CHAPTER 8: APPLICATION OF SETTINGS POTT SIGNALING SCHEMEL60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 8-1188.2.9.4 Phase distance zone 3If a remote backup philosophy is followed, then the reach of this element must be set to account for any infeed at theremote bus, plus the impedance of the longest line which terminates on this remote bus. The time delay must coordinatewith other time-delayed protections on any remote line. Circuit loading limitations created by a long zone reach can beovercome by using lens or quadrilateral characteristics and/or a load encroachment supervising characteristic. Consideralso a situation where the load impedance can enter into the relay characteristic for a time longer than the chosen timedelay, which can occur transiently during a system power swing. For this reason, use the power swing blocking function.8.2.10 Ground distance8.2.10.1 Neutral current supervisionThe current supervision for the ground distance elements responds to an internally calculated neutral current (3 × I_0).Base the setting for this element on twice the zero-sequence line capacitance current or the maximum zero-sequenceunbalance under maximum load conditions. Do not use this element to prevent an output when the load impedance isinside the distance characteristic on a steady state basis.8.2.10.2 Ground distance zone 1The zone 1 reach must be set so that nominally instantaneous operation does not extend beyond the end of the protectedline. However this can be more complicated than for the phase elements, because of zero sequence mutual induction withan adjacent parallel line, possibly carried on the same tower, which can be out of service and grounded at multiple points.A fault beyond 100% of the protected line can cause overreach unless the reach is reduced significantly, sometimes as lowas 65% of the line length. If the line being protected does not have a significant interaction with an adjacent circuit, thenthe typical 80% setting can be used. If there is significant mutual coupling between the parallel lines, then the mutualcompensation feature of the ground distance elements can be used instead of a drastic reduction in the reach.However, even in this case, there is more uncertainty compared with the phase distance elements because the zero-sequence impedance of the line and thus the zero-sequence-compensating factors can vary significantly due to weatherand other conditions.8.2.10.3 Ground distance zone 2To ensure that zone 2 can see 100% of the line, inter-circuit mutual effects must be considered, as they can contribute to asignificant under-reach. Typically this occurs on double circuit lines, when both lines carry the same current. Conduct ananalytical study to determine the appropriate reach setting.The main purpose of this element is to operate for faults beyond the reach of the local zone 1 element, and therefore atime delay must be used similar to the phase fault case.8.2.10.4 Ground distance zone 3This remote back up function must have a reach that is set to account for any infeed at the remote bus, plus theimpedance of the longest line that terminates on this remote bus. Similar to the phase fault case, a zone 3 element mustbe time coordinated with timed clearances on the next section.8.3 POTT signaling scheme8.3.1 OverviewThis scheme is intended for two-terminal line applications.This scheme uses an over-reaching Zone 2 distance element to essentially compare the direction to a fault at both theends of the line.Ground directional overcurrent functions available in the relay can be used in conjunction with the Zone 2 distanceelement to key the scheme and initiate its operation. This provides increased coverage for high-resistance faults.
