Harmonic voltage
BSRIA Power quality guide (AG 2/2000) was written by C C Pearson and V Uthayanan and published by BSRIA in July 2000. It states:
Distortion is usually measured using a value called the total harmonic distortion (THD). The THD reports the percent of the harmonic components relative to the fundamental. Generally the fundamental component is of constant amplitude. During disturbances the fundamental may drop or increase significantly so that the same level of harmonic becomes a different percentage of the fundamental. In the US standard, IEEE 519, the nominal voltage value is used as the fundamental. An alternative approach is to record the harmonic rms value (hrms), defined as the square root of the sum of the squares of all harmonic components at or above the second component.
Two methods can be used to calculate the hrms value. The most direct way is to determine the value of each harmonic component at or above the second component, sum the squares of each component’s magnitude and then take the square root. However, a more computationally efficient method is to first find the total rms value (which is being calculated anyway), square it, subtract the squares of the fundamental and dc components, and take the square root. This method is equivalent to the direct method since the total rms values are equal to the square root of the sum of the squares of the Fourier coefficients. The formula for hrms is:
--BSRIA
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