Research On Dead-time Compensation Strategy In Three-level T-type Inverters

Under high-speed and high-quality development environment,although traditional fuel vehicles have been in service for centuries,their shortcomings of pollution to the environment and consumption of non-renewable resources are becoming more and more obvious.With the characteristics of high efficiency,energy saving and environmental protection,electric vehicles have been widely concerned and expected.The drive system is a very important part of the performance of electric vehicles and the inverters are the key in the drive systems.Three-level inverters possess the characteristics of higher voltage withstand,lower loss,lower harmonic and electromagnetic interference than two-level inverters.Nevertheless,it is necessary to solve the problems of output waveform distortion caused by dead time insertion.This paper mainly focuses on the dead-time distortion effect of three-level inverters.After deep analysis of the structure and performance characteristics of three-level T-type inverters,this paper subdivides and quantifies the causes of output distortion,including dead time insertion,IGBT and diode voltage drop and parasitic capacitance etc.Besides,the influence of neutral potential fluctuation is also considered.On the premise of modeling of terminal voltage,the expression of compensation is derived based on the principle of volt-second balance.The reason of neutral point potential fluctuation caused by NTVPWM modulation algorithm is explained and introduce the influence to terminal voltage.Based on the modulation characteristics of NTVPWM,this paper proposes a current polarity detection method through sampling the output voltage of the inverter,which can accurately judge the current polarity.Aiming at the problem of compensation time acquisition,a method of obtaining compensation time by sampling the actual duty cycle of three-level inverters with the universal double-threshold method is proposed,and the compensation time is corrected on the principle of parasitic capacitance effect.The dead-time effect can be accurately compensated by online feedback of system variables and compensation voltage feedforward.The validity of the proposed double-threshold method to obtain and modify compensation time is verified by simulation results.Meanwhile,it is necessary to consider the neutral point potential fluctuation factor in the dead-time effect study of three-level inverters.The results show that the proposed method can effectively improve the output waveform of the inverters.The key points of voltage sampling to judge current polarity and double-threshold method to obtain and correct compensation time proposed in this paper are verified.Experiments show that the proposed method can accurately judge the current polarity in the whole PWM cycle and obtain effective compensation time.Through comparing different compensation methods,the validity of the proposed method and the necessity of considering the neutral point potential fluctuation are confirmed again.The methods of current polarity judgment and compensation time acquisition are based on voltage dividing sampling.Therefore,the dead-time compensation method proposed in this paper can abandon the burden of current sensors.At the same time,the simplified hardware circuit makes the proposed method have remakeable advantages for open-loop and closed-loop systems.