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Unified Power Flow Controller (UPFC) is one of the typical FACTS devices capable of instantaneous control of three system parameters. In this paper a novel configuration of UPFC which consists of a nine-switch ac/ac converter parallel with a series capacitor has been proposed to inject desired series voltage. It means that operation of nine-switch ac/ac converter in this configuration is the same as combination of two dc/ac converter in conventional UPFC. However, proposed configuration needs fewer power electronics switches and gate drive circuits and control scheme becomes simpler than conventional UPFC configuration. The main reasons for using series capacitor parallel with nine–switch ac/ac converter are reduction of injection voltage’s THD, elimination of output passive filter, and reduction of converters power rating in comparison with conventional UPFC consisted of series and shunt converters. The proposed UPFC is simulated using MATLAB/SIMULINK software and simulation results are presented to indicate well-performance of the novel configuration of UPFC.

This paper proposes a new topology of DC-DC converters with high gain of boost. The suggested circuit consists of Z-source inverter, high frequency transformer and diode rectifier. In this topology the output voltage amplitude is not limited to DC voltage source and transformer turn ratio similar to traditional high frequency DC-DC converters and can be regulated with Z network shoot-through state control. Besides, it is more reliableagainst short circuit. By using high frequency modulation, the size of transformer is reduced and it is proper in isolation between high voltage and low voltage circuits. The performance of suggested inverter and switching algorithm are validated with simulation results using MATLAB/SIMULINK software.Key words: DC-DC Converter; high frequency transformer; Z-source inverter; boost factor

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 three-phase 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.

This paper proposes a new isolated multi transformer cell inverter employing low-frequency single-phase transformers and two DC voltage sources. This inverter includes several transformer cells. Some of cells contain two unidirectional switches that generate two voltage levels and only one of cells has six switches that provide five voltage levels. Also, the multi transformer cell inverter provides a high number of output levels, symmetric and asymmetric states, reduction of DC sources and high modularity. The operation and performance of the suggested inverter has been proved by simulation and experimental results. Keywords: Multi transformer cell inverter, Cascaded transformer multilevel inverter, Reduction of DC sources and components