Power System Protection - CDEEP · Power System Protection S.A.Soman Abnormality and Faults Abnormal State Sources of Fault Current Introduction to ... The transformer short circuit

Power SystemProtection

S.A.Soman

Abnormalityand FaultsAbnormal State

Sources of FaultCurrent

Introduction toFuse

Fuse Charac-teristicsThermalCharacteristics

InterruptingCharacteristics

Classificationof FusesNon-Current LimitingFuses

Current LimitingFuse

Power Class andDistribution ClassFuses

Physics of ArcInterruption

Summary

Power System Protection

S.A.Soman

Department of Electrical EngineeringIIT Bombay

Fuse Protection


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

1 Abnormality and FaultsAbnormal StateSources of Fault Current

2 Introduction to Fuse

3 Fuse CharacteristicsThermal CharacteristicsInterrupting Characteristics

4 Classification of FusesNon-Current Limiting FusesCurrent Limiting FusePower Class and Distribution Class Fuses

5 Physics of Arc Interruption

6 Summary


S.A.Soman



Introduction toFuse







Summary

Fuse Protection






6 Summary


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Abnormal State

Pertains to deviation from the rated operating point.Operation in this state for a long time can lead todamage of the equipment.It is also unsafe to operate in this region.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection






6 Summary


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Sources of Fault CurrentIn a radial system with single source, the magnitude of faultcurrent depends upon the following:

Source contribution.

Transformer impedance.

Motor contribution

Distance of fault from the source.

The transformer short circuit current is given by,

Isc = Transformer Full Load Current × 100%Z


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Introduction to FuseA fuse is a device that opens a circuit with a fusible partwhich is heated and severed by current flowing through it.Itsoperation involves two phases.

Melting of the fuse element when the current has largeenough magnitude.Current interruption when the arc stuck is de-ionized.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Fuse Characteristics

Thermal Characeristics

Current rating.Melting characteristics.

Interrupting Characteristics

Voltage rating.Interrupting rating.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection


Thermal CharaceristicsCurrent rating.Melting characteristics.


Voltage rating.Interrupting rating.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection


Thermal CharaceristicsCurrent rating.Melting characteristics.

Interrupting CharacteristicsVoltage rating.Interrupting rating.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection






6 Summary


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Thermal Characteristics

The melting time of fuseis inversely proportionalto the square ofmagnitude of current.

This relationship is givenby its melting timecurrent characteristics.

Fuse has very inversetime currentcharacteristics.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection






6 Summary


S.A.Soman



Introduction toFuse







Summary

Fuse Protection


During arcing period, fuse must withstand transientvoltage condition and steady state recovery voltage.

Because of the stored energy in the inductive elementsof the power system, for large currents, arcing time islarge.

Addition of melting time and arcing time gives the totalclearing time.

Total clearing curve (TCC) gives this information.

These characteristics are used to coordinate fuse withovercurrent relays.

Voltage rating of a fuse is the highest voltage at whichfuse is designed to operate.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Total Clearing Curve


S.A.Soman



Introduction toFuse







Summary

Fuse Protection






6 Summary


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Non-Current Limiting Fuse

Expulsion Fuse

It is used in overhead circuits and equipments whereexpulsion gases causes no problem.Due to high temperature of the arc, special gasgenerating materials like fiber, melanine,boric acid, oiletc rapidly create gases.These gases help to reduce the arc channel anddeionize the arc.This leads to rapid build up of dielectric strength thatcan withstand the transient recovery voltage.

Vacuum Fuse

Its design, operation and current-voltage-timecharacteristics are similar to that of expulsion fuse.It is a completely sealed unit.There is no expulsion action.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection


Expulsion FuseIt is used in overhead circuits and equipments whereexpulsion gases causes no problem.Due to high temperature of the arc, special gasgenerating materials like fiber, melanine,boric acid, oiletc rapidly create gases.These gases help to reduce the arc channel anddeionize the arc.This leads to rapid build up of dielectric strength thatcan withstand the transient recovery voltage.

Vacuum Fuse

Its design, operation and current-voltage-timecharacteristics are similar to that of expulsion fuse.It is a completely sealed unit.There is no expulsion action.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection


Expulsion FuseIt is used in overhead circuits and equipments whereexpulsion gases causes no problem.Due to high temperature of the arc, special gasgenerating materials like fiber, melanine,boric acid, oiletc rapidly create gases.These gases help to reduce the arc channel anddeionize the arc.This leads to rapid build up of dielectric strength thatcan withstand the transient recovery voltage.

Vacuum FuseIts design, operation and current-voltage-timecharacteristics are similar to that of expulsion fuse.It is a completely sealed unit.There is no expulsion action.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Current-Voltage-Time Relationship of Expulsion Fuse


S.A.Soman



Introduction toFuse







Summary

Fuse Protection






6 Summary


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Current Limiting Fuse

The basic principle is to insert a high resistance in thefault circuit which is otherwise inductive.

This makes zero crossing of current and voltage inphase.

Thus temporary arc is extinguished and dielectricmedium will be quickly deionized.

The fusible element used is very long and surroundedby silica sand.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Current Limiting Fuse


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Current-Voltage-Time Relationship of Current Limiting Fuse


S.A.Soman



Introduction toFuse







Summary

Fuse Protection






6 Summary


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Classification Based on Domain of Application

Power class fuses which are used in or near togenerating station or substation for three phase circuits.

Distribution fuses have specifications closely matchingto distribution system and are away from source orsubstation on a single phase or a three phase system.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Physics of Arc Interruption

The differential equation governing the simplified faultcurrent circuit is given by,

v(t)− earc(t) = Ldidt

i(t) =1L

∫(v(t)− earc(t))dt

earc(t) is the arc voltage and v(t) is the source voltage.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

From the equation it can be seen that

The current is proportional to the area under thedifference of source and arc voltage.

A higher source voltage will adversely affect theinterruption of current.

A high fuse arc voltage will help in limiting the faultcurrent.

The fuse arc voltage sustained over a time has apositive impact on limiting fault current.

Lower the inductance, higher the available prospectivefault current.


S.A.Soman



Introduction toFuse







Summary

Fuse Protection


S.A.Soman



Introduction toFuse







Summary

Fuse Protection

Summary

Abnormality and faults.

Sources of fault currents.

Fuse for overcurrent protection.

Characteristics of fuse.

Classification of fuse.

Physics of arc interruption.

Documents

Power System Protection - CDEEP · Power System Protection S.A.Soman Abnormality and Faults Abnormal State Sources of Fault Current Introduction to ... The transformer short circuit