Functions
2.12 Unbalanced Load (Negative Sequence) Protection (ANSI 46)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
92
Cool Down
A cool-down time with adjustable parmeters starts as soon as the constantly permissible unbalanced load I2>
is undershot. The tripping drops out on dropout of the pickup. However, the counter content is reset to zero with
the cooling time parameterized at address 1705 T COOL DOWN. In this context, this parameter is defined as
the time required by the thermal replica to cool down from 100 % to 0 %. The cool-down time depends on the
construction type of the generator, and especially of the damper winding. Preloading is taken into consideration
when unbalanced loading occurs again during the cool-down period. The protective relay will thus trip in a
shorter time.
Tripping Stages
Figure 2-23 Tripping Zone of the Unbalanced Load Protection
Definite Time Tripping Stage
High negative phase sequence currents can only be caused by a two-pole power system short circuit which
must be covered in accordance with the network grading plan. For this reason, the thermal characteristic is cut
by a selectable, independent negative phase-sequence current stage (parameters 1706 I2>> and 1707 T
I2>>).
Please also observe the instructions regarding phase sequence changeover in Sections 2.5 and 2.47.
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