Kindly someone enlighten me on Hot-spot temp limit of Transformer compared
with Oil&Winding. temp limit. Should hot-spot value same as Winding
temp.value? It anyway affects the Tank design?
S RAVIManager-Engineering
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Mohammad Shoaib
Mohammad Shoaib
Head of Power Transformer Design Dept. at Crompton Greaves Power Systems - Saudia
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if transformer is oil type (not dry type) then allowed hotspot limit is 13 degC more than the winding average
temperature according to standards.
I dont think it will affect the tank design. Special care needs to be taken in winding design to reduce hotspot!
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S RAVI
S RAVI
Manager-Engineering
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Mr.Mohammad,
What standard? IEC?
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Mohammad Shoaib
Mohammad Shoaib
Head of Power Transformer Design Dept. at Crompton Greaves Power Systems - Saudia
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yes. check:
60076-2 &
60076-7
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P Ramachandran
P Ramachandran
Technical Advisor at ABB Ltd
As per IEC60076-2, the maximum permissible winding hot spot temperature rise is 78 C, over a max ambient of
40C, monthly max average of 30C over the hottest month, max 20C of yearly average. If any of the above
ambient temperatures exceed for the site you are referring to, the winding hot spot temperature rise shall be
reduced to that extent.
As per ANSI/IEEE C57.12.00, the max winding hot spot temperature rise is 80C over a max ambient of 40C and
average max daily ambient of 30C.
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The above temperature rises are for oil immersed transformers.
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K A Radhakrishnan
K A Radhakrishnan
GM ENERGY
Dear Mr Ravi
Copper losses in both primary and secondary winding's, and core losses are converted into heat in a transformer.
These losses are all measured in watts. This heat must be conducted out of the winding's and core and dissipated
to the surrounding, or ambient air. There's a time delay involved in getting this heat out; as a result, the
transformer temperature increases. This temperature will continue to increase until a condition of equilibrium is
reached, one where the amount of heat generated in the transformer equals the amount of heat being dissipated.
The difference in temperature between a non operating transformer ("cold" condition) and one at full load
equilibrium point ("hot" condition) is called temperature rise. It usually is measured in degrees Centigrade.
Measuring temperature rise
The core temperature is measured with a thermometer, with readings taken with the transformer "cold" and
"hot." With these two readings, the temperature rise is calculated. For example, if we have a reading of
25C"cold" and 75C "hot," then the temperature rise is 50C.
The average winding temperature rise is determined by measuring the resistance of a winding when it's "cold"
and again when the winding temperature has stabilized under full load. From the difference in the resistance
readings, the average temperature is calculated for each winding.
Hot spot temperature
Although, the resultant temperature rise is averaged over the whole winding, the inside of a winding is hotter
than its outside, in reality. The hottest spot is at some point inside the coil having the longest thermal paths to the
outside air. This hot spot temperature differential is determined by the manufacturer on prototype units; it's
usually expressed as a temperature increase over the average temperature.
The hot spot temperature differential is defined by industry standards (NEMA and ANSI) for each insulation
class (type and temperature rating of insulation used on windings). Obviously, the hot spot temperature is the
limiting temperature for a transformer's insulation system. In other words, the temperature rise must be limited
by design and application so that the total temperature does not exceed the temperature rating of the insulation
used.
For example, if an insulation is rated at 105C maximum, the manufacturer must allow a 10C differential
between average and hot spot temperatures in a winding. If the room ambient is 40C, then the allowable
temperature rise is as follows.
Average rise = 105C(hot spot) minus 10C minus 40C(ambient) = 55C
Thus the transformer must be capable of withstanding a 55C average temperature rise.
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Radhakrishnan K A
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P Ramachandran
P Ramachandran
Technical Advisor at ABB Ltd
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Tank design has nothing to do with winding hot spot temperature.Generally we can measure only average
winding temperature by measuring the winding resistance drop after a shut down, running after at rated full load
current. This average temperature will be at the middle of the winding. Oil to winding gradient is assumed to be
same through out the heigt of the winding.Top winding temperature is estimated by adding this gradient to top
oil temperature in the winding/tank. But hot spot will be more than this due to fringing of leakage flux at the
ends of winding which will increase the eddy loss in copper conductor at the end of winding by 10 -20 % due to
radial flux impinging the width side of the conductor. This will result in oil to hot spot temperature gradient to go
up by 20-50 % and standards specify 30 % as norm toi be used.
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Naveen Jain
Naveen Jain
Technical Sales Manager at Cargill India Pvt Ltd
The hotspot and winding rise as well as top oil rise is dependent on type of fluid used. Now there are various
fluids available for transformer applications like Natural esters, synthetic esters and silicon. IEC has come out
with another transformer standard 60076-14 last last year for such fluids as they have capability to handle more
heat and temp. Like in IEEE, kraft paper with ester fluid insulation system is rated thermal class 120 against
same kraft paper with mineral oil as 105 thermal class.
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As per IEC, these fluids can have top oil rise upto 90 C against 60 C for mineral oil. If higher top oil rise is
allowed, tank design / size may change.
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JOHN KWEKU AMOO-OTOO, Phd(2012),P.E
JOHN KWEKU AMOO-OTOO, Phd(2012),P.E
EXEC.DESIGN CONSULTING AND POWER SYSTEMS ENGINEER at ABB
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Hot Spot temperature Limit and Winding and Oil temperature limit are different phenomenon associated with
power transformers.
For oil temperatures we have the top oil temperature associated with the top of the winding and bottom oil
temperature associated with the bottom of the winding. For Winding we have the average winding temperature.
As you know a transformers winding is either made of copper or aluminum and during operation the load current
that is taken by these windings ends up in what is know as copper losses or aluminum losses. The losses for
copper are much lower than aluminum. The heat gained by the mineral oil as a result of these losses is dissipated
by natural convection through the radiators's etc. The increase in temperature of the oil comparable to the
ambient temperature is usually designated as the temperature rise. The maximum Hot spot rise recommended for
a lasting insulation is 80C. If the hot Spot temperature is exceeded usually through overloading then the
insulation material will be degraded due to thermal degradation and that will affect the life span of the
transformer. Hence IEEE recommends a maximum Hot Spot temperature rise of 80C at a 30C ambient
temperature which corresponds to a total temperature of 110C. There is a margin which is built into the
calculation of the maximum Hot spot temperature. We have a 55/65C rise for transformer designs. Thus for a
30C ambient temperature, the Hot spot margin allowed for 65C temp transformer is 15C to achieve a 110C
Maximum Hot Spot temperature.
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Alan Sbravati
Alan Sbravati
Dielectric Fluids Specialist at Cargill
Dear Mr. S Ravi,
Trying to keep it simple, the average winding, as a heat run test result, is the average temperature measured by
resistance method. It is calculated based on the average resistance of all components between the measured
terminals, represeting an average temperature of this circuit.
By other side, the hotspot temperature should be the hottest point of each winding, which is the critical point for
the solid insulation aging process.
Since the hotspot is, typically, not a direct measurement (one can claim the optic fiber allows direct
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measurement, but this assumes the fiber position is the hottest one...), the standards, such as IEC 60076-2, IEEE
C57.91, and, the new ones IEC 60076-14 and IEEE C57.154, have defined limits for the average winding, which
will, indirectly and associated with the top oil, limit the hotspot temperature.
The average winding temperature rise and the hotspot temperature rise are defined based on an yearly average
temperature, since aging is a long term process. For reaching the limit for your case you should:
- verify the type of solid insulation material (kraft paper, thermally upgraded paper, aramid etc.);
- verify the type of insulation for the winding (homogeneous, hybrid etc.);
- verify the insulation fluid (the thermal class of the solid insulation is different with mineral oil or with natural
esters - for example, the thermal class of TUK is 20 degrees higher when immersed in natural esters in
comparison to mineral oil);
- reduce the temperature rise limit for the same amount of degrees that the yearly average of the installation
location is above the the reference standard.
In addition, each company has some loading and assets management strategy, which can affect the selection of
temperature rise limits.
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Harald Cuber
Harald Cuber
Regional Sales Manager bei Maschinenfabrik Reinhausen
The paper insulation has an ageing factor of 1 at 98C total, which roughly equals an expected transformer life
time of 28 years. In Network transformers the usual load should be below 100% which will increase the lifetime
of a transformer to the widely known 40 years.
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I personally highly recommend to let the transformer manufacturer design the transformer according to this
rules, but let them use thermally upgraded paper instead of the Standard paper insulation. This will give you an
additional gap of 10K at a low increased cost for the complete transformer (the upgraded paper should be around
10% costlier than the standard paper).
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S RAVI
S RAVI
Manager-Engineering
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I have a client who need 50 deg c is the max temp. rise allowed considering outside temp.of 55 deg C(gulf
country). The Transformer insulation class limit of 105deg C.
Hence, we have to keep hot-spot temp. rise limit to 50deg C.
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P Ramachandran
P Ramachandran
Technical Advisor at ABB Ltd
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No as per IEC, you need to limit to only 68C since IEC max ambient is 40C.But see other two ambient figures
also and in case any of them exceed more tahn 10C for your site, to that extent you have to reduce all
temperature rises. Your max ambient is 55C and why client is considering 50C.
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S RAVI
S RAVI
Manager-Engineering
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Considering Transformer insulation class with limit of 105 deg.C
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P Ramachandran
P Ramachandran
Technical Advisor at ABB Ltd
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Not exactly .You have mentioned that outside temperature is 55C.Then maximum ambient should be 55C and
not 50C. For insulation class of 105C , there is no problem in operating above that tempertaure. For example, as
per IEC, at max ambient of 40C ,hot spot temperature that paper will see is 40+78=118C.The normal life
estimation hot spot temperature is based on annual weighted average ambient temperature ie 20+78 =98 <105C
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S RAVI
S RAVI
Manager-Engineering
Not clear.
What client need is Transformer Temp.rise (avg)= 50 and ambient Temp.(max)= 55,
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Total 50+55=105 which is the Transformer insulation class limit.
Therefore, Transformer should be designed for Max Temp.Rise (hot-spot) of 50.
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Harald Cuber
Harald Cuber
Regional Sales Manager bei Maschinenfabrik Reinhausen
Dear S Ravi,
what you need is the average temperature over the year to calculate the permitted hot spot temperature of the
transformer. The peak temperature will be interesting for you to understand if there will be any Problems for a
used part in the transformer when it reaches the Maximum temperature.
For example:
In Europe the average temperature over the year used is 20C, the Maximum hot spot temperature is 98C, so the
allowed temperature rise of the Hotspot is 78K.
In Dubai for example, the average temperture over the year is 35C, the maximumg hot spot temperature is still
98C, the allowed temperature rise of the hotspot is therefore 63K.
Now for example, if you use gaskets in your transformer which decombust at maybe 100C and have a peak
temperature of 55C in that area with maybe a top oil rise of 60K, you will get into trouble.
So:
average temperature over the year - hot spot calculation
Maximum temperature - check used equipment if it can survive the peaks.
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Yours,
Harald
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S RAVI
S RAVI
Manager-Engineering
Thanks Mr. Harald,
I understand but, my client is insisting 50 limit on any(oil/Wdg/Hotspot) Temp Rise and NOT year average
temp.
We are advising our Transformer suppliers (korea,Japan/Germany) to design with 50 deg C
as limit.
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Alan Sbravati
Alan Sbravati
Dielectric Fluids Specialist at Cargill
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Dear S. Ravi,
We have had this same discussion inside IEC TC14 group. In some cases the customer just inform the max
ambient. In such cases the recommendation should be to reduce the limits according the extra degrees in
comparison to the reference max ambient (despite the correct would be to use the average ambient, as stated
previously).
For your case, if the reference is the 55/55/65 (IEEE limits for materials thermal class 105), the final limits
should be reduced by the difference 55C - 40C = 15C. This would result in a hotspot limit of 50C, but also on a
av wind and top oil limits of 40C (temperature rise).
The temperature limits of 40/40/55 are quite low, but possible (extra costs for the transformer).
My suggestion would be to consider would be the use of Natural Esters. For such very high ambient conditions,
the use of materials with higher thermal class is highly recommended.
Using Natural Ester + TUK (thermal class 140), the possible temperature limits could be defined by reducing 15
degrees from the IEEE C57.154 limits (table 4 specifies limits of 85/90/100), resulting in:
Av wind temp rise = 70
Top Oil temp rise = 75
Hotspot temp rise = 85
Possibly the final cost of the transformer would be lower, with the advantages of using a K class fluid,
environmental friendly.
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Harald Cuber
Harald Cuber
Regional Sales Manager bei Maschinenfabrik Reinhausen
Dear S Ravi,
it adds a nice portion of life time to the transformer, from a technical point of view a nice thing.
Harald
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Laszlo Kadar
Laszlo Kadar
Associate at Hatch
Hot-spot, by definition, is a localized point where the temperature is the highest. It can be winding, core, etc. A
transformer expected life time is based on this value, since at this temperature degradation of materials in contact
with this hot-spot is a boundary value for the life of the transformer. To do not reduce the expected life time of
the transformer this value shall not be exceeded under all operating conditions (ambient temperature, loading,
etc.) However, finding the location and measuring this value is all the time a challenge.
Based on the transformer type (dry, oil), material used in it (high temperature fluids, high temperature materials),
operating conditions and desired expected life time of the transformer one can define this value. There are
standards which provide such values for the different transformer designs, which were described by previous
contributors. Also, these standards provide indication how to relate the hot-spot temperature values to the
average temperature values (oil, winding, core, etc.), which values can be measured more easily.
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Regarding the last part of the question on the tank design: usually not, but in same cases it may - for example I
can think about tank wall around a high current bushing when one needs to be careful.
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S RAVI
S RAVI
Manager-Engineering
Thanks to all !
