Low Voltage AC Microgrid Research Articles

Virtual Impedance Control for Load Sharing and Bus Voltage Quality Improvement in Low-Voltage AC Microgrid

Virtual Impedance Control for Load Sharing and Bus Voltage Quality Improvement in Low-Voltage AC Microgrid

State-of-the-Art Literature Review of Power Flow Control Methods for Low-Voltage AC and AC-DC Microgrids

The development of AC distribution systems provides for the seamless integration of low-voltage microgrids with distributed energy resources (DERs). This poses new challenges for the control of normal, emergency, and post-emergency states of microgrids, calling for the creation and development of information and communications technology infrastructure. Power converters/inverters that are used to integrate renewable DERs lack inertia. Along with them, fossil fuel-fired generation units are also being integrated into microgrids. These include gas generator sets, diesel generator sets, and microturbines, having small (up to 1–2 s) values of mechanical inertia constants—Tj. This leads to an increase in the rate of transients by a factor of 5–10. Under these conditions, the technical requirements for the speed of automatic power flow control systems, as well as the methods they rely on, have to be reconsidered. Microgrids include DC microgrids, AC microgrids, and hybrid (AC-DC) microgrids. In the case of hybrid microgrids, DERs are connected to the DC grid and are integrated into the AC grid through a common inverter. The complexity of the task of microgrid control is due to the need to choose properly the type and extent of control actions so as to prevent the emergence and development of accidents. The employed control methods must ensure the reliable power supply to consumers and the quality of power in microgrids, as well as the reliable operation of the external distribution systems into which they are integrated. The article gives an overview of control methods for low-voltage AC and AC-DC microgrids, which allow one to tackle effectively solve the tasks.

Machine Learning Based Protection Scheme for Low Voltage AC Microgrids

The microgrid (MG) is a popular concept to handle the high penetration of distributed energy resources, such as renewable and energy storage systems, into electric grids. However, the integration of inverter-interfaced distributed generation units (IIDGs) imposes control and protection challenges. Fault identification, classification and isolation are major concerns with IIDGs-based active MGs where IIDGs reveal arbitrary impedance and thus different fault characteristics. Moreover, bidirectional complex power flow creates extra difficulties for fault analysis. This makes the conventional methods inefficient, and a new paradigm in protection schemes is needed for IIDGs-dominated MGs. In this paper, a machine-learning (ML)-based protection technique is developed for IIDG-based AC MGs by extracting unique and novel features for detecting and classifying symmetrical and unsymmetrical faults. Different signals, namely, 400 samples, for wide variations in operating conditions of an MG are obtained through electromagnetic transient simulations in DIgSILENT PowerFactory. After retrieving and pre-processing the signals, 10 different feature extraction techniques, including new peaks metric and max factor, are applied to obtain 100 features. They are ranked using the Kruskal–Wallis H-Test to identify the best performing features, apart from estimating predictor importance for ensemble ML classification. The top 18 features are used as input to train 35 classification learners. Random Forest (RF) outperformed all other ML classifiers for fault detection and fault type classification with faulted phase identification. Compared to previous methods, the results show better performance of the proposed method.

Decentralized Virtual Impedance- Conventional Droop Control for Power Sharing for Inverter-Based Distributed Energy Resources of a Microgrid

The work presents the power-sharing in a standalone low voltage AC microgrid consisting of three parallel grid supporting inverters using a virtual impedance-based droop system. Typically, isolated microgrids suffer unique challenges regarding voltage, current, frequency regulation, power flow control, and power-sharing due to the absence of a stiff AC grid source. This work investigated the power flow and sharing technical challenges using three inverter-based distributed energy sources, three static loads, and one dynamic load. These distributed energy sources are interconnected with the loads using low voltage line impedance within the microgrid to implement the uneven power distribution. Furthermore, a distributed grid supporting control utilizing virtual impedance is proposed in this work to improve power-sharing. The microgrid model is developed with the MATLAB Simulink environment and validated using the Opal RT OP4510 simulator. The proposed technique ensured improved power-sharing and mitigated the effect of voltage drops introduced through a virtual impedance using combined positive and negative virtual.

An Improved Virtual Inertia Control Strategy for Low Voltage AC Microgrids with Hybrid Energy Storage Systems

This paper proposes a novel virtual inertia control (VIC) method based on a feedforward decoupling strategy to address the low inertia issue of power-converter-interfaced microgrids. The feedforward control scheme is employed to eliminate the coupling between active and reactive power caused by line impedance. The active power-voltage droop can be applied to the battery converter in the hybrid energy storage system (HESS). A novel VIC method is developed for the supercapacitor (SC) converter of HESS to increase the inertia of the microgrid. Detailed small-signal modeling of the SC converter with the proposed VIC was conducted, and the transfer function model was obtained. Parameter analysis of the virtual inertia and virtual damping was carried out with the pole-zero map method, and the step response characteristic of output voltage amplitude with power variation was analyzed in detail, deriving the parameter design principle. The simulation study verifies the effectiveness and validity of the proposed control strategy. The proposed feedforward decoupling method and VIC can be widely applied in microgrids to enhance inertia and improve their power quality.

Combined Voltage and Frequency Control for Diverse Standalone Microgrid Networks Using Flexible IDC with Novel FOC: A Real-Time Validation

In this paper, a maiden application of combined voltage and frequency control ensures voltage, frequency, and power regulation in diverse standalone low voltage AC microgrid (MG) networks by combining flexible improved droop control (IDC) with a novel fractional-order controller (FOC) proposed. An effort is made to design an IDC into the control loop based on virtual power, virtual negative resistance, inductance, and FOC to ensure power decupling and compensate voltage source converter (VSC) based distributed generators feeder voltage drop. A maiden proportional fractional integral derivative filter plus double derivative controller is used to precisely monitor VSC voltage and current reference value for reducing the control errors. The controller robustness regarding system parameters uncertainties, unmodeled dynamics, and disturbance in the MG system is ensured by a new memetic salp swarm algorithm by finding optimal controller parameters. Several simulation studies in the MATLAB Simulink environment were conducted to verify the efficacy of the proposed technique. The suggested control methodology is also investigated by hardware in the loop using the OPAL-RT real-time simulator platform to demonstrate the controllers practical functional realization. The findings confirmed the efficacy of the proposed strategy.

Online pricing for demand‐side management in a low‐voltage resistive micro‐grid via a Stackelberg game with incentive strategies

It has been demonstrated that online pricing mechanisms are a viable solution for demand side management in power systems. This study deals with the analysis and design of a droop-controlled low-voltage resistive AC micro-grid network system. Such a system is subjected to a dynamic demand obtained from an online pricing mechanism, which is proposed as a novelty in the study of micro-grids. This mechanism is derived from a variation of the Stackelberg game, which includes the use of incentive strategies. First, a configuration in which a supplier announces an incentive function and n-consumers’ reaction to the resulting personalised price is presented. Then, a detailed stability analysis of the micro-grid is presented as a result of the interaction with the aforementioned online pricing mechanism. The units of the micro-grid (generators as the supplier and loads as the consumers) operate under either conventional or bounded droop control. The novelty of the approach is that it bridges the gap between the physical and the market layers of the problem. The ways in which the existence of multiple equilibrium points is guaranteed for both the consumer's load and the supplier's announced incentive are shown. A detailed design process for the profit functions of the players is shown in conjunction with the parameter selection for their implementation. Finally, simulations that demonstrate the system stability and its convergence to different equilibria are implemented under scenarios with one and multiple consumers.

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.

Early Identification and Location of Short-Circuit Fault in Grid-Connected AC Microgrid

With the rapid development of microgrid and large-scale grid-connected operation, the detection and location of short-circuit faults in microgrid has become a bottleneck. In this paper, a simulation model of short-circuit fault in low-voltage AC microgrid is built on PSCAD/EMTDC. The characteristics of current wavelet energy spectrum under various short-circuit fault states are studied and the early detection of faults is realized. Based on the relationship between the first peak value of fault current wavelet energy spectrum and the location of short-circuit points, a method for early detection and area location of short-circuit faults in microgrid is proposed. The simulation and experimental results show that this method can detect and locate the short-circuit fault area of microgrid quickly and accurately.

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.

Wavelet-Based Frequency Tracking Monitor Applied for Low-Inertia AC Microgrids

This article proposes a wavelet-based frequency tracking monitor (WB-FTM) applied for low-voltage ac microgrids. It is based on a modified continuous wavelet transform (CWT) algorithm that makes it feasible for real-time applications (RT-CWT). The WB-FTM provides the frequency estimation through the point of common coupling voltage measurements and can be employed in single-phase or three-phase microgrids. Also, this estimation approach overcomes the main drawbacks of the standard techniques presented in the literature. The effectiveness of the WB-FTM was evaluated in comparison with the discrete Fourier transform, augmented complex least mean square, and second-order generalized integrator based on a frequency-locked loop based standard frequency estimation methods. Experimental results obtained from an ac microgrid laboratory setup, under different operational scenarios, demonstrated that WB-FTM overcomes conventional estimators' limitations, with faster convergence time, which is essential for frequency restoration systems applied for low-inertia ac microgrids.

Protection strategy for fault detection in inverter-dominated low voltage AC microgrid

This paper presents a protection strategy based on active power flow direction, current magnitude and voltage sags to determine the existence of low impedance faults in inverter-dominated low voltage AC microgrids. Different fault situations are considered, concerning each element of the microgrid (line section, load, node and energy sources). Simulation is done in MATLAB/Simulink software. Microgrid was modelled based on CIGRE benchmark for network integration of renewable and distributed energy resources, considering the low voltage european distribution network characteristics. Inverter-based distributed generation units were modelled considering a control scheme composed by natural reference frame control and RMS current limiter. The results obtained from simulations indicate that the proposed strategy is able to protect the microgrid in any topology configuration or mode of operation, for all type of low impedance faults.

A Virtual Complex Impedance based P — V Droop Method for Parallel-connected Inverters in Low-voltage AC Microgrids

Due to the high R/X ratio and mismatched feeder impedance of low-voltage microgrids, conventional droop method is no longer able to decouple the active and reactive power of distributed generators and the powersharing accuracy is degraded. In this article, a virtual complex impedance based P - V̇ droop method is proposed to decouple the powers and improve the power-sharing accuracy among DGs. With the virtual impedance method, the equivalent impedance between virtual power source and point of common coupling is shaped to be purely resistive. Then, a P - V̇ strategy is adopted to alleviate the effect of mismatched line impedance, where the virtual powers rather than the ordinary P/Q are used in the droop equation. In case the output voltage violates the operation code, a restoration mechanism is proposed to reset V̇ to zero. Compared with existing virtual impedance and Q - V̇ droop methods, the proposed method combines the advantages of both. Besides, a modified P - V̇ strategy is also presented to accelerate the restoration process and improve the active power-sharing accuracy at the same time. Simulation results validate the effectiveness of the proposed method.

A Novel Strategy Based on Linear Active Disturbance Rejection Control for Harmonic Detection and Compensation in Low Voltage AC Microgrid

In an AC microgrid, harmonic distortion is mainly caused by power electronic equipment and nonlinear loads. In this paper, linear active disturbance rejection control (LADRC) is used to control the fundamental current at the point of common coupling (PCC). Meanwhile, an active power filter (APF) is added to eliminate the harmonic current generated by the nonlinear loads. The tracking differentiator (TD) in active disturbance rejection control (ADRC) serves as a low-pass filter (LPF) in the harmonic detection algorithm of APF. Compared to traditional harmonic detection algorithms, the improved strategy solves the contradiction between rapidity, accuracy, and overshoot of filtering. LADRC has good performance of disturbance rejection, internal decoupling, and accessible parameters tuning. It can observe the internal uncertainty and external disturbance of the system as the total disturbance through the extended state observer (ESO), and compensate it in time through state feedback to make the system achieve the desired performance. The abilities of resonance suppression for LCL-type filter and internal decoupling of LADRC demonstrates its advantages through frequency domain analysis and simulation. The proposed strategy was simulated in MATLAB/SIMULINK and realized in the experimental hardware platform, and the effectiveness of the proposed strategy is approved.

Optimal Multiobjective Control of Low-Voltage AC Microgrids: Power Flow Regulation and Compensation of Reactive Power and Unbalance

The presence of single-phase distributed generators unevenly injecting active power in three-phase microgrids may create undesired upstream current unbalance. Consequently, voltage asymmetry and even active power curtailment may occur in such networks with negative economic impact. Thus, this paper proposes an optimal multiobjective approach to regulate the active and reactive power delivered by distributed generators driven by a three-layer hierarchical control technique in low-voltage microgrids. This method does not require previous knowledge of network parameters. The multiobjective algorithm is implemented in the secondary level achieving optimal dispatch in terms of maximizing the active power generation, as well as minimizing the reactive power circulation and current unbalance. By the existence of a utility interface three-phase converter placed at the point-of-common-coupling, the proposed control can regulate the power circulating among the microgrid phases, and the microgrid structure can withstand grid-connected and islanded operating modes. The path for interphase power circulation through the DC-link of the utility interface allows the multiobjective algorithm to achieve better results in terms of generation and compensation compared to the system without utility interface. The proposed method is assessed herein by computational simulations in a three-phase four-wire microgrid under realistic operational conditions.

Distributed Control of Low-Voltage Resistive AC Microgrids

This paper proposes a distributed control strategy for coordination of distributed energy resources (DERs) in low-voltage resistive microgrids. The proposed framework consists of two level structures; primary and secondary control. Unlike the existing distributed control solutions, the proposed method is based upon the practical assumption of network impedance being resistive. The primary control level consists of a V-I droop mechanism, where GPS timing is used to synchronize the control agents. A consensus-based distributed secondary control method is introduced to improve the voltage regulation and load sharing accuracy of the V-I droop method. In the proposed approach, the d-axis component of the voltage is altered so as to regulate the average microgrid voltage to the rated value while guarantying proper sharing of active power among the DERs. Additionally, the q-axis component of voltage is adjusted to perform proper current and, accordingly reactive power sharing. The proposed control methodology accounts for the distribution line impedances. It features a plug-and-play environment; prior system knowledge is not required, and an arbitrary DER can enter the microgrid without any need for additional synchronization. An ac microgrid is prototyped to experimentally demonstrate the efficacy of the proposed method.

Distributed Finite-Time Convergence Control of an Islanded Low-Voltage AC Microgrid

The control of an islanded AC microgrid with high penetration level of renewable generations is complicated and challenging due to its low inertia and fast changing operating conditions. To address the drawbacks of traditional droop control with low-voltage AC microgrid, an adaptive virtual impedance control is proposed to improve the active/reactive power sharing accuracy. To deal with the frequency and voltage deviation caused by the primary control while satisfying the fast response requirements and various users' preferences, a flexible distributed control strategy is proposed as the secondary control. The proposed control strategy can restore the frequency and voltage magnitude to the desired values in a finite-time manner, using only local and neighboring information. The finite-time convergence of the proposed distributed control algorithm is guaranteed through rigorous analysis. Simulation studies validate the effectiveness and scalability of the proposed distributed control strategy.

Power Flow Analysis for Low-Voltage AC and DC Microgrids Considering Droop Control and Virtual Impedance

In the low-voltage (LV) ac microgrids (MGs), with a relatively high R/X ratio, virtual impedance is usually adopted to improve the performance of droop control applied to distributed generators (DGs). At the same time, LV dc MG using virtual impedance as droop control is emerging without adequate power flow studies. In this paper, power flow analyses for both ac and dc MGs are formulated and implemented. The mathematical models for both types of MGs considering the concept of virtual impedance are used to be in conformity with the practical control of the DGs. As a result, calculation accuracy is improved for both ac and dc MG power flow analyses, comparing with previous methods without considering virtual impedance. Case studies are conducted to verify the proposed power flow analyses in terms of convergence and accuracy. Investigation of the impact to the system of internal control parameters adopted by DGs is also conducted by using proposed method.

Design and Control of Parallel Three Phase Voltage Source Inverters in Low Voltage AC Microgrid

Design and hierarchical control of three phase parallel Voltage Source Inverters are developed in this paper. The control scheme is based on synchronous reference frame and consists of primary and secondary control levels. The primary control consists of the droop control and the virtual output impedance loops. This control level is designed to share the active and reactive power correctly between the connected VSIs in order to avoid the undesired circulating current and overload of the connected VSIs. The secondary control is designed to clear the magnitude and the frequency deviations caused by the primary control. The control structure is validated through dynamics simulations.The obtained results demonstrate the effectiveness of the control structure.

Simplified and Automated Fault-Current Calculation for Fault Protection System of Grid-Connected Low-Voltage AC Microgrids

Abstract Fault currents inside a grid-connected AC microgrid are significantly varied because fault current contributions of the main grid and DG units are different from each other due to various fault locations, fault types, and high penetration of inverter-based distributed generators (IBDGs) and rotating-based distributed generators (RBDGs). A traditional fault-analysis method cannot be sufficiently applicable for AC microgrids with the presence of both rotating-based distributed generators and inverter-based distributed generators. From the above viewpoint, this paper proposes a simplified and automated fault-current calculation approach for grid-connected AC microgrids to quickly and accurately calculate fault-current contributions from IBDGs and RBDGs as well as the grid fault-current contribution to any faulted microgrid sections. The simplified and automated fault-current calculation approach is mainly focused on grid-connected and small-sized low-voltage AC microgrids with the support of communication system. Under the grid-connected microgrid operation mode, fault-tripping current-thresholds of adaptive overcurrent relays are properly adjusted thanks to the proposed fault analysis method. Relying on fault-current distribution-coefficients of IBDGs, RBDGs, and the utility grid, the setting values of adaptive overcurrent relays in a low-voltage AC microgrid are effectively self-adjusted according to various microgrid configurations and the operation status of DG units during the grid-connected mode.