Droop Control Loop Research Articles

An Adaptive Droop Control for Power Sharing Between Three Phase Parallel Inverter in Autonomous Microgrid

The development of an adaptive droop controller is for power sharing between three-phase parallel inverters. Traditionally, droop controllers have been employed in conventional power systems to allocate the load among generators with varying capacities. Our approach involves the utilization of an adaptive control technique. This method is designed for a setup involving three-phase parallel inverters linked to an AC microgrid, to enhance the proportional distribution of active power despite changes in feeder network characteristics and loads. The proposed control mechanism automatically tunes the frequency droop parameter to address mismatch frequencies. The system includes two sets of three-phase inverters regulated by a combination of P-F droop control, an enhanced droop control loop, and PI controllers. The adaptive droop coefficient control is capable of addressing problems where power distribution inaccuracies arise due to different line loads as well as different inverter capacities.

Novel Fuzzy Logic Controls to Enhance Dynamic Frequency Control and Pitch Angle Regulation in Variable-Speed Wind Turbines

This study introduced a novel control approach based on fuzzy logic control (FLC) to enhance the frequency regulation capacity of variable-speed wind turbines (VSWTs). The proposed method integrates FLC within droop and inertia control loops. Real-time measurements of the system frequency and the rate of change of frequency (ROCOF) serve as inputs to the FLC, enabling the method to improve the frequency response by VSWTs. In addition, the method employs FLC for pitch angle frequency control, optimizing reserve power for frequency regulation under varying wind speed levels. The innovative aspect of this study lies in the simultaneous application of FLC to pitch angle frequency control and droop/inertia control, leading to the enhanced frequency regulation capability of VSWTs and smoother operation across a range of wind speeds. Compared with traditional methods, the proposed approach provides a comprehensive and effective solution to the challenges associated with frequency regulation in VSWTs. Through simulations across different wind speed scenarios, the proposed control method demonstrated the best performance among various mature methods, highlighting the efficacy of the proposed method on the frequency regulation of VSWTs under different wind speeds. This study’s findings highlight the potential of the proposed FLC-based method to optimize frequency regulation and contribute to more reliable and efficient wind energy systems.

Design, Modeling, and Validation of Grid-Forming Inverters for Frequency Synchronization and Restoration

This paper focuses on the modeling, analysis, and design of grid-forming (GFM) inverter-based microgrids (MGs). It starts with the development of a mathematical model for three-phase voltage source inverters (VSI). The voltage and current controllers consist of two feedback loops: an outer feedback loop of the capacitance-voltage and an inner feedback loop of the output inductance current. The outer voltage loop is employed to enhance the controller’s response time. The inner current loop is used to provide active damping for the resonance created by the LCL filter. A two-layer control scheme is adopted for the GFM inverter control. The primary decentralized control uses droop control and virtual impedance loops to share active and reactive power. Simultaneously, the centralized secondary control addresses frequency and amplitude deviations induced by the droop control. Additionally, a synchronization loop is proposed for seamless reconnection of GFM inverters to the MG and to connect the GFM-controlled MG to the main grid. It has the advantage that the inverter operates in GFM mode even after the synchronization has occurred. The simulation results have shown that the voltage controller ensures a 0.005 s settling time and maintains the steady-state error at its minimum value of 0.1 V. Similarly, the current controller ensures a 0.006 s settling time with a 10−5 steady-state error. The system with the designed controller has a low total harmonic distortion (THD) of 1.46% and improved power quality of the output voltage. Furthermore, a quick restoration time is observed during load steps and tripping events, with a restoration time of 1 s with 10−10 steady-state error. Synchronization is achieved within 0.8 s for the incoming inverters and requires 3 s to synchronize the MG with the main grid, maintaining a steady-state error of 10−9.

Economic Optimization Control Method of Grid-Connected Microgrid Based on Improved Pinning Consensus

For the sake of reducing the total operation cost of grid-connected microgrids, an improved pinning consensus algorithm based on the incremental cost rate (ICR) is proposed, which defines ICR as the state variable. In the algorithm, the power deviation elimination term is introduced to rapidly eliminate the total power deviation, and the pinning term is brought to realize the fast convergence to reference value. By computing the optimal ICR of the system, the optimal active output reference value of each distributed generation (DG) is obtained when the system realizes the economic optimization operation. In addition, an economic optimization control method of grid-connected microgrids, based on improved pinning consensus, is proposed. By utilizing the method, the economic optimization operation of the system is attained by basing on the established distributed hierarchical architecture and by sending the reference value of optimal active output of each DG to the P-f droop control loop. Finally, a simulation model of parallel operation system of six DGs is established. The impact of grid electricity price, pinning coefficient and other factors on the operation state of the system is analyzed and simulated. The simulation results show that the economic distribution of active output is completed. The proposed method can make the microgrid rapidly enter the economic optimization state, and can still reduce the total operation cost and possess the faster response speed under the conditions of changing electricity price, low algebraic connectivity topology, DG plug-of-play, dynamic line rating (DLR) constraint, etc.

Secondary-Frequency and Voltage-Regulation Control of Multi-Parallel Inverter Microgrid System

As an important form of distributed renewable energy utilization and consumption, the multi-parallel inverter microgrid system works in both an isolated and grid-connected operation mode. Secondary-frequency and voltage-regulation control are very important in solving problems that appears in these systems, such as the distributed secondary-frequency regulation real-time scheme, voltage and reactive power balancing, and the secondary-frequency regulation control under the disturbances and unbalanced conditions of a microgrid system. This paper introduces key technologies related to these issues, such as the consensus algorithm and event-triggered technique, the dynamic and adaptive virtual impedance technique, and the robust and self-anti-disturbance control technique. Research and design methods such as small-signal state-space analysis, the Lyapunov function design method, the impedance analysis method, μ-synthesis design, and the LMI matrix design method are adopted to solve the issues in secondary-frequency regulation and voltage regulation. As the number of inverters increases, the structure of the microgrid becomes more and more complex. Suggestions and prospects for future research are provided to realize control with low-communication technology and a distributed scheme. Finally, for the case study, the droop-control model and primary frequency/voltage deviation of a multi-parallel inverter microgrid system is analyzed, and a state-space model of a multi-parallel inverter microgrid system with a droop-control loop is established. Then, the quantitative relationship between the primary frequency/voltage deviation and the active and reactive power output in the system is discussed. The methods and problems of centralized and decentralized secondary-frequency regulation methods, secondary-frequency regulation methods based on a consensus algorithm and an event-triggered mechanism, reactive power and voltage equalization, power distribution, and small-signal stability of the multiple parallel inverter microgrid system regarding the virtual impedance loop are analyzed.

Revisiting Grid-Forming and Grid-Following Inverters: A Duality Theory

Power electronic converters for integrating renewable energy resources into power systems can be divided into grid-forming and grid-following inverters. They possess certain similarities, but several important differences, which means that the relationship between them is quite subtle and sometimes obscure. In this article, a new perspective based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">duality</i> is proposed to create new insights. It successfully unifies the grid interfacing and synchronization characteristics of the two inverter types in a symmetric, elegant, and technology-neutral form. Analysis shows that the grid-forming and grid-following inverters are <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">duals</i> of each other in several ways including a) synchronization controllers: frequency droop control and phase-locked loop (PLL); b) grid-interfacing characteristics: current-following voltage-forming and voltage-following current-forming; c) swing characteristics: current-angle swing and voltage-angle swing; d) inner-loop controllers: output impedance shaping and output admittance shaping; and e) grid strength compatibility: strong-grid instability and weak-grid instability. The swing equations are also derived in dual form, which reveal the dynamic interaction between the grid strength, the synchronization controllers, and the inner-loop controllers. Insights are generated into cases of poor stability in both small-signal and transient/large-signal. The theoretical analysis and simulation results are used to illustrate cases for single-inverter systems, two-inverter systems, and multi-inverter networks.

Distributed secondary controller to ensure proportional sharing of reactive power in AC microgrid

In an ac microgrid, reactive power sharing accuracy is affected due to unequal values of interconnecting cable impedances. To resolve this issue, secondary controllers are used to compensate the effect of cable impedances. Various types of secondary controllers are suggested in the literature which includes linear proportional plus integral (PI) controllers to minimize the difference in reactive power sharing created by conventional E−Q droop law. The reference value of the PI controller is the average value of reactive powers supplied by the sources. When the PI controller is used to minimize the difference between the actual and reference value of reactive power, the difference between the algebraic sum of reactive powers supplied by the sources and the reactive powers demanded by the loads has a nonzero value. Therefore, the performance of these controllers becomes poor in the case of ac microgrid having sources of unequal ratings. To resolve this issue, a proportional reactive power-sharing (PRPS) controller-based distributed secondary controller is proposed in this paper. The PRPS controller modifies the droop gain of the E−Q droop control loop of each source in such a way that each source supplies reactive power equal to its proportional value of reactive power. The proportional value of reactive power supplied by each source is the power when the equivalent output impedance of all sources as seen by the loads are identical. The key advantage offered by the proposed controller is the accurate reactive power-sharing in the case of ac microgrid having sources of unequal ratings. The proposed controller ensures zero value of the difference between the algebraic sum of reactive powers supplied by the sources and the reactive powers demanded by the loads. This ensures accurate sharing of reactive power among the sources. The validation of the proposed controller is carried out for islanded mode of operation of ac microgrid. Further, the proposed controller requires of low bandwidth communication for its implementation. The effect of the proposed controller on the stability of the system is demonstrated using reduced order small-signal model. The effect of communication delay on the performance of the system is analyzed with the help of roots locus plots. To validate the efficacy of the proposed controller, detailed simulation studies are carried out in Matlab/Simulink.

Robust Power Sharing and Voltage Stabilization Control Structure via Sliding-Mode Technique in Islanded Micro-Grid

With a focus on the problems of active power sharing and voltage deviation of parallel-connected inverters in an islanded micro-grid (MG), in this study, the power-voltage droop controller and the inner voltage regulator are redesigned based on a total sliding-mode control (TSMC) technique. As for the power-voltage droop control loop, a droop control relation error between the active power and the point-of-common-coupling (PCC) voltage amplitude is defined. Then, the TSMC scheme is adopted to reach the new droop control relation, where the active power sharing and voltage amplitude recovery can be achieved simultaneously. Owing to the faster dynamic response and strong robustness provided by the TSMC framework, high-precision active power sharing during transient state also can be ensured without the influence of line impedances. The power allocation error can be improved by more than 81.2% and 50% than the conventional and proportional-integral (PI)-based droop control methods, respectively, and the voltage deviation rate can be reduced to 0.16%. Moreover, a small-signal model of the TSMC-based droop-controlled system is established, and the influences of control parameters on the system stability and the dynamic response are also investigated. The effectiveness of the proposed control method is verified by numerical simulations and experimental results.

A novel droop control method to achieve maximum power output of photovoltaic for parallel inverter system

In general, the output power of PV inverter should be coordinated with the load. In the initial stage of the design, when the PV inverter operating in islanded mode, the load power is supposed to be no more than the maximum PV output power with the purpose to stabilize the system. However, it may waste PV cells' energy. Consequently, more and more solar panels are linked to other energy sources in order to create a parallel inverter system, which makes it feasible to fit the power output of each source. The power distribution of parallel inverter is achieved by the use of droop control in microgrid system. For such a parallel inverter system consisted of PV inverters and non-regeneration energy sources inverters without energy storage devices in islanded mode, if the shared load power is no more than the available maximum PV inverter output power, there is a power waste for PV inverter; Moreover, due to the intermittency of PV sources, the system may become unstable if the shared load power is more than the available maximum output power of PV inverter. However, the maximum utilization of the PV output energy cannot be achieved automatically with the traditional methods to add virtual impedance or adjust the droop coefficients. Thus, in order to avoid power waste and potential instability caused by insufficient PV power by traditional droop control, this paper recommends an improved droop control scheme to maximize the power output of PV units. Required by the load, the remaining power is composed of the other inverters, which could effectively improve the utilization rating of renewable energy sources and system stability. At the same time, according to the system stability analysis based on the small signal modeling, it has been designed about the droop coefficients of the improved droop control loop. In the end, the experimental results have showed that the suggested scheme has a varied validity and robustness.

Robust Hierarchical Control of VSC-Based Off-Grid AC Microgrids to Enhancing Stability and FRT Capability Considering Time-Varying Delays

In this article, a new robust hierarchical control scheme is proposed for off-grid (autonomous) voltage-sourced converter (VSC)-based alternative current (ac) microgrids (MGs). The main focus of this article is devoted to the primary layer, including the cascade structure of inner (current), outer (voltage), the virtual impedance, and droop control loops. In the inner control loop, a robust controller is designed based on an adaptive backstepping integral nonsingular fast terminal sliding mode control (ABINFTSMC) strategy to regulate and track the reference of current signals in the presence of unknown bounded uncertainties and external disturbances. The outer loop is designed based on a mixed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{2}/H_\infty $ </tex-math></inline-formula> control strategy by utilizing the state feedback control law to generate the inner-loop reference signal and to achieve stability and robustness versus perturbations, and sufficient conditions are derived based on linear matrix inequalities (LMIs). The performance of the controllers is improved to consider time-varying delay (TVD). The droop control and virtual impedance loops are applied to enhance the power-sharing quality of the system. Also, a distributed consensus-based protocol is used in the secondary layer. Finally, to evaluate the performance of the control laws and to demonstrate the effectiveness of the proposed robust control scheme, offline digital time-domain simulation studies are carried out in MATLAB/Simulink software environment. The obtained simulation results and comparison with previous work prove that the proposed robust hierarchical control scheme can effectively, and robustly enhance the transient response and steady-state performance, and fault ride-through (FRT) capability of MG when faced with small- and large-signal disturbances, and show better robust performance over conventional proportional-integral (PI)-based nested-loop control strategies.

Cascaded controller for a standalone microgrid‐connected inverter based on triple‐action controller and particle swarm optimisation

In this study, power quality (PQ) improvement has been addressed, in the form of total harmonic distortion (THD) minimisation, as well as, voltage regulation using two cascaded schemes. The first scheme is the optimised triple action controller (TAC)-based pulse width modulated voltage source inverter. TAC consists of a proportional resonant controller, selective harmonic compensator, and a new added current-assisted feed forward controller. The second scheme is the optimised cascaded dual-level control of a standalone microgrid. Cascaded level control consists of droop, secondary, and synchronisation control loops. The two approaches have been optimised for best parameter selection out of the possible solution space using a particle swarm optimisation algorithm to satisfy the study objectives. The optimisation objectives/constraints were to minimise THD and minimise overshoot/undershoot, rise time, and steady-state error for voltage compensation under two disturbance scenarios, sudden load change, and voltage flicker injection as a power frequency disturbance. These research results have been compared to other existing simulation and experimental work. The results proved to be better in output voltage, frequency, response time, and THD. Furthermore, the proposed schemes ensure power factor improvement, high efficiency, overall system PQ, and reliability at various load conditions.

Controlling DC microgrids in communities, buildings and data centers

Microgrid technology is poised to transform the electricity industry. In the context of commercial/domestic buildings and data centers, where most loads are native direct current, DC microgrids are in fact a natural choice. Voltage stability and current/power-sharing between sources within a DC microgrid have been studied extensively in recent years. DC voltage droop control is known to have its drawbacks in that current or power-sharing is relatively poor. To eliminate this drawback, some have proposed to add a communication-based consensus control in addition to the primary voltage droop control loop. The current sharing performance is improved, however, the voltage deviation inherent in droop control requires a further, slower control to achieve voltage quality control. To overcome this complication, and reduction in response time, a low latency communication-based control technique that achieves proportional current sharing without significant voltage deviations is proposed in this work. The stability of the proposed control technique is compared to state-of-the-art using eigenvalue and transient analyses. The negative impact of communication delays on proposed control is discussed in detail.

Intelligent coordinated control for improved voltage and frequency regulation with smooth switchover operation in LV microgrid

Abstract During demand changes and incidence of grid faults, the microgrid system is subjected to an imbalance between the generation and demand. The maintenance of power balance under such dynamic conditions is a major concern for the proper functioning of the microgrid network. During those dynamic periods, achieving voltage and frequency regulation, in particular, during islanded/standalone mode of operation is a challenging task. For reliable operation, in addition to power balance, voltage and frequency regulation, reduced switchover transients and fast switchover operations are necessary. Conventional coordinated controllers comprising of droop, voltage and current control loops, aim at achieving stability of the microgrid network but fails to ensure a quick and smooth response. This work proposes an Adaptive Neuro-Fuzzy Inference System based intelligent coordinated control strategy by combining control techniques namely, inverse droop control, virtual impedance control and current control. The proposed coordinated control scheme provides improved voltage and frequency regulation, reduced switching transients and quick adaptation of the system during demand changes and fault events. The proposed system uses an inverse droop control technique to achieve power decoupling. Additionally, a virtual impedance-based voltage control loop is implemented which ensures voltage regulation of the microgrid and a feed-forward current control loop is developed to minimize the transients during load switching and switchover operations. The reverse droop based PLL strategy is implemented for grid synchronization and smooth switchover operations. The proposed system is simulated in a 20-kVA grid-tied microgrid system in MATLAB/SIMULINK R2018b and tested in real-time of 5-kVA grid-tied solar PV system which demonstrated that the proposed approach is effective under varying irradiance, changing demand conditions and switchover operations. The experimental results prove that the proposed scheme is performing better under different system conditions with fast switching response when compared with other control algorithms.

Equivalence Analysis of Virtual Synchronous Machines and Frequency-Droops for Inertia Emulation in Power Systems with Converter-Interfaced Renewables

In converter-based AC microgrids, the frequency-droop is widely applied. As a considerable share of installed capacity is replaced by renewable generation, the virtual synchronous machine (VSM) algorithm is presented to address stability issues caused by low-inertia converter interfaces. Recently, droop control loops are also modified to improve stability. The VSM and the frequency-droop are both derived from the property imitation of synchronous generators (SGs) while researched independently. In this paper, by simplifications in the steady state, the expression of the inertia emulation of the VSM algorithm based on the second-order SG model is linearized as a first-order ordinary differential equation. In frequency-droop loops, as the instantaneous power components are commonly passed through a low-pass filter for disturbance rejection, the system function of the droop control can also be transformed into a first-order ordinary differential equation, which is equivalent to that of the VSM under certain conditions. In terms of the inertia constant, the swing equation is normalized under per unit representation. By the same simplification in the VSM linearization, the equivalence between the linearized VSM and the SG is also demonstrated. Therefore, the equivalence among SGs, the VSM, and the frequency-droop are summarized, which provides insights for coordinative control of power converters integrated into current SG dominating power systems. For a specific synchronous generator, the equivalence conditions are verified by simulation.

A novel cascaded control to improve stability and inertia of parallel buck-boost converters in DC microgrid

To deal with the issues caused by the high integration of renewable resources, the parallel operation of the converters is generally required to improve the system stability and reliability. To support the parallel operation of buck-boost DC/DC converters, they are generally regulated by a cascaded control strategy, which includes droop control loop, and nested proportional-integral (PI) based voltage and current control loops. However, it can provide barely inertia support and needs precise system model to reach the desired performance. In order to overcome these drawbacks, this paper proposes a novel cascaded control strategy for parallel operation of buck-boost DC/DC converters. The proposed cascaded control strategy involves a voltage synchronization loop, a virtual inertia control (VIC) loop, and two nested NTSMC based voltage and current control loops. The voltage synchronization controller can enable the plug and play requirements by synchronizing the output voltage of the converter with the voltage at the point of common coupling (PCC) before connecting a DC/DC converter to the DC microgrid. The VIC can improve the inertia of the converter-interfaced DGs. The nested-loop NTSMC based voltage and current controller can improve the robustness of converter system. Hardware experiments demonstrate the improved performance of the proposed control strategy.

Voltage and frequency regulation in an islanded microgrid with PEM fuel cell based on a fuzzy logic voltage control and adaptive droop control

Nowadays, the demand for electrical energy is increasing, and conventional power systems will face major problems in the future if they cannot generate extra demand for electrical energy. One of the best solutions for solving extra demand for electrical energy is the use of renewable resources to produce energy. The main aim of this study is designing and controlling the microgrid voltage and frequency. This study proposed the voltage and frequency control of an islanded microgrid based on fuzzy logic controller. The system considered for this study consists of Proton-Exchange Membrane Fuel Cell (PEMFC) that connected to parallel inverters to convert DC voltage to AC voltage. The control structure is based on adaptive droop control and fuzzy voltage control loop. Also, the proposed structure controls active and reactive powers and decreases power losses of the microgrid. Simulation results are presented to show the effectiveness and robustness of the proposed control structure over the conventional proportional-integral controller. It is shown that the proposed controller has good transient response under the output voltage PEMFC variation and load changes as disturbance.

Adaptive fractional‐order control of power system frequency in the presence of wind turbine

Inertial and droop control are common methods for improving grid frequency control. Using the first-order derivative of the frequency in the inertial control loop only investigates the rate of frequency variation, while other useful information is not taken into consideration. Accordingly, in this study, the application of a fractional-order derivative (between 1 and 2) in the inertial control loop is proposed to have more useful frequency-based information. To achieve this goal, firstly continued fraction approximations and secondly, the discretization is adopted by using the Tustin Method with Prewarping. Inertia loop and droop loop gain have a significant impact on the operation of the wind turbines and the power system frequency control. Therefore, in this paper, a novel method for the adaptive adjusting of droop and inertia control loops gains in DFIG is proposed. To utilize more frequency error information, fractional-order adaptive law is proposed to update the loop gains and the inertia control loop. In order to have a robust system, which is permanently stable for every fractional order (FO), is used parameter space method. Simulation results demonstrate the proper performance of the FO-based adaptive approach to increase the frequency nadir (FN) and decrease the frequency variations.

Determination of Optimal Q-V Droop Controller Parameters for Three-Phase Modular UPS System Based on Genetic Algorithm (GA)

This paper presents an optimized reactive power-voltage (Q-V) droop controller parameters for three-phase modular uninterruptible power supply (UPS) based on genetic algorithm (GA). The main objective of this paper is to minimize the voltage regulation error when there are changes in loads. In addition to select the control parameters carefully to promote the performance of the system against disturbances. This is substantial to attain power sharing between different UPS units to balance sudden disturbances that happen when there are changes in loads. We propose a discrete proportional-integral controller which compares the output voltage with the reference value given by the droop control loops then generates suitable voltage vector signal to pulse-width modulation (PWM) inverter. The fitness function for the optimization problem is formulated by considering the sum of square errors in voltage. This fitness function converges to local minima during the iterative operation of the genetic algorithm. Thus, the optimal parameters of the voltage droop control can be discovered with great probability. Simulation of two parallel-connected three-phase UPS units are carried out using MATLAB/SIMULINK R2018a. Comparison between the conventional trial and error tuning methods and GA optimization is performed. The results show that GA succeeded to minimize the output voltage root mean square error (RMSE) and to enhance the power sharing capability when there is variation in loads.

Virtual-battery based droop control and energy storage system size optimization of a DC microgrid for electric vehicle fast charging station

DC microgrid is supposed to be a feasible solution to reduce the negative impact of electric vehicle (EV) fast charging on the electric grid and improve the penetration of photovoltaics (PV) generation. In this paper, an improved decentralized Virtual-battery based droop control with the capability of bus voltage maintenance, load power dispatch and SOC balance of the energy storage system (ESS) is proposed to ensure the autonomous and stable operation of the DC microgrid. The reference output voltage and virtual resistance in the droop control loop are altered dynamically based on the Virtual-battery model of the ESS. The coordinated control among the PV-ESS-Grid integrated system is realized through the primary Bus-Signaling control, where the reference voltages at which the control modes of the PV array and the grid are switched are designed based on the VirtualOCV of the ESS. The effectiveness of the proposed control strategy is validated in MATLAB/Simulink environment with an equivalent bus capacitance-based model where the EV charging profile is obtained from real-world charging data of a fast charging station. The merits of the control strategy including higher PV utilization, less frequent connection of the grid and more precise voltage tracking are highlighted in comparison with the conventional droop control strategy. Finally, the sizing of the ESSs is optimized based on the total cost of the DC microgrid, including the daily electricity cost purchased from the grid and the depreciation cost of the ESSs based on the expanded capacity degradation model of Li-ion batteries.

Active Power Sharing and Frequency Restoration in an Autonomous Networked Microgrid

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.