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Programmable logic controllers (PLCs), used extensively in manufacturing and process industries for many years, offer the electric utility engineer a reliable and cost-effective means of implementing some relay and control systems in power substations. The single greatest advantage of using a PLC is that the control logic required for the application is executed as software inside the PLC. The PLC user can therefore customize his control logic to make his system do exactly what he wants it to do. Should the operation or the system requirements change, the user can normally change the software without making any major hardware changes.
The smaller cost and size of most PCs permits a saving in panel space and materials cost over an equivalent system composed of discrete relays and timers. A PLC system is also more easily expandable than a discrete relay system because the hardware expansion of a PLC system often consists of simply plugging an extra Input/Output (I/O) module into the PC rack. Many PLCs available today can communicate with machines of their own kind, larger master PLCs or computers. Several PLCs linked together in a communications network permit data from various points in the system to be collected and analyzed, increasing system efficiency and reliability and aiding the engineer or technician in solving system problems.
Although PLC-based systems offer several desirable advantages over conventional relay systems, the technology as it exists today requires the system designer to use some tempered judgment as he applies PLCs in electric utility substations. In fact, there can be a number of application constraints associated with the particular PLC chosen for the system. All PLCs are available in a basic model requiring an ac power source, and some models are available for operation from a dc power source. Some control systems, such as an automatic carrier tester, may not require power from the station battery permitting the PLC to be powered from a 120/240-V ac source such as station service. Other systems, such as station transfer systems, require power from the station battery. Since the dc version of most PLCs permits operation only at 24-V dc, a voltage divider or dc/dc converter is required to power the PLC from the 48-, 125- or 250-V station battery.
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