Shuai Zhang

Longitudinal T-shaped finned tubes are important heat-transfer components in spacecraft thermal control systems, and their forming accuracy and quality during bending have a significant influence on heat-transfer efficiency. Therefore, the analysis and control of nonuniform sectional deformation and nonlinear springback are essential for precision forming. In this study, a semi-analytical springback correction model coupling neutral-axis shift and equivalent unloading modulus degradation was proposed based on axial force equilibrium and sectional stress integration. Based on numerical simulations and experiments, the relationship between free-bending die eccentricity and formed curvature was established, and the effects of forming radius and bending angle on springback were analyzed. The results show that the formed curvature increases approximately linearly with die eccentricity, the radius springback increases with forming radius, and the springback angle first increases and then decreases with increasing bending angle. The constant-modulus model systematically underestimated springback, whereas the equivalent unloading modulus correction improved agreement with experimental measurements. Furthermore, a coupled angle-curvature inverse compensation method was proposed to determine the pre-compensated forming radius, free-bending die eccentricity, and over-bending angle from the target geometry. After compensation, the forming accuracy reached ±0.28° for bending angle and ±0.33 mm for bending radius, compared with ±3.16° and ±2.51 mm before compensation, respectively. The proposed coupled prediction-compensation framework provides an effective approach for precision free bending of longitudinal T-shaped finned tubes.

Grid-forming type control has become an important control mode of renewable energy inverters in practice, since it makes renewable energy could provide the dynamic frequency support for power system like conventional synchronous generator. However, experiments shows that its dynamic frequency support response characteristics are contrary to the original design intention of grid-forming type control. For example, the inertia output doesn’t increase to the maximum value as expected at the moment of disturbance. Instead, it follows a trend of initially increasing and then decreasing. This paper conducts the relative analyse research. It reveals the reason causing the above problem is the grid-forming type renewable energy does not sample the grid voltage frequency in real time for closed-loop control, but instead by constructing an intrinsic equivalent frequency. Furthermore, it constructs one frequency support response model taking into account the impact of non-real-time frequency sampling, and systematically explains the impact mechanism of non-real-time frequency sampling on the dynamic frequency support response of grid-forming type renewable energy, and the impact mechanism of damping coefficient on the primary frequency regulation response of grid-forming type renewable energy. Finally, the correctness of the analysis conclusions is verified through simulation.

With the increasing integration of large‐scale renewable energy sources into the power grid, the issue of sub‐synchronous oscillation (SSO) has become more pronounced, posing a significant threat to the safe and stable operation of the power system. This paper proposes a new method for the early and rapid detection of SSO using deterministic learning theory. A comprehensive database of oscillation patterns is constructed by simulating a variety of conditions that can induce grid oscillations, including wind speed and control parameters. A learning estimator is then developed to identify stable and oscillation dynamics. Furthermore, a knowledge bank for SSOs detection and isolation is also established, using the minimum residual principle, oscillation patterns matching those in the knowledge bank can be detected. Then, based on an example of grid‐connected sub‐synchronous oscillations in wind turbines are utilized to show the effectiveness of the proposed method. The detection accuracy of SSO is calculated as 94%, and the detection time in this paper is reduced to about 0.023 s.

Hybrid multilevel converters have gained more popularity due to their unique superiority. Thus, the reduced switch hybrid multilevel (RSHMC) converter is adopted in this article which consists three H-bridge cells and a Vienna rectifier. The RSHMC converter combines the advantages of these two topologies and can output higher voltage level by adopting fewer switching devices compared with other multilevel converters. However, the Vienna rectifier of the RSHMC converter has the inherent current zero-crossing distortion (CZCD) problem, which will increase the total harmonic distortion of the grid-side current. Thus, a modulation method based on phase disposition-pulsewidth modulation is proposed to eliminate the CZCD of the RSHMC converter by selecting redundant switching states. Besides, in order to ensure the normal operation of the RSHMC converter, the proposed modulation method can also realize the voltage balance control of the floating capacitor and the neutral point which is also realized by selecting the switching states to charge or discharge. Consequently, the RSHMC converter can operate normally with balanced voltage and without CZCD after adopting the proposed modulation method. Finally, the feasibility and correctness of the proposed modulation method are verified by simulation and experimental results.

Hybrid multilevel converters have gained more popularity due to their unique superiority. Thus, the T ² C-HB converter is adopted in this article which consists a T-type converter (T ² C) and three H-bridge (HB) cells. This hybrid converter combines the advantages of these two topologies and can output the same voltage level by adopting fewer switching devices compared with other multilevel converters. However, due to the high switching frequency of the HB switches, they may fail in practice which will cause serious deterioration in the output waveform and compromise the reliability of the entire system. Thus, a fault-tolerant control method based on floating capacitor voltage balance is proposed in this article. First, based on the level-shifted pulse width modulation (LS-PWM) method, the fault-tolerant control is accomplished by modifying the three-phase reference voltage. Second, a strategy which bypasses the HB equivalently when the absolute value of output voltage exceeds the peak value of the dc-link voltage is proposed to balance the floating capacitor voltage. Besides, the three-phase reference voltage is modified again utilizing the similar idea in order to balance the line voltage. Consequently, the line voltage remains unchanged during postfault operation and the floating capacitor voltage can restore balance without excessive fluctuation. Finally, the feasibility and correctness of the proposed method are verified by simulation and experimental results.

This paper aims to assess the efficacy of the Harris Hawk Optimization (HHO) algorithm within the domain of photovoltaic (PV) power-generation systems. The focus lies in elucidating how the HHO algorithm optimizes maximum-power-point tracking (MPPT) and augments the performance of grid-connected PV systems. Initially, in the MATLAB/Simulink environment, a comparison is made between the HHO algorithm and two other extensively utilized methods for maximum-power-point tracking (MPPT): Perturb and Observe (P&O) and Particle Swarm Optimization (PSO). Preliminary findings indicate the HHO algorithm’s notable advantages in efficiency and speed over the other algorithms. Furthermore, by establishing a practical experimental platform and synchronously verifying outcomes through simulation, we conducted a comprehensive assessment of the HHO algorithm on a single-phase full-bridge-inverter grid-connected system. Results show the HHO algorithm’s exceptional optimization capabilities, which displays superior adaptability and ability to adjust to varying external conditions.