1. GAS INSULATED
SUBSTATION

2. CONTENTS
 Introduction
 Conventional substation
 Limitation of AIS
 Need for GIS
 Introduction to GIS
 Components of GIS substation
 Advantages of GIS over conventional
open air substation
 Disadvantages of GIS
 Calculation and suppression of VFTO

3. INTRODUCTION
 For maintaining proper voltage levels at
transmission and distribution level and for
providing greater stability a number of
electrical transformation and switching
setups have to be created in between
generating stations and consumer ends.
These transformation and switching setups
are generally known as Electrical
Substations.
 Based on nature of dielectric medium a
substation is of two types as following :
 1.AIS(Air insulation substation)
 2.GIS(Gas insulation substation)

4. AIS substation

5. LIMITATION OF AIS
 Physical factor
 Infrastructure factor
 Social & Environmental factor
 Commercial factor

6. NEED OF GIS
 Non availability of sufficient space. It is very much
required to establish a substation at load center.
Establishing a substation load center is quite
economical and profitable in following ways:
A. Reduction in length of feeders
B. Improvement of quality of voltage regulation due
to short length feeders.
 Difficult climatic and seismic conditions at site, high
altitude and atmospheric pollution.
 Aesthetically “superior” to air insulated substations.
 The higher the voltage, the more favorable gas
insulated technology becomes.
 Overcomes or decreases the magnitude of limitations
of AIS site selection

7. INTRODUCTION OF GIS
 GIS is a compact multi component assembly
enclosed in a ground metallic housing in which
the primary insulating medium is compressed
sulphurhexaflouride(SF6) gas.
 The increase in demand for electricity and
growing energy density in metropolitan cities
have made it necessary to extend the existing
high voltage networks right up to the
consumers. Stepping down the voltage from
transmission to distribution level at the
substation located near the actual consumers
not only produces economic advantages but
also ensures reliable power supply.

8. Gas Insulated Substation

9. Continuation…
 Life of GIS is affected by several factors :
 A. Conductive particles
 B. Particle discharges
 C. Contamination
 The GIS require less number of lightning
arresters than a conventional one. This
is mainly because of compactness.
 Gas insulated substations are in service
up to the highest voltage of 800 kV,
meeting almost all the requirements in
urban, industrial as well as rural areas.

10. Essential Parts of GIS
1. Bus bar
2. Circuit Breaker
3. Disconnector
4. Earthing switch
5. Current transformer
6. Voltage transformer
7. Feeder Disconnector
8. Feeder Earthing switch
9. Lightnining /Surge Arrester
10. Cable termination
11. Control Panel

12. ADVANTAGES OF GIS OVER
CONVENTIONAL OPEN AIR
SUBSTATION

 Area and volume saving in construction for
over or underground applications.
 Greatly improved safety and reliability due to
earthed metal housing of all high voltage
parts and much higher intrinsic strength of
SF6 gas as insulation.
 High service reliabilty.
 Reduction in radio interference with the use
of earthed metal enclosures.
 More optimal life cycle costs.
 Not necessary that high voltage switchgear
has to be installed outdoors.

13. DISADVANTAGE OF GIS
 Switching operation generate Very
Fast Transients Over
Voltages(VFTOS).
 VFTOS may cause secondary
breakdown inside a GIS and Transient
Enclosure Voltages(TEV) outside the
GIS.

14. Problems with VFTOS
 Flash over to ground at the disconnecter
switch contacts.
 Failure of electronic control circuits connected
to GIS due to electro magnetic interference of
VFTOS.
 Dielectric strength is reduced under VFTOS if
non uniform electric field is formed by the
particles.
 Effect of equipment such as bushing, power
transformer and instrument transformer.
 Transient Enclosure Voltage(TEV) on external
surface of the sheath, this may cause
flashovers to nearby grounded objects.

15. Calculation & Suppression of
VFTOS
 For these we have to consider the variations of
VFTOS magnitudes at different points in 500kV and
750kV GIS during different points have been
calculated and compared by using
MATLAB/SIMULINK.
 Very fast transients belong to highest frequency range
of transients in power systems.
 The transients are originated within a GIS at any time
when there is an instantaneous change in voltage.
This generally occurs as the result of opening or
closing of a disconnecter switch, but other events
such as the operation of circuit breaker, the closing
of ground switch can also cause VFT.