ABB RED615 Technical Manual page 325

1YHT530004D05 D
615 series
Technical Manual
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 ohm) and the magnitude of the
network is 100 A (Rf = 0 ohm). 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:
(
(
I + ⋅ j I ⋅ − 1
Rcc eTot
Yo ≈
U
ph
(
( )
15 A + ⋅ j 100 A ⋅ − 1 1 1 .
=
15 k k V 3
GUID-8763BA04-22DC-4B93-B52D-1E8FD44A68B9 V1 EN
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.
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.
)
)
K − I
eFd
)
− 10 A
( )
≈ 1 73 . − ⋅ j 2 31 . milliSiemens
Section 4
Protection functions
(Equation 27)
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