A block diagram of the terminal voltage
transducer and the load compensator is shown in Fig 2. These model
elements are common to all excitation system models described in this
document.
Figure 2-Terminal Voltage Transducer
and Optional Load Compensation Elements
It is realized that, for some systems,
there may be separate and different time constants associated with
the functions of voltage sensing and load compensation. The
distinction is not recognized in this model, in which only one time
constant, TR, is used for the combined voltage sensing and
compensation signal.
When load compensation is not employed
(RC=XC= 0), the block diagram reduces to a simple sensing circuit.
The terminal voltage of the synchronous machine is sensed and is
usually reduced to a dc quantity.
While the filtering associated with the
voltage transducer may be complex, it can usually be reduced, for
modeling purposes, to the single time constant, TR, shown. For many
systems, this time constant is very small, and provision should be
made to set it to zero.
The terminal voltage transducer output,
VC, is compared with a reference that represents the desired terminal
voltage setting, as shown on each of the excitation system models.
The equivalent voltage regulator reference signal, VREF, is
calculated to satisfy the initial operating conditions.
It will, therefore, take on a value
unique to the synchronous machine load condition being studied. The
resulting error is amplified as described in the appropriate
excitation system model to provide the field voltage and subsequent
terminal voltage to satisfy the steady-state loop equations.
Without load compensation, the
excitation system, within its regulation characteristics, attempts to
maintain a terminal voltage determined by the reference signal. When
compensation is desired, the appropriate values of RC and XC are
entered.
In most cases, the value of RC is
negligible. The input variables of synchronous machine voltage and
current must be in phasor form for the compensator calculation. Care
must be taken to ensure that a consistent per unit system is utilized
for the compensator parameters and the synchronous machine current
base.
This type of compensation is normally
used in one of the following two ways:
1) When units are bused together with
no impedance between them, the compensator is used to create an
artificial coupling impedance so that the units will share reactive
power appropriately. This corresponds to the choice of a regulating
point within the synchronous machine. For this case, RC and XC would
have positive values.
2) When a single unit is connected
through a significant impedance to the system, or when two or more
units are connected through individual transformers, it may be
desirable to regulate voltage at a point beyond the machine
terminals.
For example, it may be desirable to
compensate for a portion of the transformer impedance and effectively
regulate voltage at a point part way through the step-up transformer.
For these cases,
RC and XC would take on the appropriate
negative values.
Some compensator circuits act to modify
terminal voltage as a function of reactive and real power, instead of
reactive and real components of current. Although the model provided
will be equivalent to these circuits only near rated terminal
voltage, more precise representation has not been deemed worthwhile.
Thank you very much for your clear description! It is very helpful!
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