GE L60 Instructions Manual page 640

DISTANCE/BACKUP SUPERVISION
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 following figure, 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 that can be quite
different due to line capacitance, reactors on the line, and so on. The situation worsens when there is a tapped load off the
protected line that can infeed/outfeed fault current. The same checks are required to coordinate Protection #2 FDH with
Protection #1 FDL for fault F2.
F2
Taking into consideration the points indicated above, the procedure for choosing FDL/FDH settings and checking
sensitivity is as follows:
1. Calculate pickup settings for FDL and FDH as per the recommendations above.
2. For all internal faults on the line, perform a check 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.
3. 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 can 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
distance or simple undervoltage protection, can be assigned via the FDL AUX or FDH AUX settings to boost 87PC
carrier start and trip supervision.
4. Once the FDL and FDH settings are selected, check for FDL and FDH coordination at opposite line terminals as
indicated in the preceding figure.
– 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
least 20%.
– If there is not enough sensitivity, a reverse looking distance zone or neutral/negative-sequence directional
8 overcurrent element can be assigned to the
– Perform similar checks for the F2 fault.
In some applications, for example a radial line terminated with autotransformer or terminal with a weak source, where
there can be issues with sensitivity of FDL and FDH, additional elements have to be assigned to the
settings. These elements include distance, negative-sequence overvoltage, zero-sequence overvoltage, and
FDH AUX
positive-sequence undervoltage.
The phase comparison process of exchanging with HF signals is ready when the operating current (mixed or zero-
sequence) is above 2%. If an internal fault is detected, the challenge is to provide starting transmitting signals at both ends
and to have enough operating quantity for arming FDH to operate and to allow tripping.
8-8
Figure 8-6: Coordination between FDH and FDL at opposite ends of the line
A
I (I_1, I_2, I_0)
F
1
FDH
B
2
I
F
FDL
setting to secure FDL operation during external faults.
FDL AUX
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
CHAPTER 8: APPLICATION OF SETTINGS
F1
and
87PC FDL AUX 87PC