Aiming at solving the problem of insufficient damping and resonance for two-stage PV converter systems with LCL filters under weak grid conditions, a novel bilateral active damping control strategy based on the feedbacks of both DC capacitor voltage and AC filter capacitor current is proposed. The capacitor current of the LCL filter is used as feedback to realize the zero pole cancellation of the transfer function, reduce the order of system equivalent model, track the migration of the resonant point, and suppress the resonant peak. A state observer is used to measure the filtered capacitor current instead of a sensor to reduce the system cost. Furthermore, the capacitor voltage of the inverter DC side is used as feedback to the inverter voltage control loop. Therefore, the fluctuation of the inverter DC voltage is suppressed and the resonant risk can be reduced when the PV output varies. The resonant characteristics of a PV two-stage LCL converter system with the proposed damping strategy are analyzed under different weak grid situation. Theory analyses, simulations, and experiment results all verify that when compared to the traditional unilateral damping strategy of capacitive current feedback, the proposed bilateral active damping strategy has a better damping effect.

Utility harmonic impedance estimation is critical for power quality assessment and improvement. Noninvasive methods without injecting harmonics are widely adopted to estimate utility harmonic impedance using natural load variations. However, background harmonic voltage fluctuations and abrupt harmonic impedance changes can lead to significant errors in utility harmonic impedance estimation. In this paper, a new noninvasive method is proposed to solve the above problem. To overcome the errors in harmonic impedance estimation caused by background harmonic voltage fluctuations, a time series clustering (TSC) method based on the cross-correlation principle is proposed to filter harmonic data. Moreover, an improved Pettitt method is proposed to identify the change points of harmonic impedance. Finally, the self-born weighted least squares (SBWLS) method is used to calculate harmonic impedance by iteratively weighting the anomalous data to weaken its influence, thereby improving the accuracy of the utility harmonic impedance. Simulation and field results validate the proposed method.

ObjectiveIn the microgrid under islanding mode, factors such as local load shedding or variations in line impedance cause differences in system equivalent impedance, which makes it challenging for parallel inverters using droop control to achieve accurate reactive power sharing. A reactive power sharing control strategy for islanded microgrids with both frequency and amplitude compensation functions is proposed to address this issue.MethodsFirst, analyzing the power transmission characteristics of the droop-controlled parallel inverter indicated that the active power output of the inverter was not affected by the line impedance. When the active power droop coefficient was inversely proportional to the inverter capacity, active power sharing was achieved. However, the reactive power output of the inverter was not only related to the reactive power droop coefficient but also affected by the line impedance. Due to the low voltage level of the microgrid and the non-negligible line resistance, there was a power coupling problem when using inductive droop control for parallel inverters. Therefore, a virtual impedance was introduced in the dual-loop control to make the system equivalent impedance inductive, thus realizing the decoupled control of active/reactive power in low-voltage microgrids. Secondly, the analysis of the reactive power sharing condition exhibited that the accuracy of reactive power equalization was closely related to the line impedance and the output voltage of the inverter. When the impedance of the line did not match the capacity of the inverter, it was difficult to evenly distribute the reactive power output of the inverter based on the capacity. Although introducing appropriate virtual impedance matched the line impedance with reactive power capacity and improved the accuracy of reactive power sharing, the accuracy was affected by the variation of equivalent impedance and caused additional bus voltage drop. Therefore, this study introduced an integral correction term in the reactive power control loop and adopted the method of regulating the output voltage of the inverter to achieve reactive power sharing. Based on the three-dimensional schematic diagram of the effect of power sharing on the amplitude of the inverter output voltage, it was observed that by introducing a reactive power correction term to adjust the amplitude of the inverter output voltage, it was distributed on the line where power sharing occurred, thus achieving reactive power sharing. The reactive power correction term was set to an adaptive form, driven by reactive power deviation, to adjust the inverter output voltage and achieve reactive power sharing to adapt to changes in line impedance and local load switching scenarios. The accuracy of reactive power sharing was not affected by changes in line impedance. However, introducing reactive power correction caused changes in the inverter output voltage, which in turn affected the amplitude of the bus voltage. Therefore, it was necessary to consider measures for compensating the bus voltage. In addition, droop control simulated the droop characteristics of synchronous generators. When there were large-scale load changes, the inverter output voltage frequency and amplitude deviated significantly from the rated values, which also affected the quality of the bus voltage. Frequency and amplitude compensation terms were introduced in the active and reactive control loops, respectively, to suppress the voltage frequency and amplitude deviation caused by large-scale load shedding. The selection principles of frequency and amplitude compensation coefficients were analyzed from the perspective of the dynamic response of the power control loop. Introducing frequency compensation suppressed frequency offset; however, the analysis of the active power transmission characteristics after the introduction of frequency compensation indicated that a substantial frequency compensation coefficient reduced the accuracy of active power sharing. An active power correction term was introduced into the active control loop to suppress frequency deviation while ensuring the accuracy of active power sharing. The power transmission characteristics indicated that the introduction of an active power correction term eliminated the active power deviation caused by frequency compensation.Results and DiscussionsThe proposed strategy was compared to traditional droop control and virtual impedance-based droop control. Simulation results showed that the proposed control achieved adaptive reactive power sharing in the scenario of system equivalent impedance variation, and the accuracy of reactive power sharing was not affected by the variation of system equivalent impedance. In the scenario of plug-and-play for inverters, the proposed strategy allocated power reasonably based on the status of the inverters. The proposed strategy in both scenarios reduced the magnitude of bus voltage and frequency deviation while achieving active and reactive power sharing, improving the quality of bus voltage.ConclusionsA self-adaptive reactive power-sharing control strategy is proposed to address the challenges posed by system equivalent impedance variation and bus voltage amplitude and frequency deviation caused by large-scale load switching on the reactive power-sharing performance of droop-controlled parallel inverters. This strategy integrates voltage frequency and amplitude compensation functions. The proposed strategy has reference value for research on improving the power equalization accuracy of isolated microgrids while suppressing voltage frequency and amplitude deviations.

Abstract In response to the issue of harmonic data anomalies affecting harmonic impedance estimation, a method based on improved rank regression is proposed, building upon the foundation of rank estimation. This method utilizes harmonic data sampled from the point of common coupling, treating harmonic impedance as regression parameters. Initially, the least squares method is employed to solve for regression parameters. Subsequently, Bayesian optimization is applied to refine these parameters. The optimized parameters are then incorporated into the rank estimation function derived from the residual rank matrix for weighted iteration. The final calculation results are determined through this iterative process. Simulation analysis demonstrates that this method can effectively mitigate the impact of outliers, yielding more accurate harmonic impedance values.

With the continuous increase in photovoltaic (PV) access capacity, voltage sag in the grid when the PV is off-grid can adversely impact the stable operation of the system. Therefore, it is imperative for the PV system to possess low-voltage ride-through (LVRT) capability. However, the existing PV LVRT strategy, which is based on a fixed DC bus voltage, indirectly adjusts PV output power according to changes in the DC bus voltage, resulting in a slow dynamic response. A two-stage PV LVRT control strategy based on the PV power–voltage (P–U) characteristic curve directly controls PV output power based on the inverter’s output active power during a fault. However, a drawback is that the PV voltage and current reference values must be obtained through model solving, and the model accuracy is susceptible to the completeness of the PV nameplate and irradiance. Moreover, the LVRT effect under partial shadow shading scenarios has not been considered. To address these challenges, a dynamic current reference value-based LVRT control strategy for two-stage grid-connected PV systems is introduced, specifically designed for partial shading scenarios. Initially, a mathematical model is established based on the characteristics of the two-stage grid-connected PV system and PV cells. The strengths and weaknesses of existing LVRT control strategies are analyzed. Subsequently, a dynamic current reference value with adaptive convergence characteristics is constructed, and its convergence under scenarios of uniform illumination and local shadow shading is examined. The pre-stage boost circuit is employed to control PV output current using the set dynamic current reference value. This adjustment of the PV operating point accelerates the dynamic response of the PV system and mitigates errors caused by model solutions. Additionally, a fault decoupling module is incorporated into the maximum power tracking algorithm, enabling the system to lock the maximum power point tracking output voltage reference value by switching input quantities during a fault. This facilitates a quick system recovery to the maximum power point after the fault concludes. Finally, the proposed strategy is compared with fixed DC bus voltage control and PV P–U curve-based LVRT control strategies under various environmental conditions through simulation. The results indicate that the proposed strategy exhibits a faster dynamic response compared to the fixed DC bus voltage control strategy. Moreover, in comparison to the control strategy based on the P–U characteristic curve of PV, the proposed strategy effectively achieves LVRT under different irradiance levels, particularly in partial shadow shading conditions, making it more adaptable to varying environmental conditions.

弱电网环境下,锁相环会对并网逆变器的稳定性造成威胁,建立考虑锁相环影响的逆变器输出阻抗模型和传统同步参考坐标锁相环的传递函数,基于阻抗分析法分析采用传统锁相环时逆变器的稳定性.随后,研究了一种基于降阶广义积分和二阶广义积分的复合广义积分锁相环,并给出参数设计方法.采用谐波线性化的方法获取复合广义积分锁相环的传递函数,分析弱电网环境下采用新型锁相环时逆变器的稳定性.考虑到系统频率偏移对锁相精度的影响,设计频率自适应结构.理论分析和仿真结果表明,采用复合广义积分锁相环,并网逆变器在高电网阻抗下也能稳定运行,且并网电流质量高.

The selection of the size of the injected harmonic signal is important for the impedance measurement accuracy and normal operation of the system. The system noise and background harmonic interfere greatly with the limitation of the injected harmonic signal amplitude. The noise of the Lee-HVDC converter station and the system background harmonic are analysed, the upper limit of impedance measurement error is preset to 2%. The threshold value of the injected harmonic voltage when considering the noise and the system background harmonic is calculated by changing the system impedance parameters. The threshold selection method is used to find the system harmonic impedance, and polynomial The threshold selection method is used to find the system harmonic impedance and reduce the overall error of the measured harmonic. Through Matlab/Simulink simulation analysis and verification, it is demonstrated that the proposed method can effectively weaken the influence of noise and background harmonic on the harmonic impedance measurement accuracy.

The high proportion of new energy sources such as wind power and photovoltaic (PV) connected to the grid, coupled with a large number of nonlinear users entering the grid on a large scale, leads to increasingly prominent power quality problems and stability problems caused by harmonics in the new grid. As a result, accurate system harmonic impedance measurements are an important means of solving some of these problems. A non-characteristic harmonic current injection method is proposed to obtain the harmonic impedance value with high accuracy at each frequency and to confirm the series-parallel resonance points in the system. The simulation results show that the scheme achieves good results in the ideal case, the case containing background noise, and the case containing noise.

针对变压器铁芯多点接地故障,对接地电流进行研究,提出了 一种实际应用上更简便的均匀化建模方法,并验证方法的准确性.同时通过建立变压器铁芯多点接地故障有限元模型,在Maxwell三维瞬态模式下对变压器多点接地故障进行有限元分析计算,探究了变压器铁芯多点接地故障下短路电流特征.

To solve the problem of large friction caused by the normal force in the single-side transverse flux linear motor, meanwhile, to improve space utilization, two dual-side-type transverse flux linear motors are introduced in this paper. The permanent magnets are fixed on the shallow slot of both primary and secondary sides, and the proposed linear motor’s thrust force density is excellent among existing transverse flux linear motors. Firstly, the basic topology, structural dimensions, and principle of two dual-side-type linear motors are introduced, respectively. Then, some electromagnetic characteristics, e.g., PM-flux linkage, back electromotive force, detent force, electromagnetic thrust force, inductance, and overload capability of two linear motors are analyzed by the three-dimensional finite-element method. At last, the advantages and disadvantages of the two structures are concluded.

在LCL型并网逆变器中,为了减少传感器的使用,并网电流反馈的闭环控制得到了广泛的应用.然而一方面LCL型并网逆变器自身存在谐振现象,限制了电流控制器的设计;另一方面,由于弱电网中电网阻抗的存在,使该控制性能下降,对系统稳定产生不利影响.文中针对上述问题进行改进:采用特定的并网电流反馈有源阻尼控制器来虚拟电网侧的串联阻抗,即基于有源阻尼的虚拟阻抗法来抑制谐波尖峰;采用相位超前补偿的方法,增大逆变器输出阻抗的相角,极大地减少不稳定区域.保证了当电网阻抗变化时,该系统仍具有较强的稳定性.最后在MATLAB/Simulink上进行仿真,验证了所提方法的有效性和可行性.

To address the problems of inverter overcurrent and DC bus voltage fluctuation during voltage dips in two-stage grid-connected PV systems, this paper proposes a new power adaptive LVRT control strategy based on the front-to-back power relationship and U-I curve of PV arrays. Without adding additional hardware circuitry and the need of fitting the PV power curve, the control of PV output voltage is only based on the desired output power as well as the actual PV output current during voltage sag. And then, the output power of the PV array can be flexibly adjusted. Moreover, a fault decoupling step is added in the classical disturbance observation algorithm, which greatly saves the transient recovery time after low voltage ride-through and the PV system can transit to normal operating conditions more quickly after the voltage sag ends. Finally, the effectiveness of the proposed control strategy under different environmental conditions is verified by simulation. Compared with the LVRT control strategy based on the power curve of the PV arrays, the proposed strategy is more suitable to variable environmental conditions and has shorter transient recovery time, which improves the operational efficiency of the PV system.

以新能源为主的微电网惯量小、阻尼弱,而虚拟同步发电机(VSG)是增强惯性和阻尼以提高微电网稳定性的有效手段,但同时引入同步机类似的功角振荡问题.该文提出一种优化VSG惯量和阻尼的自适应控制策略,改善小干扰稳定和暂态稳定.首先,建立虚拟同步发电机控制模型,在虚拟惯量控制中引入双曲正弦函数(tanh)优化惯量,通过等面积定则分析,验证优化惯量控制的正确性;在虚拟阻尼中引入灵活切换暂态阻尼和稳态阻尼控制环节,采用根轨迹分析论证优化阻尼方法的有效性.然后,根据频域特性指标,采用主极点法设计控制参数.最后,通过Simulink仿真,验证提出的优化方法对提高小干扰稳定和暂态稳定的有效性.

Aiming at the problems of easily falling into local optimum, slow convergence and low solution accuracy of standard Slime Mould Algorithm （SMA） , , an Improved Slime Mould Algorithm with Multi-Strategy fusion （MSISMA） , was proposed. Firstly, Brownian motion and Levy flight were introduced to enhance the search ability of the algorithm.Secondly, according to different stages of the algorithm, the location update formula of the slime mould was improved to increase the convergence speed and accuracy of the algorithm. Thirdly, the Interval Adaptative Opposition-Based Learning （IAOBL） , strategy was adopted to generate the reverse population, with which the diversity and quality of the population were improved, as a result, the convergence speed of the algorithm was improved. Finally, a convergence stagnation monitoring strategy was introduced, which would make the algorithm jump out of the local optimum by re-initializing the positions of some slime mould individuals. With 23 test functions selected,the proposed MSISMA was tested and compared with Equilibrium Slime Mould Algorithm （ESMA） , , Slime Mould Algorithm combined to Adaptive Guided Differential Evolution Algorithm （SMA-AGDE） , , SMA, Marine Predators Algorithm （MPA） , and Equilibrium Optimizer （EO） , . Moreover, the Wilcoxon rank-sum test was performed on the running results of all algorithms. Compared with the above algorithms, MSISMA achieves the best average value on 19 test functions and the best standard deviation on 12 test functions, and has the optimization accuracy improved by 23. 39% to 55. 97% on average. Experimental results show that the convergence speed, solution accuracy and robustness of MSISMA are significantly better.

In the multi-paralleled grid-connected inverters system, harmonic oscillations may be deteriorated or even lead to system instability by the coupling between the inverters and the grid impedance. This paper presents a quantify method to determine which inverters contribute more to the harmonic instability so that the corresponding inverters can be removed quickly or targeted oscillation suppression measures can be taken to restore the system to stable operation. In the method, a contribution factor (CF $_{\mathrm {i}}$ ) is defined which can quantitatively represent the responsibility of each inverter to system instability. The CFi are calculated by analyzing the variation of system stability margin after different inverters are removed based on the bode diagram, therefore, according to the corresponding CFi, which inverters have a significant impact on harmonic instability can be determined under different grid impedances. Besides used in the parallel system with different types of inverters (PSDI), the CFi can also be used for the parallel system with identical inverters (PSII) to determine the number of inverters that need to be removed to restore the system to stable operation. The reasonableness and effectiveness of the proposed method are verified by simulation results.

受多变量和接触非线性的影响,如何延长高强度螺栓疲劳寿命依然是亟待解决的难题.为准确预测螺栓的疲劳寿命,文章将经典参数分析方法与机器学习技术相结合,首先根据数值分析结果对螺栓疲劳寿命参数的影响因素进行降维处理,然后使用多项式回归(PR)和多层感知(MLP)回归的机器学习模型建立螺栓应力幅与影响因素间的映射关系,最后将机器学习模型与图形化编程语言LabVIEW相结合,设计一套能够准确分析高强度螺栓连接系统应力幅值并预测其疲劳寿命的窗口化分析工具.实验结果表明,PR模型得到的预测值与数值模拟计算值的误差低于2％,MLP回归模型得到的预测值与数值模拟计算值的误差低于4％.

In this paper, the open-circuit flux density and cogging torque of spoke-type permanent-magnet machine(PMM) with eccentric pole. The open-circuit magnetic field in the air gap is calculated according to conformal transformation. Based on Maxwell stress theory and the air-gap open-circuit flux density the cogging torque is calculated. The research results show that the use of appropriate eccentric poles in the spoke-type PMM can greatly reduce the total harmonic distortion of the radial magnetic flux density and cogging torque, and fundamental wave of radial flux density is changed a little at the same time. According to the finite element results, the analytical model to predict open-circuit flux density and cogging torque has good accuracy.

孤岛检测是分布式并网发电系统的基本功能,快速准确的孤岛检测是分布式发电系统安全运行的必要前提.为了应对可能出现的孤岛运行,提出了一种基于双谐波电网阻抗测量的孤岛检测技术,即通过并网逆变器向电网中周期性地注入高频谐波电流,检测并网点的并网电流与电压并作离散傅里叶变化,获得包含注入谐波频次的谐波分量,计算得到并网点的阻抗值.仿真结果表明与传统的单谐波电流注入法相比,双谐波电流注入法电网阻抗检测精度更高,该方法能够快速有效地实现检测且具有较高的工程实践实用价值.

对于因线路阻抗的不同而引起的并联逆变器输出无功功率不能实现精确均分的问题,提出了一种基于虚拟阻抗法的改进自适应下垂控制技术.此方法采用了一种新型的复合型虚拟阻抗,增加了下垂控制微调补偿和自适应下垂控制系数两个环节.通过引入此虚拟阻抗,不仅可以消除各支路阻性阻抗的影响,也能使得各支路阻抗感性成分达到近似相同,从而减小并联支路之间的阻抗差异.同时,改进的下垂环节可以进一步降低并联逆变器之间的无功误差.最后,仿真结果验证了此方法可实现快速和稳定的无功功率均分,减小了环流,提升了对无功功率均分的精确度,提高了系统的动态响应速度,使得并联逆变器稳定运行.