Remote Terminal Unit (RTU) Overview
Remote terminal units (RTUs) are used in commercial and industrial systems throughout the world. Typical applications include the supervisory control and data acquisition (SCADA) systems used by electric, water and wastewater utilities. Units can also be found in refineries, food processing plants and automobile factories.
An RTU is an apparatus used to monitor and control various devices. In regard to the RTU meaning of the word “remote,” it simply means that the location makes it impractical to hardwire the unit to a central processing computer. The unit could be located in an adjacent room or on an off-shore oil rig. Most units can be thought of as self-contained computers because of the way that they incorporate microprocessors.
Comparison to Other Devices
Remote terminal units are sometimes referred to as remote telecontrol units or remote telemetry units. Although they can be compared to programmable logic controllers (PLCs), most are considered to be superior in autonomy, power options, and environmental tolerances. A select number of units can be programmed through a simple web interface and operated in a cloud environment. Some have backup batteries charged by solar power so that the unit can continue to operate in the event of a power outage.
RTU System Design
RTU units are designed to be operated as a component within a larger system. The units collect data from various digital and analog inputs and send that data to a central processing station. The software contained within the unit is used to initialize variables, define protocols and solve problems. Units vary in complexity from small control cabinets containing a single circuit card to larger units with various sections containing multiple cards. In addition to a communications interface, cards typically provide one or more of the following functions:
- Analog input (AI)
- Digital input (DI)
- Digital or control relay output (DO/CO)
- Analog output (AO)\
In addition to monitoring input data and relaying it to a CPU, the remote units also control various functions, such as switches, pumps, and gates. Feedback control systems are typically incorporated to fine-tune the operation of sensitive mechanisms. For example, water pumps automatically turn on and refill an overhead storage tank when the water level inside the tank drops to a predetermined level.
RTU units must be able to communicate with the central processing station in all types of environments ranging from -50 to 70 degrees Celsius. They typically use RS-232, RS-422 or RS-485 protocols in addition to wireless links in a multi-drop configuration. Some of these are designed to operate at low power levels and may be solar-powered. Some of the more sophisticated systems use cellular, satellite and Ethernet to provide end-to-end cloud-based solutions.
Electric transmission and distribution systems sometimes use power line carrier to operate their SCADA systems. Communication speeds can be improved by performing certain operations during off-peak times. For example, large data dumps are often performed shortly after midnight. Additionally, the pre-processing of data can improve data quality and boost throughput speeds.
AC power is typically used to power circuit breakers and some cards. Functions that require DC power depend on an AC to DC converter or battery power. In some applications, AC power is used for the sole purpose of charging a battery bank, and the batteries supply DC power to all the controls and cards. Advantages of this type of system include steady DC voltages and the ability to operate through short AC power interruptions. In extremely remote locations where there is no grid access, units must be able to operate on batteries, solar power or a combination thereof.
Analog versus Digital Inputs and Outputs
Analog inputs typically consist of electrical currents and voltages that use minimum to maximum values in order to monitor variables. For example, a current that varies from 0 mA to 1 mA could be used to represent water tank levels that range from empty to full. A current value of 0.7 mA, then, would indicate that the water tank is 70 percent full. Analog outputs can be used to control devices with variable outputs, such as strip charts.
Digital inputs are used to indicate the status of such things as a switch, breaker, valve position or alarm. Digital outputs are used to operate any device that has two states, such as a switch. They can also be used to control other devices through relays or transistors.
Modern units often contain software that allows them to operate in an autonomous fashion. In some cases, this is critical because certain functions must be performed even when communications have been lost. For example, a circuit breaker on a high-voltage transmission line must be capable of instantaneously clearing a fault without SCADA assistance. The same can be said for the detection of toxic gases, fire, and smoke.