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132 kv gss summer training report from CPWD vidhyadar nagar jaipur
GRID SUB STATION, CPWD JAIPUR
In partial fulfilment
For the award of the Degree of
Bachelor of Technology
In Department of Electrical Engineering
Supervisor Submitted By:
Er. Mukesh Kumar RAMESH KUMAWAT
Designation Roll No.: 15EEAEE036
GOVT. ENGINEERING COLLEGE, AJMER
(Run under society act)
Badliya circle, NH-08, Ajmer
www.ecajmer.comTelephone: 0145-2671800, 2671801
Rajasthan Technical University
I would like to express my deep gratitude to Er. Mukesh Kumar (Assistant
Engineer) for giving me honour to work with this esteem organization.
First of all I would like to convey my sincere thanks to Dr. K.G SHARMA
(Head of Department , Electrical Engineering) and Ms. INDIRA
RAWAT(Associate Professor) of Electrical Department for giving me
permission for training program at R.R.V.P.N.L.,132 KV GSS CPWD JAIPUR,
Ajmer I would also like to thanks to Mr. AJAY AGGRAWAL(Assistant
Professor) and all faculty members of the Department, who had helped and gave
guidance to me for preparing the report.
CHAPTER NO. PAGE NO
LIST OF CONTENT
INDEX OF FIGURES
Chapter 1: THE GLANCE OF GSS
1.2 The Grid Sub Station (G.S.S.)
1.3 The Equipments In A Substation
1.4 132kv Grid Substation, CPWD, Jaipur
1.4.1 Incoming Feeder (132 KV)
1.4.2 Outgoing Feeders (33 KV)
Chapter 2: LIGHTNING ARRESTER
2.3 Installation Location
2.6 Arrester Voltage
2.8 Types Of Arrestors
2.8.1 Rod/Sphere Gap
2.8.2 Expulsion Type La
2.8.3 Valve Type La
Chapter 3: BUS BARS AND ISOLATORS
3.2 Bus Bar Arrengement Adopted By R.R.V.P.N.L.
3.2.1 Single Bus Bar Arrangement
3.2.2 Double Bus Bar Arrangement
3.2.3 Double Bus Bar Arrangements Contains Main Bus With Auxilary Bus
3.4 Applications Of Isolators
Chapter 4: INSULATOR
4.2 Type Of Insulators
4.2.1 Pin Type
4.2.2 Suspension Type
4.3.3 Strain Insulator
4.2.4 Shackle Insulator
Chapter 5: CONTROL ROOM
5.3 Measuring Instrument Used
5.3.1 Energy Meter
5.3.3 Frequency Meter
5.3.6 Maximum Demand Indicator
5.3.7 Mvar Meter
5.4 Control Panels
Chapter 6: POWER LINE CARRIER COMMUNICATION
6.2 Basic Principle Of Plc
6.3 Wave Trap
6.4 Coupling Capacitors
6.5 Drainage Coils
6.6 Block Diagram
6.10 Future Scope
Chapter 7: CURRENT TRANSFORMER
7.2 Funtion Of Current Transformer
7.3 Use Of Current Transformer
7.4 Safety Of Current Transformer
Chapter 8: CAPACITIVE VOLTAGE TRANSFORMER
8.2 Components Of Capacitive Voltage Transformer
8.3 Frequency Response
Chapter 9: CIRCUIT BREAKER
9.3 Arc Interruption
9.4 Short Circuit
9.5 Types Of Circuit Breaker
9.5.1 High Voltage Circuit Breaker
9.5.2 Sf6 Circuit Breaker
Chapter 10: RELAYS
10.2 Basic Design And Operation
10.3 Types Of Relays
10.3.1 Latching Relay
10.3.2 Coaxial Relay
10.3.3 Time Delay Relay
10.3.4 Buchholz Relay
10.4 Pole And Throw
INDEX OF FIGURES
Figure No. Figure Name Page no.
Figure-1.1 132 KV GSS CPWD, jaipur 3
Figure-2.1 L.A. At 132kv GSS CPWD 7
Figure-3.1 Isolators 10
Figure-4.1 Pin Type Insulators 13
Figure-4.2 Suspension Type Insulator 13
Figure-4.3 Strain Type Insulator 14
Figure-4.4 Shackle Type Insulator 14
Figure-5.1 Control Room 16
Figure-6.1 Wave Trap 20
Figure-6.2 Block Diagram Of Plc 21
Figure-7.1 Current transformer 28
Figure-7.2 Function of current transformer 30
Figure-8.1 A capacitor voltage transformer 32
Figure-9.1 High Voltage Circuit Breaker 39
Figure-9.2 SF6 Circuit Breaker 40
A substation is an assembly of apparatus, which transform the characteristics of electrical
energy from one form to another say from one voltage level to another level. Hence a
substation is an intermediate link between the generating station and the load units. There is
one bus bars in a 132 KV yard and two bus-bars in 33kv yard. The incoming feeders are
connected to bus-bar through lighting arrestors, capacitive voltage transformer, line isolator,
circuit breakers, current-transformers, line isolator etc. The bus-bars are to have an
arrangement of auxiliary bus.
In the 132KV GSS the income 132 KV supply is stepped down to 33Kv with the help of
transformers which is furthers supplied to different sub-station according to the load. 132 Kv
G.S.S. has a large layout consisting of 2 Nos. of (40/50MVA and 20/25MVA) transformers,
with their voltage ratio respectively 132/33Kv.
At "132kv CPWD,Jaipur" the separate control room switches and fuses are available. There
are meters for reading purpose. A circuit concerning the panel is shown on the panel with
standard color, provided for remote protection of 132KV switch yards transformer incoming
feeder, outing feeders. The control panel carries the appropriate relays. The training at grid
substation was very helpful. It has improved my theoretical concepts of electrical power
transmission and distribution. Protection of various apparatus was a great thing. Maintenance
of transformer, circuit breaker, isolator, insulator, bus bar etc was observable. I had a chance
to see the remote control of the equipments from control room itself, which was very
THE GLANCE OF GSS
“Rajasthan State Electricity Board” started working form 1 July, 1957. When India
becomes independent its overall installed capacity was hardly 1900 MW. During first year
plan (1951-1956) this capacity was only 2300 MW. The contribution of Rajasthan state
was negligible during 1 & 2 year plans the emphasis was on industrialization for that end it
was considered to make the system of the country reliable. Therefore Rajasthan
state electricity board came into existence in July 1957.The aim of RSEB is to supply
electricity to entire Rajasthan state in the most economical way. Government of Rajasthan on
19th July 2000, issued a gazette notification unbundling Rajasthan State Electricity Board
1. Rajasthan Rajya Vidyut Utpadan Nigam Ltd (RRVUNL), the generation Company;
2. Rajasthan Rajya Vidyut Prasaran Nigam Ltd, (RRVPNL), the transmission Company
And the three regional distribution companies namely
1. Jaipur Vidyut Vitran Nigam Ltd, (JVVNL)
2. Ajmer Vidyut Vitran Nigam Ltd (AVVNL) and
3. Jodhpur Vidyut Vitran Nigam Ltd (JdVVNL).
The Generation Company owns and operates the thermal power stations at Kota and
Suratgarh, Gas based power station at Ramgarh , Hydel power station at Mahi and mini hydel
stations in the State. The Transmission Company operates all the 400KV, 220 KV, 132 KV
and 33KV electricity lines and system in the State. The three distribution Companies operate
and maintain the electricity system below 66KV in the State in their respective areas. Power
obtain from these stations is transmitted all over Rajasthan with the help of grid stations. .
Depending on the purpose, substations may be classified as
1. Step up substation
2. Primary grid substation
3. Secondary substation
4. Distribution substation
5. Bulky supply and industrial substation
6. Mining substation
7. Mobile substation
8. Cinematograph substation
Depending on constructional feature substations are classified as
1. Outdoor type
2. Indoor type
3. Basement or Underground type
4. Pole mounting open or kilos type
1.2 THE GRID SUB STATION (G.S.S.)
The assembly of apparatus used to change some characteristics (e.g. voltage ac / dc, freq., p.f.
etc) of electric supply is called sub-station.Electrical power is generated, transmitted in the
form of alternating current. The electric power produced at the power stations is delivered to
the consumers through a large network of transmission & distribution. The transmission
network is inevitable long and high power lines are necessary to maintain a huge block of
power source of generation to the load centers to inter connected. An electrical substation is a
subsidiary station of an electricity generation, transmission and distribution system where
voltage is transformed from high to low or the reverse using transformers. Electric power
may flow through several substations between generating plant and consumer, and may be
changed in voltage in several steps.
Substations have switching, protection and control equipment and one or more transformers.
In a large substation, circuit breaker are used to interrupt any short-circuits or overload
currents that may occur on the network.
Figure-1.1 132KV CPWD,GSS
Depending on the constructional feature, the high voltage substations may be further
1. Outdoor substation
2. Indoor substation
3. Base or Underground substation
Following are the feeders established in a substation.
1. Tie Feeders
2. Radial Feeders
1.3 THE EQUIPMENTS IN A SUBSTATION
2. Power Transformers;
4. Circuit Breakers ( 132KV AND 33 KV);
5. Isolators or Isolating Switches ( 132KV AND 33 KV);
6. Earthing Switches;
8. Power and Control Cables;
9. Control Panel;
10. Lightning Protection − Surge Arrestors;
11. Instrument Transformers (Current and Power Transformers, I.E., CTs and PTs);
12. Earthing Arrangements;
13. Reactive Compensation;
14. DC Supply Arrangement;
15. Auxiliary Supply Transformer; and
16. Fire-Fighting System.
1.4 132KV GRID SUBSTATION, MADAR
It is part of RRVPNL. It is situated at JAIPUR. The power mainly comes from 220KV GSS
Chomu and 220 KV GSS Kalwar. The substation is equipped with various equipments and
there are various arrangements for the protection purpose. The equipments in the GSS are
listed previously. 132 KV GSS CPWD is an outdoor type primary substation and distribution
as well it has not only step down but the distribution work.
The electrical work in a substation comprises to
1. Choice of bus bar arrangement layout.
2. Selection of rating of isolator.
3. Selection of rating of instrument transformer.
4. Selection of rating of C.B.
5. Selection of lighting arrester (LA)
6. Selection of rating of power transformer
7. Selection of protective relaying scheme, control and relay boards.
8. Selection of voltage regulator equipment.
9. Design a layout of earthing grids and protection against lightening strokes.
1.4.1 Incoming Feeder (220 KV)
1.4.2 Outgoing Feeders (33 KV)
1. Vidyadhar nagar
A lightning arrester is a device connected between line and earth i.e. in parallel with the over
headline, HV equipments and substation to be protected. It is a safety valve which limits the
magnitude of lightning and switching over voltages at the substations and provides a low
resistance path for the surge current to flow to the ground. The practice is also to install
lightning arresters at the incoming terminals of the line.
All the electrical equipments must be protected from the severe damages of lightning strokes.
The techniques can be studied under:-
1. Protection of transmission line from direct stroke.
2. Protection of power station and sub-station from direct stroke.
3. Protection of electrical equipments from travelling waves.
The ThyriteAlugard Lightning arrester consists of a stack of one or more units connected in
series depending on the voltage and the operating condition of the circuit three single pole
arresters are required for 3-phase installation. The arresters are single pole design and they
are suitable for indoor and out-door service.Each arrester unit consists essentially of
permanently sealed Porcelain housing equipped with pressure relief and containing a number
of thyrite value-element discs and exclusive locate gaps shunted by Thyrite resistors metal
fitting cemented of the housing provide means for bolting arrester units into a stack. Each
arrester unit is shipped assembled. No charging or testing operation is required before placing
them in service.
2.3 INSTALLATION LOCATION
Install arrester electrically as close as possible to the apparatus being protected Line and
ground connections should be short and direct.
Figure 2.1: L.A. At 220 KV GSS Madar
The arrester ground should be connected to the apparatus grounds and the main station
ground utilizing a reliable common ground network of low resistance. The efficient operation
of the Lightning arrester requires permanent low resistance grounds Station class arresters
should be provided with a ground of a value not exceeding five ohms.
These are given on the drawings. These are the maximum recommended. The term
‘clearance’ means the actual distance between any part of the arrester or disconnecting device
at line potential, and any object at ground potential or other phase potential.
2.6 ARRESTER VOLTAGE
The Thyrite station-class arrester is designed to limit the surge voltages to a safe value by
discharging the surge current to ground; and to interrupt the small power frequency follow
current before the first current zero. The arrester rating is a define limit of its ability to
interrupt power follow current. Therefore it is important to assure that the system power
frequency voltage from line to ground under any condition, switching, fault, overvoltage
never exceeds the arrester’s rating.
Lightning, is a form of visible discharge of electricity between rain clouds or between a rain
cloud and the earth The electric discharge is seen in the form of a brilliant arc, sometimes
several kilometers long, stretching between the discharge points How thunderclouds become
charged is not fully understood, but most thunderclouds are negatively charged at the base
and positively charged at the top However formed, the negative charge at the base of the
cloud induces a positive charge on the earth beneath it, which acts as the second plate of a
huge capacitor. When the electrical potential between two clouds or between a cloud and the
earth reaches a sufficiently high value (about 10,000 V per cm or about 25,000 V per in), the
air becomes ionized along a narrow path and a Lightning flash results .
2.8 TYPES OF ARRESTORS
2.8.1 Rod/Sphere Gap
It is a very simple protective device i.e. gap is provided across the stack of Insulators to
permit flash-over when undesirable voltages are impressed of the system.
2.8.2 Expulsion Type La
It have two electrodes at each end and consists of a fiber tube capable of producing a gas
when is produced. The gas so evolved blows the arc through the bottom electrode.
2.8.3 Valve Type La
It consists of a divided spark-gap in series will a non linear resistor. The divided spark gap
consists of a no. of similar elements.
BUS BARS AND ISOLATORS
Bus Bars are the common electrical component through which a large no of feeders operating
at same voltage have to be connected.If the bus bars are of rigid type (Aluminum types) the
structure height are low and minimum clearance is required. While in case of strain type of
bus bars suitable ACSR conductor are strung/tensioned by tension insulators discs according
to system voltages. In the widely used strain type bus bars stringing tension is about 500-900
Kg depending upon the size of conductor used.Here proper clearance would be achieved only
if require tension is achieved. Loose bus bars would affect the clearances when it swings
while over tensioning may damage insulators. Clamps or even affect the supporting structures
in low temperature conditions.The clamping should be proper, as loose clamp would spark
under in full load condition damaging the bus bars itself.
3.2BUS BAR ARRENGEMENT ADOPTED BY R.R.V.P.N.L.
3.2.1 Single Bus Bar Arrangement
This arrangement is simplest and cheapest. It suffers, however, from major defects.
1. Maintenance without interruption is not possible.
2. Extension of the substation without a shutdown is not possible
3.2.2 Double Bus Bar Arrangement
1. Each load may be fed from either bus.
2. The load circuit may be divided in to two separate groups if needed from operational
consideration. Two supplies from different sources can be put on each bus separately.
3. Either bus bar may be taken out from maintenance of insulators.
The normal bus selection insulators cannot be used for breaking load currents. The
arrangement does not permit breaker maintenance without causing stoppage of supply.
3.2.3 Double Bus Bar Arrangements Contains Main Bus With Auxilary Bus
The double bus bar arrangement provides facility to change over to either bus to carry out
maintenance on the other but provide no facility to carry over breaker maintenance. The main
and transfer bus works the other way round. It provides facility for carrying out breaker
maintenance but does not permit bus maintenance. Whenever maintenance is required on any
breaker the circuit is changed over to the transfer bus and is controlled through bus coupler
“Isolator" is one, which can break and make an electric circuit in no load condition. These are
normally used in various circuits for the purposes of Isolation of a certain portion when
required for maintenance etc.
Switching Isolators are capable of
1. Interrupting transformer magnetized currents
2. Interrupting line charging current
3. Load transfer switching
Figure-3.1 Isolators At 220 KV GSS Madar
3.4 APPLICATIONS OF ISOLATORS
Its main application is in connection with transformer feeder as this unit makes it possible to
switch out one transformer, while the other is still on load. The most common type of
isolators is the rotating centre pots type in which each phase has three insulator post, with the
outer posts carrying fixed contacts and connections while the centre post having contact arm
which is arranged to move through 90` on its axis.
The following interlocks are provided with isolator
1. Bus 1 and2 isolators cannot be closed simultaneously.
2. Isolator cannot operate unless the breaker is open.
3. Only one bay can be taken on bypass bus.
4. No isolator can operate when corresponding earth switch is on breaker.
The insulator for the overhead lines provides insulation to the power conductors from the
ground so that currents from conductors do not flow to earth through supports. The insulators
are connected to the cross arm of supporting structure and the power conductor passes
through the clamp of the insulator. The insulators provide necessary insulation between line
conductors and supports and thus prevent any leakage current from conductors to earth. In
general, the insulator should have the following desirable properties:
1. High mechanical strength in order to withstand conductor load, wind load etc.
2. High electrical resistance of insulator material in order to avoid leakage currents to earth.
3. High relative permittivity of insulator material in order that dielectric strength is high.
4. High ratio of puncture strength to flash over.
These insulators are generally made of glazed porcelain or toughened glass. Poly come type
insulator [solid core] are also being supplied in place of hast insulators if available
indigenously. The design of the insulator is such that the stress due to contraction and
expansion in any part of the insulator does not lead to any defect. It is desirable not to allow
porcelain to come in direct contact with a hard metal screw thread.
4.2 TYPE OF INSULATORS
4.2.1 Pin Type
Pin type insulator consist of a single or multiple shells adapted to be mounted on a spindle to
be fixed to the cross arm of the supporting structure. When the upper most shell is wet due to
rain the lower shells are dry and provide sufficient leakage resistance these are used for
transmission and distribution of electric power at voltage up to voltage 33 KV. Beyond
operating voltage of 33 KV the pin type insulators thus become too bulky and hence
Figure-4.1 Pin Type Insulators
4.2.2 Suspension Type
Suspension type insulators consist of a number of porcelain disc connected in series by metal
links in the form of a string. Its working voltage is 66KV. Each disc is designed for low
voltage for 11KV.
Figure-4.2 Suspension Type Insulator
4.2.3 Strain Insulator
The strain insulators are exactly identical in shape with the suspension insulators. These
strings are placed in the horizontal plane rather than the vertical plane. These insulators are
used where line is subjected to greater tension. For low voltage lines (< 11KV) shackle
insulator are used as strain insulator.
Figure-4.3 Strain Type Insulator
4.2.4 Shackle Insulator
In early days, the shackle insulators were used as strain insulators. But now a day, they are
frequently used for low voltage distribution lines. Such insulators can be used either in a
horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt
or to the cross arm.
Figure-4.4 Shackle Type Insulator
Control panel contain meters, control switches and recorders located in the control building,
also called the dog house. These are used to control the substation equipment to send power
from one circuit to another or to open or to shut down circuits when needed.
To remote control of power switch gear requires the provision of suitable control plates
located at a suitable point remote from immediate vicinity of CB’s and other equipments.
In GSS the separate control room provided for remote protection of 132KV switch yards
transformer incoming feeder, outing feeders. Bus bar has their own control plant in their
control rooms. The control panel carrier the appropriate relays. Necessary meters indicating
lamp control switches and fuses. There are meters for reading purpose. A circuit concerning
the panel is shown on the panel with standard color.
On each panel a control switch is provided for remote operation of circuit breaker. There are
two indicators which show that weather circuit breaker is closed or open. A control switch for
each insulator is also provided. The position indicator of isolator is also done with the help of
single lamp and indicator. The color of signal lamps are as follows:-
1. Red:- for circuit breaker or isolator is close option
2. Green - for circuit breaker is in open position.
3. Amber - indicates abnormal condition requiring action.
In addition to used indication, an alarm is also providing for indicating abnormal condition
when any protective relay or tripping relay has operated. Its constants energies on auxiliary
alarm, Relay which on operation completes the alarm belt circuit.
There is a hinged Synchronizing panel mounted at the end of control panel, before coupling
any incoming feeders to the bus bar. It is just be synchronized with switches. When the
synchronous copy shows zero we close the circuit breaker. Synchronous scope is used to
determine the correct instant of closing the switch which connects the new supply to bus bar.
The correct instant of synchronizing when bus bars incoming voltage:
1. Are in phase
2. Are equal in magnitude
3. Are in some phase sequence
4. Having same frequency
5. The voltage can be checked by voltmeter the function of synchronoscope is to indicate the
difference in phase and frequency.
Figure-5.1 Control Room At 220 KV GSS MADAR
5.3.1 Energy Meter
To measure the energy transmitted energy meters are fitted to the panel to different feeders
the energy transmitted is recorded after one hour regularly for it MWHr, meter is provided.
It is attached to each feeder to record the power exported from GSS
5.3.3 Frequency Meter
To measure the frequency at each feeder there is the provision of analogor digital frequency
It is provided to measure the phase to phase voltage .It is also available in both the analog and
digital frequency meter.
It is provided to measure the line current. It is also available in both the forms analog as well
5.3.6 Maximum Demand Indicator
There are also mounted the control panel to record the average power over successive
5.3.7 Mvar Meter
It is to measure the reactive power of the circuit.
5.4 CONTROL PANELS
Control panels installed within the control building of a switchyard provide mounting for
mimic bus, relays, meter, indicating instruments, indicating lights, control switches, test
switches and other control devices. The panel contains compartments for incoming lines,
outgoing lines, bus-bars with provision for sectionalizing, relays, measuring instruments, etc.
The panel is provided with
1. Suitable over-current and earth fault relays to protect the equipment against short circuit
and earth faults; and
POWER LINE CARRIER COMMUNICATION
Power line communication or power line carrier (PLC), also known as Power line Digital
Subscriber Line (PDSL), mains communication, power line telecom (PLT), or power line
networking (PLN), is a system for carrying data on a conductor also used for electric power
transmission. Broadband over Power Lines (BPL) uses PLC by sending and receiving
information bearing signals over power lines.Electrical power is transmitted over high
voltage transmission lines, distributed over medium voltage, and used inside buildings at
lower voltages. Power line communications can be applied at each stage. Most PLC
technologies limit themselves to one set of wires (for example, premises wiring), but some
can cross between two levels (for example, both the distribution network and premises
wiring). Typically the transformer prevents propagating the signal so multiple PLC
technologies are bridged to form very large networks.
All power line communications systems operate by impressing a modulated carrier signal on
the wiring system. Different types of power line communications use different frequency
bands, depending on the signal transmission characteristics of the power wiring used. Since
the power wiring system was originally intended for transmission of AC power, in
conventional use, the power wire circuits have only a limited ability to carry higher
frequencies. The propagation problem is a limiting factor for each type of power line
communications. A new discovery called E-Line that allows a single power conductor on an
overhead power line to operate as a waveguide to provide low attenuation propagation of RF
through microwave energy lines while providing information rate of multiple GBPS is an
exception to this limitation .
6.2 BASIC PRINCIPLE OF PLCC
In PLCC the higher mechanical strength and insulation level of high voltage power lines
result in increased reliability of communication and lower attenuation over long distances.
Since telephone communication system cannot be directly connected to the high voltage
lines, suitably designed coupling devices have therefore to be employed. These usually
consist of high voltage capacitors or capacitor with potential devices used in conjunction with
suitable line matching units (LMU’s) for matching the impedance of line to that of the
coaxial cable connecting the unit to the PLC transmit-receive equipment.
Also the carrier currents used for communication have to be prevented from entering the
power equipment used in G.S.S as this would result in high attenuation or even complete loss
of communication signals when earthed at isolator. Wave traps usually have one or more
suitably designed capacitors connected in parallel with the choke coils so as to resonate at
carrier frequencies and thus offers even high impedance to the flow of RF currents.
In PLCC system the following Equipments are used
1. PLCC Station
2. Line matching Unit
4. Earth Switch
5. Lightening Arrestor
6. Wave Trap
7. Co axial cable
6.3 WAVE TRAP
Line trap also is known as Wave trap. What it does is trapping the high frequency
communication signals sent on the line from the remote substation and diverting them to the
telecom/teleprotection panel in the substation control room (through coupling capacitor and
Figure-6.1 Wave Trap
This is relevant in Power Line Carrier Communication (PLCC) systems for communication
among various substations without dependence on the telecom company network. The signals
are primarily teleprotection signals and in addition, voice and data communication signals.
The Line trap OFFERS HIGH IMPEDANCE TO THE HIGH FREQUENCY COMMUNICATION
SIGNALSthus obstructs the flow of these signals in to the substation bus bars. If there were not
to be there, then signal loss is more and communication will be ineffective/probably
6.4 COUPLING CAPACITORS
The coupling capacitor is used as part of the tuning circuit. The coupling capacitor is the
device which provides a low.Impedance path for the carrier energy to the high voltage line
and at the same time, it blocks the power frequency current by being a high impedance path
at those frequencies. It can perform its function of dropping line voltage across its
capacitance if the low voltage end is at ground potential. Since it is desirable to connect the
line tuner output to this low voltage point a device must be used to provide a high impedance
path to ground for the carrier signal and a low impedance path for the power frequency
current. This device is an inductor and is called a drain coil.
It is desirable to have the coupling capacitor value as large as possible in order to lower the
loss of carrier energy and keep the bandwidth of the coupling system as wide as possible.
However, due to the high voltage that must be handled and financial budget limitations, the
coupling capacitor values are not as high as one might desire. Technology has enabled
suppliers to continually increase the capacitance of the coupling capacitor for the same price
thus improving performance.
6.5 DRAINAGE COILS
The drainage coil has a pondered iron core that serves to ground the power frequency
charging to appear in the output of the unit. The coarse voltage arrester consists of an air gap,
which sparks over at about 2 KV and protects the matching unit against line surges. The
grounding switch is kept open during normal operation and is closed if anything is to be done
on the communication equipment without interruption to power flow on the line. The
matching transformer is isolated for 7 to 10 KV between the two winding and former two
functions. Firstly it isolates the communication equipment for the power line. Secondly it
serves to match the characteristic impedance of the power line 400-600 ohms to that of the
co-axial vacuum arrester (which sparks) is over at about 250 V is provided for giving
additional protection to the communication equipment.
6.6 BLOCK DIAGRAM
Figure-6.2 Block Diagram Of Plc
This block diagram explanation will give the basic idea about the flow of data fromone user
to another. It will give the general idea about the processing that the dataunderwent from one
section to another. Following is the end to end jest of the project. Wefirst generated a
sequence of random bits with the use of Lab view. This will act as thedata given by the user
which is to be transmitted to the receiver. Random bits aregenerated for illustration purpose.
The functioning of the receiver side blocks is the exact opposite as that of transmitterside.
The coupling circuitry on the receiver side not just provides the isolation but alsoacts as a
tuned circuit will allows only the high frequency in a selected band i.e. between800Hz to
3000Hz to enter the receiver side.
1.Transmission & Distribution Network
PLCC was first adopted in the electricaltransmission and distribution system to transmit
information at a fast rate.
2.Home control and Automation
PLCC technology is used in home control andautomation. This technology can reduce the
resources as well as efforts for activities likepower management, energy conservation,
etc.Home automation or also known as SmartHome technology is a collection of systems and
devices in a home that have an ability tointeract with each other or function individually in
order to be optimized in best way.
PLCC is used to distribute the multimedia content throughout thehome.
Data transmission for different types of communications liketelephonic communication,
audio, video communication can be made with the use ofPLCC technology.
In monitoring houses or businesses through surveillance cameras,PLCC technology is far
useful. The surveillance cameras connected over Power Systemin a light bulb is unique;
simply screw it into any light socket. Hidden inside the "bulb" isa sophisticated Low-Light
Monochrome Camera, coupled with PLCC circuitry. Once the Decoder is plugged in and
connected, live video is delivered to the TV or VCR.It's simple and easy. There are no wires
to run, no holes to drill, and no antennas orcomplicated "tuning" is required.
6.Automatic Meter Reading
Automatic Meter reading applications use the PLCCtechnology to send the data from home
meters to Host Central Station. Automatic MeterReading using PLCC technology is quite
useful as it saves a lot of human efforts and alsomakes the whole system more efficient. The
automatic meter reading system consists ofthree components, namely, Multifunction Node
(MFN), Concentrator & CommunicationNode (CCN) and Operation & Management System
1. No separate wires are needed for communication purposes as the power lines themselves
carry power as well as the communication signals. Hence the cost of constructing separate
telephone lines is saved.
2. When compared with ordinary lines the power lines have appreciably higher mechanical
strength. They would normally remain unaffected under the condition which might
seriously damage telephone lines.
3. Power lines usually provide the shortest route between the power stations.
4. Power lines have large cross-sectional area resulting in very low resistances per unit
length. Consequently the carrier signal suffers lesser attenuation than when travel on
usual telephone lines of equal lengths.
5. Power lines are well insulated to provide negligible leakage between conductors and
ground even in adverse weather conditions.
6. Largest spacing between conductors reduces capacitance which results in smaller
attenuation at high frequencies. The large spacing also reduces the cross talk to a
7. PLCC uses existing power line for communication so it provides many advantages
overtraditionally used telephone models and other communication systems. The
mainadvantage of power line carrier communication is cost on infrastructure is reduced to
alarge extent. Consider any building or company office as an example. To
supplyelectricity for the whole structure first Mains cables are dropped throughout the
buildingand it bears all the load thrusters by fridge , fans and all the other equipments or
devicesrun on electricity. If PLCC modem is used in such an area, no additional wiring
orcabling is required as PLCC uses this power line only. Data is sent on this
withequipotential coupling circuitry using modulator and it's retrieved at the receiver's
sideusing demodulator. Thus elimination of wiring or cabling is the biggest bliss
incommunication system. This is also a flexible type of service which can have
differentformulations as per the need or application. Half duplex PLCC or Full Duplex
PLCCmodems are available. Thus they are now greatly used in houses and small
officenetworks. It is also considered as a replacement to intercom as it doesn't need any
extracabling.The biggest relief to the customers is when you talk about installation
charges andservice tax. Apart from initial installation charges, user doesn't need to pay
service taxand government taxes for it as he's using existing power line only for
8. With built-in Error Checking and direct interface with uc as an ad-on , it's the
biggestsource of Research and Development since last 50 years. With other applications
likeAutomatic meter reading , Fire & Security Alarm Systems and Lighting Control
thisserves as the major Integration for all the tasks that can be computed easily. With
theinvention of new modems such as PLC Modem from Sunrom Technologies, care has
been taken that though using a same platform for transmitter and receiver side ,
highpower side and low power side are isolated so chances of shock hazards are reduced
tolarge extent and device is made more user friendly.
1. Proper care has to be taken to guard carrier equipment and persons using them against
high voltage and currents on the line.
2. Reflections are produced on spur lines connected to high voltage lines. This increases
attenuation and create other problems.
3. High voltage lines have transformer connections, which attenuate carrier currents. Sub-
station equipments adversely affect the carrier currents.
4. Noise introduced by power lines is much more than in case of telephone lines. This due to
the noise generated by discharge across insulators, corona and switching processes.
6.10 FUTURE SCOPE
Data transmission for different types of communications liketelephonic communication,
audio, video communication can be made with the use ofPLCC technology. The user will be
free to choose the necessary mode of communication.
In an industrial environment the PLC communicationnetworks can be used to give electric
energy related services, such as meter reading,demand management and remote billing but
also to give value added services like remotecontrol and security, automation or even,
education, information and e businessopportunities. On the other hand it can also offer
telecommunication services such astraditional telephony and Internet.
The communication link can be used to transit control signals thatmay be used to protect the
system. For example, PLC can be successfully used in order todetect is landing operation of
Current PLC networks are able to reach speeds of200Mbps. Telephony and Internet services
can be delivered at high speed throughbroadband PLC networks. Traditional telephony uses
Plesiochronous Digital Hierarchy ,PDH. PDH uses Time Division Multiplexing, TDM. One
possibility is to send the TDMframe over IP, and the voice over TDM, VoTDM. However,
this service shouldaccomplish the quality and reliability criteria, like Bit Error Rate, timing
and latency, andunfortunately the delay in Vo TDM transmissions exceeds 25ms.
Nevertheless, it ispossible to give a good telephony service over IP. Over TCP/IP, VoIP.
A current transformer (CT) is a type of transformer that is used to measure AC Current. It
produces an alternating current (AC) in its secondary which is proportional to the AC current
in its primary. Current transformers, together with voltage transformers (VTs) or potential
transformers (PTs), which are designed for measurement, are known as an Instrument
The main tasks of instrument transformers are:
i. To transform currents or voltages from a usually high value to a value easy to handle for
relays and instruments.
ii. To insulate the metering circuit from the primary high voltage system.
iii. To provide possibilities of standardizing the instruments and relays to a few rated currents
When the current to be measured is too high to measure directly or the system voltage of the
circuit is too high, a current transformer can be used to provide an isolated lower current in its
secondary which is proportional to the current in the primary circuit. The induced secondary
current is then suitable for measuring instruments or processing in electronic equipment.
Current transformers have very little effect on the primary circuit.
Current transformers are the current sensing units of the power system. The output of the
current transformers are used in electronic equipment and are widely used for metering
and protective relays in the electrical power industry.
Fig. 7.1 Current transformer
7.2 FUNTION OF CURRENT TRANSFORMER
Like any transformer, a current transformer has a primary winding, a core and a secondary
winding, although some transformers, including current transformers, use an air core. In
principle, the only difference between a current transformer and a voltage transformer
(normal type) is that the former is fed with a 'constant' current while the latter is fed with
a 'constant' voltage, where 'constant' has the strict circuit theory meaning.
The alternating current in the primary produces an alternating magnetic field in the core,
which then induces an alternating current in the secondary. The primary circuit is largely
unaffected by the insertion of the CT. Accurate current transformers need close coupling
between the primary and secondary to ensure that the secondary current is proportional to the
primary current over a wide current range. The current in the secondary is the current in the
primary (assuming a single turn primary) divided by the number of turns of the secondary. In
the illustration on the right, 'I' is the current in the primary, 'B' is the magnetic field, 'N' is the
number of turns on the secondary, and 'A' is an AC ammeter.
Current transformers typically consist of a silicon steel ring core wound with many turns of
copper wire as shown in the illustration to the right. The conductor carrying the primary
current is passed through the ring. The CT's primary therefore consists of a single 'turn'. The
primary 'winding' may be a permanent part of the current transformer, i.e. a heavy copper bar
to carry current through the core. Window-type current transformers (aka zero sequence
current transformers, or ZSCT) are also common, which can have circuit cables run through
the middle of an opening in the core to provide a single-turn primary winding. To assist
accuracy, the primary conductor should be centered in the aperture.
CTs are specified by their current ratio from primary to secondary. The rated secondary
current is normally standardized at 1 or 5 amperes. For example, a 4000:5 CT secondary
winding will supply an output current of 5 amperes when the primary winding current is 4000
amperes. This ratio can also be used to find the impedance or voltage on one side of the
transformer, given the appropriate value at the other side. For the 4000:5 CT, the secondary
impedance can be found as ZS = NZP = 800ZP, and the secondary voltage can be found as
VS = NVP = 800VP. In some cases, the secondary impedance is referred to the primary side,
and is found as ZS’ = N2ZP. Referring the impedance is done simply by multiplying initial
secondary impedance value by the current ratio. The secondary winding of a CT can have
taps to provide a range of ratios, five taps being common.
Shapes and sizes vary depending on the end user or switch gear manufacturer. Low-voltage
single ratio metering current transformers are either a ring type or plastic molded case.
Split-core current transformers either have a two-part core or a core with a removable section.
This allows the transformer to be placed around a conductor with minimum disturbance.
Split-core current transformers are typically used in low current measuring instruments, often
portable, battery-operated, and hand-held (see illustration lower right).
Fig.7.2 Function of current transformer
7.3 USE OF CURRENT TRANSFORMER
Current transformers are used extensively for measuring current and monitoring the operation
of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's
watt-hour meter on virtually every building with three-phase service and single-phase
services greater than 200 amperes.
High-voltage current transformers are mounted on porcelain or polymer insulators to isolate
them from ground. Some CT configurations slip around the bushing of a high-voltage
transformer or circuit breaker, which automatically centers the conductor inside the CT
Current transformers can be mounted on the low voltage or high voltage leads of a power
transformer. Sometimes a section of a bus bar can be removed to replace a current
Often, multiple CTs are installed as a "stack" for various uses. For example, protection
devices and revenue metering may use separate CTs to provide isolation between metering
and protection circuits, and allows current transformers with different characteristics
(accuracy, overload performance) to be used for the devices.
The burden (load) impedance should not exceed the specified maximum value to avoid the
secondary voltage exceeding the limits for the current transformer. The primary current rating
of a current transformer should not be exceeded or the core may enter its non linear region
and ultimately saturate. This would occur near the end of the first half of each half (positive
and negative) of the AC sine wave in the primary and would compromise the accuracy.
7.4 SAFETY OF CURRENT TRANSFORMER
Current transformers are often used to monitor hazardously high currents or currents at
hazardously high voltages, so great care must be taken in the design and use of CTs in these
The secondary of a current transformer should not be disconnected from its burden while
current is in the primary, as the secondary will attempt to continue driving current into an
effective infinite impedance up to its insulation break-down voltage and thus compromise
operator safety. For certain current transformers this voltage may reach several kilovolts and
may cause arcing. Exceeding the secondary voltage may also degrade the accuracy of the
transformer or destroy it. Energizing a current transformer with an open circuit secondary is
the dual of energizing a voltage transformer (normal type) with a short circuit secondary. In
the first case the secondary tries to produce an infinite voltage and in the second case the
secondary tries to produce an infinite current. Both scenarios can be dangerous and damage
CAPACITIVE VOLTAGE TRANSFORMER
A capacitor voltage transformer (CVT or CCVT), is a transformer used in power systems to
step down extra high voltage signals and provide a low voltage signal, for metering or
operating a protective relay.
Fig. 8.1 A capacitor voltage transformer
8.2 COMPONENTS OF CAPACITIVE VOLTAGE TRANSFORMER
line signal is split, an inductive element to tune the device to the line frequency, and a voltage
transformer to isolate and further step down the voltage for metering devices or protective
relay. In its most basic form, the device consists of three parts: two capacitors across which
The tuning of the divider to the line frequency makes the overall division ratio less sensitive
to changes in the burden of the connected metering or protection devices. The device has at
least four terminals: a terminal for connection to the high voltage signal, a ground terminal,
and two secondary terminals which connect to the instrumentation or protective relay.
In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in
series. This provides a large voltage drop across C1 and a relatively small voltage drop across
C2. As the majority of the voltage drop is on C1, this reduces the required insulation level of
the voltage transformer. This makes CVTs more economical than the wound voltage
transformers under high voltage (over 100kV), as the latter one requires more winding and
"Capacitive voltage transformers exist and are used by utilities for high-voltage (greater than
66 kV) metering. They have a capacitive voltage divider but also have a dual-winding
transformer to couple the divided voltage to the metering circuit. They tend to have lower
allowable burdens than a wound transformer but can be made economically at higher voltage
ratings. Another difference is that even though they decrease voltage, they do not increase
current as found in wound electromagnetic transformers - an ampere drawn by the load is an
ampere drawn from the primary circuit. And of course they can only reduce voltage, not
The above is a part of a Wikipedia write-up. I might sum up the definition of a capacitor
voltage transformer as a step down transformer with a convenient node of a series-connected
capacitor network connected in series with the primary winding. The free end of the capacitor
and the free end of the transformer primary constitute the primary terminals. This device is
presently used as a potential transformer to monitor high voltages. Of course ordinary step
down transformer does not employ series capacitor in the primary.
8.3 FREQUENCY RESPONSE
With the rated load at the voltage transformer secondary side, The output voltage of CVT
initially decrease a little bit, then reaches the resonance peak at around 800 Hz. Then it
decreases drastically and remains almost level out after 2000hz. The C2 current is linear with
frequency. The frequency response for voltage transformer current has a resonance peak at
around 800 Hz. C2 current is substantially larger than voltage transformer current.
The bus voltage in frequency domain can be calculated by summing the voltages on C1 and
C2. From the calculation result it can be seen that the bus voltage only relates to C2current,
voltage transformer current and their ratios.This result is helpful to reconstruct the bus
voltage with the C2 current, voltage transformer current. For the ratio, it can be achieved by
using a summing amplifier.
A circuit breaker is an automatically operated electrical switch designed to protect
an electrical circuit from damage caused by excess current, typically resulting from
an overload or short circuit. Its basic function is to interrupt current flow after a fault is
detected. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can
be reset (either manually or automatically) to resume normal operation. Circuit breakers are
made in varying sizes, from small devices that protect low-current circuits or individual
household appliance, up to large switchgear designed to protect high voltage circuits feeding
an entire city. The generic function of a circuit breaker, RCD or a fuse, as an automatic
means of removing power from a faulty system is often abbreviated as ADS (Automatic
Disconnection of Supply).
All circuit breaker systems have common features in their operation, but details vary
substantially depending on the voltage class, current rating and type of the circuit breaker.
The circuit breaker must firstly detect a fault condition. In small mains and low
voltage circuit breakers, this is usually done within the device itself. Typically, the heating
and/or magnetic effects of electric current are employed. Circuit breakers for large currents or
high voltages are usually arranged with protective relay pilot devices to sense a fault
condition and to operate the opening mechanism. These typically require a separate power
source, such as a battery, although some high-voltage circuit breakers are self-contained
with current transformers, protective relays, and an internal control power source.
Once a fault is detected, the circuit breaker contacts must open to interrupt the circuit; This is
commonly done using mechanically stored energy contained within the breaker, such as a
spring or compressed air to separate the contacts. Circuit breakers may also use the higher
current caused by the fault to separate the contacts, such as thermal expansion or a magnetic
field. Small circuit breakers typically have a manual control lever to switch off the load or
reset a tripped breaker, while larger units use solenoids to trip the mechanism, and electric
motors to restore energy to the springs.
The circuit breaker contacts must carry the load current without excessive heating, and must
also withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts
are made of copper or copper alloys, silver alloys and other highly conductive materials.
Service life of the contacts is limited by the erosion of contact material due to arcing while
interrupting the current. Miniature and molded-case circuit breakers are usually discarded
when the contacts have worn, but power circuit breakers and high-voltage circuit breakers
have replaceable contacts.
When a high current or voltage is interrupted, an arc is generated. The length of the arc is
generally proportional to the voltage while the intensity (or heat) is proportional to the
current. This arc must be contained, cooled and extinguished in a controlled way, so that the
gap between the contacts can again withstand the voltage in the circuit. Different circuit
breakers use vacuum, air, insulating gas, or oil as the medium the arc forms in. Different
techniques are used to extinguish the arc including:
i. Lengthening or deflecting the arc
ii. Intensive cooling (in jet chambers)
iii. Division into partial arcs
iv.Zero point quenching (contacts open at the zero current time crossing of the AC waveform,
effectively breaking no load current at the time of opening. The zero crossing occurs at twice
the line frequency; i.e., 100 times per second for 50 Hz and 120 times per second for 60 Hz
v. Connecting capacitors in parallel with contacts in DC circuits.
Finally, once the fault condition has been cleared, the contacts must again be closed to restore
power to the interrupted circuit.
9.3 ARC INTERRUPTION
Low-voltage miniature circuit breakers (MCB) use air alone to extinguish the arc. These
circuit breakers contain so-called arc chutes, a stack of mutually insulated parallel metal
plates which divide and cool the arc. By splitting the arc into smaller arcs the arc is cooled
down while the arc voltage is increased and serves as an additional impedance which limits
the current through the circuit breaker. The current-carrying parts near the contacts provide
easy deflection of the arc into the arc chutes by a magnetic force of a current path,
although magnetic blowout coils or permanent magnets could also deflect the arc into the arc
chute (used on circuit breakers for higher ratings). The number of plates in the arc chute is
dependent on the short-circuit rating and nominal voltage of the circuit breaker.
In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of
oil through the arc.
Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic
field, and then rely upon the dielectric strengthof the sulfur hexafluoride (SF6) to quench the
Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the
contact material), so the arc quenches when it is stretched a very small amount (less than 2–
3 mm (0.079–0.118 in)). Vacuum circuit breakers are frequently used in modern medium-
voltage switchgear to 38,000 volts.
Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts
are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing
out the arc.
Circuit breakers are usually able to terminate all current very quickly: typically the arc is
extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending
upon age and construction of the device. The maximum current value and let-through energy
determine the quality of the circuit breakers.
9.4 SHORT CIRCUIT
Circuit breakers are rated both by the normal current that they are expected to carry, and the
maximum short-circuit current that they can safely interrupt. This latter figure is the ampere
interrupting capacity (AIC) of the breaker.
Under short-circuit conditions, the calculated maximum prospective short circuit current may
be many times the normal, rated current of the circuit. When electrical contacts open to
interrupt a large current, there is a tendency for an arc to form between the opened contacts,
which would allow the current to continue. This condition can create conductive ionized
gases and molten or vaporized metal, which can cause further continuation of the arc, or
creation of additional short circuits, potentially resulting in the explosion of the circuit
breaker and the equipment that it is installed in. Therefore, circuit breakers must incorporate
various features to divide and extinguish the arc.
The maximum short-circuit current that a breaker can interrupt is determined by testing.
Application of a breaker in a circuit with a prospective short-circuit current higher than the
breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt a
fault. In a worst-case scenario the breaker may successfully interrupt the fault, only to
explode when reset.
Typical domestic panel circuit breakers are rated to interrupt 10 kA (10000 A) short-circuit
Miniature circuit breakers used to protect control circuits or small appliances may not have
sufficient interrupting capacity to use at a panel board; these circuit breakers are called
"supplemental circuit protectors" to distinguish them from distribution-type circuit breakers.
9.5 TYPES OF CIRCUIT BREAKER
9.5.1 High Voltage Circuit Breaker
Electrical power transmission networks are protected and controlled by high-voltage
breakers. The definition of high voltage varies but in power transmission work is usually
thought to be 72.5 kV or higher, according to a recent definition by the International
Electrotechnical Commission (IEC). High-voltage breakers are nearly always solenoid-
operated, with current sensing protective relays operated through current transformers.
In substations the protective relay scheme can be complex, protecting equipment and buses
from various types of overload or ground/earth fault.
High-voltage breakers are broadly classified by the medium used to extinguish the arc:
i. Bulk oil
ii. Minimum oil
iii. Air blast
Due to environmental and cost concerns over insulating oil spills, most new breakers use
SF6 gas to quench the arc.
Circuit breakers can be classified as live tank, where the enclosure that contains the breaking
mechanism is at line potential, or dead tank with the enclosure at earth potential. High-
voltage AC circuit breakers are routinely available with ratings up to 765 kV. 1,200 kV
breakers were launched by Siemens in November 2011, followed by ABB in April the
High-voltage circuit breakers used on transmission systems may be arranged to allow a single
pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults
this improves the system stability and availability.
High-voltage direct current circuit breakers are still a field of research as of 2015. Such
breakers would be useful to interconnect HVDC transmission systems.
Fig.9.1 High Voltage Circuit Breaker
9.5.2 Sf6 Circuit Breaker
A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to
quench the arc. They are most often used for transmission-level voltages and may be
incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or
de-rating of the circuit breakers may be required due to liquefaction of the SF6 gas.
A relay is an electrically operated switch. Many relays use an electromagnet to mechanically
operate a switch, but other operating principles are also used, such as solid-state relays.
Relays are used where it is necessary to control a circuit by a separate low-power signal, or
where several circuits must be controlled by one signal. The first relays were used in long
distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit
and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges
and early computers to perform logical operations.
A type of relay that can handle the high power required to directly control an electric motor
or other loads is called a contactor. Solid-state relayscontrol power circuits with no moving
parts, instead using a semiconductor device to perform switching. Relays with calibrated
operating characteristics and sometimes multiple operating coils are used to protect electrical
circuits from overload or faults; in modern electric power systems these functions are
performed by digital instruments still called "protective relays".
Magnetic latching relays require one pulse of coil power to move their contacts in one
direction, and another, redirected pulse to move them back. Repeated pulses from the same
input have no effect. Magnetic latching relays are useful in applications where interrupted
power should not be able to transition the contacts.
Magnetic latching relays can have either single or dual coils. On a single coil device, the
relay will operate in one direction when power is applied with one polarity, and will reset
when the polarity is reversed. On a dual coil device, when polarized voltage is applied to the
reset coil the contacts will transition. AC controlled magnetic latch relays have single coils
that employ steering diodes to differentiate between operate and reset commands.
10.2 BASIC DESIGN AND OPERATION
A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core (a
solenoid), an iron yoke which provides a low reluctance path for magnetic flux, a movable
iron armature, and one or more sets of contacts (there are two contacts in the relay pictured).
The armature is hinged to the yoke and mechanically linked to one or more sets of moving
contacts. The armature is held in place by a spring so that when the relay is de-energized
there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in
the relay pictured is closed, and the other set is open. Other relays may have more or fewer
sets of contacts depending on their function. The relay in the picture also has a wire
connecting the armature to the yoke. This ensures continuity of the circuit between the
moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via
the yoke, which is soldered to the PCB.
When an electric current is passed through the coil it generates a magnetic field that activates
the armature, and the consequent movement of the movable contact(s) either makes or breaks
(depending upon construction) a connection with a fixed contact. If the set of contacts was
closed when the relay was de-energized, then the movement opens the contacts and breaks
the connection, and vice versa if the contacts were open. When the current to the coil is
switched off, the armature is returned by a force, approximately half as strong as the
magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity
is also used commonly in industrial motor starters. Most relays are manufactured to operate
quickly. In a low-voltage application this reduces noise; in a high voltage or current
application it reduces arcing.
When the coil is energized with direct current, a diode is often placed across the coil to
dissipate the energy from the collapsing magnetic field at deactivation, which would
otherwise generate a voltage spike dangerous to semiconductor circuit components. Such
diodes were not widely used before the application of transistors as relay drivers, but soon
became ubiquitous as early germanium transistors were easily destroyed by this surge. Some
automotive relays include a diode inside the relay case.
If the relay is driving a large, or especially a reactive load, there may be a similar problem of
surge currents around the relay output contacts. In this case a snubber circuit (a capacitor and
resistor in series) across the contacts may absorb the surge. Suitably rated capacitors and the
associated resistor are sold as a single packaged component for this commonplace use.
If the coil is designed to be energized with alternating current (AC), some method is used to
split the flux into two out-of-phase components which add together, increasing the minimum
pull on the armature during the AC cycle. Typically this is done with a small copper "shading
ring" crimped around a portion of the core that creates the delayed, out-of-phase
component, which holds the contacts during the zero crossings of the control voltage.
10.3 TYPES OF RELAYS
10.3.1 Latching Relay
A latching relay (also called "impulse", "keep", or "stay" relays) maintains either contact
position indefinitely without power applied to the coil. The advantage is that one coil
consumes power only for an instant while the relay is being switched, and the relay contacts
retain this setting across a power outage. A latching relay allows remote control of building
lighting without the hum that may be produced from a continuously (AC) energized coil.
In one mechanism, two opposing coils with an over-center spring or permanent magnet hold
the contacts in position after the coil is de-energized. A pulse to one coil turns the relay on
and a pulse to the opposite coil turns the relay off. This type is widely used where control is
from simple switches or single-ended outputs of a control system, and such relays are found
in avionics and numerous industrial applications.
Another latching type has a remanent core that retains the contacts in the operated position by
the remanent magnetism in the core. This type requires a current pulse of opposite polarity to
release the contacts. A variation uses a permanent magnet that produces part of the force
required to close the contact; the coil supplies sufficient force to move the contact open or
closed by aiding or opposing the field of the permanent magnet. A polarity controlled relay
needs changeover switches or an H bridge drive circuit to control it. The relay may be less
expensive than other types, but this is partly offset by the increased costs in the external
In another type, a ratchet relay has a ratchet mechanism that holds the contacts closed after
the coil is momentarily energized. A second impulse, in the same or a separate coil, releases
the contacts. This type may be found in certain cars, for headlamp dipping and other
functions where alternating operation on each switch actuation is needed.
A stepping relay is a specialized kind of multi-way latching relay designed for early
automatic telephone exchanges.
An earth leakage circuit breaker includes a specialized latching relay.
Very early computers often stored bits in a magnetically latching relay, such as ferreed or the
later remreed in the 1ESS switch.
Some early computers used ordinary relays as a kind of latch—they store bits in ordinary
wire spring relays or reed relays by feeding an output wire back as an input, resulting in a
feedback loop or sequential circuit. Such an electrically latching relay requires continuous
power to maintain state, unlike magnetically latching relays or mechanically racheting relays.
In computer memories, latching relays and other relays were replaced by delay line memory,
which in turn was replaced by a series of ever-faster and ever-smaller memory technologies.
10.3.2 Coaxial Relay
Where radio transmitters and receivers share one antenna, often a coaxial relay is used as a
TR (transmit-receive) relay, which switches the antenna from the receiver to the transmitter.
This protects the receiver from the high power of the transmitter. Such relays are often used
in transceivers which combine transmitter and receiver in one unit. The relay contacts are
designed not to reflect any radio frequency power back toward the source, and to provide
very high isolation between receiver and transmitter terminals. The characteristic
impedance of the relay is matched to the transmission line impedance of the system, for
example, 50 ohms.
10.3.3 Time Delay Relay
Timing relays are arranged for an intentional delay in operating their contacts. A very short (a
fraction of a second) delay would use a copper disk between the armature and moving blade
assembly. Current flowing in the disk maintains magnetic field for a short time, lengthening
release time. For a slightly longer (up to a minute) delay, a dashpot is used. A dashpot is a
piston filled with fluid that is allowed to escape slowly; both air-filled and oil-filled dashpots
are used. The time period can be varied by increasing or decreasing the flow rate. For longer
time periods, a mechanical clockwork timer is installed. Relays may be arranged for a fixed
timing period, or may be field adjustable, or remotely set from a control panel. Modern
microprocessor-based timing relays provide precision timing over a great range.
Some relays are constructed with a kind of "shock absorber" mechanism attached to the
armature which prevents immediate, full motion when the coil is either energized or de-
energized. This addition gives the relay the property of time-delay actuation. Time-delay
relays can be constructed to delay armature motion on coil energization, de-energization, or
Time-delay relay contacts must be specified not only as either normally open or normally
closed, but whether the delay operates in the direction of closing or in the direction of
opening. The following is a description of the four basic types of time-delay relay contacts.
First we have the normally open, timed-closed (NOTC) contact. This type of contact is
normally open when the coil is unpowered (de-energized). The contact is closed by the
application of power to the relay coil, but only after the coil has been continuously powered
for the specified amount of time. In other words, the direction of the contact's motion (either
to close or to open) is identical to a regular NO contact, but there is a delay in closing
direction. Because the delay occurs in the direction of coil energization, this type of contact is
alternatively known as a normally open, on-delay:
10.3.4 Buchholz Relay
A Buchholz relay is a safety device sensing the accumulation of gas in large oil-
filled transformers, which will alarm on slow accumulation of gas or shut down the
transformer if gas is produced rapidly in the transformer oil. The contacts are not operated by
an electric current but by the pressure of accumulated gas or oil flow.
10.4 POLE AND THROW
Since relays are switches, the terminology applied to switches is also applied to relays; a
relay switches one or more poles, each of whose contacts can be thrown by energizing the
coil. Normally open (NO) contacts connect the circuit when the relay is activated; the circuit
is disconnected when the relay is inactive. Normally closed (NC) contacts disconnect the
circuit when the relay is activated; the circuit is connected when the relay is inactive. All of
the contact forms involve combinations of NO and NC connections.
The National Association of Relay Manufacturers and its successor, the Relay and Switch
Industry Association define 23 distinct electrical contact forms found in relays and switches.
Of these, the following are commonly encountered:
(i) SPST-NO (Single-Pole Single-Throw, Normally-Open) relays have a single Form
A contact or make contact. These have two terminals which can be connected or
disconnected. Including two for the coil, such a relay has four terminals in total.
(ii) SPST-NC (Single-Pole Single-Throw, Normally-Closed) relays have a single Form
B or break contact. As with an SPST-NO relay, such a relay has four terminals in
(iii) SPDT (Single-Pole Double-Throw) relays have a single set of Form C, break before
make or transfer contacts. That is, a common terminal connects to either of two
others, never connecting to both at the same time. Including two for the coil, such a
relay has a total of five terminals.
(iv) DPST – Double-Pole Single-Throw relays are equivalent to a pair of SPST switches
or relays actuated by a single coil. Including two for the coil, such a relay has a total
of six terminals. The poles may be Form A or Form B (or one of each; the
designations NO and NC should be used to resolve the ambiguity).
(v) DPDT – Double-Pole Double-Throw relays have two sets of Form C contacts. These
are equivalent to two SPDT switches or relays actuated by a single coil. Such a relay
has eight terminals, including the coil
(vi) The S (single) or D (double) designator for the pole count may be replaced with a
number, indicating multiple contacts connected to a single actuator. For example,
4PDT indicates a four-pole double-throw relay that has 12 switching terminals.
Training at 132KV GSS CPWD,JAIPUR gives the insight of the real instruments used. There
are many instruments like transformer, CT, PT, CVT, LA, relay, PLCC, bus bars, capacitor
bank, insulator, isolators, Battery etc. What is the various problem seen in substation while
handling this instruments. There are various occasion when relay operate and circuit breaker
open, load shedding, shut down, which has been heard previously.To get insight of the
substation, how things operate, how things manage all is learned there. Practical training as a
whole proved to be extremely informative and experience building and the things learnt at it
would definitely help a lot in snapping the future ahead a better way.Transmission and
distribution stations exist at various scales throughout a power system. In general, they
represent an interface between different levels or sections of the power system, with the
capability to switch or reconfigure the connections among various transmission and
distribution lines. The major stations include a control room from which operations are
coordinated. Smaller distribution substations follow the same principle of receiving power at
higher voltage on one side and sending out a number of distribution feeders at lower voltage
on the other, but they serve a more limited local area and are generally unstaffed. The central
component of the substation is the transformer, as it provides the effective in enface between
the high- and low-voltage parts of the system. Other crucial components are circuit breakers
and switches. Breakers serve as protective devices that open automatically in the event of a
fault, that is, when a protective relay indicates excessive current due to some abnormal
condition. Switches are control devices that can be opened or closed deliberately to establish
or break a connection. An important difference between circuit breakers and switches is that
breakers are designed to interrupt abnormally high currents (as they occur only in those very
situations for which circuit protection is needed), whereas regular switches are designed to be
operable under normal currents. Breakers are placed on both the high- and low-voltage side
of transformers. Finally, substations may also include capacitor banks to provide voltage
1. “Electronic Communication System”, George Kennedy, Brendan Devis, S.R.M.Prasanna,
TataMcGraw Hills, Third Edition, 2010.
2.“Carrier Commnucation Over Power Limes”,
3. “Principle of Electrical Transmission Line in Power and Communication”,J.H.Gridley,
P.Hammond , Elsevier Science Publication, Fourt Edition,2011
5. Manuals present at RRVPNL, JAIPUR
6. According training notes