Allen-Bradley 1407-CGCM-DLR User Manual page 63

CGCM-DLR Unit Operation Chapter 3
Over-frequency (81O)
When generator frequency exceeds the over-frequency setpoint for a specified
amount of time, a definite time over-frequency fault occurs. Once the frequency
drops below the threshold, the over-frequency fault timer is reset. If this fault
occurs, tag OvrFreqFlt = 1 in the Scheduled Read table.
Under-frequency (81U)
When generator frequency drops below the under-frequency setpoint for a
specified amount of time, a definite time under-frequency fault occurs. This
function is disabled during soft start timing, when no voltage is present on the
generator voltage sensing inputs, or when the EngineIdle tag is set. Once the
frequency rises above the threshold, the under-frequency fault timer is reset. If
this fault occurs, tag UndrFreqFlt = 1 in the Scheduled Read table.
Reverse Power Protection (32R)
When generator reverse power exceeds the reverse power setting for a specified
amount of time, a reverse power fault occurs. Once the reverse power drops below
95% of the threshold, the reverse power fault timer is reset. If this fault occurs, tag
RevPwrFlt = 1 in the Scheduled Read table.
Rotating Diode Failure
The Rotating Diode Monitor can detect one or more open or shorted diodes in
the generator rotor. If a failed diode is detected, a fault occurs.
The CGCM-DLR unit monitors specific harmonic components present in the
field current. The frequency of the harmonics is proportional to the system
frequency and the ratio between the main and exciter field poles.
For example, during normal operation at 60 Hz, a 3-phase exciter bridge produces
a ripple current frequency of 1080 Hz.
1080 Hz = 6 * 60 Hz * (12 exciter poles / 4 main poles)
A shorted diode produces increased ripple current at 1/6 of the normal ripple
frequency or 180 Hz. Similarly, an open diode shows increased current at 1/3 of
the normal ripple frequency or 360 Hz. The CGCM-DLR unit senses harmonics
in the 1/6 and 1/3 harmonic levels to provide protection for these conditions.
Rockwell Automation Publication 1407-UM002B-EN-P - February 2019 63