Bus bar arrangements
Chapter two
Chapter two
Substation Switching Schemes
(Bus bar arrangements)
- 41 -
Bus bar arrangements
Chapter two
Substation Switching Schemes
Bus bar arrangements
Switching scheme of substation determines the electrical and physical
arrangement of the switching equipment. Different switching schemes can
be selected as emphasis is shifted between the factors of:
1.
2.
3.
4.
5.
6.
7.
8.
Security of supply.
Extendibility.
Maintainability
Operational flexibility.
Protection arrangement.
Short circuit limitations.
Cost.
Other factors as: Land area, Safety, Simplicity dictated by function
and Importance of the substation.
1. Security of supply:
Security of supply or substation service continuity is the main factor in
selecting the switching scheme. Complete security of supply may be
achieved by duplicating all circuits and substation equipment such that
following a fault or during maintenance connections remain available.
This would be extremely costly.
Considering that line or transformer faults destroy service continuity on
the affected circuits, substation service continuity could be categorized
into four categories, as described below:
 1st category: No outage necessary within the substation for either
maintenance or fault; e.g. the 1 ½ breaker scheme under
maintenance conditions in the circuit breaker area.
 2nd Category: Short outage necessary to transfer the load to an
alternative circuit for maintenance or fault conditions; e.g. the
- 41 -
Bus bar arrangements
Chapter two
double bus bar scheme with bypass disconnect switch and buscoupler switch under fault or maintenance conditions in the circuit
breaker or bus bar area.
 3rd Category: Loss of a circuit or section; for example the single
bus bar with bus section circuit breaker scheme for a fault in the
circuit breaker or bursar area. The single feed scheme also comes
under category 3 service continuity and for this arrangement the
addition of incoming circuit breakers, bus bar and transformer
circuit breakers does not improve the classification.
 4th Category: Loss of substation; for example the single bus bar
scheme without bus sectionalization for a fault in the bus bar area.
2. Extendibility:
The design should allow for future extendibility. Adding bays of
switchgear to a substation is normally possible and care must be taken to
minimize the outages and outage durations for construction and
commissioning.
3. Maintainability:
The switching scheme must take into account the electricity supply
company system planning and operations procedures together with
knowledge of reliability and maintenance requirements for the proposed
substation equipment. The need for circuit breaker disconnect switch
bypass facilities may therefore be obviated by an understanding of the
relative short maintenance periods for modern switchgear.
4. Operational flexibility:
The switching scheme must permit the required power flow control of
individual circuits and groups of circuits. In a two transformer substation
operation of either or both transformers on one in feed together with the
facility to take out of service and restore to service either transformer
without loss of supply would be a normal design consideration. In
general a multiple bus bar arrangement will provide greater flexibility
than a ring bus bar.
- 41 -
Bus bar arrangements
Chapter two
5. Protection arrangements:
The switching scheme must allow for the protection of each system
element by provision of suitable CT locations to ensure overlapping of
protection zones. The number of circuit breakers that require to be
tripped following a fault, the auto-reclose arrangements, the type of
protection and extent and type of mechanical or electrical interlocking
must be considered. For example a 1½ breaker substation layout
produces a good utilization of switchgear per circuit but also involves
complex protection and interlocking design which all needs to be
engineered and thus increases the capital cost.
6. Short circuit limitations:
In order to keep fault levels down parallel connections (transformers or
power sources feeding the substation) should be avoided. Multi-bus bar
arrangements with sectioning facilities allow the system to be split or
connected through a fault limiting reactor. It is also possible to split
system using circuit breakers in a mesh or ring type substation layout
although this requires careful planning and operational procedures.
7. Cost:
A satisfactory cost comparison between different substations switching
scheme is extremely difficult because of the differences in performance
and maintainability. It is preferable to base a decision for a particular
scheme on technical grounds and then to determine the most economical
means of achieving these technical requirements.
The following arrangements are commonly used:
1. Single bus arrangement.
2. Single Bus System with Bus Sectionalizing circuit breaker
arrangement.
3. Main and Transfer Bus arrangement.
4. Double bus–single breaker arrangement.
5. Double bus single breaker with extra transfer bus arrangement.
6. Double bus–double breaker arrangement.
7. Breaker-and-a-half arrangement.
8. Ring bus arrangement.
9. Single feed arrangement.
- 41 -
Bus bar arrangements
Chapter two
10.Double feed arrangement.
1. Single Bus Arrangement:
Advantages of this arrangement:
1. This is the simplest bus arrangement, a single bus and all
connections directly to one bus.
2. Cost of a single bus arrangement is relatively low, but also is
the operational flexibility; for example, transfer of loads from
one circuit to another would require additional switching
devices outside the substation.
Disadvantages of this arrangement:
1. Reliability of the single bus configuration is low: even with
proper relay protection, a single bus failure on the main bus or
between the main bus and circuit breakers will cause an outage
of the entire facility.
2. With respect to maintenance of switching devices, an
outage of the line they are connected to is required.
3. For a bus outage due to fault or for maintenance the entire
facility must be de-energized. This requires standby generation
or switching loads to adjacent substations, if available, to
minimize outages of loads supplied from this type of facility.
4. Due to the low reliability, significant efforts when performing
maintenance, and low operational flexibility, application of the
single bus configuration should be limited to facilities with low
load levels and low availability requirements.
Since single bus arrangement is normally just the initial stage of a
substation development, when laying out the substation a designer
should consider the ultimate configuration of the substation, such as
where future supply lines, transformers, and bus sections will be
added. As loads increase, substation reliability and operational
abilities can be improved with step additions to the facility, for
- 41 -
Bus bar arrangements
Chapter two
example, a bus tie breaker to minimize load dropped due to bus
outages.
Single bus single breaker arrangement
Single bus single breaker operating procedures:
1. In case of connecting the system: start with the disconnecting
switches.
2. In case of disconnecting the system: start with the circuit breakers.
3. In normal operation all Disconnecting switches and Circuit
breakers are closed.
2. Single Bus System with Bus sectionalizing circuit breaker:
Some advantages are realized if a single bus bar is sectionalized with
circuit breaker. If there are more than one incoming and the incoming
sources and outgoing feeders are evenly distributed on the sections,
interruption of system can be reduced to a good extent.
- 41 -
Bus bar arrangements
Chapter two
Advantages of this arrangement:
The reliability of this type is low but beater than single bus. If any of
the sources is out of system, still all loads can be fed by switching on
the sectional circuit breaker or bus coupler breaker. If one section of
the bus bar system is under maintenance, part load of the substation
can be fed by energizing the other section of bus bar.
Disadvantages of this arrangement:
1. As in the case of single bus system, maintenance of equipment of
any bay cannot be possible without interrupting the feeder or
transformer connected to that bay.
2. The use of isolator for bus sectionalizing does not fulfill the
purpose. The isolators have to be operated „off circuit‟ and which
is not possible without total interruption of bus – bar. So
investment for bus-coupler breaker is required.
Single bus single breaker with sectionalizing circuit breaker
operating procedures:
1. In case of connecting the system: start with the disconnecting
switches.
2. In case of disconnecting the system: start with the circuit
breakers.
3. In normal operation all Disconnecting switches and Circuit
breakers are closed.
4. In case of a fault at any bus section, open the tie breaker to
isolate the faulty bus section.
- 02 -
Bus bar arrangements
Chapter two
Single bus single breaker with sectionalizing circuit breaker
3. Main and Transfer Bus Arrangement:
The main and transfer bus configuration connects all circuits between the
main bus and a transfer bus sometimes referred to as an (inspection bus).
Some arrangements include a bus tie breaker and others simply utilize
switches for the tie between the two buses.
Advantages of this arrangement:
It has the same advantaged of single bus single breaker arrangement
but it provides more flexibility during maintenance.
Disadvantaged of this arrangement:
1. This configuration is similar to the single bus arrangement; in that
during normal operations, all circuits are connected to the main
bus. So the operating reliability is low.
2. Any main bus fault will de-energize all circuits However; the
transfer bus is used to improve the maintenance process by moving
the line of the circuit breaker to be maintained to the transfer bus.
- 04 -
Bus bar arrangements
Chapter two
3. This arrangement is slightly more expensive than the single bus
arrangement.
4. The area required for a low profile substation with a main and
transfer bus scheme is also greater than that of the single bus, due
to the additional switches and bus.
Operating procedures to replace the line circuit breaker to be
maintained by the tie breaker:
1. When maintenance work is necessary, the transfer bus is energized
by either closing the tie breaker, or when a tie breaker is not
installed, closing the switches connected to the transfer bus. With
these switches closed, the breaker to be maintained can be opened
along with its isolation switches.
2. Then the breaker is taken out of service. The circuit breaker
remaining in service will now be connected to both circuits
through the transfer bus. This way, both circuits remain energized
during maintenance.
3. Since each circuit may have a different circuit configuration,
special relay settings may be used when operating in this abnormal
arrangement. When a bus tie breaker is present, the bus tie breaker
is the breaker used to replace the breaker being maintained, and
the other breaker is not connected to the transfer bus.
4. A shortcoming of this scheme is that if the main bus is taken out of
service, even though the circuits can remain energized through the
transfer bus and its associated switches, there would be no relay
protection for the circuits. Depending on the system arrangement,
this concern can be minimized through the use of circuit protection
devices (reclosure or fuses) on the lines outside the substation.
5. If the main bus is ever taken out of service for maintenance, no
circuit breakers remain to protect any of the feeder circuits.
Failure of any breaker or failure of the main bus can cause
complete loss of service of the station.
- 00 -
Bus bar arrangements
Chapter two
Main and Transfer Bus Arrangement
4. Double bus single breaker:
The double bus–single breaker arrangement connects each circuit to two
buses, and there is a tie breaker between the buses. With the tie breaker
operated normally closed (it means it is opened during operation), it
allows each circuit to be supplied from either bus via its switches. This
providing increase in operating flexibility and improve reliability. For
example, a fault on one bus will not impact the other bus. Operating the
bus tie breaker normally open (it means it is closed during operation)
eliminates the advantages of the system and changes the configuration to
a two single bus arrangement.
Advantages of this arrangement:
 The double bus–single breaker arrangement with two buses and a
tie breaker provides for some ease in maintenance, especially for
bus maintenance, but maintenance of the line circuit breakers
would still require switching and outages as described above for
the single bus arrangement circuits.
- 02 -
Bus bar arrangements
Chapter two
Disadvantages of this arrangement:
1. Relay protection for this arrangement will be complex with the
flexibility of transferring each circuit to either bus.
2. Operating procedures would need to be detailed to allow for
various operating arrangements, with checks to ensure the inservice arrangements are correct.
3. A bus tie breaker failure will cause an outage of the entire station.
4. The cost of this arrangement would be more than the single bus
arrangement with the added bus and switching devices. Once
again, low-profile configuration of this arrangement would require
more area.
Application of this arrangement is best suited where load transfer and
improved operating reliability are important. Though adding a transfer
bus to improve maintenance could be considered, it would involve
additional area and switching devices, which could increase the cost of
the station.
Double bus single breaker arrangement
- 01 -
Bus bar arrangements
Chapter two
Double bus single breaker operating procedures:
 To stat the system operation on the working bus:
1. Close the disconnecting switches (DL's 1, 3, 5, 7, 9, and 10).
2. Close the circuit breakers (CB's 1, 2, 3, and 4).
3. All other DSs and CBs are opened.
 The function of Bus coupler (Tie breaker):
1. It is used to check the status of the standby bus before
transferring.
2. It‟s used instead of C.B1 or C.B2 during maintenance by
using breaker bypass disconnecting switch.
 Sequence of transferring from working bus to stand by bus:
1.
2.
3.
4.
5.
6.
Close DL 11 and DL 12
Close CB 5
If CB5 is not tripped so, the standby bus is ready to transfer
Then open CB5
Close DL‟s 2, 4, 6 and 8.
Open D.L‟s 1, 3, 5 and 7.
5. Double bus single breaker with extra transfer bus:
This configuration is similar to the double bus single breaker but it
provides more flexibility for maintenance of any line breaker:
 In normal operation:
1. D.L‟s 8, 1, 3 and 5 are closed.
2. C.B's 1 and 2 are closed.
3. All other D.L‟s and C.B‟s are open.
- 01 -
Bus bar arrangements
Chapter two
Operating procedures for replacing C.B1 by stand by C.B4
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Check the status of the transfer bus
Close D.L‟s 10, 11
Close C.B 4
If C.B4 is not tripped so, the transfer bus is ready to transfer
Then open C.B4
Close D.L‟s 9
Close C.B4.
Now you have two parallel paths.
Open C.B1.
Open D.L 1, 8.
Now C.B1 is isolated.
Double bus single breaker with extra transfer bus
- 01 -
Bus bar arrangements
Chapter two
6. Double Bus–Double Breaker Arrangement:
The double bus–double breaker arrangement involves two breakers
and two buses for each circuit.
Advantages of this Arrangement
1. With two breakers and two buses per circuit, a single bus failure
can be isolated without interrupting any circuits or loads.
2. A circuit failure of one circuit will not interrupt other circuits or
buses. Therefore, reliability of this arrangement is extremely high.
3. Maintenance of switching devices in this arrangement is very easy,
since switching devices can be taken out-of-service as needed and
circuits can continue to operate with partial line relay protection
and some line switching devices in-service, i.e., one of the two
circuit breakers.
Disadvantages of this Arrangement:
1. With double the amount of switching devices and buses, cost will
be substantially increased relative to other more simple bus
configurations.
2. Relaying is more complicated and more land is required,
especially for low-profile substation configurations.
Operating procedures to transfer from working bus to
standby bus
1. Check the status of the standby bus.
2. Close D.L‟s 2, 4.
3. Close C.B 2.
4. If C.B2 is not tripped so, the standby bus is ready to transfer.
5. Then Close D.L‟s 6, 8.
6. Close C.B 4.
7. Now you have two parallel paths.
8. Open C.B's 1, 5.
9. Open D.L's 1,3,5,7.
10. Now the working bus isolated.
- 01 -
Bus bar arrangements
Chapter two
7. Breaker and half arrangement:
The breaker-and-a-half scheme is configured with a circuit between two
breakers in a three-breaker line-up with two buses; thus, one-and-a-half
breakers per circuit. In many cases, this is the next development stage of
a ring bus arrangement.
Advantages of this arrangement:
1. High security against loss of supply.
2. Similar to the ring bus, this configuration provides good
reliability; with proper operating relay protection, a single
circuit failure will not interrupt any other circuits. Furthermore,
a bus section fault unlike the ring bus will not interrupt any
circuit loads.
3. Maintenance as well is facilitated by this arrangement, since an
entire bus and adjacent breakers can be maintained without
transferring or dropping loads.
Disadvantages of this arrangement:
1. Additional costs of circuit breakers are involved together with
complex protection arrangements.
2. The circuit breakers and other system components must be rated
for the sum of the load currents of two circuits.
3. Relay protection is similar to the ring bus, and due to the
additional devices, is more complex and costly than most of the
previously reviewed arrangement since the middle breaker must
be responsive to either of its associated circuits.
- 01 -
Bus bar arrangements
Chapter two
Breaker and half arrangement
8. Ring Bus (Mesh) Arrangement:
As the name implies, all breakers are arranged in a ring with circuits
connected between two breakers. From a reliability standpoint, this
arrangement affords increased reliability to the circuits, since with
properly operating relay protection, a fault on one bus section will only
interrupt the circuit on that bus section and a fault on a circuit will not
affect any other device.
Advantages of this arrangement:
1. From a maintenance point of view, the ring bus provides good
flexibility. A breaker can be maintained without transferring or
dropping load, since one of the two breakers can remain in-service
and provide line protection while the other is being maintained.
- 01 -
Bus bar arrangements
Chapter two
2. Similarly, operating a ring bus facility gives the operator good
flexibility since one circuit or bus section can be isolated without
impacting the loads on another circuit.
3. The ring bus arrangement is applicable to loads where reliability
and availability of the circuit is a high priority.
The disadvantages of this arrangement:
1. A “stuck breaker” event could cause an outage of the entire
substation depending on the number of breakers in the ring,
2. Expansion of the ring bus configuration can be limited due to the
number of circuits that are physically feasible in this arrangement,
and
3. Circuits into a ring bus to maintain a reliable configuration can
cause extensive bus and line work. For example, to ensure service
reliability, a source circuit and a load circuit should always be
next to one another. Two source circuits adjacent to each other in
a stuck breaker event could eliminate all sources to the station.
Therefore, a low-profile ring bus can command a lot of area.
4. Cost of the ring bus arrangement can be more expensive than a
single bus, main bus and transfer, and the double bus–single
breaker schemes since two breakers are required for each circuit,
even though one is shared.
5. Protective relaying for a ring bus will involve more complicated
design and, potentially, more relays to protect a single circuit.
Keep in mind that bus and switching devices in a ring bus must all
have the same ampacity, since current flow will change depending
on the switching device‟s operating position.
- 22 -
Bus bar arrangements
Chapter two
Ring Bus (Mesh) arrangement
Comparison between the substation switching schemes
- 24 -
Bus bar arrangements
Chapter two
Substation Interlocking Schemes:
To ensure that operators do not compromise the integrity of the
Transmission System by incorrect or inadvertent operation of equipment.
Substations shall be provided with a full interlocking scheme to ensure
that all disconnecting switches, fixed earthing switches (or other
interlocked earthing devices) and where required circuit breakers, are
operated in the correct sequence.
Interlocking schemes shall cover the following conditions:
1. Interlocking between circuit breakers and disconnecting switches
to ensure disconnecting switches do not make or break load
currents.
2. Interlocking between disconnecting switches and earthing switches
to ensure that earthing switches cannot be closed on to a locally
energized circuit and cannot be energized, when closed, by
operation of disconnecting switches.
3. Interlocking between disconnecting switches and adjacent earthing
switches to permit operation of the disconnecting switch when
earthing switches are closed on both sides of the disconnecting
switch. Such interlocking is not required for equipment rated at
123 kV and below.
4. To ensure correct sequence of on load bus bar transfer switching
operations at multiple bus bar substations.
5. To ensure that a bus-coupler or bus-section circuit breaker is only
closed with its associated disconnecting switches both open or both
closed.
Types of interlocking systems:
Interlock systems may be classified into three main divisions based on
the type of interconnection between associated devices:
- 20 -
Bus bar arrangements
Chapter two
1. Mechanical interlocks consist of a bar, chain, gear or other
mechanical arrangement between associated devices.
2. b) Electrical interlocks consist chiefly of switches and/or solenoids
arranged at the associated devices and connected by electrical
conductors. Application is limited to devices adjacent to a
satisfactory electrical power source.
3. Key interchange interlocks consist of self–contained individual
locking units located at associated devices which permit a desired
operation only when conditions are correct for that operation.
Substations interlock rules:
1. For feeder bay the disconnecting switches Q1, Q2, Q9 can be
operated only when the circuit breaker Q0is open. And for
transformer and bus coupler the disconnecting switches Q1, Q2can
be operated only when the circuit breaker Q0is open.
2. The circuit breaker of feeder bay Q0 can not be closed when the
disconnecting switches Q1, Q2, Q9 are in the intermediate
position. And for transformer and bus coupler bay the circuit
- 22 -
Bus bar arrangements
Chapter two
breaker Q0can not be closed when the disconnecting switches Q1,
Q2 are in the intermediate position.
3. For feeder and transformer the disconnecting switches Q1 , Q2 are
mutually interlocked so that only one can be closed at time ,but
they can be closed together only when the bus coupler circuit
breaker and disconnecting switches are closed.
4. For feeder and transformer the disconnecting switches Q1, Q2can
be operated only if the related bus bar earthing switch Q15, Q25 is
open.
5. The line feeder disconnecting switch Q9 can be operated only
when earthing switch Q8 is open.
6. The earthing switch Q8 can be operated only when The line feeder
disconnecting switch Q9 is open.
7. The bus coupler circuit breaker can not be opened (tripped) if
there are two disconnecting switches for same feeder or
transformer closed together.
8. The bus bar earthing switches Q15, Q25can not be operated if any
disconnecting switch (Q1 , Q2 ) at the same bus bar are closed (for
feeder and transformer bay).
9. The CB of transformer medium voltage side (MV income) can not
be closed if transformer feeder CB Q0 is opened.
10. If transformer feeder CB Q0 is manual opened, the CB of
transformer medium voltage side (MV income) is also opened
(tripped).
The SF6 circuit breaker control circuit:
- 21 -
Bus bar arrangements
Chapter two
Original Situation of Circuit Breaker scheme
Where:



M: Motor.
Y1: ON Coil & Y2: OFF Coil.
K1: Anti pumping Relay.
K2: Auxiliary Relay to block ON and OFF processes if the
Pressure is not sufficient.
 PS: Pressure Switch (SF6).
 S1: Auxiliary Switches from C.B.
 S2: Limit Switch from the spring.
Original Situation of Circuit Breaker:
 Circuit Breaker OFF.
 DC ON.
 Spring Charged (S2 Opened).
 Gas Pressure Sufficient for ON/OFF operations (PS is Opened).
 Circuit Breaker is ready for ON.
- 21 -
Bus bar arrangements
Chapter two
The cases of different conditions of spring, pressure of SF6 gas, and C.B.
states (on/off) :
1st Case :-( Spring is Uncharged and Gas Pressure Insufficient).
2nd Case:- (Spring is Charged and Gas Pressure Insufficient).
3rd Case:- (Spring is Charged and Gas Pressure Sufficient).
4th Case:- (Circuit Breaker is ON).
5th cCase:- (Circuit Breaker is OFF).
1st Case :-( Spring Uncharged and Gas Pressure Insufficient):
1. Circuit Breaker OFF:
 S1_1 Closed.
 S1_2 Opened.
 S1_3 Opened.
2. DC ON
3. Spring Uncharged:
 S2_1 Closed.
 S2_2 Closed.
4. Gas Pressure is Insufficient for ON/OFF operations:
 PS Closed.
 K2 Energized.
 C.B. is not ready for ON.
- 21 -
Bus bar arrangements
Chapter two
2nd Case:- (Spring Charged and Gas Pressure Insufficient):
1. Circuit Breaker OFF
 S1_1 Closed
 S1_2 Opened
 S1_3 Opened
2. DC ON
3. Spring Charged.
 S2_1 Opened.
 S2_2 Opened.
4. Gas Pressure Insufficient for ON/OFF operations.
 PS Closed
- 21 -
Bus bar arrangements
Chapter two
 K2 Energized
 Circuit Breaker is not ready for ON.
3rd Case:- (Spring Charged and Gas Pressure Sufficient)
1. Circuit Breaker OFF
 S1_1 Closed.
 S1_2 Opened.
 S1_3 Opened.
2. DC ON
3. Spring Charged
 S2_1 Opened.
 S2_2 Opened.
4. Gas Pressure Sufficient for ON/OFF operations
 PS Opened.
 K2 De-energized.
- 21 -
Bus bar arrangements
Chapter two
 Circuit Breaker is ready for ON
4th Case:- (Circuit Breaker ON)
1. Circuit Breaker ON
 S1_1 Opened
 S1_2 Closed
 S1_3 Closed
2. DC ON
3. Spring Charged
 S2_1 Opened
 S2_2 Opened
4. Gas Pressure Sufficient for ON/OFF operations
 PS Opened
 K2 De-energized
- 21 -
Bus bar arrangements
Chapter two
 Circuit Breaker is ready for OFF
5th Case:- (Circuit Breaker OFF)
1. Circuit Breaker OFF
 S1_1 Closed.
 S1_2 Opened.
 S1_3 Opened.
2. DC ON
3. Spring Charged
 S2_1 Opened.
 S2_2 Opened.
4. Gas Pressure Sufficient for ON/OFF operations
 PS Opened
 K2 De-energized
- 12 -
Bus bar arrangements
Chapter two
 C.B.is ready for ON Again
Disconnecting switch control circuit:
Disconnecting switches are usually interlocked with the associated circuit
breaker to prevent any attempt being made to interrupt load current.
Disconnecting switches are not designed to break fault current although
some designs will make fault current.
- 14 -
Bus bar arrangements
Chapter two
- 10 -