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Phase-locked loop (PLL) and grid impedance easily negatively affect the stability of the grid-connected inverter system and even cause the system oscillation. To suppress this oscillation, a stability enhancing voltage feed-forward inverter control method is proposed to reduce the effects of PLL and grid impedance. Then, considering the effects of the frequency coupling characteristic, the admittance model of the grid-connected inverter with the proposed stability enhancing control method is established, and the admittance characteristic and the system stability of the inverters with the traditional control method and the proposed stability enhancing control method are compared and analyzed. Analysis results indicate that the proposed stability enhancing control method improves the system stability in different short-circuit ratio (SCR) conditions. What is more, the system with the proposed stability enhancing control method keeps stable under a relative larger PLL bandwidth, while the system with the traditional control method oscillates. Finally, the effectiveness of the proposed stability enhancing control method is verified by experiments.

The premise that the virtual synchronous generator (VSG) can actively support the weak grid is that the VSG can operate stably. In this paper, the small-signal sequence impedance models of the voltage-controlled VSG and the current-controlled VSG are built and the sequence impedance characteristics of these two types of VSGs are compared and analyzed. The sequence impedance of the voltage-controlled VSG, being consistent with the grid impedance characteristics, is generally inductive. By contrast, the sequence impedance of the current-controlled VSG is mainly negative impedance in the middle- and high-frequency area and the impedance amplitude is quite low. Based on the built sequence impedance models and the impedance stability criterion, the influence of the weakness of the grid on the stability of these two types of VSG grid-connected system is analyzed. The results of stability analysis show that the current-controlled VSG is prone to instability while the voltage-controlled VSG remains stable. Therefore, the voltage-controlled VSG is more suitable than the current-controlled VSG for grid-connected renewable energy generation in an ultraweak grid from the point of view of system stability. Finally, experiment results validate the stability analysis.

In this work, with the aim of delivering the raw materials cost savings, P-containing Finemet-type Fe73(Si0.64B0.27P0.09)23+xCu1Nb3-x (x = 0, 1 and 2) amorphous alloys were fabricated by the single roller melt spinning technique to decrease Nb content. The dependence of (Si + B + P)23+x/Nb3-x ratio on crystallization behaviors and magnetic properties were investigated. The decrease of (Si + B + P)23+x/Nb3-x ratio reduced the onset temperature of ɑ-(Fe,Si) and boride phase, the interval of two onset crystallization temperatures decreases slightly, however, the crystallized alloys show a small coercivity (Hc) and moderate saturation magnetization (Ms) after annealed at the temperature higher or lower about 30 or 40 °C than the first crystallization peak temperature (Tp1-30 °C

Adaptive modification design occupies a large proportion in modern product design, which is essentially a coordination process. Due to the conduction phenomenon, it often leads to the problem that the solution space is too large or even unsolvable. Aiming at the problems caused by the conduction phenomenon, an optimal coordination path selecting method for conduction transformation based Floyd algorithm is proposed. The product conduction transformation coordination process is considered as a path by combining the product graph theory model. The selection criteria of the start and end nodes is given. The functional contribution index, modified adaptation index and conduction impact index are described in detail. The calculation of the comprehensive coordination factor composed of them is given and used as the evaluation index of the conduction transformation coordination path. Floyd algorithm is adopted to coordination path optimization selection model, case study and system development, in order to verify the method proposed in this paper.

In the maritime VSC-HVDC system supplying power to passive network on the island, the dc side of the system is prone to oscillation and even instability, which is resulted from the negative incremental input resistance characteristic of the inverter station. First, a virtual phase-lead impedance stability control strategy aiming at rectifier station is proposed. This control strategy can effectively mitigate the dc-side oscillation of the VSC–HVDC system without affecting the load performance of the inverter station. Then, the dc impedance model of the VSC–HVDC system is built, including the output impedance of the rectifier station, dc cable impedance, and the input impedance of the inverter station. In addition, the oscillation mechanism of the VSC–HVDC system is analyzed by impedance-based Nyquist stability criterion. The reason why rectifier station fails to mitigate the dc-side oscillation using traditional virtual resistance stability control is that the output impedance of the rectifier station exhibits negative damping characteristic outside the control bandwidth of voltage outer loop. For this issue, the proposed control strategy can correct the output impedance of the rectifier station to exhibit positive damping characteristic outside the control bandwidth of voltage outer loop, thus mitigating the dc-side oscillation of the VSC–HVDC system. Finally, simulation and experiment results validate the proposed control strategy and analysis.

In Tahe Oilfield, Tarim Basin, NW China, although the carbonate reservoir displays poor porosity and permeability in matrix, the fractured-vuggy system distributes in a random, discreet and discontinuous way, which could yield high oil rates once the fractured-vuggy system is connected by production wells. The fractured-vuggy carbonate reservoirs consist a group of large caves which are inter-connected with high permeable fractures. Due to the lack of powerful and suitable IOR (Improved Oil Recovery) strategies, the depletion recovery is usually applied at the beginning for a fractured-vuggy carbonate reservoir. During the depletion process, the elastic energy of the cave formation starts to decrease, which will cause insufficient liquid supply. Then, the water was injected to improve the oil recovery when the energy (pressure) is the main reason for the insufficient production performance of the well. However, if the high water cut is the main reason, alternative IOR techniques such as the N2 huff and puff method could be utilized. This paper mainly researches the application of N2 huff and puff in a fracture-vuggy carbonate reservoir, where the critical parameters for designing the N2 huff and puff process are proposed. In addition, the proposed method is applied to Well SX to serve as a practical example. Based on the three-dimensional (3D) quantitative sculpture geological model and the dynamic production analysis, the critical parameters for N2 huff and puff are obtained by numerical simulation. In addition, the obtained parameters are validated by the field data, which proves that the proposed method can be applied to design the critical parameters for simulating the N2 huff and puff process in a fractured-vuggy carbonate reservoir.

This paper intends to comparatively study the stabilities of grid-connected inverters with three closely related controllers: Quasi-proportional resonance (quasi-PR), proportional integral (PI), and proportional resonance (PR) under the weak grid. Firstly, considering the influence of frequency coupling characteristic, a frequency coupling admittance model of quasi-PR controlled inverter is established. Then, the admittance characteristics of the quasi-PR, PI and PR controlled inverters are compared. Admittance characteristics of the PI and PR controlled inverters are similar while the quasi-PR controlled inverter is quite different: The amplitude of the quasi-PR controlled inverter is larger than that of the PI controlled inverter and the phase difference between the two inverters is obvious in the mid-high frequency areas, which are mainly caused by the resonance bandwidth of the quasi-PR controller. Furthermore, the stabilities of the quasi-PR, PI and PR controlled inverters are analyzed. The stabilities of the PI and PR controlled inverters are similar but the quasi-PR controlled inverter is more sensitive to weak grid and high inverter output power. To achieve the same system stability, the voltage outer-loop bandwidth of the quasi-PR controlled inverter should be designed narrower than that of the PI and PR controlled inverters. Finally, experiments verify the correctness of the analyses.

BackgroundAntiviral therapies against the novel coronavirus SARS-CoV-2, which has caused a global pandemic of respiratory illness called COVID-19, are still lacking.MethodsOur study (NCT04252885, named ELACOI), was an exploratory randomized (2:2:1) controlled trial assessing the efficacy and safety of lopinavir/ritonavir (LPV/r) or arbidol monotherapy for treating patients with mild/moderate COVID-19.FindingsThis study successfully enrolled 86 patients with mild/moderate COVID-19 with 34 randomly assigned to receive LPV/r, 35 to arbidol and 17 with no antiviral medication as control. Baseline characteristics of the three groups were comparable. The primary endpoints, the average time of positive-to-negative conversion of SARS-CoV-2 nucleic acid and conversion rates at days 7 and 14, were similar between groups (all P>0.05). There were no differences between groups in the secondary endpoints, the rates of antipyresis, cough alleviation, or improvement of chest CT at days 7 or 14 (all P>0.05). At day 7, eight (23.5%) patients in the LPV/r group, 3 (8.6%) in the arbidol group and 2(11.8%) in the control group showed a deterioration in clinical status from moderate to severe/critical(P =0.206). Overall, 12 (35.3%) patients in the LPV/r group and 5 (14.3%) in the arbidol group experienced adverse events during the follow-up period. No apparent adverse event occurred in the control group.ConclusionsLPV/r or arbidol monotherapy present little benefit for improving the clinical outcome of patients hospitalized with mild/moderate COVID-19 over supportive care.FundingThis study was supported by project 2018ZX10302103-002, 2017ZX10202102-003-004 and Infectious Disease Specialty of Guangzhou High-level Clinical Key Specialty (2019-2021).

In Tahe Oilfield, Tarim Basin, NW China, the carbonate reservoir displays poor porosity and permeability in its matrix, while the fractured-vuggy system is distributed in a random, discreet and discontinuous way, which yields significant oil rates once the fractured-vuggy system is connected by production wells. The fractured-vuggy carbonate reservoirs are consisting of a group of large carves and inter-connected by high permeability fractures. Due to lack of powerful and suitable EOR (Improved Oil Recovery) strategies, depletion-drive recovery is commonly adopted for practical production at the beginning. After a period of production, the elastic energy of the cave formation weakens, causing insufficient liquid supply, the water injected for water flooding is used to improve the oil recovery of the fractured-vuggy carbonate reservoirs. However, the water injection curves of the carbonate reservoir are different from that of the sandstone reservoir as their fundamental flow mechanisms are different. For the fractured-vuggy reservoirs with high permeability, the water injection curves can be adopted to calculate the volumes in place of crude oil and formation water in a single or multi-caves. This paper presents a new model of injection curves. Using the proposed model with multi-round water injection curves, we can initially judge reservoir type and determine the volumes of cave, crude oil and water in the caves reasonably. Besides, as testified by the practical case, the parameters interpreted by the new models are proved to be reasonable and reliable. This new model is a useful tool to estimate the crude oil reserves, to demonstrate how much remaining oil in the caves, and to identify where the water-oil contact is.

In LCL-type grid-connected inverter, an optimized design method for grid-current-feedback active damping (AD) is proposed to improve the system dynamic characteristic. By analyzing the equivalent virtual impedance model, the essential relationship between the system dynamic characteristic indicators and damping control parameters is obtained, and the damping control parameters are contradictory that difficult to optimize. Then, a parameters optimization design method combining zero-pole model and Routh stability criterion is proposed to reduce the complexity of the parameters design and improve the system dynamic. Moreover, the influence of the control delay on system stability and parameters design is analyzed in the discrete domain. Simulation and experimental results verified the validity of the proposed method.

In a single-phase grid-connected inverter, because the constructed orthogonal signals are not strictly orthogonal in broadband and the control of dq-axis is asymmetric, when the voltage disturbance with frequency of f p is injected in point of common coupling (PCC), not only the response current with frequency of f p will be generated, but also the response currents with frequencies of f p –2 f1 and f p +2 f1 will be coupled. In order to accurately establish the broadband impedance model of the single-phase grid-connected inverter, the frequency coupling mechanism is analyzed in this paper. Considering the influence of the constructed orthogonal signal, current loop, phase-locked loop (PLL) and frequency coupling, a broadband impedance model of the single-phase grid-connected inverter is established. Its broadband impedance characteristics are analyzed. The simulation results verify the accuracy of the established model.

With the continuous expansion of new energy grid-connected power generation, the utilization of a large number of power electronic inverters results in the aggravating interaction between new energy power generation and the grid, which led to critical wide-bandwidth oscillation. Therefore, this paper proposes a simplified wide-bandwidth generalized impedance model by modeling the single-phase grid-connected inverter in the polar coordinate system. Firstly, the small-signal dynamic model of the inverter in the polar coordinate system based with second-order generalized integrator (SOGI) is obtained. Then a frequency division basis of 10 Hz to 1 kHz is proposed because the dynamic response of each link varies with frequency. Finally, generalized impedance models of the inverter in the different frequency band are established respectively. The good agreement of the simulation result and the theoretical model verifies the feasibility and correctness of the model. The proposed model lays a good foundation for wide-bandwidth modeling of the three-phase system.