The Impact of Mains Impedance on Power Quality

Mains impedance is a critical parameter to consider when diagnosing load generated power quality problems. High impedance can contribute to voltage sags, low voltage conditions, high frequency noise, transient impulses, and harmonic voltages. These problems can be created by the critical load, or by other loads powered from the same electrical source. This paper presents the concept of mains impedance. It starts with a simple impedance model (resistive) and the resultant effect on AC voltage at the fundamental frequency (50 / 60 Hz). A more complex model is presented, addressing inductive and capacitive components of the mains, and the impact of these on harmonic voltages and transient voltages. The concept of non-linear impedance, as often seen at the output of power conditioning devices, is introduced. Techniques used to control impedance are presented, including selecting voltage levels, applying transformers, selecting conductors, distributing loads, and applying power conditioning equipment. Finally, case studies of impedance problems found in the field are highlighted.