Power Electronic Interface in a 70 kW Microturbine- Based Distributed Generation Mahmoud Ranjbar, Saeed Mohaghegh, Mehdi Salehifar, Hassan Ebrahimirad, and AmirPirooz Ghaleh Power Electronics Department, Niroo Research Institute (NRI), Tehran, Iran E-mail: m_ranjbar@ieee.org Abstract—This paper deals with required power electronics interface for a 70 kW microturbine-based distributed Generation system. First, an introduction of microturbine technology and its applications is presented, and then the power electronics interface that is required to transfer microturbine’s output power to an islanded local load or an electric power grid is described. Different control strategies for controlling the inverter in stand-alone and grid connected modes and the seamless transfer between two modes are described in this paper. Also, the required power converter protection system is presented. To show effectiveness of the designed inverter and its digital controller the experimental results based on a constructed 70 kW semi-industrial setup has been demonstrated. Keywords-microturbine; distributed generation (DG); grid-tied inverter; islanding; digital controller I. INTRODUCTION ISTRIBUTED Generation systems such as microturbines, fuel cells, wind turbines and photovoltaic systems are expected to represent a large portion of power generation capacity, especially in future [1]. In particular, Micro-Turbine Generator (MTG) is witnessed to be capable of delivering clean energy from a wide variety of fuels with superior safety and low emissions. The capacity of MTG can be ranged from several kilowatts up to megawatts. As it is often sited dispersedly near the industrial load, it is deemed as a category of distributed generations nowadays [2]. A direct merit exhibited by such electric power generation is to provide the utility a way to defer power plant construction, while offering customers a clean resource at reasonable cost. Compact design and low maintenance are some advantages of microturbine in comparison with old electricity generators such as Diesel generators. Nowadays, the demand for connection of small electricity generators to the distribution grid increased. Electric companies are interested in developing small cogeneration systems in respect to reduce the cost and increase the efficiency [3]. There are essentially two types of micro turbine designs. One is a split shaft design that uses a power turbine rotating at 3600 rpm and a conventional generator (usually induction generator) connected via a gearbox. The power inverters are not needed in this design. Another is a high-speed single-shaft design with the compressor and turbine mounted on the same shaft as the permanent magnet (PM) synchronous generator. The advantages of the high-speed permanent-magnet generator are its compact size, low-mass design and the elimination of the gearbox, resulting in reduction and simplification of the generating package. The use of power electronics enhances the system performance because of the asynchronous operation of the gas turbine, with the gas turbine speed independent of the grid frequency. It enables the gas turbine speed control to adjust for optimal gas turbine efficiency [4]. A general view of microturbine system is shown in Fig. 1. The configuration of an MTG system is composed of a gas turbine, a compressor, and an AC generator. They are inertia welded on a single shaft to simplify the mechanical structure. When this shaft turns at the speed of the turbine, the generator would provide high-frequency AC electricity that requires a rectifier (Converter 1 in Fig. 1) and an inverter (Converter 2 in Fig. 1) to interface with utility network. By employing different control strategies, the MTG can operate as a power conditioner for the grid-connected operation to improve the quality of supplying power or sever from the grid as an emergency generator [5], [6]. The grid-connected inverter in MT’s power electronic interface should operate in grid-tied and off-grid modes in order to provide power to the emergency load during system outages. Moreover, the transition between the two modes should be seamless to minimize any sudden voltage change across the emergency load or any sudden current change to the grid. A seamless transfer between both modes has been proposed in [7]. However, the grid current controller and the output voltage controller must be switched between the two modes, so the outputs of both controllers may not be equal D Figure 1. Block diagram of power electronic interface configuration for a MT. 2011 2nd Power Electronics, Drive Systems and Technologies Conference 978-1-61284-421-3/11/$26.00 ©2011 IEEE 111