Neutral-point Voltage Balance Control For NPC Three-level Inverter Based On Hybrid SVPWM Strategy

Multilevel converters have attracted more and more attention in high-powerapplications due to low voltage across the power device, improved output voltagequality and low dv/dt. Among them, the neutral-point-clamped (NPC) three-levelinverter is a most widely used multilevel converter topology, which has low hardwarerequirement. The disvantage of NPC three-level inverter is the presence oflow-frequency voltage oscillations on the NP under the conditions of high modulationindex and low power factor. And the ripple amplitude increases and the ripplefrequecncy changes under unbalanced load. This paper explores above issues.This paper introduces the topology and the principle of NPC three-level inverter,and analyzes the vector distribution based on the space vector diagram. Then thenearest-three-vector (NTV) technique is described and the NTV methods before andafter adding balance control are compared. The nearest-three-virtual-vector (NTV2)technique is introduced based on the definition of virtual vector.Based on the analysis of the duty-cycle coefficients of the redundant smallvectors, the intervals where NTV can control the NP voltage balance in a fundamentalcycle are obtained. This paper combines NTV2with NTV and proposes a hybridSVPWM method. This method can limit the NP ripple amplitude and reduce theswitching frequency by extending the NTV action area. A proportional parameter isproposed which represents the combination of the two techniques. The systemperformance depends on the proportional parameter and this paper gives therelationship between the performance and the proportional parameter to provide thebasis for selecting the combination of the two modulation methods.The impact of unbalanced load on the NP voltage balance is analyzed and theconclusion that the unbalanced load can increase the ripple amplitude and change theripple frequency is obtained. Then the intervals where NTV can control the NPvoltage balance under unbalanced load are explored, and the hybrid SVPWM methodwhich is applicable to the unbalanced load is presentd.Last, the maximum and minimum values of the NP current during the wholefundamental cycle are explored. Base on that, the NP current, when the balancecontrol is put into the NTV method, can be obtained. Various extension modes areanalyzed and the optimal extension mode is presented. Then, this paper compares five typical extension modes. This paper explains the asymmetry of the output phasevoltage, and the reason that the output voltage level of inverter is not constant isindicated by comparing the ideal operation condition and the practical operationcondition.