Distribution Systems.pptx

Unit – 5
Distribution Systems
1

Introduction
• The energy is neither be created nor be destroyed
but it can be converted from one form to another.
• The various energy sources
1. Burning coal oil
2. Natural gas
3. Water stored in dams
4. Diesel oil
5. Nuclear power
6. Other non conventional energy sources
2

Introduction
• Depending upon the source of energy used, these
stations are called thermal power station,
hydroelectric power station, diesel power station,
nuclear power station etc.
• The generated electric power is to be supplied to the
consumers.
• Generally the power stations are located too far away
from the town and cities where electrical energy is
demanded.
• Hence there exists a large network of conductors
between the power stations and the consumers.
• The network is broadly classified into two parts
1. Transmission
2. Distribution
3

A Typical Transmission and Distribution
Scheme
• The flow of electrical power from the generating
station to the consumer is called an electrical
power system or electrical supply system.
• It consists of the following important components
1. Generating station
2. Transmission network
3. Distribution network
4

Structure of electrical power system
6

Components of Distribution
• The distribution scheme consists of following
important components.
1. Substation
2. Local distribution scheme
3. Feeders
4. Distributors
5. Service mains
7

Interconnection of feeders,
distributors and service mains
8

Types of transmission and distribution system
• The transmission and distribution systems can be
classified as,
1. A.C. systems
2. D.C. systems
10

A.C. systems
• The a.c. system which is very commonly used for
the transmission of power till substations and
local distribution centre is three phase three wire
system.
• While for the secondary distribution, the
universally adopted a.c. system is three phase
four wire system.
11

• Three phase three wire system
12

• Three phase four wire system
13

• Single phase 2-wire a.c. system with one
conductor earthed
14

• Single phase 2-wire system with mid-point
earthed
15

• Single phase, 3-wire system
16

• Two phase, 4-wire a.c. system
17

• Two-phase, 3-wire system
18

Advantages of A.C. system
• It is possible to build up high voltage levels, using
high speed a.c. generators of large capacity.
• The cost of such a.c. generators is very low.
• The a.c. voltages can be raised or lowered as per
the requirement. The voltage levels of 11 kV , 22
kV etc. are raised upto 220 kV for transmission
purpose. This is not possible in case of d.c.
• The maintenance of a.c. substations is very easy
and cheaper.
19

Disadvantages of A.C. system
• The construction of a.c. transmission line more
complicated than d.c. line.
• The resistance of a.c. line is higher due to skin
effect causing more voltage drop.
• The drop is also due to the inductance of a.c. line
causing loss of power.
• The copper requirement for a.c. line is more than
a d.c. line.
• The a.c. lines are more sensitive to corona than a
d.c. line. 20

D.C. systems:
• Though a.c. is extensively used everywhere, there
are few application like operation of d.c. motors,
batteries, charging where d.c. supply is must.
• It can be obtained by using rectifiers or by d.c.
generators at substations.
• The d.c. systems are further classified as,
1. Two wire d.c. system
2. Two wire with midpoint earthed d.c. system
3. Three wire d.c. system
21

• Two-wire d.c. system
22

• Two-wire d.c. system with mid point earthed
23

• Three wire d.c. system
24

Advantages of D.C. system:
• As the frequency of d.c. is zero, there is no
inductance or capacitance associated with the line.
Hence the power losses and voltage drops are much
less compared to a.c. system.
• Due to the reduced voltage drop, the voltage
regulation is better.
• Absence of skin effect makes use of entire cross-
section of conductor.
• For the same voltage level, the voltage stress on the
insulation is less in d.c. system. Hence the insulation
required in case of d.c. is less compared to a.c.
system
25

• In a three phase a.c. system, three lines are
necessary. As against this only two are sufficient
for a d.c. system. So copper requirement is less.
Disadvantages of D.C. System:
• The power generation is not possible at high d.c.
voltage levels due to communication problem.
• The transformer works on a.c. and not d.c. supply.
So the d.c. voltage levels cannot be stepped up or
lowered as per the requirement.
• Obtaining a.c. from d.c. is not easy in practice.
26

Requirements of a good distribution system
• The continuity in the power supply must be
ensured. Thus system should be reliable.
• The specified consumer voltage must not vary
more than the prescribed limits. (±5%)
• The efficiency of the lines must be as high as
possible.
• The system should be safe from consumer point
of view. There should not be leakage.
• The lines should not overloaded.
• The layout should not affect the appearance of
the site or locality.
• The system should be economical. 27

DC Distribution System
• Though the a.c. transmission and distribution is
used, still for certain applications such as d.c.
motors, electro-chemical work, batteries, electric
traction etc. the dc supply is must.
• Hence along with a.c., d.c. distribution is also
equally important. In a d.c. distribution, d.c.
generators are used in the generating stations or
a.c. is converted to d.c. using the converters like
mercury arc rectifiers, rotary converters etc. at the
substations.
• Then the d.c. supply is distributed to the
consumers as per the requirement.
28

General DC Distribution System
29

Types of DC Distribution System
The various schemes of distribution system are,
1. Radial distribution system
2. Ring main distribution system
3. Ring main distribution system with
interconnector
4. Interconnected system
30

• Radial Distribution System
31

32

Advantages of Radial system:
• Simplest as is fed at only one end.
• The initial cost is low.
• Useful when the generation is at low voltage.
• Preferred when the station is located at the centre
of the load.
33

Disadvantages of Radial system:
• The end of distributor near to the substations gets
heavily loaded.
• When load on the distributor changes, the
consumers at the distant end of the distributor
face serious voltage fluctuations.
• As consumers are dependent on single feeder and
distributor, a fault on any of these two causes
interruption in supply to all the consumers
connected to that distributor.
34

• Ring Main Distribution System:
35

Advantages of Ring Main Distribution System:
• The feeder get equally loaded.
• If fault develops on one of the feeder then
consumer gets continuous supply from the other
part of the feeder.
• It eliminates the possibility of the voltage
fluctuations.
• Easy from the maintenance and repair point of
view without interrupting the supply to the
consumers.
• Great saving in copper required. 36

• Ring Main Distributor with Interconnector:
37

• The points D and G are joined by an interconnector.
• Such a case is generally analysed using Thevenin's theorem.
• Let us briefly revise the steps to use the Thevenin's
theorem.
The steps to use Thevenin's theorem :
1. Remove an interconnector DG.
2. Find the voltage VDG without an interconnector, which is
Thevenin's voltage denoted as Eo.
3. Determine the equivalent resistance as viewed through the
terminals D and G, i.e. where an interconnector is to be
connected. This is Thevenin's equivalent resistance denoted
as RTH.
4. Knowing the resistance of an interconnector DG, the
Thevenin's equivalent can be drawn as shown in the figure.
38

• The current I through an interconnector then can
be obtained as,
• Once this current is known, current in all the
sections and the voltages at load points can be
determined.
39

• Interconnected Distribution System:
40

Advantages of interconnected system
• Reliability of supply increases. In case of fault on
one source, supply can be continued with the
help of other sources.
• Additional load demand in one area can be fed
from other source where load demand is less. This
reduces the reverse power capacity and improves
the efficiency of the distribution system.
41

Types of D.C. Distributors
• The most general method of classifying d.c.
distributors is the way they are fed by the feeders.
On this basis, d.c. distributors are classified as:
(i) Distributor fed at one end
(ii) Distributor fed at both ends
(iii) Distributor fed at the centre
(iv) Ring distributor.
42

• Distributor fed at one end.
• Distributor fed at both ends.
43

• Distributor fed at the centre.
• Ring Distributor.
44

• D.C. Distributor Fed at one End—Concentrated Loading
45

• D.C. Distributor Fed at one End—Concentrated Loading
46

• Distributor Fed at Both Ends — Concentrated Loading
• The two ends of the distributor may be supplied with (i) equal
voltages (ii) unequal voltages.
(i) Two ends fed with equal voltages.
47

(ii) Two ends fed with unequal voltages.
49

• Uniformly Loaded Distributor Fed at One End
50

• Uniformly Loaded Distributor Fed at One End
51

Uniformly Loaded Distributor Fed at Both Ends
• We shall now determine the voltage drop in a
uniformly loaded distributor fed at both ends.
There can be two cases viz. the distributor fed at
both ends with (i) equal voltages (ii) unequal
voltages. The two cases shall be discussed
separately.
52

(i) Distributor fed at both ends with equal voltages.
• Consider a distributor AB of length l metres, having
resistance r ohms per metre run and with uniform
loading of i amperes per metre run as shown in Fig.
• Let the distributor be fed at the feeding points A and
B at equal voltages, say V volts. The total current
supplied to the distributor is i l.
• As the two end voltages are equal, therefore, current
supplied from each feeding point is i l/2 i.e.
• Current supplied from each feeding point =i l /2
53

(ii) Distributor fed at both ends with unequal voltages.
• Consider a distributor AB of length l metres
having resistance r ohms per metre run and with a
uniform loading of i amperes per metre run as
shown in Fig.
• Let the distributor be fed from feeding points A
and B at voltages VA and VB respectively.
• Suppose that the point of minimum potential C is
situated at a distance x metres from the feeding
point A. Then current supplied by the feeding
point A will be i x.
56

Types of Transmission
• In general two types of systems are used for the
transmission,
1. Overhead system
2. Underground system
59

• Overhead System
60

• Underground System
63

S.No. Underground System Overhead System
1. Transmission is by using cables Transmission is by using the transmission
lines
2. All the cables must be properly
insulated from each other
The appropriate spacing provided between
the conductors acts as an insulation. No
external insulation is necessary
3. The insulation cost is very high No transmission cost as air acts as an
insulator
4. Transmission over long distance is not
possible as laying of cables is difficult,
costly and complicated
Transmission over long distance is possible
with the help of transmission lines.
5. The voltage level used is below 66 kV
due to insulation difficulties
The voltage level used can as high as 400 kV
6. The maintenance cost is less The maintenance cost is high
7. The faults due to lightning, short circuit,
storms etc. are eliminated
The occurrence of faults due to lightning,
short circuits, storms and abnormal
weather conditions are possible.
8. It is very safe due to insulation used The conductors are bare without insulation
hence dangerous
64

S.No. Underground System Overhead System
9. Maximum stress is on the insulation
between the conductors
Maximum stress is between conductor and
earth
10. The beauty of the area, towns etc. is
well maintained
The beauty of the area gets affected due
to overhead lines. Sometimes trees are
required to be cut
11. The size of the cables is high The size of the conductors is less
12. The voltage drop is less The voltage drop is more
65

Advantages of High Transmission Voltage
• Reduces volume of conductor material.
• Increases transmission efficiency.
• Decreases percentage line drop.
66

67

68

69

70

Disadvantages of High Voltage
• Corona loss and radio interference
• Line supports
• Erection difficulties
• Insulation needs
• The cost of transformers, switchgear equipments
and protective equipments increases with
increase its cost
• The HV line generates electrostatic effects which
are harmful to human beings and animals
71

EHVAC Transmission
• In recent years the electrical energy is generated
and consumed at a very high rate throughout the
world.
• There are many new trends and developments
which have occurred in the field of transmission
of electric power which leads to use of high
voltages extensively.
• Currently large amount of power is transmitted
over medium and long transmission lines at the
voltage of 300 kV and above.
72

EHVAC Transmission
• As per the terminology, voltages which are less
than 300 kV are termed as high voltages.
• The voltages which are in the range of 300 kV and
765 kV are called Extra High Voltage (EHV) where
as the voltages 765 kV are termed as Ultra High
Voltages (UHV).
73

Necessity of EHVAC Transmission
• With the increase in transmission voltage, for
same amount of power to be transmitted current
in the line decreases which reduces I2R losses (or
copper losses). This will lead to increase in
transmission efficiency.
• With decrease in transmission current, size of
conductor required reduce which decreases the
volume of conductor.
74

• The transmission capacity is proportional to
square of operating voltages. Thus the
transmission capacity of line increases with
increase in voltage. The costs associated with
tower, insulation and different equipments are
proportional to voltages rather than square of
voltages. Thus the overall capital cost of
transmission decreases as voltage increases.
Hence large power can be economically
transmitted with EHV or UHV.
75

• With increase in level of transmission voltage, the
installation cost of the transmission line per km
decreases.
• It is economical with EHV transmission to
interconnect the power systems on a large scale.
• The number of circuits and the land requirement
for transmission decreases with the use of higher
transmission voltages.
• Large amounts of power over long distance is
technically and economically feasible only at
voltages in EHV and UHV range. Thus economics
can be achieved in power generation.
76

Configuration of EHVAC Transmission
• EHV AC transmission line requires minimum two
parallel three phase transmission circuits to
ensure reliability and stability during a fault on
any phase of the three phase lines.
• Similarly EHV line also requires one or more
intermediate substations for installing series
capacitors, shunt reactors, switching and
protection equipment. Generally an intermediate
substation is required at an interval of 250 to
300 km.
77

Configuration of EHVAC Transmission
78

Advantages of EHV Transmission System
• Reduction in the current
• Reduction in the losses
• Reduction in volume of conductor material
required
• Decrease in voltage drop and improvement of
voltage regulation
• Increase in transmission efficiency
• Increased power handling capacity
• The number of circuits and the land requirement
reduces as transmission voltage increases
79

Advantages of EHV Transmission System
• The total line cost per MW per km decreases
considerably with the increase in the voltage
• The operation with EHV AC voltage is simple and
can be adopted easily and naturally to the
synchronously operating a.c. systems.
• The equipments used in EHV AC system are
simple and reliable without need of high
technology.
• The lines can be easily tapped and extended with
simple control of power flow in the network.
80

Disadvantages of EHV Transmission System
• Corona loss and radio interference.
The corona loss is greatly influenced by choice of
transmission voltage. If weather conditions are not proper
then this corona loss further increases. There is also
interference in radio and TV which causes disturbance.
• Line supports
In order to protect the transmission line during storms and
cyclones and to make it wind resistant, extra amount of metal
is required in the tower which may increase the cost
• Erection difficulties
There are lot of problems that arise during the erection of
EHV lines. It requires high standard of workmanship. The
supporting structures are to be efficiently transported
81

Disadvantages of EHV Transmission System
• Insulation needs
With increase in transmission voltage, insulation
required for line conductors also increases which
increase its cost.
• The cost of transformers, switchgear equipments
and protective equipments increases with
increase in transmission line voltage
• The EHV lines generates electrostatic effects
which are harmful to human beings and animals.
82

Standard Rated voltage of EHVAC Lines
Description HV EHVAC UHVAC
Rated voltage (Nominal) in kV
(rms Ph to Ph)
132 220 345 400 500 750 1000 1150
Highest voltage in kV (rms ph to
ph)
145 245 362 420 525 765 1050 1200
In EHVAC lines additional parallel three phase
line is always provided to maintain continuous
flow of power and stability of transmission line.
83

High Voltage Direct Current Transmission (HVDC)
• Principle of HVDC Transmission System Operation
84

Advantages of HVDC Transmission
• These systems are economical for bulk
transmission of power for long distances as the
cost of conductor reduces since d.c. system
requires only two conductors or even one if
ground is used as return. Similarly the cost of
supporting towers and insulation is also reduced.
Also the transmission losses are reduced.
• There are no stability problems with d.c. system.
Hence asynchronous operation of transmission
link is possible.
85

• The line length is not limitation as there is no
charging current in d.c. systems. Cables in d.c.
system does not suffer from high dielectric loss.
The skin effect is also low in d.c. system.
• Greater power transmission per conductor is
possible with d.c. system.
• There are no serious problems of voltage
regulation as there is no reactance drop that exist
in d.c. at steady state.
• The corona loss is low in d.c. systems. The radio
interference with HVDC is less.
86

• The losses are less in transmission with d.c.
• The fault level increases with interconnections of
ac grids through ac lines whereas interconnection
of ac grids through dc links does not increase fault
level to that extent.
• With HVDC link there is easy reversibility and
controllability of power flow.
• Shunt compensation is not required in d.c. lines.
• Intermediate substations are not required with
HVDC transmission.
87

• During fault with HVDC system, the grid control of
the converter reduces the fault current
significantly.
• The transient stability of the power system can be
improved by making parallel connection of HVAC
and HVDC lines.
88

Disadvantages of HVDC Transmission
• The power transmission with HVDC is not
economical if length of transmission is less than
500 km as HVDC system additionally requires
converters, inverters and filters.
• With multiterminal d.c. the circuit breaking is
difficult and expensive.
• Considerable reactive power is required by
converter stations.
• Harmonics are generated with d.c. system hence
filteration is necessary.
• Overload capacity of HVDC converters is low.
89

Disadvantages of HVDC Transmission
• There should be local supply of reactive power if
required as HVDC will not transmit reactive
power.
• The maintenance of insulator in HVDC system is
more.
• There are additional losses in converters
transformers and valves. These losses are
continuous. Hence cooling system must be
effective to dissipate the heat.
90

Types of HVDC System (HVDC Links)
• Depending on the arrangement of pole and earth
return, HVDC systems are classified in different
types. The pole is nothing but the path of direct
current which has same polarity with respect to
earth.
• Monopolar HVDC transmission line
• Bipolar HVDC transmission line
• Homopolar HVDC transmission line
• Back to back HVDC coupling system
• Multiterminal HVDC system
91

• Monopolar HVDC transmission system
92

• Bipolar HVDC transmission line
93

• Homopolar HVDC transmission line
94

• Back to back coupling system
In this system there is no dc transmission line but
the rectification and inversion is done in the same
substation.
95

• Multiterminal HVDC system
• Three or more terminals connected in parallel, some feed
power and some reactive power from HVDC bus.
• Provide interconnection among three or more AC
networks.
96

Standard rated voltages for HVDC system
97

Comparison between HVDC and HVAC Transmission
S.No. HVDC HVAC
1. Economical over long distances as it
requires only two conductors
Cost is more as it requires three
conductors
2. Ground can be used as return conductor
where only one conductor is required. This
is because ground impedance is negligible.
The ground impedance is high which
causes telephonic interference when
high ground current flow. Thus high
ground current is objectionable in
steady state and hence avoided.
3.
The line length is not the limitation due to
the absence of charging current. Hence
power carrying capacity does not depend
on distance of transmission.
The power transfer depends on the
angle between sending end and
receiving end voltages denoted as δ.
This δ depends on distance of line. Thus
power carrying capacity depends on
distance of transmission.
4. No reactance drop hence voltage
regulation is better
Due to reactance voltage drop, voltage
regulation is poor than d.c. transmission
5. Corona loss and the radio interference are
less.
Corona loss and the radio interference
are more and depends on choice of
voltage level. 98

S.No. HVDC HVAC
6. The voltage level cannot be changed using
transformers.
The voltage level can be raised or
lowered using transformers.
7.
The cost of the terminal equipments
converters, inverters and filters is much
more
The converters, inverters and filters are
not required
8.
The fault level of short circuit current is
less if d.c. links are used to interconnect
a.c. lines.
The seriousness of short circuit current
fault level increases with
interconnections of a.c. grids using a.c.
links.
9. It does not require compensation.
To overcome line charging and stability
problems, shunt and series
compensation is required.
10. The maintenance of insulators and other
equipments is more.
The maintenance is low compared to
d.c.
11.
Considerable reactive power is required by
converter stations but line itself does not
require reactive power control.
The line itself requires reactive power
control to keep constant voltage at the
two ends. The reactive power control
increases with line length of the line.
99

S.No. HVDC HVAC
12. Intermediate substations are not required Intermediate substations are required
13. For a given power level cost of conductors
is less and require cheaper towers.
Coat of towers and conductors is high
for a given power level.
14. Insulation required is less. Insulation required is more and
increases with increased voltage level
15. Skin effect is absent hence power loss is
less
Higher power loss due to presence of
skin effect.
100

101

Flexible AC Transmission System (FACTS)
• In the modern power systems, the power in the
transmission lines can be controlled with the use
of power electronics.
• FACTS technology increases load on the line upto
thermal limits without having compromise with
the reliability.
• The line capacity is thus increased which improves
reliability of the system.
• FACTS based controllers gives instantaneous
control of transmission voltage and increases
capacity providing larger flexibility in bulk power
transmission.
102

Flexible AC Transmission System (FACTS)
• Tennessee Valley Authority (TVA) has installed the
first Static Synchronous Compensator (STATCOM)
in the year 1995.
• The concept of FACTS was first defined in 1988 by
N.G. Hingorani.
103

Advantages of Flexible AC Transmission System (FACTS)
• In controls line impedance angle and voltage
which helps in controlling the power flow in
transmission lines.
• The power flow in the transmission lines can be
made optimum.
• It helps in damping out the oscillations and avoids
damage of various equipments.
• It supports the power system security by
increasing the transient stability limit. It also limits
overloads and short circuit current.
• It limits the impact of faults and equipment
failure.
104

Advantages of Flexible AC Transmission System (FACTS)
• The reactive power flow in the lines can be
decreased and the lines are made to carry more
active power.
• There is a increase in utilization of low cost
generation due to cost effective enhancement of
transmission line capacity.
105

Objectives of Flexible AC Transmission System (FACTS)
• The power transfer capability of transmission
systems is to be increased.
• The power flow is to be kept over the designated
routes.
106

Types of FACTS Controllers
• Basic symbol of FACTS controller
107

• Series controller
108

• Shunt controller
109

• Combined series series controller
110

• Combined series-shunt controllers
111

FACTS Devices
• Static Synchronous Compensator (STATCOM)
• Static Synchronous Generator (SSG)
• Static VAR Compensator (SVC)
• Thyristorized switched or controlled reactor
(TSR/TCR)
• Thyristor switched capacitor
• Static VAR Generator or absorber (SVG)
• Static VAR System (SVS)
• Thyristor Controlled Braking Resistor (TCBR)
• Static Synchronous Series Compensator (SSSC)
112

FACTS Devices
• Interline Power Flow Controller (IPFC)
• Unified Power Flow Controller (UPFC)
• Thyristor Controlled Phase Shifting Transformer
(TCPST)
• Interphase Power Controller (IPC)
• Thyristor Controlled Voltage Limiter (TCVL)
• Thyristor Controlled Voltage Regulator (TCVR)
113

Static Synchronous Compensator (STATCOM)
114

Static VAR Compensator (SVC)
115

Thyristor Controlled Series Capacitor (TSCS)
116

Unified Power Flow Controller (UPFC)
117

Substation
• A substation may be defined as an assembly of
apparatus which is used to change the
characteristics of supply system such as voltage,
frequency, a.c. to d.c. or power factor.
118

Substation
• It should be located at a proper site. As far as
possible, it should be located at the centre of
gravity of load.
• It should provide safe and reliable arrangement.
• It should be easily operated and maintained.
• It should involve minimum capital cost.
119

Location of Substation
• The location of substation is governed by the voltage level,
voltage regulation and cost of primary and secondary
distribution system.
• In deciding the location, the following rules should be followed.
 Substations are placed near to the load centre, thereby meeting
the load demand properly with respect to the distance.
 Substations should be located in such a way that properly
voltage regulation can be achieved.
 Number of incoming and outgoing lines can be properly
provided to locate the substation.
 Extra area should be provided for future expansion.
 The substation to be located to satisfy the consumer by
providing continuity of supply.
120

Major components of Substation
• Transformers
• Circuit breakers
• Isolators
• Load break switch
• Instrument transformers
• Bus bars
• Protective relays
• Lightning arresters or surge arresters
• Earthing switch
• Shunt capacitors
• Earthing
• Station battery and charging equipment
121

Classification of Substation
• Service
1. Static(A.C)
2. Converting (A.C. to D.C.)
• Function
1. Extra high voltage transmission
2. Distribution
3. Industrial
4. Power Factor correction
5. Frequency changer
6. Direct current for light and power
122

• Type of apparatus
1. Transformer
2. Rotary converter
3. Rectifier
4. Motor generator
5. Frequency changer
• Control
1. Manual
2. Automatic
3. supervisory
123

• Mounting
1. Indoor (upto 66 kV)
2. Outdoor (beyond 66 kV)
3. Under ground
4. Pole mounted (11 kV to 33 kV)
124

Pole-Mounted Sub-Station (200 kVA)
125

S.No. Point Indoor Outdoor substation
1. Fault location Difficult Easy as all equipments are
within view
2. Time required for erection More Less
3. Future expansion Difficult Easy
4. Amount building material
required
Large Small
5. Capital cost and cost of
switchgear installation
More Less
6. Construction work required
to be done
More Less
7. Operation Easy Difficult
8. Performance of all the
operations, maintenance and
supervision
In closed atmosphere In open air in all kinds of
weather
9. Space required Less More
128

Air Insulated Switchgear Substation (AIS)
• In this type of switchgear substation, atmospheric
air is used as the phase to ground insulation for
the switchgear of an electrical substation. The
installation of AIS are generally outdoor.
• These type of substation are popular in 400 kV
substations.
• All equipments in AIS are exposed to weather
conditions. The bus bars and equipment
terminations normally open to air. The insulation
properties of ambient air are used for insulation
to ground.
129

Advantages:
• This arrangement is best for low voltage rating
substations and where plenty of space is available for
installation.
• The expansion for future is easier.
• Time required for erection is less.
• The propagation of fault from one point to another is
less likely as the equipments are spaced away from
each other sufficiently.
• Location of fault is easier and all the equipments in
AIS switchyard are within view.
• The construction work to be carried out is
comparatively less. 130

Limitations:
• Space requirement for erection is large
• As the equipments are exposed to atmosphere,
outdoor switch yards are more vulnerable to
faults.
• Lesser reliability as it is exposed to lightning
strokes and other external condition such as
winds, rains and cyclones.
• Periodic maintenance is required due to exposure
of this type of installation to outside
environment.
131

Gas Insulated Switchgear Substation (GIS)
• GIS make use of SF6 (Sulfur hexafluoride) gas.
• SF6 gas has excellent dielectric properties when
used at moderate pressure for phase to phase
and phase to ground insulation.
• The high voltage conductors, circuit breaker
interrupters, current transformers, voltage
transformers, switches and lightning arresters are
enclosed in SF6 gas inside grounded metal
enclosures.
• SF6 gas has higher dielectric strength than air.
132

• The overall size of each equipment and complete
substation is significantly reduced to about 10%
in comparison with conventional AIS substations.
• GIS substations can be a preferable choice in
large cities and towns, mountains and valley
regions, underground substations and power
stations and in highly polluted and saline
environment.
133

Advantages:
• These are safe since the metal enclosures are
earthed, the operating personal are protected.
They are reliable.
• It takes lesser space as compared with
conventional AIS substation
• It has long service life with less maintenance
requirement.
• It has lower weight due to aluminium enclosures.
134

Disadvantages:
• It is costlier as compared to AIS installation
• Supply of SF6 gas at the site and its procurement
is sometimes poses problems.
• It requires separate building due to indoor nature
of these substations.
• Dust or moisture inside the compartments may
cause flash overs.
• The outages period under the event of fault is
more with severe damage.
135

AIS GIS
Outdoor substation Indoor substation
During maintenance, disconnect
contacts must be cleaned
regularly
Switchgear an important role for
maintenance
Land area required is large Land area required is less
compared to that of AIS
Cost is less compared to that of
GIS
Cost is high compared to that of
AIS
Frequent maintenance should be
done
Long term maintenance should
be done
Considerable dismantling may be
required if a main element fails
Manufacturer supervision will be
required for the 20 year full
overhaul
136

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