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.

AbstractIn recent years, many meta‐heuristic algorithms have been investigated to estimate the parameters of photovoltaic (PV) models. However, the accuracy of the estimated parameters still needs to be concerned, especially for some complex PV models with many unknown parameters. In order to estimate the unknown parameters of the PV models more precisely and reliably, an efficient hybrid algorithm based on particle swarm optimisation and teaching‐learning‐based optimisation (PSOTLBO) is proposed in this paper. In PSOTLBO, inspired by the learner phase of teaching‐learning‐based optimisation (TLBO), an improved learner phase is designed and introduced into the basic PSO to enhance the global search ability and the ability to get rid of local optimum. The improved learner phase divides the population into four groups according to three values, which are the average fitness values of the overall population, the population in the first half of the fitness ranking and the population in the second half of the fitness ranking. Typically, each group has its particular movement pattern concentrating on exploration or exploitation respectively to improve the search efficiency of the algorithm. Furthermore, to deal with individuals beyond the boundary, a new designed probabilistic rebound strategy is introduced, which increases the diversity of population and avoids population aggregation at the search boundary. Then, the proposed PSOTLBO is applied to estimate the parameters of the single diode model, double diode model and PV module model. The comparative results between PSOTLBO and other 14 advanced algorithms show that the average root mean square error values of different PV models obtained by PSOTLBO are 9.86021878E−04, 9.82630511E−04, 2.42507487E−03, 1.72981371E−03, and 1.66006031E−02, respectively, which indicate that PSOTLBO can provide more accurate and stable parameter estimation results than other compared algorithms. Furthermore, the convergence experimental results demonstrate that PSOTLBO has outstanding performance in convergence speed and stability.

In this paper, we propose a sensorless control strategy for permanent magnet synchronous motors (PMSMs) based on an adaptive-gain sliding mode observer (ASMO) with precise compensation. Firstly, the observer adopts a saturation function with a continuous boundary layer as the switching function to avoid the delay effect of the low-pass filter. In addition, the adaptive-gain method is designed based on a conventional sliding mode observer (SMO) with constant-gain and a saturation function. The adaptive-gain mathematical model is simplified by establishing a nonlinear feedback channel of an SMO. Secondly, in order to improve the practicability and facilitate debugging, the explicit stability condition of an ASMO is deduced according to Lyapunov's second method. According to the proposed adaptive method, the chattering caused by the mismatch of the sliding-mode gain is suppressed, and the observable speed range is also improved. Thirdly, the position estimation delay problem of an SMO with a continuous switching function is analyzed in detail from the perspective of frequency characteristics. Then, a precise compensation method is proposed for the delay problem, which greatly improves the position estimation accuracy and the control performance. Finally, the correctness of the theory and the feasibility of the ASMO with precise compensation for the sensorless control of PMSMs are verified by experiments.

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.

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.

The harmonic problem of the power grid is getting more and more serious as a significant number of power electronic devices and associated control technologies are used to new energy generation and load control. Accurate measurement of the harmonic impedance of the power system is important for the classification of harmonic responsibility, harmonic control, research of harmonic penetration characteristics and the design and operation of the power grid itself. To enhance the conventional capacitor switching method, wave trap and windowed Fourier transform algorithm are introduced. Wavelet transform theory is used to reduce noise, and a better denoising effect is obtained while taking into account the impact of background harmonic and noise of the actual power grid. The simulation experiment findings demonstrate that even when dealing with background harmonic and noise influences, the enhanced approach is still capable of achieving excellent measurement accuracy.

In order to complete the preliminary calculation of axial flux permanent magnet synchronous motor(AFPMSM) quickly and accurately, a simplified model is proposed in this paper to calculate the magnetic field and basic performance of AFPMSM. Firstly, the three-dimensional model of AFPMSM is transformed into a multi-layer linear motor model. Secondly, the two ends of the linear motor model are closed and converted into a traditional radial motor. The three-dimensional model of AFPMSM is transformed into a simplified model of a multilayer two-dimensional radial flux permanent magnet synchronous motor. Finally, taking a 10 poles/12 slots dual stator single rotor AFPMSM as an example, the two models were simulated and compared using finite element software. The results show that the simulation results of the simplified model in this paper are similar to those of AFPMSM, and the time consumption is shorter, which is more conducive to the optimization design of this type of motor.

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.

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.

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.

为解决微网逆变器的解耦问题,提高其抗干扰能力,文章提出一种双模式自抗扰切换控制策略(SADRC).文中分析了逆变器的主拓扑结构,建立了在dq轴的数学模型,文中介绍了非线性自抗扰(Unlinear Active Disturb-ance Rejection Control,NLADRC)和线性自抗扰(Linear Active Disturbance Rejection Control,LADRC)控制器的特性,将耦合、内外部扰动作为总扰动,设计双模式自抗扰切换控制策略跟踪扰动、估计对象状态,进行控制和补偿,并给出参数的整定方法.MATLAB仿真平台也充分表明所提策略的可行性和正确性,抗干扰能力和解耦性能较好.

With the increasing proportion of renewable energy and power electronic equipment, the characteristics of traditional power system have changed greatly. Virtual synchronous generator (VSG) is a control strategy that simulates the rotor motion and provides voltage and frequency support for power system. Although virtual synchronous generator can solve the low inertia and weak damping problem caused by high permeability of renewable energy, the influence of harmonic dynamic characteristics on VSG can not be ignored. In order to study the harmonic dynamic characteristics of virtual synchronous generator, it is necessary to establish a model to describe the multi-frequency dynamic characteristics. Based on the theory of harmonic state space (HSS), a small signal model of virtual synchronous generator is established in dq frame. Finally, the HSS model calculation results and MATLAB/Simulink simulation results are provided to validate the proposed model.

In order to overcome the spectral interference of the conventional Fourier transform in the International Electrotechnical Commission framework, this paper introduces a Bregman-split-based compressive sensing (BSCS) method to estimate the Taylor–Fourier coefficients in a multi-frequency dynamic phasor model. Considering the DDC component estimation, this paper transforms the phasor problem into a compressive sensing model based on the regularity and sparsity of the dynamic harmonic signal distribution. It then derives an optimized hybrid regularization algorithm with the Bregman split method to reconstruct the dynamic phasor estimation. The accuracy of the model was verified by using the cross entropy to measure the distribution differences of values. Composite tests derived from the dynamic phasor test conditions were then used to verify the potentialities of the BSCS method. Simulation results show that the algorithm can alleviate the impact of dynamic signals on phasor estimation and significantly improve the estimation accuracy, which provides a theoretical basis for P-class phasor measurement units (PMUs).

为了提升配电网中分布式光伏最佳接纳能力,提出一种将光电站和配电网的联合效益(包含环境效益)最大化作为目标,考虑电网安全运行电压和功率约束等的分布式光伏发电最佳接纳模型.定量分析光电并网对光电站和电网的成本效益和环境效益,以光电站和配电网的联合经济效益最优为目标,考虑电网安全稳定运行中电压和功率等约束建立分布式光伏发电最佳接纳模型;采用鸡群算法对模型进行求解分析,以分布式光伏接入IEEE 33节点配电网为例,证明了模型的有效性,得出经济效益最高时的光伏接纳能力.

微网中各并联逆变器与公共交流母线之间的线路阻抗存在差异,使得逆变器的输出功率不能精确均分,从而导致系统环流的产生.为此,提出了一种在分频域中引入虚拟阻抗的方法.在基波频域中,使用二阶广义积分算法构造的感性虚拟阻抗使得等效线路阻抗在基波频域为感性特性;在谐波频域中,使用一种根据谐波电流有效值来实时调节的阻性虚拟阻抗使得等效线路阻抗在谐波频域呈现阻性特性,以达到降低系统谐波电流和系统环流的目的;在总频域中,使用虚拟负阻抗来抵消线路中阻性阻抗的影响,减少由线路阻抗引起的电压降落.仿真结果验证了分频域虚拟阻抗的引入减小了线路阻抗之间的差异,提高了功率均分的精度,降低了系统环流,实现了逆变器稳定运行.