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Introduction: Virtual Synchronous Generators (VSGs) are used in Voltage Source Converter-based Multi-Terminal High-Voltage Direct Current (VSC-MTDC) systems to enhance power system stability. However, the MTDC framework can lead to instability due to reduced inertia in certain grid areas, especially during load switching at VSC stations. This instability is exacerbated by untimely adjustments of the VSG's power setpoint, leading to voltage and frequency oscillations. Methods: This study introduces a cooperative control approach for the DC voltage of the VSG, employing a consensus algorithm and Model Predictive Control (MPC). This method aims to achieve incremental power for the VSG and provide interactive power commands for both the grid side and the wind farm side. The consensus algorithm ensures coherent system adjustments, while the MPC algorithm tracks DC-side voltage changes in real time. Results: The application of this cooperative control approach significantly enhances DC voltage regulation performance. It effectively reduces the extent of frequency drops and mitigates secondary frequency drop (SFD) issues, particularly those arising from the use of wind farms for frequency regulation and the associated speed recovery in wind turbine units. Discussion: The increase in supplemental power effectively utilizes the energy stored in DC-side capacitors for power balance regulation and introduces additional inertial power into the system. Electromagnetic transient simulations have confirmed the effectiveness of the Consensus MPC-VSG method, demonstrating its ability to optimize the dynamic performance of VSC-MTDC systems and promote stability in DC voltage and frequency. Conclusion: The findings suggest that employing the Consensus MPC-VSG method offers a promising solution for enhancing the stability and operational efficiency of VSC-MTDC systems, addressing the challenges posed by the inherent segmentation of the grid and the integration of renewable energy sources like wind farms. [ABSTRACT FROM AUTHOR]

Transient stability and frequency stability of virtual synchronous generator (VSG)-controlled converters during grid faults are critical for the stable operation of high-share renewable energy power systems. However, the existing transient stability methods often overlook the consideration of frequency stability. This paper presents a method to improve both frequency stability and transient stability of VSGs during grid faults. The influence mechanism of inertia and damping on transient stability and frequency stability is comprehensively analyzed under two types of faults, with or without stable equilibrium points. To address the conflict between frequency stability and transient stability of VSGs with large inertia under the first type of fault, an improved VSG control with angular frequency deviation feedback is proposed. The transient response of this control method is analyzed using small-signal and large-signal models. Furthermore, to achieve optimal dynamic performance, this improved VSG control introduces the optimal damping ratio and presents a parameter design method. Finally, the correctness of the proposed method and its theoretical analysis is verified through experimental results. [ABSTRACT FROM AUTHOR]

A virtual synchronous generator (VSG) strategy can introduce the rotational inertia and damping characteristics of the synchronous generator to the static inverter, e.g., PV, wind generation, and ESS, which are used to enhance the system frequency support characteristics of the micro-grid. However, under various operations of the VSG, the inertia and damping support capabilities are different, and the conventional VSG strategy with a fixed control coefficient sacrifices a certain degree of dynamic regulation performance with less robustness. This research proposes an improved VSG strategy with adaptive inertia and damping coefficients to increase the flexibility of VSG. To this end, a mathematical model is first established to analyze the impact of various parameters on the characteristics of VSG, and then the root trajectory is used to explore the impacts of various rotational inertia and damping coefficients on the stability ofthe VSG system. Second, an improved control strategy with adaptive inertia and damping coefficients is proposed based on the characteristics of the second- order system and the system frequency deviations. Finally, a simulation system with the VSG is constructed based on MATLAB/Simulink. The effectiveness of the proposed control strategy is verified by comparing the simulation results to the conventional control strategy with the fixed control coefficients under over- frequency and under-frequency disturbances. [ABSTRACT FROM AUTHOR]

In response to the inability of the flexible DC transmission system connected to the AC grid under conventional control strategies to provide inertia to the system as well as to participate in frequency regulation, a virtual synchronous generator (VSG) control strategy is proposed for a voltage source converter (VSC)-based multi-terminal high-voltage direct current (VSC-MTDC) interconnection system. First, the virtual controller module is designed by coupling AC frequency and active power through virtual inertia control, so that the VSC-MTDC system can provide inertia response for AC grid frequency. Second, by introducing the power margin of the converter station into the droop coefficient, the unbalanced power on the DC side is reasonably allocated to reduce the overshoot of the DC voltage in the regulation process. Finally, the power regulation capability of the normal AC system is used to provide power support to the fault end system, reducing frequency deviations and enabling inter-regional resource complementation. The simulation model of the three-terminal flexible DC grid is built in PSCAD/EMTDC, and the effectiveness of the proposed control strategy is verified by comparing the conventional control strategy and the additional frequency control strategy.

The “3060” plan fully demonstrates the determination of China’s economic and social green and clean development. As an important secondary energy source, hydrogen has the advantages of high energy density, green and pollution-free, etc. As an ideal energy source in the future, green hydrogen is the final state of hydrogen energy prepared by electrical energy through electrolysis of water. The development of renewable energy hydrogen production will certainly help to achieve the goal of carbon peaking and carbon neutral. Therefore, the grid-connected control strategy of hydrogen production unit for weak grid is worthy of in-depth study. The research work in this paper is as follows: firstly, based on the analysis of the converter, a grid-connected control method of electric hydrogen production unit using virtual synchronization generator(VSG) control strategy is proposed. Secondly, a control method without phase-locked loop is adopted for the grid-connected phase synchronization part. Then, the stability of the proposed control strategy is demonstrated by using root locus plot. Finally, experimental verification is carried out. The experimental results show that the proposed control strategy makes the hydrogen production unit friendly to the weak grid with good adaptability, and at the same time, as a load, it can perform adaptive power regulation very well.

Virtual Synchronous Generator (VSG)-based grid-forming converters (GFC) increasingly serve as interfaces between renewable energy resources and the power grids with high penetration. Although the emulation of the characteristics of Synchronous Generator (SG) enables GFC to provide virtual inertia and oscillation damping under the VSG control, it remains challenging to replicate the short-circuit behaviors of SGs under the grid voltage sag/recovery conditions. The existing technologies are limited and lack full functionalities in terms of power synchronization, overcurrent suppression and capability of dealing with asymmetric grid-faults. This paper presents a Virtual Impedance Voltage Control (VIVC) strategy to enhance the fault-ride-through capability of the converter under a variety of fault conditions. In the case of asymmetric faults, a negative-sequence current control is performed to mitigate the negative-sequence voltages and achieve the grid current balance. The fault current is restrained and the power synchronization is improved by the virtual reactance without oversizing the GFC. The simulation and experimental results validate the effectiveness of the proposed control strategies.

With the high ratio of distributed generation (DG) linked to power system via converters, power system stability is degraded in terms of inertia and damping. In recent years, the virtual synchronous generator (VSG) has become a potential solving method to deal with these issues. However, the power coupling problem exists in the traditional VSG control method, which severely impacts the VSG power control performance. The impedance characteristics of the transmission line are closely related to the effect of the power decoupling control of VSGs. To solve the voltage sag and poor robustness issues resulting from traditional virtual impedance control, a virtual complex impedance power decoupling and sharing control strategy for VSGs based on coordinate transformation is proposed. Combining the characteristics of traditional virtual impedance control and virtual power control, the power coupling mechanism in parallel VSGs caused by line impedance is analyzed. The virtual complex impedance control loop is introduced in VSG control to realize the matching of the equivalent line impedance and the precise decoupling of VSG power output. Then the load can be accurately allocated in the light of the proportion of VSG capacity.

Unlike Synchronous Generators (SGs), Virtual Synchronous Generators (VSGs) inertia is not fixed once it is manufactured and only has an upper limit defined by its energy storage components. In this paper, a novel Enhanced Virtual Inertia Controller (EVIC) is proposed. The proposed controller alters the VSG inertia coefficient between two limiting levels in response to a grid transient. The key difference between the proposed controller and the variable inertia controller is that the proposed EVIC causes a smooth transition in the inertia coefficient while the variable inertia controller causes a discontinuous jump in it. The proposed EVIC guarantees an adaptive response to grid dynamics, such that a negligible change occurs at small disturbances and a linear and smooth increase occurs at moderate disturbances. For large disturbances, the proposed controller smoothly oscillates the inertia between two saturation levels, which then quickly returns the converter to its steady-state operating point with minimum oscillations. A qualitative study of the performance and stability margin of the proposed controller was conducted using a large signal model (nonlinear model) of VSG connected to a microgrid. The large signal model provided a complete description of the converter's behaviour under large disturbances, which is the area of interest of the proposed controller. It also contained the small-signal dynamics (linear dynamic) within the vicinity of the equilibrium (steady-state) point. Thus, a complete description of the proposed controller dynamics is conveyed to prove its validity and adaptability. [ABSTRACT FROM AUTHOR]

Introduction: Virtual Synchronous Generators (VSGs) are used in Voltage Source Converter-based Multi-Terminal High-Voltage Direct Current (VSC-MTDC) systems to enhance power system stability. However, the MTDC framework can lead to instability due to reduced inertia in certain grid areas, especially during load switching at VSC stations. This instability is exacerbated by untimely adjustments of the VSG's power setpoint, leading to voltage and frequency oscillations.Methods: This study introduces a cooperative control approach for the DC voltage of the VSG, employing a consensus algorithm and Model Predictive Control (MPC). This method aims to achieve incremental power for the VSG and provide interactive power commands for both the grid side and the wind farm side. The consensus algorithm ensures coherent system adjustments, while the MPC algorithm tracks DC-side voltage changes in real time.Results: The application of this cooperative control approach significantly enhances DC voltage regulation performance. It effectively reduces the extent of frequency drops and mitigates secondary frequency drop (SFD) issues, particularly those arising from the use of wind farms for frequency regulation and the associated speed recovery in wind turbine units.Discussion: The increase in supplemental power effectively utilizes the energy stored in DC-side capacitors for power balance regulation and introduces additional inertial power into the system. Electromagnetic transient simulations have confirmed the effectiveness of the Consensus MPC-VSG method, demonstrating its ability to optimize the dynamic performance of VSC-MTDC systems and promote stability in DC voltage and frequency.Conclusion: The findings suggest that employing the Consensus MPC-VSG method offers a promising solution for enhancing the stability and operational efficiency of VSC-MTDC systems, addressing the challenges posed by the inherent segmentation of the grid and the integration of renewable energy sources like wind farms.

A virtual synchronous generator (VSG) strategy can introduce the rotational inertia and damping characteristics of the synchronous generator to the static inverter, e.g., PV, wind generation, and ESS, which are used to enhance the system frequency support characteristics of the micro-grid. However, under various operations of the VSG, the inertia and damping support capabilities are different, and the conventional VSG strategy with a fixed control coefficient sacrifices a certain degree of dynamic regulation performance with less robustness. This research proposes an improved VSG strategy with adaptive inertia and damping coefficients to increase the flexibility of VSG. To this end, a mathematical model is first established to analyze the impact of various parameters on the characteristics of VSG, and then the root trajectory is used to explore the impacts of various rotational inertia and damping coefficients on the stability of the VSG system. Second, an improved control strategy with adaptive inertia and damping coefficients is proposed based on the characteristics of the second-order system and the system frequency deviations. Finally, a simulation system with the VSG is constructed based on MATLAB/Simulink. The effectiveness of the proposed control strategy is verified by comparing the simulation results to the conventional control strategy with the fixed control coefficients under over-frequency and under-frequency disturbances.

The proportion of distributed generation systems in power grids is increasing, leading to the gradual emergence of weak grid characteristics. Moreover, using voltage-sourced grid-connected inverters can enhance the stability of a weak grid. However, due to the presence of background harmonics in weak grids, the grid voltage can cause significant distortions in the grid-connected current, which adversely affects the quality of the grid-connected current. This paper begins by briefly introducing the principle of the virtual synchronous generator (VSG). Then, the output current of the voltage source inverter is analyzed to elucidate the mechanism of harmonic current generation. Considering the distortion in the grid-connected current of the voltage source grid-connected inverter caused by background harmonics in the grid voltage, a harmonic current suppression strategy based on an adaptive virtual harmonic resistor is proposed. The proposed strategy employs a signal separation module based on multiple second-order generalized integrators connected through a cross-feedback network. This module effectively separates the fundamental and harmonic currents from the grid-connected current, extracts the amplitudes of the fundamental and harmonic currents through coordinate transformation, and adaptively adjusts the virtual harmonic resistance magnitude through the negative feedback control of the harmonic content (the ratio of the harmonic current amplitude to the fundamental current amplitude). These measures are used to enhance the quality of the grid-connected current. Additionally, the stability of the system is analyzed using the root locus of the open-loop transfer function. Finally, the effectiveness of the proposed method is validated through a combination of MATLAB/Simulink simulations and experimental results. [ABSTRACT FROM AUTHOR]

The constant increase of energy demand and the widespread adoption of renewable energy sources have led to the significant interest in micro-grids, which are small-scale power systems that utilize distributed energy resources. However, micro-grids face synchronization challenges in terms of stability and reliability, and virtual synchronous generator technology has become the key to addressing this issue. This study focuses on the pre synchronization control strategy of virtual synchronous generators in micro-grids, aiming to solve the potential surge current problem that virtual synchronous generators may generate when switching from off grid mode to grid connected mode through innovative methods. This strategy employs a phase difference acquisition method that eliminates the need for a phase-locked loop. It calculates virtual power by utilizing the output voltage of the virtual synchronous generator and the grid side voltage, and obtains the phase difference through calculation. The resulting phase difference is then used as input for the frequency phase synchronization unit in pre-synchronization. By adjusting the diagonal frequency, the phase difference $\Delta \theta $ can be modified to achieve pre-synchronization before connecting the virtual synchronous generator to the power grid. This method notably reduces the dependence on the phase-locked loop and enhances the stability and reliability of the system. The paper has effectively solved the synchronization problem of virtual synchronous generators in micro-grids through innovative pre synchronization control strategies and improved secondary frequency regulation strategies. It is hoped that these research results will provide new ideas and methods for the further development and application of micro-grid technology.

For the pre-synchronization method of the virtual synchronous generator (VSG), only the case of a single VSG pre-synchronization and grid connection has been studied and analyzed. If the phase synchronization of multiparallel VSGs in a decentralized microgrid is realized by controlling a VSG, power circulation exists in the pre-synchronization process. Firstly, the structure and control mode of the microgrid are introduced. For the decentralized structure, a control mode switching mechanism under distributed communication is designed, and a pre-synchronization method of parallel VSGs based on control mode switching is proposed. Secondly, a theoretical analysis of the active power circulation problem is performed by an equivalent frequency model for parallel VSGs in the pre-synchronization process. Finally, the simulation results show that the proposed pre-synchronization method can reduce the dynamic power circulation of parallel VSGs by 83%, and improve the pre-synchronization speed by 75%. [ABSTRACT FROM AUTHOR]

With the high penetration of renewable energy, new challenges, such as power fluctuation suppression and inertial support capability, have arisen in the power sector. Battery energy storage systems play an essential role in renewable energy integration. In this paper, a distributed virtual synchronous generator (VSG) control method for a battery energy storage system (BESS) with a cascaded H-bridge converter in a grid-connected mode is proposed. The VSG is developed without communication dependence, and state-of-charge (SOC) balancing control is achieved using the distributed average algorithm. Owing to the low varying speed of SOC, the bandwidth of the distributed communication networks is extremely slow, which decreases the cost. Therefore, the proposed method can simultaneously provide inertial support and accurate SOC balancing. The stability is also proved using root locus analysis. Finally, simulations under different conditions are carried out to verify the effectiveness of the proposed method.

Unlike Synchronous Generators (SGs), Virtual Synchronous Generators (VSGs) inertia is not fixed once it is manufactured and only has an upper limit defined by its energy storage components. In this paper, a novel Enhanced Virtual Inertia Controller (EVIC) is proposed. The proposed controller alters the VSG inertia coefficient between two limiting levels in response to a grid transient. The key difference between the proposed controller and the variable inertia controller is that the proposed EVIC causes a smooth transition in the inertia coefficient while the variable inertia controller causes a discontinuous jump in it. The proposed EVIC guarantees an adaptive response to grid dynamics, such that a negligible change occurs at small disturbances and a linear and smooth increase occurs at moderate disturbances. For large disturbances, the proposed controller smoothly oscillates the inertia between two saturation levels, which then quickly returns the converter to its steady-state operating point with minimum oscillations. A qualitative study of the performance and stability margin of the proposed controller was conducted using a large signal model (nonlinear model) of VSG connected to a microgrid. The large signal model provided a complete description of the converter’s behaviour under large disturbances, which is the area of interest of the proposed controller. It also contained the small-signal dynamics (linear dynamic) within the vicinity of the equilibrium (steady-state) point. Thus, a complete description of the proposed controller dynamics is conveyed to prove its validity and adaptability.

ObjectiveTo determine whether vertical sleeve gastrectomy (VSG) attenuates fibrosis in mice on a high-fat high-cholesterol (HFHC) diet.BackgroundBariatric surgery mitigates non-alcoholic steatohepatitis in 85-90% of obese patients. While animal models demonstrate similar results on a high-fat diet, none have observed the effects of bariatric surgery on a combined HFHC diet.MethodsMice on a HFHC diet were used to confirm the development of hepatic fibrosis at 8 (n = 15) and 24 (n = 15) weeks. A separate cohort of mice on a HFHC diet for 12 weeks was subjected to either VSG (n = 18) or sham (n = 12) operations and remained on a HFHC diet for an additional 20 weeks. Changes in weight, dyslipidemia, and the development of steatosis and fibrosis were documented. Serum was obtained for bile acid analysis by liquid chromatography and mass spectrometry, while hepatic gene expression by RT-PCR was performed to evaluate intrahepatic lipid metabolism.ResultsHepatic steatosis and fibrosis developed after 8 weeks on the HFHC diet. After VSG, mice demonstrated a sustained decrease in weight with a significant decrease in fibrosis compared to sham mice. Serum total cholesterol, HDL, and LDL were significantly reduced following surgery, while serum bile acids were significantly elevated. Intra-hepatic cholesterol excretion was not upregulated based on hepatic gene expression of CYP7A1 and ABCG5/8.ConclusionsVSG attenuates the development of hepatic fibrosis in diet-induced obese mice, presumably through enhancement of cholesterol elimination at the intestinal level.

BACKGROUND AND AIMS: Bariatric surgery, such as vertical sleeve gastrectomy (VSG), is the most effective long-term treatment for obesity. However, there are conflicting reports on the effect of bariatric surgery on inflammatory bowel disease (IBD). Bariatric surgery increases bile acid concentrations, which can decrease inflammation by signaling through the bile acid receptor, TGR5. TGR5 signaling protects against chemically induced colitis in mice. VSG increases circulating bile acid concentrations to increase TGR5 signaling, which contributes to improved metabolic regulation after VSG. Therefore, we investigated the effect of VSG on chemically induced colitis development and the role of TGR5 in this context. METHODS: VSG or sham surgery was performed in high fat diet-fed male Tgr5+/+ and Tgr5-/- littermates. Sham-operated mice were food restricted to match their body weight to VSG-operated mice. Colitis was induced with 2.5% dextran sodium sulfate (DSS) in water post-operatively. Body weight, energy intake, fecal scoring, colon histopathology, colonic markers of inflammation, goblet cell counts, and colonic microRNA-21 levels were assessed. RESULTS: VSG decreased body weight independently of genotype. Consistent with previous work, genetic ablation of TGR5 increased the severity of DSS-induced colitis. Notably, despite the effect of VSG to decrease body weight and increase TGR5 signaling, VSG increased the severity of DSS-induced colitis. VSG-induced increases in colitis were associated with increased colonic expression of TNF-α, IL-6, MCP-1, and microRNA-21. CONCLUSIONS: While our data demonstrate that TGR5 protects against colitis, they also demonstrate that VSG potentiates chemically induced colitis in mice. These data suggest that individuals undergoing VSG may be at increased risk for developing colitis; however, further study is needed.

With the increase of photovoltaic penetration, the power-electronic converter (PEC)-interfaced generators with traditional control strategy may cause severe power oscillations after being disturbed due to lack of inertia and damping. The photovoltaic power plant controlled by virtual synchronous generator (VSG) technology, while introducing virtual inertia and virtual damping to improve the stability of the system, will also inherit its oscillation characteristics and cause a new oscillation mode. Aiming at this problem, this paper analyzes the difference between the system low-frequency oscillation (LFO) modes before and after the VSG is connected to the two-area system by the modal analysis. It is pointed out that the PEC-interfaced renewable energy generators based on VSG control technology could change the oscillation mode of the original system. The influence of VSG's main parameters and capacity on system dynamics is analyzed. Finally, a synchrophasor-based inter-area oscillation suppressed comprehensive control strategy is proposed to suppress the LFO of active power on the tie line, and the superiority of the strategy is verified by DIgSILENT /PowerFactory. [ABSTRACT FROM AUTHOR]

Grid-Tied Converters (GTCs) serve a vital role in integrating renewable energy generation into power networks with high penetration rates. During a grid voltage sag or recovery, the existing LVRT technologies still suffer from overcurrent caused by fault detection delay under the control of the Virtual Synchronous Generator (VSG). This vulnerability may result in trip-off occurrences or damage to the power electronic components. At present, VSG-related research mainly focuses on control and stability analysis, etc. There is a relative lack of work on the fault current-voltage (I-V) characteristics of VSG during grid voltage dips, which is urgently needed. Due to this indistinct nonlinear characteristics, the grid-forming GTC either loses the voltage source characteristics of the VSG or deteriorates the transient stability against deep asymmetrical grid dips. As a result, the conventional solutions provide limited assurance of transient stability and inadequate management of asymmetric grid disturbances. The analysis in this paper begins with a dimensional examination of the I-V characteristics of the VSG (IV-VSG) during grid contingencies. Subsequently, this research presents an improved control strategy for increasing the LVRT capability of GTCs under different grid-fault circumstances. Additionally, a rigorous analysis of transient stability is performed to uphold the synchronization between the VSG and the grid during severe grid disturbances. Both the simulation and experimental verification demonstrate that fault currents are restricted at a rapid rate, power synchronization is maintained and the need for an enlarged GTC and current limiting strategies are eliminated. In addition, the transient stability of the VSG is validated under a variety of fault conditions. • A novel low-voltage ride-through (LVRT) control scheme is proposed to fully comply with the grid codes. • The LVRT strategy effectively restricts the fault currents while simultaneously maintaining the characteristics of the VSG. • The transient stability is rigorously analysed to ensure the synchronism under a severe grid voltage dip. • Sophisticated gain tuning, specific fault-current limiter or additional control is not required for the implementation. [ABSTRACT FROM AUTHOR]

The proportion of distributed generation systems in power grids is increasing, leading to the gradual emergence of weak grid characteristics. Moreover, using voltage-sourced grid-connected inverters can enhance the stability of a weak grid. However, due to the presence of background harmonics in weak grids, the grid voltage can cause significant distortions in the grid-connected current, which adversely affects the quality of the grid-connected current. This paper begins by briefly introducing the principle of the virtual synchronous generator (VSG). Then, the output current of the voltage source inverter is analyzed to elucidate the mechanism of harmonic current generation. Considering the distortion in the grid-connected current of the voltage source grid-connected inverter caused by background harmonics in the grid voltage, a harmonic current suppression strategy based on an adaptive virtual harmonic resistor is proposed. The proposed strategy employs a signal separation module based on multiple second-order generalized integrators connected through a cross-feedback network. This module effectively separates the fundamental and harmonic currents from the grid-connected current, extracts the amplitudes of the fundamental and harmonic currents through coordinate transformation, and adaptively adjusts the virtual harmonic resistance magnitude through the negative feedback control of the harmonic content (the ratio of the harmonic current amplitude to the fundamental current amplitude). These measures are used to enhance the quality of the grid-connected current. Additionally, the stability of the system is analyzed using the root locus of the open-loop transfer function. Finally, the effectiveness of the proposed method is validated through a combination of MATLAB/Simulink simulations and experimental results.

Virtual synchronous generator (VSG)-based control approaches for AC machine drivers have attracted much attention in recent years. The contradiction between the strong motor loading capacity and the small DC voltage fluctuation is disregarded in previous studies. A larger virtual inertia is required to raise the torque gain and drive the motor with heavier loads, thus resulting in greater DC voltage variations. A systematic method is proposed for the optimal design of the virtual inertia for VSG-based motor starting systems to achieve the maximal motor loading capacity and the low DC voltage fluctuation in this paper. Firstly, the relationship between the virtual inertia and the loading capacity is revealed on the basis of the closed-loop analysis. Considering the DC voltage variation, the constraint on the maximal value of the virtual inertia is proposed from the energy-balance perspective despite the unknown contribution of the DC capacitors to the virtual inertia. To fully exploit the energy storage capacity of the DC capacitors and raise the upper limit of the virtual inertia under the same permitted DC voltage variation, a modified DC voltage reference is introduced. Extensive simulation and experimental results are presented to validate the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

For the pre-synchronization method of the virtual synchronous generator (VSG), only the case of a single VSG pre-synchronization and grid connection has been studied and analyzed. If the phase synchronization of multiparallel VSGs in a decentralized microgrid is realized by controlling a VSG, power circulation exists in the pre-synchronization process. Firstly, the structure and control mode of the microgrid are introduced. For the decentralized structure, a control mode switching mechanism under distributed communication is designed, and a pre-synchronization method of parallel VSGs based on control mode switching is proposed. Secondly, a theoretical analysis of the active power circulation problem is performed by an equivalent frequency model for parallel VSGs in the pre-synchronization process. Finally, the simulation results show that the proposed pre-synchronization method can reduce the dynamic power circulation of parallel VSGs by 83%, and improve the pre-synchronization speed by 75%.

ABSTRACT In the mammalian host, Trypanosoma brucei is coated in a single-variant surface glycoprotein (VSG) species. Stochastic switching of the expressed VSG allows the parasite to escape detection by the host immune system. DNA double-strand breaks (DSB) trigger VSG switching, and repair via gene conversion results in an antigenically distinct VSG being expressed from the single active bloodstream-form expression site (BES). The single active BES is marked by VSG exclusion 2 (VEX2) protein. Here, we have disrupted monoallelic VSG expression by stably expressing a second telomeric VSG from a ribosomal locus. We found that cells expressing two VSGs contained one VEX2 focus that was significantly larger in size than the wild-type cells; this therefore suggests the ectopic VSG is expressed from the same nuclear position as the active BES. Unexpectedly, we report that in the double VSG-expressing cells, the DNA sequence of the ectopic copy is lost following a DSB in the active BES, despite it being spatially separated in the genome. The loss of the ectopic VSG is dependent on active transcription and does not disrupt the number or variety of templates used to repair a BES DSB and elicit a VSG switch. We propose that there are stringent mechanisms within the cell to reinforce monoallelic expression during antigenic variation. IMPORTANCE The single-cell parasite Trypanosoma brucei causes the fatal disease human African trypanosomiasis and is able to colonize the blood, fat, skin, and central nervous system. Trypanosomes survive in the mammalian host owing to a dense protective protein coat that consists of a single-variant surface glycoprotein species. Stochastic switching of one VSG for an immunologically distinct one enables the parasite to escape recognition by the host immune system. We have disrupted monoallelic antigen expression by expressing a second VSG and report that following DSB-triggered VSG switching, the DNA sequence of the ectopic VSG is lost in a transcription-dependent manner. We propose that there are strict requirements to ensure that only one variant antigen is expressed following a VSG switch, which has important implications for understanding how the parasite survives in the mammalian host.

The Trypanosoma (T) brucei life cycle alternates between the tsetse fly vector and the mammalian host. In the insect, T. brucei undergoes several developmental stages until it reaches the salivary gland and differentiates into the metacyclic form, which is capable of infecting the next mammalian host. Mammalian infectivity is dependent on expression of the metacyclic variant surface glycoprotein genes as the cells develop into mature metacyclics. The VEX complex is essential for monoallelic variant surface glycoprotein expression in T. brucei bloodstream form, however, initiation of expression of the surface proteins genes during metacyclic differentiation is poorly understood. To better understand the transition to mature metacyclics and the control of metacyclic variant surface glycoprotein expression we examined the role of VEX1 in this process. We show that modulating VEX1 expression leads to a dysregulation of variant surface glycoprotein expression during metacyclogenesis, and that following both in vivo and in vitro metacyclic differentiation VEX1 relocalises from multiple nuclear foci in procyclic cells to one to two distinct nuclear foci in metacyclic cells - a pattern like the one seen in mammalian infective bloodstream forms. Our data suggest a role for VEX1 in the metacyclic differentiation process and their capacity to become infectious to the mammalian host.

In recent years, virtual synchronous generator (VSG) control algorithm has been widely studied in distributed grid-connected power inverters to deal with active and reactive power adjustment by simulating the operation principle of the actual synchronous generator. Nevertheless, once the current sensor fails, the grid-connected power inverter will lose its stability. To enhance the fault-tolerant operation ability of the grid-connected power inverter, a new inverter side current sensor-less model predictive control method based on sliding mode observer (SMO) is proposed. The stability of the SMO is analyzed by using the Lyapunov stability criterion. The gains of the SMO are then designed by analyzing their influences on the current and voltage observation based on the closed-loop transfer functions. Besides, to show the influences of the capacitance changes on the current observation, a detailed parameter sensitivity analysis is also carried out. Then, by using the estimated inductance current from the SMO, a current sensor-less model predictive control method is finally proposed. The main novelty of the proposed method is that both the output voltage of the inverter and the inverter side inductance are not required in the proposed SMO. So, the accuracy of current sensor-less control is improved. To verify the effectiveness of the proposed method, detailed experimental studies are carried out based on a Typhoon and PE-Expert 4 based experimental platform.

• Development of a new modified pre-synchronization approach for the VSG inverter for smooth and efficient switching. • The operation of the VSG inverter is implemented in islanding and islanding-to-grid connected modes with and without the pre-synchronization process. • Seamless switching between islanding and grid-connected mode. • Fast switching from the islanding to grid mode after the pre-synchronization conditions are met. • Enhanced system stability against the injected load perturbations. • An improved frequency transient response against the grid frequency perturbations. • The proposed approach is better at ensuring the regulation of voltage, frequency, and phase of the VSG as compared to the traditional approaches. The growing integration of renewable energy sources and distributed generators (DGs) significantly reduces the system inertia and damping for microgrid operation and control, thereby affecting grid stability. A virtual synchronous generator (VSG) offers a promising solution to enhance power system stability by emulating the behavior of synchronous generators (SGs). A new modified control strategy for seamless switching is introduced in this study for the VSG inverter during the transition from off-grid to on-grid mode. The operation of the VSG inverter has been proposed for both islanding and grid-connected modes. To reduce the impact of inrush current during synchronization, a new modified pre-synchronization process based on voltage and frequency compensating signals has been proposed. A detailed stability analysis is also conducted for the VSG controller to verify the effectiveness of the proposed approach. A model with particular consideration of seamless switching is developed and analyzed in islanding and grid-connected modes with and without the pre-synchronization process. The proposed modified control strategy has been simulated in MATLAB and the results demonstrate an improved frequency transient response against the grid frequency perturbations along with fast pre-synchronization-based switching from islanding to grid-connected modes. The proposed approach efficiently addresses instantaneous inrush current during synchronization, successfully reducing it from 120A to its nominal value. Furthermore, the proposed pre-synchronization technique has been compared with other traditional methods, highlighting its superiority in terms of active power, reactive power, and frequency stability. This research indicates the efficacy of the control approach in enabling seamless and smooth switching of the VSG inverter from off-grid to on-grid modes. [ABSTRACT FROM AUTHOR]

This paper researched on the pre-synchronization grid connection method of a virtual synchronous generator (VSG), and proposed a fast pre-synchronization method based on the virtual power loop. According to the self-synchronization characteristics of the VSG power loop, the double power loop design was adopted in this paper. The two power loops had the same structure, which the power loop 1 was used to implement the basic VSG function, and the power loop 2 was used to achieve rapid pre-synchronous control. Therefore, the mathematical model of the VSG was similar to the synchronous generator in the pre-synchronization control. This method did not require a phase-locked loop and VSG has a good inertia and Damping performance. Moreover, this method was suitable for the networking of multiple VSGs.

Abstract Background Gold standard microscopic examination of Plasmodium falciparum intraerythrocytic stage remains an important process for staging and enumerating parasitized erythrocytes in culture; however, microscopy is laborious and its accuracy is dependent upon the skill of the examiner. Methods In this study, ViSafe Green (VSG), which is a nucleic acid-binding fluorescent dye, was used for assessing in vitro development of P. falciparum using flow cytometry. Results Fluorescence intensity of VSG was found to depend on the developmental stage of parasites. Specifically, multiple-nuclei-containing schizonts were observed in the VSGhigh population, and growing trophozoites and ring-shaped forms were observed in the VSGintermediate and VSGlow populations. The efficacy of VSG-based assay was found to be comparable to the microscopic examination method, and it demonstrated an ability to detect as low as 0.001% of the parasitaemia estimated by Giemsa staining. Moreover, when applying VSG for anti-malarial drug test, it was able to observe the growth inhibitory effect of dihydroartemisinin, the front-line drug for malaria therapy. Conclusions Taken together, the results of this study suggest the VSG-based flow cytometric assay to be a simple and reliable assay for assessing P. falciparum malaria development in vitro.

Abstract Suramin was introduced into the clinic a century ago and is still used to treat the first stage of acute human sleeping sickness. Due to its size and sixfold negative charge, uptake is mediated through endocytosis and the suramin receptor in trypanosomes is thought to be the invariant surface glycoprotein 75 (ISG75). Nevertheless, we recently identified a variant surface glycoprotein (VSGSur) that confers strong in vitro resistance to suramin in a Trypanosoma brucei rhodesiense line. In this study, we introduced VSGSur into the active bloodstream expression site of a T. b. brucei line. This caused suramin resistance and cross resistance to trypan blue. We quantified the endocytosis of different substrates by flow cytometry and showed that the expression of VSGSur strongly impairs the uptake of low‐density lipoprotein (LDL) and transferrin, both imported by receptor‐mediated endocytosis. However, bulk endocytosis and endocytosis of the trypanolytic factor were not affected, and the VSGSur‐expressors did not exhibit a growth phenotype in the absence of suramin. Knockdown of ISG75 was synergistic with VSGSur expression, indicating that these two proteins are mediating distinct suramin resistance pathways. In conclusion, VSGSur causes suramin resistance in T. brucei bloodstream forms by decreasing specific, receptor‐mediated endocytosis pathways.

This paper proposes an inductor current sensorless control strategy based on modified virtual synchronous generator (VSG) method for single-phase inverter-interfaced microgrid application. Firstly, the outer power loop with Q-U integral term is presented for output reactive power enhancement, and an inner voltage regulation loop based on luenberger state observer is proposed to reduce the inductor current sensor. Meanwhile, in order to improve the response speed of the observer, the luenberger observer is designed based on the optimal pole assignment method. Secondly, a seamless switching strategy based on the modified VSG method is proposed, which can realize the seamless transition between the island mode and grid-connected mode. Finally, a single-phase 3kW VSG prototype is built, and the effectiveness and correctness of the proposed control strategy is verified by the simulation and experimental results.

Trypanosoma brucei is the causative agent of human sleeping sickness. The parasites’ variant surface glycoprotein (VSG) enables them to evade adaptive immunity via antigenic variation. VSG comprises 10% of total cell protein and the high stability of VSG mRNA is essential for trypanosome survival. To determine how VSG mRNA stability is maintained, we used mRNA affinity purification to identify all its associated proteins. CFB2 (cyclin F-box protein 2), an unconventional RNA-binding protein with an F-box domain, was specifically enriched with VSG mRNA. We demonstrate that CFB2 is essential for VSG mRNA stability, describe cis acting elements within the VSG 3'-untranslated region that regulate the interaction, identify trans-acting factors that are present in the VSG messenger ribonucleoprotein particle, and mechanistically explain how CFB2 stabilizes the mRNA of this key pathogenicity factor. Beyond T. brucei, the mRNP purification approach has the potential to supply detailed biological insight into metabolism of relatively abundant mRNAs in any eukaryote.

In this study, virtual excitation emulation (VEE) is proposed along with the virtual inertia emulation (VIE) for a virtual synchronous generator (VSG) in a standalone microgrid to improve the AC voltage and frequency profile through the control of power electronic DC–AC converter in the microgrid. The VIE emulates the inertia and damping characteristics of a synchronous generator by mimicking the swing equation. The VEE is proposed to emulate the behaviour of the field circuit and exciter system of a synchronous generator. In addition, a restoration control is also added with the inertia and excitation emulation to restore the magnitude and frequency of the output voltage at the rated value and achieve isochronous control. A linearised small-signal model of the VSG with the proposed control technique is also presented to analyse the system stability and design the control parameters. The standalone microgrid is simulated in MATLAB/Simulink environment to show the effectiveness of the proposed control technique. From the simulation results, it is reflected that the proposed control technique improves the voltage and frequency response during transient and steady-state including the frequency nadir of the microgrid.

ABSTRACT Lucy Glover’s research focuses on the role of DNA repair and recombination in antigenic variation in the parasite Trypanosoma brucei, the causative agent of both human and animal African trypanosomiasis. In this mSphere of Influence article, she reflects on how “A CRISPR-based approach for proteomic analysis of a single genomic locus” by Z. J. Waldrip, S. D. Byrum, A. J. Storey, J. Gao, et al. (Epigenetics 9:1207–1211, 2014, https://doi.org/10.4161/epi.29919) made an impact on her research by taking the precision of CRISPR-Cas9 and repurposing it to look at single-locus proteomics. By using this technology in trypanosomes, Dr. Glover and her colleagues could study the dynamic accumulation of repair proteins after specific damage and gain insight into how the location of a double-strand break (DSB) dictates repair pathway choice and how this may influence immune evasion in these parasites.

The grid-connected inverter with virtual synchronous generator (VSG) control technology can improve the friendliness of a distributed power supply to the power grid. However, its low-voltage ridethrough (LVRT) capability is insufficient, which results in difficulties in limiting the current and provide reactive power support. A new LVRT control strategy based on the smooth switching is proposed in this paper. In this strategy, the voltage source mode of VSG is transformed into current source mode to limit the output current and provide reactive power support through the proportional resonance current control algorithm under grid fault. Furthermore, the feedback tracking synchronization strategy of the phase angle is employed to realize the smooth switching between two modes. When the grid fault recovers, it can directly switch back to grid-connected operation mode through a delay module without an additional algorithm. The simulation results verify the correctness and feasibility of the proposed control strategy.

Considering that the low‐voltage and small capacity ratings of the traditional distributed energy storage system (ESS) cannot meet the increasing capacity of distributed generations, this study describes a cascaded multilevel converter‐based ESS. In addition, the virtual synchronous generator (VSG) as a promising solution to enhance the system's inertia is studied. However, some studies indicate that VSG suffers from the inherent resonance near synchronous frequency under weak grid conditions, which may result in power oscillation and even deteriorate the system stability. In this work, the resonance mechanism is investigated with an established wide‐band small‐signal dynamic model. It is revealed that the resonance poles caused by the small impedance ratio on the AC side introduce a phase lag jump of 180°, which decreases the phase margin of VSG, leading to synchronous power oscillation (SPO). The influence of control parameters on the power oscillation is investigated with the eigenvalue analysis method. Accordingly, a differential feedforward compensation‐based SPO suppression strategy is proposed. The stability margins are significantly enhanced with this method. The validity of the SPO analysis and suppression method is verified with simulation and a 10 kW experimental prototype. [ABSTRACT FROM AUTHOR]

Background: Gold standard microscopic examination of Plasmodium falciparum intraerythrocytic stage remains an important process for staging and enumerating parasitized erythrocytes in culture; however, microscopy is laborious and its accuracy is dependent upon the skill of the examiner. Methods: In this study, ViSafe Green (VSG), which is a nucleic acid-binding fluorescent dye, was used for assessing in vitro development of P. falciparum using flow cytometry. Results: Fluorescence intensity of VSG was found to depend on the developmental stage of parasites. Specifically, multiple-nuclei-containing schizonts were observed in the VSGhigh population, and growing trophozoites and ring-shaped forms were observed in the VSGintermediate and VSGlow populations. The efficacy of VSG-based assay was found to be comparable to the microscopic examination method, and it demonstrated an ability to detect as low as 0.001% of the parasitaemia estimated by Giemsa staining. Moreover, when applying VSG for anti-malarial drug test, it was able to observe the growth inhibitory effect of dihydroartemisinin, the front-line drug for malaria therapy. Conclusions: Taken together, the results of this study suggest the VSG-based flow cytometric assay to be a simple and reliable assay for assessing P. falciparum malaria development in vitro. [ABSTRACT FROM AUTHOR]

Abstract Background Trypanosoma cruzi and Trypanosoma brucei are protozoan parasites causing Chagas disease and African sleeping sickness, displaying unique features of cellular and molecular biology. Remarkably, no canonical signals for RNA polymerase II promoters, which drive protein coding genes transcription, have been identified so far. The secondary structure of DNA has long been recognized as a signal in biological processes and more recently, its involvement in transcription initiation in Leishmania was proposed. In order to study whether this feature is conserved in trypanosomatids, we undertook a genome wide search for intrinsic DNA curvature in T. cruzi and T. brucei. Results Using a region integrated intrinsic curvature (RIIC) scoring that we previously developed, a non-random distribution of sequence-dependent curvature was observed. High RIIC scores were found to be significantly correlated with transcription start sites in T. cruzi, which have been mapped in divergent switch regions, whereas in T. brucei, the high RIIC scores correlated with sites that have been involved not only in RNA polymerase II initiation but also in termination. In addition, we observed regions with high RIIC score presenting in-phase tracts of Adenines, in the subtelomeric regions of the T. brucei chromosomes that harbor the variable surface glycoproteins genes. Conclusions In both T. cruzi and T. brucei genomes, a link between DNA conformational signals and gene expression was found. High sequence dependent curvature is associated with transcriptional regulation regions. High intrinsic curvature also occurs at the T. brucei chromosome subtelomeric regions where the recombination processes involved in the evasion of the immune host system take place. These findings underscore the relevance of indirect DNA readout in these ancient eukaryotes.

Suramin was introduced into the clinic a century ago and is still used to treat the first stage of acute human sleeping sickness. Due to its size and sixfold negative charge, uptake is mediated through endocytosis and the suramin receptor in trypanosomes is thought to be the invariant surface glycoprotein 75 (ISG75). Nevertheless, we recently identified a variant surface glycoprotein (VSGSur) that confers strong in vitro resistance to suramin in a Trypanosoma brucei rhodesiense line. In this study, we introduced VSGSur into the active bloodstream expression site of a T. b. brucei line. This caused suramin resistance and cross resistance to trypan blue. We quantified the endocytosis of different substrates by flow cytometry and showed that the expression of VSGSur strongly impairs the uptake of low‐density lipoprotein (LDL) and transferrin, both imported by receptor‐mediated endocytosis. However, bulk endocytosis and endocytosis of the trypanolytic factor were not affected, and the VSGSur‐expressors did not exhibit a growth phenotype in the absence of suramin. Knockdown of ISG75 was synergistic with VSGSur expression, indicating that these two proteins are mediating distinct suramin resistance pathways. In conclusion, VSGSur causes suramin resistance in T. brucei bloodstream forms by decreasing specific, receptor‐mediated endocytosis pathways. [ABSTRACT FROM AUTHOR]

• The microgrid lacks the necessary inertia and damping to reduce stability, so an VSG is used for control. • The neural network algorithm is used to adaptively adjust the inertia and damping of the virtual synchronous machine, which can flexibly adapt to the changes of the microgrid. • In order to improve the stability of the microgrid, P-f and Q-U double sag stability control is realized by adjusting the parameters. In microgrid systems, traditional virtual synchronous generator (VSG) face challenges in adaptively converting the virtual inertia J and damping coefficient D following system changes. This can lead to significant system frequency and voltage deviations, resulting in unreasonable power distribution. This paper proposes an artificial neural network (ANN)-based VGS dual droop control strategy tailored for microgrid systems. The study initially analyzes the influence of moment of inertia and damping on VSG, and subsequently establishes adaptive rules through angular frequency deviation and its change rate. By harnessing ANN algorithm, virtual coefficients are adjusted under different working conditions. This ensures stable f-p and Q-U dual droop control, minimizes frequency and voltage deviations, and facilitates optimal power allocation. The effectiveness of the proposed strategy is verified through MATLAB simulations. The microgrid system architecture predominantly consists of components such as a photovoltaic power generation unit, a battery energy storage unit, an inverter module, an LC filter module, and a VSG control module. [Display omitted] [ABSTRACT FROM AUTHOR]

Trypanosoma brucei causes human African trypanosomiasis and regularly switches its major surface antigen, Variant Surface Glycoprotein (VSG), to evade the host immune response. Such antigenic variation is a key pathogenesis mechanism that enables T. brucei to establish long-term infections. VSG is expressed exclusively from subtelomere loci in a strictly monoallelic manner, and DNA recombination is an important VSG switching pathway. The integrity of telomere and subtelomere structure, maintained by multiple telomere proteins, is essential for T. brucei viability and for regulating the monoallelic VSG expression and VSG switching. Here we will focus on T. brucei TRF and RAP1, two telomere proteins with unique nucleic acid binding activities, and summarize their functions in telomere integrity and stability, VSG switching, and monoallelic VSG expression. Targeting the unique features of TbTRF and TbRAP1′s nucleic acid binding activities to perturb the integrity of telomere structure and disrupt VSG monoallelic expression may serve as potential therapeutic strategy against T. brucei.

Recently, the virtual synchronous generator (VSG) has drawn considerable attention for its function of simulating the electromechanical transient of SG to make converters equip with the characters of inertia, damping, voltage regulating, and frequency regulating. Energy storage systems (ESSs), which are connected to the converter by a DC/DC circuit, are the basis of VSG. The output active power of VSG is restricted by the capacity margin of the grid-side converter and DC/DC circuit, and the output energy is bounded by the energy margin of ESS, which means that VSG is restrained by treble constraints: DC side power, DC-side energy, and AC-side power. A small signal model of VSG is built, and the method of getting the treble constraints is proposed. Through the analysis of the dynamic response characteristics of the VSG, the operating zone of VSG under constraints is confirmed. Simulations in power system computer aided design (PSCAD)/electromagnetic transient including DC (EMTDC) verified the theoretical analysis and the transient characteristics of VSG when the constraints are broken and simulated.

Modular multilevel converter-battery energy storage system (MMC-BESS) has a good engineering application. When MMC-BESS is connected to the grid, the real-time phase angle of grid is an important parameter. When MMC-BESS is connected to the grid, a simulation model based on virtual synchronous generator (VSG) is built in MATLAB. The results show the control algorithm based on VSG can improve the dynamic response of the system and stably detect the phase angle of the grid.

Since the concept that inverters can be controlled to behave like a synchronous generator has been proposed, the virtual synchronous generator (VSG) started to become the mainstream control of distributed energy resources connected distributed system. This concept simulates the frequency and voltage regulation of synchronous generator to improve the overall stability of the medium- and low-voltage power system. Although significant research works have considered the application of VSGs in active and reactive power sharing, it seems to be that the issue of the proportional power sharing of VSGs of different capacities has not considered much in the previous works, especially when the distribution lines have increased resistive components. Such resistive disturbs the power flow and causes the power coupling. When focusing eyes on the parallel operation, VSGs need to coordinate parallel operation to increase the microgrid power quality, reliability, and for easy maintenance. Based on an improved droop control and virtual complex impedance, this study adjusts PI controller parameters to implement the proportional power sharing and decoupling for VSGs of different capacities. Simulation results are shown to verify the feasibility and validity of the control method.

Recently, the virtual synchronous generator (VSG) has drawn considerable attention for its function of simulating the electromechanical transient of SG to make converters equip with the characters of inertia, damping, voltage regulating, and frequency regulating. Energy storage systems (ESSs), which are connected to the converter by a DC/DC circuit, are the basis of VSG. The output active power of VSG is restricted by the capacity margin of the grid-side converter and DC/DC circuit, and the output energy is bounded by the energy margin of ESS, which means that VSG is restrained by treble constraints: DC side power, DC-side energy, and AC-side power. A small signal model of VSG is built, and the method of getting the treble constraints is proposed. Through the analysis of the dynamic response characteristics of the VSG, the operating zone of VSG under constraints is confirmed. Simulations in power system computer aided design (PSCAD)/electromagnetic transient including DC (EMTDC) verified the theoretical analysis and the transient characteristics of VSG when the constraints are broken and simulated. [ABSTRACT FROM AUTHOR]

Modular multilevel converter-battery energy storage system (MMC-BESS) has a good engineering application. When MMC-BESS is connected to the grid, the real-time phase angle of grid is an important parameter. When MMC-BESS is connected to the grid, a simulation model based on virtual synchronous generator (VSG) is built in MATLAB. The results show the control algorithm based on VSG can improve the dynamic response of the system and stably detect the phase angle of the grid. [ABSTRACT FROM AUTHOR]