DC Microgrid System Research Articles

DC Link Voltage Enhancement in DC Microgrid Using PV Based High Gain Converter with Cascaded Fuzzy Logic Controller

Renewable-based sources can be interconnected through power electronic converters and connected with local loads and energy storage devices to form a microgrid. Nowadays, DC microgrids are gaining more popularity due to their higher efficiency and reliability as compared to AC microgrid systems. The DC Microgrid has power electronics converters between the DC loads and renewable-based energy sources. The power converters controlled with an efficient control algorithm for maintaining stable DC bus voltage in DC microgrids under various operating modes is a challenging task for researchers. With an aim to address the above-mentioned issues, this study focuses on the DC link voltage enhancement of a DC Microgrid system consisting of PV, DFIG-based wind energy conversion system (WECS), and battery Energy Storage System (ESS). To elevate PV output voltage and minimize the oscillations in DC link voltage, a high-gain Luo converter with Cascaded Fuzzy Logic Controller (CFLC) is proposed. Droop control with virtual inertia and damping control is proposed for DFIG-based WECS to provide inertia support. Artificial Neural Network (ANN) based droop control is utilised to regulate the ESS’s State of Charge (SOC). The effectiveness of the proposed converter and its control algorithms for maintaining stable DC bus link voltage has been analysed using MATLAB/Simulink and experimentally validated using a prototype model and FPGA Spartan 6E controllers.

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.

An Imbalance-Status-Enabled Autonomous Global Power-Sharing Scheme for Solid-State Transformer Interconnected Hybrid AC/DC Microgrids

This paper proposes a novel imbalance-status (IS)-enabled autonomous global power-sharing scheme for a solid-state transformer (SST) interconnected hybrid AC / DC microgrid system. Firstly, the proposed scheme globally estimates instantaneous active power imbalances according to the existing droop regulation characteristics of each sub-microgrid, and normalizes these imbalances by introducing the definition of IS. Next, three novel IS-enabled controllers are designed for the SST’s three core converters, to autonomously share the power imbalances in the entire system. Then, the control parameters of the proposed scheme are optimized through small-signal analysis. The control effectiveness and flexibility of the proposed scheme in realizing global power-sharing function are verified by controller hardware-in-loop experiments in RT-LAB platform.

Coordinated Distributed Predictive Control for Voltage Regulation of DC Microgrids With Communication Delays and Data Loss

This paper is concerned with the voltage tracking problem of DC microgrids subject to communication delays and packet losses, for which existing work commonly adopts passive fault-tolerant approaches. To accurately compensate for the above communication constraints experienced by DC microgrids, a coupled discrete microgrid model is developed on the basis of the physical laws of actual DC microgrids. Unlike the practice of passively tolerating communication delays, through the physical model established, this paper suggests a consensus-based proportional-integral predictive control strategy, which can actively compensate for communication delays and consecutive packet dropouts encountered by DC microgrids. Under the observer-based distributed networked predictive control framework, each distributed generation subsystem in the microgrid exchanges its own measurements over the network and integrates information from controllers of other units to achieve output voltage consensus in the presence of time delays and packet losses. Furthermore, to demonstrate the generality of the proposed method, the sufficient and necessary conditions for the DC microgrid system to accomplish voltage tracking are given. These conditions are rendered in the form of matrix eigenvalues associated with the physical connections and communication couplings between distributed generation units. Besides, the stability and convergence of the closed-loop microgrid system are given in the form of linear matrix inequality based on the Lyapunov function. Finally, the performance of the proposed control scheme is evaluated in terms of its convergence, robustness to load variations, and plug-and-play functionality through the built photovoltaic cell-based (with battery banks) DC microgrid hardware system.

Blockchain Protocol-Based Secondary Predictive Secure Control for Voltage Restoration and Current Sharing of DC Microgrids

DC microgrids with distributed architectures inevitably encounter the threats of communication constraints and cyber attacks, and there is little research to address these two communication irregularities simultaneously. To provide DC microgrids with maximum resistance to communication constraints and cyber attacks, a distributed predictive secure control method based on blockchain protocol is proposed in this paper. The proposed control approach is the first attempt to address cyber attacks with a hybrid of data and models, improving the resilience and robustness of DC microgrids. With the help of practical Byzantine ideas, the proposed strategy preserves the resilience strengths of the blockchain against corruption and discards its shortcomings of low real-time performance, significantly enhancing the security without compromising the dynamic performance of the system by active intervention. Subsequently, a bound on the communication delay induced by the blockchain network under the expected security index is given based on the theoretical analysis. Unlike the existing practice of passively tolerating communication delays and packet dropouts, the predictive control method proposed in this paper actively compensates for them experienced by DC microgrids. Then, an analytical model of the closed-loop DC microgrid system is developed, and stability analysis is presented accordingly. Finally, several experimental tests performed on a PV-based DC microgrid hardware system are presented to demonstrate the effectiveness of the proposed control strategy.

Robust adaptive composite control of DC–DC boost converter with constant power load in DC microgrid

Power electronic loads are penetrating in DC microgrid. These loads are usually regarded as constant power loads (CPLs) with negative impedance characteristics, which seriously affect the dynamic performance of DC bus voltage and may result in instability of the DC microgrid system. To improve the performance and stability of a boost converter with CPL, proposed herein is a robust adaptive composite control strategy, which mainly includes two control loops. An affine nonlinear mathematical model is established. Based on the differential geometry theory, the minimum phase problem is analyzed under different output functions. In the inner control loop, the inductor current is linearized by using exact feedback linearization theory to a linear system. Meanwhile, sliding mode control method is studied to control the linear system, and an adaptive law is designed to update the sliding mode switching gain. In the outer control loop, the output voltage is regulated by proportional–integral control technology to achieve no steady-state error for the DC bus voltage. Furthermore, the robustness of the closed-loop system is verified, and the global asymptotic stability of the control system is proved. Compared with the existing method, both simulation results and experimental results verify the validity and superiority of the presented control strategy.

Large-signal modeling and stable region estimation of DC microgrid with virtual DC machine control

In order to enhance the anti-disturbance capability of a DC microgrid, the virtual DC machine control (VDMC) is usually utilized. However, the existing literature on VDMC mainly focuses on the small-signal modeling based on linearization technique, which cannot be used to describe the system dynamics under large disturbance. Furthermore, the power electronics-based loads have the features of constant power characteristic, which increases the nonlinear of the DC microgrid. In such case, it becomes difficult to obtain the system stability region accurately via the small-signal methods. To settle such problem, the simplified large-signal model of the DC microgrid system with VDMC is established via the TS fuzzy modelling method. Thereupon, an approach to estimate the asymptotic stability region of the system is put forward. The region of asymptotic stability can be used to estimate the maximum undertaking changes of both the DC bus voltage and load power. Finally, the accuracy of the proposed model and the effectiveness of the presented methods are verified by simulations and experiments.

Optimized power flow management based on Harris Hawks optimization for an islanded DC microgrid

Abstract This article presents an energy management system (EMS) in a DC microgrid (MG) operating in an islanded mode to control the power flow in the distribution network. The microgrid system considered in this research consists of distributed generation sources like a solar photovoltaic system, a fuel cell energy system, and an energy storage system controlled by an optimized energy management system. As the distributed energy sources used are primarily renewable, unpredictable weather conditions may cause irregular energy generation. These variations impact the power flow in the DC bus, making it challenging to maintain a supply and demand balance. Therefore, an intelligent energy management system using the Harris Hawks Optimization (HHO) is implemented to enhance the microgrid’s performance and efficiency. The HHO algorithm is based on the hunting nature of the Harris Hawks, and the EMS is developed to maintain the optimal power flow and to handle the constraints. The performance of the presented system is analyzed with the particle swarm optimization (PSO) based Proportional Integral (PI) controller in different operating scenarios to validate the effectiveness of the DC microgrid system.

Dual-Terminal Voltage Feedforward Based Direct Power Control Scheme and Stability Analysis of Dual Active Bridge Converter in DC Microgrid Systems

The dual active bridge (DAB) dc-dc converter plays an important role in energy exchange between DC microgrids integrated with energy storage systems. To directly control the power flow between the adjacent DC microgrids rapidly and accurately, a dual-terminal voltage feedforward based direct power control scheme (DVF-DPC) with single-phase-shift control is proposed for the DAB dc-dc converter. The strategy makes the converter realize rapid and precise output power regulation under the following conditions, including the start-up process, load step-change, the desired output power step-change, and voltage fluctuations. An accurate discrete-time model is applied to analyze the system's dynamic characteristics, and a discrete-time small-signal model of the system is established to design the controller's parameters. In addition, a comprehensive and systematic analysis of all the possible unstable phenomena for adjacent DC microgrids is conducted based on the discrete-time model. Accordingly, the underlying mechanisms are revealed by analyzing the relationships among the state variables, the Floquet multipliers, and system parameters. Hence, the stability-oriented parameter design method is given which can provide guidance in practice. The theoretical analysis is verified in both the time domain and frequency domain. Finally, by comparing the DVF-DPC method with the traditional PI feedback control, the superiority of the proposed strategy is verified through theoretical analysis and experimental results, which demonstrates its application prospect in the field of electric energy dispatch control of the DC power supply network.

Optimum Sizing and Modeling of Stand-Alone DC Microgrid With Hybrid Energy Storage System for Domestic Applications: Cost of Energy, Net Present Cost, and Feasible Configurations

In this article, optimum sizing and modeling of a stand-alone dc microgrid (DCMG) system for domestic applications with hybrid storage system is been proposed. The hybrid storage system consists of a lithium-ion battery (LIB) and supercapacitor (SC). The DCMG is designed to meet the load requirements of a house located in Aruppukottai, in the southern region of Tamil Nadu, India. The daily energy demand is estimated as 7.77 kWh with a peak load of 1.19 kW. This DCMG is designed and simulated by using Homer Pro software. The optimum sizing of solar photovoltaics (PVs), wind turbine (WT), LIB, and SC is evaluated based on cost of energy (CoE) and net present cost (NPC), and all feasible configurations are discussed. The feasible configurations include PV+LIB, PV+WT+LIB, WT+LB, PV+WT+SC, PV+SC, and WT+SC and are compared in terms of CoE and NPC. The results show that the PV+LIB architecture is the optimum configuration, with a CoE of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\$}$</tex-math></inline-formula> 0.2/kWh and an NPC of $7,334.

Optimal Scheduling of DC Microgrids based on MOPSO Algorithm

The DC microgrid system contains a variety of distributed power sources, but the distributed power sources are uncertain and intermittent. In order to improve the economy of DC microgrid operation, mainly to reduce the microgrid operation cost and environmental management cost, and at the same time the largest proportion of scenery consumption, the optimization algorithm is effective for microgrid for optimal scheduling. A multi-objective optimal scheduling model for microgrids is established. A microgrid model with wind, photovoltaic, micro gas turbine and energy storage is constructed and solved using a multi-objective particle swarm algorithm. The multi-objective particle swarm algorithm is invoked for a typical 24h daily isolated micro-grid system in a certain region. The simulation results show that the optimal dispatching scheme can effectively improve the economy of the microgrid, reduce its operating costs as well as the ability to consume renewable energy scenery power.

Optimized Bidirectional DC-DC Converter Adapted to High Voltage Gain and Wide ZVS Range

As a key component connecting dc microgrid and energy storage system, dual active bridge (DAB) converter has some inherent problems under high gain conditions. To solve the problems, this article proposes a half-bridge cascaded DAB converter with partial switch sharing. The converter combines the advantages of high voltage gain, wide soft switching range and less reactive backflow loss, under a hybrid modulation strategy based on pulsewidth modulation control and phase-shift control. In this article, the output power, soft switching characteristic and reactive backflow loss are analyzed in detail. By using the Karush–Kuhn–Tucker function method to achieve multiobjective optimization, the system can achieve the zero voltage switch (ZVS) turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> and reduce the reactive backflow loss. Moreover, the coupled relationships among variables are clarified clearly, and the system can achieve high voltage gain at low turns ratio. A 500 W, 100 kHz experimental prototype is built to verify the correctness of the theoretical analysis.

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.

PID Controller for DC-DC Converter under Dynamic Load Change in Photovoltaics based Low-Voltage DC Microgrid

Today, DC Microgrid gain more attraction due to increasing electronic digital devices application such smart-phones, smart-tvs, and other digital apparatus which are operated in DC form. In the common grid, the electric power from AC source is converted to DC voltage for powering the digital devices as load. Hence, there are power conversions from AC-DC and potentially loss energy during conversions. DC Microgrid consisted of sources, loads, distribution lines and energy storages. In small capacity DC Microgrid, the stability of the system is vulnerable by dynamic load change. During load demands fluctuations, the DC Microgrid voltage is also dynamically fluctuated and can reach over the designated rate. To solve this problem, the PID controller is introduced in the DC-DC converter for maintaining the voltage rate at designated value regardless the load demands. In this paper, the DC Microgrid is consisted of photovoltaics as DC sources, XL-6019 as DC-DC converter, Arduino as controller, voltage and current sensors, distribution lines and loads. The proposed method is evaluated via experimental results. The responses of the proposed method in the DC Microgrid system are presented, evaluated, discussed, and compared between with and without applied method. The experimental results indicate that the proposed method has ability to reduce the voltage profile fluctuations during load demands changes and in short time.

Guest editorial: Application of cloud energy storage systems in power systems

Guest editorial: Application of cloud energy storage systems in power systems

Coupled Inductor Based Soft Switched High Gain Bidirectional DC-DC Converter With Reduced Input Current Ripple

Bidirectional dc–dc converter (BDC) is an integral part of energy storage interface, where high efficiency, high voltage transfer ratio and small input ripple current are essential for application, such as dc microgrid and electric vehicle. In this article, a new approach is proposed to achieve reduced input ripple current at the low voltage side, which potentially replaces the necessity of using interleaved circuit structure. Parallel coupled inductor and capacitor circuit are utilized for charging and discharging the two parallel paths in a complementary fashion using a <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> . The proposed solution helps to increase the voltage conversion ratio unlike interleaved structure. Additionally, in proposed BDC, zero voltage switching turn <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> operation of all active switches are possible utilizing synchronous rectification principle in both the operating modes. Parallel path structure not only reduce the input current ripple, but also helps to share the input current, thereby reducing the coil size and associated losses. A 250 W BDC is designed to validate performance in 1kW dc microgrid system

Robust Nonsingular Terminal Sliding Mode Control of a Buck Converter Feeding a Constant Power Load

In recent years, DC microgrid systems feeding constant power loads (CPLs) have been given a particular focus due to their effect on the overall system stability caused by their electrical characteristics behaving as negative incremental impedance. To address this issue, this paper investigates the stabilization of a DC bus voltage in a DC microgrid (MG) feeding a CPL. The output voltage of the main DC bus is stabilized by using a robust nonsingular terminal sliding mode controller that is characterized by the elimination of the singularity problem that arises from the conventional terminal sliding mode controller. The CPL is emulated by a boost converter where its output voltage is tightly regulated. The system is investigated in terms of voltage following and disturbance rejection. The robustness and effectiveness of the proposed control strategy are assessed against input voltage fluctuations and power demand variations. The proposed controller is validated through simulations and an experimental setup.

Source-load-variable voltage regulated cascaded DC/DC converter for a DC microgrid system

&lt;span lang="EN-US"&gt;Solar energy is available abundantly, the utilization of solar energy is developing rapidly and the photovoltaic based direct current (&lt;/span&gt;&lt;span lang="EN-US"&gt;DC) microgrid system design is under demand but the stability of the DC voltage is of most important issue, as the variation of the output DC voltage is a common problem when the load or source voltage varies, hence a regulated DC output voltage converter is proposed. This paper presents source-load-variable (SLV) voltage regulated cascaded DC/DC converter which is used to obtain regulated output voltage of 203.1 V DC at 0.4 duty ratio with ±2% voltage fluctuations for the variation in the input source voltage and ±1.5% voltage fluctuations for the variation in load resistance of the nominal value with lower output voltage ripple and without use of sub circuits. A simulation model of SLV voltage regulated cascaded DC/DC converter in LTspice XVII software environment for the assessment of converter performance at different input source voltages and load resistances are verified.&lt;/span&gt;

Mixed-potential-function-based large-signal stability analysis of DC microgrid with constant power loads

With the development of power grid technology, a large number of constant power loads (CPL) are introduced into the power grid system. The negative damping characteristic of CPL poses a great challenge to the stability of power grid under the condition of large signal disturbance. This paper focuses on the large signal stability and dynamic response characteristics of DC microgrid with CPL. The mixed potential function (MPF) method is adopted to model the large signal characterstic of load converter controlled by the traditional PI control strategy and the new virtual DC motor (VDM) control strategy. The large signal stability criteria and the asymptotic stability region of the DC microgrid system under the two control strategies are derived. The dynamic response characteristics of the DC microgrid are compared under three large signal operation conditions of the load step, load linear change and the load fault. The results show that VDM control strategy can effectively improve dynamic response characteristics of the DC microgrid system compared with those of PI control, and enhance the stability and anti-interference ability of the DC microgrid system.

Optimized sensor charge controller for bus voltage stabilization in hybrid battery-supercapacitor fed islanded microgrid system

This paper proposes a novel Energy Storage Device (ESD) charge controller for integrating multiple hybrid ESD (battery-supercapacitor) stacks within the DC microgrids. A low-cum-medium power islanded DC microgrid system, consisting of Distributed Energy Resources (DERs), loads and isolated/non-isolated multiple hybrid ESDs is configured for this purpose. The hybrid ESDs tackle the source-load and DC bus voltage fluctuations smoothly, while multiple ESDs foster system redundancy. As opposed to conventional ESD controllers where sensory requirements increase proportionally with the number of ESDs converters connected, the proposed algorithm uses just one voltage sensor placed at the DC link bus. The Power Management Strategy (PMS) is designed accordingly, considering the system power flow status and State of Charge (SOC) of each ESD. The minimum use of sensors leads to simplified control circuitry, better system stability (less control error tuners required), ease of hardware implementation and makes system economical for market outreach. The viability of the proposed ESD controller for multiple hybrid isolated/non-isolated ESDs converter has been tested through the simulation platform during the transient conditions. The power exchange between different ESDs in factors of 1:1 and 1:2 is verified. The DC link voltage is stabilized with the ripple factor of <1.0 %. For experimental verification, scaled-down laboratory prototype of DC microgrid (≈ 0.25 to 0.5 kW) is made and tested in ESD charging-discharging modes. A fast transient response slope of up to 2.0 kW/s. is tested using the proposed controller on the experimental platform. Since the proposed controller is less computation intensive, a low-cost microcontroller serves the purpose.