electric power protection

1. EE448-ELECTRICAL POWER ENGINEERING
DR. MONA IBRAHIM

2. LECTURE (14) –TRANSMISSION
LINE PROTECTION USING
DISTANCE RELAYS

3. CONTENTS What are Distance
Relays?
Protection Zones of
Distance Relays
Examples

4. WHAT ARE DISTANCE RELAYS?
As the length of electrical
transmission line is generally long
enough and it runs through open
atmosphere, the probability of
occurring fault in electrical power
transmission line is much higher
than that of transformers and
alternators .
That is why a transmission line
requires much more protective
schemes than a transformer and an
alternator.

5. Features of protection of transmission line
During fault, the only circuit breaker closest to the fault point
should be tripped.
If the circuit breaker closest the faulty point, fails to trip the
circuit breaker just next to this breaker will trip as back up.
The operating time of relay associated with protection of line
should be as minimum as possible in order to prevent unnecessary
tripping of circuit breakers associated with other healthy parts of
power system.
WHAT ARE DISTANCE RELAYS?

6. A distance protection element measures the quotient V/I (impedance),
considering the phase angle between the voltage V and the current I.
In the event of a fault, sudden changes occur in measured voltage and current,
causing a variation in the measured impedance.
The measured impedance is then compared against the set value.
Distance element will trip the relay (a trip command will be issued to the CB
associated with the relay) if the measured value of the impedance is less then the
value set.
WHAT ARE DISTANCE RELAYS?

7. WHAT ARE DISTANCE RELAYS?
 The voltage element of the relay is
excited through a potential
transformer (P.T.) from the line to be
protected.
 The current element of the relay is
excited from a current transformer
(C.T.) in series with the line.
 The portion AB of the line is the
protected zone.
 Under normal operating conditions,
the impedance of the protected zone
is Z.
 Under fault condition, the relay is so
designed that it closes its contacts
whenever impedance of the protected

8. PROTECTION ZONES OF DISTANCE
RELAYS
 The need for zones arises from
the need of selective protection;
i.e. the distance element should
only trip faulty section.
 We can set the distance element
to only trigger a trip signal for
faults within a certain distance
from the relay, which is called the
distance element reach.
 The setting impedance is
represented by Zr= hs. ZL , where
ZL is the line impedance.
 The distance element will only
trip when the measured

9. PROTECTION ZONES OF DISTANCE
RELAYS

10. PROTECTION ZONES OF DISTANCE
RELAYS

11. PROTECTION ZONES OF DISTANCE
RELAYS

12. PROTECTION ZONES OF DISTANCE
RELAYS

13. PROTECTION ZONES OF DISTANCE
RELAYS

14. PROTECTION ZONES OF DISTANCE
RELAYS

15. EXAMPLE 1

16. EXAMPLE 2

17. EXAMPLE 3

18. EXAMPLE 4

19. EXAMPLE 5