Overcurrent Coordination Basics Capacitors

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The purpose of this guide is to provide a basic overcurrent protection philosophy for capacitors.
  Transformer Overcurrent Protection
Time-current curve (TCC) landmarks (figure 1)
  •   Full load amps (FLA) – located in the upper decade
•   Case rupture curve – located in all 5 decades
  TCC regions (figure 2)  
  •   Equipment operating region – located at and to the left of the FLA marker
•   Equipment damage region – located to the right and above the case rupture 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
•   Set protection below the case rupture curve
  •   If current penetrates the limits of the case rupture curve, the capacitor enclosure will fail. Failure of the enclosure will cause a discharge of liquid and may cause damage to adjacent capacitor units.  
  Fig. 1 Capacitor TCC landmarks  
  Fig. 2 Capacitor TCC regions  
  Fig. 3 Capacitor 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.
  The latest revision of:  
  •   IEEE Std C37.99, IEEE Guide for the Protection of Shunt Capacitor Banks
•   IEEE Std 18, IEEE Standard for Shunt Power Capacitors
•   IEEE Std 1036, IEEE Guide for the Application of Shunt Power Capacitors
•   NEMA Std CP-1, Shunt Capacitors