Transformer_Cooling 2015 Doble LOAT Conference

©2015 Doble Engineering Company. All Rights Reserved 1
Life of a Transformer™ Seminar
February 9 - 13, 2015 | San Antonio, Texas USA
Training: Transformer Cooling
Basics and Cooling Type Selection
Krzysztof Kulasek
ABB Inc.

• Thermal Stresses
– Losses are always generated during transformer operation
– Hot spot temperature will determine the loss of life
– Insulation ageing accelerates with loading above the nameplate rating
• Mechanical Stresses
– Between conductors, windings insulation structure and the core, leads and windings due to
overcurrents or fault currents caused by short circuits and inrush currents
• Dielectric Stresses
– Due to system overvoltage, transient impulse
conditions or internal resonance of windings
Stresses Acting on Power Transformers
Good design work is done when all limits are met and margins
for thermal, dielectric and mechanical stresses optimized

Loss of life
The Arrhenius equation - the life of a transformer insulation is reduced by a factor of two with ~ 8 ºC
rise in temperature.
Cellulose Fiber Chain
The degree of polymerization (DP) is a measure of the number of intact chains in a cellulose fiber.
DP of transformer insulation is approx. 1,000 at the start of life and approx. 200 at the end of life.
0.001
0.01
0.1
1
10
100
1000
60 70 80 90 100 110 120 130 140 150 160 170 180
IEEE 57.91-2011 Per Unit Life
Per Unit Life at 117 ºC = 9.8 ∗ 10−18
∗ 𝑒
15000
117+273 = 0.4953

Sources of heat in a transformer
No-load (energization of a transformer) Load (loading of a transformer)
Windings
I2R-loss
Eddy current loss
Metallic parts
Eddy current loss will have
its origin in metallic parts
exposed to flux generated
by the load current in
windings and leads.
Hysteresis loss (remagnetization)
• Quality
• Frequency
• Flux density
Eddy current losses
• Thickness
• Resistivity
• Frequency
• Flux density
F
i
D
B
C
A
B
Ic
Transformer losses

Transformer heat dissipation
Conduction - Transfer of heat energy resulting from differences in temperature between materials in contact
(strand to strand in a winding coil, strands to paper insulation)
Convection - a heated surface immersed in a fluid transfers heat by particle movement; an increase in the fluid (or gas)
temperature decreases its density producing circulation and flow (winding to oil, oil to radiators, coolers to air)
Heat Transfered ≈
Thermal conductivity ∗ Surface Area ∗ ∆T
Material Thickness
Heat Transfered ≈ 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 ∗ 𝐸𝑚𝑖𝑠𝑖𝑣𝑖𝑡𝑦 𝐹𝑎𝑐𝑡𝑜𝑟 ( 𝑇𝑜𝑏𝑗𝑒𝑐𝑡- 𝑇𝑎𝑚𝑏𝑖𝑒𝑛𝑡) ∗ 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎
Heat Transfered ≈ 𝐻𝑐 ∗ Tobject− Tfluid ∗ Surface Area
𝐻𝑐- convective heat transfer coefficient is dependent on the type of media,
gas or liquid, the flow properties such as velocity, viscosity and other
flow and temperature dependent properties.
Radiation - transfer of heat energy by electromagnetic waves, the
amount of radiated energy depends on the temperature
and type of the object surface (tank walls, cover)

An example of the three loss types
in a winding
DC-loss (grey); Axial eddy loss (blue)
Radial eddy loss (red)
Winding Load Losses Distribution and Hot Spot
Hot spot calculation
Cooler
oil in
Winding
oil out
Winding
hot spot
Top oil
hot spot factor
Winding
average
Copper
over oil
Ambient Oil out coolerWinding
Temperature
Temperature
distribution of the oil
in the winding

Winding hot spot calculations
Winding hot spot
Oil in the top of the windingLocal losses in the discs
Steps
1. Calculate the oil temperature
distribution in windings
2. Determine the local losses by
means of a FEM program
3. Calculate the winding hot spot
temperature on the basis of the local
losses and the oil temperature in the
winding
IEEE C57.12.00-2010 – 5.11.1. 1 c
“Calculations of the temperatures throughout each active winding and all leads. The calculation method
shall be based on fundamental loss and heat transfer principles and substantiated by tests on production or
prototype transformers or windings.”

Winding cooling
Heat transfer by conduction from conductor to conductor, from conductor to insulation
Example of a multi-strand conductor (CTC)
paper insulation
strand insulation – enamel and epoxy
Thermal conductivity W/m*K
- Copper ~380
- Aluminum ~220
- Paper; pressboard oil-treated ~0.17-0.25
CTC without paper insulation

Oil flow in windings - convection
Guided oil flowNon guided oil flow
Oil flow is blocked and heat transfer
is limited by spacers, sticks and other insulating
elements which are required by dielectric and
mechanical design
Washer
(guides)
controlling
oil
Heat transfer is dependent on the flow properties such as velocity, oil viscosity, vertical
or horizontal cooling surface

Losses and Cooling - Transformer model law
CuFem SJSBtfIEPowerRatedTransfomer ******44.4*~ 
S2 [MVA]
S1[MVA]
Assuming the same use of the active materials (current
density, core flux density) we can derive the following relation
for transformers with rated power S1 and S2
0.5
S1PowerRated
S2PowerRated
A1SurfaceCooling
A2SurfaceCooling







0.75
S1PowerRated
S2PowerRated
L1LossesTrafo
L2LossesTrafo







Losses increase faster with a change in the rated power
than the winding effective cooling surface

Losses and Cooling
• More oil flow required to dissipate
the heat from the core and windings;
larger external cooling system
• Pumps and fans may be required for
larger transformers
ONAN
OFAF
ODAF
Total losses for a very large unit can
be in excess of 2000 kW (2 MW!)
Very low losses comparing to the
available cooling surface

• Heat run test
• Overload test
• Extended Over excitation
Special testing
- Fiber optics can be used to measure winding hot spot rise and top
oil temperatures in the windings.
- Thermocouples can be installed to measure stray loss heating.
- Thermovision scanning can be done to verify tank wall heating
especially for a generator step-up transformer.
Together with the gas analysis this would validate the calculated
temperature rises and confirm the transformer can readily accept
the required base load and overload
Testing
Fibre optics installed in a winding
Example of a thermovision results

• The cooling design together with dielectric and
short circuit consideration is critical for transformer
performance in service
• Winding hot spot is a dimensioning factor; increase
of ~ 6-8 deg. due to a cooling system failure,
material problems, design or production mistakes
will reduce transformer life by a factor of ~ 2
• Cooling system selection and design should be
optimized for expected service conditions (ambient,
loading, footprint) and validated during the factory
acceptance tests
• Load and temperature monitoring (possibly with a
direct measurement by fiber optics) gives users a
possibility to precisely track the loss of life
Transformer Cooling Basics - Summary

Life of a Transformer™ Seminar
February 9 - 13, 2015 | San Antonio, Texas USA
Transformer Cooling Basics
Vasanth Vailoor & Kevin Riley
Trantech Radiator Products, Inc.

Types of Transformer Cooling Systems
• ONAN – Oil Natural Air Natural
• ONAF – Oil Natural Air Forced
• OFAF – Oil Forced Air Forced
• OFWF – Oil Forced Water Forced
• OD Applications - are always forced oil flow that is
directed through predetermined paths in the
transformer winding.

ONAN - Transformer Cooling Systems
• ONAN – Formerly known as OA.
• Natural convection flow of hot oil is
utilized to dissipate heat.
• Radiator or tube applications are
predominately used.
• Least expensive form of cooling if
application and cooling levels are
met.

Pros
• No Pumps or Fans
• Long Life Expectancy
• Very Low Maintenance
• Available in various coatings
• Ability to add pumps or fans for
additional cooling capacity
• Low Noise
Cons
• Consumes more space than some
other applications.
• Requires the most connection
points to transformer.
ONAN - Transformer Cooling Systems

ONAF - Transformer Cooling Systems
• ONAF – Formerly known as FA.
• Natural convection flow of hot oil is
utilized in conjunction with cooling
fans.
• Heat dissipation is increased
across cooling surfaces by air
movement.
• Most common application in the
field.
• Offers dual ratings as an
ONAN/ONAF system.

Pros
• No Pumps needed
• Long Life Expectancy
• Low maintenance on radiators
while fans are considered
consumables.
• Can be dual rated as heat transfer
application depending on fans
being used or turned off.
Cons
• Reduced capacity if auxiliary
power is lost.
• Fans require maintenance and
replacement during transformers
life.
• Noise levels increased as
compared to ONAN applications.
ONAF - Transformer Cooling Systems

OFAF - Transformer Cooling Systems
• OFAF – Formally known as FOA.
• Heat dissipation is increased
across cooling surfaces by air
movement while oil is circulated
with pumps.
• Can cool higher load levels than
ONAN or ONAF applications.
• Higher heat dissipation within a
smaller space / footprint.

OFAF with Radiators
• Fans and pumps work to cool
radiators plates as needed.
• Can be set-up as multiple stage
cooling system.
• Can be used as ONAN system at
lower temperatures.
• Most flexible means of cooling
transformers rated at 30 MVA or
higher.
Fans
Pumps

Pros
• System Flexibility
• Long Life Expectancy with
preventative maintenance.
• Easier maintenance and cleaning
of cooling surfaces.
• Energy consumption of system
can be optimized compared to
other forced oil applications.
Cons
• Auxillary power required.
• Fans and pumps need monitoring
and maintenance.
• Potential issues in certain
environments as with any forced
air application.
OFAF with Radiators

OFAF with Coolers
• Variety of cooling surfaces
and fin styles to meet
applications.
• Requires fans and pumps
operating to achieve cooling
demand.
• Less connections to tank than
other applications.
• Proven technology.

Pros
• High cooling capacities.
• Less space than ONAN / OFAF
with radiator applications.
• Lower Top Oil Temperature in
general use.
Cons
• Consumes more energy and higher
noise levels.
• Fans and pumps require maintenance
and replacement during transformers
life.
• Potential issues in certain
environments as with any forced air
application.
• No cooling capacity with loss of
auxiliary power.
OFAF with Coolers

• Most cooling capacity.
• Oil is cooled by forced water
application.
• Least amount of connections
to tank.
• Requires water source unless
a closed looped system is
used.
• Small and compact but higher
maintenance.
OFWF - Transformer Cooling Systems

Pros
• Highest potential cooling
capacities of all applications.
• Least amount of space.
• Method of cooling has been used
for many years.
• Maintenance personnel are
generally familiar with technology.
• Little or no extra noise.
Cons
• Oil and water pumps can fail.
• Possibilities of oil contamination
by water upon failure.
• Tubes foul and must be cleaned.
• Water source is needed in most
high heat applications.
• Bacteria hazards if water system
is not a closed loop system.
OFWF – Transformer Cooling Systems

• Existing cooling systems can be
adversely effected by new
construction of ballistic barriers and
protective structures.
• New transformer cooling system
designs must take into consideration
these potential air flow obstructions.
• Indoor and underground applications
are becoming more common.
Site Constraints

• Firewall installations or relocation of
transformers can lead to a need for
cooling systems redesign or
enhancement.
• Lack of auxiliary power or water
requirements when relocating
transformers may drive a need for cooling
system changes.
• Noise and seismic requirements can be
factors either by equipment relocation or
regulatory changes.
Site Constraints (cont.)

Cooling System Maintenance (Past)
• Cooling systems historically have seen little or no maintenance until failures
occur.
• Generally visual inspections detect issues.
• With deregulation more transformers are running at higher loads with no
upgrades to cooling capacities or systems.
• Sometimes cooling systems are replaced to original specifications when
cooling upgrades are actually needed.
• Environmental and regulatory constraints sometimes overlooked during
maintenance or transformer relocation.

Cooling System Maintenance (Present)
• Cooling systems designed for OEM and replacement applications should be
viewed as part of the essential component life of the overall transformer.
• Preventative and predictive maintenance applications are essential.
• Lean methodologies used in the manufacturing industry are being
implemented throughout the utility industry. Example: TPM (Total Productive
Maintenance).
• Proper cooling surface protection and maintenance can extend the life of
your cooling system up to 30%.
• Asking the cooling industry experts about what the best system for your
application is instead of repeating old specifications is a must.

Cooling System ‘Musts’:
• At a minimum, complete semi-annual inspections of external surfaces for
corrosion protection. This includes valves, radiators, piping, sheet metal
surfaces, tubes, pumps and any other surface. Just because it is galvanized
or stainless does not always make it less susceptible to damage from
certain elements.
• Complete periodic maintenance on all moving components within the
system. Vibration, thermal scans and sound are still the best ways but you
must document and compare.
• Clean cooling surfaces of build-up, debris and fouling.
• Remember that cleaning is inspection!

Cooling System ‘Musts Not's’:
• Spraying water over the transformer or cooling system as a means to reduce
temperatures for extended periods.
• Using any forced air application in caustic, sea water or chemical environments
without the proper base material and coatings. Ask the heat exchanger supplier for
recommendations of materials.
• Not documenting maintenance inspections and findings.
• Attempting repairs to existing cooling equipment components without proper
materials or knowledge of procedures.
• Allowing leaks in the system to continue without remediation.
• Remember that all transformer components deteriorate and fail. Limited or No
periodic maintenance to the cooling system accelerates system failures.

Factors affecting Heat Transfer
• Width (9”, 12”, 15”, 520mm)
• Length (Custom – up to 15 feet)
• # of Plates
• Hydraulic Diameter
• Oil Flow Rate (ON or OF)
• Air Flow Rate (AN or AF)
• Plate Spacing (1.77”, 2”, 2 ¼”)
•Surrounding environment / constraints
How does it influence
cost-economics?

Case Study – I
90 MVA GSU

Case Study – I (cont’d)
Rusty water cooler removed from transformer

Radiator bank designed with fans

0
5
10
15
20
25
30
35
40
8/2/06 8/4/06 8/6/06 8/8/06 8/10/06
Hourly readings
T,WindingHotSpotminus
Ambient[°C]
10
15
20
25
30
35
40
2/2/06 2/4/06 2/6/06 2/8/06 2/10/06
Hourly readings
T,WindingHotSpotminus
Ambient[°C]

Case Study – II
Copper Mine –
Salt Lake City –
1958 – 90MVA

Case Study – II

Case Study – III (Identification)

Case Study – III (Proposal)

Case Study – III (Field Testing)

Case Study – III (Completion)

Case Study – Tube Replacement

Cooler Design and Selection
• Selection and Matching
- Air or Water flow capacity
- Oil flow requirement
- Operating Conditions

Cooler Design and Selection (cont’d)

• Before
Case Study – Cooler Replacement
Leaking coolers and pumps.
Unit losing up to 30 gallons per
week. Cost of waste oil
remediation, replacement oil
and labor had become a daily
issue.

• After
Case Study – Cooler Replacement
Leaks removed and
environmental issues resolved.
Units designed for 10% safety
factor rating over original
cooling system rating.

Conclusions
• Simple design process using heat transfer analysis but can get
complex especially for Forced Convection Systems.
• Optimization must focus on entire Transformer Cooling System
– not in parts.
• Periodic maintenance is critical to the overall cooling system.
• The cooling system is not an independent system.

Transformer_Cooling 2015 Doble LOAT Conference

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