1 Practical power control design of a NPC multilevel inverter for grid connection of a renewable energy plant based on a FESS and a Wind generator Omar Bouhali Bruno François Christophe Saudemont El Madjid Berkouk L2EP, LCP & LAMEL L2EP L2EP LCP Central school of Lille Central school of Lille Hautes Etudes d’Ingénieur Polytechnic National School BP 48 Cité Scientifique BP 48 Cité Scientifique 13, rue de Toul, 10, hassen Badi 59651 Lille CEDEX, France 59651 Lille CEDEX, France 59046 Lille, France El Harrach, Algeria bouhali.omar@ec-lille.fr bruno.francois@ec-lille.fr christophe.saudemont@hei.fr emberkouk@yahoo.fr Abstract – In this paper we present a modelling of power flows into a power station. This one is based on a variable speed wind generator coupled with a flywheel energy storage system. A multilevel NPC converter is used for the grid connection. The design of the power controller is obtained by a dedicated inversion of the used power model. It includes the strategy for regulating the energy storage in order to supply required active and reactive power references. I. INTRODUCTION The main problem of renewable energy based generators is that they operate as random power sources. For the grid manager they are considered as passive sources since they do not provide ancillary services; as the regulation of the rms voltage in accordance with reactive power generation and the automated active power generation in accordance with a frequency deviation characteristic. The regulation of the voltage and the frequency is consequently deferred on the traditional alternators. In consequence the penetration ratio of the decentralized production, i.e. the generated power compared to the total consumed power, must then be limited in order to guarantee the "stability" of the electrical network. Certain experience feedbacks in wind generator indicate that for penetration ratio higher than 20 or 30%, management problems of the production consumption balance can appear. The use of a storage unit can be a solution to achieve a large scale integration of renewable energy based generators, which then evolve to multi source power stations. A local energy management has to balance the variations of the random production (from the renewable primary source, wind, sun …) in order to implement a production forecast [1], [2]. That is possible while storing in case of overproduction from the renewable energy and then to use later this stored power in the event of a production lack. In this paper the multi source power station under study is composed by: _ a Permanent Magnet Synchronous Machine (PMSM) based wind generator driven by a two level PWM AC/DC converter _ a Flywheel Energy Storage System (FESS) based on a squirrel-cage induction machine (IM) driven by a two level PWM AC/DC converter [3], [4]. Both units are connected to a common DC voltage bus. A three level NPC multilevel inverter and a filter are used for the connection to the grid. Multilevel converters have many advantages for power conversion applications. The first one is to use a high DC bus by means of a series connection of DC capacitors. We will demonstrate in this paper that this enables to supply higher power to the grid and the rated power characteristic will be determined. More over the various DC capacitor points of the DC bus make possible the connection of many low power sources. The second advantage is the better quality of generated grid currents. In the past the modelling, the control and the self regulation of the DC bus voltages have been studied [5]. In this paper we present the practical power control design of this multilevel converter for the grid connection of a multi source power station. Grid vm 1 PMSM AC/DC Converter DC/AC multilevel converter IM Grid connection Rs,Ls is1 is2 3 AC/DC Converter i e i st Fig. 1: Studied system. II. MODELLING OF THE GRID CONNECTION A. DC/AC multilevel converters A L filter is used to attenuate adequately generated current harmonics. Then an equivalent mean modelling of the power converters is sufficient for the study (fig. 2) [5]. It represents voltage/current fundamental components of: _ modulated phase to phase voltages ( 13 m u and 23 m u ) and _ two modulated currents ( 1 m i and 2 m i ) . Rs, Ls C um 23 um 13 is 2 is 1 vs 2 vs 1 vc 1 Grid vm 2 vm 1 vm 3 ic 1 vc 2 C im 1 im 2 ie+ist Fig. 2: Equivalent continuous electrical model of the grid connection. Mean values of modulated quantities can be expressed as:      + = + = 2 22 1 21 23 2 12 1 11 13 s s m s s m u m u m u u m u m u (1) With: 2 1 1 c c s v v u + = and 2 2 c s v u = . 2 22 1 12 2 2 21 1 11 1 , s s s s i m i m i i m i m i m m + = + = (2) 4291 1-4244-0136-4/06/$20.00 '2006 IEEE