Abstract This paper proposes multilevel inverters for use in single-phase distributed generation systems. It consists of H-bridge converters and cascaded transformers. To produce high-quality voltage waves, it synthesizes a large number of output voltage levels using cascaded transformers, which have a series-connected secondary. By a suitable selection of the secondary turn-ratio of the transformer, the amplitude of an output voltage appears at the rate of an integer to an input dc source. It can reduce EMI and THD problems and save number of switching devices compared with the conventional multilevel approaches. Operational principles and analysis are illustrated in depth. The validity of the proposed system is verified through computer-aid simulations generating output voltages of a 9, 27 and 81 level, and PWM 5, 11 and 29 levels, respectively, and their result are compared with conventional counterparts. Keywords multilevel inverters, grid-connected, harmonic distortion, electromagnetic interference I. INTRODUCTION ECENTLY, due to the growing energy consumption around the world and the eminent exhaustion of fossil- fuel reserves, a great interest on alternative energy sources can be noticed nowadays. The threat of electrical energy rationing, blackouts, the wariness of the public to nuclear power plants in reaction to the leak of radioactive material in Russia and Japan, and in addition to increased environmental awareness, the requirement of research on alternative renewable energy system (RES) has drastically increased. Among the clean and green power sources are photovoltaic panels, wind turbine and fuel cells [1]-[3]. The distributed generation (DG) concept emerged as a way reducing emissions, and providing additional power quality benefits [4]. As the RES generated to the DG, a power conditioning system is also required, in order to adjust the T. Unchim is with the Center of Excellence in Electric Energy, Smart Materials, and Health Science, School of Electrical Engineering, Suranaree University of Technology, Nakhon Ratchasima, Thailand. (e-mail: tapekung13th@hotmail.com) P. Pao-La-Or is with the Center of Excellence in Electric Energy, Smart Materials, and Health Science, School of Electrical Engineering, Suranaree University of Technology, Nakhon Ratchasima, Thailand.(e-mail: padej @ sut.ac.th) A. Oonsivilai, corresponding author, is with Center of Excellence in Electric Energy, Smart Materials, and Health Science, Postharvest Technology Research Center, School of Electrical Engineering, Institute of Engineering, Suranaree University of Technology, Nakhon Ratchasima, 30000 Thailand. (phone: 66-81-548-7728, fax: 66-44-224601; e-mail: anant@sut.ac.th) frequency and voltage to a suitable level on the utility grid and allow the parallel connection. In addition, the RES must present some features standards, such as IEEE Std. 929-2000, IEEE Std. 519-1992 and IEC 61727, that supports the connection of the RES to the utility grid [5]-[7]. Converters are employed to connect these energy sources to the grid, as shown in Fig. 1. Fig. 1 General scheme of a DG unit connected to the grid. Generally, the DG is generated with only one voltage source. Amplitude of voltage produced by DG is low. Thus, there is a need for additional boost converter or a step-up transformer to obtain high-output voltage converting power from ac to ac. This is commonly connected in series with a pulse-width modulated (PWM) inverter. However, a somewhat high switching frequency of PWM inverter and its dv/dt stress, results in low efficiency and occasionally electromagnetic- interference (EMI) problems. In addition, an output filter is required to reduce high-switch frequency components which produces high power losses [8]-[10]. In points of alleviating these problems, multilevel inverters can substitute the conventional PWM inverter. Generally, multilevel topologies have been studied to increase the power-reducing voltage stress on the power switching devices and provide high-quality output voltage [11]-[15]. Four presentable multilevel topologies can be considered such as diode-clamped, flying capacitor and cascade H-bridged cell with separate dc sources [16]- [17]. Theoretically, they can synthesize an infinite output voltage level. By increasing the number of output levels, the output voltages have more steps generating a staircase waveform, which has reduced harmonic distortion. However, to increase output level, they need a lot switching devices, clamping diodes, or other passive elements. It, unfortunately, causes an increase in the complexity of switching patterns. This paper presents a new multilevel inverter scheme for application to single-phase grid-connected photovoltaic S. Janjamraj, P. Pao-La-Or., and A. Oonsivilai Multilevel Inverters Suitable for the Use of Single- Phase Distributed Generation Systems A Study of Wind Speed Characteristic in PI R World Academy of Science, Engineering and Technology 60 2011 1129