Input filter pre-charge scheme for high-power PWM-current source rectifiers connected to a weak utility supply

High-power PWM current source rectifiers are normally connected to the ac grid by means of a lightly damped LC input filter. Pre-charge schemes employing pre-insertion resistors are typically used to mitigate the over-voltage across the filter capacitors during the connection to the grid. However, noticeable distortion of the grid voltage may still occur during the insertion of the input filter to the grid. This is particularly true at low ratios of the supply fault level to the converter-rated power and cannot be avoided when conventional pre-charge schemes are used. This study proposes a pre-charge scheme that yields a smooth and controllable connection of the input filter to the grid along with total absence of both over-voltage across the input filter capacitors and voltage distortion on the grid. This is novel and it is of particular interest whenever PWM-CSRs are connected to relatively weak grids. Pre-charge is carried out by the main PWM-CSR converter, which is in turn supplied on its dc-side by a small low-power low-voltage pre-charge converter. This completely eliminates the need for any pre-charge components on the ac-side [which can be bulky and expensive at medium voltage (MV) levels]. Experimental results obtained from a 150 kW PWM-CSR are included in the study.