Optimal Connection Voltage of Soft Open Point and Shore-to-Ship Power Converters for Enhanced Capacitive Reactive Power Operation

Soft Open Point and Shore-to-Ship Power applications permit to deploy smart grid infrastructure into distribution and port networks. Both are based on power electronics converters that require an optimal integration to maximize capacitive reactive power limits at minimum impact over costs and efficiency. To meet this objective, the paper defines a methodology to calculate the optimal connection voltage of power converters used in these applications. The proposed methodology is based on three novelties related to capacitive reactive power limits definition. Firstly, all affecting variables from positive and negative sequences are jointly considered completing partial approaches identified in previous works. Secondly, the impact of negative sequence affecting multiple converter terminals is considered, adding accuracy to reactive power limit calculation of Soft Open Point converters. Thirdly, the influence of two main constraints affecting modulation limits in real converters is described, quantifying its impact over maximum capacitive reactive power limits. These constraints are related to semiconductor characteristics and to digital implementation; usually not considered for dimensioning purposes at application level, but which can have a key impact on capacitive operation. At this point, the authors provide explicit details of industrially available converter designs to support calculations and evaluate its impact over the final solution. Considering the above, the proposed methodology is implemented on a real 20 MW 6.6 kV medium voltage converter design and results are quantified and compared from a reactive power capacity, efficiency and cost perspective.