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 more local microprocessors, as well as the communications equipment, power supplies, and
terminal strips necessary to connect the equipment being monitored and controlled. There may
be a local CRT display and keyboard interface with the microprocessors and even a small data
printer and an,alarm printer. These, however, are usually used for troubleshooting and varying
the system configuration, rather than for daily operational use. The various RTUs and central
computers are connected by a variety of methods; a simple twisted pair cable, multi-pair cables,
fiber optic cables, by carrier communications over the power conductors, by radio transmissions,
and even by satellite transmissions. The selection of the transmission medium depends on the
needs of the user, the location of the central station and RTUs, the volume of data, the need for
data security, system reliability goals, and the cost.
9.2 CONTROL CIRCUITS AND DEVICES. Control circuits for electrical distribution systems
traditionally consisted of discrete electromechanical relays that were hard wired to produce the
desired end result, based on a predetermined set of input conditions. There are now two major
types of control circuits; the Programmable Logic Controller (PLC) and the integrated
microprocessor control. The following two subparagraphs briefly describe the major features of
both types of control devices.
9.2.1 Programmable Logic Controllers. PLC systems have been in use for about fifteen
years; their development paralleling the ongoing development of integrated circuits. These
systems consist of many logic circuits that may be configured by the user to any number of
configurations to accommodate the size of the equipment purchased and the complexity of the
task to be performed. PLC systems were first used in the automotive industry and developed to
replace the large electromechanical relay systems that were used to control large automated
factory processes. The automotive industry faced the yearly task of either rewiring or replacing
relay control panels used to control various parts of the auto assembly lines. PLCs, as the name
implies, are programmable. The logic is, therefore, not hard-wired and can be changed by the
end user as often as desired without changing any physical connection.
With the achievement of new communications technology, it is sometimes difficult to
differentiate between a SCADA system and a system of PLCs communicating via a LAN. Some
manufacturers offer a hierarchy of products utilizing PLC to control the final end devices. The
PLCs are generally connected to the SCADA system via RTUs, which have limited control
capability. other manufacturers, without strong PLC product lines, have developed SCADA
systems with stronger local control capability built into the RTUs. In the electrical distribution
system, PLCs are most often used for motor control logic, load shedding schemes, motor
reacceleration schemes, and for other utilization device control systems.
Originally, PLCs were developed for complex systems involving hundreds or even thousands of
discrete relays. They are available today, however, for systems that require as few as four to six
traditional relays. PLCs are available with time-delay relay and analog capabilities, although
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