Dr.G.Jayakrishna, K.Vijayalakshmi 71 International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 10, Issue. 9, Oct. 2014 Capacitor Voltage Balancing for Five level Converter Using Space Vector PWM * Dr.G.Jayakrishna 1 , K.Vijayalakshmi 2 Mail id : g.jayakrishna25@gmail.com Abstract- This paper proposes a Space Vector Modulation (SVM)-based approach for the switching state redundancy of an n -level Flying Capacitor Converter(FCC) to carry out the voltage balancing task within the switching states. Capacitor voltage deviations are minimized to choose the proper redundant switching states among the available switching states. The performance of a grid-connected Five-level FCC under the proposed SVM strategy for various operating conditions is simulated and evaluated based on time-domain simulations in the MATLAB/SIMULINK. The simulation results demonstrate the capability of the proposed SVM strategy to regulate the capacitor voltages at their nominal reference values. Keywords: Capacitor voltage balancing, flying capacitor converter, multilevel converter, space vector modulation. I. INTRODUCTION The multilevel flying capacitor inverter (MFCI), a relatively new type of power converter topology, has attracted worldwide attention for high power applications such as static power conditioners and large motor drives[1-2]. Each phase limb consists of a series of connected cells nested inwardly toward the load from the DC link. Each cell has a capacitor for clamping the node voltage and two bidirectional power switches that operate in a complementary fashion [3]. A greater number of possible output voltage levels require more cells in each phase limb and a greater total capacitor count. An important advantage of this circuit is that many switch state combinations produce the same voltage level. This gives flexibility in choosing switching control strategies for optimized output performance. It also has a simple arrangement with modular building blocks employing fewer switching devices, and snubber less operation is possible [4]. In ensuring proper operation of MFCI, the main challenge is to maintain the correct voltage across the floating or cell capacitor [5]. It is essential to appreciate that the MCFI is inherently stable under constant (positive resistance) load conditions [6]. Closed-loop control of capacitor voltages is not fundamentally necessary, because, for a steady load, they automatically settle at a stable fraction of the DC-link voltage. However, closed-loop control can be used to reduce the capacitor voltage swings and to improve response speed at transient load changes [7]. This paper proposes a closed-loop space vector modulation (SVM)-based capacitor voltage balancing method, which exploits the SVM switching state redundancy to regulate the capacitor voltages at their reference values. The section II describes Operation of FCC and grid connected FCC with its operation and SVM, section III describes the control strategy and mathematical modeling of grid connected FCC, section IV presents simulation of FCC connected to balanced and unbalanced grids, section V presents simulation results of FCC and section VI presents the conclusion. II. PROPOSED FIVE LEVEL FLYING CAPACITOR CONVERTER Several researchers analyzed the voltage balance dynamics in FCCs using frequency domain methods. Recently, considerable research attention has been devoted to switched systems, i.e. systems composed of several subsystems, and a switching law that determines which subsystem is active at every time instant. Several strategies for addressing the balancing problem in diode clamped DC/AC MCs with passive front ends have been proposed. More generally, voltage balancing is regarded as the most important problem in the field of high voltage MCs. The flying capacitor converter (FCC) is a multilevel pulse width modulated (PWM) converter whose internal architecture automatically guarantees the voltage balancing property for passive loads. Furthermore, a single. leg FCC may be used for both DC/DC and DC/AC conversion [8]. This is so because a DC current at a clamping point permanently charges/discharges the DC bus capacitors, so no voltage balance is possible in principle. Even for the case of DC/AC conversion, significant low frequency clamping point voltage oscillations may appear for some operating conditions. A. Structure of Grid-Connected FCC System Fig.3.1 shows a schematic diagram of a four-level FCC which is connected to a constant dc bus. The ac terminal of the FCC is connected to a utility grid through a series-connected filter. Although, in this paper, a four-level FCC is considered, the analysis and the proposed strategies are general and applicable to an n-level FCC. Fig.1. Circuit diagram of a grid-connected four-level FCC.