DGC Brushless Excitation - Emerson Process Management

DGC Brushless Excitation
System Description
When the DGC is in OFF or TEST modes, the regulator output signal is held at 0%, thus the
excitation demand is entirely controlled by the Base signal.
The Regulator Output signal is produced by the DGC. The DGC is usually configured as a
proportional only controller, receiving a positive setpoint from the Voltage Adjuster (+) and a
negative feedback signal from the generator. The two signals are added together to produce an
“error signal” which can be positive or negative. Integral control can be utilized if desired, usually
seen when a numeric setpoint is provided. In lieu of DGC integral control, the Base following action
acts as an integrator.
If the generator voltage is too low, then the error signal is positive, calling for more excitation. If the
generator voltage is too high, then the error signal is negative, calling for less excitation. If the
generator voltage is equal to the setpoint, then the regulator output is zero. The Regulator Output is
capable of providing an excitation demand from –100% to +100%.
When the DGC operational mode implements the AC or DC control type, the Regulator Output is
permitted to pass. If the regulator output signal is zero, then the excitation demand is equal to the
Base signal. If the regulator output goes positive, then the excitation demand increases above the
base. If the regulator output goes negative, then the excitation demand decreases to a value below
the base.
If the operator wishes to raise the output voltage of the generator, then he raises the voltage
adjuster. The setpoint signal is greater than the feedback signal so the error signal is positive. The
regulator output signal goes positive, adding to the base signal, causing the excitation demand to
be increased. Excitation current increases and generator voltage goes up.
If the operator wishes to lower the output voltage of the generator, then he lowers the voltage
adjuster. The setpoint signal becomes less than the feedback signal so the error is negative. The
regulator output signal goes negative, subtracting from the base signal, causing the excitation
demand to be reduced. Excitation current decreases and generator voltage goes down.
Due to the inherent voltage drop of the generator, as the generator is loaded, the output voltage will
drop correspondingly. If the generator voltage drops below the setpoint, then the error signal is
positive. If the generator is unloading the output voltage will rise correspondingly. If the generator
voltage rises above the setpoint, then the error signal is negative.
While in automatic, the Base Adjuster is not able to change terminal voltage because the regulator
will fight against it. For example, if the operator raises the Base Adjuster, the excitation demand will
increase. Excitation current increases and generator voltage goes up. The feedback goes up
which makes the regulator output signal move in the negative direction. The reduction in regulator
output causes the excitation demand to return to its initial starting point. Excitation current will drop
back to its starting point and the generator output voltage will return to its starting point.
4.3 Automatic AC Voltage Control Software
The term AC control is applied when the controlled variable is measured from the generator
terminals and is thus an AC quantity. This is most often the terminal voltage, but reactive power
and power factor angle can also be the measured variable. The following paragraphs provide
greater detail for each of the features of the software.
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