Control Of DC Microgrids Research Articles

Model-Free Approach to DC Microgrid Optimal Operation under System Uncertainty Based on Reinforcement Learning

There has been tremendous interest in the development of DC microgrid systems which consist of interconnected DC renewable energy sources. However, operating a DC microgrid system optimally by minimizing operational cost and ensures stability remains a problem when the system’s model is not available. In this paper, a novel model-free approach to perform operation control of DC microgrids based on reinforcement learning algorithms, specifically Q-learning and Q-network, has been proposed. This approach circumvents the need to know the accurate model of a DC grid by exploiting an interaction with the DC microgrids to learn the best policy, which leads to more optimal operation. The proposed approach has been compared with with mixed-integer quadratic programming (MIQP) as the baseline deterministic model that requires an accurate system model. The result shows that, in a system of three nodes, both Q-learning (74.2707) and Q-network (74.4254) are able to learn to make a control decision that is close to the MIQP (75.0489) solution. With the introduction of both model uncertainty and noisy sensor measurements, the Q-network performs better (72.3714) compared to MIQP (72.1596), whereas Q-learn fails to learn.

Dynamic event-triggered fuzzy non-fragile control of DC microgrids

This paper studies the event-triggered fuzzy non-fragile control of uncertain DC microgrids subject to false data injection (FDI) attacks, controller saturation, network delays and premise mismatching. Firstly, a dynamic event-triggered mechanism (ETM) is proposed, which can save more communication bandwidth than the static ETMs, and remove the complex Zeno-free computation required by the continuous-time ETMs. Secondly, a fuzzy time-delay closed-loop system model is established, which provides a unified framework to study the effects of the dynamic ETM, FDI attacks, uncertainties, saturation, delays and premise mismatching. Thirdly, mean-square exponential stability criteria are established, and co-design method for the saturated fuzzy non-fragile (SFNF) controller and the dynamic ETM is presented. Simulation results confirm that the SFNF controller can stabilize the unstable DC microgrid, while the dynamic ETM significantly reduces the triggering rate by 84.98%. Comparisons show that the proposed controller performs better than the non-fragile controller, fuzzy controller and robust linear controller, and the dynamic ETM achieves a lower triggering rate than the static ETMs.

On the stability of distributed secondary control for DC microgrids with grid-forming and grid-feeding converters

This paper investigates the stability problem of a single-bus DC microgrid with mixed grid-forming/grid-feeding converters and the constant impedance, constant current, and constant power (ZIP) loads. Considering the different output properties of grid-forming and grid-feeding converters, distributed secondary controllers with mixed V–I droop and I–V droop controllers are designed to achieve accurate bidirectional current sharing and voltage regulation. As few converters have access to the information of the common bus, an observer is designed for each local controller to estimate the bus voltage. Then, the observer state is used to generate voltage correction term in the feedback loop, which gets potential faster regulation compared with the existing approaches that use pinning gains. Based on the rank-one perturbation analysis, a stability criterion of DC microgrid is given for the case of ZI load, where global voltage regulation and current sharing are guaranteed. Then, the stability criterion is extended to the case of ZIP load, where local stability of the DC microgrid is guaranteed. Both numerical examples and experimental tests are given to verify the effectiveness of the theoretical results.

Capacitor Current Control Based Virtual Inertia Control of Autonomous DC Microgrid

Virtual inertia (VI) control of DC microgrids (DC MG) is a potential solution to the voltage stability issue caused by the intermittency of loads and renewable sources. Existing VI strategies for DC MG rely on a first-order differential equation (FODE) relating voltage (speed) with current (torque) to control the grid-forming converters that are crucial in an autonomous DC MG. However the output impedance of these converters can distort the inertial response. Existing research works overcome this by using a feed-forward controller (FFC) necessitating an accurate system model for proper compensation. Hence, in this paper, a novel VI scheme based on capacitor current control which does not rely on any differential equation is proposed. The proposed VI scheme employs a static gain to restrict the capacitor current for inertia emulation without any additional FFC. Furthermore, the proposed VI scheme is extended to parallel-connected converters to study their steady-state and transient coordination. Finally, the proposed strategy is validated in simulation using MATLAB/Simulink and is also experimentally verified in a laboratory prototype using the TMS320F280049C controller.

Circulating current minimization based adaptive droop control for grid-connected DC microgrid

Circulating current minimization based adaptive droop control for grid-connected DC microgrid

Hierarchical Hybrid Control for Scaled Consensus and Its Application to Secondary Control for DC Microgrid.

This article addresses the scaled consensus problem for a class of heterogeneous multiagent systems (MASs) with a cascade-type two-layer structure. It is assumed that the information of the upper layer state components is intermittently exchangeable through a strongly connected communication network among the agents. A distributed hierarchical hybrid control framework is proposed, which consists of a lower layer controller and an upper layer one. The lower layer controller is a decentralized continuous feedback controller, which makes the lower layer state components converge to their target values. The upper layer controller is a distributed impulsive controller, which enforces a scaled consensus for the upper layer state components. It is proved that the two layer controllers can be designed separately. By considering the dwell-time condition of impulses and the feature of the strongly connected Laplacian matrix, a novel weighted discontinuous function is constructed for scaled consensus analysis. By using the Lyapunov function, a sufficient condition for scaled consensus of the MAS is derived in terms of linear matrix inequalities. As an application of the proposed distributed hybrid control strategy, a relaxed distributed hybrid secondary control algorithm for dc microgrid is obtained, by which the balance requirement on the communication digraph is removed, and an improved current sharing condition is obtained.

Distributed consensus control for voltage tracking and current distribution in DC microgrid

DC microgrids are extensively researched because of advancements in power electronics, penetration of DC loads, and the growing demand for DC power. The DC microgrid is also broached as a multi-agent system (MAS) because of its distributed nature. In this paper, a distributed consensus controller for DC microgrids is proposed that has basically two main objects; the first object is to track the desired reference voltage for the distributed generation units while maintaining the voltage stability at the point of common coupling (PCC), whereas the second object is to distribute current proportionally among distributed loads. Furthermore, a linear distributed model is constructed in which the dynamics of the net current for the distributed generation units are established using the directed graph of power lines. Moreover, local information from neighboring agents is utilized to construct a distributed linear controller, and the consensus control problem is solved for voltage tracking and current distribution of the DC microgrid. Furthermore, the control gains for the proposed controller are optimized using closed-loop stability analysis. Finally, simulation results are illustrated for both open-loop and closed-loop systems, and the effects of the directed power line graph and the undirected communication graph are analyzed. The effects of parameter uncertainties on the performance of the proposed controller are investigated through simulations. The results show that the distributed controller efficiently solves the consensus control problem for the MAS.

Optimal power distributed control of the DC microgrid in meshed configuration

This paper proposes a Lyapunov-based power sharing control scheme and a fixed-time-based distributed optimization algorithm to achieve optimal power sharing of sources in a DC microgrid. The Lyapunov-based controller is designed based on so-called ratio consensus protocol, where it drives the sources to a desired proportional power sharing by regulating the voltage profile of the DC microgrid. The distributed optimization optimizer is established by integrating a finite-time weighted consensus algorithm with an iterative algebraic operation, where it calculates the optimal power dispatch on the target of minimizing the generation cost. The optimizer receives the current output power of the controlled DC microgrid and sends the obtained power dispatch to the power sharing controller as the proportionality coefficients. Both the controller and optimizer are carried out in a fully distributed way. Under the framework of the Lyapunov method, stability analysis of the DC microgrid with the proposed control scheme, as well as convergence and optimality analysis of the distributed optimization algorithm, is provided. However, the influence of the time delay of the controller on the system remains to be further investigated in future work.

Energy coordinated control of DC microgrid integrated incorporating PV, energy storage and EV charging

The power of photovoltaic (PV) and electric vehicles (EV) charging in integrated standalone DC microgrids is uncertain. If no suitable control strategy is adopted, the power variation will significantly fluctuate in DC bus voltage and reduce the system’s stability. This paper investigates the energy coordination control strategy for the standalone DC microgrid integrated with PV, energy storage, and EV charging. The specific research contents include the following: The maximum power point tracking (MPPT) control of the variable-step perturbation observation method is proposed to shorten the regulation time of the PV unit to reach the maximum power point; an improved droop control is designed based on the state of charge (SOC) of the batteries, and the power equalization and bus voltage recovery secondary controls are introduced to enhance the regulation effect of the DC bus voltage; a coordination control strategy based on the power difference is formulated to improve further the transient and steady-state performance of the DC bus voltage. The proposed coordination control strategy is applied to the integrated standalone DC microgrid model built by MATLAB/Simulink. The simulation results show that the proposed coordination control strategy can not only effectively improve the stability of the DC microgrid system but also reduce the capacity redundancy of the energy storage device.

Distributed finite-time event-triggered current sharing and voltage control of DC microgrids

This paper proposes a distributed finite-time event-triggered secondary control for islanded DC microgrids (MGs). The proposed controller can provide accurate current sharing and critical bus voltage regulation in finite time without the need for periodic communications. The finite-time feature improves the convergence speed and transient response of the secondary control while the event-triggered behavior reduces the burden on the communication network. The proposed secondary control scheme is validated in MATLAB/Simulink and HIL testbed utilizing OPAL-RT and Raspberry Pis.

Energy management optimization strategy of DC microgrid based on consistency algorithm considering generation economy

Renewable and clean energy power generation has effectively reduced the carbon emissions of thermal power generation. Vigorously developing renewable energy power generation is an important way to achieve the dual carbon goal. The energy dispatching of DC microgrids is flexible and fast and has gradually become an important part of the distribution network. However, the distributed renewable energy generation in the DC microgrid is intermittent and random, resulting in large output fluctuations. To address the energy coordination control of DC microgrid distributed generation units, a distributed consistency algorithm-based energy optimization strategy that takes into consideration power generation costs is proposed. Graph theory is used to construct the multi-agent communication architecture of the DC microgrid for wind turbines, diesel storage, and charging. With the control goal of reducing bus voltage fluctuation and power-producing cost, distributed micro source and load power balancing equations were created. The micro source constraint equations are used to construct the equal consumption micro increment function. The microgrid utilizes the distributed consistency approach to enhance the power distribution of wind turbine diesel storage and charging. The simulation results demonstrated that the proposed strategy maximizes distributed renewable energy generation capacity, decreases DC bus voltage fluctuation, and achieves power balance and optimal control of a DC microgrid.

Fully Distributed Dynamic Edge-Event-Triggered Current Sharing Control Strategy for Multibus DC Microgrids With Power Coupling

Although the current sharing control of dc microgrids has been widely studied, the high communication bandwidth and global communication network structure information demands hander the renewable energy consumption. Thus, this article proposes a fully distributed dynamic edge-event-triggered current sharing control strategy for multibus dc microgrids with power coupling. First, the system model with power coupling is built, which is further switched to the linear heterogeneous multiagent systems with unknown disturbance. It is an indispensable preprocessing for controller design. Furthermore, the fully distributed current sharing control strategy is proposed through adaptive coupling weights. Note that the global communication network structure information demand is eliminated. Moreover, the fully distributed dynamic edge-event-triggered mechanism is proposed to reduce communication bandwidth. Compared with the previous dynamic event-triggered mechanisms applied into dc microgrids, the continuous communication between neighboring agents is avoided, and controller updating frequency is reduced. Finally, the simulation and experimental results verify the proposed control performance.

A robust autonomous sliding-mode control of renewable DC microgrids for decentralized power sharing considering large-signal stability

DC microgrids (MGs) are providing a pathway toward a zero-carbon-based future. The intermittent renewable energy sources (RESs) and non-linear constant power loads (CPLs) thrive in DC MGs, craving for effective coordinative control solutions to ensure the stability of DC MGs. In this paper, a robust autonomous sliding mode control (SMC) scheme is proposed for achieving a globally stable and decentralized power sharing operation of multiple dispatchable units (DUs) in CPL-integrated DC MGs. Firstly, by using the high-order finite-time observer (HOFTO) technique, the disturbances, such as the power coupling and parameter uncertainties between different DU-interfaced converters, are self-eliminated within a finite time without any output current sensor and communication link. Secondly, a decentralized control scheme synthesizing the robust SMC and droop control algorithm is proposed to achieve proportional power sharing among paralleled DUs and precise DC bus voltage regulation. The proposed control guarantees the global system’s large-signal stability by ensuring the local stability of an individual converter, offering a simple yet effective stable coordinative solution. Finally, simulation and experimental results verify the effectiveness of the proposed strategy.

A Novel Reconfigurable Coordinated Control for Autonomous Low Voltage DC Microgrid

DC microgrids with a small electrical network of locally available energy resources and distributed loads are becoming popular in real-life applications, especially in remote areas and in some specific urban applications. This, in turn demands a serious insight into the control and reliability enhancement of such systems so that the availability of secure and safe energy can be assured to the users. But the integration of intermittent renewable energy resources in dc microgrids lead to a serious concern regarding the stability of the microgrid bus voltage under continuous change in available source power and the loading conditions. While the microgrid management is still a challenge under steady state operating conditions, it becomes much complex under some emergency events like faults, failure of controllers/sources and overloads etc. This paper proposes a novel reconfigurable coordinated control architecture for a Low Voltage DC (LVDC) microgrid comprises of solar photovoltaic, fuel cell based renewable energy sources supported Energy Storage System (ESS) formed with battery and ultra-capacitor banks. The reliability of the system is ensured in the proposed reconfigurable coordinated control with three layered hierarchy viz. Emergency control layer sandwiched between the supervisory and local control layers. The proposed control strategy is implemented through a new Microgrid Bus Controller (MGBC) strengthened by an “emergency” control layer which handles the responsibility in emergency conditions to coordinate the energy sources, loads and controllers. The MGBC facilitates the fault identification and isolation to ensure the reliable operation of the healthy microgrid sections and speedy fault recovery. The MGBC is dedicated for observing the bus parameters and identify the fault location by monitoring the bus branching currents. Some of the key hardware and simulations results validating the proposed architecture along with the details of the control implementation are presented in this paper.

Distributed Fixed Time Control for DC Microgrid with Input Delay

This paper develops a distributed fixed-time quadratic coordinated control strategy for isolated DC microgrids with time delays. The proposed control scheme based on the droop control strategy is intended to realize bus voltage recovery and current balance distribution under time delays. We first model the isolated DC microgrids as a single integrator system with input delays by the Artstein’s transform. The current balance and voltage recovery are achieved by using fixed time control, which does not depend on the initial state of the system. Based on the Lyapunov functional method, the stability analysis criterion of the system is derived. In order to verify the effectiveness of the proposed control method, the DC microgrid system is simulated in Matlab/Simulink.

Modeling and control of islanded DC microgrid fed by intermittent generating resources

In this paper an islanded microgrid fed through the wind and solar energy resources is presented. The power flow within the microgrid is controlled using a Neutral Point Clamped Dual Active Bridge (NPC-DAB) converter. In the proposed dc microgrid, the solar energy source is associated at the low voltage (LV) bus and the wind energy source is connected at the high voltage (HV) bus. A permanent magnet synchronous generator (PMSG) machine is used in wind energy conversion system. The real time solar radiation and wind speed data of Rupangarh, Rajasthan, India is used as an input for renewable energy resource. The NPC-DAB will work as a power electronics juncture for expediting the energy exchange in the islanded DC Microgrid. The proposed closed loop controller based on the capacitor voltage and load voltage will expedite a complete automatic operation of the islanded DC-microgrid considering various load changes. The system is studied without storage element as the automatic control of energy generation and load feeding is carried out by the NPC-DAB, also this makes the scheme cost effective. The optimum duty ratios for NPC-DAB operation are obtained and thus the increased load demand is met. The modeling of PMSG, NPC-DAB and wind energy system is discussed in details in this work. The proposed system is studied in MATLAB/Simulink environment and results are obtained for different load variations. All the wind control parameters, NPC-DAB waveforms, load waveforms are also plotted using MATLAB.

Affine projection‐like optimal control of UIPC in networked microgrids with controllable loads

AbstractThe upcoming generation of power systems would be outlined as a system of systems in which many AC and DC microgrids are interconnected to exchange power in a controlled manner. In this work, a new platform for the interconnection of multiple microgrids using a unified interphase power controller (UIPC) is presented. The networked microgrids operate in an islanded mode where voltage control in DC microgrids, and voltage and frequency control in AC microgrids are demanding. Therefore, we develop a new structure for the UIPC according to a current‐controlled four‐leg power converter configuration which can satisfy voltage and frequency support at the AC side. Due to many power and voltage oscillations, UIPC control is challenging in this heterogeneous environment. Therefore, the UIPC is equipped with a new affine projection‐like optimal control (APLOC) strategy which enables the microgrids to safely exchange power when necessary. Further, each AC or DC microgrid contains a new controllable load model which is commanded by the UIPC according to the power management paradigm. Simulation results proved the sufficiency of the proposed networked microgrids model in both controlling the exchanged power among microgrids and improving power quality problems.

Sliding Mode Control of Ship DC Microgrid Based on an Improved Reaching Law

The bus voltage of the ship DC microgrid is sensitive to the change of loads, which has an influence on the power supply quality. This paper introduces a hybrid energy storage system (HESS) that is composed of a battery set and a supercapacitor set, and further studied the control method of HESS. First of all, the topological structures of the ship DC microgrid and HESS are described. Second, combined with the frequency division droop control and voltage PI control, a sliding mode control (SMC) method is proposed to control the charge and discharge of HESS based on an improved reaching law. Finally, the simulation model of the ship DC microgrid is established for the verification of the control method. Simulation results show that: (1) HESS can overcome the shortage of the dynamic response ability of the diesel rectifier generator to the steep change of load power. The supercapacitor set and the battery set successfully respond to the high-frequency and low-frequency components of the differential power in the system, respectively. (2) Compared with the traditional PI control method, SMC can reduce the current chattering of HESS and the voltage fluctuation amplitude of the DC bus. The proposed SMC method can provide a reference for the stable and reliable operation of the ship DC microgrid.

Distributed event-triggered control of DC microgrids with output saturation constraint

To achieve both voltage restoration and proportional power sharing, we propose a novel distributed event-triggered secondary control for islanded DC microgrids that is subject to output saturation. Unlike the time-triggered control, the event-triggered control only updates its control input when the trigger condition is met, instead of updating the control signal periodically. As a result, the number of controllers’ updates and the burden of computation will be significantly reduced. Output saturation is an unavoidable limitation in almost every physical system which determines that sensor measurements cannot measure any arbitrary value. Thus, the present paper investigates how to achieve consensus of event-triggered control in the presence of output saturation constraint compared with the case where there is no output saturation. The proposed controller achieves voltage restoration and power sharing, whereas by using output saturation, the number of data transmissions among distributed generators reduces. Our proposed event-triggered condition is analyzed by Lyapunov synthesis to make sure it is stable and Zeno-free. As a final step, the proposed controller’s effectiveness is evaluated using the MATLAB/SimPowerSystems toolbox for an islanded DC microgrid.

Privacy-Informed Consensus-Based Secondary Control in Cyber-Physical DC Microgrids

This letter proposes a privacy-informed consensus-based distributed secondary control strategy for cyber-physical dc microgrids. The proposed secondary control approach relies on output masks that transform the physical states of distributed generation (DG) units to some auxiliary states by adding local perturbation signals, whose functional form and parameters are local and chosen independently by each DG unit. The proposed privacy-preserving secondary control scheme ensures voltage balancing and proportional current sharing in dc microgrids without disclosing the physical states of distributed generation units to their neighbors. Simulation case studies confirm the theoretical results of this letter.