2-22 Specifications Date Code 20000421
SEL-251, -2, -3 Instruction Manual
The DEMR setting is entered with the SET G command. See Section 3: Communications for
more details on the SET, COPY, GROUP, and METER RD commands.
CIRCUIT BREAKER MONITOR
The SEL-251 Relay detects every circuit breaker trip operation. It designates each trip as one
caused by the relay or an external device and maintains a running count of each.
The relay also maintains a running sum of the interrupted current in each circuit breaker pole for
relay and external trips. Running sums for relay trips use the current present one cycle after the
trip output contacts assert. Running sums for external trips use the currents present when the cir-
cuit breaker status input indicates that the circuit breaker has opened.
You can access the circuit breaker operation data using the BREAKER commands. See
Section 3: Communications for more details on these commands.
METERING
The SEL-251 Relay provides complete voltage and current metering. It also determines real and
reactive power values, demand values, peak demand values, and negative and zero-sequence
components of the voltages and currents.
If voltage is measured at the bus and there are current-limiting reactors on the feeder, the relay
can derive the voltage on the load-side of the reactors for metering and fault locating purposes
(see Figure 2.12).
You can access and reset metering data using different METER commands. See Section 3:
Communications for more information.
SERIAL INTERFACES
Port 1 and Port 2 are EIA-232 serial data interfaces.
Port 1 is located on the rear panel and is generally used for remote communications via a modem,
an SEL-PRTU, or an SEL-2020.
Port 2 has connectors on both the front and rear panels, designated Port 2F and Port 2R, respec-
tively. Port 2F has priority over Port 2R. These ports are generally used for local communica-
tions. Port 2R is typically connected to a printer, SEL-RD, or SEL-DTA. Port 2F is typically
used for temporary communications via a portable terminal.
When a device is plugged into Port 2F, the relay automatically begins addressing Port 2F and
discontinues communication with Port 2R. When a device is unplugged from Port 2F, the relay
automatically resumes communication with the device connected to Port 2R.
The baud rate of each Port is set by jumpers near the front of the main board. You can access
these jumpers by removing either the top cover or front panel. Available baud rates are 300, 600,
1200, 2400, 4800, or 9600.
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