The dc-link capacitors in back-to-back (BTB) converters perform as a necessary dc-link voltage supporter and power buffer for both sides. However, the dc-link capacitor is normally over-designed due to performance considerations, causing a larger volume and higher cost. This article proposes a more comprehensive and accurate design scheme of the dc-link capacitance for the BTB three-level neutral-point-clamped (NPC) converter, achieving the dc bus lightweight design. Multidimensional aspects are considered when quantifying the capacitance constraints, including small-signal interactive stability, dc-link voltage fluctuation facing a given power step change from both ac sides, and switching ripple filtering performances. Corresponding to the above multidimensional aspects, the stability constraint, response speed constraint, and filtering performance constraint of the dc-link capacitance and the control parameter value are analyzed. In addition, the active capacitor is adopted to suppress the NP voltage low-frequency oscillation and to avoid the unfavorable boundary reduction of the dc-link capacitance selection. The effectiveness of the proposed lightweight dc-link capacitance design scheme is validated through simulations and experimental results. The experimental results indicate that the dc-link capacitance of each converter can be reduced to 10 & micro;F in the 2-kW experimental BTB converter, realizing over 60% volume reduction of the dc bus compared with conventional designs

In the low-voltage distribution network (LVDN), the large-scale integration of single-phase load and new energy leads to continuous three-phase voltage unbalance. Phase switch devices (PSDs) and static var generators (SVGs) are two effective devices for voltage unbalance mitigation. However, few studies have reported that via SVGs and PSDs together to reduce the unbalance of LVDN. In this paper, a coordinated optimization strategy integrating PSDs and SVGs is proposed based on the non-dominated sorting genetic algorithm II, aiming to mitigate voltage unbalance in the LVDN. The strategy simultaneously optimizes the phase sequence and the SVG compensation current to achieve a trade-off between the three objectives: minimizing the average voltage unbalance factor (VUF), reducing phase-switching times, and minimizing the total SVG output capacity. Simulation results on an IEEE-18 node system show that the proposed method can effectively mitigate multi-node voltage unbalance while reducing both PSD switching operations and SVG capacity occupation, which demonstrate that the optimization approach effectively enhances voltage balance and operational efficiency.

In contemporary distributed power systems, the proliferation of power electronics and dispersed non-linear loads causes increasingly severe harmonic issues. The traditional point-to-point compensation method requires the deployment of numerous control devices, leading to high costs and suboptimal system-wide mitigation outcomes. To overcome these limitations, this paper proposes a dual time-scale coordinated harmonic mitigation strategy employing a few active power filters (APFs). On a short time-scale, a multi-bus harmonic mitigation model is developed and solved using nonlinear programming and graphical auxiliary analysis. This yields the current references for the APFs, ensuring that the harmonic distortion of each bus remains below predefined limits. On a long time-scale, a multi-objective genetic algorithm is employed to dynamically determine specific, system-wide limits for each bus, optimizing harmonic voltage distortion, harmonic power loss, and total APF capacity. Finally, simulation results based on an 18-bus system verify that the strategy achieves well-balanced multi-objective mitigation performance and dynamic response capability.

The software-based overcurrent capability enhancement for the grid-connected converter is essential in actual engineering applications. An optimal operation parameter determination methodology for most efficiently enhancing overcurrent capability is proposed in this article, utilizing the DC-link voltage, switching frequency, and the modulation strategy as variables. Firstly, the mechanism of the non-monotonic relationship between overcurrent capability and switching frequency/DC-link voltage is elucidated, from the perspective of the output current switching ripple and the thermal distribution. Secondly, the parallel-computable loss distribution calculation model is designed, considering the output current switching ripple, ensuring both accuracy and speed in computation. Thirdly, the optimal operation determination methodology is introduced based on the above loss distribution calculation model. Combined with the actual analysis case, the optimal DC-link voltage and the switching frequency are presented by visualizing the variation behavior of the maximum loss concerning the operation parameters. Finally, the simulation and the experimental results verity the precision of the trend of the maximum loss variation and the optimality of the selected operation parameters. Based on the verification case, an improvement of 50% in overcurrent capacity can be achieved by adopting the proposed method.

With the increased penetration of renewable energy sources, the grid-forming (GFM) energy storage (ES) has been considered to engage in primary frequency regulation (PFR), often necessitating the use of a frequency deadband (FDB) to prevent excessive battery charging cycling and miti-gate frequency oscillations. Implementing the FDB is relatively straightforward in grid-following (GFL) control. However, implementing the FDB in GFM control presents a significant challenge since the inverter must abstain from providing active power at any frequency within the FDB. Therefore, in this paper, the performance of PFR control in the GFM-ES inverter is analyzed in detail first. Then, the FDB is implemented for GFM inverters with various types of synchronization methods, and the need for inertia response is also considered. Moreover, given the risk of oscillations near the FDB boundary, different FDB setting methods are proposed and examined, where an improved triangular hysteresis method is proposed to realize the fast response and enhanced stability. Finally, the simulation and experiment results are provided to verify the effectiveness of the above methods.

With the advancement of power electronics, power systems exhibit a “double-high” trend of highpenetration power electronics. Among them, the converters serve as the foundation for integrating multiple energy sources. However, conducting a thorough analysis of its wide-frequency characteristics presents a significant challenge. Accurate widefrequency coupling models are essential for analyzing the ability of power electronic equipment to withstand wide-frequency perturbations and determining the acceptance of new energy sources in the grid. Yet system complexity and device diversity require different modeling approaches, which makes traditional methods impractical due to their complexity. To achieve a balanced compromise between model relatability and accuracy, this paper proposes a backpropagation neural network with the embedded adaptive denoising training mechanism (EADT) method for harmonic disturbance coupling analysis in a singleconverter scenario. The approach eliminates reliance on detailed system parameters, enabling direct harmonic disturbance analysis via trained models to determine current coupling characteristics.

The DC bus neutral point of the three-level neutral point clamped (3L-NPC) converter is always connected to the neutral wire of the power system due to the requirement of zero-sequence output current, based on which the additional common resonance loop via the neutral wire will be introduced. The analysis of this introduced common-mode resonance is presented in this article, and the neutral point voltage fluctuation model is established considering the coupling of multi-frequency. The proposed neutral point voltage fluctuation model reveals that the output current of the 3L-NPC causes the fluctuation excitation sources of different frequencies, and the resonance will occur when the DC capacitors and the line inductance form a resonance loop at exactly this frequency. An additional extra active filter circuit is proposed in the article to suppress the fluctuation excitation sources, and the common-mode resonance won't be motivated as a matter of course. The control strategy and the stability are analyzed based on the structure of the proposed active filter circuit. Finally, simulation results verify the accuracy of the derived neutral point voltage fluctuation model and the effectiveness of the proposed active filter circuit.

The dynamic interaction between voltage-source converters (VSC) and the grid may lead to high-frequency destabilization (HFDS). It has been shown that HFDS can be effectively avoided if the VSC input admittance (VIA) is compensated to be passive at all the frequencies below Nyquist frequency (NF). The effective approaches, auxiliary current feedback and advanced control sequence (i.e., oversampling or delayed sampling), are limited for cost-sensitive applications. Without the above approaches, full-range passive VIA is hard to achieve, even if existing self-tuning active damping methods are used. Therefore, this article proposes an adaptive parameter switching voltage-feedback active damping control (AFVC) method. Both positive and negative feedback are used to design preset AFVC parameters, making VIA passive in different ranges, which overlap and cover the frequencies below NF. Parameter switching is triggered by fast Fourier transformation (FFT). The data sequence length of FFT is designed to balance the speed and frequency spectrum resolution. Sampling frequency aliasing and PWM sideband frequency coupling challenge the admittance/impedance based stability analysis around NF, so they are modeled and proved to be negligible for high-frequency stability. The effectiveness of the proposed method is verified by both simulation and experiment.

The widespread presence of single-phase loads and increasing integration of distributed generation resources in local distribution networks lead to significant voltage unbalance (VU). Phase-switching schemes are constrained by the compensation capacity dependency on connected singlephase loads, while SVG-based active compensation demonstrate inadequate capability in addressing multi-bus VU exceeding limits with high investment costs. Hence, this paper proposes a novel coordinated control strategy of PSDs and SVGs aiming at multi-bus voltage unbalance mitigation. This strategy synergizes the load-side mitigation capability of phase-switching with the dynamic compensation advantages of SVG. The method effectively addresses multi-bus voltage unbalance while reducing PSD switching frequency and SVG output capacity requirements.

The hybrid soft open point(HSOP) adopts the structure of mechanical switch and soft open point(SOP) in parallel, which can reduce the capacity of the converter. It’s commonly used for power flow control and load transfer between two distribution networks. However, when one end of the distribution network loses power, the operation delay of the mechanical switch will cause the load power failure during the load transfer. In view of this, a novel HSOP consisting of back-to-back voltage source converters(BTB-VSC), a mechanical switch, and an anti-parallel thyristor is proposed. The proposed HSOP can avoid the load power failure caused by the mechanical switch operation delay compared to the traditional HSOP, and its feasibility is verified by simulation.

The decentralized distribution of nonlinear harmonic loads drives the trend of harmonic control from local compensation to system-level mitigation. Based on the active power filter (APF) with multibus harmonic detections, comprehensive control of harmonic distortions in multiple buses achieves, which is so-called system-level harmonic mitigation. However, existing system-level harmonic mitigation methods require high-speed data exchange for real-time phase synchronization among multiple buses, which reduces the feasibility of the methods. To reduce the burden of communication, a system-level harmonic mitigation method without interbus phase synchronization is proposed in this article. First, the phase synchronization problem is analyzed, and the control system of multibus detected APF is proposed, in which the reference phase of each detector could be set locally. Then, a new online system-level harmonic mitigation model is introduced and solved to obtain the optimal APF current reference. Subsequently, the closed-loop control strategy of multibus detected APF is raised and four control stages are designed to cope with different situations. Finally, an eight-bus network with dynamic harmonic loads is designed in simulation and experiment. According to the results, multibus harmonic voltage distortions in the network could meet the standard. The proposed method contributes to achieving good harmonic compensation performance without multibus phase synchronization.

The monitoring and estimation of harmonic is of great significance to maintain the stable operation of distribution networks. In order to determine the harmonic state level in network and reduce the investment and difficulty of installation, it is necessary to carry out research on the optimization of meter configuration. Therefore, this paper proposes a method of optimal meter configuration combining harmonic state estimation model and binary particle swarm optimization, which aims at estimating all harmonic state variables in the network while using fewer meters. Harmonic state estimation is the process of estimating the harmonic voltage and current state with limited data collected by meters according to the harmonic state estimation model. The rules of state estimation model are established based on the topology connection in network and Kirchhoff's law. IEEE-18 bus network is employed to apply the state estimation model and the result of optimization algorithm shows that only six meters are required to measure and estimate all the harmonic state variables. Finally, the real harmonic values of this network are obtained from Simulink/MATLAB to compare with the estimation values, which verifies the effectiveness of the proposed method.

With the massive and distributed access of nonlinear loads to distribution networks, traditional point-to-point harmonic compensation method becomes inefficient. Meanwhile, the demand for system-level harmonic control by few shunt active power filters (SAPFs) has emerged, which lays emphasis on the optimal allocation and coordination of SAPFs, in terms of location, capacity, and output. Existing allocation algorithms suffer from local minimum, characterized by excessive locations and unreasonable capacity assignment of SAPFs. Therefore, this article introduces a new nonlinear loads partition model and a cluster-based SAPFs allocation algorithm. First, the existing basic SAPFs allocation model is reviewed for problem description. By reconstructing the existing model, a three-level nonlinear loads partition model is built, in which the essence of system-level harmonic control is revealed and the harmonic compensation distance between SAPF and the nonlinear load is defined. Based on this distance, a modified K-means clustering algorithm is implemented to partition nonlinear loads and obtain the optimal allocation of SAPFs. Finally, the IEEE 18-bus standard system and its simplified eight-bus network are designed, respectively, in simulation and experiment. Results verify that the proposed SAPFs allocation method has the advantages of less location, less total capacity, and good harmonic suppression performance.

Conventional linearized-model-based system-level control strategy of active power filter (APF) has poor dynamic control performance when load changing happens. Therefore, a four-layer neural network is built to learn the convergence behavior of the linearized system-level harmonic mitigation model. Then, a deep-learning-based control strategy is proposed to achieve fast mitigation of multi-bus harmonic voltages by single APF. Finally, an eight-bus system with distributed harmonic loads is built in simulation. Simulation results proves the good dynamic performance of the proposed method. Moreover, compared with conventional implementation of deep learning method in system-level harmonic control, the proposed method benefits in lower data demand and simplified training process.

With the rapid increase of the type and quantity of nonlinear loads in networks, the required amount of shunt active power filters (SAPFs) becomes large. To reduce the maintenance cost of harmonic compensation, the optimal installation and system-level control of SAPF is proposed to achieve the comprehensive harmonic mitigation in a distribution network. Firstly, a SAPF extended-range compensation model is built to evaluate the harmonic compensation distance and harmonic compensation sensitivity of multi-bus harmonic disturbance sources. Then, the optimal installation algorithm and system-level control scheme of SAPF are designed. Finally, the simulation result based on the IEEE 18-bus network with distributed nonlinear loads verifies the effectiveness of the proposed SAPF installation strategy in good harmonic voltage suppression performance and small installation capacity.

With the development of high-proportion renewable energy systems, the number of distributed nonlinear loads has been increasing, which causes a growing harmonic problem in the distribution network. In this situation, the traditional point-to-point harmonic compensation method is inefficient, since it needs to configure plenty of control equipment. This article presents a multi-objective optimal location algorithm of multiple shunt active power filters (SAPFs), considering that some harmonic sources are unknown in the distribution network. Firstly, based on the measurement equations, the least-square method is used to estimate the unmeasured power quality data, so as to improve the accuracy of the multi-bus harmonic mitigation model. Secondly, a multi-objective genetic algorithm is proposed for determining the Pareto-optimal set of the equipment layout to suppress multi-bus harmonic distortion. Finally, a standard IEEE-18 buses simulation model with 6 nonlinear loads is built to verify the proposed optimal location strategy. The simulation results show that the proposed algorithm can ensure that harmonic distortion indexes of each bus meet the recommended standards with few devices and total capacity.

Recently, with the development of the smart distribution network with the advanced metering infrastructure, the requirement of comprehensive evaluation with mass data and global improvement of power quality has emerged. However, most comprehensive evaluation methods only focus on the score of power quality without considering the role of comprehensive evaluation in the global improvement strategy of power quality control devices. This paper proposes a complete global configuration strategy of shunt active power filters (SAPFs), which utilizes the evaluation scores as a part of the objective function. Then, an IEEE 18-bus test system with non-linear loads and unbalance loads is employed in simulation to verify the feasibility of the proposed comprehensive evaluation algorithm and global configuration strategy. After configuring SAPFs according to the result, total power quality indicators in the system can meet the recommended standards.

With the increase of photovoltaic (PV) penetration, the power beyond the demand may cause the voltage violation problem in distribution networks. On the other hand, due to the regulation ability of reactive power to voltage, this problem can be solved based on PV residual capacity. However, as long as one single PV inverter reaches the upper limit of power capacity, although other PV inverters have reactive power margin, the grid will still have the problem of voltage violation. Therefore, a reactive power margin management method of multiple PV inverters is proposed to comprehensively improve the effect of voltage regulation. Firstly, a high PV penetration distribution network model is built to analyze the relationship between PV reactive power margin and power flow. Then, the local PV control method and the reactive power margin optimization method are combined to realize voltage regulation. Finally, a five-bus distribution network is built in simulation and the results verify the rationality of reactive power margin management and the effectiveness of voltage regulation.

随着我国的能源转型进入关键时期,分布式光伏在配电网的大规模应用已成为重要趋势.然而在光伏功率过剩时可能引起配电网过电压、逆潮流等问题,威胁配电网的安全稳定运行.传统配电网调压方法如有载调压、定点无功补偿、大规模弃光等,存在调压性能有限、经济性差、光伏利用率低等缺陷,不再适用于含大量分布式光伏的配电网.因而本文充分发掘分布式光伏的调控潜力,从单机调控与多机协调两方面优化配网台区潮流,从而在提升光伏发电利用率的同时,实现对配电网过电压平抑.文章在配电网电压灵敏度分析的基础上,首先对分布式光伏系统运行的多种模态进行划分,在优化了传统的光伏无功控制方案基础上,进一步加入了有功功率削减模式,实现单机光伏系统对连接点电压的控制与优化;同时考虑单机调控能力受限时,依据灵敏度排序进行分布式光伏间的协调优化,通过单机与多点多机两个层面的协调优化与综合调控的配电网台区的电压优化控制.经验证,本文所提的分布式光伏系统优化协调算法,能够提高电压调节的效率,并且有效减少分布式光伏系统的弃光率.