DSE 8610 MKII, Operator'S Manual

Page: 152 / 188

Protections
057-254 ISSUE: 2 Page 152 of 188
6.7.2 INVERSE DEFINITE MINIMUM TIME (IDMT) ALARM
If the Over Current IDMT Alarm is enabled, the controller begins following the IDMT ‘curve’ when the
current on any phase passes the Trip setting.
If the Trip is surpassed for an excess amount of time, the IDMT Alarm triggers (Shutdown or Electrical
Trip as selected in Action).
The larger the over circuit fault, the faster the trip. The speed of the trip is dependent upon the fixed
formula:
=
− 1
Where:
is the tripping time in seconds
is the actual measured current of the most highly loaded line (L1, L2 or L3)
is the delayed trip point setting in current
is the time multiplier setting and also represents the tripping time in seconds at twice full
load (when
= 2
).
The settings shown in the example below are a screen capture of the DSE factory settings, taken from
the DSE Configuration Suite PC Software for a brushless alternator.
These settings provide for normal running of the generator up to 100% full load. If full load is
surpassed, the Immediate Warning alarm is triggered and the set continues to run.
The effect of an overload on the generator is that the alternator windings begin to overheat; the aim of
the IDMT Alarm is to prevent the windings being overload (heated) too much. The amount of time that
the alternator can be safely overloaded is governed by how high the overload condition is.
The default settings as shown above allow for an overload of the alternator to the limits of the Typical
Brushless Alternator whereby 110% overload is permitted for 1 hour or 200% overload is permitted for
36 seconds.
If the alternator load reduces, the controller then follows a cooling curve. This means that a second
overload condition may trip soon after the first as the controller knows if the windings have not cooled
sufficiently.
For further details on the Thermal Damage Curve of your alternator, refer to the alternator
manufacturer and generator supplier.
t
(time multiplier setting)
I
T
(trip point setting in current)