Power-mains transients from connection of resistive loads and a possible capacitive mitigation

The voltage transient disturbances produced in the power mains by the connection of resistive loads are analyzed theoretically and experimentally. The effect of a series inductance is included in the circuit model. A fast unidirectional (exponential) waveshape that reduces the instantaneous phase-to-neutral voltage is demonstrated. Differential- and common-mode components are separated, and the basic quantitative formulas are given. An additional interesting case is analyzed, i.e., the switching of resistive loads when a capacitor is installed across the line on the mains side of the switch. The presence of such a capacitor results in the appearance of double-exponential transients that are considerably smoothed with a maximum rate of change reduced by orders of magnitude in comparison with the case without the capacitor. Low capacitance values are shown to be adequate to give substantial smoothing, and this technique appears suitable to mitigate the rate-of-change related effects of the resistive-load switching. To that purpose, a quantitative analysis is given with simple design equations. The analytical derivations are based on a three-wire circuit representation of the power line described in a previous paper. This different application and the agreement with the experimental results confirm the reliability of that lumped-constant model for the analysis of the low-frequency disturbances.