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INTRODUCTION
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Structure of Electric Power Systems
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Power System Components
Generators
Transformers
Transmission Lines
Feeders
Motors
Capacitor Banks
Bus Bars
Reactors
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Abnormal Conditions In A Power System
Short circuits in the transmission or distributionlines
Over-voltages due to switching or lightning
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Over-speeding of generators or motors
Loss of excitation of the machines
Overheating of the stator and rotor of themachines
Low oil levels in the transformers and circuit
breakers
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Need For Power System Protection
To detect abnormalities (faults)
To eliminate such abnormality by isolating
smallest portion of the system in a shortestperiod of time
To prevent injury to personnel
To prevent damage to equipment
Enable continuous service in undamaged part ofthe network
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Protective RelayWhat should it do ?
Monitor system parameters continuously (V, I,
P, F)
Operate quickly when necessary(Dependability)
Should not operate wrongly (Stability,
Discrimination)
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Essential Qualities Of Protective Relaying
Reliability
Selectivity and Discrimination
Speed and Time
Sensitivity
Stability
Adequateness
Simplicity and Economy
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Zones of Protection
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Development of Protective Relays
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Static Relays
The relays which do not use moving parts and
use the solid state electronic components are
called static relays.
Comparators, level detectors, zero crossing
detectors etc are used for measurement and
comparison of electrical quantities.
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Basic Elements of A Static Relay
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Limitations of Static Relays
They are temperature sensitive
They are sensitive to voltage transients
They need an auxiliary power supply Overload capacity
Reliability
Cost
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Summation
It is the combination of a number of electrical
quantities into a single quantity through static
equipment.
Single Input Devices
Two Input Devices
Multi-input Devices
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Single Input Devices
Non-critical repeat function (All or nothing
relays)
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Critical or measuring function
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Fixed time or definite time function
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Input dependent time function
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Two Input Devices
Comparators
Level Detectors
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Comparators:
Amplitude comparator
Phase comparator
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Amplitude comparator
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Phase comparator
Cosine type
Sine type
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Level Detector
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Multi-input Devices
