In section 1, where 0.0 < I
b
/I
n
<
End section 1
, the differential current required for
tripping is constant. The value of the differential current is the same as the
Low
operate value
setting selected for the function block. The
Low operate value
setting
allows for small inaccuracies of the current transformers but it can also be used to
influence the overall level of the operating characteristic.
Section 2, where
End section 1
< I
b
/I
n
<
End section 2
, is called the influence area
of the setting
Slope section 2
. In this section, variations in
End section 2
affect the
slope of the characteristic, that is, how big the change in the differential current
required for tripping is in comparison to the change in the load current. The
End
section 2
setting allows for CT errors.
In section 3, where I
b
/I
n
>
End section 2
, the slope of the characteristic can be
set by
Slope section 3
that defines the increase in the differential current to the
corresponding increase in the biasing current.
The required differential current for tripping at a certain stabilizing current level can
be calculated using the formulae:
For a stabilizing current lower than
End section 1
I I Set Lowoperate value
doperate r
[% ] =
(Equation 63)
For a stabilizing current higher than
End section 1
but lower than
End section 2
I I Lowoperate value I Ir End section Slope
doperate r b
[% ] ( [% ] )= + − ⋅1 ssection 2
(Equation 64)
For higher stabilizing current values exceeding
End section 2
I I Lowoperate value End section End section S
d operate r
[% ] ( )= + − ⋅2 1 llope section I I End section Slope section
b r
2 2 3+ − ⋅( [% ] )
(Equation 65)
When the differential current exceeds the operating value determined by the
operating characteristics, the OPR_LS output is activated. The OPERATE output is
always activated when the OPR_LS output activates.
The operate signal due to the biased stage can be blocked by the activation of the
BLK_OPR_LS or BLOCK input. Also, when the operation of the biased low stage is
blocked by the waveform blocking functionality, the INT_BLKD output is activated
according to the phase information.
The phase angle difference between the two currents I_A1 and I_A2 is theoretically
180 electrical degrees for the external fault and 0 electrical degrees for the internal
fault conditions. If the phase angle difference is less than 50 electrical degrees or
if the biasing current drops below 30 percent of the differential current, a fault
has most likely occurred in the area protected by MPDIF. Then the internal blocking
signals (CT saturation and DC blocking) of the biased stage are inhibited.
Protection functions
1MRS757644 H
506 620 series
Technical Manual
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