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 they have not found much utilization to perform actual control of the electrical system. Large
PLCs were historically developed for hundreds of points at one location, which did not fit the
need for the typical electrical distribution system. Modern PLCs can handle less points,
however, the more recent development of integrated microprocessor controls by switchgear and
circuit breaker manufacturers provided the same capability and were designed specifically to
meet the need of the typical electrical distribution system. PLCs and integrated microprocessor
controls are essentially the same product, both consisting of the same subcomponent.
9.2.2 Microprocessor Controls. Microprocessor controls are one of the newest technologies
to be applied to the electrical distribution system.
Microprocessors are now being used for such applications as: protective relaying and tripping
functions in circuit breakers and fuse-like switching devices; electronic meters that provide all of
the voltage; current, power, energy consumption, demand, power factor, frequency, and for
gathering other information that previously required up to eight separate metering devices;
dedicated controls for complex machinery, gas turbines, diesel engines, compressors, generators,
adjustable speed drives; desk top computer control and monitoring systems; and automated
protective device testing. Microprocessor based protection modules are now being installed in
molded case circuit breakers and low voltage power circuit breakers to control the operation of
the direct acting trip units. These trip units are used to provide long time, short time,
instantaneous, and ground fault overcurrent protection, as well as undervoltage protection.
Additionally, microprocessor based protective relays are also being used to replace traditional
electro-mechanical protective relays that have traditionally been used for low and medium
voltage switchgear installations. The new devices offer improved protection of equipment by
allowing more accurate protection settings paralleling equipment needs. The new devices also
offer better troubleshooting diagnostics, on-line test features, and communication capabilities to
allow remote trouble reporting. The new protective devices are being equipped with metering
capabilities, which may allow elimination of separate voltmeters, ammeters, and wattmeters;
often used on feeder and utilization circuits to provide operating load information.
Electronic metering devices are now available in one package to replace kilowatt-hour meters,
demand attachments, power meters, power factor meters, and kilovar-hour meters. The new
devices have bidirectional power flow monitoring capabilities; allowing one device to replace
four conventional billing meters when a site has two way power flow with a power factor
adjustment clause. The new devices have multiple rate and time of day usage meters with
built-in communication capabilities to allow remote meter reading by SCADA or other systems,
such as a personal computer with a modem and software (i.e., spreadsheet program) for analysis
of the data. These new metering devices will display power factor, voltage, current, and power
flows (both real and reactive); providing the possibility of elimination of even more of the old
indicating meters. Use of modern metering equipment allows significant size reduction in certain
applications where many discrete meter devices were used on a lineup of feeder switchgear
equipment. A typical metering unit consists of a discrete kilowatt demand meter, a discrete
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