Overcurrent Coordination Basics Transformer

  
     
  The information presented in this application guide is for review, approval, interpretation and application by a registered professional engineer only. SKM disclaims any responsibility and liability resulting from the use and interpretation of this information.

Reproduction of this material is permitted provided proper acknowledgement is given to SKM Systems Analysis Inc

Purpose

The purpose of this guide is to provide a basic overcurrent protection philosophy for transformers.		  
  Transformer Overcurrent Protection
Time-current curve (TCC) landmarks (figure 1)		  
  •   All landmarks are based on the nominal kVA rating of the transformer
•   Full load amps (FLA) – located in the upper decade
•   Thermal damage curve – located in the upper 3 decades
•   Mechanical damage curve – located in the middle decade
•   Inrush points defined @ 12 x FLA and 0.1 seconds, and 25 x FLA and 0.01 seconds 		 
  TCC regions (figure 2)  
  •   Equipment operating region – located at and to the left of the FLA and inrush points
•   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 protection above the full load amps and inrush points
•   Set protection below the damage curves		  
  Comments  
  •   If current penetrates the limits of the thermal damage curve, insulation damage may occur.
•   If current penetrates the limits of the mechanical damage curve, cumulative mechanical damage may occur. 		 
   
  Fig. 1 Transformer TCC landmarks  
   
  Fig. 2 Transformer TCC regions  
   
  Fig. 3 Transformer 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 C37.91, IEEE Guide for Protective Relay Applications to Power Transformers
•   IEEE Std C57.12.59, IEEE Guide for Dry-Type Transformer Through-Fault-Current Duration
•   IEEE Std C57.109, IEEE Guide for Liquid-Immersed Transformer Through-Fault-Current Duration 		 
     
     
  back to Application guides