Hybrid PWM Scheme For Grid-connected PV Inverter

PWM scheme is one of the key technologies for grid-connected inverter. Compared with sinusoidal PWM(SPWM) and space-vector PWM(SVPWM), the selected harmonic elimination PWM(SHEPWM) strategy has several advantages, such as lower equivalent switching frequency, higher quality output voltage.This thesis studies SHEPWM modulation scheme applied on T-type three-level PV inverter. Firstly, analysis and implementation of SHEPWM is presented. Then, power quality and efficiency are analysed and compared between SHEPWM and SVPWM.Furthermore, several factors which influence SHEPWM performance, including dead-time effect, bus voltage ripple and grid harmonic, are taken into account. The performance of SHEPWM will be affected by these factors, thus decreasing the efficiency and power quality of grid-connected PV inverter. Some measures are proposed to solve these problems after that.The dynamic performance of SHEPWM is relatively poor, which makes it not suitable for certain occasions that need good dynamic performance such as low voltage ride through(LVRT). Thus, a hybrid PWM scheme, combining the merits of SVPWM and SHEPWM, is presented for PV inverter. By adopting the hybrid PWM scheme, both the steady-state performance and the dynamic performance can be improved compared with single PWM scheme. Besides, several factors are taken into account during transition to make sure the transition is smooth and fast enough as well.The analysis and proposed methods are validated by the experiment on a lOkW T-Type three-level prototype.