Current Differential Relay
GARD 8000 SYS RFL Electronics
June 20, 2014 13-74 973.334.3100
13.5.2.6.3 Communications Failure
A communications failure is declared whenever the communications channel is determined to be
unusable. The following conditions will generate a comms failure.
Detection of a comms hardware failure within the GARD chassis (or the bus not configured).
Address mismatch.
Failing six successive ping-pong tests.
Severely degraded comms.
When the communications channel fails the relay will go into backup mode (if enabled).
13.5.2.6.4 Ping-Pong and Ping-Pong Alarm
The relay constantly monitors the communications channel delay using a ping-pong measurement.
The measurement is calculated by generating a message at the local end, the remote end responds to
the message with a reply—included in the reply is the amount of time between the remote end
receiving the message and sending out the reply. The local end knows when the first message was
transmitted and when the reply was received, after compensating for the delay at the remote relay the
round-trip communications time is calculated. The reported ping-pong delay is ½ of the calculated
round-trip time.
The contributors to the ping-pong alarm include:
Failure of one or more ping-pong tests.
If the measured ping-pong exceeds 28ms.
If the ping-pong measured at the local and remote relay do not match.
If the ping-pong changes by more than 3ms.
These criteria are deliberately made sensitive and are required to remain active for a prolonged period
prior to being reported to avoid nuisance errors.
The ping-pong 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) the user can be
notified.
Neither the ping-pong 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.
13.5.2.7 TRANSFERRED STATUS BITS
The relays are constantly communicating with each other passing data regarding the measured
currents, addressing, ping-pong, etc. Messages are also included that communicate the status of eight
bits received from the logic bus in the chassis. The data is transferred to the remote relay and then
placed onto the logic bus in the remote chassis.
Under normal steady-state conditions the bits are updated approximately 30 times per second. During
trip conditions, however, the status bit messages may be preempted by higher priority protection
messages, resulting in lengthened transfer times.
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