Adaptive Compensation of Reactive, Unbalanced and DC Currents of AC Arc Furnaces

The arcs in AC arc furnaces at the furnace calm operation can be regarded as bi-directional and the arc currents of the furnace are symmetrical. This could be not true in the first, uneasy phase of the furnace operation The extinction of the arc in one direction causes the arc current asymmetry and a DC component occurs in the arcs’ currents. This DC component shifts the working point of the furnace transformer towards its saturation which causes additional distortion of the current and it contributes to energy loss in the transformer. Since AC arc furnaces can stand for one of the highest power loads in distribution systems, thus with very high disturbance capacity, compensation of the DC current could be important both for technical and economic reasons. The DC current can be compensated, along with the reactive and unbalanced currents, by an adaptive reactance compensator built of thyristor switched inductors with thyristors fired asymmetrically. The article presents a rationale for the selection of thyristors’ firing angles that enables the reduction of the DC component of the AC arc furnace current along with its reactive and unbalanced currents. The presented rationale is illustrated with an example of such compensation.