ABB RED615 Technical Manual page 185

1YHT530004D05 D
615 series
Technical Manual
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 IED 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 the fictive voltage is unusable, the fault direction cannot be
determined. The fictive voltage can be unusable for two reasons:
• The fictive voltage is discarded after Voltage Mem time
• The phase angle cannot be reliably measured before the fault situation.
Section 4
Protection functions
179