GE MiCOM P40 Agile, Technical Manual, Page: 254 / 962

6 LOSS OF LOAD SCHEME
The Loss of Load Scheme provides fast unit protection performance for most fault types occurring on a double-
end fed line or feeder, but it does not need communications.
It is used on circuits that are designed for three-pole tripping, and provides protection for faults involving one or
two phases. It is not suitable for single-pole tripping applications, and it cannot protect against three-phase faults.
The scheme does not require communications, but it can be used alongside carrier aided schemes if
communications are available.
The Loss-of-Load scheme can be permanently enabled, permanently disabled, or enabled if communication failure
compromises channel-aided scheme operation. If used in conjunction with a channel-aided scheme, loss of load
can be set to be enabled if only Channel 1 fails, or if only Channel 2 fails, or if both channels fail, or if either channel
fails. Aided scheme communication failure is detected by permissive scheme unblocking logic, or the presence of a
Channel Out of Service (COS) input.
V02740
Zone 2 IED1
Zone 1 IED 1
Zone 1 IED 2
Zone 2 IED 2
IED 1 IED 2
Figure 115: Loss of load accelerated trip scheme
Any fault in the reach of Zone 1 results in fast tripping of the local circuit breaker. For an end zone fault for IED 1
(near IED 2) with remote infeed (from IED 2), the remote breaker is tripped in Zone 1 by the remote device at IED 2.
The local device (IED 1) can recognise this by detecting loss of load current in the healthy phases. This condition, in
conjunction with operation of a Zone 2 comparator at IED 1P, can be used to trip the local circuit breaker.
Before an accelerated trip can occur, load current must be detected before the fault. The loss of load current
opens a window during which time a trip occurs if a Zone 2 comparator operates. A typical setting for this window
is 40ms as shown in the figure below, although this can be altered in the LoL Window setting. The accelerated trip
is delayed by 18ms to prevent initiation of a loss of load trip due to circuit breaker pole discrepancy occurring for
clearance of an external fault. The local fault clearance time can be deduced as follows:
t = Z1d + 2CB + LDr + 18ms
where:
● Z1d = Maximum downstream zone 1 trip time
●
CB = Breaker operating time
● LDr = Upstream level detector (LOL <1) reset time
For circuits with load tapped off the protected line, care must be taken in setting the loss of load feature to ensure
that the undercurrent level detector setting is above the tapped load current. When selected, the loss of load
feature operates with the main distance scheme that is selected. This provides high speed clearance for end zone
faults when the Basic scheme is selected or, with permissive signal aided tripping schemes, it provides high speed
back up clearance for end zone faults if the channel fails.
Note:
Loss of load tripping is only available where three pole tripping is used.
Chapter 9 - Non-Aided Schemes P446SV
222 P446SV-TM-EN-1