GE Multilin L60 Line Phase Comparison System 9-79 APPLICATION OF SETTINGS 9.2 DISTANCE BACKUP/SUPERVISION99.2.7 SENSITIVITY ISSUESPhase comparison is fundamentally dependent on the coincidence of local and remote squares to ensure a correct trippingdecision. However, correct starting by the FDL detector and arming action by the FDH detector should be ensured to allowthe coincidence detector operate correctly. Therefore, some precautions are necessary when choosing settings for FDLand FDH or assigning auxiliary elements to compliment those detectors.Even if direct coordination between FDL and FDH at opposite ends of the line is not required, FDH and FDL must haveenough security margin. This is especially critical when the blocking scheme is used. It is not advisable to set the FDHpickup at one end of the line close to or lower than FDL at other end of the line. FDL at the remote terminal should alwaysbe more sensitive and reach further to external faults behind remote bus. In the figure below, it is critical to ensure that forany fault F1 beyond terminal B, where FDH of Protection #1 still operates, the FDL at terminal B is sensitive enough andhas at least 20% margin for operation. The fault current must also be considered, even for through faults which might bequite different due to line capacitance, reactors on the line, etc. The situation worsens when there is a tapped load off theprotected line which can infeed/outfeed fault current. The same checks are required to coordinate Protection #2 FDH withProtection #1 FDL for fault F2.Figure 9–6: COORDINATION BETWEEN FDH AND FDL AT OPPOSITE ENDS OF THE LINETaking into consideration the points indicated above, the procedure for choosing FDL/FDH settings and checking sensitivityis as follows:1. Pickup settings for FDL and FDH are calculated per the recommendations above.2. For all internal faults on the line, a check is performed to ensure a minimum 20% margin in sensitivity for both FDL andFDH detectors at all terminals, according to the operating quantity formula for all system configurations.3. If there is not enough margin in sensitivity, then steps must be taken to provide carrier start and trip permission. It ispreferable to employ built-in functionality, as it provides reliable and deterministic coordination between FDL and FDHat opposite ends of the line. The following options can be employed:• Lowering the K factor in the composite signals. As such, detectors are less dependent on load current and can beset to be more sensitive to asymmetrical faults. However, this may affect sensitivity to three-phase faults and mustbe addressed by using supplementary protection functions (see below).• Assigning supplementary protection elements. Protection elements, like forward-looking overreaching phase dis-tance or simple undervoltage protection, can be assigned via the FDL AUX or FDH AUX settings to boost 87PCcarrier start and trip supervision.4. Once the FDL and FDH settings are selected, check for FDL and FDH coordination at opposite line terminals as indi-cated in the figure above.• For an external fault at the adjacent line (fault F1 for protection at terminal A), determine the sensitivity of FDH.• For protection at terminal B, ensure FDL sensitivity by a margin that overlaps the Protection A FDH zone by atleast 20%.• If there is not enough sensitivity, a reverse looking distance zone or neutral/negative-sequence directional over-current element might be assigned to the FDL AUX setting to secure FDL operation during external faults.• Similar checks must be performed for the F2 fault.In some applications, for example, radial line terminated with autotransformer or terminal with a weak source, where theremight be issues with sensitivity of FDL and FDH, additional elements have to be assigned to 87PC FDL AUX and 87PCFDH AUX settings. These elements include distance, negative-sequence overvoltage, zero-sequence overvoltage, posi-tive-sequence undervoltage.1 2BF1I (I_1, I_2, I_0)FFDHFDLAIFF2
