DGC Brushless Excitation - Emerson Process Management

DGC Brushless Excitation
System Description
The Voltage Feedback Signal is modified by a multiplication factor. The multiplication factor is
equal to 1.0 when there is no VAR loading on the generator, thus the value of the feedback signal is
unchanged.
As VAR loading increases in the VARS OUT direction, the compensator multiplication factor rises
above unity, causing the perceived generator voltage to be higher than actual. As VAR loading
increases in the VARS IN direction, the compensator multiplication factor drops below unity,
causing the perceived generator voltage to be lower than actual.
The compensation factor is applied proportionally based on the percentage of VAR loading and the
Compensation setpoint.
4.3.e Line Compensation
Line Compensation is used when the voltage at the end of a long transmission-line is of concern.
The compensation is used to overcome the voltage drop that occurs as the line is loaded with real
(resistive) and reactive current.
For Reactive load (MVAR) on the line, the effect of the compensation is to make the apparent
terminal voltage lower as the reactive loading of the line increases in the VARS OUT direction. This
causes the DGC to increase excitation, boosting the voltage at the end of the line.
For Real load (MW) on the line, the effect of the compensation is to make the apparent terminal
voltage lower as the megawatt loading of the line increases. This causes the DGC to increase
excitation, boosting the voltage at the end of the line.
Each of these calculations develops a compensation factor that modifies the feedback signal.
The multiplication factor is equal to 1.0 when there is no load on the generator, thus the value of the
feedback signal is unchanged. As loading of the line increases with real load or VARS Out reactive
load, the compensator multiplication factor drops below unity causing the perceived generator
voltage to be lower than actual. The compensation factor is applied proportionally based on the
percentage of VAR loading and the compensation setpoints.
4.4 Controller Characteristics
The controller is a Proportional plus Integral controller with transient gain reduction (TGR) via rate
feedback. The setpoint to the controller is the voltage adjuster value (va_volts_value) and the
measured variable is the compensated feedback from the generator PT’s (fb_comp). The rate
feedback signal, the limiter outputs, and the power system stabilizer output (if installed) contribute to
the difference signal. The difference (auto_error) is applied to the proportional and integral sections
of the controller. The proportional (p_output) and intregral (i_output) outputs are summed to
produce the PID controller output (ctlr_out).
If the Regulator Mode is OFF or TEST then the controller output is held at zero.
4.4.a Proportional Action
The Proportional Action section of the controller multiplies the error signal by a gain factor. The
value of the gain factor can be dynamically adjusted as a function of the megawatt load on the
generator, using a gain (gain_curve) vs. load curve (load_curve). Proportional control alone cannot
reduce the error signal to zero. This is not usually an issue unless setpoint matching is needed. In
most cases, the setpoint (va_volts_value) is adjusted by the operator to get the desired output
without knowledge of the setpoint value. The fact that the controller error is not zero is of no
concern to the operator.
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