Quantcast Chapter 5. Power System Instruments and Meters

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5.1 INSTRUMENTATION AND METERING. This chapter discusses instruments and meters
used in power distribution systems. Metering and instrumentation are essential for satisfactory
power system operation. Instruments and meters are needed for monitoring system operating
conditions and for allocating the costs of electrical energy purchase, generation, and distribution.
5.1.1 Basic Objectives. Instrumentation and metering assists operators in the operation of the
power systems. Information relative to the magnitude of loads, energy consumption, load
characteristics, load factor, power factor, voltage, and other parameters are required for proper
operation. Certain checks are required on electric equipment, prior to placing it in service, to
ensure that the insulation is adequate for application of voltage and that the connections have
been properly made. After equipment is in service, certain periodic checks are necessary to
ensure that the equipment remains in proper operating condition. Instruments and meters are
used to perform these and many other important functions.
5.1.2 Definitions. The following two paragraphs broadly define power system instruments
and meters. Instruments. An instrument is defined as a device for measuring the value of the
quantity under observation. Instruments may be either of the indicating or recording type. Meters. An electric meter is generally thought of as a device that measures and
records watts with respect to time or watt-hours. The term meter is also commonly used in a
general sense, as a suffix, or as part of a compound word (such as voltmeter, frequency meter)
even though these devices are, more correctly classified as instruments.
5.1.3 Alternating Current Measurements. Alternating current instruments and meters are
designed to measure the root mean square values at the frequency specified and are calibrated for
a pure sine wave. Any deviation in frequency or wave shape will result in a decrease in the
accuracy of the instrument or meter. A large variety of instruments and meters are available to
measure alternating current system parameters. In most cases their current coils are rated 5 A
and their potential coils are rated 120 V. Instrument current and potential transformers are used
when the current and voltage of the circuit exceed the rating of the instruments. Current Transformers. Current transformers isolate instrument circuits from the
primary circuits and reduce current through instruments to values within individual instrument
measuring element ratings. The current transformer ratio selected should be minimized, without
exceeding rated current in the secondary winding. A ratio that will give a normal current reading
at about one-half to three-quarter scale on the instrument should be used if possible.


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