ABB RELION 611 Series Technical Manual page 115

1MRS757454 D
611 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.
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
109