Wenhua Wu

The class VII flextensional transducer (FT) with giant magnetostrictive material (GMM) featuring high power density, low resonant frequency, high source level (SL), and strong static water pressure bearing capacity has broad application prospects in underwater detection and communication. For the structural parameters of the transducer shell, considering the prestress characteristics of GMM, with SL, resonant frequency, operating depth, and weight as optimization objectives, this article proposes a multiobjective optimization design method based on the Multiobjective Sooty Tern Optimization Algorithm (MOSTOA). First, the lumped parameter-assisted finite element model (LP-FEM) suitable for multiphysics coupling of transducers was constructed. Second, sensitivity analysis was then employed to eliminate nonsignificant variables, thereby enhancing optimization efficiency. Then, the response surface method was combined with the MOSTOA to obtain a multiobjective optimization solution set. By comparing and analyzing the optimization results of multiobjective and single-objective methods, as well as different algorithms, the optimal shell structure parameters were obtained. Finally, to verify the proposed method, a prototype was fabricated, and an experimental platform was established. The experimental results indicate that the measured results agree well with the designed results and LP-FEM, which illustrate the effectiveness and precision of the method proposed in this article.

Wideband admittance measurement devices are essential tools for validating and correcting the admittance model of inverter-based resources (IBRs). During the measurement process using the low-voltage wideband admittance measurement device, the perturbation injection unit (PIU) exhibits multi-frequency voltage ripples on its DC-side. Such pronounced ripple voltage fluctuations not only increase protection tripping risks but also constrain the allowable perturbation amplitude, leading to degrades the accuracy of admittance measurement results. To address this issue, this paper first establishes the mathematical model of DC-side ripple voltage during perturbation injection. The effects of the perturbation voltage on the DC-side ripple voltage are analyzed, revealing the boundary of the perturbation amplitude considering the allowable DC-side voltage fluctuation limits. Then, considering the perturbation amplitude boundary, spectral leakage (SL), and noise interference, an adaptive perturbation voltage injection method with integrated high-precision signal extraction is proposed. The proposed method adaptively adjusts the perturbation amplitude based on the PIU’s amplitude boundary and the SNR requirements, ensuring both measurement accuracy and the stability of the DC-side voltage within safe operating limits. Finally, a 380V/10kVA low-voltage wideband admittance measurement device is developed. The effectiveness of the proposed method is verified by experiments.

The impedance or admittance characteristics of the grid-connected inverter play an important role in its stability analysis. The traditional sequence-admittance measurement methods of the grid-connected inverter mostly rely on special admittance measurement equipment or adjacent grid-connected inverters for disturbance injection. Because of high hardware cost or inconvenience, the abovementioned methods are not suitable for price-conscious testers to some extent. To address the abovementioned issues, this article proposes a sequence-admittance measurement method of the grid-connected inverter with its control system disturbance. First, the sequence-admittance model of the measured grid-connected inverter when small-disturbance signal is superimposed on its modulation signal is deduced and the system equivalent circuit model is built. Then, the flow chart of the proposed sequence-admittance measurement method is designed. According to the injected small-disturbance signal, the output voltage and inductor current of the grid-connected inverter, the sequence-admittance characteristics in sub/supersynchronous frequency area can be obtained. The measurement process does not need the cooperation of external disturbance source, which saves costs and improves the measurement convenience. Finally, experiments verify the effectiveness of the proposed sequence-admittance measurement method.

Due to the dynamic interaction between the inverter and the grid, the inverter may have power oscillation problems under weak grid. In this paper, considering frequency coupling, the more precise broadband sequence admittance model of the current-controlled virtual synchronous generator (CVSG) is established, and the admittance characteristics of CVSG and traditional grid-connected inverter (TGCI) are compared and analyzed. The amplitude of the sequence admittance of CVSG is generally larger than that of TGCI in the low frequency region, and in the entire frequency sweep range, the coupling admittance amplitude of CVSG is larger than that of TGCI. In the low frequency region, the coupling admittance has a great impact on the stability of CVSG and TGCI, so the coupling admittance cannot be neglected when analyzing the stability of the system. The generalized Nyquist stability criterion is used to compare and analyze the influence of power grid strength, phase-locked loop (PLL) bandwidth and output power on the stability of CVSG and TGCI according to the established admittance model. It can be seen from the stability analysis results that, compared with TGCI, when the PLL bandwidth is relatively large, CVSG has stronger adaptability to weak grid and power changes. Finally, experiments verify the correctness of the theoretical analysis.

Due to the multifrequency coupling effect ( m -FCE), precise modeling and effective stability assessment of an asymmetric grid-following converter (GFC) based ac system are complex to achieve. Traditional small-signal modeling methods and recently reported order-reduction methods could establish multi-input and multi-output (MIMO) and equivalent single-input and single-output (SISO) models to characterize m -FCE effectively. However, revealing the physical implications of coupling components and the generation mechanism of m -FCE are still challenging. Meanwhile, although the amplitude and phase dynamics have been considered in the existing modeling, a concrete conclusion illustrating these dynamics’ impact on m -FCE is still missing. To fill the gaps, this article thoroughly considers the impacts of amplitude and phase dynamics and finds that phase dynamics form additional positive feedback loops to enhance m -FCE. Based on this cognition, an accurate complex circuit modeling framework is proposed. It reveals that m -FCE is generated by the admittance mismatching between conjugated responses and ’chain reaction’ among coupling components. The proposed method has two merits: 1) it is a SISO modeling approach and helps reduce analysis complexity; 2) its graphical form helps provide a deep physical insight into m -FCE. Simulations and experiments verified the correctness of the proposed method.

The influence of parameters on the damping characteristics in a multi-inverter-paralleled system is different from that in a single-inverter system, because of inverter interactions in a nonideal grid. To quantify the interaction degree and assess its impact on damping characteristics, this paper proposes a closed-loop interconnected model for multi-inverter-paralleled systems. The model can explain the impact mechanism of inverter interactions on system stability. Capacity expansion of inverter-based renewable distributed generation is taken as the background in this paper, and then, the proposed model is applied to assessing the influence of the collecting line length of the newly installed inverters on the overall system stability. The conclusion is that the damping of a multi-paralleled-inverter system in the medium frequency range degrades with the approaching of the lengths of the newly added line and the existing line. Therefore, to avoid damping degradation in capacity expansion scenarios, the length of the newly added line should be different from the length of existing lines or the parameters of new inverters should be readjusted. Simulation results are presented to verify the effectiveness of the proposed model and stability enhancement approaches.

For the distributed generation units (DGs) using virtual synchronous generator (VSG) control algorithm connected to the islanded microgrid with high penetration of nonlinear load, there is always a trade-off between harmonics sharing accuracy and voltage quality when using conventional harmonics sharing methods. To address this issue, an impedance-based adaptively reshaping method is proposed in this paper, which is based on harmonic current feedforward compensation control and impedance reshaping factor adaptive control, assigning a variable harmonic impedance reshaping factor for each VSG to improve the controllability of VSG harmonic impedance, thus simultaneously enhancing nonlinear load sharing and voltage quality. Moreover, a droop-based methodology for impedance reshaping factor tuning is presented to adaptively reshape harmonic impedance for each VSG without acquiring the line parameters in advance. The proposed method also enjoys the advantage of avoiding introducing additional virtual harmonic impedance, and thus it is immune from the inverter closed-loop gain throughout the dominant harmonic frequencies. Furthermore, the stability analysis and the key control parameter design are discussed in detail. Finally, the effectiveness of the proposed method is verified by simulation and experimental results.

Impedance-based approachis generally studied for stability analysis of the power electronic converter-based system. Considering multiple-frequency coupling effects, in this article, we establish the harmonic transfer function (HTF) based αβ -frame impedance models of droop inverters based islanded microgrid with unbalanced loads. Therein, the HTF-based αβ -impedance modeling methods of unbalanced loads, as well as droop inverters in parallel, are proposed. Afterward, the HTF-based αβ -frame impedance models are equivalent to single-input single-output (SISO) impedances for simpler impedance measurement and stability analysis, preserving the αβ -frame coupling and multiple-frequency coupling characteristics. Finally, the experimental measurement results verify the accuracy of the αβ -frame SISO impedances. Besides, the stability analysis based on the SISO impedance models accurately reveals low-frequency oscillations of the islanded microgrid with unbalanced loads, further verifying the effectiveness of the SISO impedance models.

In order to meet the power supply requirement of short-term high-energy pulse load, a high overload power supply system and its energy synchronization control method are proposed, which can provide three times the rated capacity under short-term overload conditions. In terms of power supply structure, the diesel generator and the internal uninterruptible power supply (UPS) are cleverly utilized to form three energy supply channels for meeting multiple overload requirements, without increasing the rated capacity of the diesel generator. Meanwhile, it is found that the low-frequency power oscillation will occur once the high-energy pulse loads are put in. That is because the angular accelerations of the diesel generator and the internal converters turn to be inconsistent during the dynamic process. Therefore, an enhanced virtual synchronous generator (VSG) control with self-adaptive dynamic synchronous torque is proposed to suppress the low-frequency power oscillation by narrowing the acceleration gap between the diesel generator and the internal converters during the dynamic adjustment period. Then, the synchronous power supply for the three power supply channels is realized under high-energy pulse load conditions. Finally, the experimental results verify the effectiveness of the proposed overload power supply system and its energy synchronization control method.