Research On Current Harmonic Optimization Of Three-level Inverter In Electric Locomotion

Nowadays rail transport system in China plays a key important role, and Harmony CRH EMU emergence of a key role in promoting China's railway development. The traction converter is a part of EMU traction, which plays an impotent role in EMU's key technology system. The structure consists of an AC- DC- AC converter system by traction rectifier, DC link and the traction inverter.On high-capacity AC drivers, the switching frequency is limited under a certain level,typically below 1kHz in order to reduce switching loss. Due to that, traction inverter output waveforms contain a lot of harmonics.Firstly, the principle and composition of electric locomotive's traction drive system are analyzed in this paper. Then analyzed the principle of traction rectifier, middle filter and traction inverter in CRH2 electric locomotive. The occurring of the harmonic current introduced, and its harms to electric locomotive are analyzed. The harmonic current is affected by the switching angle.Secondly, the topologies of multi-level inverter are introduced, such as diode clamped,flying- capacitor and cascade multi-level inverter. All kinds of topology's characteristics and application situation are discussed. How to select a topology and why are summarized in this paper. This paper selected the Three-level neutral point clamped(NPC) inverter for high power ac drive such as electric locomotives. The modulation methods for NPC are introduced, such as SPWM and SVPWM, etc. For traditional modulation strategy's complexity, a simplified three-level inverter modulation strategy is proposed. Three level inverter space voltage vector is equivalent to 6 two level inverter space voltages in the strategy. The reference vector is synthesized from two level inverter voltages. Tested by simulation and experiments, the experimental results show that the proposed modulation strategy is correct, the advantages of digital implementation is simple.Finally, the principle of NPC three-level inverter's harmonic current is analyzed. The mathematical model of an optimal PWM method is built, and the goal function for harmonic current is derived. THD of phase currents is constituted focusing on the three-level inverter used on the high power electric locomotive. The narrow pulse elimination algorithm is applied while using MMT-OA to solve the model. The solution of the optimization function on whole modulation range in various pulse wave is completed.MMT-PWM has a good result of harmonic optimization compared with SPWM method. It can reduce the traction inverter harmonic current. All design thoughts and theoretical areverified by emulation and experiment.