Distributed generation system control strategies with PV and fuel cell in microgrid operation

Control strategies of distributed generation (DG) are investigated for different combination of DG and storage units in a microgrid. In this paper the authors proposed a microgrid structure which consists of a detailed photovoltaic (PV) array model, a solid oxide fuel cell (SOFC) and various loads. Real and reactive power (PQ) control and droop control are developed for microgrid operation. In grid-connected mode, PQ control is developed by controlling the active and reactive power output of DGs in accordance with assigned references. Two PI controllers were used in the PQ controller, and a novel heuristic method, artificial bee colony (ABC), was adopted to tune the PI parameters. DGs can be controlled by droop control both under grid-connected and islanded modes. Droop control implements power reallocation between DGs based on predefined droop characteristics whenever load changes or the microgrid is connected/disconnected to the grid, while the microgrid voltage and frequency is maintained at appropriate levels. Through voltage, frequency, and power characteristics in the simulation under different scenarios, the proposed control strategies have demonstrated to work properly and effectively. The simulation results also show the effectiveness of tuning PI parameters by the ABC.     

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.      

MAS Based Distributed Automatic Generation Control for Cyber-Physical Microgrid System

The microgrid is a typical cyber-physical microgrid system (CPMS). The physical unconventional distributed generators (DGs) are intermittent and inverter-interfaced which makes them very different to control. The cyber components, such as the embedded computer and communication network, are equipped with DGs, to process and transmit the necessary information for the controllers. In order to ensure system-wide observability, controllability and stabilization for the microgrid, the cyber and physical component need to be integrated. For the physical component of CPMS, the droop-control method is popular as it can be applied in both modes of operation to improve the grid transient performance. Traditional droop control methods have the drawback of the inherent trade-off between power sharing and voltage and frequency regulation. In this paper, the global information (such as the average voltage and the output active power of the microgrid and so on) are acquired distributedly based on multi-agent system (MAS). Based on the global information from cyber components of CPMS, automatic generation control (AGC) and automatic voltage control (AVC) are proposed to deal with the drawback of traditional droop control. Simulation studies in PSCAD demonstrate the effectiveness of the proposed control methods.      

Selective compensation of voltage harmonics in grid-connected microgrids

In this paper, a novel approach is proposed for selective compensation of main voltage harmonics in a grid-connected microgrid. The aim of compensation is to provide a high voltage quality at the point of common coupling (PCC). PCC voltage quality is of great importance due to sensitive loads that may be connected. It is assumed that the voltage harmonics are originated from distortion in grid voltage as well as the harmonic current of the nonlinear loads. Harmonic compensation is achieved through proper control of distributed generators (DGs) interface converters. The compensation effort of each harmonic is shared considering the respective current harmonic supplied by the DGs. The control system of each DG comprises harmonic compensator, fundamental power controllers, voltage and current proportional-resonant controller and virtual impedance loop. Virtual impedance is considered at fundamental frequency to enhance power control and also at harmonic frequencies to improve the nonlinear load sharing among DGs. The control system design is discussed in detail. The presented simulation results demonstrate the effectiveness of the proposed method in compensation of the voltage harmonics to an acceptable level.     

孤岛微电网异构电池储能系统的分布式有限时间次级控制

针对孤岛微电网异构电池储能系统频率、电压以及电池能量的一致性问题,考虑初级下垂控制,提出一种新的分布式有限时间次级控制策略.采用所提出的控制方案能够在有限时间内实现系统频率、电压恢复一致到额定值,并获得电池能量等级的均衡一致和期望的有功功率分配.该控制方法的优势在于整定时间的上界独立于系统的任意初始条件,能够保证微电网有限时间控制的及时性,同时基于Lyapunov方法分析控制策略的一致性收敛特性.最后,通过Matlab/Simulink仿真实验,结果进一步佐证了分布式有限时间次级控制策略的有效性.     

Distributed Secondary Control and Optimal Power Sharing in Microgrids

We address the control problem of microgrids and present a fully distributed control system which consists of primary controller, secondary controller, and optimal active power sharing controller. Different from the existing control structure in microgrids, all these controllers are implemented as local controllers at each distributed generator. Thus, the requirement for a central controller is obviated. The performance analysis of the proposed control systems is provided, and the finite-time convergence properties for distributed secondary frequency and voltage controllers are achieved. Moreover, the distributed control system possesses the optimal active power sharing property. In the end, a microgrid test system is investigated to validate the effectiveness of the proposed control strategies.      

微电网的电流均衡/电压恢复自适应动态规划策略研究

含多类型分布式电源的微电网已经成为了未来电力系统的重要发展方向,其中风能和光能在降低化石能源消耗和二氧化碳排放等方面有着极大优势,考虑二者之间强互补性的协同调度已被广泛研究.但风/光协同调度的微电网多关注分钟级的调度或优化问题而非风/光波动下秒级的实时电流按容量比例精准分担,简称电流均衡,而精准电流均衡有助于可再生能源的高比例消纳.因此,本文提出了基于自适应动态规划的微电网电流均衡和电压恢复控制策略.首先,构建包含风电整流型电能变换器和光电升压型电能变换器的广义风光拓扑同胚升压变换器模型,其提供了后续控制器设计的模型基础.其次,本文将电流均衡和电压恢复问题转化为最优控制问题,基于此,每个能源主体的目标函数转化为获取最优控制变量和最小电压/电流控制偏差,进而转化为求解哈密顿?雅克比?贝尔曼(Hamilton-Jacobi-Bellman,HJB)方程问题.基于此,提出了基于贝尔曼准则的分布式自适应动态规划控制策略以求取HJB方程的数值解,最终实现电流均衡和电压恢复.最后仿真结果验证了所提分布式自适应动态规划控制策略的有效性.     

微电网的电流均衡/电压恢复自适应动态规划策略研究

含多类型分布式电源的微电网已经成为了未来电力系统的重要发展方向,其中风能和光能在降低化石能源消耗和二氧化碳排放等方面有着极大优势,考虑二者之间强互补性的协同调度已被广泛研究.但风/光协同调度的微电网多关注分钟级的调度或优化问题而非风/光波动下秒级的实时电流按容量比例精准分担,简称电流均衡,而精准电流均衡有助于可再生能源的高比例消纳.因此,本文提出了基于自适应动态规划的微电网电流均衡和电压恢复控制策略.首先,构建包含风电整流型电能变换器和光电升压型电能变换器的广义风光拓扑同胚升压变换器模型,其提供了后续控制器设计的模型基础.其次,本文将电流均衡和电压恢复问题转化为最优控制问题,基于此,每个能源主体的目标函数转化为获取最优控制变量和最小电压/电流控制偏差,进而转化为求解哈密顿?雅克比?贝尔曼(Hamilton-Jacobi-Bellman,HJB)方程问题.基于此,提出了基于贝尔曼准则的分布式自适应动态规划控制策略以求取HJB方程的数值解,最终实现电流均衡和电压恢复.最后仿真结果验证了所提分布式自适应动态规划控制策略的有效性.     

基于分布式策略的直流微电网下垂控制器设计

本文研究了分布式控制策略下直流微电网的负荷分配和电压平衡问题. 给出一种新的基于分布式策略的下垂控制器设计方法, 能够在统一的框架下实现直流微电网负载共享和电压平衡. 首先,将直流微电网的负载共享和电压平衡问题转化为多目标优化问题, 其性能指标与微源的容量密切相关. 然后, 通过求解多目标优化问题获得实现负载共享和电压平衡的集中式控制策略, 并给出下垂控制器的设计方法. 为了降低系统的通信负担, 给出一种新的只需与邻居节点交换信息的分布式控制策略, 通过理论分析可知该分布式控制策略能够收敛到多目标优化问题的最优解. 最后, 通过对新能源汽车充换电站系统的仿真验证了本文提出的方法的有效性.     

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.     

Study impact of various load models on DG placement and sizing using backtracking search algorithm

In this article, a meta-heuristic technique based on a backtracking search algorithm (BSA) is employed to produce solutions to ascertain distributed generators (DGs). The objective is established to reduce power loss and improve network voltage profile in radial distribution networks by determining optimal locations and sizes of the DGs. Power loss indices and bus voltages are engaged to explore the initial placement of DG installations. The study cares with the DG type injects active and reactive power. The proposed methodology takes into consideration four load models, and their impacts are addressed. The proposed BSA-based methodology is verified on two different test networks with different load models and the simulation results are compared to those reported in the recent literature. The study finds that the constant power load model among various load models is sufficed and viable to allocate DGs for network loss and voltage studies. The simulation results reveal the efficacy and robustness of the BSA in finding the optimal solution of DGs allocation.     

Robust Distributed Cooperative Controller for DC Microgrids with Heterogeneous Sources

In a DC microgrid, a conventional solution for the primary control is droop control, which is achieved by imposing a virtual impedance on each converter. Its main drawbacks are the current sharing inaccuracy and the voltage deviation. To address these problems, we propose a robust distributed cooperative controller, which is an alternative to the droop mechanism. The controller is distributed in the sense that each one exchanges information with its neighboring converters, and hence no central controller is required. It is robust in the sense that parameters of power sources or load are not directly used to determine the controller parameters. A rigorous mathematical analysis is presented to guarantee the stability of the proposed controller, and simulation results are included to validate it.

     

Distributed Secondary Control of AC Microgrids With External Disturbances and Directed Communication Topologies: A Full-Order Sliding-Mode Approach

This paper addresses the problem of distributed secondary control for islanded AC microgrids with external disturbances.By using a full-order sliding-mode(FOSM)approach,voltage regulation and frequency restoration are achieved in finite time.For voltage regulation,a distributed observer is proposed for each distributed generator(DG)to estimate a reference voltage level.Different from some conventional observers,the reference voltage level in this paper is accurately estimated under directed communication topologies.Based on the observer,a new nonlinear controller is designed in a backstepping manner such that an FOSM surface is reached in finite time.On the surface,the voltages of DGs are regulated to the reference level in finite time.For frequency restoration,a distributed controller is further proposed such that a constructed FOSM surface is reached in finite time,on which the frequencies of DGs are restored to a reference level in finite time under directed communication topologies.Finally,case studies on a modified IEEE 37-bus test system are conducted to demonstrate the effectiveness,the robustness against load changes,and the plug-and-play capability of the proposed controllers.     

环形直流微电网的二次控制与稳定性分析

本文阐述了一种环形直流微电网系统分布式二次控制的稳定性分析方法,实现了微网系统的电压调控和电流分配.首先,借助多智能体系统的一致性算法,设计了局部观测器来估计所有分布式能源节点的平均电压.然后,基于观测器状态和邻居节点的电流信息设计了融合误差和动态反馈控制器,并通过解耦潮流代数方程得到了关于融合误差的闭环系统.进一步基...     

Backstepping and Sliding‐Mode Techniques Applied to Distributed Secondary Control of Islanded Microgrids

In this paper, secondary voltage control scheme for a droop‐controlled inverter‐based islanded microgrid is addressed. Motivated by the cooperative control theory of multi‐agent systems, the proposed secondary voltage control is a distributed scheme which uses a simple communication network. Two nonlinear control algorithms are applied to restore the voltage of each distribution generation (DG) to the reference value. The simulation results verify the effectiveness of the proposed secondary voltage control for a prototype microgrid system.     

微电网经济运行的分布式二次电压–频率恢复控制

由于传统下垂控制的特点,微电网中分布式能源的输出电压和频率易受负荷变化的影响.为此,本文提出了分布式二次电压–频率恢复控制算法,在二次控制层实现电压–频率恢复与同步的同时,提高微电网系统运行的经济性,并减少由于运行时间尺度不一致造成的供需不匹配.此外,本文提出的控制算法能实现孤岛模式向并网模式转变的无缝切换,能有效应对通讯故障,在极端情况下系统仍能保持稳定.最后,本文对所提出的算法进行了稳定性证明,并利用仿真和实验验证该算法的有效性和可靠性.     

新趋近律的直流微电网母线电压滑模控制设计

随着直流微电网技术的快速发展,直流微电网中母线电压的稳定性控制作为直流微电网正常平稳运行的关键,也成为了研究热点之一;针对直流微电网母线电压外环易受参数摄动和负荷扰动的影响,提出了一种扩张状态扰动观测器ESO(Extend state observer)与新型趋近律的滑模控制器NRSMC(New reaching law sliding mode control)相结合的复合控制器ESO-NRSMC;该控制器采用了一种新型趋近律方法来解决抖振现象与滑模面趋近时间之间的矛盾;同时扩张状态观测器将观测到的扰动值补偿到滑模控制器中,进一步提高控制器的抗扰动能力;还构造了Lyapunov函数来验证所设计的母线电压外环闭环控制系统的稳定性;最后通过仿真证明控制器ESO-NRSMC在微电网系统参数发生变化以及负载扰动的情况下,依然能够实现对直流微电网母线电压的稳定控制,具有较强的优越性和鲁棒性能。     

Power quality improvement of solar power plants in grid connected system using novel Resilient Direct Unbalanced Control (RDUC) technique

Distributed Generation systems (DGs) using solar power is one of the new trends in power generation. These distributed generating units are integrated to form a micro grid to serve the loads among the locality, which is in connection with the utility grid for power transmission. The elimination of the harmonics in the grid and the usage of solar energy resources in the power electronics applications become famous worldwide. In this work, a Resilient Direct Unbalanced Control (RDUC) algorithm is used to improve the performance of the controller by achieving optimal numerical parameters for photovoltaic power generation - Unified Power Quality Conditioner (PV-UPQC). Then the voltage sag, swell and elimination of current harmonics are used to study the effects of proposed RDUC algorithm for photovoltaic feed UPQC system. According to the evaluations, the proposed unified power quality conditioner eliminates both the supply current distortion caused by a non-linear load and the load voltage distortion introduced after adding fifth and seventh harmonics to the Alternating Current (AC) mains voltage. To validate the simulation results of Resilient Direct Unbalanced Control scheme, tests are performed under various operating conditions. Test results show the satisfactory behavior under steady state, and dynamic conditions such as load unbalance, insolation variation, voltage sag and swell. Finally, Total Harmonic Distortions (THDs) of proposed optimization-based grid current and grid voltages found within limits of the IEEE standard.     

Weighted and Constrained Consensus for Distributed Power Dispatch of Scalable Microgrids

Microgrids with distributed energy generators, controllable appliances, electric vehicle charging infrastructures, and energy storage systems introduce new technical challenges in the management of distribution networks. This paper introduces a new framework for power flow control based on the emerging consensus control for networked systems. Due to unique features of power systems, the consensus control problem becomes weighted and constrained, beyond the typical consensus formulation. Using only neighborhood communication for each bus on the microgrid, the consensus control achieves global weighted load balancing. Algorithms are introduced and their convergence properties are established. It is shown that the algorithms have the fastest convergence rates in terms of consensus error variances, by achieving asymptotically the Cramér‐Rao lower bound, and hence is optimal among all algorithms. Examples and case studies demonstrate convergence, robustness, and scalability of the methodology and feasibility in distribution networks. Practical issues are further discussed.     

LMI‐Based Robust PID Controller Design for Voltage Control of Islanded Microgrid

This paper presents the design of a robust proportional integral derivative (PID) controller for the control of a single phase microgrid voltage. A microgrid consists of loads, distributed generation units and several power‐electronics interfaced LC filter and voltage source inverter. These loads are unknown and parameters are uncertain which produce unmodeled load dynamics. This unmodeled load dynamics reduces the voltage tracking performance of the microgrid. The proposed controller gives the robustness of the system with unmodeled load dynamics. Under different kinds of uncertainties, PID controller guarantees the stability and provides zero steady‐state error and fast transient response. The robustness and optimal performance of the controller is obtained by using linear matrix inequality approach. The performance of the controller under different uncertainties is studied. Results indicate the robustness and high voltage tracking performance of the microgrid system.