Introduction of Visited Substation
Substation
Location: Division-1, Electric Supply, Amberkhana,
Sylhet
Substation
Rating:
33/11kv
Substation
Type:
Outdoor
Distributed
Area:
Amberkana, Shahi-Eidgah, Dorga-Mohalla, Mirermoydan, SubidBazar, Patantula,
Ahakalia, Airport, Baghbhari.
Sub-station
A
substation may be defined as “assembly of apparatus which transfers
characteristics of electrical energy from one to another for example from
alternating current to direct current or from one to another.” Every electrical
circuit needs a switching device and protective device. They are together
called Switchgear. These are located in the substation.
Why we need Sub-station
For
economical transfer electrical energy or to meet various demand of the load, we
require sub-station.
Fig:
an outdoor Substation
Schematic Power System Diagram
Fig:
Schematic Power System Diagram
Why we need to know about Switchgear
All
electrical equipment and circuits have to be protected by protective devices
against damages arising abnormal conditions.
Switchgear equipment which is used in Amberkhana Substation
High & Low voltage switchgear panel
Ø Current Transformer
Ø Potential Transformer
Ø Circuit Breaker
o
Vacuum
Breakers
o
SF6
Breakers
Ø Air break Switch
Ø Isolator
Ø Oil switch
Ø Relays
Incoming & Outgoing switchgear
Ø Surge Arrestor
Ø Fuses
Current Transformer (CT)
A
current transformer is an instrument transformer, used along with measuring or
protective devices, in which the secondary current is proportional to the
primary current (under normal conditions of operation) and differs from it by
an angle which is approximately zero.
Fig:
Current Transformer
Why we use C.T in a sub-station
Current T/F is used for protection and metering
purposes. It steps down the current level of the line for following purposes.
Ø Measurement:
ammeter, kw meter, kwh meter, power factor meter
Ø Control and protection:
current when exceeds the set value operates the relay which senses the faulty
condition.
Potential Transformer (PT)
Potential
Transformer or Voltage Transformer gets used in electrical power system for
stepping down the system voltage to a safe value which can be fed to low
ratings meters and relays. Commercially available relays and meters used for
protection and metering, are designed for low voltage.
Fig:
Potential Transformer
Why we use P.T in a sub-station
It steps down the voltage level of line for -
Ø Measurement:
voltmeter, frequency meter , kw meter, kwh meter , power factor meter
Ø Control & protection:
used in directional over current protection and under frequency relay operation (48.8 Hz , df/dt
relay)
Circuit Breakers
A
circuit breaker is equipment which can open or close a circuit under all
conditions, such as: no loads, full loads or fault condition.
Circuit breakers inter locking system at normal condition
When
closing:
Ø Open
earthlings switch
Ø Close
isolator
Ø Then
close circuit breaker
When
opening
Ø First
to open circuit breaker
Ø Next
to open isolator
Ø Then
the earthlings switch to close
Vacuum breakers
Vacuum
circuit breakers are used mostly for low and medium voltages. Vacuum
interrupters are developed for up to 36 kV and can be connected in series for
higher voltages. The interrupting chambers are made of porcelain and sealed.
They cannot be open for maintenance, but life is expected to be about 20 years,
provided that the vacuum is maintained. Because of the high dielectric strength
of vacuum, the interrupters are small. The gap between the contacts is about 1
cm for 15 kV interrupters, 2 mm for 3 kV interrupters.
Fig:
Vacuum breakers
Advantage
of Vacuum Circuit Breaker
Ø no
oil is required for are quenching
Ø least
maintenance
Ø compact
size
Ø light
in weight
Sulfur Hexafluoride (SF6) Circuit
Breaker
Sulfur
Hexafluoride (SF6) is an excellent gaseous dielectric for high voltage power
applications. It has been used extensively in high voltage circuit breakers and
other switchgears employed by the power industry. Applications for SF6 include
gas insulated transmission lines and gas insulated power distributions. The
combined electrical, physical, chemical and thermal properties offer many
advantages when used in power switchgears. Some of the outstanding properties
of SF6 making it desirable to use in power applications are:
Ø High
dielectric strength
Ø Unique
arc-quenching ability
Ø Excellent
thermal stability
Ø Good
thermal conductivity
Fig: Sulfur Hexafluoride (SF6) Circuit Breaker
Air Breaker Switch
An
"air break switch' is a switching device that uses air the dielectric. Air
break switches can be single pole or 'gang operated', and can be operated
manually (using either a handle/ratchet mechanism or an insulated 'hook stick'
made either of wood or fiberglass). Air break switches are designed for
switching under load, but there is often a noticeable arc associated with
switching. Air break switches can be found either in substations or out on the
distribution system - either pole top or in pad-mounted metal enclosures. Air-break
switches at currents up to 750 kV, which are generally used at high-voltage
power plants and substations.
Ø It
is operated on No Load Only.
Ø Rated
Voltage :- 12 KV
Ø Normal
Current :- 400 A
Fig:
Air Breaker
Advantage:
Ø Air-break
switches lie in the fact that they are fireproof and explosion proof.
Ø Have
rapid connect and disconnect operation, and are relatively simple in design.
Disadvantage
Ø The
presence of equipment for the production and storage of compressed-air supplies
Isolator
An
isolator is a non load-breaking switch, and is provides a visible means of isolating
a component, such as a circuit breaker, transformer, etc., from the high-voltage
lines, whenever it is necessary to perform maintenance of that component.
Normally, isolators come in pairs, with one on each side of the component to be
isolated. Isolators are only opened after the load current has been broken
using a circuit breaker, and must be closed before the circuit breaker is re-closed. To work on, say, a h.v. circuit breaker, the breaker must be tripped,
the isolators on either side must be opened and locked off, temporary earths
attached to either side of the circuit breaker- to-work card, detailing the
maintenance work, must be issued to the crew by the supervising engineer.
Fig:
Isolator
Why we use it in Sub-station
Ø Its
prime purpose is operator safety.
Ø It
is physical and visual disconnection of the circuit from power supply.
Ø Isolators
are usually used on both sides of the C.B in order to repair or replace the
C.B.
Ø It
is used to separate any section from rest part of the power supply.
Maintenance of isolator
Ø oiling
&greasing of moving parts
Ø check
contact firmness
Ø check
open / close trials
Ø Earthlings
switch
Oil Switch
An
oil switch is a high-voltage switch whose contacts are opened and closed in oil.
The switch is actually immersed in an oil bath and contained in a steel tank.
The reason for placing high-voltage switches in oil is that the oil will break
the circuit when the switch is opened with high voltages. A separation of the
switch contacts does not always break the current flow because an electric arc
forms between the contacts if the contacts are opened in oil. The oil will
quench the arc. Further more if an arc should form in the oil. It will
evaporate part of the oil because of the high temperature and will partially
fill the interrupters surrounding the switch contacts with vaporized oil. This
vapor develops a pressure in the interrupters which assists in quenching or
breaking the arc by elongating the arc.
Fig:
Oil switch
Relays
Relay
is a device when detects the fault supplies information to the breaker for
circuit interruption.
Working principal of relay
Ø Under
normal load current a small current flows in the relay operating coil so breaker
remains unchanged.
Ø When
fault occurs a large amount of current flows in the relay operating coil and it
energized the trip coil of the breaker. Hence the breaker is open.
Fig: Scheme of Relay
Why we use it in Sub-station
Ø Reliability
Ø Sensitivity
Ø Selectivity
Ø Quickness
Ø Non interference with future extension
Ø Protection for transformer
Surge arrester
Lightning
Arresters or Surge Arresters are always connected in Shunt to the
equipment to be protected; they provide a low impedance path for the surge
current to the ground. Wave trap is a parallel tuned inductor - capacitor
'tank' circuit made to be resonant at the desired communication frequency.
It is the effort to utilize the same transmission line between two substations
for the purpose of communications. At this communication frequency the tank
circuit provides high impedance and does not allow passing through them
& onto the substation bus & into transformers.
Ø Lighting
arrestor is used for lightning surge protection.
Ø Lighting
arrestor is used in line side as well as T/F HV & LV side.
Fig:
Surge arrester
Fuses
Fuse
is a safety device. It is connected in series with the circuit and protects the
electrical apparatus and equipment from damage, when excess current flows. It
is one of the simplest protective devices and is used as circuit interrupting
device under short circuit condition.
Fig:
Fuse
Types of Fuse element
Ø Rewire
able fuse
Ø Cartridge
type fuse
Ø Drop
out fuse
Ø High
capacity H.R.C. fuse
Ø High
voltage H.R.C. fuse
Drop Out Fuse Specification
Ø 11
KV, 50 HZ.
Ø Rated
Voltage :- 12 KV
Ø Rated
Normal Current:- 100 A
Reference
Ø Training Institute for Chemical
Industries(TICI)
o Electrical Substation & Distribution
System
o Electrical Protective Devices &
Electrical Symbols
Ø http://www.electrical4u.com/
Ø https://answers.yahoo.com
Ø http://wiki.answers.com/
Ø http://www.slideshare.net/
