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    Protection of Alternators

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    Parameshwar Tummeti
    Protection of alternators is important as they are costly pieces of equipment. Some key faults that can occur include stator faults like phase-to-phase or phase-to-earth faults, rotor faults, and abnormal operating conditions. Differential protection using circulating currents is commonly used to protect the stator winding from faults. Additional schemes like restricted earth fault protection are used to protect against costly internal earth faults while limiting fault currents. Negative sequence relays provide protection against unbalanced loads or phase-to-phase faults which can cause overheating.

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    Protection of Alternators

    Uploaded by

    Parameshwar Tummeti
    3K views20 pages
    Protection of alternators is important as they are costly pieces of equipment. Some key faults that can occur include stator faults like phase-to-phase or phase-to-earth faults, rotor faults, and abnormal operating conditions. Differential protection using circulating currents is commonly used to protect the stator winding from faults. Additional schemes like restricted earth fault protection are used to protect against costly internal earth faults while limiting fault currents. Negative sequence relays provide protection against unbalanced loads or phase-to-phase faults which can cause overheating.

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    Protection of alternators

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    Protection of alternators

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    Protection of alternators is important as they are costly pieces of equipment. Some key faults that can occur include stator faults like phase-to-phase or phase-to-earth faults, rotor faults, and abnormal operating conditions. Differential protection using circulating currents is commonly used to protect the stator winding from faults. Additional schemes like restricted earth fault protection are used to protect against costly internal earth faults while limiting fault currents. Negative sequence relays provide protection against unbalanced loads or phase-to-phase faults which can cause overheating.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    3K views20 pages

    Protection of Alternators

    Uploaded by

    Parameshwar Tummeti
    Protection of alternators is important as they are costly pieces of equipment. Some key faults that can occur include stator faults like phase-to-phase or phase-to-earth faults, rotor faults, and abnormal operating conditions. Differential protection using circulating currents is commonly used to protect the stator winding from faults. Additional schemes like restricted earth fault protection are used to protect against costly internal earth faults while limiting fault currents. Negative sequence relays provide protection against unbalanced loads or phase-to-phase faults which can cause overheating.

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    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    of 20

    Protection of Alternators

    • The generating units, especially the larger ones, are relatively few in
    number and higher in individual cost than most other equipments.
    • Therefore, it is desirable and necessary to provide protection to
    cover the wide range of faults which may occur in the modern
    generating plant.
    • Some of the important faults which may occur on an alternator
    are
    (i) stator faults
    a) phase to phase faults b)phase to earth faults (c)inter turn faults
    (ii) Rotor faults
    (iii) abnormal conditions
    (i) failure of prime-mover (ii) failure of field (iii) over loading
    (iv) Over speeding (v) overvoltage (vi) unbalanced loading
    (vii) cooling system failures
    Differential Protection of Alternators
    • The most common system used for the protection of stator
    winding faults employs circulating-current principle
    • Under normal operating conditions, these currents are equal but
    may become unequal on the occurrence of a fault in the protected
    section.
    • The difference of the currents under fault conditions is arranged
    to pass through the operating coil of the relay.
    • The relay then closes its contacts to isolate protected section
    from the system.
    • This form of protection is also known as Merz-Price circulating
    current scheme
    Merz-Price circulating current scheme
    Modified Differential Protection for Alternators
    Merz-Price protection of star connected
    alternator
    Restricted Earth Fault Protection of Generator
    • Generally Merz-Price protection based on circulating current
    principle provides the protection against internal earth faults.
    • But for large generators, as there are costly, an additional protection
    scheme called restricted earth fault protection is provided.
    • The percentage of windings protected depends on the value of
    earthing resistance and the relay setting.
    • In this scheme, the value of earth resistance, relay setting, current
    rating of earth resistance must be carefully selected.
    • The earth faults are rare near the neutral point as the voltage of
    neutral point with respect to earth is very less.
    • But when earth fault occurs near the neutral point then the
    insufficient voltage across the fault drivers very low fault current
    than the pick up current of relay coil.
    • Hence the relay coil remains unprotected in this scheme. Hence it is
    called restricted earth fault protection. It is usual practice to protect
    85% of the winding
    Restricted Earth Fault Protection of Generator

    • Hence practically 15% of winding from the neutral point is kept


    unprotected, protecting the remaining 85% of the winding against
    phase to earth faults.
    Effect of Earth Resistance on % of Winding Unprotected
    • If the resistance R is very small i.e. the neutral is almost solidly
    grounded, then the fault current is very high.
    • But high fault currents are not desirable hence small R is not
    preferred for the large machines.
    •        For low resistance R, the value of R is selected such that full
    load current passes through the neutral, for a full line to neutral
    voltage V.
    •        In medium resistance R, the earth fault current is limited to
    about 200A for full line to neutral voltage V, for a 60 MW
    machine.
    •        In high resistance R, the earth fault current is limited to about
    10 A. This is used for distribution transformers and generator-
    transformer units.
        
    • Now higher the value of earth resistance R, less is the earth fault
    current and less percentage of winding gets protected. Large
    percentage of winding remains unprotected.
    • Let                  V= Full line to neutral voltage
                            I = Full load current of largest capacity generator
                            R = Earth resistance
           The value of the resistance R is,
                             R = V/I
           And the percentage of winding unprotected is given by

    Wher Io = Maximum operating current in the primary of C.T.


       If relay setting used is 15% then Io is 15% of full load current of
    the largest machine and so on.Greater percentage of windings of
    small capacity machines running parallel get protected.
    Unrestricted Earth Fault Protection
    • The unrestricted earth fault protection uses a residually connected
    earth fault relay. It consists of three C.T.s, one in each phase.
    • The secondary windings of three C.T.s are connected in parallel.
    • The earth fault relay is connected across the secondaries which
    carries a residual current.
    •  Where there is no fault, under normal conditions, vector sum of
    the three line currents is zero. Hence the vector sum of the three
    secondary currents is also zero.
    • So if Irs, Iys and Ibs C.T. secondary currents then under normal
    conditions we can write,
              Īrs+ Īys + Ībs = 0
    •        The sum of the three currents is residual current IRs which is
    zero under normal conditions.
    • The earth fault relay is connected in such a way that the residual
    current flows through the relay operating coil.
    • Under normal condition, residual current is zero so relay does not
    carry any current and is inoperative.
    • the earth fault at any location near or away from the location of
    C.T.s can cause the residual current.
    • Hence the protected zone is not definite. Such a scheme is hence
    called unrestricted earth fault protection.
    Balanced Earth Fault Protection for Generator
    In practice for small rating alternators, the neutral ends of the three
    phases are connected to a single point. Hence it is not possible to
    introduce C.T. in each phase on neutral side as required in Merz-
    Price Protection. In such cases, the balanced earth fault
    protection can be used.
    •  At the outgoing side, the three C.T.s are connected in parallel as
    are connected in unrestricted earth fault protection.
    • A single C.T. is connected on the neutral side in the pilot wire
    connecting neutral of alternator to earth. The neutral earthing is
    achieved through the earthing resistance.
    • A relay is connected across the C.T. secondaries.
    •  Under normal conditions, the alternator line currents add to zero.
    • Hence the vector sum of the currents through the secondaries of
    C.T. is also zero.
    • Thus no currents flows through the relay and as neutral carries
    zero current hence no current is supplied to relay from neutral
    side. So relay is inoperative.
    • By this scheme, the protection against earth faults is restricted to
    the region between the neutral and the position where line current
    transformers are installed
    Stator Protection Against Interturn Faults
    • The Merz-Price protection system gives protection against phase
    to phase faults and earth faults. It does not give protection against
    interturn faults.
    • The interturn fault is a short circuit between the turns of the same
    phase winding.
    • Thus the current produced due to such fault is a local circuit
    current and it does not affect the currents entering and leaving the
    winding at the two ends, where C.T.s are located.
    • Hence Merz-Price protection can not give protection against
    interturn faults.
    • Such an interturn fault protection system is extremely sensitive
    but it can be applied to the generators having doubly wound
    armatures
    Stator Protection Against Interturn Faults
    Rotor Earth Fault Protection
    • The rotor circuit of the alternator is not earthed and d.c. voltage is
    imposed on it.
    • hence single ground fault in rotor does not cause circulating current to
    flow through the rotor circuit.
    • Hence single ground fault in rotor does not cause any damage to it.
    • But single ground fault causes an increase in the stress to ground at other
    points in the field winding when voltage are induced in the rotor due to
    transients.
    • Thus the probability of second ground fault increases part of the rotor
    winding is bypassed and the currents in the remaining portion increases
    abruptly.
    • This cause the unbalance of rotor circuit and hence the mechanical and
    thermal stresses on the rotor.
    • Due to this, rotor may get damaged. Sometimes damage of bearings and
    bending of rotor shaft take place due to the vibrations.
    • Hence the rotor must be protected against earth fault.
    Rotor Earth Fault Protection
    Negative Sequence Relays
    • The negative relays are also called phase unbalance relays because
    these relays provide protection against negative sequence component of
    unbalanced currents existing due to unbalanced loads or phase-phase
    faults.
    • The unbalanced currents are dangerous from generators and motors
    point of view as these currents can cause overheating.
    • Negative sequence relays are generally used to give protection to
    generators and motors against unbalanced current.
    •  A negative sequence relay has a filter circuit which is operative only
    for negative sequence components.
    • Low order of over current also can cause dangerous situations hence a
    negative sequence relay has low current settings.
    • The earth relay provides protection for phase to earth fault but not for
    phase to phase fault.
    • A negative sequence relay provides protection against phase to phase
    faults which are responsible to produce negative sequence components
    Negative Sequence Relays

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