Alstom MiCOM P54 Series, Technical Manual

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P54x/EN OP/La4 Operation
(OP) 5-18
MiCOM P543, P544, P545 & P546
OP
ta
tp1
t A 1
t A 2
t A 3
t A4
t A5
t A 6
t A*
t B 3 *
t B 1
t B 2
t B 3
t B4
t B5
t B 6
t B*
td
tc
tp2
Re l a y B Re l a y A
P100 4E Na
P100 4E Na
Figure 5 Data transmission
Note :
Relay A can measure the total elapsed time = (tA* - tA1). This equals
the sum of the propagation delay times tp1 and tp2, the delay in
sending out the initial message ta, and the delay time tc+td at end B.
Hence
tp1 + tp2 = tA* - tA1 - ta - tc - td
However, because of the GPS synchronization of the re-sampling instants, tA3 is at the
same instant as tB3 (therefore tB3* = tA3) we can use this knowledge to calculate the
receive path delay
tp2= tA* - tA3 – td
And, by the same process the relay can also calculate tp1.
In the event of the GPS synchronizing signal becoming unavailable, the synchronization of
the re-sampling instants at the different ends will be lost and the sampling will become
asynchronous. However, time alignment of the current data will now be performed, by using
the memorized value of propagation delay times prior to the GPS outage (tp2 in relay A and
tp1 in relay B -Figure 4). Each relay also keeps measuring the overall propagation delay,
tp1+tp2. As long as the overall propagation delay does not exceed the setting value under
PROT COMMS/IM64/Comm Delay Tol, it is considered that the communication path has not
been switched, tp2 and tp1 at the two ends remains valid and the differential protection
remains active. If the overall propagation delay exceeds the above mentioned setting, the
differential protection will be inhibited. This patented “fallback” strategy ensures protection
continuity even in the event of antenna vandalism, maintenance error, extremely adverse
atmospheric conditions etc – all of which could result in GPS outage.
Note:
tp1 and tp2 do not need to be equal for the fallback strategy to
become operational.