Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-105
OP
One of the checks performed on the communications link is a check on the calculated
propagation delay for each data message. During normal operation the difference in
calculated time should be minimal (possible delays being introduced by multiplexers or other
intermediary communication equipment). If successive calculated propagation delay times
exceed a user settable value (250 – 1000 s). The P54x raise a comm delay alarm and
initiate a change in relay setting for a short time period (Char Mod Time setting) to overcome
any switching delay. This change in setting is shown in Figure 74 whereby the relay bias
setting, k1, is increa
sed to 200%. This characteristic provides stability for all load conditions
and will still allow tripping for most internal fault conditions.
Figure 73 sh
ows a possible scenario for a switched network. Initially the P54x relays are
communicating via path 1. The go and return times for this path are 2 ms and hence the
calculated propagation delay is (2 + 2)/2 = 2 ms. When the channel is switched to path 2, a
small time period exists where the P54x’s could be sending messages via path 1 and
returning via path 2.
The calculated propagation delay will now be (2 + 5)/2 = 3.5 ms. The resultant 1.5 ms error
at each line end may cause the relay to maloperate due to incorrect time alignment of
current vectors (see section 1.1.1.1). After a short delay, both ‘
go’ and ‘return’ paths will
follow route 2 and the calculated propagation delay will be (5 + 5)/2 = 5 ms. The relay will
now be stable, as correct current vector time alignment exists at each line end.
The Char Mod timer is started when a change in propagation delay is detected. Any
subsequent change during this period will cause the timer to restart. In the above example
the timer will start for the first change (2 to 3.5 ms). The second change (3.5 ms to 5 ms) will
cause the timer to restart, therefore allowing for multiple switching between communication
paths.
A change in propagation delay may result in a temporary failure of the protection
communications channel. If this occurs, the propagation delay change may not be detected
by the relay. To overcome this problem, the Char Mod Timer is re-started when the channel
recovers from a protection communications channel failure if the Char Mod Timer was
running when the channel failure occurred.
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