RFL 9300 RFL Electronics Inc.
August 25, 2000 3 - 16 (973) 334-3100
3.11 RECLOSE BLOCKING FOR THREE-PHASE FAULTS
Because the RFL 9300 is a per-phase (or "segregated phase") scheme, fault types are always correctly identified.
This means that reclosing can be blocked for three-phase faults, if desired. (The output is always furnished; the user
may choose to connect this output to the reclosing relay's block input.)
Protection schemes that are not per-phase will sometimes mistake DLG (double line-to-ground) faults for three-phase
faults. This may occur when there is a very strong zero-sequence source at one end and the positive-sequence
source is at the other end. (See Figure 3-16.)
The unusual current line-up in Figure 3-16 will fool a non-segregated phase scheme at the local station into thinking
there is a three-phase fault. Also, these schemes probably will interpret the fault at the remote station as SLG, instead
of DLG. The RFL 9300 will correctly recognize this condition as a BC-G fault at both stations.
In the DLG fault condition shown in Figure 3-16, the Phase B and Phase C current line-ups at the local and remote
stations are 60 degrees out of phase. This still provides correct "nesting," but the margin for incorrect channel com-
pensation is reduced from a nominal 90 degrees (about 4 ms) to only 30 degrees. However, this is not a problem for
tripping; if the error in channel delay compensation hurts one of the faulted phases, it will actually help the other
faulted phase. (This happens because the out-of-phase relationship between I
B
at the local station and I
B
at the re-
mote station is opposite to that of I
C
at the local station and I
C
at the remote station). This is one of the advantages of
per-phase charge comparison.
Also in Figure 3-16, there is no 3I
0
current at the local station. Because of this, the 3I
0
phase controller in the RFL
9300 at the local station will operate in its weak-feed mode during a DLG fault.
The RFL 9300 calculates a trip independently on each phase. If a long channel delay exists or high-speed breakers
are used, the fault could be cleared before all three phases have calculated a trip. Because of this, reclose block on
modem-equipped RFL 9300's is not recommended.
3.12 UNDERGROUND CABLES
The distributed wire-to-ground shunt capacitance of an underground cable is very high - usually 10 to
20 times that of overhead lines of equal length. This is because the phase wires in an underground ca-
ble are closer to earth ground. Also, the phase-to-phase capacitance of an underground cable is usually
very high, depending upon cable construction. (The exception is where the phase cables are buried or
laid separately. This is done in some submarine installations.)
Shunt capacitive currents are sometimes referred to as "line charging currents." These are "false differential" currents
that can fool a current-based protection scheme into thinking there is a low-grade internal fault.
The RFL 9300 can be applied to underground cables by setting the phase bias and ground bias levels high enough to
provide security in the presence of line charging currents (or the discharging currents present during a fault). The
minimum bias setting is 1.0 amperes; this allows for up to 0.5 amperes of line charging current. If the cable's line
charging current is higher than 0.5 amperes, a higher bias setting must be used.
A false differential can also be created by transient in-rush to a cable when the line is energized. The amount of in-
rush is a function of source impedance and cable capacitance. The RFL 9300 can be made secure during the tran-
sient in-rush period by using its Bias Control feature. This raises the RFL 9300's bias level by an additional 1.5 am-
peres for a preset time (up to 1 second maximum, as selected by the user).
The RFL 9300 has a programmable parameter called "OVHD/UVG." This sets it for use with either overhead (OVHD)
or underground (UVG) cables. When this parameter is set to UVG, the incremental bias is increased to 3.0 amperes.
Weak Feed Boost
When a phase controller is configured with Bias Control enabled and is set to protect for underground cables
(UVG) a weak feed boost feature is automatically enabled. In this case a remote current greater than 3 Arms
+ bias setting is required to cause a weak feed trip signal to be returned to the remote station if the local sta-
tion breaker is open. This prevents the weak feed trip function from tripping the remote station at a fault cur-
rent lower than its Bias Control setting.
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