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Electrical Substation Equipments - GIS components |
Electrical Substation Equipments | A substation is an assemblage of
electrical apparatus. These include bus-bars, circuit breakers, isolator with
earthing switch, insulator, surge arresters, CTs, PTs, Line trap unit coupling
capacitors, compensation equipment, and power transformers.
1. Transformers
Transformers are necessary for a substation for stepping –
up and stepping down of ac voltage besides the transformers the electrical substation
has several others electrical equipments including bus-bars, circuit breakers, isolators, surge arresters, CTs, VTs, shunt Reactors and Shunt Capacitors.
2. Circuit breakers
A Circuit breaker is a device capable of breaking an electric
circuit under normal and abnormal conditions such as short and open circuits.
The breaker can operate automatically and clear fault currents safely and
quickly. They are the switching and current interrupting devices. Generally, a
circuit breaker comprises of fixed and movable contacts. Using the operating
mechanism coils can be separated.
The separation of current carrying contacts produces an arc. The
arc is extinguished by a suitable medium such as dielectric oil, vacuum, SF6
gas. The circuit breakers are necessary at every switching point in the electrical
substation.
3. Isolators
A switching device which can be opened or closed only under no
current condition. It provides isolation of a circuit for the purpose of
maintenance. Isolators are disconnecting switches which can be used for
disconnecting circuit breakers. An isolator can be opened after the circuit
breaker. After opening the isolator the earthing switch can be closed to
discharge the trapped electrical charges to the ground.
4. Current Transformers and Potential Transformers
The current transformers and voltage transformers are used for transforming
the electrical current and electrical voltage to a lower value for the purpose
of measurement, protection, and control.
- Current Transformer (CT)
The current ratio of current transformers is generally high
(500A/5A) and volt-ampere capacity is relatively low (50 VA) as compared with
that of the power transformers.
Voltage Transformer (PT)
The volt-ampere capacity of a potential transformer is low(100VA)
and the voltage ratio is relatively high (132kv/100v). The protective relays
are connected in the secondary circuits of CTs and PTs.
5. Bus-bar
Bus-bars are conductors to which several circuits are connected
Bus-bar is either flexible or rigid. Flexible bus-bars are made of ACSR
conductors and are supported on strain insulators. Rigid bus-bars are made up of
aluminum tubes and are supported on post insulators.
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Single Bus Bar - Substation Equipments |
6. Earthing Switch
It is a switch which connects a conductor to the earth so as to
discharge the charges on the conductor to the earth. Earthing switches are
generally installed on the frames of the isolators.
7. Relay
It is an automatic device, which closes its contacts when the
actuating quantity/quantities reach a certain predetermined magnitude/phase.
8. Lighting Arrester (Surge Arrester)
The substation equipment connected between the conductor
and ground to discharge the excessive voltages to earth. Surge arresters divert
the over-voltages to earth and protect the electrical substation
equipment from over-voltage surges.
9. Auto-reclosures
Automatic closing of the circuit breaker after auto-reclouseres
opening. Auto-reclosure is provided to restore the service continuity after
interrupting transient fault. High voltage circuit breakers used for
controlling overhead transmission lines and distance protection scheme for line
protection are provided with such a feature.
10. HRC Fuse
High rupturing capacity cartridge fuse is used for over-current
protection of low voltage and high voltage circuit.
11. Protective Scheme
A selected set of protective systems which protect one or two
components of the power systems against abnormal conditions. Eg: generator
protection scheme transformer protection scheme.
12. Structure
Galvanized steel structures are made of bolted/welded structures
of angles/channels/pipes. These are used for towers gantries equipment support
structures. Galvanized structures provide rigid support to the various equipment
and insulators. The design should be safe and economical. Compensating
substations are installed at an interval of 300km along EHV – AC lines for
feeding reactive power VAR to line.
13. Following Compensation Equipment is Necessary for Voltage Control
- Series capacitors are sometimes installed in series with long EHV AC Transmission lines to compensate line reactance.
- Shunt capacitors are installed near load points in distribution substations receiving substations for improvement of power factor. Shunt capacitors are switched on during high inductive loads. They are switched off during low loads. Shunt capacitors are also included in static Var sources (SVS).
- Shunt Reactors are necessary with long EHV transmission lines to compensate for the reactive power of the line capacitance during low loads.
- Static Var Sources (SVS).These are thyristor controlled shunt capacitors and shunt reactors which give rapid, step less control of reactive power VAR. These are connected in receiving stations and distribution systems.
- Power Line Carrier Current Equipment (PLCC) is necessary for transmitting/receiving high-frequency signals over the power line (transmission line) for the following: Voice communication, data transmission, protection signaling, control signaling.
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110kV/11kV Substation Components |
Electrical Substation Equipment Function
1. Bus-bar:
Incoming and outgoing circuits connected bus-bars
2. Circuit-breakers:
Automatic switching during normal or abnormal conditions
3. Isolators:
( Disconnections) Disconnection under no-load condition for safety isolation
and maintenance
4.
Earthing switch: To discharge the voltage on dead lines to earth
5. Current
Transformer: To step-down currents for measurement control and
protection
6. Voltage
Transformer: To step down currents for measurement control and
protection
7. Lighting
Arrester (surge arrester): To discharge lightning over voltages an
switching over voltages to earth.
8.
Shunt Reactor: To provide reactive power compensation during low loads
9. Series
Reactors: To reduce the short circuit current or starting currents.
10. Neutral-Grounding
Resistor: To limit the earth fault current.
11. Coupling
capacitor: To provide connection between high voltage line and power
line carrier current equipment.
12. Line
trap: To prevent high frequency signals from entering other zones.
13. Shunt
capacitors: To provide compensations to reactive loads of lagging power
factors
14. Power
Transformer: To step-up or step-down the voltage and transfer power
from one ac voltage to another ac voltage at the same frequency.
15.
Series Capacitors: Compensation of long lines.
Now-a-days, generation and transmission is almost exclusively
three-phase. The secondary transmission is also 3-phase whereas the
distribution to the ultimate customer may be 3-phase or single-phase depending
upon the requirements of the customers.
C.S. represents the central
station where power is generated by 3-phase alternators at 6.6 or 11 or 13.2 or
even 33 kV. The voltage is then stepped up by suitable 3-phase transformers for
transmission purposes. Taking the generated voltage as 11 kV, the 3-phase
transformers step it up to 132 kV as shown. Primary or high-voltage transmission is carried out at 132 kV.
The three phase, three wire overhead high voltage transmission
line next terminates in step down transformers in a electrical substation
known as Receiving Station (R.S.) which usually lies at the outskirts of
a city because it is not safe to bring high-voltage overhead transmission lines
into thickly populated areas. So, the voltage is stepped down to 33 kV.
From the Receiving Station, power is next transmitted at 33 kV
by underground cables or by overhead lines to various sub-stations (S S)
located at various strategic points in the city. This is known as secondary or
low-voltage transmission. From now on wards starts the primary and secondary
distribution. At the sub-station (S S) voltage is reduced from 33kV to
3.3kV 3-wire for primary distribution.
Consumers whose demands exceed 50 kVA are usually supplied from SS
by special 3.3 kV feeders. The secondary distribution is done at 400/230 V
for which purpose voltage is reduced from 3 .3kV to 400 V at the distribution
sub-stations. Feeders radiating from distribution sub-station supply power to
distribution networks in their respective areas. The most common system for
secondary distribution is 400/230V, three phase four wire system. The single
phase building lighting load is connected between any one line and the neutral.
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Final Word
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