17 Generator and Generator Transformer Protection

1 Winding winding Protection ofdirect-connected Protection of Generator Transformer earth fault forward power/reversepower against inadvertent energisation Protection faults of excitation Protection slipping Protection faults Generator Protection generation of Generator Protection 17 Generator andGenerator Transformer INTRODUCTIONThe core of an electric power system is the the exception of emerging fuel cell and solar-celltechnology for power systems, the conversion of thefundamental energy into its electrical equivalentnormally requires a 'prime mover' to develop mechanicalpower as an intermediate nature of this machine depends upon the source ofenergy and in turn this has some bearing on the designof the Generator .

2 Generators based on steam, gas, wateror wind turbines, and reciprocating combustion enginesare all in use. Electrical ratings extend from a fewhundred kVA (or even less) for reciprocating engine andrenewable energy sets, up to steam turbine setsexceeding and medium sized sets may be directly connectedto a power distribution system. A larger set may beassociated with an individual Transformer , throughwhich it is coupled to the EHV primary transmissionsystem. Switchgear may or may not be provided between thegenerator and Transformer .

3 In some cases, operationaland economic advantages can be attained by providinga Generator circuit breaker in addition to a high voltagecircuit breaker, but special demands will be placed onthe Generator circuit breaker for interruption ofgenerator fault current waveforms that do not have anearly zero unit Transformer may be tapped off theinterconnection between Generator and Transformer forthe supply of power to auxiliary plant, as shown inFigure The unit Transformer could be of the orderof 10% of the unit rating for a large fossil-fuelled steamset with additional flue-gas desulphurisation plant, butit may only be of the order of 1% of unit rating for ahydro set.

4 17 Generator andGenerator- Transformer ProtectionNetwork Protection & Automation Guide 281 Industrial or commercial plants with a requirement forsteam/hot water now often include generating plantutilising or producing steam to improve overalleconomics, as a Combined Heat and Power (CHP)scheme. The plant will typically have a connection to thepublic Utility distribution system, and such generation isreferred to as embedded generation. The generatingplant may be capable of export of surplus power, orsimply reduce the import of power from the Utility.

5 Thisis shown in Figure modern generating unit is a complex systemcomprising the Generator stator winding, associatedtransformer and unit Transformer (if present), the rotorwith its field winding and excitation system, and theprime mover with its associated auxiliaries. Faults ofmany kinds can occur within this system for whichdiverse forms of electrical and mechanical Protection arerequired. The amount of Protection applied will begoverned by economic considerations, taking intoaccount the value of the machine.

6 And the value of itsoutput to the plant following problems require consideration from thepoint of view of applying electrical electrical of of of prime oil in expansion between rotating andstationary lamination Generator EARTHINGThe neutral point of a Generator is usually earthed tofacilitate Protection of the stator winding and associatedsystem. Earthing also prevents damaging transientovervoltages in the event of an arcing earth fault HV generators, impedance is usually inserted in thestator earthing connection to limit the magnitude ofearth fault current.

7 There is a wide variation in the earthfault current chosen, common values (low impedance earthing) (high impedance earthing)The main methods of impedance-earthing a generatorare shown in Figure Low values of earth faultcurrent may limit the damage caused from a fault, butthey simultaneously make detection of a fault towardsthe stator winding star point more difficult. Except forspecial applications, such as marine, LV generators arenormally solidly earthed to comply with safetyrequirements.

8 Where a step-up Transformer is applied, 17 Generator and Generator - Transformer ProtectionNetwork Protection & Automation Guide 282 GeneratorMain transformerHVbusbarsUnit transformerAuxiliarysupplies switchboardFigure : Generator - Transformer unitUtilityPCCI ndustrial plantmain busbarPlant feeders - totaldemand:xMWWhen plant Generator is running:If y>x, Plant may export to Utility across PCCIf x>y, Plant max demand from Utility is reducedPCC:Point of Common CouplingGeneratorRating: yMWFigure : Embedded generationthe Generator and the lower voltage winding of thetransformer can be treated as an isolated system that isnot influenced by the earthing requirements of thepower earthing Transformer or a series impedance can beused as the impedance.

9 If an earthing Transformer isused, the continuous rating is usually in the range 5-250kVA. The secondary winding is loaded with a resistorof a value which, when referred through the transformerturns ratio, will pass the chosen short-time earth-faultcurrent. This is typically in the range of 5-20A. Theresistor prevents the production of high transientovervoltages in the event of an arcing earth fault, whichit does by discharging the bound charge in the circuitcapacitance. For this reason, the resistive component offault current should not be less than the residualcapacitance current.

10 This is the basis of the design, andin practice values of between 3-5 Icoare is impw (u83 18 is is t0/ G(e used.)TP is is t0/ l)' r704 Tm 999 TD nted; os irwisea vaery nd esirablConventional Generator Protection systems would beblind to an interturn fault, but the extra cost andcomplication of providing detection of a purely interturnfault is not usually justified. In this case, an interturnfault must develop into an earth fault before it can becleared. An exception may be where a machine has anabnormally complicated or multiple windingarrangement, where the probability of an interturn faultmight be STATOR WINDING PROTECTIONTo respond quickly to a phase fault with damaging heavycurrent, sensitive, high-speed differential Protection isnormally applied to generators rated in excess of large generating units, fast fault clearance will alsomaintain stability of the main power system.