Substation automation and communications for controlling primary equipment (switches, breakers, etc.)

Remote Terminal Unit

For many years, Remote Terminal Unit (RTU) has been the automation system’s traditional heart. RTU has been a backbone of distributed automation and SCADA systems. This paper focuses on substation applications, although much of the information presented is also true for other power industries.

Figure 1 shows RTUs suitable for deployment in substations.

The main requirements for an RTU are a communications interface and the ability to monitor digital status points and analog values (currents, voltages, etc.). The RTU is important in widely spaced geographic regions. Its main function is to provide information about the power system to a central control system through a communications interface and to provide remote control of switches and circuit breakers. Many RTUs are also used to monitor current and voltage and to calculate power.

For maintenance purposes, it is simpler to connect these signals to a substation RTU rather than another IED or microprocessor relay.

The overwhelming popularity of the microprocessor relay has led many to challenge the place of the RTU in the modern substation. The argument against the RTU is that the increased computational power, abundance of physical input/output (IO), and the required instrument transformer connections (for current and voltage readings) already exist in the protective relays and therefore should be reused in the SCADA system for monitoring.

While the initial savings on physical hardware and decreased wiring may be appealing, the engineer must consider the maintenance and operational environment that the system must perform within. The use of an RTU in a substation often simplifies design, commissioning, and maintenance.

Data Concentrators

Ăe data concentrator aggregates information and provides a subset of that information to another device or devices. It is similar in function to the RTU and then can be the same device. The main difference in the terms is that a data concentrator does not necessarily have physical interfaces to monitor contact statuses and analog values.

The data concentrator uses communication protocols to acquire data from other devices rather than through a direct connection.

Substation Gateways

The term “gateway” is unfortunately applied in a couple of similar, although distinctly different applications in the substation. The first and simplest is the router. In IP networking, the gateway is a device that allows communication between different subnets. This is called layer switching or routing. This terminology is common when dealing with information technology (IT) departments who spend their days and nights fixing IP networks.

The substation gateway can be thought of as a superset of the data concentrator and the RTU (although the RTU can be a separate device, it does not have to be in most substation applications).

Figure 2 shows a substation gateway diagram.

Protocol Convertors

Although not ideal, protocol convertors can be used to solve the problem of two devices that do not speak the same “language.” The protocol convertor can be a simple two-port device providing conversion between protocols such as a Modbus-TCP (networked) to Modbus-RTU (serial) convertor.

They can also be as intricate as a large-scale RTU or data concentrator that converts many protocols simultaneously on different ports. The standalone protocol convertor is not preferred as it adds another possible point of failure to the system.

Remote Input/Output Devices

Remote IO units are of particular interest in very large applications. The cost of the copper cabling and the pulling, and terminating of those cable can be very high. For this reason there is a lot of interest in distributing IO devices throughout a station and communicating to them through a network. The remote IO device presents a number of challenges to the integrator.

When IO is distributed, the time stamping of signals can be problematic. It may be necessary (depending on the region and local regulations) to have a very high resolution and accurate time stamp recorded with any status change. This means that the remote IO unit must have some way to synchronize with an external clock.

The remote IO unit may require more intelligence that its name suggests. Generic object-oriented system event (GOOSE) message IO devices, for example, require some sort of intelligence (timers) to ensure that a loss of communications does not permanently close a contact.

For this reason, careful attention should be paid to the protocols selected.