Analysis Of A 3-level-vsi Neutral-point-control For Fundamental Frequency Modulated Svc-applications

This paper presents a new method to control the neutral-point (NP) of a 3-level-VSI in SVC-applications with fundamental frequency modulation (FFM) or optimized pulse patterns. In a first step the systems equations for a 3-level-VSI connected to a symmetrical, ideal sinusoidal load are derived. This includes a general equation for the NP-current. Based on this, investigations on the self-balancing abilities are performed. It is shown that, in the presence of a resistance in the system, the 3-level VSI is self-balancing for all pulse patterns symmetrical to a quarter of a period and for all power factors on the AC-side. However, in high power applications the self-balancing is very slow due to very small resistances in the system. This is verified by SABER simulations for an SVC-application. In a further step, a NP-control principle for fundamental frequency modulated SVC-applications is deduced on a simplified model. The validity of this scheme for a wide operation range is verified by mathematical analysis and simulation results with Matlab. In a further step, this analysis is extended by including all harmonics of the 3 level-VSI output currents, which shows their influence on the control scheme. Simulation results with Matlab and SABER proof the validity of the theoretical investigations. Finally the control method is adapted to PWM-SVC-applications.