POWER SYSTEM PROTECTION relays, types of relays, comparison, operation of relay switch gears. protection,Circuit Breakers, types, circuit, breakers, electrical engineering, electrical, switch, switch gears. protection, relay, relays,

2.  Introduction
 Protection of transformers
 Protection of generators
 Protection of transmission and distribution
lines
 Bus protection

4. A part of the system protected by a certain protective
scheme is called as protective zone or zone of
protection.
The entire part of a power system is covered by several
protective zones and no part of the system is left
unprotected.
The protective zones are shown in figure.
Overlapping of zones is done to ensure safety of each
element in the system.

5. Primary Protection –This is the first protection to act. This is
responsible to protect power system element from all types of
faults .If primary protection fails, then back up protection comes
into action and removes the faulty part from healthy system.
Backup Protection- This protection comes into action when primary
protection fails.
If due to some reason (such as fault in CT,PT,trip circuit etc.) the
primary protection fails to operate, there should be additional
protection to take care of fault otherwise the fault will not be
cleared and may damage the equipments .
The maintenance and testing work is carried on primary protection
system. Then primary protection is not operative due to
maintenance work. Then back up protection acts as main
protection.

7. If fault occurs beyond C region ,Circuit breaker of C region operates
as primary protection.
In the meanwhile relays at B and A may also start operating.
They ar given time lag.
If breaker at C does not operate then after 0.6 seconds breaker at B
will operate and act as backup protection.
Breaker at A will operate if breaker at B fails to operate.
In this way back up protection is provided.

15.  When earth current flows through earth return path,
the fault is called earth fault. The earth faults are
frequent. So earth fault protection is important.
 Residual Earth Fault Relay:
 When earth fault does not take place, the sum of three
secondary currents of CT is zero i.e. IR +IY+IB =0
 The sum of all these currents is called residual current.
When earth fault develops, the sum of three currents is
not zero, the residual current flows through earth fault
relay.
 This protection is also called unrestricted earth fault
protection.

20. It consists of a domed vessel. There are two mercury switches.
The upper mercury switch is attached to a float. The lower
mercury switch is attached to a flap and the mechanism is
hinged as shown in figure.
Alarm contacts are brought to upper mercury switch and trip
contacts are brought to lower mercury switch.
The upper mercury switch operates during incipient(less sever
faults like overloads) faults and the lower mercury switch
operates in case of severe faults.

26. When the fault current flows through earth return path, the
fault is called earth fault. The earth faults are frequent.
So earth fault protection is important.
Residual earth fault relay-
When earth fault does not take place, the sum of the three
currents of CT is zero.
IR+IY+IB=0
The sum of these currents is called residual current. When
earth fault develops, the sum of three currents is not
zero, the residual current flows through earth fault
relay.
This protection is also called unrestricted earth fault
protection. In other scheme, the relay is connected in the
neutral wire of the transformer.

27. RESIDUAL EARTH FAULT
RELAY
RELAY IN NEUTRAL
WIRE

30. This scheme is used for multi
turn alternators.
For multi turn alternators, the
winding of each phase is
divided into two equal
sections (S1 and S2) as
shown in figure.
Two identical CTs are connected
in respective windings. The
CT secondary are connected
in phase opposition and a
relay R is connected in CT
secondary loop.

38. Distance protection scheme uses distance relay. The relay
operation is based on the impedance (or
distance)between the relay and point of fault. This
scheme of protection does not require any pilot wire.
The voltage element of impedance receives supply from
PT secondary and current element receives supply
from CT secondary.

42. Bus bar Protection by Over Current Relays