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.

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.

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.

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.

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.

Distributed Secondary Control Based on Dynamic Diffusion Algorithm for Current Sharing and Average Voltage Regulation in DC Microgrids

This paper introduces a distributed secondary control scheme for achieving current sharing and average voltage regulation objectives in a DC microgrid.The proposed scheme employs a dynamic diffusion algorithm(DDA)instead of the consensus algorithm to enable distributed communication among converters.To help understand DDA,the relation of DDA and other diffusion algorithms is discussed in detail and its superiority is shown by comparison with diffusion and consensus algorithms.Furthermore,considering the discrete nature and different sampling time of the digital controller and communication network,a z-domain model of the entire DC microgrid is established.The influence of communication and secondary control parameters on the system stability is investigated.Based on the established model,the tolerable communication rates are obtained.Real-time simulations conducted on the OPAL-RT platform validate the effectiveness of the proposed scheme,showcasing its advantages in terms of convergence speed and stability.

Robust backstepping global integral terminal sliding mode controller to enhance dynamic stability of hybrid AC/DC microgrids

In this paper,a Backstepping Global Integral Terminal Sliding Mode Controller(BGITSMC)with the view to enhancing the dynamic stability of a hybrid AC/DC microgrid has been presented.The proposed approach controls the switch-ing signals of the inverter,interlinking the DC-bus with the AC-bus in an AC/DC microgrid for a seamless interface and regulation of the output power of renewable energy sources(Solar Photovoltaic unit,PMSG-based wind farm),and Battery Energy Storage System.The proposed control approach guarantees the dynamic stability of a hybrid AC/DC microgrid by regulating the associated states of the microgrid system to their intended values.The dynamic stabil-ity of the microgrid system with the proposed control law has been proved using the Control Lyapunov Function.A simulation analysis was performed on a test hybrid AC/DC microgrid system to demonstrate the performance of the proposed control strategy in terms of maintaining power balance while the system’s operating point changed.Furthermore,the superiority of the proposed approach has been demonstrated by comparing its performance with the existing Sliding Mode Control(SMC)approach for a hybrid AC/DC microgrid.

Improving the Performance of DC Microgrids by Utilizing Adaptive Takagi-Sugeno Model Predictive Control

In naval direct current(DC)microgrids,pulsed power loads(PPLs)are becoming more prominent.A solar sys-tem,an energy storage system,and a pulse load coupled directly to the DC bus compose a DC microgrid in this study.For DC mi-crogrids equipped with sonar,radar,and other sensors,pulse load research is crucial.Due to high pulse loads,there is a possibility of severe power pulsation and voltage loss.The original contribution of this paper is that we are able to address the nonlinear problem by applying the Takagi-Sugeno(TS)model formulation for naval DC microgrids.Additionally,we provide a nonlinear power observer for estimating major disturbances affecting DC microgrids.To demonstrate the TS-potential,we examine three approaches for mitigating their negative effects:instantaneous power control(IPC)control,model predictive control(MPC)formulation,and TS-MPC approach with compensated PPLs.The results reveal that the TS-MPC approach with adjusted PPLs effectively shares power and regulates bus voltage under a variety of load conditions,while greatly decreasing detrimental impacts of the pulse load.Additionally,the comparison confirmed the efficiency of this technique.Index Terms-DC microgrids(MG),model predictive control(MPC),pulsed power loads(PPLs),nonlinear power observer,Takagi-Sugeno(TS)fuzzy model.

A Voltage-supportive Controller for Ultra-fast Electric Vehicle Chargers in Islanded DC Microgrids

This paper presents a controller for fast and ultrafast electric vehicle(EV)charging stations.Without affecting the charging efficiency,the proposed controller enables the charger to provide support to the interconnection voltage to counter and damp its transients.Existing solutions are either hardware-based such as using supercapacitors and flywheels which increase the cost and bulkiness of the charging station,or software-based such as P/V droop methods which are still unable to provide a robust and strong voltage support.This paper proposes an emulated supercapacitor concept in the control system of the ultra-fast EV charger in an islanded DC microgrid.Thus,it converts the EV from a static load to a bus voltage supportive load,leading to reduced bus voltage oscillations during single and multiple ultra-fast EV charging operations,and rides through and provides supports during extreme external disturbances.Detailed analysis and design guidelines of the proposed controller are presented,and its effectiveness and improved performance compared with conventional techniques are shown for different case studies.

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.

Decentralized control of two DC microgrids interconnected with tie-line

This paper examines the interconnection of two DC microgrids(MGs) with tie-line. The voltages at respective MG buses are controlled to manage the powerflow across the tie-line. Formation of such a DC MG cluster ensures higher reliability of power supply andflexibility to manage distributed energy resources and loads in the system. Two MGs consist of photovoltaic and battery units interfaced by power electronic converters. The bus voltages of two DC MGs act as an indicator for the powerflow monitoring the supply-demand balance. A decentralized control approach is proposed to control each MG and bus voltage fluctuation in an allowable range. Furthermore,a mode adaptive decentralized control approach is proposed for seamless mode transition in order to assign microgrid operation modes and for the power management of DC MGs. The effectiveness of the proposed concept is validated by simulation and experimental results.

Analysis of the Transition Between Multiple Operational Modes for Hybrid AC/DC Microgrids

There are four basic operational modes for the hybrid AC/DC microgrid,including AC grid-connected while interconnecting,both off-grid while interconnecting,AC gridconnected without connection,and both off-grid without connection.How to achieve a seamless operational mode transition is an urgent technical need to overcome.First,this paper describes the typical structure of the hybrid microgrid,and places a detailed focus on the power balance and transition strategy.Secondly,it takes the master-slave control structure as an example,and designs the transition logic for different operational modes,and then a method for selecting the slack bus and transition time-sequence is proposed.Based on the different roles that the interlinking converter(IC)plays in the process of modes transition,a voltage-power(U-P)control method for a hybrid AC/DC microgrid is proposed,and the exchanged power is calculated based on the voltage deviation between the rating value and measured value.Finally,a control flowchart for the transition between the four operational modes in transition is designed.Using the PSCAD/EMTDC platform,this paper takes a typical seven-point microgrid structure as an example,the proposed transition strategy is carried out,and the results show that the transition method and time sequence can achieve smooth transition between different operational modes.

Double-uncertainty optimal operation of hybrid AC/DC microgrids with high proportion of intermittent energy sources

This paper applies double-uncertainty optimization theory to the operation of AC/DC hybrid microgrids to deal with uncertainties caused by a high proportion of intermittent energy sources.A fuzzy stochastic expectation economic model for day-ahead scheduling based on uncertain optimization theory is proposed to minimize the operational costs of hybrid AC/DC microgrids.The fuzzy stochastic alternating direction multiplier method is proposed to solve the double-uncertainty optimization problem.A real-time intra-day unbalanced power adjustment model is established to minimize real-time adjustment costs.Through comparative analysis of deterministic optimization,stochastic optimization and fuzzy stochastic optimization of day-ahead scheduling and real-time adjustment,the validity of fuzzy stochastic optimization based on a fuzzy stochastic expectation model is proved.

Review of Operational Control Strategy for DC Microgrids with Electric-hydrogen Hybrid Storage Systems

Hydrogen production from renewable energy sources(RESs)is one of the effective ways to achieve carbon peak and carbon neutralization.In order to ensure the efficient,reliable and stable operation of the DC microgrid(MG)with an electric-hydrogen hybrid energy storage system(ESS),the operational constraints and static dynamic characteristics of a hydrogen energy storage system(HESS)needs to be fully considered.First,different hydrogen production systems,using water electrolysis are compared,and the modeling method of the electrolyzer is summarized.The operational control architecture of the DC MG with electric-hydrogen is analyzed.Combined with the working characteristics of the alkaline electrolyzer,the influence of hydrogen energy storage access on the operational mode of the DC MG is analyzed.The operational control strategies of the DC MG with electric-hydrogen hybrid ESS are classified and analyzed from four different aspects:static and dynamic characteristics of the hydrogen energy storage system,power distribution of the electric-hydrogen hybrid ESS and the efficiency optimization of hydrogen energy storage.Finally,the advantages of a modular hydrogen production system(HPS)are described,and the technical problems and research directions in the future are discussed.

Mutual Interactions and Stability Analysis of Bipolar DC Microgrids

This paper presents an Multi-Input Multi-Output(MIMO)analysis to investigate the mutual interactions and small-signal stability of bipolar-type dc microgrids.Since bipolar dc microgrid is replete with power-electronic converters,its dynamics can not be understood unless the interactions among control systems of converters are properly investigated.To tackle the challenge,each converter in microgrid is modeled via an MIMO transfer matrix.Then,the MIMO models are combined together based on the interactions among the control systems of source and load converters.From this integrative MIMO model,the mutual interactions between various input-output pairs are quantified using Gershgorin Band theorem.Also,Singular Value Decomposition(SVD)analysis is carried out to estimate the frequency of unstable poles.Test results not only successfully validate the effectiveness of the MIMO method but also show that the control system of voltage balancer has a major impact on the overall stability of bipolar dc microgrid,making it a suitable location for applying damping systems.

Hierarchical Control of DC Microgrids Robustness and Smartness

This paper investigates the hierarchical control of DC microgrids.Compared to AC microgrids,DC microgrids encounter complicated converter-level control,and simplified system-level management.To address these characteristics,a new three-level control hierarchy is introduced.The converter control level encapsulates sophisticated converter topologies and inner control loops into a black-box representation.The voltage coordination level uses DC voltage signals to coordinate both static and transient power sharing.The energy management level optimizes the power flow and power quality in a broader scope through communication.This architecture lowers the focus of control to bottom levels.More functions are allocated to the converter control and voltage coordination levels.They can maintain basic microgrid performance with fully local control,thereby ensuring reliable power supply in case of communication failures.Moreover,taking advantage of DC microgrids’simplified system-level operation patterns,the energy management level uses straightforward algorithms to achieve intelligent functions.As a result,this architecture achieves both robust and smart control by exploring DC microgrids’critical features.

Dynamic Improvement of DC Microgrids Using a Dual Approach Based on Virtual Inertia

In this paper,inspired by the concept of virtual inertia in alternating current(AC)systems,a virtual impedance controller is proposed for the dynamic improvement of direct current microgrids(DCMGs).A simple and inexpensive method for injecting inertia into the system is used to adjust the output power of each distributed generation unit without using additional equipment.The proposed controller consists of two components:a virtual capacitor and a virtual inductor.These virtual components can change the rate of change of the DC bus voltage and also improve the transient response.A small-signal analysis is carried out to verify the impact of the proposed control strategy.Numerical simulation studies validate the effectiveness of the proposed solution for increasing the inertia of DCMGs.

Distributed Optimal Power Flow of DC Microgrids:A Penalty Based ADMM Approach

The popularity of direct current(DC)networks have made their optimal power flow(OPF)problem a hot topic.With the proliferation of distributed generation,the many problems of centralized optimization methods,such as single point failure and slow response speed,have led to utilization of measures such as distributed OPF methods.The OPF problem is non-convex,which makes it difficult to obtain an optimal solution.The second-order cone programming(SOCP)relaxation method is widely utilized to make the OPF problem convex.It is difficult to guarantee its exactness,especially when line constraints are considered.This paper proposes a penalty based ADMM approach using difference-of-convex programming(DCP)to solve the non-convex OPF problem in a distributed manner.The algorithm is composed of distributed x iteration,z iteration and A,/i iteration.Specifically,in the distributed z iteration,the active power flow injection equation of each line is formulated as a difference of two convex functions,and then the SOCP relaxation is given in a different form.If the SOCP relaxation is inexact,a penalty item is added to drive the solution to be feasible.Then,an optimal solution can be obtained using a local nonlinear programming method.Finally,simulations on a 14-bus system and the IEEE 123-bus system validate the effectiveness of the proposed approach.

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.