What is a relay?
• Relay is a sensing device which senses
abnormal voltage and current conditions in
power system and sends signal to circuit
breaker to remove faulty part from rest of the
• First line of defense is provide by the back up
relaying equipment. Back up protection
operates only when primary protection has
Electromagnetic Induction Principle
• The relay operating on this principle can be used
for A.C. quantity only.
• In this relay, first alternating flux produces eddy
currents in aluminium disk.
• The second alternating flux interact with these
eddy currents and thus produces torque.
• Similarly, Eddy currents produced by second flux
interacts with the first flux and torque is
• Net torque produces rotating torque in the disc.
• Over Current Relay1
• Directional Relay2
• Distance Relay6
TYPES OF RELAY
• Instantaneous Over-Current Relays
• Inverse-time Current Relay
• Inverse Definite Minimum Time Current Relay
• Very Inverse Relay
• Extremely Inverse Relay
TYPES OF OVER CURRENT
Feeder can be long, medium or short transmission line and it may also be a
Feeder can be protected by various method:
1) Over Current Protection
2) Distance Protection
3) Pilot Wire Protection
Over Current Protection though simple and
economical, require re-adjustment if change in circuit
Only two phase and one earth fault relay is required for
complete protection of three phase line.
Distance protection is costly so far distribution
network, electric utilities and on sub transmission lines over
current protection is used.
SELECTIVITY IN OVER CURRENT PROTECTION
• Whenever over-current relay is not selective
or circuit requirements change, then distance
protection is preferred.
• Problem with over current relaying is that
fault current depends upon ckt condition and
• If these conditions changes, then overall
protection needs to be changed.
• Transformers need to be protected against
short circuits and over-heating.
• For low KVA ratings, over current relays are
used, but in case of large transformers with
higher KVA ratings, differential protection is
• For this purpose, Buchholz Relay is used.
• Gas actuated relay immersed installed in oil immersed
transformer for protection against all kinds of faults.
• In the field of electric power distribution and
transmission, a Buchholz relay is a safety device
mounted on some oil-filled
power transformers and reactors, equipped with an
external overhead oil reservoir called a conservator.
• The Buchholz Relay is used as a protective device
sensitive to the effects of dielectric failure inside the
Circuit breaker is a piece of
equipment which can:-
Make or break a circuit either
manually or by remote control
under normal conditions
Break a circuit automatically
under fault conditions
Make a circuit either manually
or by remote control under
Consists of fixed and moving contacts called
Under normal conditions, these contacts remain closed
& will not open until and unless the system becomes
When faults occurs in any part of the system, the trip
coils of the circuit breaker get energized and the
moving contacts are pulled apart by some
mechanism, thus opening the circuit.
When a short circuit occurs, a heavy current flows through
the contacts of CB before they are opened.
At the instant when the contacts begin to separate, the
contacts area decreases rapidly and large current increases
current density thus increasing the temperature.
Heat produced in the medium[Oil or Air] is sufficient to
The ionise air or vapour of oil acts as a conductor and an
arc get sustain in the contacts.
Factors responsible for sustenance of Arc.
Potential difference between the contacts.
Ionised particles between contacts.
Methods of Arc Extinction
• High Resistance Method
• Low Resistance Method
OIL CIRCUIT BREAKER
Transformer oil used as arc quenching medium
It insulates between phases and the ground, and it provides
the medium for the extinguishing of the arc.
When electric arc is drawn under oil, the arc vaporizes the
oil and creates a large bubble that surrounds the arc.
The gas inside the bubble is around 80% hydrogen, which
The oil surrounding the bubble conducts
the heat away from the arc and thus also
contributes to deionization of the arc.
• It absorbs the arc energy to decomposes the oil into
gases which have excellent cooling properties
• It acts as an insulator and permits smaller clearance
between live conductors and earth components.
• The surrounding oil presents cooling surface enclose
proximity to the arc.
It is inflammable and there is a risk of a fire.
It may form an explosive mixture with air.
The arcing products (eg. Carbon) remains in
the oil and its quality get deteriorated with
Air-Blast Circuit Breaker
These breakers employ a high pressure air-blast as an arc
The contacts are opened in a flow of air-blast established by
the opening of blast valve.
The air-blast cools the arc and sweeps away the
arcing products to the atmosphere. This rapidly
increases the dielectric strength of the medium
between contacts and prevents from re-
establishing the arc.
The risk of fire is eliminated.
The arcing time is very small due to rapid movement
of the contacts.
The arcing products are completely removed by the
The Air has relatively inferior arc extinguishing properties
Vacuum Circuit Breaker
• When the contacts of the breaker are opened in
vacuum (10−7 to 10−5 torr), an arc is produced
between the contacts
• Vacuum is used as an arc quenching medium.
• Have greatest insulating strength.
• The arc is quickly extinguished because the metallic
vapours, electrons and ions produced during arc rapidly
condense on the surfaces of the circuit breaker
contacts, resulting in quick recovery.
• Compact, reliable and have longer life.
• No fire hazards.
• No generation of gas during and after operation.
• Can interrupt any fault current.
• No noise is produced while operating.
• Require less power for control operation.
The Sulphur hexaflouride SF6 is an electro-negative gas
and has a strong tendency to absorb free electrons.
The contacts of the breaker are opened in a high
pressure flow of SF6 gas and an arc is struck between
SF6 has excellent insulating strength.
When a free electron collides neutral gas molecule a
negative ion is formed and the electron is absorbed by
neutral gas molecule.
SF6 + e SF6
Negative ions formed are relatively higher compared to
Sulphur Hexaflouride (SF6) Breakers
ADVANTAGES OF SF6 C.B.
Current chopping is minimized at low pressure and
Eliminates moisture problem.
Gives noiseless operation.
No carbon particle is formed during arcing.
Atmospheric conditions does not affect Circuit
DISADVANTAGES OF SF6 C.B.
SF6 breakers are costly due to the high cost of SF6
Since SF6 gas has to be reconditioned after every
operation of the breaker, aditional equipment is
• Instrument transformers convert the currents
and voltages of the H.V. lines to values, which
are measurable by meters and protections
• Two types:
– Current Transformer
– Potential/Voltage Transformer.
• A current transformer (CT) is used for measurement of
electric currents. Current transformers, together
with voltage transformers (VT) (potential
transformers (PT)), are known as instrument
• When current in a circuit is too high to directly apply to
measuring instruments, a current transformer
produces a reduced current accurately proportional to
the current in the circuit, which can be conveniently
connected to measuring and recording instruments.
• A current transformer also 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