132 33kv substation documentation

CHAPTER- 2
CLASSIFICATION OF SUBSTATIONS

CHAPTER-3
SINGLE LINE DIAGRAM (SLD)

CHAPTER- 4
BRIEF DISCRIPTION OF
INSTRUMENT IN THE SUBSTATION

CHAPTER- 5
PROTECTION FOR VARIOUS
EQUIPMENTS

A
Mini Project Report on
OPERATION AND MAINTENANCE OF 132/33KV
SUBSTATION
Mini Project Submitted in Fulfillment of The Requirements
For The Award of The Degree
BACHELOR OF TECHNOLOGY
IN
ELECTRICAL AND ELECTRONICS ENGINEERING
Submitted By
G.RAVI KUMAR 116U1A0213
R.TRIVENI 116U1A0242
I.VENKATESWARLU 116U1A0217
G.RAJASHEKAR REDDY 116U1A0216
1
Under the guidance of
Ms.R.RAMADEVI B.Tech
Asst.Professor, EEE Department
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
SreeKavitha Educational Society’s
SREE KAVITHA INSTITUTE OF SCIENCE & TECHNOLOGY
(Approved by AICTE –New Delhi & Affiliated to JNTU- Hyderabad )
KRISHNAPURAM (V), MADHIRA (M), KHAMMAM-507203(T.S)

SreeKavitha Educational Society’s
SREE KAVITHA INSTITUTE OF SCIENCE & TECHNOLOGY
(Approved by AICTE –New Delhi & Affiliated to JNTU- Hyderabad )
KRISHNAPURAM (V), MADHIRA (M), KHAMMAM-507203(T.S)
CERTIFICATE
This is certify that the mini project report Entitled
OPERATION AND MAINTENANCE OF 132/33KV
SUBSTATION
Is a bonafide record of work carried out by
We here by accord my approval of it as a mini project report carried out and presented in a
manner required for its acceptance in fulfillment for award of degree of Bachelor of Technology
in Electrical & Electronics Engineering in Jawaharlal Nehru Technological University,
Hyderabad.
PROJECT GUIDE HEAD OF THE DEPARTMENT
Ms.R.RAMADEVI,B.Tech Mr.G.VENKAT,M.Tech
Assistant Professor Assistant professor.

DECLARATION
We declare that the project report entitled is done by us, submitted in partial fulfillment of
the requirements for the award of the degree in BACHELOR OF TECHNOLOGY.
G.RAVI KUMAR 116U1A0213
R.TRIVENI 116U1A0242
I.VENKATESWARLU 116U1A0217
G.RAJASHEKAR REDDY 116U1A0216
1
PLACE : KRISHNAPURAM

ABSTRACT
A substation receives electrical power from generating station via incoming transmission
line and delivers electrical power through feeders and this is used for controlling the power on
different routes. Substations are integral part of a power system and form important part of
transmission and distribution network of electrical power system.Their main functions are to
receive energy transmitted at high voltage from the generating stations, reduce the voltage to a
value appropriate for local distribution and provide facilities for switching some sub-station are
simply switching stations different connections between various transmission lines are made,
others are converting sub-stations which either convert AC into DC or vice-versa or convert
frequency from higher to lower or vice-versa.The various circuits are joined together through
these components to a bus-bar at substation. Basically, sub-station consists of power
transformers, circuit breakers, relays, isolators, earthing switches, current transformers, voltage
transformers, synchronous condensers/ capacitor banks etc.This mini project covers the
important equipments & their function in a substation. And also an attempt is made to cover
the general maintenance of substation and checks the observations to be made by shift engineer.
As a part of case study we are going to visit a 132/33Kv TRANSCO substation in
Khammam.

INDEX
CHAPTER TITTLE PAGE NO
LIST OF FIGURES
LIST OF ABBREVATIONS
CHAPTER-1 INTRODUCTION
1.1 Introduction of Substation ` 1
1.2 Construction of A Substation 1
1.2.1 Selection of Site 1
CHAPTER-2 CLASSIFICATION OF SUBSTATION
2.1 According To The Requirements 3
2.2 According To The Constructional Features 3
CHAPTER-3 SINGLE LINE DIAGRAM
3.1 Single line diagram 4
3.2 Feeder Circuit 4
CHAPTER-4 BRIEF DISCRIPTION OF INSTRUMENTS IN THE
SUBSTATION
4.1 Lightning Arrester 5
4.1.1 The Action of The Lightning 5
4.2 Earthing 7
4.2.1 In All Substations There Shall Be Provision
For Earthing The Following 7
4.3 Capacitor Voltage Transformers (CVT) 7
4.3.1 Specifications of CVT 9
4.4 Wave trap 9
4.5 Instrument Transformer 10
4.5.1 Current Transformer (C.T) 10
4.5.1.1 Basic Design Principle of C.T 11
4.5.1.2 Simple Line Diagram of C.T 12
4.5.1.3 Tests Generally To Be Conducted on C.T 12
4.5.1.4 Specifications of HVCT 12

4.5.1.5 Specifications of LVCT 13
4.5.2 Potential Transformers 14
4.5.2.1 Basic Design Principle of Voltage Transformers 15
4.5.2.2 Simple Line Diagram of Voltage Transformers 15
4.5.2.3 Tests Generally To Be Conducted on The P.T’s 15
4.5.2.4 General Checks For P.T 16
4.6 Circuit Breaker (C.B) 16
4.6.1 SF6 Circuit Breaker 18
4.6.2 Vaccum Circuit Breaker 18
4.6.3 Name Plate Details of 132KV SF6 C.B 18
4.6.4 Name Plate Details of 33KV Vaccum C.B 19
4.7 Bus 19
4.8 Transformer 20
4.8.1 Basic Principle 20
4.8.2 Induction Law 21
4.8.3 Specifications of 132KV/33KV Auto T/F 23
4.9 Capacitor Bank Attached To The Bus 23
4.9.1 Capacitor Control is Usually Done To Achive
The Following Goals 24
CHAPTER-5 PROTECTION FOR VARIOUS EQUIPMENTS
5.1 Transformer Protection 25
5.2 Feeder Protection 25
5.3 Important Points To Be Kept In View While Laying 26
Out The Substation
CONCLUSION 27
REFERENCES 28

LIST OF FIGURES
Fig:1.2.1 Diagram of Substation
Fig.4.1.1.1 (i) Surge Diverter
(ii)Characteristics of The Non Linear Resister
Fig: 4.1.1.2 Lightning Arrester
Fig: 4.3.1 Circuit Diagram of CVT.
Fig: 4.3.2 Capacitor Voltage Transformer.
Fig:4.4.1 Wave Trap
Fig:4.5.1.1.1 Current Transformer
Fig: 4.5.1.2.1 Line Diagram of C.T
Fig: 4.5.2.1.1 Potential Transformer.
Fig: 4.5.2.2.1 Line Diagram of V.T
Fig: 4.6.1 Circuit Breaker
Fig: 4.8.1 Electrical Transformer.
Fig: 4.8.1.1 Ideal Transformer.
Fig: 4.8.2.1 Mutual Induction.
Fig: 4.8.2.2 Three Phase 50MVA Auto Transformer.
Fig: 4.9.1 Capacitor Bank In The Distribution System.
Fig: 4.9.1.1 Reactive Losses.

LIST OF ABBREVIATIONS
EHV –Extra High Voltage
SLD – Single Line Diagram
PT – Potential Transformer
CT – Current Transformer
HVCT - High Voltage CT
LVCT – Low Voltage CT
CVT – Capacitor Voltage Transformer
LA – Lightening Arrestors
ES - Earth Switches
CB – Circuit Breaker
HV side – High Voltage Side
LV side – Low Voltage Side
PLCC - Power Line Carrier Communication
OLTC –On load Tap Changer
HG Fuse -Horn Gap Fuse
OTT –Oil Temperature Indicator
WTI – Winding Temperature Indicator
IDMT Characteristics – Inverse Definite Minimum Time Characteristics.

OPERATION AND MAINTENANCE OF 132/33KV SUBSTATION
DEPT OF EEE, SKIT. Page 1
1.1 INTRODUCTION OF SUBSTATION
The present-day electrical power system is A.C. i.e. electric power is generated,
transmitted and distributed in the form of alternating current. It is delivered to the
consumers through a large network of transmission and distribution. At many places in
the line of the power system, it may be desirable and necessary to change some
characteristic (e.g. voltage, A.C. to D.C., frequency, Power factor etc.) of electric supply.
This is accomplished by suitable apparatus called sub-station. For example, generation
voltage (11KV or 6.6KV) at the power station is stepped up to high voltage (say
132KV or 220KV) for transmission of electric power. The assembly of apparatus (e.g.
transformer etc.) used for this purpose is the sub-station. Similarly, near the consumer’s
localities, the voltage may have to be stepped down to utilization level. This job is again
accomplished by a suitable apparatus called ‘substation.
1.2 CONSTRUCTION OF A SUBSTATION
At the time of constructing a substation, we have to consider some factors which
affect the substation efficiency like selection of site.
1.2.1 Selection of Site
Main points to be considered while selecting the site for EHV Sub-Station are as
follows:
 The site chosen should be as near to the load centre as
possible.
 It should be easily approachable by road or rail for transportation of
equipments.
 Land should be fairly levelled to minimize development cost.
 The sub-station site should be as near to the town / city but should be clear
of public places, aerodromes, and Military / police installations.

 The land should be have sufficient ground area to accommodate substation
equipments, buildings, staff quarters, space for storage of material, such as store
yards and store sheds etc. with roads and space for future expansion.
 Set back distances from various roads such as National Highways,
State
 While selecting the land for the substation preference to be given to the
Govt. land over Private land.
Fig:1.2.1.1 Diagram of Substation

There are several ways of classifying sub-stations. However, the two most important
ways of classifying them are according to (1) service requirement and (2) constructional
features.
2.1 ACCORDING TO THE REQUIREMENT
A sub-station may be called upon to change voltage level or improve power factor or
convert A.C. power into D.C. power etc. According to the service requirement, sub-stations
may be classified into:
1 Transformer sub-stations
2 Switching sub-stations
3 Power factor correction sub-stations
4 Frequency changer sub-stations
5 Converting sub-stations
6 Industrial sub-stations
2.2 ACCORDING TO THE CONSTRUCTIONAL FEATURES
A sub-station has many components (e.g. circuit breakers, switches, fuses, instruments
etc.) which must be housed properly to ensure continuous and reliable service. According to
constructional features, the sub-stations are classified as
 Indoor sub-station
2 Outdoor sub-station
 Underground sub-station
 Pole-mounted sub-station

3.1 SINGLE LINE DIGRAM
A Single Line Diagram (SLD) of an Electrical System is the Line Diagram of the
concerned Electrical System which includes all the required electrical equipment connection
sequence wise from the point of entrance of Power up to the end of the scope of the mentioned
Work. As in the case of 132KV Substation, the SLD shall show Lightening Arrestor,
C.T/P.T Unit, Isolators, Protection and Metering P.T & C.T. Circuit Breakers, again Isolators
and circuit Breakers, Main Power Transformer, all protective devices/relays and other special
equipment like CVT, GUARD RINGS, etc as per design criteria. And the symbols are
shown below. There are several feeders enter into the substation and carrying out the
power. As these feeders enter the station they are to pass through various instruments.
3.2. FEEDER CERCUIT
1 Lightening Arrestors
2 CVT
3 Wave trap
4 Isolators with Earth Switch
5 Current Transformer
6 Circuit Breaker
7 Feeder Bus Isolator
8 BUS
9 Potential Transformer in the Bus with a Bus Isolator.

4.1 LIGHTENING ARRESTERS
Lightening arresters are the instruments that are used in the incoming feeders so that to
prevent the high voltage entering the main station. This high voltage is very dangerous to the
instruments used in the substation. Even the instruments are very costly, so to prevent any
damage lightening arresters are used. The lightening arresters do not let the lightening to fall
on the station. If some lightening occurs the arrestors pull the lightening and ground it to the
earth. In any substation the main important is of protection which is firstly done by these
lightening arrestors. The lightening arresters are grounded to the earth so that it can pull the
lightening to the ground.
These are located at the entrance of the transmission line in to the substation and as near
as possible to the transformer terminals.
The lightning arresters or surge diverters provide protection against such surges. A
lightning arrester or a surge diverter is a protective device, which conducts the high voltage
surges on the power system to the ground.
Fig.4.1.1.1 (i) Surge Diverter
(ii)Characteristics of The Non Linear Resister

Fig: 4.1.1.2 Lightning Arrester
4.1.2 The Action of The Lightning Arrester or Surge Diverter is as Under
 Under normal operation, the lightning arrester is off the line i.e. it conducts no current
to earth or the gap is non-conducting.
 On the occurrence of over voltage, the air insulation across the gap breaks down and
an arc is formed providing a low resistance path for the surge to the ground. In
this way, the excess charge on the line due to the surge is harmlessly conducted through
the arrester to the ground instead of being sent back over the line.
 It is worthwhile to mention the function of non-linear resistor in the operation of
arrester. As the gap sparks over due to over voltage, the arc would be a short circuit on
the power system and may cause power-follow current in the arrester. Since the
characteristic of the resistor is to offer low resistance to high voltage (or current), it
gives the effect of short circuit. After the surge is over, the resistor offers high resistance
to make the gap non conducting
The LA voltage rating corresponding to the system voltages are indicated below
Rated system
Voltage (KV)
Highest system
Voltage (KV)
Arrester rating in KV
Effectively earthed systems
11 12 9
33 36 30
66 72.5 60
132 145 120/132 (latex)
220 245 198/216 (latex)
400 420 336
Table:4.1.2.1 LA Voltage Rating

4.2 EARTHING
The earthing practice adopted at generating stations, sub-stations and lines should be in
such a manner as to provide in units of ohms
 Safety to personnel
 Minimum damage to equipment as a result of flow of heavy fault currents
Improve reliability of power supply
 Large sub-stations- 1
 Small sub-stations-2
 Power stations -0.5
 Distribution transformer stations- 5
4.2.1 In All Sub-Stations There Shall be Provision For Earthing The Following
 The neutral point of earth separate system should have an independent earth, which
in turn should be interconnected with the station grounding mat
 Equipment frame work and other non-current carrying parts.
 All extraneous metallic frame work not associated with equipment (two connections)
 The earth conductor of the mat could be buried under earth to economical depth of
burial of the mat 0.5 meters.
4.3 CAPACITOR VOLTAGE TRANSFORMER (CVT)
A capacitor voltage transformer (CVT) is a transformer used in power systems to
step-down extra high voltage signals and provide low voltage signals either for measurement
or to operate a protective relay
These are high pass Filters (carrier frequency 50KHZ to 500 KHZ) pass carrier
frequency to carrier panels and power frequency parameters to switch yard. In its most basic
form the device consists of three parts: two capacitors across which the voltage signal issplit, an
inductive element used to tune the device and a transformer used to isolate and further step-
down the voltage.

Fig: 4.3.1 Circuit Diagram of C.V.T
The device has at least four terminals, a high-voltage terminal for connection to the
high voltage signal, a ground terminal and at least one set of secondary terminals for
connection to the instrumentation or protective relay. CVTs are typically single-phase devices
used for measuring voltages in excess of one hundred KV where the use of voltage transformers
would be uneconomical. In practice the first capacitor, C1, is often replaced by a stack of
capacitors connected in series. This results in a large voltage drop across the stack of
capacitors, that replaced the first capacitor and a comparatively small voltage drop across the
second capacitor, C2, and hence the secondary terminals.
Fig: 4.3.2 Capacitor Voltage Transformer.

4.3.1 Specifications of CVT
CVT type : CVEB/245/1050
Weight : 665 kg
Total output simultaneous : 250 VA
Output maximum : 750 VA at 50O
C
Rated voltage : A-N, 220/√3
Highest system voltage : A-N, 245/√3
Rated frequency : 50Hz
Nominal intermediate voltage : A1-N, 20/√3 KV
Voltage factor : 1.2Cont. 1.5/30 sec
‘HF’ capacitance : 4400pF +10% -5%
Primary capacitance C1 : 4840pF +10% -5%
Secondary capacitance C2 : 48400 pF +10%-5%
Voltage ratio : 220000/√3/ 110/√3/110-110/√3
Voltage : 110/√3 110-110/√3
Burden : 150 100
Class : 0.5
4.4 WAVE TRAP
Wave trap is an instrument using for trapping of the wave. The function of this
wave trap is that it traps the unwanted waves. Its shape is like a drum. It is connected to the
main incoming feeder so that it can trap the waves which may be dangerous to the
instruments in the substation. Generally it is used to exclude unwanted frequency components,
such as noise or other interference, of a wave.
Note: Traps are usually unable to permit selection of unwanted or interfering signals.

Line trap also is known as Wave trap. What it does is trapping the high frequency
communication signals sent on the line from the remote substation and diverting them to the
telecom/tele protection panel in the substation control room through coupling capacitor.
Fig:4.4.1 Wave Trap
This is relevant in Power Line Carrier Communication (PLCC) systems for
communication among various substations without dependence on the telecom company
network. The signals are primarily tele protection signals and in addition, voice and data
communication signals. The Line trap offers high impedance to the high frequency
communication signals thus obstructs the flow of these signals in to the substation bus bars. If
these are not present in the substation, then signal loss is more and communication will be
ineffective/probably impossible.
4.5 INSTRUMENT TRANSFORMERS
“Instrument Transformers are defined as the instruments in which the secondary current or
voltage is substantially proportional to the primary current or voltage and differs in phase from
it by an angle which is approximately zero for an appropriate direction of connection”.
Direct measurement of current or voltage in high voltage system is not possible
because of high values and insulation problems of measuring instruments they cannot be
directly used for protection purposes.
Instrument transformers are of two types:
 Current Transformers
 Voltage Transformers
4.5.1 Current Transformers:
Current transformer is a current measuring device used to measure the currents in
high voltage lines directly by stepping down the currents to measurable values by means
of electromagnetic circuit.

4.5.1.1 Basic Design Principle of Current Transformers
The basic principle induced in designing of current transformers is
Primary ampere turns = Secondary ampere turns
Ip  Np = Is  Ns
Where, Ip - Primary current
Np - Primary Winding Turns
Is - Secondary Current; Ns - Secondary Winding Turn
 Ampere turns plays very important role in designing current transformers.
 Current transformers must be connected in series only.
 Current transformer has less no of turns in primary and more no of turns in
secondary.
 The secondary current is directly proportional to primary current.
 The standards applicable to CT's are IEC-60044-1 and IS – 2705.
Fig:4.5.1.1.1 Current Transformer

4.5.1.2 Simple Line Diagram of Current Transformer
The line diagram of a current transformer contains different components:
S
Fig: 4.5.1.2.1 Line Diagram of C.T
4.5.1.3 Tests generally to be conducted on CT
 Insulation resistance values (IR values): Primary to earth, primary to
secondary core1, primary to secondary core2, core1 to earth, core2 to earth and
core1 to core2. Primary to earth and primary to secondary cores are to be checked
with 5KV motor operated insulation tester (megger) and secondary to earth values
are to be checked with 1000V insulation tester or preferably with 500V insulation
tester.
 Ratio test: Primary injection test is to be conducted for this purpose
 TAN-DELTA test: on 132KV CTs and above
 Secondary and lead resistance check
 Secondary injection check
 Primary injection check
4.5.1.4 Specifications of HVCT
Type : IT-245
Frequency : 50 Hz
H.S.V : 245 KV
BIL : 460/1050KV Oil
weight : 360kgs
Total weight : 1250kgs

RATIO
800-600-400/1-1-1-1-1
CORE NUMBER 1 2 3 4 5
RATED PRIMARY
CURRENT (A)
800
RATED
SECONDARY
CURRENT(A)
1 1 1 1 1
OUTPUT(VA) -------- ----------
--------
-----
------- - 30
ACCURACY CLASS PS PS PS PS 0.5
I.S.F/A.L.F ---- --- --- --- <=5
TURN RATIO 2/1600 1200 800
RCT at 75 C AT 800/1
(ohms)
6 6 6 ---
Table: 4.5.1.4.1 Specifications of HVCT.
At the rate of LV (132KV) side we can use 1:3 core CT. The specifications of LVCT
are given below:
4.5.1.5 Specifications of LVCT
Type : IT-145
Frequency : 50 Hz
HSV/NSV : 145/132 KV
BIL : 650/275 KV Oil
weight : 75Kg
Total weight : 550Kg

RATIO
500/1-1, 0.66-1
CORE NUMBER 1 2 3
RATED PRIMARY
CURRENT (A)
500
PRIMARY &
SECONDARY
CONNECTION
500/1 500/1 500/0.66 500/1
1s1-1s2 2s1-2s2 2s1-2s3 3s1-3s2
RATED SECONDARY
CURRENT(A)
1 1 0.66 1
OUTPUT(VA) 20 ------- ------------ 20
ACCURACY CLASS 5p PS 0.2
I.S.F/A.L.F 20 -------- --------- <=5
Rct at 75o
C (Ohms) -------- <=5 --------- -------------
Table:4.5.1.5.1 Specifications of LVCT
NOTE
 CT secondary circuit and PT primary should never be open circuited. It is
vulnerable to the CT/PT
 CT primary circuit and PT secondary should never be short circuited.
4.5.2 Potential Transformers (PT)
An instrument transformer in which the secondary voltage, in normal conditions of use,
is substantially proportional to the primary voltage and differs in phase from it by an angle
which is approximately zero for an appropriate direction of the connections.

4.5.2.1 Basic Design Principle of Voltage Transformer’s
The basic principle involved in the designing of Voltage Transformer is
Voltage Ratio = Turns Ratio
VP / VS = NP / NS
Thus NS  VP = NP  VS
As heavy primary voltages will be reduced to low secondary voltages, it will have more
turns in the primary & less turns in the secondary. It must always be connected in parallel only.
Even if we connect it directly from high voltage to earth, it is not going to be a short circuit as its
primary winding has very high resistance. Its core is a set of assembled laminations. It operates
at constant flux density. The standards are IEC – 600044 – 2 and IS – 3156.
Fig: 4.5.2.1.1 Potential Transformer.
4.5.2.2 Simple Line Diagram of Voltage Transformer
Fig: 4.5.2.2.1 Line Diagram of VT.
4.5.2.3 Tests generally to be conducted on the PTs
 Insulation resistance values (IR values): primary to earth, primary to
secondary core-1, primary to secondary core-2, core1 to earth, core 2 to earth
and core-1 to core-2. These values are to be checked with 1000V insulation
tester (megger) or preferably with 500V insulation tester.

 Ratio Test: By applying single phase voltage across primary the voltage
induced in the secondary winding is to be measure. This is approximately equal to
voltage applied in the primary winding or voltage ratio of the PT.
 Polarity test: at the time of commissioning (at least on the PTs connected to
revenue meters)
 PT secondary injection check
4.5.2.4 General checks for PT
 Mechanical alignment for PT power jaws
 PT primary winding star earthing
 Tightness of all connections
 Primary/secondary fuse ratings
 PT specifications
In PTs no of secondary cores is 1 or more than 1 based on the requirement. Generally
in 11KV or 33KV bus PTs, there is one secondary winding which is used both for protection
and metering and in 132KV and above, there are two secondary cores. First core is of metering
core with 1.0 or 0.5 or 0.2 accuracy classes. This will be used metering, directional over current
protection and distance protection.
4.6 CIRCUIT BREAKER
The circuit breakers are used to break the circuit if any fault occurs in any of the
instrument. These circuit breaker breaks for a fault which can damage other instrument in the
station. For any unwanted fault over the station we need to break the line current. This is only
done automatically by the circuit breaker.
 Operation mechanism function,
 Arc quenching function.

Fig: 4.6.1 Circuit Breaker

 various operating mechanisms
1 Spring charge mechanism,
2 Pneumatic mechanism,
3 Hydraulic Mechanism.
 Arc quenching medium
1 Bulk oil (called bulk oil circuit breakers-BOCB)
Minimum oil (called minimum oil circuit breakers-MOCB)
 Natural air (called air circuit breakers-ACB) (415v)
 Forced air (called air blast circuit breaker-ABCB)
 Vacuum (called vacuum circuit breaker-VCB)
 SF6 gas (called Sulphur Hexafluoride-SF6 gas CB)
The present trend is up to 33KV, VCBs are preferred and beyond 33KV, SF6 gas
circuit breakers are preferred.
There are mainly two types of circuit breakers used for any substations. They are
(a) SF6 circuit breakers,
(b) Vacuum circuit breakers.

4.6.1 SF6 Circuit Breakers
Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and arc
extinguishing properties. The dielectric strength of the gas increases with pressure and is more
than the dielectric strength of oil at 3 kg/cm2. SF6 is now being widely used in electrical
equipment like high voltage metal enclosed cables; high voltage metal clad switchgear,
capacitors, circuit breakers, current transformers, bushings, etc. The gas is liquefied at certain
low temperature, liquidification temperature increases with the pressure.
Some of the properties of SF6 are,
 Very high dielectric strength
 High thermal and chemical inertia
 Superior arc extinguishing capability

 Low decomposition by arcing
4.6.2 Vacuum Circuit Breakers
Vacuum type of circuit breakers is used for small KV rated stations below 33KV.
They are only used in low distribution side.
4.6.3 Name Plate Details of 132KV SF6 CB
Type : 200-SFM-40A
Rated Voltage : 145KV
Lightining Impulse Withstand : 650KV(Peak)
Rated Frequency : 50HZ
Normal Current : 1600A
Rated Short Circuit Breaking Current
Symmetrical : 31.5KA
Asymmetrical : 37.2KA
Rated Short Circuit Making Current : 80KA(Peak)
Out-Of-Phase Breaking Current : 7.9KA
Rated Break Time : 60ms(3 Cycles)
Rated Short Time Current : 40KA For 3 Sec

Operating Sequence : 0-0.03s-C0-3min-CO
Total Mass OF SF6 Gas : 8.7kg
SF6 Gas Pressure AT 20c,1013hpa : 6.3bar
Total Mass Of The Circuit Breaker : 1300kg
Reference Standard : IEC-56
4.6.4 Name Plate Details of 33KV Vacuum CB
Voltage : 36KV
Frequency : 50HZ
Normal Current : 800A
SYM Breaking Capacity : 25KA
Short Time Current : 25KA
Duration : 3sec
Making Capacity : 63KA(peak)
P.F Withstand : 70KV
Impulse : 170KV(peak)
Shunt Trip coil : 220VDC
Spring RELCoil : 220VDC
Total Weight : 2000kg
Operating Sequence : 0-3MIN-CO-3MIN-CO
Type : VN36 3AF
4.7 BUS
The bus is a line in which the incoming feeders come into and get into the instruments
for further step up or step down. The first bus is used for putting the incoming feeders in la
single line. There may be double line in the bus so that if any fault occurs in the one the other
can still have the current and the supply will not stop. The two lines in the bus are separated by
a little distance by a conductor having a connector between them. This is so that one can work
at a time and the other works only if the first is having any fault.

4.8 TRANSFORMERS
Transformers come in a range of sizes from a thumbnail-sized coupling transformer
hidden inside a stage microphone to huge units weighing hundreds of tons used to
interconnect portions of national power grids. All operate with the same basic principles,
although the range of designsis wide. While new technologies have eliminated the need for
transformers in some electronic circuits, transformers are still found in nearly all electronic
devices designed for household ("mains") voltage. Transformers are essential for high
voltage power transmission, which makes long distance transmission economically practical.
Fig: 4.8.1 Electrical Transformer.
4.8.1 Basic Principle
The transformer is based on two principles: firstly, that an electric current can produce
a magnetic field (electromagnetism) and secondly that a changing magnetic field within a coil
of wire induces a voltage across the ends of the coil (electromagnetic induction).
Changing the current in the primary coil changes the magnetic flux that is developed.
The changing magnetic flux induces a voltage in the secondary coil.

Fig: 4.8.1.1 Ideal Transformer.
An ideal transformer is shown in the adjacent figure; Current passing through the
primary coil creates a magnetic field. The primary and secondary coils are wrapped
around a core of very high magnetic permeability, such as iron, so that most of the magnetic
flux passes through both primary and secondary coils.
4.8.2 Induction law
The voltage induced across the secondary coil may be calculated from Faraday's
law of induction, which states that, where VS is the instantaneous voltage, NS is the number
of turns in the secondary coil and Φ equals the magnetic flux through one turn of the coil.
If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the
product of the magnetic field strength and the area A through which it cuts. The area is
constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic
field varies with time according to the excitation of the primary.
Fig: 4.8.2.1 Mutual Induction.

Since the same magnetic flux passes through both the primary and secondary coils in an
ideal transformer, the instantaneous voltage across the primary winding equals Taking the ratio
of the two equations for VS and VP gives the basic equation for stepping up or stepping down
the voltage Ideal power equation The ideal transformer as a circuit element.
If the secondary coil is attached to a load that allows current to flow, electrical power is
transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is
perfectly efficient; all the incoming energy is transformed from the primary circuit to the
magnetic field and into the secondary circuit. If this condition is met, the incoming electric
power must equal the outgoing power.
Giving the ideal transformer equation Transformers are efficient so this formula is a
reasonable approximation. If the voltage is increased, then the current is decreased by the same
factor. If an impedance ZS is attached across the terminals of the secondary coil, it appears to
the primary circuit to have an impedance of ZS = (VS/IS).
Fig: 4.8.2.2 Three Phase 50MVA Auto Transformer.

4.8.3 Specifications of 132KV/33KV Auto Transformer
Rated MVA : 50MVA
No of phases : 3
Insulation level : HV LI 900 AC 395
: HVN LI 95 AC 38
IV LI 550 AC 230
LV LI 170 AC 70
Type of cooling : ONAN DNAF
Rated MVA : 75 100
Rated KV at no load : HV 220KV --
: IV 132KV --
LV 11KV --
Line Amperes : HV 196.8 262.4
IV 328.0 437.4
LV 1299.0 1732.1
Temperature Rise oC : Top oil - 50oC
Avg.WDG : - 55OC
Impedance volts : HV-IV 7.667 10.222
Normal Tap conditions) : HV-LV 24.55 32.72
: IV-LV 17.69 23.59
4.9 CAPACITOR BANK ATTACHED TO THE BUS
The capacitor banks are used across the bus so that the voltage does not get down till at
the require place. A capacitor bank is used in the outgoing bus so that it can maintain the
voltage level same in the outgoing feeder.
Fig: 4.9.1 Capacitor Bank In The Distribution System.

4.9.1 Capacitor Control Is Usually Done To Achieve The Following Goals
 Reduce losses due to reactive load current;
 Reduce KVA demand, decrease customer energy consumption,
 Improve voltage profile, and increase revenue.
 Indirectly capacitor control also results in longer equipment lifetimes because
of reduced equipment stresses.
Experience shows that switched feeder capacitors produce some of the fastest returns
on equipment investment Sources of Energy Loss. Energy losses in transmission lines and
transformers are of two kinds: resistive and reactive. The former are caused by resistive
component of the load and cannot be avoided. The latter, coming from reactive component of
the load, can be avoided.
Fig: 4.9.1.1 Reactive Losses.

5.1 TRANSFORMER PROTECTION
 Station Transformer: HG Fuse protection on HV side and fuse protection on LV side
and Vent pipe.
 Power transformers up to 7.5MVA:
HV side: O/L & Directional E/L protection with highest element in O/L relays.
LV side: O/L & E/L protection Buchholz Relay OLTC Buchholz Relay
OTI and WTI
 Power transformers from 8.0MVA and above: HV side O/L & Directional E/L
protection with high set element in O/L relays. LV side O/L & E/L
protection: differential protection Buchholz Relay OLTC Buchholz Relay OTI, WTI
and PRV.
 Power transformers from 31.5MVA and above: Over flux protection & LV WTI in
addition to protection.
5.2 FEEDER PROTECTION
 33KV feeders: Non directional O/L & E/L protection with highest and IDMT
characteristics.
 132KV feeders: Main protection: Distance protection. Back up protection: Directional
O/L & E/L protection.
 220KV feeders: Main-1 protection: Distance protectionMain-2protection: Distance
protection, LBB protection, pole discrepancyRelay.

5.3 IMPORTANT POINTS TO BE KEPT IN VIEW WHILE LAYING
OUT THE SUBSTATION
Substations are important part of power system. The continuity of supply depends to a
considerable extent upon the successful operation of sub-stations. It is, therefore, essential to
exercise utmost care while designing and building a substation.
The following are the important points which must be kept in view while laying out a
substation:
 It should be located at a proper site. As far as possible, it should be located at the
centre of gravity of load.
 It should provide safe and reliable arrangement. For safety, consideration must be
given to the maintenance of regulation clearances, facilities for carrying out repairs
and maintenance, abnormal occurrences such as possibility of explosion or fire etc.
For reliability, consideration must be given for good design and construction,
the provision of suitable protective gear etc.
 It should be easily operated and maintained.
 It should involve minimum capital cost

CONCLUSION
Transmission and distribution stations exist at various scales throughout a power
system. In general, they represent an interface between different levels or sections of the
power system, with the capability to switch or reconfigure the connections among various
transmission and distribution lines.
The major stations include a control room from which operations are coordinated.
Smaller distribution substations follow the same principle of receiving power at higher
voltage on one side and sending out a number of distribution feeders at lower voltage
on the other, but they serve a more limited local area and are generally unstaffed.
The central component of the substation is the transformer, as it provides the effective
in enface between the high- and low-voltage parts of the system. Other crucial
components are circuit breakers and switches. Breakers serve as protective devices that
open automatically in the event of a fault, that is, when a protective relay indicates
excessive current due to some abnormal condition. Switches are control devices that can
be opened or closed deliberately to establish or break a connection.
An important difference between circuit breakers and switches is that breakers are
designed to interrupt abnormally high currents (as they occur only in those very
situations for which circuit protection is needed), whereas regular switches are
designed to be operable under normal currents. Breakers are placed on both the high- and
low-voltage side of transformers. Finally, substations may also include capacitor banks to
provide voltage support

REFERENCES
[1] Principles of Power Systems by V.K. Mehtha
[2] Electrical Power Systems by C.L. Wadhwa
[3] Power System Engineering by ML. Soni
[4] Electrical & Electronics Measurement &Instruments by A.K.Sawhney Dhanpat Rai

132 33kv substation documentation

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