Inverter-dominated Microgrids Research Articles

A new current-based protection system for inverter-dominated microgrids

A new current-based protection system for inverter-dominated microgrids

Frequency and Voltage Control Techniques through Inverter-Interfaced Distributed Energy Resources in Microgrids: A Review

Microgrids (MG) are small-scale electric grids with local voltage control and power management systems to facilitate the high penetration and grid integration of renewable energy resources (RES). The distributed generation units (DGs), including RESs, are connected to (micro) grids through power electronics-based inverters. Therefore, new paradigms are required for voltage and frequency regulation by inverter-interfaced DGs (IIDGs). Notably, employing effective voltage and frequency regulation methods for establishing power-sharing among parallel inverters in MGs is the most critical issue. This paper provides a comprehensive study, comparison, and classification of control methods including communication-based, decentralized, and construction and compensation control techniques. The development of inverter-dominated MGs has caused limitations in employing classical control techniques due to their defective performance in handling non-linear models of IIDGs. To this end, this article reviews and illustrates advanced controllers that can deal with the challenges that are created due to the uncertain and arbitrary impedance characteristics of IIDGs in dynamics/transients.

Constrained Predictive Control Based on a Large-Signal Model for a Three-Phase Inverter Connected to a Microgrid

Small-signal models are mostly used to model and design inverter-dominated microgrids. Conversely, this article proposes a large-signal model for grid-forming inverters connected to a microgrid based on the active and reactive power dynamic equations. In this work, it is proposed to use this nonlinear model to develop a constrained predictive control for the inverters connected to a microgrid. The main features of this proposed control are follows: first, direct voltage control (in amplitude and frequency) is not necessary; second, stability is guaranteed under a wide range of line impedances (hence, the virtual impedance is not needed); third, the proposed control can operate with unbalanced and nonlinear loads. Moreover, a theoretical stability analysis is presented. Selected experimental results show that the proposed control operates satisfactorily in case of a load step change, a load imbalance, and nonlinear loads.

A Two-Level Simulation-Assisted Sequential Distribution System Restoration Model With Frequency Dynamics Constraints

This paper proposes a service restoration model for unbalanced distribution systems and inverter-dominated microgrids (MGs), in which frequency dynamics constraints are developed to optimize the amount of load restoration and guarantee the dynamic performance of system frequency response during the restoration process. After extreme events, the damaged distribution systems can be sectionalized into several isolated MGs to restore critical loads and tripped non-black start distributed generations (DGs) by black start DGs. However, the high penetration of inverter-based DGs reduces the system inertia, which results in low-inertia issues and large frequency fluctuation during the restoration process. To address this challenge, we propose a two-level simulation-assisted sequential service restoration model, which includes a mixed integer linear programming (MILP)-based optimization model and a transient simulation model. The proposed MILP model explicitly incorporates the frequency response into constraints, by interfacing with transient simulation of inverter-dominated MGs. Numerical results on a modified IEEE 123-bus system have validated that the frequency dynamic performance of the proposed service restoration model are indeed improved.

A Distributed Secondary Control Approach for Inverter-Dominated Microgrids With Application to Avoiding Bifurcation-Triggered Instabilities

The distributed secondary control is applied in an inverter-dominated microgrid (MG) for compensating the voltage and frequency deviations caused by the operation of the primary droop-based control. Despite the flexibility and reliability of the controllers, the low-inertia nature of voltage source inverters threatens the system stability against probable disturbance. Therefore, it is necessary to analyze the stability of islanded MG by focusing on the hierarchical control structure. The effect of the distributed secondary control on the stability margin of the MG using bifurcation analysis when parameters change is investigated in this article. To this end, this article firstly derives a precise dynamic model of the MG with the secondary control level. Next, the eigenvalues of the system state matrix are analyzed to determine the dominant parameters affecting the stability. Then, the bifurcation theory is used to predict the parameter stability margin with/without secondary control. By changing the parameters, the Hopf and saddle-node bifurcations are detected in the system. Finally, the simulation and experimental results confirm the effectiveness of the distributed secondary control in preventing the occurrence of bifurcation.

Dynamic adaptability of model predictive control for power converters in inverter dominated microgrids

This paper presents the design and development of Dynamic Adaptability (DA) feature, added to a multi-objective Model Predictive Control (MPC) of a basic grid-tied inverter with a secondary function for switching frequency reduction. The classical MPC is enhanced with dynamically varying secondary function weightage in accordance with the demand of the operating condition for self-reliant change-over across grid-tied mode and grid-forming modes in an Inverter Dominated Microgrids (IDM) environment. A customized DA database has been developed through prior investigation on the IDM, considering possible operating conditions with diverse combinations of the sampling frequency and the secondary function weight. Investigations unveiled that there is a unique value of secondary weight for every operating condition, beyond which the reference tracking is lost. This necessitated the dynamic assignment of the weight to achieve a desirable inverter performance. The dynamic weightage assignment is accomplished by accessing the DA database through the priority signals for harmonic standards, inverter losses and reference tracking, respectively. The DA imparted MPC is tested for its mode transfer capability with desired performance under all operating modes of IDM in simulations with a 15 kVA grid-tied/forming inverter and the results are presented. Further, the experimental investigations are conducted with a 10 kVA inverter on a 5-bus laboratory scale IDM emulator along with hardware-in-loop. Both inter-mode and intra-mode transitions were initiated in the IDM and the inverter performance are presented here. Both the simulation and the hardware results yielded competent values of reference tracking accuracy and mode transition times.

Non-Pilot Protection of the Inverter- Dominated Microgrid

Without utilizing costly communication systems, the existing protection strategies fail to reliably detect the occurrence and direction of faults in the inverter-dominated microgrid. To address this issue, this paper introduces a selective and reliable non-pilot protection strategy for the inverter-dominated microgrid. The proposed protection strategy (i) does not require communication signals, (ii) incorporates phase- and sequence-domain protective elements for reliable detection of symmetrical and asymmetrical faults, (iii) improves the existing sequence-domain directional elements and effectively utilizes them for accurate determination of the fault direction in the presence of inverter-interfaced distributed energy resources, (iv) selectively protects the inverter-dominated microgrid against internal and external faults, (v) is robust against the grid-connection mode of the microgrid, and (vi) enables fuse protection of laterals and non-critical circuits. The acceptable performance of the proposed protection strategy is verified through comprehensive fault studies conducted on a realistic study system simulated in the PSCAD/EMTDC software environment. This paper also demonstrates that the proposed protection strategy can be implemented using an off-the-shelf digital relay.

Design of a distributed non‐linear stabiliser in MG using synergetic control theory

Stability of inverter-dominated microgrids (MGs) faces many challenges such as inertia shortage, mutual interactions among converters and sudden external disturbances. To enhance the stability of MGs, a distributed non-linear stabiliser is designed in this study based on synergetic control theory. First, inverter interfaces are uniformly modelled as virtual synchronous generators, whose small-signal stability is then analysed with the state-space average method. Also, the objective of the following control strategy, i.e. the system unstable domain, is obtained. Second, macro-variables applicable to droop control are constructed based on the typical dynamic model and then the analytic expression of the controller output could be derived. When external disturbances occur, or droop coefficients with high gains are chosen, power system stability could be guaranteed by adding a non-linear stabiliser. Finally, taking a 9-node MG as an example, simulation verifies the feasibility of the proposed strategy in cases of small disturbances, large disturbances and mode transition.

N-version programming-based protection scheme for microgrids: A multi-agent system based approach

Abstract Islanded mode and grid-connected mode are the two operational modes of a microgrid (MG). It is essential to develop a suitable protection scheme for an MG network to protect it against all types of faults in both of the modes. The fault current in an inverter-dominated MG is limited by the ratings of the semiconductor devices. Only traditional over-current protection schemes are not adequate for such inverter-based grids. In order to cover the drawbacks of conventional protection schemes, an N-version programming-based protection scheme has been proposed in this paper. An N-version software unit with N = 3 has been developed in MATLAB Simulink and all the agents in the developed multi-agent scheme have been equipped with this unit. The three developed protection versions are Clark’s transformation-based current protection, positive sequence phase differential-based protection and conventional over-current-based protection scheme. The software unit makes the final decision about the fault through a polling process. Different types of faults have been tested in both the islanded and grid-connected modes. The proposed protection scheme has given desired results in all the tested cases and hence proved to be a reliable and accurate method for fault identification in microgrid networks. Finally, a framework has been presented for possible realization of the proposed protection scheme through existing IEC 61850-based intelligent electronic devices (IEDs).

Improvement of small signal stability margin and transient response in inverter-dominated microgrids

Abstract Inverter-based distributed generators (DGs) based on renewable sources are widely used in microgrids. Most of these sources operate in droop control mode for effective load sharing. Higher droop is desired in these systems to improve dynamic power sharing, however, such systems suffer from stability issues. Stability margin of a system having different combinations of inverter and synchronous generator-based sources is compared in this work. A microgrid is modeled here with three DGs on a modified IEEE 13-bus system using state space approach. The study showed that lack of stability of inverters restricts the use of higher droop gains, resulting in poor power-sharing dynamics. A modified droop control is proposed to improve transient response and stability margin in such cases. The results are validated with time domain simulations using Simulink/MATLAB.

Dynamic Phasors-Based Modeling and Stability Analysis of Droop-Controlled Inverters for Microgrid Applications

System modeling and stability analysis is one of the most important issues of inverter-dominated microgrids. It is useful to determine the system stability and optimize the control parameters. The complete small signal models for the inverter-dominated microgrids have been developed, which are very accurate and could be found in literature. However, the modeling procedure will become very complex when the number of inverters in microgrid is large. One possible solution is to use the reduced-order small signal models for the inverter-dominated microgrids. Unfortunately, the reduced-order small signal models fail to predict the system instabilities. In order to solve the problem, a new modeling approach for inverter-dominated microgrids by using dynamic phasors is presented in this paper. Our findings indicate that the proposed dynamic phasor model is able to predict accurately the stability margins of the system, while the conventional reduced-order small signal model fails. In addition, the virtual ω-E frame power control method, which deals with the power coupling caused by the line impedance X/R characteristic, has also been chosen as an application example of the proposed modeling technique.

Traveling Wave-Based Protection Scheme for Inverter-Dominated Microgrid Using Mathematical Morphology

Inverter-dominated microgrids impose significant challenges on the distribution network, as inverters are well known for their limited contribution to fault current, undermining the performance of traditional overcurrent protection schemes. This paper introduces a new protection scheme based on the initial current traveling wave utilizing an improved mathematical morphology (MM) technology, with simplified polarity detection and new logics introduced for meshed networks and feeders with single-end measurement. The proposed protection scheme provides ultrafast response and can be adapted to varied system operational modes, topologies, fault conditions, and load conditions. Only low-bandwidth communication is required to achieve high-speed operation and adequate discrimination level in meshed networks. Simulation in PSCAD/EMTDC verifies both the sensitivity and stability of the proposed protection scheme under different microgrid operational scenarios.

New Power Sharing Control for Inverter-Dominated Microgrid Based on Impedance Match Concept

Power flow control is one of the most important issues for operating the inverter-dominated autonomous microgrid. A technical challenge is how to achieve the accurate active/reactive power sharing of inverters. P-F and Q-V droop control schemes have been widely used for power sharing in the past decades. But they suffer from the poor power sharing in the presence of unequal line impedance. In order to solve the problem, a comprehensive analysis of the power droop control is presented, and a new droop control based on the impedance match concept is proposed in this paper. In addition, the design guidelines of control coefficients and virtual impedance are provided. Finally, the performance evaluation is carried out, and the evaluation results verify the effectiveness of the proposed method.