Saturation Compensation Strategy for Grid Connected Converters Based on Line Frequency Transformers

This paper deals with the compensation of magneticsaturation in line frequency transformer coupled, grid-connectedpower converters. The developed strategy originates from thechoice of controlling the actual current injected in the grid (outputof the line frequency transformer) instead of the input current ofthe transformer, as usually proposed. As a matter of fact, betterpower quality cannot neglect the direct control of the injected gridcurrent. An effective solution providing zero steady-state error atgrid frequency and superior disturbance rejection involves the useof a d–q current controller synchronous with the grid voltage. Forthis reason, the proposed saturation compensation strategy hasbeen implemented inside the d–q reference frame. The downsideis that closing the current feedback at the transformer output rendersimpossible to control the dc current at transformer input,leading to core saturation. A core saturation compensation strategy,based only on the monitoring of the output current distortionaround voltage zero crossings, is here presented. Moreover, sincethe proposed solution does not rely on dc measures, offset problemsare avoided. Experimental results confirm the effectiveness of theproposed control architecture and of the strategy adopted to avoidtransformer core saturation.