1.7 Bus Bar System & Single Line Diagram

Single Line Diagrams
• The single line diagram is a graphical representation of a three phases

electrical installation on a single phase basis, and showing the main electrical
elements and their interconnections.
• The electrical elements are represented by graphical symbols and their main
electrical characteristics are shown on the drawing.
• IEC Standard 60617 suggests graphical symbols to use for the elaboration of a
single line diagram.
• IEEE std C37.2 is also used for the device function number and contact
designation for metering and protection diagrams.
• Many utilities used their own standards.
Substation Erection, Construction, Operation Maintenance & Testing and Commissioning Training © CHMC, 2023
Busbar Systems in Substations
BUS BAR SCHEMES:

The commonly used bus bar schemes at Sub Stations are:
a) Single bus.
b) Main and Transfer bus.
c) Double bus.
d) Double Main and transfer bus.
e) One and a half breaker scheme.
a) Single Bus
Single busbar system is simplest
switching scheme
• Lowest cost;
• Small land area;
• Easily expandable;
• Simple in concept and operation and
for the application of protective
relaying.
Disadvantage
• Arrangement having a low reliability;
• Loss of entire substation caused by a
bus fault;
• Involve fairly extensive outages for
busbar and busbar disconnector
maintenance.
Single busbar system is very little used
in the main HV grid, however common in
rural distribution systems.
Figure 1 Typical single Bus arrangement is shown
in Fig.1.
b) Single Bus with Transfer Bus
• Reasonable in cost
• Fairly small land area
• Easily expandable
Disadvantage
• Additional circuit breaker
needed for bus tie;
• Protection and relaying becoming
complicated;
• Main Bus fault causes loss of
the entire substation.
• the substation operation cannot
be sectionalized.
This bus arrangement has been
extensively used in 132 kV
Substations.
Figure 2 Typical Single Bus with Transfer
Bus arrangement is shown in Fig.2.
c) Double Bus Double Busbar System
Double busbar system is especially

applicable at substations where
sectionalizing is necessary and
changing this sectionalizing has to
be possible.
• It is also possible to make one
of the busbars dead during
operation.
• In case the outage caused by a
fault has to be short or in case
circuit breaker maintenance must
not cause an outage, by-pass
disconnectors are used to by-pass
the faulted circuit breaker or
circuit breaker under
maintenance.
Typical Double Bus arrangement is
Figure 3 shown in Fig.3.
d) Double Bus and Transfer bus
Double Main Busbar and Transfer
Busbar System
Its benefits are:
• operation can be sectionalized
• two of the busbars (one main and
the transfer) can be made dead
simultaneously
• any circuit-breaker can be by-
passed
• feeders can be connected to by-
pass the switchgear.
Disadvantage
• more expensive solution than the
latter one
This Bus bar arrangement was
generally used in earlier 220 kV sub
stations.
Figure 4 Typical Double Bus Bar arrangement
is shown in Fig.4.
e) Double Bus Double Breaker
• Flexible operation and very

high reliability Isolation of
either bus or any breaker
without disrupting service
• Double feed to each circuit
• No interruption of service to
any circuit from a bus fault
• Suitable for substations
handling large amount of power
Disadvantage
High cost – 2 breakers per
circuit
Figure 5
f) Ring bus
• Flexible operation;
• High reliability;
• Double feed to each circuit;
• No main bus;
• Isolation of bus section and
circuit breakers for maintenance
without circuit disruption.
Disadvantage
• During fault, splitting of ring
may leave undesirable circuit
combination;
• Limited number of circuit
positions and limited extension
possibilities .
Figure 6
Breaker and Half System
g) Breaker-and-a-Half In this scheme, three circuit breakers
are used for controlling two circuits
which are connected between two bus
bars. Normally, both the bus bars are
in service.
•Flexible operation and high
reliability;
•Isolation of either bus without
service disruption;
•Isolation of any breaker for
maintenance without service
disruption;
•Double feed to each circuit;
•Bus fault does not interrupt service
to any circuits;
•More complicated relaying;
This scheme has been used in the 400
kV substations.
Suitable for substations handling
Figure 7 large amount of power.
Following table contains unavailability and relative cost
and reliability values of the busbar systems as well as
summarized the flexibility (load re-arrangements) and
extension possibilities of the systems:
Service continuity
When choosing a specific arrangement, one of the prime

consideration should be the effect of the loss of plant
due to fault considerations or for maintenance. It
includes:
• Loss of generating plant;
• Loss of transmission;
• Loss of supply to customers;
Reliability calculations for schemes are complex.
Quantitative assessment of the effect of repair or
maintenance outage.
This assessment of continuity is divided in 4 categories
Service continuity
Category 1
No outage necessary within the substation for either
maintenance or fault
Category 2
Short outage (4 hours) necessary to transfer the load to
an alternative circuit for maintenance or fault
Category 3
Loss of circuit or section until the repair is completed
Category 4
Loss of substation
Analysis of service continuity
Type of Arrangement Service Continuity Category

C B area Bus area
Single bus 3 4
Single bus with 2 2
transfer facility
Ring bus 1 or 2 Not applicable
Double bus double 1 or 2 1
breaker
Breaker-and-a-half 1 or 2 1
Breaker-and-a-half 1 or 2 1
plus double breaker
Category 1 applies for maintenance conditions and category 2 for

fault conditions
• SUBSTATION TYPES
Technology: AC and DC
AC Substations
Distribution
• Step-down HV to MV
Transmission
• Step-up MV to HV/EHV ( generating station )
• Step-down EHV to HV, EHV to MV
• Switching, HV, EHV
• Interconnection, generally HV or EHV
DC Substations
Converters stations
Transmission of energy from one station to the other
Rectifiers
Inverters
Network Interconnection
Back-to-back
DC line link
Submarine Cable Transmission
AC Substations
AIS = AIR INSULATED SUBSTATION
-INDOOR
-OUTDOOR BOX TYPE
-OUTDOOR OPEN STRUCTURE
-OUTDOOR LOW PROFILE
-METAL ENCLOSED INDOOR/OUTDOOR
GIS = GAS INSULATED SUBSTATION

-INDOOR
-OUTDOOR
Prefabricated MV Substation

1.7 Bus Bar System & Single Line Diagram

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Single Line Diagrams

• The single line diagram is a graphical representation of a three phases

• Many utilities used their own standards.

BUS BAR SCHEMES:

Double busbar system is especially

• Flexible operation and very

When choosing a specific arrangement, one of the prime

This assessment of continuity is divided in 4 categories

Type of Arrangement Service Continuity Category

Category 1 applies for maintenance conditions and category 2 for

GIS = GAS INSULATED SUBSTATION

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