RFL 9300 RFL Electronics Inc.
October 29, 2001 2 - 17 (973) 334-3100
2.7.3 WEAK-FEED TRIP (WFT) ALGORITHM
In 2-terminal systems the SFT (Strong-Feed Trip) algorithm requires a received CCD or WCM message to be
compared with a local CCD half-cycle integral value for the corresponding interval. In 3-terminal systems the
SFT algorithm requires a composite CCD value derived from a validated pair of received CCD values, a pair of
received WCM values, or a combination of CCD and WCM values to be compared with a local CCD half-cycle
integral value for the corresponding interval. However, if the local station current is less than 0.5 Arms, there will
be no stored half-cycle integral value for comparison. Also, if the fault at the local station is weak enough, there
may be no fault detector activity. The WFT (Weak Feed Trip) algorithm would then be used to enable tripping for
weak internal faults. (Refer to Figure 2-5.)
A weak-feed trip will be enabled if both of the following conditions are met:
1. The CCD value (2-terminal) or composite CCD value (3-terminal) received by the phase controller is
greater than 1.5A rms plus the bias level setting. Note that if the bias control function is enabled for un-
derground lines, the CCD value (2-terminal) or composite CCD value (3-terminal) received by the phase
controller must be greater than 3A rms plus the bias level setting.
The composite CCD value used in the WFT algorithm for 3-terminal applications is calculated as fol-
lows:
a) If a CCD value received from one remote terminal is validated by a WCM from the second remote
terminal the CCD value along with its adjusted received time-tag is used as is.
b) If a pair of CCD values received from the remote terminals validate each other the adjusted re-
ceived time-tag will be the average of the two. The CCD values will be resolved to this time-tag and
summed to calculate the composite value used in the algorithm.
2. The current value recorded locally is less than 1.5 Arms.
If these conditions are satisfied and there is no composite fault detector, a weak-feed trip will not be enabled at
the local station. Instead, a weak-feed trip signal will be sent to the remote station. If the remote station has the
required number of active fault detectors, it will issue a trip signal and transmit a CCT-U signal back to the local
station.
Receiving a UHST signal without the necessary fault detector conditions being satisfied, also will result in a
WFT signal being echoed back to the sending RFL 9300. This will result in a faster trip response time, since the
remote RFL 9300 does not have to wait for the zero crossing of the current waveform before it transmits the
UHST signal.
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