2.10 Unbalanced Load (Negative Sequence) Protection (ANSI 46)
75
7UM61 Manual
C53000-G1176-C127-3
This value T COOL DOWN is set at address 1705.
Definite-Time Trip-
ping Characteristic
Asymmetrical faults also cause high negative phase-sequence currents. A definite-
time negative phase-sequence current stage characteristic 1706 I2>> can thus
detect asymmetrical power system short circuits. A setting between 60 % and 65 %
ensures trip always occurs in accordance with the thermal characteristic 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 more
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 phase-to-ground fault with current I corresponds to a negative sequence current:
A phase-to-ground fault with current I corresponds to a negative sequence current:
With an isolated starpoint, the I current value is particularly low and can be neglected.
With a low-resistance grounding, however, it is determined by the ground resistance.
2.10.3 Settings
Addr. Parameter Setting Options Default Setting Comments
1701 UNBALANCE LOAD OFF
ON
Block relay
OFF Unbalance Load Protection
1702 I2> 3.0 .. 30.0 % 10.6 % Continously Permissible Current
I2
1703 T WARN 0.00 .. 60.00 sec; ∞ 20.00 sec Warning Stage Time Delay
1704 FACTOR K 2.0 .. 100.0 sec; ∞ 18.7 sec Negativ Sequence Factor K
1705 T COOL DOWN 0 .. 50000 sec 1650 sec Time for Cooling Down
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