This paper presents a high-performance bimorph MEMS cantilever harvester with partially covered asymmetrical functional piezoelectric layers on both sides of a beryllium bronze foil. The optimal PZT coverage length is employed to improve the output power density. The two piezoelectric layers with the controlled different thicknesses for the top and bottom PZT layers can provide three different output powers. Furthermore, a proof mass tungsten bonded at the free end of the cantilever can reduce the resonant frequency. The experimental results showed that the serial-connected bimorph piezoelectric energy harvester can obtain the maximum power density of 26.90 mW/cm³ at the matched loading resistance of 660 kΩ under 1.5-g acceleration at the resonant frequency of 38.89 Hz. The normalized power density of the harvester can reach 307.4 μW·g⁻² · cm⁻³. Meanwhile, the harvester provides good stability under 1.5-g acceleration amplitude.

Grid-forming (GFM) converters have received much attention in renewable energy applications because of their voltage support capability. Existing studies generally assume constant dc-link voltage for cascaded GFM converters, which may lead to the stability challenges under some occasions with dc-link dynamics. Thus, this article first analyzes the small-signal stability of GFM converters considering dc-link dynamics. It is found that dc-link exhibits inherent instability risk due to the nonminimum phase characteristics of the dc-link capacitor, which may lead to subsynchronous oscillations on the ac side. To solve this issue, dc-link dynamics are introduced into both active and reactive power loops through proportional controllers. This method does not increase the order of the controller, and it is easy to implement digitally. Compared with the conventional damping method that only introduces dc-link dynamics into the active power loop, the proposed method can dampen the subsynchronous oscillations caused by dc voltage fluctuations, and it can enhance the transient response performances of ac side. The experimental results validate the correctness of the small-signal model and the effectiveness of the proposed damping method.

The existing dual-coil driving topology mostly adopts two inverters or additional switching circuit, which increases the cost and size, and the complexity of the wireless power transfer (WPT) system. In this letter, a novel inverter topology with independent dual-coil driving capability is proposed for WPT applications, where the main switch and auxiliary switch can drive two independent coils. Moreover, this topology not only offers wide zero voltage switching range and compact size, but also improves the output power and coil utilization. Meanwhile, a quadruple D square (QDS) coil with uniform magnetic field is discussed with the dual-coil driving topology. The detailed parameters design of the proposed topology is discussed and the QDS coil is compared with other coils in terms of magnetic flux intensity. Finally, a 1MHz and 220W wireless power transfer system is built to verify the proposed topology and coil structure.

The integration of large-scale photovoltaics induces significant challenges of low inertia and weak damping within power grids. To effectively address these issues, grid-forming (GFM) strategies have been adopted in photovoltaic systems (PV). In such systems, the front-stage converter is tasked with maximum power tracking, while the back-stage adopts dc-link voltage-synchronized control. However, conventional modeling of the photovoltaic and front-stage dynamics via a constant power source (CPS) model tends to neglect the intrinsic impedance of the front-stage converter, leading to inaccuracies in stability identification. To rectify this oversight, this paper introduces an equivalent circuit model of the PV front-stage converter, which features a power source combined with parallel impedance (PPI), precisely capturing the PV front-stage dynamics and facilitating the stability analysis of the back-stage converter. Furthermore, to address the issue of the low-frequency oscillation induced by the conventional matching control, an enhanced dc-link voltage synchronization strategy is proposed in this paper. This strategy integrates a proportional loop in parallel with a low-pass filter loop. The dc-link voltage regulation, grid strength adaptability, and grid support capability of the PV-GFM system with the proposed strategy are quantified analytically and verified by case studies.

As the penetration of photovoltaic (PV) sources in the power system continues to rise, it is important for PV converters to operate in grid-forming (GFM) mode. Since the dc voltage is not controlled by the PV side, GFM PVs should have capabilities for dc voltage regulation and ac side support. However, most research on GFM PVs has concentrated on the stability related to dc-link voltage synchronization control (DVSC), neglecting the impact of ac voltage amplitude control (AVC) on the stability of GFM PVs. In this article, a new low-frequency oscillation issue in GFM PVs is first identified. To investigate the destabilization mechanism of this phenomenon, a small-signal model is derived, revealing that the low-pass filter of the reactive power introduced in the AVC loop and the AVC loop coupled into the DVSC loop are two key factors leading to instability. This article proposes an effective approach to suppress the low-frequency oscillations, which feeds forward reactive power dynamics into the DVSC loop through a proportional gain. This strategy not only addresses low-frequency oscillations but also improves the transient performance of the GFM PVs. Moreover, this strategy enables the GFM converter to operate stably across a wide range of short-circuit ratios. Finally, simulation and experimental results validate the accuracy of the theoretical analysis and the effectiveness of the proposed method.

The neutral-point potential balance issues in three-level converters have obtained great attention. The popular view thinks that the neutral-point voltage deviation can be suppressed by regulating the injected zero-sequence component, whether via carrier modulation or space vector modulation techniques. However, this paper presents a novel finding: the efficacy of different frame controllers on the self-balancing of neutral-point potential in three-level converters differs when a comprehensive analysis of zero-sequence dynamics, including neutral-point current and PWM modulation, is conducted. That is, the proportional-resonant (PR) controller in the abc frame effectively introduces a zero-axis PR control of the zero-sequence component, which subsequently degrades the stability of neutral-point potential self-balancing. In contrast, the PI control in the dq frame does not incorporate any additional control of the zero-sequence component, thereby enhancing the self-balancing capability of the neutral-point potential. To substantiate this novel finding, a series of simulations and experimental validations were performed.