Research On The T-type Three Level Inverter

With the problem of energy crisis and environmental pollution, the utilization of renewableenergy including PV generation has been attracted more and more attention. The hybrid AC/DCmicrogrid technology is an important direction in the development of renewable energy. As thetransmission medium between AC and DC bus, both the topology and control strategy of thebi-directional AC/DC converter need to be researched. Compared with the traditional NPCthree-level inverter topology, the T-type three-level inverter topology has the advantages of fewerswitching devices and loss distribution balance. Therefore, the research of this paper is based onthe T-type three-level inverter.Firstly, the operation principle of the single phase T-type three-level inverter is analyzed, andboth the loss of model and distribution are studied. An experimental prototype is built up with theresults given. Secondly, the mathematical model of the three phase three-level inverter is derived. Amodified SVPWM algorithm is utilized to reduce the common mode voltage of the inverter. Thesmall voltage sector is divided in two different ways, the method is analyzed which is moreeffective to suppress leakage currents. Thirdly, the equivalent model of two paralleled converters isstudied. Leakage currents are affected by both the pulsation amplitude and frequency of thecommon mode voltage. Therefore, the carrier is phase shifted180degree between the paralleledinverters to suppress leakage currents of the paralleled system. In order to meet the demand ofdifferent working conditions, the bi-directional control stragety of the paralleled AC/DC convertersis researched. The consistent power flow of paralleled converters is achieved by utilizing acombined positive and negative dc bus voltage regulators, and the grid-tied current of theparalleled AC/DC converters are well distributed by the dc bus voltage droop control.Finally, the prototype of the paralleled T-type three phase three-level inverters is built up. Thedesign process of power circuit, drive and protection circuit are given. Both the steady state anddynamic experimental results of a three phase inverter and paralleled AC/DC converters arepresented. Experimental results verify the effectiveness of both the theoretical analysis andsimulation results.