Communication-Less Power Sharing Strategy for Microgrids Using Oscillations Generated by Inertia-Enabled Power Sources

Distributed generation (DG) sources together with loads, storage devices and a controller form a micro-grid (MG). The micro-grid can operate in grid-connected or islanded mode. A controller is needed for islanded mode operation to ensure voltage and frequency stability and proper power sharing among DG units. Droop, average current-sharing, and master-slave controls are among approaches that have been developed. Droop control has recently gained popularity due to the fact that it does not require a communication path, has high stability, and is relatively inexpensive. Therefore, this paper aims to offer a comparative study of three droop control techniques, namely, the traditional, arctan and universal droop control approaches. Using MATLAB/SIMULINK software, a simple MG made of two parallel-connected inverters with resistive-inductive load is used to compare the performance of these three methods. Results showed that the universal droop control (UDC) provides the best voltage regulation at 0.45% followed by the traditional droop control (TDC) at 3.37% and lastly the arctan droop control (ADC) at 3.85%. Also, regarding real power sharing between parallel connected inverters, UDC again provides the performance (49.36%, 49.53%) followed by TDC (46.30%, 46.30%) and lastly ADC (46.31%, 46.11%). For reactive power sharing, UDC has again the best performance (49.13%, 49.33%) followed by ADC (50.03%, 50.43%) and lastly TDC (50.43%, 41.83%). Finally, ADC provides the best frequency regulation at 0.02% followed by TDC at 0.34% and lastly UDC at 0.36%. Therefore, according to the results a hybrid of the universal and arctan droop control is recommended to gain the performance of two controller for the future works.

Voltage-source converter (VSC)-based multiterminal dc grids are receiving widespread acceptance as an enabling technology to integrate renewable energy sources, energy storage units, and modern dc-type loads into existing ac grids. Droop control is a common power-sharing strategy to facilitate autonomous power sharing among different terminals in dc grids. However, the dynamics and stability of a gird-connected VSC with dc power sharing droop control can be affected by several important factors that are not addressed in the current literature. Important among these are: 1) ignoring the effect of the outer droop loop on the dc-link voltage dynamics when the dc-link voltage controller is designed, which induces destabilizing dynamics particularly under variable droop gain needed for optimum economic operation, energy management, and successful network operation under converter outages and contingencies; 2) uncertainties in the dc grid parameters (e.g., passive load resistance and equivalent capacitance as viewed from the dc side the VSC); and 3) disturbances in the dc grid (i.e., power absorbed or injected from/to the dc grid), which change the operating point and the converter dynamics by acting as a state-dependent disturbance. To overcome these difficulties, this paper presents a robust power sharing and dc-link voltage regulation controller for grid-connected VSCs in multiterminal dc grids applications. A detailed dynamic model that considers the droop controller dynamics and the impact of the effective dc-side load parameters and disturbances imposed on the VSC is developed. Then, a robust controller that preserves system stability and robust performance is developed. Theoretical analyses, including the MIMO dynamic analysis of a multiterminal dc grid with the proposed controller, as well as simulation and experimental results are provided to show the effectiveness of the proposed controller.

The droop control scheme based on Q − ω and P − V characteristics is conventionally employed to share the load power among sources in an islanded low-voltage microgrid with resistive line impedances. However, it suffers from poor active power sharing, and is vulnerable to sustained deviations in frequency and voltage. Therefore, accurate power sharing and maintaining the frequency and voltage in the desired ranges are challenging. This paper proposes a novel microgrid control strategy to address these issues. The proposed strategy consists of a virtual flux droop and a model predictive control, in which the virtual flux is the time integral of the voltage. Firstly, the novel virtual flux droop control is proposed to accurately control the power sharing among DGs. Then, the model predictive flux control is employed to generate the appropriate switching signals. The proposed strategy is simple without needing multiple feedback control loops. In addition, pulse width modulation is not required and tuning challenges for PI regulators are avoided. In order to evaluate the effectiveness of the proposed microgrid control strategy, simulation analysis is carried out in Matlab/Simulink software environment. The results show that accurate power sharing is achieved while a good dynamic response is provided. Furthermore, the voltage and frequency deviations are significantly improved.

Microgrid (MG) usually operates in medium/low-voltage systems, where the line impedance parameters are mainly resistive, and traditional P-f/Q-U droop control is no longer applicable. When the virtual complex impedance method is adopted, the resistance component of line impedance can be counteracted by a virtual negative resistance. Unfortunately, the improper design of the virtual negative resistance will result in an unstable system due to the problem of line impedance parameter drift and estimation error. According to the line parameters characteristics of the off-grid MG with medium/low voltage, the P-U/Q-f droop control is adopted in this study, where the virtual complex impedance composed of a virtual negative inductance and a virtual resistance is introduced in the control loop. The virtual negative inductance is used to reduce the power coupling caused by the inductive component of the system impedance. The virtual resistance is implemented to enhance the resistive component and adjust the impedance matching degree for raising the accuracy of power sharing. However, the power sharing is still affected by the system hardware parameters; meanwhile, the voltage deviation caused by the droop control and the virtual impedance exists. In this study, a novel voltage stabilization and power sharing control method based on the virtual complex impedance is investigated to achieve accurate power sharing without the impact of hardware parameters variations and to improve the voltage quality. Moreover, the small-signal model of the inverter-based off-grid MG with the proposed controller is established, which can be utilized to analyze the stability and dynamic performance of the system. Meanwhile, the control parameters can be sequentially determined. Analysis shows that the strategy is robust against the line-impedance parameter drift and the estimation error and has a large stability margin and fast dynamic-response speed. Finally, numerical simulations and experimental results are provided to verify the effectiveness of the proposed control method in comparison with traditional frameworks.

To address inaccurate power sharing problems in autonomous islanding microgrids, an enhanced droop control method through online virtual impedance adjustment is proposed. First, a term associated with DG reactive power, imbalance power, or harmonic power is added to the conventional real power-frequency droop control. The transient real power variations caused by this term are captured to realize DG series virtual impedance tuning. With the regulation of DG virtual impedance at fundamental positive sequence, fundamental negative sequence, and harmonic frequencies, an accurate power sharing can be realized at the steady state. In order to activate the compensation scheme in multiple DG units in a synchronized manner, a low-bandwidth communication bus is adopted to send the compensation command from a microgrid central controller to DG unit local controllers, without involving any information from DG unit local controllers. The feasibility of the proposed method is verified by simulated and experimental results from a low-power three-phase microgrid prototype.

Mathematical modeling and stability analysis of DC microgrid using SM hysteresis controller

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.

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.

DC microgrids look attractive in distribution systems due to their high reliability, high efficiency, and easy integration with renewable energy sources. The key objectives of the DC microgrid include proportional load sharing and precise voltage regulation. Droop controllers are based on decentralized control architectures which are not effective in achieving these objectives simultaneously due to the voltage error and load power variation. A centralized controller can achieve these objectives using a high speed communication link. However, it loses reliability due to the single point failure. Additionally, these controllers are realized through proportional integral (PI) controllers which cannot ensure load sharing and stability in all operating conditions. To address limitations, a distributed architecture using sliding mode (SM) controller utilizing low bandwidth communication is proposed for DC microgrids in this paper. The main advantages are high reliability, load power sharing, and precise voltage regulation. Further, the SM controller shows high robustness, fast dynamic response, and good stability for large load variations. To analyze the stability and dynamic performance, a system model is developed and its transversality, reachability, and equivalent control conditions are verified. Furthermore, the dynamic behavior of the modeled system is investigated for underdamped and critically damped responses. Detailed simulations are carried out to show the effectiveness of the proposed controller.

With the growing interest in microgrids to support renewable-energy sources, such as wind power and photovoltaic systems, control of microgrid components, such as inverters, is of increasing importance. Droop controls are methods that without communication lines can stabilize power sharing and grid formation for power generators. A virtual synchronous generator (VSG) is a control method for inverters that mimics both steady-state droop characteristics and dynamic characteristics of synchronous generators (SGs) to enhance the inertia of microgrids. In this paper, some representative VSG control methods are presented and compared. Some characteristics that are not described clearly in the literature are analyzed, such as the different forms of damping effects in VSGs and the necessity of an inner voltage loop. A comparison of VSGs and droop controls leads to the conclusion that a VSG can be regarded as a special type of droop control that mimics the rotor inertia and damping effect of SGs. This is done by changing the compensator parameters of a droop control. Furthermore, in the droop control, if the compensator is designed properly, the dynamic performance of the system can be even better than SGs. Simulations and experimental results are provided to support the conclusions.

In a microgrid with distributed generation (DG) units, proper control of the grid interfacing inverter is critical to facilitate safe and stable operation of the system. The droop control mechanism is a widely accepted control method in a microgrid operation. However, a droop controller is unable to provide inertia support in such systems, and thus, cannot contribute in primary frequency control during disturbances. The virtual synchronous generator (VSG) method was introduced in 2007 for the grid interfacing inverter control, this approach can provide virtual inertia support along with emulation of steady-state droop characteristics of synchronous generators. In this paper, a fuzzy secondary controller (FSC) based VSG control scheme is proposed for voltage and frequency regulation in microgrids. The proposed control scheme shows significantly improved dynamic performance when compared with conventional droop and conventional VSG control through several case studies. Its effectiveness and robustness is further evaluated through sensitivity studies.

Microgrid (MG) is emerging as a modern power system which can be operated in islanded as well as grid-connected modes. The evolution of power electronic technology in the power system gives forward direction to smart grid and microgrid. A major challenge in a microgrid is the absence of rotational inertia as distributed energy resources (DER) are interfaced with the grid through power electronic converters. Converters are operated under droop control for power-sharing conventionally. The concept of the virtual synchronous generator(VSG) can be a suitable approach for controlling these converters in low inertia microgrids. VSG implementation with a solar photovoltaic system is called photovoltaic synchronous generator. This paper discusses the trade-off between conventional droop control and VSG control of photovoltaic generation (PV).

For inverter control, droop control is an effective control scheme, but this scheme does not have the inertia and damping of the synchronous generator, resulting in insufficient frequency stability. In response to this problem, some scholars have proposed a virtual synchronous generator (VSG) control method to simulate the operating characteristics of traditional synchronous generators. In order to analyze the advantages and disadvantages of the two control strategies, this paper first established a small signal model of the inverter based on droop control and VSG control. The dynamic response of the time is compared. We can conclude that the droop control and the VSG control have their own advantages and disadvantages, and cannot meet the strict dynamic characteristics of the GC and SA states. Then, based on the comparison between the virtual synchronous generator and the droop control, the traditional droop control is improved. On this basis, an adaptive control is proposed to improve the response characteristics of the output frequency and active power when the power command changes in the grid-connected mode. In the GC state, the IDC-based adaptive control method can improve the output tracking of the input active power, reduce frequency fluctuations, and ensure frequency stability when the power reference value changes. In the SA state, the IDC-based adaptive control method can provide sufficient inertial characteristics. Finally, the simulation results verify the effectiveness of the proposed control strategy.

This paper proposes a new control strategy of microgrids for the improved voltage quality. In the existing control techniques, the droop control is commonly adopted as a decentralized power sharing method at the cost of voltage deviations. Besides, the conventional cascaded control featuring relatively slow dynamic response shows difficulties in handling the fluctuation of renewable energy outputs, leading to further voltage quality deterioration. In this paper, an advanced model predictive power control strategy by considering the battery constraints is proposed for the bidirectional dc–dc converters to smooth the solar photovoltaic (PV) outputs and stabilize the dc-bus voltages. A model predictive voltage control scheme taking into account the voltage changing trend is then developed to control the distributed inverters to improve the output ac voltages. Furthermore, a washout-filter-based power sharing approach with the plug-and-play capability is adopted to achieve a proper load sharing among parallel inverters and mitigate the voltage deviation. The proposed control strategy is numerically simulated in MATLAB/Simulink and experimentally verified by hardware-in-the-loop tests under the condition of fluctuating PV outputs and variable power demands. (This paper is accompanied by a video showing the hardware-in-the-loop test.)

Microgrid (MG) concept is considered as the best solution for future power systems, which are expected to receive a considerable amount of power through renewable energy resources and distributed generation units. Droop control systems are widely adopted in conventional power systems and recently in MGs for power sharing among generation units. However, droop control causes frequency fluctuations, which leads to poor power quality. This paper deals with frequency fluctuation and stability concerns of f-P droop control loop in MGs. Inspired from conventional synchronous generators, virtual damping is proposed to diminish frequency fluctuation in MGs. Then, it is demonstrated that the conventional frequency restoration method inserts an offset to the phase angle, which is in conflict with accurate power sharing. To this end, a novel control method, based on phase angle feedback, is proposed for frequency restoration in conjunction with a novel method for adaptively tuning the feedback gains to preserve precise active power sharing. Nonlinear stability analysis is conducted by drawing the phase variations of the nonlinear second-order equation of the δ-P droop loop and it is proved that the proposed method improves the stability margin of f-P control loop. Simulation results demonstrate the effectiveness of the proposed method.