DC microgrids (MGs) represent self-contained systems for distributing and managing electricity locally, offering advantages in terms of efficiency and control. This article proposes the optimization of redundancy-based dc MG, emphasizing the integration of an intelligent algorithm to minimize operational costs under real-time pricing conditions and also increase the efficiency operation. The redundancy-based dc MG consists of a cascaded bidirectional Cuk converter (CBC) linked to a cascaded bidirectional boost converter, sharing two battery energy storage system (BESS) units as common inputs. Thereby, two boost converters interface the dc utility and fuel cell (FC) to the dc-link of the CBC. The redundancy-based dc MG operates with a coordinated and decentralized control among the dc utility, FC and two BESS units. The energy management system (EMS) employs a droop control for the dc utility and FC, while BESS units adhere to a state-of-charge (SoC)-sharing function. In addition, the particle swarm optimization dynamically adjusts EMS parameters for optimal real-time performance. Then, a stability analysis determines MG operational boundaries, and the effectiveness of the method is confirmed through experiments using Typhoon HIL and dSPACE setups. Finally, comparative evaluations against traditional SoC-based droop and genetic algorithms demonstrate the cost reduction of the proposed approach.

The fast-growing integration of renewable energy sources into the utility grids jeopardizes the system’s performance and stability at risk. Particularly, the increasing tendency of power electronics converters in the current renewables-based power generation and their integration to utility grids through long sub-sea cables compromises the grid strength and amplifies the risk of system instability during disturbances. To sustain grid stability and ensure effective regulation during transients, grid-following (GFL) and grid-forming (GFM) control approaches have been extensively proposed for power systems with inverter-based resources (IBRs). The former approach is solely based on a phase-locked loop (PLL) to track the phase angle of grid voltage, which reduces the system stability margin, particularly in weak-grid scenarios. Consequently, grid-forming control is increasingly recognized for its ability to maintain stability and ensure reliable operation under weak-grid conditions. Droop control is one of the most widely used grid-forming control strategies owing to its capability to emulate the behavior of synchronous machines, achieve autonomous power sharing, and ensure stable voltage and frequency regulation even under varying grid conditions. This paper aims to evaluate the impact of grid impedance and its characteristics (i.e., resistive or inductive grid impedance) on the dynamic performance of a droop control GFM grid-connected converter. To that end, first, a detailed MATLAB/Simulink model of a voltage source converter implementing the proposed droop-based GFM control is developed. Then, the overall system will be validated by performing on distinct case studies including weak and stiff power grids with inductive, resistive and nonlinear impedances in response to various grid disturbances.

Cascaded multilevel converters based on medium-frequency (MF) AC-links have been proposed as alternatives to the traditional low-voltage inverter, which uses a bulky low-frequency transformer step-up voltage to medium voltage (MV) levels. In this paper, a three-phase cascaded DC-AC-AC converter with AC-link for medium-voltage applications is proposed. Three stages integrate each DC-AC-AC converter (cell): a MF square voltage generator; a MF transformer with four windings; and an AC-AC converter. Then, k DC-AC-AC converters are cascaded to generate the multilevel topology. This converter’s topological structure avoids the per-phase imbalance; this simplifies the control and reduces the problem only to solve the per-cell unbalance. Two sets of simulations were performed to verify the converter’s operation (off-grid and grid-connected modes). Finally, the papers present two reduced preliminary laboratory prototypes, one validating the cascaded configuration and the other validating the three-phase configuration.

In order to address power imbalance in large-scale PV systems, this paper presents a multiport isolated medium-frequency (MF) link to process different power levels from PV arrays, using current-fed Z-source inverters (CZSI) modules to drive the energy from the PV array to the MF link in a single power stage. The MF link provides the galvanic isolation required by many codes and standards to integrate a system to the grid. The multiport configuration of the MF link allows the system to process different power levels on each primary port and, being all ports magnetically coupled, provide balanced power levels on secondary ports which can be used for multilevel converters applications. Additionally, the current-fed version of the Z-source topology offers natural protection against short-circuit damage enhancing reliability of the system. The analysis of CZSI cells and the study of asymmetrical power levels transference, as well as simulation and experimental results are presented.

Large-scale PV plants (hundreds of MW) are a reality and are being used to generate high-power levels in medium-voltage (MV) utilities. To achieve this goal, some new power electronics systems based on medium-frequency AC-links (MFAC) and multilevel output configurations have been proposed to interfacing with the MV utility. Most of these systems rectify the MFAC voltage, generating a DC bus and then invert the DC voltage to generate a line frequency AC voltage. This paper presents a three-phase cascaded multilevel converter topology with reduced-energy conversion stage and multiples AC medium-frequency (MF) links. The proposed converter is composed by modules connected in cascaded. Each module has three conversion stages: input converter (DC-AC), isolation stage (medium-frequency transformer) and output converter (AC-AC). In order to validate the proposed topology simulations of a 100 kW system are performed, taking into account balanced and unbalanced DC source cases. The proposed system is validated through a 1.5 kW single-phase five-level laboratory prototype.

Solar-photovoltaic (PV) is a fast growing renewable energy source. For its a abundance and maturity, utility scale PV farms are getting integrated to electrical systems in many countries. For PV energy integration to grid, bulky line frequency transformers are used to meet isolation requirements. This work proposes a medium-frequency (MF) isolated PV system for medium-voltage (MV) utility scale applications, based on current fed Z-source inverter (CZSI) and a cascaded multilevel inverter for MV side. The proposed PV system uses a common magnetic MF link for isolation and a CZSI for low voltage boost in a single stage enhancing efficiency as well as individual maximum power point tracking (MPPT). CZSI offers natural overlap protection in H-bridge and a filtered input current which enhance sensing for MPPT algorithm. Design equations for the CZSI, simulations and experimental validation are presented.