Introduction to substation (AIS)
Introduction to GIS
Limitations of AIS
The need for GIS
Properties of SF6 Gas
Future trends in GIS
A substation is a part of an electrical generation, transmission,
and distribution system. Substations transform voltage from high to low,
or the reverse, or perform any other important functions. Between the
generating station and consumer, electric power may flow through several
substations at different voltage levels.
Air insulated substation (AIS) :
The AIS uses air as the primary dielectric for phase to phase, and phase to
Actually, most substations across all regions are AIS. For e.g. In rural area
where space, weather conditions, seismic occurrences, and environmental
concerns are not an issue, mostly AIS are being installed.
The indoor AIS version is only used in highly polluted areas, and saline
conditions, as the air quality is compromised.
4. An assembly of apparatus installed to control
transmission and distribution of electric power.
A:Primary power lines' side
B:Secondary power lines' side
1.Primary power lines
4.Transformer for measurement
of electric voltage
12.Secondary power lines
5. Introduction to GIS
A gas insulated substation (GIS) is a high voltage substation in which
the major structures are contained in a sealed environment with sulfur
hexafluoride gas as the insulating medium. GIS is now in widespread
use in the electrical power industry .
Basic Principle :
1. The high-voltage current- carrying parts are within a metal enclosure
and are held in a concentric configuration by cast epoxy spacer
insulators and faraday cage.
2. The space between the conductor and the enclosure is filled with
sulfur hexafluoride gas under moderate pressure.
6. Limitations of AIS
More space is required for outdoor substation when compared to
indoor gas insulated substation (GIS)
Due to poor dielectric strength of air it requires more amount of air
for insulation which leads to more space requirement.
As AIS are exposed to outside environment, so AIS is very sensitive to
pollution, salt, sand , large amounts of snow etc. and easily got
affected. Deposition of saline particles on insulators can cause
They are also easily affected by direct lightning strikes and other
external events such as heavy winds, rains and cyclones.
Regular maintenance of substation is required, but still reliability is
Life of steel structures is reduced.
7. The need for GIS
Non availability of sufficient space. It is very much required to establish a substation at
load center. Establishing a substation at load center is quite economical and profitable
in following ways :
Reduction in length of feeders
Improvement of the quality of voltage regulation due to short length feeders
Flood areas, wetlands, challenging soil conditions.
Difficult climatic and seismic conditions at site like high altitude and atmospheric
Special air conditions (salty, dusty, polluted air etc.)
The footprint of 765kV conventional substation is enormous, but GIS allows a significant
reduction in size
8. Properties of SF6 Gas
SF6 is used in GIS at pressures from 400 to 600 kPa absolute. The
pressure is chosen so that the SF6 will not condense into a liquid at the
lowest temperatures the equipment experiences.
Insulating properties 2.5 times more than air.
SF6 is about 100 times better than air for interrupting arcs.
Non-toxic, Colorless, heavier than air & chemically very stable.
Lifetime – Very long (800 to 3200 years!).
Almost water insoluble.
10. GIS ASSEMBLY
Main components of GIS are following :
1. Electrical bus bars
An electrical conductor that carries a large current, especially one that is part of
a power distribution system: typically a thick strip, or a tube of copper or
2. Electrical isolators or disconnectors
An isolator is a manually operated mechanical switch which separates a part of
the electrical power.
Its main purpose is to isolate one portion of the circuit from the other and is not
intended to be opened while current is flowing in the line.
11. 3. Circuit breakers
An electrical circuit breaker is a switching device which can be operated
manually and automatically for controlling and protecting an electrical
Under short circuit conditions, however current can reach tens of thousand
of amperes at a power factor as low as 0.1. It is the duty of circuit
disconnectors to interrupt such high current as soon as possible to avoid
SF6 Circuit Breaker : Due to its low gaseous viscosity, SF6 gas can
efficiently transfer heat by convection. So due to high dielectric strength
and high cooling effect, SF6 gas is approximately 100 times more effective
arc quenching media than air.
4. Current transformers
A CT is an instrument transformer in which the secondary current is
substantially proportional to primary current and differs in phase from it by
ideally zero degree.
CT are used for installation and maintenance of the current relays in
substation for protection.
12. 5. Voltage transformers
A VT is a sensor device which is used to measure high voltage in transmission.
It’s main purpose is to reduce the voltage to the lower value and provide the
isolation b/w high voltage power network and the relays.
6. Earth switches
slow operating switch are used for protection purpose.
The fast closing earthing switch can close against full voltage and short circuit
The high-speed earthing switch is achieved by means of a spring-closing
7. Surge arrestors or lightning arresters
Electrical surge are generated mainly due to
lightning impulses and switching impulses.
Electrical surge produces a large transient over
voltage in electrical network and system.
13. shape of the transient over voltage
lightning arrester or surge arrester are used for protecting electrical
equipment from these surge.
Due to non linear resistance characteristics of ZnO, stack of ZnO disc are used for
14. 8. Local Control Cubicle (LCC)
LCC is the interface cubicles to all secondary system of a substation
which represent a station control and protection.
LCC includes control and alarm functions as well as the correct
distribution of auxiliary power supply for the relevant GIS .
In general, each cabinet should contain the following equipment for
control, indication and protection of switches, circuit breaker, and
1. One control switch for each three-phase circuit breaker.
2. One remote local switch for each three-phase circuit breaker.
3. One open close control switch for each motor operated grounding
4. One or two red light-emitting diodes and one green for each circuit
breaker, each disconnecting and grounding switch.
5. Control switches for ac and dc supply to each compartment.
The main benefit of GIS over AIS is that SF6 has dielectric constant 2.5
(approximately) times higher than that of air, which leads to a significant
Space requirement is roughly 10% to that of AIS. So in urban areas where
land cost is high GIS is more preferable.
GIS technology is popular in desert and arctic areas because it can be
enclosed in a building with environmental control, leading to less exposure
The average installation time is reduced by approximately 30% with a GIS
installation, due to the fact smaller size and less weight.
GIS requires less maintenance and are also more reliable . On an average it
requires only visual inspection every after four or more years.
For higher voltage transmission GIS is more preferred.
16. Future Trends in GIS
Use of vacuum circuit breaker cells in the medium high voltage GIS.
Optimization of GIS design to allow easier maintenance.
Development of DC GIS for incorporating into expanding national/international
Search for replacement gases for SF6.
The most promising - an 80%/20% N2/SF6 mixture.
Replacement of existing AIS by GIS will accelerate especially near urban
Excessive damage happens in case of internal fault.
Diagnosis of internal fault and rectifying takes very long time (high outage
SF6 gas pressure must be monitored in each compartment.
Maintaining Cleanliness is very important. Dust or moisture inside the
compartment causes the flash overs
Reduction in the pressure of the SF6 gas in any module results in flash over and
SF6 causes ozone depletion and global warming. SF6 is a gas specifically
mentioned in Kyoto protocol. Search is on for a replacement gas or gas
GIS – necessary for Extra HV & Ultra HV electrical power
Some important areas to be studied include:
More conservative design.
Improved gas handling.
Decomposition product management techniques.
Achieving & maintaining high levels of availability require – more integrated
approach to quality control by both users and manufactures.