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5. Thermal protection of the power transformer, and phase and earth protections are to be done.
Below calculations and settings are done for this purpose.
Making of Iθ> Thermal Overload Protection Settings
It is assumed that the thermal constant for the power transformer is Te = 20 minutes
according to the information supplied by the manufacturer. Take trip threshold translation
constant k as 1.10, thermal overload trip heating as %ΘTrip = %100 and thermal
overload alarm heating as %ΘAlarm = %80.
Power transformer nominal current is:
Power transformer nominal current / Protection CT primary nominal current ratio:
Make the setting below so as to activate thermal overload protection at 1.00 I
rTR
:
Making of I> Phase Overcurrent Protection Settings
a. Around 1.00 I
rTR
, the PT is protected from overloads. Setting I> threshold at 1.3 I
rTR
is
suitable according to the PT ratings. Threshold set current in terms of I
n
of CT:
b. According to the calculation above, do the following settings:
Notes:
If there are loads of high inrush pick-up current, fine adjustment of the TMS parameter
might be needed.
If the Thermal Overload Protection is not applied / made passive, set the I> (Phase
Overcurrent Protection Threshold) as 1.0 I
rTR
(0.70 I
n
) instead of 1.3 I
rTR
(0.91 I
n
).
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