Siemens SIPROTEC 7UT613 series, Руководство пользователя

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2.8 Unbalanced Load Protection
213
7UT613/63x Manual
C53000-G1176-C160-2
The unbalanced load protection must have been assigned to a side of the main pro-
tected object or another 3-phase current measuring location (Subsection 2.1.4 under
margin heading „Further 3-Phase Protection Functions“). Consider also the assign-
ment of the measured current inputs of the device against the measuring locations
(current transformer sets) of the power plant (section 2.1.4 under margin heading „As-
signment of 3-phase Measuring Locations“).
In address
4001 UNBALANCE LOAD
the function can be set to
ON
or
OFF
. The option
Block relay
allows to operate the protection but the trip output relay is blocked.
Note
If the unbalanced load protection is assigned to a side of the main protected object,
the current values are set referred to the rated current of that side
I
/
I
NS
, as stated in
section 2.1.4. In other cases, current values are set in amps.
Definite Time
Stages
I
2
>>,
I
2
> (O/C)
The two-stage characteristic enables the user to set the upper stage (address
4011
or
4012 I2>>
) with a short time delay (address
4013 T I2>>
) and the lower stage
(address
4014
or
4015 I2>
) with a slightly longer time delay (address
4016 T I2>
).
Stage
I
2
>, for example, can be used as alarm stage, and stage
I
2
>> as trip stage.
In most cases, stage
I
2
>> is set in such manner that it does not pick up in case of
phase failure. Setting
I2>>
to a percentage higher than 60 % ensures that no tripping
is performed with stage
I
2
in case of phase failure.
If power supply with current
I
is provided via just two phases, the following applies for
the inverse current:
On the other hand, with more than 60 % negative sequence current, a two-phase fault
in the system may be assumed. Therefore, the delay time
T I2>>
must be coordinat-
ed with the time grading of the system.
On line feeders, unbalanced load protection may serve to identify low-current unsym-
metrical faults below the pickup values of the time overcurrent protection. The follow-
ing applies
a phase-to-phase fault with current
I
corresponds to a negative sequence current:
a phase-to-earth current
I
corresponds to a negative sequence current:
With more than 60% unbalanced load, a two-phase fault can be assumed. The delay
time thus needs to be coordinated with the system grading for phase-to-phase faults.
If, for example, the asymmetrical load protection has been assigned to an outgoing
feeder, the asymmetrical load protection can be set to very sensitive. However, it must
be ensured that no asymmetrical load stage can be picked up be operationally permis-
sible asymmetries. With the preset values and secondary rated current 1 A the follow-
ing fault sensitivities are obtained:
for 2-pole faults:
I2>
= 0.1 A, i.e. fault current as from approx. 0.18 A,