ABB RELION REX640, Техническое руководство, Страница: 586 / 1960

defined as the vector sum of fundamental frequency phase voltage phasors divided
by three.
(Equation 100)
When the magnitude of U
o
exceeds setting
Voltage start value, an earth fault is
detected and
EF_IND
output is set to TRUE. Setting value
Voltage start value
is given in per unit format with system nominal phase-to-earth value (U
n_PE
) as
reference.
IFPTOC supports fundamental frequency residual voltage monitoring based on
measured U
o
from open-delta winding or internally calculated U
o
derived from
connected phase-to-earth voltages.
Input
EXT_RELEASE
is alternative method for internal GFC and validity
check modules to indicate the presence of earth fault in the protected
feeder and to release the calculation of IFPTOC function. Activation of
EXT_RELEASE
will also override earth-fault validity check criterion i.e.
earth-fault current estimate is considered as valid when
EXT_RELEASE
is activated. It is therefore mandatory that
EXT_RELEASE
is based on
start of directional earth-fault detection function, such as MFADPSDE or
DEFxPDEF.
The setting
Voltage start value defines the basic sensitivity of the IFPTOC function
in terms of fault resistance. Fault detection sensitivity in terms of fault resistance
can be estimated using the equation below:
(Equation 101)
where U
PE
is the system phase-to-earth voltage [V],
I
d
is the total system damping
[A],
I
v
is the network detuning value [A] and
U
o_pu
is setting
Voltage start value in per
unit.
In compensated networks the minimum value for the setting
Voltage start value can
be easily selected based on the resonance curve calculated by the coil controller
considering different network topologies and conditions, refer to
Figure 336
.
Protection functions 1MRS759142 F
586 REX640
Technical Manual