The Evolution of RTU and PLC Technology
By Jerry Locsin, Senior Editor
The traditional substation RTU is in transition with the advent of intelligent electronic devices (IED). Because of their intelligent nature (i.e. microprocessor-based), IEDs collect much of the same data which has been traditionally gathered by RTUs. IEDs are now being interfaced to RTUs through RS232/RS485 serial channels or LAN networks to collect data which was previously collected by RTU I/O modules. Thus, the RTU is in transition from a data I/O unit to a data concentrator function integrating multiple IEDs into a single database structure.
Another key issue which has to be addressed by the utility engineer is the heterogeneous protocols found when purchasing IEDs from different manufacturers which would be a typical scenario when one wishes to purchase “Best in Class.” The RTU now not only acts as a data concentrator but as a protocol convertor, acting as an interface between the master to RTU protocol and the multitude of IED protocols found in a typical substation.
Although, RS232/RS485 serial channels are typically used as the method of communications between RTUs and IEDs, many utilities are discovering the response times for control and data collection are inadequate. A number of substation automation vendors are now integrating IEDs onto a high-speed LAN, such as Ethernet. This approach not only increases data throughput and control response times, but allows the utility engineer to configure IEDs from a central or remote location.
Many utilities are now adding a PC-based graphical user interface (GUI) to the RTU/data concentrator that provides a method of presenting information in a modern graphical representation to the substation operator/engineer. The GUI replaces the older mimic panels, panel meters, switches, etc. found in a traditional substation control panel.
What the industry is starting to see is the integration of multiple IEDs–and in this context, IEDs are legacy RTUs, programmable logic controllers (PLC), relays, meters, etc.–onto a single high-speed LAN with all data and control being concentrated in an intelligent RTU/data concentrator, or as this device is now called, “The Substation Controller.” The figure is a schematic representation of a modern integrated substation control system (iSCS).
Economics of scale can be achieved by connecting all of the IEDs into a single iSCS. Specifically, introduction of fully integrated control systems can lead to further streamlining of redundant equipment, as well as reduced costs for wiring communications, maintenance and operation, and improved power quality and reliability.
Even though the benefits are well worthwhile, the integrated control system approach to substation automation has made little headway in North America, largely because hardware interfaces and protocols for IEDs are not standardized. Protocols are as numerous as the vendors, and in fact, more so since products even from the same vendor often have different protocols.
One solution to this problem is to install what is called a gateway to act as a hardware and protocol interface between the IEDs and a network. The gateway allows the utility to run a common communications network and protocol throughout the substation in order to integrate a multiplicity of these devices. It provides a physical interface between the IEDs and the network`s electrical standards, and it also functions as a protocol converter between the devices and the standard substation network. The gateway makes all IEDs look identical as far as communications over the substation network are concerned.
With the integration of all substation data concentrated into a single “virtual database” in an iSCS with powerful computer horsepower, a few vendors are beginning to incorporate powerful applications software onto their iSCS platforms to perform the following functions:
Fault detection and isolation;
Transformer load balancing; and
Power quality monitoring.
In summary, an iSCS supplies a platform for full integration of a utility substation into a single, intelligent automation system. Utilities adapting to a competitive environment will gain several advantages from it, including savings in operation and maintenance costs; increased reliability; and the modularity and flexibility of an iSCS architecture, which promotes fast response to customer needs and provides for customized services.
The first programmable controller–commonly known as a PLC–spawned a major shift in how control was performed in the factory and utility, a shift that`s continuing even today. The form may have changed dramatically since the first model, but not the function. Any automation system will require some kind of control function. How that function is performed and the additional capabilities now available are a testament to the evolution of technology.
Every function of a current PLC-based control system is embedded in, or created, through software. There are essentially no functions that are strictly hardware based. The control logic is software, and the software is the function, so all the functionality is given today to the product via software. Software becomes synonymous with function, execution and performance.
There are four major size classifications for programmable controllers: micro (up to 32 I/O), small (32 to 128 I/O), medium (128 up to 512 I/O) and large (512 I/O and higher). Fixed programmable controllers are available with a fixed combination of discrete inputs and outputs. A modular approach allows the user greater flexibility to match the I/O requirements of the application for the ideal solution.
Programmable controllers are typically used in control applications where the proportion of analog to digital is as much as 50:50. They are ideal for applications that are more sequential in operation, require fast system response, involve batch control, require daily shutdowns and startups and have more budget limitations.
With today`s advanced technology, micro-PLCs have increased in sophistication and decreased in size and cost to the point where they are a cost-effective and efficient method of control. System designers evaluating control methods for their less complex machines and processes are finding that, when taking into account both functionality and cost, micro-PLCs are a good fit for their applications. Processes that feature repetitive, time- or event-driven operations, high-speed control and limited requirements for data acquisition and manipulation can also benefit from micro-PLC technology.
New RTU, PLC and IED technologies are on the horizon from such companies as GE-Harris, Square D, Advanced Control Systems and Motorola. Whether these devices are used for distribution management or plant operations, they will continue to be important communication links to the future of utility operations.
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General overview of a substation control panel using GE-Harris` D200 controller alongside several mounted IEDs.