ABB RELION REX640, Техническое руководство, Страница: 590 / 1960

fault current due to total network phase-to-earth capacitance value and the network
damping (
I
d
) value includes losses due to the neutral point resistor.
Estimation of earth-fault current and methods for ensuring its validity are explained
next.
Fault current estimation
In IFPTOC the estimation of earth-fault current flowing at the fault location is done
utilizing changes in phase currents measured due to an earth fault.
To estimate the earth-fault current magnitude, change in threefold negative-
sequence component due to earth fault is calculated (phase A as reference, phase
rotation: ABC):
(Equation 109)
(Equation 110)
(Equation 111)
Where
= Earth-fault current estimate (phasor)
= Earth-fault current estimate (magnitude)
= change of phase A current phasor due to earth fault
= change of phase B current phasor due to earth fault
= change of phase C current phasor due to earth fault
= change of negative sequence-current phasor due to earth fault
= Phase rotation operator = cos(120°) + j·sin(120°)
Applying the “delta” calculation i.e. change in phase currents due to earth fault
removes the healthy-state negative-sequence component from earth-fault current
estimate, which may exist due to the practical unbalances in load currents and
capacitive charging currents. Pre-fault negative-sequence current value is typically
rather low in magnitude and constant in time domain, which enables good
estimation accuracy regardless of load current level variations, or in case of
uncertainty in the exactness of the pre-fault value (e.g. during auto-reclosing cycle).
The validity of memorized pre-fault negative-sequence component value that is
utilized in delta calculation becomes poorer in time. There is thus a maximum
time defined by the user that delta calculation is valid. This is given with setting
Protection functions 1MRS759142 F
590 REX640
Technical Manual