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To support the “carbon peak and carbon neutrality” goal, new energy is poised to explode, and new energy power generation converter is simultaneously facing new challenges. The conventional current-controlled new energy converter can quickly transmit active power on the DC bus to the power grid. However, for the weak grid, the stability margin of the converter grid-connected system is reduced on the one hand, which can easily cause resonance oscillation, on the other hand, the current controlled converter cannot actively respond to system frequency and voltage fluctuation to offer support. The voltage controlled virtual synchronous generator (VVSG) is used to improve system small signal stability and frequency stability, however, its power response speed is too slow to meet the requirements of fast following power command. Although a voltage/current dual-mode switching control scheme is put forward to achieve characteristics complementary of current controlled converter and voltage-controlled converter, the control structure switching and intermediate variable following is required to realize mode switching, which is prone to large power shocks and switching failures. In view of the problem, a direct power control strategy based on VVSG is proposed. The control structure is raised based on a conventional VVSG outer active power control loop, and the output active power and frequency characteristics are analyzed. Compared with the voltage and current dual-mode control, VVSG with direct power control can perform large inertia characteristic in the weak grid and fast power following characteristics in the strong grid by adjusting λ, and without control structure switching and intermediate variable following. Moreover, the two characteristics can be smoothly transited. In addition, the active support ability of voltage source can be maintained under both characteristics. Finally, the effectiveness of the proposed control strategy is verified through simulation results.

With increasing penetration of renewable energy sources into distributed power systems, multi-parallel inverters are commonly employed in the interface to the utility grid, giving rise to potential resonance problems. Aiming at this issue, common-mode resonance of single inverter and multi-parallel inverters are analyzed. First, single inverter common-mode circuits model is built. Common-mode paths are found in it and the admittances of each path are analyzed. Based on it, multi-parallel inverters equivalent circuits are modeled and analyzed. In order to investigate common-mode resonance characteristics, self-coupled and mutual-coupled paths are analyzed and compared to single inverter's path. To get more accuracy resonance characteristics, a crossover filter function is introduced and used to separate output common-mode currents at different frequencies for two-parallel inverters. The impact of parasitic capacitance on resonance is examined. Experimental results validate the developed models and the proposed resonance characteristics.

With the growth of renewable energy, grid-connected inverter as the interface has gradually increased its application. However, with the large-scale connection of inverters, resonance and stability problems have also been brought to distributed systems. Aiming at this issue, in this paper, a multi-parallel equivalent small signal model based on the structure of the multi-inverter grid-connected system is established firstly, and the expression of the output current of each inverter is obtained. According to the phase characteristics of the inverter’s output at different frequency bands, the output current expressions for synchronized and interleaved control period are further obtained. On this basis, influences of penetration, control gain and delay on harmonic or resonance are analyzed. Finally, a platform of four 30kW inverters is setup, and "resonance stability margin" method is proposed to verify the harmonic resonance characteristics of the inverter output current under high penetration.

In recent years, virtual inertia algorithms are mostly applied to inverters of high-penetration distributed generation systems to improve system equivalent inertia. When distributed generators (DG) based on traditional first-order virtual inertia (TFVI) algorithm operates in weak grid connected mode, there is contradiction between its output active power steady state and dynamic characteristics adjustment. Although the virtual inertia with improved structures such as first-order virtual inertia based on differential compensation (DCFVI) and second-order virtual inertia (SVI) have better active power control accuracy and power oscillation damping ability simultaneously, its frequency support ability in high permeability distributed generation system is weaker due to its fast frequency response and small inertia in the initial stage of response, which is easier to cause under-frequency load shedding. In this paper, firstly, the frequency characteristics of the DG based on the above virtual inertia algorithms operating in islanded mode are analyzed, and the active power steady-state and dynamic characteristics of the DG operating in grid connected mode are studied as well. Afterwards, based on the active power steady-state and dynamic characteristics comparison and summary of DG grid-connected system with each improved virtual inertia algorithm, through optimization of the second-order virtual inertia structure, an optimized second-order virtual inertia (OSVI) algorithm is proposed, which can not only maintain the active power control accuracy and suppress power oscillation, but also can provide larger frequency support capability. Finally, the effectiveness of the proposed control strategy is verified by simulation.

The virtual inertia and virtual damping affect both the dynamic stability of the virtual synchronous generator(VSG) and the configuration of energy storage, but there is a conflict between them while selecting the virtual inertia and virtual damping. An optimal coordination control strategy of micro-grid inverter and energy storage based on variable virtual inertia and damping is proposed to mitigate this conflict. With the integrated optimal constraint of the VSG frequency variation and the energy storage capacity, the virtual inertia and damping of VSG are configured dynamically, adopting linear quadratic optimal control. It can suppress the oscillations of the active power and frequency to improve the stability of VSG and optimally configure the energy storage capacity of VSG simultaneously. The proposed strategy aims to improve the control performance of micro-grid inverter and the system economy. The simulation and experimental results verify the effectiveness of the strategy.

This paper investigates the use of a virtual synchronous generator (VSG) to improve frequency stability in an autonomous photovoltaic-diesel microgrid with energy storage. VSG control is designed to emulate inertial response and damping power via power injection from/to the energy storage system. The effect of a VSG with constant parameters (CP-VSG) on the system frequency is analyzed. Based on the case study, self-tuning algorithms are used to search for optimal parameters during the operation of the VSG in order to minimize the amplitude and rate of change of the frequency variations. The performances of the proposed self-tuning (ST)-VSG, the frequency droop method, and the CP-VSG are evaluated by comparing their effects on attenuating frequency variations under load variations. For both simulated and experimental cases, the ST-VSG was found to be more efficient than the other two methods in improving frequency stability.

Mega-scale solar projects are turning to string inverters nowadays. Multi-parallel string inverters are employed as the interface to the utility grid, giving rise to potential unstable problems. Aiming at this issue, the relationship between number of inverters, output power and grid impedance is analyzed. Based on this, according to different topology, control structures and filter parameters, four types of Standard Equivalent Model (SEM) for multi-inverters system are established. By analyzing the stability margin of the four SEMs and the sensitivity of their control parameters, the difference of stability between the string inverters and the central inverters is compared. Finally, the established SEM and the comparison results of the string and central inverters are verified by simulation and experimental results.

In view of the contradiction between active power steady-state and dynamic characteristics regulation of VSG based on the existing first-order virtual inertia algorithm operating in grid- connected mode and multi-paralleled mode, a virtual inertia control strategy based on multi-stage lead-lag compensation is proposed in this paper. By cascading a multi-stage lead-lag link in the first -order virtual inertia forward channel, the control freedom of VSG grid-connected and paralleled system is increased, so that the zero and poles can be freely configured. Furthermore, a first-order lead-lag link is taken as an example to analyze its effect on the output active power steady-state and dynamic characteristics of VSG grid-connected system and paralleled systems. Through reasonable configuration of the second-order virtual inertia parameters, the system damping ratio can be increased and the power oscillation can be eliminated while its steady-state characteristics are not changed under the circumstance of a large inertia. Experimental results verify the effectiveness of the proposed strategy.

For the problem that paralleled inverters cannot share reactive power proportional to their power capacity in islanding microgrid. Originally, the effect of inverter output impedance and line impedance to reactive power sharing with conventional droop control is analyzed. Following, a method adjusting the output reactive power by paralleling capacitor at the inverter output is proposed, and the relationship between paralleled capacitors, line impedance and reactive power is analyzed. On the basis, virtual capacitor method is realized to improve reactive power balance. Finally, the proposed method and analysis are verified through simulation.

Stochastic modeling is a rapidly evolving, popular approach to the study of the uncertainty and heterogeneity of groundwater systems. However, the use of Monte Carlo-type simulations to solve practical groundwater problems often encounters computational bottlenecks that hinder the acquisition of meaningful results. To improve the computational efficiency, a system that combines stochastic model generation with MODFLOW-related programs and distributed parallel processing is investigated. The distributed computing framework, called the Java Parallel Processing Framework, is integrated into the system to allow the batch processing of stochastic models in distributed and parallel systems. As an example, the system is applied to the stochastic delineation of well capture zones in the Pinggu Basin in Beijing. Through the use of 50 processing threads on a cluster with 10 multicore nodes, the execution times of 500 realizations are reduced to 3% compared with those of a serial execution. Through this application, the system demonstrates its potential in solving difficult computational problems in practical stochastic modeling.