GE P741 Technical Manual page 185

Application Notes
MiCOM P741, P742, P743
Since it is likely that slower baud rates will be selected, the choice of signaling mode
becomes significant. However, once the signaling mode has been chosen it is necessary to
consider what should happen during periods of noise when message structure and content
can be lost. If "Blocking" mode is selected, only a small amount of the total message is
actually used to provide the signal, which means that in a noisy environment there is still a
good likelihood of receiving a valid message. In this case, it is recommended that the "IM#
Fallback Mode" is set to "Default" with a reasonably long "IM# FrameSyncTim". A typical
default selection of Default = 1 (blocking received substitute) would generally apply as the
failsafe assignment for blocking schemes.
If "Direct Intertrip" mode is selected, the whole message structure must be valid and
checked to provide the signal, which means that in a very noisy environment the chances of
receiving a valid message are quite small. In this case, it is recommended that the "IM#
Fallback Mode" is set to "Default" with a minimum "IM# FrameSyncTim" setting i.e. whenever
a non-valid message is received, InterMiCOM will use the set default value. A typical default
selection of Default = 0 (intertrip NOT received substitute) would generally apply as the
failsafe assignment for intertripping schemes.
If "Permissive" mode is selected, the chances of receiving a valid message is between that
of the "Blocking" and "Direct Intertrip" modes. In this case, it is possible that the "IM#
Fallback Mode" is set to "Latched". The table below highlights the recommended "IM#
FrameSyncTim" settings for the different signaling modes and baud rates:
"IM# FrameSyncTim" Setting
Baud
Rate
Direct Intertrip
600
1200
2400
4800
9600
19200
Note:
A setting of 25% is recommended for the communications failure alarm.
Minimum Recommended
Blocking
Mode Mode
100 250
50 130
30 70
20 40
10 20
10 10
No recommended setting is given for the Permissive mode since it is
anticipated that "Latched" operation will be selected.
"Default mode" is selected, the "IM# FrameSyncTim" setting should be
set greater than the minimum settings listed above.
FrameSyncTim" setting is set lower than the minimum setting listed
above, there is a danger that the relay will monitor a correct change in
message as a corrupted message.
Minimum Setting Maximum Setting
(ms)
100
50
30
20
10
10
P74x/EN AP/Na7
(AP) 6-19
AP
(ms)
1500
1500
1500
1500
1500
1500
However, if
If the "IM#