Research On Dead-time Compensation And Power Losses For Three-level Inverters

The three-level inverter is suitable for applications requiring less harmonic,high-voltage and high-power fields. This paper introduces the working principle,mathematical model, controller design and several common modulation methods ofdiode clamp three-level inverter.The paper makes a detailed analysis of the mechanism and impact of dead-timeeffect of diode clamp three-level inverter and makes a deep research on the currentdead compensation strategies and dead-time elimination control strategy ofthree-level inverter and the existing problems.A dead compensation method basedof current forecast without delay for three-level cconverters is proposed by thispaper. According to the failure of current direction detection because of the currentcrossing over the zero point repeatedly and sampling error, a novel method byforecasting the actual current to judge the direction of current is proposed.Dead-time elimination control strategy is used in the interval when the current cannot cross over the zero point repeatedly, but near the zero point the dead-time isinserted. According to relay of traditional dead-compensation, a compensationmethod on the rising edge and falling edge of triangular wave is put up. Simulationresults demonstrates that the forecast current by this method and the actual currentwaveform are consistent with each other. The experimental results verify that thedead-time compensation method is reasonable and effective.With the rise of converters power,the power losses of devices increaseddramatically. The efficiency has become one of the key issues in the process ofresearch and development of high-power three-level inverter.This paper studies amethod based on the theory of curve fitting for calculating the power losses ofdiode clamping three-level inverter.Firstly, according to the relationship of currentand voltage, the conduction regular will be known;Secondly,the duty cycle of thedevice in different switch conduction state is calculated;Thirdly, function fitting isused to get linear equations of cconduction voltage drop and switching losses bycurrent of the diode and IGBT; Finally, the power losses are calculated through twodifferent methods continuous modeling method and discrete modeling method byMATLAB.About continuous modeling method, first of all, these expressions relatedby various parameters of condition losses and switching losses arededuced;Secondly, the losses calculation model was established by MATLAB, then the curves of the three-level power losses on different parameters are drawn.This approach helps to analyze the relationships of power losses with m, fc, cosφ ordevice parameters. However, due to the using of a function fitting devicecharacteristic curve, the error is relatively large, and by the limitations of themodulation strategy, we must know the modulation wave equation. A discretemodeling method is proposed due to the defects of the above methods. In order toimprove accuracy, these devices characteristic curve equations are get by quadraticfunction fitting by using MATLAB.A modulation wave period Tris discretized inaccordance with a certain time Ts,then the power loss in a discrete period Tsiscalculated, divided by the modulation wave cycle Trto obtain average power losses.This modeling method accuracy is high, combined with zero-sequence componentinjection modulation, power losses of different modulation strategies can beanalyzed.