SCADA in a power system

SCADA (Supervisory Control And Data Acquisition) is a computer-based system that collects real-time data from power stations and substations, shows it to operators at a control centre, and lets them control equipment remotely.

Architecture

      +-------------------------------+
      | Master station (control ctr)  |
      | SCADA/EMS servers, HMI, data  |
      | historian, alarm printer      |
      +---------------+---------------+
                      | communication
       (fibre, PLCC, microwave, radio, GSM)
         +------------+------------+
         |            |            |
     +---+---+    +---+---+    +---+---+
     | RTU 1 |    | RTU 2 |    | RTU 3 |
     +---+---+    +---+---+    +---+---+
         |            |            |
   CT, PT, relays, CB aux contacts, OLTC,
   transducers in plant / substations

Components

1. Field devices: CTs, PTs, transducers, breaker status contacts, tap-position indicators, intelligent electronic devices (IEDs, numerical relays).
2. Remote Terminal Unit (RTU): installed at each substation/plant; acquires analog values (V, I, MW, MVAr, f) and digital status (CB open/closed, alarms), converts them to digital data and executes control commands from the master.
3. Communication system: carries data between RTUs and master using protocols such as IEC 60870-5-101/104, DNP3, Modbus, IEC 61850. Media: optical fibre (OPGW), PLCC, microwave, radio.
4. Master station: servers and software that poll RTUs, process data, raise alarms, store history and run applications.
5. HMI: operator consoles with single-line diagrams, trends and alarm lists.

Implementation steps

1. Survey the plants/substations and list the points (analog, digital, control) to be monitored.
2. Install transducers/IEDs and RTUs in each station and wire them to CTs, PTs, CBs and isolators.
3. Build the communication network (e.g. OPGW fibre with PLCC as back-up) and choose the protocol.
4. Set up the master station with redundant servers, database of all points and HMI displays.
5. Test point-to-point, then commission and train operators.

Functions in a power system

• Data acquisition: voltage, current, power, frequency, energy.
• Supervisory control: open/close CBs, change transformer taps, start/stop units.
• Alarm and event recording with time stamps (sequence of events).
• Trend and historical data for analysis.
• EMS applications: automatic generation control (load–frequency), economic dispatch, state estimation, load shedding.
• Distribution automation: fault location, isolation and restoration.

Advantages

• Fast, accurate information and quick fault response.
• Fewer staff at unmanned substations.
• Better reliability, economy and security of supply.

In Nepal, the NEA Load Dispatch Centre at Siuchatar uses SCADA to monitor and control the integrated grid.