Neutral-Point Potential Balancing Problem Research Based On Double-Modulation-Wave Carrier-Based PWM Strategy

Neutral-point potential balancing (NPPB) problem, an inherent problem in three-level neutral-point-clamped (NPC) inverter, is a bottleneck of three-level NPC inverter widely application. Neutral-point potential unbalance will lead to partial power switches turn off voltage increase or overvoltage, and output harmonics increase. Conventional carrier-based pulse width modulation (PWM), which has only one modulation wave per phase and implements NPPB control universally by inserting zero-sequence voltage, has the problem that neutral-point potential low-frequency ripple (LFR) can not be completely eliminated under some modulation index and power factor. NPPB methods based on space vector PWM (SVPWM) also exist either LFR problem or dc unbalance or high switching frequency problem on some conditons. In addition, NPPB control often conflicts with some special PWMs on switching sequences, which causes these special PWMs can not be applied in three-level NPC inverter, or can not achieve expected performance.The NPPB problem of three-level NPC inverter is researched from the angle of double modulation wave and coordination control. Double-modulation-wave carrier-based PWM, double-group double-modulation-wave carrier-based PWM and NPPB coordination control method are developed in this paper. Neutral-point potential dc unbalance and LFR problem are solved under all modulation index and power factor. Switching sequence conflict problem are also solved.Fundamental principle of three-level NPC inverter is analyzed firstly and mathematical model of average neutral-point current is established. Essential reason of neutral-point potential LFR under conventional single-modulation-wave carrier-based PWM (SMWPWM) on some conditions is analyzed. A sufficient condition for completely eliminating LFR is proposed. On this basis, a double-modulation-wave carrier-based PWM (DMWPWM) in which each reference voltage is decomposed into two of positive and negative modulation waves to implement modulation is proposed based on voltage equivalent and NPPB principle. Free degree of'0'state duty cycle is increased in DMWPWM. Neutral-point potential dc unbalance and LFR are completely eliminated at all modulation index and power factor, which solves the LFR problem which can not be conquered under conventional SMWPWM on some conditions.On the basis of DMWPWM, relation of double-modulation-wave waveforms and switching frequency is furtherly researched, and a double-group double-modulation-wave carrier-based PWM (DGDMWPWM) with low switching frequency is proposed. Two groups of double-modulation-waves which produce equal and opposite average neutral point current are established by inserting zero sequence voltage into modulation waves of DMWPWM, and used to implement switch modulation based on direction of neutral point potential in DGDMWPWM. DGDMWPWM can completely eliminate LFR, and has lower swiching frequency compared with DMWPWM with same carrier frequency.To solve switching sequence conflict problem between NPPB control and some special PWMs, a NPPB coordination control method which is suitable for back-to-back three-level NPC inverter is proposed. The conflict problem is solved by implementing NPPB control only through one side of rectifier or inverter, and implementing PWMs which have special switching sequence requirements at another side. 4 NPPB coordination control methods based on DGDMWPWM and SVPWM are developed to solve switching sequence conflict problem between NPPB control and harmonic optimal SVPWM as an illustration. The corresponding coordination control algorithms and areas where neutral-point potential LFR can be completely eliminated are presented, which effectively provide guidelines to select control strategy and PWMs for back-to-back three-level NPC inverter.Finally, a single-carrier modulation method which is equivalent and easy to implementing double-carrier modulation for DMWPWM and DGDMWPWM is developed, and experimental system of three-level NPC inverter with DSP and CPLD controller is established. Experimental research on DMWPWM and DGDMWPWM is conducted. Experimental results verified that DMWPWM and DGDMWPWM are correct and effective.