Optimal Cooperative Secondary Control for Islanded DC Microgrids via a Fully Actuated Approach

DC-DC converter-based multi-bus DC microgrids(MGs) in series have received much attention, where the conflict between voltage recovery and current balancing has been a hot topic. The lack of models that accurately portray the electrical characteristics of actual MGs while is controller design-friendly has kept the issue active. To this end, this paper establishes a large-signal model containing the comprehensive dynamical behavior of the DC MGs based on the theory of high-order fully actuated systems, and proposes distributed optimal control based on this. The proposed secondary control method can achieve the two goals of voltage recovery and current sharing for multi-bus DC MGs. Additionally, the simple structure of the proposed approach is similar to one based on droop control, which allows this control technique to be easily implemented in a variety of modern microgrids with different configurations. In contrast to existing studies, the process of controller design in this paper is closely tied to the actual dynamics of the MGs. It is a prominent feature that enables engineers to customize the performance metrics of the system. In addition, the analysis of the stability of the closed-loop DC microgrid system, as well as the optimality and consensus of current sharing are given. Finally, a scaled-down solar and battery-based microgrid prototype with maximum power point tracking controller is developed in the laboratory to experimentally test the efficacy of the proposed control method.

Research on the Stability Analysis Method of DC Microgrid Based on Bifurcation and Strobe Theory

During the operation of a DC microgrid,the nonlinearity and low damping characteristics of the DC bus make it prone to oscillatory instability.In this paper,we first establish a discrete nonlinear system dynamic model of a DC microgrid,study the effects of the converter sag coefficient,input voltage,and load resistance on the microgrid stability,and reveal the oscillation mechanism of a DC microgrid caused by a single source.Then,a DC microgrid stability analysis method based on the combination of bifurcation and strobe is used to analyze how the aforementioned parameters influence the oscillation characteristics of the system.Finally,the stability region of the system is obtained by the Jacobi matrix eigenvalue method.Grid simulation verifies the feasibility and effectiveness of the proposed method.

Automatic SOC Equalization Strategy of Energy Storage Units with DC Microgrid Bus Voltage Support

In this paper,an improved sag control strategy based on automatic SOC equalization is proposed to solve the problems of slow SOC equalization and excessive bus voltage fluctuation amplitude and offset caused by load and PV power variations in a stand-alone DC microgrid.The strategy includes primary and secondary control.Among them,the primary control suppresses the DC microgrid voltage fluctuation through the I and II section control,and the secondary control aims to correct the P-U curve of the energy storage system and the PV system,thus reducing the steady-state bus voltage excursion.The simulation results demonstrate that the proposed control strategy effectively achieves SOC balancing and enhances the immunity of bus voltage.The proposed strategy improves the voltage fluctuation suppression ability by approximately 39.4%and 43.1%under the PV power and load power fluctuation conditions,respectively.Furthermore,the steady-state deviation of the bus voltage,△U_(dc) is only 0.01–0.1 V,ensuring stable operation of the DC microgrid in fluctuating power environments.

基于非线性扰动观测的储能双向DC-DC变换器非奇异终端滑模控制策略

在光储直流微电网系统中,储能双向DC-DC变换器在储能设备与直流母线之间起着关键的接口作用,其在光伏输出功率及负载大信号扰动下的性能优劣至关重要,将直接影响着直流母线电压的稳定运行。为此,提出了一种基于非线性扰动观测器(nonlinear disturbance observer,NDO)的非奇异终端滑模控制策略。首先采用基于微分几何理论的精确反馈线性化方法,将双向DC-DC变换器系统的状态空间模型转化为布鲁诺夫斯基标准式,并利用非线性扰动观测器对系统扰动量进行估计和补偿。在此基础上设计非奇异终端滑模控制器,并根据李雅普诺夫稳定性理论,证明所提控制策略的稳定性;进而分析所提控制策略的收敛性。最后,通过仿真验证所提控制策略的有效性。仿真结果表明,与文献中典型非线性控制策略相比,本文所提控制策略在光伏输出功率和负载有较大扰动时响应时间较快,并具有电压过充小、稳态误差小的优点。

基于双重移相控制的双向全桥DC-DC变换器最小回流功率分段优化控制

双向全桥DC-DC变换器以其重量轻、高功率密度、能量双向流动等优点成为直流微电网中不可或缺的一部分。但双向全桥DC-DC变换器在传统的单重移相控制下存在回流功率和电流应力等问题,尤其当输入电压和输出电压不匹配时,回流功率和电流应力会显著增加。针对输入电压和输出电压不匹配的情况,该文提出了一种双向全桥DC-DC变换器在双重移相下的最小回流功率控制策略。该控制策略通过对回流功率进行分段优化,得到了变换器在不同传输功率范围下的最优移相角。通过将所提控制策略和传统的双重移相控制进行对比分析,发现该文所提控制策略具有更小的回流功率和电流应力,提升了变换器的效率。最后基于所提控制策略搭建了实验样机,实验结果验证了控制策略的正确性和有效性。

Research on Virtual DC Generator-Based Control Strategy of DCMicrogrid with Photovoltaic and Energy Storage

With the penetration of a large number of photovoltaic power generation units and power electronic converters,the DC microgrid shows low inertia characteristics,which might affect the stable operation of the microgrid in extreme cases.In order to enhance the“flexible features”of the interface converter connected to the DC bus,a control strategy of DCmicrogrid with photovoltaic and energy storage based on the virtual DC generator(VDCG)is proposed in this paper.The interface converters of the photovoltaic power generation system and the energy storage system simulates the inertia and damping characteristics of the DC generator to improve the stability of the DC bus voltage.The impedance ratio of DC microgrid was obtained by establishing the small-signal model of photovoltaic power generation system and energy storage system,and the Nyquist curves was applied to analyze the small-signal stability of the system.Finally,the simulation results were verified with MATLAB/Simulink.The results show that the proposed control strategy can slow down the fluctuation of bus voltage under the conditions of photovoltaic power fluctuation and load mutation,thus enhancing the system stability.

Attack-resilient control for converter-based DC microgrids

In light of the growing integration of renewable energy sources in power systems,the adoption of DC microgrids has become increasingly popular,due to its simple structure,having no frequency,power factor concerns.However,the dependence of DC microgrids on cyber-networks also makes them susceptible to cyber-attacks.Potential cyberattacks can disrupt power system facilities and result in significant economic and loss of life.To address this concern,this paper presents an attack-resilient control strategy for microgrids to ensure voltage regulation and power sharing with stable operation under cyber-attack on the actuators.This paper first formulates the cyber-security problem considering a distributed generation based microgrid using the converter model,after which an attack-resilient control is proposed to eliminate the actuator attack impact on the system.Steady state analysis and root locus validation illustrate the feasibility of the proposed method.The effectiveness of the proposed control scheme is demonstrated through simulation results.

基于VDCG的直流微电网退役锂电池SOC均衡方案

为增加系统惯性,直流微电网内锂电池控制器常采用虚拟直流发电机(virtual DC generator,VDCG)控制方案,但该方案无法使锂电池荷电状态(state of charge,SOC)自均衡。现有基于VDCG的锂电池SOC均衡方案仅能实现电压等级一致的非等容锂电池SOC均衡,而退役锂电池储能系统(retire lithium battery energy storage systems,RLBESS)的容量和电压等级均难以保持一致。针对此问题,提出了一种基于VDCG的适用于不同电压等级及容量的RLBESS组间SOC均衡方案。该方案在传统VDCG的基础上建立U-P_(m)关系式并引入SOC均衡因子,能够根据初始SOC状态自动调节锂电池的功率分配,并保持良好的电压质量。建立了所提方案的小信号模型,分析了关键控制参数对系统稳定性的影响。最后,利用Matlab/Simulink仿真软件对不同工况进行有效性验证。仿真结果表明:所提方案能够在锂电池电压等级不一致工况下实现RLBESS的SOC均衡,具有良好的可扩展性。

基于模型预测控制的Buck型双向DC-DC变换器动态补偿控制

针对直流微电网中负载投切、功率波动和二倍频注入导致的直流电压波动问题,提出了一种基于模型预测控制的Buck型双向DC-DC变换器动态补偿控制策略。首先,建立Buck型双向DC-DC变换器的离散状态空间矩阵,并以输入电流为扰动量。其次,设计Buck型双向DC-DC变换器的基于模型的内环电流预测控制和外环电压控制。然后,针对电流扰动设计基于残差生成器的动态补偿控制结构,并求解动态补偿控制器Q(z),同时利用最小二乘递推算法进行参数辨识,减少模型不确定性对动态补偿控制策略的影响。最后,在PSCAD/EMTDC仿真平台上设计对比实验,验证所提控制策略的有效性。通过实验结果可知,该补偿控制结构在不改变原预测控制的前提下,可以有效地解决直流母线电压波动问题,增强整个系统的鲁棒性。

直流微网储能DC/DC变换器的自适应虚拟直流电机控制

电力电子化的直流微电网自身缺乏惯性,当功率发生波动时,直流母线电压会产生较大突变,不利于其稳定运行。为了解决这一问题,虚拟直流电机控制被应用于直流变换器中来模拟直流电机的外特性,进而为直流微电网提供惯性支撑。但传统参数固定的虚拟直流电机控制在提供惯性的同时会牺牲系统的动态响应速度。针对这一问题,提出了参数自适应虚拟直流电机控制,并将它应用于储能端推挽式DC/DC变换器中。建立了系统的小信号模型,分析了转动惯量参数变化对系统的影响,并给出了参数的自适应调节原则。最后,搭建了仿真模型对不同控制方法进行了对比分析。仿真结果表明所提控制策略在为系统提供较大惯性支撑的同时,系统仍具有较快的动态响应速度。

基于模糊神经网络的储能侧双向DC-DC变换器自抗扰控制策略

在直流微网中,为确保储能装置与母线间能量流动的平滑稳定,解决由于工况的不确定性而导致控制效果恶化的问题,在建立双向DC-DC变换器小信号数学模型的基础上,设计一种基于模糊神经网络的双向DC-DC变换器自抗扰控制策略,利用线性扩张状态观测器实现对总扰动的估计补偿,结合模糊神经网络实现控制器参数的在线优化。通过分析对比不同工况中该文所提出的控制策略、线性自抗扰控制与比例积分控制下的直流母线侧电压波形,证明了所提自抗扰控制策略的正确性与有效性。最后,通过参数摄动下的蒙特卡洛实验验证,该控制方法具有较好的鲁棒性。

三角形模块化多电平DC/DC变换器研究

DC/DC变换器是双母线直流微网中的接口变流器,在能量转换中起重要作用。因传统DC/DC变换器具有器件应力高,电压增益低等缺点,导致其不适用于中高功率场合。模块化多电平DC/DC变换器通过模块串并联方式降低器件应力,提高电压增益,适用于中高功率场合。本文以三角形模块化多电平DC/DC变换器作为研究对象,对电路拓扑结构与工作原理进行分析,该电路所有模块可实现均压均流,电路功率均分至各个模块。针对电路中变量相互耦合的关系,研究设计解耦控制策略,实现两级间的变量解耦;针对非交错调制方式下电压纹波大的特点,提出不同级间交错调制方式,在所提方式下电路拥有良好的动静态性能,并且可以有效降低输出电压纹波。通过MATLAB/Simulink搭建仿真模型,仿真结果验证了解耦控制策略及调制方式的可行性及有效性,提高了电路动态特性及输出直流电能质量。

Distributed Periodic Event-Triggered Optimal Control of DC Microgrids Based on Virtual Incremental Cost

This article presents a distributed periodic eventtriggered(PET)optimal control scheme to achieve generation cost minimization and average bus voltage regulation in DC microgrids.In order to accommodate the generation constraints of the distributed generators(DGs),a virtual incremental cost is firstly designed,based on which an optimality condition is derived to facilitate the control design.To meet the discrete-time(DT)nature of modern control systems,the optimal controller is directly developed in the DT domain.Afterward,to reduce the communication requirement among the controllers,a distributed event-triggered mechanism is introduced for the DT optimal controller.The event-triggered condition is detected periodically and therefore naturally avoids the Zeno phenomenon.The closed-loop system stability is proved by the Lyapunov synthesis for switched systems.The generation cost minimization and average bus voltage regulation are obtained at the equilibrium point.Finally,switch-level microgrid simulations validate the performance of the proposed optimal controller.

A Grid Interface Current Control Strategy for DC Microgrids

In this paper,a grid interface current control strategy is presented for a DC microgrid,which aims to reduce the disturbance from PV generation and the load variation to the main grid without a grid interface converter.The grid interface current is directly controlled by a battery DC-DC converter within the DC microgrid.Based on a comprehensive analysis of the battery DC-DC converter and interface current control,the control system has been mathematically modelled.This enabled two transfer functions to be derived that reflect the dynamic response of the inductor current to the duty cycle variation(inner loop),and the dynamic response of the grid interface current to the inductor current variation(outer loop).Experimental study has been done to assess the effectiveness of the proposed control strategy.The experimental results indicate that the proposed control strategy has a good performance to control the grid interface current without an interface converter,regardless the variations of both PV and the load conditions.

Adaptive Decentralized Output-Constrained Control of Single-Bus DC Microgrids

A single-bus DC microgrid can represent a wide range of applications. Control objectives of such systems include high-performance bus voltage regulation and proper load sharing among multiple distributed generators(DGs) under various operating conditions. This paper presents a novel decentralized control algorithm that can guarantee both the transient voltage control performance and realize the predefined load sharing percentages. First, the output-constrained control problem is transformed into an equivalent unconstrained one. Second, a two-step backstepping control algorithm is designed based on the transformed model for bus-voltage regulation. Since the overall control effort can be split proportionally and calculated with locally-measurable signals, decentralized load sharing can be realized. The control design requires neither accurate parameters of the output filters nor load measurement. The stability of the transformed systems under the proposed control algorithm can indirectly guarantee the transient bus voltage performance of the original system. Additionally, the high-performance control design is robust, flexible, and reliable. Switch-level simulations under both normal and fault operating conditions demonstrate the effectiveness of the proposed algorithm.

基于平坦理论的直流微电网双向DC-DC变换器改进滑模自抗扰控制

针对直流微电网系统中分布式电源功率波动、负载频繁投切以及不确定性扰动造成的母线电压波动问题,以直流微电网储能单元双向DC-DC变换器为研究对象,提出了一种基于平坦理论的改进滑模自抗扰控制策略。该方法采用双闭环控制结构,其中,电流内环采用微分平坦控制,提高系统的动态响应速度。在考虑系统抗扰性能和高频噪声影响的基础上,利用增阶滤波扩张状态观测器对外环模型总扰动进行估计。并根据状态量和扰动量估计信息设计了电压外环改进滑模自抗扰控制器,进一步提升系统的鲁棒性能。最后利用Matlab/Simulink软件进行仿真,验证了所提控制策略的有效性及优越性。

基于自适应PSO的微电网双向DC-DC变换器前馈自抗扰控制

针对传统PI控制光储微电网系统双向DC-DC变换器存在的直流母线电压波动大、充放电有效性差、抗干扰能力弱等问题,设计了一种基于自适应粒子群优化(APSO)的双闭环控制策略。首先,建立双向DC-DC变换器的数学模型。其次,设计了包括电压环线性自抗扰控制(ADRC)、电流环PI控制的双闭环控制系统,并在电压环中加入前馈控制以增强控制系统的鲁棒性。然后,针对自抗扰控制器参数难以整定的问题,提出了一种基于APSO算法的参数优化系统,该算法引入了自适应惯性权重因子,使惯性权重在粒子群迭代过程中可以动态调整以获得更佳的寻优效果。最后,设计一种带罚函数的时间乘以误差绝对值积分(ITAE)指标作为适应度函数,实现了前馈线性自抗扰控制(FF-LADRC)系统控制参数的自主寻优。MATLAB仿真结果表明,所提控制策略能够有效减小直流母线电压波动,提升储能系统的充放电性能,解决了线性自抗扰控制器参数整定问题。

基于ZVS的双向全桥DC-DC变换器最小回流功率双重移相分段控制

双向全桥DC-DC(DAB)变换器在直流微电网作为可再生能源与储能设备的能量传输环节,通过实现零电压开关(ZVS)和最小回流功率,可显著地提升DAB变换器的性能,从而提高直流微电网的稳定性。但同时实现ZVS和最小回流功率控制在理论上是相互制约的。针对这一问题,该文提出一种基于ZVS的最小回流功率双重移相分段控制策略。通过对双重移相下的传输功率进行分段,前段将ZVS作为约束条件,得到基于ZVS条件下的最小回流功率控制策略,后段通过KKT条件法实现在给定传输功率下的最小回流功率控制策略。通过将所提控制策略和传统的双重移相控制对比分析,发现所提控制策略具有更小的回流功率和电流应力,提高了变换器的性能。最后,基于所提控制策略搭建实验样机,验证了控制策略的正确性和有效性。

基于NLESO的交直流微电网双向AC/DC换流器改进滑模稳压控制

针对交直流混合微电网中双向AC/DC换流器在外界扰动下出现的直流母线电压波动问题,设计了一种应用于双向AC/DC换流器的非线性扩张状态观测器(nonlinear extended state observer,NLESO),以实现对分布式电源功率波动和负荷投切变化等不确定因素的快速追踪与补偿,保证了在不同扰动下交直流混合微电网的稳定性。进一步提出了基于NLESO的改进积分滑模控制方法,提高了直流母线电压的控制精度。结合非线性光滑函数设计了滑模趋近律,消除了传统滑模控制中的高频抖振现象。通过Lyapunov理论对系统的稳定性进行分析验证,仿真结果表明该控制方法响应速度快、控制精度高、抗扰动能力强并且无抖振现象。

Performance Analysis of Grid Connected and Islanded Modes of AC/DC Microgrid for Residential Home Cluster

This paper presents performance analysis on hybrid AC/DC microgrid networks for residential home cluster. The design of the proposed microgrid includes comprehensive types of Distributed Generators (DGs) as hybrid power sources (wind, Photovoltaic (PV) solar cell, battery, fuel cell). Details about each DG dynamic modeling are presented and discussed. The customers in home cluster can be connected in both of the operating modes: islanded to the microgrid or connected to utility grid. Each DG has appended control system with its modeling that will be discussed to control DG performance. The wind turbine will be controlled by AC control system within three sub-control systems: 1) speed regulator and pitch control, 2) rotor side converter control, and 3) grid side converter control. The AC control structure is based on PLL, current regulator and voltage booster converter with using of photovoltaic Voltage Source Converter (VSC) and inverters to connect to the grid. The DC control system is mainly based on Maximum Power Point Tracking (MPPT) controller and boost converter connected to the PV array block and in order to control the system. The case study is used to analyze the performance of the proposed microgrid. The buses voltages, active power and reactive power responses are presented in both of grid-connected and islanded modes. In addition, the power factor, Total Harmonic Distortion (THD) and modulation index are calculated.