Research On The Key Techniques For The NPC In Verters

Compared with conventional two-level inverters,three-level inverters have an advantage of low output harmonic value and less device switches stress,etc,and are widely used in high-voltage power conversion areas.Three-level diode clamped inverters(NPC)were first proposed and applied among the many multi-level inverters.However,the key issues,the unbalance of dividing capacitor midpoint voltage and large device losses affect the performance of the inverters seriously,such as harmonics and life,etc.This paper focused on these two issues of the unbalance of dividing capacitor midpoint voltage and large device losses about the three-level NPC inverters and explored methods and techniques to overcome or weaken the negative impact on NPC.Firstly,this paper has analyzed the three-level NPC inverter working principle and sine space vector pulse width modulation(SVPWM)strategy and achieved the SVPWM modulation strategies with software.This content included sector division,the duration calculations of output sequence of a basic vector modulation and determination of the switch vector output sequence.Secondly,this paper has analyzed the mechanism of three-level NPC inverter midpoint voltage unbalance and pointed that the fundamental reason of the midpoint voltage fluctuation was the inequality in charge and discharge current.Furthermore,the factors that affect the balance of the midpoint voltage vector SVPWM,DC side capacitance value,load and voltage modulation coefficient were analyzed,based on the principle of current charge and discharge inequality.Based on the above,it gived a midpoint voltage balance control algorithm of closed-loop control and adjusted coefficient of generalized zero vector of small vector of three-level SVPWM seven-nine hybrid modulation strategy.This paper has built a three-level NPC inverter midpoint voltage balance simulation model and programmed a balancing algorithm on MATLAB / Simulink platform.From the aspects of the influence of different factors on the midpoint voltage,comparing with the traditional method,has verified the validity and rationality of the proposed algorithm in this paper.Finally,this paper has analyzed loss calculation method of power devices three-level NPC inverters and the effect of the power device loss in three-level NPC inverter operation mode.In order to reduce the loss power devices,this paper has proposed a polynomial fitting method to calculate the device losses,with a three-level SVPWM seven-nine hybrid modulation strategy by adjusting generalized zero vector coefficient and making the switch do not operate at max current area on the condition of meeting the objective function,reasonable adjustment of generalized zero vector coefficient.The simulation model analyzing from load power factor and voltage modulation factor,comparing with the traditional method,gived results that this method can effectively reduce the inverter device overall losses.