Himoinsa CEM7 - Page 48

CEM7 CONTROL UNIT ALARMS | PAGE 48
EXAMPLE CALCULATION OF THE IDMT CURVE TIME CONSTANT (T)
For a genset current (I) with a value of 110% of the nominal value, the desired
tripping time (td) is 1 hour (3,600 seconds).
From the IDMT curve equation, we obtain the time constant:
For a genset current value of 110% of In, the value is set to the ratio
Therefore, the value of the time constant for a trip time of 3,600 sec for a current
110% of the nominal is established as:
The high temperature alarm for PT100 level 2 probes 1 to 4 is set by default
(Alarms table, parameter 149, 152, 155 and 158) to perform no action (warning).
IDMT ALARM
The CEM7 control unit’s IDMT alarm is associated to the measurement of the
genset current. The alarm detection time (td) ts the curve given by the formula:
where the time constant parameter T is dened in seconds (Times table,
parameter 34, default value 36 seconds) and a percentage of the maximum
overload current of genset I (Thresholds table, parameter 51, default value
200%). With these default values, the base of dened times is equal to one hour
for a circulating current corresponding to 110% of the genset’s overload threshold
(Thresholds table, parameter 7).
The purpose of the IDMT alarm is to avoid heating as a result of the ow of
current in the installation components (contactors, cables etc.). Therefore, the
trip time of the alarm is a function of the current owing where a higher current in
the genset corresponds to a shorter alarm trip time. In addition, this condition
implies that the IDMT alarm can trip repeatedly in less time than is established
(Times table, parameter 34) as a result of not being given sufcient cooling time
when the genset current is below the overload current threshold.
The cooling time is determined by the formula:
Detection of the IDMT alarm is set by default (Alarms table, parameter 180) to
be activated: Never.
Once an IDMT alarm has been generated, the genset current must be less than
the nominal current dened by the IDMT curve (Thresholds table, parameter 51)
for a programmable interval of time (debounce) (Alarms table, parameter 181).
The genset power alarm is set by default (Alarms table, parameter 182) to
perform no action (warning).
Value 0 for the time constant (Times table, parameter 34) or nominal current
(Thresholds table, parameter 51) of the IDMT curve disable the management of
this alarm.