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Synchronous Machines

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    الصورة الرمزية نجرو555
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    Synchronous Machines

    Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power

    Largest single-unit electrical machines in production (up to 2000 MVA)

    Synchronous generators are the primary source of electrical energy we consume today

    Large ac power networks rely almost exclusively on synchronous generators

    Synchronous motors are built in large units compare to induction motors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drives
    Constructional Features of Synchronous Machines
    Various Types

    Salient-Pole Synchronous Generator

    Cylindrical-Rotor Synchronous Generator
    Operation Principle
    The rotor of the generator is driven by a prime-mover


    A dc current is flowing in the rotor winding which produces a rotating magnetic field within the machine


    The rotating magnetic field induces a three-phase voltage in the stator winding of the generator
    Electrical Frequency
    Generated Voltage
    Equivalent Circuit_1
    The internal voltage Ef produced in a machine is not usually the voltage that appears at the terminals of the generator.
    The only time Ef is same as the output voltage of a phase is when there is no armature current flowing in the machine.
    There are a number of factors that cause the difference between Ef and Vt:

    The distortion of the air-gap magnetic field by the current flowing in the stator, called the armature reaction

    The self-inductance of the armature coils.

    The resistance of the armature coils.

    The effect of salient-pole rotor shapes.
    Equivalent Circuit_2
    Phasor Diagram
    Three-phase equivalent circuit of a cylindrical-rotor synchronous machine
    The voltages and currents of the three phases are 120o apart in angle, but otherwise the three phases are identical.

    Example 8.2.1 (pp.358)
    The per-phase synchronous reactance of a three-phase, Y-connected, 2.5-MVA, 6.6-kV, 60 Hz turboalternator is 10 W. Neglect the armature resistance and saturation. Calculate the voltage regulation when the generator is operating at full-load (a) with 0.8 power factor lagging, and (b) with 0.8 power factor leading.
    Determination of the parameters of the equivalent circuit from test data
    The equivalent circuit of a synchronous generator that has been derived contains three quantities that must be determined in order to completely describe the behaviour of a real synchronous generator:

    The saturation characteristic: relationship between If and f (and therefore between If and Ef)
    The synchronous reactance, Xs
    The armature resistance, Ra

    The above three quantities could be determined by performing the following three tests:

    Open-circuit test
    Short-circuit test
    DC test
    Open-circuit test
    The generator is turned at the rated speed
    The terminals are disconnected from all loads, and the field current is set to zero.
    Then the field current is gradually increased in steps, and the terminal voltage is measured at each step along the way.
    It is thus possible to obtain an open-circuit characteristic of a generator (Ef or Vt versus If) from this information
    Short-circuit test
    Adjust the field current to zero and short-circuit the terminals of the generator through a set of ammeters.
    Record the armature current Isc as the field current is increased.
    Such a plot is called short-circuit characteristic.
    DC Test
    The purpose of the DC test is to determine Ra. A variable DC voltage source is connected between two stator terminals.
    The DC source is adjusted to provide approximately rated stator current, and the resistance between the two stator leads is determined from the voltmeter and ammeter readings
    Determination of Xs
    For a particular field current IfA, the internal voltage Ef (=VA) could be found from the occ and the short-circuit current flow Isc,A could be found from the scc.
    Then the synchronous reactance Xs could be obtained using
    Xs under saturated condition
    Short-circuit Ratio
    Another parameter used to describe synchronous generators is the short-circuit ratio (SCR). The SCR of a generator defined as the ratio of the field current required for the rated voltage at open circuit to the field current required for the rated armature current at short circuit. SCR is just the reciprocal of the per unit value of the saturated synchronous reactance calculated by
    Example 8.3.1 (pp.361)
    Concept of the infinite bus
    Active and reactive power-angle characteristics
    P>0: generator operation
    P<0: motor operation
    Positive Q: delivering inductive vars for a generator action or receiving inductive vars for a motor action
    Negative Q: delivering capacitive vars for a generator action or receiving capacitive vars for a motor action
    Active and reactive power-angle characteristics
    The real and reactive power delivered by a synchronous generator or consumed by a synchronous motor can be expressed in terms of the terminal voltage Vt, generated voltage Ef, synchronous impedance Zs, and the power angle or torque angle d.
    Referring to the fig, it is convenient to adopt a convention that makes positive real power P and positive reactive power Q delivered by an overexcited generator.
    The generator action corresponds to positive value of d, while the motor action corresponds to negative value of d.
    Active and reactive power-angle characteristics
    Active and reactive power-angle characteristics
    Active and reactive power-angle characteristics
    The above two equations for active and reactive powers hold good for cylindrical-rotor synchronous machines for negligible resistance
    To obtain the total power for a three-phase generator, the above equations should be multiplied by 3 when the voltages are line-to-neutral
    If the line-to-line magnitudes are used for the voltages, however, these equations give the total three-phase power
    Steady-state power-angle or torque-angle characteristic of a cylindrical-rotor synchronous machine (with negligible armature resistance).
    Steady-state stability limit
    Pull-out torque
    Parallel operation of synchronous generators
    There are several major advantages to operate generators in parallel:

    Several generators can supply a bigger load than one machine by itself.
    Having many generators increases the reliability of the power system.
    It allows one or more generators to be removed for shutdown or preventive maintenance.
    Before connecting a generator in parallel with another generator, it must be synchronized. A generator is said to be synchronized when it meets all the following conditions:
    The 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 equal.
    The oncoming generator frequency is equal to the running system frequency.

  2. [2]
    al-shereiqi
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    Thanks alot my friend

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    mnci
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    mnci
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    الكتاب اعلاه ممتاز جدا

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    حسين الخزرجي
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    مشكوووووووووووور أخي الكريم

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    احمد الششتاوى
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    جزاك الله كل الخير انشاء الله

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    alsotari
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    السلام عليكم ..
    انا مهتمة بال sync,gen لقدرة متوسطة ... تقدر ب 3.7 ا... اريد معرفته اي معلومات تفيدني ..
    من توصيلة ... و حسابات ... و سعره ... اي موقع او كتاب يفيدني ...

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  8. [8]
    salemop31
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    كل الشكر لهذا الطرح الموجز لكن نقصه synchronous compensators وههو مهم جدا لانه يعمل كماتور تارة وكمولد تارة اخرى لتعويض النقص في ractive power اوسحب الزيادة للمحافظة على استقرارية الشبكة و P.F
    ودائما يذكر مع المواضيع المشابهة

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  9. [9]
    mahmoud_thehero
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    thanks

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  10. [10]
    engdoly
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    thank u very much i realy need it

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