Memory Polarization - GE L90 Instruction Manual

CHAPTER 10: THEORY OF OPERATION
The fast distance algorithm is activated upon detection of the system disturbance and is active for three cycles only. The
pickup and operate operands of the fast distance elements are internally OR-ed with regular distance elements, to achieve
integrated distance element optimal operating speed.
Note that fast distance is not active for the following applications:
• Use of the dynamic reach control for series-compensated line applications by selecting a non-zero value for the
voltage level setting in the Distance elements
• Non-directional option for the zone 1 and 2 Direction setting
• Phase Distance applications through power transformers, when the XFMR VOL CONNECTION or XFMR CUR
CONNECTION setting is other than "None"

10.3.5 Memory polarization

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 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.
The same desirable effect of memory polarization applies to the directional comparator of the quadrilateral characteristic.
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
Figure 10-10: Dynamic shift of the mho characteristic
DISTANCE ELEMENTS
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