P44x/EN AP/Hb
MiCOM P40 Agile P442, P444
(AP) 5-
‘DIST. UNB CR’ = ‘DIST. Chan Recv’ (CR unblocked when “distance carrier received” is
present),
‘DEF. UNB CR’ = ‘DEF. Chan Recv’ (DEF CR unblocked when “DEF carrier received” is
present).
3.2.6.2 Loss of Guard Mode
This mode 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 Power Line Carrier (PLC)
communications are transmitted over the power line that 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 input, the second is designated Loss
of Guard (the inverse function to guard received). The function logic is summarised in Table
5: .
System
Condition
Permissive
Channel
Received
Loss of
Guard
Permissive Trip
Allowed
Alarm
Generated
Healthy Line No No No No
Internal Line Fault Yes Yes Yes No
Unblock No Yes
Yes, during a
150ms window
Yes, delayed on picku
by 150ms
Signalling Anomaly Yes No No
by 150ms
Table 5: Logic for the loss of guard function
The window of time during which the unblocking logic is enabled starts 10ms after the guard
signal is lost, and continues for 150 ms. The 10 ms delay gives time for the signalling
equipment to change frequency as in normal operation.
For the duration of any alarm condition (DDB: ‘COS Alarm’), the “zone 1 extension” trip logic
will be invoked if the option ‘Distance schemes / Z1 Ext on Chan. Fail’ has been enabled.
Figure 37: Loss of guard logic
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