978-1-4244-2800-7/09/$25.00 ©2009 IEEE ICIEA 2009 Distributed Generation With Parallel Connected Inverter M.A.A. Younis 1 1 Dept. of Electrical Engineering, UNISEL, Selangor, Malaysia E-Mail: mahmoud@unisel.edu.my N. A. Rahim 2 and S. Mekhilef 3 2, 3 Dept. of Electrical Engineering, University of Malaya, Kuala Lumpur, Malaysia. Abstract— This paper presents the design and analysis of a new configuration of parallel connected inverter suitable for Distributed Generation Application. The configuration consists of dc/dc converter and parallel three-phase dc/ac inverter. Series resistors added to the inverter output to maintain same current in each inverter of the two parallel inverters, and to reduce the circulating current in the parallel inverters to the minimum. Third harmonic injection PWM (THIPWM) reduces the total harmonic distortion and to make maximum use of the voltage source. DSP was used to generate the THIPWM and the control algorithm for the converter. PID controller is applied on the full- bridge dc/dc converter side to maintain the ac voltage to the required level. Selected experimental results have been shown to validate the proposed system. Index Terms—Three-phase inverter, Third harmonic injection PWM, inverters parallel connection, Distributed Generation. I. Introduction Distributed Generation (DG) have become increasingly more accepted since the demand for reliable and secure power systems with high power quality increases, [1]; especially after producing alternative energy resources (such as fuel cell, wind-turbine, bio mass, micro-turbine and solar-cell systems) with lower costs. To connect Distributed Energy Resources (DERs) to the existing three- phase power systems effectively and efficiently, power electronics- based power conversion systems need to be developed to deliver a proper energy [2]. Through the control of the Power Conversion System, benefits such as increased reliability, security, and fewer downtimes can enhance the utility grid without having to add or replace the existing transmission/distribution system. The concept of DG has been recently become commercially extensive. Distributed generation is the interconnection of alternative energy resources to the utility grid system close to the load Point to alleviate the demand for and expansion of the electric transmission system [3] [4]. DG is meant to shift the structure of the utility system from a centralized, radial system to energy source connected on the distribution level. The penetration of distributed generation into the energy market has some advantages includes [5] [6]. • Distributed generation reduce the dependency on fuel sources. • Enable renewable energy usage which has minimum impact on the environment and it offers free replacement of prime fuel sources. • Electrify the rural areas, which give chance of development for those areas • Reduce the need for transmission lines extension or construction, and it can be used to support demand during peak periods]. • Effects power management concept that utilizes local resources nationally Fuel cells and photovoltaic cells are currently considered attractive power sources for portable power applications in civilian arenas because of their superior power densities, silent operation and non-polluting nature. However, Fuel cells provide unstable dc voltage which needs to be stabilized and converted to ac voltage to be suitable for ac load application. With PWM control technologies, ac side of the grid connected inverter has the abilities of controllable power factor, sinusoidal output currents and bi-directional power transfer [7] [8]. The third harmonic injection method to control the power factor of the inverter output current used for three-phase inverter. However, it is very difficult to generate the right third harmonic amplitude [9] [10]. In hysteresis control the switching frequency varies significantly according to the power level and the dc link [11] [12]. The fuel cells system typically consists of a dc/dc converter to boost the source voltage to a higher level and a dc/ac inverter. Several variations of this scheme have been proposed in the literature [13] [14], each presenting advantages and disadvantages. Isolated dc/dc converter became a hot topic in the last decade because of their wide application in green power application which includes solar, wind, geothermal, ocean energy and fuel cell. The front end of the isolated dc-dc converter, which is full bridges have been seen in most applications. Compared with half bridge, full bridge will have less current stress and doubled input voltage utilization. II. SYSTEM BLOCK DIAGRAM The block diagram of the system is shown in Figure 1. The dc voltage produced by the dc power supply is converted to a high-frequency train of positive and negative pulse pairs by the full-bridge converter, which is controlled by the phase- 2935