The document discusses basic design practices for transformer cooling. It describes how heat generated in transformers needs to be dissipated to prevent high temperatures that accelerate aging. Both passive cooling systems using natural convection and active systems using fans or pumps are discussed. The key factors in transformer cooling system design are minimizing temperature, maximizing heat transfer area, and increasing fluid flow rates and temperature differences. Both radiator and cooler style heat exchangers are addressed.

1. BASIC DESIGN PRACTICES for TRANSFORMER COOLING
Vasanth Vailoor & Kevin Riley
Trantech Radiator Products Inc.
ABSTRACT
Transformer losses due to electro-magnetic conversions produce heat in the coils and the cores. In
medium and large transformers, the heat needs to be dissipated quickly to prevent high temperatures due
to thermal resistance. Elevated temperatures accelerate aging of insulation paper. The life of a
transformer is measured using the degree of polymerization (DP) of the insulating paper and aging, or the
remaining life of a transformer is measured as the inverse of DP. Temperature affects life and this is given
by the Arrhenius equation that shows that the life of a transformer is approximately reduced by 50% with
every 8ºC rise in temperature. High temperature also accelerates aging of insulating paper in the
presence of moisture. Thus elevated temperature is a common enemy of transformers.
INTRODUCTION
Transformers dissipate heat by all three modes of heat transfer – conduction, convection and radiation.
All modes of heat transfer occur simultaneously when temperature rises. Depending on the size of a
transformer and the density of losses, conduction or convection could be the controlling factor. Radiation
usually plays a minor role due to the operating range of temperatures encountered in transformers. Small
transformers have a relatively large surface area per unit volume and the temperature rise in the materials
used for construction is well within regulatory specifications and justifies considerations of market
economy and risk of failure. However, larger transformers with higher electrical load density need
additional external cooling systems. Liquid cooled transformers can be designed to meet the electrical
loads and also limit the temperatures to safe operating conditions with passive or active cooling systems.
When losses occur in the core and coils of a transformer, the heat must be dissipated to the surroundings
with minimal temperature rise. In a liquid filled transformer, the heat or the thermal energy is passed on to
the fluid in the cooling system – this is the transformer oil, be it mineral oil, Silicone oils or natural oil
esters. The oil in turn must transfer the heat to the ambient, either air or water. Heat transfer occurs either
by natural convection due to density changes with temperature or may be enhanced with active
components like fans and pumps. Natural convection is the choice of design when reliability is critical and
radiators are installed. However, when space around a transformer is constrained, fans and pumps may
be used to develop a compact cooling system using radiators or coolers. Coolers typically need less real
estate, cost less initially and take up less foot-print space due to their secondary cooling surface that
enhances the heat transfer capability but need more attention in the form of regular maintenance. In all
such liquid filled transformers with radiators or coolers, convection is the primary mode of heat transfer.
BACKGROUND
Convective heat transfer is governed by Newton’s Law of cooling – in equation form,
Q = U * A * ∆T - - - - - - - - - - (1)
Where Q is the quantity of heat transferred
U is the overall heat transfer coefficient
A is the heat transfer area and
∆T is the effective temperature difference between the hot and the cold fluid
‘A’, the heat transfer area can be easily measured. The effective temperature difference ‘∆T’, is the log-
mean-temperature (LMTD) given by,
∆T = ((TTop,Oil – THot,Air) – (TBottom,Oil – TAmbient))/(ln((TTop,Oil – THot,Air)/(TBottom,Oil – TAmbient))) - (2)
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2. Due to the practical limitations of measuring these parameters accurately, the effective temperature
difference is approximated with the maximum temperature, given by,
∆T ≈ ∆TMax = TTop,Oil – TAmbient - - - - - - - - (3)
This is a reasonably good approximation for transformer cooling systems and is justified due to the
relatively small temperature differences involved in this application.
The overall heat transfer coefficient ‘U’ is a characteristic of the heat transfer equipment dependent on
heat exchanger geometry and flow characteristics of the fluids exchanging heat. This is a very complex
parameter to calculate and so is usually determined by experimentation in transformer applications. Most
radiator manufacturers experimentally determine a standard ‘U’ for a normal operating condition and use
empirical correction factors for other operating conditions. In addition, a standard ‘U’ must be established
for each radiator geometry.
Cooling systems are usually classified as radiators or coolers. Coolers have a high ratio of secondary
cooling surface or fins to primary cooling surface compared to radiators.
It is also to be noted that the life of transformer, determined by insulation aging is most critical at the hot
spot temperature. However, this parameter is relatively difficult to measure due to lack of access and
sometimes, even to determine its location. So the hot spot temperature is estimated and modeled with
great effort. But the top oil temperature is a good reliable indicator of the hot spot temperature depending
on transformer oil channel design, oil flow velocity and oil characteristics like viscosity and heat capacity.
The choice of active and passive cooling systems is abundant. Active systems with fans and pumps are
smaller in size but need additional power for operations and are favored when space around the
transformer is at a premium. Passive systems, on the other hand, need more space but need no external
power and maintenance needs are considerably less – and it comes with an upfront price tag. Due to a
high fin density or secondary cooling surface, coolers are more compact in size and initially less
expensive. Depending on the operating environment, over time, the fins can accumulate dust and debris
that reduces the heat transfer coefficient due to thermal resistance or fouling and need careful
maintenance to maintain its thermal performance.
Another significant difference between active and passive coolers is their temperature profile at part load
operations as shown in Figure 1.
Figure 1
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3. DESIGN APPROACH
Due to the inherent nature of manufacturability, the design approach to the choice of radiators and
coolers are different.
Since the oil must flow through the transformer and the radiator, it is preferable to minimize the number of
connectors and potential sources of leaks in the joints. So the radiator is made to fit the available
dimension provided on a transformer. However when an oil pump is used to enhance thermal dissipation
by increasing oil flow in a radiator or a cooler, it would be preferred to have vibration isolators between the
transformer and the cooling system. Due to the additional connectors available, the length of a radiator or
the dimensions of a cooler is less constraining and gives the designer the opportunity to optimize cost by
selecting standard sizes.
In passively cooled radiator applications, with natural convection on the air and oil side, the radiator is
made to fit the I/O (inlet and outlet) ports available on the transformer. Thus once the number of plates is
designed and manufactured, the heat dissipation capacity is influenced by only one variable parameter -
the temperature gradient between the two fluids - air and the transformer cooling oil.
In actively cooled systems, air flow and/or oil flow rates may be changed independently. Thus such
actively cooled systems are rated at manufacturer’s standard operating condition, which may be different
from the user’s standard or recommended operating conditions. This could potentially cause the same
cooling system to be rated differently by the manufacturer and the user, unless other independent
operating conditions are also specified.
Additional modifications are necessary when different transformer fluids are used due to differences in
viscosity, other thermal characteristics and permissible operating conditions that may change in the near
future.
HARDWARE CONSIDERATIONS
There are many types of hardware considerations when calculating needed heat dissipation in various
transformers. The most common approach is the simplest approach which is through natural convectional
flow or an ONAN method. This application can be uprated or enhanced by adding fans in which case the
method changes to an ONAF configuration. In taking away the heat from the radiator plate surface by
means of forced air this will increase heat dissipation. As we move into the next category of cooling
methods we must employ pumps in order to accelerate oil circulation therefore increasing heat
dissipation.
The OFAF w/radiators method is the most flexible system as it can provide a triple rating. By this system
ratings of an ONAN, ONAF and OFAF can be provided based upon transformer demand. OFAF w/cooler
systems provides a means of significant cooling when space is limited or directed flow rates within the
core winding design require constant flow and efficient cooling.
OFWF systems utilize water flow to cool the transformer oil. As it is known that the ambient temperature
of water is less than atmospheric air in the same weather this is the most efficient form of cooling but with
generally shorter life spans and higher maintenance.
Any system coded as an OD application regardless of whether the oil is of the thermosiphon or pumped
movement is a system that is designed to circulate oil along pre-determined paths with the transformer’s
main winding.
When considering any of these systems for new installation or replacement it is necessary to understand
the historical and current requirements of your transformer in order to meet desired cooling efficiencies.
What worked as original factory specifications for and older model transformer may not meet the current
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4. needs or loads for a replacement or retrofit application. Environmental and footprint conditions play the
biggest factor in how new transformer cooling systems should be approached when cooling efficiency
needs are established. In order to help identify these various cooling system types listed on your
transformer a listing of code letters has been provided in Table 1.
Table 1
Internal to
Transformer
First Letter
(Cooling medium)
O Transformer Oil Application
Second Letter
(Cooling
mechanism)
N Natural convection through cooling equipment and windings
F
Forced circulation through cooling equipment, natural
convection in windings
D
Forced circulation through cooling equipment, directed flow in
man windings
External to
Transformer
Third letter
(Cooling medium)
A Air
W Water
Fourth letter
(Cooling medium)
N Natural convection
F Forced circulation
CONCLUSION
In conclusion, there are many factors to consider when designing or replacing a transformer cooling
system. Functionality, design constraints, footprint and future maintenance are just a few. Lowering the
operating temperature of the transformer, thereby prolonging the functional life of the insulating paper, is
critical. And finally – the cost-economic considerations of the entire transformer system is necessary to
design the appropriate cooling system since optimization of a part of a system can rarely give an
optimized solution of the whole.
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5. FIELD APPLICATIONS
Example -1
Here is an application where a water cooled cooling system was replaced with a bank of radiators at this
aging plant that was on the verge of shutting down due to environmental issues of oil leaks into the river.
From the heat run test, an appropriate radiator cooling system was designed to replace the water coolers.
The two GSUs had three single phase transformers each. Initially, one transformer cooling package was
replaced and after passing the hot summer months, monitored results showed an average of 5.6ºC lower
operating temperature than the other transformers (see the bold dark line in Figure 2).
All transformers were refitted with radiator cooling systems by the end of the year (see Figure 3a and 3b).
Before and After – Replacement Cooling System for 1948 GSUs
Figure 3a Figure 3b
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6. Example -2
This is an application where a cooler was replaced with a bank of radiators at the Copper mines. Due to
the severe dusty environment, the coolers were getting clogged too frequently and maintaining the
cooling system was a nightmare. Again with the help of the heat run test report, appropriate size of a
replacement radiator cooling system was designed (see Figure 4a and 4b).
Before and After – Replacement Cooling System for the Mining Project
Figure 4a Figure 4b
At this site, the oil temperature was monitored with high temperature alarms. Prior to installation of the
radiator cooling system the high temperature alarm system would trigger frequently but after the
replacement there have been no such calls. Note that the additional blowers were abandoned after the
replacement.
Example -3
Here is another example of a cooling system replacement (Figures 5a to 5d).
Site is identified at a Hydro Facility Proposal for Customer Approval
Figure 5a Figure 5b
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7. Testing of One Transformer Cooling
System Replacement
Replacement on all Transformers
Completed
Figure 5c Figure 5d
BIOGRAPHY
Vasanth Vailoor is a Mechanical Engineer from the Indian Institute of Technology with specialization in
Heat Transfer and Fluid Dynamics. He has now been with Trantech Radiators as the Engineering
Manager for over ten years. Before that he has been associated with industries as diverse as Aviation,
Cryogenics and Renewable Energy focused on modeling, simulation and testing. His wide range of
interests and depth of knowledge in various topics help bridge diverse technologies leading to simple
optimized solutions. He has published over ten papers while still maintaining an active commercial career.
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8. Kevin Riley is a Mechanical Engineer and certified Six Sigma Black Belt. He has been with Trantech
Radiator Products for 6 years and is the Quality and Product Development Manager. Kevin has worked in
the heat exchanger industry for over 10 years holding positions at Young Touchstone and Trantech within
senior engineering and operations for products as diverse as Cuprobraze, Fin and Tube and Plate
Radiator technologies. Kevin has also worked with Electric Utility and Generation customers for over 12
years in the Fleet, Genset and T&D sectors. He currently works on cooling systems development and
components with OEM and utility customers for new and replacement applications covering all types of
transformers and equipment. Before beginning his career in the private sector with Caterpillar, Kevin
served as an officer in the United States Navy.
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