Three-Level Midpoint Potential Balance and Harmonic Suppression Strategy Under Low Carrier Ratio

Abstract

An improved virtual space vector modulation method is proposed to address the issues of output voltage distortion and high harmonic content caused by unbalanced midpoint potential in the application of diode midpoint clamped three-level inverters in high-speed motor drive scenarios. Firstly, based on the potential difference of the voltage divider capacitor, the average value of the midpoint current that needs to flow out is determined to allocate the action time of the positive and negative virtual small vectors, so as to make the potential difference of the voltage divider capacitor approach zero and achieve midpoint potential balance control. Secondly, the virtual vector losing its midpoint potential balance suppression effect due to angle and sampling delay in high-speed permanent magnet synchronous motors, and considering that traditional methods do not use virtual small vectors in the high modulation ratio range (0.667<m<1), this paper re divides the reference voltage vector interval to avoid the voltage reference vector falling into the high modulation ratio range and causing the midpoint potential to lose balance. Finally, a simulation model was constructed to validate the proposed strategy, and the simulation results showed that the proposed method can effectively reduce the harmonic content of line voltage and current by suppressing the midpoint potential balance.