GE L60 Instructions Manual page 634

87PC PHASE COMPARISON ELEMENT
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 can be required to definitely avoid FDL pickup during normal load condition.
• If the 3I_0 operating signal has been chosen, set FDL as FDL=1.1 × I1L where I1L is a maximum line load current
Note that in some cases a channel can also perform other functions if it is objectionable to key the transmitter constantly.
In such cases, FDL can 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
does not operate on maximum load. Also FDH must be set high enough to reset itself in the presence of heavy loads
following clearing of 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 the coincidence detector to start comparing the local and received signals and to
make trip decision if FDH is not picked up.
If the 3I_0 operating signal has been chosen, set FDH at most 0.66 times but preferably 0.5 times the minimum internal
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,
and ground directional. Be aware that auxiliary element assigned with FDL AUX does not provide any tripping function, but
only starts the carrier to send pulses to remote terminal and allow the coincidence detector to make trip/block decisions.
87PC FDH AUX
This setting is provided for cases (similar to FDL AUX) when sensitivity of the built-in FDH operating on the I_2 – K × I_1
quantity is not sufficient. FDH AUX is connected in parallel with the built-in FDH fault detector and is intended to be
assigned with elements, such as distance, negative-sequence overvoltage and overcurrent, and ground directional. FDH
(along with FDH AUX) provides arming action for phase comparison trip decisions, allowing trips only when the coincidence
detector detects internal fault conditions and FDL (or FDL AUX) operates.
87PC CH1 SYMMETRY and 87PC CH2 SYMMETRY
These settings are used to make the local squared signal and the received signal from the remote terminal symmetrical. To
8
set it properly, keying of the remote transmitter and an oscilloscope are required. If the received signal is ideally
symmetrical with respect to the MARK and SPACE signals, use a value of 0 ms (set as default). If for example, measured
length of the MARK is longer than SPACE for 4.0 ms, setting –2.0 ms to be entered. If the measured length of the MARK is
shorter than SPACE for 3.0 ms, setting +1.5 ms is to be entered. As the sum of MARK and SPACE signals equals the length of
the power cycle, correspondingly scale the signals for off nominal system frequencies.
Negative setting time is needed if the receiver elongates the received signal, and positive setting time is needed if the
receiver shortens the signal.
The L60 allows you to check and set channel symmetry without using an oscilloscope, by means of FlexLogic operands
and applying the corresponding current to the relays, which in turn key the PLC and consequently measure MARK and
SPACE signals on the oscillography. Moving cursors and measuring an average from a few points time, you can determine
and enter the setting.
8-2
CHAPTER 8: APPLICATION OF SETTINGS
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL