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.
DPH3xPDOC can be forced to non-directional operation with the
NON_DIR
input.
When the
NON_DIR
input is active, DPH3xPDOC operates as a non-directional
overcurrent protection regardless of the
Directional mode setting.
1MRS757644 H
Protection functions
620 series
Technical Manual
301