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Purpose

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

Comments

• 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

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.

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

 

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