C57.12.00 Sect 5 Proposed Changes to Temperature Rise
Proposal 1 REVISED March 2004
Sect 5.11.1.2
Other winding rises
It is proposed to add before the first paragraph the following
“If specified, the average winding temperature rise above ambient shall not exceed 70°C at rated kVA for
transformers who’s windings are cooled using only forced-directed oil. These transformers are identified
as OD . . , and typically are ODAF. The maximum hot spot rise shall not exceed 80°C and the top oil rise
shall not exceed 60°C. The tendered losses and measured losses shall be corrected to 90°C in place of
85°C reference temperature.”
Existing paragraph:
“Other winding rises may be recognized for unusual ambient conditions or for special applications. These are in
appropriate applications or in certain product standards”
Background:
When the transformer is cooled using oil pumps and the oil is forced and directed through at least the main
windings as defined in note 2 in section 5.1 of C57.12.00-2000, the top oil rise will be substantially less than for
ONAF cooling. This will vary between different manufacturers and sizes of transformers. For ONAF cooling the top
oil temperature rise limit is 65°C. Most transformers in practice have top oil temperature rises around 60°C with
ONAF cooling. For ODAF cooling the top oil rise will typically be 50°C or less. Thus the oil surrounding the
uppermost part of the winding and leads may be 10°C cooler and thus the maximum (hottest-spot) winding
temperature may be 10°C less than the 80°C allowed for the thermally upgraded insulation system.
Thus, for these types of transformers it is possible to increase the average temperature rise of the windings
that are cooled using oil pumps with directed oil flow to a higher value than 65°C and still maintain an 80°C hottestspot winding temperature. Many world standards, IEC 60076-2 and CAN/CSA-C88-M90 recognize an average
temperature rise of 70°C for OD . . type cooling.
CSA-C88 section 9.2 note (2) comments on this subject which we may want to consider for inclusion into
C57.12.00.
(This comment below was not recommended by the task force. JDMarlow)
“The temperature-rise limits of the windings (measured by the resistance method) are chosen to give the same hot-spot
temperature rise with different types of oil circulation. The hot-spot temperature rise cannot normally be measured
directly. Transformers with forced-directed oil flow have a difference between the hot-spot and the average
temperature rise in the windings which is smaller than that in transformers with natural or forced but not directed oil
flow. For this reason, the windings of transformers with forced-directed oil flow can have temperature-rise limits
(measured by the resistance method) which are 5°C higher than in other transformers”
CSA –C88 section 9.2-note (3) also recommends that the losses may be corrected to 90°C if so specified by the
user. We may want to include a similar note for C57.12.00
(This comment below was recommended by the task force with some different wording. JDMarlow)
“In cases where an ODAF transformer is normally loaded close to the full output rating, and where the load losses are
to be capitalized and used in bid comparison or for guarantee purposes, or both, and where these losses are based on
full output rating of the transformer, the user should consider specifying that the tendered losses and measured losses
be corrected to 90°C in place of 85°C.”
This proposal will thus harmonize C57.12.00 with IEC for this type of cooling
The present loading guide C57.91-1995 uses top oil rise and winding temperatures in determining
permissible overloads. There will be some work necessary on this standard because of the two different
designations OFAF and ODAF previously was FOA. Any changes that may be necessary to reflect the 70°C
temperature rise for ODAF cooling should be revised when C57.91 is updated.
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Proposal 2 REVISED March 2004
SECTION 5.11.1
Winding temperature rises
It is proposed to add a new section after the first paragraph per the following:
“In transformers with concentric winding arrangement, two or more separate windings may be situated one
above the other. In this case, the winding temperature limit shall apply to the average of the readings for
the stacked windings if they are of equal size and kVA rating and similar design. If they are not, the
evaluation should be subject to agreement between the user and the manufacturer. For all rated loading
conditions that are evaluated, a hot spot temperature rise limit of 80°C must still apply to all windings”
Background Information
For many years transformers, for step-down (substation) and step-up service (GSU) were occasionally specified to
be supplied with dual LV windings that generally are of equal kVA and voltage ratings. Recently, this type of
application is becoming more common, especially for GSU service. For this type of application, one winding
arrangement commonly used is to locate the LV windings one above the other as shown in fig.1 This arrangement
is called a split LV, Jones connection or stacked LV.
Another possible winding arrangement would be to locate the LV windings in separate radial locations per fig. 2.
This is in common use when the LV windings are of unequal kVA ratings or special impedances are required to
each individual LV winding. Each winding has its own oil flow. The hot spot temperature rises are essentially the
same as the windings in fig.1
From fig. 1, It can be seen that the cooling liquid flows through the LV windings in series. Although the average
temperature rise of the upper LV winding may be higher than the guaranteed average temperature rise, the top oil
rise for the upper LV winding is the same as the HV winding. Thus the hot spot rise for the upper LV winding is the
same as if the LV windings were connected in series. The hot spot rise limit for the HV is essentially the same for
the upper LV or a single LV. From the example shown it can be concluded that although the average winding
temperature rise of the upper LV winding is 68°C that the hot-spot temperature rise still meets the 80°C limit. The
average temperature rise of the upper and lower LV is 62°C and maximum hot spot is 80°C just as if it was
a single LV winding.
Fig.1 Split LV Winding
Fig.2 Double Concentric LV Windings
Core
57°C
top oil
Upper
LV X
H 45°C
V mean oil
LV Y
Lower
LV Y
33°C
Bottom oil
HV Winding
Liquid Temp Rise
LV X
Hot Spot
Gradient
Hot spot
rise
Winding
Avg.
Gradient
Mean oil
rise
Avg.
Winding
rise
Max Oil
rise
Hot Spot
Gradient
Hot spot
rise
51
39
45
DOUBLE CONCENTRIC LV WINDINGS FIG 2
Max Oil
rise
17
17
19
HV
Avg.
Winding
rise
Avg.
Gradient
Upper X
Lower Y
HV
Mean oil
rise in
winding
Winding
SPLIT LV WINDINGS FIG 1
HV Taps
68
56
64
57
45
57
6
6
4
80
68
80
LV X
LV Y
HV
17
17
19
45
45
45
62
62
64
57
57
57
6
4
4
80
78
80
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