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The modelling of a three-phase four-leg four-wire grid-forming inverter in a low voltage distribution system 18-bus European Cigre under unbalanced conditions in an autonomous distribution network is presented. The case study has two types of inverters control strategy: (i) grid-forming to supply all the system demand in the interval of the intentional supply interruption and (ii) grid-following to integrate photovoltaic renewable energy resources into power systems. The model suggests a control scheme with two loops: An inner current loop with a proportional-integral controller and an outer voltage loop with a proportional controller, both in the synchronous reference frame (dq0), in which dq-axis are decomposed in positive and negative sequences. Simulation results, carried out using the PSCAD software, showed the effectiveness of the suggested control strategy with smooth synchronization where the grid-forming inverter was able to form a network with an unbalanced degree lower than 2%, sinusoidal voltage and frequency within standard limits 49.5-50.5 Hz.

Wave energy is one of the emerging sustainable energy resources due to its high energy density and vast untapped areas. Nevertheless, the intermittency of wave resources is a significant challenge for the grid operator. Additionally, a varying grid demand is an added complexity to wave resources. Energy storage systems such as batteries and supercapacitors can be used to smoothen the power injection into the grid and match the available energy with the grid demand. Thus, this study compares battery energy storage with the hybrid energy storage system and proposes a control strategy to reduce the power fluctuations on the battery, allowing power delivery for varying grid demands. The study found that the hybrid system increases the system efficiency, in addition to the reduction in power fluctuations and enhancement of battery's performance, among other gains when the proposed control is implemented.