Abstract:

In this work, a new five level inverter topology is presented using an existing design of six switch, three leg, three-phase inverter and transformer. This works depends on the cascaded connection of two of the inverter legs in such a manner, so that the third leg is commonly used by remaining two phases or legs. It is done by an integrated transformer setup which is in series with the inverter legs. In this method, a conventional three phase, three leg inverter and a three phase transformer are modified into an secluded single phase MLI (Multi Level Inverter) based on an distinctive arrangement and switching technique. The proposed inverter is simulated in MATLAB/Simulink software.

Key words—Multi Level Inverter, Integrated Transformer, Modulation scheme, Voltage mode controller.

INTRODUCTION:

In past decades, the MLI (Multi Level Inverters) are developed into many unique topologies, overcoming the conventional design of inverters. This leads to the increase in power quality, reduction in power losses and handling of high voltage and power with low rating devices. The most commonly used MLI topologies are Flying capacitor, Cascaded H-Bridge and Neutral Point Clamped inverter topologies. The main deciding features of the selection of MLI topology are number of load voltage levels and number of switching devices. And voltage stress across the power electronic switches and usage of separate voltage sources are also main factors in deciding the MLI topologies. In some applications such as PV (Photo Voltaic) systems or BESS (Battery Energy Storage System), the isolation of source and load is essential. Either high frequency or low frequency transformers are utilized as high frequency transformer involves two stage architectures in which the independent control of dc or ac variables is possible and in case of low frequency transformer, provides isolation between source and load in single stage architectures and also to increase the load side voltage (step-up mode). Also cascaded topologies are introduced by connecting two or more conventional topologies in series at the load side to get increased number of output voltage levels but this causes increase in the number of isolation transformers which leads to more losses as well as size of the inverter circuit. The transformers are replaced by integrated coupled magnetic inductors which also provide isolation and reduce number of components. Still, more number of coupled inductors are required in order to further increase the number of voltage levels. Several power electronic converters are connected to single transformer by using multiple parallel magnetic flux paths. Hence more inverters are connected in series using single integrated transformer structure. This is used in both single and three phase applications of cascaded structures and achieves multi level voltage at the load side. Various modulation schemes and control strategies are proposed such as Multilevel SPWM, SHE/SHM, and space-vector modulation techniques in order to control and improve the power quality standards. Multi Module Converters (MMC) are also used to get multi level voltages and in this, the voltage stress across individual switches are greatly reduced. The common mode current flow is a common issue in MMCs but it is reduced by introducing coupling inductors for each phase. The increase in number of output voltage levels leads to increase in switch losses which are reduced by introducing soft switching techniques among the DAB converters.

PROBLEM STATEMENT:

In this, a conventional three-phase three leg inverter is used along with a three-phase transformer in order to provide a isolated single-phase mli. This is done by providing two of the inverter legs for generation of three voltage levels and the third leg is a common neutral point for return path of the current. The inverter legs are connected to the windings of three-phase transformer in two parallel paths. A switching strategy is proposed to get a multilevel voltage across the load over the third transformer winding. The filtering of the output current is done by the transformer leakage inductances.