Sunday, November 18, 2018

Key Features and Applications of Remote Terminal Units (RTU)

Remote Terminal Units also called Remote Telemetry Units or Remote Telecontrol Units are Microprocessor controlled devices that interfaces in the physical world to either SCADA (Supervisory Control and Data acquisition) system or DCS (Distributed and Control system).  They transmit data to a master system and uses messages from the master supervisory system to control objects connected to the system.
They are designed for use in applications in remote locations unattended. These locations may have limited to no power, hence RTUs are designed to consume low power than DCS and PLC & this enables operation on solar power and batteries.
In application where supervision is done from distant central location, the SCADA software sits in the central office connected over a backhaul network typically using radio communication to the RTUs located far away and in most cases geographically spread out. The communication may be interrupted for long periods of time therefore RTUs have on-board  data storage continuing local data collection for more than a month if backhaul communication is lost as well as “history backfill’’ uploading this data once the connection is established again. Report by exception communication mechanisms are often used to minimize backhaul communication using Wide Area Networks e.g. Mobile, Microwave, Satellite.
RTU in Multidrop Communication System

Remote Terminal Units (RTU) Configuration
The RTU configuration software is separate from the HMI (Human Machine Interface) software from a third-party manufacturer i.e. two separate databases. RTU is configured first; next the OPC server is configured. For a native OPC server this happens automatically, but for OPC server from a third-party, manual data mapping is required which can be time-consuming and error prone requiring thorough testing. In most cases native OPC server is preferred. To finalize, the HMI database has to be configured for graphics, alarms, and trends etc.
The 4-20 mA AI and AO cards for a RTU optionally support native HART pass through hence separate HART multiplexer (MUX) hardware and associated work is not required. Native HART pass through AI and AO cards are much easier to integrate and should be specified if 4-20 mA is used. Since RTUs are generally used in very slow monitoring applications that don’t require fast control, some applications do not use the real-time analog 4-20 mA but only the digital HART communication multi-drop topology. This means the field instruments draw less than 4 mA instead of up to 20 mA hence further reducing the overall power consumption.
Applications of Remote Terminal Units (RTU)
Remote terminal units are commonly used in the following applications:
  • Electrical Power Transmission Networks and Associated Equipment.
  • Remote Monitoring of Functions and the whole Instrumentation Network in Oil and Gas (offshore platforms, onshore oil wells, Pump Stations on Pipelines)
  • Water and Wastewater collection and supply networks including the Pumping stations.
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