载波移相SPWM开关频率的选取

Selection of phase-shift SPWM switching frequency

载波移相正弦脉冲宽度调制(SPWM)应用于高压大容量链式多电平逆变器场合时,若载波频率太低,各载波初相位对输出电压相位影响较大,可能造成链式逆变器各单相桥输出电压基波初相位存在明显差异,引起电容电压不平衡。推导了载波移相SPWM输出电压的数学表达式,分析了低开关频率下输出电压的谐波特性及其对逆变器运行的影响,提出应选择较高的载波频率。数字仿真和物理实验结果表明,在一相串联桥数大于4时,选择高于300Hz的载波频率,可保证各桥输出电压初相位差异小于0.5°,对电容电压的影响可忽略。

PS-SPWM（Phase- Shift Sinusoidal Pulse- Width Modulation ） is frequently applied in cascaded multi- level inverter such as STATCOM or DSTATCOM. If its switching frequency is too low,the base voltage of single Hbridge will be highly sensitive to the carrier wave phase- shift,which might cause unbalanced voltage on its DC links. The mathematical expressions of PS- SPWM output voltage are deduced,and special phenomena of PS- SPWM with low switching frequency is introduced,especially the influence of base voltage phase on the voltage balancing of DC links. It is pointed out that the switching frequency should be sufficient high. Numerical simulations and prototype experiments validate that,when there are more than 4 H- bridges in a phase, the switching frequency higher than 300 Hz can promise the phase difference less than 0.5°,and its influence on DC capacitor voltage can be ignored.