ABB RELION REX640, Техническое руководство, Страница: 588 / 1960

where t
FLT
(1) equals the first time instant, when earth fault is detected by the GFC-
module. Pre-fault time moment is at least
Revert time before earth fault occurrence
moment.
Revert time setting should be set high enough (default value is 300 ms), because
during a high(er) ohmic fault the increase speed (gradient) of the magnitude of
U
o
may be slow, refer to example in
Figure 337
, where earth-fault occurs at time 0
ms. Earth fault is detected by GFC module slightly after that, at time instant t
FLT
(1)
marked with red dot, when magnitude of U
o
exceeds setting
Voltage start value. As
an indication of detected earth-fault output
EF_IND
is activated.
Figure 337: Illustration of operation of General Fault Criterion (GFC) module and the meaning of
setting Revert time
In case of secondary testing of IFPTOC function, the pre-fault time in the
test file must be longer than
2xRevert time. For example, if Revert time =
300 ms, then pre-fault time in the test file must be longer than 600 ms.
Indication of detected earth fault is available in Recorded data:
EF
indication.
Fault current estimation and validity check
Operation of IFPTOC is based on estimating the magnitude of earth-fault current
flowing in the fault spot, or on touch voltage magnitude derived from the earth-
fault current estimate.
In compensated network, the magnitude of fundamental frequency earth-fault
current depends on the network damping (
I
d
), detuning (
I
v
), voltage (
U
PE
) and fault
resistance (
R
F
) and can be written as:
Protection functions 1MRS759142 F
588 REX640
Technical Manual