Memory Polarization - GE D90 Plus Instruction Manual

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THE D90 DISTANCE ELEMENTS

Memory polarization

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All distance functions use memory polarization. The positive-sequence voltage, either
memorized or actual, is used as a polarizing signal. The memory is established when the
positive-sequence voltage remains above 80% of its nominal value for five power system
cycles. The memory voltage is a two-cycle old voltage.
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
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expires, the D90 checks the magnitude of the actual positive-sequence voltage. If it is
greater than 10% of nominal, the actual voltage is used; if it is less than 10% of nominal,
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 illustrated below.
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 465: Dynamic shift of the memory-polarized mho characteristic
The same desirable effect of memory polarization applies to the directional comparator of
the quadrilateral characteristic.
Figure 466: Dynamic shift of the memory-polarized quadrilateral characteristic
Mutual zero-sequence compensation may raise concerns regarding directional integrity
on reverse faults in the situation when the relay gets overcompensated. This problem does
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not affect the D90 because its ground distance elements use zero-sequence and
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D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 15: THEORY OF OPERATION