DC-link capacitor sizing of three-level neutral point clamped voltage source inverters for variable speed drives

Ogan, Serhat Emir
A three-level neutral point clamped (NPC) voltage source inverter (VSI) topology can be advantageous in electric vehicles with a high DC-link voltage and a high switching frequency due to the flexibility in power switch selection. However, the potential oscillations in neutral point must be handled to guarantee an efficient operation of this topology. A bulky DC-link capacitor is not an option for EV applications; thus, the sizing of the DC-link capacitor considering the traction system characteristics is an important design step. This thesis investigates the effect of PWM modulation methods on the DC-link capacitor size of a three-level neutral point clamped voltage source inverter. Five PWM methods, from which three methods have an active neutral point potential control, are compared in terms of their neutral point potential (NPP) oscillations at various operating conditions. Since the operating conditions in a vehicle traction system depend on the traction motor, the worst-case NPP oscillation condition of each PWM method is determined based on the operation characteristics of a reference electric machine. Then, the size of the DC-link capacitor is determined for each PWM method so that NPP ripple is kept under desired limits under its worst-case operating condition. It is shown that both the modulation technique and the electric machine characteristics are effective on the capacitor size. A 1 kVA prototype three-level neutral point clamped inverter is designed and used to experimentally verify the results of capacitor sizing. Carrier-based PWM method required significantly smaller capacitors than other methods; however, this method had disadvantage of higher switching losses.


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Citation Formats
S. E. Ogan, “DC-link capacitor sizing of three-level neutral point clamped voltage source inverters for variable speed drives,” M.S. - Master of Science, Middle East Technical University, 2021.