Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual

Page: 760 / 824

I.2
SEL-700G Relay Instruction Manual Date Code 20170814
M
IRRORED
B
ITS
Communications
Operation
Transmitting at longer intervals for data rates above 9600 avoids overflowing
relays that receive M
IRRORED
B
ITS
at a slower rate.
Message Reception
Overview
During synchronized M
IRRORED
B
ITS
communications with the
communications channel in normal state, the relay decodes and checks each
received message. If the message is valid, the relay sends each received logic
bit (RMBn, where n = 1 through 8) to the corresponding pickup and dropout
security counters, that in turn set or clear the RMBnA and RMBnB relay
element bits.
Message Decoding
and Integrity Checks
Set the RX_ID of the local SEL-700G to match the TX_ID of the remote
SEL-700G. The SEL-700G provides indication of the status of each
M
IRRORED
B
ITS
communications channel with Relay Word bits ROKA
(receive OK) and ROKB. During normal operation, the relay sets the ROKc
(c = A or B). Upon detecting any of the following conditions, the relay clears
the ROKc bit when:
➤
The relay is disabled.
➤
M
IRRORED
B
ITS
are not enabled.
➤
Parity, framing, or overrun errors.
➤
Receive message identification error.
➤
No message received in the time three messages have been sent
when PROTO = MBc, or seven messages have been sent when
PROTO = MB8c.
➤
Loopback is enabled.
The relay asserts ROKc only after successful synchronization as described
below and two consecutive messages pass all of the data checks described
previously. After ROKc is reasserted, received data may be delayed while
passing through the security counters described below.
While ROKc is deasserted, the relay does not transfer new RMB data to the
pickup-dropout security counters described below. Instead, the relay sends one
of the user-definable default values to the security counter inputs. For each
RMBn, use the RXDFLT setting to determine the default state the M
IRRORED
B
ITS
should use in place of received data if the relay detects an error
condition. The setting is a mask of 1s, 0s, and/or Xs (for RMB1A–RMB8A),
where X represents the most recently received valid value. The positions of
the 1s and 0s correspond to the respective positions of the M
IRRORED
B
ITS
in
the Relay Word bits (see Appendix J: Relay Word Bits). Table I.2 is an extract
of Appendix J: Relay Word Bits, showing the positions of the M
IRRORED
B
ITS
.
Table I.1 Number of M
IRRORED
B
ITS
Messages for Different Data Rates
Data Rate Transmission Rate of M
IRRORED
B
ITS
Packets
2400 15 ms
4800 7.5 ms
9600 4 times a power system cycle (automatic pacing mode)
19200 4 times a power system cycle (automatic pacing mode)
38400 4 times a power system cycle (automatic pacing mode)
NOTE:
Exercise caution when
applying a M
IRRORED
B
ITS
channel to
relays that protect systems that may
not be synchronized, as the
automatic pacing modes operate
under the assumption that both
relays are protecting systems of
similar frequency. To maintain
M
IRRORED
B
ITS
channel dependability
for this application, it is
recommended that you use a data
rate of 2400 or 4800.