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Thermal Overload Protection of Power Transformers Operating Theory and Practical Experience

2005 Georgia Tech Protective Relaying Conference

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Symbols

Some of the functions described in this paper are non-linear in nature. Linear Boolean logicsymbols commonly used in system protection diagrams are not adequate to truly represent these

functions. The process control industry has developed symbols and diagramming formats torepresent non-linear processes. The symbols and diagramming format are commonly known as

SAMA diagrams, as they were originally a standard developed by the Scientific ApparatusMakers Association. Though the Scientific Apparatus Makers Association has declared the

original standard obsolete, and no longer permits the direct association of the organization namewith the industry standard, these symbols, and the term SAMA diagram, are still in common

use in the process control industry.. It is the opinion of the authors that protection engineers needto become familiar with these symbols, as protection systems migrate from traditional protection

and control to automatic process control.

Measuring

Symbol Function

Setpoint

Automatic Signal Processing

Final Controlling

A Analog

f(x) Non-linear function

H/ High signal monitor

K Proportional

DI Time delay on initiation

DT Time delay on termination

X2m

X

Symbol Function

Dividing

Exponential

Multiplying

Summing





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References

[1] IEEE Guide for Loading Mineral-Oil Immersed Power Transformers, IEEE StandardC57.91, Institute of Electrical and Electronic Engineers, New York NY, 1995.

[2] L. L. Grigsby, editor, The Electric Power Engineering Handbook , CRC Press, Boca Raton,FL, 2001.

[3] Adaptive Transformer Thermal Overload Protection, Final Report of IEEE Power System Relaying Committee Working Group K3, IEEE Power Engineering Society Power System

Relay Committee Report, January, 1999.[4] IEEE Guide for Protective Relay Applications to Power Transformers, IEEE Standard

C57.91, Institute of Electrical and Electronic Engineers, New York NY, 2000.[5] G. Swift, D. Fedirchuk, Z. Zhang, A New Relaying Principle for Transformer Overload

Protection, 52nd

Annual Georgia Tech Protective Relaying Conference, May 6-8, 1998.[6] G. Swift, T. S. Molinski, W. Lehn, A Fundamental Approach to Transformer Thermal

Modeling Part I: Theory and Equivalent Circuit , IEEE Transactions On Power Delivery,Vol. 16, No. 2, April 2001, pp. 171 175.

[7] G. Swift, T. S. Molinski, R. Bray, R. Menzies, A Fundamental Approach to Transformer Thermal Modeling Part II: Field Verification, IEEE Transactions On Power Delivery, Vol.

16, No. 2, April 2001, pp. 171 180.[8] SIPROTEC 7UT612 Differential Protection Relay Instruction Manual , Siemens AG,

Nuremburg, Germany, 2002.[9] T-PRO Transformer Protection Relay User Manual Version 3.3 Rev 1, NxtPhase T&D

Corporation, Vancouver, BC, 2003.[10] Functional Diagramming of Instrument and Control Systems, The Measurement, Control

& Automation Association, Williamsburg, VA, 1981.

Special thanks to Wayne Hartmann as the original author of some of the drawings.





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About the Authors

Mike Giordano, P.E. is presently an Electrical Engineer in the Distribution Engineering &

Design Protection and Control department of Northeast Utilities Systems, responsible for substation controls design, transformer protective relay scheme design, distribution protection

design, and distribution automation system design. Mike has also extensive experience in theElectrical Test and Engineering group of NU. In addition, Mike is a Certified Firefighter I and

Incident Safety Officer from the National Fire Academy, and a licensed Professional Engineer inthe State of Connecticut.

Rich Hunt, M.S., P.E. is presently a Senior Field Application Engineer for NxtPhase T&D

Corporation, responsible for technical marketing, technical sales, and technical support of opticalcurrent and voltage sensors, and protective relays and fault recorders. Rich has over 10 years of

utility experience at Virginia Power and the University of North Carolina, as well as over 6 yearsof industry experience. Rich earned his M.S.E.E at Virginia Tech under the guidance of Dr. Arun

Phadke, with a thesis topic entitled Hidden Failure in Protective Relays: Supervision andControl. Rich is a member of the Main Committee of the IEEE PSRC, and is a Professional

Engineer licensed in the Commonwealth of Virginia.

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