A Multi-level Inverter System Design with Multi-winding Transformer Türev Sarıkurt, Ceyhun Sezenoğlu, Abdulkadir Balıkçı Gebze Institute of Technology, Dept. Of Electronics Engineering turev@gyte.edu.tr, sezenoglu@gyte.edu.tr, a.balikci@gyte.edu.tr Abstract Because of environmental factors and sustainability concerns, renewable energy sources are the state-of-art today. In energy conversion it is important, the output of the system to be suitable with standard applications, for both economical and compatibility issues. To fit the system output voltage and frequency with the international standards, battery packs and transformers are widely used. For modularity the major problem in this area is large physical dimensions. In this study a high frequency multi-winding transformer is used to reduce the size. Also a simple multi-level inverter which has fewer switching elements is proposed by taking account the same concern. And calculation of switching angles for the multi- level inverter is handled by a trigonometric method in order to reduce output harmonics. Keywords: Multi-level Inverter, Multi-winding transformer, Renewable energy systems. 1. Introduction In most applications renewable energy sources are used to produce AC power. Therefore, for both economical reasons and compatibility issues the output voltage of the system needs to be available for standard applications [1]. Since most of the renewable energy sources are intermittent and have variable outputs, a storage equipment is required. For this purpose, it is a common method to use the panels with a battery pack in order to generate a DC bus. These kind of complicated and hybrid topologies are more popular than the conventional multi-level inverter systems [1, 2]. In this study a system is proposed in order to maximize the operating frequency of transformer hence minimizes the size. At the input side of the system, a chopper structure is used and for the output side, a simplified inverter structure with less switching elements is proposed. For the selection of switching angles a simple trigonometric equation is proposed in order to lessen computational work. The load is chosen as pure resistance. The proposed model is simulated with MATLAB and PowerSIM software. The results of simulations, the numbers of used switching elements and the physical dimensions of system are compared with the other studies. 2. The Proposed System As mentioned, in most energy conversion systems, the battery is considered as a constant DC source. In some of the studies capacity elements are used to divide the source [1, 3], in which DC/AC conversion is handled by a conventional multi-level inverter. But the control of converters and balancing the capacitors occur as vital problems. Another alternative is dividing the bus voltage by using a multi-winding transformer [1, 3, 4]. In these systems the large size of the transformer becomes a problem and the output signal of the transformer is transmitted directly to the load [1, 4]. Because of this reason the operating frequency of the transformer have to be chosen 50 Hz for utility applications which increase the transformer size. In another study the outputs of transformer are rectified [2] which brings flexibility of selecting the output frequency. The use of h-bridges to invert bus voltage for transformer and the use of conventional multi-level inverter at the output of the system, require many switches. Also if the transformer is operated in high frequencies, restrictions of the switching elements will cause problems. In the proposed system, the DC input signal is inverted to a high frequency AC signal by a chopper as seen in Figure 1. Chopper is consist of a single switching element instead of H-bridge circuits mentioned in literature [1, 3, 4] which allows proposed topology to use fewer switching elements. Also by this simplification the system will be partially protected from the problems occur due to high switching. A multi-winding transformer is used to divide the input voltage in to various voltage levels in the proposed system. The physical dimensions of a transformer are the function of saturation flux density of the core material and maximum allowable core. Saturation flux density is inversely proportional with frequency. Therefore the size of the transformer decreases as the operating frequency increases [5].