Switchgear and Power
System Protection
An Introduction and
Course Outline
Why we need Power
System Protection….?
Why we need Power System
Protection….?
Electrical apparatus operates at various voltage levels and may be enclosed
or placed in open. Under abnormal operating conditions protection is
necessary for:
• Safety of electrical equipment.
• Safety of human personnel.
Power System Operation
Normal
Abnormality
Operation Faults
Power
Series Swing
Open Frequency
Conductor drift
Voltage
Shunt
Unbalance
Ground
Lightning
Fault
Phase Fault
Simultaneous
Faults Causes
Electrical Mechanical
Flash Over Falling Branch
Insulation
Subjective Faults
Failure
Mechanical damage
of equipment
What is System Protection?
System protection is the art and science
of detecting problems with power system
components and isolating these
components. Problems on the power
system include:
1. Short circuits
2. Abnormal conditions
3. Equipment failures
Basic Protection Philosophy
• Implementing a system adopting components, which
should not fail and requires the least or nil
maintenance to maintain the continuity of service. This
is very important terms nowadays in deregulated
markets.
• The main idea is to restrict the disturbances during
such failures to a limited area and continue power
distribution in the balance areas.
Protective relays monitor
the current and/or voltage
of the power system to
detect problems with the
power system. Currents
and voltages to relays are
supplied via CT’s and PT’s
Basic construction of protection system
What is a Circuit Breaker?
• A circuit breaker is basically
a switch to interrupt the flow
of current.
• It opens on relay command.
• It has to handle large
voltages and currents.
• The inductive nature of power
system results in arcing
between the terminals of a
CB.
• CBs are categorized based
on the interrupting medium
used. 115 kV circuit breaker
Generating
Area ZA
Zs
Prot.
relay
Generating
Area ISC
Zs
VSC
Prot.
Relay
Generating
Area
Zs
Prot.
Relay
Generating
Area
Zs
Prot.
Relay
Protection system operation
What is a Relay?
A relay is a logical element which process the inputs (mostly voltages
and currents) from the system and issues a trip decision if a fault within
its zone is detected. Inputs to a relay are:
Current from a current transformer.
Voltage from a voltage transformer.
Logic of relay operation
Generations of Relays
• Electromechanical Relays
• First generation of relays.
• Uses the principle of
electromechanical energy
conversion.
• Immune to
electromagnetic
interference and rugged
Electromechanical Relay
