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In recent years, with a large number of distributed energy sources connected to the grid, the distribution network has shown a clear trend towards the use of power electronics. As a power conversion interface, the stability of the converter's interaction with the grid has received considerable attention. Adequate admittance (impedance) modeling of the three‐phase grid‐connected converter is a prerequisite for analyzing the stability and resonance characteristics of the grid‐connected system. Understanding the internal mechanisms and proposing effective solutions are crucial to enable power systems to effectively cope with the impact of power electronics. Therefore, by studying the admittance equivalence relationship and equivalent circuit of three‐phase AC systems in various coordinate systems, this paper focuses on solving the non‐uniqueness problem of impedance admittance modeling of three‐phase converter system. It is proposed that the input admittance of the three‐phase asymmetrical system in the dq and fb rotating coordinate systems is a 2‐order matrix and equivalent. The input admittance of the three‐phase unbalanced system in the αβ and “12” static coordinate systems is a 4‐order matrix and equivalent. From both theoretical analysis and simulation verification, it is shown that the number of eigenvalues of the three‐phase asymmetrical system in the stationary coordinate system is twice that in the rotating coordinate system, and the overall damping of the system is improved. The control strategy based on the stationary coordinate system will make the system more stable. The second contribution of this paper is to establish the equivalent circuit of the three‐phase symmetric converter system by the node admittance matrix and extend it to the multi‐machine system to explain the system instability mechanism by the circuit resonance mechanism. Finally, the frequency coupling phenomenon is studied for the three‐phase asymmetrical converter system, and it is explained that it cannot establish an equivalent circuit similar to the three‐phase symmetrical converter system. [ABSTRACT FROM AUTHOR]

The application of virtual synchronous generators (VSGs) in power systems is gradually increasing to address the low inertia issue caused by the high penetration of renewable energy sources. Besides, the stability of microgrids (MGs) is threatened by constant power loads (CPLs) consisting of power electronic converters with closed‐loop controllers. To investigate the stability of the MG composed of VSG and CPL, this paper analyzes the operational principles of VSG and CPL, revealing that only the bipolar switch logic function is suitable for establishing a closed‐loop model if the output of the inner‐loop controller is considered as the duty cycle of the PWM signal. Furthermore, the closed‐loop impedance models of VSG and CPL were established in the dq frame. Impedance‐based analysis results show that the MG is prone to oscillation in the mid‐band. Therefore, the active power loop parameters of the VSG have a small impact on the stability of the MG, while the LC filter and inner loop controllers have a greater impact on the stability. Based on the oscillation mechanism and passivity theory, the impedance reshaping strategy for VSG and CPL is proposed. It is verified that the proposed control strategy can significantly improve the stability of the MG through the comparison of simulation results and theoretical analysis. [ABSTRACT FROM AUTHOR]

Summary: The large integration of distributed generation (DG) and electric vehicles (EVs) facilitates daily life and reduces carbon emissions. However, it brings problems such as increased uncertainty and power electronic permeability to the distribution network. In this context, it is important to solve the line loss optimization problem as closely as possible to the real situation. Therefore, this paper solves the line loss optimization problem based on a probabilistic model of three‐phase unbalanced modern power electrical distribution network. Firstly, the paper proposes a frequency domain model of the distribution network and the optimization problem model. Then, to deal with the uncertainty of the system, the point estimation method (PEM) and Monte Carlo Simulation (MCS) are used. Finally, this paper proposes two optimization schemes considering the power quality when single and multiple DGs are integrated. The simulation results demonstrate that the PEM using the Gram–Charlier series expansion of the fourth‐order statistical moments to estimate the probability density function (PDF) is significantly less time consuming than the traditional MCS method while maintaining accuracy. The comparative analysis reveals that under the optimization scheme with single DG integration, the expected value of system line loss and expected line loss rate are reduced from 283.95 kW to 218.96 kW (22.89% reduction) and from 13.44% to 10.68%, respectively. Under the dual DG integrated optimization scheme, the expected value of line losses and expected line loss rate are reduced from 283.95 kW to 208.88 kW (26.4% reduction) and from 13.44% to 10.25%, respectively. However, after taking the uncertainty into account, the operational reliability of the system has been enhanced under different scenarios. In addition, it is found that the optimization approach incorporating multiple DG units can effectively mitigate the system line loss under diverse operational scenarios while enhancing the system's DG integration capability. [ABSTRACT FROM AUTHOR]

SUMMARY The cascaded H-bridge (CHB) multilevel inverter is being recognized as the most suitable topology for high-power medium-voltage power quality conditioning applications. This paper presents mathematical modeling and effective controller design methodology for the CHB-based active power filters (APFs), which achieves dynamic reactive power and harmonic compensation. The most crucial problems in CHB-APF control are the simultaneous requirements of both accurate harmonic current compensation and the dc-link voltage stabilization among the H-bridges, which is the prerequisite for the stable operation of CHB-APF. To achieve dc-link stabilization, a novel voltage balancing algorithm is proposed by splitting the dc-link voltage control task into two parts, namely, the average voltage control and the voltage balancing control, where the sine and cosine functions of the phase angle of the fundamental component of the grid voltage are used, respectively. To ensure accurate phase tracking, a novel phase-locked loop (PLL) is proposed by using the adaptive linear neural network (ADALINE), where the grid voltage background distortion is also taken into account. The superior performance of the ADALINE-PLL is validated by comparison with the existing PLLs in literatures. Furthermore, the proportional-resonant (PR) controller is used for the reference current tracking. A separate ADALINE algorithm is applied for reference current generation (RCG) for the CHB-APF. The excellent performance of the ADALINE-based RCG scheme is verified by comparison with the existing RCG schemes, namely, the low-pass filter approach and the single-phase p − qmethod. The experimental results on the three modules CHB-APF are presented, which verifies the effectiveness of the proposed control algorithms. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]