Section 4
1MAC050144-MB C
Protection functions
374
615 series ANSI
Technical Manual
and the fundamental frequency component of the neutral current (IG) flowing in the
conductor between the transformer or generator's neutral point and ground. The
differential current is calculated as the absolute value of the difference between the
residual current (the sum of the fundamental frequency components of the phase currents
I_A, I_B and I_C) and the neutral current. The directional differential current ID_COSPHI
is the product of the differential current and cos
φ
. The value is available through the
Monitored data view.
(Equation 47)
Residual current
φ
Phase difference between the residual and neutral currents
Neutral current
A ground fault occurring in the protected area, that is, between the phase CTs and the
neutral connection CT, causes a differential current. The directions, that is, the phase
difference of the residual current and the neutral current, are considered in the operation
criteria to maintain selectivity. A correct value for
CT connection type
is determined by
the connection polarities of the current transformer.
During a ground fault in the protected area, the currents
Σ
I and IG are directed towards the
protected area. The factor cos
φ
is 1 when the phase difference of the residual current and
the neutral current is 180 degrees, that is, when the currents are in opposite direction at the
ground faults within the protected area. Similarly, ID_COSPHI is specified to be 0 when
the phase difference between the residual current and the neutral current is less than 90
degrees in situations where there is no ground fault in the protected area. Thus tripping is
possible only when the phase difference between the residual current and the neutral
current is above 90 degrees.
The stabilizing current IB used by the stabilizing current principle is calculated as an
average of the phase currents in the windings to be protected. The value is available
through the Monitored data view.
(Equation 48)
The current transformer ratio mismatch between the phase current
transformer and neutral current transformer (residual current in the analog
input settings) is taken into account by the function with the properly set
analog input setting values.
ID COSPHI
I I
_
(
) cos
=
−
×
Σ
0
Σ
I
I
0
IB
I A
I B
I C
=
+
+
_
_
_
3
Содержание 615 Series ANSI
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