ABB RELION REX640, Техническое руководство, Страница: 592 / 1960

(Equation 112)
When setting
Enable harmonics = “Enable”:
(Equation 113)
Where is the magnitude of the n
th
harmonic negative-sequence current
component (n = 1, 2, 3, 5, 7 and 9).
In case of frequency adaptive system measurements, only 2
nd
, 3
rd
and 5
th
harmonics are calculated.
In case there is fault resistance included in the fault current path, the effect of fault
resistance is to decrease the value of earth-fault current as shown by
Equation 106
.
The downscaling of fault current magnitude can be estimated to be proportional
to the magnitude of per unit value of residual voltage, refer to
Equation 106
. With
setting
Ena RF Compensation = “Enable”, user can compensate the effect of fault
resistance and then earth-fault current estimate becomes independent of actual
fault resistance value. This is done by dividing the fault current estimate with per
unit value of residual voltage (< 1pu):
Estimate for “fault resistance compensated” earth-fault current, :
(Equation 114)
This means that with setting
Ena RF Compensation = “Enable”, regardless of actual
fault resistance magnitude, the earth-fault current estimate matches the value that
would be valid if the fault would be a galvanic one (fault resistance equals zero).
Thanks to this feature the fault is always seen and treated as a galvanic fault
regardless of possible fault resistance. This functionality allows acceleration of
protection operation speed during high(er) ohmic earth fault when fault current
magnitude is low.
In case IFPTOC is used to detect high(er) ohmic earth faults, all start
thresholds (
Voltage start value, EF current Str Val or Touch Vol Str Val)
must be set according to sensitivity requirements. Otherwise
Ena RF
Compensation may not be effective during actual high(er) ohmic earth
fault.
In case of setting
Ena RF Compensation = “Enable”, the effect of
harmonics are not taken into account i.e. fault current estimation is
based only on fundamental frequency component.
Figure 339
illustrates this concept: In the left-hand column is shown residual
voltage, phase currents and fault current estimate during a galvanic earth fault (R
F
= 0 ohm). During a galvanic fault U
o
equals system nominal phase-to-earth voltage
Protection functions 1MRS759142 F
592 REX640
Technical Manual