Rockwell Automation 1407-CGCM Combination Generator Control Module User Manual
Page 53
Rockwell Automation Publication 1407-UM001G-EN-P - April 2013
53
CGCM Unit Operation Chapter 3
The CGCM unit also uses two counters, the reset counter and the time limit
counter. The counters are used to prevent excessive heating of the exciter field
that can be a result of repeated over-excitation. The time limit counter monitors
the duration of an over-excitation condition. The reset counter counts backward
from either the high OEL time setting or the sum of the high and medium OEL
times, depending on the value of the time limit counter.
If, during an OEL cycle, excitation current returns below the low current value,
the reset counter begins counting backwards from its present value. If it reaches
zero, the time limit counter is reset to zero and a new OEL cycle can then occur.
If the reset counter does not reach zero before the excitation current rises above
the low current value, the time limit counter begins counting where it stopped
when the excitation current last fell below the low current value. If the time limit
counter is greater than the programmed high OEL time, the excitation current is
limited to the medium current value. This prevents repeated cycling of the exciter
field at its highest possible current value.
When the excitation current exceeds the OEL limit, the OEL alarm tag
OEL_Active = 1. In FCR mode, OEL limiting is not active although the tag is
set. This tag is in the Scheduled Read table. The OEL function meets
ANSI/IEEE C50.13.
Under-excitation Limit
Under-excitation limiting (UEL) operates in all modes except FCR mode. UEL
senses the leading var input of the generator and limits any further decrease in
excitation to prevent loss of synchronization and excessive end-iron heating
during parallel operation. UEL operates in the area below the Armature Core
End Iron Heating Limitation curve in the generator capability curve.
A customizable UEL limiting curve is defined by a piecewise linear curve
specified by five points you select as shown in the
diagram.
When the excitation current is less than the UEL curve, the UEL alarm tag
UEL_Active
= 1. In FCR mode, UEL limiting is not active although the tag is
set. This tag is in the Scheduled Read table.
TIP
The UEL function is not designed to prevent the loss of excitation
function from operating.