A SCADA system consists of a master
station that communicates with remote terminal units (RTUs) for the
purpose of allowing operators to observe and control physical plants.
Generating plants and transmission substations certainly justify
RTUs, and their installation is becoming more common in distribution
substations as costs decrease. RTUs transmit device status and
measurements to, and receive control commands and setpoint data from,
the master station.
Communication is generally via
dedicated circuits operating in the range of 600 to 4800 bits=s with
the RTU responding to periodic requests initiated from the master
station (polling) every 2 to 10 s, depending on the criticality of
the data.
The traditional functions of SCADA
systems are summarized:
. Data acquisition: Provides
telemetered measurements and status information to operator.
. Supervisory control: Allows operator
to remotely control devices, e.g., open and close circuit breakers. A
‘‘select before operate’’ procedure is used for greater
safety.
. Tagging: Identifies a device as
subject to specific operating restrictions and prevents unauthorized
operation.
. Alarms: Inform operator of unplanned
events and undesirable operating conditions. Alarms are sorted by
criticality, area of responsibility, and chronology. Acknowledgment
may be required.
. Logging: Logs all operator entry, all
alarms, and selected information.
. Load shed: Provides both automatic
and operator-initiated tripping of load in response to system
emergencies.
. Trending: Plots measurements on
selected time scales.
Since the master station is critical to
power system operations, its functions are generally distributed
among several computer systems depending on specific design. A dual
computer system configured in primary and standby modes is most
common.
SCADA functions are listed below
without stating which computer has specific responsibility.
. Manage communication circuit
configuration
. Downline load RTU files
. Maintain scan tables and perform
polling
. Check and correct message errors
. Convert to engineering units
. Detect status and measurement changes
. Monitor abnormal and out-of-limit
conditions
. Log and time-tag sequence of events
. Detect and annunciate alarms
. Respond to operator requests to:
– Display information
– Enter data
– Execute control action
– Acknowledge alarms
. Transmit control action to RTUs
. Inhibit unauthorized actions
. Maintain historical files
. Log events and prepare reports
. Perform load shedding
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