Based On The Three-level Three-phase Inverter Modulation Strategy

Three-level inverters have been becoming increasingly popular in recent years in medium-high voltage ac drive systems and FACTS equipments, mainly due to their high voltage-capability, high output quality, low switching losses and low EMC concerns. The paper primarily makes an in-depth research on the multilevel inverter topological structure and modulation strategy, and then the key problems concerning multilevel converter technique are described and analyzed. Firstly, the development of medium-voltage three-level inverter is introduced ; based on the basic topology of three-level neutral-point-clamped inverter, the basic principle of Space Vector Pulse Width Modulation for three-level inverter is described. A novel Space Vector Modulation algorithm is proposed by using the nearest three vectors as reference voltage vector. The judging rules of the small sectors and the out voltage vector sequence are presented as well as the producing of the signals for SVPWM. The main factors for the neutral-point imbalance are discussed . Also, the dwelling time of each vector is computed. The analysis of other sectors is realized by transforming the sectors.Passive "control" , where the positive and negative small vector is selected alternatively in each new switching cycle. This method can work only in the case of perfectly balanced load and perfectly balanced PWM scheme, which is unlikely to happen in practice. This method would have difficulties to recover from line or load transients. Still, this "control" method can be used to establish a benchmark for NP controller performance. This benchmark can then be used to evaluate the performance of other NP control methods. Hysteresis type control is perhaps the simplest and most popular closed loop NP control scheme. This method requires the knowledge of the current direction in each phase. Based on that information, the small vectors that will move the NP voltage in the direction opposite from the direction of unbalance can be selected. The downside of this method is the current ripple at half the switching frequency. Active control schemes that control the current modulation indexesm_s0 and m_s1. Ingeneral, these schemes require measurement of the voltage unbalance in the NP, and often require measurement of the amplitudes of the phase currents as well. The benefits of these schemes is that they do not have the ripple at half the switching frequency, and some variations of these control schemes can balance the NP exactly. Unfortunately, they all increase the switching losses due to introduction of additional switching states, and may be less robust than the hysteresis type control schemes.In the digital realization of SVPWM modulation in three-level inverter, the design scheme of control system is provided, system hardware structure and software flowchart is designed separately, mainly includes the design of double DSP and CPLD. The realization manner of SVPWM is analyzed in detail, program and experimental results are analyzed and compared. The feasibility and superiority of SVPWM in three-level inverters are proved.