Large consumers of electrical energy are generally billed to cover the operating costs of energy production and also to recover the high capital costs associated with the delivery of the highest energy demand. The former are calculated from results of energy measurements such as a watt-hour meter and the latter from a maximum VA demand registration (short term average, e.g. 15 minutes) as determined by VA demand meters which are designed to measure the apparent power S.
The apparent power is defined as: EQU S=V.sub.rms .times.I.sub.rms ( 1)
where V.sub.rms and I.sub.rms are the rms values of the supply voltage and line current. However, two commonly used approximate formula for apparent power are: EQU S'=V.sub.rect .times.I.sub.rect .times.(1.11).sup.2 ( 2)
where V.sub.rect and I.sub.rect are the average of the rectified values of the voltage and current waveforms and EQU S"=.sqroot.P.sup.2 +Q.sup.2 ( 3)
where P and Q are the active and reactive powers of the waveforms. The term "1.11" in equation (2) is the form factor of a sine wave representing the ratio of the rms and average values of a sine wave.
Meters measuring the true value of apparent power S are relatively expensive to manufacture and, as a result, over the years manufacturers have built a number of VA meters operating according to the approximate equations (2) or (3). However, these two approximate formulas can lead to errors if the waveforms for voltage and current are not sinusoidal waveforms.
One type of VA demand meter is a rectifier type instrument which measures KVA demand by applying rectified signals to a thermal element. This meter responds to the product of the average rectified values of the applied a.c. voltage and currents with its reading made proportional to the form factors squared of pure sinusoidal waveforms i.e. 1.11.times.V.sub.avg .times.1.11.times.I.sub.avg. This type of meter is sensitive to the form factors of the waveforms. Since rectifiers are used in both the voltage and current circuits of the meter, if either the voltage or current waveform is distorted the meter will not accurately indicate the kVA demand. Another type of meter for a 3-phase system consists of two 3 element inductive watt-hour meters with microprocessor assisted calculations. The energy (watt-hours) is measured by a conventional inductive watt-hour meter whereas the second inductive meter is connected to register a quantity called a "Q-hour" rather than the reactive power. The latter measurement requires voltage phasor phase shifting by 90.degree. while the former uses only a 60.degree. phase shift. The microprocessor calculates kVA from the watt-hour (Wh) and Q-hour (Qh) registration as ##EQU1## which is the same as that calculated according to equation (3). Again, this type of meter will give inaccurate readings for distorted waveforms since the Q-hour meter will not respond properly to higher harmonics and, in addition, approximate equation (3) is not correct for distorted waveform conditions.
Large consumers of electric energy subject the electric grid system to electrical loads which result in harmonics being introduced into the system and therefore distortions of the current and voltage waveforms. In recent years, the growing presence of converting apparatus has led to a steady increase in the level of these harmonics and, since revenue meters are designed to operate under sinusoidal conditions, this has raised questions concerning the accuracy of these meters in field conditions where harmonics are present. The amount and type of distortion will vary for different consumers depending on their type of load.
The present techniques of analyzing field conditions for possible difficulties with meters is to first collect samples of on site voltage and current waveforms which are then analyzed to determine their harmonic content. These field conditions are reproduced in a laboratory and field meters of different types are then compared to the performance of known standards. There are major disadvantages to this approach in that the results are not available on site and a large investment in equipment is necessary. Furthermore, after weeks of laboratory test, often expensive field trips are organized to investigate the situation only to find that no problems actually existed with that particular consumer. Therefore, there is a need for an instrument which can determine errors of different types of meters, particularly VA meters, under actual field conditions when connected to real loads where the voltage and current waveforms may be distorted. The type of meter most suitable for that particular field condition can then be determined. These errors in the VA meter readings appear to be more severe when three phase meters are connected in a three phase distribution system.
USPTO Backgrounds
USPTO Backgrounds