ABB 615 series Technical Manual page 240

Section 4
Protection functions
234
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 trip current setting. The minimum amplitude level for the
polarizing quantity (voltage) is set with the Min trip 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 trip 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.
67/51P, 67/50P is provided with a memory function to secure a reliable and correct
directional protection relay operation in case of a close short circuit or a ground 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 trip 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 trip 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 trip 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 trip voltage and hysteresis. The fictive voltage is also discarded if the
measured voltage stays below Min trip voltageand 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 trip voltage and hysteresis and the fictive voltage is unusable, the fault
direction cannot be determined. The fictive voltage can be unusable for two reasons:
1MAC059074-MB A
615 series ANSI
Technical Manual