1668 Five-Level Virtual-Flux Direct Power Control for the Active Neutral-Point Clamped Multilevel Inverter L. A. Serpa * , P. M. Barbosa * , P. K. Steimer † and J. W. Kolar ‡ * ABB Corporate Research Power Electronics Applications, Dätwill, Switzerland Email: leonardo-augusto.serpa@ch.abb.com † ABB Switzerland Ltd. Power Electronics and MV Drives, Turgi, Switzerland ‡ Swiss Federal Institute of Technology - ETH Power Electronics Systems Laboratory, Zürich, Switzerland Abstract—Although multilevel inverters present numerous advantages such as high quality waveform, low switching losses, high voltage capability and low electromagnetic compatibility concerns, some drawbacks are evident. They require a higher number of semiconductors and either multiple isolated dc sources or a bank of series connected capacitors. Conse- quently, the control complexity increases considerably, since more switching devices normally result in a higher number of possible combinations and the balance of the capacitors has to be guaranteed. But on the other hand, multilevel inverters create an extra degree of freedom due to existing redundant voltage vectors, which produce the same output phase voltage level but with diverse effect on the dc-link and floating capacitors. Among the existing control techniques the Virtual-Flux Direct Power Control (VF-DPC) has showed to be very suitable for grid connected systems since it controls the active and reactive powers directly without any internal current control loop or PWM modulator. However, in order to adapt the VF-DPC for multilevel systems, specifically for the recently proposed five-level Active Neutral-Point Clamped converter, additional features must be included and/or modified in the inner main control loop. In order to allow the controller to select a higher number of available voltage vectors, the active and reactive power hysteresis strategies are modified. Additionally, a method to balance the dc-link and floating capacitor voltages by applying available redundant states is implemented, based on the actual condition of the voltage across the lower dc-link and floating capacitors, as well output phase currents direction. The proposed five-level VF-DPC has been implemented using a 6kW five-level prototype and has shown good static and dynamic performance. I. I NTRODUCTION Important improvements concerning voltage and current ratings have been achieved over the last decade for fast semiconductors like IGBTs. This permits the utilization of these devices in high power and high voltage applications, employing either the conventional two-level topology or the industry standard three-level NPC converter. However, the use of fast switching devices under high voltage may gener- ate high dv/dt to the load. Additionally, the switching losses are increased significantly, reducing the overall efficiency. In order to bring the stress on the switching components back to acceptable values, a new branch of multilevel converter is quickly emerging, namely five-level topologies. Furthermore, due to the ability of generating a higher number of output voltage levels, this group of multilevel converters can further reduce the voltage and current harmonic contents when compared against three-level topologies. Traditionally, five-level structures are obtained as natural extension of existing three-level topologies, such as the Neutral-Point Clamped (NPC) [1] and Floating-Capacitors (FC) [2] converters. Additional semiconductors and energy storage elements are included into the classical three-level topologies to produce extra levels. However, both topologies reveal technical difficulties which complicate their applica- tion by the industry. A higher number of clamping diodes and flying capacitors is required considering all components with the same voltage rating [3]. Another important aspect to be considered is the complex- ity to balance the dc-link capacitor voltages of the NPC, and flying capacitors of the FC. In the five-level NPC the dc-link is subdivided into four equal voltage levels by using a split bank with four capacitors instead of only two capacitors as for the three-level approach. The number of flying capacitors is also significantly increased for the five-level converter, demanding a dedicated control technique. Both difficulties are avoided in the Cascaded H-Bridge multilevel topology, which is characterized by series con- nection of single-phase H-bridge converters. However, the use of this strategy is limited in some applications due to the necessity of isolated dc power sources. Recently, a new multilevel topology has been introduced [4] to overcome some of the above mentioned limitations while achieving all the advantages of multilevel converters. The five-level active neutral-point clamped (ANPC) inverter combines characteristics of the neutral-point clamped and floating-capacitor inverters. However, at the same time the five-level ANPC inverter brings numerous benefits, it also demands especial and dedicated control strategies. If the balance of the capacitor voltages are not guaranteed, a group of switching devices experience a higher voltage applied across their terminals and extra distortion may occurs in the load voltages and currents. Additionally, the controller shall allow the inverter to switch between different voltage levels in order to take advantage of the higher number of voltage vectors available in a five-level structure. Therefore, existing control tech- niques, namely voltage oriented control, hysteresis, direct 978-1-4244-1668-4/08/$25.00 ©2008 IEEE Authorized licensed use limited to: ETH BIBLIOTHEK ZURICH. Downloaded on December 1, 2008 at 05:22 from IEEE Xplore. Restrictions apply.