Functions
2.12 Unbalanced Load (Negative Sequence) Protection (ANSI 46)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
96
Cool-down Time
The parameter 1705 T COOL DOWN establishes the time required by the protection object to cool down under
admissible unbalanced load I2> to the initial value. If the machine manufacturer does not provide this informa-
tion, the setting value can be calculated by assuming an equal value for cool-down time and heatup time of the
object to be protected. The formula below shows the relation between the K asymmetry factor and the cool-
down time:
Example:
With an asymmetry factor of K = 20 s and an admissible continual unbalanced load I
2
/I
N
= 11 %, the following
cool-down time results.
This value T COOL DOWN is set at address 1705.
Definite-Time Tripping Characteristic
Asymmetrical faults also cause high negative phase-sequence currents. A definite-time negative phase-se-
quence current stage characteristic 1706 I2>> can thus detect asymmetrical power system short circuits. A
setting between 60 % and 65 % ensures that tripping always occurs in accordance with the thermal character-
istic in case of a phase failure (unbalanced load continually below 100/√3
%, i.e. I
2
< 58 %). On the other hand,
a two-pole short circuit can be assumed for an unbalanced load of between 60 % and 65 %. The delay time T
I2>> (address 1707) must be coordinated with the system grading of phase-to-phase short circuits.
Contrary to time-overcurrent protection, the I2>> stage is able to detect fault currents at nominal current. The
following conditions apply:
a two-phase fault with fault current I produces a negative sequence current
a single-phase fault with fault current I produces a negative sequence current
With an isolated starpoint, the I current value is particularly low and can be neglected. With a low-resistance
earthing, however, it is determined by the ground resistance.
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