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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