Switchgear Equipment in a Substation

1. Department of Electrical & Electronic Engineering
Assignment on
Switchgear Equipment in a Substation
Course Code: EEE-4229
Course Title: Power System Protection
Submission Date: 15.05.2014
Presented By
Md. Atiqur Rahman
Department of Electrical & Electronic Engineering
Leading University

2. 2
Contents
Introduction of this Substation...............................................................................................................3
Sub-station..............................................................................................................................................4
Why we need Sub-station...................................................................................................................4
Schematic Power System Diagram .........................................................................................................5
Why we need to know about Switchgear...............................................................................................6
Switchgear equipment which is used in Amberkhana Substation..........................................................6
High & Low voltage switchgear panel.................................................................................................6
Incoming & Outgoing switchgear........................................................................................................6
Current Transformer (CT)........................................................................................................................7
Why we use C.T in a sub-station.........................................................................................................7
Potential Transformer (PT) .....................................................................................................................8
Why we use P.T in a sub-station.........................................................................................................8
Circuit Breakers.......................................................................................................................................9
Circuit breakers inter locking system at normal condition.................................................................9
Vacuum breakers ................................................................................................................................9
Sulfur Hexafluoride (SF6) Circuit Breaker .........................................................................................10
Fig: Sulfur Hexafluoride (SF6) Circuit Breaker ...................................................................................10
Air Breaker Switch.................................................................................................................................11
Isolator..................................................................................................................................................12
Oil Switch ..............................................................................................................................................13
Relays....................................................................................................................................................14
Surge arrester .......................................................................................................................................15
Fuses .....................................................................................................................................................16
Reference..............................................................................................................................................17

3. 3
Introduction of this 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.

4. 4
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

5. 5
Schematic Power System Diagram
Fig: Schematic Power System Diagram

6. 6
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
 Current Transformer
 Potential Transformer
 Circuit Breaker
o Vacuum Breakers
o SF6 Breakers
 Air break Switch
 Isolator
 Oil switch
 Relays
Incoming & Outgoing switchgear
 Surge Arrestor
 Fuses

7. 7
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.
 Measurement: ammeter, kw meter, kwh meter, power factor meter
 Control and protection: current when exceeds the set value operates the relay which
senses the faulty condition.

8. 8
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 -
 Measurement: voltmeter, frequency meter , kw meter, kwh meter , power factor
meter
 Control & protection: used in directional over current protection and under
frequency relay operation (48.8 Hz , df/dt relay)

9. 9
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:
 Open earthlings switch
 Close isolator
 Then close circuit breaker
When opening
 First to open circuit breaker
 Next to open isolator
 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

10. 10
Advantage of Vacuum Circuit Breaker
 no oil is required for are quenching
 least maintenance
 compact size
 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:
 High dielectric strength
 Unique arc-quenching ability
 Excellent thermal stability
 Good thermal conductivity
Fig: Sulfur Hexafluoride (SF6) Circuit Breaker

11. 11
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.
 It is operated on No Load Only.
 Rated Voltage :- 12 KV
 Normal Current :- 400 A
Fig: Air Breaker
Advantage:
 Air-break switches lie in the fact that they are fireproof and explosion proof.
 Have rapid connect and disconnect operation, and are relatively simple in design.
Disadvantage
 The presence of equipment for the production and storage of compressed-air supplies

12. 12
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 reclosed. 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 breake- 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
 Its prime purpose is operator safety.
 It is physical and visual disconnection of the circuit from power supply.
 Isolators are usually used on both sides of the C.B in order to repair or replace the
C.B.
 It is used to separate any section from rest part of the power supply.
Maintenance of isolator
 oiling &greasing of moving parts
 check contact firmness
 check open / close trials
 Earthlings switch

13. 13
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

14. 14
Relays
Relay is a device when detects the fault supplies information to the breaker for circuit
interruption.
Working principal of relay
 Under normal load current a small current flows in the relay operating coil so breaker
remains unchanged.
 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
 Reliability
 Sensitivity
 Selectivity
 Quickness
 Non interference with future extension
 Protection for transformer

15. 15
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.
 Lighting arrestor is used for lightning surge protection.
 Lighting arrestor is used in line side as well as T/F HV & LV side.
Fig: Surge arrester

16. 16
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
 Rewire able fuse
 Cartridge type fuse
 Drop out fuse
 High capacity H.R.C. fuse
 High voltage H.R.C. fuse
Drop Out Fuse Specification
 11 KV, 50 HZ.
 Rated Voltage :- 12 KV
 Rated Normal Current:- 100 A

17. 17
Reference
 Training Institute for Chemical Industries(TICI)
o Electrical Substation & Distribution System
o Electrical Protective Devices & Electrical Symbols
 http://www.electrical4u.com/
 https://answers.yahoo.com
 http://wiki.answers.com/
 http://www.slideshare.net/