Table of Contents
Advertisement
Time for Cool Down
Definite-Time
Tripping
Characteristic
2.10.2.1 Settings of the Unbalanced Load Protection
Addr.
Setting Title
1701
UNBALANCE LOAD OFF
1702
I2>
7UM62 Manual
C53000-G1176-C149-3
The parameter 1705 T COOL DOWN is defined as the time required by the thermal
image to cool down from 100 % to 0 %. If the machine manufacturer does not provide
this information, the setting value can be calculated by assuming an equal value for
the cool-down time and the heating time of the object to be protected. The formula
below shows the relation between the K asymmetry factor and the cool-down time:
K
---------------------------------
t
=
Cooldown
(
I
2 perm
Example:
The following cool-down time results for a K = 20 s and a permissible constant
unbalanced load I
/I
= 11 %.
2
N
20 s
t
------------------- -
=
Cooldown
(
,
)
0 11
This value T COOL DOWN is set at address 1705.
Asymmetrical system faults also cause high negative phase-sequence currents. A
definite-time negative phase-sequence current–stage 1706 I2>> can detect
asymmetrical power system short circuits. A setting to about 60 % to 65 % ensures
that a trip is always performed according to the thermal characteristic in case of a
phase failure (unbalanced load always less than 100/√3 %, i.e. I
other hand, a two-pole short circuit can be supposed for an unbalanced load of more
than 60 % to 65 %. The delay time T I2>> (address 1707) must be coordinated with
the system grading of phase-to-phase short circuits.
Contrary to the time-overcurrent protection, the I2>> stage is able to detect fault
currents under nominal current. The following conditions are valid:
− A phase-to-phase fault (I) results in the following negative sequence current
1
------ - I ⋅
0.58 I ⋅
I
=
=
2
3
− A phase-to-ground fault I) corresponds to the following 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.
Setting Options
ON
Block relay for trip com-
mands
3.0..30.0 %
Unbalanced Load (Negative Sequence) Protection (ANSI 46)
2
⁄
)
I
N
≈
1650 s
2
1
-- - I ⋅
0.33 I ⋅
I
=
=
2
3
Default Setting
OFF
10.6 %
< 58 %). On the
2
Comments
Unbalance Load Protection
Continously Permissible Cur-
rent I2
69
Advertisement
Table of Contents
