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| Substation Maintenance |
Index
1. SUBSTATION
1.1 Introduction
1.2 Classification
1.3 Indoor Substation
1.4 Outdoor Substation
1.5 Selection and Location of Site
1.6 Equipment of a substation
1.7 Substation Auxiliaries
1.8 Earthing
2. SWITCH
GEAR AND PROTECTION
2.1CIRCUIT
BREAKERS
2.1 1Introduction
2.1 2Arc phenomenon
2.1 3Principles of arc extinction
2.1 4Classification of circuit breakers
2.1 7Types of circuit breakers
2.2Protective
Relays
2.2.1 Introduction
2.2.2Electromagnetic Relays
2.2.3Induction Relays
2.2.6Time/TSM curves
2.2.8Functional Relay type
2.2.9 Induction type over current relay
2.2.10Induction Type Directional over current relay
Introduction-
Substations serve as source of power for the local areas of
distribution in which these are located. Their main function is to receive
power transmitted at high voltage from generating stations and reduce the
voltage appropriate for local utilization and provide facilities for switching.
Some substation are simply switching substations where different connections
between various transmission lines are made, others are converting substations
which convert either ac to dc or vice versa or change the frequency of the
supply. Substations provide additional features for safety devices. Voltage of
the outgoing feeders can be regulated at a substation. A substation is an ideal
place for installation of synchronous condensers at the end of the transmission
line for the purpose of power factor improvement and to make measurements of
the various operating parts of the interconnected system.
Classification of
substations-
1. Classification on the basis of nature of duties-
1.1Step-Up or Primary substation
1.2Primary grid substation
1.3Step down or distribution substation
2. Classification on the basis of service rendered
2.1Transformer substation
2.2Switching substation
2.3Converting substation
3. Classification based on the operating voltage
3.1High voltage substation
3.2Extra high voltage substation
3.3Ultra high voltage substation
4. Classification on the basis of Importance
4.1Grid substation
4.2Town substation
5. Classification on the basis of design
5.1Outdoor substation
5.2Indoor substation
Indoor Substation-
In these substations the apparatus installed are within the
substation building. Such substation are erected for a voltage level of 11Kv
but can be erected for 33Kv and 66Kv as well, when the surrounding environment
iscontaminated with impurities such as metal corroding,gasses and fumes. The
switchgear on the primary side of the supply will consist mainly of Oil circuit
breakers only. The high voltage supply is given to transformer and different
feeders emerge out of the busbar.The panel for each feeder consists of isolator
switch and circuit breakers, in addition to isolators and circuit breakers the panel
also consists of different measuring instruments. Theauxiliaries of indoor
substation are Dc supply and different fire extinguishing arrangements
Outdoor
substation-
Outdoor substation are of two types 1) pole mounted and 2)
foundation mounted substation
1) Pole-Mounted substation:-Such substation are erected for
mounting distribution oftransformers of capacity of 250 kVA.Such substations
are cheapest simplest and smallest of any kind of substations. All the equipment
are mounted on the supporting structure of HT distribution line. Triple pole
mechanically operated switch is used for ON and OFF control of HT line.ht fuse
is used, to control LT side iron clad low tension switch is used for suitable capacity.
The substation is switched at two or more points.
2) Foundation mounted substation:-these substation are built
entirely I open and fenced from the point of view of safety. Substation for
primary and secondary transmission and for secondary distribution, (above
250kVA) are foundation substation .since instruments required for such
substations are heavy ,therefore sites selected for such substation must
have good transport facility. Owing to
exposed bus bars and other associated equipment the clearance and spacing is
important and are not only governed by the operating voltage but also by the
encroachment from the outside.
Selection and Location of
Site
The locational requirements for substations are site specific, and are
determined by practicalities of engineering constraints, connection costs,
environmental issues and impacts on social attributes. Generally, the location
in which a substation is to be sited is dictated by load location and
transmission line arrangements for connection of the substation to the grid,
and the substation to the place where the connection is required. Access to the
substation sites must be capable under all but the most extreme conditions.
The following key issues are taken into
consideration:-(1)General location;(2)Development;(3)Physical;(4)Surrounding land usage;(5)Accessibility;(6)Cost;(7)Customer requirements;(8)Environmental
considerations; and (9)Community
considerations.(10)General access to
the substations for construction, operation and maintenance.
Equipment of a substation
1)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.
2)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.
3) 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.
4)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.
5)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.
6)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.
7)Isolator-The use of this isolator is to
protect the transformer and the other instrument in the line. The isolator
isolates the extra voltage to the ground and thus any extra voltage cannot
enter the line. Thus an isolator is used after the bus also for protection.
8)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.
9)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
Substation
Auxiliaries
1)
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.
2)Auxiliary
Transformers
Auxiliary transformers shall be
adequately rated to suit the requirements of the substation.The type, number,
vector group and rating shall be determined by the designer to suit the Specific
site requirements. The auxiliary transformer shall not be more than 500kVA.
Substation
Earthing
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.
Circuit Breaker
Introduction-circuit breaker is a switching device which can be operated manually as well
as automatically for controlling and protection of electrical power system
respectively. As the modern power system deals with huge currents, special attention
should be given during designing of circuit breaker to safe interruption of arc produced
during theoperation
of circuit breaker. This was the basic definition of circuit breaker.
Arc Phenomenon-During opening
of current carrying contacts in a circuit breaker the medium in between opening contacts become highly ionized
through which the interrupting current gets low resistive path and continues to
flow through this path even the contacts are physically separated. During the
flowing of electric current from one contact to other the path becomes so
heated that it glows. This is called arc.
Whenever,
on load current contacts of circuit breaker open there is an arc in circuit breaker,
established between the separating contacts. As long as this arc is sustained
in between the contacts the current through the circuit breaker will not be
interrupted finally as because arc is itself a conductive path of electricity.
For total interruption of current the circuit breaker it is essential to quench
the arc as quick as possible. The main designing criteria of a circuit breaker
is to provide appropriate technology of arc quenching in circuit breaker to fulfill quick
and safe current interruption
Principles
of arc extinction-
1) Heat Loss from Arc-heat loss from arc in circuit
breaker is taken place through conduction, convection as well as radiation. In
circuit breaker with plain break arc in oil, arc in chutes or narrow slots
nearly all the heat loss due to conduction.
2)Deionization of Gas due to Increasing Pressure-If
pressure of the arc path increases, the density of the ionized gas is increased
which means, the particles in the gas come closer to each other and as a result
the mean free path of the particles is reduced.
3)Deionization of Gas due to Decreasing Temperature-The
rate of ionization of gas depends upon the intensity of impact during collision
of gas particles. The intensity of impact during collision of particles again
depends upon velocity of random motions of the particles.
Types of circuit breaker
.According to their arc quenching media the circuit breaker can be
divided as-
1) Oil circuit breaker.
2) Air circuit breaker.
3) SF6 circuit breaker.
4)Vacuum circuit breaker.
According to their services the circuit breaker can be divided as-
1) Outdoor circuit breaker
2) Indoor breaker.
According to the operating mechanism of circuit breaker they can
be divided as-
1) Spring operated circuit breaker.
2) Pneumatic circuit breaker.
3) Hydraulic circuit breaker.
According to the voltage level of installation types of
circuit breaker are referred as-
1) High voltage circuit breaker.
2) Medium voltage circuit breaker.
3) Low voltage circuit breaker.
Oil Circuit Breaker-
Mineral oil has better insulating
property than air. In oil circuit breaker the fixed
contact and moving contact are immerged inside the insulating oil. Whenever
there is a separation of electric current carrying contacts in the oil, the arc in circuit
breaker is initialized at the moment of separation of
contacts, and due to this arc the oil is vaporized and decomposed in mostly
hydrogen gas and ultimately creates a hydrogen bubble around the arc. This
highly compressed gas bubble around the arc prevents re-striking of the arc
after current reaches zero crossing of the cycle. The oil
circuit breaker is
the one of the oldest type of circuit breakers.
Air
Circuit Breaker-
This type of
circuit breakers, is those kind
of circuit
breaker which operates in air at atmospheric pressure.
After development of oil circuit breaker,
the medium voltage air circuit breaker (ACB) is replaced completely by oil circuit
breaker in different countries. But in countries like
France and Italy, ACBs are still preferable choice up to voltage 15 KV. It is
also good choice to avoid the risk of oil fire, in case of oil circuit
breaker. In America ACBs were exclusively used for the
system up to 15 KV until the development of new vacuum and SF6circuit
breakers.
Vacuum Circuit Breaker-
A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is
suitable for mainly medium voltage application. For higher voltage vacuum
technology has been developed but not commercially viable. The operation of
opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the
breaker which is called vacuum interrupter. The vacuum interrupter consists of
a steel arc chamber in the center symmetrically arranged ceramic insulators.
The vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6 bar. The material used for current
carrying contacts plays an important role in the performance of the vacuum circuit breaker. CuCr is the most ideal material
to make VCB contacts. Vacuum interrupter technology was first introduced in the
year of 1960. But still it is a developing technology. As time goes on, the
size of the vacuum interrupter is being reducing from its early 1960’s size due
to different technical developments in this field of engineering. The contact
geometry is also improving with time, from butt contact of early days it
gradually changes to spiral shape, cup shape and axial magnetic field contact.
The vacuum circuit breaker is today recognized as
most reliable current interruption technology for medium
voltage switchgear. It requires minimum maintenance compared to other
circuit breaker technologies.
Sulphur
Hexafluoride Circuit Breaker-
A
circuit breaker in which the current carrying contacts operate in sulphur hexafluoride or SF6 gas is
known as anSF6 circuit breaker.SF6 has excellent insulating property. SF6has high electro-negativity. That means it has high
affinity of absorbing free electron. Whenever a free electron collides with the
SF6 gas molecule, it is absorbed by that gas
molecule and forms a negative ion.The attachment of electron with SF6 gas molecules may occur in two
different ways,
Hence,
for heavier and less mobile charged particles in SF6 gas,
it acquires very high dielectric strength. Not only the gas has a good
dielectric strength but also it has the unique property of fast recombination
after the source energizing the spark is removed. The gas has also very good
heat transfer property. Due to its low gaseous viscosity (because of less
molecular mobility) SF6 gas can efficiently
transfer heat by convection. So due to its high dielectric strength and high
cooling effect SF6 gas is approximately 100
times more effective arc quenching media than air. Due to these unique
properties of this gas SF6 circuit
breaker is used in complete range of medium voltage and high
voltage electrical power system.
RELAY
INTRODUCTION
A protective relay is a device that detects the fault and
initiates the operation of the circuit breaker to isolate the defective element
from the rest of the system.
The relay detects the abnormal conditions in the electrical
circuits by constantly measuring the electrical quantities which are different
under normal and fault conditions. The electrical quantities which may change
under fault conditions are voltage,current,frequency,phase angle. Through the
changes in one or more of these quantities, the fault signals their presence,
type and location to the protective relays. Having detected the fault, the
relay operates to close the trip circuit of the breaker. This results in the
opening of the breaker and disconnection of the faulty circuit.
A typical relay circuit is shown below:
This diagram shows one phase of 3-phase system for
simplicity. The relay circuit connections can be divided into three parts viz.
1) The primary winding of a current transformer connected in
series with line to be protected.
2) Secondary winding of current transformer and the relay
operating coil
3) Tripping circuit which may be either a.c or d.c it
consists of a source of supply, the trip coil of the circuit breaker and the
relay stationary contacts.
When a short circuit occurs on the transmission line, the
current flowing in the line increases to an enormous value. This results in a
heavy current flow through the relay coil, causing the relay to operate by
closing its contacts. This in turn closes the trip circuit of the breaker,
making the circuit breaker open and isolating the faulty section from the rest
of system. Thus ensuring safety of circuit equipment from damage.
Electromagnetic
Relay
Electromagnetic relay operate by virtue of an armature being
attracted to the poles of an electromagnet or a plunger being drawn into a
solenoid. Such relays may be actuated by d.c. or a.c. quantities.it consists of
three types.
1)
Attracted armature type relay:
It consists of a laminated electromagnet carrying a coil and
a pivoted laminated armature. The armature is balanced by a counterweight and
carries a pair of spring contact fingers at its free end. Under normal
operating condition the current through the relay coil is such that
counterweight holds the armature in the position shown. However when a short
circuit occurs the current through the relay coil increases sufficiently and
the relay armature is attracted upwards. The contacts on the relay armature
bridge a pair of stationary contacts attached to the relay frame. This
completes the trip circuit which results in the opening of the circuit breaker
and therefore in the disconnection of the faulty circuit
2) Solenoid type
relay:
It consists of a solenoid and movable iron plunger. Under
normal operating conditions, the current through the relay coil c is such that
it holds the plunger by gravity or spring in the position shown. However, on
the occurrence of a fault, the current through the relay coil becomes more than
the pickup value, causing the plunger to be attached to the solenoid. The
upward movement of the plunger closes the trip circuit, thus opening the
circuit breaker and disconnecting the faulty circuit.
3) Balanced beam
type relay:
It consists of an iron armature fastened to a balanced beam.
Under normal operating conditions the current through the relay coil is such
that the beam is held in the horizontal position by the spring. However when a
fault occurs, the current through the relay coil becomes greater than the
pickup value and the beam is attracted to close the trip circuit. This causes
the opening of the circuit breaker to isolate faulty circuit.
Induction
Relay
Electromagnetic induction relay operate on the principle of
induction motor and are widely used for protective relaying purposes involving
a.c. quantities. They are not used with d.c. quantities owing to the principle
of operation.an induction relay essentially consists of a pivoted aluminum disc
placed in two alternating magnetic fields of the same frequency but displaced
in time and space. The torque is produced in the disc by the interaction of one
of the magnetic fields with the currents induced in the disc by the other.
φ1 = Φ1 sin ωt
φ2 = Φ2 sin (ωt + θ),
Where θ is the phase angle by which ø2 leads ø1. It may be
assumed with negligible error
That the paths in which the rotor currents flow have
negligible self-inductance, and hence
That the rotor currents are in phase with their voltages:
iφ1 α dφ1/dtα Φ1 cos ωt
iφ2 α dφ2/dt α Φ2 cos
(ωt + θ)
We note that the two forces in opposition, and consequently
we may write the
Equation for the net force (F) as follows:
F = (F2 – F1) α (φ2iφ1 – φ1iφ2) (1)
Substituting the values of the quantities into equation 1, we
get:
F α Φ1Φ2 [sin (ωt + θ) cos ωt – sin ωt cos (ωt + θ)] (2)
Which reduces to:
F α Φ1Φ2 sin θ
1)
Shaded-pole structure:
the general arrangement consists of a pivoted aluminum disc
free to rotate in the air-gap of an electromagnet.one-half of each pole of the
magnet is surrounded by a copper band known as shading pole. The alternating
flux in the shaded portion of the pole will owing to the reaction of the
current induced in the ring, lag behind the flux in the unshaded portion by an
angle α.thses two a.c. fluxes differing in phase will produce the necessary
torque to rotate the disc.
2) Watt-hour
meter structure:
This structure gets its name for the fact that it is used in
watt-hour metres.it consists of a pivoted aluminum disc arranged to rotate
freely between the poles of two electromagnets. The upper electromagnet carries
two windings; the primary and secondary. The primary windings carries the relay
current while the secondary winding is connected to the lower magnet. The
primary current induces the e.m.f in the secondary turns so circulates a
current in it. The flux induced in the lower magnet by the current in the
secondary winding of the upper magnet will lag behind by an angle @.the two
fluxes differing in phase by @ will produce a driving torque on the disc.
3)
Induction cup structure:
It resembles an induction motor, except that the rotor iron
is stationary, only the rotor conductor portion being free to rotate.
The moving element is a hollow cylindrical rotor which turns
on its axis. The rotating field is produced by two pairs of coils wound on four
poles. The rotating field induces current in the cup to provide the necessary
driving torque.
Important terms
1) Current
setting:
It is often desirable to add just the pick-up current to any
required value. This is known as current setting and is usually achieved by the
use of tapings on the relay operating coil. The taps are brought out to a plug
bridge. The plug bridge permits to alter the number of turns on the relay coil.
This changes the torque on the disc and hence the time of operation of the
relay. The values assigned to each tap are expressed in terms of percentage
full-load rating of C.T. with which the relay is associated and represents the
value above which the disc commences to rotate and finally closes the trip
circuit.
Pick up
current=rated current of c.t. * current setting
2) Plug-setting
multiplier (P.S.M):
It is the ratio of fault current in relay coil to the pick-up
current i.e.
P.S.M = (fault current in relay
coil)/(pick-up current)
3) Time setting
multiplier:
A relay is generally provided with control to adjust the time
of operation. This adjustment is known as time-setting multiplier. The time
setting dial is calibrated from 0 to 1 in steps of 0.05 sec. These figures are
multipliers to be used to convert the time derived from time/P.S.M. curve into
the actual operating time.
Time/P.S.M.
curve
The above figure shows the curve between time of operation
and plug setting multiplier of a typical relay.the horizontal scale is marked
in terms of plug-setting multiplier and represents the number of times the
relay current is in excess of the current setting. The vertical scale is marked
in terms of the time required for relay operation.
Functional relay types
Most of the relays in service on power system today operate
on the principle of electromagnetic attraction or electromagnetic induction.
Regardless of the principle involved, relays are generally classified according
to the function they are called upon to perform in the protection of electric
power circuits. The important types of relays are mainly:
a) Induction type overcurrent relay
b) Induction type reverse power relay
c) Distance relay
d) Differential relay
Induction type overcurrent relay
This type of relay works on the induction principle and
initiates corrective measures when current in the circuit exceeds the
predetermined value. The actuating source is a current in the circuit supplied
to the relay from a current transformer. These relays are used on a.c. circuits
only and can operate for fault current flow in either direction.
Constructional
details:
It consists of a metallic disc which is free to rotate in
between the poles of two electromagnets. The upper electromagnet has a primary
and a secondary winding. The primary is connected to the secondary of a C.T. in
the line to be protected and is tapped at intervals. Thetapings are connected
to a plug-setting bridge by which the number of active turns on the relay
operating coil can be varied, thereby giving the desired current setting. The
secondary winding is energized by induction from primary and is connected in
series with the winding on the lower magnet. The controlling torque is provided
by the spiral spring. The spring of the disc carries a moving contact which
bridges two fixed contacts when the disc rotates through a pre-set angle. This
angle can be adjusted to any value between 0 and 360 degrees.by adjusting this
angle, the travel of the moving contact can be adjusted and hence the relay can
be given any desired time setting.
Operation:
The driving torque on the aluminum disc is set on the
induction principle. This torque is opposed by the restraining torque provided
by the spring. Under normal operatingconditions, restraining torque is greater
than the driving torque produced by the relay coil current. Therefore the
aluminum disc remains stationary. However,if the current in the protected
circuit exceeds the pre-set value, the driving torque becomes greater than the
restraining torque. Consequently the disc rotates and the moving contact
bridges the fixed contacts when the disc has rotated through a pre-set angle.
The trip circuit operates the circuit breaker which isolates the faulty section
Induction
type directional overcurrent relay
The directional power relay discussed above is unsuitable for
use as a directional protective relay under short-circuit conditions. When a
short circuit occurs, the system voltage falls to a low value and there may be
insufficient torque developed in the relay to cause its operation. This
difficulty is overcome in the directional over current relay which is designed
to be almost independent of the system voltage and power factor.
Constructional
details:
It consists of two relay elements mounted on a common case a)
directional element b) non-directional element
1) Directional element:
It is essentially a directional power relay which operates
when power flow in a specific direction. The potential coil in connected
through a potential transformer to the system voltage. The current coil of the
element is energized trough a c.t. by the circuit current. This winding is
carried over the upper magnet of the non-directional element. The trip contacts
of the directional element are connected in series with the secondary circuits
of the over current element. Thus the latter element cannot start to operate
until its secondary circuit is completed.
2) Non-directional element:
The spindle of the disc of this element carries a moving
contact which closes the fixed contact after the operation of the directional
element. Plug setting bridge is also provided in the relay for current setting.
The tapings are provided on the upper magnet of overcurrent element and are
connected on the bridge.
Operation:
Under normal operating condition power flows in the normal
direction in the circuit protected by the relay. Therefore, directional power
relay does not operate, thereby keeping the over current element un energized.
However when a short-circuit occurs there is a tendency for the current or power
to flow in the reverse direction. Should this happen, the disc of the upper
element rotates to bridge the fixed contacts. This completes the circuit for over current element. The disc of this element rotates and the moving contact
attached to it closes the trip circuit. This operates the circuit breaker which
isolates the faulty section
Index
1. SUBSTATION
1.1 Introduction
1.2 Classification
1.3 Indoor Substation
1.4 Outdoor Substation
1.5 Selection and Location of Site
1.6 Equipment of a substation
1.7 Substation Auxiliaries
1.8 Earthing
2. SWITCH
GEAR AND PROTECTION
2.1 CIRCUIT
BREAKERS
2.1.1 Introduction
2.1.2 Arc phenomenon
2.1.3 Principles of arc extinction
2.1.4 Classification of circuit breakers
2.17 Types of circuit breakers
2.2 Protective
Relays
2.2.1 Introduction
2.2.2 Electromagnetic Relays
2.2.3 Induction Relays
2.2.6 Time/TSM curves
2.2.8 Functional Relay type
2.2.9 Induction type over current relay
2.2.10 Induction Type Directional over current relay
Introduction-
Substations serve as source of power for the local areas of
distribution in which these are located. Their main function is to receive
power transmitted at high voltage from generating stations and reduce the
voltage appropriate for local utilization and provide facilities for switching.
Some substation are simply switching substations where different connections
between various transmission lines are made, others are converting substations
which convert either ac to dc or vice versa or change the frequency of the
supply. Substations provide additional features for safety devices. Voltage of
the outgoing feeders can be regulated at a substation. A substation is an ideal
place for installation of synchronous condensers at the end of the transmission
line for the purpose of power factor improvement and to make measurements of
the various operating parts of the interconnected system.
Classification of
substations-
1. Classification on the basis of nature of duties-
1.1 Step-Up or Primary substation
1.2 Primary grid substation
1.3 Step down or distribution substation
2. Classification on the basis of service rendered
2.1 Transformer substation
2.2 Switching substation
2.3 Converting substation
3. Classification based on the operating voltage
3.1 High voltage substation
3.2 Extra high voltage substation
3.3 Ultra high voltage substation
4. Classification on the basis of Importance
4.1 Grid substation
4.2 Town substation
5. Classification on the basis of design
5.1 Outdoor substation
5.2 Indoor substation
Indoor Substation-
In these substations the apparatus installed are within the
substation building. Such substation are erected for a voltage level of 11Kv
but can be erected for 33Kv and 66Kv as well, when the surrounding environment
is contaminated with impurities such as metal corroding,gasses and fumes. The
switchgear on the primary side of the supply will consist mainly of Oil circuit
breakers only. The high voltage supply is given to transformer and different
feeders emerge out of the busbar. The panel for each feeder consists of isolator
switch and circuit breakers, in addition to isolators and circuit breakers the panel
also consists of different measuring instruments. The auxiliaries of indoor
substation are Dc supply and different fire extinguishing arrangements
Outdoor
substation-
Outdoor substation are of two types 1) pole mounted and 2)
foundation mounted substation
1) Pole-Mounted substation:-Such substation are erected for
mounting distribution oftransformers of capacity of 250 kVA.Such substations
are cheapest simplest and smallest of any kind of substations. All the equipment
are mounted on the supporting structure of HT distribution line. Triple pole
mechanically operated switch is used for ON and OFF control of HT line.ht fuse
is used, to control LT side iron clad low tension switch is used for suitable capacity.
The substation is switched at two or more points.
2) Foundation mounted substation:-these substation are built
entirely I open and fenced from the point of view of safety. Substation for
primary and secondary transmission and for secondary distribution, (above
250kVA) are foundation substation .since instruments required for such
substations are heavy ,therefore sites selected for such substation must
have good transport facility. Owing to
exposed bus bars and other associated equipment the clearance and spacing is
important and are not only governed by the operating voltage but also by the
encroachment from the outside.
Selection and Location of
Site
The locational requirements for substations are site specific, and are
determined by practicalities of engineering constraints, connection costs,
environmental issues and impacts on social attributes. Generally, the location
in which a substation is to be sited is dictated by load location and
transmission line arrangements for connection of the substation to the grid,
and the substation to the place where the connection is required. Access to the
substation sites must be capable under all but the most extreme conditions.
The following key issues are taken into
consideration:-(1)General location;(2)Development;(3)Physical;(4)Surrounding land usage;(5)Accessibility;(6)Cost;(7)Customer requirements;(8)Environmental
considerations; and (9)Community
considerations.(10)General access to
the substations for construction, operation and maintenance.
Equipment of a substation
1)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.
2)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.
3) Instrument
Transformer-Instrument
transformers are used to step-down the current or voltage to measurable values.
They provide standardized, usable 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.
4)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.
5)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.
6)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.
7)Isolator-The use of this isolator is to
protect the transformer and the other instrument in the line. The isolator
isolates the extra voltage to the ground and thus any extra voltage cannot
enter the line. Thus an isolator is used after the bus also for protection.
8)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.
9)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
Substation
Auxiliaries
1)
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.
2)Auxiliary
Transformers
Auxiliary transformers shall be
adequately rated to suit the requirements of the substation.The type, number,
vector group and rating shall be determined by the designer to suit the Specific
site requirements. The auxiliary transformer shall not be more than 500kVA.
Substation
Earthing
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.
Circuit Breaker
Introduction-circuit breaker is a switching device which can be operated manually as well
as automatically for controlling and protection of electrical power system
respectively. As the modern power system deals with huge currents, special attention
should be given during designing of circuit breaker to safe interruption of arc produced
during the operation
of circuit breaker. This was the basic definition of circuit breaker.
Arc Phenomenon-During opening
of current carrying contacts in a circuit breaker the medium in between opening contacts become highly ionized
through which the interrupting current gets low resistive path and continues to
flow through this path even the contacts are physically separated. During the
flowing of electric current from one contact to other the path becomes so
heated that it glows. This is called arc.
Whenever,
on load current contacts of circuit breaker open there is an arc in circuit breaker,
established between the separating contacts. As long as this arc is sustained
in between the contacts the current through the circuit breaker will not be
interrupted finally as because arc is itself a conductive path of electricity.
For total interruption of current the circuit breaker it is essential to quench
the arc as quick as possible. The main designing criteria of a circuit breaker
is to provide appropriate technology of arc quenching in circuit breaker to fulfill quick
and safe current interruption
Principles
of arc extinction-
1) Heat Loss from Arc-heat loss from arc in circuit
breaker is taken place through conduction, convection as well as radiation. In
circuit breaker with plain break arc in oil, arc in chutes or narrow slots
nearly all the heat loss due to conduction.
2)Deionization of Gas due to Increasing Pressure-If
pressure of the arc path increases, the density of the ionized gas is increased
which means, the particles in the gas come closer to each other and as a result
the mean free path of the particles is reduced.
3)Deionization of Gas due to Decreasing Temperature-The
rate of ionization of gas depends upon the intensity of impact during collision
of gas particles. The intensity of impact during collision of particles again
depends upon velocity of random motions of the particles.
Types of circuit breaker
.According to their arc quenching media the circuit breaker can be
divided as-
1) Oil circuit breaker.
2) Air circuit breaker.
3) SF6 circuit breaker.
4)Vacuum circuit breaker.
According to their services the circuit breaker can be divided as-
1) Outdoor circuit breaker
2) Indoor breaker.
According to the operating mechanism of circuit breaker they can
be divided as-
1) Spring operated circuit breaker.
2) Pneumatic circuit breaker.
3) Hydraulic circuit breaker.
According to the voltage level of installation types of
circuit breaker are referred as-
1) High voltage circuit breaker.
2) Medium voltage circuit breaker.
3) Low voltage circuit breaker.
Oil Circuit Breaker-
Mineral oil has better insulating
property than air. In oil circuit breaker the fixed
contact and moving contact are immerged inside the insulating oil. Whenever
there is a separation of electric current carrying contacts in the oil, the arc in circuit
breaker is initialized at the moment of separation of
contacts, and due to this arc the oil is vaporized and decomposed in mostly
hydrogen gas and ultimately creates a hydrogen bubble around the arc. This
highly compressed gas bubble around the arc prevents re-striking of the arc
after current reaches zero crossing of the cycle. The oil
circuit breaker is
the one of the oldest type of circuit breakers.
Air
Circuit Breaker-
This type of
circuit breakers, is those kind
of circuit
breaker which operates in air at atmospheric pressure.
After development of oil circuit breaker,
the medium voltage air circuit breaker (ACB) is replaced completely by oil circuit
breaker in different countries. But in countries like
France and Italy, ACBs are still preferable choice up to voltage 15 KV. It is
also good choice to avoid the risk of oil fire, in case of oil circuit
breaker. In America ACBs were exclusively used for the
system up to 15 KV until the development of new vacuum and SF6circuit
breakers.
Vacuum Circuit Breaker-
A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is
suitable for mainly medium voltage application. For higher voltage vacuum
technology has been developed but not commercially viable. The operation of
opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the
breaker which is called vacuum interrupter. The vacuum interrupter consists of
a steel arc chamber in the center symmetrically arranged ceramic insulators.
The vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6 bar. The material used for current
carrying contacts plays an important role in the performance of the vacuum circuit breaker. CuCr is the most ideal material
to make VCB contacts. Vacuum interrupter technology was first introduced in the
year of 1960. But still it is a developing technology. As time goes on, the
size of the vacuum interrupter is being reducing from its early 1960’s size due
to different technical developments in this field of engineering. The contact
geometry is also improving with time, from butt contact of early days it
gradually changes to spiral shape, cup shape and axial magnetic field contact.
The vacuum circuit breaker is today recognized as
most reliable current interruption technology for medium
voltage switchgear. It requires minimum maintenance compared to other
circuit breaker technologies.
Sulphur
Hexafluoride Circuit Breaker-
A
circuit breaker in which the current carrying contacts operate in sulphur hexafluoride or SF6 gas is
known as anSF6 circuit breaker.SF6 has excellent insulating property. SF6has high electro-negativity. That means it has high
affinity of absorbing free electron. Whenever a free electron collides with the
SF6 gas molecule, it is absorbed by that gas
molecule and forms a negative ion.The attachment of electron with SF6 gas molecules may occur in two
different ways,
Hence,
for heavier and less mobile charged particles in SF6 gas,
it acquires very high dielectric strength. Not only the gas has a good
dielectric strength but also it has the unique property of fast recombination
after the source energizing the spark is removed. The gas has also very good
heat transfer property. Due to its low gaseous viscosity (because of less
molecular mobility) SF6 gas can efficiently
transfer heat by convection. So due to its high dielectric strength and high
cooling effect SF6 gas is approximately 100
times more effective arc quenching media than air. Due to these unique
properties of this gas SF6 circuit
breaker is used in complete range of medium voltage and high
voltage electrical power system.
RELAY
INTRODUCTION
A protective relay is a device that detects the fault and
initiates the operation of the circuit breaker to isolate the defective element
from the rest of the system.
The relay detects the abnormal conditions in the electrical
circuits by constantly measuring the electrical quantities which are different
under normal and fault conditions. The electrical quantities which may change
under fault conditions are voltage,current,frequency,phase angle. Through the
changes in one or more of these quantities, the fault signals their presence,
type and location to the protective relays. Having detected the fault, the
relay operates to close the trip circuit of the breaker. This results in the
opening of the breaker and disconnection of the faulty circuit.
A typical relay circuit is shown below:
This diagram shows one phase of 3-phase system for
simplicity. The relay circuit connections can be divided into three parts viz.
1) The primary winding of a current transformer connected in
series with line to be protected.
2) Secondary winding of current transformer and the relay
operating coil
3) Tripping circuit which may be either a.c or d.c it
consists of a source of supply, the trip coil of the circuit breaker and the
relay stationary contacts.
When a short circuit occurs on the transmission line, the
current flowing in the line increases to an enormous value. This results in a
heavy current flow through the relay coil, causing the relay to operate by
closing its contacts. This in turn closes the trip circuit of the breaker,
making the circuit breaker open and isolating the faulty section from the rest
of system. Thus ensuring safety of circuit equipment from damage.
Electromagnetic
Relay
Electromagnetic relay operate by virtue of an armature being
attracted to the poles of an electromagnet or a plunger being drawn into a
solenoid. Such relays may be actuated by d.c. or a.c. quantities.it consists of
three types.
1)
Attracted armature type relay:
It consists of a laminated electromagnet carrying a coil and
a pivoted laminated armature. The armature is balanced by a counterweight and
carries a pair of spring contact fingers at its free end. Under normal
operating condition the current through the relay coil is such that
counterweight holds the armature in the position shown. However when a short
circuit occurs the current through the relay coil increases sufficiently and
the relay armature is attracted upwards. The contacts on the relay armature
bridge a pair of stationary contacts attached to the relay frame. This
completes the trip circuit which results in the opening of the circuit breaker
and therefore in the disconnection of the faulty circuit
2) Solenoid type
relay:
It consists of a solenoid and movable iron plunger. Under
normal operating conditions, the current through the relay coil c is such that
it holds the plunger by gravity or spring in the position shown. However, on
the occurrence of a fault, the current through the relay coil becomes more than
the pickup value, causing the plunger to be attached to the solenoid. The
upward movement of the plunger closes the trip circuit, thus opening the
circuit breaker and disconnecting the faulty circuit.
3) Balanced beam
type relay:
It consists of an iron armature fastened to a balanced beam.
Under normal operating conditions the current through the relay coil is such
that the beam is held in the horizontal position by the spring. However when a
fault occurs, the current through the relay coil becomes greater than the
pickup value and the beam is attracted to close the trip circuit. This causes
the opening of the circuit breaker to isolate faulty circuit.
Induction
Relay
Electromagnetic induction relay operate on the principle of
induction motor and are widely used for protective relaying purposes involving
a.c. quantities. They are not used with d.c. quantities owing to the principle
of operation.an induction relay essentially consists of a pivoted aluminum disc
placed in two alternating magnetic fields of the same frequency but displaced
in time and space. The torque is produced in the disc by the interaction of one
of the magnetic fields with the currents induced in the disc by the other.
φ1 = Φ1 sin ωt
φ2 = Φ2 sin (ωt + θ),
Where θ is the phase angle by which ø2 leads ø1. It may be
assumed with negligible error
That the paths in which the rotor currents flow have
negligible self-inductance, and hence
That the rotor currents are in phase with their voltages:
iφ1 α dφ1/dtα Φ1 cos ωt
iφ2 α dφ2/dt α Φ2 cos
(ωt + θ)
We note that the two forces in opposition, and consequently
we may write the
Equation for the net force (F) as follows:
F = (F2 – F1) α (φ2iφ1 – φ1iφ2) (1)
Substituting the values of the quantities into equation 1, we
get:
F α Φ1Φ2 [sin (ωt + θ) cos ωt – sin ωt cos (ωt + θ)] (2)
Which reduces to:
F α Φ1Φ2 sin θ
1)
Shaded-pole structure:
the general arrangement consists of a pivoted aluminum disc
free to rotate in the air-gap of an electromagnet.one-half of each pole of the
magnet is surrounded by a copper band known as shading pole. The alternating
flux in the shaded portion of the pole will owing to the reaction of the
current induced in the ring, lag behind the flux in the unshaded portion by an
angle α.thses two a.c. fluxes differing in phase will produce the necessary
torque to rotate the disc.
2) Watt-hour
meter structure:
This structure gets its name for the fact that it is used in
watt-hour metres.it consists of a pivoted aluminum disc arranged to rotate
freely between the poles of two electromagnets. The upper electromagnet carries
two windings; the primary and secondary. The primary windings carries the relay
current while the secondary winding is connected to the lower magnet. The
primary current induces the e.m.f in the secondary turns so circulates a
current in it. The flux induced in the lower magnet by the current in the
secondary winding of the upper magnet will lag behind by an angle @.the two
fluxes differing in phase by @ will produce a driving torque on the disc.
3)
Induction cup structure:
It resembles an induction motor, except that the rotor iron
is stationary, only the rotor conductor portion being free to rotate.
The moving element is a hollow cylindrical rotor which turns
on its axis. The rotating field is produced by two pairs of coils wound on four
poles. The rotating field induces current in the cup to provide the necessary
driving torque.
Important terms
1) Current
setting:
It is often desirable to add just the pick-up current to any
required value. This is known as current setting and is usually achieved by the
use of tapings on the relay operating coil. The taps are brought out to a plug
bridge. The plug bridge permits to alter the number of turns on the relay coil.
This changes the torque on the disc and hence the time of operation of the
relay. The values assigned to each tap are expressed in terms of percentage
full-load rating of C.T. with which the relay is associated and represents the
value above which the disc commences to rotate and finally closes the trip
circuit.
Pick up
current=rated current of c.t. * current setting
2) Plug-setting
multiplier (P.S.M):
It is the ratio of fault current in relay coil to the pick-up
current i.e.
P.S.M = (fault current in relay
coil)/(pick-up current)
3) Time setting
multiplier:
A relay is generally provided with control to adjust the time
of operation. This adjustment is known as time-setting multiplier. The time
setting dial is calibrated from 0 to 1 in steps of 0.05 sec. These figures are
multipliers to be used to convert the time derived from time/P.S.M. curve into
the actual operating time.
Time/P.S.M.
curve
The above figure shows the curve between time of operation
and plug setting multiplier of a typical relay.the horizontal scale is marked
in terms of plug-setting multiplier and represents the number of times the
relay current is in excess of the current setting. The vertical scale is marked
in terms of the time required for relay operation.
Functional relay types
Most of the relays in service on power system today operate
on the principle of electromagnetic attraction or electromagnetic induction.
Regardless of the principle involved, relays are generally classified according
to the function they are called upon to perform in the protection of electric
power circuits. The important types of relays are mainly:
a) Induction type overcurrent relay
b) Induction type reverse power relay
c) Distance relay
d) Differential relay
Induction type overcurrent relay
This type of relay works on the induction principle and
initiates corrective measures when current in the circuit exceeds the
predetermined value. The actuating source is a current in the circuit supplied
to the relay from a current transformer. These relays are used on a.c. circuits
only and can operate for fault current flow in either direction.
Constructional
details:
It consists of a metallic disc which is free to rotate in
between the poles of two electromagnets. The upper electromagnet has a primary
and a secondary winding. The primary is connected to the secondary of a C.T. in
the line to be protected and is tapped at intervals. The tapings are connected
to a plug-setting bridge by which the number of active turns on the relay
operating coil can be varied, thereby giving the desired current setting. The
secondary winding is energized by induction from primary and is connected in
series with the winding on the lower magnet. The controlling torque is provided
by the spiral spring. The spring of the disc carries a moving contact which
bridges two fixed contacts when the disc rotates through a pre-set angle. This
angle can be adjusted to any value between 0 and 360 degrees.by adjusting this
angle, the travel of the moving contact can be adjusted and hence the relay can
be given any desired time setting.
Operation:
The driving torque on the aluminium disc is set on the
induction principle. This torque is opposed by the restraining torque provided
by the spring. Under normal operating conditions, restraining torque is greater
than the driving torque produced by the relay coil current. Therefore the aluminium disc remains stationary. However,if the current in the protected
circuit exceeds the pre-set value, the driving torque becomes greater than the
restraining torque. Consequently the disc rotates and the moving contact
bridges the fixed contacts when the disc has rotated through a pre-set angle.
The trip circuit operates the circuit breaker which isolates the faulty section
Induction
type directional overcurrent relay
The directional power relay discussed above is unsuitable for
use as a directional protective relay under short-circuit conditions. When a
short circuit occurs, the system voltage falls to a low value and there may be
insufficient torque developed in the relay to cause its operation. This
difficulty is overcome in the directional over current relay which is designed
to be almost independent of the system voltage and power factor.
Constructional
details:
It consists of two relay elements mounted on a common case a)
directional element b) non-directional element
1) Directional element:
It is essentially a directional power relay which operates
when power flow in a specific direction. The potential coil in connected
through a potential transformer to the system voltage. The current coil of the
element is energized trough a c.t. by the circuit current. This winding is
carried over the upper magnet of the non-directional element. The trip contacts
of the directional element are connected in series with the secondary circuits
of the over current element. Thus the latter element cannot start to operate
until its secondary circuit is completed.
2) Non-directional element:
The spindle of the disc of this element carries a moving
contact which closes the fixed contact after the operation of the directional
element. Plug setting bridge is also provided in the relay for current setting.
The tapings are provided on the upper magnet of over current element and are
connected on the bridge.
Operation:
Under normal operating condition power flows in the normal
direction in the circuit protected by the relay. Therefore, directional power
relay does not operate, thereby keeping the over current element un energized.
However when a short-circuit occurs there is a tendency for the current or power
to flow in the reverse direction. Should this happen, the disc of the upper
element rotates to bridge the fixed contacts. This completes the circuit for over current element. The disc of this element rotates and the moving contact
attached to it closes the trip circuit. This operates the circuit breaker which
isolates the faulty section
Index
1. SUBSTATION
1.1 Introduction
1.2 Classification
1.3 Indoor Substation
1.4 Outdoor Substation
1.5Selection and Location of Site
1.6Equipment of a substation
1.7 Substation Auxiliaries
1.8Earthing
2. SWITCH
GEAR AND PROTECTION
2.1CIRCUIT
BREAKERS
2.1.1Introduction
2.1.2Arc phenomenon
2.1.3Principles of arc extinction
2.1.4Classification of circuit breakers
2.17Types of circuit breakers
2.2Protective
Relays
2.2.1 Introduction
2.2.2Electromagnetic Relays
2.2.3Induction Relays
2.2.6Time/TSM curves
2.2.8Functional Relay type
2.2.9 Induction type over current relay
2.2.10Induction Type Directional over current relay
Introduction-
Substations serve as source of power for the local areas of
distribution in which these are located. Their main function is to receive
power transmitted at high voltage from generating stations and reduce the
voltage appropriate for local utilization and provide facilities for switching.
Some substation are simply switching substations where different connections
between various transmission lines are made, others are converting substations
which convert either ac to dc or vice versa or change the frequency of the
supply. Substations provide additional features for safety devices. Voltage of
the outgoing feeders can be regulated at a substation. A substation is an ideal
place for installation of synchronous condensers at the end of the transmission
line for the purpose of power factor improvement and to make measurements of
the various operating parts of the interconnected system.
Classification of
substations-
1. Classification on the basis of nature of duties-
1.1Step-Up or Primary substation
1.2Primary grid substation
1.3Step down or distribution substation
2. Classification on the basis of service rendered
2.1Transformer substation
2.2Switching substation
2.3Converting substation
3. Classification based on the operating voltage
3.1High voltage substation
3.2Extra high voltage substation
3.3Ultra high voltage substation
4. Classification on the basis of Importance
4.1Grid substation
4.2Town substation
5. Classification on the basis of design
5.1Outdoor substation
5.2Indoor substation
Indoor Substation-
In these substations the apparatus installed are within the
substation building. Such substation are erected for a voltage level of 11Kv
but can be erected for 33Kv and 66Kv as well, when the surrounding environment
iscontaminated with impurities such as metal corroding,gasses and fumes. The
switchgear on the primary side of the supply will consist mainly of Oil circuit
breakers only. The high voltage supply is given to transformer and different
feeders emerge out of the busbar.The panel for each feeder consists of isolator
switch and circuit breakers, in addition to isolators and circuit breakers the panel
also consists of different measuring instruments. Theauxiliaries of indoor
substation are Dc supply and different fire extinguishing arrangements
Outdoor
substation-
Outdoor substation are of two types 1) pole mounted and 2)
foundation mounted substation
1) Pole-Mounted substation:-Such substation are erected for
mounting distribution oftransformers of capacity of 250 kVA.Such substations
are cheapest simplest and smallest of any kind of substations. All the equipment
are mounted on the supporting structure of HT distribution line. Triple pole
mechanically operated switch is used for ON and OFF control of HT line.ht fuse
is used, to control LT side iron clad low tension switch is used for suitable capacity.
The substation is switched at two or more points.
2) Foundation mounted substation:-these substation are built
entirely I open and fenced from the point of view of safety. Substation for
primary and secondary transmission and for secondary distribution, (above
250kVA) are foundation substation .since instruments required for such
substations are heavy ,therefore sites selected for such substation must
have good transport facility. Owing to
exposed bus bars and other associated equipment the clearance and spacing is
important and are not only governed by the operating voltage but also by the
encroachment from the outside.
Selection and Location of
Site
The locational requirements for substations are site specific, and are
determined by practicalities of engineering constraints, connection costs,
environmental issues and impacts on social attributes. Generally, the location
in which a substation is to be sited is dictated by load location and
transmission line arrangements for connection of the substation to the grid,
and the substation to the place where the connection is required. Access to the
substation sites must be capable under all but the most extreme conditions.
The following key issues are taken into
consideration:-(1)General location;(2)Development;(3)Physical;(4)Surrounding land usage;(5)Accessibility;(6)Cost;(7)Customer requirements;(8)Environmental
considerations; and (9)Community
considerations.(10)General access to
the substations for construction, operation and maintenance.
Equipment of a substation
1)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.
2)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.
3) 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.
4)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.
5)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.
6)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.
7)Isolator-The use of this isolator is to
protect the transformer and the other instrument in the line. The isolator
isolates the extra voltage to the ground and thus any extra voltage cannot
enter the line. Thus an isolator is used after the bus also for protection.
8)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.
9)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
Substation
Auxiliaries
1)
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.
2)Auxiliary
Transformers
Auxiliary transformers shall be
adequately rated to suit the requirements of the substation.The type, number,
vector group and rating shall be determined by the designer to suit the Specific
site requirements. The auxiliary transformer shall not be more than 500kVA.
Substation
Earthing
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.
Circuit Breaker
Introduction-circuit breaker is a switching device which can be operated manually as well
as automatically for controlling and protection of electrical power system
respectively. As the modern power system deals with huge currents, special attention
should be given during designing of circuit breaker to safe interruption of arc produced
during the operation
of circuit breaker. This was the basic definition of circuit breaker.
Arc Phenomenon-During opening
of current carrying contacts in a circuit breaker the medium in between opening contacts become highly ionized
through which the interrupting current gets low resistive path and continues to
flow through this path even the contacts are physically separated. During the
flowing of electric current from one contact to other the path becomes so
heated that it glows. This is called arc.
Whenever,
on load current contacts of circuit breaker open there is an arc in circuit breaker,
established between the separating contacts. As long as this arc is sustained
in between the contacts the current through the circuit breaker will not be
interrupted finally as because arc is itself a conductive path of electricity.
For total interruption of current the circuit breaker it is essential to quench
the arc as quick as possible. The main designing criteria of a circuit breaker
is to provide appropriate technology of arc quenching in circuit breaker to fulfil quick
and safe current interruption
Principles
of arc extinction-
1) Heat Loss from Arc-heat loss from arc in circuit
breaker is taken place through conduction, convection as well as radiation. In
circuit breaker with plain break arc in oil, arc in chutes or narrow slots
nearly all the heat loss due to conduction.
2)De ionization of Gas due to Increasing Pressure-If
pressure of the arc path increases, the density of the ionized gas is increased
which means, the particles in the gas come closer to each other and as a result
the mean free path of the particles is reduced.
3)De ionization of Gas due to Decreasing Temperature-The
rate of ionization of gas depends upon the intensity of impact during collision
of gas particles. The intensity of impact during collision of particles again
depends upon velocity of random motions of the particles.
Types of circuit breaker
.According to their arc quenching media the circuit breaker can be
divided as-
1) Oil circuit breaker.
2) Air circuit breaker.
3) SF6 circuit breaker.
4)Vacuum circuit breaker.
According to their services the circuit breaker can be divided as-
1) Outdoor circuit breaker
2) Indoor breaker.
According to the operating mechanism of circuit breaker they can
be divided as-
1) Spring operated circuit breaker.
2) Pneumatic circuit breaker.
3) Hydraulic circuit breaker.
According to the voltage level of installation types of
circuit breaker are referred as-
1) High voltage circuit breaker.
2) Medium voltage circuit breaker.
3) Low voltage circuit breaker.
Oil Circuit Breaker-
Mineral oil has better insulating
property than air. In oil circuit breaker the fixed
contact and moving contact are immerged inside the insulating oil. Whenever
there is a separation of electric current carrying contacts in the oil, the arc in circuit
breaker is initialized at the moment of separation of
contacts, and due to this arc the oil is vaporized and decomposed in mostly
hydrogen gas and ultimately creates a hydrogen bubble around the arc. This
highly compressed gas bubble around the arc prevents re-striking of the arc
after current reaches zero crossing of the cycle. The oil
circuit breaker is
the one of the oldest type of circuit breakers.
Air
Circuit Breaker-
This type of
circuit breakers, is those kind
of circuit
breaker which operates in air at atmospheric pressure.
After development of oil circuit breaker,
the medium voltage air circuit breaker (ACB) is replaced completely by oil circuit
breaker in different countries. But in countries like
France and Italy, ACBs are still preferable choice up to voltage 15 KV. It is
also good choice to avoid the risk of oil fire, in case of oil circuit
breaker. In America ACBs were exclusively used for the
system up to 15 KV until the development of new vacuum and SF6circuit
breakers.
Vacuum Circuit Breaker-
A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is
suitable for mainly medium voltage application. For higher voltage vacuum
technology has been developed but not commercially viable. The operation of
opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the
breaker which is called vacuum interrupter. The vacuum interrupter consists of
a steel arc chamber in the center symmetrically arranged ceramic insulators.
The vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6 bar. The material used for current
carrying contacts plays an important role in the performance of the vacuum circuit breaker. CuCr is the most ideal material
to make VCB contacts. Vacuum interrupter technology was first introduced in the
year of 1960. But still it is a developing technology. As time goes on, the
size of the vacuum interrupter is being reducing from its early 1960’s size due
to different technical developments in this field of engineering. The contact
geometry is also improving with time, from butt contact of early days it
gradually changes to spiral shape, cup shape and axial magnetic field contact.
The vacuum circuit breaker is today recognized as
most reliable current interruption technology for medium
voltage switchgear. It requires minimum maintenance compared to other
circuit breaker technologies.
Sulphur
Hexafluoride Circuit Breaker-
A
circuit breaker in which the current carrying contacts operate in sulphur hexafluoride or SF6 gas is
known as an SF6 circuit breaker. SF6 has excellent insulating property. SF6 has high electro-negativity. That means it has high
affinity of absorbing free electron. Whenever a free electron collides with the
SF6 gas molecule, it is absorbed by that gas
molecule and forms a negative ion.The attachment of electron with SF6 gas molecules may occur in two
different ways,
Hence,
for heavier and less mobile charged particles in SF6 gas,
it acquires very high dielectric strength. Not only the gas has a good
dielectric strength but also it has the unique property of fast recombination
after the source energizing the spark is removed. The gas has also very good
heat transfer property. Due to its low gaseous viscosity (because of less
molecular mobility) SF6 gas can efficiently
transfer heat by convection. So due to its high dielectric strength and high
cooling effect SF6 gas is approximately 100
times more effective arc quenching media than air. Due to these unique
properties of this gas SF6 circuit
breaker is used in complete range of medium voltage and high
voltage electrical power system.
RELAY
INTRODUCTION
A protective relay is a device that detects the fault and
initiates the operation of the circuit breaker to isolate the defective element
from the rest of the system.
The relay detects the abnormal conditions in the electrical
circuits by constantly measuring the electrical quantities which are different
under normal and fault conditions. The electrical quantities which may change
under fault conditions are voltage,current,frequency,phase angle. Through the
changes in one or more of these quantities, the fault signals their presence,
type and location to the protective relays. Having detected the fault, the
relay operates to close the trip circuit of the breaker. This results in the
opening of the breaker and disconnection of the faulty circuit.
A typical relay circuit is shown below:
This diagram shows one phase of 3-phase system for
simplicity. The relay circuit connections can be divided into three parts viz.
1) The primary winding of a current transformer connected in
series with line to be protected.
2) Secondary winding of current transformer and the relay
operating coil
3) Tripping circuit which may be either a.c or d.c it
consists of a source of supply, the trip coil of the circuit breaker and the
relay stationary contacts.
When a short circuit occurs on the transmission line, the
current flowing in the line increases to an enormous value. This results in a
heavy current flow through the relay coil, causing the relay to operate by
closing its contacts. This in turn closes the trip circuit of the breaker,
making the circuit breaker open and isolating the faulty section from the rest
of system. Thus ensuring safety of circuit equipment from damage.
Electromagnetic
Relay
Electromagnetic relay operate by virtue of an armature being
attracted to the poles of an electromagnet or a plunger being drawn into a
solenoid. Such relays may be actuated by d.c. or a.c. quantities.it consists of
three types.
1)
Attracted armature type relay:
It consists of a laminated electromagnet carrying a coil and
a pivoted laminated armature. The armature is balanced by a counterweight and
carries a pair of spring contact fingers at its free end. Under normal
operating condition the current through the relay coil is such that
counterweight holds the armature in the position shown. However when a short
circuit occurs the current through the relay coil increases sufficiently and
the relay armature is attracted upwards. The contacts on the relay armature
bridge a pair of stationary contacts attached to the relay frame. This
completes the trip circuit which results in the opening of the circuit breaker
and therefore in the disconnection of the faulty circuit.
2) Solenoid type
relay:
It consists of a solenoid and movable iron plunger. Under
normal operating conditions, the current through the relay coil c is such that
it holds the plunger by gravity or spring in the position shown. However, on
the occurrence of a fault, the current through the relay coil becomes more than
the pickup value, causing the plunger to be attached to the solenoid. The
upward movement of the plunger closes the trip circuit, thus opening the
circuit breaker and disconnecting the faulty circuit.
3) Balanced beam
type relay:
It consists of an iron armature fastened to a balanced beam.
Under normal operating conditions the current through the relay coil is such
that the beam is held in the horizontal position by the spring. However when a
fault occurs, the current through the relay coil becomes greater than the
pickup value and the beam is attracted to close the trip circuit. This causes
the opening of the circuit breaker to isolate faulty circuit.
Induction
Relay
Electromagnetic induction relay operate on the principle of
induction motor and are widely used for protective relaying purposes involving
a.c. quantities. They are not used with d.c. quantities owing to the principle
of operation.an induction relay essentially consists of a pivoted aluminum disc
placed in two alternating magnetic fields of the same frequency but displaced
in time and space. The torque is produced in the disc by the interaction of one
of the magnetic fields with the currents induced in the disc by the other.
φ1 = Φ1 sin ωt
φ2 = Φ2 sin (ωt + θ),
Where θ is the phase angle by which ø2 leads ø1. It may be
assumed with negligible error
That the paths in which the rotor currents flow have
negligible self-inductance, and hence
That the rotor currents are in phase with their voltages:
iφ1 α dφ1/dtα Φ1 cos ωt
iφ2 α dφ2/dt α Φ2 cos
(ωt + θ)
We note that the two forces in opposition, and consequently
we may write the
Equation for the net force (F) as follows:
F = (F2 – F1) α (φ2iφ1 – φ1iφ2) (1)
Substituting the values of the quantities into equation 1, we
get:
F α Φ1Φ2 [sin (ωt + θ) cos ωt – sin ωt cos (ωt + θ)] (2)
Which reduces to:
F α Φ1Φ2 sin θ
1)
Shaded-pole structure:
the general arrangement consists of a pivoted aluminium disc
free to rotate in the air-gap of an electromagnet.one-half of each pole of the
magnet is surrounded by a copper band known as shading pole. The alternating
flux in the shaded portion of the pole will owing to the reaction of the
current induced in the ring, lag behind the flux in the unshaded portion by an
angle α.these two a.c. fluxes differing in phase will produce the necessary
torque to rotate the disc.
2) Watt-hour
meter structure:
This structure gets its name for the fact that it is used in
watt-hour metres.it consists of a pivoted aluminium disc arranged to rotate
freely between the poles of two electromagnets. The upper electromagnet carries
two windings; the primary and secondary. The primary windings carries the relay
current while the secondary winding is connected to the lower magnet. The
primary current induces the e.m.f in the secondary turns so circulates a
current in it. The flux induced in the lower magnet by the current in the
secondary winding of the upper magnet will lag behind by an angle @.the two
fluxes differing in phase by @ will produce a driving torque on the disc.
3)
Induction cup structure:
It resembles an induction motor, except that the rotor iron
is stationary, only the rotor conductor portion being free to rotate.
The moving element is a hollow cylindrical rotor which turns
on its axis. The rotating field is produced by two pairs of coils wound on four
poles. The rotating field induces current in the cup to provide the necessary
driving torque.
Important terms
1) Current
setting:
It is often desirable to add just the pick-up current to any
required value. This is known as current setting and is usually achieved by the
use of taping on the relay operating coil. The taps are brought out to a plug
bridge. The plug bridge permits to alter the number of turns on the relay coil.
This changes the torque on the disc and hence the time of operation of the
relay. The values assigned to each tap are expressed in terms of percentage
full-load rating of C.T. with which the relay is associated and represents the
value above which the disc commences to rotate and finally closes the trip
circuit.
Pick up
current=rated current of c.t. * current setting
2) Plug-setting
multiplier (P.S.M):
It is the ratio of fault current in relay coil to the pick-up
current i.e.
P.S.M = (fault current in relay
coil)/(pick-up current)
3) Time setting
multiplier:
A relay is generally provided with control to adjust the time
of operation. This adjustment is known as time-setting multiplier. The time
setting dial is calibrated from 0 to 1 in steps of 0.05 sec. These figures are
multipliers to be used to convert the time derived from time/P.S.M. curve into
the actual operating time.
Time/P.S.M.
curve
The above figure shows the curve between time of operation
and plug setting multiplier of a typical relay.the horizontal scale is marked
in terms of plug-setting multiplier and represents the number of times the
relay current is in excess of the current setting. The vertical scale is marked
in terms of the time required for relay operation.
Functional relay types
Most of the relays in service on power system today operate
on the principle of electromagnetic attraction or electromagnetic induction.
Regardless of the principle involved, relays are generally classified according
to the function they are called upon to perform in the protection of electric
power circuits. The important types of relays are mainly:
a) Induction type over current relay
b) Induction type reverse power relay
c) Distance relay
d) Differential relay
Induction type over current relay
This type of relay works on the induction principle and
initiates corrective measures when current in the circuit exceeds the
predetermined value. The actuating source is a current in the circuit supplied
to the relay from a current transformer. These relays are used on a.c. circuits
only and can operate for fault current flow in either direction.
Constructional
details:
It consists of a metallic disc which is free to rotate in
between the poles of two electromagnets. The upper electromagnet has a primary
and a secondary winding. The primary is connected to the secondary of a C.T. in
the line to be protected and is tapped at intervals. The taping are connected
to a plug-setting bridge by which the number of active turns on the relay
operating coil can be varied, thereby giving the desired current setting. The
secondary winding is energized by induction from primary and is connected in
series with the winding on the lower magnet. The controlling torque is provided
by the spiral spring. The spring of the disc carries a moving contact which
bridges two fixed contacts when the disc rotates through a pre-set angle. This
angle can be adjusted to any value between 0 and 360 degrees.by adjusting this
angle, the travel of the moving contact can be adjusted and hence the relay can
be given any desired time setting.
Operation:
The driving torque on the aluminium disc is set on the
induction principle. This torque is opposed by the restraining torque provided
by the spring. Under normal operating conditions, restraining torque is greater
than the driving torque produced by the relay coil current. Therefore the aluminium disc remains stationary. However,if the current in the protected
circuit exceeds the pre-set value, the driving torque becomes greater than the
restraining torque. Consequently the disc rotates and the moving contact
bridges the fixed contacts when the disc has rotated through a pre-set angle.
The trip circuit operates the circuit breaker which isolates the faulty section
Induction
type directional over current relay
The directional power relay discussed above is unsuitable for
use as a directional protective relay under short-circuit conditions. When a
short circuit occurs, the system voltage falls to a low value and there may be
insufficient torque developed in the relay to cause its operation. This
difficulty is overcome in the directional over current relay which is designed
to be almost independent of the system voltage and power factor.
Constructional
details:
It consists of two relay elements mounted on a common case a)
directional element b) non-directional element
1) Directional element:
It is essentially a directional power relay which operates
when power flow in a specific direction. The potential coil in connected
through a potential transformer to the system voltage. The current coil of the
element is energized trough a c.t. by the circuit current. This winding is
carried over the upper magnet of the non-directional element. The trip contacts
of the directional element are connected in series with the secondary circuits
of the over current element. Thus the latter element cannot start to operate
until its secondary circuit is completed.
2) Non-directional element:
The spindle of the disc of this element carries a moving
contact which closes the fixed contact after the operation of the directional
element. Plug setting bridge is also provided in the relay for current setting.
The taping are provided on the upper magnet of over current element and are
connected on the bridge.
Operation:
Under normal operating condition power flows in the normal
direction in the circuit protected by the relay. Therefore, directional power
relay does not operate, thereby keeping the over current element un energized.
However when a short-circuit occurs there is a tendency for the current or power
to flow in the reverse direction. Should this happen, the disc of the upper
element rotates to bridge the fixed contacts. This completes the circuit for over current element. The disc of this element rotates and the moving contact
attached to it closes the trip circuit. This operates the circuit breaker which
isolates the faulty section
