Nowadays, various Sliding Mode Control (SMC) methods have been successfully applied to digitally controlled Grid-Connected Inverter (GCI) with LCL filter. However, how to design of the Pulse Width Modulation based SMC (PWM-SMC) controller need be further explored, especially upon the large variation of parameters drift and the delay issue. In this paper, the essence of two classic SMC methods used in power converter area is first analyzed in detail. Thus, a novel design of PWM-SMC controller using linearized model for three-phase GCI with LCL filter is proposed. Based on this, a three-loop step-by-step design of the PWM-SMC controller is developed, by using the closed-loop pole locations. Robust analysis against parameters drift is also studied. In addition, a discrete state observer is adopted to reduce the number of sensors. Further, a discussion between the proposed control strategy with the existing SMC methods and the full-state feedback controller is carried out. Finally, a 3 kW lab device designed on the dSPACE controller is constructed to verify the feasibility of the proposed strategy and the correctness of theoretical analysis.

A magnetic lead screw (MLS), that not only has enough maximum thrust force but also has good controllable performance, is a perfect linear actuator that can convert low-speed linear motion into high-speed rotary motion and apply for wave energy conversion system. In this paper, an improved transient sub-domain analytical model (SDAM) is proposed to analyze the performance of a novel proposed hybrid excitation magnetic lead screw (HEMLS) for the WEC system. The proposed HEMLS not only can have enough maximum thrust through the PM is arranged on both sides of the rotor and mover, but also can be controllable through two-phase windings that carry opposite DC and placed in slots on the mover. The proposed SDAM gives the analytical expressions of trust and torque. The hybrid exciting source and slot effect can be considered in the proposed SDAM. The proposed SDAM is validated by the finite element analysis method under both load and no-load conditions. Finally, a laboratory prototype and the corresponding test platform are developed to verify the effectiveness of the proposed HEMLS and its SDAM.

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.

Grid-connected voltage source inverters (VSIs) with LCL filters have been widely used for distributed generation systems (DGs). Various control methods have been studied to achieve good performance. Among them, sliding mode control has been applied to LCL-filtered grid-connected VSIs for its fast, dynamic response and strong robustness. However, LCL networks can easily cause instability problems under weak grid conditions such as grid impedance variation. At the same time, the stability design of sliding mode control applied for LCL-filtered grid-connected inverters are important, but they lack detailed parameters design in recent papers. In this paper, a design of observer-based sliding mode control to improve the stability of three-phase LCL-filtered grid-connected VSIs was proposed. The theoretical stability analysis was developed to consider the effect of the system discretization and grid impedance variations. Finally, a 3-kW, 110-V, 50-Hz experimental setup has been built to demonstrate the validation of the proposed method.

The passivity-based control (PBC) with Euler–Lagrange (EL) model has been used for LCL-filtered grid-connected converter. However, the impacts of the control delay and the interaction resonances under capacitive grid impedance are rarely considered for PBC method. Nonlinear Lyapunov’s stability theory is hard to estimate the stability of the system and accurately design the controller parameters considering the control delay. This paper derived the impedance model of LCL-filtered grid-connected inverter with grid current control based on PBC method. The passive theory and the zero-pole maps are applied to reveal the influence of the digital delay and differential item of inverter current loop. A new three-level cascaded control method is then proposed based on traditional PBC method to achieve stability and high robustness in complex grid. Furthermore, controller parameters are designed accurately to maintain passivity within switching frequency ranges and parameter variations of the LCL filter are investigated. Final validation of the effectiveness and performance of the proposed control structure is performed by simulation and experimentation.

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.