Conventional and reverse droop control in islanded microgrid: Simulation and experimental test

This paper proposes a control strategy for sharing load requirements in a low voltage islanded microgrid with converter interfaced distributed energy resources. In high voltage lines with large X/R ratio, real and reactive power are decoupled and thus conventional droop control can effectively share the power needs of the load. However, the same strategy cannot be applicable in dominantly resistive remote microgrids where, X/R ratio is very small. In order to ensure accurate sharing of load power, this paper adopts cut in contrast of conventional angle droop control, where active power sharing is achieved by controlling voltage and reactive power by angle droop method. In addition, virtual resistive output impedance is utilized to effectively decouple real and reactive power. Moreover, this strategy includes influence of line impedance in load power sharing and offers an added advantage of effective overall system damping. Third harmonic injection based pulse width modulation scheme is used for each distributed generators(DGs), which provides better utilization of DC link voltage and suppress neutral currents under unbalanced conditions.

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.

This paper presents primary and secondary control of islanded microgrid. In conventional primary droop control, there is a problem of poor voltage regulation of microgrid. The drawback of conventional primary droop control is improved by voltage shifting based primary control. Voltage shifting based primary control is not completely satisfied by the voltage regulation of microgrid. Also, there is current sharing problem in both types of primary control. So, in secondary control, both current sharing and voltage regulation are achieved by slope adjusting and voltage shifting simultaneously. In this secondary control method, average value of current, voltage, and droop resistance of two neighboring converters is calculated then controlled by additional layer of distributed secondary control over both primary controls. By adjusting droop coefficient of different converters making same value of output impedance of converter, current sharing and voltage regulation are achieved. Also, in transient state, current sharing is achieved by using this secondary control technique. The performance of proposed microgrid is verified by MATLAB simulation.

- 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

The paper deals with the control of low-voltage microgrids with master/slave architecture. Distributed energy resources (DERs) are interfaced to the grid by means of conventional inverters (slave units), and a microgrid master controller (master unit) governs the interaction between the utility and the microgrid at the point of common coupling with the main grid. The power sharing among the available resources is achieved by the power-based control, a technique which allows to pursue both local (DER level) and global (microgrid level) optimization goals. The paper overviews and analyzes the control algorithm and focuses on how the considered approach can be employed to attain effective modes of operation in microgrids. Simulation results supporting the proposed approach are finally provided.

The large penetration of Distributed Energy Re- sources (DERs), especially PhotoVoltaic (PV), in Low Voltage (LV) distribution grids has raised the concern of unintentional islanding operations. The probability of unintentional islanding is increased due to the newly introduced standards with extended frequency and voltage thresholds and additional grid support functions, such as active and reactive power regulation using P/f and Q/V droop characteristics, which can make ineffective the anti-islanding provisions each PV is equipped with. Moreover, islanding operation due to faulty events at the Medium Voltage (MV) distribution level may be dangerous in presence of auto- matic reclosing procedure leading to out of phase reconnections. In this paper, an analysis of this phenomena is provided using a simplified characterization of the generators and the loads. Simulation results based on Matlab and DIgSILENT tools on a simplified LV distribution network are reported together with preliminary experimental results on a lab-scale prototype.

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.

In order to achieve the flexible and efficient utilization of distributed energy resources, microgrids (MGs) can enhance the self-healing capability of distribution systems. Conventional primary droop control in microgrids exhibits deviations in voltage and frequency and lacks research on voltage–frequency control during network reconfiguration. Therefore, this paper investigates the control strategy of secondary control for voltage and frequency during the process of reconstructing distribution networks to operate in the form of microgrids after faults. Firstly, the mathematical model of three-phase voltage-source inverters with droop control is analyzed. Then, inspired by the generator’s secondary frequency control strategy, an improved secondary control method based on droop control is proposed, and the effectiveness of the improved droop control strategy is verified. Finally, simulation examples of distribution networks with various dynamic boundary topology changes are designed to simulate a relatively stable control result, validating the proposed strategy.

Conventional droop control is widely used for achieving proportional power sharing in islanded microgrids. The active power sharing becomes proportional to the ratings of the inverters by applying the conventional droop control. However, reactive power sharing is not proportional because of resistive feeder impedance, unequal feeder lengths, and asymmetric load distribution in the network. In this article, an existing control strategy that has been proposed for improving reactive power sharing is enhanced by removing its dependency on communication. The required synchronization for executing the correction is generated by each inverter locally using a load change detection algorithm. This modification also makes the original control strategy immune to load changes happening during the correction process. An almost accurate reactive power sharing is achieved by applying the proposed modification even for highly resistive networks. Also, a tradeoff required in tuning the controller gain is discussed. The efficacy of the proposed modification has been validated by performing detailed simulation studies and by carrying out exhaustive experimental studies on a laboratory prototype.

Improving power sharing in islanded networked microgrids using fuzzy-based consensus control

With conventional droop control, the droop relationship between the voltage and reactive power is not purely linear because of the filter reactance. This paper focuses on the theoretical analysis to account for this characteristic and presents a precise power flow method for conventional droop control. The proposed method is universal, which can handle not only conventional droop control, but also other control strategies, such as robust droop control, constant power control, and constant voltage–frequency control. It can also handle frequency-dependent active and reactive power loads and is adapted for islanded and grid-connected systems. The proposed method extends the applicability of conventional power flow methods to microgrids so that the framework of the method is generic; any conventional power flow algorithm can be adapted to this framework. Compared with the time-domain simulation method, the proposed method is accurate, simple, and easy to implement for industrial applications.

In this paper, an improved load sharing strategy is proposed for distributed generation units (DGs) connected in a microgrid. Conventional frequency and voltage droop control result in unacceptable active and reactive power sharing. The proposed method formulates a suitable algorithm for load sharing in the islanded microgrid. The feeder power loss and the line impedance voltage drops are minimized so as to regulate the voltage at the point of common coupling (PCC) at its nominal value. The desired DG output voltages are calculated and a linear relationship is obtained between the shared active and reactive powers and the DG output voltages. A master DG controller sets the frequency which is followed by other DG units. The reference powers for the DG units are adjusted so as to maintain the rated PCC voltage. The proposed strategy is verified taking into account the DG ratings, unequal line impedance drops, feeder losses, change in system impedance and effect of DG local loads and formulates an improved power sharing strategy that also facilitates PCC voltage regulation under variable loading conditions. Simulation and experimental results are presented to verify the effectiveness of the proposed method.

Determining P-Q Droop Coefficients of Renewable Generators for Voltage Regulation in an Islanded Microgrid

For distributed energy resources (DERs) in islanded microgrid (MG), to improve the control effect on the output voltages and frequency from droop control, a novel distributed control strategy is proposed in this article based on the concept of cyber physical system (CPS). The novel control structure is consisted by the cyber and the physical layers. In the cyber layer: firstly, the communication structures of overall microgrid and single DER are constructed; then, based on the communication structures, the influence of time-delay (delay) and packet loss (loss) on consensus control is analyzed in this article; And then, an event-triggered mechanism combined with adapt virtual leader-following consensus control (AVLFCC) and predictive compensation is proposed in this layer. It is used to solve the problem of time-varying delay and loss rate in the process of consensus control. In the physical layer: firstly, the droop control is used as the primary control on the output voltages and frequency of DERs; then, a novel secondary control method is proposed by using the event-triggered mechanism. And finally, the simulation results confirm the effectiveness of the novel distributed control strategy under communication problems (delay and loss) and during plug-and-play operation.

In the present paper, a Multi-Segment P/f Droop Control Strategy is developed to achieve localized control, coordination and power management of different sources in an islanded microgrid which consists of two PV/Battery storage Hybrid units and a droop unit. Separate control loops are designed, each for PV power production control, Battery charging and discharging level control, and Droop unit operation control based on multi-segment P/f Droop characteristics. The conventional PI controllers can be used in the control loop design as they are very simple for implementation and give a better dynamic response. But their performance deteriorates when the complexity in the system increases due to disturbances like load variations and intermittent sources. Fuzzy Logic Control (FLC) technique is model-independent and a flexible tool. It can deal with complex systems such as microgrids with different types of imprecise inputs and variables particularly if the power is supplied by intermittent sources and is also consumed by varying and unpredictable loads during sudden disturbances. The Membership functions (MF) of a conventional FLC are determined by trial and error method. This method is time-consuming and does not guarantee an optimal controller. In the present paper, a Self-Adaptive Fuzzy Logic Droop Controller (SAFLDC) is proposed, whose input–output MFs are made adaptive by obtaining the cluster centers using Self-Organizing Maps (SOM) Algorithm. The paper aims to follow a systematic approach to propose an effective SAFLDC with optimal MFs whose parameters are tuned using Hybrid GA-PSO. The developed strategy has been validated using detailed switching models in MATLAB/SIMULINK tool and the behavior of key variables in the strategy is illustrated.