GEK-113188A
MIGII Digital Protection for Electrical Machines
2-9
2 PRODUCT DESCRIPTION
2.6 THERMAL IMAGE ELEMENT (49)
2
2.6THERMAL IMAGE ELEMENT (49)
This unit provides protection against overheating due to overloading conditions. The operating time curve is set from the
time curve family, as a function of the time constant
T
1 (settable between 3 and 600 minutes). The cooling time constant
T
2
can be set between 1 and 6 times the heating time constant
T
1. Refer to
Appendix A
for a detailed explanation on the
operating principles of this element.
The thermal element measures the three phase currents of the motor. The algorithm to calculate the thermal image value is
based on the positive and negative sequence values, I
1
and I
2
as follows:
Where K1 is a constant that overvalues the effect of the negative sequence I2 component, and is selectable between 1 and
8.
The negative sequence is included in the above formula in order to protect the generator from the effects caused by the
system light unbalanced currents, such as the ones produced by load unbalance. High negative sequence values, such as
those produced by uncleared external faults (phase-to-phase or phase-to-ground), long lasting loss of a phase, etc. will be
detected by function 46 in a faster way, as it works with a different algorithm. The phenomena that cause a supplementary
overheating in the machine are described in section
The resulting time for reaching an overheating condition due to Ieq including both overload and unbalance is defined by the
following equation:
Where:
T
Generator’s time constant at In
T
Operation time at Ieq
Summary of Contents for Multilin MIGII
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