1. Field of the Invention
This invention relates to the field of electric power measuring devices. More specifically, this invention relates to the accumulating and recording of watt-hours.
2. Description of the Prior Art
Electric power is ordinarily delivered to residences, commercial facilities, and industrial facilities as an Alternating Current (AC) voltage that approximates a sine wave with respect to time, and ordinarily flows through consumer premises as an AC current that also approximates a sine wave with respect to time. Ordinarily, a watt-hour meter is used to charge for the power that is consumed.
In an AC power distribution system, the expected frequency of voltage or current (usually 50 Hertz, 60 Hertz, or 400 Hertz) is usually referred to as the fundamental frequency, regardless of the actual spectral amplitude peak. Integer multiples of this "fundamental" are usually referred to as harmonic frequencies, and spectral amplitude peaks at frequencies below the fundamental are often referred to as "sub-harmonics", regardless of their ratio relationship to the fundamental.
It is widely recognized that loads which draw harmonic currents place an increased economic burden on the power distribution system by requiring derating of transformers and increased conductor area. If the non-fundamental currents, harmonics or sub-harmonics, are large relative to the impedance of the distribution system, they can induce harmonic voltages in the voltage delivered to other loads that share the distribution system. It is also possible for a load to accept power at the fundamental frequency and simultaneously act as a power source at a harmonic frequency. Under these circumstances, accurate watt-hour measurements fail to accurately measure the economic impact of the load.
For example, an accurate watt measurement treats the following two loads identically: a load that consumes 80 kilowatts at the fundamental frequency, and a load that consumes 100 kilowatts at the fundamental frequency and sources 20 kilowatts at the fifth harmonic. The latter, almost certainly has an adverse economic impact on the distribution system which traditional watt meters fail to measure.
Harmonic adjustments to watt-hour measurements can provide a better estimate of the economic impact of a non-linear load, and can encourage behavior by electric power consumers that match the goals of the electric power provider.