SUBSTATIONS
A.Nagaraja Sekhar
Functions of a Substation
1 - Supply of required electrical power.
2 - Maximum possible coverage of the supply network.
network
3 - Maximum security of supply.
4 - Shortest possible fault-duration.
5 - Optimum efficiency of plants and the network.
6 - Supply of electrical power within targeted frequency limits, (49.5
Hz and 50.5 Hz).
)
7 - Supply of electrical power within specified voltage limits.
8 - Supply of electrical energy to the consumers at the lowest cost.
Classification Of substations:The substations may be classifi
in numerous ways
y such as on the basis of
(i)
(ii)
(iii)
(iv)
( )
(v)
Nature of duties
Service rendered
Operating voltage
Importance and
Design
i
The substations, on the basis of nature-of-duties, may be classified
i
into
the
h ffollowing
ll i three
h
categories:
i
Step-up
p p or p
primary
y substations.
Such substations are usually associated with generating stations.
The generated voltage, which is usually low (3.3, 6.6, 11 or 33 kV), is
stepped up to primary transmission voltage so that huge blocks of
power can be
b transmitted
t
itt d over long
l
distances
di t
to
t the
th lload
d centres
t
economically.
Primary grid substations.
Such substations are located at suitable load centres along the
primary transmission lines.
In these substations, the primary transmission voltage is stepped
down to different suitable secondary voltages.
The secondary transmission lines are carried over to the secondary
substations situated at the load centres where the voltage is further
stepped down to sub-transmission or primary distribution voltages.
1.
Step-down or distribution substations.
Such substations are located at the load centres, where the subtransmission/primary distribution voltage is stepped down to
secondary
d
di
distribution
t ib ti voltage
lt
(400/230V).
(400/230V) th
these are th
the
substations which feed the consumers through distribution
network and service lines.
2 Classification of Substations on the basis of service rendered
rendered. The
substations, according to service rendered are:
1 T
1.
Transformer
f
substations.
b t ti
T
Transformers
f
are installed
i t ll d on such
h
substations to transform the power from one voltage level to
another level as per needs.
2. Switching substations. Such substations are meant for switching
operation of power lines without transforming the voltage. At
each substation different connections are made between various
transmission lines.
3. Converting substations. Such substations are meant for either
converting ac to dc or vice-versa or converting frequency from
higher to lower or vice-versa.
Classification of Substations on the basis of operating
p
g voltage:
g
The substations, according to operating voltage, may be categorized
as
1. High voltage substations (HV Substations): involving voltages
between 11 KV and 66 kV
2. Extra High Voltage Substations (EHV Substations). Involving
voltages
g between 132 kV and 400 kV.
3. Ultra High Voltage Substations (UHV Substations). Operating on
Voltage above 400 kV
kV.
Classification of Substations on the basis of importance.
1. Grid Substations. These are the substations from where bulk
power is transmitted from one p
p
point to another p
point in the ggrid.
These are important because any disturbance in these
substations may cause the failure of the grid.
2. Town Substations. These substations step-down the voltages at
33/11 kV for further distribution in the towns and any failure in
such
h substations
b t ti
results
lt iin the
th ffailure
il
off supply
l for
f whole
h l off the
th
town.
Classification of Substations on the basis of Design.
1 Indoor Type Substations
1.
Substations.
In such Substations the apparatus is installed within the
substation building. Such substations are usually for a voltage
upto 11kV but can be erected for the 33 kV and 66 kV when the
surrounding atmosphere is contaminated with impurities such as
metal corrodingg ggases and fumes,, conductive dust etc.
2. Outdoor Substations. These substations are further subdivided
into:
(a) Pole mounted substations.
substations Such substations are erected for
distribution of power in localities. Single stout pole or H-pole and
4 pole structures with suitable platforms are employed for
transformers
f
off capacity
i up to 25 kVA,
kVA 125 kVA and
d above
b
125
kVA (but upto 250 kVA) respectively.
((b)) Foundation mounted substations: for transformers of capacity
p
y
above 250 kVA the transformers are too heavy for pole mounting.
Such substations are usually for voltages of 33,000 volts and
above
The outdoor substations have the followingg main advantages
g over
indoor substations
(i) All the equipment is within view and therefore the fault locat
is easier.
easier
(ii) The extension of the installation is easier, if required
(iii) The time required in erection of such substations is lesser.
(iv)
i
The smaller amount of building
i i materials
i (steel-concrete) is
i
required.
((v)) The construction work required
q
is comparatively
p
y smaller a
cost of the switchgear installation is low.
(vi) There is practically no danger of a fault which appears at o
point being carried over to another point in the installation beca
the apparatus of the adjoining connectios can be spaced libera
without any appreciable increase in costs.
( ii) Repairing
(vii)
R
i i work
k is
i easy.
The disadvantages of outdoor installations in comparison of indoor
installations are:
(i) The various switching operations with isolators as well as
supervision and maintenance of the apparatus is to be performed in
the open air during all kinds of weather.
(ii) More space is required for the substation.
(iii) Protection devices are required to be installed for protection
against lighting surges.
(iv) The length of control cables required is more.
(v) The influence of rapid fluctuation in ambient temperature and
dust and dirt deposits upon the outdoor substation equipment makes
it necessary to install apparatus specially designed for outdoor
service and, therefore, more costly.
Notwithstanding the disadvantages, outdoor substations are very
widely used in power systems.
systems
SELECTION AND LOCATION OF SITE FOR A SUBSTATION
T The following factors are considered while making site selection
for a substation
1. Type of substation. The category of substation is important for
its location.
For example a step-up substation, which is generally a point
where power from various sources (generating machines or
ggenerating
g stations)) is p
pooled and stepped
pp up
p for long
g distance
transmission, should be located close to the generating stations as
possible to minimize the transmission losses, cost of distribution
system and better reliability of supply
supply.
2. Availability of suitable and sufficient land.
The land proposed for s substation should be normally level and
open from all sides. It should not be water logged particularly in
rainy season. The site selected for s substation should be such that
According to the latest practice the land required for various types of
substations are given below.
Type of substation
Area required
(a) 400 kV substation
50 acres
(b) 220 kV substation
25 acres
((c)) 132 kV Substation
10 acres
The places nearer to the aerodrome, shooting practice grounds etc.,
should be avoided.
3.
Communication facility. Suitable communication facility is
proposed
p
substation both during
g and after its
desirable at a p
construction. It is better, therefore, to select the site along side on
existing road to facilitate an easier and cheaper transportation.
4. Availability of essential amenities to the staff. The site
should be such where staff can be provided essential amenities
like school, hospital, drinking water, housing etc.
5. Drainage facility. The site selected for the proposed
substation
b t ti should
h ld h
have proper d
drainage
i
arrangementt or
possibility of making effective drainage arrangement to avoid
pollution of air and growth of micro-organisms detrimental to
equipment and health.
Power transformers are connected between two voltage levels.
The main connection scheme is drawn keeping in view the following factors:
(i) General bus-bar arrangement
(ii) Operating voltage
(iii) Number
N b off iincoming
i and
d outgoing
i li
lines
(iv) Number of transformers
(v) Safety of equipment
(vi) Safety to operating personnel
(vii) Future extension requirement.
The main connection diagram drawn for substation shows the arrangement of
all the circuits with the main bus-bars.
bus bars.
For simplicity and to facilitate reading, all the electrical connections of a
substation can be represented by a single line diagram.
A single line diagram will indicate all the main elements of any given
installation such as generators,
generators bus
bus-bars,
bars power transformers,
transformers
circuit breakers, isolators, series and shunt capacitors, fuses, CTs
and PTs, diode or thyristor rectifiers, static VAR sources, harmonic
filters, surge arresters etc.
Connections may be divided as incoming (power feed connections)
tie (lines interconnecting two substations or switchgear installations,
each of which is fed through its own incoming feeder connections),
outgoing (feeder connections for feeding other subsequent
substations or switchgear installations), power transformers
(
(connections
ti
made
d in
i given
i
substation),
b t ti ) voltage
lt
transformers
t
f
(connected for control and metering).
The components in series with the main circuit of power flow are:
Bus-bars, power transformers, circuit breakers, isolators, fuses, CTs,
series capacitors, series reactors, diode or thyristor rectifiers etc.
The components in the shunt circuits connected between phase and
ground
d are:
Shunt capacitors, shunt reactors, static VAR sources, harmonic
filters, pTs, lightning or surge arresters etc.
Special type of apparatus such as isolators (or disconnecting
switches), circuit breakers, instrument transformers etc. are used for
interconnecting high voltage power lines (overhead or cable), with
the main bus
bus-bars
bars in the substations.
substations
Circuit breaker is connected between the bus-bar and each incoming
and
d outgoing
i circuit.
i i
Isolator is provided on each side of the circuit breaker.
CTs are p
provided for measurement and p
protection.
PTs are generally connected to bus-bars and on incoming line side.
side
Lightning or surge arrester are connected phase to ground at the
i
incoming
i line
li as the
th first
fi t apparatus
t and
d also
l att the
th terminal
t
i l off
transformer, terminal of the capacitor bank, terminal of shunt
generator, terminal of large
g motor to divert
reactor, terminal of g
switching/lightning surges to ground.
Main Electrical Connections
The electrical power in substations and switchgear installations is
received and distributed by means of the main bus
bus-bars
bars to which the
equipment is connected according to some given main circuit
scheme.
It is necessary to distinguish two fundamental types of power station
and substation circuit arrangements.
((1)) The main connections of p
primary
y ((or p
power)) circuits.
The secondary (or control) circuit arrangements.
Primary circuit main connections are represented by either single
line or three line schematic or elementary diagrams.
The first type of diagram shows all three-phases
three phases of each circuit as a
single line while the second type of diagram shows each phase of
every circuit as a separate line, i.e., shows each three-phase circuit as
three lines.
lines
The single line diagram represents the electrical circuit arrangement
of any electrical power installation in its basic form.