Zhiwei Xie

During low voltage ride through (LVRT) period, variable damping significantly affects the transient synchronous stability (TSS) of phase-locked loop-synchronized voltage-source converters (PLL-VSCs). However, in previous studies on TSS analysis under asymmetric faults, completely ignoring and insufficiently considering variable damping both increase the misjudging risk or yielding conservative estimation results. To fill this gap, this paper proposes an improved equal area criterion (iEAC) considering variable damping to qualitatively analyze the TSS of PLL-VSC and quantitatively estimate the less-conservatism current feasible region under asymmetric faults. Firstly, the sequence-domain transient synchronization model with PLL dynamics is developed to reveal the synchronization characteristics of PLL-VSC. Then, an improved equal area criterion considering variable damping is proposed to qualitatively analyze the effect of variable damping on TSS. Compared to traditional methods, the proposed iEAC improves both accuracy and conservatism on TSS analysis. Moreover, an iterative estimation method based on the iEAC is put forward to construct a less-conservatism sequence-domain current feasible region (SCFR), which provides a quantitative design for current injections under asymmetric faults to achieve system LVRT. The expansion feasibility of the proposed modeling and TSS analysis method to the multi-VSCs system is fully discussed. Finally, hardware-in-loop experiments validate the effectiveness and superiority of the theoretical analysis.

Conventional methods for achieving zero-voltage switching (ZVS) with minimized reactive power under phase-shift modulation overlook the inevitable changes in coupling coefficient and load during the practical charging process. Analyzing ZVS operation only from a static perspective. To fill the research gap, this article develops a time-domain model that takes into account variations of bilateral detuning factors, mutual inductance, and load. Their impact on ZVS and the system characteristics are analyzed. Furthermore, a collaborative tuning control strategy with switch-controlled capacitors is proposed for adaptive tracking of the minimum turn-off current under ZVS and wide output voltage regulation in dynamic scenarios with parameter variations. In addition, complex mathematical calculation is not required to implement control. Finally, a 960 W experimental prototype was established, and the results showed that the ZVS current remained the minimum across a wide voltage gain range of 0.5–1.2 and loads of 110–960 W. The maximum efficiency after tuning reached 94.7%.

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.

Transient synchronization stability (TSS), current limitation and reactive power support (RPS) are the key objectives for fault ride-through (FRT) that grid-forming converters (GFMCs) must satisfy. However, the complex coupling restraints among these objectives, along with the fact that GFMC cannot directly control current due to its voltage-source nature, present significant challenges for FRT. To address these issues, this paper first analyzes the coupling relationships among the three objectives, and reveals the formations and control mechanisms of phase current, reactive current, and fault inrush current in GFMC. Inspired these insights, an adaptive FRT strategy is designed by cooperatively adjusting the active power and voltage references, as well as the adaptive virtual resistance, which satisfies FRT multi-objective constraints even if the voltage sags to 0. And it is straightforward to implement. Furthermore, the proposed FRT strategy is shown to exhibit superior TSS and RPS performance, as demonstrated through region of attraction and voltage phase portraits. Finally, simulation and experiment results validate the effectiveness and superiority of the proposed FRT strategy.

Active power oscillation in the hybrid grid-connected system with grid-forming (GFM) and grid-following (GFL) inverters can threaten its safe operation. In this paper, considering the coupling between GFM and GFL inverters, the small signal model of the heterogeneous system is established. Based on the model, the influence mechanism of control parameters on the power oscillation under varying short-circuit ratio (SCR) is qualitatively and quantitatively revealed. In particular, the damping of the GFM inverter has a dominant effect compared with other parameters. Therefore, dual mutual damping control (DMDC), containing angular frequency and active power mutual damping, is integrated into the GFM inverter to enhance system damping. Its stability analysis and key parameters design are also presented. Finally, the experimental results validate the effectiveness of the DMDC, demonstrating a reduction in the interaction power oscillation amplitudes by 38.5% and 29.4% under strong and weak grid conditions, respectively.