TM
5-811-1/AFJMAN
32-1080
aging is a cumulative process and there is a time
temperature rise at rated kVA and to have an
lag in insulation temperature rise. Table 8-2
insulation upgraded to 65 degrees C (55 degrees/65
indicates loads consistent for normal life expect-
degrees rise) can carry continuously 122 percent of
ancy at a 30 degrees C average ambient tempera-
rated kVA. Likewise ventilated dry-type trans-
ture. Table 8-3 may be used for approximating the
formers with 220 degrees C insulation having 80
loads permitted at other ambient temperatures, or
degrees C temperature rise can carry continuously
the designer may use the tables in the applicable
135 percent of rated kVA Thermal upgrading
ANSI standards. Correction factors apply only
provides increased temperature rating or extended
from 0 degrees to 50 degrees C. Temperatures not
life at rated capacity. Thermal upgrading will not
in this range must be checked with the manufact-
be specified for dry-type transformers and will be
urer. For further discussion on loading of trans-
required for liquid-filled transformers only at main
formers, see ANSI standards shown in table 8-3
electric supply stations.
(b) Ambient. Since the temperature classifi-
plus IEEE Std C57.91 for 65 degrees C operation
of overhead transformers.
cation of the insulation system is a function of the
(2) Temperature ratings. Transformers are
ambient temperature, the ambient temperature
rated for a hottest-spot temperature which will
very largely determines the load which can reason-
give normal life expectancy based on rated kVA
ably be carried by transformers in service. An
loading.
average ambient temperature of 30 degrees C is
(a) Temperature rise. The temperature rise
used as the basis for nameplate ratings. Average
specified will be 65 degrees C for liquid-filled
ambient temperatures for the actual cooling air
transformers having a 120 degree C insulation and
are then based on daily operation and the maxi-
150 degrees C for ventilated dry-type transformers
mum ambient operating temperature in any
having a 220 degree C insulation. If thermally
24-hour period can be no more than 10 degrees C
upgraded insulation (greater winding temperature
above the average ambient temperature. The Us-
rise rating) is specified, then the transformer will
ing Agency will be contacted to determine the
have a longer life at rated capacity and will have
average ambient temperature if ANSI load factors
more overload capacity. For example, liquid-filled
are taken into account in transformer loadings.
transformers specified to have a 55 degrees C
Most load estimates are sufficiently conservative
Table 8-2. Daily Allowable Peak Loads for Normal Life Expectancy."
Mineral-oil, self-cooled
Dry-type, self-cooled,
(65 C Rise)
(150 C Rise)
Peak
load
Allowable peak load in percent of rated capacity following and followed by the indicated percent constant loadb
duration
hours
50%
70%
90%
50%
70%
90%
l/2
200.............
195 . . . . . . . . . . .
173
149.............
143.............
133
1
183 . . . . . . . . . . . .
172.............
153
128.............
125.............
121
2
157 . . . . . . . . . . . . .
150.............
134
116.............
115.............
114
4
134 . . . . . . . .
130.............
120
110.............
110.............
109
8
118.............
116.............
110
105
106.............
106.............
"This material is reproduced with the permission of the American National Standards Institute from the ANSI Standards entitled
"Guide for Loading Mineral Oil-Immersed Power Transformers Up To and Including 100 MVA", IEEE Std C57.92, copyright 1981
and "Guide for Loading Dry-Type Distribution and Power Transformers", IEEE Std C57.96, copyright 1989.
b
At 30C ambient temperature.
Table 8-3. Loading on the Basis of Ambient Temperatures.a
For each degree C that the average ambient temper-
ature is above or below 30 C the percent of rated
kVA shall be
Increased for
Decreased for
Transformer
lower ambient
higher ambient
Mineral-oil-immersed, self-cooled (65 C Rise)
1.0
1.5
Dry-type, self-cooled ventilated (150 C Rise)
0.35
0.35
a
This material is reproduced with the permission of the American National Standards Institute from the ANSI Standards entitled
"Guide for Loading Mineral-Oil-Immersed Power Transformers Up To and Including 100 MVA", IEEE Std C57.92, copyright 1981
and "Guide for Loading Dry-Type Distribution and Power Transformers", IEEE Std C57.96, copyright 1989.
8-9
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