9-8 D30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
DISTANCE ELEMENTS CHAPTER 9: THEORY OF OPERATION
9
Once established, the memory is applied for the user-specified time interval. The memory timer is started when the
voltage drops below 80% of nominal or when the user-programmable condition is asserted to force memory polarization.
After the memory expires, the relay checks the magnitude of the actual positive-sequence voltage. If it is higher than 10%
of nominal, the actual voltage is used; if lower, the memory voltage continues to be used.
A provision is added to force self-polarization from any user-programmable condition.
The memory-polarized mho has an extra directional integrity built-in as shown in the figure. The self-polarized mho
characteristic is shifted in the reverse direction for a forward fault by an amount proportional to the source impedance,
and in the forward direction for a reverse fault.
Figure 9-2: Dynamic shift of the mho characteristic
The same desirable effect of memory polarization applies to the directional comparator of the quadrilateral characteristic.
Figure 9-3: Dynamic shift of the memory-polarized directional characteristic
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