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The development in distributed energy resources technology has led to a significant amount of non-linear power electronics converters to be integrated in the power system. Although this leads to a more sustainable system, it also can have adverse impacts on system stability and energy power quality. More importantly, the majority of the distribution power systems currently are unbalanced (with asymmetrical voltages) due to load unbalance, while the most common fault types are unbalanced grid faults that can have many adverse effects on distributed resource operations. In that regard, proper control of the grid connected converters in active unbalanced distribution systems will become very important. This paper aims to present the behavior of the advanced grid connected converter control technique under different voltage states at the point of common coupling (according to the ABC classification). The main insufficiencies of the classical control technique will be highlighted, while the paper will propose an appropriate solution for mitigation of negative sequence currents under asymmetrical voltages at the point of common coupling. An extensive experimental verification of the proposed techniques is performed using an advanced laboratory prototype for research in grid integration of distributed resources. The experimental verification clearly demonstrates the benefits offered by the advanced control strategy.

The paper presents an overview of the electric distribution test grids issued by different technical institutions. They are used for testing different scenarios in operation of a grid for research, benchmarking, comparison and other purposes. Their types, main characteristics, features as well as application possibilities are shown. Recently, these grids are modified with inclusion of distributed generation. An example of modification and application of the IEEE 13-bus for testing effects of faults in cases without and with a distributed generator connection to the grid is presented.

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Summary Considering the recent trend in energy sector transformation towards high share of renewable energy sources, it has become very hard to imagine modern power system without the integration of power electronic devices. A grid‐connected converter will be on the forefront of future energy trading, while simultaneously striving to offer good dynamic behaviour and operation in full accordance with the relevant grid requirements. The control algorithm of the grid‐connected converter has to be capable of achieving the stable steady state operation even during the most severe faults in the system. More importantly, the power quality of the injected currents (and thus the power) needs to be kept at the maximum possible level. This paper presents the control strategy for the grid‐connected converter that offers the possibility of symmetrical grid current injection at the point of common coupling even during unbalanced grid conditions. Proposed control strategy uses delay signal cancellation in the negative sequence synchronous rotating reference frame for the mitigation of the respective current components. The negative influence of asymmetrical grid voltages, present at the point of common coupling as a result of unbalanced grid loads or faults, will be shown within the paper. The key features of the improved control method are outlined, with a special reference to basic theoretical background. The proposed method is experimentally verified using sophisticated research and development station for control of grid‐connected converter.

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