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Dr. Samer AlsadiDepartment of Electrical Engineering &

TechnologyPTU-Kadoorie

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Electrical Energy is Generated at few kV and

stepped up. Transmitted through AC andHVDC lines.Stepped down and distributed at loadcenters.

Its natural mode of synchronous operationknits the system together.

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Why do we need protection?

Electrical apparatus operates at variousvoltage levels and may be enclosed orplaced in open. Under abnormal operatingconditions protection is necessary for

Safety of electrical equipments.Safety of human personnel.

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Types of Protection1.Apparatus Protection

a) Transmission Line Protectionb) Transformer Protection

c) Generator Protectiond) Motor Protectione) Busbar Protection2.System Protectiona) Out-of-Step Protectionb) Under-frequency Relaysc) Islanding Systemsd) Rate of Change of Frequency Relays

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What is a Relay?A relay is a logical element which process theinputs (mostly voltages and currents) from thesystem and issues a trip decision if a faultwithin its jurisdiction is detected.

Inputs to a relay areCurrent from a current transformer.Voltage from a voltage transformer.

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Evolution of RelaysIf we zoom into a relay, we can see three

different types of realization:1) Electromechanical Relays.2) Solid State Relays.3) Numerical Relays.

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Electromechanical RelaysFirst generation of relays.

Uses the principle ofelectromechanical energyconversion.Immune to

electromagneticinterference and rugged.

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Solid State RelaysThese relays were developed with the advent oftransistors, operational amplifiers etc. Theirfunctionality is through various operations likecomparators etc.

Their advantages are1) More flexible.2) Self checking facility.3) Less power consumption and low burden.

4) Improved dynamic performancecharacteristics.5) High seismic withstand capacity.6) Reduced panel space.

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Numerical RelaysOperation of a numerical relay involves analogto digital conversion of voltage and currentsobtained from VT and CTs. These samples arefed to the microprocessor or DSP where theprotection algorithms process these signalsand necessary decisions are taken.

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The advantages of Numerical Relays are1) Maximum flexibility.2) Provides multiple functionality.3) Self checking and communication facility.4) It can be made adaptive.

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What is a Circuit Breaker?A circuit breaker is basically a switch tointerrupt the flow of current. It opens on relaycommand.

It has to handle large voltages and currents.The inductive nature of power system resultsin arcing between the terminals of a CB.CBs are categorized based on the interruptingmedium used.

1) Air CB.2) SF6 CB.3) OIL CB4) VACUM CB

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Summary

Necessity of a protection system.Types of Protection.Three generations of relays.

Role of circuit breakers.