INDEX LAYOUT pg. no1. INTRODUCTION [SUB STATION] -12. BUS BARS -43. ISOLATORS -74. PROTECTIVE RELAYS -115. CIRCUIT BREAKERS -166. POWER TRANSFORMER -257. CURRENT TRANSFORMER -318. CAPACITIVE VOLTAGE TRANSFORMERS -339. TRASNFORMER OIL AND ITS TESTING -3610. LIGHTENING ARRESTORS -3811. CONTROL ROOM -4112. EARTHING OF THE SYSTEM -4413. POWER LINE CARRIER COMMUNICATION -4614. CORONA -47 CONCLUSION REFERENCES
INTRODUCTIONElectric power is generated, transmitted and distributed in the form of alternatingcurrent. The electric power produced at the power stations is delivered to theconsumers through a large network of transmission & distribution.The transmission network is inevitable long and high power lines are necessary tomaintain a huge block of power from source of generation to the load centers tointer connected. Power house for increased reliability of supply greater.The assembly of apparatus used to change some characteristics (e.g. voltage, ac todc, frequency, power factor etc.) of electric supply keeping the power constant iscalled a sub-station.Depending on the constructional feature, the high voltage sub-stations may befurther subdivided: a) Out door substation. b) Indoor substation. c) Basement or Underground substation. 2 Dept. of Electrical Engg.
fig 1220 KV G.S.S. SANGANER 1) It is an outdoor type substation. 2) It is primary as well as distribution substation. 3) One and half breaker scheme is applied. 3 Dept. of Electrical Engg.
The power mainly comes from HIRAPURA-1 and HIRAPURA-2 & KOTATHERMAL .Out going feeders1) One feeder of 220kv to KTPS2) One feeder of 220kv to Sakatpura.3) One feeder of 220kv to KTPS24) One feeder of 220kv to Phulera5) One feeder of 400kv to Merta6) One feeder of 220kv to SanganerAt this substation following feeders are established. 1. TIE FEEDERS. 2. RADIAL FEEDERS. TIE FEEDERS: There are 220KV tie feeders as follows. 1.220 KV KOTA-JAIPUR 1st & 2nd 2. Inter state 220KV KOTA –DELHI 3. Tie from 220 KV Heerapura. 4. 220KV KTPS first & second. RADIAL FEEDERS 4 Dept. of Electrical Engg.
1. 220 KV JAIPUR –KOTA 1st & 2nd feeders 2. 132KV KOTA –BUNDI 1st 3. 132KV KOTA –SAWAI MADHOPUR 1st & 2nd 4.132 KV KOTA –SANGOD 5. 132 KV KOTA –MORAR BUS BARS Bus Bars are the common electrical component through which alarge no. of feeders operating at same voltage have to be connected. If the bus bars are of rigid type (Aluminum types) the structure heights are lowand minimum clearance is required. While in case of strain type of bus barssuitable ACSR conductors are strung / tensioned by tension insulator discsaccording to system voltages. In the widely used strain type bus bars stringingtension is about 500 - 900 kg depending upon the size of conductor used.Here proper clearance would be achieved only if require tension is achieved. Loosebus bars would effect the clearances when it swings while over tensioning maydamage insulators. Clamps or even effect the supporting structures in lowtemperature conditions.The clamping should be proper, as loose clamp would spark under in full loadcondition damaging the bus bars itself. 5 Dept. of Electrical Engg.
BUS BAR ARRANGEMENT MAY BE OF FOLLOWING TYPES WHICHARE BEING ADOPTED BY R.R.V.P.N.L1.) Single bus arrangement.2.) Double bus bar arrangement. a) Main bus with transformer bus. b.) Main bus-I with Main bus-II.3.) Double bus bar arrangement with auxiliary bus.DOUBLE BUS BAR CONTAINING MAIN BUS I WITH MAIN BUS II:1. Each load may be fed from either bus.2. The load circuits may be divided in two separate groups if needed fromoperational consideration. Two supplies from different sources can be put on eachbus separately.3. Either bus bar may be taken out from maintenance and cleaning of insulators.This arrangement has been quite frequently adopted where the loads and continuityof supply is necessary. In such a scheme a bus coupler breaker is mostly providedas it enables on load change over from one bus bar to other.The normal bus selection isolators cannot be used for breaking load currents. Thearrangement does not permit breaker maintenance without causing stoppage ofsupply.DOUBLE BUS BAR ARRANGEMENTS CONTAINS MAIN BUS WITHAUXILIARY BUS: 6 Dept. of Electrical Engg.
The double bus bar arrangement provides facility to change over to either bus tocarry out maintenance on the other but provide no facility to carry over breakermaintenance. The main and transfer bus works the other way round .It providesfacility for carrying out breaker maintenance but does not permit busmaintenance. Wherever maintenance is required on any breaker the circuit ischanged over to the transfer bus and is controlled through bus coupler breaker. fig 2 7 Dept. of Electrical Engg.
ISOLATORS Isolators which are also called disconnect switches or air breakswitches after the assembly as per drawings on the leveled structures theadjustment of connecting pipes, moving and fixed contacts is done so that all thethree phase of the isolator close and open simultaneously and there is a full surfacecontact between moving and fixed contacts. Such switches are generally used onboth sides of equipment in order that repairs and replacement of the equipment canbe made without any danger. They should never be opened until the equipment inthe same circuit has been turned off and should always be closed before theequipment is turned on.The adjustment of the tendon pipes leveling of post insulator, stop holts in thefixed contacts etc. is done for smooth operation of insulator. Following type ofinsulator are being used in R.S.E.B- a) Isolator without earth blades. b) Isolator with earth blade. 8 Dept. of Electrical Engg.
c) Tendon isolator. INSULATORSThe insulators for the overhead lines provide insulation to the power conductorsfrom the ground so that currents from conductors do not flow to earth throughsupports. The insulators are connected to the cross arm of supporting structure andthe power conductors passes through the clamp of the insulator. The insulatorsprovide necessary insulation between line conductors and supports and thusprevent any leakage current from conductors to earth. In general, the insulatorsshould have the following desirable properties: 1. High mechanical strength in order to withstand conductor load, wind load etc. 2. High electrical resistance of insulator material in order to avoid leakage currents to earth. 3. High relative permittivity of insulator material in order that dielectric strength is high. 4. The insulator material should be non porous, free from impurities and cracks otherwise the permitivity will be lowered. 5. High ratio of puncture strength to flash over.These insulators are generally made of glazed porcelain or toughened glass. Polycome type insulators [solid core] are also being supplied in place of hast insulatorsif available indigenously. The design of the insulator is such that the stress due tocontraction and expansion in any part of the insulator does not lead to any defect. It 9 Dept. of Electrical Engg.
is desirable not to allow porcelain to come in direct contact with a hard metalscrew thread.TYPES OF INSULATORS:There are three types of insulators used for overhead lines: 1. Pin type- pin type insulator consists of a single or multiple shells adapted to be mounted on a spindle to be fixed to the cross arm of the supporting structure. When the upper most shell is wet due to rain the lower shells are dry and provide sufficient leakage resistance. These are used for transmission and distribution of electric power at voltage up to voltage 33KV. Beyond operating voltage of 33KV the pin type insulators thus become too bulky and hence uneconomical. Fig 3.1 2. Suspension type- suspension type insulators consist of a number of porcelain disc connected in series by metal links in the form of a string. 10 Dept. of Electrical Engg.
Its working voltage is 66KV. Each disc is designed for low voltage for 11KV. Fig 3.23. Strain insulator- the strain insulators are exactly identical in shape with the suspension insulators. These strings are placed in the horizontal plane rather than the vertical plane. These insulators are used where line is subjected to greater tension. For low voltage lines (<11kV) shackle insulators are used as strain insulator. 11 Dept. of Electrical Engg.
Fig 3.3 PROTECTIVE RELAYSA Protective relay is a device that detects the fault and initiates the operation of thecircuit breaker to isolate the defective element from the rest of the system. The relays detect the abnormal condition in the electrical circuits by constantlymeasuring the electrical quantities i.e. voltage, current, frequency, phase anglewhich are different under normal and fault conditions. Having detected the fault,the relay operates to close the trip circuit of the breaker, which results in openingof the breaker and disconnection of the faulty circuit.Relay circuit connections can be divided in three parts:1.) Primary winding of a C.T. that is connected in series with the line to be 12 Dept. of Electrical Engg.
protected.2.) Secondary winding of C.T. and the relay operating coil.3.)Third part is the tripping circuit, which may be either a.c. or d.c. . It consists of asource of a supply, the trip coil of a circuit breaker and the relays stationarycontacts. When a short circuit occurs at point F on the transmission line the currentincreases to enormous value. This results in a heavy current flow through the relaycoil, causing the relay to operate by closing its contacts. This in turn closes the tripcircuit of the breaker, making the C.B. open and isolating the family section fromthe rest of the system. In this way, the relay ensures the safety of the circuitequipmentfrom damage and normal working of the healthy portion of the system. 13 Dept. of Electrical Engg.
Fig 4Basic qualities that a protective relay must possess are:1.) Selectivity2.) Speed3.) Sensitivity4.) Reliability5.) Simplicity6.) EconomyDIFFERENTIAL RELAYS 14 Dept. of Electrical Engg.
A differential relay is one that operates when the phasor difference of two or moresimilar electrical quantities exceeds a predetermined value.Thus the current differential relay is one that compares the current entering andcurrent leaving the section. Under normal operating conditions, the two currentsare equal but as soon as fault occurs, this condition is no longer applied.The difference between the incoming and outgoing currents is arranged to flowthrough the operating coil of the relay. If this differential current is equal to orgreater than the pick up value, the relay will operate and open the C.B. to isolatethe faulty section.BUCHHOLZ RELAYIt is a gas-actuated relay installed in oil immersed transformers for protectionagainst all kinds of faults. it is used to give an alarm in case of incipient (i.e. slowdeveloping)faults in the transformer and to disconnect the transformer from thesupply in the event of severe internal faults. it is usually installed in the pipeconnecting the conservator to the main tank. It is a universal practice to useBUCHHOLZ relay on all such oil immersed transformers having ratings in excessof 750kVA.CONSTRUCTIONIt takes the form of a domed vessel pipe between the main tank and the 15 Dept. of Electrical Engg.
conservator. The device has two elements. the upper element consistsof a mercury type switch attached to a float. The lower element containsa mercury switch mounted on a hinged type flap located in the direct pathof the flow of oil from the transformer to the conservator. the upper elementcloses an alarm circuit during incipient faults whereas the lower element isarranged to trip the circuit breaker in case of server internal faults.OPERATIONThe operation of Buchholz relay is as follows:(i)In case of incipient faults within the transformer, the heat due to faultcauses the decomposition of some transformer oil in the main tank theproducts of decomposition contain more than 70% of hydrogen gas. thehydrogen gas being light tries to go into the conservator and in the processgets entrapped in the upper part of the relay chamber. when a pre determinedamount of gas gets accumulated, it exerts sufficient pressure on the float tocause it tilt and close the contacts of the mercury switch attached tom it.This completes the alarm circuits to to sound an alarm.(ii)If a serious fault occurs in the transformer, enormous amount of gasis generated in the main tank. The oil in the main tank rushes to theconservator via the Buchholz relay and in doing so tilts the flap to closethe contacts of the mercury switch. This completes the trip circuit to openthe circuit breaker controlling the transformer.ADVANTAGES 16 Dept. of Electrical Engg.
(i) It is the simplest form of transformer protection.(ii) It detects the incipient faults at a stage much earlier than possible withother forms of protection.DISADVANTAGES(i) It can only be used with oil immersed transformers equipped with conservatortanks.(ii) The device can detect only faults below oil level in the transformer. thereforeseparate protection is needed for connecting cables. CIRCUIT BREAKERS 17 Dept. of Electrical Engg.
Thus circuit breakers are used for switching & protection of various parts of powersystem. Circuit breaker is a piece of equipment, which can 1) Make or break a circuit either manually or automatically under normal condition. 2) Break a circuit automatically under fault condition 3) Make a circuit either manually or by remote control under fault conditions.OPERATING PRINCIPLES A C.B. consists of fixed and moving contacts called electrodes. Undernormal operating conditions, these contacts remain closed and will not openautomatically until and unless the system becomes faulty. When a fault occurs onany part of the system, the trip coils of the circuit breaker get energised and themoving contacts are pulled apart, thereby opening the circuit. When the contacts of the C.B. are seperated under fault conditions, an arc isstruck between them. The current is thus able to continue until the dischargeceaeses. The production of arc not only delays the current interruption process butit also generates enormous heat which may cause damage to the system or to theC.B.It is thus necessary to extinguish the arc within the shortest possible time so thatthe heat generated by it may not reach a dangerous value. 18 Dept. of Electrical Engg.
ARC PHENOMENONWhen a short circuit occurs, a heavy current flows through the contacts of the C.B.before they are opened by the protective system. At the instant when the contactsbegin to separate, the contact area decreases rapidly and large fault current causesincreased current density and hence rise in temperature. The heat produced in themediumbetween contacts is sufficient to ionize the arc or vaporize and ionize the oil. Theionized air or vapour acts as conductor and an arc is set between the contacts. Thepotential difference between the contacts is quite small and is sufficient to maintainthe arc. the arc provides a low resistance path and as a result the current in thecircuit remains uninterrupted so long as the arc persists. During the arcing period the current flowing between the contacts dependson the arc resistance. The greater the arc resistance, the smaller the current thatflows between the contacts. The arc resistance depends upon:(i) Degree of ionization.(ii) Length of arc.(iii) Cross section of arc.CLASSIFICATION OF THE CIRCUIT BREAKERS: 19 Dept. of Electrical Engg.
There are several ways of classifying the circuit breakers. However, the mostgeneral way of classification is on the basis of medium used for arc extinction.The medium used for arc extinction is usually oil, air, sulphur hexafluoride (SF6)or vacuum. Accordingly, circuit breakers may be classified into:They are generally classified on the basis of the medium used for arc elimination(i) Oil circuit breakers, which employ some insulating oil for arc extinction.(ii) Air-blast circuit breakers in which high pressure air blast is used forextinguishing the arc.(iii) Sulphur hexa fluroide C.B. in which SF6 gas is used for arc extinction.(iv) Vacuum C.B. in which vacuum is used for arc extinction. SULPHUR HEXAFLOURIDE (SF6) CIRCUIT BREAKER In such breakers, sulphur hexaflouride (SF6) gas is used as the arc quenchingmedium. The sf6 is an electro-negative gas and has a strong tendency to absorbfree electrons. The SF6 circuit breakers have been found to be very effective forhigh power and high voltage service.CONSTRUCTION 20 Dept. of Electrical Engg.
The cylindrical large size steel tanks are mounted horizontally parallel toeach other. Each tank consists of SF6 under pressure. The interruption is of multibreak type & is placed along the axis of each tank. The interruption assembly issupported inside the tank by the vertical bushing, which are mounted near the endof each tank. Gas at high pressure is supplied to the interrupter from a gasreservoir. The bushing are also insulated with SF6 the conductor is in the from ofcopper tube supported at both end by porcelain shields. SF6 gas is supplied fromthe high pressure tanks. Shields are provided with gasket seals to eliminate leakageof gas from beginnings. 21 Dept. of Electrical Engg.
22 Dept. of Electrical Engg.
Fig 5WORKINGIn the closed position of the breaker the contacts remain surrounded by SF6 gas ata pressure of about 2.8 kg/sq cm. When the breaker operates, the moving contact ispulled apart and an arc is struck between the contacts. Themovement of the moving contact is synchronised with the opening of a valvewhich permits SF6 gas at 14kg/sq cm pressure from the reservoir to the arcinterruption chamber. the high pressure flow of SF6 rapidly absorbs the freeelectrons in the arc path to form immobile negative ions which are ineffective ascharge carriers. The result is that the medium between the contacts quickly buildsup high dielectric strength and causes the extinction of the arc. Afterthe breaker operation the valve is closed by the action of a set of springs.400 KV SF6 C.B. [RATINGS]: -Manufacture: BHEL Hyderabad.Type: HLR245/2503 B.S.Rated voltage: Normally 420 KV, maximum 440 KV.Rated frequency: 50 HZ.Rated power frequency: voltage: 520 KVRated Impulse withstand voltage: Lightning: 1425KV Switching: 1050KV 23 Dept. of Electrical Engg.
Normal current ratingAt 50 c ambient: 2240AmpsAt 40 c ambient: 2500AmpsShort time current rating: 40 KA for 3 sec.Rated operating duty: 0 to 0.3 sec. c-0-3min-mb.Rated short circuit duration: 1 sec.BREAKING CAPACITY [BASED ON SPECIFIED DUTY CYCLE]:(a) Capacity at rated voltage: 29000MVA [440KV].(b) Symmetry current: 40 KA.(c) Asymmetry current: 49 KA.Making capacity: 100KA [peak]Rated pressure of hydraulic operating (gauge): 250-350bar.Rated pressure of SF6 gas at degree: 7.5bars.Weight of complete breaker: 11700 Kg.Weight of SF6 gas: 76.5Kg.Rated trip coil voltage: 220 V. AC.Rated closing voltage: 220 V. DC. 24 Dept. of Electrical Engg.
First poll to clear factor: 1.3 ADVANTAGES OF SF6 CIRCUIT BREAKER: 1. Due to the superior arc quenching property of SF6, such circuit breakers have very short arching time. 2. Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents. 3. The SF6 circuit breakers gives noiseless operation due to its closed gas circuit and no exhaust to the atmosphere unlike the air blast circuit breaker. 4. The closest gas enclosure keeps the interior dry so that there is no moisture problem. 5. There is on risk of fire in such breakers because SF6 gas is not inflammable. There are no carbon deposits so that tracking and insulation problems are eliminated. 6. The SF6 breakers have low maintenance cost, light foundation requirement and minimum auxiliary equipment. 7. Since SF6 breakers are totally enclosed and sealed from atmosphere they are particularly suitable where explosion hazard exists e.g., coal mines. 25 Dept. of Electrical Engg.
DEMERITS OF SF6 CIRCUIT BREKER: 1. Sealing problems arise due to the type of the construction used. 2. The presence of moisture in the system is very dangerous to SF6 circuit breaker. 3. Arced Sf6 gas is poisonous & should not be let out. 4. The double pressure SF6 CB is cost liner due to complex gas system. 5. The internal parts should be cleaned thoroughly during periodic maintenanceunder clean dry environment. 6. Dust of Teflon & sulfide should be removed. 7. Special facilities are needed for transporting the gas.APPLICATIONSSF6 C.B. have been developed for voltages 115 KV to 230 KV, power ratings 10MVA to 20 MVA and interrupting time less than 3 cycles.S.N I.E. MAK TYPE VOLTAGE CURREN STC SF6/HYO E T D1 552A 3AT3 3AT3 420/520 2000A 40KA/S 7.5/3502 552T DO DO DO DO DO DO3 552B MG FAR2 DO 3150A DO 7/3004 452T NGEF S2M420 420/610/1425 2000A DO 8/355 252A BHEL 3AT3 420/520/1050 DO DO 7.5/3506 252B ABB EL(V) 420/1050 3150 40KA/3S 7/31.5 26 Dept. of Electrical Engg.
POWER TRANSFORMERThe transformer is a static apparatus, which receives power/energy at it, one circuitand transmits it to other circuit without changing the frequency. With this basicconception we can use the voltages at our desired level while utilizing the power.As, the voltage used to generate at modern power houses at 11 KV or so andafterwards we get it step up at a level of 33 KV, 66 KV, 132 V, 220 KV or 400KV, 750 KV for transmission to minimize the distribution losses. Again we get itstep down with the help of transformer to use at our wishes at 11 KV, 6.6 KV oreven 415, 230 volts at our houses.BASIC PARTS OF TRANSFORMERThe following are the inherent parts of a modern day transformer: 27 Dept. of Electrical Engg.
1. Primary and secondary coils (circuit) or windings.2. Core3. Main Tank4. Conservator5. Breather6. Radiator7. Buchholz relay8. Explosive vent9. Bushings (HT & LT) (Primary or secondary)10. Cooling fans11. Tap changer (on load and off load)12. NGR (Neutral Grounding Resistance) to minimize the earth fault current Fig 6.1DESCRIPTION OF PLANT: 28 Dept. of Electrical Engg.
The three transformer are oil immersed with rating of 250 MVA& one with 315 MVA. However a synchronous loading of 100MVA at 0.8 powerfactor (lag) and 18 MVA 0.8 pf (lag) on the tertiary can also be loaded to 20MVAloading with 100MVA 0.8 pf on LV without exceeding the generated temperaturerise. The transformer is also provided with a separate bank ofradiation, fans, and associated control equipments. The control equipments arehoused in a tank mounted miscalling. Fig 6.2RATING DATAS.Type of cooling: ONAN / ONAF/ ODAF 29 Dept. of Electrical Engg.
MVAHV: 189 / 252 / 315IV: 189 / 252 / 315LV: 63 / 84 / 105VOLTSHV: 400 KVIV: 220 KVLV: 33 KVLINE AMPERESHV: 273 / 364 / 455IV: 497 / 662 / 828LV: 1104 / 1471 /1839IMPEDANCE VOLTAGEHV to IV 12.65% on 315 MVA BaseHV to LV 39.16 % on 315MVA BaseIV to LV 26.66 % on 315 MVA BaseNUMBER OF PHASESThree HV, LV, IVFREQUENCY IN Hz50 Hz 30 Dept. of Electrical Engg.
YEAR OF MANUFACTURE: 1985 Mass of Core & Windings: 1,32,000 kg Mass of Oil: 65,150 kg Total weight: 261,200 kg Oil in tank: 73,200 kg Oil in radiator: 8400 kg Oil in tap changer: 83,850 kg Transportation mass: 168,000 kg Unmaking height: 7760 mm Unmaking mass 18000 Kg Guaranteed maximum temperature rise of: Oil 45ºC Winding 50ºCCOOLING FANS:Rating: 2000 m3 of air per minute.Type: 915 mm dial GEC (India) make.Numbers per transformer: two 31 Dept. of Electrical Engg.
Fan motor: direct on line starts weather proof. Squirrel cage IM 1400 W 400/440 Volt 3-φ , 50 Hz 720 rpmPUMPS:Rating: 1818 liters per minute.Type: a landless A to 8c sentiment.Number of pump per transformer: one working, one standby.Pump motor: direct on line starts weather proof. Squirrel cage IM 32 Dept. of Electrical Engg.
CURRENT TRANSFORMERThese transformers are used with low range ammeter to measure currents in highvoltage alternating current circuits where it is not practicable to connectinstruments and meters directly to lines. In addition to insulating the instrumentfrom the high voltage line, they step down the current in the known ratio. Thecurrent (or series) transformers has a primary coil of 1 or more turns of thick wiresconnected in series with the line whose current is to be measured. The secondaryconsist of a large number of turns of fine wire and is connected across the ammeterterminals (usually of 5 amp bracket should be removed or 1 amp range) 33 Dept. of Electrical Engg.
Fig 7 POTENTIAL TRANSFORMERThese transformers are extremely accurate ratio step down transformers and areused in conjunction with standard low range voltmeter (usually 150 volt) whosedeflection when divided by voltage transformation ratio, gives the true voltage onthe high voltage side. In general, they are of the shell type and do not differ muchfrom the ordinary two winding transformer, except that their power rating isextremely small. Up to voltage of 5000 potential transformers are usually of drytype, between 5000 and 13800 volts, they may be either dry type or oil immersedtype, although for voltage above 13800 they are oil type. Since their secondarywindings are required to operate instruments or relays or pilot lights, their ratingsare usually 42 to 100 watts. 34 Dept. of Electrical Engg.
CAPACITIVE VOLTAGE TRANSFORMERS (CVT) Capacitive voltage transformers are special kind of powertransformers using capacitors to step down the voltage.DESCRIPTION: 35 Dept. of Electrical Engg.
The capacitive voltage transformer comprises of a capacitor dividerwith its associated electromagnetic unit. The divider provides an accurateproportioned voltage, while the magnetic unit transforms this voltage, in bothmagnitude and phase to convenient levels suitable for measuring, metering,protection etc. all WSI capacitor units has metallic bellows to compensate thevolumetric expansion of oil inside. The porcelain in multi unit stack, all thepotential points are electrically tied and suitably shielded to overcome the effectof corona RIV etc. Capacitive voltage transformers are available for systemvoltages of 33 KV to 420KV. Fig 8APPLICATION:1. Capacitive voltage transformers can be effectively as potential sources for measuring ,metering, protection, carrier communication and other vital functions of an electrical network. 36 Dept. of Electrical Engg.
2. CVT are constructed in single or multi unit porcelain housing with there associated magnetic units. For EHV systemcuts are always supplied in multi unit construction. 3. In case of EHV cuts the multi unit system has many advantage easy to transport and storing, convenience in handling. RATING OF CVT Voltage: 22/sqrt 3 KV Total o/p: 500MVA Operating voltage: 400/sqrt 3 max. Voltage factor: 1.5/30 sec. Test voltage: 630 KV for 1 min Impulse withstands voltage: 1.2/ 50 µs. 1425KV max. Frequency : 50Hz High frequency capacitance: 4400pF Primary capacitance: 4657pF Secondary capacitance: 80000 pFS no Ie Make Ratio Burden Class Sec cap1 Bassi Wsi/cve/420 400 200,200, 3p,3p,0.5 80000pf /1425 1002 Bassi 2 Wsi/cve/420 400 200,200, 3p,3p,0.5 80000pf /1425 1003 Bus 1 Wsi/cve/420 400 200,200, 3p,3p,0.5 80000pf 37 Dept. of Electrical Engg.
/1425 1004 Bus 2 Wsi/cve/420 400 200,200, 3p,3p,0.5 80000pf /1425 100 TRANSFORMER OIL & ITS TESTINGThe prime function of oil is to convey the heat from the core and winding to thetank where it can be dissipated. Besides these, the oil provides additional insulationbetween primary and secondary windings. So, the oil must be completely free fromdirt, moisture and other un-wanted solid matter. The oil used in the transformer isnatural mineral oil and should undergo the following tests if required:BREAKDOWN VOLTAGE: The voltage at which the oil breaks down when subjected to an electric field.FLASH POINT:The temperature, at which the oil gives off so many vapors, when mixed with airforms an ignitable mixture and gives a momentary flash with small pilot flame. 38 Dept. of Electrical Engg.
For checking above values, various tests are done. These arecategorized as: 1. Physical test. 2. Chemical test. 3. Electrical test. The results must be close to standard results that are follows- S.N TYPE OF TEST STD. RESULTS 1. Density (gm/cubic cm.)at 27°C .85 to .89 2. Flash point >125°C 3. B.D.V Test K.V (rms.) >50 KV 4. Tan delta at 90°C < 20% 5. Water content (PPM.) 25(max.)above 145KV 6. Gas contents (PPM.) (a) Hydrogen 100 to150 (b) Methane 50 to 70 (c) Ethane 30 to 50 (d) Ethylene 100 to 150 (e) Acetylene 20 to 30 (f) Carbon dioxide 3000 to 3500 (h) Carbon mono-oxide 200 to300 39 Dept. of Electrical Engg.
LIGHTENING ARRESTORSAn electric discharge between cloud and earth, between cloud and the chargecenters of the same cloud is known as lightening.The earthing screens and the ground wires can well protect the electrical systemagainst direct lightening strokes but they fail to provide protection againsttravelling waves which may reach the terminal apparatus. The lightening arrestorsor the surge diverters provide protections against such surges.THYRITE TYPE: Ground wire run over the tower provides an adequate protectionagainst lighting and reduce the induced electrostatic or electromagnetic voltage butsuch a shield is inadequate to protect any traveling wave, which reaches theterminal of the electrical equipment, and such wave can cause the followingdamage. 40 Dept. of Electrical Engg.
1 the high peak of the surge may cause a flashover in the internal wiring thus it may spoil the insulation of the winding . 2 the steep wave front may cause internal flash over between their turns of transformer. 3 The stop wave front resulting into resonance and high voltage may cause internal or external flashover causing building up the oscillator is the electrical operation.Lightening arrestors are provided between the line and earth provided theprotection against traveling wave surge the thyrite lightening arrestor are providedat GSS. This type of LA has a basic cell made of thirties, which is a particular typeof clay, mixed with carborendum. Thirties has a particular property of beinginsulator one voltageAt high voltage It will behave like a conducting material the electrical resistance ofthyrite depends upon the voltage each time the voltage is made twice the resistancedecrease in such a manner as to allow an increased current of 12.5 times thechange in current is independent of rate of application voltage and its instantaneousvalue.The above law is followed by this material without any limit on the voltageincrease and after the surge has passed the thyrite againretain its original propertyA standard cell is rated for 1KV and is formed into a disc, which is sprayed onboth the sides of to give good contact with each disc. The dimensions of the discsare stacked i.e. 16 cm in diameter and 17.5 cm thick these discs are stacked oneupon each other and they are further placed in to a porcelien container with asuitable arrangement of gap between them. 41 Dept. of Electrical Engg.
These gaps serves as the purpose of preventing any current flow duringnormal operating voltage in case of any transients the gap are punctured. TheThyrite type arrestor will discharge several thousands ampere without the slightesttendency of flashover on the edges of most important of the advance is that there isabsolutely no time lag in its performance.400KV LIGHTNENIG ARRESTORmanufacture: English electric companyno of phases: onerated voltage: 360 KVnominal discharge current (8×20µs) 10KAhigh current impulse(4×110 µs ) 100KAlong duration rating(200 µs) 500KASno Ie Make Type Current Voltage1 Bassi1 Wsi Cpl 10KA 360KV2 bassi2 Elpro Alugard2 10KA 360KV3 ILT1 Elpro Alugard2 10KA 360KV4 ILT2 Elpro Alugard2 10KA 360KVh5 ILT3 WSI CDV303 10KA 398KV6 ILT4 WSI CDV03 10KA 398KV 42 Dept. of Electrical Engg.
CONTROL PANELThe diagram made on the control panel is known as mimic diagram.COLOUR CODING* 33KV GREEN* 132 KV BLACK* 220KV BROWN* 440 VOLTS VOILET/INDIGO* 110 VOLTS ORANGE REACTORIt is used to lower the over excited capacitor. Capacitor bank is connected in shuntover the reactor. Capacitors main purpose is to boost up the voltage. so when wewant to lower the voltage we use reactors. it is also use to stop the sudden change.the commonly used reactor is NGR(Neutral ground reactor). 43 Dept. of Electrical Engg.
CIRCUIT BREAKERThere is a one and half breaker scheme i.e. 3 breakers for 2 buses used in 400 KVG.S.S. BUS COUPLERSIt is used to equalize the load on both Bus bars. DISTURBANCE RECORDERIt records the distance & fault on graph with voltage w.r.t time. EVENT LOGGERit monitors as well as provides the details as a printed material.These details may contain the sequence of operation, switching time, closing timeetc. ON LOAD TAP CHANGER (OLTC)In this method a number of tappings are provided on the secondary of thetransformer. The voltage drop in the line is supplied by changing the secondaryemf of the transformer through the adjustment of its number of turns by usingtransition resistorwhich are placed in between each tapping. 44 Dept. of Electrical Engg.
In supply system, tap changing has to be performed on load so that here is nointerruption to supply. By using transition resister therefore shut down is notrequired. Fig 11 NO LOAD TAP CHANGER (NLTC)in this we change the tap manually for which we have to shut down thetransformer.When the load increases the voltage across the primary drops but the secondaryvoltage can be kept at the previous value by placing the movable arm on to ahigher stud. Whenever a tapping is to be changed in this type of transformer, theload is kept off and hence the name off load tap-changing transformer. SYNCHRONOSCOPEA synchronoscope is used to determine the correct instance of closing the switchwith connect the new supply to bus bar the correct instance of synchronizing isindicated when bus bar and incoming voltage 45 Dept. of Electrical Engg.
* are equal in magnitude* are equal in phase* have the same frequency the phase sequence is sameEARTHING OF THE SYSTEM:The provision of an earthling system for an electric system is necessary by thefollowing reason. 1 In the event of over voltage on the system due to lightening discharge or other system fault. These parts of equipment, which are normally dead, as for as voltage, are concerned do not attain dangerously high potential. 2 In a three phase, circuit the neutral of the system is earthed in order to stabilize the potential of circuit with respect to earth.The resistance of earthling system is depending on 1 Shape and material of earth electrode used. 2 Depth in the soil 3 Specific resistance of soil surrounding in the neighborhood of system electrodes.PROCEDURE OF EARTHING:Technical consideration the current carrying path should have enough capacity todeal with more faults current. The resistance of earth and current path should below enough to prevent voltage rise between earth and neutral. The earth electrodemust be driven into the ground to a sufficient depth to as to obtain lower value ofearth resistance. To sufficient lowered earth resistance a number of electrodes areinserted in the earth to a depth they are connected together to form a mesh. Theresistance of earth should be for the mesh in generally inserted in the earth at 0.5m 46 Dept. of Electrical Engg.
depths the several point of mesh then connected to earth electrode or groundconduction. The earth electrode is metal plate copper is used for earth plate.Neutral Earthing: Neutral earthing of power transformer all power system operates withgrounded neutral. Grounding of neutral offers several advantages the neutral pointof generator transformer is connected to earth directly or through a reactance insome cases the neutral points is earthed through an adjustable reactor of reactancematched with the line. The earthling is one of the most important feature of systemdesign for switchgear protection neutral grounding is important because: 1 The earth fault protection is based on the method of neutral earthling. 2 The neutral earthling is associated switchgear. 3 The neutral earthling is provided for the purpose of protection arcing grounds unbalanced voltages with respect to protection from lightening and for improvement of system. 47 Dept. of Electrical Engg.
POWER LINE CARRIER COMMUNICATIONAs electronics plays a vital role in the industrial growth, communication is also abackbone of any power station, communication between various generating andreceiving station is very essential for proper operation of power system. This ismore so in case of a large interconnected system where a control load dispatchstation has to coordinatethe working of various units to see that the system is maintained in the optimumworking condition, power line communication is the most economical and reliablemethod of communication for medium and long distance in a power network.PLCC system in Rajasthan: -1 HEERAPURA: JAIPUR, AJMER, BYAWAR, BHILWARA, PALI, JODHPUR2 HISSAR: KHETRI, HEERAPURA, KOTA, RAPP3 HEERAPURA: KOTS, JSP, RPS, GSD4 BHILWARA: RPS5 PALI: FALANA6 HEERAPURA: ALWAR, BHARATPUR7 NEEMUCH: DEBARI8 DEBARI: SIROHI 48 Dept. of Electrical Engg.
9 DEBARI: ZAWAR MINES10 HEERAPURA: SIKAR, RATANGARH, BIKANER11 HANUM, ANGARH: HISSAR, SHRIGANGANAGAR12 HEERAPURA: BADHERPUB CORONA EFFECTWhen an alternating potential difference is applied across two conductors whosespacing is as large as compared to their diameters, there is no apparent change inthe condition of atmospheric air surrounding the wires if the applied voltage is low.However when the applied voltage exceeds a certain value called criticaldisruptive voltage, the conductors are surrounded by a faint violet glow calledcorona. The phenomenon of corona is accompanied by a hissing sound, productionof ozone, power loss and radio interference. The higher the voltage is raised, thelarger and higher the luminous envelope becomes, and greater are the sound, thepower loss and the radio noise. If the applied voltage is increased to breakdownvalue, a flash over will occur between the conductors due to the breakdown of airinsulation. The phenomenon of violet glow, hissing noise and production of ozone gasin an overhead transmission line is known as corona. 49 Dept. of Electrical Engg.
If the conductors are polished and smooth, the corona glow will be uniformthroughout the length of the conductors, otherwise the rough points will appearbrighter. The positive wire has uniform glow about it, while the negativeconductors has spotty glow.FACTORS AFFECTING CORONAThe phenomenon of corona is affected by the physical state of the atmosphere aswell as by the conditions of the line. The following are the factors on which coronadepends: 1. Atmosphere. In the stormy weather, the number of ions is more than normal and as such corona occurs at much less voltage as compared with fair weather. 2. Conductor size. The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage. 3. Spacing between conductors. Larger space between conductors reduces the electro-static stresses at the conductor surface, thus avoiding corona formation. 4. Line voltage. If the line voltage is low, there is no chance in the condition of air surrounding the conductors and hence no corona is formed. 50 Dept. of Electrical Engg.
ADVANTAGES AND DISADVANTAGES OF CORONACorona has many advantages and disadvantages. In the correct design of a high voltage overhead line, a balance should be struck between the advantages and disadvantages.Advantages 1. Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electro-static stresses between the conductors. 2. Corona reduces the effect of the transients produced by surges.Disadvantages 1. Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line. 2. Ozone is produced by corona and may cause corrosion of the conductor due to chemical action. 3. The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines. 51 Dept. of Electrical Engg.
CONCLUSION A technician needs to have not just theoretical but practical as well and soevery student is supposed to undergo a practical training session after III yearwhere I have imbibed the knowledge about transmission, distribution, generationand maintenance with economical issues related to it. During our 30 days training session we were acquainted with the repairing ofthe transformers and also the testing of oil which is a major component oftransformer. At last I would like to say that practical training taken at 220KV GSS hasbroadened my knowledge and has widened my thinking as a professional. 52 Dept. of Electrical Engg.
REFERENCES:Principles of Power System-by V.K.MEHTAElectrical Power System-by C.L.WADHWA REPORT BY- Kapil Kumar SKIT,JAIPUR 53 Dept. of Electrical Engg.