Low-inertia Microgrids Research Articles

An Adaptive Reclosing Scheme for Preserving Dynamic Security in Low-Inertia Microgrids

Conventional reclosing schemes with fixed dead times can threaten the dynamic security of low-inertia microgrids ( <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> Gs) in case of permanent faults. This paper proposes an adaptive reclosing approach for restoring the interrupted feeder operation and maintaining the dynamic security within the shortest time. The proposed approach is outlined in three stages: First, a secure zone based on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mu}\text{G}$ </tex-math></inline-formula> operation conditions is determined at the pre-fault stage for reclosing under permanent faults. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mu }\text{G}$ </tex-math></inline-formula> dynamic security can be retrieved if the fault is cleared in secure zone. Second, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mu }\text{G}$ </tex-math></inline-formula> state variables are monitored at the during-fault stage to obtain associated temporal characteristics. At the third stage, post-fault samples are examined as appropriate candidates to reclose faulty feeders. Here, samples are forecasted considering a sufficient readiness lead time. For forecasted samples, the locus of state variables for reclosing under permanent fault is predicted using the characteristics attained at the second stage. Here, reclosing decisions are made by comparing the locus of state variables with secure zone boundaries. Comprehensive simulations presented in this paper confirm effectiveness of the proposed adaptive reclosing method.

Analysis of Inertia Characteristics of Direct-Drive Permanent-Magnet Synchronous Generator in Micro-Grid

Micro-grid has received extensive attention as an effective way to absorb new energy. Compared to large power systems, the micro-grid system consisting of power electronics is relatively weak due to the lack of support for synchronous machines. In this paper, the direct-drive wind turbine (WT) is connected to the low-inertia micro-grid as the research background. Based on the virtual inertia control of the WT, the inertia source and the physical mechanism of the WT connected to the micro-grid system are studied. The inertia characteristics of the rotor of the WT on the electromechanical time-scale, the DC side capacitor on the DC voltage time-scale, and the simulated grid under the droop control are analyzed. The research results show that under the control of the system, the inertia of the system is derived from the WT, DC capacitor, and the micro-grid simulated by droop control converter. The equivalent inertia of each link is determined by the control parameters, steady-state operating point, and structural parameters. The resulting inertia characteristics will have frequency domain characteristics under control. Finally, the correctness of the system inertia analysis conclusion is verified by simulation and experiment.

Modeling Load Dynamics to Support Resiliency-Based Operations in Low-Inertia Microgrids

Microgrids have repeatedly demonstrated the ability to provide uninterrupted service to critical end-use loads during normal outages, severe weather events, and natural disasters. While their ability to provide critical services is well documented, microgrids present a more dynamic operational environment than grid-connected distribution systems. The electrodynamics of a microgrid are commonly driven by the high inertia of rotating generators, which are common in many microgrids. In such high-inertia systems, the impact of end-use load electromechanical dynamics are often not examined. However, with the increased penetration of inverter-based generation with little or no inertia, it is necessary to consider the impact that the dynamics of the end-use loads have on the operations of microgrids, particularly for a resiliency-based operation. These operations include, but are not limited to, switching operations, loss of generating units, and the starting of induction motors. This paper examines the importance of including multi-state electromechanical dynamic models of the end-use load when evaluating the operations of low inertia microgrids, and shows that by properly representing their behavior, it is possible to cost effectively size equipment while supporting resilient operations of critical end-use loads.

Algebraic connectivity conditions for synchronization in low-inertia microgrids with adaptive droop-controlled inverters

It is shown that a network of inverters equipped with adaptive droop controllers can be cast as a nonuniform model of phase-coupled oscillators. Necessary and sufficient conditions are derived for the global asymptotic synchronization of these droop-controlled oscillators coupled through the parameters of a dynamic Kuramoto network. For an electrical network with general interconnections and line conditions, where the coupling weight of oscillators varies with the electrical parameters of the connection lines, it is shown that the gain can be scaled with an adaptive droop coefficient. The synchronization condition is found to depend on algebraic connectivity, which must be larger than a critical value. In contrast to grid-connected inverters, which assume a priori network operation in a quasi-stationary sinusoidal steady state, adaptive droop is a control strategy that globally stabilizes a desired sinusoidal steady state. Implementing this control law in model reference adaptive control numerically validates the analytical results and demonstrates system performance in primary and secondary frequency control.

Disturbance-Observer-Based Frequency Regulation Scheme for Low-Inertia Microgrid Systems

This paper presents a disturbance observer approach for frequency regulation for a low-inertia microgrid system consisting of various distributed energy resources (DERs). The mismatch in load and generation is estimated using the disturbance observer and is used as a supplement to the classical integral controller to vary the set points of various controlled DERs. Since the disturbance-observer-based approach is an algorithm based on local frequency measurement, it reduces the capital expenditure in implementing additional sensors. Further, in case of microgrids, the communication infrastructure is mostly built with public networks, which can introduce random delays. Therefore, the performance of the proposed scheme is also tested in the presence of random communication delays. A sensitivity analysis is also presented to show that the performance of the proposed control scheme is much better than the integral controller even when there are variations in the system parameters. Finally, the proposed approach is tested in a laboratory-scale microgrid setup in which the frequency of a synchronous generator is controlled using the disturbance-observer-based approach. The results obtained in the experimental setup show that the proposed approach can be easily implemented in a real system in order to achieve adequate frequency control.

Application of Multi-Resonator Notch Frequency Control for Tracking the Frequency in Low Inertia Microgrids Under Distorted Grid Conditions

Frequency variations are much faster and more significant in microgrids with low-inertia inverters and other control devices for distributed generation. Accordingly, it is imperative to utilize a concise frequency estimation method for analyzing the transient behavior of microgrids. This paper proposes a robust algorithm which is based on multi-resonator and adaptive notch filter and is referred to as the multi-resonant notch frequency control for improving the frequency estimation of perturbed power system. In general, power system distortions such as harmonics, inter-harmonics, and faults may affect the accuracy of frequency estimation. In contrast to conventional methods for frequency estimator, the proposed method is less sensitive to distortions and unbalanced power system conditions. The proposed method uses the information in all three-phase signals which is proved to be more robust against signal variations than single-phase methods. Simulation results demonstrate the accuracy and the speed of the proposed method for frequency estimation in both grid-connected and islanded mode of low inertia microgrids under grid faults.

Robust Virtual Inertia Control of a Low Inertia Microgrid Considering Frequency Measurement Effects

Virtual inertia emulation could be regarded as an inevitable component of microgrids with renewable energy, enhancing microgrid inertia and damping properties. In applying this control technique, a phase-locked loop (PLL) is necessary to obtain the estimation of the system frequency data. However, the employment of PLL could cause larger frequency oscillation to the microgrid due to its dynamics. This issue would be exacerbated in a low-inertia microgrid driven by high renewable penetration, severely deteriorating the frequency stability. Thus, the effect of PLL with measurement delay is a critical issue in utilizing the virtual inertia control. To overcome such problem, this paper proposes a robust virtual inertia control for a low-inertia microgrid to minimize the undesirable frequency measurement effects, improving the microgrid frequency stability. The robust H ∞ control design using a linear fractional transformation (LFT) technique is used to develop the virtual inertia control loop, considering the dynamics of PLL with measurement delay and the uncertainties of system inertia and damping. The efficacy of the proposed H ∞ control method is compared to the conventional and optimum proportional-integral (PI)based inertia control. The results show that the H ∞ -based robust virtual inertia control is superior to both conventional virtual inertia control and optimum PI-based virtual inertia control against a wide range of microgrid operating conditions, disturbances, and parametric uncertainties.

Voltage Waveform Transient Identification for Autonomous Load Coordination

Significant electrical loads such as HVAC systems can be made “aware” of the operation of other loads nearby in the electric grid. Local examination of the utility voltage waveform can provide this awareness without the need for a dedicated communication network. This is particularly true in low-inertia microgrids and “soft” sections of a utility network. This paper presents techniques for extracting frequency and voltage harmonic transients corresponding to individual load events. With data collected from a microgrid energized by diesel generators, we demonstrate the ability to identify the operation of HVAC units and generator dispatch events from their transient effects on the voltage using a cross-correlation based scoring algorithm. Ultimately, incorporating such awareness into load controllers allows loads to autonomously meet system-level objectives in addition to their individual requirements. For example, HVAC units could maintain occupant comfort while also reducing the utility’s peak aggregate electrical demand by consuming electricity on a schedule interleaved with the operation of other nearby HVAC units.

Self-Adaptive Virtual Inertia Control-Based Fuzzy Logic to Improve Frequency Stability of Microgrid With High Renewable Penetration

Maintaining frequency stability of low inertia microgrids with high penetration of renewable energy sources (RESs) is a critical challenge. Solving this challenge, the inertia of microgrids would be enhanced by virtual inertia control-based energy storage systems. However, in such systems, the virtual inertia constant is fixed and selection of its value will significantly affect frequency stability of microgrids under different penetration levels of RESs. Higher frequency oscillations may occur due to the fixed virtual inertia constant or unsuitable selection of its value. To overcome such a problem and provide adaptive inertia control, this paper proposes a self-adaptive virtual inertia control system using fuzzy logic for ensuring stable frequency stabilization, which is required for successful microgrid operation in the presence of high RESs penetration. In this concept, the virtual inertia constant is automatically adjusted based on input signals of real power injection of RESs and system frequency deviations, avoiding unsuitable selection and delivering rapid inertia response. To verify the efficiency of the proposed control method, the contrastive simulation results are compared with the conventional method for serious load disturbances and various rates of RESs penetration. The proposed control method shows remarkable performance in transient response improvement and fast damping of oscillations, preserving robustness of operation.

Enabling Resiliency Operations Across Multiple Microgrids With Grid Friendly Appliance Controllers

Changes in economic, technological, and environmental policies are resulting in a re-evaluation of the dependence on large central generation facilities and their associated transmission networks. Emerging concepts of smart communities/cities are examining the potential to leverage cleaner sources of generation, as well as integrating electricity generation with other municipal functions. When grid connected, these generation assets can supplement the existing interconnections with the bulk transmission system, and in the event of an extreme event, they can provide power via a collection of microgrids. To achieve the highest level of resiliency, it may be necessary to conduct switching operations to interconnect individual microgrids. While the interconnection of multiple microgrids can increase the resiliency of the system, the associated switching operations can cause large transients in low inertia microgrids. The combination of low system inertia and IEEE 1547 and 1547a-compliant inverters can prevent multiple microgrids from being interconnected during extreme weather events. This paper will present a method of using end-use loads equipped with grid friendly appliance controllers to facilitate the switching operations between multiple microgrids; operations that are necessary for optimal operations when islanded for resiliency.

Synchronous islanded operation of an inverter interfaced renewable rich microgrid using synchrophasors

This study describes a novel strategy for microgrid operation and control, which enables a seamless transition from grid connected mode to islanded mode, and restoration of utility supply, without loss or disruption to loads sensitive to frequency or phase angle dynamics. A simulation study is conducted on a microgrid featuring inverter connected renewable generation, and power electronic interfaced loads. Therefore, the microgrid inherently has low inertia, which would subsequently affect the dynamic characteristics of the microgrid, in particular during mode transition. The microgrid is controlled by means of synchrophasor data to achieve synchronous island operation, enabling the microgrid to track the utility frequency and phase angle. The simulation includes synchrophasor acquisition and telecoms delays, allowing for detailed investigation of the microgrid dynamics under various mode transition scenarios, including the risk of commutation failure of the inverter sources. The proposed method is demonstrated to successfully maintain a microgrid in synchronism with the main utility grid after the transition to islanded mode without significant impact on various equipment connected to the microgrid. Thus, synchronous island operation of low inertia microgrids is feasible. This study also showed that utility supply could be seamlessly restored if the microgrid is operated as a synchronous island.

Determining optimal virtual inertia and frequency control parameters to preserve the frequency stability in islanded microgrids with high penetration of renewables

Preserving the frequency stability of low inertia microgrids (MGs) with high penetration of renewables is a serious challenge. To rise to this challenge, the inertia constant of MGs would be virtually increased using energy storages. However, it is important to determine the suitable value of inertia constant for these systems such that the frequency stability is preserved with a lower cost. Frequency droop coefficient of distributed energy resources (DERs) and load frequency controllers’ parameters would also affect the frequency response of MGs. Hence, in this paper, inertia constant is tuned together with frequency droop coefficient of DERs and load frequency controllers’ parameters. Determining these parameters is modeled as a multi-objective optimization problem and, because the number of objectives is higher than three, the problem is solved by a many-objective optimization algorithm. Comparative simulation studies have been done on an MG with different types of DERs to prove that using the proposed strategy for tuning the MG parameters not only the frequency deviation is highly decreased but also the amount of load shedding is considerably diminished. This would increase the customers’ satisfaction. Moreover, considering the inertia constant as a minimization objective, frequency stability would be preserved with a lower cost.

Protection of Autonomous Microgrids Using Agent-Based Distributed Communication

This paper presents a real-time implementation of autonomous microgrid protection using agent-based distributed communication. Protection of an autonomous microgrid requires special considerations compared to large-scale distribution networks due to the presence of power converters and relatively low inertia. In this paper, we introduce a practical overcurrent and a frequency-selectivity method to overcome conventional limitations. The proposed overcurrent scheme defines a selectivity mechanism considering the remedial action scheme of the microgrid after a fault instant based on feeder characteristics and the location of the intelligent electronic devices (IEDs). A synchrophasor-based online frequency-selectivity approach is proposed to avoid pulse loading effects in low inertia microgrids. Experimental results are presented for verification of the proposed schemes using a laboratory-based microgrid. The setup was composed of actual generation units and IEDs using the IEC 61850 protocol. The experimental results were in excellent agreement with the proposed protection scheme.

A Novel Protection Method for Single Line-to-Ground Faults in Ungrounded Low-Inertia Microgrids

This paper proposes a novel protection method for single line-to-ground (SLG) faults in ungrounded low-inertia microgrids. The proposed method includes microgrid interface protection and unit protection. The microgrid interface protection is based on the difference between the zero-sequence voltage angle and the zero-sequence current angle at the microgrid interconnection transformer for fast selection of the faulty feeder. The microgrid unit protection is based on a comparison of the three zero-sequence current phase directions at each junction point of load or distributed energy resources. Methods are also included to locate the minimum fault section. The fault section location technology operates according to the coordination of microgrid unit protection. The proposed method responds to SLG faults that may occur in both the grid and the microgrid. Simulations of an ungrounded low-inertia microgrid with a relay model were carried out using Power System Computer Aided Design (PSCAD)/Electromagnetic Transients including DC (EMTDC).

Microgrid Ramp Rates and the Inertial Stability Margin

Variable power sources are becoming increasingly common in low-inertia microgrids. Inadequate amounts, however, can disrupt electric service if ramped events occur too quickly. The authors introduce an approach to predict how long microgrids can withstand ramped events as a function of local inertia. Although microgrids can ride through disturbances using local inertia and other technologies, it is not clear for how long disturbances can be tolerated—particularly when in progress. The estimated reaction times are presented, first, as stability margins to illustrate how ramp-rate magnitudes relate to local inertia. Then, use of the reaction times is demonstrated as a forward-looking capability to anticipate frequency deviation times as a microgrid model undergoes large solar ramp rates. It is shown that the available remaining times can be estimated even as disturbances and remedial actions take place.