Thermal Plant Ropar 1260MW

About Powercom and Transcom Govt. of Punjab unbundled Punjab State Electricity Board into two companies Punjab State Power Corporation Ltd. (POWERCOM) Punjab State Transmission Corporation Ltd. (TRANSCOM) In exercise of the Power conferred by article -45 of the Articles of Association of Punjab State Power Corporation Limited and Punjab State Transmission Corporation Ltd.and all other Powers enabling him on this behalf, Governor of Punjab is appoint the following Chairman cum Managing Directors and Directors of the above Corporations with immediate effect. The appointment will be initially for a period of one year from the date of joining:POWERCOM 1. Er.K.D.Chowdhry 2. Sh.G.S.Bachi 3. Er.Arun Kumar 4. Er.G.S.Chhabra 5. Sh.S.C.Arora 6. Sh.H.C.Seth Chairman cum Managing Director Director/Administration Director/Distribution Director/Generation Director/Finance Director/H.R TRANSCOM 1. Sh.Anurag Agarwal 2. Er.Ravinder Singh 3. Sh.Uma kanta Panda Chairman cum Managing Director Director/Technical Director/Finance

Prior to this unbundling the organization was PSEB.

PEC University of technology

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The Punjab State Electricity Board (PSEB) was a statutory body formed on 1-21959 under the Electricity Supply Act.1948. Subsequently with the reorganization of the erstwhile State of Punjab under the Punjab Re-organization Act 1966 this form came into existence w.e.f. 1st May, 1967. Starting with the modest installed capacity of 62 MW, the PSEB grew up by leaps and bounds with generating capacity 6841 MW as on 31-3-2009 from all sources, including share from Central Sector Projects. The Board's gross generation during the year2008-09 was 38880 Million Units. PSEB operated its own Generation Power Plants and also got power as its share from BBMB. The PSEB also constructed and maintained its Transmission and Distribution system for providing efficient services to the various categories of electricity consumers in the state. PSEB proudly serving more than 66.31 lakhs consumers comprising of approximate 54.86 lakhs. General, 1.12 lakhs Industrial and 10.33 lakhs Agricultural connections till 31.3.2009


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Introduction To GGSSTP, ROPAR Plant the ever increasing gap between power demand and its availability was one of the main reasons for envisaging the Guru Gobind Singh Super Thermal Power Plant for the state of Punjab after Guru Nanak Dev Thermal Power Plant, Bhatinda.

Salient Features of GGSSTP, Ropar :-

1.

Location

ROPAR,

about

50

km

from

Chandigarh on Chandigarh Nangal Highway no.21 Near Ghanauli

Village railway station. 2. 3. No. of Houses No. of Units 6 6


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4. 5. 6.

Total Generator Capacity Source of Water Supply Fuel used

6210 MW = 1260 MW From Nangal Hydel Channel Coal from coal fields of Bihar, West Bengal & Madhya Pradesh more than 50 sources calked Collieries.

Distance of these sources is between 1417 Km & 1560 Km. 7. Turbine 210 MW 3 Cylinder mixed flow tandem coupled 3000 rpm BHEL make. 8. Generator 247 MVA, 15.75 KV, 9050 A at 0.82 lag, 50 Hz, double star two pole 9. Commissioning Unit 1 = 26.9.84 Unit 2 = 29.3.85 Unit 3 = 31.3.88 Unit 4 = 29.1.89 Unit 5 = 29.3.92 Unit 6 = 30.3.93 10. Cost of project Stage-I Stage-II Stage-III 11. Total annually 12. Cost per unit Rs 1.84 energy contribution 6942 MUs Rs 380 Crores Rs 438 Crores Rs 599 Crores

1.WORKING OF STEAM POWER PLANT :-


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Steam Power Plant basically operates on the Rankine Cycle. Coal is burnt in a boiler, which converts water into steam. The steam is expanded in a turbine, which produces mechanical power driving the alternator coupled to the turbine. The steam after expansion in prime mover (Turbine) is usually condensed in a condenser to be fed into the boiler again. In a practice, however, a large number of modifications and improvements have been made so as to effect economy and improve thermal efficiency of the Plant. The working of a modern coal-fired steam power plant can be studied conveniently with the help of various cycles. Schematic arrangement of a modern coal-fired steam power plant is shown in Fig. the entire arrangement for thew sake of simplicity may be divided into four main circuits namely (i) Fuel and Ash ciruit. (ii) Air and Fuel gas circuit. (iii) Feed water and Steam circuit. (iv) Cooling water circuit.

1. Fuel and Ash ciruit :Coal is delivered from the supply points to the storage site by road, rail or water. By road coal is transferred in trucks and for small stations such transport may be enough. Where power plants are situated close to a water way, such as a canal, river or sea transportation b boat or ship may be quite effective. However, in most cases transportation of coal by rail, road is the most common. In the case of small power plants the quantity of coal being small, manual unloading from rail car may be used but for large power stations unloading from the railway siding is done with the help of wagon tripplers and then the coal is


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conveyed to the coal handling plant. From the coal handling plant, the coal having been good enough to be burnt in boilers is taken into the boiler bunkers by means of bucket conveyers. The coal is thus stored in the bunkers from where it falls into the hoppers by gravity and finally the requisite quantity of coal either goes on falling directly on the grate, or where the coal spreaders are provided, coal is spread in the grate up to the rear end. When use of spreaders is made, most of the coal burns in air and remaining falls at the rear end of the grate. Any unburnt coal particles in the middle of the grate are collected in a pipe and are again refired by cinder-refiring fan. The grate in such types of boilers, where use of spreaders are made move from rear and to front end, and without spreaders, the movement of the grate is from front end to rear end. Combustion is controlled by controlling the grate speed, quantity of coal entering the grate, the damper openings. The ash resulting after complete combustion of fuel collects at the back of the boiler and is removed to the ash storage by means of scrap conveyers.

2. Air and Fuel gas circuit :Air is drawn from the atmosphere by a forced draught fan or induced draught fan through the air-preheater, in which it is heated by the flue gases passing to the chimney and then admitted to the furnaces. The flue-gases after passing around boiler tubes and superheaters tubes are drawn by the induced draught fan through dust collector (or precipitator), economizer and air-preheater and finally exhausted to the atmosphere through cimney.


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3. Feed water and Steam circuit :The steam coming out of the turbine is condensed and the condensate is extracted from the condenser by the condensate extraction pump and is forced to the low pressure feed water heaters ( usually three in numbers ) where its temperature is raised by the heat from bled system (steam extracted from the lowest pressure extraction point of the turbine). The feed water is now pumped through deaerator to high pressure feed water heaters (usually 2 or 3 in numbers) where it gets heated by the heat from bled steam extracted at suitable point of the steam turbine. The function of deaerator is to reduced dissolved oxygen content in the condensate (i.e. in the feed water). The feed water is then pumped into boiler through economizer in which it is further heated by the heat of the flue gas passing through it on the way to chimney. A small part (about 1 %) of steam and water in passing through the different components of the system is lost. Therefore, the water is added in the feed water system as makeup water, as shown in Fig. in boiler water is converted into high pressure steam, which is wet. Wet steam is passed through super-heater, where it is dried and further superheated, and then supplied to the steam turbine through the main valve. After giving out its heat energy to the turbine it is exhausted to the condenser where its latent heat is extracted and steam is converted into feed water. At one or more stages a quantity of steam is bled or withdrawn for heating of feed water. Making up water for boiler is taken through the evaporator, where it is heated by low pressure steam extracted at the suitable point of the turbine.

4. Cooling water circuit :PEC University of technology Page 7

Cooling water is supplied from a natural source of supply such as river, canal, sea or lake or cooling towers through screens to remove the matter, that might take choke the condenser tubes. It is circulated through the condenser for condensating the steam and finally discharged to the suitable position near the source of supply. During the passage its temperature rises and in the case of cooling towers the heat must be extracted before the water is again pumped to the condenser. The circulation of cooling water to the condenser help in maintaining a low pressure in the condenser.


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1. Introduction :-

1. Turbo Generator :A turbo generator is a turbine directly connected to electrical generator for the generation of electric power. An electrical generator is a machine which converts mechanical energy toelectrical energy.

2. History of Turbo Generator :Generators are based on the theory of electromagnetic induction, which was discovered byMichael Faraday in 1831, a British Scientist. Faraday discovered that if an electric conductor,like a copper wire, is moved through a magnetic field, electrical current will flow(be induced) inthe conductor. So the mechanical energy of the moving wire is converted into the electric energyof the current that flows in the wire.

3. Principle of Operation:Turbo generator or A.C. generators or alternators operates on the fundamentalPrinciples of FARADAYS LAWS OF ELECTROMAGNETIC INDUCTION. In them thestandard construction consists of armature winding mounted on stationary element called stator and field windings on rotating element called rotor . The stator consists of a cast-iron frame,which supports the armature core, having slots on its inner periphery for housing the armatureconductors. The rotor is like a flywheel having alternating north and south poles fixed to its outer rim. The magnetic poles are excited with the help of an exciter mounted on the shaft of alternator itself. Because the field magnets are rotating the current is supplied through two slip rings. Asmagnetic poles are alternately N and S, they induce an e.m.f and hence current in armatureconductors. The frequency of e.m.f depends upon the no.of N and S poles moving past a conductor in 1 second and whose direction is given by Flemings right hand rule.


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2. Generator :-

Salient Features of Generator (210 MW) :-

1. Stator Frame :-

The stator body with core and stator winding form the heaviest component of the entire Turbo generator. The active parts to be accommodated and the forces and torques arising during operation call for a rigid and strong stator shell. Moreover, it is designed to withstand high internal pressure, which may arise due to unlikely event of explosion of hydrogen air mixture without any residual deformations. Stator body is a totally enclosed in a gas tight fabricated structure made-up of high quality mild steel and austenitic steel. It is suitably ribbed with annular rings called inner walls to ensure high rigidity and strength. Inner and side walls are suitably blanked to house four longitudinal hydrogen gas cooler in side the stator body.

2. Pipe connections :-

To attain a good aesthetic look the water connection to gas cooler is done by routing stainless steel pipes inside the stator body which emanate from bottom and emerge out at side walls. These stainless steel pipes serve as inlet and outlet for gas coolers. From side wall these are connected to gas coolers by means of eight U-tubes outside the stator body. For filling the generator with Hydrogen a perforated manifold is provided at the top inside the stator body. The feed and


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vent terminating flanges for hydrogen, carbon dioxide and air are provided at the bottom of stator body. Manhole is provided at the bottom to inspect inside of the generator if required.

3.

Terminal Box :-

The beginnings and ends of three phases of stator winding are brought out to the slipping end of the stator body and brought out through a terminal bushings in the terminal box.

The terminal box is a welded construction of (Non-magnetic) austenitic steel plates. This material eliminates stray losses due to eddy currents, which may result in excessive heating.

4.

Testing of Stator Body :-

On completion of manufacture of stator body ,it is subjected to a hydraulic pressure of 8kg/cm2 for 30 minutes for ensuring that it will be capable of withstanding all explosion pressure which might arise on account of hydrogen air mixture explosion . At this stage, leakage in weld seams, if any ,is detected and rectified immediately.

Complete stator body is then subjected to gas tightness test by filling in compressed air.

5.

Stator Core :-


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A rotating magnetic flux threads with the core. In order to minimize the magnetizing and eddy current losses in this active portion of the stator the entire core is built up of thin laminations. For reasons of manufacture, each lamination layer is made-up of a number of individual segments.

The segments are stamped out with accurately finished die from sheets of cold rolled high quality silicon steel. Before insulating with varnish each segment is carefully deburred. The stator body is turned on end while the core is stacked with lamination segments in individual layers. The segments are assembled in an inter leaved manner from layer to layer so that a monolithic core of high mechanical strength and uniform permeability to magnetic flux is obtained. The stampings are held in position by twenty core bars having dovetail section. Insulating paper press boards are also put between the layer of stampings to

provide additional insulation and to localise short circuit which may occur due to failure of varnish insulation of sheet stamping . To ensure tight monolithic core ,the stampings are hydraulically compressed during the stacking procedure at different stages when a certain heights of stack are reached forming different pockets. Between two packets, one layer of ventilating segments is provided. The steel spacers are spot welded on stamping.

These spacers from ventilating ducts from where the cold hydrogen from gas coolers enter the core radially inwards there-by taking away the heat generated due to eddy current losses. The pressed core is held in pressed condition by means of two massive nonmagnetic steel castings of press ring. The press rings are bolted to the ends of core bars. The pressure of press ring is transmitted to stator core stampings through press fingers of non-magnetic steel and duralumin placed adjacent to press rings. The non-magnetic steel press fingers extend up to the tip of stamping teeth so as to ensure the firm compression of the teeth part of the corePEC University of technology Page 12

portions too. The stepped arrangement of the stampings towards the bore at the two ends provides an efficient support of the tooth portion and contributes to a reduction of eddy current losses and local heating in this range in addition to the provision of more area of cross section for gas flow. To avoid heating of press rings due to end leakage flux two rings made of copper sheet are used as flux shield . The rings screen off the flux by short circuiting .To monitor the formation of hot spots, resistance temperature detectors are placed along the bottom of slots.

6. Stator Winding :-

6.1 General :-

The stator has a three phase, double layer, short pitched and bar type of windings having two parallel paths .Each slot accommodates two bars. The slot lower bars and the slot upper bars are displaced from each other by one winding pitch and connected at their ends so as to form coil groups . The coil groups are connected together by bus inside the stator frame.

6.2 Conductor Construction :-

Each bar consist of 26 solid as well as 14 hollow conductors with cooling water passing through the latter. Alternate arrangements of hollow and solid conductors ensure an optimum solution for increasing current and to reduce losses.

The conductors of small rectangular cross section are provided with glass lapped strand insulation. These are arranged side by side in two layers. The


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individual layers are insulated from each other by a separator. In the straight slot portion the strands are transposed by 360 o to reduce the eddy losses.

The transposition provides for a mutual neutralization of voltages induced in the individual strands due to the slot cross-field and end winding field and ensures that no circulating currents will arise.

The current flowing through the conductor is thus uniformly distributed over the entire bar cross section so that the current dependent losses are reduced.

6.3 Corona Prevention :-

To prevent corona discharges between insulation and the wall of the slot, the insulation in slot portion is coated with semi conducting varnish. At the transition from slot to over hang winding a stress grading varnish is coated to ensure a uniform control of the electric field and to prevent the formation of creep age sparks during high voltage test.

In the course of manufacture the bar is subjected to number of tests to ensure proper quality. The various test which are performed are:-

a) Inter turn insulation test on stack after consolldation to ensure absence of inter turn short. b) Each bar is subjected to hydraulic test to ensure the strength of all joints. c) Flow test is performed on each bar to ensure that there is no reduction in cross section area of the ducts of the hollow conductor. d) Leakage test by means of air pressure is performed to ensure gas tightness of all joints. e) High voltage test to prove soundness of insulation.PEC University of technology Page 14

f) Dielectric loss factor measurement to establish void free insulation.

6.4 Laying of Stator Winding :-

The stator winding is placed in open rectangular slots of the stator core which are uniformly distributed on the circumference. A semi conducting spacer is placed in bottom of slot to avoid any damage to bar due to any projection etc. any manufacturing tolerances are compensated by driving in semi conducting filler strips between bar and slot wall, which also ensures good contact between semi conducting coating and slot wall. After laying top bar, slot wedges, high strength glass texolite spacers are put to have proper tightness. In between top and bottom bars, spacers are also put. These measures prevent vibrations which may be set up by the bar currents.

6.5 Electrical & Water Connection :-

Electrical connection between top and bottom bar is made by putting copper ferrule over the two limbs of coil lug. In between , copper wedges are inserted and then soldering is done. Solder is so poured that no air and void remains inside. After that joint is subjected to ultrasonic testing.

Water connection on Exciter side is done by simply connecting copper tube in two lugs. On turbine side, each lug is connected through a Teflon hose to inlet/outlet header. Bus bars and the terminal bushings are also provided with water connections by copper tubes.


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Ring type water headers, made of copper are provided separately for distillate inlet and outlet in the stator on turbine side. The headers are supported on insulators and isolated from stator body .The end connection of top and bottom bar is done by putting copper ferrule over the two opening of lugs by connecting a copper tube to the two opening of lugs at exciter ends. At turbine side, each individual bar is connected with inlet/outlet headers by P.T.F.E hoses. The bar heads are insulated by fiber moulded covers filled with putty.

The vent pipe connections are provided at the top of both inlet and outlet header to expel air during filling these headers with distillate. These vent pipes can be connected to gas trap device to measure the extent of hydrogen leaking into water circuit.

6.6 Terminal Bushing :-

Three phases and six neutral terminals are brought out from the stator frame through bushings, which are capable of withstanding high voltage, and provided with gas tight joints. For this purpose, the bushings are bolted to

the bottom plate of the terminal box, with their mounting flanges. The terminal box, which is welded underneath the stator frame at exciter end, is made of nonmagnetic steel to avoid eddy current losses and thus any inadmissible temperature rise. Provision is made for fixing the external bus ducts with terminal plate. The conductor of the bushing is made of high conductivity copper tube on which silver plated terminal plates are brazed at both ends. A copper pipe is connected to circulate water for cooling .The terminal bar

conductor is housed in porcel in insulator, which can be mounted on the terminal box by means of ring.


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The bushing is assembled and tested for flow, leakage to ensure tightness and continuous flow of water.

The bushing is connected to terminal bus bar by means of flexible copper leads for making the electrical connections conveniently.

To make the stator body gas tight at the two ends, two end shields are fitted with the help of bolts. Gas tightness is achieved by putting a rubber sealing cord.

The end shields are made-up in two halves for convenience during erection and inspection. To avoid leakage of gas through the split surface rubber sealing is put between two halves of end shields.

A chamber is provided near the internal diameter to collect oil, which might enter from shaft seal. This chamber is connected to liquid leakage detector,

which gives an alarm for presence of any liquid.

Aluminum alloy casting of fan shield is supported on end shields to direct the gas flow from the propeller fan. Shaft seal and oil catcher are also mounted on end shields.

7. Rotor :-

The rotor comprises of following components:-

1.-

Rotor ShaftPage 17


2.3.4.5.6.-

Rotor Winding Rotor Wedges & other locating parts for winding. Retaining ring Fans Field Lead connections.

7.1 Rotor Shaft :-

The rotor shaft is a long forging measuring more than 9 meters in length and slightly more than one meter in diameter.

On 2/3 of its circumference approximately, the rotor body is provided with longitudinal slots to accommodate field windings .The slot pitch is selected in such a way that two solid poles displaced by 180o are obtained.

The solid poles are provided with additional slots in short lengths of two different configurations, one type of slots served as an outlet for hydrogen which has cooled the overhung winding and other type used to accommodate finger of damper segments acting as damper winding. Within the barrel portion the rotor slot wedges act as damper winding.

7.2 Rotor Winding :-


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The field winding consists of several coils inserted into the longitudinal slots of the rotor body .The coils are wound around the poles so that one north magnetic pole and one south magnetic pole are obtained on shaft.

7.3 Cooling of Winding :-

The rotor winding is cooled by means of direct cooling method of gap pick-up method. In this type of cooling the hydrogen in the gap is sucked through the elliptical holes serving as scope on the rotor wedges and is directed to flow a along lateral vent ducts on rotor cooper coil to bottom of the coil. The gas picked up by the wedge scoopes in the inlet zones due to the pressure created under the scoopes by rotation corresponding duct on the other side and flows outwards and thrown into the gap in the outlet zones. In this machine, these are zones which result in multi jet flow of hydrogen exposing large amount of rotor winding copper to the cooling medium, thus creating very effective cooling and enabling a very low ratio of maximum to average copper temperature rise becomes independent of length of rotor.

The overhang portion of the winding is cooled by axial two-flow system and sectionalized into small parallel paths to minimize the temperature rise. Cold gas enters the overhang from under the retaining rings through special chamber in the end shields and ducts the fan hub and gets released into the air gap at the rotor barrel ends.


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7.4 Rotor Wedges :-

For protection against the effect of centrifugal force the winding is secured in the slots by slot wedges. The wedges are made from Duralumin, an alloy of copper Magnesium and Aluminum having good electrical conductivity and high mechanical strength. The slot wedges behave as damper winding bars also under unbalanced operation of generator.

7.5 Retaining Ring :-

The overhang portion of field winding is held by non-magnetic steel forging of retaining ring against centrifugal forces. They are shrink fitted to the ends of the rotor body barrel at one end, while the other side of the retaining ring does not make contact with the shaft thus ensuring an unobstructed shaft deflection at the end winding and eliminating the chances of fretling corrosion.

To reduce stray losses, the retaining rings are made of non-magnetic, austenitic steel and cold worked, resulting in high mechanical strength.

7.6 Fans :-

The generator cooling gas is circulated by two single stage axial flow propeller type fans. The fans are shrink fitted on either sides of rotor body.


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7.7 Field Lead Connections :-

The field lead connections have the following components: -

7.7.1 Slip Rings :-

The slip rings consist of helically grooved alloy steel ring shrunk on the rotor body shaft and insulated from it. For convenience in assembly both the rings are mounted on a single common steel bush which has an insulating jacket premoulded on it. The complete bush with slip rings is shrunk on the rotor shaft.

7.7.2 Field Lead :-

The slip rings are connected to the field winding through semi flexible copper leads and current carrying bolts placed radially in the shaft made-up semi flexible of thin copper sheets silver plated and copper leads are made-up insulated by glass cloth impregnated, with epoxy resin for low resistance and ease of assembly. Two semicircular hard copper bars insulated from each other and from rotor shaft are placed in central bore of rotor joining two sets of current carrying bolts with special profiled precision conical threads.

The radial holes with current carrying bolts in the rotor shafts are effectively sealed to prevent the escape of hydrogen.


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The connection between current carrying bolts and fields winding is done by a field lead bar which has similar construction as that of semiflexible copper leads.

7.7.3 Brush Gear :-

The rotor winding is solidly connected to the slip rings by means of field lead bars, current carrying bolts, field lead core bar and flexible leads. The field current to the rotor winding is provided through Brush Gear .

7.7.4 Gas Cooler :-

Hot hydrogen is cooled by four gas coolers mounted longitudinally inside the generator stator body. Gas coolers consist of cooling tubes made of admiralty brass with coiled copped wire wound on them to increase the surface area of cooling. Cooling water flows through the tubes while hydrogen flowing across the cooler comes into contact with the external surface of the cooling tubes. The tubes are arranged in a staggered form so as to provide water tight joints.

Water chambers are bolted to the tube plates on either end through rubber gaskets. The outside flange of water chamber on slip ring side is elastically fixed to the stator body with the help of moulded rubber gasket to allow free expansion of cooler.

Whereas on the turbine side ,it is fixed rigidly to the stator. Flexible rubber sheets are provided at all four corners of the gas cooler which are pressedPEC University of technology Page 22

against the enclosure in stator frame to provide effective sealing for uncooled hydrogen from flowing past the cooler. End covers of water chambers are removable without purging the hydrogen from the generator.This enables cleaning of the tubes of coolers while the generator is running at partial load . The inlet and outlet water pipes to gas coolers are routed through inside the stator body and welded to start or end walls on turbine side. These tubes are connected to gas coolers by means of additional semicircular bent pipes.

In order to remove air from gas coolers while filling them with water ,vent pipes are provided on slip ring side .For a alignment of the coolers in the stator while insertion ,the bolts are provided at each end. The rollers in the gas cooler facilitate easy insertion of cooler into the stator frame.


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8. Operating Values of Generator :-

Sr. No. 1 2 3 4 5 6 7 8 9 10 11 12 13

Parameters Active power Reactive power(lag) Reactive power (lead) Generator voltage Load current Power Factor Frequency Excitation current on No Load Excitation current on Full load Excitation current on Field forcing Excitation voltage on No load Excitation voltage on Full load Excitation voltage on Field forcing

Max. Value 210 MW 172 MVAR 75 MVAR

15.75 K.V 9050 Amps 0.85 pf (lag) 50 Hertz

917 Amps 2600 Amps 3900 Amps 102 Volts 310 620 Volts Volts

8.1 Operating Limits of Generator :

1. The generator is capable of delivering 247 MVA, continuously at 15.75 K.V terminal voltage,9050 Amps of stator current and hydrogen gas pressure 3.5 kg/cm2 with hydrogen cold end temperature 44oC and distillate temperature at inlet of stator winding not exceeding 45oC . 2. The generator can develop rated power at the rated power factor .When the stator voltage varies + 5% the stator current be charged accordingly -+ 5%PEC University of technology Page 24

Terminal Voltage Output in MVA Stator current Amps

16.54 247 8598

15.75 247 9050

14.96 247 9053

3. In the extreme conditions of operation depending upon system requirements, the generator may be operated on +10% and accordingly the output may be 217 MVA (on +10% voltage) and 232 MVA (-10% voltage). While

operating generator with voltage variation more than + 5% frequency variation. 4. Turbo-generator is capable of operating continuously on unbalanced load provided that continuous negative sequence current shall not exceed 5% rated stator current. 5. Asynchronous operation of machine causes extra heating of rotor surface ,stator end zone and sometimes they are accompanied by extra surges in field winding .

In case of sudden loss of field machine operates as asynchronous generator and machine to over speed .During the failure of field ,active load on generator shall be decreased to 60% of the rated value within 30 seconds and to 40% in the following 40 sec. The generator can operate at 40% load asynchronously for a period of 15 minutes from the instance of field failure. Within this period, steps should be taken to restore the field . If excitation can not be restored within 15 minutes ,set should be taken out from the Grid.

Caution:


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The generator should not be operated under this asynchronous conditions as far as possible. The reduction of load and period should be ensured either by automatic control or other method. The power system should also permit such conditions.

8.2 Technical Data :-

Table 8.2 :- Technical Data for Generator : Electrical Data Generator Sr. Technial Particulars No. 1. Resistance of stator winding per phase at 20C 2. Resistance of rotor winding per phase at 20C 3. Capacitance of stotor winding to earth condition (Estimate value) 4. Line charging capacity 5. Basic impulse insulation level with respect to body 6. Basic impulse insulation level between turns

Values 0.00155 0.0896 0.72 F 75 MVAR 49000 V (peak) 49000 V (peak) 2.0 1.5 10 sec 4.0 ms 0.214 p.u. 0.179 p.u. 0.266 p.u.Page 26

Generator Excitation Data 1. Ratio of the ceiling voltage to the rated excitation voltage 2. Ratio of the field forcing current to the nominal current 3. Duration of field forcing 4. Response time Generator Reactances 1. Sub-transient direct axis reactance Xd (unsaturated) 2. Sub-transient direct axis reactance Xd (saturated) 3. Transient direct axis reactance X d (unsaturated)PEC University of technology

4. 5. 6. 7.

Transient direct axis reactance X d (saturated) Steady state direct axis reactance Xd (unsaturated) Steady state direct axis reactance Xd (saturated) Sub-transient quadrature axis reactance Xq (unsaturated) 8. Sub-transient quadrature axis reactance Xq (saturated) 9. Transient quadrature axis reactance X q (unsaturated) 10. Negative phase sequence X2 (unsaturated) 11. Negative phase sequence X2 (saturated) 12. Zero phase sequence X0 (unsaturated) 13. Zero phase sequence X0 (saturated) 14. Potier reactane Xp 15. Leakage reactance XL (stator) X1 16. Leakage reactance XL (rotor) X1 17. Zero sequence resistance R0 18. Effective surge impedance to neutral per phase 19. Synchronous impedance 20. Effective negative sequence resistance Generator Time Constants 1. Field time constant with open circuit stator winding (Tdo) 2. Field time constant with 3-ph short circuit stator winding (Td3) 3. Field time constant with 3-ph short circuit stator winding (Td2) 4. Field time constant with 3-ph short circuit stator winding (Td1) 5. Time constant of the aperiodic component for 3-ph s.c. (Ta3) 6. Time constant of the aperiodic component for 3-ph s.c. (Ta2) 7. Time constant of the aperiodic component for 3-ph s.c. (Ta1) 8. Time constant of the periodic component of the subtransient current for 3-ph, 2-ph, 1-ph short circuit Td 9. Direct axis sub-transient open circuit time constant (Tdo)PEC University of technology

0.234 p.u. 2.225 p.u. 2.05 p.u. 0.2454 p.u. 0.1969 p.u. 2.11 p.u. 0.252 p.u. 0.217 p.u. 0.105 p.u. 0.084 p.u. 0.213 p.u. 0.179 p.u. 0.0946 p.u. .00193 p.u. 70 2.06 0.02 7 sec 0.84 sec 1.38 sec 1.62 sec 0.29 sec 0.29 sec 0.24 sec 0.03 sec 0.04 sec

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10.

Direct axis sub-transient open circuit time constant (Tqo) 11. Direct axis sub-transient open circuit time constant (T qo) Short Circuit Currents 1. Sub-transient current of 3-ph short circuit 2. Transient current of 3-ph short circuit 3. Steady-state current of 3-ph short circuit Rated Parameters ( at the rated hydrogen pressure and cooling water temperature) 1. Maximum continous KVA rating 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. Maximum continous KW rating Rated terminal voltage Rated stator current Rated power factor Excitation current at MCR condition Excitation voltage at MCR condition Rated speed Rated frequency Efficiency at MCR condition SCR (short ckt ratio) Negative sequence current capability Direction of rotation when viewed from slipring end Phase connection No. of terminals

0.2 sec 2.5 sec

9.7 p.u. 3.8 p.u. 1.4 p.u.

247,000 KVA 210,000 KW 15,750 V 9050 A 0.85 lag 2600 A 310 V 3000 rpm 50 Hz 98.55 % .49 I22t = 8 Clockwise Double Star 9 (6 neutral & 3 phase)

4. Turbogenerator :-

4.1 Seal oil System ( Ring Type ) :PEC University of technology Page 28

Generator shaft seals are supplied with pressurized seal oil to prevent hydrogen escape at the shaft and the ingress of air into the generator . As long as , the seal oil pressure in the annular gap exceeds the gas pressure in the generator , no hydrogen will escape from the generator housing . The shaft seals are supplied with required seal oil from an exclusive closed loop oil circuit . The oil in the seal oil system is the same as that used in the Turbine/ Turbogenerator journal bearings and the turbine governing system. During normal operation A.C.Seal oil pump draws the seal oil from the seal oil tank and feeds it to the shaft seals through coolers and filters .The seal oil supplied to the shaft seals is drained towards hydrogen and air side through the annular gap between the shaft and the seal ring. The air side seal oil is returned directly in to the seal oil tank through a float valve. The oil drained on the hydrogen side first flows into the pre-chambers and then flows into the intermediate oil tank before returning into seal oil tank.

Most of the components of seal oil system are mounted on seal oil unit except pre-chambers and seal oil storage tank which form part of drain circuit. The centralised arrangement of main components in the seal oil unit provides for easy and simple operation /supervision of the seal oil system.

The seal oil in the seal oil tank is kept under vacuum to prevent deterioration of the hydrogen purity in the generator housing .The gases entrapped in the oil are thus removed so that the seal oil pumps draw largely degassed oil only . For oil circulation two 100% capacity screw type pumps, one A.C motor driven and other D.C. motor driven are provided. A vacuum pump is provided for maintaining specified vacuum in seal oil tank and is kept in operation when either of the seal oil pumps is running.PEC University of technology Page 29

Upon failure of normal A.C seal oil pump due to mechanical or electrical fault, the standby D.C. seal oil pump automatically takes over the supply. Upon failure of both the pumps, the seal oil is taken over by the turbine governing oil system without any interruption. The seal oil system is thus self contained. The additional facility for connection to the turbine governing oil system provides a high reliability since all measures aiming at a constant turbine oil supply are also available. This provision is not available in stage II & III. The seal oil is kept at a higher pressure then the gas pressure in the generator by specified differential for reliable functioning of shaft seals. With the seal oil pumps in operation, the seal oil pressure is controlled by differential pressure regulating valve (DPR)A. Depending on the differential pressure setting and the signal oil and gas pressures prevailing, a larger or smaller amount of oil is returned into the seal oil tank so that the required seal oil pressure is

established at the shaft seals. Due to the fact that the gas pressure and signal oil pressure act in opposite direction in the valve actuator, the valve stem is moved upwards or downwards when these pressures are no longer properly balanced. The valve cone is arranged so that the valve closes further at a downward movement of the valve stem (occurs at rising gas pressure or falling seal oil pressure). This oil flow throttling results in a rise of the seal oil pressure at the shaft seals. Setting of the desired differential pressure (set value) to be maintained by the valve is carried out by corresponding preloading of the mainbellows. The preloading is applied through a compression spring, the upper end of which is rigidly connected to the valve yoke, while its lower free end is attached to the valve stem by means of an adjusting nut.


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The two seal oil coolers each of 100% capacity are provided, one of which is always in operation , while the other serves as standby .The seal oil flow can be changed over from one cooler to the other by operation of associated isolating valves.

The seal oil filter duplex type is arranged directly after the seal oil coolers. It consists of 3x100% capacity mesh type filters and serves for screening any foreign particle entrapped in the oil and thus preventing any damage to seal babbitt and shaft. By means of change over valve assembly provided at the filters any one filter can be taken out of service for cleaning without any interruption of the oil flow to the shaft seals.

The oil drains from the shaft seals in two parts, one towards the hydrogen side and the other towards the air side. The oil drained towards the hydrogen side is at first passed into the prechambers at both ends. These prechambers serve for calming down the oil, permitting the escape of entrained gas bubbles and defoaming of oil. Downstream of the pre-chambers the two oil flows combine together and flow into the intermediate oil tank . The oil from the intermediate oil tank, is continuously returned into the seal oil tank together with the oil drained on the air side through float valves. One float valve provided on intermediate oil tank maintains a fixed level in this tank , so that the hydrogen in the drain lines does not escape into drain system .The float valve provided on the seal oil tank permits the in-flow of oil such that the level of oil does not go above high level. Any surplus oil which is not accepted by the seal oil tank is returned to the seal oil storage tank and finally to main turbine oil tank .In case of any short fall in the oil level in the seal oil tank the oil flows from the seal oil storage tank automatically and maintains the required constant level.


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The seal oil tank is evacuated of gases by a rotary vane vacuum pump. The vacuum pump is kept in operation by providing suitable interlock as long as either of the seal oil pumps is working.

When the shaft seal oil supply is obtained from the governing oil system the pressure is regulating by means of differential pressure regulating valve B This arrangement is available in stage-I&II, except for the valve seat and valve cone this valve is of the same design as A valve. Due to the different valve cone arrangement the valve B opens at a downward movement of valve stem (occurs at falling oil pressure in the seal oil system). The regulated oil further takes the same path to the shaft seals through the coolers and filters. Since the oil is not drawn from the seal oil tank the float valve of seal oil tank will not open and the drain oil will flow towards seal oil storage tank, from where it further flows into the main turbine oil tank. In this case, a slow deterioration of hydrogen purity in the generator will take place necessitating the scavanging of hydrogen to improve its purity.

The small amount of hydrogen entrapped with oil does not pose any danger to the generator surroundings, since the seal oil storage tank is provided with 2x100% duty vapour extractors to ensure continuous venting of this tank. Moreover an additional vapour extractor is provided on turbine oil tank to ensure its continuous venting.

To ensure the free floating condition of sealing ring in the seal body even at high machine gas pressures, the shaft seals are provided with ring relief oil. The ring relief supply is obtained from governing oil supply line through a simplex filter and is admitted on the side of both shaft seals.


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5. TURBO-GENERATOR STATOR WATER COOLING SYSTEM :One of the efficient ways of taking away the losses from the windings of any electrical machine is by direct cooling using water. The optimum design of a large capacity Turbogenerator, as a rule, envisages water-cooled stator windings. The 200 MW / 210 MW / 235 MW Turbogenerator employ a closed loop circulation of high quality deminderalised water through the stator windings made of hollow & solid conductors.

The generator is capable of delivering its rated load only when the stator watercooling system is functioning properly. Therefore, it is necessary that highest attention is paid for proper operation & maintenance of all the equipments in this system. The heat losses arising in the stator winding, main terminal bushings & phase connectors are removed by demineralised water coming into direct contact with high voltage winding. The cooling water must have an electrical conductivity of less than 2.5 micro mho/cm.

The Demineralized (DM) water supply system comprises of following main components:1) 2) 3) 4) 5) 6) Centrifugal pump 2x100% duty Distilled water coolers 2x100% duty Polishing unit Mechanical filter 1x100% duty Magnetic filter 1x100% duty Gas trap device


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7) 8) 9)

Expansion tank Water jet ejector Valves & associated instrumentation

5.1 Cooling of stator Water :-

The Cooling circuit makes use of either of the following water supplies , free from oxygen.

1. Distilled water 2. Fully Demineralised water from boiler feed water treatment plant. 3. Condensate

Fully demineralised water from the boiler feed water treatment plant condensate may only be used if no chemicals , such as ammonia ,hydrazine , phosphates etc. are present in the water or condensate . A part of water is by passed , and is treated in mixed bed ion exchanger, connected in parallel to the stator winding, magnetic filter and expansion tank and returned in to suction side of the water pump, thus, maintaining conductivity of closed loop circulating water within permissible limit.

The DM water pumps are of single stage type with spiral casing and overhung impeller. The pump is connected to a three phase AC Motor.


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Failure of the working pump due to fault or power supply failure results in an automatic change-over to standby pump the criterion for such change over is the falling water pressure is sensed by means of a pressure switch.

The heat absorbed by the DM water in the generator is dissipated to the secondary coolant in the primary water cooler.

5.2 Polishing Unit :-

The water Treatment Plant (Polishing Unit) provided across the stator winding, essentially comprises an exchanger tank filled with anion and cation resins in the form of a mixed bed ion exchanger.

The base substances of the exchanger resins are normally polymers containing groups of diverse character featuring exchange activity. In this way , the synthetic resins are capable of cation and anion exchange.

The cation exchanger contains highly acidic groups, while the anion exchanger comprises highly basic groups.

The exchanger resins are thus capable of accepting ions from the DM water, while releasing equipment amounts of other ions (hydrogen ions from the cationPEC University of technology Page 35

exchanger and hydroxyl ions from the anion exchanger) to the DM water at the same time. This exchange can only take place between equally charged ions. Therefore, cation exchanger exchange cation and exchanger exchange anion. This process takes place within a very short time while the DM water is passed through the ion exchanger.

A combination of highly acidic cation exchanger & highly basic anion exchanger constitutes a multi- tube of small demineralisation units resulting in a high purity deionate.

5.3 Detection of Hydrogen Leakage :

The mechanical filter eliminates any foreign particles in the water, which may choke, erode the hollow conductor. Difference of pressure at inlet and outlet of the filter is indicative of the degree of choking. The choked filter may be cleaned after using the stand by without affecting the system working. The magnetic filter prevents any magnetic particle from entering the generator.

Any accidental leakage of hydrogen into the main distilled water stream is detected by gas trap device. The water from the outlet of the stator winding collects in an overhead expansion tank which provides a constant level of water during normal running of the system. The hot water enters the tank through perforated pipe in the form of spray thus releasing any entrapped gas.

A water jet ejector is connected to expansion tank for creating vacuum for the purpose of removing any traces of oxygen/hydrogen gas which may be presentPEC University of technology Page 36

as a result of hydrogen leakage into the DM water stream. Level signaling device provided in the expansion tank monitors the high and low level of distilled water and initiates a tripping command for stator water pump at low level in the expansion tank. Make-up DM water to the system is provided at expansion tank through a float operated level regulator.

5.4 Flow Measurement :-

The quantity of DM water flowing though the windings is measured by a system of orifice plate, flow transducers, flow indicator and recorder. Signaling/ contacts are available in flow switch the indicator / recorder which are set to annunciate at low flow through the windings, and initiates tripping of the machine at emergency flow on the principle of two out of three. Continuous monitoring of the conductivity of water is done by conductivity cell and suitable indicator and recorder which annunciates alarm at conductivity high set value. At very high conductivity the unit is tripped automatically.

In addition to above, the system also provides for necessary instrumentation for indicating/ signaling of the temperatures and pressures at various points of the system. Pressure interlocking of the pumps is also provided.

5.5 Stator Water Pump Interlocks :PEC University of technology Page 37

Stator water for cooling the stator winding is circulated in a closed loop. To ensure the continuous generator operation, without any interruption two full capacity pumps are provided. The pumps have equal priority and are alternatively brought into operation, a change-over being possible at any time without interruption. The standby pump is kept ready for service and is automatically started in the event of the failure of stator water supply, which results in fall of the pressure below the set value in pressure switch installed in the circuit.

The pump is connected to 3 phase 415Volt A.C.Motor by a coupling covered by the coupling guard spacer. Water pressure normally should be 0.2 kg/cm2 less than the hydrogen pressure in generator casing . The stator water flow should be maintained at 25/27 M3/Hr.

5.6 Stator Water Coolers :-

The stator water cooler is shell and tube type heat exchanger suitable for vertical mounting, in which the D.M.water flows through the shell. Shell is made of non-magnetic stainless steel. Cooling water flows through the tubes. Tubes are made of stainless steel.

To have effective heat transfer between the coolant and cooled water , baffles made of brass are provided across the tube nest to guide the water to be cooled in a zing zap fashion. The cooling water is fed in and out of the cooler from the flannel provided at the bottom of the cooler. Four numbers of dial type

thermometers have been provided for the following purpose: PEC University of technology Page 38

1.- One thermometer each at inlet and outlet of primary water to measure inlet and outlet temperatures. 2.One thermometer each at inlet and outlet of secondary cooling water to measure the inlet and outlet temperature.

5.7 Stator Water Filter : -

The filter consists of cylindrical body having

separate inlet and outlet

chambers. A perforated cylindrical body having holes all along its length and periphery is surrounded by an assembly of 45 filter elements. The filter elements are kept pressed by the cover. The filter elements consist of sound corrugated brass sheet with a central hole and two sizes of wire mesh apertures, 0.244 mm and 0.5 mm held in one assembly by the inner and outer brass rings.

The material of construction of body cover, perforated cylinder is stainless steel whereas that of filtering elements is brass.

The water after passing through the filtering elements enters the perforated cylinder from where it flows to the outlet chamber. Mechanical impurities get collected on the wire meshes as the liquid passes through the mesh holes from the inlet chamber to the delivery chamber through the perforated cylinder. The filter is provided with vent and drain plugs. The normal pressure drop, across the filter corresponding to the rated flow is of the order of 0.1 to 0.2 kg/cm2 (g). As the particles get depositing on the wirePEC University of technology Page 39

meshes, the resistance to flow increases and consequently pressure drop across the water filter increases. When the pressure drop increases to 0.4 kg/cm2 (gauge) cleaning is recommended.

5.7.1 Technical Particles :Table 5.7.1 :- Technical Particles for Stator Water Filter 1. 2. 3. 4. 5. Type Rated discharge Maximum pressure rated Minimum size of particles that can be filtered Wire mesh type 30 M3/hr. 012 kg/cm2 0.224 mm Disc

Pressure drop across filter at which cleaning is 0.4 kg/cm 2 gauge recommended 0.1 to 0.2 kg/cm 2 gauge Stainless steel Brass 71.5 0.224 and 0.5 38 kg/cm2 (g) for 15 minutes

6. 7. 8. 9. 10. 11.

Pressure drop across filter during Normal running Material of body and perforated Cylinder Material of filtering elements Weight (Kg.) Size of wire mesh (Aperture) mm Hydraulic test pressure

5.8 Magnetic Filter :-

The magnetic filter is of permanent type designed to trap ferromagnetic (strong and average magnetic) particles, if present in the water system. The capacity of


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the magnetic filter is 36M3/hr.maximum strength of magnetic field in the circular air gap is of the order of 4000 gauss.

The filter mainly consists of an upper and a lower body , housing the permanent magnet system , comprising of the magnets , pole shoes and pole rings as shown in the figure.

The demineralised water before entering the winding passes through the magnetic filter, where it successively passes through two annular air gaps in

which ferromagnetic particles are filtered out because the later are attracted to the pole shoes and pole ring points in the zone high magnetic field strength. The filter is installed in the vertical position and is connected to the corresponding piping flanges by means of bolts.

5.8.1 Technical Particulars :Table 5.8.1 :- Technical Particulars for Magnetic Filter 1. 2. 3. 4. 5. 6. 7. 8. 9. Magnetic Filter type YO-36 Magnet type Permanent magnet Maximum induction density 4000 Gauss Capacity 36 M3/hr Weight 113 kg 2 Pressure rating (Kg/Cm ) Not more than 10 Type of particles, which can be Highly & medium filtered. magnetic Hydraulic test pressure 16 Kg/Cm2 for 10 min. Pressure drop Negligible


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5.9 Expansion Tank :-

The Expansion tank is hermetically sealed container made of stainless steel (see diagram). The DM water from the stator winding outlet header discharger in to a section of the tank through a perforated pipe. The tank is under a vacuum of 250-300 mm of mercury under the influence of water jet ejector. In case of stage-I & II and stage-III, the vacuum is created with the help of vacuum pump. In the process of the entrapped air/gas in DM water, entering the tank, if any, gets liberated and ejected out. The tank is provided with a made up connection from the power station DM Water source. The inlet of make up water in to the tank is controlled by means of a float operated level regulator, which opens or closes depending on the level in the tank.

The stator water pump draws water from the bottom of the tank for recirculation. The tank is provided with the following instruments / components:

i. - A gauge glass to indicate the level of the water in the tank. ii. -Two nos. of transducers for level signaling device to give high and low level alarm signals. iii. - A vacuum gauge to indicate vacuum inside the tank. iv. - A float operated level regulator to regulate the make up water. v. - A drain valve to facilitate draining of the complete tank in case of necessity.


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5.10. Maintaining Vacuum in Extension Tank :-

In Stage III, liquid Ring vacuum Pump is used to create vacuum in expansion tank. It also builds up vacuum stator water cooling system during starting. In case of stage I & II , the vacuum is maintained with the help of water jet Ejectors of Raw Water.

5.11. Gas Trap Device :-

Gas trap device is used for detecting the presence of hydrogen gas in the water employed for cooling stator windings. Appearance of hydrogen in the water stream is possible due to loose/leak of joints, as the water pressure in the winding is kept lower than the hydrogen pressure in the generator casing. This device is to be used continuously.

5.12. Predicted parameters of Stator Water System :Table 5.12 :- Predicted Parameters of Stator Water System 1. 2. 3. Nominal Gauge Pressure at inlet of stator 3.3 Kg/Cm2 windings Flow of distillate through stator winding 27 m3/hr. +3 m3/hr Nominal temperature of distillate at inlet to the 45 o C windingPage 43


For Stator Water Coolers 1. Primary Water inlet temperature 2. Primary water outlet temperature 3. Pressure drop in primary water 4. 5. 6. 7. Cooling Water flow Maximum cooling water inlet temperature Cooling water outlet temperature Pressure drop in cooling water

72.6 o C 45o C 6.5 MWC 110 m3/hr 36 o C 42.2 o C 0.8 MWC

6 Generator Gas System :-

6.1 Gas Scheme :The scheme is suitably designed to carry out the following principal functions: -

1.- Filling and purging of hydrogen safety without bringing it in contact with air. 2. -Maintaining gas pressure inside the machine at the desired value at all the times. 3. -Indicating to the operator at all the times the condition of the gas inside the machine, i.e. its Pressure , temperatures and purity.


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4. -Continuous circulation of gas inside the machine through gas driers in order to remove any moisture that may be present in it and keep the level of

moisture content below the maximum permissible limit. 5. - Indication of presence of liquid in generator housing.

6.2 Major Parts of the system :-

Following are the basic components of the Generator Gas System.

6.2.1 Gas Control Stand :-

Gas Control stand consists of hydrogen and carbon-di-oxide manifolds suitable mounted on stands with provision for holding and connecting hydrogen and carbon-dioxide cylinders. The hydrogen manifold is of tubular construction having provision for connecting the hydrogen cylinders to the manifold at a time. The hydrogen cylinders are provided with suitable pressure regulators to reduce the pressure of hydrogen supply. The manifold is filled with pressure relief valve, which is set to blow off at pressure exceeding 6 kg/cm2.

A pressure gauge is fitted to the manifold to indicate manifold pressure. Another pressure guage is also mounted at the stand to indicate the hydrogen gas pressure inside the generator casing.


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The carbon-dioxide manifold is similar to the hydrogen manifold and has provision for connecting seven carbon-dioxide cylinders at a time; the manifold has a relief valve set at 5.5kg/Cm2 pressure.

6.2.2 Hydrogen Gas Driers (Stage-I&II) :-

There are two in number the drying is done by the silica gel, which can be reactivated with the help of built in heaters.

The drier consists of a gas tight cylindrical body with silica gel in a perforated container at the top. A heater is provided in the central portion of the drier,

which is completely isolated from hydrogen zone. A thermostat is embedded which cuts off the supply to heater at 120oC automatically indicating thereby that the drier is ready for reuse, the colour of the silicagel can be observed through two glass windows provided at the top.

Normally out of the two driers, one remains in operation and other is under reactivation. The continuous circulation of hydrogen through drier is facilitated by the differential pressure created by fan mounted on generator shaft (turbine side) across which the drier is connected. The hydrogen from the inlet connection enters silicagel from bottom and moisture is given-up to silicagel while it passes through the dried hydrogen is led away to outlet connection from the outer annular space in the drier.


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As silica-gel becomes saturated i.e. it changes colour from blue to pink it needs re-activation. For this, the inlet and outlet valves of the drier are closed and the heater is switched on.

At this time other re-activated drier is taken in service by opening its outlet and Inlet valves. After re-activation, the silicagel becomes blue. The moisture which is collected inside the outer cylinder that trickles down to the bottom of drier is drained off through a drain for this purpose. When the temperature of silicagel reaches 120oC , the thermostat cuts off the supply to heater automatically indicating that the drier is ready for reuse.

6.2.3 Hydrogen Dryer Installed in Stage-III :-

In the stage-III the Hydrogen dryer of refrigerated type has been installed. It consists of a hermetically sealed Compressor, Condenser, Expansion Valves, Evaporator, and Separator system, Heat Exchanger, Automatic Derivator and Condensate Chamber. The function of hydrogen dryer is to extract moisture from circulating hydrogen gas in Turbo-generator, which is present in the form of the water vapour or is locally present as condensed water from surface gas cooler.

This consists of FREON 12 gas as refrigerant material & is in closed circuit. The hydrogen gas at a temperature of 40 to 60o o o

C is

fed into the hydrogen

dryer when speed of T.G. rotor is 3000 rpm, when it is chilled to the controlled temperature between 3 humidity to C to 6 C. At these dew temperature, the absolute becomes 5.9 gm/M3 & 7.3 gm/M3

hydrogen gas

respectively. This saturated hydrogen gas between 3o C to 6oC is passed throughPEC University of technology Page 47

the separator system where saturated water from hydrogen gas is removed. Condensate is automatically drained to the system by an automatic Derivator without letting hydrogen gas to go out into the atmosphere. Gas so dried is fed back to turbo-Generator. Principle of operation of this hydrogen dryer is shown in the flow Diagram.

CAUTION: -

When the TG Rotor is on barring gear, the supply of the hydrogen dryer should be switched off as there is no flow of hydrogen to the hydrogen dryer.

6.2.4 Liquid leakage Detector :-

The Liquid leakage detectors are used for detecting the presence of liquid such as oil/ water in the generator casing and the end shields. It also initiates signal for taking corrective action.

The liquid leakage detector consists of two main parts namely switch and the float chamber. The float assembly is provided with top and bottom flanged ends for direct mounting on pipelines. A sight glass is provided for visual indication of presence of liquid and its identification in the liquid leakage detector. Whenever the liquid comes in the float chamber, the position of the float changes due to which the contact in the switch is made which gives a signal in the control room indicating presence of liquid in the detector.


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These are three in number and installed near the seal oil system under the generator.

6.2.5 Control Panels :-

It is installed near the generator. The gas system control panel comprises of two panels:

i. - Gas Panel ii. - Hydrogen and Seal Oil Signaling panel

6.2.6 Equipment Mounted :-

i.- On Gas Panel : Gas analyzer receiving equipment. ii.-Hydrogen and seal oil signalling panel. y Gas Analyser indicator. y Manifold and Generator casing gas pressure indicators. y H2 Seal Oil differential pressure indicator. y Annunciation system equipment. y Multiplier relays, control devices etc. y Power Units. y Alarm Units.PEC University of technology Page 49

6.2.7 Hydrogen & Carbon Dioxide Gas Safety Valves :-

The Gas safety valves are designed for automatic relief when working pressure exceeds pre-determined values, the valves are tight with respect to external medium.

Safety valves are mounted on hydrogen and carbon dioxide manifolds and set at 6 and 5.5. Kg/Cm2 respectively so that in case pressure in the manifold exceeds the set limit, they blow off thereby preventing the gas system from getting subjected to high pressure. Another safety valves set at 4 Kg/Cm2 is provided on low-pressure side of pressure regulators. All the Safety valves are of spring loaded type.

6.3 TECHNICAL DATA :-

Table6.3 :- Technical Data of Hydrogen Gas

A .1. 2.

GENERATOR VOLUME & FILLING QUANTITIES Total volume of generator Carbon Dioxide required for 56 M3 expelling 120 M3 expelling 168 M3

hydrogen with unit at stand still. 3. Carbon dioxide required for

Hydrogen when unit is under rolling 4. Hydrogen filling quantity required with unit at 300 M3


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stand still. 5. Hydrogen filling quantity required when unit 336 M3 is under rolling 6. Quantity of compressed air required when unit 170 M3 is at stand still 7. Quantity of compressed air required when unit 200 M3 is running.

B .1. 2.

PRESSURE & PURITY OF HYDROGEN Nominal hydrogen pressure Permissible variation in Hydrogen pressure: 3.5 Kg/cm2 +0.2 Kg/cm2 3.3 Kg/cm2 3.7 Kg/cm2 97 % 1.2 % 15 gm/M3 of hydrogen. Should not be more than 30 %

a.- Minimum value b.- Maximum value 3. 4. 5. 6. Minimum purity of Hydrogen Oxygen content in hydrogen Moisture content in hydrogen Relative humidity content in hydrogen

7.

Cold gas temperature of hydrogen: Minimum Normal Maximum 20 o C 44 o C 45 o C 75oC

8.

Hot gas temperature value


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6.4 Gas Filling :-

1. Prior to operating the system, ensure the following: -

i. - The Generator along with the complete piping and other elements should have been certified to be gas tight. ii. Never use fire or an open flame (welding, flame, cutting, smoking etc. ) in the vicinity of the generator and the hydrogen system at any time even when starting with the preparations for generator filling. iii .- The Seal oil system should have been commissioned and should be in operation before charging the machine with carbon-dioxide for subsequently hydrogen gas filling. Centrifugal fan should be in operation. iv. - The power and control supplies necessary for operating the system should be available. v.- Do not perform any filling or purging operations with the generator at higher speeds than turning at barring-gear operation . Filling the generator can carried out with machine at rest or on barring gear while purging out of hydrogen may be done at rest or at any speed depending upon exigencies.

vi- Ensure that the hose between compressed air supply and valve A-1 is not connected. It should be connected only when compressed air from the station air system is required for removing the carbon dioxide from the generator or for performing a leakage test.

Caution: It is not permitted to run the generator in air except for drying out or balancing as per the regimes spelt out else where in the manual.PEC University of technology Page 52

2. In order to avoid the formation of an explosive mixture of hydrogen and air in the generator at any time either when the generator is being filled with hydrogen prior to being put into service or when hydrogen is being removed from the generator prior to opening the generator for repairs, carbon-dioxide is used as it is the inert gas.

2.1. Carbon-dioxide is admitted from carbon-dioxide manifold to the generator casing at the bottom by opening the valves of 2-3 cylinders at a time , the rate of admission being controlled by normally regulating the valves on the manifold as indicated by (CPG-I ) pressure gauge shoul not be more than 3 to 3.5 kg/cm2 In order to avoid excessive frosting on the pipe lines

connecting cylinders and manifold, the flow rate of carbon dioxide must be carefully regulated as otherwise the copper tubing might get chocked. The frosting if any can be removed by pouring warm water on the pipelines or simply by wiping with waste cotton.

2.2. Check zero setting of purging gas analyzer. Set range selector switch to position 0-100 % air in carbon dioxide. Open valve H-26 and close H-25.

2.3. - Filling with Carbon-dioxide may be terminated when a purity in excess of 95% oxygen in air (5% air in carbon-dioxide) has been reached. If the open carbon-dioxide bottles are not yet completely empty, the residual carbondioxide should also be admitted into the generator.

2.4. - Prior to starting with hydrogen filling, check to ensure that sufficient amount of carbon- dioxide is available for the next generator purge.


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3.

Start with Hydrogen filling.

3.1. Open close the valves as per schematic diagram (filling the system with hydrogen). Each Hydrogen cylinder is provided with pressure regulator to reduce pressure from 150 to 5 kg/cm2. Pressure after pressure reducers PR6/PR-7 must be set at rated pressure of 3.5 Kg/cm2.

3.2.- Turn range selection of purge gas analyzer (KT-2) to positions 0.100% Hydrogen in carbon- dioxide. Open valve H-25 and close H-26 increase in purity can be seen at the indicator since the purity meter system continues to measure the vent gas purity. In case the indicator shows Hydrogen purity about 85 % in carbon dioxide bring Kathero Meter KT-I in circuit. Close valve H-25 & open H-24. Continue hydrogen filling until indicator gives a reading of 98% Hydrogen in air close valve C-10

NOTE:- During the process of filling with hydrogen the carbon dioxide mixture is discharged through carbon mixture feed pipe.

3.3. - Check that the pressure switches PS-I to PS-5 are set at operating setting valves.

4. - Continue filling generator casing until required pressure of hydrogen is achieved.

5. - The Hydrogen purity meter cause a small gas loss since the measuring gas is vented to the atmosphere. For this reason, sufficient Hydrogen must always be available.


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6.5 Removal of Gas From Generator :-

1. - Prior to removing the Hydrogen from the generator housing the unit must be at stand still or on the turning gear. However in emergency conditions, hydrogen may have to be purged at coasting down of machine. 1.1. - The hydrogen pressure should be reduced to 0.2 Kg/Cm2 before admitting CO2 by opening valve. 2. - Hydrogen is purged out of the generator by admitting carbon dioxide.

2.1 Set purity meter KT-2 to position 0-100% hydrogen in carbon-dioxide by opening valve H-26.

2.2.- The valve setting for this operation should be done as per figure . The casing through the CO2 feed pipe and the hydrogen is discharged to the atmosphere through hydrogen feed pipeline.

2.3.- carbon dioxide should be admitted until a carbon-dioxide concentration in excess of 95% of carbon dioxide in Hydrogen ( 5% Hydrogen in carbon dioxide is obtained in the discharged gas.

2.4. - When the Carbon dioxide concentration in the carbon dioxide , hydrogen mixture reaches about 95% carbon dioxide filling can be terminated.


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3. - After terminating the carbon- dioxide filling, air filling can be started to expel the carbon dioxide.

3.1- Set purity meter KT-2 to position 0-100% air in carbon- dioxide by opening valve H-25 & closing H-26. 3.2- The compressed air should be admitted into the generator through valve A-1. 3.3- Check the percentage of air in carbon- dioxide when the analyzer shows 98% air in carbon dioxide, stop filling the air. 3.4- After about 15 minutes close all valves pertaining to the gas supply that are still open. With the generator rotor at standstill (Zero rpm) the seal oil supply to the shaft seals may like wise be shutdown.

6.6 Gas Drying ( With Duplex Dryer ) ( In case of Stage I & II only ) :-

1. - The cold gas from the generator flows through dryer inlet valve into the gas dryer chamber from where it passes through the absorbent material and then returns into the generator through dryer outlet valve. 1.1- The continuous circulation of hydrogen through the gas dryer is facilitated by the differential pressure created by the rotor fan on the turbine side. Normally one dryer is in operation while the other dryer is either under reactivation or ready for use after reactivation. 1.2- Usually the operator has to frequently observe the colour of the silica gel in the dryer under service through the glass provided on top of the dryer.


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1.3- Once the colour indicates saturation with mixture by changing from deep blue to pink, dryer has to be taken out of service and the standby dryer is to be switchedin as per the following sequence. 2. i .ii.Close the inlet and outlet valves of the working dryer. Open the inlet and outlet valves of the stand-by dryer. by the thermostat fitted in the dryer when the temperature of 120 oC is achieved. iv.Drain off the water collected . The dryer is again ready for reuse.

iii.- Switch on the heater of the first dryer for reactivation which is terminated

3.-

During the operation of the generator, the following parameters are to be closely watched and recorded;-

i.ii.iii.iv.-

Hydrogen pressure Hydrogen purity Cold Hydrogen temperature Presence of water / oil in the generator casing with the help of induction liquid indicator / liquid leakage detectors.

v.vi.-

Pressure of Hydrogen & Carbon dioxide in manifolds Operation of safety valves.

vii.- Differential pressure between seal oil and hydrogen. viii.- Condition of silicagel during operation of dryer and during re-activation of dryer. ix.Humidity of Hydrogen.

6.7 FAULT REMOVAL :PEC University of technology Page 57

1.0. - Hydrogen Consumption increases beyond permissible limits This may be due to the following:i. Gas tightness of machine or i.- check all the joints & valves for system got hampered. leakage and rectify the defective ones. Change the defective valves etc. if required, by suitable isolating it from the system. ii. - Leakage into gas cooler or stator ii. - In case abnormal leakage persists winding. take shut down for rectification of the leakage point after identification. iii. Float valve of intermediate oil iii.-Close the valve at the outlet of tank permit oil to flow out even intermediate oil tank. Allow the oil in closed position. level to rise up to the middle of the gauge glass. Then open the valve and regulate such that the level in IOT remains constant. Watch the level constantly and attend to the defect at the next immediate shut down. Note:- Immediately high order. Purge shut-down the

machine , if the consumption is of very Hydrogen and

identify leakage by air tightness test. 2. - Hydrogen Purity inside the casing This may be due to the following:i.Excessive flow of seal oil i.-Scavange by fresh hydrogen till the purity of not less than 97% is achieved.

towards hydrogen side. ii.-

Suction of air into the suction ii.- Restore tightness in the suction


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pipes of oil pumps. iii.- Faulty indication of purity meter.

pipes of pumps. iii.-Check instrument for calibration, zero setting and proper line resistance.

iv.- No indication on gas purity iv.-Check main voltage , fuses, check meter. electronic power block for simulated operation. Check that the voltage of receiver and indicating block are in phase. 3.0 Too Frequent Saturation of silicagel in Hydrogen dryer. This may be due to the following:Only) i.Less absorption capacity of still i.-Replace the silicagel crystals by fresh cagel crystals. ii.good quality silicagel. ( In case of Stage I & II

Leakage of water from gas ii.-Arrest water entry inside TG during coolers or stator winding operation of set or by taking shut down as appropriate for rectification of leakage points.

iii.- Water entry inside TG casing iii.-Check the functioning of vapour through shaft seals due to the extractors and periodically check the oil presence of moisture in seal oil quality for its moisture content . If moisture is present treat the oil for its properties given for lub oil system of Turbine. 4.0 Low Hydrogen pressure in Generator Casing This may be due to the following:i.Excessive Leakage at the i.-Restore Pressure by filling make quantity of hydrogen . In case pressure continues to all reduce loading of

generator or system piping


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machine and identify leakage source. For details, follow operating

instructions TG-6-00037-01 for leakage tests.

5.0 Reactivation of silicagel in gas Dryer not taking place ( For Stage I & II Only ) i.Loss of heater Voltage or burnt i.- Check & if required replace fuses / heater coil. ii.Failure in circuitory coil ii.- Investigate and identify the cause and ensure proper reactivation operation. 6.0.- Liquid in Generator casing Annunciation comes even when there is no liquid in the liquid leakage Detector.

i.-

Liquid

leakage

detector i.- Check for proper wiring and working of the relays etc.

malfunctioning 7.0.- Low Hydrogen manifold pressure.

i.-

The hydrogen cylinder connected i.- connect new H2 Cylinder. is empty.


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7. Static Excitation Equipment :7.1 Introduction :At present various types of excitation systems, such as conventional DC, High frequency AC, Static & Brushless are being adopted in India and abroad. The conventional DC exciter was the un-challenged source of generator Excitation for nearly fifty years till the rating of turbo-generators reached around 100 MW. In the last two decades, alternative arrangements have been widely adopted because of limitations of the DC exciters. With increase in generator ratings , it is no longer enough to consider the exciter as earlier. Instead the performance of the whole excitation system including the automatic voltage regulator and the response of the main generator have to be considered. Techno economic considerations, grid requirements, reliability and easy maintenance have become of prime considerations.

7.2 Types of Excitation Systems :Following are the types of excitation systems available: -

1. Conventional D.C :The earliest AC Turbine generators obtained their excitation supply from the Power Station direct current distribution system. Each machine had a rheostat in series with its field winding to permit adjustment of the terminal voltage and reactive load. This method was suitable for machine which needed small field power and low internal reactance. As generator sizes increased excitation power requirements also increased and it became increasingly desirable for each unit to be self sufficient for excitation and thus the shaft driven DC exciter was introduced.


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2. AC (High Frequency ) Excitation System :-

This system was developed to avoid commutator and Brush Gear Assembly. In this System, a shaft driven AC pilot exciter, which has a rotating permanent magnetic field and a stationary armature , feeds the DC field current of the main high frequency AC exciter through controlled rectifiers . The high frequency output of the stationary armature is rectified by stationary diodes and fed via slip-rings to the field of the main turbogenerator . A response ratio of about two can be achieved.

3.Brushless System :-

Supply of high current by means of slip rings involves considerable operational problems and it requires suitable design of slip rings and brush gear. In brushless excitation system diode rectifiers are mounted on the generator shaft and their output is directly connected to the field of the alternator, thus eliminating brushes and slip rings. This arrangement necessitates the use of a rotating armature and stationary field system for the main AC exciter. The voltage regulator final stage takes the form of a thyristor bridge controlling the field of the main AC exciter, which is fed from PMG on the same shaft. The response ratio of brushless excitation system is normally about two.

4. Static System :-


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In order to maintain system stability, it is necessary to have fast excitation system for large synchronous machines which means the field current must be adjusted extremely fast to changing operational conditions. Besides maintaining the field current and steady state stability the excitation system is required to extend the stability limits. It is because of these reasons the static excitation system is preferred to conventional excitation systems.

In this system, the AC power is tapped off from the generator terminal stepped down and rectified by fully controlled thyristor bridges and then fed to the generator field there by controlling the generator voltage output. A high control speed is achieved by using a free control and power electronic system. Any deviation in the generator terminal voltage is sensed by an error detector and causes the voltage regulator to advance or retard the firing angle of the thyristors there by controlling the field excitation of the alternator.

Figure shows a block diagram for a statistic excitation system. Static excitation system can be designed without any difficulty to achieve high response ratio, which is required by the system. The response ratio of the order of 3 to 5 can be achieved by this system.

This equipment controls the generator terminal voltage, and hence the reactive load flow by adjusting the excitation current. The rotating exciter is dispensed with and silicon controlled rectifier (SCRs) are used which directly feed the field of the alternator.

7.3 Components of Static Excitation System :PEC University of technology Page 63

The Static Excitation system installed at GGSSTP, Ropar has the following components:-

1.- Rectifier Transformer. 2.- SCR output stage 3.- Excitation start-up & field discharge equipment. 4.- Regulator and operational control circuits.

7.3.1. Rectifier Transformer :-

The excitation power is taken from generator output and fed through the excitation (rectifier) transformer, which steps down to the required voltage, for the SCR Bridge and then fed through the field breaker to the generator field. The rectifier transformer used in the SEE should have high reliability, as failure of this will cause shutdown of power station dry type cast coil transformer are suitable for static excitation applications. The transformer is selected such that it supplies rated excitation current at rated voltage continuously and is capable of supplying ceiling current at the ceiling excitation for a short period of ten seconds.

7.3.2 SCR Output Stage :-


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The SCR output stage consists of a suitable number of bridges connected in parallel. Each thyristors bridge comprises of six thyristors, working as a six pulse fully controlled bridge current carrying capacity of each depends on the rating of individual thyristor. Thyristors are designed such that their junction temperature is well within its specified rating. By changing the firing angle of the thyristors, variable output is obtained. Each bridge is controlled by one final pulse stage and is cooled by a fan.

These bridges are equipped with protection devices and failure one bridge causes alarm. If there is failure of one more thyristor bridge the excitation current will be limited to a predetermined value lesser than the normal current. However, failure of the third bridge results in tripping and rapid de-excitation of the generator.

7.3.3 Excitation start-up and field discharge Equipment :-

For the initial build-up of the generator voltage, field-flashing equipment is required. The rating of this equipment depends on the no-load excitation requirement and field time constant of the generator. From the reliability point of view, provisions for both the AC & DC field flashing are provided.

The field breaker is selected such that it carries the full load excitation current continuously and also it breaks the maximum field current when the three-phase short circuit occurs at the generator terminals.


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The field discharge resistor is normally of non-liner type for medium and large machines i.e. voltage dependent resister.

To protect the field winding of the generator against over voltages, an over voltage protector along with a current limiting resistor is used to limit the over voltage across the field winding. The OVP operates on the insulation break over principle. The voltage level at which OVP should operate is selected based on insulation level of field winding of the generator.

7.3.4. Control Electronics :-

Regulator is the heart of the System. This regulates the generator voltage by controlling the firing pulses to the thyristors.

7.3.4.1 Error Detector & Amplifier :-

The Generator terminal voltage is stepped down by a three phase PT and fed to the AVR. The A.C input thus obtained is rectified filtered and compared against a highly stabilized reference value and the difference is amplified in different stages of amplification. The AVR is designed with highly stable elements so that variation in ambient temperature does not cause any drift or change in the output level. Three CTs sensing the output current of the generator feed proportional current across variable resistors can be added vectorially either for compounding or for transformer drop compensation.


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7.3.4.2 Grid Control Unit :-

The output of the AVR is fed to a grid control unit; it gets its synchronous a.c. reference through a filter circuit and generates a double pulse spaced 60o apart whose position depends on the output of the AVR, i.e. the pulse position varies continuously as a function of the control voltage. Two relays are provided by energizing, which the pulses can be either blocked completely or shifted to inverter mode of operation.

7.3.4.3 Pulse Amplifier :-

The pulse output of the Grid Control Unit is amplified further at intermediate stage amplification. This is also known as pulse intermediate stage. The Unit has a D.C. power supply which operates from a three phase 380 V supply and delivers + 15V, - 15 V, + 5V and a coarse stabilized voltage V1 . A built in the relay is provided which can be used for blocking the 6 pulse channels. In a twochannel system (Like Auto and Manual), the change over is effected by energizing de-energizing the relay.

7.3.4.4 Pulse Final Stage :-

This Unit receives input pulses from the pulse amplifier and transmits them through pulse transformers to the gates of the thyristors. A built in power supply provides the required D.C supply to the final pulse and amplifier. Each thyristor bridge has its own final pulse stage. Therefore, even if a thyristor bridge failsPEC University of technology Page 67

with its final pulse stage, the remaining thyristors bridges can continue to cater to full load requirement of the machine and there by ensure (n-1) operation.

7.3.4.5 Manual Control Channel :-

A separate manual control channel is provided where the controlling D.C signal is taken from a stabilized D.C voltage through a motor operated potentiometer, the D.C signal is fed to a separate grid control unit whose output pulses after being amplified at an intermediate stage can be fed to a separate grid control unit whose output pulses after being amplified at an intermediate stage can be fed to the final pulse stage. When one channel is working , generating the required pulses, the other remains blocked. Therefore, a changeover from Auto to Manual control or vice-verse is

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Thermal Plant Ropar 1260MW