Research On Neutral Point Potential Balance Strategy Of Three Level NPC Inverter Based On Neutral Point Current

Compared with two-level inverter,the three-level neutral-point clamped(NPC)inverter has the advantages of lower total harmonic distortion(THD),lower voltage stress on power switching devices and lower du/dt values.It has been widely used in new energy power generation,active power filter,flexible AC transmission,motor drive and other medium-high voltage and high power applications.Theoretically,the capacitor voltage of the three-level NPC inverter is half of the DC bus voltage.But due to the non-ideal factors and the inherent characteristics of the modulation strategy,the voltage error between the two capacitors appears in practical system.The neutral point portential unbalance problem will affect the quality of output waveforms,which makes the output voltage containing low-frequency harmonics.At the same time,it will also increase the voltage stress on individual switching devices,or even worse,it can cause serious damage to the switching devices and disturbs the normal operation of the system.In order to solve the neutral point portential unbalance problem for the three-level NPC inverter,a novel voltage balance control strategy based on judging the polarity of the neutral-point current is proposed in this paper.The proposed method,using level shifted pulse width modulation(LS-PWM)strategy,can effectively avoid the influence on the waveform quality of the output voltage by adding the high frequency voltage regulating signal on the three phase modulation signals.The feasibility of the proposed control algorithm is studied via the seven-segment switching sequences and detailed mathematical calculation.The voltage balance characteristics of different modulation index and power factor are derived,which revealed the effective range and the influence factors of the method.Further,in order to suppress the AC unbalance of the neutral point potential,the zero sequence component superimposed on the three-phase modulation signals is obtained based on the polarity judgment of the neutral point current's mean value in one switching period.The proposed method can effectively suppress the low frequency fluctuation of the neutral point potential without changing the characteristics of output line voltage of the inverter.Finally,simulation and experimental platforms of three-phase three-level NPC inverter are built to test the control strategy of neutral point potential balance based on neutral point current.The results are presented to verify the validity of the theoretical analysis and the effectiveness of the proposed strategy.