In the field of power electronics and power systems, electrical signals, such as voltage and current, are expressed by trigonometric functions. Some functions are sinusoidal functions with periodically changing amplitude, which are expressed by the product of two sinusoidal functions with different frequencies. In this paper, we describe the effective value of the functions. The error of the effective value becomes large when the frequency of the functions is close to each other. However, the effective value is equal to the product of the effective values of the two functions when the frequency of the functions is far from each other. The method is simple and straightforward, and can be applied to the calculation of an effective value of ripple current and voltage flicker in power electronics equipment.
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