TABLE OF CONTENTS Assume the input power of a load with power factor (lagging ) between 0.6 and 1.0 is measured with a CT, an ammeter, a PT, and a voltmeter, as shown in Fig. A6.1. The ammeter and the voltmeter have an error of ? A and ? V, respectively. The CT has a ratio correction factor RCF CT and a phase angle shift ? measured in minutes. The PT has a ratio correction factor RCF PT and a phase angle shift ? measured in minutes [97 100, Chapter 8].
The input power is expressed as
The measurement error of the input power is given by
For power factors ranging from 0.6 to 1.0 (lagging, based on consumer notation), the worst case in power measurement error is when cos ? = 0.6, therefore,
where
According to American National Standard [97, Chapter 8], transformer correction factors TCF CT and TCF PT ( ? CT and ? PT) can directly be obtained based on the accuracy class from the device name-plates. Normally, the maximum absolute errors for ammeters and voltmeters are based on the full-scale errors. For instrument transformers, the maximum absolute errors ( ? CT and ? PT) are proportional to the actually measured values of currents and voltages, as derived above.
Note that the maximum...
