Distributed Optimal Control of DC Microgrids with Communication Delays

In this paper, a novel distributed control algorithm for current sharing and voltage regulation in DC microgrids is proposed. The DC microgrid is composed of several distributed generation units, including buck converters and current loads. The considered model permits an arbitrary network topology and is affected by an unknown load demand and modeling uncertainties. The proposed control strategy exploits a communication network to achieve proportional current sharing using a consensus-like algorithm. Voltage regulation is achieved by constraining the system to a suitable manifold. Two robust control strategies of sliding mode type are developed to reach the desired manifold in a finite time. The proposed control scheme is formally analyzed, proving the achievement of proportional current sharing, while guaranteeing that the weighted average voltage of the microgrid is identical to the weighted average of the voltage references.

In this letter we propose a new distributed control scheme, achieving current sharing and average voltage regulation in direct current (dc) microgrids. The considered dc microgrid is composed of several distributed generation units (DGUs) interconnected through resistive-inductive power lines. Each DGU includes a generic energy source that supplies a local current load through a dc-dc buck converter. The proposed distributed control scheme achieves current sharing and average voltage regulation, independently of the initial condition of the controlled microgrid. Moreover, the proposed solution requires only measurements of the generated currents, and is independent of the microgrid parameters and the topology of the used communication network, facilitating plug-and-play capabilities. Global convergence to a desired steady state is proven and simulations indicate a good performance.

The integration of renewable energy sources in DC microgrids presents significant challenges in maintaining stable voltage regulation and efficient power management. Existing control approaches struggle to handle the inherent variability of renewable sources while maintaining optimal system performance. This paper presents an Adaptive Sinh Cosh Optimizer (ASCHO) integrated with cascade Fractional Order Proportional-Integral (FOPI) control to address these challenges. ASCHO enhances the original SCHO algorithm through dynamic parameter updating and improved exploration-exploitation mechanisms. The proposed control system demonstrates significant performance improvements, with ASCHO achieving a fitness value of superior to conventional approaches' values. In practical implementation, the system maintains voltage regulation within ±5V of the reference value and achieves a 70\% reduction in steady-state error (3.15V vs 10.39V) compared to SCHO. The method was extensively validated through 23 benchmark functions comprising unimodal, multimodal, and fixed-dimension multimodal functions, with ASCHO outperforming other algorithms in 17 functions. Comprehensive testing validates the system's robustness under varying solar irradiance, wind speed, and load demands. The results demonstrate faster rise times (0.00208s vs 0.00209s) and improved power management capabilities, while maintaining battery state of charge variations within 0.003%. The successful implementation establishes ASCHO as an effective solution to optimize DC microgrid control systems with high penetration of renewable energy.

In dc microgrids, load power sharing and bus voltage regulation are two common control objectives. In this article, a consensus-based algorithm is presented to achieve proportional power sharing and regulation of weighted geometric mean of bus voltages in dc microgrids with ZIP (constant impedance, constant current, and constant power) loads simultaneously. By using the virtue of the Laplacian matrices of undirected connected graphs, a lemma is derived to assist the stability analysis of the dc microgrids. Thus, a sufficient condition that stabilizes the system with ZIP loads is established. In addition, with the help of a distributed voltage regulation error estimator, the tuning of the bus voltages can be realized without the requirement on initial voltage conditions. Through the Lyapunov synthesis, the large-signal stability of the closed-loop system is theoretically ensured for a wide range of load conditions. Finally, simulation studies are performed to validate the merits of the proposed consensus-based algorithm.

Notice of Violation of IEEE Publication Principles: <br><br>“Washout Filter-Based Distributed Resilient Current Control for Cyber-Physical Microgrids," <br>by X. Lu and J. Lai, <br> in IEEE Systems Journal, Early Access <br><br>After careful and considered review of the content and authorship of this paper by a duly constituted expert committee, this paper has been found to be in violation of IEEE’s Publication Principles. <br><br>This paper contains significant portions of original text from the paper cited below. The original text was copied without attribution (including appropriate references to the original author(s) and/or paper title) and without permission. <br><br>"A Combined Distributed Cooperative Method and Washout Filter-Based Method for Power Sharing and Voltage Balancing in Bipolar DC Microgrids" <br> by Ramin Babazadeh-Dizaji, Mohsen Hamzeh, Nima Mahdian Dehkordi, <br>submitted to IEEE Transactions on Energy Conversion, August 2020. <br><br> <br/> This article proposes a washout filter-based distributed resilient current control scheme that can achieve current sharing of distributed generations (DG) and voltage restoration of dc bus almost surely under communication channel noise interferences. Since the cyber channels are exposed to inherent noise interferences, the stability and reliability of cyber-physical dc microgrids may terribly be reduced. To dispel the adverse influences of noise interferences, a novel discrete time washout filter is developed. Accordingly, a discrete-time-distributed noise-resiliency control algorithm is developed for dc microgrids based on washout filters, in which only the current state variable is allowed to exchange among DGs with the local decentralized voltage controller. This makes the proposed control mode different from the conventional parallel mode usually requiring a leader–follower consensus-based voltage control by exchanging two state variables (i.e., terminal voltage and current output). Thus, the proposed control mode decreases the complexity to the formulation and enhances the robustness to the leader communication failure. By introducing stochastic theory and Lyapunov stability function, the sufficient conditions considering noise interferences is derived to ensure the stability operation of the whole closed-loop dc network. The obtained results show the effectiveness of the proposed strategy by a tested dc microgrid in OPAL-RT real-time simulator.

In this article, we propose a novel four-stage distributed controller for a dc microgrid that achieves proportional power sharing and average voltage regulation for the voltages at actuated and unactuated buses. The controller is presented for a dc microgrid comprising multiple distributed generation units (DGUs) with time-varying actuation states, dynamic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$RLC$</tex-math> </inline-formula> lines, nonlinear constant impedance, current, and power (ZIP) loads, and a time-varying network topology. The controller comprising a nonlinear gain, proportional–integral (PI) controllers, and two dynamic distributed averaging stages is designed for asymptotic stability. This constitutes deriving passivity properties for the dc microgrid, along with each of the controller subsystems. Thereafter, design parameters are found through a passivity-based optimization using the worst-case subsystem properties. The resulting closed loop is robust against DGU actuation changes, network topology changes, and microgrid parameter changes. The stability and robustness of the proposed control are verified via simulations.

The increasing penetration of dc distributed energy resources (DERs) and dc loads has motivated the utilization of dc microgrids (MGs). In this paper, a new two-level control scheme is proposed for accurate power sharing and appropriate voltage regulation in dc MGs during islanded operation mode. In the primary control level, a P-\dot V droop method is proposed to eliminate the dependency of power sharing among DERs on line resistances. Since the P V droop deviates the voltage derivative to a nonzero value, a voltage derivative restoration mechanism is adopted in the secondary control level to provide voltage stability. The secondary control level also compensates the voltage deviations by cascading an outer voltage control loop with the inner voltage derivative restoration loop. The secondary control signal is broadcasted to each DER via a unidirectional low-bandwidth communication link. Small signal stability of the proposed scheme is analyzed by studying the locus of the system eigenvalues. To verify the efficacy of the proposed method and compare it with the conventional scheme, real-time simulations in OPAL-RT lab setup are provided.

A novel distributed secondary layer control strategy based on average consensus and fractional‐order proportional‐integral (FOPI) local controllers is proposed for the regulation of the bus voltages and energy level balancing of the energy storage systems (ESSs) in DC microgrids. The distributed consensus protocol works based on an undirected sparse communication network. Fractional‐order local controllers increase the degree of freedom in the tuning of closed‐loop controllers, which is required for DC microgrids with high order dynamics. Therefore, here, FOPI local controllers are proposed for enhanced energy balancing of ESSs and improved regulation of the bus voltages across the microgrid. The proposed control strategy operates in both islanded and grid‐connected modes of a DC microgrid. In both modes, the average voltage of the microgrid converges to the microgrid desired reference voltage. The charging/discharging of ESSs is controlled independent of the microgrid operating mode to maintain a balanced energy level. The performance of the proposed distributed control strategy is validated in a 38‐ V DC microgrid case study, simulated by Simulink real‐time desktop, consisting of 10 buses and a photovoltaic renewable energy source.

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The inaccessibility of energy to remote communities is a great concern due to economical and feasibility constraints in the conventional electrification approach. The reliable, efficient, autonomous, and sustainable solar driven system having the potential to overcome challenges of expansion of conventional utility grid. The solar PV-based distributed generation could address these issues significantly as the cost is declined. The low-voltage DC-microgrid (DCMG) is a prominent solution to achieve autonomy in generation, cost-effectiveness, efficiency, and reliability. This article brings forward an arrangement of a PV-fed two-stage DC/DC conversion stage with battery storage. This provides better control over MPPT and voltage regulation in DCMG. Also, capable of handling uncertainties in source due to weather intermittency and load. The system performance is analyzed in MATLAB/ Simulink under different irradiance and load conditions. The simulation results describe the effectiveness of the proposed system.

Direct Current (DC) microgrids are expected to become larger due to the rapid growth of DC energy sources and power loads. As the scale of the system expends, the importance of voltage control will be increased to operate power systems stably. Many studies have been performed on voltage control methods in a DC microgrid, but most of them focused only on a small scale microgrid, such as a building microgrid. Therefore, a new control method is needed for a middle or large scale DC microgrid. This paper analyzes voltage drop problems in a large DC microgrid and proposes a cooperative voltage control scheme with a distributed generator (DG) and a grid connected converter (GCC). For the voltage control with DGs, their location and capacity should be considered for economic operation in the systems. Accordingly, an optimal DG allocation algorithm is proposed to minimize the capacity of a DG for voltage control in DC microgrids. The proposed methods are verified with typical load types by a simulation using MATLAB and PSCAD/EMTDC.

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.

Direct current (DC) microgrid is being increasingly investigated in modern power grid. An important issue in DC microgrid operation is to ensure proper current sharing among converters. While this has been addressed through droop control, the resulting voltage deviation in DC bus has to be compensated. To solve this problem, a new distributed secondary control scheme is presented in this paper for both current sharing and voltage restoration. A key part of the presented scheme is the integration of a new parameter `virtual voltage drop' defined from droop gain and line resistance. Since the DC bus voltage is not required as a feedback signal, the proposed secondary control is simple and easy to design and implement. In addition, as the proposed scheme has no requirement for the loads, it can handle both resistance loads and constant power loads (CPLs). Simulations as well as experimental studies are carried out to demonstrate the effectiveness of the proposed scheme.

Advancements in power conversion efficiency and the growing prevalence of DC loads worldwide have underscored the importance of DC microgrids in modern energy systems. Addressing the challenges of power-sharing and voltage stability in these DC microgrids has been a prominent research focus. Sliding mode control (SMC) has demonstrated remarkable performance in various power electronic converter applications. This paper proposes the integration of universal droop control (UDC) with SMC to facilitate distributed energy resource interfacing and power-sharing control in DC microgrids. Compared to traditional Proportional-Integral (PI) control, the proposed control approach exhibits superior dynamic response characteristics. The UDC is strategically incorporated prior to the SMC and establishes limits on voltage variation and maximum power drawn from the DC–DC converters within the microgrid. A dynamic model of the DC–DC converter is developed as the initial stage, focusing on voltage regulation at the DC link through nonlinear control laws tailored for Distributed Generation (DG)-based converters. The UDC ensures voltage stability in the DC microgrid by imposing predetermined power constraints on the DGs. Comparative evaluations, involving different load scenarios, have been conducted to assess the performance of the proposed UDC-based SMC control in comparison to the PI control-based system. The results demonstrate the superior efficiency of the UDC-based SMC control in handling dynamic load changes. Furthermore, a practical test of the proposed controller has been conducted using a hardware prototype of a DC microgrid.

DC microgrid small-signal stability and control: Sufficient stability criterion and stabilizer design