Concerns with Use of the IEEE Loading Guide and Monitoring Devices
for Autotransformers with Load Tap Changing
Donald W. Platts
Abstract:
Utilities rely on the equations in the IEEE Loading Guide, C57.91, for several reasons: to calculate loading
capabilities for planning studies, and contingency operation; to calculate the expected temperatures in a transformer
when analyzing an unusual loading event; many rely on monitoring equipment installed on the transformer. In most
cases the temperature algorithm used by the monitor manufacturer follows these same equations. Equations in the
Loading Guide and LTC Winding Connections will be discussed, with examples that demonstrate that it is crucial to
understand the construction of the unit and the specific details of the temperatures and the location of the hot spot.
The paper demonstrates how users would typically use the data from test report, and the differences that would be
produced process compared to the design data provided. The paper demonstrates how the CT currents used by
standard temperature monitoring devices will often produce different temperatures from the design data.
Background:
Utilities and other transformer owners rely on the equations in the IEEE Loading Guide, C57.91,
(Loading Guide) for several reasons.
• Many calculate a loading capability to determine prudent overload limits for their transformers.
During a contingency event when other system equipment is removed or has tripped off line, the
additional load a transformer must carry should not be allowed to exceed that limit.
• Many attempt to verify the expected temperatures in a transformer during an unusual loading
event, or with indications of abnormal operation. An example would be when the gauges indicate
that the oil is hot, or initiate an alarm - given the loading and ambients, are the gauges showing an
expected temperature?
• Many rely on monitoring equipment installed on the transformer for an indication of the top oil
and winding hot spot temperatures. In most cases the temperature algorithm used by the monitor
manufacturer follows the same equations as found in the guide. A few of those owners will use
hot spot monitoring equipment that provides a direct reading of the measured temperature at the
location of the probe. But even in this case, if there is intelligence in the device to provide an
alarm, it too will use a similar algorithm to determine whether the reading is reasonable or
unexpected.
• Some will utilize the equations to verify the output of the temperature indicators, since they
utilize those devices to dynamically load the unit.
• Some will utilize the equations in an attempt to calculate the accumulated insulation aging
experienced throughout the operation of the unit.
The autotransformer examples shown in this paper will demonstrate potential problems for all of the
applications described above.
Summary of the Application of the Loading Guide:
• Transformer test reports provide the information from the loss testing and the top oil and winding
hottest spot and temperatures from either a factory heat run, or calculated from the
manufacturer’s design data. This data represents the inputs for loading evaluations.
• When the transformer loading changes, the losses change, and that produces different heatingresulting in different temperatures in the windings. As the ambient changes, the oil and winding
temperatures will eventually change by equal amounts.
• Equations in the Loading guide will calculate new oil and winding temperatures to account for
the different heating conditions produced by the variation in losses for different loading
conditions, and as the ambient varies.
Donald W Platts
October 30, 2007
Page 1 of 8
•
These equations provide reasonably accurate predictions of temperatures, and when combined
with the users limiting criteria, provide a reasonable estimate of the loading capability of a
particular unit.
The guide also has equations to allow the owner to adjust the test (or design) input temperatures to
account for the different losses that result when using the transformer on a different set of taps from those
used in the factory tests. Some users will take advantage of this, particularly if the de-energized tap
changer (DETC) is never used on the higher loss connection required for the thermal data. That type of
adjustment provides a higher calculated safe loading capability.
Autotransformer vs. Transformer Basics:
In a 2 winding transformer each of the primary and secondary windings carries the full rated volt
amperes. Changing load results in a proportional change in the current in both windings, and the losses in
both will increase or decrease together. The loading guide equations will predict the resulting
temperatures quite well.
In an autotransformer, the series and common winding each carry a portion of the full rated volt amperes.
The HV line current must flow through the series winding. The current induced in the common winding
sums with the series winding current to produce the output current.
Series Winding
Common Winding
100 V
2A
2A
4A
50 V
The autotransformer provides a more economical design, as each winding can be built with the optimally
sized conductors required for the portion of the current that it must carry, and each can be optimally
insulated for the voltage exposure.
LTC Winding connections
In these illustrations I will assume we are only dealing with step down autotransformer applications. The
location of the regulating voltage (RV) winding determines the magnitude of the current that flows thru
this winding. There are 4 common locations used to connect the RV winding for an LTC to an
autotransformer.
1. It can be placed in the series winding to adjust for varying input voltages.
2. To adjust the output, it is often placed in series with the common winding, at the neutral end of
the winding. This is probably the most common connection. However, it can present problems if
there is a tertiary winding. With this configuration, as the taps are changed, the volts/turn for all
of the transformer windings vary, therefore the rated voltage of every one of the windings is
affected.
Donald W Platts
October 30, 2007
Page 2 of 8
3. It can be placed between the series and the common winding to form the connection. The
unusual effect of this connection is that a portion of the RV winding will carry the current of the
series winding, while the remainder carries the common winding current. The current thru a tap
section will change dramatically with just a single tap change when the LTC is connected directly
to it, then moves on to another tap.
4. The RV winding can be connected between the auto connection point and the low voltage
bushings (commonly referred to as a line end LTC). In this case, it will carry the current of the
low voltage output.
The following figures are single phase representations of a Wye connected
Autotransformer with LTC
#1
#2
#3
#4
5. Other options do exist, and can be used. Also in each of the cases shown above, except #3, where
the RV is used to connect the series and the common, a variation of the connection is available.
The RV winding can be connected in phase to add voltage to the winding as shown. It could also
be built using an LTC with a reversing switch, so that fewer RV winding tap connections are
required. The polarity of the winding can be reversed, so that for ½ of the positions it adds to the
output voltage, and for the other ½, it subtracts voltage from the output.
Example 1:
This example shows that it is crucial to understand both the construction of the unit and the specific
details of the location of the hot spot as well as its temperature. The following table of data was presented
at a design review meeting.
On a standard test report, the temperature rise test results are reported for the combination of connections
and taps that give the highest average winding temperature rise. Here that tap would be the maximum of
both the de-energized tap changer (DETC) and LTC. The report would show:
Average Winding Rises
HV
LV
Top Oil Rise Hottest Spot Rise
45.3°
42°
47.5°
69.8°
Donald W Platts
October 30, 2007
Page 3 of 8
Autotransformer 230-138 kV 204/ 272 /340 MVA with 17 position, +10%, -5% LTC in secondary
and +5%, -5% DETC in primary. LTC connected per diagram #4 above.
Connection MVA
HV
241.5
230
218.5
241.5
230
218.5
241.5
230
218.5
LV
151.8
151.8
151.8
138
138
138
131.1
131.1
131.1
340
340
340
340
340
340
340
340
340
241.5 151.8
241.5 138
241.5 131.1
204
204
204
Regulating
Winding
Rise
48.7
47.6
46.3
46.9
45.7
44.4
34.8
33.5
31.9
H.S.
69.8
68.7
67.0
67.6
66.1
64.7
50.8
49.1
47.4
Common
Winding
Rise
42.0
41.0
36.1
44.0
41.5
39.8
45.1
42.9
40.4
H.S.
61.3
60.4
53.4
64.6
61.2
59.2
66.4
63.3
60.3
Series
Winding
Rise
45.3
44.0
43.2
45.0
44.0
42.8
45.3
43.9
42.4
H.S.
61.3
59.9
58.9
61.4
60.1
58.8
62.1
60.2
58.7
Winding Currents Amps
HSrise
Top
/TO
Oil
Deg C Regulating Common* Series LV Line Deg C
47.5
1293
480
813
1293
22.3
46.3
1293
440
853
1293
22.4
44.8
1293
395
898
1293
22.2
48.4
1422
610
813
1422
19.2
46.8
1422
569
853
1422
19.3
45.5
1422
524
898
1422
19.2
49.4
0
684
813
1497
17.0
47.7
0
644
853
1497
15.6
46.0
0
599
898
1497
14.3
42.7 54.4 42.4 54.7 43.5 56.3
44.9
775.9
288.2
487.7
775.9
11.4
43.5 55.7 43.7 56.7 43.7 54.0
46.2
0.0
410.7
487.7
898.4
10.5
* The common winding current values shown are those that would be derived from bushing CTs interconnected to
simulate the winding current. It actually is the sum of the currents induced in the common winding and those
induced in the regulating winding. The column Regulating shows the total current flow through the RV winding.
Adjustment of Reported Data for Operation on a different De-energized Tap:
To use the loading guide with this example, without additional information, most users would assume that
the hottest spot rise would be in the winding that has the hottest average winding rise, the Series, or HV.
If he wanted to adjust for a connection at a different DETC tap, he would adjust the losses, and the series
winding currents based on the new tap.
In this case, the adjustment would change to a lower voltage and a higher current. The adjustment
equations would produce a higher hot spot rise over top oil. A review of the data table shows the hot spot
rise should actually decrease slightly, or remain virtually unchanged.
In this particular design, the hotspot for the maximum output rating is in the RV winding. This is in
series with the output current. The current thru that winding does not change with changes to the DETC,
therefore, the hottest spot rise over top oil remains constant at all DETC taps. For this particular case
study, any calculations done by the user to adjust the input data should not change the hot spot rise over
top oil for any change to the DETC taps.
As with any transformer, a change of the DETC which produces lower currents through the winding will
reduce the I2R losses in that winding. This results in a small reduction in the transformer top oil rise. For
this case study, transformer top oil rise reduction is 1.2 to 1.5°C throughout the range.
Results of Operation of the LTC:
The loading guide does not attempt to provide a methodology to predict the changes to the hot spot
temperature based on operation of the LTC. The user must evaluate his application of the data and the
calculated temperatures and make decisions about the level of conservatism he should have in his
predicted outputs. For instance, since the hottest spot temperature is reported based on the connection
that produces the highest temperature, the user can assume that on any other LTC tap, the temperatures
will be less than those predicted by the loading guide equations. Therefore, if he does not attempt to
adjust for the losses of a different tap, then his results will be somewhat more conservative than the real
application. This would be appropriate for planning studies. It will however, cause problems for monitor
manufacturers.
Donald W Platts
October 30, 2007
Page 4 of 8
This paper doesn’t advocate trying to make thermal adjustments for changes to the LTC tap connections.
However, if you are using any type of temperature monitoring, even a mechanical temperature gauge, and
you actually rely on the output to be accurate, you need to do another evaluation for changes in the LTC
connection.
The problem is that in this particular case study, the hottest spot temperature which is in the RV winding
does not increase with an increase in the current flowing through that winding.
The winding temperature simulator will be calibrated using the data from the test report and will be based
on the HS rise over top oil with XXX amps of current flowing from the sensing CTs. This will be based
on the tap connection used for the temperature rise test (the tap with the greatest winding HS rise over top
oil).
In Example #1, when you change the LTC position from the maximum to the nominal voltage (110% to
100%), for a given output MVA, the line current will increase by 10%, as shown in the table. If the CT is
installed to measure the line current - the increase will be 10%. However, if it is connected to measure
the common winding current, it will measure an increase of 27 to 33%, depending on the DETC
connection. The increased measured current will lead to an increase in the temperature output reading of
the winding temperature indicator (WTI).
The equation for the variation of the hot spot rise over top oil is
Hot Spot (ultimate) = Hot Spot (rated) K2m.
[1]
Where: K
is the ratio of the new load to the rated -- [For this example -it is the ratio of the current
on the new tap vs. the old tap]
m
is the hot spot exponent (0.8 for ONAF cooling)
If the WTI uses a heated thermal well, the increased current will produce heating in the well to raise the
simulated winding temp. If the WTI calculates the hot spot, the equation above will lead to higher hot
spot rise over top oil.
The WTI predicted percentage increase in hot spot rise over top oil depends on the
current being measured and the DETC connection in use
Line
116%
116%
116%
Common DETC
146%
1
151%
3
157%
5
However, when you examine the temperature data, you will see that the transformer’s hottest spot
temperature (and HS rise over top oil) actually decreases. Therefore, any one relying on the output of a
WTI or monitor will be mislead.
In this case, on the nominal DETC connection, the table shows that the design value of the hot spot rise
decreases by 3.1°C, but a WTI connected to measure the common winding current would produce an
increase of 11.4°C. The overestimation (or total error) in reported hot spot temperature would be 14.5°C.
For most of the applications outlined at the start of the paper, an error this large in the reported hot spot
temperature presents significant operations problems.
The author does not yet have a reasonable solution to propose for this application problem.
Donald W Platts
October 30, 2007
Page 5 of 8
In contrast, the technical explanation for this phenomenon is fairly simple. [Remember this LTC has a
+10% and -5% range]. The current has increased by 10%, and the I2 component of the load losses has
increased by 21%. However, the portion of the RV winding that actually carries current is now only 1/3
of the total. The resistance of the winding tap sections that are still in the circuit is 33% of the prior value.
(0.9 I)2 x 0.333R = 0.4 I2R
Therefore, the total I2R loss is reduced to just 40% of the prior value. The reduced loss produces a lower
hot spot rise.
ONAN Operation:
When this autotransformer is operating at low loads, with no cooling operating, the limited data available
at this time shows that the hot spot is never located in the RV winding. With the lower loading, the losses
are significantly less. The cooling is different, since the fans are not running, and therefore the difference
between the top oil and bottom oil temperatures will be less. The rate of oil flow thru the windings will
be different. These factors apparently combine in this particular case to produce changes in the location
of the hottest spot temperatures. There is too little information available to draw firm conclusions.
Example 2:
The following table of data was presented at a design review meeting.
Autotransformer 230-69 kV 102/136 /170 MVA with 17 position +7.5%, -7.5% LTC in secondary
and +5%, -5% DETC in primary. LTC connected per diagram #3 above.
Connections MVA Regulating
Common
Series
Degrees
Winding Currents Amps
Winding
Winding
Winding
C
HSrise/TO
DETC
LTC
1
3
5
1
3
5
1
3
5
8R
8R
8R
N
N
N
8L
8L
8L
Rise
170
170
170
170
170
170
170
170
170
H.S. Rise
51.9
50.4
48.8
55.9
54.4
52.8
47.9
46.7
46.3
H.S.
54.7
53.0
51.4
59.1
57.5
55.9
64.0
62.1
60.8
Rise
H.S.
52.8
52.6
52.5
54.8
54.7
54.7
56.8
56.3
56.7
Top Oil
40.8
39.7
38.5
42.9
41.8
40.7
44.8
43.4
42.7
Common
916.8
896.5
874.0
1016.0
995.7
973.3
1131.4
1111.1
1088.6
Series LV Line
406.4 1323.2
426.7 1323.2
449.2 1323.2
406.4 1422.5
426.7 1422.5
449.2 1422.5
406.4 1537.8
426.7 1537.8
449.2 1537.8
Deg C
13.9
13.3
14.0
16.2
15.7
15.2
19.2
18.7
18.1
In this case, the hottest spot at rated load is found in the common winding for all tap connection
combinations except for the tap combination with the lowest voltage tap on the DETC, and the highest
voltage tap on the LTC (DETC 5, LTC 8R). When used on that tap combination, the ratio of series
current to common winding current is the largest of these 9 combinations, so the losses in the series
winding will be proportionally higher. The total losses on this tap combination are the lowest. With
those differences, the hottest spot will be found in the series winding.
When the hottest spot is located in the common winding, as you reduce the output voltage with LTC
changes, the load current increases, and the common winding current also increases. The CTs will
measure the increased current, and produce an increase in the hot spot rise over top oil, and increase the
output of the WTI. If the CTs are measuring the winding current and not the bushing current, the output
should be reasonably accurate.
The only problem will occur on the tap combination where the hot spot has moved to the series winding.
In this case, example #2, the design values of the winding hot spots vary by only 1.1°C, so the indicator
display error may exceed the theoretical error in the measurement.
Donald W Platts
October 30, 2007
Page 6 of 8
Conclusion:
While a tap changer in a 2 winding transformer will modify the current and the I2R losses in the winding,
it is unlikely that the hottest spot will move from one winding to another. In an autotransformer however,
it appears that the variation in the winding currents will actually lead to sufficient changes in the losses
within various windings, so that the hottest spot will move from one winding to another.
The most extreme example of erratic hot spot temperature changes presented in this paper, (example 1)
probably results from the fact that the hottest spot was designed to be in the RV winding. Other designs
should be more predictable (like example 2).
When the hottest spot location moves from one winding to another, the only way to accurately predict the
temperature of a winding utilizing the equations of the loading guide, is to treat each of these cases as if
they were separate transformers and separate evaluation cases.
It appears to be prudent for the purchaser of new autotransformers to request additional thermal
evaluation and additional design data. (Particularly the location and the value of the hottest spot
temperatures across the range of all tap changers).
As an additional recommendation, it appears that it would also be prudent to require information on the
expected deviations that will be seen between the output of the winding temperature indicator that was
specified and the transformer design thermal data.
Donald W. Platts, PE
PPL Electric Utilities.
Senior Engineer - Substation Maintenance Engineering Transmission & Substations.
He has a BSEE degree from Lafayette College, and is a Registered Professional Engineer in Pennsylvania.
He has experience in the Substation Engineering, Substation Component Engineering, and Substation Standards
Engineering groups. He is a member of the IEEE Transformers Committee, and has served as the chair of the
Insulation Life Subcommittee since 2000.
References:
[1] IEEE Guide for Loading Mineral-Oil-Immersed Transformers, IEEE Standard C57.91 –
1995
Donald W Platts
October 30, 2007
Page 7 of 8
Annex
Example Data Sets with added detail
Example 1
Autotransformer 230-138 kV 204/ 272 /340 MVA with 17 position, +10%, -5% LTC in secondary
and +5%, -5% DETC in primary. LTC connected per diagram #4 above.
Connection
HV
241.5
230
218.5
241.5
230
218.5
241.5
230
218.5
LV
151.8
151.8
151.8
138
138
138
131.1
131.1
131.1
Description
MVA
DETC Max, Nom
& Min with
Max LTC Tap
DETC Max, Nom
& Min with
Nominal LTC Tap
DETC Max, Nom
& Min with
Min LTC Tap
340
340
340
340
340
340
340
340
340
241.5 151.8 DETC Max, with
241.5 138 Max, Nom & Min
241.5 131.1
LTC Tap
204
204
204
Regulating Winding
Common Winding
Rise Oil top duct
42.0
52.8
41.0
52.0
36.1
47.3
44.0
55.0
41.5
52.2
39.8
50.7
45.1
55.4
42.9
53.5
40.4
51.1
Series Winding
H.S.
61.3
60.4
53.4
64.6
61.2
59.2
66.4
63.3
60.3
Rise Oil top duct
45.3
59.9
44.0
58.5
43.2
57.8
45.0
59.3
44.0
58.4
42.8
57.1
45.3
59.3
43.9
58.1
42.4
56.5
Degrees C
Rise Oil top duct
48.7
42.5
47.6
40.5
46.3
40.9
46.9
40.6
45.7
39.2
44.4
37.4
34.8
39.9
33.5
38.3
31.9
36.0
H.S.
69.8
68.7
67.0
67.6
66.1
64.7
50.8
49.1
47.4
42.7
43.1
54.4 42.4
49.3
54.7 43.5
53.0
56.3
44.9
35.9
43.5
42.3
55.7 43.7
51.0
56.7 43.7
53.1
54.0
46.2
36.7
Winding Currents Amps
H.S. Top Oil Mean Oil Regulating Common Series LV Line
61.3 47.5
32.8
1293.1
480.3
812.8 1293.1
59.9 46.3
31.6
1293.1
439.7
853.5 1293.1
58.9 44.8
30.4
1293.1
394.7
898.4 1293.1
61.4 48.4
33.2
1422.5
609.6
812.8 1422.5
60.1 46.8
31.9
1422.5
569.0
853.5 1422.5
58.8 45.5
30.7
1422.5
524.1
898.4 1422.5
62.1 49.4
33.8
0.0
684.5
812.8 1497.3
60.2 47.7
32.5
0.0
643.8
853.5 1497.3
58.7 46.0
30.9
0.0
598.9
898.4 1497.3
775.9
853.5
0.0
288.2
365.8
410.7
487.7
487.7
487.7
775.9
853.5
898.4
HSrise/TO Total loss
Deg C
kW
22.3
536.0
22.4
517.0
22.2
496.5
19.2
543.6
19.3
521.3
19.2
500.4
17.0
554.2
15.6
530.8
14.3
504.2
11.4
0.0
10.5
234.3
Example 2:
Autotransformer 230-69 kV 102/136 /170 MVA with 17 position +7.5%, -7.5% LTC in secondary
and +5%, -5% DETC in primary. LTC connected per diagram #3 above.
Connection
241.5 - 74.175
230 - 74.175
218.5 - 74.175
241.5 - 138
230 - 138
218.5 - 138
241.5 - 63.825
230 - 63.825
218.5 - 63.825
Donald W Platts
Description
DETC Max, Nom
& Min with
Max LTC Tap
DETC Max, Nom
& Min with
Nominal LTC Tap
DETC Max, Nom
& Min with
Min LTC Tap
HV
LV
MVA Regulating
Winding
kV
kV
Rise
241.5
230
218.5
241.5
230
218.5
241.5
230
218.5
74.2
74.2
74.2
69.0
69.0
69.0
63.8
63.8
63.8
October 30, 2007
170
170
170
170
170
170
170
170
170
Common
Winding
H.S. Rise
51.9
50.4
48.8
55.9
54.4
52.8
47.9
46.7
46.3
H.S.
54.7
53.0
51.4
59.1
57.5
55.9
64.0
62.1
60.8
Series
Winding
Rise
H.S.
52.8
52.6
52.5
54.8
54.7
54.7
56.8
56.3
56.7
Degrees
C
Top Oil
40.8
39.7
38.5
42.9
41.8
40.7
44.8
43.4
42.7
Page 8 of 8
Winding Currents Amps
Common
916.8
896.5
874.0
1016.0
995.7
973.3
1131.4
1111.1
1088.6
Hsrise
/TO
Total
loss
Series LV Line Deg C
406.4 1323.2
13.9
426.7 1323.2
13.3
449.2 1323.2
14.0
406.4 1422.5
16.2
426.7 1422.5
15.7
449.2 1422.5
15.2
406.4 1537.8
19.2
426.7 1537.8
18.7
449.2 1537.8
18.1
kW
482
465
448
513
496
480
542
520
510