GE Multilin D30 Line Distance Protection System 8-78 THEORY OF OPERATION 8.1 DISTANCE ELEMENTS8The fast distance algorithm is activated upon detection of the system disturbance and is active for three cycles only. Thepickup and operate operands of the fast distance elements are internally OR-ed with regular distance elements, to achieveintegrated 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 volt-age 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 CON-NECTION setting is other than "None"8.1.5 MEMORY POLARIZATIONAll distance functions use memory polarization. The positive-sequence voltage – either memorized or actual – is used as apolarizing signal. The memory is established when the positive-sequence voltage remains above 80% of its nominal valuefor 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 voltagedrops below 80% of nominal or when the user-programmable condition is asserted to force memory polarization. After thememory expires, the relay checks the magnitude of the actual positive-sequence voltage. If it is higher than 10% of nomi-nal, 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 illustrated below. The self-polarized mho character-istic is shifted in the reverse direction for a forward fault by an amount proportional to the source impedance, and in the for-ward direction for a reverse fault.Figure 8–2: DYNAMIC SHIFT OF THE MHO CHARACTERISTICThe same desirable effect of memory polarization applies to the directional comparator of the quadrilateral characteristic.RMemory-polarized mho(Reverse fault)Memory-polarized mho(Forward fault)X837719A3.CDRSelf-polarized mho
