2. 1
A
PRACTICAL INTERNSHIP REPORT
On
Transmission & Distribution of Electrical Power
taken at
220KV GSS RRVPNL, AJMER (Raj.)
Session 2015-16
Submitted to Submitted by
Prof. S.P. Srivastava ASAFAK HUSAIN
HOD of Electrical Deptt. B.Tech 3rd
year, Electrical Science
IIT Roorkee IIT ROORKEE
Enrollment no. 12115026
Time Period: 14 May to 28 June (45 Days)
DEPT. OF ELECTRICAL SCIENCE
IIT ROORKEE, ROORKEE (U.K.)-247667
3. 2
Acknowledgement
I would like to take this opportunity to express my heartfelt words for the
people who were part of this training in numerous ways, people who gave
me unending support right from the beginning of the training.
I am really grateful to training incharge, Mr. Tarun Issrani (JEN &
executive AEN) and Mr. M. L. Jarwal (XEN) for giving proper and valuable
guidance to make the project that successful.
I also would love to thank all my batch mates there in Madar. They are really
helpful and curios about the subject and responsible to create a learning
environment.
4. 3
Abstract
This huge 220KV Grid substation is at Madar in Ajmer (Rajasthan). This
Substation is quite far from the city about 10km south. Like most of the
substations in Rajasthan, here also the main source of power is Thermal
Power Plant, Kota & Suratgarh, Nuclear power plant Ravatbhata and there
are some private power companies as well.
The incoming supplies comes from two other substations Kishangarh and
Beawar. Interestingly Madar also supplies to Kishangarh that is nothing but
an example of ring type power system.
The substation has seven power transformers as its heart. Two are at
incoming side of rating 160MVA 220KV/132KV/11KV (EMCO), one is of
rating 40/50MVA 132KV/33KV (BBL), other two are 20/25MVA
132KV/33KV (IMP), and other two are of rating 7.5MVA
132KV/33KV/11KV (Westing House). The insulating and cooling oil flows
in the transformers like blood in the veins. Rest of the equipment and
accessories are either supportive or protective. In the following document I
am trying to explain my learnings from my internship.
5. 4
CONTENT
Introduction
1. Operation Of Substation
1.1 Type of substation
1.2 Operation of Substation
2. List Of Equipment And Their Ratings, General
Purposes
3. Relays
3.1 Mechanical Relays
3.1.1 Buckolz Relay
3.1.2 Oil Surge Relay
3.2 Electrical Relay
3.2.1 Overcurrent Relays
3.2.2 Earth Fault Relay
3.2.3 Distance Relay
3.2.4 Differential Relay
3.2.5 Auxiliary Relay
3.2.6 Digital Relay
3.2.7 Over Flux Relay and Flux Setting Relay
3.3 Thermal Relay
3.3.1 over current thermal relay
3.3.2 CTR relay
4. Transformers
4.1 List of transformers
4.2 Challenges with power transformer
4.3 Stationary parts
4.3.1 Main Tank
4.3.2 Conservative Tank and Breather
4.3.3 Core and Windings
4.3.4 Tertiary windings
4.3.5 Bushing
4.3.6 Earthing
6. 5
4.4 Active parts
4.4.1 Transformer oil
4.4.2 Cooling assembly
4.4.3 Relays
4.5 Associate parts
4.5.1 On Load Tap Changer (OLTC)
4.5.2 Fire Protection System
5. Current Transformer
6. Potential Transformers
7. Circuit Breakers
8. Capacitor Bank
9. Insulators
10. Other Equipment
10.1 Lightening Arrester
10.2 Bus-Main And Auxiliary Bus, Bus Coupler
10.3 Isolators
10.4 Fuses- D.O. and HRC
10.5 Station Transformers
10.6 Concrete And Trenches
10.7 Earthing
11. Control Room
12. PLCC Rooms
Safety Measures
Conclusion
References
Appendix A
Appendix B
Appendix C
7. 6
INTRODUCTION
This GSS was installed in the year 1962, with the aim to supply according to load
capacity of Ajmer. It is of 220KV presently upgraded from 132KV. The input supply
is indirectly coming from TPP Kota and Suratgarh,NPP Ravatbhata andfrom some of
the private sector companies.
Initially there used to be only one board in Rajasthan for everything concerned to
electricity, Rajasthan State Electricity Board (RSBE) but because of its complete failure
state government formed 5 new companies for each sector
1) Rajasthan Rajya Vidyut Utpadan Nigam Limited ( RRVUNL)- Generation
2) Rajasthan Rajya Vidyut Prasaran Nigam Limited ( RRVPNL)-Transmission
3) Vidyut Vitaran Nigam Limited ( VVNL) -Distribution
4) Ajmer Vidyut Vitaran Nigam Limited - Distribution in Ajmer region
5) Jodhpur Vidyut Vitaran NigamLimited- Distribution in Jodhpur region
Since all the GSS and Substations are a part of Transmission that’s why they fall under
the care of RRVPNL which has its Load Dispatch Center(LDC) in Heerapura, Jaipur.
This LD takes data from all the substations and GSS of the state and dispatch an
instruction to the same to keep the entire power system stable.
“Transformers are the heart of the substation and the oil used as coolant and
insulator is the blood runs in its veins. Rest is either to associate or to protect
transformers.”
This is all I learnt in my 45 days internship. There are many other equipment like CB,
CT, PT, Insulator, Relays, LA, Wave Trap, Buses etc.
In the following report I will try to lighten the working andconstruction ofallimportant
equipment and moreoverI willto try fix the importance and positionofeach equipment
under the operation of GSS.
At the end I will try to lighten importance of this GSS in the prospective of Ajmer.
About 220KV Madar Substation:
This substation has 220KV input from two feeders- Kishangarh and Beawar, is also
supplies 132 KV to Kishangarh that is example of rings type power system.
8. 7
This substation has two sister EMCO manufactured transformers of rating
220KV/132KV/11KV, but like all high rating power transformers, tertiary winding
11KV is not used, it has some advantages that’s why it’s provided. There is new yard
had been made few years, before that it a 132KV substation.
132KV yard have three feeders- Bherunda, Kishangarh and Saradhna. This yard has 5
transformers, among them RVVPNL is planning to remove 2 Westinghouse
132//33/11KV, 7.5MVA transformer as they hardly carry any load.
33KV yard have 14 feeders and this is the one shared maximum load among all. It also
have five capacitor banks to maintain the voltage level.
11KV yard has 4 feeders. And a station transformerthatis like distribution transformer
and an Earthing transformer.
In the substation I/C side have some lighteningarresters and PTs. Fromthere
this line is comes to set of isolators, CTs, Circuit breakers and isolators.
PTs and CTs are to measure the bus voltage and feeder currents respectively and also
they are integral part of switchgear.
Circuit breakerare to cut the faulty of unhealthy section off from the power system.
9. 8
1.OPERATION OF SUBSTAION
Before going to the operation,it’s necessary to get idea about the significance and
location of a substation.Substations are broadly can be classified as three types
according to its location
1.1 Type of Substations:
1.1.1 GeneratingSubstation:
Generation of electrical poweris not yet possible beyond 11KV in our country and to
reduce the transmission losses, voltage should be much higherthan this level. To have
a better economy, this voltage level is raised up to 765KV in India. To increase this
voltage level, A substation is made at the sending end.
These are also called primary substations.
1.1.2 Transmission Substation:
This kind of substations are also Grid substation.There are somewhere in between the
generating plant and the consumers. Although industrialclass of loads are also supplied
by these substations.
220KV Madaris fall under this category. These are also called secondary substations.
1.1.3 Distribution Substation:
These are at the tail of transmission,generally having a rating of 11KV/ 400V. These
are meant to supply powerresidential, industrial loads etc.
1.2 Operation of Substation:
1.2.1 At Normal condition:
Operation ofa substation unaffecteduntilthere is no fault and overcurrent.But at the
normaloperation as well properand regular inspections and maintenance are essential
to keep the substation healthy.Like
o Cleaning of various equipment
o Testing of insulation and Earthing
10. 9
o Checking of transformeroil
o Inspection ofleakage current
o Inspection ofall the alarms and annunciators
o PLCC maintenance
o
1.2.2 At Faulty Condition:
If there is any fault occurs in transmission line or there is overloading, then there are
protective overcurrent relays to sense that fault and these relays are connected to
secondary of CT and secondary of PT. Once they relay senses the abnormality,it give
command to Master relay or 86 relay, that is one which energizes the tripping coil of
circuit breaker and cut off that faulty feeder or section.
This master relays also give command to alarms, annunciators and hooter which are
placed in control room, so that concerned personnel notice it.
If there is any internal fault like insulation fault in transformers that is sensed by
Buchholz relay, OSR relay or CTR relay and these relay and these relays produces the
alarm and do their task to protect transformer.
11. 10
2.LISTING OF EQUIPMENT
Table 2.1 Various type instruments in switchyard
S.N. Name of
Equipment
Qnt Remark on
Location
Application
1. Transformer 7 b/w two buses To step down the voltage
2 Relay variable Protection of various equipment by
tripping and alarming
3 CT 43 b/w Isolator and
Circuit Breaker
Protection and
Measurement ofcurrent
4 PT 6 At main bus Protection and
Measurement ofvoltage
5 Circuit
breaker
b/w CT and Bus Protection (to break the
Circuit)
6 Station
transformer
3 Under 11KV and
33KV
400v & 230v supply for
Controlroom and panels
7 Capacitor
bank
5 Near 33KV and
11KV buses
To boost up the bus voltage
8 Insulator -- Almost
everywhere
To provide properinsulation
9 Lightening
Arrester
11 Near buses on
Insulator String
To equalize the string voltage
10 Isolator -- Between bus and
CT
To isolate the circuit and transferthe
load to auxiliary bus
12. 11
3. PROTECTIVE RELAYS
Relay are protective devices widely used in power system that senses any abnormality
with the associated equipment and if there is any, relays gives a command to circuit
breaker to break the unhealthy device.
All of the relays have three basic elements
i) Sensing element: responds to the change in the actuating quantity, the
current in a protected system in case of over-current relay.
ii) Comparingelement:serves to compare the action ofthe actuating quantity
on the relay with a pre-selected relay setting.
iii) Control element: on a pick of the relay, accomplishes a sudden change in
the control quantity such as closing of the operative current circuit.
Relays may be classified as to which kind of physical quantity the sensing element
respond. Above concept of classification broadly results in below three types
i) Mechanical: actuated by pressure,velocity, of outflow of a liquid or gas etc.
ii) Electrical: actuated by some electrical quantity such as current, voltage,
power etc. Further these are divided into two parts
a) Electromagnetic: there are moving parts.
b) Static: no moving parts
iii) Thermal: actuated by heating effect.
In power system most of the relays being used are of electrical type.
In today’s scenario more advanced programmable Digital relay are being preferred in
place of conventionalinduction type relays. In Indian power system scenario Various
Relays are used in protection of Transmission lines and transformers. Protection of
large transformers is governed by many relays and as the rating decreases number of
relays also decreases. With largertransformerthere has to beseparate relays forWinding
temperature and Oil temperature. Similarly transmission line also being protected by
relays. These are tabulated below
13. 12
Table 3.1 Relays for Transmission & Distribution Lines Protection
S.N. Line to be protected Relays to be used
1. 220 KV
Transmission Line
Main-I:Non switched distance scheme (Fedfrom
Bus PTs)
Main-II:Switched distance scheme (Fed from line
CVTs)
With a changeoverfacility from bus PT to line CVT
and vice-versa.
2. 132 KV
Transmission Line
Main Protection:Switcheddistance scheme (fed
from bus PT).
Backup Protection:3 Nos. directionalIDMT O/L
Relays and
1 No. DirectionalIDMT E/L relay.
3. 33 KV lines Non-directionalIDMT 3 O/L and 1 E/L relays.
4. 11 KV lines Non-directionalIDMT 2 O/L and 1 E/L relays.
Most of the relays are situated in controlrooms itself, from where they operates,which
has incoming leads to sense some parameter like temperature, current or voltage etc.,
now the sensed input trip the relay that consequently awake to master 86 relay to give
a command to Circuit breaker to get disconnected.. Various relays operating in the
substation are tabulated below
Table 3.2 Various relays in substation
S.N. Name of Relay Function Principle
1. Buchholz To detect internalfaults in
transformer
Pressure sensor
2. Oil surge relay To detect fault in OLTC Pressure sensor
3. CTR relay To isolate main tank from
conservator
Temperature
sensor
4. Overcurrent Protection of transformer
from excessive current
Induction type,
EM attraction type
5. Earth fault Induction disc
14. 13
6. Distance Locate the fault
7. Differentialrelay
8. 86 relay CPU of entire protective
instruments
9. Auxiliary To associate other relays Hinged armature
EM attraction type
10. Digital
11. Overflux To prevent over fluxing in
transformer
12 Flus setting relay Pre over fluxing alarm
13. Thermal Conjunction with other
instantaneous relays
Thermal expansion
of bimetallic sheet
In a GSS most ofthe relays are electrical type.Digitalrelays are newerto the this family
of relays but because of their static position they are preferred sometimes but mostly
Induction disc type relays are part of protection system in Substation. Principles and
working of these relays are as explained below.
3.1.Mechanical Relay
3.1.1. Buckolz relay:
It is a gas and oil operated mechanical device
installed in the pipework between the top of the
transformer main tank and the conservator. The
function ofthe relay is to detect an abnormalcondition
within the tank and sendan alarm ortrip signal. Under
normal conditions the relay is completely full of
oil. Operation occurs when floats are displaced by an
accumulation of gas, or a flap is moved by a surge of
oil. Almost all large oil-filled transformers are equipped
with a Buchholz relay. Fault conditions within a
transformer produce gases such as carbon monoxide,
hydrogen and a range of hydrocarbons.
Figure 3.1 Buchholz relay
mechanism (Source:
www.electrical4u.com)
15. 14
A small fault produces a small volume of gas that is deliberately trapped in the gas
collection chamber built into the relay. Typically, as the oil is displaced the upper float
and a switch operates - normally to send an alarm.
A large fault produces a large volume of gas which drives a surge of oil towards the
conservator. This surge moves the lowerfloat in the relay to operate a switch and send
a trip signal. A severe reduction in the oil level will also result in a float falling. Where
two floats are available these are normally arranged in two stages, alarm followed by
trip.
Figure 3.2 Buchholz relay and CTR relay these are situated between main tank and
conservative tank
3.1.2. Oil Surge relay:
Oil surge relay also has same principle as the Buchholz relay but as far as the function
is concerned theyare differ.OLTC chamberhas movingparts so there is some sparking
that produces gases which further causes pressure in the chamber this excessive
16. 15
pressure is sensed by Oil surge relay. If the relay is tripped it breaks the circuit breaker
and isolates the transformer from the rest of the system.
3.2.ELECTRICAL
3.2.1. Overcurrent Relays:
Wheneverthere is any excessive current in the lines these relays sense this abnormality
and trip energizes which in return gives commandto circuit breaker. Mainly electrical
overcurrent relay are based on two principles electromagnetic attraction type and
induction disc type.Thermalrelay also applicable for overcurrent protectionbut it has
limit when we use it in power system.
Electromagnetic Attraction type
These are simplest type of relays that work on the principle of electromagnetic
attraction. They are also can be of Plunger, hinged, balanced beam, polarized moving
iron type. All these relays operates on same principle.
In such relays the operation is obtained by virtue of an armature being attracted to the
poles of an electromagnet or a plunger being drawn into a solenoid. The
electromagnetic force exerted on the moving element is proportionalto the square of
flux in the air gap or the square of current flowing through the coil. It is basically single
actuated relay. Such relays respond to both ac and dc, with dc they produce constant
torque but with ac this torque consists of both ac as well as dc components
For dc 𝐹𝑒 = 𝐾𝐼 𝑑𝑐
2
= 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝐹𝑜𝑟𝑐𝑒
For ac 𝐹𝑒 = 𝐾(𝐼 𝑚𝑎𝑥 𝑠𝑖𝑛𝜔𝑡)2
=
1
2
𝐾 [𝐼 𝑚𝑎𝑥
2
− 𝐼 𝑚𝑎𝑥
2
cos 2𝜔𝑡]
To get a constanttorque with ac current, flux is splitted into two paths having a phase
difference of 90°
or by adding a shading rings on the poles of electromagnet.
Induction type Non-directional overcurrent relay:
Electromagnetic-induction relays use the principle of the induction motor whereby
torque is developed by induction in a rotor; this operating principle applies only to
relays actuated by alternatingcurrent,and in dealing with those relays we shallcall them
simply "induction-type" relays.
17. 16
Figure 3.3 Induction type relay operating principle (Source: www.elprocus.com)
An induction relay works only with alternating current.It consistsofan electromagnetic
system which operates on a moving conductor,generally in the form of a disc or cup,
and functions through the interactionofelectromagnetic fluxes with the parasitic Fault
currents which are induced in the rotor by these fluxes. These two fluxes, which are
mutually displaced both in angle and in position,producea torque thatcan be expressed
by
T= Κ1.Φ1.Φ2 .sin θ,
Where Φ1 and Φ2 are the interacting fluxes.
θ is the phase angle between Φ1 and Φ2.
It should be noted that the torque is a maximum when the fluxes are out of phase by
90º, and zero when they are in phase.Electromagnetic forces in induction relays It can
be shown that
Φ1= Φ1sin ωt, and Φ2= Φ2 sin (ωt+ θ),
Where θ is the angle by which Φ2 leads Φ1.
Thus: F= (F1 - F2) α Φ2 Φ1 sin θ α T
18. 17
Induction relays can be grouped into three classes as set out below.
a) Shaded pole type
b) Wattmeter type
c) Cup type
These relays are also used in limiting the power supplied to any particular feeder.
3.2.2. Earth-Fault Relay :
Earth fault relays are based on the principle of electromagnetic induction but here
ratherthan phase current zero sequence current provides bettersensitivity as ground
resistance is quite so that current is small and generally does not get sensed.
Earth current relays are used in following configuration where three overcurrent
relays are connected in series with Earth fault relay.
Figure 3.4 (a) Earth fault relay with
Overcurrent relays
Figure 3.4(b) Connection diagram foe
earth fault relay with over current relays
Source: www.eBlogBD.com
19. 18
3.2.3. Distance relays:
The working principle of distance
relay or impedance relay is very
simple. There is one voltage
element from potentialtransformer
and an current element fed from
current transformer of the system.
The deflecting torque is produced
by secondary current of CT and
restoring torque is produced by
voltage of potentialtransformer.In
normal operating condition,
restoring torque is more than
deflecting torque. Hence relay will
not operate.But in faulty condition,
the current becomes quite large
whereas voltage becomes less.
Consequently, deflecting torque
becomes more than restoring
torque and dynamic parts of the
relay starts moving which ultimately
close the No contact of relay. Hence
clearly operation orworking principle of
distance relay, depends upon the ratio of system voltage and current. As the ratio of
voltage to current is nothing but impedance a distance relay is also known as
impedance relay.
Impedance is thus measured as Impedance =
𝑉𝑜𝑙𝑡𝑎𝑔𝑒
𝑐𝑢𝑟𝑟𝑒𝑛𝑡
And the distance of fault from the substation α Impedance
Distance is relay is of three types
a) Admittance type
b) Impedance type
c) Reactance relay
Figure: 3.5 Distance relay principle
(Source: www.electrical4u.com)
20. 19
3.2.4. Differential relay:
Generally Differential protection is provided in the electrical power transformer rated
more than 5MVA. The Differential Protection of Transformer has many advantages
over other schemes of protection.
Principle of Differential Protection scheme is one simple conceptual technique. The
differential relay actually compares between primary current and secondary current of
powertransformer,if any unbalance found in between primary and secondary currents
the relay will actuate and inter trip both the primary and secondary circuit breaker of
the transformer.
1) The faults occur in the transformer inside the insulating oil can be detected by
Buchholz relay. But if any fault occurs in the transformerbut not in oil then it cannot
be detected by Buchholz relay. Any flash over at the bushings are not adequately
covered by Buchholz relay. Differentialrelays can detect such type of faults. Moreover
Buchholz relay is provided in transformer for detecting any internal fault in the
transformer but Differential Protection scheme detects the same in faster way.
2) The differential relays normally response to those faults which occur in side the
differential protection zone of transformer.
General connection diagram of a differential relay is as follow
21. 20
Figure 3.6 Differential protection scheme
Source: www.openelectrical.org
3.2.5. Auxiliary Relay:
Auxiliary relays are associative relay that assist in the functioning of other protective,
tripping relays. These are just supportive relays. Generally these are static type. There is
a central relay named as Master relay or 86 Relay that receives signal from other relay
and give command to tripping coil of circuit breaker.
Master relay is necessary in a substation as if we directly connected overcurrent relays
from the tripping coil, then this relay will take around 3 seconds that is undesirable but
master relay take seconds to do same. Apart from this master relay give command to
alarm, annunciator and hooter.
3.2.6. Digital Relay:
A digitalrelay consists of the following main parts: processor,analogue input system,
digital output system and independent power supply. The main difference between
digital and conventional relays pertains to the method of input signal processing.
22. 21
In the case of digital relays, input
signals are converted into digitalform
within the analogue input system
before being analyzed by the
processor. Digital relays possess
advanced programmable functionality
providing high performance level,
flexibility as well as additional
monitoring capabilities. At present,
their application is mainly in
transmission system and generation
unit protection.
Currently in Indian power system, there are being used instead of induction
type relays.
3.2.7. Overflux relay and flux setting relay:
Over flux relay senses excessive flux in the core of the transformer.Excessive flux is a
serious dangerto the transformerbut for theirmagnetic utilization operating flux is
kept near to rated flux. So over fluxing may occur at any stage and to prevent it over
flux relay is used. These generate an alarm if flux go pass a certain flux setting that is
more than the rated.
Flux setting relay is associative relay to over flux relay and perform almost same
function but here flux setting below the rated,it acts as early protective relay.
3.3.THERMAL RELAY
3.3.1. Bimetallic thermal relay
These relays operates on the principle of thermal effect of electric current. It consists
of bimetallic strips which are used in small sizes and are heated by heating coils orstrips
supplied through a current transformer. Under normal operating condition the strip
remains straight but underthe action offault the strip is heatedandbentandthe tension
Figure 3.6 Digital relay block diagram
(Source: www.electrical-engineering-portal.com)
23. 22
of the spring is released. Thus the relay contacts are closed which energizes the trip
circuit.
Thermal relays are not suitable for short-circuit as it will burn the element sufficiently
before the strip may deflect so as to close the contacts. This type of relay is used in
conjunction with instantaneous short-circuit relays of high setting or suitably graded
time limit fuses.
Figure 3.7 (b) Thermal relay at overcurrent operation (Source: www.electrical4u.com)
3.3.2. CTR Relay
Figure 3.87(a) Bimetallic thermal relayat normal operation
24. 23
This relay is situated just below the conservatortank.
It has function to isolate conservatortank from tank during fire. If seen from outside
in the yard, it’s a red cubical box just below the conservatortank before the buchholz
relays.
Its principle is based on thermalexpansion of liquid. There are some tube filled with
expandable liquid that expands after certain temperature, it consequently blast the
tube that produces an alarm signal for fire.
25. 24
4.TRANSFORMERS
A transformer is the heart of a substation. This substation has four voltage levels,
Incoming voltage from Beawarand Kishangarh 220KV,Step Down/Outgoing feeder
Voltages 132KV, 33KV, 11KV and according to load requirement, there are some
feeders from these voltage levels, which goes to distribution network.
There is mainly two type oftransformers Powertransformers and station transformers.
Powertransformers are used to much known purpose to reduce downthe transmission
voltage to lower value. Station transformers are used to obtain 400v, 230v voltage for
local use of the substation like for control panels, battery chargers.
4.1.List of Transformers
Table 4.1 Transformers list
S.
N.
Name of
Equipment
type Qn
t.
Ratings Location Purpose
1. Transformer
EMCO
Power 2 160MVA
220kv/
132kv/11kv
220KV yard To step down
i/c voltage
2. Transformer
BBL
power 1 40/50 MVA
132kv/33kv
132 KV yard To step down
Voltage
3. Transformer
IMP &
EMCO
power 2 20/25 MVA
132kv/33kv
132kv yard To step down
Voltage
4. Transformer
Westing
house
power 2 7.5MVA
132kv/33kv/
11kv
33kv yard To step down
Voltage
5. Station Tr.-1 Station 1 250KVA
33kv/400v
33kv yard To yield 230
Volts
6. Station Tr-2 Station 1 100KVA
33kv/400v
33kv yard --do--
7. Station Tr-3 Station 1 100KVA
11kv/400v
33kv yard --do--
26. 25
4.2.Challengeswith power transformers
As the rating of electrical increases, challenges with its safe operation also increases. In
this GSS there are 2 huge 160 MVA 220/132 KV transformers which provides us better
opportunities to understand about the Earthing, Cooling, and Maintenance of power
transformers. Main problems are listed below
Due to core losses, copper losses etc. transformer oil also heats up and this
weakens insulation as its dielectric strength decrease,to overcome this problem
cooling of transformer oil became a vital issue with power transformers.
This problem is relatively more challenging in case powertransformers because
they are designed to operate at high magnetizing flux.
These transformers are costlier, so their protection is major concern. Various
protective accessories is needed in order to keep them safe if there any
abnormality occurs.
Transformer oil is blood of transformer, is has to be pure (moisture free), for
this purpose transformer is associated with many accessories.
“In this report 220KV transformers will be described in detail because they are associate with most
protective,cooling scheme and accessories,other are also operate almost similar to these but with lesser
accessories and schemes.
Power transformers parts are
classified in three categoriesbasedon
their dynamic position. Power
transformers are associated with
many viable equipment, some of
them are stationary, some are
dynamic or do possesses anything
that have motion inside it, and third
one is other associates like fire
protection and OLTC which are
equally important.”
Figure 4.1: 160 MVA, 220KV power transformer in GSS, Madar
27. 26
4.3.Stationary parts
4.3.1. Main tank:
Main tank of transformer keep windings, transformer oil etc. inside, are made up of
stainless steel that provide a robust mechanicalstructure for the same. This tank has a
size of our two hostel rooms and made highly mechanically robust and varnished for
external protection.
Bushing are mountedon themain tank andthere is Earthing transformeras well. Apart
from these this tank also contains various protective devices that helps in functioning
or relays etc.There is Nitrogen gas pipes are connectedto this tank forfire protection.
Fortransformeroilcirculation there are radiators connectedvia two separate valve one
at bottom and other at top.
Apart from these there is inspection window which is devoted for maintenance
purpose.
Main tank is connected to conservative tank through buchholz relay and CTR relay
and some valves for oil supply. If oil level decreases in main tank conservative tank
provides oil to it and if decrease conservative tank assist it by tanking some of oil.
4.3.2. Conservative tank and Silicon breather:
Conservative tank is a cylindrical tank above the main tank. Whenever is there is
expansion of oil, some oil transfers from main tank to conservative tank and if there
is contraction it provide some oil to main tank, it act like reservoir of oil.
There is two type of conservative tanks are available for power transformers
Atmoseal Type Conservator
In this type conservatoroftransformer,an aircell made ofNBR materialis fitted inside
the conservatorreservoir. The silica gel breather is connectedat the top of this air cell.
The oil level in the power transformer rises and falls according to this air cell deflated
and inflated. When the air cell gets deflated the air inside the air cell comes out via
breather and on the other hand if the cell is inflated the outside air comes in through
breather.
28. 27
This arrangement prevents direct
contact ofoilwith air,thereby reduces
ageing effect of oil.
The space available outside the cell in
conservatortank is totally filled by
oil. Air vents are provided on the top
of the conservatorfor venting
accumulated air outside the air cell.
The pressure inside the air cell must be maintained 1.0 PSI.
MOG:
This device is used to indicate the
position of transformer insulating oil level in
conservator of transformer. There is a PRV
Relay in the conservative tank that provides
output as liquid level. This relay is float type.
Magnetic oil level indicator of transformer
consists of mainly three parts-
1. One float,
2. Bevel gear arrangement and
3. An indicating dial.
Diaphragm Sealed Conservator:
Here diaphragm is used as a barrier between transformer oil and atmospheric air. In
this case the conservator of transformer is made of tow semicircular halves as shown
below. The diaphragm is held between the two halves and bolted.
Figure 4.2 (a) Atmoseal type of breather (source:
www.electrical4u.com)
Figure 4.2 (b) Magnetic oil level
gauge (PRV Relay)
(Source: electrical4u.com)
29. 28
As oil expands it pushes up the
diaphragm. The position of the
diaphragm is indicated by the oil
level indicator i.e. magnetic oil
gauge as the rod of this MOG is
connectedto the diaphragm. When
the oil level falls down in the
conservator,the diaphragmdeflects
and the atmospheric air fills the
vacant place. This air is sucked
through silica gel breather which is
connected to the top middle of
conservator tank of transformer.
This type of conservator has
advantage overair cell conservator.
Silica gel
It is nothing but a pot of silica gel through which, air passes during breathing of
transformer.The silica gel is a very good absorberofmoisture. Freshly regenerated gel
is very efficient, it may dry down air to a dew point of below − 40°C.
Silica gel crystal has tremendous capacity of absorbing moisture. When air passes
through these crystals in the breather; the moisture of the air is absorbed by them.
Therefore,the air reaches to the conservatoris quite dry, the dust particles in the air get
trapped by the oil in the oil seal cup.
The color of silica gel crystal is dark blue but, when it absorbs moisture; it becomes
pink. When there is sufficient difference between the air inside the conservatorand the
outside air, the oil level in two components of the oil seal changes until the lower oil
level just reaches the rim of the inverted cup, the air then moves from high pressure
compartment to the low pressure compartment of the oil seal. Both of these happen
when the oil acts as core filter and removes the dust from the outside air.
Figure 4.2 (c) structure ofdiaphragm conservative tank
(Source: electrical4u.com)
30. 29
Figure 4.3 (a) Silica gel- blue colored is fresh and Pink is one that absorbed moisture (b) Oil
cup seal filled with oil partially.
4.3.3. Core and windings
Core and windings are common to any kind of transformer. The performance of a
transformer mainly depends upon the flux linkages between these windings. For
efficient flux linking between these windings, one low reluctance magnetic path
common to all windings should be provided in the transformer. This low reluctance
magnetic path in transformer is known as core of transformer.
Core of the transformer is made up of CRGOS or Cold Rolled Grain Oriented
Silicon Steel.
Here also the core is laminated to minimize eddy current and hysteresis losses.
High ratingpower transformersan additional, tertiary winding is also provided
because of some advantage of it.
During core manufacturing in factory some factors are taken into consideration,
1. Higher reliability.
2. Reduction in iron loss in transformer and magnetizing current.
3. Lowering material cost and labor cost.
4. Abatement of noise levels.
4.3.4. Tertiary windings
31. 30
In some high rating transformer,one winding in addition to its primary and secondary
winding is used. This additionalwinding, apart from primary and secondary windings,
is known as Tertiary winding of transformer. Because of this third winding, the
transformer is called three winding transformer or 3 winding transformer.
1.1 Advantages of Using Tertiary Winding in Transformer
Tertiary winding is provided in electrical powertransformer to meetone ormore ofthe
following requirements-
1. It reduces the unbalancing in the primary due to unbalancing in three phase load.
2. It redistributes the flow of fault current.
3. Sometime it is required to supply an auxiliary load in different voltage level in
addition to its main secondary load.This secondary load can be taken from tertiary
winding of three winding transformer.
4. As the tertiary winding is connectedin delta formation in 3 winding transformer,it
assists in limitation of fault current in the event of a short circuit from line to
neutral.
4.3.5. Bushings:
In electric power,a bushing is an insulated device that allows an electrical conductorto
pass safely through a (usually) earthed conducting barrier such as the wall of a
transformer or circuit breaker.
All materials carrying an electric charge generate an electric field. In the case of DC the
field remains either positive or negative, in the case of AC, the field is alternating
between positive and negative.
When an energized conductoris near any material at earth potential, its can cause very
high field strengths to be formed.
In this substation,porcelain bushing are used and HV side have slightly biggerbushing
than LV side.
32. 31
Figure 4.4 HV and LV bushings
4.3.6. Earthing:
Equipment Earthing is a connection done through a metal link between the body of any
electrical appliance, or neutral point, as the case may be, to the deeper ground soil. The
metal link is normally of MS flat, CI flat, GI wire which should be penetrated to the
ground earth grid.
In this GSS the Earthing mesh is damaged few year back so they earthed equipment
using a grounded delta shaped wire.
4.4.Active parts
4.4.1. Transformeroil:
Generally there are two types of transformerOil used in transformer,
1. Paraffin based transformer oil
2. Naphtha based transformer oil
Naphtha oil is more easily oxidized than Paraffin oil. But oxidation product i.e. sludge
in the naphtha oilis more soluble than Paraffin oil. Thus sludge of naphtha basedoilis
not precipitated in bottom of the transformer. Hence it does not obstruct convection
circulation of the oil, means it does not disturb the transformercooling system.But in
33. 32
the case of Paraffin oil although oxidation rate is lowerthan that ofNaphtha oilbut the
oxidation product or sludge is insoluble and precipitated at bottom of the tank and
obstruct the transformer cooling system. Although Paraffin based oil has above
mentioned disadvantage but still in our country it is generally used because of its easy
availability. Another problem with paraffin based oil is its high pour point due to the
wax content,but this does not affect its use due to warm climate condition of India.
Some specific parameters of insulating oil should be considered to determine the
serviceability of that oil.
1. Electrical parameters: – Dielectric strength,specific resistance,dielectric dissipation
factor.
2. Chemical parameter: - Water content, acidity, sludge content.
3. Physical parameters: - Inter facial tension, viscosity, flash point, pour point.
4.4.2. CoolingAssembly:
The main source of heat generation in transformer is its copper loss or 𝐼2
𝑅 loss.
Although there are other factors contribute heat in transformer such as hysteresis &
eddy current losses but contribution of I2
R loss dominate them. If this heat is not
dissipated properly,the temperature ofthe transformerwill rise continually which may
cause damages in paperinsulation and liquid insulation medium of transformer.So it is
essentialto controlthe temperature with in permissible limit to ensure the long life of
transformer by reducing thermal degradation of its insulation system.
In electrical power transformer we use external transformer cooling system to
accelerate the dissipation rate of heat of transformer.
Different Transformer CoolingMethods
ONAN Cooling of Transformer:here naturalflow of using radiators and
atmosphere airare the coolant.
ONAF Cooling of Transformer:Fans are used to cool the oil in the radiator.
OFAN:here pump is used.
OFAF: both pump and fans are used. This type of cooling is used for 220KV
transformerwhenever oil temperature exceeds normal operating temperature.
4.4.3. Relays:
34. 33
Buchholz relay, OSR relay, CTR relays are main relays associated directly with
transformers. “These are already explained.” Following table shows protective relays for
transformer protection
Table 4.2 Transformerprotectionrelays
S.N. Relay on HV
side
Relays on LV
side
Common relays
1.
2 nos O/L
Relay & 1 E/L
Relay
2/3 nos O/L
Relays & 1 no
E/L Relay
Buchholz Relay,
OLTC Buchholz relay,
PRV relay,
OT Trip Relay,
WT Trip Relay,
Overflux Relay,
Differential Relay
4.5.Associated parts
Apart from these equipment and assembly of powertransformers there are few
more devices that are equally important in function of transformer.
4.5.1. OLTC:
In larger electrical powertransformer,forproper voltage regulation oftransformer, on
load tap changer is required. As there is no permission of switching off the
transformer during tap changing. The tapping arrangement, is placed in separate
diverter tank attached to electrical power transformer main tank.Inside this tank, the
tap selectors are generally arranged in a circular form. The diverter switches have
contacts operating in rapid sequence with usually four separate make and break units.
4.5.2. Fire protection system:
There is nitrogen gas based fire protection system.There is a valve at the main tank
body for entrance of nitrogen in case of fire. Nitrogen is stored in the underground
tanks.
35. 34
5. POTENTIAL
TRANSFORMERS
A voltage transformer theory or potential transformer theory is just like a theory of
generalpurpose step downtransformer.Primary ofthis transformeris connectedacross
the phase and ground. Just like the transformer used for stepping down purpose,
potential transformer i.e. PT has
lower turns winding at its secondary.
The system voltage is applied across
the terminals of primary winding of
that transformer, and then
proportionate secondary voltage
appears across the secondary
terminals of the PT.
In this yard PTs are placed near the
bus barbecause they have to measure
the bus voltage. PTs step down bus
voltage (220KV, 132KV, 33KV,
11KV) to much lower voltage 110V,
220V voltage which is fed to various
relays and measuring instruments.
Table 5.1 Details of PT in substation
S.
N.
Equipment Qnt. Ratings Location Application
1 PT-1 1 (220/√3)kv/(220/√3)v,
220v
Near 220kv
main bus
Voltage
Measurement
2 PT-2 1 (132/√3)kv/(110/√3)v,
110v
Near 132kv
main bus
--do--
3 PT-3 3 33/√3)kv/(110/√3)v,
110v
Near 33 kv main
bus
--do--
4 PT-4 1 11kv/110v Near 11 kv main
bus
--do--
Figure 5.1 220KV PT
36. 35
6. CURRENT TRANSFORMERS
A current transformer is used for measurement ofalternating electric currents. Current
transformers, together with voltage (or potential) transformers, are known
as instrument transformers.
When current in a circuit is too
high to apply directly to measuring
instruments, a current transformer
produces a reduced current
accurately proportional to the
current in the circuit, which can be
conveniently connected to
measuring and recording
instruments. A current transformer
isolates the measuring instruments
from what may be very high voltage
in the monitored circuit. Current
transformers are commonly used in
metering and protective relays.
There are numerous CTs in the
yard those generally step down the
current from several hundred
ampere to 1A or 5A. Apart from
this these CTs are rated like
150A/1-1-1-1A. This secondary
CT current is fed to various
tripping relays and meters because they are all operate on this range only.
In the yard CT were placed before the circuit breakerbecause they are associated in the
breaking of CB and before CTs, there were isolators.
Figure 6.1 current transformerin the 220KV yard
37. 36
7. CIRCUIT BREAKERS
Definition ofcircuit breaker:- Electrical circuit breaker is a switching device which can
be operated manually as well as automatically for controlling and protection of electrical
powersystem respectively. As the modern powersystem deals with huge currents, the
specialattention should be given during designingof circuit breaker to safe interruption
of arc produced during the
operation of circuit breaker.
The modern power system deals
with huge power network and
huge numbers of associated
electrical equipment. During short
circuit fault or any other types of
electrical fault these equipment as
well as the powernetwork suffer a
high stress of fault current in them
which may damage the equipment
and networks permanently. For
saving these equipment and the
power networks the fault current
should be cleared from the system
as quickly as possible.
Again after the fault is cleared, the system must come to its normalworking condition
as soon as possible forsupplying reliable quality powerto the receiving ends. Inaddition
to that for proper controlling of power system, different switching operations are
required to be performed.So for timely disconnectingand reconnectingdifferent parts
of power system network for protection and control, there must be some special type
of switching devices which can be operated safely under huge current carrying
condition.
During interruption of huge current, there would be large arcing in between switching
contacts, so care should be taken to quench these arcs in circuit breaker in safe manner.
The circuit breaker is the specialdevice which does allthe required switching operations
during current carrying condition. This was the basic introduction to circuit breaker.
Figure 7.1 Circuit breaker in the yard between CTs
and isolators
38. 37
Accordingto their arc quenchingmedia the circuit breaker can be divided as-
7.1 Air-Break Circuit Breakers
These circuit breakers are suitable for high current interruption at low voltage, this type
of circuit breakeruses air at atmospheric pressure as an quenching medium.It employs
two pairs of contact main contact and the arcing contacts. They have low contact
resistance.The main contact carries the current when breaker is at the closed position.
When contacts are opened,the main contacts separate first,the arcing contacts remain
in closed position. Therefore the current is shifted from main contacts to the arcing
contacts. The arcing contacts separate later on the arc is drawn between them. The
principle of high resistance is employed for arc interruption, the arc resistance is
increased by lengthening,splitting and cooling the arc. The arc interruption is assisted
by current zero in case of air break circuit breakers, high resistance is obtained near
current zero. These circuit breakers are available in the voltage 400 to 12kv. They are
widely used in the low and medium voltage system. The Figure (6) of air break circuit
breaker is given below. Figure 6: Air break circuit breaker.
7.2 Oil Circuit Breaker
Mineral oil is the best insulator than air and it has good cooling properties. So, This is
employed in many electrical equipment as, well as circuit breakers. But these type of
circuit breakers are not suitable for heavy current interruption at low voltages due to
carbonization of oil.
7.3 Air Blast Circuit Breakers In the air blast circuit breakers, compressed air at
pressure of 20-30kg/cm2 is employed as, an arc quenching medium. Air blast circuit
breakers are suitable for operating voltage of 132kv and above.The main advantage of
using them is their cheapness andfree availability of the interrupting medium,chemical
stability and inertness of air, high speed operation.
7.4 SF6 Circuit Breaker
These type ofcircuit breakers have good dielectric strength andexcellent arc quenching
property.It is an inert, non-toxic,non-flammable and heavy gas. As circuit breakers are
39. 38
totally enclosed and sealed from atmosphere so it is very careful where explosion
hazards exist.At atmospheric pressure, its dielectric strength is about 2.35 times that of
air. At normal conditions it is chemically inert, these properties ofsf6 has made it
possible to design circuit breakers with smalleroverall dimensions, shortercontactgaps,
which help in the constructionsofoutdoorbreakers with fewerinterruptsandevolution
of metalclad. It is particularly suitable for metalclad switch-gear. It is suitable for the
range 3.3kv to 765kv. They are preferred for voltages 132kv and above.
7.4 Vacuum Circuit Breaker
The dielectric strength and interrupting ability of high vacuum is superior to those of
porcelain, oil, air and SF6 at atmospheric pressure. Its construction is very simple as,
compared to other circuit breakers. When contacts are separated in high vacuum, an
arc is drawn between them. The arc does not take place on the entire surface of the
contacts but only a few spots. The contactsurface is not perfectly smooth.It has certain
microprojections.At the time of contact separation, these projections form the last
point of separation. The current flows through these points of separation resulting in
the formation of a few hot spots, these spots emit electrons and act as cathode spots.
It’s enclosure is made up of insulating material such as, glass, porcelain or glass fiber
reinforced plastic. The vapor condensingshield is made up of synthetic resin. Vacuum
CB is now very popular for voltage rating up to 36kv.
40. 39
8. CAPACITOR BANK
Most of the domestic and industrialload are consumers of reactive powerthat’s why
there is voltage dip in the powersystem.To provide the necessary reactive power
capacitorbank are attachedto the respective buses.
In the yard these capacitors are delta connectedto provide more capacitive
reactance with same number of banks.
The capacitorhas following functions:
1. Voltage rise
2. Energy store
3. Powerfactorimprovement
It has two components associatedwith it:
8.1 SERIES REACTOR: During parallel operation of capacitor bank it is
necessary to limit inrush current to a safe limit depending upon circuit breaker
Figure 8.3 Series reactor
41. 40
capability, this is done by series reactor.It provides additionalinductive reactance in
circuit
8.2 RESIDUAL VOLATGE RANSFORMER : It provides protection to
capacitor bank and for fast charging of the same. RVT is connected across the
capacitor bank. It has dual secondary windings. One is for metering and another is
for protection.
Figure 8.1 Capacitor bank
Figure 8.2 RVT
42. 41
9. INSULATORS
Electrical Insulator must be used in electrical system to prevent unwanted flow of
current to the earth from its supporting points. The insulator plays a vital role in
electrical system. Electrical Insulator is a very high resistive path through which
practically no current can flow. In transmission and distribution system, the overhead
conductors are generally supported by supporting towers or poles. The towers and
poles both are properly grounded. So there must be insulator between tower or pole
body and current carrying conductors to prevent the flow of current from conductor
to earth through the grounded supporting towers or poles.
In high power application these type of insulators are most preferred according to the
requirement
Figure 9.1 Pin type Insulator Figure 9.2 Post type Insulator
Figure 9.3 Strain and string type insulators in 220KV yard
43. 42
10. OTHER EQUIPMENT
10.1. Lightening Arrester:
An electrical surge can be occurred in an electrical power transmission system due to
various reasons. Surge in electrical system originated mainly due to lightning impulses
and switching impulses. Electrical surge produces a large transient over voltage in the
electrical network and system. The shape of the transient over voltage has a steeply
rising front with slowly decaying tail as shown in the figure below. This steep voltage
wave travels through the electrical network and causes over voltage stresses on all the
electrical insulators and equipment come under its travelling path.
That is why all electrical equipment and
insulators of power system must be
protected against electrical surges. The
method of protecting system from surge
is normally referred as surge protection.
The main equipment commonly used for
this purpose is lightning arrester or
surge arrester. There are two types of
surges one comes externally from
atmosphere such as atmospheric
lightning. Second type is originated from
electrical system itself, such as switching
surges.
10.2. Main bus, auxiliary bus and bus coupler
Main Bus: In powersystem,there is a common reservoir needed to outlet the feeder
of that voltage level, so incoming supply is connectedto a bus that provides supply to
various feeder.
Figure 10.1 Lightening arrester
44. 43
Auxiliary Bus: If there is any maintenance work is carried out at main bus then
auxiliary bus take over all the feeders. Generally main bus is one that supplies power
to distribution network.
As the voltage level of bus increases strands of conductoralso increases, so there are
few type of conductorgenerally used in powersystem. The most commonconductor
in use for transmission today is aluminum conductorsteelreinforced (ACSR).
220KV and above- Zebra conductor
132 KV- Panther
33KV- Dog
11KV- Weasel
Bus Coupler: This additionalfeature is required to charge the auxiliary bus form
main bus.
10.3. Isolator:
Circuit breaker always trip the circuit but open contacts of breaker cannot be visible
physically from outside of the breaker and that is why it is recommended not to touch
any electrical circuit just by switching off the circuit breaker. So for bettersafety there
must be some arrangement so that one can see open condition of the section of the
circuit before touching it. Isolatoris a mechanicalswitch which isolates a part of circuit
from system as when required. Electrical isolators separate a part of the system from
rest for safe maintenance works.
Figure 10.2 Isolators
45. 44
10.4. Fuses:
In electronics and electrical engineering, a fuse is a type of low resistance resistor that
acts as a sacrificialto provide overcurrent protection,ofeitherthe load orsource circuit.
Its essentialcomponent is a metalwire or strip that melts when too much current flows
through it, interrupting the circuit that it connects. Short circuits, overloading,
mismatched loads, or device failure are the prime reasons for excessive current. Fuses
are an alternative to circuit breakers.
10.4.1 Drop Out Fuse:
In electrical distribution, a fuse cutout or cut-out fuse is a
combination ofa fuse and a switch, used in primary overhead
feeder lines and taps to protect distribution
transformers from current surges and overloads. An
overcurrent caused by a fault in the transformer or customer
circuit will cause the fuse to melt, disconnecting the
transformer from the line. It can also be opened manually
by utility linemen standing on the ground and using a long
insulating stick called a "hot stick".
10.4.2 High rupture capability fuse:
In normalworking condition ofelectricalnetwork,
the current flows through the network is within
the rated limit. If fault occurs in the network
mainly phase to phase short circuit fault or phase
to ground fault, the network current crosses the
rated limits. This high current may have very high
thermal effect which will cause a permanent
damage to the valuable equipment connected in
the electrical network. So this high fault current
should be interrupted as fast as possible. This is
what an electrical fuse does. A fuse is a part of the
circuit which consists of conductor which melts
easily and breaks the connection when current exceeds the predetermined value. An
electrical fuse is a weakest part of an electrical circuit which breaks when more than
predetermined current flows through it.
Figure 10.3 Drop out
Fuse (Source:
www.isotechindia.tradeindia
.com)
Figure 10.4 HRC fuse
Source: www.wikipedia.com
46. 45
10.5. Station Transformer:
Station Transformers are employed for supplying powerto plant auxiliary loads during
the event of starting of the plant or when generating unit is not generating power.
Station Transformers are connected to the switchyard bus. LV side of the station
transformer is connected to the auxiliary load buses.
10.6. Concrete And Trenches:
Concrete structure is used in switchyard to have lesserstep voltage.Because ofthis step
voltage, it is advised to take shorter step in yard.
Trenches are made for underground cabling from yard to control room.
10.7. Earthing:
All the electrical appliances are needed to be grounded.In the same mannerequipment
in the switchyardalso needed to be earthed. In the yard220KV powertransformerhave
five Earthing, two from bothwindings (as it’s 𝑌 − 𝑌),twofrom body,onefrom tertiary.
The Earthing is broadly divided as
a) System Earthing: Connectionbetween parts ofplant in an operating systemlike
LV neutral of a powertransformer winding and earth. All the towers are grounded by
common conductor.
b) Equipment Earthing(safety grounding): connectingbodies ofequipment
(like electric motor body,transformertank, switchgear box,operating rods ofair break
switches, LV breaker body, HV breaker body, feeder breaker bodies etc.) to earth.
Equipment Earthing
1. Earth grid: A System of grounding electrodes consisting of interconnected
connectors buried in the earth to provide a commongroundfrom electricaldevices
and metallic structures.
2. Earth mat:A grounding systemformed by a grid of horizontally buried conductors
- Serves to dissipate the earth fault current to earth and also as an equipotential
bonding conductor system.
47. 46
11. CONTROL ROOM
Although switchyard is almost the entire area of a substation that contains all the high
powerrating equipment but controland protection ofthe power system is even more
important.
Almost all the relays, like overcurrent,digital, directional, Earth fault relay, master
relays are located in the controlroom itself.
These relays take input signalfrom switchyard through wires and they provides tripping
command to master relays which ultimately operate the circuit breaker.
Apart from relays there are various measuring devices. Most of the measuring devices
are of digitaltype but in the old section (11KV) there is still indicating devices are used.
Ammeter, Voltmeter, pf meter, frequency meter and energy meter are most common
meters in the control panels.
Since frequency is very important parameter to check the stability of power system so
there is a separate meter for frequency measurement just in the position where it can
easily be seen.
There are Hooter as well that get active whenever there is any fault and once the
corresponding person listen it, he shut it.
Figure 11.1 Inside the control panels.
48. 47
There separate panel for each bus and power transformers and they are marked
accordingly.And there are various screens that shows status ofvarious equipment.For
clearance of fault there is switch that can operate the circuit breaker.
These panel also need DC supply to work so the control room is also equipped with
few battery sets of 110V and 54V. These battery set are series combination of2V cells.
And all these cells are connected with metallic strips.
These batteries are charged by a charger that comprises of rectifier.
49. 48
12. PLCC ROOMS
Power line carrier communication system differs in method of calling. The Power
supply or in the modulation system,each endofpoweris provided withidenticalcarrier
equipment consisting oftransmitter, receiver,line turning unit, masteroscillator, power
amplifier etc. In this GSS carrier frequency of PLCC is 472 KHz (50-500 KHz).
Figure 12.1 PLCC Communication schematic
Brief illustration of PLCC system is given in the section
1.1 PLCC ELEMENTS
i) COUPLING CAPACITOR
50. 49
The carrier equipment is connected to transmission line through coupling capacitor,
whose capacitance offers low reactance to carrier frequency (472 KHz) but high
reactance to power frequency (50 Hz).
Coupling capacitors allow carrier signal to enter the equipment but does not allow 50
Hz power frequency to enter the carrier equipment.
ii) LINE TRAP UNIT
Line trap unit is also known as Wave trap. The line trap offers high impedance to the
high frequency communication signalthus obstructs the flow of these signals into the
substation bus bars. If these were not be there, then signal losses will be more and
communication will be ineffective/probably impossible. It is inserted in between bus
bar and connection of coupling capacitor to the line. It is parallel tuned circuit
comprising L & C. It has a low impedance 50 HZ and high Impedance to carrier
frequency. This unit prevents the high frequency signalfrom entering the neighboring
line and carrier currents flows.
iii) LINE MACHING UNIT
This is situated near current transformer and devoted for line matching.
There is a coupling device as well that is devoted forimpedance matching as mismatch
in impedance will lead to high transmission losses.
Like all the substation,here also PLCC is used for following purpose
i. Data transmission: The information of load is to be transferred to a
LOAD DISTAPTCH CENTER Heerapura Jaipur, from where it receives
certain instructions to keep the entire power system stable.
ii. Protection: Whenever some ambiguity, faults etc. occurs anywhere in
power system,that affect the nearby substation first then it spreads to others.
By PLCC firstly affected substation can inform the nearby substations about
the fault, this transmission takes 3 ms to inform other substation so they can
take proper steps to protect the power system.
iii. Local communication: There is a Telephone Exchange in PLCC room
that provides facility to call another substation or even to make trench call
using hot line. Even if there is no power flow through transmission line, hot
51. 50
line still works. This get disable only when line breaks. Now a days RRVPNL
has facilitate Jen with separate Mobile phones so this application is no longer
a vital one. PLCC is also used for faxing in substations.
In the Urban cities like Noida, instead of conventionalmetering smart metering is
being used and PLCC also used for this purpose.So in near future we can expect that
PLCC can be used for smart metering as well.
2.1.3 ADVANTAGE OF PLCC
No separate wires are neededforcommunicationpurpose,as the powerline itself carry
power as well as communication signals. Hence the cost of constructing separate
telephone line is saved.
1. When compared with the ordinary lines, power lines have appreciable
higher mechanical strength. They would normally remain unaffected
under the conditions, which might seriously damage telephone lines.
2. Powerlines usually provide the shortestroute betweenthe powerstations.
Power lines have large cross sectional area which results in very low
resistance perunit length.Consequently,the carriersignalsuffermuch less
attenuationduring they travelled on usual telephone lines of equal length.
3. Full bandwidth (300 to 3400Hz), high quality speech cum fax channels.
4. Data transmission at 1200bps; for Computer-Networking and SCADA
applications.
5. Transmission of trip signals from distance protection scheme (network)
for the protection of High Voltage Transmission Lines.
52. 51
SAFETY MEASURES
Earthing of non-current part from the point of view of safety of personnel.
Yard must be layered with stone gravellayer of 100-150 mm thick to minimize
the step voltage.
Shrubs, grass and trees etc. should not be allowed to develop in the yard.
Electrical checking of PRD, Buckolz relay, OLTC surge relay and replacement
of the gaskets of the boxes.
IR measurement of windings.
Both auxiliary and main should not be in operation at the same time.
Tightening of nuts, bolts, clamps, fixtures etc.
Checking of arcing horn gap-setting of bushing.
Checking of oil level.
Checking of break down voltage (BDV) of transformer oil.
According to BDV test,transformeroilshould be regularly sent to the laboratory
and necessary steps should be taken like change of oil if BDV is below 30 KV.
Checking of alarm/indicator circuit and control and relay armlet wiring.
Checking of air/ 𝑆𝐹6 leakage.
Checking of jumpers and bus connections.
Use of rubber mat in control room.
Use of insulated shoes and gloves.
The fences must be checked.
Safety of consumers and maintenance staff from hazard of electrical shock.
53. 52
Conclusion
The summer training at 220KV GSS RVPNL Ajmer has proven out to be good
exposure to practical aspects of concepts that we learnt in past 3 years. Although the
complete operation of a GSS toggle around transformers. In the theory we just learn
about the concept of transformers but here in this practical arena we had learnt that
how difficult it is to deal with such a huge power transformers, importance of oil for
transformers was new thing to learn. This oil do not only work as a coolant but also
provide necessary insulation in the transformers. The developmentofthis transformers
is an isolated engineering to me and this fascinated me a lot.
Apart from transformerCircuit breaker,CT, PT and relays plays an important role in a
GSS. These are devoted for proper working of transformer, their working and
maintenance is completely new to all the trainee. Principle and associate problems with
relays can only be fully understand in a practical study, this GSS provide a good
opportunities to achieve it.
54. 53
References
1. ‘ A Course in Electrical power’ by J B Gupta
2. Manuals of various equipment
3. EMCO transformers manual, charts and plates on the transformers.
4. “Electrical Relays Principles and Applications” by Vladimir Gurevich
5. “Electrical Machinery” by P. S. Bhimbhra
6. “ Principles of Electronic Materials and Devices” by S O Kasap
7. “ A Course in Electrical and electronic Measurements and instrumentation” by A K
Sawhney
8. “Switchgear and protection” by U A Bakshi and M V Bakshi
9. http://www.electrical4u.com
10.http://www.rvpnl.co.in
11.http://www.wikipedia.org
12.http://www.youtube.com
13.http://www.electricaleasy.com
14.http://www.transformerworld.co.uk
15.http://www.nptel.com
16.http://www.eblogBD.com
17.http://seminarprojects.com
55. 54
Author
This report is submitted by Asafak Husain. This is an
internship report submitted to department of Electrical
Engineering, IIT Roorkee. This report is based on the
learnings and experiences from the internship taken at Grid
substation, Madar.
Author is 4rd
year B.Tech student,Electrical engineering,IIT
Roorkee.
56. 55
Appendix A
List of device numbers and acronyms ANSI
1 – Master Element
2 – Time Delay Starting or Closing Relay
3 – Checking or Interlocking Relay
4 – Master Contactor
5 – Stopping
6 – Starting Circuit Breaker
7 – Rate of Change Relay
8 – Control Power Disconnecting Device
9 – Reversing Device
10 – Unit Sequence Switch
11 – Multi-function Device
12 – Over speed Device
13 – Synchronous-speed Device
14 – Under speed Device
15 – Speed – or Frequency, Matching
Device
16 – Data Communications Device
17 – Shunting or Discharge Switch
18 – Accelerating or Decelerating Device
19 – Starting to Running Transition
Contactor
20 – ElectricallyOperated Valve
21 – Distance Relay
22 – Equalizer Circuit Breaker
23 – Temperature Control Device
24 – Volts Per Hertz Relay
25 – Synchronizing or Synchronism-
Check Device
26 – Apparatus Thermal Device
27 – Under voltage Relay
28 – Flame detector
29 – Isolating Contactor or Switch
30 – Annunciator Relay
31 – Separate Excitation
32 – Directional Power Relay or Reverse
Power Relay
33 – Position Switch
34 – Master Sequence Device
35 – Brush-Operating or Slip-Ring Short-
Circuiting Device
36 – Polarity or Polarizing Voltage
Devices
37 – Undercurrent or Under power Relay
38 – Bearing Protective Device
39 – Mechanical Condition Monitor
40 – Field (over/under excitation) Relay
41 – Field Circuit Breaker
42 – Running Circuit Breaker
43 – Manual Transfer or Selector Device
44 – Unit Sequence Starting Relay
45 – Abnormal Atmospheric Condition
Monitor
46 – Reverse-phase or Phase-Balance
Current Relay
47 – Phase-Sequence or Phase-Balance
Voltage Relay
48 – Incomplete Sequence Relay
49 – Machine or Transformer, Thermal
Relay
50 – Instantaneous Overcurrent Relay
50G- Instantaneous Earth Over Current
Relay (Residual Method)
57. 1
50N- Instantaneous Earth Over Current
Relay (Neutral CT Method)
51 – AC Inverse Time Overcurrent Relay
51G- AC Inverse Time Earth
Overcurrent Relay(Residual Method)
51N- AC Inverse Time Earth
Overcurrent Relay(Neutral CT Method)
52 – AC Circuit Breaker
52a- AC Circuit Breaker Position (Contact
Closed when Breaker Closed)
52b- AC Circuit Breaker Position
(Contact Open when Breaker Closed)
53 – Exciter or DC Generator Relay
54 – Turning Gear Engaging Device
55 – Power Factor Relay
56 – Field Application Relay
57 – Short-Circuiting or Grounding
Device
58 – Rectification Failure Relay
59 – Overvoltage Relay
60 – Voltage or Current Balance Relay
61 – Density Switch or Sensor
62 – Time-Delay Stopping or Opening
Relay
63 – Pressure Switch
64 – Ground Detector Relay
64R- Restricted earth fault
65 – Governor
66 – Notching or Jogging Device
67 – AC Directional Overcurrent Relay
68 – Blocking Relay
69 – Permissive Control Device
70 – Rheostat
71 – Liquid Level Switch
72 – DC Circuit Breaker
73 – Load-Resistor Contactor
74 – Alarm Relay
75 – Position Changing Mechanism
76 – DC Overcurrent Relay
77 – Telemetering Device
78 – Phase-Angle Measuring Relay or
"Out-of-Step" Relay
79 – AC Reclosing Relay (Auto Reclosing)
80 – Flow Switch
81 – Frequency Relay
82 – DC Reclosing Relay
83 – Automatic SelectiveControl or
Transfer Relay
84 – Operating Mechanism
85 – Communications, Carrier or Pilot-
Wire Relay
86 – Lockout Relay/Master Trip
87 – Differential Protective Relay
88 – Auxiliary Motor or Motor Generator
89 – Line Switch
90 – Regulating Device
91 – Voltage Directional Relay
92 – Voltage and Power Directional Relay
93 – Field Changing Contactor
94 – Tripping or Trip-Free Relay
95 – For specific applicationswhere other
numbers are not suitable
96 – Bus bar Trip Lockout relay
97 – For specific applicationswhere other
numbers are not suitable
98 – For specific applicationswhere other
numbers are not suitable
99 – For specific applicationswhere other
numbers are not suitable
150 – Earth Fault Indicator
AFD – Arc Flash Detector
CLK – Clock or Timing Source
58. 2
DDR – Dynamic Disturbance Recorder
DFR – Digital Fault Recorder
DME – Disturbance Monitor Equipment
ENV – Environmental Data
HIZ – High Impedance Fault Detector
HMI – Human Machine Interface
HST – Historian
LGC – Scheme Logic
MET – Substation Metering
PDC – Phasor Data Concentrator
PMU – Phasor Measurement Unit
PQM – Power Quality Monitor
RIO – Remote Input/output Device
RTU – Remote Terminal Unit/Data
Concentrator
SER – Sequence of Events Recorder
TCM – Trip Circuit Monitor
LRSS - LOCAL/REMOTE SELECTOR
SWITCH
SOTF - Switch On To Fault
59. 1
APPENDIX B
Equipment earthing based on IS: 3043-1987 Standard
1. Classification of electrical equipment IS: 9409-1980
2. Important rules for safety and earthing practice is based on IE rules 1956
3. Guide on effects of current passing through human body – IS:8437-1997
4. Protection of buildings and structures from lightning – IS:2309-1969
5. Earth: The conductive mass of the earth, whose electric potential at any point is conventionally
assumed and taken as ZERO.
6. Earth electrode: A Conductor or group of conductors in intimate contact with and providing as
electrical connection to earth.
7. Earth electrode resistance: The electrical resistance of an earth electrode to the general mass of
earth.
8. Earthing Conductor: A protective conductor connecting the main Earthing terminal to an earth
electrode or other means of earthing.
9. Equipotential Bonding: Electrical connection putting various exposed conductive parts and
extraneous conductive parts at a substantially equal potential.
10. Example: Inter connect protective conductor, earth continuity conductors and risers of AC/HV
systems if any.
11. Potential gradient: The potential difference per unit length measured in the direction in which it
is max.
12. Touch Voltage: TheP.D. betweena grounded metallicstructureanda point on theearth’ssurface
separated by a horizontal reach of one Meter.
13. Step voltage: The P.D. between two points on the earth’s surface separated by a distance one
pace (step) assumed to be one Meter.
14. Earth grid: A System of grounding electrodes consisting of interconnected connectors buried in
the earth to provide a common ground from electrical devices and metallic structures.
15. Earth mat: A grounding system formed by a grid of horizontally buried conductors - Serves to
dissipate the earth fault current to earth and also as an equipotential bonding conductor system.
Transformer BDV testing
60. 2
Appendix C:Relay protection of protection of transformers
1. No Buchholz relay for transformers below 500 KVA capacity.
2. Transformers up to 1500 KVA shall have only Horn gap protection.
3. Transformers above 1500 KVA and up to 8000 KVA of 33/11KV ratio shall have
one group controlbreaker on HV side and individualLV breakers if there is more
than one transformer.
4. Transformers above 8000 KVA shall have individual HV and LV circuit breakers.
5. The relays indicate above shall be provided on HV and LV.
6. LAs to be provided on HV & LV for transformers of all capacities and voltage
class.
7. OLTC out of step protection is to be provided where masterfollower scheme is in
operation.
8. Fans failure and pumps failure alarms to be connected.
9. Alarms for O.T., W.T., Buchholz (Main tank & OLTC) should be connected.
