Abstract
This thesis presents a survey on reduced common-mode voltage pulse width modulation techniques suitable for grid-connected photovoltaic converters. As the conversion efficiency, weight and size are critical concerns, the non-isolation (i.e., transformerless) photovoltaic system is preferred. However, the common-mode current flowing through stray capacitances is a critical safety issue due to the lack of galvanic isolation. Therefore, reducing common-mode current magnitude by using PWM methods is the target of this survey. Due to common-mode voltage magnitude has great influence on common-mode current magnitude, conventional PWM methods (Space Vector PWM and Discontinuous PWM1) and reduced common-mode voltage PWM methods (Active Zero State PWM, Near State PWM) are experimentally tested. The performance of all PWM methods has been investigated with laboratory test bench. It can be shown that the common-mode current performance with reduced common-mode voltage PWM methods implemented are much better than that with conventional PWM methods due to the avoidance of using zero state vectors. The differences on common-mode current performance among reduced common-mode voltage PWM methods are analysed either in time domain or frequency domain. In particular, Near State PWM yields the best overall performance including low ground leakage current, low inverter output (phase current) and input (DC link current) ripple, and low switching losses. When accounting for the system resonance, the resonant frequency in the photovoltaic system can be predicted by system parameters. The predicted resonant frequency can be an index when choosing switching frequency for grid-connected photovoltaic converters avoiding of triggering system resonance, making common-mode current performance worse.