Rockwell Automation 1407-CGCM Combination Generator Control Module - Manual (Page 58)

58 Rockwell Automation Publication 1407-UM001G-EN-P - April 2013
Chapter 3 CGCM Unit Operation
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 is capable of detecting one or more open or shorted
diodes in the generator’s rotor. If a failed diode is detected, a fault occurs.
The CGCM 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 * 60Hz * (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 unit senses harmonics in
the 1/6 and 1/3 harmonic levels to provide protection for these conditions.
When the ripple current at one of these frequencies exceeds the applicable user
specified threshold, a timer is started. Once the time delay is exceeded, a rotating
diode fault occurs. If the ripple current falls below the threshold (configured as
percent of measured excitation current) before the timer expires, the timer is
reset. If this fault occurs, tag RotDiodeFlt = 1 in the Scheduled Read table.