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# Excitation Systems by huanghengdong

VIEWS: 6 PAGES: 7

• pg 1
```									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
Capacitive - Current leads voltage –
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
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
- 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