THE ELECTRIC SUB STATION
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THE TRANSFORMER BAY
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 INTRODUCTION
THE ELECTRIC SUB STATION
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M L SHESHADRI
The present electrical power system is a
complex interconnection of Generating
stations-Transmission systemsReceiving stations- Distribution systems
and Load points.
In all the above phases of power flow, the
transfer of electrical energy takes place in
the electric sub stations.
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 INTRODUCTION
THE ELECTRIC SUB STATION
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M L SHESHADRI
 Basically an electrical sub station consists
of a number of incoming circuits and out
going circuits connected to common bus
bar systems.
 Bus bars are conducting bars to which a
number of incoming or out going circuits
are connected.
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 INTRODUCTION
THE ELECTRIC SUB STATION
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Each circuit connected to the bus bar will have
certain electrical component such as circuit
breakers, isolators, earth switches, current
transformers and voltage transformers.
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These components are connected in a definite
sequence such that a circuit can be switched off
during normal operation by manual command
and also automatically during abnormal
conditions such as short circuits.
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 INTRODUCTION
THE ELECTRIC SUB STATION
A sub station receives electrical power from
generating station through incoming
transmission lines and delivers electrical
power through the out going transmission
lines.
Sub station is an integral part of a power
system and is an important link between
the generating stations, transmission
systems, distribution systems and load
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points.
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THE ELECTRIC SUB STATION
 DESIGN CONSIDERATIONS
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M L SHESHADRI
 The sub station is designed with an
objective to provide maximum
reliability, flexibility, continuity of
service and to meet these objectives
with the lowest investment costs that
satisfy system requirement.
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 DESIGN
THECONSIDERATIONS
ELECTRIC SUB STATION
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System requirements include the selection of
optimum voltage levels depending on the load
requirements and the transmission distances
involved. Generally, the generating source will be
far away from the load centers. The advantage of
capitalizing on low site costs, availability of ample
cooling water supply, economical fuel supply and
less stringent environmental considerations
compel construction of generating source far away
from load centers, there by, increasing
transmission distances.
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 DESIGN CONSIDERATIONS
THE ELECTRIC SUB STATION
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 The system requirement must be met with minimum
costs as the cost of equipment, labor, land and site
treatment is increasing every day.
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 Hence, to transmit power over long distances the
transmission voltage is to be increased and in our
country 400 kV is becoming common and higher
voltages for transmission is being explored. Many
factors such as voltage level, load capacity, site space
limitations, transmission line right of way requirement
and environmental considerations influence the design
of sub stations.
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THE ELECTRIC SUB STATION
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 BUS LAYOUT AND SWITCHING ARRANGEMENT
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 Since the major sub station costs are reflected in the
power transformers, circuit breakers and disconnecting
switches, the bus layout and switching arrangement
selected will determine the number of switches and
power circuit breakers required.
 A number of factors must be considered in the selection
of bus layouts and switching arrangements for a sub
station to meet system and station requirements.
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 A sub station must be reliable, economical, safe,
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 SUB STATION LAYOUT AND BUS BAR SCHEMES
THE ELECTRIC SUB STATION
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M L SHESHADRI
 The term layout denotes the physical
arrangement of various components in the
sub station relative to one another. The
layout is significant as it influences the
operation, maintenance, cost and
protection of the sub station. These
aspects are considered while designing
the sub station layout.
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 SUB STATION LAYOUT AND BUS BAR SCHEMES
THE ELECTRIC SUB STATION
9/21/2019
M L SHESHADRI
With the given number of incoming lines, out
going lines, transformers, etc., the sub station
can be designed in several alternative ways.
 The physical arrangement of the equipment is
called the layout of the sub station. The layout is
illustrated by means of single line diagrams.
 The design of sub station layout need careful
consideration of several aspects such as:
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 SUB STATION LAYOUT AND BUS BAR SCHEMES
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THE ELECTRIC SUB STATION
Switching requirement for normal operation.
Switching requirement during abnormal
conditions like short circuits and overloads.
Degree of flexibility in operations, simplicity.
Freedom from total shutdowns.
Maintenance requirements, space for
approaching various equipment for
maintenance.
Road/ rail for transporting main and auxiliary
equipment.
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 SUB STATION LAYOUT AND BUS BAR SCHEMES
THE ELECTRIC SUB STATION
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 Safety of personnel.
 Protective zones for main and back up
protection.
 Provision for bye pass facilities and for
extensions, space requirements.
 Technical requirements such as ratings,
clearances, earth system, lightning protection.
 Requirement for SCADA and communication.
 Compatibility for local and ambient condition.
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 SUB STATION LAYOUT AND BUS BAR SCHEMES
THE ELECTRIC SUB STATION
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 The choice of bus bar schemes for ac yards
depend upon several factors mentioned above.
The important bus bar schemes are:
 Single bus bar
 Double bus bar with one breaker per circuit
 Double bus bar with two breaker per circuit
 Main and transfer bus
 Ring bus or Mesh scheme
 Breaker and a half(1 1/2 breaker) arrangement 14
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 SUB STATION LAYOUT AND BUS BAR SCHEMES
THE ELECTRIC SUB STATION
 The degree of reliability is evaluated by
determining continuity of service under
anticipated operating conditions and possible
faults.
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 The basis of comparison is generally the
degree of reliability & economic justification.
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The various schemes are generally compared to
emphasize their advantage and limitations.
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 VARIOUS
BUS BARSUB
SCHEMES
THE ELECTRIC
STATION
ISOLATOR
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BUS
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 SINGLE BUS BAR SCHEME
BREAKER
CURRENT TRANSFORMER
POWER TRANSFORMER
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THE ELECTRIC SUB STATION
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 SINGLE BUS BAR SCHEME
–Lowest cost
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 Advantage
 Disadvantage:
Maintenance without interruption of supply is not possible.
Sub station can not be extended without completely deenergizing the sub station
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Can be used only where loads can be interrupted or have17
other supply arrangements. Least flexibility.
 VARIOUS BUS BAR SCHEMES
THE ELECTRIC SUB STATION
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 SINGLE BUS BAR SCHEME WITH BUS
SECTIONALISER
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BUS SECTION-1
BUS SECTION-2
SECTIONALISER
ISOLATOR
BREAKER
CURRENT TRANSFORMER
POWER TRANSFORMER
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THE ELECTRIC SUB STATION
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Sectionalizing the single bus improves slightly the
reliability if the incoming and out going circuits are
distributed evenly on both the sections.
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 SINGLE BUS BAR SCHEME WITH BUS SECTIONALISER
 Where double feed is provided for any single load it is
preferable to have one circuit from each section.
 In this arrangement each section behaves as a separate
bus bar and any outage can be confined to one section
of the bus bar.
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 Only the faulty section will be tripped by bus differential
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 DOUBLE BUS BAR WITH ONE BREAKER PER CIRCUIT
THE ELECTRIC SUB STATION
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BUS-2
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BUS-1
BUS COUPLER
BREAKER
BREAKER
POWER TRANSFORMER
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THE ELECTRIC SUB STATION
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 DOUBLE BUS BAR WITH ONE BREAKER PER
CIRCUIT
 This arrangement has the following advantages:
 Each load may be fed from either bus.
 Operational flexibility may be increased by grouping the
incoming and out going feeders in separate groups.
 Either bus bar can be taken out for maintenance.
 Bus coupler helps in ‘on load change over ‘from one bus
to the other.
 Adopted where load and continuity justify additional
cost.
 A major disadvantage is that the breaker can not be 21
taken out for maintenance without interrupting supply to21
the concerned circuit.
 DOUBLE BUS BAR WITH ONE BREAKER PER
CIRCUITTHE ELECTRIC SUB STATION
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 Bus protection scheme may cause loss of sub
station when it operates if all circuits are
connected to that bus.
 High exposure to bus faults.
 Line breaker failure takes all circuits connected
to that bus out of service.
 Bus tie breaker failure takes the entire sub
station out of service.
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DOUBLE MAIN BUS & CB BYPASS ISOLATOR SYSTEM
T/F-1
BAY2
BAY6
BAY4
BAY7
BUS COUPLER
BAY1
T/F-2
BUS-1
BUS-2
BAY3
BAY5
FOR ANY CB PROBLEM OR FOR
PREVENTIVE
MAINTANENCE, SUCH
FEEDER CAN BE SHIFTED TO ANOTHER
BUS AND THE BYPASS ISOLATOR IS
CLOSED,
THEN
PROTECTION
IS
TRANSFERRED TO BUS COUPLER AND
THE FAULTY CB CAN BE ISOLATED.
FEEDER1
FEEDER2
FEEDER3
FEEDER4
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 MAIN AND TRANSFER BUS
THE ELECTRIC SUB STATION
MAIN BUS
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BREAKER
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TIE BREAKER
TRANSFER BUS
LINE
LINE
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 MAIN AND TRANSFER BUS
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THE ELECTRIC SUB STATION
 This is an alternative to double bus single
breaker arrangement which provided for change
over to either bus for carrying out maintenance
on other bus. But it provided no facility for
breaker maintenance without interrupting power
supply to the concerned circuit.
 The main and transfer bus works the other way
round.
 This arrangement provides facility for carrying
out breaker maintenance but does not permit
bus maintenance.
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THE ELECTRIC SUB STATION
 Wherever maintenance is required on any breaker, the circuit is
changed over to the transfer bus and controlled through the bus
coupler breaker.
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 Any breaker can be taken out of service for maintenance.
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 MAIN AND TRANSFER BUS
 Potential devices may be used on the main bus for relaying.
 The cost is increased due to use of an extra isolator for each circuit
and providing interlock for bus coupler and circuit isolators.
 Relaying sensitivity decreases as the same bus coupler is used to26
energize the concerned circuit for all the circuit breakers whenever26
they are taken out for maintenance.
DOUBLE MAIN BUS & TRANSFER BUS SYSTEM
T/F-1
T/F-2
BUS-2
BAY3
BAY1
BAY2
FEEDER1 FEEDER2
BAY4
TRANSFER BUS
BAY5
TRANSFER BUS
COUPLER
BUS COUPLER
BUS-1
BAY6
BAY7
BAY8
FEEDER3 FEEDER4
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BUS BAR ARRANGEMENTS
• Double Bus
Transfer Bus.
Bar
Arrangement
with
This arrangement provides more additional
flexibility, continuity of Power Supply,
permits periodic maintenance without total
shut down as the two main buses can be
operated independently with the same
redundancy.
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 DOUBLE BUS BAR WITH TWO BREAKER
PER CIRCUIT
THE ELECTRIC SUB STATION
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BREAKER
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BUS-1
BUS-2
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LINE
LINE
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 DOUBLE BUS BAR WITH TWO BREAKER PER
THE ELECTRIC SUB STATION
CIRCUIT
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M L SHESHADRI
 Each circuit has two dedicated breakers.
 Has flexibility in permitting feeder circuits to be
connected to either bus.
 Any breaker can be taken out of service for
maintenance.
 High reliability.
 Most expensive. Used only in large generating
stations where security of connection is paramount.
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THE ELECTRIC SUB STATION
• RING BUS OR MESH SCHEME
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ISOLATOR
BREAKER
LINE
POWER TRANSFORMER
LINE
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 RING BUS OR MESH SCHEME
THE ELECTRIC
STATION
 In this scheme
the breakers SUB
are arranged
in a ring with
 During normal operation, all breakers are closed. For a
circuit fault, two breakers are tripped, and in the event
one of the breaker fails to operate to clear the fault, an
additional circuit will be tripped by operation of breakerfailure back up relays.
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 There are the same number of circuits as there are
breakers.
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circuits connected between breakers.
 During breaker maintenance, the ring is broken, but all 32
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lines remain in service.
THE ELECTRIC SUB STATION
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 RING BUS OR MESH SCHEME
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 The circuits connected to the ring are arranged so that
sources are alternated with loads.
 For an extended circuit outage, the line isolator may be
opened and the ring can be closed.
 No changes to protective relays are required for any of
the various operating conditions or during maintenance.
 The ring bus scheme is economical in cost, has good
reliability, is safe for operation, is flexible, and is
normally considered suitable for important sub stations33
up to a limit of five circuits.
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THE ELECTRIC SUB STATION
• One and a Half Breaker Arrangement.
This arrangement provides three circuit
breakers for every two circuits. It gives
high security against loss of supply but
higher cost is involved. Hence this is
provided for important 400/220 KV Sub
Stations.
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THE ELECTRIC SUB STATION
BREAKER -AND -A -HALF SCHEME
ISOLATOR
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BUS-1
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BREAKER
LINE
TIE BREAKER
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BUS-2
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I-CONFIGUARATION
FEEDER11
DIA5
DIA6
BAY2
BAY5
BAY8
BAY11
BAY14
BAY17
BAY3
BAY6
BAY9
BAY12
BAY15
BAY18
BAY1
DIA4
FEEDER9
BAY16
DIA3
BAY4
DIA2
FEEDER7
BAY13
BUS-1 DIA1
FEEDER5
BAY10
FEEDER3
BAY7
FEEDER1
BUS-2
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FEEDER2
FEEDER4
FEEDER6
FEEDER8
FEEDER10
FEEDER12
D-CONFIGUARATION
FEEDER10
BAY14
BAY13
BAY7
DIA1
FEEDER9
BAY8
BAY3
BAY1
BAY2
FEEDER6
DIA3
BAY15
FEEDER5
FEEDER2
BAY9
FEEDER1
DIA5
BUS-1
BAY5
FEEDER3
BAY11
FEEDER4
FEEDER7
DIA6
BAY18
BAY16
DIA4
BAY12
BAY10
DIA2
BAY6
BAY4
BUS-2
BAY17
FEEDER8
FEEDER11
FEEDER12
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• BREAKER -AND -A -HALF SCHEME
THE ELECTRIC SUB STATION
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M L SHESHADRI
• The breaker and a half scheme, some times
called the three switch scheme, has three
breakers in series between the main buses.
• Two circuits are connected between the three
breakers, hence the term breaker and a half.
• This pattern is repeated along the main buses so
that one and a half breakers are used for each
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circuit.
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THE ELECTRIC SUB STATION
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 BREAKER -AND -A -HALF SCHEME
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 Under normal operating conditions all breakers are
closed and both buses are energized.
 A circuit is tripped by opening the two associated circuit
breakers.
 Tie breaker failure will trip one additional circuit, but no
additional circuit is lost if a line trip involves failure of a
bus breaker.
 Either bus may be taken out of service at any time with
no loss of service.
 With sources connected opposite loads, it is possible to
operate with both buses out of service.
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 Breaker maintenance can be done with no loss of
service, no relay changes, and simple operation of the 39
breaker isolators.
• BREAKER -AND -A -HALF SCHEME
THE ELECTRIC SUB STATION
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M L SHESHADRI
• The breaker-and-a-half arrangement is more
expensive than other schemes, except the
double-breaker-double-bus scheme.
• However, the breaker-and-a-half scheme is
superior in flexibility, reliability, and safety.
• Protective relaying and automatic re-closing
schemes are more complex than for other
schemes and hence costly.
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THANK YOU
9/21/2019
• ANY QUESTIONS ?
M L SHESHADRI
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