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Cool Down
A settable cool-down time starts as soon as the constantly permissible unbalanced
load I2> is undershot. The tripping drops out on dropout of the pickup threshold drop-
out. 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 image to cool down from 100 % to 0 %. The cool-down time
depends on the construction type of the generator, and especially on the damper wind-
ing. Preloading is taken into consideration when unbalanced loading occurs during the
cool-down period. The protective relay will thus trip in a shorter time.
Tripping Stages
Figure 2-16
Definite Time Trip-
High negative phase sequence currents can only be caused by a phase-to-phase
ping Stage
short circuit in the system which must be covered in accordance with the network
grading plan. For this reason, the thermal characteristic is cut by a selectable, definite
time negative phase-sequence current stage (parameters 1706 I2>> and 1707 T
I2>>).
Please also observe the instructions regarding phase sequence changeover in Sec-
tions 2.3 and 2.33.
Logic
The following figure shows the logic diagram for the unbalanced load protection. The
protection may be blocked via a binary input (">BLOCK I2"). Pickups and time
stages are reset and the metered values in the thermal model are cleared. The binary
input ">RM th.rep. I2" only serves to clear metered values of the thermal char-
acteristic.
7UM61 Manual
C53000-G1176-C127-3
2.10 Unbalanced Load (Negative Sequence) Protection (ANSI 46)
Tripping Zone of the Unbalanced Load Protection
71
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