Maintenance, erection and Operation of
An Electrical substations
Importance:
The EHT Electrical
sub-Stations are most important to transmit the power by transmission line to
the load center without more deviation of the quality of the Power supply i.e
to avoid voltage drop, Power loss due to high currents.
Aim:
Transmission of the
power supply from generating stations to Load centers for long distances also without
much voltage drop and power loss, with optimum quality to the consumer.
Requirement:
The LT voltage useful
at consumer end is 3Ø 415 to 440V or 1Ø
220 to 250V, But the generating stations are generating the power with 3h. 11KV
which is to be transmitted to the far away to load centers. Due to high load
current of the 11KV/440V Sub-Stations directly under the generating stations, the
I2R i.e heat loss is more and voltage drop is very high. The
consumer end voltage may drop very high level order of 40%. The devices of the
consumer may not be operative. Hence the 11KV Power generated up to 400KV, to
be transmitted and step down to deferent voltage levels [e.g 220KV à132KVà33KVà11KVà415 to440V] as per
the distances and variation of the load or demand at various areas. Hence it is
required to construct the electrical sub-stations at various voltage levels to
transmit the power by lines.
Electrical
sub-station: the electrical sub-station mainly consists of Transformers for
step up at generating station and step down the voltage at load centers and in
between. The transformers feeds the number of feeders with secondary voltage
with the LV current=Ratio XHV current. Here Mainly Transformers step down the
voltage without much change its input power to output power and frequency with
small transformer losses i.e about 0.3% of copper and Iron or no load loss out
of its input power.
**[Important note: The DC voltage transmission is not
possible by any transformer as it does not works with zero frequency. The
transformer works with the principle of Faraday's loss of electromagnetic induction
i.e rate of change of electromagnetic flux which not possible in DC i.e without
frequency.]
Brief explanation of EHT Sub-stations
[220/132KV and 132/33K Sub-Stations
EHV (Extra High Voltage) Sub-Station
forms an important link between Transmission network and Distribution network.
It has a vital influence of reliability of service. Apart from ensuring
efficient transmission and Distribution of power, the sub-station configuration
should be such that it enables easy maintenance of equipment and minimum
interruptions in power supply. Flexibility for future expansion in terms of
number of circuits and transformer MVA Capacity also needs to be considered
while choosing the actual configuration of the substation.EHV Substation
is constructed as near as possible to the load centre. The voltage level
of power transmission is decided on the quantity of power to be transmitted
to the load centre. Generally, the relation between EHV Voltage level and the
power to be transmitted is as follows:
1)
Up to 80MVA to 132KV/33KV.2) From
100MVA to 300MVA 220KV/132KV.) 300 MVA to 1000 MVA 400 KV/220KV.
*SIGNIFICANT
NOTES TO BE CONSIDERED DURING LAYING
OUT OF THE EHT SUBSTATION*
Substations are
important part of power system. The continuity of supply depends to a
considerable extent upon the successful operation of sub-stations. It is, therefore,
essential to exercise utmost care while designing and building a substation.
The following are the important points which must be kept in view while laying
out a sub-station
It should be located at a good
extendable site for future expansions as the demand increases day by day. As
far as possible, it should be located at the centre of gravity of load.
It should provide
reliable and safety arrangement likes firefighting equipment, first aid
arrangements, hospitality. For safety, consideration must be given to
the maintenance of proper clearances between HT element to ground and other equipment
with highly solid earthing of the body of the EHT equipment. The Facilities for
carrying out repairs and maintenance, abnormal occurrences such as
possibility of explosion or fire etc. for reliability must be given for good
design and construction, and the provision of suitable protective switch gear
etc.
·
It should be easily operated and
maintained.
·
It should involve low capital cost.
·
It should be out of polluted area as
the metal parts like supporting CPL and tower structures rust, dielectric level
of the insulators may decrease.
·
The site should be selected for future
expansion as every year the load demand increases by 15% of the previous year
demand.
·
The transportation facility like road,
rail should consider as heavy electrical equipment like power transformers and
breakers to be transported to the site and repair centers during the
construction and replacement of the failed equipment.
·
The site should suitable for using
optimum floor area.
Schematic Diagram of 132KV/33KV Sub-Station
Schematic Diagram of 132KV/33KV Sub-Station
conclusion
Transmission and
distribution stations exist at various scales throughout
a power system. In general, they represent an interface between different levels or
sections of the power system, with the capability to switch or reconfigure the connections
among various transmission and distribution lines. The major stations include a
control room from which operations are coordinated. Smaller distribution
substations follow the same principle of receiving power at higher
voltage on one side and sending out a number of
distribution feeders at lower voltage on the other, but they serve a more
limited local area and are generally unstaffed. The central component of the
substation is the transformer, as
it provides the effective interface between the high- and low-voltage parts of the system.
Other crucial components are controlling panels, Relays, circuit breakers and
instrument transformers act as protective devices that open the
faulty section of the circuit automatically in the event of a fault, that is, when a protective
relay find excessive secondary current according to the primary current of the
current transformer connected in EHT line or equipment. An important difference
between circuit breakers and switches is
that breakers are designed to interrupt abnormally high range
of currents i.e. from 0Amps to few Kilo Amps, whereas regular switches are
designed to be operable under no loads at high voltage levels >11KV,
and with small currents at voltage levels <11KV. Breakers are placed
on both the high- and low-voltage side of transformers
to arrange internal and external protection of the transformers. . Finally, substations may also
include capacitor banks to provide lagging Mvar control further small voltage improvement.
Continue with explanation
of all equipment erection, maintenance, precautions and safety.
