Separate exciter. The most common way of supplying dc current to the rotor winding without brushes and sliprings is shown in Fig. 5.17.
Fig. 5.17 Separate brushless exciter
The output of the AVR drives a dc current If through the
pole windings of the exciter, which are mounted in a stator frame. The poles
produce a stationary field which induces a voltage in the exciter rotor winding
as it turns.
The ac voltage produced by the rotor winding of the exciter
is converted to dc by a bridge rectifier which is also mounted on the rotor
shaft.
The dc output of the rectifier is connected to the main
rotor windings by conductors laid in a slot along the rotor shaft. The
inductance of the main generator rotor coils is usually sufficient to smooth
out the ripple in the bridge rectifier output.
The advantage of the pilot exciter is that the generator has
a source of power available once the shaft is turning; the voltage supplied to
the AVR is completely independent of generator load and there is no reliance on
residual flux in the magnetic circuit of the main generator to start the
self-excitation process.
The pilot exciter also enables the generator to supply
current to a connected network even when a short circuit occurs, enabling the
high current to be detected by protection relays which will then disconnect the
faulty circuit.
If the AVR is supplied from an excitation winding in the
main generator stator, the supply voltage is very small when the stator
windings experience a short circuit, and the AVR is unable to drive an adequate
rotor excitation current.
One manufacturer uses two excitation windings to provide a
voltage from the AVR under short-circuit conditions, so that sufficient current
is supplied into the fault to trip the protection system.
During a short circuit the air gap flux density in these
machines shows a pronounced harmonic component. This component induces voltage
in coils of one of the excitation windings, which are short pitched and
therefore deliver a voltage to the AVR even under short-circuit conditions.
The second excitation winding is fundamental-pitched and
provides the major drive for the AVR under normal operating conditions. It is
claimed that the performance of this system is comparable to a machine using a
permanent-magnet exciter.
No comments:
Post a Comment