Conventional PFC systems quickly reach their limits when they have to deal with fast changing loads. Applications like rolling mills, steel presses, wind turbines, container cranes and large buildings may require a reactive power adjustment on the ms scale. Spot welding, production equipment, elevators, chillers, and other electric devices not only require such dynamic reactions of the power factor compensation equipment, they also lead very soon to a total number of switchings that exceeds the specifications of standard electromechanical contactors by far. Standard contactors are used to switch capacitor steps on and off, built for 100 000 to 200 000 switching operations in total during their life time. Burnt main contacts may produce oscillation or “unclean” (re-bouncing) switching operations. This massive over-load not only shortens the life expectancy of the capacitor, but also increases the risk of premature failure and in the worst case represents a potential safety risk.

In dynamic PFC systems, the capacitor contactors are replaced by thyristor modules that are suitable for nearly unlimited number of switching operations as there is no mechanical wear-off. Thyristor modules feature electronic semiconductor switches that are able to react to a changing reactive power demand on the ms scale and that can switch capacitors without additional stress. It keeps the capacitors at the peak value of the grid voltage and connect them only when the grid reaches this peak voltage value. Thus the capacitors are switched current free and inrush currents that can reach values of 200 times the nominal current for conventional contactors are avoided.