Islanded AC Microgrid Research Articles

A communication‐less distributed frequency restoration and economical active power control scheme for islanded AC microgrids

AbstractIn this article, a novel distributed cooperative control scheme is presented to restore the microgrid frequency to the nominal value and achieve economical active power sharing. Based on hierarchical control structure, a distributed event‐triggered economic dispatch algorithm is designed to calculate optimal active power reference, and a decentralized frequency controller is developed to restore the microgrid frequency and track the optimal active power reference. The presented scheme is globally distributed for tertiary power optimization and locally decentralized for secondary frequency control. By employing event‐triggered communication mechanism and decentralized secondary frequency control, the communication burden can be significantly reduced. Furthermore, exponential convergence rate of the presented scheme is rigorously proved by Lyapunov approach. Finally, several simulation examples are provided to illustrate the effectiveness of the presented scheme.

Distributed Finite-Time Secondary Frequency and Voltage Restoration Control Scheme of an Islanded AC Microgrid

To solve the problems of frequency and voltage deviation caused by the droop control while meeting the requirements of rapid response, a distributed finite-time secondary control scheme is presented. Unlike the traditional cooperative controllers, this scheme is fully distributed; each unit only needs to communicate with its immediate neighbors. A control protocol for frequency restoration and active power sharing is proposed to synchronize the frequency of each unit to the reference value, and achieve accurate active power distribution in a finite-time manner as well. The mismatch of the line impedance is considered, and a consensus-based adaptive virtual impedance control is proposed. The associated voltage drop is considered to be the compensator for the voltage regulation. Then, a distributed finite-time protocol for voltage restoration is designed. The finite-time convergence property and the upper bound of convergence times are guaranteed with rigorous Lyapunov proofs. Case studies in MATLAB are carried out, and the results demonstrate the effectiveness, the robustness to load changes, plug-and play capacity, and better convergence performance of the proposed control scheme.

Improved droop control strategy for parallel connected power electronic converter based distributed generation sources in an Islanded Microgrid

The control and protection are critically essential to facilitate the stable operation of an island microgrid. To accomplish the exact power sharing in an islanded AC microgrid, the frequency and the voltage restoration need to be sustained. In this regard, a robust control strategy pooling an improved droop with a virtual impedance control based droop control for power sharing with f/V restoration is proposed in this article. The proposed droop controller state-space small signal modeling and an investigation for the total microgrid system are presented in this article. Meanwhile, the robustness of the strategy is escalated for a step change in the system power loads. The overall controller performance has been tested in the hardware-in-loop (HIL) test-bed.

Design of Decentralized Adaptive Sliding Mode Controller for the Islanded AC Microgrid With Ring Topology

Sliding mode control can restrain the perturbations generated from the intermittence of the renewable energy generation and the randomness of local loads when microgrids are operating in islanded mode. However, the microgrid consists of several subsystems and the interactions among them will cause the chattering problems under the overall sliding mode control. In this paper, the chattering restraint issues for voltage control of the islanded microgrid with a ring topology structure are investigated based on the decentralized adaptive sliding mode control strategies. Firstly, we construct a tracking error system with interconnections considering the power transmission among subsystems and nominal values of system states. Secondly, we design linear matrix inequalities (LMIs) according to the H∞ attenuation performance of the system external disturbances. Then, the tracking error performance and the control precision are guaranteed via the asymptotic stabilities of integral sliding mode surfaces. Adaptive laws are utilized to address the chattering problems of the sliding mode control. Finally, simulation results verify the effectiveness of the proposed decentralized control methods.

Modeling and Stability Analysis of a Novel Voltage-Oriented Power Coordination Controlled Constant-Frequency AC Microgrid System

The conventional AC microgrid power regulation method is achieved using dual undulations of the grid-side voltage and frequency, whose complex variables and processes make the system less efficient and stable. For this problem, this paper proposes a novel two-hierarchical small signal model stability analysis for islanded AC microgrid systems with a designed power regulation algorithm under the constant frequency, mainly aimed at the continual switching microgrids, and considers the future expansion. MATLAB/Simulink simulations and experiments are conducted to validate the feasibility. It is found that there is a strong consistency between the stability of primary and overall systems, and that two key parameters affect the stability properties. Discussing the mutual influence of various parameters, k1 is positively correlated with control intensity and k2 determines the proportion of reactive power distribution in each power generation. Contrasting with the previous complex processes, the unique points of this method are the simplicity of calculation, parameter induction, response testability, strong operability, and system extensibility. The conclusion is that this constant frequency power control is simple, feasible, and stable under several specified conditions.

A Novel Power Sharing Strategy Based on Virtual Flux Droop and Model Predictive Control for Islanded Low-Voltage AC Microgrids

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.

An enhanced current sharing strategy for islanded ac microgrids based on adaptive virtual impedance regulation

In islanded mocrogrids, the mismatch among the equivalent impedances (sum of output impedance and wire impedance) degrades the current sharing performance. It is an essential mission to eliminate this mismatch to achieve accurate current sharing in different conditions. About this issue, several aspects should be considered. Firstly, for a general microgrid, the inverter equivalent impedances are unknown, both the resistance and reactance should be considered. Secondly, all the balanced, unbalanced and harmonic components of load current should be shared according to the power ratings of distributed generators. Besides, not only the current sharing in steady states but also during transients should be achieved. Herein, a low-bandwidth communication-based distributed current sharing strategy for parallel-connected inverters is proposed in this paper. The virtual resistance and reactance are adaptively regulated according to the magnitude error and phase angle error of the inverter uniform output current respectively. Then they can be used to reshape the inverter equivalent impedance at fundamental positive-sequence frequency, fundamental negative-sequence frequency and harmonic frequencies. With the proposed method, all the three aspects mentioned above can be guaranteed without knowing inverter wire impedances in advance. The simulation and experiments on an islanded microgrid prototype are developed to verify the effectiveness of the proposed strategy.

Intelligent control of islanded AC microgrids based on adaptive neuro-fuzzy inference system

In inverter-based AC microgrids, active power sharing between Distributed Generation (DG) units is achieved by introducing a droop frequency control for the inverter. The purpose of the secondary controller is to regulate the frequency deviation caused by the primary controller while maintaining the droop characteristic of active power sharing among DG units. In this paper, Adaptive Neuro-Fuzzy Inference System (ANFIS) is employed to effectively control frequency and power sharing among DG units in a 100% renewable based microgrid while working in different areas of the operating points. Rules and configuration of ANFIS are introduced to handle the complexity of larger microgrids. The effectiveness of the proposed method is demonstrated in two case studies. The simulation results suggest that the proposed ANFIS controller, compared to the conventional controllers, is able to satisfy the power sharing among DG units while better regulating the frequency.

A Finite-Time Robust Distributed Cooperative Secondary Control Protocol for Droop-Based Islanded AC Microgrids

In this research work, a resilient finite-time consensus-based distributed secondary control protocol is presented for droop-based distributed generating (DG) units of an islanded AC microgrid (MG). Through a multi-agent control structure, the DG units of the microgrid adjust their active power outputs so that they reach an agreed-upon value in a finite time. Concurrently, all the DG units are forced to operate with their frequencies regulated to the reference MG frequency in a finite time, despite time-varying load perturbations. Each DG unit is provided with a hierarchical control architecture, where the primary control is achieved using the droop control method, while the secondary control is established through the proposed distributed control protocol. The communication between DG units takes place over a sparse communication network. The proposed control protocol is robust to both small and sufficiently large communication latencies and it supports the plug-and-play feature of DG units. Different time-domain-based numerical simulations are carried out on a small as well as large microgrid testbenches in Matlab/Simulink and demonstrate the correctness and effectiveness of the proposed distributed control protocol. A comparative study is also presented with the existing distributed control protocol, and it is found that the proposed strategy is superior in its performance.

Synthetical Optimal Design for Passive-Damped LCL Filters in Islanded AC Microgrid

In AC microgrid systems, technical issues resulting from the power control effect are the power control method efficiency and the filter resonance since voltage-frequency interference and filtering resonance may cause significant control input reduction and oscillation. A simple and robust solution is to regulate active power flow by adding an optimal passive damping resistor. The optimal LCL filter parameterization process in this paper provides an innovative approach to obtain the optimal quality factor and the optimal damping resistor. The amplitude responses of resonant frequency and high frequency are involved in the cost function. Simulations of various cases are conducted in MATLAB/Simulink. The key advantages of this procedure include flattened admittance peak, weakened high frequency decay, and less damping loss. Through the method in this paper, the optimal passive-damped filter parameters can be calculated in an efficient manner, while the power can be regulated and tracked in a simplified way, and the performance of the control strategy is improved.

Support Vector Machine Based Fault Location Identification in Microgrids Using Interharmonic Injection

This paper proposes an algorithm for detection and identification of the location of short circuit faults in islanded AC microgrids (MGs) with meshed topology. Considering the low level of fault current and dependency of the current angle on the control strategies, the legacy overcurrent protection schemes are not effective in in islanded MGs. To overcome this issue, the proposed algorithm detects faults based on the rms voltages of the distributed energy resources (DERs) by means of support vector machine classifiers. Upon detection of a fault, the DER which is electrically closest to the fault injects three interharmonic currents. The faulty zone is identified by comparing the magnitude of the interharmonic currents flowing through each zone. Then, the second DER connected to the faulty zone injects distinctive interharmonic currents and the resulting interharmonic voltages are measured at the terminal of each of these DERs. Using the interharmonic voltages as its features, a multi-class support vector machine identifies the fault location within the faulty zone. Simulations are conducted on a test MG to obtain a dataset comprising scenarios with different fault locations, varying fault impedances, and changing loads. The test results show that the proposed algorithm reliably detects the faults and the precision of fault location identification is above 90%.

Optimal allocation of active and reactive power of dispatchable distributed generators in a droop controlled islanded microgrid considering renewable generation and load demand uncertainties

This paper proposes a novel sequential optimization strategy (SOS) for optimal allocation of both active and reactive power among dispatchable distributed generator (DDG) units present in a droop controlled islanded AC microgrid. Active power is optimally dispatched based on the simultaneous satisfaction of an economic objective of total operational cost (TOC) minimization, along with an environmental objective of total operational emission (TOE) minimization, as well as a network-related objective of total active power loss (Ploss) minimization. A fuzzy-embedded multi-objective particle swarm optimization (FMOPSO) technique is used to solve this optimization problem. Reactive power is optimally allocated depending on the objective of capacity based reactive power sharing, which is solved using the conventional particle swarm optimization (PSO) algorithm. Node voltage deviation index (NVDI) and total active power loss in the network (Ploss) are used as two main attributes to quantitatively measure the improvement in node voltage profile as a result of optimal sharing of reactive power among the dispatchable distributed generator units. The presence of local heat demand and uncertainties inherent in weather dependent load demand and renewable generation are taken into consideration. A novel mixed probabilistic–possibilistic scenario based approach (MPPSBA) is put forward to model the uncertainties associated with both the electrical and heat load demand and renewable distributed generator (RDG) output. The effectiveness of the proposed methods is demonstrated on a 33-node droop controlled islanded microgrid (DCIMG) test network.

An efficient iterative approach for power flow solution of droop-controlled islanded AC microgrids through conventional methods

This paper proposes a new method for solving the power flow problem (PFP) in droop-controlled islanded microgrids (DCIMGs). The method is utilizing the conventional power flow (CPF) solution methods. A virtual slack bus (VSB) including a dummy generator/load (DGL) is set to fulfill the necessity of using the CPF methods. This bus emulates the treatment of the main grid for fulfilling the load demand to equalize the production and consumption in the microgrid and maintaining constant the frequency. Solving the problem is done to tend injective active and reactive powers to zero by DGL, which is expressing the islanded operation properties of the microgrid which is solved by an iterative process. The system frequency and the VSB's voltage magnitude are adjusted in this iterative process simultaneously. The proposed method is applied to a small 7-bus system and a big 41-bus system. Validation of the developed method is performed using the obtained results comparing with the steady-state results of time-domain (TD) simulations by PSIM. Besides the accuracy and computational burden of the required calculation is compared with (modified Newton-Raphson) MNR and the other relevant iterative methods.

Performance improvement of an islanded AC microgrid in presence of Plug-in Hybrid Electric vehicles, load and renewable generation uncertainties

In this paper, a performance improvement scheme for an autonomous AC microgrid in the presence of dispatchable and renewable units, and Plug-in Hybrid Electric vehicles has been proposed. In a standalone mode of operation, a droop-control strategy is implemented at the primary control level for a communication-free accurate power-sharing among multiple dispatchable generators. For a network that is neither highly reactive nor highly resistive, active and reactive powers are coupled to both voltage magnitudes and phase angles. A mixed-droop feature is incorporated at the primary control level for autonomous power-sharing among the units for such networks. The mixed-droop feature is considered in this paper. Further, stochastic models of load demands, renewable generation, and plug-in hybrid electric vehicle charging loads are considered. Objectives are to reduce the cost of operation and minimize the emission of greenhouse gases from generation activities. The bi-objective problem is solved in the fuzzy domain. Performance improvement is achieved in two steps, first, by choosing optimal static droop constants using the particle swarm optimization, and then a further performance improvement is achieved by network reconfiguration. The proposed algorithm is tested on a thirty-three bus autonomous microgrid and gives satisfactory results.

Distributed security secondary control for cyber-physical microgrids systems under network DoS attacks

This paper considers the frequency regulation and power-sharing problems for cyber-physical microgrids (MGs) under network denial-of-service (DoS) attacks and time-varying communication delays. Compared to conventional distributed controllers based on a system model with ideal communication, a distributed security control protocol is designed. Benefiting from the developed security control method, the adverse effects of network DoS attacks and communication delays can be resisted. Then, a sufficient condition, which is used to achieve the achieve frequency restoration and power-sharing, is established according to the frequency and duration of DoS attacks. By using the average dwell time method and the Lyapunov stability theory, it is theoretically proved that the proposed control scheme can guarantee the stability of the overall AC microgrid system. In order to illustrate the proposed scheme, simulation studies are carried out based on an islanded AC microgrid system with four distributed generators (DGs) built in the simulation. The results do show its effectiveness and also verify the established theoretical results.

MPC-Based Arctan Droop Control Strategy of the Parallel Inverter System in an Islanded AC Microgrid

Distributed generations (DG) are one of the upcoming technologies recently used by many electric utilities in all corners of the world. Most of those DG form the microgrid (MG) to serve local loads and can be connected to the grid. This DG’s technology is enabled by utilizing renewable energy sources (REs) that are ecofriendly; however, these REs are intermittent by their nature, so controlling a power electronic device interfaced with them to be connected to the grid is another challenge. Many researchers have worked on the inverters’ control in MG. This study also elaborates on the control strategy for inverters adapted to REs for proper control of voltage and frequency used in an islanded microgrid. The study proposes a hybrid control strategy made of the virtual impedance droop control with arctan function and model predictive control. Extensive simulations have been carried out to validate the proposed control strategy’s effectiveness in terms of rapid transient response and stabilization of voltage, frequency, and power equitability among the microsources in the islanded microgrid.

An Adaptive Virtual Impedance Control for Improving Power Sharing Among Inverters in Islanded AC Microgrids

Several drawbacks of the conventional and inverse droop control based decentralized techniques for islanded microgrids are being addressed actively, over the years by various researchers. One of the prominent issues is the inaccurate power sharing (reactive power in the conventional droop and real power in the inverse droop) among the distributed generators (DGs) due to the feeder/line impedances. Virtual impedance (VI) is a popular technique to overcome impedance mismatches but the challenge is in the setting of the appropriate value. This paper presents an adaptive decentralized technique for adjusting the virtual impedance in the controller of a DG, based on its output current, without the need of communication, extra sensors or network parameter/load estimations. The methodology is tested through simulations for a wide variety of cases including unbalanced, harmonic, constant power and induction motor loads, DG plug and play functionality and performance in larger networks (modified 13 and 33 bus systems) under meshed configurations. The range for parameter stability is verified through modelling and small signal eigenvalue analyses. Further, improved power sharing performances using the proposed scheme is validated through implementation on a laboratory experimental setup.

Asynchronous Periodic Distributed Event-Triggered Voltage and Frequency Control of Microgrids

In this paper, we introduce a distributed secondary voltage and frequency control scheme for an islanded ac microgrid under event-triggered communication. An integral type event-triggered mechanism is proposed by which each distributed generator (DG) periodically checks its triggering condition and determines whether to update its control inputs and broadcast its states to neighboring DGs. In contrast to existing event-triggered strategies on secondary control of microgrids, the proposed event-triggered mechanism is able to handle the consensus problem in case of asynchronous communication. Under the proposed sampled-data based event-triggered mechanism, DGs do not need to be synchronized to a common clock and each individual DG checks its triggering condition periodically, relying on its own clock. Furthermore, the proposed method efficiently reduces communication rate. We provide sufficient conditions under which microgrid's frequency and a critical bus voltage asymptotically converge to the nominal frequency and voltage, respectively. Finally, effectiveness of our proposed method is verified by testing different scenarios on an islanded ac microgrid benchmark in the MATLAB/Simulink environment as well as a hardware-in-the-loop (HIL) platform, where the physical system is modeled in the Opal-RT and the cyber system is realized in Raspberry Pis.

Distributed Event-Triggered Control for Reactive, Unbalanced, and Harmonic Power Sharing in Islanded AC Microgrids

For several reasons, particularly due to the mismatch in the feeder impedance, accurate power sharing in islanded microgrids is a challenging task. To get around this problem, a distributed event-triggered power sharing control strategy is proposed in this article. The suggested technique adaptively regulates the virtual impedances at both fundamental positive/negative sequence and harmonic frequencies and, therefore, accurately share the reactive, unbalanced, and harmonics powers among distributed generation units. The proposed method requires no information of feeder impedance and involves exchanging information among units at only event-triggered times, which reduces the communication burden without affecting the system performance. The stability and interevent interval are analyzed in this article. Finally, experimental results are presented to validate the effectiveness of the proposed scheme.

A Robust Controller for Battery Energy Storage System of an Islanded AC Microgrid

A battery energy storage system (BESS) can play a critical role in regulating system frequency and voltage in an islanded microgrid. A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> -synthesis-based robust control has been proposed for dc link voltage regulation of BESS for achieving frequency regulation and voltage quality enhancement of islanded microgrid. Variation in the operating condition of ac microgrid affects the operating condition of the BESS’ converter. This controller synthesis accounts for such uncertain variations as parametric uncertainty. The stability and performance of the proposed controller can be guaranteed for bounded parametric variations. The bounds on parameters are selected based on practical limitations of BESS. In this article, the proposed controller's performance is tested on an islanded CIGRE TF C6:04:02 benchmark low voltage ac microgrid system. The importance of dc link voltage regulation is analyzed based on performance comparison with a benchmark controller. The controller performance is also validated using a real-time Typhoon HIL emulator.