An analog to the Fourier series based on the cosine-and-sine function, is considered. The new series is related to the Hartley transform and thus it is termed the Hartley series. Properties and applications of the Hartley series are discussed in general. It is pointed out that the technique is valuable for the assessment of power quality in industrial and commercial power distribution systems. In particular, the impact of switched and pulse-modulated loads and AC/DC converters may be analyzed. An example is presented in which an industrial distribution system impacted by a six-pulse rectifier is analyzed.