Inverter-based Microgrids Research Articles

Fault Detection and Protection Strategy for Islanded Inverter-Based Microgrids

This article proposes a fault detection and protection strategy for islanded inverter-based microgrids (IBMGs). Reliable and accurate protection is one of the main challenges in the proliferation of modern microgrids (MGs). Considering the limited fault current of the voltage–frequency-controlled inverter-based distributed energy resources (VF-IBDERs), the protection is more challenging in islanded IBMGs. In this regard, the control scheme of VF-IBDER with a current limiting strategy plays an important role. Due to the limited fault currents close to the converter nominal current, the conventional fault detection methods do not work properly. In addition, bidirectional fault currents worsen protection coordination. In this article, first, an analytical sequence network modeling for VF-IBDERs is derived to specify their behavior under fault conditions. Then, a voltage-restrained negative-sequence resistance-based fault detection approach is proposed, which is based on the derived sequence networks. This quantity inherently detects the fault and its direction and is independent of the fault current magnitude. The proposed feature can be employed in both the conventional and communication-assisted coordination strategies. In addition, a protection coordination strategy based on a definite-time grading approach is employed. Finally, the performance of the proposed scheme is demonstrated by applying different faults in a test MG in PSCAD/EMTDC environment.

A Protection Method for Inverter-based Microgrid Using Current-only Polarity Comparison

The design of an effective protection system for inverter-based microgrids is a complicated engineering challenge. This is due to the fact that inverters have limited fault current capabilities, and that the conventional overcurrent protection is not suitable for inverter-based microgrids. This paper introduces a novel protection method for inverter-based microgrid using a current-only polarity comparison. The proposed method is based on the phase difference between the pre-fault and fault current components. The method responds to faults in both grid-connected and autonomous operation modes and provides a new way to identify faulted sections. Simulations of an inverter-based microgrid with a relay model are conducted using PSCAD/EMTDC software. The results show that the proposed method can detect faults in inverter-based microgrids.

Frequency Restoration and Oscillation Damping of Distributed VSGs in Microgrid With Low Bandwidth Communication

The virtual synchronous generator (VSG) provides a promising way to increase the inertia of inverter-based microgrids. However, when multiple VSGs are parallel in the grid, severe oscillations may be incurred. To address this problem, this article proposes a secondary frequency control for distributed VSGs with low bandwidth communication. The control can damp the oscillations and restore the frequency to the rated value without changing the virtual inertia provided by the VSGs. In addition, the proportional active power sharing is achieved by event-triggered communication mechanism, thereby reducing the communication burden considerably. The stability and Zeno-free behavior of the proposed control strategy are demonstrated by the ultimately uniformly bounded (UUB) theory. Simulations are conducted to verify the effectiveness of the proposed control strategy on the PSCAD/EMTDC platform.

Optimal Frequency Control for Inverter-Based Micro-Grids Using Distributed Finite-Time Consensus Algorithms

This paper addresses the distributed frequency control and economic dispatch problem of micro-grid systems. The optimal frequency control problem is formulated considering both frequency regulation and generation cost minimization, by establishing an equivalent nonlinear second-order dynamic model. A distributed finite-time consensus protocol is proposed to solve the optimal frequency regulation problem based on the equivalent model. Despite the nonlinear dynamics, the system frequency converges to the nominal value and the economic dispatch is achieved both in finite time. It is proved that the equilibrium point of the closed-loop system is the unique optimal solution to the associated economic dispatch problem. Simulation results verify the theoretical analysis and demonstrate the effectiveness of the proposed control method.

Admittance Modeling and Stability Analysis for Inverter-Based Microgrid With Voltage-Source-Rectifier Load

Instability and low-frequency oscillation phenomenon of inverter-based microgrid (IBM) with voltage-source-rectifier load have obtained increasing concern. However, the influence of instantaneous angle of operation point on IBM is usually neglected during voltage faults, and it may generate the uncertain impact of voltage faults on the stable and safe operation of IBM. Besides, the influence of instantaneous angle of operation point may cause varying degrees of system oscillation and stability at a different angle of system states' change, including fault recovery. To solve this issue, a systematic approach of small-signal admittance modeling for IBM in the αβ frame is proposed in this article. The small-signal admittance model of each submodule (SM) of IBM is established and formed as a multi-input-multi-output (MIMO) system considering the actual closed-loop controls and decoupled strategies in the dq frame. Based on the proposed admittance models of SMs, the complete model of IBM is integrated and linearized, and the influence of both amplitude and instantaneous angle of operation point on the IBM stability is analyzed in small-signal model comprehensively. Besides, using the generalized Nyquist stability criterion (GNC), the influence of instantaneous angle in large-signal operation point on oscillation amplitude and stability of IBM is investigated. The proposed analysis approach can analyze and determine the responded oscillation and stability of the system more accurately during system states' variation, such as voltage sag and sudden changes in load. It also reduces the uncertainty of IBM behaviors during states' variation to ensure that IBM operates smoothly, flexibly, and stably. Finally, experimental results are presented to validate the correctness of the proposed analysis method.

Distributed finite-time active power sharing control with generation costs considered

With the generation costs considered, a modified distributed active power sharing control scheme for the inverter-based microgrid is proposed in this paper. Inverters are regarded as agents that can exchange information with adjacent inverters over a communication network. Since the distributed generations in the microgrid are usually of different types, the total generation cost of the microgrid must be considered. Thus, this controller is more practical than traditional power sharing control scheme. Furthermore, a distributed finite-time active power sharing control strategy is proposed to share active power in a shorter period of time. Some sufficient conditions are derived to achieve active power sharing. Finally, simulations are presented to illustrate the validity of the theoretical results.

A Novel Compact dq-Reference Frame Model for Inverter-Based Microgrids

The development and the experimental validation of a novel dynamic model of an islanded three-phase Inverter-based Microgrid (IMG) is presented in this paper. The proposed model reproduces the relevant system dynamics without excessive complexity and enough accuracy. The dynamics of the IMG are captured with a compact and scalable dynamic model, considering inverter based distributed generators with d-current droop primary and proportional resonant inner controllers. The complete development of the model, the practical assumptions, and the accurate proportional power sharing of the primary control technique are shown. The accuracy performance was verified in experiments performed at the Aalborg Intelligent Microgrids Laboratory for an islanded IMG case.

A new unified control strategy for inverter-based micro-grid using hybrid droop scheme

Micro-grid (MG) operation using voltage control methods (VCMs) has been widely recommended for parallel operation of three phase voltage source inverters, especially during islanded mode to maintain the voltage level in the MG. In contrast, this paper presents a new unified control strategy for MG parallel operation using a current control method (CCM). A hybrid control scheme which combines a universal robust droop controller (URDC) and quasi-proportional resonant (QPR) regulator is introduced. It ensures equal power sharing among parallel operated inverters during all MG's operating modes. Moreover, an improved adaptive estimator for the reference current magnitude is integrated to meet dynamic load variations. Furthermore, the scheme is enhanced with a self-synchronized capability using a simple synchronous reference frame phase locked loop (SRF-PLL). This obviates the necessity of communication links and external synchronous clocks required for synchronization process. In addition, a decentralized control action is locally managed to restore the inverter's frequency upon any reactive load change. Modeling and simulation results using MATLAB/SIMULINK software are presented to show the effectiveness of the proposed strategy.

Voltage and Frequency Recovery in an Islanded Inverter-Based Microgrid Considering Load Type and Power Factor

This paper introduces simple and complete under voltage-frequency load shedding schemes to determine proper load shedding amounts (LSAs) in an islanded inverter-based microgrids (IBMGs) considering power and power factor (PF) of various load types, such as constant power, constant current, and constant impedance loads. The proposed method employs a fast time-step simulation approach based on the complete state-space model of IBMG in order to obtain the transient and steady state responses of the system. The required power-voltage curves are obtained considering load PF and system topology. Ultimately, LSA is obtained according to the under voltage load shedding (LS) and under frequency LS values. To demonstrate the effectiveness of the proposed simple and complete LS methods, the corresponding performances are analyzed in a test islanded IBMG modeled in MATLAB. The simulation results show that unlike the other compared methods, the proposed LS approach can accurately determine less LSA while maintaining the power system stability.

Fault Ride-Through Enhancement of Grid Supporting Inverter-Based Microgrid Using Delayed Signal Cancellation Algorithm Secondary Control

The growing level of grid-connected renewable energy sources in the form of microgrids has made it highly imperative for grid-connected microgrids to contribute to the overall system stability. Consequently, secondary services which include the fault ride-through (FRT) capability are expected to be possessed characteristics by inverter-based microgrids. This enhances the stable operation of the main grid and sustained microgrid grid interconnection during grid faults in conformity with the emerging national grid codes. This paper proposes an effective FRT secondary control strategy to coordinate power injection during balanced and unbalanced fault conditions. This complements the primary control to form a two-layer hierarchical control structure in the microgrids. The primary level is comprised of voltage/power and current inner loops fed by a droop control. The droop control coordinates grid power-sharing amongst the voltage source inverters. When a fault occurs, the participating inverters operate to support the grid voltage, by injecting supplementary reactive power based on their droop gains. Similarly, under unbalanced voltage condition due to asymmetrical faults in the grid, the proposed secondary control ensures the positive sequence component compensation and negative and zero sequence components clearance using a delayed signal cancellation (DSC) algorithm and power electronic switched series impedance placed in-between the point of common coupling (PCC) and the main grid. While ensuring that FRT ancillary service is rendered to the main utility, the strategy proposed ensures relatively interrupted quality power is supplied to the microgrid load. Consequently, this strategy ensures the microgrid ride-through the voltage sag and supports the grid utility voltage during the period of the main utility grid fault. Results of the study are presented and discussed.

Droop Control in DQ Coordinates for Fixed Frequency Inverter-Based AC Microgrids

This paper presents a proof-of-concept for a novel dq droop control technique that applies DC droop control methods to fixed frequency inverter-based AC microgrids using the dq0 transformation. Microgrids are usually composed of distributed generation units (DGUs) that are electronically coupled to each other through power converters. An inherent property of inverter-based microgrids is that, unlike microgrids with spinning machines, the frequency of the parallel-connected DGUs is a global variable independent from the output power since the inverters can control the output waveform frequency with a high level of precision. Therefore, conventional droop control methods that distort the system frequency are not suitable for microgrids operating at a fixed frequency. It is shown that the proposed distributed droop control allows accurate sharing of the active and reactive power without altering the microgrid frequency. The simulation and hardware-in-the-loop (HIL) results are presented to demonstrate the efficacy of the proposed droop control. Indeed, following a load change, the dq droop controller was able to share both active and reactive power between the DGUs, whereas maintaining the microgrid frequency deviation at 0% and the bus voltage deviations below 6% of their respective nominal values.

Distributed secondary frequency control in microgrids: Robustness and steady-state performance in the presence of clock drifts

Microgrids are distributed systems with high share of inverter-interfaced renewable energy sources where stable and reliable system operation is realized by suitably controlling the inverters. In this work, we focus on secondary frequency control, which is an important ancillary service provided by the inverters. In the literature on secondary frequency control, the effect of clock drifts has often been neglected. However, clock drifts are practically unavoidable parameter uncertainties in inverter-based microgrids and we show that the most commonly employed distributed secondary frequency controllers exhibit performance deteriorations when taking clock drifts explicitly into consideration. Motivated by this, we propose a novel alternative control law called generalized distributed averaging integral (GDAI) control, which achieves the secondary control objectives of steady-state accurate frequency restoration and proportional power sharing in the presence of clock drifts. In addition, we derive a sufficient tuning criterion in the form of a set of linear matrix inequalities (LMIs) which guarantees robust stability of the closed-loop equilibrium point in the presence of uncertain clock drifts. Finally, our analysis is validated extensively via simulation with comprehensive comparisons to other related distributed control approaches.

Trends in the protection of inverter‐based microgrids

The study reviews the current trends in the protection of inverter-based microgrids. This topic is currently given significant attention, as the inverter-interfaced distributed generation is likely to become dominant in microgrids. Since low fault current is a well-known signature of microgrids, the emphasis is on the complicated fault characteristics in inverter-based microgrids, bearing in mind that low-magnitude fault current and complicated fault responses are inter-related. The fault characteristics are rooted in the inverter controllers that can control inverter voltages and currents during the fault period such that they are completely different from the conventional fault characteristics. Given that these differences could impact conventional protection schemes, challenges to different elements of fault identification, i.e. fault detection, fault polarisation, and faulted phase selection in microgrids are separately addressed in detail. The existing preliminary solutions that define the current trends are discussed and gaps are identified accordingly. The review classifies techniques into three categories: modified classical techniques, heuristic techniques, and differential-based techniques. The explanations are supported by simulation case studies to provide clarification. The purpose is to provide the current status of microgrid protection and assist in developing new effective solutions to protect these grids.

Impact of DG Connection Topology on the Stability of Inverter-Based Microgrids

In this letter, we theoretically explain that the common connection topology for inverter-based distributed generators (DGs) is harmful to the small-disturbance stability of microgrids. We first prove that the algebraic connectivity of a graph will approach zero in case of inappropriate expansion of nodes and lines. Such type of expansion is often the case in microgrids where new DGs are simply connected to the nearby nodes via single lines that leads to a tree-like structure. Furthermore, we prove that the zero algebraic connectivity leads to a zero eigenvalue in the dynamic Jacobian matrix of microgrids and, hence, deteriorates stability. The result reveals the importance of careful planning for DG connection topology especially when a large number of DGs are to be integrated. It also suggests that forming loops by linking the nodes close to feeder terminals could be an effective way for stability enhancement.

A protection strategy for inverter-interfaced islanded microgrids with looped configuration

Development of an efficient protection strategy is one of the main barriers in paving the way for the implementation of inverter-based microgrids. The limited fault current of voltage-sourced converter (VSC) units as the sole fault feeding sources in the islanded mode of operation makes the fault detection difficult for conventional overcurrent-based protection system. On the other hand, bidirectional power flow due to the presence of distributed energy resource units and looped configuration of microgrids results in miscoordination of the overcurrent relays. To address these problems, this paper proposes a protection strategy for islanded inverter-interfaced looped microgrids. By monitoring the fault-imposed component of VSC current, the proposed scheme quantifies the severity of fault condition as the fault detection criterion. To preserve the protection coordination among available overcurrent relays, the proposed protection scheme adaptively changes the limiting level of hybrid reference frame limiter of the VSC control system proportional to their distances to the fault point. The proposed protection scheme is local, and there is no need for any change in the commercially available overcurrent protection relays. The reliable performance of the proposed protection strategy is assessed on the simulation model of a looped microgrid network.

Modeling and Stability Analysis of Inverter-Based Microgrid Under Harmonic Conditions

Microgrids (MGs) operate under harmonic conditions due to the integration of nonlinear loads. The autonomous harmonic compensation control of inverter-interfaced DG has been proposed to successfully mitigate the harmonics. However, the small-signal analysis of harmonic compensation controls has not been investigated in the microgrid with multiple inverters. This paper develops the modeling and analysis of the inverter-based MGs under harmonic conditions. The concept of dynamic phasor (DP) is used to describe the fundamental and harmonic components of an ac waveform via dc variables. The developed model consists of droop-controlled distributed generators (DGs), diodes rectifiers (working as nonlinear load) and resistance loads. Virtual impedance control is considered in the droop-controlled DGs for the autonomous harmonic compensation. Based on the developed DP model, the dynamic behavior of the microgrid is investigated via small-signal analysis. It is observed that the virtual impedance for harmonic compensation brings inter-inverter oscillations on harmonic domain. Participation and eigenlocus analysis are performed to investigate the influence of parameter tuning of harmonic compensation on microgrid stability. Numerical simulations are carried out to validate the effectiveness of the proposed modeling method and the analysis results.

Experiments on a Real-Time Energy Management System for Islanded Prosumer Microgrids

This paper presents an experimental demonstration of a novel real-time Energy Management System (EMS) for inverter-based microgrids to achieve optimal economic operation using a simple dynamic algorithm without offline optimization process requirements. The dynamic algorithm solves the economic dispatch problem offering an adequate stability performance and an optimal power reference tracking under sudden load and generation changes. Convergence, optimality and frequency regulation properties of the real-time EMS are shown, and the effectiveness and compatibility with inner and primary controllers are validated in experiments, showing better performance on optimal power tracking and frequency regulation than conventional droop control power sharing techniques.

Cost-Function-Based Microgrid Decentralized Control of Unbalance and Harmonics for Simultaneous Bus Voltage Compensation and Current Sharing

Power quality of inverter-based microgrids is a challenging issue due to nonlinearities of inverters, multiple resonance modes of network impedance, and unbalanced and nonlinear loading condition. The ideal solution is to assure the best power quality at the common bus to which loads are connected, and share the negative and harmonic sequence currents between the inverters without any communication. However, it is difficult to achieve this objective with existing virtual-impedance-based methods when nonlinearities of inverters, e.g., dead time, are not negligible. In this paper, a novel cost-function-based method is proposed to solve this problem. The proposed cost functions are optimized using continuous-control-set model predictive control. As unknown nonlinearities of inverters can be observed by introducing disturbance models, even when they are not negligible, the presented method can compensate bus voltage unbalance and harmonics and share the compensated current between inverters autonomously, with neither communication nor upper level controller. Parameter tuning of the proposed method is the key to achieve the balance between bus voltage quality and current sharing, and between stability and fast response. Experimental results demonstrate that the proposed control scheme is effective even under conditions of large inverter nonlinearities.

A Design and Optimization Tool for Inverter-Based Microgrids Using Large-Signal Nonlinear Analysis

In this paper, large-signal Lyapunov-based stability analysis is performed on an inverter-based microgrid. Most microgrid stability studies have been based on small-signal stability by using linearization techniques. This paper produces the large-signal nonlinear mathematical model of an inverter-based microgrid. The inverter models include the dc-side circuits associated with dc sources. The mathematical model is then used to estimated the domain of asymptotic stability of the microgrid. A three bus microgrid system is used as a case study to highlight the design and optimization capability of such an approach. This paper explores the effect of system component sizing, load transient, and dc-side voltage source variations on the asymptotic stability of the ac microgrid. Thus, this approach quantifies the effect of dc-side variations on the transient stability of the ac-side microgrid. Time domain simulations were used to illustrate the mathematical nonlinear analysis results.

Plug-and-Play Compliant Control for Inverter-Based Microgrids

Control of inverter-based microgrids (IBMGs) with power sharing capability is a challenging task due to specific nature of microgrid instabilities. Specifically, droop-controlled microgrids prone to become unstable when the $X/R$ ratio is near unity and the lines are short. Virtual impedance is thought to be one of the promising tools to prevent this type of instability allowing for a broad range of droop coefficients for efficient power sharing between inverters. However, proper sizing of virtual impedance for particular microgrid configuration is still an open question. In this paper, a systematic method for determining the size of virtual impedance in a plug-and-play fashion is proposed for enhancing the stability of the droop modes. Based on the Lyapunov method, concise and simple stability certificates are derived to provide flexible choices of the droop gains for different inverter ratings. The implementation of the virtual impedance with the proposed approach for stability enhancement is verified by both numerical simulation through MATLAB/Simulink and experiment via a small-scale prototype for different scenarios.