Description of Operation
GARD 8000 Distance Relay
RFL Electronics Inc.
The operational logic for a two-phase element is shown in the figure below.
Figure 9-9. AB Distance Element Operational Logic
9.1.7
MEMORY VOLTAGE LOGIC
Memory voltage is only used for close-in three phase faults when the positive sequence voltage is less
that the set threshold (0.2 – 5.0 V). In addition, if the series compensated setting is used, memory
voltage is always used for three phase faults due to the possibility of voltage reversals for those
applications.
The positive sequence voltage memory buffer stores the voltage from two cycles before fault
inception, as determined by a fault detector.
The use of memory voltage is conditional; in addition, the memorized positive sequence voltage is
only used if its value is above 20 V to prevent use of memory for close-into-fault conditions with 0
pre-fault voltage. The duration of using memory voltage is determined by the setting ‘Duration
Memory’ on web page Zone 1 to Zone 4.
The memory duration is generally set relatively short, typically 4 cycles. Only if time-delayed Zone 2
operation for faults with terminal voltage below the set threshold is required would the memory
duration need to be extended to last for set Zone 2 time delay.
August 1, 2009
9-13
973.334.3100
