Benawa Institute of Higher Education
Engineering Faculty
Electricity Department
____________________________________________
Air Insulated Substation
Bus/Switching Configuration
Lecturer: Munir Ahmad Khubaib
3.1 Introduction
• Various factors affect the reliability of an electrical substation or
switchyard facility, one of which is the arrangement of switching
devices and buses. The following are the six types of arrangements
commonly used:
1. Single bus
2. Double bus–double breaker
3. Main and transfer (inspection) bus
4. Double bus–single breaker
5. Ring bus
6. Breaker-and-a-half
3.2 Single Bus Arrangement
• This is the simplest bus breakers will cause an outage of
arrangement, a single bus and all the entire facility.
connections directly to one bus
(Figure 3.1).
• 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
Cont. …
• This require standby generation or switching loads to adjacent
substations, if available, to minimize outages of loads supplied from
this type of facility.
• 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.
• 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.
3.3 Double Bus-Double Breaker Arrangement
• The double bus–double breaker
arrangement involves two
breakers and two buses for each
circuit (Figure 3.2).
• With two breakers and two buses
per circuit, a single bus failure can
be isolated without interrupting
any circuits or loads.
Cont. …
• Furthermore, a circuit failure of one circuit will not interrupt other
circuits or buses. Therefore, reliability of this arrangement is
extremely high.
• 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, one of the two circuit breakers.
• Obviously, with double the amount of switching devices and buses,
cost will be substantially increased relative to other more simple bus
configurations.
Cont. …
• In addition, relaying is more complicated and more land is required,
especially for low-profile substation configurations.
• This arrangement allows for operational flexibility; certain lines could
be fed from one bus section by switching existing devices.
• This bus configuration is applicable for loads requiring a high degree
of reliability and minimum interruption time.
3.4 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 (Figure
3.3).
Cont. …
• The operating reliability is low; a main bus fault will de-energize all
circuits.
• The cost of the main and transfer bus arrangement is more than the
single bus arrangement because of the added transfer bus and
switching devices.
• Application of this type of configuration should be limited to low
reliability requirement situations.
3.5 Double Bus-Single Breaker Arrangement
• 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 allows each
circuit to be supplied from either
bus via its switches.
• Thus providing increased
operating flexibility and improved
reliability.
Cont. …
• For example, a fault on one bus will not impact the other bus and
Relay protection for this arrangement will be complex with the
flexibility of transferring each circuit to either bus.
• The double bus–single breaker arrangement with two buses and a tie
breaker provides for some ease in maintenance, especially for bus
maintenance.
• The cost of this arrangement would be more than the single bus
arrangement with the added bus and switching devices.
• Application of this arrangement is best suited where load transfer and
improved operating reliability are important.
3.6 Ring Bus Arrangement
• As the name implies, all breakers any other device (Figure 3.5).
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
Cont. …
• 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.
• From a maintenance point of view, the ring bus provides good
flexibility.
• Cost of the ring bus arrangement can be more expensive than a single
bus.
• The ring bus arrangement is applicable to loads where reliability and
availability of the circuit is a high priority.
3.7 Breaker-and-a-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 (Figure3.6).
Cont. …
Thanks!
