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A.C. GENERATORWORKING PRINCIPLE:An A.C. generator (alternator) is a synchronous machine whose rotor must rotate at synchronous speed. This is a speed corresponding to the frequency of the A.C. supply. Its main purpose is to convert mechanical energy to electrical energy.In its simplest form it consists of a set of magnet poles, a coil of wires called as armature winding and a prime mover.The magnetic field structure can be a permanent magnet or an electro-magnet. In case of electromagnets there is a D.C. excitation system with suitable excitation arrangement. Usually field structure forms the rotating member of the machine and is rotated at synchronous speed by the prime mover.

As the magnetic pole rotates, the flux through the winding of armature coil is made to vary with time. This apparent sinusoidal variation of the flux through the winding, as the magnet rotates is obtained by proper shaping of the field poles. This change is flux linkage through the coil as the magnet rotates, generates an alternating voltage within the coil. The magnitude of this induced voltage is proportional to the number of turns of wires in the coil and the rate of change of flux through the winding. It lags the flux by 90. The mathematical relationship that expresses this behavior is called Faradays Law:e=N (d/dt) Voltse = NW Sin WtWhere: e = generated VoltageN= Number of turnsw = Angular Velocity of rotor d/dt= rate of change of flux linkage = flux linkage = Nm, Cos wt(-) sign indicates that induced voltage e is in a direction to oppose d /dt

The induced voltage can be increased by :Increasing the speed of rotation which in turn increases the d /dt.changing the strength of field (changing the field current) to change . increasing the number of turns.

Where f is the frequency in cycles/sec. or hertz.f = (Ns x P) / 120Where:Ns = Synchronous speedP = No. of poles

Three Phase Generator: A three phase generator has three separate but identical armature windings displaced from other by 120 electrical degrees in space. The induced phase emfs have a phase difference of 120, which appears at the terminals of the machine. The generated EMF can be given by:ea = Em Sin wteb=Em Sin(wt -120).ec= Em Sin(wt +120).Em= 2f Kg NPhm Volts/phaseErms= 4.44 f Kg NPhm Volts/phaseWhere: Kg = Generator constant depends on arrangement of armature winding.NPh = Number of series connected turns/phaseConstructional Aspects of AC Generator:The basic construction of an AC generator involves the stator components, the rotor components and the cooling system.Stator Components:The stator components consists of the stator core, core frame, stator winding, stator winding support, stator terminal bushings, stator winding cooling components and stator casing.Core frame is a fabricated structure of steel. The stator core is built in these frame supported at the back by the core plate assembly. The core is held in assembled and pressed conditions by non-magnetic core end plates, with separate non-magnetic fingers to support the teeth.

The stator core is built-up of CRGO silicon sheet steel laminations of 0.35mm to 0.5mm thickness. The core plates are cut to sector shaped segments and assembled in the core frame and located on dove tail shaped keys on the inside periphery of the frame. Stator slots and axial ventilation holes are punched on the laminations. The stator core is assembled in packets, with radial gaps in between created by steel spacers to provide path for radial cooling.

The insulated stator laminations reduce the eddy current losses in the core due to radial flow of fluxes. The leakage fluxes that flow axially along the core results in eddy heating of core, burning and welding of core lamination. To prevent this, the outer surfaces of core end plates are covered by conducting screens of copper or aluminum. Leakage fluxes impinging on these screens, set up circulating currents within them which prevent penetration of flux into the core axially. The core end plate assembly carries the end winding support structure.

The armature winding is housed in the slots of the armature core and held in position by insulating wedge. In 3 phase alternator there are three sets of windings one for each phase. They are displaced in space by 120 to produce 3-phase voltage at the terminals with 120 phase difference in time axis from each other.The armature winding can be connected either in Star or Delta connection. For star connection the starting ends of the 3-phases are joined together and brought out to neutral terminal and the other ends a, b & c are connected to the other phase terminals in the terminal box of the machine.For Delta connecting a & c, c & b and b& a are connected together and terminal leads are brought out from their junctions.The relationship between phase and line values for current and voltage in star and Delta connection is as given below.

ConnectionCurrentVoltageY (Star)IL = IphVL = 1.732 VphD (Delta)IL = 1.732 IphVL = Vph28The Stator has a 3 phase double layer short chorded bar type windings having two parallel paths. The conductors are made of high conductivity hard drawn copper. The copper conductors are of rectangular cross section and are made up of no of parallel strips, in order to minimize eddy current losses within the paralleled conductors, the conductor strips are transposed in the slot portion. The conductors in each coil side are glass insulated. Cooling water is passing through the solid and hollow copper conductors. To eliminate corona between slot wall and bar insulation conducting varnish (Alkali resin base with graphite mixture) is applied to the straight part of the winding bar.Terminal bushings:Water cooled terminal bushings are housed in the lower part of the stator on s1ip rings side. Porcelain insulators are provided to insulate the terminal bars from the stator body. Effective sealing is provided between terminal bushings and the stator body to avoid leakage of hydrogen. Terminal bushings are housed inside a chamber made of non-magnetic steel plates, 3-phase terminals and six neutral terminals are brought out to facilitate external connections.RotorThe rotor of an alternator which carries field windings can be of two types.a)Salient Pole Typeb)Smooth Cylindrical Type (Round Rotor Type)

Two-pole synchronous generators: (a) salient pole (b) cylindrical rotor constructionSalient Pole TypeLow and medium speed alternators (as water wheel driven) have large number of protruding (salient) poles. Their cores are bolted to a cast - iron or steel wheel. Such rotors usually have large diameter and short axial length. The poles and pole shoes are laminated to minimize heating due to eddy currents.The field winding is made out of rectangular Copper strips wound on the poles.Smooth Cylindrical Type (Round Rotor Type)The turbo generator, which runs at a very high speed, the rotor consists of smooth solid forged steel cylinder with slots milled along the periphery (parallel to the shaft) to accommodate field coils. They are designed for 2 or 4 poles and the regions of central pole axis are left un-slotted. These poles are non salient. To reduce peripheral velocity such rotor has small diameter and large axial length. Such rotor gives: (1) Quieter Operation (2) Less Windage losses.

Field WindingConcentric type field winding is employed. The winding is made from hard drawn silver bearing copper (0. 1% to 0. 6%).Addition of silver with copper improves the creep properties of copper thus avoid copper shortening. Rotor winding is held in position against centrifugal forces by duraluminium wedges in the slot portion and non-magnetic steel retaining rings in the overhand portion.

Copper segmented type damper winding is provided in the end zones of rotor in order to prevent overheating of retaining rings during asymmetrical and asynchronous operations (example unbalance loading, double phase to ground faults etc)

Retaining RingsThe end windings are held in position against centrifugal forces by these rings. The rings are non-magnetic to reduce stray loss due to leakage fluxes.

Slip RingsThe slip rings consist of helically grooved alloy steel rings shrunk on rotor shaft and insulated from it. Both the rings are mounted on a single, common steel bush, which has an insulated jacket pre-moulded on it. The complete bush with slip rings is shrunk on the rotor shaft. The slip ring surface is grooved and drilled to improve its surface cooling.

Field leadsThe slip rings are connected to the field winding through semi-flexible copper leads and current carrying bolts placed radially in the shaft. Two Semi-circular hard copper bars insulated from each other and from rotor shaft are placed in the 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 shaft are effectively sealed to prevent the escape of hydrogen.

42Brush GearIt is arranged with several brushes and brush holders. Brush holders are fixed on brass rings. The brushes are spring loaded to maintain required contact pressure. The design of brush gear permits replacement of the brushes during normal running condition. The current carrying brass rings are rigidly fixed to the bearing pedestal and insulated from it. Provision is made on these rings for connecting cables from exciter.BearingsGenerator bearings are of pedestal type, bearing bush is of cast steel lined with metal Oil under pressure is supplied as lubricant. Provision is also made to supply jacking oil at high pressure to facilitate rotation by the barring gear. To prevent shaft current slip ring side bearing and connectin8 pipes are insulated from earth.Damper WindingsIn addition to armature and field winding, alternators have damper windings. These windings which reassemble to cage rotor of induction motor, is provided at the outer periphery of rotor. Slots are made in pole shoes for receiving copper bars of damper winding. The Cu-bars are short circuited at both the ends by heavy Cu-rings. These dampers are useful in preventing hunting (momentary speed fluctuation) and to provide starting torque.

Whenever the speed of alternator is other than synchronous speed, such as during starting or at the time of transients, EMF is induced and hence current is induced in the short circuited damper winding due to induction effect.This produces torque to restore the synchronous speed.The solid field poles of turbo generator act as efficient dampers, Hence they do not have damper windings (except in special case to assist in synchronizing).

Generator Cooling:Energy conversion by an electrical M/c involves the loss of some power in coils, cores and structural past, which is dissipated as heat. To avoid overheating and damage to the machine especially to insulation, this heat must be abstracted and discarded outside the machine. This purpose is served by a cooling system and a cooling agent.Coolant is circulated in sufficient rate and quantity. The size of the machine, its rating and insulating material used are some of the main factors deciding the type of cooling to be used. Various insulators have different operating temperature limits, which is very important data while designing the cooling system.

In turbo generators of small capacity of 600MW.

Generator of large rating >600 MW water cooling for both stator and rotor is provided.Whenever generator cooling system fails, generator has to be unloaded to a lower power rating.

Shaft SealsSince hydrogen is used as coolant in large turbo-generators, to prevent escape of hydrogen along rotor shaft, seals are provided at both end shields. Shaft seal consists of seal liner pressing against collar on the shaft, Seal liner is enclosed in a seal body. Sealing is achieved by means of seal oil flow at higher pressure than H2 between seal liner by means of thrust oil pressure pumped into a chamber between seal body and the liner. The seal body, oil pipe lines etc. on the exciter side are insulated from earth to prevent the circulation of shaft currents.Excitation SystemThe arrangement provided to supply direct current required to energize the field winding of a synchronous machine is known as Excitation System.ClassificationThe excitation system can be classified as:Rotating armature type Excitation using direct current generator.Rectifier type Excitation using silicon diodes and thyristors.Rotating Machine type Excitation Systema) Direct Type:In this type of excitation the exciter (D.C. Generator) is directly coupled to the shaft of the synchronous machine. The D.C. output of the excitation is fed to the main field by brush and slip ring assembly.b) Indirect Type:In this the exciters rotor is coupled to a separate motor rather than to the shaft of main generator. From reliability considerations the turbo generator should have direct excitation system. But large machine needs large rating of excitation. It cannot use d.c. generator due to commutation problems at high speed.

Rectifier Type Excitation Systems:Self Excitation SystemThe energy required to excite the synchronous machine is drawn from main generator output via, a transformers as a.c. Which is rectified by rectifiers and supplied to the main generator field via, slip ring and brush assembly.

Separate Excitation SystemIn this system the energy required to supply the field winding comes from an a c 3 phase generator mounted on shaft of the main generator This a c is converted to d c with the help of rectifier bridge and supplied to the main generator field thro slip ring and brushes. The exciter itself may get its supply from a permanent magnet pilot exciter.Brushless Excitation SystemThe main Exciter is an a.c. generator with its armature coil on rotor and field winding on stator. The a.c. output of armature winding is rectified to d.c. by shaft mounted rectifier bridges (rotating along with shaft) and supplied to the main generator field. Thus eliminating the slip ring and brush assembly.

VOLTAGE REGULATION:If an alternator is operated at a constant speed with a fixed field excitation current, the terminal voltage will change with an increase in load current. The actual amount of change will depend on the load p.f of the circuit and the impedance of the armature winding. This change in terminal voltage from full load to no-load at a constant speed and at fixed field current is termed as voltage regulation.Percent voltage regulation of an alternator is defined as the percentage change in terminal voltage as the load current is changed from full load to zero value with speed and excitation remaining constant No-load Voltage - Full load Volt.%V.R.= ---------------------------------------- x 100 Full Load VoltAs an alternator is a constant speed M/C in order to maintain fixed terminal voltage the field excitation has to be changed. In small M/Cs it is achieved by inserting a rheostat in series with field circuit. But in case of large power rating M/C it becomes uneconomical. Hence various other methods are used Normally Auto-Voltage Regulator or Manual voltage regulators are used for voltage regulation in large M/Cs.

Parallel Operation of Generators Conditions required for parallelingThe rms line voltages of the two generators must be equal.The two generators must have the same phase sequence.The phase angles of the two a phases must be equalThe frequency of the new generator, called the oncoming generator, must be slightly higher than the frequency of the running systemCharacteristic CurvesSaturation Curve or Open Circuit CharacteristicsIn an alternator, for constant speed the generated voltage depends on the field flux. As field flux is directly proportional to the field current, a curve drawn between generated voltage and field current (with generator on no-load) gives the open circuit characteristics of generator.Initially as the field current increases, the flux increases but when magnetized iron core becomes saturated any increase in field current results in very less or no increase in flux, hence no further increase in generated voltages, It is called as saturation point and the curve is also called as saturation or magnetizing curve.

Short Circuit CharacteristicsThis is obtained by running the machine at its rated speed with its terminals short-circuited and observing the current by ammeter while excitation (F) Ampere - turns/phase is varied from zero to an amount sufficient to circulate 125% to 150% of full load current in the stator.It is a straight line (Fig 1.20)To determine an approximate value of synchronous impedance ZS = XS at a given field current. Get the internal generated voltage EA from 0CC Get Short circuit current ISC at that field current from SCC Find Xs = EA/ISC.

Power and Torque in Synchronous GeneratorA Synchronous generator is a synchronous machine used as a generator. It converts mechanical power to 3 phase electric power. The source of mechanical power may be a diesel engine, a steam turbine or water turbine.Not all the mechanical power going into a synchronous generator becomes electrical power out of machine. The difference between output power and input power represents the losses of the machine.The power flow diagram for the synchronized generator is shown.The input mechanical power is the shaft power in the generator

Short Circuit Ratio: It is the ratio of field current required for rated voltage at open circuit to the field current required for rated armature current at short circuit. It is reciprocal of per unit XS.

Synchronous Generator ratingsThe purpose of the ratings is to protect the generator from damage due to improper operation. A machine has a number of ratings listed on a name plate attached to it.Typical ratings on a synchronous machine are voltage, frequency, speed, apparent power (kilo-volt) amperes, power factor, field current. These ratings are interrelated.FrequencyThe rated frequency of synchronous generator depends on the power system to which it is connected. Once frequency is known, there is one possible rotational speed for a given number of poles given by the equation:f = PN/120where P = No. of poles N = Speed in rpm

VoltageA Generators voltage depends on the flux, the speed of rotation and mechanical construction of the machine. For a given frame size, speed, higher the desired voltage, the higher the machines required flux. However, flux cannot be increased forever, since there is always a maximum allowable field current.

Daily ChecksCleanliness of generator surroundings.No oil, dust on insulation of bearings, oil seals, slips rings and brush gear.Check H2 gas consumption. If abnormally high, check for leakage.Check the gas sample from dome type H2 leak detector, Mounted on the water header purge line to check H2 leak into water circuit.Check brush-gear for unusual sparking between brushes and slip ringsChattering of brushesDust and Oil accumulationCut pig-tailsGenerator rotor to ground leakage current (not more than 30 A).Maintenance, checks during operation of TGCheck rotor leakage current. If it is more blow with pressurized air and recheck until value comes down to the order of less than 30 A.Replace worn out brushes if required.Check brushes are sliding freely in their holders.Adjust brush pressure on the brushes so that it is uniform on all the brushes and is about 1.2 to 1.3 Kg/cm2 on each brush.Checks to be done before overhaulingAmplitude of vibration in all 3 directions i.e. axial, vertical and transverse.At maximum loadAt no load with rated stator volts across the generator terminals.At rated speed of the machine without excitation.General condition of the insulation of bearing and the shaft seal at exciter side.Impedance of rotor winding whenMachine running at 3000 rpmMachine is Stationary

Resistance measurement of rotor and stator windings at cold stage.Insulation resistance ofStator winding between phases and phase to groundRotor windingThank you