ABB RELION REX640 - Page 625

Figure 355: Simplified illustration of a cross-country fault between phases A and C,
where two simultaneous single-phase earth faults occur in two different locations
and two different phases in the network
IFPTOC has an in-build dedicated functionality for cross-county fault detection and
tripping. Detection of cross-country fault is always enabled, but tripping may be
enabled or disabled with setting Enable XC Op mode = “on” or “off”. In case multiple
instances of IFPTOC are applied in the protection configuration, only single instance
of IFPTOC requires cross-country fault tripping functionality to be enabled; in other
instances, cross-country fault tripping functionality may be disabled. As default
Enable XC Op mode = “on” i.e. tripping is enabled during cross-country faults.
There are two independent criteria for cross-country fault detection: The 1st
criterion requires that residual current exceeds a value which is higher than
maximum single-phase earth-fault current of the network, and the 2nd criterion
monitors voltage dip in any of the phase-to-phase voltages.
Residual over-current criterion
Monitoring of the magnitude of calculated residual current (I
o
) and comparing it to
setting XC stage A Str Val during a detected earth fault:
(Equation 132)
Calculated I
o
is used to avoid problems in case of possible CBCT
saturation during high current cross-country fault.
It should be noted, that setting value XC stage A Str Val is given in unit [xIn], where
In is phase current nominal value. Typical value for XC stage A Str Val in primary
amperes is few hundreds of amperes. The setting XC stage A Str Val should not be
higher than the estimated minimum cross-country fault current (I
F_XCmin
), and on
the other hand it must be checked against the uncompensated earth-fault current
of the network (I
eNet
) and setting of the low-set overcurrent stage ( 3I> ):
1MRS759142 F
Protection functions
REX640
Technical Manual
625