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Generator Protection

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    Stator Protection,over-voltage ,over speeding, failure of prime mover,overloading,unbalance loading,loss of excitation,Phase to phase faults,Inter turn faults,Generator Protection ,power generators,protection of the generator, main breaker and disconnect ,Generator faults,Stator faults,Phase to earth faults,

     

    power generators as we are well aware generators are built in an extremely wide range of sizes depending very much upon the application and the particular type of prime mover for example we may have a 500 kV a diesel driven generator which acts as standby for essential auxiliary services we may see a 10 megawatt gas turbine driven generator which is used for quick start emergency standby on the power system or we may encounter a steam turbine driven 30 megawatt generator used in an industrial installation such as a pulp and paper manufacturing process the majority of generators installed by power companies to feed into the power system are in the range of 200 to 500 megawatts and generally these are driven by steam turbines or water powered hydraulic turbines in some instances we may encounter two separate generators coupled together electrically and also through the turbine steam path the compound turbine cylinders are arranged in series as far as steam flow is concerned another arrangement has two generators in tandem driven by the same turbine and in yet another method the radial type turbine drives a generator. On either side clearly the electrical coupling of these generators requires special consideration when considering

     


    protection of the generator we must not forget the prime mover for example suppose an insulation failure occurred to the winding inside generator it would not be sufficient just to trip out the most breaker and disconnect the unit from the electrical system we might also got to trip the stop valves and pack up the first cause so as to prevent further
    damage similarly if a problem occurs within the prime mover which necessitates tripping the unit inter trips must be provided to trip out the generator breaker as well and this tape
    deals specifically with generator protection.



    when a generator is tripped from the system its excitation system must also be de energized by tripping the field breaker or equivalent there are two basic methods of connecting the generator to the power system smaller generators are generally connected through the circuit breaker directly to a common bus several other generators may also be connected to this bus the bus feeds the power system and also supplies the station service transformer which provides power for the station auxiliaries a fault occurring internally on generator one should be detected by its protection system and this will cause it's specific breaker to open and its own excitation system to de-energize now on large generators the terminal
    voltage which may be say thirteen point eight to 25 kV is stepped up by its own unit transformer to 132 kV or higher before feeding into the bus the generator breaker is located after the Delta Wye unit transformer.


    The advantage here is that the breaker is used to interrupt a lower than would be the case if the breaker were on the primary side note that any fault occurring inside the generator will open this breaker and so isolate the transformer as well in this arrangement it is usual to connect another transformer to supply auxiliary power. This unit station service transformer which is connected directly to the generator terminals feeds the unit auxiliaries the unit auxiliary bus has an alternative feed from a general station service transformer as the unit auxiliaries need to be energized during start-up. However once the unit is synchronized and online the breaker will be closed and the general service breaker opened now in this situation in the case of an internal fault developing inside the generator both the main circuit breaker and also the auxiliary circuit breaker must be opened so as to completely isolate the generator.



    Generator faults


    Generator faults can be considered as follows.


    (a) Stator faults: These include the following
    (i) Phase to earth faults.
    (ii) Phase to phase faults
    (iii) Inter turn faults

    The stator is susceptible to maximum amount of faults within the system with phase to earth fault being the foremost common. The inter turn faults and phase faults are less common but become an earth fault within the end of the day .
    (b) Rotor faults: The faults that exist in the rotor can be either earth fault or an inter turn fault. These faults are mainly caused by the mechanical and thermal stress acting upon the winding insulation. The existence of such fault could also be taken care of because the incidence of second fault may short some a part of the sector winding which might end in the asymmetrical air gap flux which can cause vibrations and end in damage to the bearings. In the era , the practice is to work the sector winding isolated from

    the earth in order that one fault between field coil and rotor body thanks to insulation breakdown are often tolerated.

    (c) Abnormal running conditions: The abnormal running conditions which will occur are:

    (i) loss of excitation,

    (ii) unbalance loading,

    (iii) overloading,

    (iv) failure of first cause ,

    (v) over speeding, and

    (vi) over-voltage.


    Field failure may occur thanks to a faulty field breaker. When a generator loses its excitation, the quantity of reactive power supplied to the system is lost. Instead it might draw excitation from the system while delivering real power at leading power factor. This results in an operation of an induction generator where the speed is slightly increased. Also thanks to loss of excitation there would be a voltage fall which might cause loss of synchronism. things can also cause overheating in rotor and damper windings.


    If there's any unbalance within the system thanks to a phase fault or thanks to the unbalance loading, it gives rise to negative sequence currents. It produces an armature reaction field which rotates during a direction opposite thereto of the rotor and hence produces a flux which is twice the frequency. These currents are linked to the rotor and damper windings which produces heating within the windings.


    When there's an overloading within the generator, it might draw more current and as a result would produce more heating loss within the stator which can damage the insulation.
    When there's a failure of first cause the important power delivering capacity is lost and instead it might draw power from the system making it to run as a motor. This affects the drive of the system thanks to opposite torque being applied on to the shaft.


    When a sudden load is removed then consistent with the AGC control, the machine goes to over speed. This happens mainly within the hydraulic generators since the water flow can't be immediately stopped for the inertia in water motion.
    Over voltages may occur thanks to the failure within the AVR control within the excitation or could also be thanks to over speeding.



    Stator Protection


    As discussed, the foremost normal fault that happens in stator is that the earth faults. Since an earth fault near the generator is extremely critical because the magnitude of the present is extremely high, therefore the current is restricted by either a resistance connected within the neutral circuit. counting on this value the present are often limited to either
    200 to 250 A which is completed by resistor earthing or 4 to 10 A by distribution transformer earthing. albeit the second method has a plus of reducing the damage on to the stator core, the practicability of this method is restricted if the transformer is connected in delta.
    In the resistor earthing, the resistance is connected between the neutral and therefore the refore the ground and the CT is mounted on the neutral with an IDMT or an instant attracted armature type relay. the utmost value of resistance is given by Where C is that the capacitance of the stator circuit to earth per introduce microfarad and f is that the system frequency.

     

    If the neutral is earthed through the primary winding of a distribution transformer, earth-fault protection is provided by connecting an over-voltage relay across its secondary, then the maximum value of resistance is equal to

     

    Reference:

    • cet.edu

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