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 CHAPTER 9. NEW AND EMERGING TECHNOLOGY.
9.1 SUPERVISORY CONTROL AND DATA ACQUISITION. Supervisory Control and Data
Acquisition (SCADA) systems have been in use for approximately twenty years in various
industries, since the inception of general purpose computer use for applications other than data
processing in offices. The advent of microprocessor technology has expanded the application of
SCADA technology through the more advanced concept of the Distributed Control System
(DCS). This section will briefly discuss the history of such systems; concentrating on the
applications of a SCADA or DCS system for electrical distribution systems.
9.1.1 SCADA Systems History. SCADA systems originated with the concept of computer
control of plant processing equipment, such as used in the paper and petroleum refining industry.
A need was recognized to improve the accuracy and timeliness of the outdated pneumatic and
hydraulic based control systems of the 1940s and 1950s. The development of the main frame
digital computer gave promise that such improvements could be made. Original control systems
consisted of a mainframe computer located at a centralized location. Communications lines had
to-be built from the centralized location to the locations of the equipment to be controlled and
monitored. Generally, the master communications unit (MCU) was the only unit capable of
communicating directly with the central Computer. The MCU was also used to poll the remote
terminal units (RTUs) on a predetermined schedule to gather system status information. Should
an RTU detect a change in status which required attention, however, it could signal the MCU and
request a status query. The early RTU units had very limited capabilities. Basically, RTUs could
pass information concerning the change in state of contact closures. The ability to manipulate
analog information was very limited, with analog/digital (A/D) converters able to manage only
limited data ranges. System capabilities were limited by the: relatively large size of the
equipment, small number of data points per RTU, slow communications speed, and the need for
the central computer to manage all information. All components were discrete components
(i.e., individual transistors, diodes, capacitors, etc.) assembled on printed circuit boards. The
communications network of the old systems managed all of the information traffic, there ' fore,
the chances of individual data items having errors (due to possible random noise bursts and
interruptions in the communications channel) was much higher. Additionally, the conventional
communications path of the older systems were made of copper telephone wire.
With the advent of integrated circuits and microprocessors, the great reduction in size and the
vast improvement in both computing and communications speed has resulted in the evolution of
the DCS. The DCS has microprocessors located remotely at locations close to the controlled
equipment, as well as at a central processing unit. Local control actions and status monitoring
are performed by the local microprocessors, enabling the central computer to be an information
manager for the system. Such a system is inherently more reliable than the old style system, in
which all information was required to flow from the remote points to the central computer and
back to the remote point for final execution. Modern microprocessors are able to manipulate
9-1
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