Protection of Lines

Probability of faults in lines is greater due to

 Greater length
 Exposure to atmosphere
 Protection of Lines

Requirements of line protection

(i) In the event of a short-circuit, the circuit breaker closest to the

fault should open, all other circuit breakers remaining in a
closed position.

(ii) In case the nearest breaker to the fault fails to open, back-up
protection should be provided by the adjacent circuit breakers.

(iii) Unnecessary tripping of circuits should be avoided in order to

preserve system stability.
 Protection of Lines

Common methods of line protection

(i) Time-graded overcurrent protection

(ii) Differential protection
(iii) Distance protection

Fig. Symbols indicating various types of relays

 Protection of Lines

Time-Graded Overcurrent Protection

In this scheme of overcurrent protection, time discrimination is incorporated.

In other words, the time setting of relays is so graded that in the event of fault,
the smallest possible part of the system is isolated.

We shall discuss a few important cases:

1. Radial feeder
2. Parallel feeders
3. Ring main system
 Protection of Lines

Time-Graded Overcurrent Protection

Radial feeder

The main characteristic of a radial system is that power can flow only in one
direction, from generator or supply end to the load. It has the disadvantage that
continuity of supply cannot be maintained at the receiving end in the event of
fault. Time-graded protection of a radial feeder can be achieved by using

(i) definite time relays and

(ii) inverse time relays.
 Protection of Lines

Time-Graded Overcurrent Protection

The time of operation of each
Radial feeder relay is fixed and is
independent of the operating
(i) Using definite time relays
current. Thus relay D has an
operating time of 0·5 second
while for other relays, time
delay* is successively increased
by 0·5 second. If a fault occurs
in the section DE, it will be
cleared in 0·5 second by the
relay and circuit breaker at D
because all other relays have
higher operating time. In this
way only section DE of the
system will be isolated. If the
* The amount of time delay depends upon the speed of relay at D fails to trip, the relay
breaker tripping. Sufficient time delay must be allowed to at C will operate after a time
permit the breaker on the faulted section to clear the fault delay of 0·5 second i.e. after 1
before the next relay in the sequence trips. The time-delay second from the occurrence of
usually varies from 0·25 second to 0·5 second. fault.
 Protection of Lines

Time-Graded Overcurrent Protection

Radial feeder Disadvantage:

(i) Using definite time relays The disadvantage of this

system is that if there are a
number of feeders in series,
the tripping time for faults
near the supply end
becomes high (2 sec in this
case). However, in most
cases, it is necessary to limit
the maximum tripping time
to 2 sec. This disadvantage
can be overcome to a
reasonable extent by using
inverse-time relays.
 Protection of Lines

Time-Graded Overcurrent Protection

Radial feeder

(ii) Using inverse time relays Here, operating time is

inversely proportional to the
operating current. With this
arrangement, the farther the
circuit breaker from the
generating station, the
shorter is its relay operating
time.
The three relays at A, B and C
are assumed to have inverse-
time characteristics. A fault in
section BC will give relay
times which will allow
breaker at B to trip out before
the breaker at A.
 Protection of Lines

Time-Graded Overcurrent Protection

Parallel feeder

Where continuity of
supply is particularly
necessary, two parallel
feeders may be installed.

The parallel feeders cannot* be protected by non-directional overcurrent relays only. It

is necessary to use directional relays also and to grade the time setting of relays for
selective trippings.

* Suppose relays at P and Q are non-directional type and their time settings are lower
than relays at A and B. When a fault occurs at the shown point, the relay at Q will
operate first and disconnect the feeder 2, and then feeder 1 will be cut off. Thus even
the sound feeder (No. 2) is isolated.
 Protection of Lines

Time-Graded Overcurrent Protection

Parallel feeder

The protection of this

system requires that

(i) each feeder has a non-

directional overcurrent
relay at the generator end.
These relays should have
inverse-time characteristic.
(ii) each feeder has a reverse power or directional relay at the sub-station end. These
relays should be instantaneous type and operate only when power flows in the reverse
direction i.e. in the direction of arrow at P and Q.
 Protection of Lines

Time-Graded Overcurrent Protection

Parallel feeder

Suppose an earth fault

occurs on feeder 1. It is
desired that only circuit
breakers at A and P should
open to clear the fault
whereas feeder 2 should
remain intact to maintain
the continuity of supply.
The above arrangement accomplishes this job. The shown fault is fed via two routes, viz.
(a) directly from feeder 1 via the relay A
(b) from feeder 2 via B, Q, sub-station and P
Therefore, power flow in relay Q will be in normal direction but is reversed in the relay P. This
causes the opening of circuit breaker at P. Also the relay A will operate while relay B remains
inoperative. It is because these relays have inverse-time characteristics and current flowing in
relay A is in excess of that flowing in relay B. In this way only the faulty feeder is isolated.
 Protection of Lines

Time-Graded Overcurrent Protection

Ring main system

In this system, various
power stations or sub-
stations are
interconnected
by alternate routes, thus
forming a closed ring. In
case of damage to any
section of the ring,
that section may be
Fig. shows the single line diagram of a typical ring main system
disconnected for repairs,
consisting of one generator G supplying four sub-stations S1,
and power will be
S2, S3 and S4. In this arrangement, power can flow in both
supplied from both ends
directions under fault conditions. Therefore, it is necessary to
of the ring, thereby
grade in both directions round the ring and also to use
maintaining continuity
directional relays.
of supply.
 Protection of Lines

Time-Graded Overcurrent Protection

Ring main system

In order that only

faulty section of the
ring is isolated under
fault conditions, the
types of relays and
their time settings
should be as follows :

(i) The two lines leaving the generating station should be equipped with non-directional
overcurrent relays (relays at A and J in this case).
(ii) At each sub-station, reverse power or directional relays should be placed in both
incoming and outgoing lines (relays at B, C, D, E, F, G, H and I in this case).
(iii) There should be proper relative time-setting of the relays. As an example, going
round the loop G S1 S2 S3 S4 G ; the outgoing relays (viz at A, C, E, G and I) are set with
decreasing time limits e.g.
A = 2·5 sec, C = 2 sec, E = 1·5 sec G = 1 sec and I = 0·5 sec
 Protection of Lines

Time-Graded Overcurrent Protection

Ring main system

Similarly, going round

the loop in the
opposite direction (i.e.
along G S4 S3 S2 S1
G), the outgoing
relays (J, H, F, D and B)
are also set with a
decreasing time limit
e.g. J = 2·5 sec, H = 2 sec, F = 1·5 sec, D = 1 sec, B = 0·5 sec.
Suppose a short circuit occurs at the point as shown in Fig. In order to ensure selectivity, it is
desired that only circuit breakers at E and F should open to clear the fault whereas other
sections of the ring should be intact to maintain continuity of supply. In fact, the above
arrangement accomplishes this job. The power will be fed to the fault via two routes viz (i)
from G around S1 and S2 and (ii) from G around S4 and S3. It is clear that relays at A, B, C
and D as well as J, I, H and G will not trip. Therefore, only relays at E and F will operate
before any other relay operates because of their lower time-setting.
 Protection of Lines

Differential Pilot-Wire Protection

Merz-Price voltage balance system

 Protection of Lines

Differential Pilot-Wire Protection

Merz-Price voltage balance system

 Protection of Lines

Differential Pilot-Wire Protection

Merz-Price voltage balance system

Aadvantages

(i) This system can be used for ring mains as well as parallel feeders.
(ii) This system provides instantaneous protection for ground faults. This decreases
the possibility of these faults involving other phases.
(iii) This system provides instantaneous relaying which reduces the amount of damage
to overhead conductors resulting from arcing faults.

Disadvantages
(i) Accurate matching of current transformers is very essential.
(ii)If there is a break in the pilot-wire circuit, the system will not operate.
(iii)
This system is very expensive owing to the greater length of pilot wires required.
(iv)In case of long lines, charging current due to pilot-wire capacitance effects may be
sufficient to cause relay operation even under normal conditions.
(v) This system cannot be used for line voltages beyond 33 kV because of
constructional difficulties in matching the current transformers.
 Protection of Lines
Distance Protection

Both time-graded and pilot-wire system are not suitable for the protection of
very long high voltage transmission lines because

 Time-graded protection gives an unduly long time delay in fault clearance at

the generating station end when there are more than four or five sections

 Pilot-wire system becomes too expensive owing to the greater length of pilot
wires required.

This has led to the development of distance protection in which the action of
relay depends upon the distance (or impedance) between the point where
the relay is installed and the point of fault.
 Protection of Lines
Distance Protection

A simple system
consisting of lines
in series such that
power can flow
only from left to
right.

The relays at A, B and C are set to operate for impedance less than Z1, Z2 and
Z3 respectively.
Suppose a fault occurs between sub-stations B and C. The fault impedance at
power station A and sub-station B will be Z1 + Z and Z respectively.
For the portion shown, only relay at B will operate. Similarly, if a fault occurs
within section AB, then only relay at A will operate.
It is not possible to obtain instantaneous protection for complete length of the line
due to inaccuracies in the relay elements and instrument transformers. Thus the
relay at A would not be very reliable in distinguishing between a fault at 99% of the
distance AB and the one at 101% of distance AB.
This difficulty is overcome by using ‘three-zone’ distance protection
 Protection of Lines

Distance Protection (Three zone distance protection)

Relay A
Zone 3
Zone 2

Time
Relay B
Zone 1 Zone 2
Zone 1
Relay C

A B C
G

In this scheme of protection, three distance elements are used at each terminal. The zone
1 element covers first 90% of the line and is arranged to trip instantaneously for faults in
this portion. The zone 2 element trips for faults in the remaining 10% of the line and for
faults in the next line section, but a time delay is introduced to prevent the line from
being tripped if the fault is in the next section. The zone 3 element provides back-up
protection in the event a fault in the next section is not cleared by its breaker.