ABB RELION REX640 - Page 593

and has thus value of 1. 0pu (xU
n_PE
, U
n_PE
= Nominal phase-to-earth voltage). In the
middle column is shown residual voltage, phase currents and fault current estimate
during a higher ohmic earth fault (R
F
= 3000 ohm). During a higher ohmic earth
fault U
o
reduces and thus also the magnitude of fault current is reduced. But the
magnitude of fault current is reduced proportionally to residual voltage. In the
right-hand column the effect of fault resistance is compensated by scaling the fault
current estimate by dividing it with the per unit value of residual voltage. With fault
resistance compensation, the operation speed of IFPTOC can be accelerated during
high(er) ohmic earth faults.
Figure 339: Illustration of fault resistance compensation functionality in IFPTOC. Left-hand column:
galvanic earth fault (R
F
= 0 ohm). Middle column: a higher ohmic earth fault (R
F
= 3000 ohm) with
setting Ena R
F
Compensation = “Disable”. Right-hand column: a higher ohmic earth fault (R
F
= 3000
ohm) with setting Ena R
F
Compensation = “Enable”.
Note that it is the “effective” earth-fault current estimate which determines
operation of IFPTOC. “Effective” earth-fault current includes the effect of user
setting Reduction factor :
Effective earth-fault current estimate, :
(Equation 115)
Setting Reduction factor is a user defined reduction factor, which can be used to
scale down the estimated earth-fault current magnitude. Setting Reduction factor
allows e.g. consideration that not all the earth-fault current (I
F
) will flow back
through “remote” earth. See illustration in Figure 7. A portion of the earth-fault
current (I
SCR
) may have alternative return paths, e.g. through cable sheaths/screens.
Only current which flows through earth (I
E
) will introduce rise of earth potential
(earth potential rise, EPR, also called as ground potential rise GPR), which must
1MRS759142 F
Protection functions
REX640
Technical Manual
593