Overcurrent Coordination Basics Generators

  
     
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Purpose

The purpose of this guide is to provide a basic overcurrent protection philosophy for generators. 		 
  Generator Overcurrent Protection
Time current curve (TCC) landmarks (figure 1)		  
  •   Full load amps (FLA) – located in the upper decade
•   Overload curve – located in the upper 1 or 2 decades
•   Short circuit withstand capability point – located in the middle decade
•   Decrement curve – located in the bottom 3 decades 		 
  TCC regions (figure 2)  
  •   Equipment operating region – located at and to the left of the FLA and the generator decrement curve in the instantaneous region of the lowest decade
•   Equipment damage region – located at to the right of the overload curve and short circuit withstand point, and above the decrement curve
•   Protective device operating region – located between the equipment operating and damage regions 		 
  Suggested overcurrent protection (figure 3)  
  •   Set protection above the full load amps and the decrement curve in the instantaneous region of the lowest decade
•   Set protection below the overload curve and the short circuit withstand point
•   Set protection to intersect with the decrement curve in the second lowest decade 		 
  Comments  
  •   If current penetrates the limits of the overload curve, insulation life is reduced.
•  If protection is set above the decrement curve, the device will never trip. 		 
   
  Fig. 1 Generator TCC landmarks  
   
  Fig. 2 Generator TCC regions  
   
  Fig. 3 Generator overcurrent protection  
     
  References  
  •   Other Application Guides offered by SKM Systems Analysis at www.skm.com
•   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 C37.102, IEEE Guide for AC Generator Protection
•   IEEE Std C37.101, IEEE Guide for Generator Ground Protection
•   ANSI C50.13, Cylindrical-Rotor Synchronous Generators
•   NEMA Std MG-1, Motors and Generators 		 
     
     
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