Overcurrent Coordination Basics Transformer

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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
  •   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  
  •   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