10 APPLICATION OF SETTINGS
Phase comparison is fundamentally dependent on the coincidence of local and remote squares to ensure a correct tripping
decision. However, correct starting by the FDL detector and arming action by the FDH detector should be ensured to allow
the coincidence detector operate correctly. Therefore, some precautions are necessary when choosing settings for FDL
and 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 have
enough security margin. This is especially critical when the blocking scheme is used. It is not advisable to set the FDH
pickup at one end of the line close to or lower than FDL at other end of the line. FDL at the remote terminal should always
be more sensitive and reach further to external faults behind remote bus. In the figure below, it is critical to ensure that for
any fault F1 beyond terminal B, where FDH of Protection #1 still operates, the FDL at terminal B is sensitive enough and
has at least 20% margin for operation. The fault current must also be considered, even for through faults which might be
quite different due to line capacitance, reactors on the line, etc. The situation worsens when there is a tapped load off the
protected line which can infeed/outfeed fault current. The same checks are required to coordinate Protection #2 FDH with
Protection #1 FDL for fault F2.
Figure 10–6: COORDINATION BETWEEN FDH AND FDL AT OPPOSITE ENDS OF THE LINE
Taking into consideration the points indicated above, the procedure for choosing FDL/FDH settings and checking sensitivity
is as follows:
Pickup settings for FDL and FDH are calculated per the recommendations above.
For all internal faults on the line, a check is performed to ensure a minimum 20% margin in sensitivity for both FDL and
FDH detectors at all terminals, according to the operating quantity formula for all system configurations.
If there is not enough margin in sensitivity, then steps must be taken to provide carrier start and trip permission. It is
preferable to employ built-in functionality, as it provides reliable and deterministic coordination between FDL and FDH
at 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 be
set to be more sensitive to asymmetrical faults. However, this may affect sensitivity to three-phase faults and must
be 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 87PC
carrier start and trip supervision.
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 at
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 there
might be issues with sensitivity of FDL and FDH, additional elements have to be assigned to 87PC FDL AUX and 87PC
FDH AUX settings. These elements include distance, negative-sequence overvoltage, zero-sequence overvoltage, posi-
I (I_1, I_2, I_0)
L60 Line Phase Comparison System
10.2 DISTANCE BACKUP/SUPERVISION
10.2.7 SENSITIVITY ISSUES