Relay Introduction Basics

2. Electrical System
G
G
220 KV 132 KV 33 KV 11 KV
GENERATORS

3. Objectives :
 During the Fault conditions, the
Relays will identify the faulty area and
will isolate the Unhealthy section from
the network.
 Ex : Motor Protection, Generator
Protection, Bus Protection,
Line Protection…

4. Protective Relaying
Primary Relaying( Main Protection):
Main relay to operate in the event
of faults
Secondary Relaying ( Backup Prot) :
If main relay fails to operate, these
relays will operate and clear the fault.
These relays are generally of time delay and
cause more disturbance to the system

5. Primary/Secondary Protection:
G
G
220 KV 132 KV 33 KV 11 KV
GENERATORS
G

6. Protective Relaying
 Relays Requires any of the following
Quantities to operate:
Magnitude ie. Current / Voltage / Power
Frequency ie. Under Freq / Over freq
Phase Angle
Duration ie. Timers
Rate of Change ie. df/dt, dZ/dt
Direction ie. Directional O/C
Harmonics ie. 3rd harmonic

7. Relay Wiring:
Transformer
CT
Breaker
PT
BUS

8. Basic Relays
 Current Relays
Instantaneous Over Current Relays
Definite Time Over Current Relay
Inverse Definite Over Current Relay
 Voltage Relays
Under Voltage Relays
Over Voltage Relays
 Impedance Relays
 Reactance Relays
 Differential Relays

9. Instantaneous Over Current Relay
 This Relay operates when ever the
current exceeds the Set value
 Application : Generator Inter turn,
GT Restricted E/F prot.
 Example : CAG Relay of EE make

10. Definite Time Over Current Relay
 This Relay operates when ever the
current exceeds the Set value and
that current shall persist for
defined time
 Application : Blocked Rotor
Protection, Over Load Alarm
etc..
 Example : CTU Relay of EE make
&
Operation
Current Time

11. Inverse Definite Time Over Current Relay
t
Current
 This Relay operates when
ever the current exceeds
the Set value. The relay
operation time is
inversely proportional to
the magnitude of Current
 Application: GT Standby
E/F prot., LT Motor
Prot.
 Example : CDG Relay of EE
make

12. Over Voltage Relay
 Whenever the Voltage exceeds the
set value, the relay operates.
 Application : Generator Over Voltage
Protection.
 Example : VAG of EE make

13. Under Voltage Relay
 Whenever the Voltage falls below the
set value, the relay operates.
 Application : Bus Under Voltage
Protection.
 Example : VAGM of EE make

14. Impedance Relays
 These relays are uses MHO principle.
If the Impedance is less than the Set
Value, the Relay operates. These
Relays are Under Impedance Relays
 Application: Line Protection,
Generator B/U Prot.
 Ex : YCG Relays of EE Make

15. Impedance Relay Characteristic.
NON - DIRECTIONAL
DIRECTIONAL
R
R
-R
-R
-X -X
X
X

16. Reactance Relays
 If the Reactance is less than the Set
Value, the Relay operates. Generally,
these Relays are Under Reactance
Relays
 Application: Line Protection,
 Ex : XCG Relays of EE Make

17. Reactance Relays
R
X

18. Differential Relays
 If there is any difference of currents,
the relay operates.
 Application :
 Transformer Differential
 Generator Differential
 Bus Bar Differential ….
 Ex : ALSTOM Make DTH Relays

19. Differential Relays
GEN
CT CT
R
Ph. Side Neutral Side

20. Differential Relays
CT CT
R
Primary Secondary