Excitation Systems

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					Excitation Systems
    Exciter – DC excitation to magnetic field of generator
    2 types
        Rotating –
              1. Brushless – similar to AC generator. Most
              common
                  Voltage induction is opposite to main
                  generator
                  Stator (stationary) Field winding
                  3 Phase Ac winding
                  DC Voltage Regulator controls
                  excitation
              2. Brush Type – high maintenance
                  Slip rings, brushes, and commutators
                  Commutators used as mechanical
                  rectifiers AC – DC voltage
                  DC voltage supplied to magnetic field
                  by brushes contacting field
                  Rings of generator rotor
Generator Control Systems – Based on Electromagnetism
    Current ---through a conductor --- produces
    magnetic field
    DC ----Rotor Coil ---- magnetic field --- induce voltage
    Strength of magnetic field of current -----coils
        Excitation controls “field Current” – controls
        generator terminal voltage
             “Reactive Power” VARs
             “True (Real) Power” Watts
             “Apparent Power” (VA) Vector sum of
             reactive and true power
                  Indication of total current through
                  stator


        Reactive Power – determined by field.
             VARs (volts ampre reactive)
             2 possibilities:
                  Inductive – Current lags Voltage –
                  “Lagging” Produces VARs
Leading Power Factor
Capacitive - Current leads voltage –
“Leading” Generator absorbs Vars from
grid. Lagging Power Factor


Power Factor is ratio of:
-True Power (watts) : Apparent Power (VA)
         -If voltage and current are in
         phase:
         Reactive Power = 0
         Power Factor = 1
         -out of phase:
         PF ranges from -1.0 to 1.0
    Generators have PF ratings: ex .85
    lagging
Excitation controls ROTOR’s Magnetic field
    This: controls VARs produced or
    absorbed by generator
         Remember: Gen is connected to
         grid
                 Watts produced is controlled by steam
                 flow to turbine
                       Governor valves control this
Generator Control
    Voltage Control:
        1. PT – Measures Generator voltage, compares
        to setpoint and adjusts current to Field windings
            Inductive – current lags –“Lagging Power” =
            Exciter increases current toField
            Capacitive – current leads – “Leading
            Power” = Exciter decreases current to field
        2. Control Panel (Rheostat) – Responds to load
        of the grid = frequency control
            Controls steam/fuel flow to turbine
        3. Voltage Regulator – controls portion of grid
        the station influences
            Sum of all generators must maintain
            needed VARs
            - Lagging PF – over-excite field and
              increase VARs
            - Leading PF – under-excite field and
              absorb VARs
            Load Angle – voltage wave of Generator
            leads voltage wave of Grid
            - To increase angle = more power
              produced by generator
            - Requires more torque by Prime mover
              (Turbine)
            Synchronous Speed – Equalibrium between
            torque of rotor and torque of
            The Grid = Attraction between the magnetic
            field of the three stator windings and the
            magnetic field of the rotor (Turbine driven)
                -Excitation attempts to keep this
            equilibrium = “Synchronism”
                     - Reduces Power Angle
MW Control (Generator Power) – is accomplished with
the help of:
    PTs
    CTs
    Governor
    Control Valves
    -Generator Power measured by PTs and CTs
        -Compare to desired setpoint
        - Governor then alters Turbine control valves =
        steam flow = prime mover
Generator Synchronizing
    Automatic:
    -PT – Monitors Generator Frequency compares to
    line (grid) signal with
        “autosynchronizer” – signals governor to alter
        steam/fuel – changes speed
        Of prime mover (turbine) – corrects generator
        frequency
    Start Up:
            - Field Breaker Closed
            - Increase Generator speed manually by
              increasing field current
            - This increases terminal voltage of the
              generator close to that of
              The Grid (slightly above)
            - Automatic voltage regulator is put into
              service
3 Requirements for Grid Synchronization:
    1. Frequencies match
    2. Voltages match
    3. In phase (voltage peaks match)
- Synchroscope needle – determines phase angle of
generator
    - will move clockwise if loading to grid
    - the slower the rotation the closer the synch
    -want the needle at the 12 o’clock setting = 0
    - once accomplished close generator circuit
    breaker
        Protective relays are set for approximately
        5 degrees of variance

				
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posted:2/9/2012
language:English
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