The Research On Neutral Point Drifting Technology For Cells Cascaded High Voltage Inverter

With the development of power electronics and computer technology,high performance high voltage inverter has been applied extensively.It can solve the problems of soft start and speed regulation on high power fan or pump,and can save energy remarkably. Because of its low THD,high power factor and perfect output wave shape,cells cascaded high voltage inverter has wide applying prospect and capacious developing space.CPS-SPWM can get the high equivalent switch frequency with low IGBT switch frequency.So the performance of output voltage of the cells cascaded high voltage inverter can be improved when CPS-SPWM is applied.The performance of output voltage has something to do with the carrier phase-shifed angleθ.The high voltage high power system demands high reliability.Especially for some important occasions such as energy,electric power,petroleum, mine and chemical industry, malfunction is forbidden,and machine unexpected stop can bing about enormous loss of property and the peoples’s life.The thesis has conducted thorough research on cells cascaded high voltage inverters,studied the Carrier Phase-Shifted SPWM technique(CPS-SPWM)and the neutral point drifting technology.The thesis include the following contents:(1)Two different carrier phase-shifted angles of 180 /N and 360 /N (N is the number of cascade unit)are analyzed by mathematics illation in this thesis. Some useful conclusions are got from the mathematics analysis and many simulations results by MATLAB/Simulink. The inverter performance has relation with the number of cascade unit.When N is odd,the difference is little.But if N is even,equivalent switch frequency,harmonic distribution and number of output voltage level will be different.The result is that the 180 /N mode is better than 360 /N.(2) Based on the analysis of the relation between carrier phase-shifted angle and harmonic distribution, neutral point drifting technology based on the same harmonic distribution is proposed in this thesis.With carrier phase-shifed angle,frequency modulation ratio optimized respectively,the harmonic distribution of three line voltages keep the same.The output voltage is increased and the THD is decreased than traditional control methods. Simulation results verify the validity of the proposed method. (3)The paper finally gives the experimental wave of a 6kV/1600kVA inverter.The experimental results have achieved the anticipated target,and also verify the validity of the academic analysis.