Cascaded multilevel inverters, such as the cascaded H-Bridge (CHB) converter, are an attractive solution for multi-string photovoltaic (PV) systems, because they enable direct connection to the medium voltage grid and maximum power point tracking of multiple strings. As a challenge of the topology, the operation with high power imbalance in the strings is constrained by the over-modulation. This limitation is analyzed for sinusoidal modulation and the impact on the maximum power imbalance is demonstrated. For increasing the operating range with maximum power tracking in the strings, a discontinuous modulation with extended maximum power imbalance and reduced losses is proposed. The method is analyzed in terms of maximum power imbalance, efficiency and power quality. In addition, the method is validated on an experimental test bench.

Direct-driven generation is considered one of the most reliable selections in wave energy conversion systems. Therefore, a direct-driven linear-rotary wave generator (LRWG), which can transfer the low-speed linear wave motion to the high-speed rotating motion of the rotor and then generate electrical power simultaneously, is proposed in this paper. The proposed LRWG comprises three parts: translator, common rotor, and stator. From the energy view, the proposed LRWG can be divided into two sections: the energy transmission section to increase the velocity of the wave and the energy conversion section to generate power. The initial design method is given to determine the main dimensions of the proposed LRWG. To meet the maximum power tracking control requirements, a multi-mode and multi-objective optimization method, which can consider both generator and motor mode, is proposed to improve the performance of the proposed LRWG by sensitivity analysis, response surface methodology, and non-dominated sorting genetic algorithm II method. Finally, a prototype and its experimental platform are developed based on one of the optimization schemes from the possible solutions to verify the proposed approach and the performance of the proposed LRWG.

Recently, the exploitation of renewable energy has been the drive for the development of novel power electronics solutions, with the goal to increase the grid hosting capacity and mitigate the issues of the power systems [1]. However, there has been a shift in the grid control paradigm. The increased adoption of power electronics interfaced sources and loads into the electrical grid changes the way by which the power flow is controlled, may affect the power quality, and could challenge the grid stability if not properly controlled. To sustain the rapid increase in the power generation and the power demand at the same time [e.g., electric vehicle (EV) charging stations], several concepts have been proposed to make the electrical grid “smarter” and more flexible. The smart transformer (ST), as one of the most promising solutions, has become increasingly popular in recent years [2]. The ST is a power electronics-based transformer that is supposed to replace the existing conventional power transformer as an ac–ac converter. Compared to conventional transformers (CTs), the ST takes advantage of electronic control, dc connectivity, smart maintenance [3], and provision of ancillary services [4], [5]. In this way, a gradual upgrade of the electrical grid with increased benefits can be foreseen as the penetration of the STs increases.

As a key element to connect renewable energy sources, grid-forming inverters (GFIs) mostly use droop control as an outer loop, but it can cause a swift change in the system frequency when disturbances arise. In the role of the inner loop control, the traditional model predictive current control (MPC) is widely employed in converters, but it is heavily dependent on model parameters. To overcome the above drawbacks, a virtual synchronous generator founded on model-free predictive control (VSG-MFPC) is proposed in this study. The scheme uses an autoregressive MFPC based on an auto-regressive with exogenous input (ARX) model as an inner-loop control structure for voltage prediction, combined with a simplified virtual synchronous generator (VSG) as an outer-loop control structure for power sharing and inertial support control systems. The implementation of MFPC improves the parameter robustness of the system. Building on this, the improvement schemes of algorithmic parameter initialization, delay compensation and current limiting capability are further investigated. Additionally, incorporating the VSG into the outer loop strengthens the stability of the system’s frequency and voltage while also enhancing the system’s dynamic characteristics. The aforementioned advantages, along with the efficacy and feasibility of the proposed control method, are validated through simulation and experimental results.

The Bidirectional-Asymmetrical-Power-Flow (BAPF) DC-DC converters, consist of the uncontrolled rectification power path dealing with large forward power and the active power path dealing with small reverse power, shows the potential for constructing simple and reliable AC-DC or DC-DC Power Electronics Transformers (PETs). However, the peak current caused by the uncontrolled rectification power path threatens the security of the active switches in the medium-voltage (MV) side and increases the power loss. This paper analyzes the active operation mode of the BAPF DC-DC converter and proposes the optimized operation method for the peak current reduction. Based on those, a down-scaled experimental prototype is developed and the correctness of the theoretical analysis and the optimization method are verified by experimental results.

To reduce the time consumption of dynamic performance analysis and optimization design for the hybrid excitation magnetic lead screw (HEMLS), a two-dimensional (2-D) mesh-based (MB) magnetic equivalent circuit (MEC) model is proposed. In the proposed model, the mesh structure of the proposed model doesn’t need to rebuild for each time instant, and only the parameter value of each element needs be updated to describe the status of HEMLS. To simply the proposed mesh-based MEC model, both PM and current exciting sources in one pole are together considered as one magnetomotive force. Meanwhile, the density of the mesh can be adjusted according to the solution region to decrease the time consumption. Finally, the proposed MB-MEC model is verified by comparing it with the static thrust force from the finite element method and prototype test under load and no-load conditions.