Operation Of DC Microgrid Research Articles

A Novel State-of-Charge-Based Droop Control for Battery Energy Storage Systems to Support Coordinated Operation of DC Microgrids

A modern dc microgrid often comprises renewable energy sources (RESs), such as photovoltaic (PV) generation units, battery energy storage systems (BESSs), and local load, and it is also connected to the utility grid through a point of common coupling (PCC). While most existing approaches have to rely on communication links to achieve the desired control performance, this article proposes a novel control strategy without resorting to the communication links. This is achieved by assigning BESSs as master units and regulating the dc bus voltage with a novel state-of-charge (SoC)-based droop control, where the BESSs coordinate the slave units (e.g., RES and utility grid) with the aid of the dc bus signaling (DBS) technique to avoid overcharging and overdischarging of these BESSs. In the proposed droop control, the reference voltage for these BESSs is designed for coordinated operation between BESSs and utility grid, and it is maintained constant in the normal SoC range, which can reduce dc voltage variation. Droop coefficients designed for SoC balance of BESSs are dynamically adjusted based on their own SoC values. Furthermore, the preset maximum deviation between the reference voltage and the dc bus voltage ensures the reliable coordinated operation. Real-time hardware-in-loop (HIL) tests considering three different scenarios are conducted to validate the effectiveness of the proposed method.

Resilient Distributed Control Against Destabilization Attacks in DC Microgrids

The distributed control provides a flexible and scalable solution to coordinate voltages and currents in DC microgrids but introduces intrusion risk into the system. Destabilization attacks can manipulate system states by modifying the transferred data, which threatens the stable operation of DC microgrids. In this paper, a resilient distributed control consisting of an integrated distributed control and an additional state limitation strategy is proposed to cope with destabilization attacks. The current-sharing regulation is integrated into the average voltage regulation in the integrated distributed control. A steady-state analysis is conducted to verify the superior performance of the integrated distributed control against destabilization attacks. As a supplement, state limitation strategies aim to actively restrict magnitudes of system states. Inspired by results in the attack analysis, virtual attacks are constructed to limit the voltages and currents within tolerant ranges. Several cases, including the simulation and the experiment, are conducted to demonstrate the effectiveness of the proposed resilient distributed control.

Distributed Mitigation Layers for Voltages and Currents Cyber-Attacks on DC Microgrids Interfacing Converters

The wide use of communication layers in DC microgrids to transmit voltage and current measurements of each distributed generator unit (DGU) increases the possibility of exposure to cyber-attacks. Cyber-attackers can manipulate the measured data to distort the control system of microgrids, which may lead to a shutdown. This paper proposes distributed mitigation layers for the false data injection attacks (FDIA) on voltages and currents of DGUs in meshed DC microgrids. The proposed control strategy is based on integrating two layers for cyber-attack detection and mitigation to immune the primary and the secondary control loops of each DGU. The first layer is assigned to mitigate FDIAs on the voltage measurements needed for the voltage regulation task of the primary control loop. The second layer is devoted to the mitigation of FDIAs on the DGU current measurements, which are crucial for the secondary control level to guarantee the proper current sharing of each DGU. Artificial neural networks (ANNs) are employed to support these layers by estimating the authenticated measurements. Different simulation and experimental case studies are provided to demonstrate the proposed mitigation layers’ effectiveness in detecting and mitigating cyber-attacks on voltage and current measurements. The simulation and experimental results are provided to evaluate the dynamic performance of the suggested control approach and to ensure the accurate operation of DC microgrids despite the existence of cyber-attacks on the measurements employed in the control strategy. Moreover, the control strategy succeeds to keep the maximum voltage error and the maximum error in current sharing within tolerance.

A new hybrid control technique for operation of DC microgrid under islanded operating mode

This study proposes a novel combined primary and secondary control approach for direct current microgrids, specifically in islanded mode. In primary control, this approach establishes an appropriate load power sharing between the distributed energy resources based on their rated power. Simultaneously, it considers the load voltage deviation and provides satisfactory voltage regulation in the secondary control loop. The proposed primary control is based on an efficient droop mechanism that only deploys the local variable measurements, so as to overcome the side effects caused by communication delays. In the case of secondary control, two different methods are devised. In the first, low bandwidth communication links are used to establish the minimum required data transfer between the converters. The effect of communication delay is further explored. The second method excludes any communication link and only uses local variables. Accordingly, a self-sufficient control loop is devised without any communication requirement. The proposed control notions are investigated in MATLAB/Simulink platform to highlight system performance. The results demonstrate that both proposed approaches can effectively compensate for the voltage deviation due to the primary control task. Detailed comparisons of the two methods are also provided.

Resilient operation of DC microgrid against FDI attack: A GRU based framework

DC microgrid is the most susceptible to cyber-attacks as the communication channel is involved for the implementation of the secondary controller. Accordingly, the false data are injected into the transmitted data (i.e., DC bus voltage) and it may lead to deteriorating the system performance. To address these issues, the gated recurrent unit (GRU) based mechanism is presented to eliminate the false data injection (FDI) attack for the resilient operation of the DC microgrid. The presented GRU-based framework is divided into two parts: 1) estimation strategy: an offline-trained GRU based network is employed herein for online evaluation of the actual DC bus voltage, and 2) mitigation strategy: GRU based trained network is exploited herein with an amalgamation of the proportional-integral (PI) controller to counteract the malicious cyber-attack. The presented GRU-based framework has several advantages such as ease of implementation and computationally efficient, unlike state-of-art methods. The sensitivity analysis is investigated herein to validate the effectiveness of the presented GRU-based framework over state-of-art techniques. Simulation results show satisfactory performance under manifold operating scenarios such as bias injection attack and time-varying attack. In addition, the quantitative and qualitative comparative performances are performed herein to demonstrate the efficacy of the presented framework.

Projections of Cyberattacks on Stability of DC Microgrids—Modeling Principles and Solution

Microgrids relying on cooperative control are supported by communications, which are highly vulnerable to cyberattacks. A significant amount of research is already carried out on the detection and mitigation of cyberattacks to secure the operation of dc microgrids. Although cyberattacks are fully capable of causing cascaded converter outages leading to full/partial system blackouts, disturbing the system stability can also be a viable target by the adversaries, which has been overlooked so far. Hence, this article focuses on addressing the instability caused by <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">stealth</i> cyberattacks, which can easily bypass the well-defined observability tests. In addition, this article also introduces a novel adaptive stabilization method to eliminate the unstable modes due to cyberattacks, which has been designed considering a previously defined cyberattack detection metric as an input. To investigate its feasibility, a detailed model of a stable dc microgrid is first developed. Then, considering <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">stealth</i> cyberattack as a nonlinear element, the describing function-based method is used to investigate system stability under attack conditions. Finally, theoretical analysis, simulation, and experimental results under various scenarios are presented to verify the effectiveness of the proposed stabilization scheme.

Optimal Operation of Standalone DC Microgrids Considering Renewable and Load Uncertainties

The rapid growth and development in DC microgrids (DCmGs), led system-level operation and control to be reformed continuously imbibing constraints and concerns that improve system operational performance. Focusing only on islanded DCmGs, quite a literature/work has been attempted in various perspectives to solve energy management problems. In light of this aspect, an optimal operation/control of standalone DCmGs, composed of the droop-regulated dispatchable type distributed generators (DGs) considering generation cost, voltage deviation, and current-sharing objectives has been proposed. The proposed work includes the uncertainties in network variables such as load demand, wind, and solar power generations. The scenario-based analytical method has been employed for modeling the uncertainties stated. For optimal operation, the droop parameters were solved by means of the heuristic-based multi-objective optimization technique, Dragonfly Algorithm (DA). The proposed methodology has been implemented on modified 6-bus and 33-bus test systems operated as DCmGs. Furthermore, the applicability of the proposed approach has been presented for the case study with battery energy storage system (BESS) scheduled DCmG network operation on a 24-hour time horizon. The obtained results have been compared with other optimization algorithms to validate the accuracy of the proposed approach.

Robust stability analysis of a novel droop‐based distributed control scheme for islanded operation of DC microgrids

DC microgrid is a high-efficiency plan for exploiting and controlling distributed generation (DG) resources that can supply local loads in grid-connected or islanded mode. Conventionally, the droop control method is utilized to attain proper load sharing among the DGs as a decentralized approach in DC microgrids. However, accurate current sharing among DG units with different line resistances is not achievable by the conventional droop control, whereas this method also increases the DC bus voltage deviation. In this paper, a new droop-based distributed control method is proposed in the secondary control level to overcome the limitations of droop control. To implement the proposed method, only the amount of precalculated virtual voltage drop of DG units is shared with adjacent DGs through a low-bandwidth communication, and other calculations are local. So, there is no need to define and share new parameters, and get feedback from the DC bus voltage. By employing the proposed method, not only accurate current sharing among the DG units is guaranteed but also DC bus voltage is restored to the nominal value. To ensure the stability of the proposed method, the robust stability and performance of the closed-loop system are analyzed by structured singular value (µ) under resistive load and communication delay changes. Finally, a DC microgrid is evaluated using MATLAB/Simulink as well as experimental tests to illustrate the validity and efficiency of the proposed method.

Fuzzy piecewise coordinated control and stability analysis of the photovoltaic‐storage direct current microgrid

In order to ensure the safe and reliable operation of photovoltaic-storage DC microgrid in islanded mode, this paper proposes a piecewise coordinated control strategy based on fuzzy logic to independently judge the working mode of microgrid. Firstly, according to the operation characteristics of microgrid, the device-level control of each unit (photovoltaic, energy storage and load) is designed successively. Then, according to the requirement of supply and demand balance of microgrid, fuzzy logic is introduced to realize the independent division of threshold nodes on the basis of piecewise control, so as to ensure the fast switching of working mode and stable operation of the system in various complex environments, that is system-level control. And the method of impedance analysis is used to analyze the stability of microgrid under different working modes. Finally, the effectiveness and feasibility of the proposed coordination control strategy are verified by MATLAB/Simulink simulation results.

Research on coordinated control strategy of isolated DC microgrid with PV/hybrid energy storage

During the operation of DC microgrid, energy storage system plays an important role in supplying the power difference between distributed generation unit and load and maintaining the voltage stability of DC bus, in recent years, hybrid energy storage technology has gradually attracted the attention of researchers. In this paper, a power distribution control strategy of hybrid energy storage system (HESS) is studied. The droop control based on virtual capacitor is used for the converter of supercapacitor (SC) to realize the power distribution in HESS, and the control strategy is improved to solve the problem that the deviation of bus voltage caused by droop control. Based on the control strategy of HESS, a coordinated control strategy of isolated DC microgrid is studied. By considering SOC of battery and the power demand of load, 3 operation modes of DC microgrid are set. DC microgrid can work in a designated mode under various conditions to achieve efficient and stable operation. Finally, the theory of the above control strategies is verified by the simulation work in RTDS and hardware-in-the-loop experiment based on DSP28335 and RTDS.

Accurate Distributed Secondary Control for DC Microgrids Considering Communication Delays: A Surplus Consensus-Based Approach

The state-of-the-art dynamic consensus-based microgrid (MG) secondary controls suffer from the communication delay effect. Specifically, the system could not converge to the desired operating points with time-delayed communications. Such deviations are hard to detect in a decentralized manner and could destabilize the system. This paper proposes an accurate distributed secondary controller for DC MGs based on the surplus consensus algorithm. The proposed controller achieves accurate proportional power sharing and average voltage regulation among distributed generators (DGs) with the presence of variable and bounded communication delays. A surplus consensus-based observer is developed. The developed observer is proved robust against variable and bounded communication delays; it tracks the average of a group of dynamic states with zero steady-state deviations, which cannot be done using the conventional dynamic consensus-based observer. The convergence speed of the developed observer is analyzed and a parameter design procedure is presented. Moreover, the delay-dependent stability analysis of DC MG operation with the proposed secondary controller is derived. The marginal delay that leads the system to instability is calculated. At last, the performance of the proposed secondary controller and the developed stability analysis are validated under various scenarios using MATLAB/Simulink.

Research on Coordinated DC voltage Control Strategy of DC Microgrid based on Photovoltaic Power Generation System

DC microgrid system is a flexible and commonly used renewable power generation system. Among them, the energy storage system (ESS) can reduce the waste light of the photovoltaic (PV) power generation system and provide stable power output for the power grid. However, a reasonable control strategy is needed to realize the energy balance of PV, grid-connected converter and ESS, so as to ensure the safe and stable operation of DC microgrid. Therefore, this paper proposes a DC voltage coordination control strategy without communication. The control strategy can automatically coordinate the working state of each converter of DC microgrid, which according to the change of DC voltage and the ESS’s state of charge (SOC), At the same time, the inertia of DC voltage in each working state is analyzed, and the stability of DC microgrid is improved through the backup inertia support of ESS. Finally, the effectiveness of the proposed method is verified by simulation and experiments.

Optimal DC Microgrid Operation with Model Predictive Control-Based Voltage-Dependent Demand Response and Optimal Battery Dispatch

Recently, the integration of optimal battery dispatch and demand response has received much attention in improving DC microgrid operation under uncertainties in the grid-connect condition and distributed generations. However, the majority of prior studies on demand response considered the characteristics of global frequency variable instead of the local voltage for adjusting loads, which has led to obstacles in operating DC microgrids in the context of increasingly rising power electronic loads. Moreover, the consideration of voltage-dependent demand response and optimal battery dispatch has posed challenges for the traditional planning methods, such as stochastic programming, because of nonlinear constraints. Considering these facts, this paper proposes a model predictive control-based integrated voltage-based demand response and batteries’ optimal dispatch operation for minimizing the entire DC microgrid’s operating cost. In the proposed model predictive control approach, the binary decisions about voltage-dependent demand response and charging or discharging status of storage batteries are determined using a deep-Q network-based reinforcement learning method to handle uncertainties in various operating conditions (e.g., AC grid-connect faults and DC sources variations). It also helps to improve the DC microgrid operation efficiency in the two aspects: continuously avoiding load shedding or shifting and reducing the batteries’ charge and discharge cycles to prolong their service life. Finally, the proposed method is validated by comparing to the stochastic programming-based model predictive control method. Simulation results show that the proposed method obtains convergence with approximately 41.95% smaller operating cost than the stochastic optimization-based model predictive control method.

Bidirectional Power Sharing of Modular DABs to Improve Voltage Stability in DC Microgrids

The dual active bridge (DAB) converters have the capability of handling high power in compact form and it is one of the key elements for interconnecting different dc microgrid clusters. A DAB also offers bidirectional power transfer with galvanic isolation between two power buses of similar or different voltage levels, i.e., low voltage and high voltage buses. In microgrid applications, bus voltage regulation is necessary for an uninterrupted power flow and reliable operation of dc microgrids. Due to the galvanic isolation and modularity nature of a DAB, the output can be easily interconnected for efficient and reliable power sharing. In this article, taking the advantage of modularity property of the DAB, an efficient and improved power sharing scheme is developed to stabilize the bus voltage in microgrid applications. A robust bidirectional power sharing control strategy is developed between DABs from different high voltage buses to low voltage buses that improves the voltage stability. An advanced supervisory controller is designed by considering the nonlinearity and uncertainty that ensures stable dc bus voltage and reliable power sharing among DABs under different operating conditions. The controller is validated theoretically and a small-scale laboratory prototype is developed. The concept is verified experimentally at different possible scenarios. From theoretical and experimental results, it is found that the proposed DAB control provides excellent performance in terms of power sharing, dc-bus voltage regulation and power quality.

A Dynamic and Cooperative Control Strategy for Multi-Hybrid Energy Storage System of DC Microgrid Based on SOC

With the increasingly serious crisis of fossil energy and environmental pollution, clean renewable energy becomes the inevitable choice of energy structure adjustment. The instability of output power of distributed renewable energy system greatly affects the operation of DC microgrid. The hybrid energy storage system (HESS) composed of High-Energy Battery (HEB) and High-Power Battery (HPB) can solve the above problems. Thus, this paper proposes a dynamic and cooperative control strategy for multi-HESS based on state of charge (SOC). Based on the traditional LPF method and droop control, this paper proposes a control strategy that requires no communication among multiple hybrid energy storage (HES) modules. This method can realize the stable control of HEB current, reduce the change times of HEB charging-discharging mode, prevent HEB from overcharging and overdischarging, prolong the service life of HEB and balance different energy storage SOC, so as to improve the operation stability and economy of DC microgrid. In addition, the method has certain robustness against sudden failures. Simulation and experiment results show the effectiveness of the proposed method.

Robust DC microgrid operation with power routing capabilities

This paper proposes a robust DC microgrid (MG) operation scheme for intentional power routing (PR) within the MG or between the MG and the utility grid (UG). The MG consists of a photovoltaic (PV) system, a battery energy storage system (BESS), and AC loads connected to the UG via a DC/AC power conditioning system. The operation is achieved by controlling the UG converter and the converter of the BESS in the slack mode with the same or different terminal voltages. In this way, the bus voltage is regulated in a variable range while the magnitude and direction of power exchange between the UG and the MG are realized at the primary voltage control level. The procedure to select the voltage references of the terminals and the control schemes are presented. The system has been implemented in PSIM environment and simulation results proves the reliability and stability of the design.

RES Based Islanded DC Microgrid with Enhanced Electrical Network Islanding Detection

An electrical islanding detection method for DC microgrid (MG) is proposed in this paper. Unlikely conventional AC MG system protection has been challenging for the DC MG system. The goal of the proposed scheme is to detect the islanding intelligently within the agent nodes. The proposed islanding detection technique detects the electrical link failure intelligently, and if any electrical link failure occurs, then the proposed technique maintains the MG operation and load sharing. Islanding detection is carried out using an injection signal and utilizes the existing communication network to check network connectivity. After checking the network connectivity, all the nodes maintain the look-up table of the connected network. This research work illustrates that the proposed electrical islanding detection technique is effective in maintaining the DC MG operation in the case of an electrical islanding/link failure scenario. The proposed scheme’s performance is checked through MATLAB/Simulink for the detection of islanding and maintaining the operation.

Multipurpose water storage tank based DC microgrid system for isolated communities

AbstractRenewable energy sources can contribute a lot to deliver reliable/resilient power to isolated communities and the world's sustainable energy future. In this paper, a novel DC microgrid system has been proposed for multipurpose use for isolated communities. The system taps the unused potential of the storage water to generate electricity. The system is configured to reduce complexity and cost by reducing the sensors in the solar pumping unit. Source, storage, and loads are the parts of an isolated system. The DC microgrid includes solar PV, battery, and domestic loads (All types' loads, that is, DC, AC, and dynamic loads). Incremental conductance algorithm has been chosen for the MPPT control, and sliding mode controls have been implemented to supply the sinusoidal supply for the AC loads. The system has included a battery unit to support the DC bus voltage and stable DC microgrid operation. The centrifugal pump has been taken to operate the pump as a turbine to extract the partial power potential from the water. The proposed system's MATLAB/Simulink model was designed for a village and presented as a case study.

Voltage-Based Segmented Control of Superconducting Magnetic Energy Storage for Transient Power Fluctuation Suppression in Island DC Microgrid

Voltage stability is one of the critical factors for the stable operation of DC microgrids (MG). For the communication free DC MG, the DC voltage is more vulnerable due to the DC voltage deviation caused by the droop characteristics. When facing the transient power fluctuation caused by multiple electric vehicles (EVs) connected to the grid, PV shedding, etc., the DC bus will have a large voltage sag. The voltage quality degradation will bring adverse effects to the DC MG. However, most energy storage systems like battery energy storage systems (BESS) are not suitable for suppressing transient large power fluctuation. In this paper, the SMES is introduced to a DC MG to form a power regulation system. A voltage-based segmented (VBS) control is designed for the SMES-based power regulation system to suppress transient large power fluctuation in the DC MG. The voltage deviation caused by power distribution can be tolerant by VBS controlled SMES. Meanwhile, the dynamic recovery capability of the DC voltage is improved. The control method of SMES is independent of other devices in DC MGs, hence it is not dependent on communication. The control strategy of the SMES is further verified by simulation results.

Cooperative Control of SFCL and SMES-Battery HESS for Mitigating Effect of Ground Faults in DC Microgrids

DC fault is one of the most important and critical challenges for stable operation of DC microgrids. Resistive type superconducting fault current limiter (R-SFCL) is an ideal protective solution to limit DC fault current. However, the effect of only using SFCL may further deteriorate the DC bus voltage stability. This paper first analyzes the effect of R-SFCL on DC microgrid under different operation modes and different fault locations, and then presents a coordinated control method of SMES-battery hybrid energy storage system (HESS) and SFCL in ground fault. This method can not only reduce the fault current level but also regulate the DC bus voltage. Finally, the comparative study is conducted in PSCAD/EMTDC, the feasibility and superiority of the proposed method is verified.