ABB RELION REX640, Техническое руководство, Страница: 498 / 1960

must therefore be based on the real part of the measured admittance,
that is, conductance. Thus, the best selectivity is achieved when the
compensated network is operated either in the undercompensated or
overcompensated mode.
For example, in a 15 kV compensated network, the magnitude of the earth-fault
current of the protected feeder is 10 A (Rf = 0 Ω) and the magnitude of the network
is 100 A (Rf = 0 Ω). During an earth fault, a 15 A resistor is connected in parallel to
the coil after a 1.0 second delay. Compensation degree is overcompensated, K = 1.1.
During an earth fault in the forward direction, that is, inside the protected feeder,
the theoretical value for the measured admittance after the connection of the
parallel resistor can be calculated.
Yo
I j I K I
U
A j A A
Rcc eTot eFd
ph
≈
+ ⋅ ⋅ −
( )
−
( )
=
+ ⋅ ⋅ −
( )
−
( )
1
15 100 1 1 1 10
15
.
kkV
j
3
1 73 2 31 ≈ − ⋅
( )
. .
milliSiemens
(Equation 68)
Before the parallel resistor is connected, the resistive part of the measured
admittance is due to the leakage losses of the background network and the losses
of the coil. As they are typically small, the resistive part may not be sufficiently large
to secure the discrimination of the fault and its direction based on the measured
conductance. This and the rating and the operation logic of the parallel resistor
should be considered when setting the admittance characteristic.
When a high sensitivity of the protection is required, the residual current
should be measured with a cable/ring core CT, that is, the Ferranti CT.
Also the use of the sensitive
Io
input should be considered. The residual
voltage measurement should be done with an open delta connection of
the three single pole-insulated voltage transformers.
The sign of the admittance characteristic settings should be considered
based on the location of characteristic boundary in the admittance plane.
All forward-settings are given with positive sign and reverse-settings
with negative sign.
Operation characteristic
After the admittance calculation is released, the calculated neutral admittance is
compared to the admittance characteristic boundaries in the admittance plane. If
the calculated neutral admittance Yo moves outside the characteristic, the enabling
signal is sent to the timer.
EFPADM supports a wide range of different characteristics to achieve the maximum
flexibility and sensitivity in different applications. The basic characteristic shape
is selected with the
Operation mode and Directional mode settings. Operation
mode defines which operation criterion or criteria are enabled and Directional mode
defines if the forward, reverse or non-directional boundary lines for that particular
operation mode are activated. The
FAULT_DIR
output indicates which operation
sector the fault is measured. The output is operational once the function is started.
Protection functions 1MRS759142 F
498 REX640
Technical Manual