Current Differential Relay
GARD 8000 SYS RFL Electronics
June 20, 2014 13-73 973.334.3100
Ping-Pong Alarm
When the ping-pong alarm (section 13.5.2.6.4) is active the relay will be in minor alarm.
Communications Failure
When the relay is in communications failure (section 13.5.2.6.3) the relay will be in minor
alarm.
13.5.2.6 COMMUNICATIONS FUNCTIONS
All communications channels suffer from occasional bit errors and message corruption. Each message
contains a data integrity check and any messages that are corrupted are discarded. Each message is
transmitted three times and the receiver performs a two-out-of-three voting logic. Thus, even if a
single instance of a message is corrupted in the communications link the message will be correctly
received by the relay.
If the errors are infrequent the relay will simply ride through them without any degradation in
performance. More frequent errors will cause individual messages to be dropped which will result in
degraded performance. Low to modest errors rates that continue for a prolonged period of time are
generally a sign of problems with the communications channel and should be investigated. When relay
detects these prolonged periods of errors, even if the relay is able to continue to function, it alarms the
user.
If high error rates are detected the relay will go into a comms failure condition.
13.5.2.6.1 Addressing
The relay implements an addressing scheme to ensure that the relay is communicating with the proper
remote relay prior to opening the communications channel. The user must set a receive (Rx) and
transmit (Tx) address in the relay.
The local Rx setting must match the Tx setting of the remote relay (and likewise in the opposite
direction).
13.5.2.6.2 Communications Alarm
The relay constantly monitors the communications channel. Minor, low-level communications
problems can be accepted by the relay without any degradation in performance. Modest
communications problems may result in slightly degraded performance and severely degraded comms
will result in the relay declaring a comms failure (see below), however, while the relay can work-
through minor or modest communications channel problems they should not go unreported. The relay
identifies these problems by setting a communications alarm which is displayed on a status web page.
The communications alarm also causes the module to go into minor alarm which goes out as a bit on
the logic bus. The system logic in the GARD controller module has a timer that monitors the minor
alarm signal. If this signal is continuously active for a prolonged period (e.g. 10 seconds) they user
can be notified.
Neither the communications alarm or module minor alarm has any direct impact the protection
functions of the relay in any way; they are intended strictly to alert the user.
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