Summer Internship Report -By Rahul Mehra

1 SUMMER TRAINING REPORT
A
Report On
Practical Training
Taken At
400/220 KV SCADA CONTROLLED SUBSTATION
OPERATED BY : POWER GRID CORPORATION OF INDIA LIMITED
(A Government of India Enterprise )
Submitted in the Month of July, 2015 by Under the Guidance of
Rahul Mehra Mr. Lokesh Singh Chundawat
Batch: 2012 – 2016 ( Sr. Engineer)
As part of Bachelor of Technology (Electrical Engineering)
Curriculum of
Rajasthan Technical University, Kota (Raj.)

ACKNOWLEDGEMENT
It is often said that life is a mixture of achievements, failures, experiences, exposures and
efforts to make your dream come true. There are people around you who help you realize
your dream. I acquire this opportunity with much pleasure to acknowledge the invaluable
assistance of all the people who have helped me through the course of my journey in
successful completion of this project.
I would like to express my deep gratitude to Mr. Lokesh Singh Chundawat, Sr. Engineer
PGCIL Kankroli Substation for their active support and continuous guidance without which it
would have been difficult for me to complete this project. They were generous enough to take
time out of their regular work to lend a helping hand whenever I needed one and enabling me
to complete this project.
I would also like to mention the generous guidance of Mr. H. H. Mahto, Chief Manager,
Kankroli Substation, Mr. Chandra Shekhar, Technician, Mr. Rameshwar Lal Balai,
Technician, whose guidance helped me settle down in the organization and successfully
complete the project within the relatively short time frame of 8 weeks, from 15th May, 2015
to 15 July, 2015. They were supporting enough to give me an opportunity to be a part of such
a prestigious organization for 2 months and learn the day to day functioning.
Last but by no means the least, I am grateful to the of Mr. Rohit Aheer (Assistant
Professor, Electrical Engineering Department, SITE Nathdwara) for providing a quick
turnaround time for all the requests.

EXECUTIVE SUMMARY
As a student of B.Tech – Rajasthan Technical University, Kota (Raj.), I got an opportunity to
do my summer internship in Power grid Corporation of India Limited, India’s Central
Transmission Utility.
Though my reporting office was the PGCIL Kankroli 400/220 KV Sub-Station situated in
Kankroli (Raj.), the authorities were generous enough to allocate me a working project that
Dealt with the study of existing infrastructure at the 400/220 KV substation which supplies
Power to Bhilwara (TEXTILE INDUSTRIES), to Kankroli (MARBLE FACTORIES), to
Debari, it also has important link between Southern and western Rajasthan (Jodhpur link),
Inter-Regional Tie between NR-I and WR-II (Zerda and Bhinmal Link), It is linked with
Rajasthan Atomic Power Project (Generation Link RAPP –I, II) The initial part of the project
Consisted of a thorough study of the equipments used in the transmission substation. The
study of existing infrastructure showed the advantage is its easy compatibility with the
Supervisory Control And Data Acquisition (SCADA) system which provide easy mechanism
for access and control.
The next phase of the project was about study of proposed plan of Static Var Compensator
(SVC) and NTAMC with an existing facility. The main advantage of SVCs over simple
mechanically switched compensation schemes is their near-instantaneous response to change
in the system Voltage. Static Var Compensator (SVC) provides fast acting dynamic reactive
compensation for voltage support during contingency events which would otherwise depress
the voltage for a significant length of time. SVC also dampens power swings and reduces
system losses by optimized reactive power control.

The emphasis on the power sector to ensure the growth in GDP has brought in many changes
in the business environment of Power Sector. The transmission sector being the integral part
of, is also facing multiple challenges like competitive bidding for transmission project,
stringent demands by the regulator etc. Thus, state of the art computerized control centers
NTAMC & RTAMC with associated telecommunication system and adapted substation for
enabling remote centralized operation, monitoring and control of POWERGRID
Transmission system has been proposed. The aim is to have completely unmanned substation
except security personnel.

Table of Content
1 Introduction..........................................................................................................................8
2 Why we need a Sub-Station? ............................................................................................ 8
3 About PGCIL Kankroli 400/220 KV Sub-Station...................................................................9
4 SLD Of PGCIL Kankroli 400/220KV Sub-Station...............................................................….10
5 Detail of erected equipments in 400/220kv kankroli sub-station…………………….……….11
6 Classification of Sub-station........................................................................................…….13
6.1 According to service requirement……………………………………………………………..…..……….13
6.2 According to constructional features ………………………………………………………………………14
6.3 According to nature of duties…………………………………………………………………………………..14
6.4 According to operating voltage……………………………………………………………………….………15
7 Sub-station site selection ………………………………………………………………………………….………15
8 Main equipments in a sub-station .................................................................................15
8.1 Power Transformer .........................................................................................................16
8.2 Instrument Transformer……………………………………………………………………………..…….…....18
8.3 Bus Bar…………………………………………………………………………………………………….………..……19
8.3.1Typical Bus Configurations……………………………………………………………………….…….…….19
8.3.1.A Single Bus……………………………………………………………………………………………………..…19
8.3.1.B Sectionalized Bus………………………………………………………………………………………..……20
8.3.1.C Main and Transfer Bus……………………………………………………………………………………..20
8.3.1.D Ring Bus…………………………………………………………………………………………………….…….21
8.3.1.E Breaker-and-a-Half……………………………………………………………………………………..…..22

8.3.1.F Double Breaker-Double Bus…………………………………………………………………………....…..23
9 Isolator…………………………………………………………………………………………………………………………24
10 Relay……………………………………………………………………………………………………..…………………….25
11 Circuit Breaker………………………………………………………………………………………..……..…………….25
11.1 SF6 Circuit Breaker………………………………………………………………….……………………….…………26
12 Wave-trap…………………………………………………………………………………………………………………….26
13 Reactor………………………………………………………………………………………………………….……...….…26
14 Lightning Arrestor…………………………………………………………………………………..……………..……..27
15 Sub-station Protection…………………………………………………………………………….………..……..…..29
16 Transformer and reactor protections…………………………………………………………….……………33
17 Circuit breaker auxiliary relays……………………………………………………………………………….37
17 Transmission line protection………………………………………………………………………..………………39
19 Bus bar protection…………………………………….………………………………….……………...………………41
20 Supervisory Control And Data Acquisition (SCADA)………………………………………..……...42
20.1 Parts of a SCADA System…………………………………………………………………….…………...……43
21 National Transmission-Asset Management Centre (NTMC) ......................................44
21.1 Operational Philosophy……………………………………………………………………………….…………..…46
21.2 Structure of NTMC..…………………………………………………………………..………..………….46
22 Static VAR compensator (SVC) ………………………………………………………………………...48
22.1 Control Concept of SVC…………………….………………………………………..……………………..…...49
22.2 The Thyristor Controlled Reactor ………………………………………………………….…………..…50
23 Conclusion…………………………………………………………………………………………….……………………52

Figure 24: SVC with control concept………………………………………………………………..………..50
Figure 25: Elementary thyristor-controlled reactor (TCR)…………………………………………….51
List of Tables
Table 1: Detail of erected equipments in 400/220kv kankroli sub-station............................ 11
Table 2: 400/220kv kankroli sub-station at a glance............................................………….……12
Table 3: Associated transmission lines with 400/220kv kankroli sub-station...................... .12

1: Introduction
Electric power is produced at the power generating stations, which are generally located far
away from the load centers. High voltage transmission lines are used to transmit the electric
power from the generating stations to the load centers. Between the power generating station
and consumers a number of transformations and switching stations are required. These are
generally known as substations. Substations are important part of power system and form a
link between generating stations, transmission systems and distribution systems. It is an
assembly of electrical components such as bus-bars, switchgear apparatus, power
transformers etc. Their main functions are to receive power transmitted at high voltage from
the generating stations and reduce the voltage to a value suitable for distribution. Some
substations provide facilities for switching operations of transmission lines, others are
converting stations. Substations are provided with safety devices to disconnect equipment or
circuit at the time of faults. Substations are the convenient place for installing synchronous
condensers for the purpose of improving power factor and it provide facilities for making
measurements to monitor the operation of the various parts of the power system. The
substations may be classified in according to service requirements and constructional
features. According to service requirements it is classified in to transformer substations,
switching substations and converting substations.
The present day electrical power system is a.c. i.e. electric power is generated, transmitted
and distributed in the form of Alternating current. The electric power is produce at the power
station, which are located at favorable places, generally quite away from the consumers. It is
delivered to the consumer through a large network of transmission and distribution. At many
place in the line of power system, it may be desirable and necessary to change some
characteristic (e.g. Voltage, ac to dc, frequency p.f. etc.) of electric supply. This is
accomplished by suitable apparatus called sub-station for example, generation voltage (11KV
or 6.6KV) at the power station is stepped up to high voltage (Say 765KV to 400KV) for
transmission of electric power. Similarly near the consumer’s localities, the voltage may have
to be stepped down to utilization level. This job is again accomplished by suitable apparatus
called sub-station.
2: Why we need a Sub-Station?
Sub-Station forms an important link between Transmission network and Distribution
network. It has a vital influence of reliability of service. Apart from ensuring efficient
transmission and Distribution of power, the sub-station configuration should be such that it
enables easy maintenance of equipment and minimum interruptions in power supply.

3: About the PGCIL Kankroli Sub-Station
Kankroli sub-station is situated 12 km milestone on Kankroli-Bhilwara highway. It is
constructed in a piece of 45.0 acres of land.
Mavli is nearest railway station which is 60 km away from sub-station and nearest
airport is Maharana Pratap Airport in (Udaipur).
The sub-station was commissioned in March 2008 with the total transformation
capcity:3x3 15 MVA= 945 MVA.
The Sub-Station acts as an inter-regional link between NR-I and WR-II plays an
important role in power system strengthening of south-western Rajasthan.
It mainly evacuates power from RAJSTHAN ATOMIC POWER PROJECT and
supply it to South-Western Rajasthan.
Double main and transfer bus scheme has been employed in 220 KV S/Y and one and
half breaker scheme (I-TYPE) has been employed in 400 KV S/Y in this Sub-Station.
Figure 1: View of Kankroli sub-station

4: Single line diagram (SLD) PGCIL, Kankroli Sub-Station
Figure 2: Single line diagram
A Single Line Diagram (SLD) of an Electrical System is the Line Diagram of the concerned
Electrical System which includes all the required ELECTRICAL EQUIPMENT connection
sequence wise from the point of entrance of Power up to the end of the scope of the
mentioned Work. As these feeders enter the station they are to pass through various
instruments. The instruments have their usual functioning.

5: DETAIL OF ERECTED EQUIPMNTS
DETAIL OF ERECTED EQUIPMNTS IN 400/220KV KANKROLI SUB-STATION
AS ON DATE-15.07.2015
S.No
CB(3 PHASE) CT CVT LA ISOLATOR EARTH SWITCH
MAKE QTY MAKE QTY MAKE QTY MAKE QTY MAKE QTY MAKE QTY
400 KV SYSTEM
1) AREVA 10 TELK 30 AREVA 12 AREVA 30 ELPRO 108 ELPRO 114
2) ABB 4 ABB 12 ABB 09 CGL 15 HIVELM 18 HIVELM 18
3) SIEMENS 1 CGL 3 0 0 CGL 3 SIEMENS 2 SIEMENS 2
TOTAL 15 45 21 48 128 134
220 KV SYSTEM
1) SIEMENS 11 TELK 33 AREVA 18 AREVA 27 ELPRO 123 ELPRO 57
2)
TOTAL 11 33 18 27 123 57
S.No Name of the Equipment Make Rating Qty.
1) ICT-I BHEL 315 MVA 1
2) ICT-II CGL 315 MVA 1
3) ICT-III BHEL 315 MVA 1
4) BUS RACTOR CGL 50 MVAR 1
5) BUS REACTOR CGL 125 MVAR 1
6) LINE REACTOR BHEL 50 MVAR 2
7) LINE REACTOR CGL 50 MVAR 2
Table 1: Detail of erected equipments in 400/220kv kankroli sub-station

Strategic Presentation
400kv D/C 400kv D/C
220kv S/C
220kv S/C
220kv S/C
220kv S/C
Figure 3: Strategic Presentation of
Associated transmission lines with 400/220kv kankroli sub-station
6: CLASSIFICATION OF SUBSTATION
6.1: According to service requirement
a) Transformer sub-station: Those sub-station which change the voltage level of electrical
supply is called Transformer sub-station.
b) Switching sub-station: This sub-station simply perform the switching operation of power
line.
c) Power factor correction S/S: This sub-station which improves the p.f. of the system are
called p.f. correction s/s. these are generally located at receiving end s/s.
400/220KV Kankroli Sub-
Station.
RAPP Generation Link
(RAPP –I,II)
Important link between
Southern and western
Rajasthan (Jodhpur link)
Inter-Regional Tie between
NR-I and WR-II ( Zerda and
Bhinmal link)
SOURCE TO BHILWARA
(TEXTILE INDUSTRIES)
SOURCE TO KANKROLI
(MARBLE FACTORIES
SOURCE TO DEBARI

d) Frequency changer S/S: Those sub-stations, which change the supply frequency, are
known as frequency changer s/s. Such s/s may be required for industrial utilization
e) Converting sub-station: That sub-station which change A.C power into D.C. power are
called converting s/s ignition is used to convert AC to dc power for traction, electroplating,
electrical welding etc.
f) Industrial sub-station: Those sub-stations, which supply power to individual industrial
concerns, are known as industrial sub-station.
6.2: According to constructional features
a) Outdoor Sub-Station: For voltage beyond 66KV, equipment is invariably installed
outdoor. It is because for such Voltage the clearances between conductor and the space
required for switches, C.B. and other equipment becomes so great that it is not economical to
install the equipment indoor. 23
b) Indoor Sub-station: For voltage up to 11KV, the equipment of the s/s is installed indoor
because of economic consideration. However, when the atmosphere is contaminated with
impurities, these sub-stations can be erected for voltage up to 66KV
Figure 6 24
c) Underground sub-station: In thickly populated areas, the space available for equipment
and building is limited and the cost of the land is high. Under such situations, the sub-station
is created underground. The design of underground s/s requires more careful consideration.
The size of the s/s should be as small as possible.
There should be reasonable access for both equipment & personal.
There should be provision for emergency lighting and protection against fire.
There should be good ventilation
6.3: According to nature of duties
a) Step-up or Primary Substations- Where from power is transmitted to various load
centers in the system network and are generally associated with generating stations.
b) Step-up and Step-down or Secondary Substations- may be located at generating points
where from power is fed directly to the loads and balance power generated is transmitted to
the network for transmission to other load centers.
c) Step-down or Distribution Substations- receives power from secondary substations at
extra high voltage (above 66 kV) and step down its voltage for secondary distribution.

6.4: According to operating voltage
a) High Voltage Substations (HV Substations) - involving voltages between 11 kV and 66
kV.
b) Extra high voltage substations (EHV Substations) - involving voltages between 132 kV
and 400 kV and
c) Ultra high voltage substations (UHV Substations) - operating on voltage above 400 kV
7: Sub-station site selection
The aspects necessary to be considered for site selection are:
Fairly level ground
Right of way around the substation yard for incoming & outgoing transmission &
distribution lines.
Preferably of soil strata having low earth resistance values
Easy approach & accessibility from main roads for Heavy equipment transportation
and routine O & M of substation.
Economy / Cost
8: MAIN EQUIPMENTS USED IN A SUBSTATION
A substation is an assembly of various electrical equipments connected to step down electric
power at higher voltages and to clear faults in the system. The various electrical equipments used
in the substation are as follows:-
1. Power Transformers (ICT)
2. Instrument Transformers i.e. CT and CVT
3. Bus Bars
4. Isolators
5. Relays
6. Circuit Breakers
7. Lightening Arrestors
8. Wavetrap
9. Reactor

8.1: POWER TRANSFORMERS (Inter Connected Transformer I.C.T)
This is the costliest equipment of substation. ICT is used to step down the EHV transmission
Voltage (400kv) to HV transmission voltage (220kv). Normally 315 MVA auto transformers
are being used. The secondary winding provides 220 KV voltages and other 33 KV voltage
(tertiary winding). Usually tertiary winding is connected in closed delta formation and can be
used for auxiliary station supply purpose. In practice, it is preferred to installed three phase
ICT as far as possible however in case of hilly terrain, where due to transportation
limitations, three single phase units are installed.
A transformer is a device that transfers electrical energy from one circuit to another through
inductively coupled conductors—the transformer's coils. A varying current in the first or
primary winding creates a varying magnetic flux in the transformer's core, and thus a varying
magnetic field through the secondary winding.
With transformers, however, the high cost of repair or replacement, and the possibility of a
violent failure or fire involving adjacent equipment, may make limiting the damage a major
objective. The protection aspects of relays should be considered carefully when protecting
transformers. Faults internal to the transformer quite often involve a few turns. While the
currents in the shorted turns are large in magnitude, the changes of the currents at the
terminals of the transformer are low compared to the rating of the transformer.
Figure 4: 315 MVA interconnected transformer ICT

There are different parts of a transformer given below:
i. Bushing: This maintains the incoming and outgoing connection of a transformer.
Figure 5: Bushing of transformer
ii. Radiator: This is used to radiate the heat of a transformer when transformer is heated up
at a certain level.
iii. Oil temperature meter: This meter indicates the temperature of transformer oil. If
temperature crosses a certain level then it makes an alarm.
iv. Temperature meter: This meter indicates the temperature of transformer windings. If
temperature crosses a certain level then it starts the winding fans.
v. Oil level meter: This meter indicates the oil level of transformer. If oil is low than a
certain amount it makes an alarm that means that transformer have to feed oil.
vi. Silica gel: It works like breathing. There have a little amount oil under the silica gel which
suck the moisture of air and further sends this air to silica gel which further sucks the rest of
the moisture of the air.
Figure 6: Silica gel in a cylinder

vii. Exchanger: Regulate voltage through winding selection between primary & secondary side.
viii. PRD (Pressure relief device): release the oil pressure by releasing oil when oil pressure is
high.
8.2: Instrument Transformer:
They are devices used to transform voltage and current in the primary system to values
suitable for measuring instruments, meters, protective relays etc. They are basically the
current transformers and voltage transformers.
Current transformers:
Current transformer is similar in construction to single phase power transformer and obeys
the same fundamentals laws but primary current of CT is not controlled by the connected
load in secondary circuit. In facts it is governed by the current in the main circuit viz.
line/transformer to which is connected. A typical 400/220 KV CT has five cores which is
used for following functions:-
Core 1: Busbar I protection
Core 2: Busbar II protection
Core 3: Metering
Core 4: Main I Protection
Core 5: Main II Protection
The metering core of CT is of accuracy class of 0.5 whereas the other cores having accuracy
of PS class which is a special protection class for which Knee point Voltage and max.
exciting current is specified.
Capacitive Voltage Transformer (CVT’s):
It is used for providing small representative voltage of primary system for metering and
protection applications. CVT consists of coupling capacitors, intermediate voltage
transformer, High frequency coupling terminal. The H.F terminal is used for PLCC purpose.
The CVT has three cores which are utilized as follows.
Core 1: Main I protection
Core 2: Main II protection
Core 3: Metering.
The accuracy class of protection core is 3P and metering core is 0.5.

8.3: BUS-BAR
In electrical power distribution, a bus bar is a thick strip of copper or aluminum that conducts
electricity within a switchboard, distribution board, substation or other electrical apparatus.
Bus bars are used to carry very large currents, or to distribute current to multiple devices
within switchgear or equipment. Bus bars are typically either flat strips or hollow tubes as
these shapes allow heat to dissipate more efficiently due to their high surface area to cross
sectional area ratio. The size of the bus bar is important in determining the maximum amount
of current that can be safely carried.
Bus bar may either be supported on insulators, or else insulation may completely surround it.
Bus bars are protected from accidental contact either by a metal enclosure or by elevation out
of normal reach. Bus bars may be connected to each other and to electrical apparatus by
bolted or clamp connections.
8.3.1: Typical Bus Configurations
8.3.1.A: Single Bus
Figure shows the one-line diagram of a single bus substation configuration. This is the
simplest of the configurations, but is also the least reliable. It can be constructed in either of
low profile or high-profile arrangement depending on the amount of space available. In the
arrangement shown, the circuit must be de-energized to perform breaker maintenance, which
can be overcome by the addition of breaker bypass switches, but this may then disable
protection systems.
Figure 7: Single Bus
Single Bus Advantages:
Lowest cost
Small land area
Easily expandable
Simple in concept and operation
Relatively simple for the application of protective relaying

Single Bus Disadvantages:
Single bus arrangement has the lowest reliability
Failure of a circuit breaker or a bus fault causes loss of entire substation
Maintenance switching can complicate and disable some of the protection schemes
and overall relay coordination
8.3.1.B: Sectionalized Bus
Figure 3 shows the layout of a sectionalized bus, which is merely an extension of the single
bus layout. The single bus arrangements are now connected together with a center circuit
breaker that may be normally open or closed. Now, in the event of a breaker failure or bus bar
fault, the entire station is not shut down. Breaker bypass operation can also be included in the
sectionalized bus configuration.
Figure 8: Sectionalized Bus
Sectionalized Bus Advantages:
Flexible operation
Isolation of bus sections for maintenance
Loss of only part of the substation for a breaker failure or bus fault
Sectionalized Bus Disadvantages:
Additional circuit breakers needed for sectionalizing, thus higher cost Sectionalizing
may cause interruption of non-faulted circuits
8.3.1.C: Main and Transfer Bus
A main and transfer bus configuration is shown in Figure 4. There are two separate and
independent buses; a main and a transfer. Normally, all circuits, incoming and outgoing, are
connection the main bus. If maintenance or repair is required on a circuit breaker, the
associated circuit can be then fed and protected from the transfer bus, while the original
breaker is isolated from the system.

Ring Bus Advantages:
Flexible operation High reliability
Double feed to each circuit
No main buses Expandable to breaker-and-a-half configuration
Isolation of bus sections and circuit breakers for maintenance without circuit
disruption
Ring Bus Disadvantages:
During fault, splitting of the ring may leave undesirable circuit combinations
Each circuit has to have its own potential source for relaying
Usually limited to 4 circuit positions, although larger sizes up to 10 are in service. 6 is
usually the maximum terminals for a ring bus
8.3.1.E: Breaker-and-a-Half
A breaker-and-a-half configuration has two buses but unlike the main and transfer scheme,
both busses are energized during normal operation. This configuration is shown in Figure 6.
For every 2 circuits there are 3 circuit breakers with each circuit sharing a common center
breaker. Any breaker can be removed for maintenance without affecting the service on the
corresponding exiting feeder, and a fault on either bus can be isolated without interrupting
service to the outgoing lines. If a center breaker should fail, this will cause the loss of 2
circuits, while the loss of an outside breaker would disrupt only one. The breaker-and-a-half
scheme is a popular choice when upgrading a ring bus to provide more terminals.
Figure 11: Breaker-and-a-Half

Breaker-and-a-Half Advantages:
Flexible operation and high reliability
Isolation of either bus without service disruption
Isolation of any breaker for maintenance without service disruption
Bus fault does not interrupt service to any circuits
All switching is done with circuit breakers
Breaker-and-a-Half Disadvantages:
One-and-a-half breakers needed for each circuit
More complicated relaying as the center breaker has to act on faults for either of the 2
circuits it is associated with
Each circuit should have its own potential source for relaying Substation
Configuration Reliability
8.3.1.F: Double Breaker-Double Bus
The final configuration shown is the double breaker – double bus scheme in figure 7. Like the
breaker-and-a-half, the double breaker-double bus configuration has two main buses that are
both normally energized. Here though, each circuit requires two breakers, not one-and-a-half.
With the addition of the extra breaker per circuit, any of the breakers can fail and only affect
one circuit. This added reliability comes at the cost of additional breakers, and thus is usually
only used at large generating stations.
Figure 12: Double Breaker-Double Bus

Double Breaker-Double Bus Advantages:
Flexible operation and very high reliability
Isolation of either bus, or any breaker without disrupting service
No interruption of service to any circuit from a bus fault
Loss of one circuit per breaker failure
All switching with circuit breakers
Double Breaker-Double Bus Disadvantages:
Very high cost – 2 breakers per circuit
9: ISOLATOR
In electrical systems, an isolator switch is used to make sure that an electrical circuit is
completely de-energized for service or maintenance. Such switches are often found in
electrical distribution and industrial applications where machinery must have its source of
driving power removed for adjustment or repair. High-voltage isolation switches are used in
electrical substations to allow isolation of apparatus such as circuit breakers and transformers,
and transmission lines, for maintenance.
An isolator can open or close the circuit when either a negligible current has to be broken or
made or when no significant voltage change across the terminals of each pole of isolator
occurs. It can carry current under normal conditions and can carry short circuit current for a
specified time. They can transfer load from one bus to another and also isolate equipments for
maintenance. Isolators guarantee safety for the people working on the high voltage network,
providing visible and reliable air gap isolation of line sections and equipment. They are
basically motorized i.e. motor does the closing and opening of the isolator.
Isolators are distinguished as “off load” and “on load” isolator.
Figure 13: Isolator in a sub-station

10: RELAYS
A relay is an electrically operated switch. Many relays use an electromagnet to operate a
switching mechanism mechanically, but other operating principles are also used. Relays are
used where it is necessary to control a circuit by a low-power signal (with complete electrical
isolation between control and controlled circuits), or where several circuits must be
controlled by one signal. 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".
Types of relays:
Electromagnetic attraction relay
Electromagnetic induction relay
Thermal relay
Buchholz relay
Numerical relay
Over current relay
11: CIRCUIT BREAKER
A circuit breaker is an automatically operated electrical switch designed to protect an
electrical circuit from damage caused by overload or short circuit. Its basic function is to
detect a fault condition and, by interrupting continuity, to immediately discontinue electrical
flow. Unlike a fuse, which operates once and then has to 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 an individual household appliance up
to large switchgear designed to protect high voltage circuits feeding an entire city.
Figure 14: Circuit Breaker in a substation

The type of the Circuit Breaker is usually identified according to the medium of arc
extinction. The classification of the Circuit Breakers based on the medium of arc extinction is
as follows:
Air break Circuit Breaker.
Oil Circuit Breaker (tank type of bulk oil)
Minimum oil Circuit Breaker.
Air blast Circuit Breaker.
Vacuum Circuit Breaker.
Sulphur hexafluoride Circuit Breaker. (Single pressure or Double Pressure).
11.1: SF6 circuit breaker
SF6 is inert gas the property of this gas the higher pressure and temperature its dielectric
strength will be SF6has two gas chamber when contract is close the pressure is two chamber
have the same pressure but when the contract is open then one of the chamber get totally
close and other remain open ,there is a narrow channel between two chamber and when
contract open the SF6 flow a plane of high pressure region to the low pressure region there
will be turbulence of SF6.At zero current the turbulence of SF6 absorb all the ions and since
it is flowing from a narrow region hence it provide high dielectric strength but there is
problem that the pressure of SF6 is not always remain fixed due to leakage in the cylinder of
SF6 so there is pressure gauge as well as alarm attached with it. Whenever pressure decreases
the alarm ringing and the gas is refilled to increase pressure.
12: Reactor
It is similar in appearance and used for absorbing the reactive power from the system. When
the system voltage is high. It has air core, oil filled ONAN type. Generally 50 and 63 MVAR
shunt reactor are used with both the LINE/BUS both non-switchable/ switchable type shunt
reactors are in use.
13: Wavetrap
It is an inductor having tuned LC circuit, which is mainly used for PLCC purpose. It offers
very high impedance to high frequency PLCC signals does not allow them to enter in S/Y and
offers very low impedance for frequency currents.

Figure 16: lightning arrester in a sub-station
15: SUBSTATION PROTECTION
Substation Grounding/Earthing
The sole purpose of substation grounding/earthing is to protect the equipment from surges
and lightning strikes and to protect the operating persons in the substation. Hence intentional
earthing system is created by laying earthing rod of mild steel in the soil of substation area.
All equipments/structures which are not meant to carry the currents for normal operating
system are connected with main earth mat .The substation earthing system is necessary for
connecting neutral points of transformers and generators to ground and also for connecting
the non current carrying metal parts such as structures, overhead shielding wires, tanks,
frames, etc to earth. Earthing of surge arresters is through the earthing system. The function
of substation earthing system is to provide a grounding mat below the earth surface in and
around the substation which will have uniformly zero potential with respect to ground and
low earth resistance.

The earthing system in a substation:
Protects the life and property from over-voltage.
To limit step & touch potential to the working staff in substation. Provides low
impedance path to fault currents to ensure prompt and consistent operation of
protective device.
Stabilizes the circuit potentials with respect to ground and limit the overall potential
rise.
Keeps the maximum voltage gradients within safe limit during ground fault condition
inside and around substation.
Earth Resistance:
Earth Resistance is the resistance offered by the earth electrode to the flow of current in to the
ground. To provide a sufficiently low resistance path to the earth to minimize the rise in earth
potential with respect to a remote earth fault. Persons touching any of the non current
carrying grounded parts shall not receive a dangerous shock during an earth fault. Each
structure, transformer tank, body of equipment, etc, should be connected to earthing mat by
their own earth connection. Generally lower earth resistance is preferable but for certain
applications following earth resistance are satisfactory
Large Power Stations – 0.5 Ohm
Major Power Stations - 1.0 Ohm
Small Substation – 2.0 Ohm
In all Other Cases – 8.0 Ohm
Step Potential and Touch Potential
Grounding system in a electrical system is designed to achieve low earth resistance and also
to achieve safe ‘Step Potential ‘and ‘Touch Potential’.
Step Potential:
Step potential is the potential difference between the feet of a person standing on the floor of
the substation, with 0.5 m spacing between the feet (one step), through the flow of earth fault
current through the grounding system.
Touch Potential:
Touch potential is a potential difference between the fingers of raised hand touching the
faulted structure and the feet of the person standing on the substation floor. The person

should not get a shock even if the grounded structure is carrying fault current, i.e. The Touch
Potential should be very small.
Usually, In a substation a surface layer of 150 mm of rock (Gravel) of 15 mm to 20 mm size
shall be used for the following reasons:
To provide high resistivity for working personnel.
To minimize hazards from reptiles.
To discourage growth of weed.
To maintain the resistivity of soil at lower value by retaining moisture in the under
laying soil.
To prevent substation surface muddy and water logged.
Figure 17: Step Potential and Touch Potential
FORMATION OF SUBSTATION EARTHING:
The main earth mat shall be laid horizontally at a regular spacing in both X & Y
direction(9m) based upon soil resistivity value and substation layout arrangement .The main
earth mat shall be laid at a depth of 600 mm from ground. The earth mat shall be connected to
the following in substation

i. Lightning down conductor, peak of lightning mast
ii. Earth point of S A, CVT
iii. Neutral point of power Transformer and Reactor
iv. Equipment framework and other non-current carrying parts.
v. Metallic frames not associated with equipments
vi. Cable racks, cable trays and cable armor.
LIGHTNING PROTECTION
The protection from the lightning is done with the help of shield wire and lightning mast
(high lattice structure with a spike on top).
Shield wire
Shield wire lightning protection system will be generally used in smaller sub stations of:
Lower voltage class, where number of bays is less, area of the substation is small, & height of
the main structures is of normal height. The major disadvantage of shield wire type lightning
protection is, that it causes short circuit in the substation or may even damage the costly
equipments in case of its failure (snapping ).
Earth wire
Overhead power lines are often equipped with a ground conductor (shield wire or overhead
earth wire). A ground conductor is a conductor that is usually grounded (earthed) at the top of
the supporting structure to minimize the likelihood of direct lightning strikes to the phase
conductors. The ground wire is also a parallel path with the earth for fault currents in earthed
neutral circuits. Very high-voltage transmission lines may have two ground conductors.
These are either at the outermost ends of the highest cross beam, at two V-shaped mast
points, or at a separate cross arm. By protecting the line from lightning, the design of
apparatus in substations is simplified due to lower stress on insulation. Shield wires on
transmission lines may include optical fibers (OPGW), used for communication and control
of the power system.
7/3.66 mm wire is used for providing earthing in lightning mast and towers. The main
function of Earth wire/Ground wire is to provide protection against direct lightening strokes
to the line conductors or towers.

List of Figures
Figure 1: View of Kankroli sub-station................................................................................... 9
Figure 2: Single line diagram Kankroli Sub-Station……….......................................................10
Figure 3: Strategic Presentation of Associated transmission lines with sub-station………….13
Figure 4: 315 MVA interconnected transformer (ICT).......................................................... 16
Figure 5: Bushing of transformer......................................................................................... 17
Figure 6: Silica gel in a cylinder............................................................................................. 17
Figure 7: Single Bus............................................................................................................... 19
Figure 8: Sectionalized Bus ................................................................................................... 20
Figure 9: Main and Transfer Bus............................................................................................ 21
Figure 10: Ring Bus………………………………………………………………………………………………….21
Figure 11: Breaker-and-a-Half………………………………………………………………………………….22
Figure 12: Double Breaker-Double Bus…………………………………………………………………....23
Figure 13: Isolator in a sub-station…………………………………………………………………………24
Figure 14: Circuit Breaker in a substation………………………………………………………………..25
Figure 15: wavetrap in a sub-station………………………………………………………….……………27
Figure 16: lightning arrester in a sub-station………………………………………………………….28
Figure 17: Step Potential and Touch Potential…………………………………………………………30
Figure 18: earth wire…………………………………………………………………………………………….32
Figure 19: Buchholz Relay………………………………………………………………………………………………….33
Figure 20: Location of buchholz relay on transformer………………………………………………………34
Figure 21: Structure of NTMC ……………………………………………………………………………46
Figure 22: Evolution of voltage level in India…………………………………………………………...47
Figure 23: Network Management System in India……………………………………………………47

16: TRANSFORMER AND REACTOR PROTECTIONS
Buchholz Relay:
This relay is located in pipe between Trf. tank and conservator and protects/warns for
incipient faults internal to the Trf. Main faults in this group are core insulation failure,
loss of oil and wdg. Turn to turn short.
The relay has two elements; upper one is a float with a mercury switch. lower elements
consists of baffle plate and a mercury switch. Due to incipient minor faults gas is
produced which in turn reduces oil level in relay and upper float sinks causing its mercury
switch to close and alarm is initiated.
In case of severe fault likes phase to earth or ph˗ph short circuit or faults in OLTC a
surge of oil results and it strikes a baffle plate which causes second mercury switch to
close and trip command to Trf. CB is initiated .
Figure 19: Buchholz Relay

Figure 20: location of buchholz relay on transformer
PRV (Pressure Relief Valve) protection:
On transformer tank two pressure relieving valves are provided which opens whenever
the pressure inside the trf .Tank increases beyond designed value.
The operation of PRV involves loss of substantial trf . oil and this protection operates
whenever heavy gas is generated inside the transformer tank because of severe insulation
failure in core or transformer windings.
A Micro switch closes on PRV operation which energizes an aux. relay whose N/O
contact closes and trips the trf. HV & LV CBs.
Inter –tripping of HV & LV CB is incorporated.
Differential protection:
Low impedance type static percentage biased restraint differential relay MBCH13 is
provided for detecting Phase to earth fault and phase to phase fault internal to trf.
and terminal faults.
The relay has an operating time of 10-25 sec. and provides high stability against
heavy thorough faults, magnetizing inrush current and over fluxing conditions.
The setting range of differential current is 10-50% of in .generally relay diff. current
is set at 20% in. The relay also has a high -set ranging feature and varies from 4 in at
normal load condition to 8 In at heavy thorough faults .
Inter -tripping of HV & LV CB is incorporated.

Back up O/C and E/F protection:
Directional O/C and E/F backup protection against external/internal short circuits and
excessive O/L is provided by CDD31 IDMT relay.
Restricted E/F protection:
CAG14 high impedance relay is employed to provide restricted to E/F protection for
transformer windings.
This is also a differential protection where 3 line CT output are paralleled and balanced
against neutral CT output .The differential current setting is kept at minimum i.e.10% In .
Inter -tripping of HV & LV CB is incorporated.
Over fluxing protection:
GTTM relay operating of V/F ration is used to protect Transformer against over fluxing
condition. this occurs when large quantum of trf. Load is disconnected which results in
the rise of transformer Primary voltage and its exciting current.
Oil temperature alarm/trip:
• Oil temperature is measured and indicated by oil temperature indicator When temperature
reaches 85degree C alarm is initiated .If left unattended and reaches 95 degree C trip
command to HV & LV CBs is given.
Winding temperature alarm/trip:
Winding temp. is indirectly measured by adding equivalent temperature generated by
flow of load current in transformer.
For this a turret CT is used to supply load current to a thermister whose resistance
changes according to temperature rise due to current flow .
Generally an aux. relay contacts are used for initiating winding temp. alarm /trip .
When temperature reaches 90degree C first aux . relay operates and its N/O contact closes
the supply ckt . of cooler fans start them.
When temperature reaches 95 degree C second aux. relay operates and its N/O contact
close the supply ckt .of cooler oil pump and pump starts.
When temperature reaches 100 degree C third aux. relay operates and its N/O contact
closes the alarm ckt.
When temperature reaches 110 degree C fourth aux. relay operates and its N/O contact
closes the trip ckt to the trf. HV & LV CBs.

Low oil level alarm:
Whenever oil level in the transformer drops below designed /specified safe operation
level an alarm is initiated through a micro switch connected with MOG installed at
transformer main tank which operates an aux. relay to initiate alarm in the control room.
The relay has stepped time distance characteristics for three independent measuring
zones, having quadrilateral shaped mho characteristics.
The max operating time of the relay for zone -1 fault is 40 ms for all types of 30-75
degrees.
The relay schemes include timers for zone -2&3 are having continuously variable setting
of 0-3&0-5 seconds respectively.
It is suitable for carrier aided tripping.
It is having power swing blocking protection for blocking the tripping in zone 2&3 but
tripping can be permitted in zone -1, if desired.
The relay has memory circuit to ensure correct operation during close 3-phase fault or
switch on to fault feature (SOFT).
The relay also provides Distance to fault measurement.
Auto –reclose Relay:
VARM relay is provided which is suitable for 1/3 phase auto -reclosing.
The relay has continuously variable Dead time setting range of 0.1-2 sec.
The relay scheme has reclaim timer setting of 25secs.
The relay scheme is of single shot type.
The scheme has provision for assigning priority in both CBs in case of 11/2 breaker
scheme to allow closing of main CB first.
The relay scheme has facility for selecting check synchronizing or dead line charging
feature for which SKD and VAG type relays provided respectively.
SKD relay have response time of 200 ms and a continuously variable timer with range
0.5-5 seconds. The max. phase angle setting is 35 degrees and max. voltage difference
setting is 10%.
VAG type relay have two sets of relay and each set is able to monitor the 3-phase voltage
where one set is connected to line CVTs with fixed setting of 20% of rated voltage and
other set is connected to bus CVTs with a fixed setting of 80% of rated voltage.
Over Voltage Relay:
3 VTU type relays provided with two stages.
1st
stage has IDMT characteristics and adjustable setting of 100-170% of rated voltage
with a timer of 1-60 seconds.
2nd
stage has instantaneous characteristics and adjustable setting of 100-170% of rated
voltage with a timer of 0-200ms.
Voltage setting of 110% &150% and Timer setting of 4 seconds & 30 ms are kept for 1st
& 2nd
stage respectively.

The relay has separate flags for operation indication of each stage.
17: CIRCUIT BREAKER AUXILIARY RELAYS
Local Breaker Back up relay:
MCTI type solid state relay is provided.
It is a fast operating relay with op. & resetting time less than 15 ms.
It has 3 O/C elements with setting range of 20-80% of rated current .The E/F elements
are set at 20%.
It incorporates timer with continuously adjustable setting range 0.1-1 sec. The timer
setting is kept as 0.2 sec.
In the event of non-operation of any CB on receipt of tripping command and
availability of 20% current through feeder CT secondary, the O/C elements of this
scheme operates and after timing bus zone CBs through B/B protection and all the
ckts connected on that particular bus are disconnected to clear the bus.
Trip Circuit Supervision Relay:
VAX type relay provided which are capable of monitoring the healthiness of each phase
trip coil and associated circuit of circuit breaker during ON and OFF conditions.
The relay initiates alarm in case of fault in trip circuit.
The relay has a time delay on drop off-of 200 ms at least and provided with flags for each
phase.
D.C. Supervision Relay:
VAA type aux relay is provided for supervising each D.C. Supply.
The relay monitors the failure of DC supply and initiates an alarm.
The relay has flag for indicating its operation.
The relay has a time delay on drop-off of 100 ms at least.
A/R Lock out (Z2&3) /Distance operated relay:
This operation of this aux .relay indicates that distance relay has operated in Zone 2/3 and
auto-reclosure feature is locked out.

Fuse failure relay:
This relay monitors all the three fuses of CVT s and its secondary cables (from S/Y to
C&R panels) for any open circuit condition.
In the event of CVT fuse failure /open circuiting of its cables, the relay operates and
inhabits trip ckts of CBs and initiates alarm.
This relay is of very fast acting type and its operating time is only 7ms.
This relay remains inoperative for system earth faults.
For Carrier aided protection following aux. relay are provided:
Carrier receive relay
Carrier healthy relay
Carrier send /aided trip relay
Direct trip received ch-1 relay
Direct trip received ch-2 relay
Pole Discrepancy Relay:
This relay is provided in Circuit Breaker. in the event of mismatch in the closing of all
three phases CB is more than 2.5 sec the relay operates and trips the closed CB and locks
out the A/R feature.
Low SF6 gas pressure protection:
SF6 pressure in the CB is kept at 7 kg /sq.cm.
When this pressure comes down due to leakage of gas an alarm is initiated at 6.5 Kg /sq.
cm.
When gas pressure comes down to 6.0kg/sq. the CB operation is locked out.
Low Air pressure protection
Air pressure in the CB is kept at 16 kg/sq. cm.
When this pressure is comes down to 14 Kg/sq.cm (due to non –functioning of
compressor) an alarm is initiated.
When this pressure comes down to 13kg/sq. cm the CB operation as well as A/R feature
are locked out.

The relay has self diagnostic features.
The relay is having stepped time distance characteristics for three independent
measuring zones, having quadrilateral shaped Mho characteristics. The max.
Operating time of the relay for zone-1 fault is 40 ms for all types of faults. The relay
has independent R/X settings and adjustable characteristics angle of 30-75 degrees.
The relay scheme has timers for Zone-2&3 are having continuously variable setting
of 0-3 & 0-5 seconds respectively.
It also has an off-set feature with 10-20% of Zone-3 to cover backward zone faults
between relay and busbar.
The relay has memory circuits to ensure correct operation during close 3-phase fault
or switch on to fault feature (SOTF).
It is suitable for carrier aided tripping.
It is having power swing blocking protection for blocking the tripping in zone 2&3
but tripping can be permitted in zone-1, if desired.
The relay also provides distance to fault measurement.
Main-II protection:
In 400KV line this protection is provided by static modular type distance scheme such
as OPTIMHO.
This relay is non-switched scheme with separate measurements for all ph-ph and ph-
earth faults.

19: BUS BAR PROTECTION
The bus bar faults generally involve one phase and earth and large no. of bus bar
faults result from human error when safety earthing done for maintenance work is not
removed before charging the bus.
The requirement of bus bar protection is high speed of operation (of the order of one
cycle) to limit the consequential damage and maintain system stability.
The other important requirement of bus bar protection is that it should be completely
stable.
To achieve this 2 independent measurement are taken by two differential relay
systems being energized from separate cores of CTs. One relay is applied to each
busbar/bus section and second relay is applied to both buses/sections and called check
system.
The tripping of busbar/section is only initiated if both i.e. its busbar/section relay has
operated and check busbar relay has also operated.
The busbar protection is based on circulating differential current measurement
principle. The CT output of all the zone/section feeders/ckts are connected in parallel
and relay measuring element is connected between phase and neutral.
The busbar protection covers phase as well as earth faults in that zone and setting for
operation is kept same for both.
The selective tripping of bus zone/bus section involved with fault is achieved through
the position of aux. contact of bus isolators of faulted feeder.
For 400KV busbar two identical low impedance biased differential protection scheme
MBCZ are employed.
It is a modular solid state relay having very fast operating time of less than 20ms.
Separate modules for feeders, tie, Bus coupler, zone measuring and bus selection
isolators are assembled to make the scheme functional.

20: Supervisory Control and Data Acquisition (SCADA)
The task of supervision of machinery and industrial processes on a routine basis can be an
Excruciatingly tiresome job. Always being by the side a machine or being on a 24x7 patrol
duty around the assembly line equipment checking the temperature levels, water levels, oil
level and performing other checks would be considered a wastage of the expertise of the
technician on trivial tasks. But, to get rid of this burdensome task, the engineers devised
equipments and sensors that would prevent or at least reduce the frequency of these routine
checks. As a result of that, control systems and it’s various off springs like SCADA systems
were formed. Supervisory Control and Data Acquisition (SCADA) offers the ease of
monitoring of sensors placed at distances, from one central location.
SCADA systems are used to monitor and control a plant or equipment in industries such as
telecommunications, water and waste control, energy, oil and gas refining and transportation.
A SCADA system gathers information, such as where a leak on a pipeline has occurred,
transfers the information back to a central site, alerting the home station that the leak has
occurred, carrying out necessary analysis and control, such as determining if the leak is
critical, and displaying the information in a logical and organized fashion. SCADA systems
can be relatively simple, such as one that monitors environmental conditions of a small office
building, or incredibly complex, such as a system that monitors all the activity in a nuclear
power plant or the activity of a municipal water system. SCADA systems were first used in
the 1960s.

20.1: Parts of a SCADA System
There are many parts of a working SCADA system. A SCADA system includes signal
hardware (input and output), controllers, networks, user interface (HMI), communications
equipment and software. All together, the term SCADA refers to the entire central system.
The central system monitors data from various sensors that are either in close proximity or
off site (sometimes miles away).
For the most part, the brains of a SCADA system are performed by the Remote Terminal
Units (sometimes referred to as the RTU). The Remote Terminal Units consists of a
programmable logic controller. The RTU are set to specific requirements, however, most
RTU allow human intervention, for instance, in a factory setting, the RTU might control the
setting of a conveyer belt, and the speed can be changed or overridden at any time by human
intervention. In addition, any changes or errors are automatically logged for and/or displayed.
Most often, a SCADA system will monitor and make slight changes to function optimally;
SCADA systems are considered closed loop systems and run with relatively little human
intervention.
One of key processes of SCADA is the ability to monitor an entire system in real time. This
is facilitated by data acquisitions including meter reading, checking statuses of sensors, etc
that are communicated at regular intervals depending on the system. Besides the data being
used by the RTU, it is also displayed to a human that is able to interface with the system to
override settings or make changes when necessary.
SCADA can be seen as a system with many data elements called points. Each point is a
monitor or sensor. Points can be either hard or soft. A hard data point can be an actual
monitor; a soft point can be seen as an application or software calculation. Data elements
from hard and soft points are always recorded and logged to create a time stamp or history.

21: NTMC & RTMC: THE NEXT LEVEL
NATIONAL TRANSMISSION- ASSET MANAGEMENT CENTRE (NTMC)
WHAT IS NTMC?
“Centralized control of entire transmission system from single point with fully automated
remote controlled sub-stations”.
The emphasis on the power sector to ensure the growth in GDP has brought in many changes
in the business environment of Power Sector. The transmission sector being the integral part
of, is also facing multiple challenges like competitive bidding for transmission project,
stringent demands by the regulator etc.
The technological development couple with falling prices of communication system and
information technology provides us the opportunity for virtual manning of Substation thereby
optimizing the requirement of skilled manpower and managing the asset with the available
skilled workforce. Thus, state of the art computerized control centers NTAMC & RTAMC
with associated telecommunication system and adapted substation for enabling remote
centralized operation, monitoring and control of POWERGRID Transmission system has
been proposed.
The aim is to have completely unmanned substation except security personnel. The
operations of the substations will be done from a remote centralized location i.e. NTAMC.
The RTAMC will co-ordinate the maintenance aspect of the substation from a centralized
location and will act as a backup to the NTACM for operation. The maintenance activities
would be carried out by maintenance service hub (MSH). One MSH will cater to the
requirements of 3-4 substations in its vicinity in coordination with the respective RTAMCs.
The substations and various control centers will be connected by redundant broadband

communication network through POWERGRID (Telecom) communication links. Telecom
Department to provide high speed communication links between NTAMC, RTAMCs and
Sub-stations. The Connectivity Status has been finalized in association with LD&C
department and NTAMC group. More links have to be planned by LD&C for total
protection. Bandwidth requirement and Connectivity Scheme finalized. At stations where this
connectivity is not possible, leased lines will be hired from other telecom operators up to the
nearest connection point.
WHY NTMC?
TECHNICAL
• With merging grids (N-E-W) & S, inter-regional power flow needs more coordination
• Long delays in layered system of manual operation.
• Delays in gathering & evaluating intelligence in standalone s/s.
ECONOMIC
• Huge Manpower Cost in Manned system
• No requirement of manning S/S operation in today’s digital, automated control system
• Low technology cost in Automation & Remote Control
• Gearing up for competitive bidding regime of transmission assets
• Stringent demands by the regulator etc.

48
Figure 22
Figure 23
SUMMER TRAINING REPORT
22: Evolution of Voltage level in indian grid
23: Network Management System in India
SUMMER TRAINING REPORT

22: STATIC VAR COMPENSATOR
A static VAR compensator (SVC) is an electrical device for providing fast-acting reactive
power compensation on high voltage transmission networks and it can contribute to improve
the voltages profile in the transient state and therefore, in improving the quality performances
of the electric services. A SVC is one of FACTS controllers, which can control one or more
variables in a power system. The dynamic nature of the SVC lies in the use of thyristor
devices (e.g. GTO, IGCT) . The thyristor, usually located indoors in a “valve house”, can
switch capacitors or inductors in and out of the circuit on a per-cycle basis, allowing for very
rapid superior control of system voltage. The compensator studied in the present work is
made up of a fixed reactance connected in series to a thyristor controlled reactor (TRC) based
on bi-directional valves- and a fixed bank of capacitors in parallel with the combination
reactance-TRC. The thyristors are turned on by a suitable control that regulates the magnitude
of the current.
Configuration of SVC
SVC provides an excellent source of rapidly controllable reactive shunt compensation for
dynamic voltage control through its utilization of high-speed thyristor switching/controlled
devices . A SVC is typically made up of coupling transformer, thyristor valves, reactors,
capacitance (often tuned for harmonic filtering).
Advantages of SVC
The main advantage of SVCs over simple mechanically switched compensation schemes is
their near-instantaneous response to change in the system voltage. For this reason they are
often operated at close to their zero-point in order to maximize the reactive power correction.
They are in general cheaper, higher-capacity, faster, and more reliable than dynamic
Compensation schemes such as synchronous compensators (condensers). In a word:

1) Improved system steady-state stability.
2) Improved system transient stability.
3) Better load division on parallel circuits.
4) Reduced voltage drops in load areas during severe disturbances.
5) Reduced transmission losses.
6) Better adjustment of line loadings.
22.1: Control Concept of SVC
An SVC is a controlled shunt susceptance as defined by control settings that injects reactive
power into the system based on the square of its terminal voltage. Fig illustrates a TCR SVC,
including the operational concept. The control objective of the SVC is to maintain a desired
voltage at the high-voltage bus. In the steady-state, the SVC will provide some steady-state
control of the voltage to maintain it the high-voltage bus at a pre-defined level. If the high-
voltage bus begins to fall below its set point range, the SVC will inject reactive power Into
thereby increasing the bus voltage back to its net desired voltage level.
If bus voltage increases, the SVC will inject less (or TCR will absorb more) reactive power,
and the result will be to achieve the desired bus voltage. From Fig. 1, +Q cap is a fixed
capacitance value, therefore the magnitude of reactive power injected into the system, Q net,
is controlled by the magnitude of reactive power absorbed by the TCR. The fundamental
operation of the thyristor valve that controls the TCR is described here. The thyristor is self
commutates at every current zero, therefore the current through the reactor is achieved by
gating or firing the thyristor at a desired conduction or firing angle with respect to the voltage
waveform.

Figure 24: SVC with control concept.
22.2: The Thyristor Controlled Reactor
The basis of the thyristor-controlled reactor (TCR) is shown in Fig. The controlling element
is the thyristor controller, shown here as two oppositely poled thyristors which conduct on
alternate half-cycles of the supply frequency. If the thyristors are gated into conduction
precisely at the peaks of the supply voltage, full conduction results in the reactor, and the
current is the same as though the thyristor controller were short circuited.

Principle of Operation
The current is essentially reactive, lagging the voltage by nearly 900. It contains a small in-
phase component due to the power losses in the reactor, which may be of the order of 0.5-2%
of the reactive power. Full conduction is shown by the current waveform. If the gating is
delayed by equal amounts on both thyristors, a series of current waveforms is obtained. Each
of these corresponds to a particular value of the gating angle α, which is measured from a
zero-crossing of the voltage. Full conduction is obtained with a gating angle of 900. Partial
conduction is obtained with gating angles between 900 and 1800. The effect of increasing the
gating angle is to reduce the fundamental harmonic component of the current. This is
equivalent to an increase in the inductance of the reactor, reducing its reactive power as well
as its current. So far as the fundamental component of current is concerned, the thyristor-
controlled reactor is a controllable susceptance, and can therefore be applied as a static
compensator.
Figure 25: Elementary thyristor-controlled reactor (TCR).

23: CONCLUSION
The past months of my training have been very instructive for me. POWER GRID
CORPORATION OF INDIA LIMITED has given me opportunities to learn and develop
myself in many areas. I gained a lot of experience, especially in the substation switchgears,
equipment and protection field. A lot of the tasks and activities that I have worked on during
my internship are familiar with what I’m studying at the moment. I worked in many areas
where I did different work.
As a bonus, I got to experience the recent projects of power grid i.e. NATIONAL
TRANSMISSION- ASSET MANAGEMENT CENTRE (NTMC) and STATIC VAR
COMPENSATOR. I learned how these projects transform existing infrastructure of the
400/220 KV substation kankroli. There is a big difference in the college projects and the
tasks and activities during the actual work. In college we learn how to describe the work in
projects, where in work you learn how to implement them in reality. This internship was
definitely an introduction to the actual work field for me.
I learned a lot from the different interns that I have been working with during my internship.
Each intern had a different educational background and that made it interesting for me. By
working with them I got to learn from them and become aware educational background.
My mentor during my internship was Mr. Lokesh Singh Chundawat (Sr. Engineer) whom
I have also learned a lot from during my internship. As a sr. engineer, he has lots of
knowledge of the working area . he was very helpful and always willing to give me advice
and feedback which I appreciate. he had always time to answer all my questions concerning
my internship.

Summer Internship Report -By Rahul Mehra

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