Distributed Generation Control Method for Active Power Sharing and Self-Frequency Recovery in an Islanded Microgrid

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.

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To manage the microgrid effectively, it is important to get the power information among multiple distributed generation (DG) units. However, when the droop control method is adopted, as the corresponding DG units cannot be simply modeled as VF or PQ buses, the conventional power flow algorithm may become inapplicable. To solve this issue, a time domain iteration (TDI) based power flow algorithm is hereby proposed. Firstly, a microgrid model is prepared, containing a network model and several DG unit models. Then, the proposed TDI is executed for power flow calculation, which mimics the real-time operation of microgrids. In each iteration, the DG unit models input voltages and currents to the network model; then the network model changes its state accordingly and feeds related parameters back to those DG unit models. As the DG unit models simulate the behavior of actual DG units, the proposed algorithm is not limited to the droop control governed microgrids. Moreover, as the TDI has definite physical meaning, convergence problems can be avoided. Finally, the validity of the proposed power flow algorithm is verified through the Matlab simulation results from an 8-bus microgrid system.

Droop control is a key control method for operating islanded microgrids (IMGs). The settings of the droop parameters for distributed generation (DG) units can considerably affect the ability of an IMG to satisfy the required voltage tolerance boundary prescribed in steady-state voltage regulation standards. This paper analyzes the complexity of voltage regulations in droop-controlled IMGs. A new algorithm is proposed to satisfy the voltage regulation requirements of IMGs. The proposed algorithm obviates the need for a centralized secondary controller, where each DG unit updates its own voltage droop parameters, autonomously, via interaction with other DG units, using a low-bandwidth, peer-to-peer communication network. To that end, a distributed constraint satisfaction approach is adopted to formulate the problem of voltage regulation in a multi-agent environment. An asynchronous weak commitment technique is proposed to solve the formulated problem. Several case studies are simulated to evaluate the performance of the proposed algorithm. The results show that the proposed algorithm can effectively mitigate the challenges of voltage regulation in IMG systems.

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.

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.

Microgrids are distributed generation structures that link groups of Distributed Generators (DGs) to either connect to the utility grid, or to operate as a standalone (islanded) system. However, the injected power from each DG must be carefully controlled to ensure reliable power distribution, balanced modular operation, and regulated voltages across the microgrid. It is commonly accepted that DG units cannot operate in current-control mode when islanded, because there is no stiff grid voltage available. Consequently they are usually operated in voltage control mode, with individual droop controllers regulating the inverter voltage magnitude and frequency to share active and reactive power. This paper presents a new approach to operate islanded microgrid DG units in current control mode, by combining linear current regulators with predictive voltage control. The result achieves a rapid load response while still maintaining well regulated voltage levels. A current magnitude droop controller is also proposed to maintain equal power sharing between the current controlled DG units.

In an islanded microgrid, while considering the complex nature of line impedance, the generalized droop control fails to share the actual real/reactive power between the distributed generation (DG) units. To overcome this power sharing issue, in this paper a new approach based on feed forward neural network (FFNN) is proposed. Also, the proposed FFNN based droop control method simultaneously controls the microgrid voltage and frequency within the limits. The proposed microgrid consists of combination of photovoltaic (PV) system and battery energy storage system (BESS) as the first DG unit and solid oxide fuel cell (SOFC) as the second DG unit. The simulation of the proposed microgrid is carried out in Matlab/Simulink environment and necessary results are compared to show the effectiveness of the proposed method.

Control strategies of a microgrid which includes both synchronous generators and converter-based distribution generation (DG) units must be designed such that effective operation of the microgrid is achieved. The main objective of this paper is to develop a high performance control strategy for an islanded medium voltage (MV) microgrid consisting of inverter and non-inverter interfaced DG units. A new control method for the synchronous generator in an islanded microgrid is proposed based on a virtual droop scheme. The proposed strategy can effectively manage the real and reactive powers of the microgrid among the inverter and non-inverter based DG units. The steady state and dynamic responses of the MV microgrid is significantly improved by using the proposed power management scheme. The performance of the proposed control strategy is verified by using digital time-domain simulation study in the PSCAD/EMTDC software environment.

With high penetration of power electronics-based distributed generation (DG) units, the decentralized control strategy based on droop control with virtual impedance has been widely studied in recent years. But the conventional virtual impedance cannot properly deal with the massively increasing of nonlinear loads. Therefore, an improved virtual impedance method based on second-order general-integrator (SOGI) is presented. Besides, a further value correction of reactive power in droop control is also proposed in this paper. The proposed approaches have been applied and verified in a parallel system formed by two DG units connected with nonlinear loads. The form of SOGI and derivative formula of corresponding virtual impedance in the system has been analyzed in detail. The obtained experimental results show the proposed approaches can achieve less output voltage and current distortion of DG units with nonlinear loads, and also realize good performance of power sharing.

- 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

This study puts forward a strategy for optimal power sharing control in a microgrid that is connected to a utility grid through a back-to-back (B2B) converter. In grid-connected mode, the B2B converter totally isolates the microgrid from the utility grid in terms of voltage and frequency. In the proposed strategy, a pre-specified amount of power exchanged between the utility grid and the microgrid is regulated via active/reactive control in the B2B converter. This regulation means that distributed generation (DG) units supply the rest of microgrid load demand and track load changes through droop control. In both the voltage source inverter (VSI) of the B2B converter and the DG units, state-feedback control is employed as an inner control loop for tracking the state variable reference signals generated by the corresponding outer loops of the variables. Microgrid stability is essential and highly affected by controller parameters, droop coefficients, and the components of inductor-capacitor-inductor filters. In this regard, control is formulated as an optimization problem, for which particle swarm optimization is used to optimally calculate the parameters of the system and the controllers. Objective functions are derived by minimizing an eigenvalue-based function. The PSCAD simulation results demonstrate the effectiveness of the proposed control strategy.

This paper presents modeling, active power management strategy, and implementation of a fuel cell, pbotovoltaic, and super-capacitor power conversion system in MATLAB. The micro-grid includes two distributed generation (DG) units. Each DG unit consists of a fuel cell (FC), photovoltaic unit (PV) and super-capacitor (SC). The super-capacitor energy storage compensates the shortage power of FC stack, PV unit and the load demand. The output of DG unit is connected to grid using inverter. Load changes are applied for balanced conditions. The frequency deviation is fed to adaptive proportional resonance controller which adapts to the changes in frequency. This results in eliminating steady state error and output impedance of DG unit is decreased thereby reducing low frequency oscillations in the power components of the DG units. The system is modeled and simulated using software. The dynamic response of the DG units to balanced load changes is investigated using software.

Line impedance mismatch and unregulated harmonic currents cause serious problems for an islanded microgrid, such as inaccurate reactive power sharing and voltage distortion at the point of common coupling (PCC). To overcome these issues, a coordinated virtual resistance and capacitance are introduced together with its control scheme for parallel distributed generation (DG) in an islanded microgrid. The proposed coordinated virtual impedance control scheme directly modifies the DG output impedance at the fundamental frequency to realise accurate current sharing among DG units. In addition, a variable harmonic impedance control loop is proposed for each DG to absorb the non‐linear load harmonic current. Thanks to the proposed control scheme, the PCC harmonic voltage distortion is successfully compensated and power‐sharing among DG units is accurately achieved. The stability of the microgrid system was analysed in detail to verify the feasibility of the coordinated virtual impedance control scheme. Experiments on a laboratory prototype microgrid are performed to validate the performance of the proposed control scheme.

For the operation of autonomous microgrids, an important task is to share the load demand using multiple distributed generation (DG) units. In order to realize satisfied power sharing without the communication between DG units, the voltage droop control and its different variations have been reported in the literature. However, in a low-voltage microgrid, due to the effects of nontrivial feeder impedance, the conventional droop control is subject to the real and reactive power coupling and steady-state reactive power sharing errors. Furthermore, complex microgrid configurations (looped or mesh networks) often make the reactive power sharing more challenging. To improve the reactive power sharing accuracy, this paper proposes an enhanced control strategy that estimates the reactive power control error through injecting small real power disturbances, which is activated by the low-bandwidth synchronization signals from the central controller. At the same time, a slow integration term for reactive power sharing error elimination is added to the conventional reactive power droop control. The proposed compensation method achieves accurate reactive power sharing at the steady state, just like the performance of real power sharing through frequency droop control. Simulation and experimental results validate the feasibility of the proposed method.