GE MiCOM P40 Agile, Technical Manual, Page: 519 / 962

5 APPLICATION NOTES
Effective communications are essential for the performance of teleprotection schemes. Disturbances on the
communications links need to be detected and reported so that appropriate actions can be taken to ensure that
the power system does not go unprotected.
5.1 ALARM MANAGEMENT
Due to the criticality of IM64 communications for correct scheme performance, there is an extensive regime to
monitor signal quality and integrity, generate and report alarms. For most applications, the alarm management
provided as standard will satisfy the needs of the scheme.
For some applications, it may be necessary customise the alarm management. You can do this with the
programmable scheme logic. This section provides a detailed explanation the communications alarm signals
integrated in this product.
5.2 ALARM LOGIC
The figures in the logic diagram section show the main alarm DDB signals associated with IM64. Some of the
signals are setting or hardware dependent. For example, Channel 2 alarms are not available on a simple two-
terminal single communications link application. This section explains the logic, allowing you to understand how
you might customise the alarm logic for your application.
The messages received on each channel are individually assessed for quality to ensure the IM64 signalling scheme
is available for use. If no messages are received for a period equal to the Channel Timeout setting or the signal
quality falls below a defined value, DDB signals are activated as shown in the IM64 channel fail and scheme fail
logic diagram.
Poor quality is indicated if the percentage of incomplete messages exceeds the IM Msg Alarm Lvl setting in a
100 ms period (rolling window), or if the communications propagation time of the IM64 message exceeds the Max
Ch PropDelay (assuming the Prop Delay Stats setting is
Enabled
), or if (in IEEE C37.94 configuration only, and not
shown on the diagram) the Ch Mux Clk flag has been raised to indicate an incorrect baud rate.
If either the Ch Timeout or the Ch Degraded signal persists in the alarmed state for more than the duration of the
Comm Fail Timer setting, according to the conditions set in the Comm Fail Mode setting, the Signalling Fail signal
is raised.
For two-ended schemes (including dual redundant schemes), the IM64 SchemeFail signal is generated at the same
time as the Signalling Fail signal. However, for three-terminal applications, the IM64 SchemeFail signal indicates
that the full set of signalling bits cannot be processed by the scheme. Due to the self-healing nature of the three-
terminal application, this occurs when both channels at any one terminal are not receiving valid signals. This
condition generates a flag in the IM64 message structure which is passed to both remote ends, as well as
generating the local IM64 SchemeFail signal. Using this method, in three-terminal applications the scheme fail
indication is raised at all three ends.
The scheme fail signalling is generated by the inability of a device to receive messages through communication
failure. The transmitting device only knows that communication to a remote device has failed if it receives
notification from the remote device. If a device in the scheme is put into test mode, the communication failure
information is not passed on to the remote ends. If the communications failure is bidirectional, there will be no
indication at the remote device. If this causes operational issues, it may be necessary to include other signals to
enable more precise indication of scheme failure.
In addition to the main IM64 channel fail and scheme fail conceptual logic, there are number of additional alarm
DDB signals associated with test modes, reconfiguration for 3-terminal schemes, and the communication mode
(‘Standard’ or ‘IEEE C37.94’) shown in the logic diagrams.
The majority of signals are associated with the ‘IEEE C37.94’ communications mode and are not activated if the
Standard
communication mode is selected. The Comms Changed DDB logic is to show that switching between
the different communication modes requires a power cycle to be performed before the change is activated.
P446SV Chapter 19 - Fibre Teleprotection
P446SV-TM-EN-1 487