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Droop control is the most common approach for controlling inverter-based micro-grids. The active power droop gain has always been considered as the main parameter for identifying the micro-grid stability margin. Increasing this margin improves the transient performance and provides robustness to the micro-grid for a wide range of operations. Previo...
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... encompassed within the blue line represents all possible microgrid stable operating points. Increasing n q results in an initial decrease in the maximum active droop gain (m p max ) followed by a significant increase in its value that eventually leads to instability at any value of m p when n q exceeds 5.9e −3 . This observation is illustrated in Fig. 8 by plotting the loci of the dominant poles as m p varies from 1.9e −6 to 3e −4 at n q of 1.3e-3. Increasing n q from 1.3e-3 to 4e-3, approximately by 200%, results in an increase in the maximum active power droop gain by 50%, i.e., m p max increased from 1.9e-4 to 3e-4. Using the proposed stability domain chart, it can be seen that the ...
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... However, this work assumes prior knowledge of load changes, which may not accurately reflect real-world load and generation variations. In [12], Lahseen et al. investigated the impact of the reactive power droop coefficient on microgrid stability and proposed an active-reactive droop coefficient selection chart. However, the microgrid small signal stability model is a prerequisite. ...
... It is realized that the change in reactive power w.r.t. the reactive power droop coefficient follows dQ dkq ∝ 1 k 2 q , that is in line with the Deduction-1. The reason for approximate fit is due to the assumption of purely resistive feeders and constant PCC voltage for the deduction of (12). Nevertheless, it is observed that as the magnitude of the reactive droop coefficient increases, the coefficient's power decoupling capability reaches a saturation point. ...
... The derivative droop was used in a hybrid cost-based droop control to achieve optimal generation while maintaining an improved stability margin [27]. The stabilizing impact of the reactive droop and reactive derivative droop was intensively studied in [28]. It was revealed that the derivative droop in the reactive path alone is capable of improving MG stability while sustaining equal power sharing. ...
... At α = β ≈ 1, the poles and zeros of the approximating transfer functions, G α and G β , are almost canceled, and the system in (26) converges to a differential algebraic system that is equivalent to the IO system in [27], [28]. Each inverter has three internal SSMs other than the power controller. ...
... Hence, the entire MG matrix (A mg ) has 8 + 3N inv rows and columns. For more information about the supporting matrices used to generate A mg , kindly refer to [27], [28], [42]. ...
Islanded operation of microgrids (MGs) with parallel-operated inverters imposes many control challenges in terms of stability and dynamic behavior, especially at contingency events. Hence, improving the dynamic performance and the stability margin is essential for robust MG operation. Therefore, the fractional-order derivative (FOD) droop controller is proposed to achieve these goals. A detailed small signal model is developed for the entire MG with the proposed controller and then used to assess the stability of the MG. The FOD and the integer-order derivative (IOD) droop controllers are applied to a benchmark MG and tuned via an optimization procedure under multiple loading conditions. The results show that the extra degrees of freedom introduced by the FOD droop facilitate pushing the dominant modes toward the required stability region. The proposed FOD droop is compared to the IOD droop, conventional droop, VOC, and virtual synchronous generator controllers under several contingency events and a reconfiguration scenario using MATLAB/SIMULINK, where the proposed controller shows superior performance. The experimental validations demonstrate the improved power-sharing performance of the proposed FOD droop controller.
... The inverter-interfaced DERs based on renewable energies are gaining much attention due to their capability to meet the operational requirements very quickly [3]. Meanwhile, for autonomous/islanded MG, droop schemes are the most common control strategy for inverter-interfaced DERs adopted to provide the operational requirements and load sharing [4]. ...
... solar and wind power, are involved in the stochastic or probabilistic model [7,8]. However, in this paper, similar to [2,4,[9][10][11][12][13][14], even non-dispatchable DERs are coupled with energy storage devices, thus enabling the participation of DERs in droop control schemes. ...
... Integration of DERs is among the main components, yet also drivers for the growing interest in MGs [4]. While the grid-connected mode is typically concerned during planning stages, islanded/autonomous modes must be considered for operational purposes. ...
This paper presents an extended model for aggregation of operational settings of droop-controlled Distributed Energy Resources (DERs) in Microgrid (MG) planning, thus deriving in a novel problem named Optimal Siting, Sizing and Setting (OSSS) of DERs. The proposed approach precisely deals with the power flow formulation for autonomous MG as an essential constraint for accurate results. Furthermore, the proposed OSSS model considers the frequency and voltage dependence characteristic of loads. The problem casts in multi-objective formulation to consider different technical, power quality and controlling aspects. This will help the designer to provide efficient decision for allocation of DERs. The effects of realistic load model as well as power sharing error index are elaborately investigated in simulation results. The model is thus further validated and its usefulness demonstrated. Finally, the performance of multi-objective OSSS model is verified by comparing the results with the previous simplified models presented in the recently published literatures.
... For example, the boundaries of the voltage magnitudes and system frequency, defined in (15) and (16), respectively, are set based on the applicable standards for voltage and frequency operation limits [29], [30]. Also, the boundaries of the droop parameters defined in (20) and (21) are set to preserve the stability of the microgrid, as illustrated in [31]- [33]. ...
The legacy of power distribution systems is evolving towards more flexible and dynamically reconfigurable microgrids, which substantially increases line-switching actions. This may introduce undesirable transients, poor reliability, deteriorated power quality, and significant wear and tear in switching devices. Frequent line switching is significant in unbalanced inverter-based islanded microgrids (UIBIM). This paper proposes an optimized unsymmetrical per-phase droop-controlled approach to mitigate the influence of line switching during UIBIM reconfiguration and partitioning. To determine the unsymmetrical per-phase Optimal Transitional Droop Settings (OTDS), a new mathematical formulation is modeled to minimize power flow at the switching location(s). Given the unbalanced nature of the system, the proposed unsymmetrical droop will be optimized for each phase independently. The activation start and end instants of the OTDS are selected to reduce the UIBIM dynamics due to the transition between the states due to switching. The superiority and effectiveness of applying the proposed unsymmetrical OTDS are assessed via Matlab/Simulink, utilizing case studies implemented on a 6-bus and an IEEE 34-bus unbalanced systems. The simulation results reveal that the proposed approach can independently minimize the current flow in each phase during the switching process by nearly 98%. Furthermore, the transient voltage during the switching process is significantly reduced.
... Compared with the GFL inverter, the grid-forming (GFM) inverter has the ability to form the grid and has stronger robustness in weak grid. GFM mode can be divided into droop control [10,11], virtual synchronous machine (VSM) control [12,13] and virtual oscillator control (VOC) [14] according to transient response. Among them, droop control is more researched by scholars. ...
... In this subsection, the primary control model incorporating a PD reactive power droop controller for the IBDG discussed in [24] is summarized. The main modifications between the primary controller presented in this paper and the primary controller given in [24] is in introducing two terms in the speed and voltage equations to take into consideration the secondary controller effects. ...
... In this subsection, the primary control model incorporating a PD reactive power droop controller for the IBDG discussed in [24] is summarized. The main modifications between the primary controller presented in this paper and the primary controller given in [24] is in introducing two terms in the speed and voltage equations to take into consideration the secondary controller effects. As a result, the updated equations and matrices are developed for the power controller, along with how they affect the overall microgrid model. ...
... As a result, the updated equations and matrices are developed for the power controller, along with how they affect the overall microgrid model. The complete details of the current controller, voltage controller, power controller and LCL filter are stated in [24] and referred to with the same notation in this paper. The primary control structure is shown in Fig. 2 a. ...
The basic objective of microgrid primary control is to preserve microgrid stability. On the other hand, microgrid secondary control has been proposed for ensuring reactive power-sharing and restoring the frequency and voltages of the microgrid. Therefore, this paper presents a complete model for evaluating the combined impact of primary and secondary control actions on microgrid domain of stability since the impact of the secondary control design on the domain of stability of the microgrid has not been addressed. Further, the developed model is used to assess the impact of reactive power-sharing, secondary controller design parameters, and communication delay on the domain of stability of the microgrid. The domain of stability is a supportive tool for determining the stable operating range for different microgrid droops. Simulation results have been obtained to validate the developed model and the domain of stability analysis. These results demonstrated that secondary control actions have a significant influence on the domain of stability of the microgrid, and thus it is important to consider the secondary control design in the determination of the microgrid’s stable range. Moreover, it has been demonstrated that primary control gains can be severely constrained by reactive power sharing gains.
... The stability and transient performance of droop-based microgrids have been studied in numerous studies [9] and [12]- [ 22]. The authors in [9] and [12] introduce a small signal model (SSM) of an islanded droop-based microgrid. ...
... In the literature, the effect of the reactive power droop gain on microgrid stability has also been investigated. In [22], eigenvalue analysis shows the direct effect of the reactive power droop gain on the microgrid stability. Moreover, a stability domain chart, which is defined as the relation between the reactive power droop gain and the maximum admissible active power droop gain that maintains the microgrid stability, is introduced. ...
... The stability methods discussed in [9]- [22] consider only the normal operation of the microgrid. The stability analysis and the transient performance analysis have been discussed for the normal operation of the microgrid. ...
The most popular approach used in controlling inverter-based microgrids is the droop control technique, characterized by its plug-and-play capability and independence from communication requirements. The control parameters, at the top of which the active and reactive droop gains,
m<sub>p</sub>
and
n<sub>q</sub>
, respectively, have been recently studied, evaluating their impact on stability. Furthermore, the stability domain chart is plotted in the
m<sub>p,max</sub>
-
n<sub>q</sub>
plane identifying the stable range of the microgrid, enabling the microgrid operator to choose the desired stable operating point based on frequency, voltage, and power-sharing requirements. However, the microgrid is prone to abnormal conditions, such as network disconnection, distributed generator (DG) disconnection, and load disconnection, that also affect microgrid stability. Hence, this can have an impact on the stability domain and, consequently, the choice of droops for stable operation. Therefore, in this paper, several stability domain charts are proposed, considering all various contingencies. The proposed network disconnection, DG disconnection, and load disconnection domain charts have been combined to generate an overall stability domain chart considering
N
- 1 contingency. The proposed
N
- 1 contingency chart is of crucial importance in considering the microgrid stability for both normal and abnormal operations.
N
- 1 contingencies can be frequent in microgrid operation due to microgrid limited space and power nature. Hereby, the inclusion of their impact on microgrid stability provides better guidance for the tuning of the control parameters. Moreover, the proposed chart is tested and validated using time-domain simulations on MATLAB, considering different operating points. The simulation results highlight the significance of the stability domain chart for selecting the DG droop gains to maintain the microgrid stability considering
N
- 1 contingency. Further, real-time validation of the proposed domain of stability through OPAL-RT setup is analyzed to confirm the ability of practical controllers to respond to microgrid variations conditions in real-time operation.
... In [33], multiple microgrid inverters are divided into the form of multiple master-slave microgrid clusters, and a control method for improving the output-side voltage quality is designed based on conservative power theory. In [34], a centralized microgrid frequency and voltage regulation scheme is designed based on model predictive control, which takes into account both frequency drops and the constraints of energy storage in the microgrid. The advantages of a centralized control scheme are that the control structure of each inverter is very simple, and there is no need to switch between the grid-connected and islanded mode during operation. ...
... Medium Low [33] Separates the inverters as microgrid clusters and increases the output voltage quality. Low High Low [34] Proposes an MPC-based centralized microgrid control method. Also considers the constraints of frequency variation and SOC of the energy storage device. ...
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.
... In an alternating current (AC) transmission and distribution system, reactive power is critical. It has a direct impact on the system's voltage stability [1][2][3]. The concerns with reactive power are gradually undermined, resulting in voltage breakdown. ...
... In the primary control, the droop control was first used for conventional generators, e.g., synchronous and induction generators [5]. As DC-AC inverter-feeding power systems became numerous, droop control was introduced for inverters [6][7][8]. Zhong's work [9] highlighted some drawbacks of the droop controller for inverters, especially compared to conventional generators. Some researchers have studied control methods that can provide virtual inertia for power grids during the last decade, called virtual synchronous generators (VSG) [10][11][12]. ...
As microgrids develop rapidly, more inverters are adopted to achieve DC/AC or AC/DC/AC conversion of distributed generators (DGs). The virtual synchronous generator (VSG) control has started to replace the traditional droop control for inverters. In order to restore the frequency to its nominal value, most existing secondary frequency control (SFC) methods are based on frequency measurements. However, while reducing the rate of change of frequency (ROCOF), virtual inertia also slows down the convergence of frequency-based SFC. Therefore, this paper proposes a new distributed hierarchical control for fast frequency restoration. Based on the real-time VSG control at the bottom level, a novel frequency restoration control is designed. The power reference values generated by the proposed control can accelerate the frequency restoration with accurate power sharing. Meanwhile, by designing event-triggering conditions, parallel inverter controllers only need to communicate with neighbors at the event-triggered moments. Simulations have been performed in MATLAB/Simulink environment. Furthermore, the proposed control has also been tested on the experiment platform, which contains practical physical circuits and real-time controllers. Both simulation and experiment results verify the effectiveness of the proposed control strategy.
