2. ELECTRICAL SUB-STATION
1.1 The substations are very much essential to
• Evacuate power from generating stations.
• Transmit to the load centers.
• Distribute to the utilities & ultimate consumers.
3. The Electrical power generation from Hydel, Thermal, Nuclear and
other generating stations has to be evacuated to load centers.
The generation voltage is limited to 15/18 KV due to the limitation
of the rotating machinery. This bulk power has to be stepped up to
higher voltages depending on quantum of power generated and
distance to the load centers.
Again the power has to be stepped down to different lower
voltages for transmission and distribution.
In between the power houses and ultimate consumers a number of
Transformation and switching stations have to be created. These
are generally known as sub-stations
Accordingly the substations are classified as
Generating substations called as step up substations
Grid Substations called as Step-Down Stations
Secondary Sub-stations/ Distribution substations
5. The generating substations
Receives electric power from a nearby generating facility and uses a
large power transformer to stepped up the voltage to primary
transmission voltage, for long distance bulk power A.C
Transmission to load Centers.
The grid substations
Created at suitable load centers along the primary transmission
lines & connect different parts of a grid and are a source for sub
transmission lines or distribution lines. The step-down substation
can change the transmission voltage to a sub transmission voltage.
The sub transmission voltage lines can then serve as a source to
distribution substations. Sometimes, power is tapped from the sub
transmission line for use in an industrial facility along the way.
Other wise, the power goes to a distribution substation.
6. Switching Stations
Provided in between lengthy primary transmission lines:
To avoid switching surges.
For easy segregation of faulty zones.
For providing effective protection to the system in the A.C. network.
The switching stations also required wherever the EHT line are to be tapped and line to
be extended to different load centers without any step down facility at the switching
The number of outgoing lines will be more than the incoming lines, depending on the
Located at actual load points along the secondary transmission lines where the voltage is
further stepped down to:
Sub transmission voltages
Primary distribution voltage.
Distribution substations are created where the sub-transmission voltage and primary
distribution voltage are stepped down to supply voltage and feed the actual consumers
through a network of distribution and service line.
7. Converting Substation
Converting substations are the ones that change alternating current to direct current in
power transmission. Alternating current is used inside the country while direct current is
sent overseas along with subsea cables.
8. Classification of Sub-Station based on voltage level
• Generally the following voltage class substations prevailing in India
Designation Description Range
LV or LT Low Voltage or Low Tension Below 1000V AC
HV or HT High Voltage or High Tension All AC Voltages Above 1000V AC
MHV Medium High Voltage Between 1KV to 33KV
HV High Voltage Between 33KV to 132KV
EHV Extra High Voltage 220KV, 400KV
UHV Ultra High Voltage 765KV, 1100KV
HVDC High Voltage Direct Current 400KV, 500KV DC
9. Types of Sub-Stations:
• Out door- Conventional Air insulated substations (AIS)
• SF6 Gas insulated substations (GIS )
• Hybrid Sub-stations
• Converting substations
The Types of Sub-Stations depends upon:
• The availability of the land in the required place.
• Environmental conditions.
10. Out door- Conventional Air insulated substations (AIS)
• In Such Substations Busbars & Connectors can be seen by naked
• All equipment's like LA, Circuit Breakers, Isolators, Transformers,
CTs, VTs etc are installed in outdoor.
• The Busbars are supported on post or strain insulators
• It has Galvanised Steel Structures for supporting the Equipment,
Insulators & Incoming & Outgoing Lines
11. A. Primary power lines' side
B. Secondary power lines' side
1. Primary power lines
2. Ground wire
3. Overhead lines
4. Voltage Transformer/ CVT
5. Disconnect switch
6. Circuit breaker
7. Current transformer
8. Lightning arrester
9. Main transformer
12.Secondary power lines
Out door- Conventional Air insulated substations (AIS)
12. Advantages of Outdoor Substation (AIS):
Cost of Switchgear installation is low
In future the extension of the substation installation is easier
The time required for the erection of air insulated substation is less compared to indoor
All the equipment in AIS switch yard is within view and therefore the fault location is easier and
related repairing work is also easy
There is practically no danger of the fault which appears at one point being propagated to
another point for the substation installation because the equipment of the adjoining connections
can be spaced liberally without any appreciable increase in the cost
Disadvantages of Air Insulated Substation (AIS):
More space is required for outdoor substation when compared to indoor gas insulated substation
Outdoor switch yards are more vulnerable to faults as it is located in outside atmosphere which
has some influence from pollution, saline environment and other environmental factors.
Deposition of saline particles on insulators can cause insulator failures. They are also vulnerable
to direct lightning strikes and other external events such as heavy winds, rains and cyclones.
Therefore reliability wise air insulated substation or outdoor substations are relatively low
compared to indoor substation
Regular maintenance is required compared to indoor substations (Maintenance for Gas Insulated
Substation is very minimal and reliability is very high) as they are exposed to outside environment
13. SF6 Gas insulated substations (GIS )
• These have been developed during 1970,s and are extremely popular in USA,
Europe, Japan, Middle east Countries etc,
• In 1989 three GIS installations have been executed in India
• They are Compact & requires Very Modest maintenance.
• In GIS various Equipment's like Circuit Breakers, Isolators, CTs, VT, Busbars &
earthing Switches are in the form of metal enclosed- SF6 gas filled Modules.
• The Modules are assembled in accordance with the required Configuration. The
various live parts are enclosed inside the enclosures and are supported on epoxy
post insulators internally.
• The hole installation is filled with SF6 gas at Pressure of about 3Kg/ Cm2
depends up on Manufactures Design.
• Such modules are dispatched in the form of major subassemblies ready to
assembly at Site
• Its only consume the 20% of floor area Conventional AIS
17. Locations where Gas Insulated Substation is preferred
• Gas Insulated Substations are preferred in the places where the land requirement for
the substation is difficult such as in populated areas and highly polluted areas where
outdoor switchyard is not preferred.
• In Gas Insulated Substations all the switching electrical equipment such as circuit
breakers, isolators, earth switches, and busbars are completely enclosed inside
modules which are filled with SF6 gas. These modules are factory made and site
assembled. Hence takes very little time for commissioning compared to air-insulated
• The main advantage of this Gas Insulated Substation is because of its compact size
due to a high dielectric strength of the SF6 gas. And the availability and reliability of
the GIS substations are more compared to air-insulated substations.
• Faults occurring chances of Air insulated substation or conventional substations are
more. Hence GIS substations are provided where the high reliability of electric
power is required such as nuclear plants and other important facilities where un-
interruption of power is more required.
18. Advantages of GIS Substation
It occupies very less space (1/10th) compared to ordinary substations. Hence these Gas
Insulated Substations (GIS) are most preferred where area for substation is small (eg:
Most reliable compared to Air Insulated Substations, number of outages due to the
fault is less
Can be assembled at the shop and modules can be commissioned in the plant easily
Disadvantages of GIS Substation
Cost is higher compared to Ordinary Conventional Substations
Care should be taken that no dust particles enter into the live compartments which
result in flashovers
When fault occurs internally, diagnosis of the fault and rectifying this takes very long
time (outage time is high)
SF6 gas pressure must be monitored in each compartment, reduction in the pressure of
the SF6 gas in any module results in flashovers and faults.
19. Converting Substations –
In such types of substations, AC power converting into DC power or
vice versa or it can convert high frequency to lower frequency or vice
The selection of switching scheme depend upon:
Availability of the space
Economics (project cost)
There can be several combinations in which the equipment's,
bus-bars, structures etc. can be arranged to achieve a particular
20. Importance of Busbar
There are several types of busbar arrangements used in power
systems. There are many factors to consider when selecting a
busbar, such as reliability, flexibility, and cost. In selecting a
particular busbar arrangement, the following factors should be
The busbar layout is simple and easy to maintain.
System maintenance did not affect its continuity.
Busbar installation is economical.
A single busbar is used in the case of small substations, where
continuity of supply is not critical. But in the large substations,
an additional bus bar is used in the system to avoid
interruption in the supply.
21. 1 2 3 n-1 n
- - - -
• Distribution and lower transmission voltage levels
• No operating flexibility
• Fault on the bus trips all circuit breakers
Initial costs are low.
Less maintenance is required.
The operation is simple.
The entire supply is disturbed when the fault occurs.
As the arrangement provides the least flexibility, it is used in small substations where continuity of
supply is not crucial.
Single bus - single breaker
22. ZONE 1
• Distribution and lower transmission voltage levels
• Limited operating flexibility
Multiple bus sections - single breaker with
23. ZONE 1
• Transmission and distribution voltage levels
• Breaker maintenance without circuit removal
• Fault on a bus disconnects only the circuits being connected to
Double bus - single breaker with bus tie
24. ZONE 1
• Increased operating flexibility
• A bus fault requires tripping all breakers
• Transfer bus for breaker maintenance
Main and transfer buses
25. ZONE 1
• Very high operating flexibility
• Transfer bus for breaker maintenance
Double bus – single breaker w/ transfer bus
26. ZONE 1
• High operating flexibility
• Line protection covers bus section between two CTs
• Fault on a bus does not disturb the power to circuits
Double bus - double breaker
27. ZONE 1
• Used on higher voltage levels
• More operating flexibility
• Requires more breakers
• Middle bus sections covered by line or other equipment
28. • Higher voltage levels
• High operating flexibility with minimum breakers
• Separate bus protection not required at line positions