Internship doc 33 11 kv substation

CHAPTER-I
INTRODUCTION
1.1 OVERVIEW ON APEPDCL
Andhra Pradesh Eastern Power Distribution Company Limited or APEPDCL is the Electricity
Distribution company owned by the Government of Andhra Pradesh for the Five Districts
of Andhra Pradesh. The Eastern Power Distribution Company of Andhra Pradesh Ltd (APEPDCL)
was incorporated under the Companies Act, 1956 as a Public Limited Company on 31-03-2000
with headquarters at Visakhapatnam. APEPDCL encompasses an area of Five districts
viz., Srikakulam , Vizianagaram , Visakhapatnam , East Godavari and West Godavari.
1.1.1 Functions & Duties:
The functions of APEPDCL are :
1) Sale of Power
2) Collection of Revenue
3) Service to the Consumers
4) O&M of Power Supply to all the Electrical Consumers
The Duties of APEPDCL are :
1
Fig 1.1 Geographical area of APEPDCL

1) To maintain un-interrupted power supply to all consumers.
2) To comply with the overall standards of performance parameters prescribed
by the Hon’ble APERC.
1.2 TYPES OF SUB-STATIONS:
Electrical power is a vital part of the industrialized world, and a large infrastructure has been
built to supply power wherever it is needed. Power lines are the most important in this
infrastructure, moving electricity from large power plants and delivering it to the consumer. High
voltages from power plants cannot be used directly in a house or business, however. The devices
built to make electricity usable for end users are called substations.
Depending on the purpose the sub-stations can be classified as:
1.2.1 Based On Nature of Duties:
1.2.1.1 Step up or primary substation:
Primary substations are associated with the power generating plants where the voltage is
stepped up from low voltage (11, 33kV ) to 220kV or 400kV for transmitting the power so that
huge amount of power can be transmitted over a large distance to load centers.
1.2.1.2 Primary Grid Substation:
Such substations are located at suitable load centers along with the primary transmission
lines. At primary Grid Power Substations the primary transmission voltage (220kV or 400kV) is
stepped down to secondary transmission voltages (110kV). This Secondary transmission lines are
carried over to Secondary Power Substations situated at the load centers where the voltage is
further stepped down to Sub transmission Voltage or Primary Distribution Voltages (11kV or
33kV).
1.2.1.3 Step Down or Distribution Substation:
Such Power Substations are located at the load centers. Here the Sub transmission
Voltages of Distribution Voltages (11kV or 33kV) are stepped down to Secondary Distribution
Voltages (400kV or 230kV). From these Substations power will be fed to the consumers to their
terminals.
1.2.2 Based on Service Rendered:
1.2.2.1 Transformer Substation:
Transformers are installed on such Substations to transform the power from one voltage
level to other voltage level.
1.2.2.2 Switching Substation:
Switching substations are meant for switching operation of power lines without
transforming the voltages. At these Substations different connections are made between various
transmission lines. Different Switching Schemes are employed depends on the application to
transmit the power in more reliable manner in a network.
1.2.2.3 Converting Substation:
Such Substations are located where AC to DC conversion is required. In HVDC
transmission Converting Substations are employed on both sides of HVDC link for converting AC
to DC and again converting back from DC to AC. Converting Power Substations are also
employed where frequency is to be converted from higher to lower and lower to higher. This type
of frequency conversion is required in connecting to Grid Systems.
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1.2.3 Based on Operating Voltage:
1.2.3.1 High Voltage Substation:
This type of Substation associated with operating voltages between 11kV and 66kV.
1.2.3.2 Extra High Voltage Substation:
This type of Substation is associated where the operating voltage is between 132kV and
400kV.
1.2.3.3 Ultra High Voltage Substation:
Substations where Operating Voltages are above 400kV is called Ultra High Voltage
Substation
1.2.4 Based On Substation Design:
1.2.4.1 Outdoor Substations:
In Outdoor Power Substations, the various electrical equipments are installed in the
switchyard below the sky. Electrical equipments are mounted on support structures to obtain
sufficient ground clearance.
1.2.4.2 Indoor Substation:
In Indoor Power Substations the apparatus is installed within the substation building. Such
substations are usually for the rating of 66kV. Indoor Substations are preferred in heavily polluted
areas and Power Substations situated near the seas (saline atmosphere causes Insulator Failures
results in Flashovers).
1.2.5 Based on Design Configuration:
1.2.5.1 Air Insulated Substation:
In Air Insulated Power Substation bus bars and connectors are visible. In this Power
Substations Circuit Breakers and Isolators, Transformers, Current Transformers, Potential
Transformers etc are installed in the outdoor. Bus bars are supported on the post Insulators or
Strain Insulators. Substations have galvanized Steel Structures for supporting the equipment,
insulators and incoming and outgoing lines. Clearances are the primary criteria for these
substations and occupy a large area for installation.
1.2.5.2 Gas Insulated Substation:
In Gas Insulated Substation Various Power Substation equipments like Circuit Breakers,
Current Transformers, Voltage Transformers, Bus bars, Earth Switches, Surge Arresters, Isolators
etc are in the form of metal enclosed SF6 gas modules. The modules are assembled in accordance
with the required Configuration. The various Live parts are enclosed in the metal enclosures
(modules) containing SF6 gas at high pressure. Thus the size of Power Substation reduces to 8% to
10% of the Air Insulated Power Substation.
1.2.5.3 Hybrid Substation:
Hybrid Substations are the combination of both Conventional Substation and Gas
Insulated Substation. Some bays in a Power Substation are Gas Insulated Type and some are Air
Insulated Type. The design is based on convenience, Local Conditions available, area available
and Cost.
1.3 CONCLUSION
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The sub-station under study is an outdoor type 33/11 kV sub-station owned by APEPDCL
and a distributes power to various major industries and distributions centres in Jaggampeta.
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CHAPTER 2
SINGLE LINE DIAGRAM
2.1 INTRODUCTION
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We visited the 33/11KV sub-station in month July with the maintenance incharge. We studied and
observed the following electrical equipment in the substation.
Fig2.1.Single line diagram of 33/11kv sub station, Jaggampeta.
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2.3 SINGLE LINE DIAGRAM DESCRIPTION
2.3.1 33 KV Side
In Jaggampeta substation we observe two incoming feeders connected to 33 kv busbar
from which they are stepped down to 11 kv using two power transformers.
The two incoming feeders are:
1. PEDDAPURAM
2. PATTIPADU (STANDBY)
2.3.2 11 KV Side
There are 7 feeders connected to 11kv bus:
1. Katravulapalli
2. Neeladriraopeta
3. Town-1
4. Town-2
5. Industrial
6. Narendrapatnam
7. Ramavaram
2.4 CONCLUSION
The substation receives power at 33 KV from two different sources and delivers power at
11KV. The power is fed to small industries, agricultural and domestic loads in and around of
jaggampeta city.
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CHAPTER-3
SWITCHYARD EQUIPMENT
The equipment used in 220/132/33kv substation is explained in this chapter starting from
incoming to outgoing as they appear in the single line diagram.
3.1 LIGHTNING ARRESTER:
A Lightning arrester is a device used on electrical power systems and telecommunication
systems to protect the insulation and conductors of the system from the damaging effects of
lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal.
Surge in an 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. This steep voltage wave travels through 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.
Fig 3.1 lightning arrester
 The fisrt and last protective equipment used in substation is lightning arrester.
3.1.1 Types of lightning arresters
 Rod arrester
 Horn gap arrester
 Multi gap arrester
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 Expulsion type arrester
 Valve type arrester
3.2 CAPACITIVE VOLTAGE TRANSFORMER:
Capacitor Voltage Transformers (CVT), are used for voltage metering and protection in
high voltage network systems. They transform the high voltage into low voltage adequate to be
processed in measuring and protection instruments secondary equipment, such as relays and
recorders.
Fig 3.2 capacitive voltage transformer
CVT’s in combination with wave traps are used for filtering high frequency communication
signals from power frequency this forms a carrier communication network throughout the
transmission network.
 CVT is works on the principle of electrostatic (capacitive action).
 CVT exist and are used by utilities for high voltage (>66kv) metering.
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The circuit diagram of simple capacitor voltage transformer
Fig 3.3 Schematic diagram of CVT
3.3 CURRENT TRANSFORMER :
The Current Transformer (C.T.), is a type of “Instrument transformer” that is designed to
produce an alternating current in its secondary winding which is proportional to the current being
measured in its primary.
Current transformers reduce high voltage currents to a much lower value and provide a
convenient way of safely monitoring the actual electrical current flowing in an AC transmission
line using a standard ammeter.
A CT functions with the same basic working principle of electrical power transformer, but
here is some difference. If electrical power transformer or other general purpose transformer,
primary current varies with load or secondary current. In case of CT primary current is the system
current and this primary current or system current transforms to the CT secondary current or
burden current depends upon primary current of the current transformer.
Fig 3.5 current transformer
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3.5 POTENTIAL TRANSFORMERS:
Potential transformer is a type of “Instrument transformer” that is designed to produce an
alternating voltage in its secondary winding which is proportional to the voltage being measured in
its primary.
These are also called as voltage transformers and conneccted in parallel with the incoming line.
Fig 3.6 Potential transformer
3.6 CIRCUIT BREAKER:
Circuit breaker is an automatically electrical switch design to protect an electrical circuit
from damage caused by an excess current.The basic function is to interrupt current flow when
protective relay detect a fault.
According to Different criteria there are different types of circuit breaker. According to their
arc quenching media the circuit breaker can be divided as :-
1. Oil circuit breaker
2. Air circuit breaker
3. SF6 circuit breaker
4. Vacuum circuit breaker
According to the operation mechanism of circuit breaker they can be divided as:-
1. Spring operated circuit breaker.
2. Pneumatic circuit breaker.
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3. Hydraulic circuit breaker.
Fig 3.7 33kv circuit breaker
3.7 ISOLATOR:
Isolator is a mechanical switch which separates a part of the electrical power. Isolators are
used to open a circuit under no load. Isolators are used on both ends of the breaker in order that
repair or replacement of circuit breaker can be done without and danger.
3.7.1 Types Of Isolators
 Double break isolator
 Single break isolator
 Pentagraph isolator
3.7.2 Depending upon the position in power system the isolators can categorized by
 Bus isolator
 Line isolator
 Transfer bus isolator
3.7.1.1 DOUBLE BREAK ISOLATOR:
The female type contacts are fixed on the top of the outer post insulators which fitted at
both sides of the central post insulator the female contacts are generally in the form of spring
loaded figure contacts the rotation of male contact in opposite direction make to it out from female
contacts and isolators becomes open.
3.7.1.2 SINGLE BREAK ISOLATOR:
The contact arm is divided into two parts one carries male contact and other female
contact the contact arm moves due to rotation of the post insulator upon which the contact arms
fitted rotation of both post insulators stacks in opposite to each other cause to close the isolator by
closing the isolator arm.counter rotation of both post insulators stacks open the contact arm.
3.7.2.1 BUS ISOLATOR:
 It disconnects the circuit from the main power supply.
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 The isolator is directly connected with main bus.
3.7.2.2 LINE ISOLATOR :
Line isolator is situated at line side of any feeder.
Fig 3.10 line isolator with double break mechanism
3.8 BUS BAR:
Fig 3.11kv bus bar
An electrical bus bar is defined as a conductor or a group of conductor used for collecting
electrical energy from the incoming feeders and distributes them to the outgoing feeders. In other
words, it is a type of electrical junction in which all the incoming and outgoing electrical current
meets. Thus, the electrical bus bar collects the electrical energy at one location.
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3.9 CAPACITOR BANK:
A capacitor bank is a identical grouping of capacitors interconnected in parallel or series
with one another. This group of capacitors are typically used to correct or counteract undesirable
characteristics, such as power factor lag or phase shifts inherent in alternating current (AC) power
supplies to increase storage energy and improve the ripple current capacity of the power supply.
Fig 3.12 capacitor bank
The use of a capacitor bank in the power supply system effectively cancels out or
counteracts these phase shifts issues, making the power supply far more efficient and cost
effective. The installation of a capacitor bank is also one of the cheapest methods of correcting
power lag problems.
3.10 POWER TRANSFORMER:
A transformer is an energy transfer device. It has an input side (primary) and an output
side (secondary). Electrical energy applied to the primary is converted to a magnetic field which in
turn, induces a current in the secondary which carries energy to the load connected to the
secondary.
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Fig 3.13 33/11kv power transformer
3.10.1 TYPES:
3.10.1 .1 Depending upon the type of construction:
1. Core type.
2. Shell type.
3.10.1.2 Depending upon type of service:
1. Power transformer.
2. Distribution transformer.
3.10.2 CONSERVATIVE TANK:
When the transformer is loaded and when ambient temperature rises, the volume of oil
inside the transformer increases. A conservator tank of the transformer provides adequate spacing
to this expanded transformer oil. It also acts as a reservoir for transformer insulating oil.
This is a cylinder shaped oil container closed from both ends. One large inspection cover
is provided on either side of the container to facilitate maintenance and cleaning inside of the
conservator.
3.10.3 BREATHER:
Breathing is the process where the transformer Breather-in or Breather-out the air from its
body due to the thermal contraction and expansion of oil mass. When the transformer is loaded or
unloaded, the oil temperature inside the transformer changes by either breathing in or breathing out
air. This phenomenon is called the “BREATHING” of the transformer.
Now, the air which is being breathed contains either dust particles or humidity that change
the dielectric strength of the oil. For proper functioning of the transformer, it is absolutely
necessary that, the air entering the transformer is free from moisture and dust particles.
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The indicating grade of SILICA GEL, which is filled in the breather is hard blue crystals,
which has considerable absorption power for moisture. Silica gel absorbs moisture signalling the
saturation degree by changing colour as follows
Fig 3.14 Breather
3.10.4 COOLING SYSTEM:
The main source of heat generation in the transformer is its copper losses.if this heat is not
dissipated properly the temperature of th transformr increases which cause damage to
transformer.so it is essential to control the temperature of the transformer.
Different methods of cooling systems are
 Oil natural air natural cooling
 Oil natural air forced cooling
 Oi forced air natural cooling
 Oil forced air forced cooing
3.10.5 BUCHHOLZ RELAY:
A buchholz relay is a safety device mounted on the oil fitted power transformers and
reactors,equipped with an external over head oil reservoir called conservator.it sense the dielectric
faiure of the equipment.
Distribution transformer:
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Fig 3.15 11kv/440v Station Transformer
.
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3.10.6 POWER TRANSFORMERS – I :
 TYPE : AUTO TRANSFORMER
 COOLING MEDIUM : Insulating Oil (ONAN)
 POWER : 8 MVA
 VOLTAGE : 33KV/11KV
 CURRENT : 242.24A/727.27A
 VECTOR GROUP:YNa0d1
 MAX TEMPERATURE: 55deg-cent.
 INSULATION LEVEL : LI 950 AC 395-AC 38(HV)
LI 550 AC 230-AC 38(LV)
3.10.7 POWER TRANSFORMERS –II :
 TYPE : AUTO TRANSFORMER
 COOLING MEDIUM : Insulating Oil (ONAN)
 POWER : 5 MVA
 VOLTAGE : 33/11KV
 CURRENT : 151.15A/454.45
 VECTOR GROUP:YNa0d1
 MAX TEMPERATURE: 55deg-cent.
 INSULATION LEVEL : LI 950 AC 395(HV)
LI 550 AC 230(LV)
Table.3.1 Total Yard Equipment Registered
EQUIPMENT NAME TOTAL
USED
POWER TRANSFORMERS 2
STATION LOAD TRANSFORMERS 1
33 KV LIGHTNING ARRESTERS 12
11 KV LIGHTNING ARRESTERS 21
11 KV LINE ISOLATER 20
33 KV LINE ISOLATER 8
33 KV LINE CT 12
33 KV DIFFERENTIAL CT 1
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11 KV LINE CT 27
11 KV DIFFERENTIAL CT 1
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3.11 BATTREY:
Generally we give D.C supply to all protective equipment like relays indicator and for
controlling devices through batteries in substations. Battery charges are basically automatically
stabilized D.C power source meant to fed the connected load and simultaneously trickle charge the
battery, there by maintaining it continuously healthy and ready to fed the load in the emergency
situation of A.C supply failure.
3.12 CONCLUSION
A detailed study on the operation and maintenance of major equipment in the substation has
been carried out. The study includes main transformer, different types of breakers and isolators,
bus sectionalizers , CT’s/PT’s and other major components used in 33/11kv Jaggampeta
substation.
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CHAPTER-4
PROTECTION SYSTEM
4.1 RELAYS:
A relay is sensing element which senses an abnormal condition of an electrical circuit and
sends signals to the circuit breaker trip circuit.
Different relays used in protection system:
 Distance relay
 Over current relay
 Differential relay
 Over fluxing relay
 Earth fault relay
 Buchholz relay
4.2 PROTECTIVE RELAYS:
For protection of electrical apparatus and transmission lines, electromechanical relays
with accurate operating characteristics were used to detect overload, short-circuits, and other
faults. While many such relays remain in use, digital devices now provide equivalent protective
functions.
4.2.1 DISTANCE RELAYS:
Distance relay respond to the voltage and current i.e., the impedance, at the relay location.
The impedance per mile is fairly constant so these relay respond to the distance between the relay
location and fault location.
4.2.2 OVER CURRENT RELAYS:
When the operating current exceeds the preset value of the relay then it will
operate.there are several types of over current relays
 Instantaneous relay
 Definite time relay
 Inverse time
 Inverse definite minimum time relay
 Very inverse definite time relay
 Extremely inverse time relay
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4.2.3 DIFFERENTIAL RELAYS:
In a differential protection, currents on both sides of the equipment are compared .Under
normal conditions, current i1 is equal to the current i2.Therfore the currents in the current
transformer secondary is also equal, and no currents flow through the current relay.
If a fault occurs inside the protection zone, the currents in the primary and secondary are
not equal, and hence the current flows through the current relay (differential relay).
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4.2.4 OVER FLUXING RELAY:
A transformer is designed to operate at or below a maximum magnetic flux density in the
transformer core.To limit the eddy current in the core and near by conducting components cause
overheating which is with in a very short time may cause severe damage.To detect this over
fluxing relay is used.
4.2.5 EARTH FAULT RELAY:
The relay is used in the case of grounded neutral system. the vector sum of the current
flowing in the three different phases will fow through the neutral conductor.under healthy
condition the resultant current is zero.in such three phase system any one of the phase is grounded
or earthed then there is a unbalance in system currents and resultant current will flow through the
neutral.if the value is above preset value then the earth fault relay will operate.
4.2.6 BUCHHOLZ RELAY:
A buchholz relay is a safety device sensing the accumulation of gas in large oil-filled
transformers, which will alarm on slow accumulation of gas or shut down the
transformer if gas is produced rapidly in the transformer oil.
4.3 CONCLUSION
The chapter provides different protection systems used in the substation. The following
are the important protection systems used in the sub-station:
Table.6.1 Specification of Relays
S.NO NAME OF THE
EQUIPMENT
NAME OF THE RELAY & PROTECTION
1 33KV FEEDER OVER CURRENT RELAY
2 11KV FEEDER OVER CURRENT RELAY
4 TRANSFORMER-1
TRANSFORMER-2
BUCHHOLZ,OVER CURRENT,DIFFERNTIAL
RELAY AND EARTH FAULT RELAY.
BUCHHOLZ AND OVER CURRENT RELAY.
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CHAPTER-5
HARDWARE SYSTEMS
5.1 CONDUCTORS:
A conductor is an object or type of material that allows the flow of an electrical current in one
or more directions.
 Conductors are the major component in over head lines.
5.1.1 A good conductor should have the following properties:
 High electric conductivity
 High tensile strength to withstand mechanical stresses
 Lower weight per unit volume
 Should not be brittle
 Low cost without compromising much of other properties
5.1.2 Different conductor materials:
 Copper
 Aluminium
 Steel cored Aluminium
 Cadmium copper
 Galvanised Steel
 Silver and other materials
5.1.3 Types of conductors:
 Solid conductor
 Stranded conductor(AAC)
 Composite stranded conductor(ACSR)
 Bundle conductors
5.2 CLAMP:
A clamp is a fastening device used to hold or secure objects tightly to prevent movement
or separation through the application of inward pressure
There are many types of clamps available for different purposes. some are temporary as
used to position components while fixing them together,others are intended to be permanent .
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Fig 8.1 clamp
Types of clamps available:
 Horse clamp
 Stud clamp
 Ct stud clamp
 Pt stud clamp
 Cone clamp
 Parallel grooved clamp
 spacer
5.3 CONCLUSION:
In this chapter we studied about conductors, clamps and other hardware equipment used in
the 220/132/33kv gajuwaka substation.
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CHAPTER-6
EARTHING SYSTEM
6.1 EARTHING:
Earthing or grounding simply means connecting the conducting part to the earth(ground).
6.2 EARTH ELECTRODE:
Earth electrode is an array of conducting elements placed in the earth or sea which
provide a low resistance path between the circuit and the earth and which is capable of carrying
continuous current for some extended period.
6.3 STATION EARTH:
Station earth is an array of conducting elements placed in earth at the substation location
and which provides connection between the earthed parts of the station equipment and the earth.
6.4 Counterpoise (Ground System):
A counterpoise is a network of suspended wires or cables used as a substitute for an earth
connection it usually consists of a single wire network of horizontal wires, parallel to the ground,
suspended above the ground under the antenna, connected to the receiver or transmitter’s ground
wire. The ground must have a low resistance, because any resistance in ground connection will
dissipate power from the transmitter. A common design for a counterpoise is a series of radial
wires suspended a few feet above the ground, extended from the base of the antenna in all the
directions in a star pattern, connected at the centre
6.5 EARTH MAT:
Fig 6.1 earth mat
In an electrical sub-station a ground (earth) mat is a mesh of conductive material installed
at places where a person would stand to operate a switch or other apparatus; it is bonded to the
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local metal structure and to the handle of the switchgear, so that the operator will not be exposed to
high differential voltage due to a fault in the sub-station.
6.6 TOUCH AND STEP POTENTIALS:
When a fault occurs ,the flow of current to ground results in voltage gradient on the
ground surface in the vicinity of grounding system.this voltage gradient may affect a person in two
ways,viz.,step or foot-to-foot contact and hand to both feet or touch contact.
6.6.1 STEP POTENTIAL:
It is the step voltage between the feet of a person standing near an energized grounded
object.it is equal to the difference in voltage given by voltage distribution curve,between two
points at different distances fro the electrode
6.6.2 TOUCH POTENTIAL:
It is the touch voltage between the energized object and the feet of a person in contact
with the object. It is equal to the difference in voltage given by voltage between object and a point
some distance away.
6.7 TRANSFERRED POTENTIAL:
A special situation arise when a person standing with in the station area touches a
conductor grounded at a remote point or a person standing at a remote point touches a conductor
connected to the station grounding grid. The special case of touch potential is known as transferred
potential and can have a very large value.it is not possible to design an earthing grid for safe
transferred potential the only remedy is to isolate such conductors and to label them as ”live
conductor”
6.8 CONCLUSION:
Earthing is very important both for statutory and safety requirements. Through this study
the importance of step potential, touch potential, transferred potential, earth mats and earthing
system used in the substation is understood.
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CHAPTER-7
OBSERVATIONS
7.1 Introduction
During training period in Jaggampeta APEPDCL, observed their work on some worst case
scenarios. And also the other branches of sub-stations.
7.2 SUBSTATION MAINTENANCE AND REPAIR:
Fault identification is becoming big challenge in distribution network. The fault
location is identified manually (line inspector check the entire line).which is time consuming
process, which lead to power interruption to consumers.
In substation major trappings are due to failure of insulators and conductors
breaking .In most of cases tripping are due to temporary faults, which creates unnecessarily
interruptions in the line.
7.2.1 TRANSFORMER MAINTENANCE:
It is one of the important factors for the successful operation of substation.
Monthly Basis Maintenance of Transformer
Monthly Basis Maintenance and Checking:
1. The oil level in oil cap under silica gel breather must be checked in one month
interval. If it is found the transformer oil inside the cup comes below the specified level,
oil to be top up as per specified level.
2. Breathing holes in silica gel breather should also be checked monthly and properly
cleaned if required, for proper breathing action.
3. If the transformer has oil filled bushing the oil level of transformer oil inside the
bushing must be individually checked in the oil gage attached to those bushing. This action
also to be done monthly basis.
If it is required, the oil to be filled in the bushing up to correct level. Oil filling to be
done under shutdown condition.
Daily Basis Maintenance and Checking:
There are three main things which to be checked on a power transformer in daily basis
and they are:
1. Reading of MOG (Magnetic Oil Gage) of main tank and conservator tank.
2. Colour of silica gel in breather.
3. Leakage of oil from any point of a transformer.
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In case of unsatisfactory oil level in the MOG, oil to be filled in transformer and also
the transformer tank to be checked for oil leakage. If oil leakage is found take required action
to plug the leakage. If silica gel becomes pinkish, it should be replaced.
Yearly Basis Transformer Maintenance Schedule:
1. The auto, remote, manual function of cooling system that means, oil pumps, air fans,
and other items engaged in cooling system of transformer, along with their control circuit
to be checked in the interval of one year. In the case of trouble, investigate control circuit
and physical condition of pumps and fans.
2. All the bushings of the transformer to be cleaned by soft cotton cloths yearly. During
cleaning the bushing should be checked for cracking.
3. Oil condition of OLTC to be examined in every year. For that, oil sample to be taken
from drain valve of diverter tank, and this collected oil sample to be tested for dielectric
strength (BDV) and moisture content (PPM). If BDV is low and PPM for moisture is
found high compared to recommend values, the oil inside the OLTC to be replaced or
filtered.
4. Mechanical inspection of Buchholz relays to be carried out on yearly basis.
5. All marshalling boxes to be cleaned from inside at least once in a year. All
illumination, space heaters, to be checked whether they are functioning properly or not. If
not, required maintenance action to be taken. All the terminal connections of control and
relay wiring to be checked an tighten at least once in a year.
6. All the relays, alarms and control switches along with their circuit, in R&C panel
(Relay and Control Panel) and RTCC (Remote Tap Changer Control Panel) to be cleaned
by appropriate cleaning agent.
7. The pockets for OTI, WTI (Oil Temperature Indicator & Winding Temperature
Indicator) on the transformer top cover to be checked and if required oil to be replenished.
8. The proper function of Pressure Release Device and Buchholz relay must be checked
annually. For that, trip contacts and alarm contacts of the said devices are shorted by a
small piece of wire, and observe whether the concerned relays in remote panel are properly
working or not.
9. Insulation resistance and polarization index of transformer must be checked with
battery operated megger of 5 KV range.
10. Resistive value of earth connection and rizer must be measured annually with clamp
on earth resistance meter.
11. DGA or Dissolve Gas Analysis of transformer Oil should be performed, annually for
132 KV transformer, once in 2 years for the transformer below 132 KV transformer and in
2 years interval for the transformer above 132 KV transformer.
The Action to be taken once in 2 years:
I. The calibration of OTI and WTI must be carried once in two years.
II. Tan & delta; measurement of bushings of transformer also to be done once in two
years.
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Maintenance of Transformer on Half Yearly Basis
The transformer oil must be checked half yearly basis that means once in 6 months,
for dielectric strength, water content, acidity, sludge content, flash point, DDA, IFT,
resistivity for transformer oil. In case of distribution transformer, as they are operating light
load condition all the time of day remaining peak hours, so there are no maintenance required.
7.3 PARALLEL CONNECTION OF TRNASFORMERS:
To improve substation power handling capabilities most of case new transformer is
installed in parallel to existing transformer instead of replacing it. In Jaggampeta circle one of
the substations required to have increased the power handling of substation. Even the required
amount of power is available, unable to provide power to consumer continuously because of
over loading of transformer, which is lead to loss of revenue. That’s why authority decided to
install new transformer with existing one, now the customers who are depended on the
substation get the power without interruptions.
7.3.1 Conditions for parallel Operation of Transformer:
a) Same voltage Ratio & Turns Ratio (both primary and secondary Voltage Rating is
same).
b) Same Percentage Impedance and X/R ratio.
c) Identical Position of Tap changer.
d) Same KVA ratings.
e) Same Phase angle shift (vector group are same).
f) Same Frequency rating.
g) Same Polarity.
h) Same Phase sequence.
7.3.1.1 Other necessary conditions for parallel operation:
a) All parallel units must be supplied from the same network.
b) Secondary cabling from the transformers to the point of paralling has approximately
equal length and characteristics.
c) Voltage difference between corresponding phase must not exceed 0.4%.
d) When the transformers are operated in parallel, the fault current would be very high
on the secondary side. Supposing percentage impedance of one transformer is say
6.25 %, the short circuit MVA would be 25.6 MVA and short circuit current would be
35 kA.
e) If the transformers are of same rating and same percentage impedance, then the
downstream short circuit current would be 3 times (since 3 transformers are in
Parallel) approximately 105 kA. This means all the devices like ACBs, MCCBs,
switch boards should withstand the short-circuit current of 105 kA. This is the
maximum current. This current will get reduced depending on the location of the
switch boards, cables and cable length etc. However this aspect has to be taken into
consideration.
30

f) There should be Directional relays on the secondary side of the transformers.
g) The percent impedance of one transformer must be between 92.5% and 107.5% of the
other. Otherwise, circulating currents between the two transformers would be
excessive.
7.3.2 Advantages of parallel operation of Transformers:
1) Maximize electrical system efficiency:
a) Generally electrical power transformer gives the maximum efficiency
at full load. If we run numbers of transformers in parallel, we can switch on only
those transformers which will give the total demand by running nearer to its full load
rating for that time.
b) When load increases we can switch no one by one other transformer
connected in parallel to fulfil the total demand. In this way we can run the system
with maximum efficiency.
2) Maximize electrical system availability:
If numbers of transformers run in parallel we can take shutdown any one of
them for maintenance purpose. Other parallel transformers in system will serve the load
without total interruption of power.
3) Maximize power system reliability:
If nay one of the transformers run in parallel, is tripped due to fault
other parallel transformers is the system will share the load hence power supply may not be
interrupted if the shared loads do not make other transformers over loaded.
4) Maximize electrical system flexibility:
a) There is a chance of increasing or decreasing future demand of power
system. If it is predicted that power demand will be increased in future, there must be
a provision of connecting transformers in system in parallel to fulfil the extra demand
because it is not economical from business point of view to install a bigger rated
single transformer by forecasting the increased future demand as it is unnecessary
investment of money.
b) Again if future demand is decreased, transformers running in parallel
can be removed from system to balance the capital investment and its return.
7.3.3 Disadvantages of parallel operation Transformers:
1. Increasing short-circuit currents that increase necessary breaker capacity.
2.The risk of circulating currents running from one transformer to another Transformer.
Circulating currents that diminish load capability and increased losses.
3.The bus ratings could be too high.
4.Paralleling transformers reduces the transformer impedance significantly, i.e. the
parallel transformers may have very low impedance, which creates the high short
31

circuit currents. Therefore, some current limiters are needed, e.g. reactors, fuses, high
impedance buses, etc
5.The control and protection of three units in parallel is more complex.
6.It is not a common practice in this industry, since Main-tie-Main is very common in
this industry.
7.4 CONCLUSION:
1. Loading considerations for paralleling transformers are simple unless kVA, percent
impedances, or ratios are different. When paralleled transformer turn ratios and
percent impedances are the same, equal load division will exist on each transformer.
When paralleled transformer kVA ratings are the same, but the percent impedances
are different, then unequal load division will occur.
2. The same is true for unequal percent impedances and unequal kVA. Circulating
currents only exist if the turn ratios do not match on each transformer. The magnitude
of the circulating currents will also depend on the X/R ratios of the transformers.
Delta-delta to delta-wye transformer paralleling should not be attempted.
In this chapter we studied about the safety precautions, maintenance, necessity of
parallel operation of transformers and different failures occurred during our training.
32

CHAPTER-8
CONCLUSION
A practical study on the operation and maintenance of a typical substation, 33/11 kV
of APEPDCL has been carried out. The study covers all the main equipment of switchyard
like Power Transformer, Circuit Breakers, CTs / PTs, Isolators including the relays, batteries,
earthing practices and safety practices. It is also observed the how the good operating and
maintenance practices will help in achieving the objectives of the APEPDCL.
The following conclusions are made during the training:
1. 33/11KV JAGGAMPETA sub-station is an outdoor type primary sub-station.
2. There are two incoming feeders at 33kv, and 7 outgoing feeders at 11kv to different
customers.
3. The main transformer is a 8MVA ONAN auto transformers along with the differential
protection scheme and another 5MVA ONAN auto transformer the associated
protection equipment.
4. Studied about different control room equipment’s such as relays, batteries, isolator
switches, sectionalizer switches and other major components and their specifications.
5. Understood the importance of DC system and working of the associated systems.
6. The importance of earthing for statutory as well as safety along with basic definitions
of step potential, touch potential and need for earth mats and earthing system is
understood.
7. Observed the importance of maintenance of transformer, and study the overall
maintenance of transformers.
8. Study about parallel operation of transformers and its importance and difficulties.
33

REFERENCES:
1. http://www.apepdcl.gov.in.
2. Mohammad Sharique Nawaz Guide- Prof. Irfan Khan “Design and Construction of
33/11 KV Line & Substation” International Research Journal of Engineering and
Technology (IRJET), Volume-03, 07 July-2016.
3. https://electricalnotes.wordpress.com/2012/07/17/parallel-operation-of-transformers/
4. https://www.electrical4u.com/maintenance-of-transformer/
34

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Internship doc 33 11 kv substation