2.11 Overcurrent protection
209
7SA522 Manual
C53000-G1176-C155-3
2.11.2 Method of Operation
Measured Values The phase currents are fed to the device via the input transformers of the measuring
input. The earth current 3 I
0
is either measured directly or calculated from the phase
currents, depending on the ordered device version and usage of the fourth current
input I
4
of the device.
If I
4
is connected in the current transformer star-point connection circuit the earth
current is directly available as a measured quantity.
If the device is fitted with the highly sensitive current input for I
4
, this current I
4
is used
with the factor ,,SK&7 (address , refer to Subsection 2.1.3 of the 36\VWHP
'DWD. As the linear range of this measuring input is severely restricted in the high
range, this current is only evaluated up to an amplitude of approx. 1.6 A. In the event
of larger currents, the device automatically switches over to the evaluation of the zero
sequence current derived from the phase currents. Naturally, all three phase currents
obtained from a set of three star-connected current transformers must be available
and connected to the device. The processing of the earth current is then also possible
if very small as well as large earth fault currents may occur.
If the fourth current input I
4
is utilized, e.g. for a transformer starpoint current or for the
earth current of a parallel line, the device calculates the earth current from the phase
currents. Naturally in this case also all three phase currents derived from a set of three
star connected current transformers must be available and connected to the device.
Definite Time
High-set Current
Stage I>>
Each phase current is compared with the setting value ,SK!! after numerical filtering;
the earth current is compared with ,!!3,&.83. Currents above the associated
pickup value are detected and signalled. After expiry of the associated time delays 7
,SK!! or 7,!! a trip command is issued. The dropout value is approximately 5 %
less than the pickup value, but at least 1.5 % of the rated current, below the pickup
value.
The following figure shows the logic diagram for the I>> stages. The stages can be
blocked via a binary input ´!%/2&.2&,!!µ. Binary input ´!2&,QVW75,3µ and
the function block “switch onto fault” are common to all stages and described below.
They may, however, separately affect the phase and/or earth current stages. This is
accomplished with the following setting parameters:
• ,!!7HOHS%, (address )determines whether a non-delayed trip of this
stage via binary input ´!2&,QVW75,3µ is possible (<(6) or impossible (12) and
• ,!!627) (address )determines whether during switching onto a fault tripping
shall be instantaneous(<(6) or not (12) with this stage.
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