Flicker Mitigation With a Hybrid Compensator

This paper concentrates on a solution for the voltage flicker problem caused by a resistance spot welding process. Resistance spot welding is characterized by high currents lasting for a few mains voltage periods and followed by idle time lasting several cycles. This causes repetitive voltage changes and creates a phenomenon called flicker. Voltage fluctuations are difficult to compensate since the length of the welding period, the delay time between cycles, and the amplitude of currents may vary substantially. In the literature, active compensators such as active filters have been found effective in compensating changing loads, providing fast compensating responses. However, they should be rated on the basis of the maximum compensating power needed. This paper examines a solution based on a hybrid compensator. The system consists of an active compensator and a fixed detuned filter. The main task of the active part is to compensate changes in the fundamental reactive power, whereas the passive part provides a reduction in the ratings of the active compensator. Since the active part incorporates an active filter, load current harmonics are also compensated as a "by-product." The system stability is analyzed, and the performance is examined in simulations and practical experiments. The resulting flicker severity indices are also assessed. The results show that with the proposed system, voltage fluctuations and flicker levels can be greatly reduced. The dynamics of the combination are found to be fast enough to compensate the rapid variations in the reactive power of the plant. A further benefit is smoothed instantaneous active power drawn from the mains.