Active power control for flicker compensation in single phase welding machines

Power quality represents a crucial issue in low voltage distribution networks. Welding equipments are among the most polluting loads because of their strong nonlinear behavior and their highly intermittent working cycle with a degree of randomness which causes the phenomenon of voltage flicker. The usual approach to the flicker mitigation consists in the introduction of static VAr compensators connected near to the polluting load. However, in low voltage network it is not always possible to adopt this approach: often the resistance of the conductors is not negligible. This means that even the active power fluctuations may result in a significant flicker contribution. In this paper the Authors will propose a flicker compensation technique which does not require the installation of an external device. It has to be noticed that virtually all the modern welding machine power supplies include a rectifier and a capacitive DC link. Basically, the solution consists in exploiting the energy stored in the DC link capacitance in order to smooth and mitigate the active power fluctuations. The large variations of the DC link voltage (hence of the energy stored in the capacitance) controlled by proper energy storage management logic, allow a short-time peak shaving which results in a consistent reduction of the voltage flicker. Finally, the performance of the flicker reduction system will be evaluated by means of numerical simulations. In particular, the short time flicker severity P_st will be computed considering a mathematical model of a manual metal arc (MMA) welding power supply with and without the proposed control strategy.