Overcurrent Coordination Basics Generators
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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
|• 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|
| • 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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