Loss Of Guard Mode; Logic For The Loss Of Guard Function; Loss Of Guard Logic - GE MiCOM P40 Agile Technical Manual

Application Notes
MiCOM P40 Agile P442, P444
'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
Healthy Line
Internal Line Fault
Unblock
Signalling Anomaly Yes
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
Permissive
Loss of
Channel
Guard
Received
No No
Yes Yes
No Yes
No
Permissive Trip Alarm
Allowed Generated
No No
Yes No
Yes, during a Yes, delayed on pickup
150ms window by 150ms
No Yes, delayed on pickup
by 150ms
P44x/EN AP/Hb6
(AP) 5-59