1. 1
Chapter: -1
INTRODUCTION TO GSS
INTRODUCTION:-
Electric power is the backbone of industrial world today. Further, comfort, convenience and
safety of large population all over the world depend upon electric power. In fact, it is no
exaggeration to say that average wage level and standard of living in any country are dependent
to a large extent on the amount of power used per capita in the industry. History, tradition,
economic circumstances (such as availability of natural resources) all govern in the development
and the use of electrical power in a country Electrical power system may be broadly categorized
as:
GENERATING STATIONS
The ever increasing use of electric power for domestic, commercial and industrial purposes
necessitates providing bulk of electric power economically. This is achieved with the help of
suitable power producing units, known as Electric Power Generating Stations.
Depending upon the type of energy converted into electrical energy, the generating stations
are classified as under:
Thermal Power Stations
Hydro Power Stations
Gas Turbine Power Pants
Nuclear Power Stations
TRANSMISSION SYSTEM
Transmit generated energy at high voltage so as to reduce the losses.
DISTRIBUTION NETWORKS
This is mainly responsible for maintaining supply to consumers without interruption .This
system mainly includes feeders, substation, distribution and service mains.
Broadly speaking the following methods of distribution of electric power are used:
RADIAL SYSTEM -: Here only one feeder supplies energy to consumers. If, for any
reason it fails supply is interrupted.
2. 2
RING MAIN SYSTEM-: In this system consumers are supplied from two feeders. In
case of failure of any feeder the supply can be continued by another feeder. So the system
is reliable.
THE ASSEMBLY OF APPARATUS USED TO CHANGE SOME
CHARACTERITICS OF ELECTRIC SUPPLY IS CALLED GRID
SUBSTATION.
History:- The present day NPH GSS was formerly a thermal power house with the
generation capacity of 3KW each. There was also a railway line for direct supply of coal
to the power house. However with due course of time this had to be abandoned because
of problems like pollution and inability to meet the power supply.
Today’s scenario:-The present day load requirement of about 200MW is being obtained
from interconnected grid system. There are two incoming feeders from 220KV basin II
phase by 3 phase, 3wire system and reduces the voltage level to 33KV and 11KV GSS.
3. 3
Chapter: -2
Single line diagram
There are two 132KV incoming lines marked interconnector 1 and interconnector2
connected to a bus bar. Such an arrangement of two incoming lines is called a double
circuit. It is 3phase, 3wire line and it is 12.5km long.
Each incoming line is capable of supplying the rated load.
Both lines may be loaded simultaneously to share grid substation load.
Thus the reliability of system increases hence assuring the continuity to supply.
The grid substation has duplicate bus bar system with one of them being main bus bar
and the other one being spare bus bar. The incoming lines can be connected to either bus
bar with the help of buss coupler which consists of a circuit breaker and isolator.
The incoming 132KV supply is stepped down to 33KV and 11KV with the help of six
power transformers. Two of these steps the voltage down to 11KV while the other four
are step it down to 33KV.
At the very beginning to protect the line and the switch yard from lightning strokes
lightning arrestors are mounted.
The incoming high tension line then culminates into two isolators situated on either side.
One end of the second isolator is connected to auxiliary bus bar.
After isolators the current transformers are mounted.
Next to follow are the circuit breakers. Both the incoming and outgoing lines are
connected through C.B.’s having isolators on either end. When repairing is to be carried
out on line towers, the line is first switched off and then earthed.
The Potential Transformers are suitably located to the point where the line is terminated.
There are other auxiliary components in substation such as:-
a. Capacitor bank
b. Earth connections
c. Local Supply Connections
d. D.C. supply connections
e. Control Room:-control and protection panels
f. Relay and metering panels
g. Shunt reactors
h. Power cables
i. Station services equipment :- auxiliary battery supply, transformer oil purification
set, compressed air system
j. Mesh Earthing System
k. Galvanized steel structure
l. Communication equipment
5. 5
FEEDER
NUMBER
FEEDER RATING
1. Kudi HB 33KV
2. Air force 33KV
3. VH/Motor merchant 33KV
4. Medical college 33KV
5. MGH/OPH 33KV
6. Engineering College 33KV
7. Basni-II 33KV
9. AIIMS 33KV
3. Heavy Industrial Area II 11KV
4. OPH+MGH 11KV
5. Heavy Industrial Area I 11KV
7. Diary 11KV
8. Milkman colony 11KV
9. Old Power House 11KV
10. BGKT 11KV
11. Pal Road 11KV
12. RSEB Colony 11KV
18. (B/G) BGKT 11KV
19. Air force 11KV
20. Diesel Shed Railway 11KV
21. Industrial area 11KV
6. 6
Chapter: -3
Salient Features
Name 132KV GSS, New Power House, Jodhpur, 342003
Circle/Division (TCC) IV/220KV GSS, Jodhpur
Date of Commissioning 30 May 1969
GSS Process Receiving supply at 132KV and distributed after transformation
(stepdown) at 33KV & 11KV
Area Feeded Jodhpur City, Nearby rural area like Boranada, Jhanwar, Pheench,
Luni, Narwa and Indroka
Total Yard Area 28600 sq. m
Total Open Space in Yard 25000 sq. m
132KV supply source 220KV GSS Basni, Jodhpur
132KV line length
conductor
(6.0km)double circuit panther
132/33KV Power
Transformer
4no’s, EMCO-1, AREVA-1, TRR-1,BBL-1
132/11KV Power
transformer
2 no’(NGEF-1) IMP-1
Substation transformer 33/0.4KV-1 no., 11/0.4KV-1 no.
GSS Capacity 132.5 MVA
33KV:/100MVA(2x25+2x25)
11KV:/32.5MVA
132KV Breakers 12 Nos.
33KV Breakers 18No.(ABB NO.9 Siemens-300 AREVA-2, G.G-18 No.(BHEL-
10, GEC-2,ABB-3)C4-2 Megasim-1 WSI-1
11KV Breakers 8 No.(D&P-1 Siemens-5, ABB-1)
BHEL-6, C.C.-1 Megasim-2 NGEF-1
33KV outgoing feeders 8 no.
7. 7
11KV outgoing feeders 13 no.
Capacitor Banks 33KV side 4x5.43MVAR
11KV side 1x5.04MVAR
Control System
Voltage
110V DC (55x2)
GSS Loading Maximum Demand-70MW
Normal-50-55 MW
Monthly Energy Export 340LU(A.V.)
Monthly Auxiliary
Consumption
0.1LU(H.V.)
Residential Quarter under
GSS
33 no’s(R-2 type-2,R3-2,F-4,G-6,H-8)
Other equipment Filter Machine(500GPH)
John &flower 1-no.’s
Mobile Oil testing Van-1 no.’s
8. 8
Chapter: -4
Equipment Details
Lines and cable network
PART-A
EHV Lines
132KV Double Circuit
Total Lines-6.02km
Plant & Machinery
PART-B
Substation Equipment:
Power Transformer with RTCC panel
132/33KV-4
20/25MVA-4
132/11KV-2
12.5/20MVA-1
10/12.5MVA-1
Station Transformer
33/0.4KV-1
250KVA
11/0.4KV-1
100KVA
CIRCUIT BREAKER
132KV-12
33KV-18
10. 10
11KV-1
Capacitor bank, Comprising of cell, Breaker, CTS,RVTs, Shunt Reactor,
F&R Panels etc.
33KV 5MVAR-4
11KV-1
STRUCTURE
Structures
Civil Foundation
CONTROL & RELAY PANEL
132KV
1xDuplex-12
33KV
Simplex-18
DC SYSTEM
1-Battery set-1
110V 200AH-1
Battery Charger-1
110V 200AH-1
Distribution Board-1 (110V)
LT PANEL
400Amps-1
Fire Fighting Equipment
Fire extinguisher-9
11. 11
Chapter: -5
Bus Bar
Bus Bar is of two types
a) Main bus bar
b) Auxiliary bus bar
When a number of generators or feeders operating at the same voltage have to be directly
connected electrically, Bus bars are used as the common electrical component. Bus bars are
copper rods or thin walled tubes and operate at constant voltage.
In Large station, it is important that breakdowns and maintenance should interfere
as little as possible with continuity of supply .In order it achieve this objective, duplicate bus –
bar system is used in important stations. One bus bar is main bus bar and another one is spare or
auxiliary bus-bar. Each generator and feeder may be connected to either bus bar with the help of
bus coupler which consists of a circuit breaker and isolator.
12. 12
Chapter: -6
Isolators
INTRODUCTION:-
Isolators are deigned to open a circuit under zero load. Its main purpose is to isolate one
portion of the circuit from the other and is not intended to be opened while current is flowing in
the line. These switches are generally used on both sides of circuit breakers on order that repairs
and replacement of circuit breakers can be made without any danger.
If we want to open the isolators then first we have to open the circuit breakers in the same circuit
and it should always be closed before the circuit breaker is closed.
CLASSIFICATION:-
OFF LOAD ISOLATOR:-
It is an isolator which is operated when it is already disconnected from all the sources of
supply & current may be due to capacitance currents of the bushing bus bar connection and very
short length of cable.
ON LOAD ISOLATOR:-
It is an isolator which is operated in a circuit when there is a parallel path off low
impedance so that no significant changes in the voltage across the terminal of each pole occur
when it is operated. There is no such distinction IS: 18:18, a typical two post single isolator.
Figure 6.1 Isolator
13. 13
Chapter: -7
Circuit Breaker
INTRODUCTION:-
A circuit breaker is a piece of equipment which can:
(1) Make or break a circuit either manually or by remote control under normal conditions.
(2) Break a circuit automatically under fault conditions.
(3) Make a circuit under either manually or by remote control under fault conditions.
7.1 OPERATING PRINCIPLE:
A circuit breaking essentially consists of fixed and moving contacts called electrodes
under normal operating conditions. These contacts remain closed and will not open automatically
until and unless the system becomes faulty. Of course the contacts can be opened manually or by
remote control whenever desired. When a fault occurs on any part of the system, the trip coils of
the breaker get energized and the moving contacts are pulled apart by some mechanism, thus
opening the circuit.
When the contacts of a circuit breaker are separated under fault conditions, an arc is
struck between them. Thus current is able to continue until the discharge ceases. The production
of arc not only delays the current interruption process but is also generates enormous heat which
may cause damage to the system or to the breaker itself .Therefore the main problem in the
circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by
it may not reach to a dangerous value.
There are two methods of extinguishing the arc in the circuit breakers.
i. High resistance method
ii. Low resistance or Current zero method
7.2 CLASSIFICATION OF CIRCUIT BREAKERS:
The most general way of classification is on the basis of the medium used for arc
extinction. Accordingly, circuit breakers may be classified into:
i. Oil Circuit Breaker
ii. Sulphur Hexa Fluoride Circuit Breaker
iii. Vacuum Circuit Breaker
14. 14
iv. Air Break Circuit Breaker
v. Air Blast Circuit Breaker
The circuit breakers used in the GSS under training were SF6 type &Vacuum
circuit breaker. We shall discuss the construction and working of these circuit breakers in detail.
7.2.1 SULPHUR HEXA FLUORIDE CIRCUIT BREAKERS:
In such breakers SF6 gas is used as the arc quenching medium. The SF6 is an electro-negative
gas and has a strong tendency to absorb free electrons. The contacts or the breakers are open in a
high pressure flow of SF6 gas and an arc is struck between them. The conducting free electrons
in the arc are rapidly captured by the gas to from relatively immobile negative ions. This loss of
conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc.
Figure 7.1 SF6 Circuit Breaker
CONSTRUCTION:-
It consists of fixed and moving contacts enclosed in a chamber (called arc interruption
chamber) containing SF6 gas. This chamber is connected to SF6 gas reservoir. When the
contacts of breaker are open, the valve mechanism permits a high pressure SF6 gas from the
reservoir to flow towards the arc interruption chamber. The fixed contact is a hollow cylindrical
current carrying contact little with an arc horn. The moving contact is also a hollow cylinder with
rectangular holes in the sides to permit the SF6 gas to let out through these holes after following
15. 15
along and across the arc. The tips of fixed contacts moving contact and arcing horn are coated
with copper -tungsten arc resistance material. Since SF6 gas is costly. It is reconditioned and
reclaimed by suitable auxiliary system after each operation of the breaker.
WORKING:
In the closed position of the breaker, the contacts remain surrounded by SF6 gas at a
pressure of about 28kg/cm2 when the pressure operates. The moving contact is pulled apart and
an arc is struck between the contacts. The movement of the moving contact is synchronized with
the opening of a valve which permits SF6 gas at 14 kg/cm2, pressure from the reservoir to the
arc interruption chamber. The high pressure flow of SF6 rapidly absorbs the free electrons in the
arc path to form immobile negative ions which are ineffective as charge carrier. The result is that
the medium between the contacts quickly builds up high dielectric strength and cause the
extinction of the arc. After the breaker operation (that is after arc extinction) the valve is closed
by the action of a set of springs.
7.2.2 VACUUM CIRCUIT BREAKER (VCB):
In such breakers vacuum is used as the arc quenching medium. When the contacts of the
breaker are opened in vacuum (10^-7 to 10^-5 Torr) an arc is produced between the contacts but
the ionization of metal vapors of contacts. However the arc is quickly extinguished because the
metallic vapors, electrons and ions produced during arc rapidly condensed on the surfaces of the
breaker contacts, resulting in quick recovery of dielectric strength.
16. 16
Figure 7.2 Vacuum Circuit Breaker
CONSTRUCTION:
It consists of fix contacts, moving contacts and arc shield mounted inside a vacuum
chamber. The moveable chamber is connected to a control mechanism by stainless steel bellows.
This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of
leak. A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevent
the deterioration of the internal dielectric strength by preventing metallic vapors falling on the
inside surface of the outer insulating cover.
WORKING:
When the breaker operates, the moving contacts separate from the fixed contact and
the arc is struck between the contacts. The production of arc is due to ionization of metal ions
and depends very much upon the material of contacts .The arc is quickly extinguished because
the metallic vapors, electrons and ions produced during arc are diffused in a short time and
seized by the surfaces of moving and fixed members and shields. Since vacuum has very fast rate
of recovery of dielectric strength, the arc extinction in vacuum breaker occurs with a short
contact separation about 0.625cm.
19. 19
Chapter: -8
Current Transformer & CVT
8.1 CURRENT TRANSFORMER:-
C.T is an instrument transformer used for protection and metering of high values of
currents. C.T is used for reducing AC from high to low value measurement/ protection control.
There are two classes of I.T. :
1. Measuring C.T
2. Protective C.T
Protective C.Ts are used for over current protection, earth fault protection, differential
protection and impedance protection etc.
Measuring C.Ts are used with ammeter, wattmeter, KVA meters and KWH meters
for reducing line currents to 1 A or 5A.
Figure 8.1 Current Transformer
Different terms related with C.T:-
RATED PRIMARY CURRENT: The value of primary current on which the primary
performance of the C.T is specified.
RATED SHORT TIME CURRENT: It is defined as rms value of AC component
which the C.T can carry without damage.
20. 20
RATED SECONDARY CURRENT: The value of the secondary current is marked on
the rating plate.
RATED EXCITING CURRENT: The rms value of current taking by secondary
winding of a C.T when sinusoidal voltage of rated frequency is applied to primary with
secondary winding open.
RATED BURDEN: The burden assigned by the manufactured at which the C.T perform
with specified accuracy the burden depend upon the number of and interconnected and
their individual burden typical values.
8.1.1 CT DETAILS:
132KV Side:
S.NO. DESCRIPTION CTR CONNECTED
RATIO
MAKE
1 132kv I/C -1st 400-200-100/5 400/5A BHEL
2 132KV I/C -2nd 400-200-100/5 400/5A BHEL
3 132KV B/C 400-200-100/5 400/5A UNIVERSAL
4 132KV O/G MBM 1st 600/1 600/1A ------
5 132KV O/G MBM 2nd 600/1 600/1A ------
6 132KV CHB 400-200-100/5 400/5A BHEL
7 132/33KV EMCO TRF 240-120/5 240/5A TELK AMEI TELK
8 132/33KV AREVA TRF 250-125/5 125/5A MEHRU
9 132/33KV T&R TRFN 250-125/5 250/5A AMEI
10 132/33KV BBL TRF 250-125/5 125/5A -------
11 132/11KV NGEF TRF 400-200-100/5 100/5A TELK
12 132/11KV IMP TRF 240-120/5 120/5A SCT LTD
22. 22
9 11KV B/C 400-200-100/5-5 400/5A -------
10 11KV O/G NO.09 400-200-100/5-5 ------- -------
11 11KV O/G NO.17 400-200-100/5-5 ------- SILKAN
12 11KV CAP. BANK NO.1 400-200-100/5-5 400/5A ---------
8.2 CAPACITOR VOLTAGE TRANSFORMER (CVT):
CVT’s are used for line voltmeters, synchroscopes, protective relays, tariff meters etc.
the performance of CVT’s affected by the supply frequency, switching transients, magnitude of
connected burden etc.
The CVT’s is more economical then an electromagnetic VT when the nominal system
voltage increases above 66 KV. The carrier current equipment can be connected via the CVT
there by no need of separate coupling capacitor. Above 66kv the cost of electromagnetic VT is
very high. The capacitor connected in series with CVT acts as P.D. the burden provided by the
capacitor is negligible. HV capacitors are enclosed in porcelain housing.
23. 23
Chapter: -9
Power Transformers
INTRODUCTION:-
Transformer is an electrical machine which works on the principle of electromagnetic
induction. It transfers electric power from one electric circuit to another electric circuit with the
help of magnetic path (flux) on constant frequency o sets of insulated windings are wounded on a
close terminated steel core. The winding which is connected to the supply is called primary
winding and that winding is connected to the load is called secondary winding.
Winding of the transformers are dipped in the oil. In these transformers silica gel is used
to absorb the moisture. In starting condition silica gel is of navy blue color. After absorbing the
moisture the color becomes pink. For reusing them we spread silica gel in sun light.
GENERAL:
The transformers are oil filled equipment conforming to ISS: 2026 with ONAN/ONAF
or ONAN/OFAF & OFWF type cooling condition. Transformer is provided with radiators, fans,
pumps, OFWF coolers depending upon their cooling conditions. Radiators are either directly
mounted on the tank or on headers. It is equipped either with OLDC or off circuit switch
depending upon requirement for variation of HV and LV voltage.
In each transformer there are 3 meters which show temperature of windings and oil.
Temperature is limited by particular value whenever it crosses limit, we have to reduce load from
transformers for its safety.
In 33 KV zone, at this GSS 4 transformers are used to step down voltage from 132KV to 33KV.
Ratings of transformers are
a) 2 transformers of 20/25MVA
b) 2 transformers of 10/12.5 MVA
Two transformer work together as 20/25 MVA & 10/12.5, since in this two transformer
rating of transformer is double from second transformer , so that means in emergency first
transformer can take load of second transformer of rating 10/12.5 MVA.
24. 24
Figure 9.1 Power Transformer
MAIN PARTS OF POWER TRAANSFORER:-
CORE: It is made up of cold rolled grain oriented(C.R.G.O), Silicon alloy steel in
which quantity of it consists of laminations made up of high grade, non-aging, cold
steel is up to 4% and thickness of laminations is of Carlit coating.
WINDING: It consists of windings of electrolytic tough copper. The most popular
alloy of copper used is its alloy with a concentration of about 0.01%. This alloy has an
increased melting temperature with same electrical conductivity. The various windings
are:-
1) H.V. Winding: This I the primary side of transformer which is fed by the incoming
132KV feeder.
2) L.V. Winding: This is the secondary wining which feed the next feeders with voltage
levels at 11KV and 33KV.
3) Tertiary Winding: This is basically used for harmonic suppression. The most common
three winding transformer is a star-star connection with tertiary provide thus providing a
path for zero sequence current. It provides a path of low reluctance for the harmonic
component of flux.
25. 25
4) Tap Changer: It is a witching device by which the transformation ratio can be changed
by changing the position of the tap changing switch. The tap changing system of
transformer at GSS is:
On Load Tap Changer: (O.L.T.C.) this is employed to change the turn ratio of
transformer to regulate system voltage while the transformer is delivering normal
load with the introduction of OLTC system the operating efficiency of electrical
system has considerably improved.
Now a day, almost all the larger power transformers are fitted with no load tap
changer. All forms of OLTC circuit possess impedance, which is introduce to prevent
short circuiting of tapped section during tap changing operation. The impedance can
either be a resistor or a center tapped reactor.
TANK: It is made up of welded mild steel plates. The tank accommodates
transformer core and winding assembly, surrounded by insulating oil filled in the
tank such arrangement protects the winding and core from external mechanical
surges. On the outside it is applied with anti-corrosive prime paint and final coat of
synthetic enamel. The cover is either bell shaped or flat. To make joints oil tight,
neoprene bonded cork or nitrite rubber gasket re used. I-directional flanged rollers
suitable for moving the transformer on rail gauge are provided.
CONSERVATOR: As the temperature of oil increases or decreases during
operation, there is a corresponding rise or volume. To account for this an expansion
vessel is connected to a transformer. The conservator is provided with magnetic oil
level gauge and oil level alarm on one end. A prismatic oil gauge is also fitted at the
other end. One the feed pipe Buchholz relay is mounted.
AIR CELL: The air cell is a flexible rubber bag it floats on the oil surface inside
the conservator. As the breathing is through air cell no moisture should come in
contact with oil, this protect the oil from deterioration air cell is made from coated
fabric with external coating resistance to transformer oil an inner coating to ozone
an weather.
BUCHHOLZ RELAY: It is a gas actuated relay installed in the pipe connecting
the conservator to the main tank, for protection against all kinds of faults. The
devices have two components the upper element consists of a mercury type switch
attached to float. The lower element consists of a mercury switch mounted on hinge
type flap located in the direct path of the flow of oil from the transformer to the
conservator. The upper element closes on alarm circuit during incipient faults
whereas the lower element is arrangement to trip the circuit breaker in case of
severe incipient fault.
DEHYDRATING BREATHER: The conservator to the air cell is connected to
atmosphere through the dehydrating breather to make sure that the air in the
conservator or the cell is dry. When the silica gel is saturated with moisture it colors
26. 26
changes to pink. By heating the Gel at 100 degree Celsius in 48 hour it can be made
reusable.
PRESURE RELEIF VALVE: In case of major fault in the transformer, like short
circuit in windings the internal pressure is build up to a very high level which may
result in rupture to tank. To avoid such a contingency a pressure relief is fitted. It is
self-sealing spring loaded type.
OIL TEMPERATURE INDICATOR: It operates on the principle of liquid
expansion. The winding temperature indicated readings are proportional to load
current plus top oil temperature. Thermometer is connected to a capillary tubing to
the local indicator and by wiring from local indicator to repair on control room
temperature are:
Alarm-90deg. Celsius
Trip-95deg. Celsius
Fans on-60deg. Celsius
Pumps on-70deg. Celsius
Ambient temperature of 45 deg. Celsius
27. 27
Chapter: -10
Transformer Oil
INTRODUCTION:
The oil is function as an insulation and coolant in the transformers. In service the insulating
liquids are subjected to thermal and electrical stresses in the presence of adverse conditions and
materials. These include air (3.5% oxygen), water, solid particles, such as corrosion products
from manufacturing and electrical equipment construction materials, fibers and decomposition
products of insulation and oil soluble constituents or impregnating varnishes and resins. These
either singly or in combination, promote degradation of the liquid with the result that soluble,
solid and gaseous products are formed which result in corrosion, impairment of heat transfer,
deterioration of electrical properties, increased dielectric action, this cycle continues and
produces an ever worsening liquid and equipment condition.
In view of the foregoing quality insulation oils are essential to ensure its expected life in
transformer where special properties like life resistance, high permittivity and gas absorbing
characteristics are used in the transformers.
10.1 TRANSFORMER OIL SPECIFICATION:
The finished oil has to meet certain specifications before its use in transformers. All
countries have formulated their own specifications and they are more or less found to be very
similar.
IS:335 is the specification number of INDIA. A survey of these national specifications
reveals that their basic requirements are common, the difference being only in the test procedure
or the relative importance of particular test. The tests laid down by these standards falls into
three categories, namely PHYSICAL, CHEMICAL and ELECTRICAL TESTS.
Table : CHARACTERISTICS AND PARAMETERS OF NEW INSULATING OIL
S.NO. CHARACTERISTICS UNIT REQUIRMENT
1. Appearance - The oil shall be
clear, transparent and
free from suspended
matter or sediment
2. Density at 29.5°C(MAX) 0.89gm/cm³
3. Kinematic Viscosity at 27°C(Max) 27cst
4. Interfacial tension at 27°C(Min) 0.04N/M
5. Flash point (Min) 140°C
6. Pour point(Max) -6°C
7. Neutralization values (Total Acidity)
(Max)
0.03
8. Corrosive Sulphur Non corrosive
28. 28
9. Electric strength (Breakdown voltage)
(Min)
a) New unfiltered oil
b) After filtration (Min.)
-
30KV(rms)
60KV(rms)
10. Dielectric dissipation factor (tan δ) at
90°C(Max)
0.002
11. Water content (Max) 50ppm
12. Specific resistance (Min) at
a) 90°C
b) 27°C
35x10¹²ohm/cm
1500x10¹²ohm/cm
13. Oxidation Stability
a) Neutralization after oxidation
value (Max)
b) Total sludge after oxidation value
(Max)
0.4mg KOH/gm
0.17 by weight
14. Aging characteristic after accelerated
aging (open breaker method with copper
catalyst)
a) Specific resistance (resistivity) at
1. 27C (Min)
2. 90C (Min)
b) Dielectric dissipation factor
tan δ at 90°C (Max)
c) Total acidity (Max)
d) Sludge content by weight
(Max)
2.5x10¹²ohm/cm
0.2x10¹²ohm/cm
0.2
0.05 mg KOH/gm
0.05%%
15. Presence of oxidation inhibitor - Oil shall contain
antioxidant
additives.
Table: SCHDULE OF CHARACTERISTICS AS PER IS: 335-1993
CHARACTERISTICS REQUIREMENT
1. Appearance The oil shall be clear, transparent & free
from suspended matter
2. Kinetic viscosity max
a) At 27°C
b) At 40°C
27 cst
Under consideration
3. Density at 29.5°C (Max) 0.89 gm/cm³
4. Interfacial tension at 27°C (Min) 0.04N/M
5. Flash point, pensky martin (closed) Min 140°C
6. Pour point (Max) -6°C
29. 29
7. Neutralization value
a) Total acidity (Max)
b) Inorganic acidity/alkalinity
0.03mg KOH/gm
Nil
8. Corrosive sulphur Non corrosive
9. Electric strength (breakdown voltage)
(Min)
a) New untreated oil
b) After treatment
30KV (rms) if the above value is not
attained the oil be treated
60KV (rms)
10. Dielectric dissipation factor
(tan δ) at 90°C , (Max)
0.002
11. Specific resistance (resistivity)
a) At 90°C (Min)
b) At 27°C (Min)
35x10¹² ohm/cm
1500x10¹² ohm/cm
12. Oxidation stability
a) Neutralization value after
oxidation (Max)
b) Total sludge after oxidation (Max)
0.40 mg KOH/gm
0.10% by weight
13. Presence of oxidation inhibitor The oil shall not contain antioxidant
additives
14. Water content (Max) 50 ppm
15. Aging characteristics
a) Specific resistance
i) at 27°C (Min)
ii) at 90°C (Min)
b) Dielectric dissipation factor at
90°C (Max)
c) Total acidity (Max)
d) Total sludge (Max)
2.5x10¹² ohm/cm
0.2x10¹² ohm/cm
0.20
0.05 mg KOH/gm
0.05% by weight
16. S.K. value Under consideration
17. Moisture content (in ppm) High value affects the insulating
properties
18. Breakdown voltage(KV) Shows ability of the oil to withstand
electrical stress
19. Dielectric Dissipation factor Indicate power loss due to impurities
20. Specific Resistance(ohm/cm) Measure of conducting contaminants
21. Sludge % by weight Measure of perceptible oxidation
products
10.2 GAS ANALYSIS OF TRANSFORMER OIL:-
Incipient faults in oil filled transformer results in electrical or thermal stress of either transformer
oil or insulting materials. It is known that such excessive stresses produce mixture of dissolved
gases in transformer oil. This gives the indication of faults.
30. 30
Gases to be analyzed and criteria for the gases found in transformer oil are tabulated:
Gases to be analyzed normally –O2, N2, H2, CO2, and CH4
Gases to estimate abnormally -H2, CH4, C2H2, C2H4 AND C2H6
Gases to eliminate deterioration –CO2, CO AND CH4
Types of faults Decomposable gases in transformer oil
1. Overheat of oil CH4, C2H4,(C2H2, C2H6, C3H88, C3H8)
2. Arcing of oil H2, C2H2,( CH4, C2H4)
3. Overheat of solid CO, CO2, C2H2, (H2, C2H4)
4. Overheat of oil and paper CH4, C2H4, CO, CO2, H2
5. Arcing of oil and paper H2, C2H2, CO,CO2, (C2H4)
( ) Shows gas contents which appear rarely.
10.3 CATEGORIES OF TEST FOR TRANSFORMER OIL:
Physical Tests:
1. Specific Gravity
2. Viscosity
3. Flash point & Fire point
4. Pour point
5. Colour
6. Interfacial Tension
7. Aniline point
Chemical Tests:
1. Neutralizations Number
2. Saponification Value
3. Copper strip corrosion
4. Oxidation Stability
5. Inorganic Chlorides & sulphates
6. Steam Emulsion Number
7. Water Content
Electrical Tests:
1. Electric strength
2. Power factor
3. Resistivity
10.4 COOLING SYSTEM:
In power transformer, the oil serves a dual purpose as an insulating medium as well as a cooling
medium. The heat generated is removed by transformer oil surrounding the winding and is
transmitted either to atmospheric air or water. This transfer of heat is essential to control the
31. 31
temperature within permissible limits for the class of insulation, thereby ensuring the longer life
due to less thermal degradation.
Types of cooling used in GSS Power Transformer
ONAN Type Cooling:
The generated heat can be dissipated in many ways. In case of smaller ratings of
transformers, tanks may be able to dissipate the heat directly to atmosphere whilst bigger rating
transformers may require additional dissipating surface in the form of tubes/radiators connected
to tank or in the term of radiator tank. In these cases the heat dissipation is from transformer oil
to atmospheric air by natural means. This form of cooling is called as ONAN (oil natural air
natural) type of cooling.
ONAF Type Cooling:
For further augmenting the rate of dissipation of heat, other, means such as fans blowing
air on to the cooling surfaces are employed. The forced air removes heat at a faster rate, thereby
giving better cooling rate than natural air. This type of cooling is called as ONAF (oil natural air
forced) type cooling.
OFAF Type Cooling:
In large capacity transformers the natural circulation of oil is not sufficient for cooling.
Hence forced circulation of oil is employed with the help of pump. The oil in the transformer
tank is cooled through a radiator; it is cooled by forced air from fans.
Cooling Arrangement With Radiator:
Radiators are commonly used for ONAN and ONAF type of cooling. Radiators consist of
elements joined to and bottom headers, made by welding two previously rolled and pressed thin
sheets to form a number of channels of flutes through which oil flows.
These radiators can be either mounted directly on the transformer tank or in the form of a
bank &connected to the tank through the pipes. The surface area available for dissipation of heat
is multiplied manifolds by using various elements in parallel as oil passes downwards either due
to natural circulation or force of a pump in the cooling circuit heat is carried away by the
surrounding atmospheric air.
32. 32
Chapter: -11
Battery System
INTRODUCTION:
The Battery charging system is intended to:
a. Keep the 100V – 200AH
Battery on trickle or boost charge as required.
b. Supply DC power to the sub-station load.
The Battery Charger mainly consists of four sections which are:
1. Float Charger Section
2. Boost Charger Section
3. Control Section
4. Alarm Circuit
The Float Charger essentially consists of a three phase transformer rectifier set for
automatic regulation of DC output. The float charger is meant to supply regulated DE voltage to
the load and keep the battery on trickle charger.
The Boost charger section essentially consists of a manually controlled (by rotary
switches) three phase transformer set. In case of float charger failure the boost charger can be put
in emergency use to supply DV voltage to the load by reducing the boost charging voltage.
The control section is made up to the solid state control circuit for:
a) Automatic voltage regulation of float charger.
b) Automatic current limiting of float charger.
c) Under voltage indication & over voltage protection of float charger.
The Alarm circuits consist of all the audiovisual alarm annunciation arrangement with
lamp test and accept facilities.
3
2
4
Figure 11.1 Battery
2
1
4
_+
5
BATTERY CELLBATTERY CELL
33. 33
1. Battery Terminal
2. Heavy duty crocodile clip
3. Heavy cell cable
4. Inter cell connector
5. Vent plug
RATINGS AND SPECIFICATIONS OF BATTERY CHARGER AT 132KV GSS:
Input Voltage: 415V, 50 Hz+10%
Output Voltage:
a) Float Charger: 110V +1%
b) Boost Charger: 99=146V
Output Current:
a) Float Charger: 20A
b) Boost Charger: 30A
Float Charger:
Line Regulation: 415+1%
Load Regulation: 0-20A+17%
Battery Room:
In battery room, I Kanhaiya Lal takes measurements of cell voltage, gravity and
temperature.
Cell Voltage : 2.2Volt
Gravity : 1192 SG (27°C)-1199.7
Temperature : 38°C
I take these measurements on 38°C, but for standard specified at 27°C measurements of
gravity, we can correct value of gravity on 27°C by this formula
Specific Gravity:
SG(27°C) = SG(t) + 0.7(t-27°C)
34. 34
Technical Specifications:
TABLE 4.1 TECHNICAL SPECIFICATIONS
Nominal input 415V AC, 50Hz
Input variation ± 10%
FLOAT - SECTION
DC Output
Output Current
Efficiency
110V ±1%
20A
Better than ±1% Not less than 75%
BOOST – SECTION
DC Output
Output Current
Efficiency
99 to 146V DC
30A
Not less than 75%
35. 35
Chapter: -12
Earthing
INTRODUCTION:
Connection of an electric equipment or apparatus to the earth with the help of a
connecting rod or wire of negligible resistance is known as earthing.
The provision of an earth electrode for an electrical system is system is necessary for the
following reasons:
All the parts of electrical equipment like casing of machines, switches & CB lead sheathing
& armoring of cable, transformer tank etc. which have to be at earth potential must be
connected to an earth electrode. The purpose of this is to protect the various parts of the
installations, as well as the persons working against damage in case the insulation of system
fails at any point.
By connecting these parts to an earth electrode, a continuous low resistance
path is available for leakage current to flow to the earth. This current operates the protective
devices & thus faulty circuit is halted in case the fault occurs.
The earth electrodes ensures that in the event of over voltage on the system due to lightning
discharging or other system faults, those parts which are normally dead as far as voltage are
concerned do not attain dangerously high potentials.
In a three phase circuit the neutral of the system is earthed in order to stabilize the potential
of the circuit with respect to earth.
In electrical insulations the following components must be earthed:
[a] The frames, tanks & enclosures of electrical machines, transformers & apparatus of
lighting & fitting.
[b] The operating mechanism of the switch gear.
[c] The frame work of the switch boards, individual panel boards, cubicles.
[d] The structural steel work of sub-station, metal cable joining boxes, the metal sheath of
the cable, the rigid metal conduit runs & similar metal work.
EARTHING ARRANGEMENTS AT 132KV GSS:
In a GSS of any magnitude the various non-current carrying equipment to be earthed
namely sub-station structures, shielding wires or mats, equipment tank etc are spread over large
area & therefore it becomes necessary to lay a grounding bus, connect the various items to be
earthed to ground bus through suitable connection to have duplicate earthing. It generally
becomes desirable to form a ring of the earthing bus which can be connected to the earthing
electrodes. In large sub-station the earthing bus itself is said to a depth of 400 to 800mm, saves
as a grounding mat & no separate earthing mat or electrodes may be required although use of
some electrodes for making use of good earth conductivity at depth unaffected by other condition
36. 36
is considered advisable particularly near lightning arrester & transformer neutral earthing point
where lighting surges are required to be discharged into earth.
A very low earthing resistance value is required in a large area occupied by a sub-station
& obviously such value can be obtained by using a number of electrodes & joining them in
parallel. A common earth electrode should be used for system earths & equipment earths. Here,
it is recommended to have common earth bus for HV & LV system.
There are manual operating levers for HV switch gears it is recommended to connect the
operating handle to the system earth electrode. To remove any voltage gradient that may exist
between the operating levers & the ground on which the operator stands, a metal grid should be
placed just below ground level & shall be connected to the system earth electrode.
METHODS OF EARTHING:
1. Pipe Earthing
2. Plate Earthing
PLATE EARTHING:
In plate earthing, plate either of Cu of dimensions 60cm x 60cm x 3.15mm or of
galvanized iron of dimensions 60cm x 60cm x6.3mm is buried into the ground with its
face vertical at a depth of not less than 3m from ground level. The earth plate is
embedded in alternate layer of coke & salt for a minimum thickness of 15cm G.I. earth
wire is used for G.I. plate earthing & Cu plate wire for Cu plate earthing is securely
bolted to an earth plate with the help of bolt-nut & washer made of material of that earth
plate.
A small masonry brick wall enclosure with a cast iron or R.C.C. pipe round the
earth plate is provided to facilitate its identification and carrying out periodical inspection
and tests.
Earth resistance depends upon the following factors:
1. Shape and material of earth electrode
2. Depth in the soil at which electrode are buried
3. Specific resistance of soil surrounding the electrodes
37. 37
Chapter: -13
Control Cables, Shunt Reactor, Metering & Indicating Instruments
CONTROL CABLES:
The control cable & conduit system is required for affecting automatic controls. The
control system generally operates at 110KV or 220KV. The cables employed for this purpose are
multicore having 10/37/61 conductors according to requirement. For laying these cables
generally ducts are run from control room basement to centrally located junction box from where
the conductors are run to the required points.
METERING & INDICATING INSTRUMENTS:
There are several metering & indicating devices e.g. ammeters, voltmeters, energy meters
etc installed in a sub-station to maintain watch over the circuit quantities. The instrument
transformer is invariably used with them for satisfactory operation.
SHUNT REACTORS:
Shunt reactors are provided at sending end & receiving end of EHV transmission line.
They are switched in when the line is to be charged or when the line is on no-load or low load
shunt capacitance predominate
& receiving end voltage is higher than the sending end voltage.
The receiving end voltage of 400KV, 100 km long line may be as high as 800 KV. The
shunt capacitance of such line is neutralized by switch in the S.R.
During high loads, the series inductive reactance of the line produces IX drop & the receiving
voltage drops, the S.R. are switched off.
S.R. may be connected to the low voltage tertiary winding of the transformer via a
suitable CB, EHV S.R. may be connected to the transmission line without any EHV CB. Usually
oil immersed magnetically shielded reactors with gapped pole are used.
38. 38
Chapter: -14
Protection and Alarms
1. Buchholz Relay
2. Excessive oil temperature
3. Excessive winding temperature
4. Oil flow failure
5. Differential pressure
6. Fan failure
7. Low oil level(conservator tank)
8. Presser relief valve
9. Differential relay
10. Over current relay
11. Earth fault
12. Inter trip, if any
13. Trip free check
39. 39
Chapter: -15
Power Line Carrier Communication
For exchange of data and transfers message between grid substation, voice
communication is necessary. For this purpose high frequency carrier current (40 to 500KC/S) is
transmitted on same transmission line on which power is transmitted. Hence such
communication is ̋power line carrier communication” (PLCC).
High frequency carried current (audio signals) are generated, transmitted and received with the
help of identical carrier current equipment provided on each end. Carrier current equipment
comprises of the following:
1) Coupling Capacitor
2) Wave Trap Unit
3) Transmitter & Receiver Unit
15.1 COUPLING CAPACITOR:-
It acts like filter. It blocks power frequency (50Hz) while offer low reactance to carrier
frequency (30-500KC/S) as, allow them to pan through because. For e.g. a 2000 pt Capacitor
offer 1.5MHz to 50Hz. While it just offer 1500 to 500 KHz.
This coupling capacitor allows carrier frequency single to enter the carrier equipment but
does not allows 50 Hz power for frequency current to enter the carrier equipment.
15.2 WAVE TRAP UNIT:-
It is parallel tuned circuit comprising of inductance. It has low impedance (less than 0.1ꭥ)
at 50Hz and high impedance at carrier frequency. Thus power frequency gets passed through
wave trap and carrier frequency passes through coupling capacitor and reaches carrier current
equipment.
Wave traps are mounted in outdoor switch yard. Wave trap mounted at GSS is “under
hung” type.
15.3 TRANSMITTER UNIT:-
Carrier current unit acts like both transmitter and receiver. Carrier frequency is generated
in master oscillator and can be tuned to a particular frequency selected in the application. Output
voltage of oscillator is fed to amplifier which increases the strength of the signal to be
transmitted to overcome the transmitted losses. Line losses vary with length of line, frequency,
weather condition, size and type of line. Losses in overhead lines are affected by weather. In fair
weather the attenuation is about 0.1 db/km at 80KHz. Rising to 0.2 db/km at 380 KHz.
15.4 RECEIVER UNIT:-
Comprise of a alternator, which reduce signal to safer value band per unit filter restricts
the acceptance of uncounted signal and matching transformer or matching element matches die
impedance of line.
40. 40
Chapter: -16
Lightning Arrestors
Lightning arrestors are used to protect the sub-station. A transmission line arrestor is
earthed. Valve type lightning arrestor is also called surge diverter. It consists of a spark gap in
series with a non linear resistor. One end of the diverter is connected to the terminal of the
equipment to be protected while other is effectively grounded. The length of the gap is so
adjusted that normal line voltage is unable to cause an arc across the gap but a dangerously high
voltage will breakdown the air insulation & form an arc.
Figure 16.1 Lightning Arrestor
The property of non linear resistance is that its resistance decreases as the voltage or
current increases and vice-versa. Operation will start when the voltage increase to 10% of the
rated voltage. As the gap sparks over due to over voltage, the arc would be a short circuit on the
power system & may cause power follow current in the arrester. Since the characteristic of the
resistor is to offer high resistance to high voltage, it prevents the effect of short circuit. After the
surge is over, the resistor offers high resistance to make the gap non-conducting.
Two important considerations:
[a] When the surge is over, the arc in gap must cease; otherwise the current would continue to
flow through the resister & both resistor and gap may be destroyed.
[b] IR drop (I is surge current) across the arrestor, when carrying surge current should not exceed
the breakdown strength of the insulation of the equipment to be protected 132KV lightning
arrestor.
41. 41
Chapter: -17
Capacitor Bank
The power factor can be improved by connecting capacitors in parallel with the equipment
operating at lagging power factor. This draws a leading current & neutralizes partly or
completely the lagging reactive component of the load current. It is collection of various
capacitors whose function is to inject or add certain voltage into a circuit so as to compensate the
IR drop in the feeders. The capacitive compensation is required to overcome following poor
effects of uncompensated power system which can be listed as:
1. Increased voltage drop resulting in poor regulation.
2. Undesired losses rendering the line efficiency to go down to a valve less than the
designed.
3. Unnecessary utilization of thermal capabilities and burdening of lines, transformers &
cables due to higher currents.
4. There are large amount of harmonics introduced in system due to pulsating loads.
5. Consumers have to pay heavy penalties on account of poor factor.
42. 42
Chapter: -18
Protective Relays
INTRODUCTION:-
In a power system consisting of generator, transformers, transmission and distribution
circuits, is inevitable that sooner or later some failure will occur somewhere in the system. When
a failure occurs on any part of system, it must be quickly detected and disconnected from the
system. There are two principal reasons for it. Firstly, if the fault is not cleared quickly, it may
cause unnecessary interruption of service to customers. Secondly, rapid disconnection of faulted
apparatus limits the amount of damage to it and prevents the effects of fault from spreading into
the system.
The detection of a fault and disconnection of a faulty section or apparatus can be
achieved by using fuses or relays in conjunction with circuit breakers. A fuse performs both
detection and interruption functions automatically but its use is limited for the protection of low
voltage circuits only. For high voltage circuits (say above 3.3KV), relays and circuit breaker are
employed to serve the desired function of automatic protective gear. The relays detect the fault
and supply information to the circuit breaker which performs the function of circuit interruption.
A protective relay is a device that detects the fault and initiates the operation of the circuit
breaker to isolate the defective element from the rest of the system.
The relays detect the abnormal condition in the electrical circuit by constantly measuring
the electrical quantities which are different under normal and fault conditions. The electrical
quantities which may change under fault conditions are voltage, current, frequency and phase
angle. Through the changes in one or more of these quantities, the fault signal their presence,
type and location to the protective relays. Having detected the fault, the relay operates to close
the trip circuit of the breaker. This results in the opening of the breaker and disconnection of the
faulty circuit.
Figure 18.1 Protective Relay
43. 43
This diagram shows one phase of 3-phese system for simplicity. The relay circuit connections
can be divided into three parts which viz.
a) First part is the primary winding of a current transformer (CT) which is connected in
series with the line to be protected.
b) Second part consists of secondary winding of CT and the relay operating coil.
c) Third part is the tripping circuit which may be either AC or DC. It consists of a source of
supply, the trip coil of the circuit breaker and the relay stationary contacts.
44. 44
CONCLUSION
A good learner needs to have not just theoretical but practical knowledge as well every aspirant
shall undergo practical training session during 3rd year as a result of which I imbibed the
knowledge about learning, enhancing my skills, getting familiar with certain aspects of industry
which were unexplored to me, which has changed my approach to think over any scientific
research and their development and different physical laws related to it.
As a trainee, I was guided by our mentor to acquire knowledge about these techniques and
maintenance strategies at the GSS. During the training, I familiarized myself with this GSS and
its major electronic and electric part and their applications.
I also learned about the engineer’s responsibility and about their hard work. The training was
not only good for personality development but also great in terms of imparting practical
knowledge. Thus, I conclude that my training was a nice and blissful experience gained at
132KV GSS, New Power House, Jodhpur, under a peaceful, kind and friendly environment.
Four year of degree course in Electrical Engineering expose one to its variable facts like
instrumentation, measurement, protection, supply system and the like. No matter, how vastly
different these may appear practically but these streams are practically related. This could be
only witnessed at Grid Sub Station where all the equipments, instruments and protection systems
are actually put to use and work continuously.
45. 45
REFERENCES
User’s Manual: Transformer and VCB
www.rrvpnl.co.in
Books:
a) V.K. Metha, Rohit Metha (Principles of Engineering in Power System)
b) J.B. Gupta (A course in Power System)
c) Ashfaq Husain (Electric Machines)
www.electrical4u.com
www.capsense.com
https://www.burnsengineering.com/tech-papers
https://www.faduooenineers.com
https://en.wikipedia.org/wiki/transformers.com
https://www.instrumentationservices.net
