Transformer Winding Hot Spot
Temperature Determination
Jean-Noël Bérubé
Jacques Aubin
W. McDermid
Neoptix, Inc.
Manitoba Hydro
Weidmann-ACTI’s Fifth Annual Technical Conference
November 2006
Albuquerque, NM
Why Monitoring Temperatures ?
„
In power transformers, winding temperatures
have a direct impact on insulation aging
„
Proper monitoring of operating temperatures
are essential to assess the value of insulation
aging and resulting remaining life
„
Allows for better asset management and
revenue generation strategies
Limitations of Transformer
Temperature Rise Tests
„
“Heat Run” tests reveal average winding temperature
under rated load
„
What is of interest for insulation aging is the winding
hottest spot temperature
„
Temperature is highest at top of winding
„
„
„
„
Oil is hotter
Stray losses in winding are higher
Winding insulation often calls for more paper to provide better
insulation against voltage surges
Correct knowledge of operating temperatures is needed
to evaluate insulation aging and remaining life
Aging Acceleration Factor
Winding insulation
sensitivity to temperature
1000
Normal Kraft
Paper (IEC)
100
10
Normal Kraft
paper (IEEE)
1
0.1
60
80
.
Thermally
Upgraded Paper
100
120
140
0.01
Hot-Spot temperature
160
180
Winding Hottest Spot
Temperature Model
Winding Hottest Spot
Temperature Model
Top oil temperature
Winding hot-spot
temperature
Winding Hottest Spot
Temperature Model
Top-Oil
Temp.
Hot-Spot
Temp.
Wi
nd
ing
Oi
l
Hot-Spot
Rise
Average
Winding
Temp.
Temperature (oC)
Winding Hottest Spot
Temperature Model
For any load level, the hottest winding temperature is
assumed to be:
Winding
hot-spot
temp.
=
Top-Oil
Temp.
+
Hot-spot rise
at rated load
* (%Load)2m
For several decades, this method was a standard feature:
• IEEE C57.91 - 1995 “IEEE Guide for Loading Mineral Oil
Immersed Transformers”
• IEC 60354 - 1991 “Loading Guide for Oil-Immersed Power
Transformers”
Winding Hottest Spot
Temperature Model
This simplified method is now regarded as inadequate
• To estimate the aging of transformers
• Given increasing occurrences of overloads
IEEE and IEC are proposing new methods to take account of
(neglected in the previous equation):
• True oil temperature in the cooling duct
• Change in winding resistance with temperature
• Change in oil viscosity with temperature
• The effect of tap changers
• Oil inertia in case of sudden overload of large magnitude
Winding Hottest Spot
Temperature Model
This evolution in calculation method indicates:
• Methods used until now are not very accurate
• New methods will require additional parameters
that are not always readily available
• New models still rely on information provided by
the transformer manufacturer and not always
validated
• Direct measurement of winding temperature with
fiber optic sensor is recognized as the best method
Direct Temperature Monitoring
„
Direct winding temperature measurement provides
valuable information for design, heat-run testing,
maximizing loading and maintenance
„
Safely maximizes transformer loading
„
Avoids catastrophic failure and emergency shutdowns
by monitoring long-term, gradual transformer
deterioration
„
Allows better timing for winding cooling requirements,
to avoid overheating around hot-spot immediate areas
„
Gathers valuable information for scheduling
maintenance and replacement of units
Introduction to Fiber Optic
„
Fiber optic (glass) is a method of carrying information,
as a copper wire. But unlike the copper wire, fibers
carry light (photons) instead of electricity (electrons)
„
Some advantages of fiber optic are compared to RTD
sensors and IR based sensors:
„
„
„
„
„
„
„
Immunity to electromagnetic fields
All dielectric material probe construction
Can be installed in harsh environments
Robust, flexible and chemically resistant probes
True intrinsic safety in explosive environments
Minimal thermal shunting
Relative ease of installation
Operating Principle
„
„
Based on a well understood and
reproducible phenomenon: the
variation in the absorption
spectrum of the semiconductor
GaAs with respect to
temperature
A direct contact temperature
sensor
System Design
„
White
Light
Source
o
Pr
be
Optical Coupler
„
Spectrometer
The system is very
simple yet elegant and
consists of a light
source, an optical
coupler, the probe
and a spectrometer.
Multi-channel models
use the same design
but the number of
component (other than
the probe) is greater.
Oil-Immersed Transformer Probe
Probe Design Details
„
„
Designed to allow complete oil impregnation,
rapid response time, high dielectric strength
and chemical resistance
GaAs based probe characteristics:
„
„
„
„
Designed for a minimum life time of 25 years
No drift
No recalibration
Ruggedized design allows for minimal probe
breakage and lost during installation
Probe Design
„
„
„
An optical fiber delivers white light to the semiconductor crystal
Some of the light is more or less absorbed--this absorption is
dependent on the temperature at the tip of the probe
The light is reflected back by a dielectric mirror and returns
through the same fiber for analysis
semiconductor crystal
fiber core fiber cladding
dielectric mirror
Fiber Optic Sensor can be in
Contact with Conductor
Fiber Optic Sensor can
Inserted in Disk Spacer
Can be inserted between
the last few disks near
the top of the winding
Location in Disk Spacer
is Adequate
Temperature (°C)
2.2
FO Sensor in contact with conductor (top of bundle)
2.0
FO Sensor in contact with conductor (bottom of bundle)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
Load
0.4
0.2
80
60
40
20
0
0
12
24
36
Time (h)
48
60
72
Load (p.u.)
FO Sensor in spacer
100
Optic Fiber Handling & Testing
During transformer
assembly, optical fibers
are safely spooled and
attached to the winding.
Bright colors, such as
orange or blue, also help
minimize breakage
during installation.
A portable test unit can
be very useful for testing
probes as they are
installed.
Feed-Through Connection
Development in
optical fiber
technology
allows for low
loss connection
and leak free
operation
(epoxy-less
design)
Temperature
sensor, inside
transformer
Tip
Extension
cable, to
monitoring
system
Feedthrough for tank wall
Interfacing & Communication
„
Systems are now available with open communication
protocols
„
„
Some popular protocols include:
„
„
„
„
„
No more proprietary communication schemes!
OPC (T/Guard+)
Modbus (T/Guard)
Others: CANopen, Profibus, Devicenet
Analog outputs (4-20mA)
Hardware interfacing is also flexible
„
„
„
„
RS232
RS422 – RS485
TCPIP (Ethernet) emulation
Analog outputs
An Example: Monitoring System
(T/Guard)
„
„
„
„
Supports up to 16 channels in a single unit
Ruggedized design for heavy industry applications
RS-232 and analog output standard
Assistant Windows™ compatible software
-Options:
-Modbus protocol
-RS485
-Supports a
network of 32
T/Guard’s
-TCPIP interface
An Example: Monitoring and
Controlling System (T/Guard+)
„
„
„
„
„
„
Supports up to 8 channels in a single unit
16 Type-C relays, can be set as type A, B or C relays)
Ruggedized design for heavy industry
applications; built with a PLC,
galvanic-isolated relays.
RS-232 and analog output standard
OPC Server built-in
Options:
„
„
Communication: CANopen Profibus,
Modbus, Devicenet and Ethernet server.
Data logging
OPC-Based Communication
„
International Industry Standard Organization
„
„
„
„
Fully supported by Microsoft (uses DCOM)
The Vision of OPC is the Adapted Standard for interOperability
„
„
„
„
„
280+ member companies
1500+ total companies build OPC products = 7500+ products
For moving information vertically from the factory floor
through the enterprise of multi-vendor systems
For moving information between devices on different
networks from different vendors
Not just data but information…
Reliable - Secure Integration is built-in
www.opcfoundation.org
OPC and the T/Guard+
„
An example: direct logging to Excel
„
Can monitor/set 500+ variables
Field Experience
–
–
–
–
Converter transformer
ODAF cooling
107 MVA
Dorsey sub, Manitoba
Hydro
Field Experience
Temperature (°C)
100
80
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
60
40
Load
20
Ambient
0
0
12
24
36
Time (h)
48
60
72
Load (p.u.)
Prediction from manufacturer using IEEE model
2.2
Measured value with FO sensor
2.0
Corrected Prediction
1.8
Economic Benefits of Accurate
Temperature Measurement
•Transformers have inherently some overloading
capability
•The loading capability is highly dependent on
winding temperature
•Dependable measurements of winding temperatures
allow to take full advantage of overloading
capability
•Market opportunities can generate important
benefits if extra load can be handled under safe and
predictable conditions
Economic Benefits, Transformer
Overloading, An Example
1.Transformer rated power (MVA)
100
2.Overloading margin made available by monitoring (%)
10
3.Probability of overloading opportunity (hr/year)
450
4.Financial benefit from energy transmitted ($/MWh)
80
Yearly benefit from extra loading (1 x 2 x 3 x 4 ) =
$360,000
Economic Benefits, Transformer
Overloading, An Example
5. Replacement cost of transformer ($)
2 000 000
6. Transformer normal life duration (hours)
150 000
7. Additional aging factor at 110% load (125°C)
3.4
Cost for additional loss of life ((5 / 6) x 7 x 3 )
=
$20,400
Net yearly benefit from overloading :
$360,000 - $20,400 = $339,600
Conclusions (1)
•For operators and utilities, overloading of transformers is
often a better alternative to more and/or larger
transformers
•Aging of power transformers is mainly driven by winding
temperature
•More frequent loading to full capacity has shown need for
better control of winding temperature
•Recent developments in IEEE and IEC loading guides have
shown that simple calculation methods used in the past
are not fully dependable
Conclusions (2)
•Fiber optic sensors have reached a level of
dependability that makes them a natural choice for
this important function
•Interfacing to utility’s computers is now easier than
ever, thanks to open communication schemes, such as
Modbus, OPC, and other non-proprietary
communication schemes.
Thanks!