ABB RELION REX640, Техническое руководство, Страница: 562 / 1960

measured residual current Io. The phase angle of operation point in the
faulted feeder can be approximated with equation
ϕ
=
+






atan
detuning
I I
I
EFFd
damping
(Equation 96)
I
EFFd
Earth-fault current produced by the protected feeder [A]
I
detuning
Detuning of the arc suppression coil [A]
I
damping
System damping [A]
Example 1:
I
EFFd
= 30 A, I
detuning
= +10 A and I
damping
= 10 A. From
Equation 96
, j = 76 deg. Thus
Tilt angle should not exceed ~10 deg.
Example 2:
I
EFFd
= 50 A, I
detuning
= +10 A and I
damping
= 5 A. From
Equation 96
, j = 85 deg. Thus
Tilt angle should not exceed ~3 deg. However, such a low Tilt angle setting may
endanger the security of protection due to measurement errors of the CBCTs. In this
case, the phase angle of operation point should be increased by considering:
1. System damping increase by increasing the current value of the parallel resistor
of the coil.
2. Reduction of the earth-fault current produced by the protected feeder by, for
example, installation of distributed compensation coils or by change of the
feeder topology.
3. Reduction of overcompensation of the network. The recommendation is to use
the undercompensation mode.
The
DIRECTION
output indicates on which operating sector the current is
measured. The
FAULT_DIR
output indicates in which operation sector the fault
current is measured. The output is operational once the function is started.
To adapt the fault direction determination to possible fault direction change during
the fault, for example, during manual fault location process, a cyclic accumulation
of sum admittance phasors is conducted. The duration of this directional evaluation
cycle is 1.2 ·
Reset delay time (minimum of 600 ms). If the fault direction based
on the cyclic phasor accumulation is opposite to the function direction output for
Reset delay time or 500 ms (minimum of 500 ms), the function is reset and fault
direction calculation of MFADPSDE is restarted.
If the earth-fault protection generates alarms, MFADPSDE also includes a
RESET
input which can be used to externally re-trigger the fault direction determination if
reevaluation of fault direction during a persistent earth fault is required.
It is also recommended to connect the start signal of non-directional earth fault
protection (EFxPTOC), set to operate in case of a cross-country fault, to
RESET
input or
BLOCK
input of MFADPSDE to reset phasor accumulation during a cross-
country fault. MFADPSDE can then adapt to a fault direction change more rapidly if a
singlephase earth fault persists in the system after the other faulty feeder has been
tripped (the cross-country fault has been transformed into a single-phase earth
fault).
The direction of MFADPSDE is supervised by a settable current magnitude
threshold, setting
Min operate current. The operate current used in the magnitude
supervision is measured with a special filtering method, which provides a stable
Protection functions 1MRS759142 F
562 REX640
Technical Manual