SCADA REQUIRES COMMUNICATION BETWEEN MASTER CONTROL STATION AND REMOTE CONTROL STATION: 

o   Leased-wire means use of a standard telephone circuit; this is a convenient and straightforward means of telemetry when it is available, although it can be unreliable, and it requires a continual outlay of leasing expenditures.

·         In addition, it is not under user control and requires careful coordination between the user and the telephone company.

·         Power-line carrier (PLC) offers an inexpensive and typically more reliable alternative to leased-wire.

·         Here,  the  transmission  circuit  itself  is  used  to  modulate  a  communication  signal at  a frequency much greater than the 60 Hz power frequency.

·         Most PLC occurs at frequencies in the  range  of  30-500  kHz.

·         The  security  of  PLC  is  very  high since  the  communication equipment is located inside the substations. One disadvantage of PLC is that the communication cannot be made through open disconnects, i.e., when the transmission line is outaged.

·         Often, this is precisely the time when the communication signal is needed most. In addition, PLC is susceptible to line noise and requires careful signal-to-noise ratio analysis.

·         Most PLC is strictly analog although digital PLC has become available from a few suppliers during the last few years.

·         Microwave radio refers to ultra-high-frequency (UHF) radio systems operating above 1 GHz.

·         The earliest microwave telemetry was strictly analog, but digital microwave communication is now quite common for EMS/SCADA applications. This form of communication has obvious advantages over PLC and leased wire since it requires no physical conducting medium and therefore no right-of-way.

·         However, line of sight clearance is required in order to ensure reliable communication, and therefore it is not applicable in some cases.

·         A more recent development has concerned the use of fiber optic cable, a technology capable of extremely fast communication speeds. Although cost was originally prohibitive, it has now decreased to the point where it is viable.

·         Fiber optics may be either run inside underground power cables or they may be fastened to overhead transmission line towers just below the lines.

·                   They may also be run within the shield wire suspended above the transmission lines.

·         One easily sees that communication engineering is very important to power system control.

·         Students specializing in power and energy systems should strongly consider taking communications courses to have this background. Students specializing in communication should consider taking power systems courses as an application area.