Modified droop control scheme for load sharing amongst inverters in a micro grid

In this study, a novel droop control method for ac microgrids is proposed to enhance the performance of power regulation, which is composed of three parts. The angle droop and the frequency droop are adopted to control the active power in coordination, while the modified voltage droop is used to control the reactive power. There are several improvements in this proposed triple droop control method compared with the conventional droop control. First, the dynamics of active power regulation are enhanced because of the angle droop. Second, the stability of the system is upgraded and the droop coefficients can vary in greater range. Third, the accuracy of reactive power regulation is improved, especially in the grid‐connected mode, the DGs can output the specified reactive power with the modified voltage droop, which benefits the economic dispatch of the microgrid. Through these three combined droop control strategies, the performance of the whole system can be improved a lot whether in the grid‐connected mode or the islanded mode. Furthermore, the stability of the proposed triple droop control is analysed, which shows that the stability of the system is upgraded. The effectiveness of the proposed control method is verified by the hardware‐in‐loop tests.

The primary function of microgrid (MG) is to share the power accurately among different distributed generation (DG) units according to the load demands. The droop control methods are normally used for appropriate power sharing. However, these control techniques suffer badly due to the feeder impedances and uncontrollable power coupling issues. Many techniques have been proposed to share power accurately, most of them rely on communication link. In this work a localized/communication free signal generating mechanism is proposed. The mechanism injects the transient reactive power in frequency droop characteristics by observing the changes in microgrid. This injection disturbs the actual active power sharing which is helpful for the computation of reactive power sharing errors. These errors are minimized by utilizing the proportional integral (PI) controller. The PI controller modifies the y-intercept and the slop of Q-E (Reactive Power –Voltage) droop characteristic for the correction of reactive power sharing error. Moreover, the parameters of droop and PI controller of inner control loop are optimized by Particle Swarm Optimization (PSO) to obtain high precision and reliability. The proposed technique has the following salient features:1. Accurately shares the reactive power without the information of feeder line impedances2. This accurate reactive power sharing scheme has been started locally without the need of communication signal from the central controller. Thus, there is no delay in monitoring the notable changes in microgrid caused by DGs. 3. The careful selection of PI & droop controller parameters through PSO makes the system more stable as compared to the conventional methods in terms of transients and steady state responses.

- The increasing penetration of inverter-based distributed energy resources has necessitated reliable and decentralized control strategies for islanded microgrid operation. This paper investigates the dynamic performance of an inverter-dominated microgrid operating in islanded mode using conventional droop control techniques. The proposed control framework employs P- and Q-V droop characteristics to achieve autonomous active and reactive power sharing among parallel inverters without relying on communication links. Detailed simulation studies are conducted to analyze system behavior under load variations and disturbances. The results demonstrate proportional active and reactive power sharing, robust voltage regulation, and stable frequency response at the point of common coupling. Transient analyses further confirm fast damping, minimal oscillations, and convergence to steady-state operation within acceptable limits. Overall, the study validates the effectiveness of droop control in ensuring stable, scalable, and resilient islanded operation of inverter-based microgrids, making it suitable for practical decentralized power system applications. Key Words: Microgrid; Droop Control; Islanded Operation; Inverter-Based Distributed Generation; Active Power Sharing; Reactive Power Control; Voltage Regulation; Frequency Stability

Power sharing and load voltage regulation requires the knowledge of feeder impedance which is one of the biggest demerit of conventional droop control. This paper proposes a new method to overcome this demerit by the use of numerical approach and a new voltage regulation technique. Modified angle droop and voltage droop is proposed for intended sharing ratio and transfer of active and reactive powers. The proposed method can work with different type of feeder impedance using a single droop law. Accurate sharing of active and reactive power while maintaining constant load voltage without the knowledge of feeder impedance is the main contribution of this paper. Simulation results are shown to verify the proposed scheme.

Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes. Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality. When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units. However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted. Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing. Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method. Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods. The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control. Finally, the future research trends on islanded microgrids are also discussed in this paper.

Modeling inverters with volt-var functions in grid-connected mode and droop control method in islanded mode

This paper presents an active-reactive power control strategy for voltage source converters (VSCs) based on derivation of the direct and quadrature components of the VSC output current. The proposed method utilizes a multivariable proportional-integral controller and provides almost completely decoupled control capability of the active and reactive power with almost full disturbance rejection due to step changes in the power exchanged between the VSC and the grid. It also imposes fast transient response and zero steady-state error as compared to the conventional power control approaches. The applicability of the proposed power control strategy for providing the robust stability of the system against the uncertainties of the load parameters is also investigated. The superiority of the proposed control strategy over conventional approaches in the new condition of supplying the load is demonstrated. The theoretical aspects of the proposed multivariable-based power control strategy and the conventional approaches are reviewed and simulation results are presented in two separate sections. MATLAB/Simulink 2009a is used to simulate different scenarios of the simulation.

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.

When the microgrid is isolated from the main grid, grid-forming units control the frequency and voltage to maintain the stability of the microgrid. Hierarchical control structure is utilized to provide these functionalities through three levels of control. Droop control is used at the primary level to sustain the frequency and voltage of the microgrid and to maintain the power sharing between units according to their power rating. Units generate active and reactive power which is appropriate for their rating by varying their terminal frequency and voltage. However, the microgrid is implemented in a distribution network where the voltage level is medium. In such voltage level, the X/R ratio approach unity hence, the frequency and voltage are related to the active and reactive power through line parameters. Therefore, to supply active power or reactive power, both frequency and voltage must be tuned accordingly. In this paper, a normalized dependency matrix is derived to identify the per unit relationship between the frequency and voltage with the active and reactive power. Then, droop control utilizes the dependency matrix to achieve an accurate relationship between frequency and voltage with active and reactive powers. To implement the dependency matrix the X/R ratio is required to calculate the line impedance phase angle. Phasor Measurement Unit (PMU) communicates the grid voltage and current through synchrophasor communication to calculate the active and reactive power consumed by of the line. Then, the line parameters are estimated and used to generate the normalized dependency matrix. The proposed droop controller is simulated in MATLAB/SIMULINK on inductive, mixed impedance, and resistive grids to verify its functionality. The results of the proposed controller are discussed in the simulation result section.

Droop control is the basic control method for load current sharing in dc microgrid applications. The conventional dc droop control method is realized by linearly reducing the dc output voltage as the output current increases. This method has two limitations. First, with the consideration of line resistance in a droop-controlled dc microgrid, since the output voltage of each converter cannot be exactly the same, the output current sharing accuracy is degraded. Second, the dc-bus voltage deviation increases with the load due to the droop action. In this paper, in order to improve the performance of the dc microgrid operation, a low-bandwidth communication (LBC)-based improved droop control method is proposed. In contrast with the conventional approach, the control system does not require a centralized secondary controller. Instead, it uses local controllers and the LBC network to exchange information between converter units. The droop controller is employed to achieve independent operation, and the average voltage and current controllers are used in each converter to simultaneously enhance the current sharing accuracy and restore the dc bus voltage. All of the controllers are realized locally, and the LBC system is only used for changing the values of the dc voltage and current. Hence, a decentralized control scheme is accomplished. The simulation test based on MATLAB/Simulink and the experimental validation based on a 2 × 2.2 kW prototype were implemented to demonstrate the proposed approach.

Microgrid (MG) control in off-grid mode of operation is one of the main challenges in MGs. Under different loading conditions, voltage and frequency deviate from their nominal limits. The droop control method that applied with the current control and voltage control loops can restore the MG voltage and frequency to their nominal limits. However, it can’t regulate these parameters to their specified values. Additionally, the droop control method can’t do well when the system supplies either nonlinear or unbalanced loads. This paper proposes a control technique for adjusting the voltage and frequency of an MG in islanded mode under different loading conditions. The proposed method is based on applying the H-infinity ($$H\infty$$) robust control as a secondary controller with the droop control method to improve its performance. The proposed method is composed of four stages including; droop control loop, voltage control loop, current control loop and control loop for inductance–capacitance–inductance (LCL) filter and coupling circuit. $$H\infty$$ control has an internal model that can control the frequency and improve the system power quality by decreasing the disturbance frequencies. Moreover, it has two weighted parameters $$\xi$$ and $$\mu$$ that must be correctly adjusted to improve the stability of the system. For a proper adjusting of these parameters, the Harmony Search (HS) optimization method is applied. To prove the effectiveness and superiority of the proposed method, it is applied on a test system with three different loading scenarios using MATLAB/Simulink program. The results are compared with those of droop controller and they confirm that the proposed control method is more effective in controlling the microgrids under islanded mode operation.

Due to the rapid increase in residential PV capacity, it gets increasingly difficult for the distribution grid to absorb power at peak hours due to overvoltage issues, causing concerns to DSOs and prosumers who are temporarily unable to inject power into the grid. In collaboration with Belgian DSOs and with realistic input data representing Belgian feeders, dedicated Monte-Carlo analyses were performed to investigate mitigation measures, among which the PQ control of residential inverters. The principle of this mitigation measure is to apply a dedicated control to the inverters since they can control their active and reactive power output to some extent. By either reducing their active power output or absorbing some reactive power in parallel of the active power production, the voltage level can be reduced and inverter tripping due to overvoltages can be avoided. Different types of controls have been assessed and their relative efficiencies compared. The inclusion of the inverters’ dynamic behaviour (i.e. disconnection and reconnection dynamic) had an important impact on the outcome of the assessment. Ultimately, the droop control on the reactive power provided the best results in terms of increased inverter production and equity among prosumers, while the droop control on the active power showed counterproductive results.

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.

Due to the increasing energy demands in microgrids (MG), the need for parallel-connected distributed generations (DG) to supply the load required by customers has been increased. An interfacing inverter connects the DGs to the microgrid. The operating performance of a microgrid is largely depends up on the interfaced inverter. For a stable and reliable operation of the microgrid, the inverters of the DGs have to be controlled for proper power-sharing. In this paper, active power-frequency(P-F)/ reactive power-voltage amplitude (Q-V) droop control is used to control the three parallel-connected DGs in a microgrid operated in islanded mode. To confirm the droop control effectiveness, three distinct types of load are examined. Such as balanced, unbalanced and non linear load. When operating in an islanded mode, the inverter droop control should maintain the frequency and voltage within the permissible ranges. Furthermore, the findings show that droop control has a substantial influence on balancing the voltage magnitude, frequency and active power-sharing. The three parallel-connected inverters share equal active power among the DGs for all types of loads. Matlab/Simulink is used to model the droop control for parallel-connected DGs and verify the effectiveness of the droop controller.

In recent researches on inverter-based distributed generators, disadvantages of traditional grid-connected current control, such as no grid-forming ability and lack of inertia, have been pointed out. As a result, novel control methods like droop control and virtual synchronous generator (VSG) have been proposed. In both methods, droop characteristics are used to control active and reactive power, and the only difference between them is that VSG has virtual inertia with the emulation of swing equation, whereas droop control has no inertia. In this paper, dynamic characteristics of both control methods are studied, in both stand-alone mode and synchronous-generator-connected mode, to understand the differences caused by swing equation. Small-signal models are built to compare transient responses of frequency during a small loading transition, and state-space models are built to analyze oscillation of output active power. Effects of delays in both controls are also studied, and an inertial droop control method is proposed based on the comparison. The results are verified by simulations and experiments. It is suggested that VSG control and proposed inertial droop control inherits the advantages of droop control, and in addition, provides inertia support for the system.