CHAPTER- 1 INTRODUCTION 1.1 Synopsis Energy is the basic necessity for the economic development of a country. Energy exists in different form in nature but the most important for is electrical energy. The conversion of energy available in different forms of nature into electrical energy is known as generation of different forms of nature into electrical energy is known as generation of electrical energy. Various sources of energy available in nature are1
Solar Energy
2
Wind Energy
3
Tidal Energy
4
Nuclear energy
A great demand of electrical energy is notable feature of modern civilization. The abun abunda danc ncee of elec electr trica icall energ energy y comp comple letel tely y chan change gess the the direc directi tion on of the the temp tempo o of civilization, living standard, vast development of rural and urban areas. Electricity has becom becomee an essent essential ial commod commodity ity.. The featur featuree of electri electrical cal energy energy not only only paraly paralyses ses industries and agriculture but also upsets the lives. The whole electrical system is classified as: 1 Generation 2 Tran ransmis smissi sio on 3
Distribution
4
Utilization
5
Switchgear and protection Turbines are moved with the help of different sources of energy. The generation is
coup couple led d with with turb turbin inee to gene genera rate te 11KV 11KV whic which h is furt furthe herr step steppe ped d up by step step up transformers to 400KV and is then distributed to various sub-stations where the voltage is reduced reduced to 220KV 220KV with the help of step down transformers. transformers. From these sub-station sub-stationss the energy is distributed to the consumers after reducing it to 33KV.
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1.2 Installation Installation The 400KV GSS Jodhpur is one of those stations, which distributes energy to the consumers. The station was installed in 2003. Jodhpur is the second largest city of the state Rajasthan. 1.2.1 The Incoming And Outgoing Feeder
The incoming supply is as follows: 1. 400 400 KV – Kank Kankro roli li 2. 400 KV – Mert Mertaa I 3. 400 KV – Mert Mertaa II II 4. 400 KV KV – Jaisalm Jaisalmer(U er(Unde nderr Constru Constructi ction) on) 5. 400 KV – Rajw Rajwest est (Und (Under er Cons Constru tructi ction) on) The outgoing feeder is as follows: 1.
220 KV – Tinwari I
2.
220 KV KV – Tinwari II II
3.
220 KV KV – Jodhpur I
4.
220 KV – Jodhpu dhpurr II
5.
220 KV – Bilara
6.
132 KV – Banar
7.
132 KV – Mathania
8.
33 KV – Salawas
9.
33 KV – Daijar
10. 10. 33 KV – JU JU All Alloy oy 11. 11. 33 KV – DRD DRDO O 1.2.2 Location And Coverage
The 400KV 400KV GSS is situat situated ed on near near SURPUR SURPURA A villag village, e, about about 14-15 14-15 KM from from Jodhpur Railway Station. It covers as area of 110-120 beegha of land.
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1.2 Installation Installation The 400KV GSS Jodhpur is one of those stations, which distributes energy to the consumers. The station was installed in 2003. Jodhpur is the second largest city of the state Rajasthan. 1.2.1 The Incoming And Outgoing Feeder
The incoming supply is as follows: 1. 400 400 KV – Kank Kankro roli li 2. 400 KV – Mert Mertaa I 3. 400 KV – Mert Mertaa II II 4. 400 KV KV – Jaisalm Jaisalmer(U er(Unde nderr Constru Constructi ction) on) 5. 400 KV – Rajw Rajwest est (Und (Under er Cons Constru tructi ction) on) The outgoing feeder is as follows: 1.
220 KV – Tinwari I
2.
220 KV KV – Tinwari II II
3.
220 KV KV – Jodhpur I
4.
220 KV – Jodhpu dhpurr II
5.
220 KV – Bilara
6.
132 KV – Banar
7.
132 KV – Mathania
8.
33 KV – Salawas
9.
33 KV – Daijar
10. 10. 33 KV – JU JU All Alloy oy 11. 11. 33 KV – DRD DRDO O 1.2.2 Location And Coverage
The 400KV 400KV GSS is situat situated ed on near near SURPUR SURPURA A villag village, e, about about 14-15 14-15 KM from from Jodhpur Railway Station. It covers as area of 110-120 beegha of land.
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Fig. 1.1
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CHAPTER- 2 IMPORTANCE OF ELECTRICAL ENERGY 2.1 Introduction Energy may be needed as heat, as light as motive power etc. the present day advancement in science and technology has made it possible to convert electrical energy into other desired form. This has given electrical energy place of pride in the modern world. In fact the advancement of country is measured in terms of per capita consumption of electrical energy. Electrical energy is superior to all other forms of energy due to the following reasons
2.2 Conventional Forms And Easy Control Electrical energy is a very convenient form of energy as it can easily converted into any form of energy like heat, light mechanical etc.
2.3 Greater Flexibility One important reason for preferring electrical energy is flexibility that it offers. It can be easily transported from one place to another.
2.4 Cheapness Electrical energy is cheaper than other forms of energy. Thus it is economical to use this form of energy in domestic commercial and industrial purpose.
2.5 Cleanliness Electrical energy is not associated with smoke, fumes or poisonous gases. Therefore its use ensures cleanliness and health conditions.
2.6 High Transmission Efficiency The consumers of electrical energy are generally situated quite away from the centers of its production. The electrical energy can be transmitted conveniently and efficiently from the center of generation to the consumers with the help of overhead conductors known as Transmission lines.
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CHAPTER-3 FUNCTION AND SPECIFICATIONS OF VARIOUS EQUIPMENTS AT 400KV GSS 3.1 Metering & Indicating Instrument There are several metering & indicating instruments e.g. Ammeters, voltmeters, energy meters etc installed in a sub-station to keep watch on circuit quantities. The transformer is invariably used with them for satisfactorily operation.
3.2 Bus Bars Bus bars are the important components in a substation. There are several bus bar arrangements the can be used in a substation. The choice of a particular arrangement depends upon various factors such as system voltage, position of substation, degree of reliability; cost etc. the following are the important bus bar arrangement used in substations: a) Single bus bar systems b) Double bus bars systems c) Duplicate bus bars systems In the 400KV GSS Surpura, Double bus bar substation and duplicate bus bar system has been installed.
3.3 Control Cables The control cable and the control system are required for officiating automatic system. The cables employed for this purpose are multi-core cables having 10 or 37 or 61 conductors are run to the required points. The conductors used in the 400 KV GSS are moose and conductors used in the 220 KV GSS are tarantula.
3.4 Power Transformers A transformer consists essentially of two or more electric circuit in the form of winding magnetically interlinked by a common magnetic circuit. An alternating voltage applied to one of the winding produces, by electromagnetic induction, a correspondence emf in the other windings & energy can be transferred from the ordinary circuit to the other circuit by means of the common magnetic flux and the principle of mutual induction. A transformer is basically a static device in which two or more stationary electric circuits are coupled magnetically, the winding being linked by a common time varying magnetic flux. Even though the static transformer is not an energy conversion 5
device & involves only the interchange of electrical energy between two or more electrical systems, it is an extremely important component in many conversation systems.
Fig. 3.1 3.4.1 Classification Of Power Transformer:
1. According to usages 2. Step-up transformer 3. Step –down transformer 4. According to type of construction used: 5. Core type 6. Shell type 7. According to number of winding 8. Two winding transformer 9. Three winding transformer 10. Multi winding transformer 3.4.2 Terms Related To Transformer 1 Primary Winding:
The winding that is excited or energized by connecting it to an input source is usually referred to as the primary winding. 2 Secondary Winding:
The winding to which the electrical load is connected and forms, which the output energy is taken, are known as the secondary winding.
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3 HV Winding:
The winding which s operated at the high voltage level is known as the HV (high voltage) winding. 4 LV Winding:
The winding which is operated at lower voltage level is known as the LV (low voltage) winding. 4 Regulating Winding:
It is winding, which is used to regulate the voltage at different levels y connecting tap changers across the winding. It consists of discrete numbers of small windings with 2 or 3 terms in each ad they being connected in series. 6 Tertiary Winding:
In addition to the tradition primary and secondary windings, a transformer can also have tertiary winding. 3.4.3
Main Parts Of Transformer Are As Follows
1.
Core:
It consist terminated silicon steel in which quantity of silicon 13up to 4% thickness of lamination is 0.35 to 0.50m. Normally the shape of core in rectangular and it has three legs. 2.
Windings:
Windings of power transformer are an important part. It consists of
super enameled copper wires. The size of wire (diameter) depends on the capacity of transformer connection of winding is r/r. 3.
Tap changer:
Tap changer is switching device by which the transformation ratio can be changed by the changing the position of tap changing the switch. Tap changing system on GSS of power transformer on-load tap changer (OLTC): On load tap changers are employed to change turn ratio of transformer to regulate system voltage while the transformer is delivering normal load with the introduction of on-load tap changing the operating efficiency of electrical system has considerably improved. Now a day, almost all the large power transformers are fitted with on load tap changer. All forms of on load tap changing circuit posse’s impedance. This is introduced to prevent short-circuiting of tapping section during tap changer operation. The impedance can be either a resist of or a centretapped reactor.
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4.
Tanks:
It is metallic tank, which is filled of insulating oil the transformer core and winding assembly are surrounding by the oil in this tank. It protects that the winding and core from the external mechanical damages. Rectangular tanks are similar in fabrication. However for large rating power transformer, shaping of tanks becomes necessary to conform to transportable profile shaping is provided by rounded corners at the ends, truncation of law portion of walls from considerable, of loading in well wagon grider and on the covers to reduce the height to minimize the tank oil, the tank profile may closely follows the electrical clearances along the coils. As is evident, shaping gives saving in tank material and oil but increases complexity and fabrication costs. Transformer tank may be classified as 1 Plain tanks. 2 Shaped tanks. 3 Belt shaped tanks. 4 Corrugated tanks. 5 Stub and type tank The transformer tank used in GSS power transformer is rectangular box (plain tank) type in shape. 5. Cooling System :
In Power transformer, the oil serves a dual purpose as an insulating medium as well as a cooling medium. The heat generated in the transformer is removed by the transformer oil surrounding the source and is transmitted either to atmospheric air or water. This transforms of heat is essential to control the temperature within permissible limits for the class of insulation, thereby ensuring longer life due to less thermal degradation. Types of cooling used in GSS power transformer: 1 ONAN type cooling: The generated heat can be dissipated in many ways. In case of
smaller rating of transformers, its tanks may be able to dissipate the heat directly to the atmospheric dry while bigger ratings may require a dditional dissipating surface in form of tubes/ radiators connected to tank or in the term of radiator tank. In these cases, the heat dissipation is form transformer oil at atmospheric air by natural means. This form of cooling is known as ONAN (Oil Natural, Air Natural) types of cooling. 2 ONAF type of cooling: For further augmenting the rate of dissipating of heat, other
means such as fans blowing air on the cooling surfaces are employed. The forced air takes away the heat at a faster rate, thereby giving better cooling rate than natural air. This type 8
of cooling called ONAF (Oil Natural Air Forced) type of cooling. In this cooling arrangement, additional raring under ONAN condition viz. after shutting off fans, is available, which is of the order of 70-75%. 5.1 Cooling Arrangements
Depending upon the typed of cooling and rating of the transformer, the cooling equipment can be arranged in various ways. 5.2 Arrangement with Radiators
Radiators are commonly used for ONAN and ONAF types cooling. Radiator consists of element joined to and bottom headers, elements are made by welding two previously rolled and pressed thin steel sheets to forms a number of channels of flutes through which oil flow. These radiators can be either mounted directly on the transformer tank or in a form of a bank or connected to the tank through the piper. The surface area available for dissipation of heat is a multiple manifold by using various elements in parallel. As oil passes downwards either due to natural circulation or force of a pump in the cooling circuits, the surrounding atmosphere air carries heat away. 5.3 Arrangements with Fans
These fans deliver large air volume at moderate speed with minimum sound and low power consumption. Ring mounted fans are designed to give maximum volume under free airflow condition and resistance up to approximately 6mm WC. These fans generally conform to IS2312 and are used for radiator cooling. Fan consists of a totally enclosed continuously rated specially designed motor with class B insulation and IP-55 class of protection to meet fan duty, impeller constructed with four broad faced. Steel sheet blades assembly on robust aluminum hub, four arms, pressed sheet mounting ring and four rubber cushions.
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Fig. 3.2 6. Temperature Meters
There are two temperatures indicating metering power transformer, which indicate the oil temperature and winding temperature. Temperature measured in degree Celsius. A complete assembly of a transformer with details of core, wingding, tank connections and major accessories. 7. Conservator and Air Cell
As the temperature of oil increases or decreases during operation there is a corresponding rise or falling volume to account for this an expansion vessel (conservator) is connected to the transformer tank. The conservator has got a capacity between the minimum to maximum oil level equal to 7.5 & of the oil in transformer. The atmoseal types conservator, it is filled with oil to level appropriate to filling temperature and in remaining portion is air cell, which is connected to atmosphere through a breather. As the breather is through air cell no moisture come in contact with oil, this protect the oil from deterioration or contamination.Air cell is a flexible separator filled inside the conservator. Oil being out of the air cell, the separator is in direct contact with the atmosphere. The advantage of air deterioration or contamination. 1. An efficient barrier between oil and air. 2. A protection against water vapors. 3. The suppression of any gas bubbles formation in the oil. Air cell is made from coated fabric with external coating resistance to transformer oil and inner to coating to ozone and weather.
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Fig. 3.3
8. Buchholz’s Relay
The transformer is fitted with a bubble float buchholz rely. It is fitted in the feed pipe from conservator to tank. Any internal fault in transformer is detected by buchholz relay the gas liberated in the transformer is divided to the buchholz relay without being trapped anywhere. 9. Dehydration Breather
The conservator is connected outside through dehydration (Silicage filled) breather to make sure that the air in conservation is dry.
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Fig. 3.4
10. Oil Temperature Indicator
Oil temperature indicator operates on the principal of liquid expansion. The OTI provided with a maximum pointer and two mercury switches are adjustable to make contact between 500 to 120 0 with the fixed differential of 10 0. the temperature for alarm and trip contact setting shall be as under:- alarm 80 0 to 900. 11. Winding Temperature Indicator
The indicator is fitted with four mercury switches, one is used for alarm, 2 nd is for tripe and 3rd is for fans on and 4 th pumps control. All the switches are adjustable. 12. Earthing
Connecting leads from core and end frame are being terminated at the top at the top of cover. By connecting them to tank cover, core and end frames being earthed. For Bank earthing two number studs have been provided on tank. 13. Terminal Bushings
It is used to isolate the leads that are coming from transformer. The size of the bushing is justified according to operation voltage of particular winding. The active part of the bushing consists of an Oil Impregnated Paper (O.I.P.) condenser core manufactured from superior grade craft paper would on aluminum tube. This bushing is voltage graded by suitably interposed aluminum foils forming condenser layers. Thus the electrical stress are controlled throughout the thickness and along the surface avoiding any highly stress concentrations. The bushing is supplied fully assembled in a wooden packing case with
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the busing supported at an angle of 10 degree to the horizontal. The bushing should never be placed horizontally. 14. Insulating Oil
The insulating oil has three functions: 1 Provides additional insulation 2 Protects the paper from dirt and moisture 3 Carries away the heat generated in the core and coils. The Insulating oil should have the following properties : i) High Dielectric Strength. ii) Free from inorganic acid, alkali and corrosive sulphur to prevent injury to the conductor or insulation. iii) Low viscosity to provide good heat transfer iv) Free from sludge under normal operating conditions. v) Free from sludge under normal operating conditions. vi) Good resistance to emulsion so that the oil may throw away any moisture that enters
the apparatus.
3.5 Lighting Arresters They are used to protect the sub-station & transmission lines arrests is earthed . Gap is adjusted in such a way that 50% over voltage is operators. We will use
value type lighting arresters this types is called non-linear diverter. In
this spark – gap & resistance disc are used . when there is less change in voltage than is not flashover in gap but when
line
there is over voltage & rapid
change in voltage then even grounding of voltage will not possible the value of flash over voltage depends on surge currents. Operation will start when voltage will increase 10% of rated voltage. 1.
Rod gap arresters
2.
Horn gap arresters
3.
Multigap arresters
4.
Expulsion type arresters
5.
Value type arresters
3.6 Circuit Breakers Classification of circuit breakers 1) Are quenching (Medium Wise) 13
a) Air Blast CB b) Oil CB c) Air Blast CB d) Vacuum CB e) SF₆ CB
2) Application wise : a) Generator CB b) Transformer Line CB c) Industrial CB d) Distribution CB 3) Voltage Level Wise: a) HV/EHV CB b) MV CB c) LV CB
4) Base on Construction: a) Dead Tank Breaker b) Live Tank Breaker G.S.S has SF₆ Circuit Breaker’s which have the following mechanism. 6.1 SF₆ Circuit Breaker: In this CB, the SF ₆ gas is used as an quenching agent. The
process of extinction by the gas is shown the below block diagram At the time of fault: Contracts of CB open
The Valve mechanism permits high pressure SF ₆ gas from the reservoir to flow towards the are interruption chamber from the Trip Valve, which is now NO-normally open.
The high pressure flow of the SF ₆ gas rapidly absorbs the free electrons in the are path to form immobile negative ions, which are ineffective are charge carriers.
As a result, the medium between the contacts quickly builds up high Dielectric Strength & causes the extinction of the arc. 14
Fig. 3.5
3.6.1.1 Electrical properties of SF ₆ Electron affinity: The excellent insulation properties of sulphur hexafluoride are
attributable to the strong electron affinity of the SF₆ molecule. This is based mainly on two mechanisms, resonance capture and dissociative attachment of electrons in accordance with the equations: 1) SF₆ +e→ SF₆ 2) SF₆ + e → SF₅ + F
The process represented by equations (1) applies to electron energies of 0.1 eV with an energy range of 0.05eV and that represented by equations (2) applies to an energy range of 0.1 eV. 1.
Are-quenching capacity: On account of its thermal properties and low
ionization temperature, sulphur hexafluoride exhibits outstanding characteristics for the extinguishing of electric arcs. The quenching time using SF₆ is about 100 times less than that using air. 2.
Dielectric Strength : The strong interaction of hi-energy electrons with the
polyatomic SF ₆ reaches that of transformer oil at pressure of only 3 bars. The breakdown 15
strength of SF₆ is independent of frequency. It is inert gas. Chemical inertness of this gas is advantageous in switchgear. The components do not get oxidized or deteriorated. The life of metallic part, Contacts is longer in SF₆ gas. Hence the maintenance requirements are reduced. 3.6.1.2 Operating Principals Of Sf ₆ Circuits Breaker:
The SF6 breaker operates on what is usually referred as the puffer principle. In puffer type SF6 circuit breaker , the entire breaker is filled with SF6 gas at single pressure of 5 Kg/cm2 or about 7 bar. The breaker is a sealed unit . during the opening stroke the SF 6 gas is compressed released through the nozzle of insulating material. The compressed gas flow through the nozzle at a high velocity and takes away the heat produced by the arc the arc is quenched at a current zero. The high dielectric strength of gas is useful in giving good with stand voltage SF6 circuit breaker are explosion free , can quench capacitive currents , short circuit current etc. Early and are used for high voltage circuit breakers for voltage above 3.3 KV During manufacture of the breaker pole it is dried internally through pumping, the breaker pole is then to pressurized and also tested against leaks inside the pile there is a absorption medium for the decomposition products of the gas. The breakers pole should only be opened by trained person at the manufacturing factory.
3.7 Isolators Then carrying out inspection or repair in a substation installation. It is essential to disconnect reliably the unit or the section, on which the work is to be done, from all other live parts of the installation in order to ensure complete safety of the working staff. To guard against mistakes it is desirable that an apparatus, which makes a visible break in the circuit, should do this apparatus is the isolating switch. It may be defined as a device used to pen ( or close ) a circuit in the voltage across the terminal e.g. each pole of the isolator will result from the operation. Isolators are classified as: 1. Off load isolator-It is an isolator which is operated when the isolator is already disconnected from all sources of supply or when the isolator is already disconnected from the supply and current may be due to capacitance current of bushings bus bar connections and very short length of cable. 2. On load isolator -It is isolator, which is operated in a circuit where there is a parallel path of low impedance so that no significant change in the voltage across the terminals of each pole occurs when it is operated .
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3.8 Instrument Transformer Is defined as a transformer intended to feed the measuring instruments, meters, relays etc. Generally
protective system are relays are connected to the secondary
of a current transformer as they cannot withstand high currents. These IT’s help in reducing these voltages & currents to acceptable level for operation
of
voltmeters & ammeters. 3.8.1 Current Transformer
A CT is an instrument transformer in which the secondary current is substantially reduced proportional to the primary current & differs from it by the angle which is approx. direction of current . These transformers are different from general power transformers.
Fig. 3.6
Shown below are the major differences that are noticeable:
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Conventional Transformer
Driving Function: voltage.
Current Transformer
Driving Function : Current
Secondary load impedance determines the CT primary current is the determining secondary current.
and predominated factor.
Corresponding to the secondary current the
Secondary current follows the primary
primary current flows.
current.
3.8.1.2 Function / Application of a CT 1. For Metering function : It transforms the high value Primary Current substantially
low value secondary current which can be fed directly to measuring instruments for measuring the current & power in the main circuit. 2. For Protection Purpose: The secondary current can also feed Protective Relays which
operate the protective system in the main circuit in case of any abnormality in the system. Definition of the different terms related with current transformers : a) Rated Primary Current : The value of primary current on which the
primary performance of the current transformers is specified. b) Rated Short Time Current : Its defined as r.m.s value of a.c component
which the CT can carry without damage. c) Rated Secondary Current : The value of secondary current marked on the
rating plate. d) Rated Exiting Current: The RMS value of current taken by the
secondary winding of a. C.T. When sinusoidal voltage of rated frequency is applied to secondary with primary winding open. e) Rated Burden: The burden assigned by the manufacturer at which C.T
performs with specified accuracy. f)
Current Error Ratio Error : The percentage error in the magnitude of
secondary current is defined in the terms of current error.
3.8.1.3 Burden on C.T.
Rated burden of CTS and VT’s referring to the maximum load in volt-amperes 9 VAO which may be applied across the secondary terminals without the ratio and phase
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angle error-exceeding the permissible limits. The burden depends upon the number of relays and instruments connected and their individuals burden typical values. 3.8.1.4 Various Types of Construction of CTs
A CT has following essential parts 1. Insulation over the core by taps 2. Secondary winding having several turns would on the insulated core. 3. Bar primary passing through the window of the core and terminals. 4. Support porcelain or epoxy insulator. 5. Synthetic resin or oil insulation. 6. 3.8.1.5 CT’s For High Voltage Installations
Separately mounted post type CT’s are suitable for outdoor service. The primary conductor is at high voltage with respect to the earth. Hence it is insulated by means of insulation column filed with dielectric oil. In high voltage CT’s the primary and secondary windings are situated at the upper end of the unit. The primary wdg. Normally being of bar type. The top – fabricated housing is at line potential and is supported on the porcelain insulator. 3.8.1.6 Specification Of 400 KV Current Transformers
1. Type – Dead tank, single phase out door, oil immersed & Hermetically sealed . 2. Manufacturer’s Designation – 420 kV CT. 3. Rated voltage ( KV ) – 420 4. Short time thermal rating for One second ( KA rms ). – 40 5. Rated dynamic current of primary 100 ( Kapeak). 6. Flux density at knee point voltage, 14.5 Wb/cm2 7. NO. of primary turns – single 8. No. of secondary turns – 200-1000-500 9. Core area , cum2 – 65.55 10. Core length, (Average magnetic path) cms. – 104.46 11. Type of primary winding – Hair pin type. 12. 1 Temperature rise (degree C) at rated continuous thermal current over max. ambient temperature at site for – i) Winding 40 ii) Oil at top 40 iii) Exposed current carrying parts 55 12.2 Temperate rise at normal rating over max. Ambient temperature site for 19
i)
Winding 40
ii)
Oil at top 40
iii)
Exposed current carrying parts.
13. Total creep age distance, mm 10500 14. Protection creep age distance 5250 15. One minute power frequency with stand test voltage ( kv rms ) 630 16. 250/2500 micro seconds switching impulse withstand voltage (kv peak) 1050 17. 1.2/50 micro second impulse withstand test voltage (kv peak) 1425 18. Power frequency withstand test 4 kv for core IV & V and Voltage on secondary (kv rms) 3 kv rms for core III 19. Weight of oil per C.T. kegs. 510 20. Governing standard for oil IS 335 21. Whether pressure relief device Yes Provided. 22. Total weight per CT kgs. 2150 23. Overall dimension .mm 1150 x 750 x 5200 24. Current density in primary winding at i)
Normal rating A/cm2 208
ii)
Thermal rating for 1 sec., A.cms 249
iii)
Dynamic rating, A/cm2 10405
25. Visual corona extinction voltage 320 (k Vrms) 3.8.2 Potential Transformers Potential transformers (PT) are mainly instrument transformers that are basically used for the following purposes : 1. For stepping down the voltage for measurement 2. As line voltmeters 3. Protective relays 4. Tariff meters 3.8.2.1 Construction
The PT is mostly step down and shell type. The secondary voltage is generally 110 V potential transformers are of two types :
a. Magnetic type
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The magnetic type PT work on the principles of power transformers. The design is particularly for the system voltage of 132 K.V. and above where it becomes increasingly more economical. Main parts of the PT are stated below :1.
Core: The core may be shell type in its construction. Shell type
core is suitable for low voltage transformers. 2.
Winding: The primary and the secondary winding are coaxial to
reduce leakage to minimum. The primary winding may be single coil but must be subdivided. 3.
Insulation: Cotton type and varnished cambric are used or soil
construction. Hard fiber separators are used between coils. At low voltages, the transformers are usually filled without above 700 volts been developed for use up to 45 K.V. b. Capacitor voltage transformers (CVT)
CVT are used for line voltmeters synchroscropes, protective relays, tariff meter etc. The supply frequency-switching transients, magnitude of connected burden etc, affect the performance of CVT. The CVT is more economical then an electromagnetic voltage transformers when the nominal system voltage increase above 66 KV. The carrier current equipment can be connected via the capacitor voltage transformers, thereby there is no need of separated coupling capacitors. The CVT are used for voltage above 66 KV and above. At such voltage the costs of electromagnetic voltage transformer is too high. The capacitor connected in series with the CVT acts like a potential divider. The burden provided by the capacitor is negligible. The construction of CTV depends on the form if the capacitors voltage divider. Generally HV capacitors are enclosed in porcelain housing. Schedule of guaranteed data and technical particulars for 4400-pf-400KV capacitor voltage transformers.
3.9 Insulators The insulators serve two purposes. They support the conductors and confine the current to the conductors. The most commonly used material for the manufacture of insulator is porcelain. There are several types of insulators and there use in the substation will depend upon the service requirement. It is stronger mechanically than glass gives less trouble from leakage & is less affected by change of temperature. Type of insulators
i) Pin type Insulator
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ii) Suspension type insulator iii) Strain Insulator iv) Shackle Insulators v) Post Insulators. Their use in the substation will depend upon the service requirement. 3.9.1 Pin type Insulator
Pin type of insulator is not economical beyond 33 KV. For high voltage (>33 kV), it is a usual practice to use suspension type insulators. They consist of a number of porcelain discs connected in series by metal links in the form of a starting. The conductor is suspended at the bottom end of the string while the other end of the string is secured to the cross-arm of the tower. Each unit or disc is designed for low voltage. If working voltage is 66kv, and then six discs in series will be provided in the string. 3.9.2 Strain Insulators
When there is a dead end of the line or there is corner or sharp curve the line is subjected to greater tension. In order the line of excessive tension are used. For high voltage transmission lines, strain insulators consists of an assembly of suspension the tension in lines is exceedingly high, 2 or more strings are used in parallel.
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CHAPTER- 4 POWER LINE CARRIER COMMUNICATION (PLCC) 4.1 Introduction For exchange of dates & transfer of message between GSS voice communication is necessary. High frequency carrying
currents audio signals is generated, transmitted &
received with the help of identical carrier current equipment provided on each
end.
Carrier current equipment comprises of following: 1.
Coupling Capacitor
It acts like a filter. It blocks power frequency (50hz) while offer low reactance to carrier frequencies as allows them to pats through because. For examples A 2000 pf capacitors offer 1.5-mega ohm to 50hz while if just offer 150 ohms to 500 coupling capacitor allows carrier frequency
kHz. Thus
signal to enter the carrier equipment bus
does not allow 50hz power frequency current to enter the carrier equipments. 2. Wave Trap Unit
It is parallel turned comprising of c & I. It has low impedance to 50hz & high impedance to carrier frequencies get passed through wave trap & carrier frequencies passes through coupling capacitor & reaches carrier current Wave traps are mounted in outdoor switchyard. Wave trap mounted at GSS is “under hung”. 3. Transmitter & Receiver Unit
Carrier current unit acts like both transmitter receiver carrier frequencies are generated in master oscillator can be tuned to a particular frequency
selected for the
application output voltage of oscillator is held constant by voltage stabilizers. Output of oscillators is fed to amplifiers, which increases the strength of signal to be transmitted to overcome the transmission losses. Line losses vary with length of line frequency type of line losses in overhead lines. Receiving unit comprises of an alternator. Band pass filter restricts the acceptance of uncounted signal & matching transformer or matching element matches the impedance of line & receiving unit block diagram of receiving of receiving unit.
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CHAPTER- 5 SUBSTATION 5.1 Introduction: Substations are important part of power system. The assembly of apparatus used to change some characteristics (e.g. voltage, arc. to o.k. frequency, p.f etc) of electrical supply is called substation.
5.2 Classification of Substation There are several ways of classifying substations. However, the two most important ways of classifying them are according to: 1) Service requirements and 2) Constructional features 1. According to service requirements: a) Transformer sub-stations b) Switching substations c) Power factor control substation d) Frequency changer sub stations e) Converting substations f) Industrial sub stations 2. According to constructional features:
a) Indoor sub-stations b) Outdoor sub-stations c) Underground sub-stations d) Pole-mounted sub-stations
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CHAPTER- 6 PROTECTIVE RELAY 6.1 Introduction In order to generate electric power and transmit to customers, millions of rupees must be spent on power system equipment. This equipment is designed to work under specified normal conditions. However a fault may occur causing the system to collapse. This fault occurs because of: 1) Over voltage due to switching. 2) Over voltage due to direct and indirect lighting strokes. 3) Bridging of conductors by birds. 4) Breakdown of insulation due to decrease of its dielectric strength. 5) Mechanical damage of equipment. These short circuits may cause heavy damage to equipment and would also cause intolerable interruption of service to customers.
6.2 Relays Relays are the devices that detect abnormal conditions in electrical circuits by constantly measuring electrical quantities, which are different under normal and fault conditions. The basic electrical quantities, which may change under fault conditions, are voltage, current, phase angle and frequency. Having detected the faults the relays operates to competent the trip circuit which result in opening of the circuits breaker and therefore in the disconnection of the faulty circuits. Basic requirements of protective relaying:
A well designed and protective relaying should have i) Speed ii) Selectivity iii) Sensitivity iv) Reliability v) Simplicity vi) Economy
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6.3 Types of Protection There are two types of protection known as primary and back up. The primary protection is the first line to defense and primary relays clear faults in the protected system as fast as possible. The reliability, not only if the protected scheme but also of the associated C.T.’s, P.T.’s and the C.B.’s cannot be guaranteed. Therefore some sort of back up protection must be provided. The backup relay operates if the primary relays fails and covers not only the local primary relays to operate. Protective relays are classified depending upon their construction and principles of operation such as:-Ordinary electromagnetic relays consisting of moving plunger, moving iron, attracted armature hinged and balanced beams types of relays are various examples, D.C. actuated such replays. Electromagnetic induction or simply induction relays use the principles of induction motors (whereby torque is developed by induction in rotor) in their operation. Such relays are actuated by A.C. quantities only. Electro thermal relays (thermal overload protection using bimetallic strip) Physic-electrical relays: Bucholy’s relays are examples of this type. Static relays employing thermionic valves, transistors or magnetic amplifiers to obtain the operating characteristics. Electro-dynamic relays operate on the same principles as moving coil instrument. The various types of relays installed at 400 KV GSS are: 1) Over current relays 2) Distance relays 3) Differential relays 4) Earth fault relays 1) Over Current Relays:
Directional type over current relays works on the induction principles and initiates corrective measures when current in the circuit. Exceed the pre-determined value. The actuating source is a current in the circuit supplied to the relay from a current transformer. These relays are used on a.c. circuits and can operate for fault flow in either direction. But their relays are unsuitable for use as a directional protective relay under short circuit conditions. When a short circuit occurs, the system value falls to a low value and there may be insufficient torque developed in the in the relays to cause its operation. This difficulty is over come in the directional over current relays, which is designed to be almost independent of system voltage and power factor.
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Operation:
Under normal operating conditions, powers flows in the normal direction in the circuit protected by the relays. Therefore, directional power relays (upper element) does not operate, thereby keeping the over current element (lower element) energized. However when a short circuit occurs, there is tendency for the current or power to flow in the reverse direction. Should this happen, the disc of the upper elements rotates to bridge the fixed contact 1 and 2. This completes the circuits for over current elements. The disc of this element rotates and the moving contact attached to it closes and the trip circuit. This operates this circuits breaker which isolates final tripping of the current by them is not made till the following conditions are satisfied: (a) Current flows in a direction such as to operate the directional element. (b) Current in the reverse direction exceed the pre-set value. Grading of the time lags of the relays, which controls a number of switches in a feeder. These relays automatically adjust their time of operation depending upon their distance from fault. There are four main elements in any distance protection as follows: (i) Operating elements “O”: The element brings protection into action whenever a fault occurs within the protected zone. (ii) Directional elements “S”: This gives directional features to the operation of the system and is useful in network having duplicate feeder. As soon as the fault current into the bus bar from the line this element operates. (iii)
Distance element “Z”: This is sensitive to the ratio of the operating voltage to
the fault current i.e. V/ if or upon fictitious impedance when looking into the system from the fault. Zf =V/1f The value of Z f is dependent upon the distance of the fault from the relays. The principle of this element is more or less like ohmmeter. (iv) Time delay element “T”: This element creates a time lag, the importance of which has already been discussed above. This time lag depends upon the distance of the fault point from the relay. 2) Distance Relay:
Distance protection is the name given to the protection, whose action depends upon the distance of the feeding point to the fault. The time of operation of such a protection is a function of the ratio of voltage and current, i.e. impedance. This impedance between the 27
relay and the fault is dependent upon the electrical distance between them. An impedance relay has an operating force proportional to the fault current and restraining force proportional to the line voltage at the relay. As soon as the ratio of this voltage to the fault current change i.e. falls below a certain value, the relay operates. This value is dependent upon the distance of the fault, which is predetermined. Hence for this reason the relay is discriminative and it does not operate for any fault occurring outside this distance. As it is very important to localize the fault, a relay of the above type is given a controlled time lag, so that the relay nearest to the fault operates first. This time lag is made proportional to the distance of the fault by so designing the relay that it has a time lag characterizes, which is dependent upon the line voltage at the relay directly. Again, the time lag characteristic is inversely proportional to the fault current that is passing through the relay. In case of a fault, there is a steady fall of voltage along the line from the feeding point to the fault. This voltage gradient can be utilized for longer be in balance. This voltage difference will cause a current to flow through the operating coil of relay, which closes the trip circuit. 3) Differential Relays:
A differential relay is one that operates when the difference of two or more electrical quantities exceeds a predetermined value. Almost any type of relay connected, in a certain way, can be made to operate as differential relays. There are two fundamental system of differential protection viz. 1) Current balance protection 2) Voltage balance protection A current balance differential relay is one that compares the current entering a section of the system of the system with the current leaving the section. Under normal operating condition no longer applies. If this differential current is equal to or greater than the pickup value, the relay will operate & open the circuit breaker to isolate the faulty section. Under healthy condition equal current flows in both primary windings. Therefore the secondary voltages are balanced against each other & no current will through the relayoperating coil. 4) Earth Fault Relays:
Directional type over current relays work on the induction principle and initiates the char-active measures. When current in the circuit exceeds the predetermined values. The actuating source is a current in the circuit supplied to the relay from a CT. these relays are unsuitable for use as directional protective relays under short-circuit conditions. When a 28
short circuit occurs, the system values falls to a low value and there may be insufficient torque developed in the relay to cause its operation. This difficult is overcome in the directional over current relay, which is designed to be almost independent of system voltage and power factor.
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CHAPTER- 7 EARTHING 7.1 Introduction Connecting of an electrical equipment or apparatus to the earth with the help connecting wire of negligible resistance
of a
is called as “Earthling or grounding”. The
provision of earth electrode for an electrical system is necessity by following reasons. 1. All the parts of an electrical equipments
like casings of machines circuit breaker,
lead sheathing & armoring of cables, tanks of transformer etc, which have to be the at earth potential, must be connected to an earth electrodes. This current operates the proactive device & thus the faulty circuits is halted in case occur. 2. The electrode ensures that in the event of over voltage of an the system due to lighting discharge or other system faults which are normally “dead” as for as voltage are concerned do not attain dangerously high potentials. 3. In a 3-phase circuit the neutral of the system is earthed in order to stabilize the potentials of the circuit with respect to
earth. In electrical installations the
following
components must be earthed: a) The flames, tanks & enclosed of electric machines transformers and apparatus,
lighting fitting. b) The operating mechanism of the switchboards control boards individual panel boards, cubicles. c) The structural steel work of sub-stations, metal cable jointing boxes, the metal sheaths of the cable s the rigid metal conduct runs & similar metal work. There are 2 methods of earthing: 1- Pipe earthing.
2- Plate earthing.
7.2 Earthing Arrangements 400 KV GSS In a GSS or any magnitude
various non current carrier equipment to be
earthed namely substation structures , shielding g wires or masts ,equipments tanks spread over large areas therefore it becomes necessary to lay a grounding bus connect the various items to be earthed to be ground bus through suitable connection to heave duplicate earthing is broken the sub - station
may remains
safe under all conditions . It generally, therefore becomes desirables to form a ring
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of the earthing
electrodes. Another way of looking into the sub - station earthing
problem is that a very low Earthing problem is that a very low earthing resistance value is required resistance in a very large low earthing value is required in a large areas occupied but the
sub - station such can only be obtained by using a number of rod &
joining them in parallel .In a sub -station the shape of grounding meet with necessary
earthing system invariably takes the
or additional rounding rods accepts in
the case of very small sub stations. Common earth electrodes should be use for both system earths & equipments earth. Here also it is recommended to have common earth bus for high voltage system. Where there are manual operating handle to the system .A typical earthing arrangements for a GSS .
7.3 Plate Earthing In plate earthing plate either of copper of dimensions 600cm * 60cm *3.15mm or of galvanized iron of dimensions 60cm * 60cm * 6.30cm s burled into the ground with its face vertical at a depth of not less that 3mt from ground levels .A small masonry brick wall enclosure with a cast iron cover or top an RCC pipe round the earth plate is provided to facilitate its identification & for carrying out periodical inspection & tests. The earth wire GI wire of GI plate earthing is securely. Bolted to an earth place with the help of a bolt nut & washer made of material & of galvanized iron in case of GI plate earthing.
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