Design and operation of multiport converters for distribution systems

Electrical power distribution systems are experiencing an increase in renewable energy integration, which is reversing the power flow during some periods and modifying their conventional operation. In this context, the curtailment of renewable distributed generation is needed to consider the electrical limitations of transformers and lines, which are mainly voltage deviations and power overloads. Multiport converters (MPC) with energy storage are presented as a potential solution for enhancing power flow operation in the distribution system and minimizing renewable generation curtailments. Alternatively, adding energy storage systems (ESS), switches, soft open points, and new lines can also be considered.

This Master's Thesis presents an optimisation-based methodology to size multiport power converters. The methodology is divided into three steps: days selection of the load and renewable resource availability profiles, storage sizing, and multiport converter sizing optimisation. The MPC sizing optimisation considers the days selected previously instead of the whole year, includes electrical constraints, and also optimises the system operation. This methodology also provides technical and economic indicators for a complete evaluation of the multiport converter integration.

The general methodology is evaluated in a case study based on a medium voltage benchmark network from CIGRE. The multiport converter solution is compared to alternative configurations with separated batteries and a simple switch interconnection, it also considers different penetrations of renewable energy. Additionally, a sensitivity analysis of economic parameters is performed on the optimisation. Overall, the objective is to identify the best scenarios where multiport converters can be considered as a potential solution.

The results conclude that the rated power of the multiport converter terminals used to exchange power between different feeders is sized as the nominal power of the lines and transformers in scenarios with unbalanced distributed generation in the feeders. Whereas in scenarios with balanced distributed generation, the size of the rated power of the multiport converter terminals is lower and has more variability. As a general conclusion, the results show that the multiport converter can reduce power curtailment in equivalent terms or even better than battery solutions, especially in scenarios with unbalanced distributed generation in the feeders.  With high penetration of renewables, all scenarios studied are economically feasible to install a multiport converter. Scenarios with unbalanced generation remain feasible if the renewable generation capacity is reduced, unless batteries are initially installed in the feeders. In general, for all scenarios, a multiport converter with a centralised battery is the best option.