4805 Figure 1. Five level diode clamped converter. An Optimized Cascaded Multilevel Static Synchronous Compensator for Medium Voltage Distribution Systems Lucas Frizera Encarnação, Emanuel Leonardus van Emmerik and Maurício Aredes Electrical Engineering Program, COPPE/UFRJ, Power Electronic Laboratory, Rio de Janeiro, Brazil Abstrac t —Problems related to energy quality in medium voltage distribution systems have severely been affecting the industrial process, especially at the industries that work with complex plant processes, causing damages to the equipments and high financial losses. The necessity of a better energy quality for industry encourages the transition of the custom power equipments, typically used in low voltage systems, to work in medium voltage systems. This recent interest has been instigating researches from the power-electronics society. The multilevel power converters represent a potential breakthrough in employing switching equipments in medium voltage applications. In this context, studies related to multilevel topologies, modulation strategies and control circuits are presented in this paper in order to achieve an optimized multilevel distribution static synchronous compensator (DSTATCOM) to improve the power quality in industrial installations. Simulation results with all the contributions show the optimized performance of the compensator. I. INTRODUCTION Multilevel power converters present several advantages when compared with conventional two-level converters [1],[2]. The principle advantages are higher output voltage levels, lower harmonic content and Electro Magnetic Interference (EMI), absence of matching transformers and passive filters. Therefore, the increase of the multilevel power converters in medium voltage distribution systems encouraged several studies related to multilevel power circuits, modulation strategies and control circuits in order to improve these advantages, optimizing the performance of the multilevel converters. This paper presents a summary of the multilevel power circuits, comparing the advantages and disadvantages of the most important topologies, in order to achieve the best performance for the multilevel converter using a minimum quantity of components. Further, this paper will present an analytical study of the modulation strategies applied in the multilevel topologies in order to propose an optimized modulation strategy for the multilevel converter. This work also exploits an independent dc voltage control circuit of the asymmetric cascaded converter used to maintain the capacitor voltages. II. POWER CIRCUIT The most import topologies of multilevel converters are the diode clamped, flying capacitor, symmetric cascaded and asymmetric cascaded [1]. Basically, two main structures exist for these multilevel topologies, the half bridge structure, used in clamped converters, and the full bridge structure, used in cascaded converters. A five level diode clamped converter and a five level cascaded converter are presented in Fig. 1 and Fig. 2, respectively. Theoretically, both structures can achieve the same harmonic performance. However, in practice, the half bridge structure uses more components to compose the same multilevel converter. The asymmetric cascaded converter presents all the advantages of the others topologies, as lower harmonic content and absence of passive filter. Furthermore, this topology has the smallest number of power component devices per output voltage level (m), as can be seen in Table I. The only drawback of this topology is the necessity of separated dc sources with different voltages for real power compensation, limiting its application, for example inhibits back-to-back configuration. However, the operation principle of a static synchronous compensator is the support of reactive power, correcting the power factor or regulating the voltage bus, leaving only a small interchange of the real power [3]-[5] to maintain the capacitor voltages, thus, this disadvantage will be discarded. The three-phase multilevel asymmetric cascaded used in this work is shown in Fig. 3. Figure 2. Five level cascaded converter. 978-1-4244-1668-4/08/$25.00 ©2008 IEEE Authorized licensed use limited to: UNIVERSIDADE DO PORTO. Downloaded on March 19,2010 at 14:25:19 EDT from IEEE Xplore. Restrictions apply.