Architecture
An RTU monitors the field digital and analog parameters and transmits data to a SCADA Master Station. It runs setup software to connect data input streams to data output streams, define communication protocols, and troubleshoot installation problems in the field. An RTU may consist of one complex circuit card consisting of various sections needed to do a custom-fitted function, or may consist of many circuit cards including a CPU or processing with communications interface(s), and one or more of the following: (AI) analog input, (DI) digital (status) input, (DO/CO) digital (or control relay) output, or (AO) analog output card(s). An RTU might even be a small process control unit with a small database for PID, Alarming, Filtering, Trending and other functions complemented with somePower supply
A form of power supply will be included for operation from the AC mains for various CPU, status wetting voltages and other interface cards. This may consist of AC to DC converters where operated from a station battery system. RTUs may include a battery and charger circuitry to continue operation in event of AC power failure for critical applications where a station battery is not available.Digital (status) inputs
Most RTUs incorporate an input section or input status cards to acquire two state real-world information. This is usually accomplished by using an isolated voltage or current source to sense the position of a remote contact (open or closed) at the RTU site. This contact position may represent many different devices, including electrical breakers, liquid valve positions, alarm conditions, and mechanical positions of devices. Counter inputs are optional.Analog inputs
An RTU can monitor analog inputs of different types including 0-1 mA, 4–20 mA current loop, 0–10 V., ±2.5 V, ±5.0 V etc. Many RTU inputs buffer larger quantities via transducers to convert and isolate real-world quantities from sensitive RTU input levels. An RTU can also receive analog data via a communication system from a master or intelligent electronic device (IED) sending data values to it. The RTU or host system translates and scales this raw data into the appropriate units such as the quantity of water left, temperature degrees, or Megawatts, before presenting the data to the user via the human–machine interface.Digital (control relay) outputs
RTUs may drive high current capacity relays to a digital output (or "DO") board to switch power on and off to devices in the field. The DO board switches voltage to the coil in the relay, which closes the high current contacts, which completes the power circuit to the device. RTU outputs may also consist of driving a sensitive logic input on an electronic PLC, or other electronic device using a sensitive 5 V input.Analog outputs
While not as commonly used, analog outputs may be included to control devices that require varying quantities, such as graphic recording instruments (strip charts). Summed or processed data quantities may be generated in a master SCADA system and output for display locally or remotely, wherever needed.Software and logic control
Modern RTUs are usually capable of executing simple programs autonomously without involving the host computers of the DCS or SCADA system to simplify deployment and to provide redundancy for safety reasons. An RTU in a modern water management system will typically have code to modify its behavior when physical override switches on the RTU are toggled during maintenance-by-maintenance personnel. This is done for safety reasons; a miscommunication between the system operators and the maintenance personnel could cause system operators to mistakenly enable power to a water pump when it is being replaced, for example. Maintenance personnel should have any equipment they are working on disconnected from power and locked to prevent damage and/or injury.Communications
An RTU may be interfaced to multiple master stations and IEDs with different communication protocols (usually serial ( RS-232, RS-485, RS-422) orIED communications
IED communications transfer data between the RTU and an IED. This can eliminate the need for many hardware status inputs, analog inputs, and relay outputs in the RTU. Communications are accomplished by copper or fibre optics lines.Master communications
Master communications usually occur between an RTU and a larger control system or a data collection system (incorporated into a larger system). Data may be moved using a copper, fibre optic or radio frequency communication system.Applications
* Remote monitoring of functions and instrumentation for: ** Oil and gas (offshore platforms, onshore oil wells, pumpstations on pipelines) ** Networks of pump stations (wastewater collection, or for water supply) **See also
* Telemetric * Digital protective relay * SCADAReferences
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