SUB-STATION EQUIPMENTS & ITS FUNCTIONS
Lightening Arrester
Lightening arrestors
are the instrument that are used in the incoming feeders so that to prevent the
high voltage entering the main station. This high voltage is very dangerous to the
instruments used in the substation. Even the instruments are very costly, so to
prevent any damage lightening arrestors are used. The lightening arrestors do
not let the lightening to fall on the station. If some lightening occurs the
arrestors pull the lightening and ground it to the earth. In any substation the
main important is of protection which is firstly done by these lightening
arrestors. The lightening arrestors are grounded to the earth so that it can
pull the lightening to the ground. The lightening arrestor works with an angle
of 30° to 45° making a cone.
C V T
A capacitor voltage
transformer (CVT) is a
transformer used in power systems to step-down extra high voltage signals and
provide low voltage signals either for measurement or to operate a protective
relay. In its most basic form the device consists of three parts: two
capacitors across which the voltage signal is split, an inductive element used
to tune the device to the supply frequency and a transformer used to isolate
and further step-down the voltage for the instrumentation or protective relay.
The device has at least four terminals, a high-voltage terminal for connection
to the high voltage signal, a ground terminal and at least one set of secondary
terminals for connection to the instrumentation or protective relay. CVTs are
typically single-phase devices used for measuring voltages in excess of one
hundred kilovolts where the use of voltage transformers would be uneconomical.
In practice the first capacitor, C1,
is often replaced by a stack of capacitors connected in series. This results in
a large voltage drop across the stack of capacitors that replaced the first
capacitor and a comparatively small voltage drop across the second capacitor, C2,
and hence the secondary terminals.
Wave Trap
Wave trap is an
instrument using for tripping of the wave. The function of this trap is that it
traps the unwanted waves. Its function is of trapping wave. Its shape is like a
drum. It is connected to the main incoming feeder so that it can trap the waves
which may be dangerous to the instruments here in the substation.
Instrument Transformer
Instrument
transformers are used to step-down the current or voltage to measurable values.
They provide standardized, useable levels of current or voltage in a variety of
power monitoring and measurement applications. Both current and voltage
instrument transformers are designed to have predictable characteristics on
overloads. Proper operation of over-current protection relays requires that
current transformers provide a predictable transformation ratio even during a
short circuit.
These are further classified
into two types which are discussed below.
a. Current Transformers
b. Potential Transformers
Current Transformer
Current transformers
are basically used to take the readings of the currents entering the substation.
This transformer steps down the current from 800 amps to 1 amp. This is done because
we have no instrument for measuring of such a large current. The main use of
this transformer is
a. Distance Protection
b. Backup Protection
c. Measurement
A current
transformer is defined as an instrument transformer in which the secondary
current is substantially proportional to the primary current (under normal conditions
of operation) and differs in phase from it by an angle which is approximately zero
for an appropriate direction of the connections. This highlights the accuracy requirement
of the current transformer but also important is the isolating function, which means
no matter what the system voltage the secondary circuit need to be insulated
only for a low voltage.
The current
transformer works on the principle of variable flux. In the ideal current transformer,
secondary current would be exactly equal (when multiplied by the turns ratio)
and opposite to the primary current. But, as in the voltage transformer, some
of the primary current or the primary ampere-turns are utilized for magnetizing
the core, thus leaving less than the actual primary ampere turns to be
transformed into the secondary ampere-turns. This naturally introduces an error
in the transformation. The error is classified into current ratio error and the
phase error
Potential Transformer
There are two potential
transformers used in the bus connected both side of the bus. The potential transformer
uses a bus isolator to protect itself. The main use of this transformer is to
measure the voltage through the bus. This is done so as to get the detail
information of the voltage passing through the bus to the instrument. There are
two main parts in it
a. Measurement
b. Protection
The standards define a voltage transformer as one in which the
secondary voltage is substantially proportional to the primary voltage and
differs in phase from it by an angle which is approximately equal to zero for
an appropriate direction of the connections. This in essence means that the
voltage transformer has to be as close as possible to the ideal transformer.
In an ideal transformer, the secondary voltage vector is exactly
opposite and equal to the primary voltage vector when multiplied by the turn’s
ratio.
In a
practical transformer, errors are introduced because some current is drawn for the
magnetization of the core and because of drops in the primary and secondary windings
due to leakage reactance and winding resistance. One can thus talk of a voltage
error which is the amount by which the voltage is less than the applied primary
voltage and the phase error which is the phase angle by which the reversed
secondary voltage vector is displaced from the primary voltage vector.
Bus Bar
The bus is a line in
which the incoming feeders come into and get into the instruments for further
step up or step down. The first bus is used for putting the incoming feeders in
la single line. There may be double line in the bus so that if any fault occurs
in the one the other can still have the current and the supply will not stop.
The two lines in the bus are separated by a little distance by a conductor having
a connector between them. This is so that one can work at a time and the other
works only if the first is having any fault.
A bus bar in electrical power
distribution refers to thick strips of copper or aluminum that conduct
electricity within a switchboard, distribution board, substation, or other
electrical apparatus. The size of the bus bar is important in determining the
maximum amount of current that can be safely carried. Bus bars are typically
either flat strips or hollow tubes as these shapes allow heat to dissipate more
efficiently due to their high surface area to cross sectional area ratio. The
skin effect makes 50-60 Hz AC bus bars more than about 8 mm (1/3 in) thick
inefficient, so hollow or flat shapes are prevalent in higher current applications.
A hollow section has higher stiffness than a solid rod of equivalent current
carrying capacity, which allows a greater span between bus bar supports in
outdoor switchyards. A bus bar may either be supported on insulators or else
insulation may completely surround it. Bus bars are protected from accidental
contact either by a metal enclosure or by elevation out of normal reach.
Neutral bus
bars may also be insulated. Earth bus bars are typically bolted directly onto
any metal chassis of their enclosure. Bus bars may be enclosed in a metal
housing, in the form of bus duct or bus way, segregated-phase bus, or
isolated-phase bus.
Circuit Breaker
The circuit breakers
are used to break the circuit if any fault occurs in any of the instrument.
These circuit breaker breaks for a fault which can damage other instrument in
the station. For any unwanted fault over the station we need to break the line current.
This is only done automatically by the circuit breaker. There are mainly two
types of circuit breakers used for any substations. They are
a. SF6 circuit breakers
b. Spring circuit breakers.
The use of SF6 circuit
breaker is mainly in the substations which are having high input kv input, say
above 220kv and more. The gas is put inside the circuit breaker by force i.e.
under high pressure. When if the gas gets decreases there is a motor connected to
the circuit breaker. The motor starts operating if the gas went lower than 20.8
bar. There is a meter connected to the breaker so that it can be manually seen
if the gas goes low. The circuit breaker uses the SF6 gas to reduce the torque
produce in it due to any fault in the line. The circuit breaker has a direct
link with the instruments in the station, when any fault occur alarm bell
rings.
The spring type of
circuit breakers is used for small kv stations. The spring here reduces the
torque produced so that the breaker can function again. The spring type is used
for step down side of 132kv to 33kv also in 33kv to 11kv and so on. They are
only used in low distribution side.
Transformer
There are three
transformers in the incoming feeders so that the three lines are step down at
the same time. In case of a 220KV or more KV line station auto transformers are
used. While in case of lower KV line such as less than 132KV line double
winding transformers are used.
The transformer is
transported on trailor to substation site and as far as possible directly
unloaded on the plinth. Transformer tanks up to 25 MVA capacity are generally
oil filled, and those of higher capacity are transported with N2 gas filled in
them +ve pressure of N2 is maintained in transformer tank to avoid the ingress
of moisture. This pressure should be maintained during storage, if necessary by
filling N2 Bushings - generally transported in wooden cases in horizontal
position and should be stored in that position. There being more of fragile
material, care should be taken while handling them. Radiators – These should be
stored with ends duly blanked with gaskets and end plates to avoid in gross of
moisture, dust, and any foreign materials inside. The care should be taken to
protect the fins of radiators while unloading and storage to avoid further oil
leakages. The radiators should be stored on raised ground keeping the fins
intact.
Oil Piping. The Oil piping should also
be blanked at the ends with gasket and blanking plates to avoid in gross of
moisture, dust, and foreign All other accessories like temperature meters, oil
flow indicators, PRVs, buchholz relay; oil surge relays; gasket ‘ O ‘ rings
etc. should be properly packed and stored indoor in store shed. Oil is received
in sealed oil barrels. The oil barrels should be stored in horizontal position
with the lids on either side in horizontal position to maintain oil pressure on
them from inside and subsequently avoiding moisture and water ingress into oil.
The transformers are received on site with loose accessories hence the
materials should be checked as per bills of materials.
Isolator
Control and Relay Panel
The control
and relay panel is of cubical construction suitable for floor mounting. All
protective, indicating and control elements are mounted on the front panel for
ease of operation and control. The hinged rear door will provide access to all
the internal components to facilitate easy inspection and maintenance.
Provision is made for terminating incoming cables at the bottom of the panels
by providing separate line-up terminal blocks. For cable entry provision is
made both from top and bottom. The control and relay panel accepts CT, PT aux 230 AC and 220V/10V
DC connections at respective designated terminal points. 220V/10V DC supply is
used for control supply of all internal relays and timers and also for
energizing closing and tripping coils of the breakers. 230V AC station
auxiliary supply is used for internal illumination lamp of the panel and the
space heater. Protective HRC fuse are provided with in the panel for P.T
secondary. Aux AC and battery supplies. Each Capacitor Bank is controlled by
breaker and provided with a line ammeter with selector switch for 3 phase
system & over current relay (2 phases and 1 Earth fault for 3 ph system).
Under voltage and over voltage relays. Neutral Current Unbalance Relays are for
both Alarm and Trip facilities breaker control switch with local/remote
selector switch, master trip relay and trip alarms acknowledge and reset
facilities.
Protective Relaying
Protective
relays are used to detect defective lines or apparatus and to initiate the
operation of circuit interrupting devices to isolate the defective equipment.
Relays are also used to detect abnormal or undesirable operating conditions
other than those caused by defective equipment and either operate an alarm or
initiate operation of circuit interrupting devices. Protective relays protect
the electrical system by causing the defective apparatus or lines to be
disconnected to minimize damage and maintain service continuity to the rest of
the system. There are different types of relays.
i. Over current relay
ii. Distance relay
iii. Differential relay
iv. Directional over current
relay
i. Over Current Relay
The over
current relay responds to a magnitude of current above a specified value. There
are four basic types of construction: They are plunger, rotating disc, static,
and microprocessor type. In the plunger type, a plunger is moved by magnetic
attraction when the current exceeds a specified value. In the rotating
induction-disc type, which is a motor, the disc rotates by electromagnetic
induction when the current exceeds a specified value.
Static
types convert the current to a proportional D.C mill volt signal and apply it
to a level detector with voltage or contact output. Such relays can be designed
to have various current-versus-time operating characteristics. In a special
type of rotating induction-disc relay, called the voltage restrained over
current relay. The magnitude of voltage restrains the operation of the disc
until the magnitude of the voltage drops below a threshold value. Static over
current relays are equipped with multiple curve characteristics and can
duplicate almost any shape of electromechanical relay curve. Microprocessor
relays convert the current to a digital signal. The digital signal can then be
compared to the setting values input into the relay. With the microprocessor
relay, various curves or multiple time-delay settings can be input to set the
relay operation. Some relays allow the user to define the curve with points or
calculations to determine the output characteristics.
ii. Distance Relay
The
distance relay responds to a combination of both voltage and current. The
voltage restrains operation, and the fault current causes operation that has
the overall effect of measuring impedance. The relay operates instantaneously (within
a few cycles) on a 60-cycle basis for values of impedance below the set value.
When time delay is required, the relays energizes a separate time-delay relay
or function with the contacts or output of this time-delay relay or function
performing the desired output functions. The relay operates on the magnitude of
impedance measured by the combination of restraint voltage and the operating
current passing through it according to the settings applied to the relay. When
the impedance is such that the impedance point is within the impedance
characteristic circle, the relay will trip. The relay is inherently
directional. The line impedance typically corresponds to the diameter of the
circle with the reach of the relay being the diameter of the circle.
iii. Differential Relay
The
differential relay is a current-operated relay that responds to the difference
between two or more device currents above a set value. The relay works on the
basis of the differential principle that what goes into the device has to come
out .If the current does not add to zero, the error current flows to cause the
relay to operate and trip the circuit.
The
differential relay is used to provide internal fault protection to equipment
such as transformers, generators, and buses. Relays are designed to permit
differences in the input currents as a result of current transformer mismatch
and applications where the input currents come from different system voltages,
such as transformers. A current differential relay provides restraint coils on
the incoming current circuits. The restraint coils in combination with the
operating coil provide an operation curve, above which the relay will operate.
Differential relays are often used with a lockout relay to trip all power
sources to the device and prevent the device from being automatically or
remotely reenergized. These relays are very sensitive. The operation of the
device usually means major problems with the protected equipment and the likely
failure in re-energizing the equipment.
iv. Directional Over current
Relay
A
directional over current relay operates only for excessive current flow in a
given direction. Directional over current relays are available in
electromechanical, static, and microprocessor constructions. An
electromechanical overcorrect relay is made directional by adding a directional
unit that prevents the over current relay from operating until the directional
unit has operated. The directional unit responds to the product of the
magnitude of current, voltage, and the phase angle between them or to the
product of two currents and the phase angle between them. The value of this
product necessary to provide operation of the directional unit is small, so
that it will not limit the sensitivity of the relay (such as an over current
relay that it controls). In most cases, the directional element is mounted
inside the same case as the relay it controls. For example, an over current
relay and a directional element are mounted in the same case, and the
combination is called a directional over current relay. Microprocessor relays
often provide a choice as to the polarizing method that can be used in
providing the direction of fault, such as applying residual current or voltage
or negative sequence current or voltage polarizing functions to the relay.
DC Power Supply
I . DC Battery and Charger
All but the
smallest substations include auxiliary power supplies. AC power is required for
substation building small power, lighting, heating and ventilation, some communications
equipment, switchgear operating mechanisms, anti-condensation heaters and
motors. DC power is used to feed essential services such as circuit breaker
trip coils and associated relays, supervisory control and data acquisition
(SCADA) and communications equipment. This describes how these auxiliary
supplies are derived and explains how to specify such equipment. It has Single
100% battery and 100% charger, Low capital cost, No standby DC System outage
for maintenance. Need to isolate battery/charger combination from load under
boost charge conditions in order to prevent high boost voltages.
I I . Battery and Charger
configurations
Capital
cost and reliability objectives must first be considered before defining the
battery and battery charger combination to be used for a specific installation.
The comparison given in Table 5.1 describes the advantages and disadvantages of
three such combinations.
Capital
cost and reliability objectives must first be considered before defining the
battery/battery charger combination to be used for a specific installation. The
comparison given describes the advantages and disadvantages of three such
combinations
III . 400V DC Battery
Make: Exide
Capacity: 300 AH at 27°
No. of Cells: 110 No.
Date of installation:
06/2001
Make: Universal,
Sr. No. : BC 1020/82
Date of manufacturing: 4/2000
Input Rating: Voltage: 415 V
+ 10 %
Output Rating : Float: 220
V, 10 Amp
Boost: 180 V, 30Amp
Functions of Associated System in Substation
Functions of Associated System in
Substation is as shown below in table-4.1
Table-4.1 Functions of Associated
System in Substation
Sr.
|
System
|
Function
|
1.
|
Substation
Earthing system
-
Earth mat
-
Earthing spikes
-
Earthing risers
|
To provide an earth mat
for connecting neutral points, equipment body, support structures to earth.
For safety of personnel and for enabling earth fault protection. To provide
the path for discharging the earth currents from neutrals, faults, Surge
Arresters, overheads shielding wires etc. with safe step-potential and touch
potential.
|
2.
|
Overhead
earth wire shielding or Lightning masts.
|
To
protect the outdoor substation equipment from lightning strokes.
|
3.
|
Illumination
system (lighting)
-
for switchyard
-
buildings
-
roads etc.
|
To provide proper
illumination to substation yard.
|
4.
|
Protection
system
-
protection relay panels
-
control cables
-
circuit breakers
-
CTs, VTs etc.
|
To provide alarm or
automatic tripping of faulty part from healthy part and also to minimize
damage to faulty equipment and associated system.
|
5.
|
Control
cable
|
For Protective circuits,
control circuits, metering circuits, communication circuits
|
6.
|
Power
cable
|
To provide supply path to
various auxiliary equipment and machines.
|
7.
|
PLCC
system
power
line carrier communication system
|
For
communication, telemetry, tele-control, power line carrier protection etc.
|
8.
|
Telephone,
telex, microwave, OPF
|
For
internal and external communication
|
9.
|
Auxiliary standby power
system
|
For supplying starting
power, standby power for auxiliaries.
|
10.
|
Fire Fighting system
- Sensors, detection
system
- water spray system
- fire port, panels, alarm
System.
- water
tank and spray system
|
To sense the occurrence of
fire by sensors and to initiate water spray, to disconnect power supply to
affected region to pinpoint location of fire by indication in control room.
|
