Time-varying loads in electric power systems: Power input, equivalent circuit elements, and disturbances

Time-varying electric loads, fluctuating around their nominal point, cause fluctuations also of current and power. The mean phasors of current and complex power differ from their nominal values, which results in a deflection of mean power input. A transformation to a time-invariant equivalent circuit is introduced representing the mean values of voltage, current and power. The variances on the other hand contribute equations for a second phenomenon of fluctuating loads: the generation of disturbances in the power supply system. Results for both phenomena are derived for single-phase loads, validated by Monte-Carlo simulations, and generalized to three important kinds of three-phase loads. Measured data from an electric arc furnace illustrate the application of ideas.