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
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