Substation
Prepared By: BN Neupane, NEA
Power System Basics
2
All power systems have three major components:
Generation, Transmission & Distribution including
load.
Generation: Creates electric power.
Transmission & Distribution: Transmits electric
power from generation to load.
Load: Consumes electric power.
3
Generator
Single Line (One
Line) Representation 10 to 30 KV
of Step-up T/F
Power System 132 KV, 220 KV, 400 KV.
Transmission EHV
Step down T/F
66 KV, 33 KV OR 11 KV
Distribution HV Receiving
Station
Step Down
To large Industries at 11 kv or 33 kv or
T/F
66 kv
consumer of HV level
440V (Utilization)
Consumers of LV level
Transmission and Distribution
5
Typical high voltage transmission voltages are 400, 220
kV, 132 kV, 66 kV (500, 345, 230, 161, 138 and 69 kV
in US standard).
Transmission tends to be a grid system or network
system, so each bus is supplied from two or more
directions.
Lower voltage lines are used for distribution, with a
typical voltage of 33 kV and 11 kV.
Distribution systems are normally radial.
Transformers in substations are used to change the
voltage.
SUBSTATION
It is the station in the power transmission system at which
electric power is transformed to a conveniently used
form. The station may consist of transformers, switches,
circuit breakers and other auxilliary equipment. Its main
function is to receive energy transmitted at high voltage
from the generating station, by either step-up or step-
down the voltage to a value appropriate for local use
and provide facilities for switching. Substations have
some additional functions. Its provide points where
safety devices may be installed to disconnect circuits or
equipment in the event of trouble.
Some substation, such as power plant switchyard are
simply switching stations where different connections can
be made between various transmission lines.
Introduction of Substation
An electrical substation takes electricity from
high voltage and lowers the voltage such as our
homes & businesses
Substation converts lower voltage to higher
voltage
It may act as converting station from AC to DC (or
DC to AC)
It may act as switching station – switching
(ON/OFF) of the feeders
It may act as voltage control station or power
factor correction station
Electrical Substation
8
Basically an electrical substation consists
of a number of incoming circuits and
outgoing circuits connected to common
busbar systems.
Busbars are conducting bars to which a
number of incoming or outgoing circuits
are connected.
Substation convert low voltage (or current)
to high voltage or vice-versa using
Transformers
Introduction of Substation Cont…
Each circuit connected to the busbar will have certain
electrical component such as circuit breakers,
isolators, earth switches, current transformers and
voltage transformers.
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.
Substation Cont…
• A substation 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 points.
Components of Switchyard of a
Substation
• CONNECT •A - Busbar
• CONNECT & DIS-CONNECT •B - Disconnector
• CONNECT, DIS-CONNECT & DETECT •C - Circuit Breaker
• DETECT & TRANSFORM •D - Current Transformer
• DETECT & TRANSFORM •E - Voltage Transformer
• PROTECT & SAFETY •F - Earthing Switch
• PROTECT •G - Surge Arrestor
Major Equipments of Substation
Power Transformers and Station Transformers
Circuit Breakers
Bus Bar
Lightning Arresters/Surge Arresters
Disconnecting Switches/Isolators
Earth Switches
Instrument Transformers (CT/PT)
Control & Relay Panels
Communication Equipment
Batteries and Battery Charger
Shunt Reactors & Capacitor Bank
Major Equipments of Substation
BUSBAR (or bus, for short) – is a term we use for a main bar or
conductor carrying an electric current to which many connection
may be made.
Buses are merely convenient means of connecting switches and
other equipment into various arrangements. The usual
arrangement of connections in most substations permit working
on almost any piece of equipment without interruption to
incoming or outgoing feeders. In the switchyard or substation,
buses are open to the air. Aluminum or copper conductors
supported on porcelain insulators, carry the electric energy
from point to point.
Busbars
(long heavy
tube type)
Busbar in AIS Substation
15
Major Equipments of Substation ...
DISCONNECTOR/Isolator/Disconnecting Switch – is an easily
removed piece of the actual conductor of a circuit. The purpose
of disconnects is to isolate equipment. Disconnects are not used
to interrupt circuits; they are no-load devices. A typical use of
disconnects is to isolate a circuit breaker by installing one
disconnect on either side of the circuit breaker (in series with the
breaker). Operation of disconnects is one of the most important
and responsible jobs of a power plant operator. One error in
isolation of equipment, or the accidental grounding of line
equipment, can be a fatal mistake.
Disconnect Switch
( moving contact rod (A) &
contacts with flexible
fingers (B) )
B
A
Isolators in HV Substations
18
b. Horizontal Double Breaking (Center
a. Central Breaking Type
Rotating) Type
Major Equipments of Substation ...
CIRCUIT BREAKER – is used to interrupt circuits while current is
flowing through them. The making and breaking of contacts in a
Oil type circuit breaker are done under oil, this oil serves to
quench the arc when the circuit is opened. The operation of the
breaker is very rapid when opening. As with the transformer,
the high voltage connections are made through bushings. Circuit
breakers of this type are usually arranged for remote electrical
control from a suitably located switchboard.
Some recently developed circuit breakers have no oil, but put
out the arc by a blast of compressed air; these are called air
circuit breakers. Another type encloses the contacts in a vacuum
or a gas (sulfur hexafluoride, SF6) which tends to self maintain
the arc.
Operating
Mechanism
Panel
Circuit Breakers
( Connected in a typical 3-
phase circuit )
Position
Indicator
Major Equipments of Substation ...
CURRENT TRANSFORMER – Current transformer are used with
ammeters, watt meters, power-factor meters, watt-hour meters,
compensators, protective and regulating relays and the trip coil
of circuit breakers. One current transformer can be used to
operate several instruments, provided that the combined
burden does not exceed that for which the transformer is
designed and compensated. The current transformer is
connected directly in series with the line.
Major Equipments of Substation ...
VOLTAGE TRANSFORMER – also know as potential
transformer, are used with volt-meters, wattmeters, watt-hour
meters, power-factor meters, frequency meters, synchroscopes
and synchronizing apparatus, protective and regulating relays
and the no-voltage and over-voltage trip coils of automatic
circuit breakers. One transformer can be used for a number of
instruments at the same time if the total current taken by the
instrument does not exceed that for which the transformer is
designed and compensated. The ordinary voltage transformer
is connected across the line, and the magnetic flux in the core
depends upon the primary voltage
Major Equipments of Substation ...
EARTHING SWITCH – also known as ground disconnect, which
used to connects the equipment to a grid of electrical
conductors buried in the earth on the station property. It is
intended to protect people working on the grounded
equipment. It does this by completing a circuit path, thereby
reducing the voltage difference between the equipment and its
surroundings. For safety reasons, it is important that ground
disconnects and all associated connections have good contact
and low resistance. It is also important that the protective
ground not be accidentally remove, that is why all the earthing
switches, disconnect switches and circuit breakers are all
interlocked to each other and proper/correct sequencing must
be followed.
Major Equipments of Substation ...
SURGE ARRESTOR – are devices used to provide the
necessary path to ground for such surges, yet prevent any
power current from following the surge. An ideal arrester must
therefore have the following properties:
1. Ability to remove the surge energy from the line in a min. time.
2. High resistive to flow of power current.
3. A valve action automatically allowing surge to pass and then
closing up so as not to permit power current to flow to ground.
4. Always ready to perform.
5. Performance such that no system disturbances are introduced by
its operation.
6. Economically feasible
Major Equipments of Substation ...
OVERHEAD GROUND WIRE - by a ground wire is meant a
wire, generally of steel, supported from the top of transmission-
line towers and solidly grounded at each tower. It is considered
a preventive device, but it does not entirely prevent the
formation of travelling waves on a line. Furthermore, those lines
which are not equipped with ground wires will be subjected to
disturbances which produce surges that must be allowed to
escaped to ground, or the apparatus connected to the line must
be strong enough to reflect or absorb these surges until they
are entirely damped out.
Classification of Substation
1. Classification based on voltage levels:
EHV (Extra High Voltage) Substation, HV (High Voltage) Substation, MV
(Medium Voltage) Substation, LV (Low Voltage) Substation, HVDC (High
Voltage DC) Substation etc.
2.Classification based on Construction:
Outdoor substation: Built under open sky.
Indoor substation: Built inside a building.
Pole Mounted Substation: Low voltage distribution substation
in which transformer is mounted on the poles.
Foundation Mounted
Underground Substation
Classification of Substation Cont…
31
3. Classification based on configuration:
Conventional air insulated outdoor substation
SF6 Gas Insulated Substation (GIS)
Composite substations having combination of the above two
4. Classification based on application
Generating Substation: Associated with generating station to step up
the voltage as the generating voltage is low.
Primary Grid Substation: Created at suitable load centre along
primary transmission lines. Primary Substations receive power at EHV
such as 400kV, 220kV, 132kV and transform the voltage to 66kV,
33kV or 11kV to suit the local requirements in respect of both load
and distance of ultimate consumers. These are also referred to ‘EHV’
Substations.
Substation Photographs – AIS Substation
Typical Gas Insulated Substations
Substation GIS Substation GIS
132 kV, 150/20 kV,
Cherry, Cyprus Thebes
GIS power GIS power
distribution distribution
center, substations
150/20 kV 150/20 kV
Vrilissia PPC’s internal
substation space
Classification of Substation Cont…
34
Secondary Substation/Distribution Substation: Along Secondary
Transmission Line, receive power at 66 kV or 33 kV which is stepped
down usually to 11kV.
Distribution Transformer Station: Usually pole mounted Substations
receiving power at 11kV and step down to a voltage suitable for LV
distribution purposes, normally at 400/230 volts
Bulk supply and industrial substation: Similar to distribution sub-
station but created separately for each consumer.
Mining Substation: Needs special design consideration because of
extra precaution for safety needed in the operation of electric supply.
Mobile Substation: Temporary requirement.
Substation Layout
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.
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
layout diagrams.
The design of sub station layout need careful consideration of
several aspects.
BUS LAYOUT AND SWITCHING ARRANGEMENT
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
substation to meet system and station requirements.
A substation must be reliable, economical, safe, and as
simple in design as possible.
Busbar Schemes
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 breaker arrangement
Factor to be considered for Substation Layout
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.
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.
Various Bus-bar Schemes
1. SINGLE BUS BAR SCHEME
BUS
ISOLATOR
BREAKER
CURRENT TRANSFORMER
POWER TRANSFORMER
39
39
SINGLE BUS BAR SCHEME Cont…
Advantage :
Lowest cost, Simplest, Least space Requirement
Disadvantage:
Maintenance without interruption of supply is not possible.
Sub station can not be extended without completely de-energizing
the sub station
Can be used only where loads can be interrupted or have other
supply arrangements. Least flexibility.
2. SINGLE BUS BAR SCHEME WITH BUS SECTIONALISER
BUS SECTION-1 BUS SECTION-2
SECTIONALISER ISOLATOR
BREAKER
CURRENT TRANSFORMER
POWER TRANSFORMER
41
41
SINGLE BUS BAR WITH BUS SECTIONALISER Cont…
• Sectionalizing the single bus improves slightly the
reliability if the incoming and outgoing circuits are
distributed evenly on both the sections.
• 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.
• Only the faulty section will be tripped by bus differential
protection. 42
3. DOUBLE BUS BAR WITH ONE BREAKER PER CIRCUIT
BUS-1
BUS-2
BUS COUPLER
BREAKER
BREAKER
POWER TRANSFORMER
43
43
DOUBLE BUS BAR SCHEME Cont…
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
taken out for maintenance without interrupting supply to
the concerned circuit.
44
44
DOUBLE BUS BAR SCHEME Cont…
Disadvantages:
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.
45
4. MAIN AND TRANSFER BUS
MAIN BUS
TIE BREAKER BREAKER
TRANSFER BUS
LINE LINE
46
46
MAIN AND TRANSFER BUS Cont …
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. 47
47
MAIN AND TRANSFER BUS Cont …
Any breaker can be taken out of service for maintenance.
Wherever maintenance is required on any breaker, the circuit is
changed over to the transfer bus and controlled through the bus
coupler breaker.
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 to
energize the concerned circuit for all the circuit breakers whenever
they are taken out for maintenance.
Failure of bus or any circuit breaker results in shut-down of entire sub
station. 48
48
5. DOUBLE BUS BAR WITH TWO BREAKER
PER CIRCUIT
BUS-1
BREAKER
BUS-2
49
LINE LINE 49
DOUBLE BUS BAR WITH TWO BREAKER PER
CIRCUIT Cont…
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.
50
50
6. BREAKER -AND -A -HALF SCHEME
BUS-1
ISOLATOR
BREAKER
LINE
TIE BREAKER
51
51
BUS-2
Breaker and a half Scheme Cont…
• 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
circuit.
52
BREAKER -AND -A -HALF SCHEME Cont …
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.
Breaker maintenance can be done with no loss of service, no
relay changes, and simple operation of the breaker isolators
53
BREAKER -AND -A -HALF SCHEME Cont …
• 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.
54
7. RING BUS OR MESH SCHEME
ISOLATOR
BREAKER
LINE
POWER TRANSFORMER
LINE
55
55
RING BUS OR MESH SCHEME Cont …
In this scheme the breakers are arranged in a ring with
circuits connected between breakers.
There are the same number of circuits as there are
breakers.
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 breaker-
failure back up relays.
During breaker maintenance, the ring is broken, but all 56
lines remain in service. 56
RING BUS OR MESH SCHEME Cont …
In this scheme the breakers are arranged in a ring with
circuits connected between breakers.
There are the same number of circuits as there are
breakers.
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 breaker-
failure back up relays.
During breaker maintenance, the ring is broken, but all 57
lines remain in service. 57
