44x/EN AP/Hb6
-68 MiCOM P40 Agile
Note: The 2 modes are blocked during Fuse failure conditions.
3.2.9.1 Weak Infeed Activation
Figure 43: Weak Infeed mode activation logic
3.2.9.2 Weak Infeed Echo
For permissive schemes, a signal would only be sent if the required signal send zone were
to detect a fault. However, the fault current infeed at one line end may be so low as to be
insufficient to operate any distance zones, and risks a failure to send the signal. Also, if one
circuit breaker had already been left open, the current infeed would be zero. These are
termed weak infeed conditions, and may result in slow fault clearance at the strong infeed
line end (tripping after time tZ2). To avoid this slow tripping, the weak infeed relay can be set
to “echo” back any channel received to the strong infeed relay (ie. to immediately send a
signal once a signal has been received). This allows the strong infeed relay to trip
instantaneously in its permissive trip zone. The additional signal send logic is (“UNB CR” for
DDB: ‘Dist UNB CR’ or DDB: ‘DEF UNB CR’):
Figure 44: Weak infeed echo
3.2.9.3 Weak Infeed Tripping
Weak infeed echo logic ensures an aided trip at the strong infeed terminal but not at the
weak infeed. The P442 and P444 relays also have a setting option to allow tripping of the
weak infeed circuit breaker of a faulted line.
Three undervoltage elements, Va<, Vb< and Vc< are used to detect the line fault at the weak
infeed terminal, with a common setting typically 70% of rated phase-neutral voltage. This
voltage check prevents tripping during spurious operations of the channel or during channel
testing.
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