Frequency Regulation In Microgrids Research Articles

Distributed model predictive based secondary control for economic production and frequency regulation of MG

This work focuses on Distributed Secondary Control (DSC) technique, for frequency regulation and Economic Load Dispatch of Microgrid (MG). The fluctuating nature and large quantity of Distributed Energy Resources (DER) in an autonomous MG result in complex control requirements, demanding fast and robust response. The contemporary DSC schemes are mostly based on Distributed Averaging Integration techniques, with slow response. This paper proposes Distributed Model Predictive based Secondary Control (DMPSC) which effectively complies with the control requirements of MG. DMPSC requires each DER-node to solve a local optimization problem with the cost function penalizing the deviation of states from their desired values and the differences between the assumed and predicted values. The desired-states are based on local intermediate-optimum values, computed using local and neighbouring information. Equality based terminal constraints are introduced to ensure the stability, where each node is forced to reach the desired-state value at the end of prediction horizon. The terminal-consensus of the network affirms convergence of the desired-states to a global optimal point of the network. The asymptotic stability of the proposed control is proved by using the sum of local cost-functions as a candidate Lyapunov function. Simulation results validate the effectiveness of the proposed control scheme.

Improving the autonomy of islanded microgrids through frequency regulation

In stand-alone operation, microgrids are susceptible to potential shortages in generation capacity, requiring a consideration of all technically feasible opportunities to conserve energy in order to keep the islanded grid operational for as long as possible. In this sense, the network autonomy capacity must be considered in the islanded microgrid frequency control formulation, alongside the traditional concern related to the accomplishment of the dynamic frequency regulation requirements. This paper proposes a new outlook for secondary frequency regulation in islanded microgrids denoted as conservation frequency reduction (CFR). The proposed approach exploits the frequency dependency characteristics of microgrids by intentionally reducing the reference setpoint of the islanded network frequency in a controllable way in order to decrease the network’s demand and improve the autonomy duration of islanded microgrids, i.e., availability to supply demand in islanded mode; while ensuring the system operation within permissible limits. The results indicate that the proposed approach is able to significantly enhance islanded microgrids autonomy capacity while guaranteeing its frequency of operation within satisfactory dynamic and steady-state limits.

Distributed Predictive Control for Frequency and Voltage Regulation in Microgrids

Distributed control schemes have transformed frequency and voltage regulation into a local task in distributed generators (DGs) rather than by a central secondary controller. A distributed scheme is based on information shared among neighboring units; thus, the microgrid performance is affected by issues induced by the communication network. This paper presents a distributed predictive control applied to the secondary level of microgrids. The model used for prediction purposes is based on droop and power transfer equations; however, communication features, such as connectivity and latency, are also included, thus making the controller tolerant to electrical and communication failures. The proposed controller considers the frequency and voltage regulation control objectives and consensus over the real and reactive power contributions from each power unit in the microgrid. The experimental and simulation results show that the proposed scheme (i) responds properly to load variations, working within operating constraints, such as generation capacity and voltage range; (ii) maintains the control objectives when a power unit is disconnected and reconnected without any user updating in the controllers; and (iii) compensates for the effects of communication issues over the microgrid dynamics.

Control strategies for frequency regulation in microgrids: A review

The electric power generation over the past decade has moved from conventional fossil fuel-fired thermal power plants to tiny-scale system generating power through distributed generation units. A group of such distributed generation units and loads are termed as microgrids. Microgrids can be located near the load centers to supply the load without any loss of power. Frequency regulation in a microgrid operating in autonomous mode is critical because of the intermittent nature of the renewable sources employed. To maintain the frequency regulation within a tolerance limit in a microgrid, proper control schemes have to be adopted in order to increase or decrease the real power generation. Hence, this article explores and presents a critical review of different types of control strategies employed for frequency regulation in microgrids.

Improvement of Frequency Regulation in VSG-Based AC Microgrid Via Adaptive Virtual Inertia

A virtual synchronous generator (VSG) control based on adaptive virtual inertia is proposed to improve dynamic frequency regulation of microgrid. When the system frequency deviates from the nominal steady-state value, the adaptive inertia control can exhibit a large inertia to slow the dynamic process and, thus, improve frequency nadir. And when the system frequency starts to return, a small inertia is shaped to accelerate system dynamics with a quick transient process. As a result, this flexible inertia property combines the merits of large inertia and small inertia, which contributes to the improvement of dynamic frequency response. The stability of the proposed algorithm is proved by Lyapunov stability theory, and the guidelines on the key control parameters are provided. Finally, both hardware-in-the-loop and experimental results demonstrate the effectiveness of the proposed control algorithm.

Practical Evaluation of Droop and Consensus Control of Distributed Electric Springs for Both Voltage and Frequency Regulation in Microgrid

Electric spring (ES) was originally proposed as a distributed demand-side management (DSM) technology for stabilizing power distribution network in the presence of intermittent power generation without using communication. This paper explores the practical use of consensus control for a cluster of ESs through a WiFi communication layer for new functions not previously realized in practice. This approach can be considered as a form of DSM for smart grid technology. A novel consensus control is introduced to enable distributed ES circuits to provide local voltage and system frequency regulations in a microgrid with shared responsibility of active and reactive power compensation. The practical implementation details of consensus control for a cluster of ESs are addressed. New plug-and-play functions of ESs are practically demonstrated for the first time under consensus control. Practical results indicate that droop control (without communication) and consensus control (with communication) are complementary. Under normal condition when the communication network is available, distributed ESs can perform with shared power compensation efforts based on consensus control. If the communication network fails, ESs can revert to perform under droop control.

Frequency Regulation in AC Microgrid with and without Electric Vehicle Using Multiverse-Optimized Fractional Order- PID controller

Frequency Regulation in AC Microgrid with and without Electric Vehicle Using Multiverse-Optimized Fractional Order- PID controller

D-PMU Based Secondary Frequency Control for Islanded Microgrids

This paper proposes a secondary frequency control approach for distributed energy resources (DERs) in networks operating in stand-alone mode; i.e., islanded microgrids. The proposed approach takes advantage of the high resolution, low latency, and time-stamped synchronized measurements of the distribution-level phasor measurement units (D-PMUs) to iteratively adjust the generators contribution, considering the D-PMUs derived active power information, in a way that improves the dynamics of the islanded microgrid frequency regulation. These adjustments are performed using a novel adaptive time-variable droop characteristic capable of significantly speeding-up the secondary frequency regulation, mitigating oscillations, and reducing the frequency nadir. The proposed secondary frequency control can be held after the stabilization of the primary control, as well as simultaneously with the primary frequency regulation. Results show the effectiveness of the proposed approach in significantly improving the frequency regulation of islanded microgrids.

Frequency regulation in islanded microgrid considering stochastic model of wind and PV

Frequency regulation in islanded microgrid considering stochastic model of wind and PV

Improving Microgrid Frequency Regulation Based on the Virtual Inertia Concept while Considering Communication System Delay

Frequency stability is an important issue for the operation of islanded microgrids. Since the upstream grid does not support the islanded microgrids, the power control and frequency regulation encounter serious problems. By increasing the penetration of the renewable energy sources in microgrids, optimizing the parameters of the load frequency controller plays a great role in frequency stability, which is currently being investigated by researchers. The status of loads and generation sources are received by the control center of a microgrid via a communication system and the control center can regulate the output power of renewable energy sources and/or power storage devices. An inherent delay in the communication system or other parts like sensors sampling rates may lead microgrids to have unstable operation states. Reducing the delay in the communication system, as one of the main delay origins, can play an important role in improving fluctuation mitigation, which on the other hand increases the cost of communication system operation. In addition, application of ultra-capacitor banks, as a virtual inertial tool, can be considered as an effective solution to damp frequency oscillations. However, when the ultra-capacitor size is increased, the virtual inertia also increases, which in turn increases the costs. Therefore, it is essential to use a suitable optimization algorithm to determine the optimum parameters. In this paper, the communication system delay and ultra-capacitor size along with the parameters of the secondary controller are obtained by using a Non-dominated Sorting Genetic Algorithm II (NSGA-II) algorithm as well as by considering the costs. To cover frequency oscillations and the cost of microgrid operation, two fitness functions are defined. The frequency oscillations of the case study are investigated considering the stochastic behavior of the load and the output of the renewable energy sources.

Distributed SMC for Secondary Voltage and Frequency Regulation of AC Microgrid Adulterated by Communication Irregularities

Distributed SMC for Secondary Voltage and Frequency Regulation of AC Microgrid Adulterated by Communication Irregularities

Voltage and Frequency Regulation of Microgrid With Battery Energy Storage Systems

This paper presents a novel primary control strategy based on output regulation theory for voltage and frequency regulations in microgrid systems with fast-response battery energy storage systems (BESS). The proposed control strategy can accurately track voltage and frequency set points while mitigating system transients in the presence of disturbance events. Therefore, it overcomes the key weaknesses of droop-based control methods such as large steady-state voltage and frequency deviations and poor transient performance. Throughout this paper, four control schemes are derived with tradeoffs between communication requirement and system dynamic performance. Their effectiveness is validated through MATLAB Simulink simulation studies involving a medium-voltage microgrid with both synchronous generation resources and BESS. Although the proposed control schemes are centralized, practical implementation is possible with available communication links in microgrids and embedded hardware technologies.

A Novel Method of Frequency Regulation in Microgrid

Owing to generation and demand mismatch, the frequency deviation and the rate of change of frequency (RoCoF) may become drastic at times in an islanded microgrid. This is due to increased penetration of noninertial renewable energy sources that brings down the system's resilience to any kind of disturbance. Therefore, efficient control algorithms are required to mimic the inertial behavior of a conventional synchronous machine to arrest/limit any sudden change. This work focuses on frequency regulation of microgrid during transient conditions by means of fast-responding external energy reserve. The characteristics of a weak grid has been studied and simulated through modeling a virtual synchronous machine. An inverter model is also developed to integrate the energy storage system (ESS) to the grid and a double second-order generalized integrator phase locked loop (DSOGI-PLL) is implemented that is capable of synchronizing the system under distorted and unbalanced situation. A new technique to estimate the frequency of the microgrid is reported. A simple proportional controller-based approach for different level of pulsed power injection (using ultracapacitors and batteries) is proposed and simulated using MATLAB. Experimental demonstration is made using batteries only but with two different current-limits. The controller is implemented using dSPACE1103.

Dynamic Characteristics Analysis and Stabilization of PV-Based Multiple Microgrid Clusters

As the penetration of photovoltaic (PV) generation increases, there is a growing operational demand on PV systems to participate in microgrid frequency regulation. It is expected that future distribution systems will consist of multiple microgrid clusters. However, interconnecting PV microgrids may lead to system interactions and instability. To date, no research work has been done to analyze the dynamic behavior and enhance the stability of microgrid clusters considering the dynamics of the PV primary sources and dc links. To fill this gap, this paper presents comprehensive modeling, analysis, and stabilization of PV-based multiple microgrid clusters. A detailed small-signal model for PV-based microgrid clusters considering local adaptive dynamic droop control mechanism of the voltage-source PV system is developed. The complete dynamic model is then used to access and compare the dynamic characteristics of the single microgrid and interconnected microgrids. In order to enhance system stability of the PV microgrid clusters, a tie-line flow and stabilization strategy is proposed to suppress the introduced interarea and local oscillations. Robustly selecting of the key control parameters is transformed to a multiobjective optimization problem which is solved by genetic algorithm. The proposed damping controller can effectively damp the power oscillations and provide robust control performance under variable operating conditions. Theoretical analysis, simulation results under various scenarios are presented to verify the effectiveness of the proposed scheme.

Enhanced Virtual Inertia Control Based on Derivative Technique to Emulate Simultaneous Inertia and Damping Properties for Microgrid Frequency Regulation

Virtual inertia control is considered as an important part of microgrids with high renewable penetration. Virtual inertia emulation based on the derivative of frequency is one of the effective methods for improving system inertia and maintaining frequency stability. However, in this method, the ability to provide virtual damping is usually neglected in its design, and hence, its performance might be insufficient in the system with low damping. Confronted with this issue, this paper proposes a novel design and analysis of virtual inertia control to imitate damping and inertia properties simultaneously to the microgrid, enhancing frequency performance and stability. The proposed virtual inertia control uses the derivative technique to calculate the derivative of frequency for virtual inertia emulation. Trajectory sensitivities have been performed to analyze the dynamic impacts of the virtual inertia and virtual damping variables over the system performance. Time-domain simulations are also presented to evaluate the efficiency of the virtual damping and virtual inertia in enhancing system frequency stability. Finally, the efficiency and robustness of the proposed control technique are compared with the conventional inertia control under a wide range of system operation, including the decrease in system damping and inertia and high integrations of load variation and renewable energy.

Hybrid energy storage system for microgrids applications: A review

Abstract Energy storages introduce many advantages such as balancing generation and demand, power quality improvement, smoothing the renewable resource’s intermittency, and enabling ancillary services like frequency and voltage regulation in microgrid (MG) operation. Hybrid energy storage systems (HESSs) characterized by coupling of two or more energy storage technologies are emerged as a solution to achieve the desired performance by combining the appropriate features of different technologies. A single ESS technology cannot fulfill the desired operation due to its limited capability and potency in terms of lifespan, cost, energy and power density, and dynamic response. Hence, different configurations of HESSs considering storage type, interface, control method, and the provided service have been proposed in the literature. This paper comprehensively reviews the state of the art of HESSs system for MG applications and presents a general outlook of developing HESS industry. Important aspects of HESS utilization in MGs including capacity sizing methods, power converter topologies for HESS interface, architecture, controlling, and energy management of HESS in MGs are reviewed and classified. An economic analysis along with design methodology is also included to point out the HESS from investor and distribution systems engineers view. Regarding literature review and available shortcomings, future trends of HESS in MGs are proposed.

Analysis of the Effect of Clock Drifts on Frequency Regulation and Power Sharing in Inverter-Based Islanded Microgrids

Local hardware clocks in physically distributed computation devices hardly ever agree because clocks drift apart and the drift can be different for each device. This paper analyzes the effect that local clock drifts have in the parallel operation of voltage source inverters (VSIs) in islanded microgrids (MG). The state-of-the-art control policies for frequency regulation and active power sharing in VSIs-based MGs are reviewed and selected, and prototype policies are then reformulated in terms of clock drifts. Next, steady-state properties for these policies are analyzed. For each of the policies, analytical expressions are developed to provide exact quantification of the impact that drifts have on frequency and active power equilibrium points. In addition, a closed-loop model that accommodates all the policies is derived, and the stability of the equilibrium points is characterized in terms of the clock drifts. Finally, the implementation of the analyzed policies in a laboratory MG provides experimental results that confirm the theoretical analysis.

Resilient distributed optimal generation dispatch for lossy AC microgrids

In this paper, we consider the problem of designing a distributed algorithm, resilient against communication delays and packet drops, for coordinating the response of distributed energy resources (DERs) in AC microgrids so as to minimize electrical line losses and generation cost, and to ensure that the microgrid network constraints are satisfied. The proposed algorithm can be utilized as a secondary or tertiary controller for frequency regulation in islanded microgrids, or to coordinate DERs for providing frequency regulation services to the bulk grid when microgrid is in grid-connected mode. We validate the practical usefulness of the theoretical results through numerical simulations involving the IEEE 39-bus system.

Dynamic Droop Control for Wind Turbines Participating in Primary Frequency Regulation in Microgrids

Wind power will provide a significant portion of electricity generation in the near future. This significant role requires wind power generators to contribute to the system frequency regulation. The droop method is one of the most popular methods to be implemented in these generators to mimic the governors of conventional generators and contribute to both transient and steady-state frequency regulation. However, the unpredictability of the variable wind speed complicates this implementation. In the present wind-based droop methods, the maximum allowable droop gain is a function of the wind speed. This dependency means either that the entire available capacity of the wind generator will not be used or that instability will threaten the implemented droop wind generators. This paper proposes the efficiency droop, a new droop-based method, which can be tuned regardless of the wind speed. Small-signal analyses are used to study the method in depth and compare its influences on both the transient and steady-state frequency performance to the influence of the present methods while adopting minimum approximation. Detailed time domain analyses are used to verify the analytical results.

Unified Probabilistic Modeling of Wind Reserves for Demand Response and Frequency Regulation in Islanded Microgrids

Islanded microgrids provide unique challenges due to the lack of transmission support, requiring local energy supply and grid regulation architecture. This paper presents a unified probabilistic assessment of wind reserves for demand response (DR) and frequency regulation in islanded microgrids. A multivariate nonparametric kernel density estimation algorithm is used to generate the probabilistic models of the wind resource, electrical demand, and predicted performance of wind generation. These models are numerically combined to evaluate the capability of wind generation to act as a dynamic reserve and predict its performance for DR, secondary generation, and frequency regulation in an islanded system. The probabilistic model captures multivariate cross-correlation, nonstationary environmental and load behavior, as well as multimodality in their underlying probability distributions. A case study is conducted to validate the proposed model, which predicts wind generation effectiveness for varying load profiles, wind profiles, and generation capacities. PLEXIM simulation software is used to implement a model microgrid to demonstrate the integration of wind generation and its regulatory capabilities. The proposed algorithm has applications in power system planning and operation, and it provides a method using probabilistic data set for long term energy management and optimization of islanded microgrids.