17-Generator and Generator Transf Prot

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    Introduction 17.1

    Generator earthing 17.2

    Stator winding faults 17.3

    Stator winding protection 17.4

    Differential protection of direct-connected generators 17.5

    Differential protection of generatortransformer units 17.6

    Overcurrent protection 17.7

    Stator earth fault protection 17.8

    Overvoltage protection 17.9

    Undervoltage protection 17.10

    Low forward power/reversepower protection 17.11

    Unbalanced loading 17.12

    Protection against inadvertent energisation 17.13

    Under/Overfrequency/Overfluxing protection 17.14

    Rotor faults 17.15

    Loss of excitation protection 17.16

    Pole slipping protection 17.17

    Overheating 17.18Mechanical faults 17.19

    Complete generator protection schemes 17.20

    Embedded generation 17.21

    Examples of generator protection settings 17.22

    1 7 G e n e r a t o r a n d G e n e r a t o r T r a n s f o r m e r P r o t e c t i o n

    Chap17-280-315 17/06/02 10:43 Page 280

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    17.1 INTRODUCTION

    The core of an electric power system is the generation.With 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 stage.

    The nature of this machine depends upon the source of energy and in turn this has some bearing on the designof the generator. 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 1200MVA.

    Small 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. 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 of generator fault current waveforms that do not have an

    early zero crossing.A unit transformer may be tapped off theinterconnection between generator and transformer forthe supply of power to auxiliary plant, as shown inFigure 17.1. 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.

    17 Generator and

    Generator-Transformer Protection

    N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e 2 8 1

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    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 is

    referred to as embedded generation. The generatingplant may be capable of export of surplus power, orsimply reduce the import of power from the Utility. Thisis shown in Figure 17.2.

    A 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 of many kinds can occur within this system for whichdiverse forms of electrical and mechanical protection are

    required. The amount of protection applied will begoverned by economic considerations, taking intoaccount the value of the machine, and the value of itsoutput to the plant owner.

    The following problems require consideration from thepoint of view of applying protection:

    a. stator electrical faultsb. overload

    c. overvoltage

    d. unbalanced loading

    e. overfluxing

    f. inadvertent energisation

    e. rotor electrical faults

    f. loss of excitation

    g. loss of synchronism

    h. failure of prime mover

    j. lubrication oil failure

    l. overspeeding

    m. rotor distortion

    n. difference in expansion between rotating andstationary parts

    o. excessive vibration

    p. core lamination faults

    17.2 GENERATOR EARTHING

    The 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 orferroresonance.

    For HV generators, impedance is usually inserted in thestator earthing connection to limit the magnitude of earth fault current. There is a wide variation in the earthfault current chosen, common values being:

    1. rated current2. 200A-400A (low impedance earthing)

    3. 10A-20A (high impedance earthing)

    The main methods of impedance-earthing a generatorare shown in Figure 17.3. 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. Where a step-up transformer is applied,

    17

    G e n e r a t o r a n d G e n e r a t o r - T r a n s f o r m e r P r o t e c t i o n

    N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e 2 8 2

    Generator Main transformer

    HV busbars

    Unit transformer

    Auxiliarysupplies switchboard

    Figure 17.1: Generator-transformer unit

    Utility

    PCC

    Industrial plantmain busbar

    Plant feeders - totaleman : xM

    When plant generator is running:If y>x, Plant may export to Utility across PCCIf x>y, Plant max demand from Utility is reduced

    PCC: Point of Common Coupling

    GeneratorRating: yMW

    Figure 17.2: Embedded generation

    Chap17-280-315 17/06/02 10:44 Page 282

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    the generator and the lower voltage winding of thetransformer can be treated as an isolated system that isnot influenced by the earthing requirements of thepower system.

    An earthing transformer or a series impedance can beused as the impedance. 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, which

    it does by discharging the bound charge in the circuitcapacitance. For this reason, the resistive component of fault current should not be less than the residualcapacitance current. This is the basis of the design, andin practice values of between 3-5 I co are used.

    It is important that the earthing transformer neverbecomes saturated; otherwise a very undesirablecondition of ferroresonance may occur. The normal riseof the generated voltage above the rated value caused bya sudden loss of load or by field forcing must beconsidered, as well as flux doubling in the transformerdue to the point-on-wave of voltage application. It is

    sufficient that the transformer be designed to have aprimary winding knee-point e.m.f. equal to 1.3 times thegenerator rated line voltage.

    17.3 STATOR WINDING FAULTS

    Failure of the stator windings or connection insulationcan result in severe damage to the windings and statorcore. The extent of the damage will depend on themagnitude and duration of the fault current.

    17.3.1 Earth Faults

    The most probable mode of insulation failure is phase toearth. Use of an earthing impedance limits the earthfault current and hence stator damage.

    An earth fault involving the stator core results in burningof the iron at the point of fault and welds laminations

    together. Replacement of the faulty conductor may notbe a very serious matter (dependent on setrating/voltage/construction) but the damage to the corecannot be ignored, since the welding of laminations mayresult in local overheating. The damaged area cansometimes be repaired, but if severe damage hasoccurred, a partial core rebuild will be necessary. Aflashover is more likely to occur in the end-windingregion, where electrical stresses are highest. Theresultant forces on the conductors would be very largeand they may result in extensive damage, requiring thepartial or total rewinding of the generator. Apart from

    burning the core, the greatest danger arising from failureto quickly deal with a fault is fire. A large portion of theinsulating material is inflammable, and in the case of anair-cooled machine, the forced ventilation can quicklycause an arc flame to spread around the winding. Firewill not occur in a hydrogen-cooled machine, providedthe stator system remains sealed. In any case, the lengthof an outage may be considerable, resulting in majorfinancial impact from loss of generation revenue and/orimport of additional energy.

    17.3.2 Phase-Phase Faults

    Phase-phase faults clear of earth are less common; theymay occur on the e