THE UNIVERSITY OF LAHORE,ISLAMABAD CAMPUS
COURSE: POWER SYSTEM PROTECTION
Lab Work Sheet 1
Student Name:________________
Reg No: _____________________
Date: _______________________
Marks obtained: ____________________
Checked date: _______________
Instructions:
 Complete all activities in lab and get checked by the lab engineer.
 Home task should be completed before next lab and must checked by the lab engineer.
 At the end of all activities and home task students advised to write a summary in their own words.
 Late submission will lead to deduction of marks.
 Date must be properly mentioned.
LAB EAXPERIMENT NO: 1
INTRODUCTION TO POWER PROTECTION SYSTEM
Objective:

To learn about electrical power protection system components.
Relay:
A relay is an electrically operated switch. It is a mechanical device. Relays are used where it is
necessary to control a circuit by a low-power signal.or where several circuit must be controlled
by one signal. Relays with calibrated operating characteristics and sometimes multiple operating
coils are used to protect electrical circuits from overload or faults; in modern electric power
systems these functions are performed by digital instruments still called "protective relays"
Transformer:
A transformer is an electrical device that transfers energy between two or more circuits through
electromagnetic induction.
Transformer types:
There are various types of transformer. Two of them are explained below;
1. current transformer (CT)
2. potential transformers (PT)
Current Transformer (CT):
A current transformer (CT) is used for measurement of alternating electric currents. Current
transformers, together with potential transformers (PT), are known as instrument transformers. A
current transformer isolates the measuring instruments from what may be very high voltage in
the monitored circuit. Current transformers are commonly used in metering and protective relays
in the electrical power industry.
Potential transformers (PT):
Potential transformers (PT) are a parallel connected type of instrument transformer, used for
metering and protection in high-voltage circuits or phasor phase shift isolation. They are
designed to present negligible load to the supply being measured and to have an accurate voltage
ratio to enable accurate metering.
A potential transformer may have several secondary windings on the same core as a primary
winding, for use in different metering or protection circuits. The primary may be connected
phase to ground or phase to phase. The secondary is usually grounded on one terminal.
Circuit breaker:
A circuit breaker is an automatically operated electrical switch designed to protect an electrical
circuit from damage caused by overload or short circuit. Its basic function is to detect a fault
condition and interrupt current flow. Unlike a fuse, which operates once and then must be
replaced, a circuit breaker can be reset manually or automatically to resume normal operation.
Circuit breakers are made in varying sizes, from small devices that protect an individual
household appliance up to large switchgear designed to protect high voltage circuits feeding an
entire city.
Types of circuit breakers:
 Low-voltage circuit breakers
Low-voltage (less than 1,000 VAC) types are common in domestic, commercial and industrial
application
 Sulfur hexafluoride (SF6) circuit breakers
A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to
quench the arc. They are most often used for transmission-level voltages and may be
incorporated into compact gas-insulated switchgear.
 Vacuum circuit breaker
A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takesplace in
vacuum. The technology is suitable for mainly medium voltage application.
 Oil circuit breaker
Mineral oil has better insulating property than air. In oil circuit breaker the fixed contact and
moving contact are immerged inside the insulating oil. Whenever there is a separation of current
carrying contacts in the oil, the arc in circuit breaker is initialized at the moment of separation of
contacts, and due to this arc the oil is vaporized and decomposed in mostly hydrogen gas and
ultimately creates a hydrogen bubble around the arc. This highly compressed gas bubble around
the arc prevents re-striking of the arc after current reaches zero crossing of the cycle. The oil
circuit breaker is the one of the oldest type of circuit breaker
Isolator:
Isolator is used to ensure that an electrical circuit is completely de - energised for service or
maintenance. Such switches are often found in electrical distribution and industrial applications,
where machinery must have its source of driving power removed for adjustment or repair. Highvoltage isolation switches are used in electrical substations to allow isolation of apparatus such
as circuit breakers, transformers, and transmission lines, for maintenance. The isolator or
disconnector is usually not intended for normal control of the circuit but only for safety isolation.
Isolators may be used in high voltage devices like transformers.
Difference between isolator and circuit breaker
• Isolator is an off-load device while, circuit breaker is an on-load device.
• Isolator is a switch operated manually, which separate the circuit from the power main and
discharges the trapped charges in the circuit.
• Circuit breakers operate automatically, triggered by electromechanical mechanism inside and
are a safety feature for abnormal loads and voltages in the circuit.
Fuse:
Fuse is a type of low resistance resistor that acts as a sacrificial device to provide over current
protection, of either the load or source circuit. Its essential component is a metal wire or strip that
melts when too much current flows through it, interrupting the circuit that it connects. Short
circuits, overloading, mismatched loads, or device failure are the prime reasons for excessive
current. Fuses are an alternative to circuit breakers.