A Seminar on “The Effect of Inrush Current on Transformer Protection”

Guided By:Mr. Virendra SangataniAssociate ProfessorDepartment of EIC

Presented By:Tushar LilaniIV Year, EE

DEPARTMENT OF ELECTRICAL ENGINEERING POORNIMA COLLEGE OF ENGINEERING

ISI-6 RIICO INSTITUTIONAL AREA, GONER ROAD, SITAPURA, JAIPUR

CONTENTS

• TRANSFORMER• TRANSFORMER FAULTS• TRANSFORMER PROTECTION• DIFFERENTIAL PROTECTION• OVERCURRENT PROTECTION• INRUSH CURRENT• HARMONIC RESTRAINT RELAY• CONCLUSION

TRANSFORMER

Transformer is an AC machine thata)Transfers Electrical Energy.b)Without the change in frequency.c) Electromagnetic Induction.d)Electric circuits linked by common magnetic

circuit.

ELEMENTARY TRANSFORMER

TRANSFORMER FAULTS

• External Faults/Through Faults : Beyond the protected zone of transformers.

• Internal Faults : Primary consideration for transformer protection. Further divided into two groupsa) Short circuit in the transformer winding

and connections.b) Incipient Faults.

TRANSFORMER PROTECTION

• The development of modern power systems has been reflected in the advances in transformer design.

• This has result in a wide range of transformers with sizes ranging from a few kVA to several hundred MVA being available for use in a wide variety of applications.

TRANSFORMER PROTECTION

The choice of protection for any given power transformer depends upon a number of factors such as

• Size• Importance• Tap Changer(On Load or Off Load)

TRANSFORMER PROTECTION

• For small size transformers up to 1 MVA simple fuses are employed.

• For medium size transformers below 10 MVA over current protection is used.

• For large size transformers above 10 MVA differential protection is used.

• Other protections are earth fault, over heating, over fluxing , parallel transformers.

DIFFERENTIAL PROTECTION

Principle of Differential Protection• Fault within an electrical equipment would

cause the current entering it to be different from the current leaving it.

• By comparing the two currents either in magnitude or in phase or in both, fault can be determined.

• It is an attractive option if both the ends of the apparatus are located near each other.

DIFFERENTIAL PROTECTION

DIFFERENTIAL PROTECTION

Differential relay responds to vector difference between two or more similar electrical quantities.

Applications• Protection of Transformers• Protection of Generators• Protection of Generator-Transformer Unit• Protection of Feeders

DIFFERENTIAL PROTECTION

OVERCURRENT PROTECTION

Principle of Over Current Protection • When the current in a system exceeds a

predetermined value, it indicates the presence of a fault.

• Relaying decision is based solely on the magnitude of current.

• Used in radial distribution systems.

OVERCURRENT PROTECTION

OVERCURRENT PROTECTION

Over current relay operates when load current exceeds a preset value. EM Relays are widely used.

Applications• Protection from Overloads.• Short-Circuit Protection.• Backup Protection with Differential Protection

OVERCURRENT PROTECTION

INRUSH CURRENT

When an unloaded transformer is switched on to normal voltage on its primary side or when a short circuit occurs on its secondary side a transient rush of current may take place this phenomenon is referred to as switching in phenomena or inrush phenomena.

INRUSH CURRENT

Factors which influence the magnitude an duration of magnetizing inrush current are

• Size of transformer.• Size of power system.• Type of magnetic material in the core.• Residual flux in transformer before switching.• How the transformer is energized.

INRUSH CURRENT

INRUSH CURRENT

INRUSH CURRENT

INUSH CURENT

Φ = Φdc - Φm cos ωtOR

Φ = Φm (1 - cos ωt)

HARMONIC RESTRAINT RELAY

Harmonic contents in the inrush current are as follows

• DC Component 40% - 60%• Second Harmonics 30% - 70%• Third Harmonics 10% - 30%Relay used is made insensitive to magnetizing

inrush current.

HARMONIC RESTRAINT RELAY

• Operating principle is to filter out the harmonics from the differential current.

• A tuned circuit is used which allows only the fundamental component of the current to pass through operating coil.

• Relay do not operates if 2nd harmonics exceeds 15% of the fundamental.

• Over current relay operates above maximum inrush current.

HARMONIC RESTRAINT RELAY