Research On The Neutral-Point Voltage Control For NPC Three-Level Inverters

Compared with two-level inverter,the neutral-point clamped(NPC)three-level inverter has such advantages,for example,low voltage stress on the switches,low electromagnetic noise and low total harmonic distortion(THD).Therefore the NPC three-level inverter has been widely used in high power conversions with medium or high voltage.However,the neutral-point voltage unbalancing problem is a typical problem in the NPC three-level inverter,which affects the output waveforms quality,and makes the output voltage containing low-frequency harmonics.This problem also increases the voltage stress on the switches,or even worse,it can cause serious damage to the switching devices and disturb the normal operation of the system.The objective of this paper is to solve the neutral-point voltage unbalancing problem,meanwhile,to increase the output waveforms quality or system efficiency.According to the mathematical expressions of the neutral-point voltage unbalancing,when using the sinusoidal pulse width modulation(SPWM)method,the mathematical relationship between the amplitude of the neutral-point voltage and the adjustment coefficient for the offset of the modulation wave is studied.At any load power factor condition,the value range of the adjustment coefficient can be obtained.Based on which,the variable modulation offset SPWM control method is proposed for the neutral-point voltage balance in this paper.This proposed method realizes controlling the neutral-point voltage balance by dynamically calculating the offset superimposed to the modulation wave based on SPWM(DCO-SPWM),according to the neutral-point voltage,the modulation index,the output current,and the load power factor.In order to verify the effictiveness of proposed methods,a experimental platform of the NPC three-level inverter is established,which is based on(Digital Signal Processor and Complex Programmable Logic Device)DSP-CPLD.The corresponding software and hardware of experimental platform is designed.This DCO-SPWM is very simple and easy for digital implementation,which has good steady-state performance and improves the output waveform quality by the simulations and experiments.The neutral-point voltage ripple problem cannot be neglected in the NPC three-level inverter when using the traditional SPWM method,which increases the output harmonics.When the neutral-point voltage is unbalancing,the neutral-point voltage ripple problem also deteriorates the neutral-point voltage unbalancing.Therefore,in order to suppress the low frequency ripple of the neutral-point voltage,a dynamic half carrier-cycle regulative discontinuous pulse width modulation(DHCR-DPWM)method is proposed,which meets the high system efficiency demand.By keeping the bridge leg voltage of a certain phase always at zero potential during each carrier-cycle,which has the highest(lowest)instantaneous voltage value amoung the three phases,the operation time of small positive and negative vectors can be equal,therefore the low frequency oscillation of neutral-point voltage is suppressed.Based on the experimental platform of the NPC three-level inverter,the experiment has been realized.The experimental results verify the effectiveness of the DHCR-DPWM method,which improves the output waveforms quality and maintains high system efficiency,compared with traditional SPWM method and 60°DPWM method.With traditional three-level space vector pulse width modulation(SVPWM),the NPC three-level has big neutral-point voltage ripple,and its the modulation wave expressions are very complex,which makes it hard to establish the module of the neutral-point voltage and design the neutral-point voltage controller.This paper proposes a three-level saddle SVPWM method based on two-level SVPWM(TBT-SAPWM).By reclassifing the main sectors and regions,and transforming the traditional three-level SVPWM into two-level SPWM method,the calculating process is more simple,and determining the main sectors or regions becomes easier with the proposed TBT-SAPWM method.Meanwhile,according to the angle range of the synthesized reference vector,the TBT-SAPWM method makes on-state duration of one selected phase equal to that of the other selected phase in a switching cycle,therefore the expressions of the modulation waves are very simple.When the range of fundamental wave modulation index is 0-1.15,the modulation waves are saddle waves.Moreover the module of the neutral-point voltage can be obtained easily,which is beneficial to design the neutral-point voltage balance controller.In order to control the neutral-point voltage balance,a neutral-point voltage balance controller is designed,which realizes controlling the neutral-point voltage balance by dynamically calculating the time offset superimposed to the switching time of the bridge arm voltage in each switching cycle.According to the experimental results in this paper,the TBT-SAPWM method has better performances in terms of the output harmonics and the neutral-point voltage ripple,compared with traditional three-level SVPWM method.When the neutral-point voltage is unbalancing,the controller has strong ability to balance the neutral-point voltage,which increases the output waveforms quality and decreases the neutral-point voltage ripple.