ABB RELION 620 Series Technical Manual page 291

1MRS757644 F
620 series
Technical Manual
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.
DPH3xPDOC 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 the 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 asymmetry 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
discarded 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 if the fault current disappears while the fictive
voltage is in use
• The phase angle cannot be reliably measured before the fault situation.
Section 4
Protection functions
285