10. overcurrent coordination basics conductors

7/27/2019 10. Overcurrent Coordination Basics Conductors

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Overcurrent Coordination Basics Conductors

The information presented in this application guide is for review, approval, interpretation andapplication by a registered professional engineer only. SKM disclaims any responsibility andliability resulting from the use and interpretation of this information.

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Purpose

The purpose of this guide is to provide a basic overcurrent protection philosophy for conductors.

Cable Overcurrent ProtectionTime-current curve (TCC) landmarks (figure 1)

Ampacity located in the upper decade Emergency overload curve located in the upper 2 decades, typically not shown

Short circuit damage curve located in the bottom 3 decades

TCC regions (figure 2)

Equipment operating region located at and to the left of the ampacity

Equipment damage region located to the right and above the damage curves

Protective device operating region located between the equipment operating anddamage regions

Suggested overcurrent protection (figure 3)

Set device long time pickup (LTPU) function at or below the ampacity Set all other device functions at or below the damage curves

Comments

If current penetrates the limits of the thermal overload curve, cable insulation life isreduced.

If the maximum through-fault current penetrates the limits of the short circuit damagecurve, cable insulation will be damaged. The through-fault current is defined as themaximum current that can flow for a fault at the load-side terminals of the feeder.


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Fig. 1 Cable TCC landmarks


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Fig. 2 Cable TCC regions


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Fig. 3 Cable overcurrent protection

OH Conductor Overcurrent Protection

TCC landmarks (figure 4)

Ampacity located in the upper decade Short Circuit Damage Curve located in the bottom 3 decades

TCC regions (figure 5)

Equipment operating region located at and to the left of the ampacity

Equipment damage region located to the right and above the damage curve

Protective device operating region located between the equipment operating anddamage regions

Suggested overcurrent protection (figure 6)

Set device long time pickup (LTPU) function at or below the ampacity Set all other device functions below the damage region

Comments

If the maximum through-fault current penetrates the limits of the short circuit damagecurve, conductor damage will occur. The through-fault current is defined as themaximum current that can flow for a fault at the load-side terminals of the conductor.


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Fig. 4 OH conductor TCC landmarks


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Fig. 5 OH conductor TCC regions


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Fig. 6 OH conductor overcurrent protection

References

Other Application Guides offered by SKM Systems Analysis at www.skm.com Aluminum Electrical Conductor Handbook, The Aluminum Association Inc., Washington,

D.C., 3rd edition, 1989 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 Industrialand Commercial Power Systems (IEEE Buff Book)

IEEE Std C37.113, IEEE Guide for Protective Relay Applications to Transmission Lines

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10. overcurrent coordination basics conductors

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