78 Electrical Power Systems Technology
receiver communication system is usually involved.
Many types of electrical and physical quantities can be monitored by
using telemetering systems. The most common transmission media for tele-
metering systems are: (1) wire-such as telephone lines, (2) superimposed
signals—which are 30- to 200-kHz signals carried on electrical power dis-
tribution lines, and (3) radio frequency signals—from AM, FM, and phase-
modulation transmitters. The block diagram of one type of telemetering
system is shown in Figure 3-15. In this type of system, a DC voltage from
the transducer is used to modulate an AM or FM transmitter. The radio
frequency (RF) signal is then received at another location and converted
back into a DC voltage to activate some end device. The end device, which
may be located at a considerable distance from the transducer, might be
a chart recorder, a hand-deflection meter, or possibly a process control-
ler. Digital telemetering is also used, since binary signals are well suited for
data transmission. In this system, the transducer output is converted to a
binary code for transmission.
Telemetering is the measurement of some quantity at an area that is
distant from its origin. For instance, it is possible, by using telemetering
systems, to monitor on one meter the power used at several different loca-
tions. Almost any quantity value, either electrical or non-electrical, can be
transmitted by using some type of telemetering system. A basic telemeter-
ing system has: (1) a transmitting unit, (2) a receiving unit, and (3) an inter-
connection method. Electrical power systems frequently utilize telemeter-
ing systems for the monitoring of power.
Figure 3-15. Block diagram of a basic telemetering system