Overcurrent Coordination Basics Conductors

  
     
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Purpose

The purpose of this guide is to provide a basic overcurrent protection philosophy for conductors.		  
  Cable Overcurrent Protection
Time-current curve (TCC) landmarks (figure 1)		  
  •   Ampacity – located in the upper decade
•   Emergency overload curve – located in the upper 2 decades, typically not shown
•   Short circuit damage curve – located in the bottom 3 decades 		 
  TCC regions (figure 2)  
  •   Equipment operating region – located at and to the left of the ampacity
•   Equipment damage region – located to the right and above the damage curves
•   Protective device operating region – located between the equipment operating and damage regions 		 
  Suggested overcurrent protection (figure 3)  
  •   Set device long time pickup (LTPU) function at or below the ampacity
•   Set all other device functions at or below the damage curves 		 
  Comments  
  •   If current penetrates the limits of the thermal overload curve, cable insulation life is reduced.
•   If the maximum through-fault current penetrates the limits of the short circuit damage curve, cable insulation will be damaged. The through-fault current is defined as the maximum current that can flow for a fault at the load-side terminals of the feeder. 		 
   
  Fig. 1 Cable TCC landmarks  
   
  Fig. 2 Cable TCC regions  
   
  Fig. 3 Cable overcurrent protection  
     
  OH Conductor Overcurrent Protection
TCC landmarks (figure 4)		  
  •   Ampacity – located in the upper decade
•   Short Circuit Damage Curve – located in the bottom 3 decades 		 
  TCC regions (figure 5)  
  •   Equipment operating region – located at and to the left of the ampacity
•   Equipment damage region – located to the right and above the damage curve
•   Protective device operating region – located between the equipment operating and damage regions 		 
  Suggested overcurrent protection (figure 6)  
  •   Set device long time pickup (LTPU) function at or below the ampacity
•   Set all other device functions below the damage region 		 
  Comments  
  •   If the maximum through-fault current penetrates the limits of the short circuit damage curve, conductor damage will occur. The through-fault current is defined as the maximum current that can flow for a fault at the load-side terminals of the conductor.
 
   
  Fig. 4 OH conductor TCC landmarks  
   
  Fig. 5 OH conductor TCC regions  
   
  Fig. 6 OH conductor overcurrent protection  
     
  References  
  •   Other Application Guides offered by SKM Systems Analysis at www.skm.com
•   Aluminum Electrical Conductor Handbook, The Aluminum Association Inc., Washington, D.C., 3rd edition, 1989
•   Electrical Transmission and Distribution Reference Book, ABB Power T&D Company, Raleigh, North Carolina, 1997
•   Protective Relaying Theory and Applications, 2nd Edition, Marcel Dekker, New York, 2004 		 
  The latest revision of:  
  •   IEEE Std 242, IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book)
•   IEEE Std C37.113, IEEE Guide for Protective Relay Applications to Transmission Lines 		 
     
     
     
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