Research On The Modulation Strategies For Three-level Converter

Compared to the two-level converter, the three-level converter has many advantages including high breakdown voltage rating,less harmonic output.High output efficiency and so on.The three-level converter has gradually replaced the two-level power as a major new energy topology,Due to its more complex topologies, so bringing more problems.We need to consider the three-level converter midpoint potential balance,rejects common-mode voltage,reduce the output harmonic content.reduce the switching losses and so.When the three-level at low DC voltage measurement,we should consider the issue overmodulation.So, we request different requirements in different applications on the three-level inverter output characteristics. The three-level converter modulation strategy directly determines the output characteristics,and we improve modulation strategy to optimize the converter’s output characteristics. In summary, Study on three-level converter modulation strategy has important theoretical and practical significance.This dissertation introduces the three-level converter basic principle,and analysis the inherent unity of vector modulation and carrier modulation. We introduces several existing typical vector modulation method of three-level modulation strategy. We mathematical analysis the three-level converter output characteristics,and substituted several typical expression modulation wave modulation strategy into the output characteristics.So we can get The comparison of different modulation strategy output characteristics.The theory also provides a theoretical basis for the three-level converter design parameters.On the basis of linear modulation research foundation, We further research the low DC voltage or motor drive weakening region need to adopt over-modulation strategy.The current over-modulation strategy are starting from a vector angle,so it is difficult to calculate and hard to achieve.To solve this problem,this dissertation proposed an over-modulation strategy on carrier wave.Also,we proposed a modulation wave splitting method to solve the problem which three-level fluctuations over-modulation region midpoint can not use the existing balance control mode to achieve the mid-point.In the last,research team set up a control center based on TMS320F28335 type DSP,and a 10KW three-level converter experimental platform in CPLD as an auxiliary control. The simulation and experimental comparison with the output characteristics, carrier modulation method realized and modulated wave splitting method to control fluctuations in the midpoint potential and verifying.