Application Notes
P44x/EN AP/E33
MiCOM P441/P442 & P444
Page 47/220
•
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:
Echo send:
WI logic
UNB_CR
&
WI_CS
(NB: For UNB_CR explanation see Unblocking logic in next section 2.9.4)
•
Weak Infeed Tripping
Weak infeed echo logic ensures an aided trip at the strong infeed terminal but not at the
weak infeed. The P441, 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.
P0481ENa
VA<_WI
WI_A
&
CB 52a_phA
CB 52a_phB
CB 52a_phC
VB<_WI
WI_B
&
VC<_WI
WI_C
&
UNB_CR
&
&
&
FLT_A
FLT_B
FLT_B
FIGURE 25 - WEAK INFEED PHASE SELECTION LOGIC
UNB_CR is used as a filter to avoid a permanent phase selection which could be maintained
if Cbaux signals are not mapped in the PSL (when line is opened).
Содержание MiCOM P441
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