The excitation system provides the DC
voltage to the field winding of the generator and modulates this
voltage for control purposes. There are many different configurations
and designs of excitation systems.
Stability programs usually include a
variety of models capable of representing most systems. These models
normally include the IEEE standard excitation system models,
described in IEEE Standard 421.5 (1992).
Reference should be made to that
document for a description of the various models and typical data for
commonly used excitation system designs. The excitation system
consists of several subsystems, as shown in Fig. 11.33.
The excitation power source provides
the DC voltage and current at the levels required by the generator
field. The excitation power may be provided by a rotating exciter,
either a DC generator or an AC generator (alternator) and rectifier
combination, or by rectifiers supplied from the generator terminals
(or other AC source).
Excitation systems with these power
sources are often classified as “DC,” “AC,” and “static,”
respectively. The maximum (ceiling) field voltage available from the
excitation power source is an important parameter.
Depending on the type of system, this
ceiling voltage may be affected by the magnitude of the field current
or the generator terminal voltage, and this dependency must be
modeled since these values may change significantly during a
disturbance.
The automatic voltage regulator (AVR)
provides for control of the terminal voltage of the generator by
changing the generator field voltage. There are a variety of designs
for the AVR, including various means of ensuring stable response to
transient changes in terminal voltage.
The speed with which the field voltage
can be changed is an important characteristic of the system. For the
“DC” and most of the “AC” excitation systems, the AVR
controls the field of the exciter.
Therefore, the speed of response is
limited by the exciter’s time constant. The speed of response of
excitation systems is characterized according to IEEE Standard 421.2
(1990).
A power system stabilizer (PSS) is
frequently, but not always, included in an excitation system. It is
designed to modulate the AVR input in such a manner as to contribute
damping to intermachine oscillations. The input to the PSS may be
generator rotor speed, electrical power, or other signals.
The PSS usually is designed with linear
transfer functions whose parameters are tuned to produce positive
damping for the range of oscillation frequencies of concern. It is
important that reasonably correct values be used for these
parameters.
The output of the PSS is limited,
usually to ±5% of rated generator terminal voltage, and this limit
value must be included in the model. The excitation system includes
several other subsystems designed to protect the generator and
excitation system from excessive duty under abnormal operating
conditions.
No comments:
Post a Comment