Four-wire Solid State Transformer to Improve Current Quality

Solid state transformers (SST) are used to perform typical functions and improve power quality in series operation of three-phase power systems. Generally, current harmonics and unbalances and neutral wire current are generated due to nonlinear, unbalance load and supply voltage distortion. In this paper operation of threephase four-wire SST to improve current harmonics, current unbalances, neutral wire current and reactive power of nonlinear load as an active filter synchronous by feeding linear load is analyzed and application of AC/DC converter with four legs and it s controller to compensation of neutral wire current and current quality in the input stage of SST is investigated. Power quality improvement with SST has been verified by simulation results using MATLAB/SIMULINK. The results show that SST and its controller has very good performances, and it can not only realize the functions of series operation, but also can prevent from harmonics injecting and unbalances by parallel operation. Besides, proposed controller makes neutral wire current of voltage source becomes almost zero in four-wire distribution system.