P54x/EN OP/La4 Operation
(OP) 5-66
MiCOM P543, P544, P545 & P546
OP
1.23.4 Permissive overreach scheme weak infeed features (Distance option only)
Weak infeed logic can be enabled to run in parallel with the POR schemes. Two options are
available: WI Echo, and WI Tripping.
(Note: Special stub-end transformer Weak Infeed is covered in section 1.37)
Wea
k Infeed Ech
o - 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
(i.e. 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
- No Distance Zone Operation, plus Channel Received.
Weak Infeed Tripping
- Weak infeed echo logic ensures an aided trip at the
strong infeed terminal but not at the weak infeed. The
MiCOM P54x also has 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. This voltage check prevents tripping during
spurious operations of the channel or during channel
testing.
The additional weak infeed trip logic is:
Weak Infeed Trip
- No Distance Zone Operation, plus V<, plus Channel
Received.
Weak infeed tripping is time delayed according to the WI Trip Delay value. Due to the use
of phase segregated undervoltage elements, single pole tripping can be enabled for WI trips
if required. If single pole tripping is disabled a three pole trip will result after the time delay.
1.23.5 Permissive scheme unblocking logic - loss of guard (Distance option only)
This mo
de is designed for use with frequency shift keyed (FSK) power line carrier
communications. When the protected line is healthy a guard frequency is sent between line
ends, to verify that the channel is in service. However, when a line fault occurs and a
permissive trip signal must be sent over the line, the power line carrier frequency is shifted to
a new (trip) frequency. Therefore, distance relays should receive either the guard, or trip
frequency, but not both together. With any permissive scheme, the PLC communications
are transmitted over the power line which may contain a fault. So, for certain fault types the
line fault can attenuate the PLC signals, so that the permissive signal is lost and not received
at the other line end. To overcome this problem, when the guard is lost and no “trip”
frequency is received, the relay opens a window of time during which the permissive scheme
logic acts as though a “trip” signal had been received. Two opto inputs to the relay need to
be assigned, one is the Channel Receive opto, the second is designated Loss of Guard (the
inverse function to guard received). The function logic is summarized in the table below.
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