Flicker compensation with combined active and passive filters

This paper concentrates on a solution for the voltage flicker problem caused by a resistance spot welding plant. Repetitive voltage changes causing the flicker are difficult to compensate, since the welding currents are high, and the length of the welding period, the delay time between the cycles, and the amplitude of the currents may vary. The solution presented in this paper is the use of an active filter together with fixed reactive power compensator capacitor banks. The simulation and measurement results presented show that voltage fluctuations and flicker levels can be greatly reduced. The dynamics of the combination is found to be fast enough to compensate the rapid variations in the reactive power of the plant. In the solution proposed the use of the passive filter provides a great reduction to the active filter ratings compared to the situation with the active filter as the only compensator. Further benefit is smoothed instantaneous active power drawn from the mains.