value of the difference between the residual current, that is, 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 cosφ. The value is available in the monitored data view.
ID COSPHI I Io_ ( ) cos= − ×Σ
ϕ
(Equation 82)
Residual current
Phase difference between the residual and neutral currents
Neutral current
An earth 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.
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.
During an earth fault in the protected area, the currents ΣI and Io are directed
towards the protected area. The factor cosφ 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 earth 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 earth 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
in the monitored data view.
(Equation 83)
Protection functions
1MRS757644 H
560 620 series
Technical Manual
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