were
greater in the months near seasonal winter than in the
seasonal summer months.
But not always was the sum total of
all of the overloads for all of the months greater with the
new methods.
Rather, in most cases, the total of all the
overloads proved to be less.
130
It was seen that there was
a consistent repetition of several months throughout the
year when the present method always yielded results that
were far below that obtained with the other two methods.
Most of these deviations were seen during April, followed
closely by October, and, somewhat less consistently, in
September and May.
From the temperature calculations it was
noted that, at the overloaded state, the temperature of the
oil calculated with the PJM method during April, was similar
to the oil temperature that, with the other two methods, was
calculated during May.
Yet, the average temperature during
May is more than 5°C higher than in April.
This was found
to be true for both the 180°C and the 150°C hot spot limited
cases.
The implication is that, with the PJM method, the
assumed ambient of 29°C could be unnecessarily high, (as
compared to the actual ambient of 10°C) and therefore understate the allowable overload by too great of a value.
It is
believed that defining April as a "winter" month will have
just the opposite extreme.
It may be best to define a new
category in the existing PJM program to handle this case so
as to obtain the most optimal loading.
An assumed temperature
for this month of, say, 10-20°C would provide additional
overload capacity for emergencies occurring that time period.
131
Although a similar adjustment could be made for October,
the lack of any outstanding temperature deviations between
the three methods would lead one to conclude that the heating
values calculated by the PJM program are not out of line.
Therefore the October calculations should remain as is.
As was seen from the various tables, the most common
cutoff in overloading was the hot spot limit of 180°C.
For
the smaller MVA units, the oil temperature unit was occasionally reached.
It seems that because of the design of these
units as well as the smaller quantity of oil, the temperature
of the oil increased to a higher value for all months and
during all three emergency periods considered.
With the
larger units, the oil temperature was lower and consequently
the hot spot limit became the limiting constraint in most
cases.
When the lower 150°C hot spot limit was imposed, the
limiting constraint became that limit.
The effect of this lower limit was to impose a 15-25%
across the board drop in overload capability of a unit.
This drop was rather consistent for all units, all emergency
132
periods, and all months throughout the year.
The oil temper-
ature dropped as well, with the coolest temperature seen for
the short two hour emergency, and warmer temperatures seen
for the longer overload periods.
The lower hot spot limit may have lowered the percent
rating above nameplate, but, from the graphs, the relative
position of the results for each month, by each of the three
methods remain the same.
In addition, the position of the
curve for each method relative to one another also remained
almost the same as in the 180°C case.
The imposition of a
new hot spot limit shifts the position of the curves along
the vertical axis, but the position of the curves relative
to one another changed little.
Hence, no method gains much
advant3ge or becomes more desirable just because of hot spot
limit changes.
With the hot spot limit being such a consistent factor
in determining the maximum transformer rating, it was seen
that real time monitoring of this temperature is highly
desirable.
At the present time., the hot spot is calculated
by adding a margin over and above the average copper temperature.
12
What is needed is detection devices that can
actually measure the hot spot temperature while the trans-
133
former is in service.
Whatever is used must not, in any
way, act as a foreign body that could allow the reliability
of the unit to decrease.
By installing such devices, the
local hot spots that exist in the transformer can be carefully
measured.
The operator can then monitor the hot spot temper-
ature on a real time basis and, knowing the actual temperature
rather than some supposed derated safe temperature, could
overload the unit to the highest thermal capacity that
results when the actual hot spot temperature of the unit is
closer to the chosen critical value.
The problem up to this point has not been developing a
technique for monitoring the temperature of the hot spot.
thermocouple
suffice.
A
installed at the hot spot location will
However, with the hot spots located in high magnetic
and electrical stress areas, the transfer of the thermocouple
information from such areas to the monitoring points outside
the transformer is difficult.
Research is presently underway to develop reliable and
economical techniques for determining and continuously
monitoring this hot spot temperature.
A number of attempts
have been tried to relay the information from the monitoring
point to the reading point outside the transformer.
134
Experi-
ments with acoustic data links
much success.
29
have been tried without too
These attempts involved converting an electrical
signal from the thermistors located on the hot spots into
acoustic waves and transmitting the acoustic waves through
the oil.
External detection and conversion devices then
yield electrical/temperature^readings.
Another technique that holds greater promise is the
spectral analysis of the acoustic variations of sensors that
are implemented in the hot spot locations.
28
In this tech-
nique, the sensor is imbedded in the turn of the coil, and
is acoustically excited via fiber wave guides.
Transducers
outside the transformer convert the received acoustic signal
into an electrical signal which, in turn, corresponds to a
temperature.
Laboratory model tests of this system have
shown that the direct measurement that is taken is accurate
to 0.01°C.
Should such hot spot detection methods become practical
for production units, the operator can then safely overload
the unit to a higher capacity without fear of exceeding the
hottest spot temperature.
135
At this point, several factors need to be mentioned.
The analysis that has been performed has strictly pertained
to just the core and coils (i.e. windings) of the transformer,
immersed in oil and in a tank.
To consider this as the
complete transformer and overload according to the calculated
ratings may cause some of auxiliary items to become dangerously
overloaded and prematurely fail.
Components such as bushings,
cables, load and no-load tap changers, and current transformers
need to be included in the overall analysis as to whether
the entire unit can be overloaded as indicated by the calculations.
In addition, the larger the MVA size of the unit, such
as the 300 MVA unit in this study, the more important becomes
the effect of stray flux.
heating of metallic parts.
Stray flux causes localized
Large units that are exposed to
high MVA loads may have stray flux heating levels that are
so high that the insulation can be damaged and oil can be
decomposed into gas.
Therefore, picking the emergency loading that a transformer can be expected to carry must include a review of the
capability of all of the components in the unit.
136
It should also be mentioned that once a rating method
is determined, switching to another method for the sake of
obtaining a higher rating is not permissible.
Such a practice
can result in excessive loss of insulation life, thereby
possibly shortening the life of the unit.
137
CHAPTER 10
CONCLUSIONS
The desirability of incorporating environmental factors
into the methodologies for calculating permissible loadings
above the transformer nameplate ratings was analyzed.
The
ambient temperature was determined to have the most pronounced
effect on the overall rating.
Two methods for incorporating
the ambient temperatures into the calculations were devised the Day/Night method and the Varying Ambient temperature
method.
Computations were made and the results of those
methods were compared with the results obtained with the
present PJM method.
The study also discussed the. results of recent investigations into the decrease of dielectric strength when the
hot spot temperature approaches 180°C.
Calculations were
made to determine what effect a 30°C lower hot spot limit
would have on the relative advantage of any of the three
methods.
As expected, certain time periods throughout the year
yielded results that were in favor of one method over another.
138
For the Day/Night method, what was not expected was 1) the
minimal total amount of overloading in favor of the Day/Night
method over the PJM method, 2) the few times during the year
that such overloading exceeded that presently allowed, and
3) the large deficit when longer emergency periods were
involved.
Although the Varying Ambient temperature method was
adjusted to yield conservative results, the amount that
these loadings underran those obtained with the PJM method
was rather large.
More advantageous results can be obtained
by positioning the peak ambient temperature with respect to
the load cycle that would more accurately reflect actual
operating conditions.
These results were true for both the 180°C and the
150°C hot spot limited cases.
By comparing the temperature results from all three
methods, the values obtained for the month of April with the
PJM method seemed out of line with those obtained with the
other two methods.
It is suggested that consideration be
given to placing April into a separate category so as to
yield perhaps a more realistic value for the limit the
139
transformer can actually carry during a short time emergency
condition.
In all, the present PJM methodology portrays quite
advantageous transformer loadings above nameplate.
It is
felt that, if necessary, it can be improved by considering
the month of April as a separate temperature period of the
year.
Although the other two methods can be implemented to
give greater overloads for any one particular time of the
year, the individual characteristics of the transformer, the
loading cycles, and the projected probability of. the occurrence of emergency situations at that particular time period
at that individual substation, must be taken into account
and analyzed on an individual basis before a decision can be
made as to the merits of any one method.
140
BIBLIOGRAPHY
Books
1.
Transformers for the Electric Power Industry, Bean, R. L.;
Chackan, N.; Moore, H. R.; and Wentz, E. C., McGraw Hill Book
Co., New York, 1959.
2.
Transformer Engineering, Blume, L. F.; Camilli, G.;
Boyajian, A.; and Montsinger, V. M., J. Wiley & Sons,
Inc., New York, 1951.
3.
Magnetic Circuits and Transformers, Massachusetts Institute
of Technology Department of Electrical Engineering Staff,
J. Wiley & Sons, Inc., New York, 1962.
4.
The J & P Transformer Book, Stigant, S. A.; and Franklin,
A. C, Butterworth, Inc., Boston, Mass., 1973.
Standards
5.
American National Standards Institute, Guide for Loading
Oil - Immersed Distribution and Power Transformers,
Appendix C57.92, New York, 1962.
141
6.
National Electrical Manufacturers Association, Guide for
Loading Oil Immersed Power Transformers with 65°C Average
Winding Rise, Publication Number TR98, New York, 19.71.
Articles and Reports
7.
"Determination of Power Transformer Ratings", Blake,
J. H.; Bast, R. R.; Donaldson Jr., R. J.; Downs, C. L.;
Mills, H. E.; Moberg, C. V.; Studeny, B. C; Wilker, S. J. ;
Wood, D. C., Pennsylvania-New Jersey-Maryland Interconnection, 1969.
8.
"Oil-Immersed Power Transformer Overload Calculations by
Computer", Blake, J. H.; Kelley, E. J., IEEE Transactions
on Power Apparatus and Systems, Vol. PAS-88, No. 8,
pp. 1205-1212, August, 1969.
9.
"The Loading of Transformer Units in Power Plants and
Networks with Regard to Ambient Temperature", Nejedly, J.,
CIGRE, (12-01), August 30 - September 7, 1978.
10.
"A Proposed Functional Life Test Model for Power Transformers", McNutt, W. J., Paper F-77-055-7 presented at
IEEE PES Winter Meeting, New York, January 30*February 4, 1977.
142
11.
"Loading of Substation Electrical Equipment with
Emphasis on Thermal Capability", Conway, B. J.; McMullen,
D. W.; Peak, A. J.; Scofield, J. M., Papers A-77-656-2
and A-77-649-7 presented at IEEE PES Summer Meeting,
Mexico City, Mexico, July 17-22, 1977.
12.
"Operation of Transformers at Loads in Excess of
Nameplate Ratings", Chew, 0., Paper presented at IEEE
PES Transformer Seminar, New Orleans, Louisiana,
March 9, 1978.
13.
"Loading Transformers by Temperature", Montsinger, V. M.,
AIEE Trans., Vol. 49, p. 776, 1930;
14.
"Effect of Color of Tank on the Temperature of SelfCooled Transformers Under Service Conditions",
Montsinger, V. M.; Wetherill, L., AIEE Trans., Vol. 49,
p. 41, 1930.
15.
"New Insuldur:
A Proved Insulation System", Westinghouse
Electric Corp., Publication #SA-9025, 1964.
16.
"Measuring Transformer Winding Temperatures Continuously
with the Transformer Carrying Load", Lockie, A. M.;
Stein, G. M., AIEE Trans., Vol. 67, p. 1608, 1948.
143
17.
"The Effect of Gassing During Overloads on the Impulse
Strength of Transformer Insulation", Heinrichs, F. W.;
Truax, D. E.; Phillips, J. D., Paper A-79-434-2 presented
at IEEE PES Summer Meeting, Vancouver, British Columbia,
Canada, July 15-20, 1979.
18.
"Transformer Invented 75 Years Ago", Halacsy, A. A.;
Von Fuchs, G. H., AIEE Trans., Vol. 80, p. 121, 1961.
19.
"The Combined Effects of Thermal Aging and Short-Circuit
Stresses on Transformer Life", McNutt, W. J.; Patel, M. R.,
IEEE Transactions on Power Apparatus and Systems, Vol. 95,
No. 4, p. 1275, July/August 1976.
20.
"Measurement of Ambient-Air Temperature During Temperature
Test on Transformers", Beavers, M. F., AIEE Trans., Vol. 79,
p. 1021, 1959.
21.
"Relative Transformer Aging Obtained Under ASA Load-Time
Recommendations When Using Different Estimating Procedures and Aging Curves", Beavers, M. F., IEEE Trans.
Power Apparatus and Systems, Vol. 83, p. 909, 1964.
144
22.
"Short Time Failure Mode Considerations Associated With
Power Transformer Overloading", McNutt, W. J.; Kaufmann,
G. H.; Vitols, A. P.; MacDonald, J. D., Paper F-79-695-8
presented at the IEEE PES Summer Meeting, Vancouver,
British Columbia, Canada, July 15-20, 1979.
23.
"Does Color Really Affect Transformer Temperature?",
Power, McGraw Hill Co., New York, December, 1957.
24.
"Loading of Power Transformers", Norris, E. T., IEEE
Proceedings, Vol. 114, p. 228, 1967.
25.
"Thermal Ratings of Transformers", Norris, E. T.,
IEEE Proceedings, Vol. 118, p. 1625, 1971.
26.
"Growth of the Electric Power Industry", Webler, R. M.,
The Line, Power System Division, McGraw Edison Co.,
3rd Quarter, 1979.
27.
"Transformer Gas Analysis Helps Trace Degradation",
Oomen, T. V., Electric Light and Power, Vol. 57, No. 11,
Nov. 1979.
145
28.
"Hot Spot Detector Helps Predict Transformer Life",
Yannucci, D. A.; Thompson, J. H., Electric Light and
Power, Vol. 57, No. 9, September, 1979.
29.
"Transformer Hot Spot Detector", Bell, R.; Bertin, M.,
Nucleohic Data Systems Report on EPRI Research Project
752, EL-573, October, 1977.
30.
"Monthly Normals of Temperature, Precipitation, and
Heating and Cooling Degree Days 1941-70", U.S. Dept. of
Commerce, National Oceanic and Atmospheric Adm.,
Environmental Data Service, August, 1973.
31.
"Determination of Ratings for Tubular Bus", Bethke, T.;
Davidson, G.; Dyraek, V.; Geiger, R.; McElhaney, R.;
Nabet, G., Pennsylvania-New Jersey-Maryland Interconnection, August, 1979.
32.
"Thermal Consideration for Outdoor Bus Design", Prager, M.;
Pemberton, D. L.; Craig, Jr., A. G.; Bleshman, N. A.; IEEE
Transactions on Power Apparatus and Systems, Vol. PAS-95,
No. 4, p. 1361, July/August 1976.
146
33.
"Gas Detection - A Key to Transformer Health", Transmission
and Distribution, Vol. 32, No. 1, January, 1980.
s-
147
APPENDIX A
Listing of Transformers Used in this Study
A.
RTE-ASEA Corporation, 10 MVA, 67-13.2 kV, OA, 55/65°C,
3 phase, Serial Number A4140.
B.
Westinghouse Electric Corporation, 15/20/25 MVA,
67-13.2 kV, OA/FA/FA, 55/65°C, 3 phase, Serial
Number RCP1653.
C.
McGraw Edison Electric Corporation, 150 MVA, 230-69 kV,
FOA, 55/65°C, 3 phase, Serial Number C-05043-5.
D.
General Electric Company, 300 MVA, 230-138 kV, FOA,
55/65°C, 3 phase, Serial Number M101504.
148
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APPENDIX C
Oil Temperature Change Equation
To calculate the increase in the transformer's top oil
temperature caused by solar radiation, the following equation
T, used.
A 14
can be
Sr
_ __a
A T
°
+ L - 6.50xl(f4
4
C + 3.88xl0~
F1,19
(T . - Tn y)1,19
1-19
0 19
v(T ol. - T,la') '
01
F
la
Where:
2
C
=
Thermal capacity - (joules/cm'/°C)/3600
F
=
L
=
Ratio of the average tank surface temperature rise
over ambient to top oil rise over ambient.
2
Transformer loss - watts/cm
r
=
Average value of the effective intensity of solar
radiation over an hour - watts/cm
S
=
Coefficient of solar absorption
T .
=
Value of top oil temperature at the beginning of
an hour - °K
AT
=
Change in top oil temperature in one hour - °K
T,
=
Average temperature of the surface absorbing heat
radiation over a one hour period - °K
01
It is assumed that the transformer is painted gray or green
giving an emissivity value of 0.95.
150
VITA
The author was born in Allentown,
Pennsylvania, on July 27, 1950.
He is the
son of Mr. and Mrs. Wasyl Chaply.
He is a graduate of the Class of 1968 from
Northampton Area High School.
In 1972, he
received a Bachelor of Science Degree in Electrical
Engineering from Lehigh University.
He joined Pennsylvania Power and Light
Company during the summer of 1971 as a Cadet
Engineer:
Upon graduation, he accepted a position
with Pennsylvania Power and Light Company in the
Substation Engineering Section.
His present position
is as Project Engineer in that section. '
The author currently resides in
Northampton, Pennsylvania.
151