A Learning Report on Bus-Bar Schemes

By
Elango Lakshmanan | LinkedIn
 List of Contents

S.No. Name of the Contents Page No.

1. Selection of Bus Bar Schemes 2
2. Types of Bus Bar Schemes` 2
3. Single Bus Bar Scheme 2
4. Main and Transfer Bus Scheme 3
5. Double Bus Bar Scheme 4
6. Double Main and Transfer Scheme 5
7. Mesh Bus Scheme 5
8. One and a Half Breaker Scheme 6
9. Double Bus and Double Breaker Scheme 7

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Bus-Bar Schemes
Selection of bus-bar scheme:
a) Factors influencing the selection of a bus-bar scheme in the substation design are
• Operational flexibility
• system safety
• Reliability
• Availability
• ability to facilitate system control and cost.
b) It is important to consider the degree of reliability of supply expected during maintenance or faults in selection
of bus-bar scheme.
c) Amount of redundancy to be provided so as to determine the equipment, which can be permitted out of use on account
of maintenance or faults, should also be taken into consideration.
d) Certain amount of sectionalisation has also to be provided in a substation so as to ensure that in the event of a fault, a
large power source does not get disconnected.
e) Future expansion of the bus-bar system at least in a foreseeable future may also be considered.

Types of Bus-bar Schemes:

Various types of bus-bar schemes generally in use are,
1. Single bus-bar, with or without sectionalizer
2. Main and transfer bus-bar
3. Double bus-bar
4. Double main and transfer bus
5. Ring bus-bar and mesh bus-bar
6. One and a half circuit breaker
7. Double bus double breaker scheme

Single Bus-Bar Scheme:

• This is the simplest scheme, in which each circuit is provided with one circuit breaker (Fig.1.1.). This arrangement offers
little security against bus bar isolator maintenance. The entire substation is lost in case of a fault on the bus bar or any
bus-bar isolator and also in case of maintenance thereof.
• Another disadvantage is that in case of maintenance of circuit breaker associated feeder has also to be shutdown.
• One of the methods for reducing the number of circuits lost in case of a bus fault is to sectionalise the bus as shown in
(Fig.1.2).
• The arrangement in (Fig.1.3.) is a improvement over that shown in (Fig.1.2), as additional bypass isolators are provided
to permit feeder circuit breakers to be taken out for maintenance without switching out the associated feeder.
• On occurrence of a fault on the feeder connected to bus bar through by-pass isolator, the other feeder on that bus section
will also be lost.
• Different bus schemes shown below include two earth switches one on bus side and the other on feeder side for feeder
breakers and one earth switch on bus side of transformer breaker.
• These earth switches are intended to discharge the concerned breakers before carrying out any works on these breakers.
In the absence of earth switches earth discharge rods may be used.

Fig.1.1. Single Bus Bar Scheme

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 Fig.1.2. Single Sectionalized Bus Bar

Fig.1.3. Single Sectionalized Bus Bar with By-pass Isolators

Main and Transfer Bus Arrangement:

• This is technically a single bus bar arrangement with an additional bus bar called “Transfer bus” energised from main bus
bars through a bus coupler circuit, (Fig.1.4.), i.e., for `n’ number of circuits it employs n+1 circuit breakers.
• The additional provision of transfer bays and bus coupler circuit facilitates taking out one circuit breaker at a time for
routine overhaul and maintenance without de-energising the circuit controlled by that breaker as that circuit then gets
energised through bus coupler breaker and transfer bus bar.
• Each circuit is connected to the main bus-bar through a circuit breaker with isolators on both sides and through an
isolator to the transfer bus-bar.
• As in the case of single bus arrangement, this scheme also suffers from the disadvantage that in the event of a fault on the
main bus bar or the associated isolator, there is a complete shutdown of the substation.
• Complete shutdown can be avoided through sectionalizing the main bus with the provision of additional one single phase
bus PT for synchronization in case of more than eight bays. This scheme has been used particularly for step-down
substations, as bus-bar faults are rare.

Fig.1.4. Main and Transfer Scheme

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 • This bus arrangement has been extensively used in 132kV and 33kV AIS Substations / Switchyard.
Double Bus-bar Scheme:
• In this scheme a double bus bar is provided and each circuit can be connected to either one of these bus-bar isolators as
shown in (Fig.1.5.).
• Bus coupler breaker is also provided so that the circuits can be switched on from one bus to the other on-load.
• The scheme suffers from the disadvantage that when the circuit breaker is taken out for maintenance, the associated
feeder has to be shutdown. This can be avoided by providing, a by-pass isolator across circuit breaker as shown in
(Fig.1.5).
• But under this condition all the circuits have to be transferred to one bus and protection of feeder has to be transferred to
bus coupler. This scheme has the limitation that only one bus is available when any breaker has to be taken out for
maintenance.
• The double bus-bar scheme with by-pass is available when any breaker has to be taken out for maintenance. The double
bus-bar scheme with by-pass isolator across circuit breakers is very suitable for large generating stations as well as large
grid substations forming part of a well inter connected system wherein a variety of grouping of circuits is required.
(Fig.1.6). shows another alternative of this scheme.
• In this alternative the by-pass isolators are connected to one of the main bus bars as shown. This scheme constitutes
double bus-bar Scheme with main reserve and transfer bus-bars.
• In both these schemes, use of temporary earthing device is called for during breaker maintenance.
• As temporary earthing drives can result in serious, accidents, if not removed, it is preferable to provide the isolators on
either side of the circuit breakers across which bypass isolators are provided with integral earthing switches having
mechanical interlocking features.

Fig.1.5. Double Bus-bar System with By-pass Isolators

Fig.1.6. Main with Reverse and Transfer Bus System

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 • This bus arrangement has been extensively used in 220kV ,132kV and 33kV GIS Substations / Switchyard.

Double Main and Transfer Bus-Bar Scheme:

• The limitation of scheme (Fig.1.6.) can be overcome by using additional transfer bus, transfer bus breaker and isolators
as shown in (Fig.1.7.).
• In this arrangement, the feeder, the breaker of which is to be maintained is transferred to the transfer bus, without
affecting the other circuits.
• This scheme has been widely used for the highly inter connected power networks were switching flexibly is important
and multiple supply routes are available.
• This scheme is also used for splitting networks, which are only connected in emergencies. to the transfer bus, without
affecting the other circuits.
• This scheme has been widely used for the highly inter connected power networks were switching flexibly is important
and multiple supply routes are available. This scheme is also used for splitting networks, which are only connected in
emergencies.

Fig.1.7. Double Main and Transfer Bus-bar Scheme

• This bus arrangement has been extensively used in 220kV AIS Substations.

Mesh Bus-Bar Scheme:

• Each circuit is controlled by two circuit breakers and therefore any one circuit breaker can be taken out for maintenance
without affecting the security of supply shown in (Fig.1.8).
• A circuit fault also is cleared by opening of two breakers. In both cases the ring is broken and the bus-bar is reduced to
sectionalised single bus-bar scheme.
• In the case of a feeder fault, the circuit isolator can be opened, the faulty feeder disconnected and both the breakers
closed which would close the ring.
• This system has got a number of advantages such as maintenance of a circuit breaker without loss of supply and without
providing by-pass facilities, loss of only the faulty feeder in case of a feeder fault, and loss of only two circuits in
case of a circuit breaker fault.
• There are, however, some problems such as occurrence of a fault when a circuit breaker is being maintained resulting in a
double break in the mesh and capacity limitation of the equipment to pass the maximum current that may flow round the
mesh. If these are provided for, it adds to cost of the station. In view of these problems, it is considered desirable to limit
the number of circuits on the mesh.

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 • The mesh scheme is very suitable where the number of circuits is comparatively small and chances of future expansion
are less such as substations associated with generating plants and also step-down substations operating at extra high
voltages.
• This scheme has been used on many of our early installations. However, during the recent past there have not been many
installations of this type as this scheme does not lend itself easily to further expansion.

Fig.1.8. Mesh Bus-bar System

One-And- A Half Breaker Scheme:

• In this scheme three circuit breakers are used for controlling two circuits as shown in Fig. (1.9.&1.10.). Normally, both
the bus-bars are in service.
• A fault on any bus is cleared by the opening of the associated circuit breakers without affecting continuity of supply.
Similarly, any circuit breaker can be taken out for maintenance without causing interruption.
• All load transfer is done by the breakers and therefore, the operation is simple. However, relaying is somewhat more
involved as the third breaker has to be responsive to troubles on either feeder in the correct sequence.
• Besides, each breaker has to be suitable for carrying the currents of two circuits to meet the requirements of various
switching operations, which may in some cases increase the cost.
• The breaker and a half scheme is suitable for those substations which handle large amounts of power on each circuit.
This scheme has been applied widely in the 420 kV systems.

Fig.1.9. One and Half Breaker Scheme (I-Configuration)

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 Fig.1.10. One and Half Breaker Scheme

• This bus arrangement has been extensively used in 765kV and 400kV AIS Substations.

Double-Bus and Double-Breaker Scheme:

• In this scheme two circuit breakers are used for controlling one circuit as shown in (Fig.1.11.). Normally both
bus-bars are in service.
• Similar to breaker and a half scheme, a fault on any bus is cleared by opening of the associated circuit
breakers without affecting continuity of supply.
• Similarly, any circuit breaker can be taken out for maintenance without causing interruption.
• All load transfer is done by breaker and therefore, the operation is simple and relaying is also simpler
compared to breaker and half scheme.
• Because of increase in number of breakers per bay and higher cost, double bus double breaker scheme may be
considered for those substations, which handle large amount of power.

Fig.1.11. Double Bus Double Breaker System

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