Chapter 8

Protection of Busbars and Lines

 8.1 Busbar Protection

• Busbars in the generating stations and sub-stations form

important link between the incoming and outgoing circuits.
• If a fault occurs on a busbar, considerable damage and
disruption of supply will occur unless some form of quick-
acting automatic protection is provided to isolate the faulty
busbar.
• The busbar zone, for the purpose of protection, includes not
only the busbars themselves but also the isolating switches,
circuit breakers and the associated connections. In the event of
fault on any section of the busbar, all the circuit equipment
connected to that section must be tripped out to give complete
isolation.
The two most commonly used schemes for busbar protection are :
(i) Differential protection (ii) Fault bus protection

(i) Differential protection. The basic method for busbar

protection is the differential scheme in which currents entering
and leaving the bus are totalized. During normal load condition,
the sum of these currents is equal to zero. When a fault occurs,
the fault current upsets the balance and produces a differential
current to operate a relay.
(ii) Fault Bus protection. It is possible to design a station so that
the faults that develop are mostly earth-faults. This can be
achieved by providing earthed metal barrier (known as fault bus)
surrounding each conductor throughout its entire length in the bus
structure. With this arrangement, every fault that might occur
must involve a connection between a conductor and an earthed
metal part. By directing the flow of earth-fault current, it is
possible to detect the faults and determine their location. This
type of protection is known as fault bus protection.
 8.2 Protection of Lines
The probability of faults occurring on the lines is much more due to
their greater length and exposure to atmospheric conditions. This has
called for many protective schemes which have no application to the
comparatively simple cases of alternators and transformers. The
requirements of line protection are :

(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) The relay operating time should be just as short as possible in

order to preserve system stability, without unnecessary tripping of
circuits.
The protection of lines presents a problem quite different from
the protection of station apparatus such as generators,
transformers and busbars. While differential protection is ideal
method for lines, it is much more expensive to use. The two
ends of a line may be several kilometers apart and to compare
the two currents, a costly pilot-wire circuit is required. This
expense may be justified but in general less costly methods are
used. The common methods of line protection are :
(i) Time-graded overcurrent protection
(ii) Differential protection
(iii) Distance protection
 8.2.1 Time-Graded Overcurrent Protection

1. 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.
(i) Using definite time relays shows the overcurrent protection of a
radial feeder by definite time relays. The time of operation of each
relay is fixed and is independent of the operating 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 relay at D fails to trip, the relay at C will operate
after a time delay of 0·5 second i.e. after 1 second from the
occurrence of fault.
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 seconds in this case). However, in most cases, it
is necessary to limit the maximum tripping time to 2 seconds. This
disadvantage can be overcome to a reasonable extent by using
inverse-time relays.
(ii) Using inverse time relays. Fig. shows overcurrent protection
of a radial feeder using inverse time relays in which 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.
2. Parallel feeders. Where continuity of supply is particularly
necessary, two parallel feeders may be installed. If a fault
occurs on one feeder, it can be disconnected from the system and
continuity of supply can be maintained from the other feeder.
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 tripping.
Fig. shows the system where two feeders are connected in parallel between the
generating station and the sub-station. 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.
Suppose an earth fault occurs on feeder 1 as shown in Fig. 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. In fact, 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.
3. 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 disconnected for repairs, and power will be supplied
from both ends of the ring, thereby maintaining continuity of
supply.
Fig. shows the single line diagram of a typical ring main system
consisting of one generator G supplying four sub-stations S1, S2, S3
and S4. In this arrangement, power can flow in both directions under
fault conditions. Therefore, it is necessary to grade in both directions
round the ring and also to use directional relays. 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 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. 23.7.
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.
 8.3 Differential Pilot-Wire Protection

• The differential pilot-wire protection is based on the principle

that under normal conditions, the current entering one end of a
line is equal to that leaving the other end.
• However, only the following two schemes will be discussed :

1. Merz-Price voltage balance system

2. Translay scheme
1. Merz-Price voltage balance system. Fig. shows the single
line diagram of Merz-Price voltage balance system for the
protection of a 3-phase line. Identical current transformers are
placed in each phase at both ends of the line. The pair of CTs
in each line is connected in series with a relay in such a way
that under normal conditions, their secondary voltages are
equal and in opposition i.e. they balance each other.
Advantages
(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
2. Translay scheme. This system is similar to voltage balance
system except that here balance or opposition is between the
voltages induced in the secondary windings wound on the
relay magnets and not between the secondary voltages of the
line current transformers. This permits to use current
transformers
of normal design and eliminates one of the most serious
limitations of original voltage balance system, namely ; its
limitation to the system operating at voltages not exceeding 33
kV.
This can be extended to 3-phase system by applying one relay at
each end of each phase of the 3-phase line. However, it is
possible to make further simplification by combining currents
derived from all phases in a single relay at each end, using the
principle of summation transformer . A summation transformer
is a device that reproduces the polyphase line currents as a
single-phase quantity. The three lines CTs are connected to the
tapped primary of summation transformer. Each line CT
energizes a different number of turns (from line to neutral) with
a resulting single phase output. The use of summation
transformer permits two advantages viz(i) primary windings 1
and 2 can be used for phase faults whereas winding 3 can be
used for earth fault (ii) the number of pilot wires required is
only two.
Advantages
(i) The system is economical as only two pilot wires are required
for the protection of a 3-phase
line.
(ii) Current transformers of normal design can be used.
(iii) The pilot wire capacitance currents do not affect the operation
of relays.
 8.4 Distance Protection

• Both time-graded and pilot-wire system are not suitable for

the protection of very long high voltage transmission lines.
• The former gives an unduly long time delay in fault clearance
at the generating station end when there are more than four or
five sections and the 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. This system provides discrimination
protection without employing pilot wires.
Fig. shows 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 and sub-station A and B will be Z1 + Z
and Z respectively. It is clear that for the portion shown, only relay
at B will operate. Similarly, if a fault occurs within section A B, then
only relay at A will operate. In this manner, instantaneous protection
can be obtained for all conditions of operation.
In actual practice, 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 A B and the one at 101% of distance A B. This
difficulty is overcome by using ‘three-zone’ distance protection
shown in Fig.
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.

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 8.5 What is islanding in power systems?

• Islanding is a condition in which a distributed generator

continues to feed the load even when the electrical grid supply
is disconnected. As we know that because of obvious reasons
the numbers of distributed generators are increasing and so is
the problem of islanding.
There are many anti islanding detection techniques by which we can detect the
given condition. Confining the study to local detection techniques there are
two methods for the purpose, namely active methods and passive methods.
Passive methods as the name suggests are the ones which don’t require any
additional circuitry for the purpose of detection. In this method we use to
measure different parameters like voltage, current, harmonics, phase jump etc.
of the PCC (point of common coupling of the load, electrical grid and
distributed generator) and compare it with the standard values, any deviation
considering the allowable margin for voltage i.e. (230 +10%) V and frequency
(50±1.5) Hz is considered as a symptom of islanding. The drawback of this
method is that it usually have a significant NDZ (non detection zone)
associated with them.
Active methods on the other hand are the ones in which a small disturbance is
injected to the PCC. The central idea behind this method is that whenever the
islanding is happening the disturbance will be amplified and the distributed
generator is stopped from delivering the power to the load. In order to create
the disturbances, additional circuitry is needed in conjunction with distributed
generator.