ABB RELION REX640 Technical Manual page 334

Section 4
Protection functions
328
The directional operation can be selected with the Directional mode setting. The user
can select either "Non-directional", "Forward" or "Reverse" operation. By setting the
value of Allow Non Dir to "True", the non-directional operation is allowed when the
directional information is invalid.
The Characteristic angle setting is used to turn the directional characteristic. The
value of Characteristic angle should be chosen in such a way that all the faults in the
operating direction are seen in the operating zone and all the faults in the opposite
direction are seen in the non-operating zone. The value of Characteristic angle
depends on the network configuration.
Reliable operation requires both the operating and polarizing quantities to exceed
certain minimum amplitude levels. The minimum amplitude level for the operating
quantity (current) is set with the Min operate current setting. The minimum amplitude
level for the polarizing quantity (voltage) is set with the Min operate voltage setting.
If the amplitude level of the operating quantity or polarizing quantity is below the set
level, the direction information of the corresponding phase is set to "Unknown".
The polarizing quantity validity can remain valid even if the amplitude of the
polarizing quantity falls below the value of the Min operate voltage setting. In this
case, the directional information is provided by a special memory function for a time
defined with the Voltage Mem time setting.
DPHxPDOC is provided with a memory function to secure a reliable and correct
directional protection relay operation in case of a close short circuit or an earth fault
characterized by an extremely low voltage. At sudden loss of the polarization
quantity, the angle difference is calculated on the basis of a fictive voltage. The fictive
voltage is calculated using the positive phase sequence voltage measured before the
fault occurred, assuming that the voltage is not affected by the fault. The memory
function enables the function to operate up to a maximum of three seconds after a total
loss of voltage. This time can be set with the Voltage Mem time setting. The voltage
memory cannot be used for the "Negative sequence voltage" polarization because it is
not possible to substitute the positive sequence voltage for negative sequence voltage
without knowing the network unsymmetry level. This is the reason why the fictive
voltage angle and corresponding direction information are frozen immediately for this
polarization mode when the need for a voltage memory arises and these are kept
frozen until the time set with Voltage Mem time elapses.
The value for the Min operate voltage setting should be carefully
selected since the accuracy in low signal levels is strongly affected by
the measuring device accuracy.
When the voltage falls below Min operate voltage at a close fault, the fictive voltage
is used to determine the phase angle. The measured voltage is applied again as soon
as the voltage rises above Min operate voltage and hysteresis. The fictive voltage is
also discarded if the measured voltage stays below Min operate voltage and hysteresis
for longer than Voltage Mem time or if the fault current disappears while the fictive
voltage is in use. When the voltage is below Min operate voltage and hysteresis and
1MRS759142 C
REX640
Technical Manual