MULTIDIMENSIONAL OPTIMAL DROOP CONTROL FOR WIND RESOURCES IN DC MICROGRIDS

In an islanded microgrid with multiple distributed generations (DGs), the difference in line impedance may cause local voltage deviation, which leads to a series of problems such as lower power allocation accuracy and bus voltage drop under traditional droop control. In this respect, a method for optimizing the droop control using an improved particle swarm optimization (IPSO) is proposed. Firstly, the microgrid structure and influence of line parameters on traditional droop control strategy is analyzed. Then, an improved particle swarm optimization is proposed. Based on the basic particle swarm optimization (PSO) algorithm, a fuzzy inference system (FIS) is introduced to dynamically adjust the particle swarm optimization, which can effectively improve the global search ability and local search ability of the algorithm. After that, the improved algorithm is applied to the droop controller, simultaneously, the range of stable operation of the system is determined by small signal analysis. Finally, the simulation and experiment results show that the proposed improved droop control strategy can achieve accurate allocation of active and reactive power effectively while maintaining bus voltage and system frequency stability, enhance the dynamic performance and transient stability of the microgrid system.

In contrast to conventional power plants, which are based on large synchronous generators with large inertia capabilities to dampen sudden disturbances, renewable energy sources, such as solar and wind, connected to the grid through power electronics converters, display low system inertia and overload limiting capabilities. Additionally, because they lack primary frequency regulation capabilities, they are unable to actively respond to the frequency response of the system. This research investigates the contribution of droop control strategies to grid resilience by focusing on their ability to maintain frequency stability during grid disturbances. The study employs a simulation-based approach using MATLAB/Simulink to model the Djoum power plant in Cameroun and implement droop control algorithm. The methodology involves designing and analyzing the system's response under sudden load changes using droop and supervisory control strategies. Parameters such as droop coefficients and control bandwidths were systematically varied to analyze their impact on frequency regulation and grid resilience. Results show that droop control maintains system operation by adjusting frequency and voltage under disturbance, while supervisory control acts as a secondary layer to fully restore parameters to their reference values thereby ensuring reliable operation during grid disturbances.

The primary goal of the research presented in this dissertation is the development of a decentralized control method for both dc and ac inverter islanded microgrids with high penetration of stochastic renewable sources that is an effective alternative to a centralized, optimal solution. One of the contributions of this work is the development of the Decentralized Mode Adaptive (DMA) guidance law for a Hamiltonian-based controller of an islanded, N-source, dc microgrid. The DMA guidance law reduces the gap between droop control and centralized control by providing a method that can operate autonomously in the event of source and bus load fluctuations as well as reduce communication requirements of centralized controllers, and thus increase resiliency and scalability. This is a significant trait, as stochastic renewable sources are prone to sizable fluctuations or even suddenly dropping offline. The high penetration of stochastic renewable sources mandates controls that can handle such swings. The proposed DMA guidance law is compared to the centrally-controlled optimal exergy destruction (OXD) power apportionment strategy, as well as a Hamiltonian-based droop control strategy. After adding a guidance law to the Hamiltonian-based droop controller to allow it to handle the removal of a source, both the DMA and droop controller can maintain a constant bus voltage. The DMA guidance law makes an improvement over droop control in that it is able to track OXD steady state performance when droop control cannot. Finally, it is shown that for a sufficiently large number of converters supplying a microgrid, the presented decentralized, mode-adaptive strategy provides an efficient and practical alternative to both droop control and centralized control schemes. Another contribution of this work is development of the Optimal Exergy Destruction (OXD) and Decentralized Power Apportionment (DPA) feedforward control law for a Hamiltonian-based controller for islanded, N-source ac inverter microgrids. The importance of the OXD guidance law is that it is an analytical, closed-loop optimal feedforward controller that was developed utilizing exergy analysis to minimize the exergy destruction in the microgrid. As in the dc microgrid case, the OXD outer loop controller requires a star or fully connected topology whereas the DPA operates with no communication among the inverters. The decentralized power apportionment feedforward controller eliminates some critical drawbacks to conventional P-ω/Q-V droop control. Unlike conventional P-ω/Q-V droop control, the DPA control scheme holds the bus frequency constant and is able to return the bus voltage to the reference value after large load or

Due to the uneven distribution of virtual resistance and line impedance in the DC micro-grid, the traditional droop control has the contradiction between current sharing and voltage regulation. In this paper, the droop control based on the discrete consensus algorithm is adopted. Each power generation unit adopts adjacent communication, and the reference value of the DC bus voltage is corrected according to the mean value after the iterative of the system voltage, thereby compensating for the voltage drop caused by the droop control. The photovoltaic (PV) cell adopts the maximum power tracking (MPPT) control. The battery adopts the constant voltage control with current threshold, which can keep the DC bus voltage stable, and the outlet current is prevented from being too large to shorten the service life of the battery. Stable operation of the DC micro-grid is realized. Finally, the PV/battery DC micro-grid model is built in Simulink. When the environmental conditions and load change, the micro-grid can ensure the DC bus voltage constant and the active power balance, which verifies the effectiveness of the proposed control strategy.

In direct current (DC) microgrids, traditional droop control methods facilitate power sharing and stabilize bus voltage. However, this communication‐less approach is constrained by challenges including insufficient voltage restoration and imbalances in battery state of charge (SOC). This paper introduces a novel communication‐less control strategy designed to address these limitations in photovoltaic energy storage system (PV‐ESS) DC microgrids. This method assigns the energy storage unit (ESU) closest to the critical load as the ‘master unit’ setting its output voltage to nominal levels. This master unit injects an alternating current (AC) signal into the DC bus, with the signal frequency modulated by its SOC and output current. Additional ESUs, referred to as ‘slave units’ adjust their output currents based on the frequency of the AC signal and their local SOC. Furthermore, the master unit utilizes a logical signal based on its SOC to switch the photovoltaic module between droop control mode and maximum power point tracking (MPPT) mode. Stability analysis verifies the robustness of the proposed method, and simulation results demonstrate that it surpasses conventional droop control approaches. Specifically, the proposed method enhances load‐sharing precision, minimizes voltage deviation, and ensures SOC equalization across the DC microgrid without the need for communication links. © 2026 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

An optimal droop control optimization method is proposed of the DC microgrid. First, a modeling of the DC micro-grid is made based on the idea of model equivalence. And then, the traditional particle swarm optimization (PSO) is improved by introducing adaptive learning factors and inertial weights. Finally, based on the improved PSO algorithm, the droop parameters are optimized to optimize the system performance. The simulation results show that even under the dual effects of line impedance and local load, the system can still achieve a high accuracy of current sharing and has a significant suppression effect on the bus voltage deviation.

This work presents a perfomance study of a dc microgrid when it is used a voltage droop technique to regulated the grid voltage and to control the load sharing between different sources. A small model of a dc microgrid comprising microsources and loads was implemented in the Simulink/Matlab environment. Some aspects about centralized (master-slave) and descentralized (voltage droop) control strategies as well as the procedures to design the controllers, with and without droop control, are presented and discussed. Simulation results obtained with the digital model of the dc microgrid with three microsources will be presented to validate the effectiveness of the voltage droop strategy, applied to proportional and proportional-integral controllers, to regulate the microgrid voltage.

The paradigm shift in electrical power grids and the increased interest towards decentralisation has opened a new window in the design, control and theoretical analysis of small scale power systems, i.e. microgrids, which aim at the integration and utilisation of renewable energy sources, energy storage systems and responsive loads at a local scale. Given their DC nature, DC microgrids have attracted significant interest as they provide a natural interface to the main grid by avoiding additional conversion steps. The aim of this thesis is to design and analyse novel hierarchical control schemes, both at the primary and secondary control level, that guarantee tight voltage regulation, accurate power sharing, current or voltage limitation, and present a straightforward approach towards deriving stability conditions for DC microgrids that incorporate nonlinear loads. As the microgrid configuration is paramount in the theoretical analysis, a rigorous method of computing the admittance matrix is developed that facilitates the stability analysis of DC microgrid systems supplying a constant impedance (Z), constant current (I) or constant power (P) load. This method is particularly useful as it permits the factorisation of the admittance matrix, while separating the singular matrices. In this way, the closed-loop asymptotic stability proof can be more easily approached by isolating the singularities and then, employing straightforward linear algebra tools, such as quadratic eigenvalue problem (QEP) theory, to derive the stability conditions. As stated in the technical specifications of every source, a crucial issue is represented by the ability to protect itself and its interface device (power converter) during faults, transients and unrealistic power demands. That is why current-limiting control is often combined with the primary control, i.e. often droop control, in DC microgrids to ensure the desired unit protection. In this thesis, novel current-limiting droop controllers areformulated for DC microgrids consisting initially of multiple unidirectional DC/DC boost converters and, later on, bidirectional DC/DC boost and three-phase AC/DC converters, that integrate different distributed generation units with the local grid. In contrast to the traditional approaches that use small-signal modelling, here, the accurate nonlinear models of the converter units are taken into account to prove the boundedness and the current-limiting property, by employing Lyapunov methods and the ultimate boundedness theory. Exponential stability at the desired equilibrium point is mathematically guaranteed, and further analysed from a graphical perspective, providing insights of the load’s effect onto the system performance and stability. To address the shortcomings that conventional droop control approaches introduce, i.e. inaccurate power sharing and significant load voltage drop, a DC microgrid architecture that takes into account the power converter dynamics of distributed generation units is deployed under decentralised primary and distributed secondary control scheme, in a hierarchical control framework. At the primary control layer, a novel current-limiting droop control scheme is implemented in a decentralised manner, whereas at the secondary control layer a fully distributed controller that performs a voltage restoration and improves the power sharing is deployed using a nearest-neighbour coupling communication network. By investigating for the first time both the dynamics of the converters with the nonlinear load and the two-layer control, singular perturbation theory is applied to analytically guarantee the stability of the entire DC microgrid. Finally, apart from the desired overcurrent protection, since DC capacitors are customary used at the output of each converter unit to stabilise the output voltage, they also introduce a maximum voltage limit. Hence, as the need for protection against overvoltages has emerged, droop controllers with inherent overvoltage protection are also proposed for parallel and meshed configuration networks. The upper limit of the voltage of each source is rigorously proven using ultimate boundedness theory. Asymptotic stability to the desired equilibrium for the closed-loop system is analytically guaranteed, and detailed conditions are derived to guide the control design. In order to validate the effectiveness of the different control methodologies developed in this thesis, both simulation and experimental testing are performed for each one of the methods, and are also compared with the conventional approaches to highlight their superiority.

A novel droop controller for DC microgrid systems, consisting of multiple paralleled sources feeding a constant power load (CPL), is proposed to achieve the desired voltage regulation and accurate load power distribution, while ensuring an overvoltage protection for each source. CPLs are well-known to exhibit negative impedance characteristic due to their nonlinear behaviour, which may cause instability to a DC microgrid if the necessary impedance inequality criteria are not satisfied. In this paper, a new droop control scheme is proposed to limit the voltage of each source below a desired bound, achieve tight voltage regulation and power sharing, and guarantee closed-loop system stability with the existence of a CPL. The upper limit of the voltage of each source is rigorously proven using ultimate boundedness theory, while after a suitable manipulation of the admittance matrix of the microgrid, analytic conditions of stability are obtained to guide the control parameters design. To validate the theoretical design and analysis, a detailed simulation is performed of a DC microgrid equipped with the proposed controller.

For maintaining the stability of voltage and frequency, the droop control is often adopted when micro-grid is in islanding operational mode. Although this method can control the fluctuation of voltage and frequency, when the loads change, its implementation effect is weak In order to solve the mentioned problem, an improved dynamic self-adjusting droop control strategy is proposed: on the basis of the traditional one, PI control of voltage and frequency is introduced to control flow. The principle and implementation steps of this method are analyzed. Finally, the operating characteristics of overloading connection, overloading cutting-off and loads disturbance are simulated and discussed based on an islanding mode built in Matlab/Simulink. The results show the proposed method can control the voltage and frequency more effectively and faster than traditional droop control, and can enhance the stability of micro-grid in islanding operational mode.

Constant power loads (CPLs) introduce negative impedance in direct current microgrids (DCMGs), which is a major challenge. This negative impedance can significantly reduce the overall damping of the system, making it less stable and harder to control. To address this issue, output virtual resistance (VR) shaping is commonly employed to enhance system damping and improve power‐sharing amongst distributed generators (DGs). The technique proposed in this work involves an adaptive variation of the DG virtual output resistance () linearly with the output current. This shows improved power sharing between sources. The work compares the small signal stability criteria and the minor loop gain methods for linear, non‐linear, and inverse droop controllers to determine the controller parameters with constant power loads. The control scheme is extensively tested through simulations for four different droop control schemes. The work also validates the DCMG performance when the DERs work with different droop controllers (heterogenous of controllers) to assess constant power load penetration, performance in meshed configurations, and DG plug‐and‐play operations. Additionally, improved power sharing performance was validated through a controller hardware in the loop (CHIL) based implementation.

Stability analysis of islanded DC microgrid for the proposed distributed control strategy with constant power loads

In the operation of DC micro grid, the uniform load distribution is beneficial to the stable operation of the system and prolongs the service life of the system. In order to solve the influence of line impedance on traditional droop control in DC micro-grid, an impedance detection method based on ripple analysis and the structural characteristics of the DC micro-grid system is proposed. The high-frequency harmonic component of microgrid output current and voltage is mainly from the transistor switching frequency. By using the Fourier transform to analyze the high-frequency harmonic component of the voltage and current detected by the converter, the impedance information is obtained, and then the droop control is compensated base on it. In this paper, the compensation method is extended to the application of multiple devices in parallel. This method does not directly operate the system, so it has done no harm to the power quality. Finally, the feasibility of the proposed method in normal working conditions and its effectiveness in the case of communication interruption and load change is verified by RT-LAB hardware in loop experiment.

Droop control has drawn widespread attention and various nonlinear droop characteristics have been developed in dc microgrids. This article proposes an improved nonlinear droop control strategy, which uses the difference between the squared nominal voltage and the squared dc voltage as the droop input and generates the ac current reference directly from the droop characteristic. Compared with the traditional dual-loop droop control strategy, this method can provide faster transient response due to the elimination of the external dc voltage or current loop. And the bus voltage can be regulated very compactly, which is an optimization of steady-state performance. In this article, the stability assessment that violates the constraint conditions of the conventional impedance-based stability criterion is investigated, which considers the number of right-half-plane poles of the open-loop transfer function. The Nyquist criterion of generalized Bode plot is used to analyze the system stability. An adaptive current sharing strategy is proposed to improve the current sharing accuracy in multisource operation. This article also discusses the comprehensive design process for determining droop gain. Finally, experimental results validate the theoretical analysis.

Cost-based droop schemes for microgrids (MG) have been developed to achieve cost reduction; meanwhile, microgrid stability depends on the droop control design and its parameters. This paper proposes a hybrid cost-based droop control that achieves both optimal economical operation and stability preserving for autonomous microgrids. The impact of cost-based droop schemes on stability is investigated utilizing a modified cost-based small-signal linearized model. Low-frequency eigenvalues tend to migrate towards instability when the load increases if a cost-based droop scheme is adopted. The proposed hybrid droop control manages to achieve the optimal generation by incorporating the incremental cost in the active power droop while ensuring stable performance by utilizing active and reactive power derivative controllers. The reactive power derivative controller is utilized to suppress the migration of the low-frequency eigenvalues towards instability. The active power derivative controller is utilized to damp the oscillations of the active power-sharing among the distributed generators (DG). The effectiveness of the proposed droop control to ensure an optimal and stable operation is validated on Matlab/Simulink.