TYPES AND FUNCTIONS OF SUB-STATION
Types of Sub-station
Substations are of three
types. They are:
>Transmission
Substation
>Distribution
Substation
>Collector Substation
Transmission
Substation
A
transmission substation connects two or more transmission lines. The simplest case
is where all transmission lines have the same voltage. In such cases, the
substation contains high-voltage switches that allow lines to be connected or
isolated for fault clearance or maintenance. A transmission station may have
transformers to convert the voltage from voltage level to other, voltage
control devices such as capacitors, reactors or Static VAR Compensators and
equipment such as phase shifting transformers to control power flow between two
adjacent power systems. The largest transmission substations can cover a large
area (several acres/hectares) with multiple voltage levels, many circuit breakers
and a large amount of protection and control equipment (voltage and current Transformers, relays and SCADA
systems). Modern substations may be implemented using International Standards
such as IEC61850.
Distribution
Substation
A
distribution substation transfers power from the transmission system to the distribution
system of an area. It is uneconomical to directly connect electricity
consumers to the
high-voltage main transmission network, unless they use large amounts of power.
So the distribution station reduces voltage to a value suitable for local distribution.
The input for a distribution substation is typically at least two transmission or
sub transmission lines. Input voltage may be, for example, 400KV or whatever is
common in the area. Distribution voltages are typically medium voltage, between
33 and 66 kV depending on the size of the area served and the practices of the
local utility. Besides changing the voltage, the job of the distribution
substation is to isolate faults in either the transmission or distribution
systems. Distribution substations may also be the points of voltage regulation,
although on long distribution circuits (several km/miles), voltage regulation
equipment may also be installed along the line.
Complicated
distribution substations can be found in the downtown areas of large cities,
with high-voltage switching and, switching and backup systems on the
low-voltage side. Most of the typical distribution substations have a switch,
one transformer, and minimal facilities on the low-voltage side.
Collector substation
In
distributed generation projects such as a wind farm, a collector substation may
be required. It somewhat resembles a distribution substation although power
flow is in the opposite direction. Usually for economy of construction the
collector system operates around 35 KV, and the collector substation steps up
voltage to a transmission voltage for the grid. The collector substation also
provides power factor correction, metering and control of the wind farm.
Functions of the
substation
a. To Change voltage from
one level to another.
b. To Regulate voltage to
compensate for system voltage changes.
c. To Switch transmission
and distribution circuits into and out of the grid system.
d. To Measure electric power
quantity flowing in the circuits.
e. To Connect communication
signals to the circuits.
f. To Eliminate lightning
and other electrical surges from the system.
g. To Connect electric
generation plants to the system.
h. To Make interconnections
between the electric systems of more than one utility.
Substation
Transformer Type
Further, transmission
substations are mainly classified into two types depending on changes made to
the voltage level. They are:
a. Step-Up Transmission
Substations.
b. Step-Down Transmission
Substations.
a. Step-Up Transmission
Substation
A step-up
transmission substation receives electric power from a nearby generating
facility and uses a large power transformer to increase the voltage for
transmission to distant locations.
There can
also be a tap on the incoming power feed from the generation plant to provide electric
power to operate equipment in the generation plant.
b. Step-Down Transmission
Substation
Step-down
transmission substations are located at switching points in an electrical grid.
They connect different parts of a grid and are a source for sub transmission
lines or distribution lines.
Layout
a. Principle of Substation
Layouts
Substation
layout consists essentially in arranging a number of switchgear components in
an ordered pattern governed by their function and rules of spatial separation.
b. Special Separation
i. Earth Clearance: This is
the clearance between live parts and earthed structures, walls, screens and
ground.
ii. Phase Clearance: This is
the clearance between live parts of different phases.
iii. Isolating Distance:
This is the clearance between the terminals of an isolator and the connections.
iv. Section Clearance: This
is the clearance between live parts and the terminals of a work section. The
limits of this work section, or maintenance zone, may be the ground or a
platform from which the man works.
c.
Separation of maintenance zones
Two methods are available for separating equipment in a
maintenance zone that has been isolated and made dead.
i.
The provision of a section clearance
ii.
Use of an intervening earthed barrier The choice between the two methods depends on the voltage and
whether horizontal or vertical clearances are involved.
i. A
section clearance is composed of the reach of a man taken as 8 feet plus an
Earth clearance.
ii.
For the voltage at which the earth clearance is 8 feet the space required will
be the same whether a section clearance or an earthed barrier is used.
Maintenances
Maintenance
plays a major role in increasing the efficiency and decreasing the breakdown.
The rules and basic principle are discussed.
Separation by earthed
barrier = Earth Clearance + 50mm for barrier + Earth Clearance Separation by
section clearance = 2.44m + Earth clearance
i. For vertical clearances
it is necessary to take into account the space occupied by the equipment and
the need for an access platform at higher voltages.
ii. The height of the
platform is taken as 1.37m below the highest point of work.
Maintenance is done
through two ways:
a. By Establishing
Maintenance Zones.
b. By Electrical
Separations.
a. Establishing Maintenance
Zones
Some
maintenance zones are easily defined and the need for them is self evident as
in the case of a circuit breaker. There should be a means of isolation on each
side of the circuit breaker, and to separate it from adjacent live parts when
isolated either by section clearances or earth barriers.
b. Electrical Separations
Together
with maintenance zoning, the separation, by isolating distance and phase clearances,
of the substation components and of the conductors interconnecting them constitute
the main basis of substation layouts. There are at least three such electrical separations
per phase that are needed in a circuit:
i. Between the terminals of
the bus bar isolator and their connections.
ii. Between the terminals of
the circuit breaker and their connections.
iii. Between the terminals
of the feeder isolator and their connections.
