10 APPLICATION OF SETTINGS
10 APPLICATION OF SETTINGS 10.1PHASE COMPARISON ELEMENT 87PC
The L60 Phase Comparison relay is designed to provide high-speed protection of transmission lines against all phase and
ground faults when operated in the "mixed-excitation" mode. The term "mixed-excitation", when applied to phase compari-
son, describes a scheme that first mixes different sequence quantities in a given proportion and phase angle, then performs
a phase-comparison based on this mix.
A complete explanation of the Phase Comparison Element operation principles can be found in Chapter 8.
A mixed I_2 – K × I_1 signal, mixed I_1 + I_2 / K signal, or 3I_0 signal can be chosen as the operating signal for FDH and
FDL excitation. In mixed excitation mode, the relay provides high-speed protection of transmission lines against all phase
and ground faults. However, if a user wants the relay to operate only during ground faults, the 3I_0 mode can be chosen.
87PC MIXED SIGNAL K:
The K factor must be chosen for the mixed excitation operating signal. As indicated in chapter 8: Theory of operation, best
results are obtained using a value of 0.2 (the default setting). The selected K value can range from 0.00 to 0.25. Setting
K = 0 makes a phase comparison on the basis of negative-sequence excitation only. In such a scheme, the relay protects
against all unbalanced faults; a suitable phase-distance relay should be used to protect against three-phase faults.
The user must remember that K is an important tool to set FDL and consequently FDH at the lower setting, especially in the
cases when the margin between the maximum load current and the minimum fault current is very small. Reducing K to 0.15
or 0.10 makes phase comparison protection less sensitive to load current which in turn allows the user to provide enough
sensitivity to the fault current. From the other hand it makes protection less sensitive to the balanced three-phase fault
which in fact occurs very rarely.
87PC FDL PICKUP:
The main function of FDL is keying the transmitter. FDL pickup must be set above the K × I_1 output of the mixing network
for the maximum expected load. The recommended FDL setting is as follows;
FDL = 1.1 × K × I1L where I1L is the maximum line load current and K is a mixed signal factor as described above.
Higher margin may be required to definitely avoid FDL pickup during normal load condition.
If the 3I_0 operating signal has been chosen, FDL should be set as FDL=1.1 ⋅ I1L where I1L is a maximum line load
It must be noted that in some cases a channel may also perform other functions if it is objectionable to key the transmitter
constantly. In such cases, FDL could be set well below the K × I_1 value of the mixing network resulting from maximum
load but not less than 0.05 pu which defines the minimum required current from current transformers.
87PC FDH PICKUP:
The main function of FDH is to permit tripping as it arms the tripping output. FDH pickup must be set high enough so that it
will not operate on maximum load. Also FDH must be set high enough to reset itself in the presence of heavy loads follow-
ing clearing an external fault. The recommended setting for FDH is as follows:
FDH = (4/3) × FDL + 0.375 × Ic1
where Ic1 is total positive-sequence charging current under normal conditions.
A distance relay is recommended as an external fault detector if the minimum internal three-phase fault is less than twice
the maximum load current. It allows coincidence detector to start comparing the local and received signals and to make trip
decision if FDH is not picked up.
If 3I_0 operating signal has been chosen, FDH should be set at most 0.66 times but preferably 0.5 times the minimum inter-
nal ground fault current to provide reliable sensitivity of the fault.
87PC FDL AUX:
This setting is provided for cases when sensitivity of the built-in FDL operating on the I_2 – K × I_1 quantity is not sufficient.
This can happen on the weak terminal of the heavily loaded line. FDL AUX is connected in parallel with built-in FDL fault
detector and is intended to be assigned with elements such as distance, negative-sequence overvoltage and overcurrent,
L60 Line Phase Comparison System
10.1 PHASE COMPARISON ELEMENT 87PC
10.1.2 USE OF SETTINGS