Research On Single-phase MMCC-SVG Based On Hybrid Modulation Technology

The development of power system has brought convenience to us,at the same time,as long as a large amount of non-linear and high voltage grade load are connected to the power grid,power quality problems come out.Static var generator(SVG)who has a superior performance in the absorption and compensation of reactive power,has become the focus of many scholars and enterprises.For the reason that SVG has developed to the trend of high voltage and large capacity,needs of applying research on modular cascade multilevel converter(Modular Multilevel Cascade Converter,MMCC)are coming.When need to improve the voltage level and increase capacity,just increasing the number of modules.At the same time,increasing module number,will make the output waveform closer to the sine wave,and thus reduce harmonic.First of all,this article introduced the working principle of MMCC,deduced mathematical model of MMCC.Compared carrier phase shifted PWM(Carrier Phase Shifted PWM,CPS-PWM)and nearest level modulation(Nearest Level Modulation,NLM)carefully,and these two modulations were researched popular in these years.Also,to prepare for PWM+NLM hybrid modulation technology,the article researched the control methods of voltage balancing of sub-modular under the two modulations.Then,it introduced the method for controlling reactive power based on chasing of momentary value especially for single-phase MMCC-SVG,however,this method is definitely different from the way for three-phase.Secondly,the paper put forward PWM+NLM hybrid modulation technology for MMCC field,through combination of CPS-PWM and NLM.The new modulation technology absorbs the advantages of lower harmonic in output voltage of PWM and lower switching frequency of NLM.More than this,it reduces the disadvantage of too high switching frequency of PWM and bad performance of NLM.Also,there are two different voltage balancing methods in PWM+NLM hybrid modulation technology:voltage sorting method and PI adjusting method.The paper deduced the control method to apply PWM+NLM hybrid modulation technology for single-phase MMCC-SVG by theoretical analysis and formula derivation.After deep research on the working principle and control method,through brought in the state of 00,the article put forward a non-complementary conduction way between the up and down IBGT of sub-modular.Different form the old and obsolescence complementary conduction way,it avoided the obstacle of the point when current pass zero,and reduced the switching frequency.Finally,implement the PWM+NLM hybrid modulation technology and non-complementary conduction way by building a 5-level MMCC-SVG in PSCAD/EMTDC.Further implementation of the two new ways through 5-level MMCC-SVG hardware platform also,research the dynamic response of MMCC-SVG and effect of reactive current controlling.