Rockwell Automation Publication 1407-UM002B-EN-P - February 2019
55
CGCM-DLR Unit Operation
Chapter
3
Volts/Hertz Limit
The Volts/Hertz limit acts to reduce the generator output voltage by an amount
proportional to generator frequency. Generator output voltage reduction is done
to help protect the generator from overheating and reduce the impact on the
prime mover when adding a large load.
When the generator frequency drops, the CGCM-DLR unit adjusts the voltage
setpoint automatically so that generator voltage follows the under-frequency
slope.
The CGCM-DLR unit provides two configurable knee frequencies and two
configurable slopes that let you define the Volts/Hz characteristic. The slopes are
expressed in PU volts / PU Hertz. For a nominal 60 Hz, 120V system, a slope of
one corresponds to 2V per Hz. The generator output voltage is maintained at the
configured level for any frequency at or above the configured knee frequency up
to 90 Hz. Excitation is inhibited when the frequency is at or below the 10 Hz
cutoff frequency.
The
Under-frequency Slope and Knee Voltages
graph shows a typical Volts/Hz
characteristic as displayed in the Logix Designer application CGCM-DLR unit
configuration screen.
The Volts/Hertz limit is automatically enabled in AVR mode and limits the
voltage increase in Soft Start mode.
Figure 31 - Under-frequency Slope and Knee Voltages
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
Underfrequency Slope
Frequency (Hz)
V
oltage (%)