Frequency Stability Of Microgrid Research Articles

Robust control method to improve microgrid frequency stability in islanded operation

Islanded microgrids have low inertia due to their small size as well as integration of inverter-based renewable energy resources. Hence frequency stability is one main concern regarding their operation. Load step pre-announcement and a bang–bang controller were proposed and applied in a test islanded microgrid, consisting of a conventional generator, a photovoltaic (PV) generator and a lumped load, to improve the frequency response. In this study, a possible control architecture for the proposed method is illustrated. A sensitivity analysis is conducted to determine the influence of two parameters, preset and total time, on the performance of the proposed control method. Furthermore, the effectiveness of the implementation of load step pre-announcement and bang–bang controller in the test microgrid with different shares of PV is evaluated through simulation results.

Load frequency control enhancement of islanded micro-grid considering high wind power penetration using superconducting magnetic energy storage and optimal controller

This article proposes a robust load frequency control using a new optimal proportional–integral–derivative controller–based genetic moth swarm algorithm for islanded microgrids considering high wind power penetration. In such microgrids, the replacement of conventional generator units with a large number of renewable energy sources reduces the system inertia, which in turn causes undesirable influence on microgrid frequency stability, leading to weakening of the microgrid. Furthermore, sudden load shedding, load restoring, and short circuits caused large frequency fluctuations which threaten the system security and could lead to complete blackouts as well as damages to the system equipment. In order to solve this challenge, this study proposes a new coordinated optimal load frequency control plus modified control signal to superconducting magnetic energy storage for compensating the microgrid frequency deviation (∆ f). To prove the effectiveness of the proposed coordinated control strategy, an islanded microgrid was tested for the MATLAB/Simulink simulation. The physical constraints of the turbines such as generation rate constraints and speed governor dead band are considered in this study. The results confirmed the effectiveness and robustness of the proposed coordination performance against all scenarios of different load profiles, wind power fluctuation, and system uncertainties in microgrid integrated with high penetration of wind farms. Moreover, the results have been compared with both: the optimal load frequency control with/without the effect of conventional superconducting magnetic energy storage.

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.

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.

Microgrid dynamic security considering high penetration of renewable energy

This paper presents a coordination strategy of Load Frequency Control (LFC) and digital frequency protection for an islanded microgrid (MG) considering high penetration of Renewable Energy Sources (RESs). In such MGs, the reduction in system inertia due to integration of large amount of RESs causes undesirable influence on MG frequency stability, leading to weakening of the MG. Furthermore, sudden load events, and short circuits caused large frequency fluctuations, which threaten the system security and could lead to complete blackouts as well as damages to the system equipment. Therefore, maintaining the dynamic security in MGs is one of the important challenges, which considered in this paper using a specific design and various data conversion stages of a digital over/under frequency relay (OUFR). The proposed relay will cover both under and over frequency conditions in coordination with LFC operation to protect the MG against high frequency variations. To prove the response of the proposed coordination strategy, a small MG was investigated for the simulation. The proposed coordination method has been tested considering load change, high integration of RESs. Moreover, the sensitivity analysis of the presented technique was examined by varying the penetration level of RESs and reducing the system inertia. The results reveal the effectiveness of the proposed coordination to maintain the power system frequency stability and security. In addition, the superiority of the OUFR has been approved in terms of accuracy and speed response during high disturbances.

Digital coordination strategy of protection and frequency stability for an islanded microgrid

This study presents a digital coordination strategy of load frequency control (LFC) and Over/Under Frequency Relay (OUFR) protection for an isolated microgrid (MG) considering high penetration of renewable energy sources (RESs). In such MGs, the reduction in system inertia due to the replacement of traditional generating units with a large amount of RESs causes undesirable influence to MG frequency stability, leading to weakening of the MG. Furthermore, sudden load change and short circuits caused large frequency fluctuations which threaten the system security. In order to handle these challenges, this study proposes a specific design of the digital OUFR, which will operate for both conditions of over and under frequency in coordination with digital PID controller based on mapping technique in discretization process to protect the MG against high-frequency variations. To prove the effectiveness of the proposed digital coordination strategy, a small MG was investigated for the simulation considering load change, varying the penetration level of RESs and the system inertia. The results reveal the robustness of the proposed coordination to maintain the MG frequency stability and security. In addition, the superiority of the digital OUFR has been approved in terms of accuracy and speed response during high disturbances.

Robust Virtual Inertia Control of an Islanded Microgrid Considering High Penetration of Renewable Energy

This paper presents robust virtual inertia control of an islanded microgrid considering high penetration of renewable energy sources (RESs). In such microgrids, the lack of system inertia due to the replacement of traditional generating units with a large amount of RESs causes undesirable influence to microgrid frequency stability, leading to weakening of the microgrid. In order to handle this challenge, the $H_{\mathbf {\infty }}$ robust control method is implemented to the virtual inertial control loop, taking into account the high penetration of RESs, thus enhancing the robust performance and stability of the microgrid during contingencies. The controller’s robustness and performance are determined along with numerous disturbances and parametric uncertainties. The comparative study between $H_{\mathbf {\infty }}$ and optimal proportional-integral (PI)-based virtual inertia controller is also presented. The results show the superior robustness and control effect of the proposed $H_{\mathbf {\infty }}$ controller in terms of precise reference frequency tracking and disturbance attenuation over the optimal PI controller. It is validated that the proposed $H_{\mathbf {\infty }}$ -based virtual inertia controller successfully provides desired robust frequency support to a low-inertia islanded microgrid against high RESs penetration.

Coordinated sectional droop charging control for EV aggregator enhancing frequency stability of microgrid with high penetration of renewable energy sources

For microgrids with high penetration of renewable energy sources (RESs) and electric vehicles (EVs), the stochastic charging/discharging of the EVs would result in a large impact on the secure and stable operation of the microgrids. Therefore, the coordinated control between EVs and RESs becomes an important challenge for keeping the microgrid stable. In this paper, a coordinated sectional droop charging control (CSDCC) strategy is proposed for an EV aggregator that participates in the frequency regulation of the microgrids with high penetration RESs. All EVs are controlled as grid-friendly loads, and the CSDCC strategy operates as a virtual synchronous generator by only controlling the charging power of the EVs. Because the discharging of an EV is not required, the CSDCC strategy has no detrimental effects on the EV battery life. The inertia damping characteristic of a synchronous generator is modelled as a virtual inertia factor, which can eliminate charging power vibration, and can improve the system inertia. Finally, the validity of the proposed strategy in enhancing frequency regulation is verified by demonstrating a set of comparative cases.

Coordination of EVs Participation for Load Frequency Control in Isolated Microgrids

Increasing the penetration levels of renewable energy sources (RESs) in microgrids (MGs) may lead to frequency instability issues due to intermittent nature of RESs and low inertia of MG generating units. On the other hand, presence of electric vehicles (EVs), as new high-electricity- consuming appliances, can be a good opportunity to contribute in mitigating the frequency deviations and help the system stability. This paper proposes an optimal charging/discharging scheduling of EVs with the goal of improving frequency stability of MG during autonomous operating condition. To this end, an efficient approach is applied to reschedule the generating units considering the EVs owners’ behaviors. An EV power controller (EVPC) is also designed to determine charge and discharge process of EVs based on the forecasted day-ahead load and renewable generation profiles. The performance of the proposed strategy is tested in different operating scenarios and compared to those from non-optimized methodologies. Numerical simulations indicate that the MG performance improves considerably in terms of economy and stability using the proposed strategy.

Dynamic reverse droop power sharing in microgrid based on neural networks

This paper presents a dynamic reverse droop power-sharing scheme to control frequency of micro grids based on neural networks. Micro grid is included undispatchable distributed generation (DG) such as wind turbine and dispatchable voltage source converter DG which is considered as auxiliary generation. Power sharing control strategies of distributed generation (DG) units without communication are based on the droop concept; while for the DG units in current control mode that applied in most renewable resources such as the wind, the droop control may not be implemented directly since the output of droop control is voltage amplitude and frequency. Also, the droop coefficients are constant and determined as a function of DG unit capacity. However, the generation capacity of the wind turbine is not constant. Therefore, in the proposed scheme first, the reverse droop method is provided for the wind turbine power sharing as its outputs are active and reactive power reference. Then, the dynamic coefficients are modified as a function of generalized regression neural networks. Finally, the control strategy is presented for coordinate control of wind turbine and auxiliary generator (AG) to ensure frequency stability of micro grid. The proposed control strategy is validated through extensive simulation results using MATLAB/Simulation software.

Game-theoretic approach to cooperative control of distributed energy resources in islanded microgrid considering voltage and frequency stability

A microgrid (MG) comprises a low-voltage network with several microsources, critical and noncritical loads, and energy storage systems (ESSs). It can operate in the grid-connected or islanded modes. In islanded mode, the voltage and frequency of the MG should be controlled by different distributed energy resources (DERs). This paper focuses on the analysis of frequency stability in an autonomous MG with renewable energy sources. In this paper, colonial competitive algorithms are used to design the DERs controllers in a cooperative manner that these controllers can keep the MG stable. Artificial neural network tool trained by Levenberg---Marquardt algorithm is used to generate controlling signal for every controller to keep the frequency and voltage in a desired range. This paper investigates a new optimal control solution for maintaining the frequency stability in the MG, by using a combination of an ESS and load-shedding procedure. The cooperative game theory is used in this paper to model the interaction among different DERs, ESSs, and loads.

Cooperative Control to Enhance the Frequency Stability of Islanded Microgrids with DFIG-SMES

The issue of frequency stability of microgrids under islanded operation mode and mode transfer has attracted particular attention recently. In this paper, a cooperative frequency control method, which consists of a microgrid central control (MGCC) and microgrid local control (MGLC), is proposed to achieve a seamless transfer from grid-connected to islanded mode, and hence increase the frequency stability of islanded microgrids during both primary and secondary frequency control. A power deficiency prediction and distribution method is proposed in MGCC to effectively distribute and utilize the power and loads, and accomplish the cooperative control of all microgrid units. With regards to MGLC, a Hopfield fuzzy neural network control (HFNNC) is applied to make the corresponding frequency control of DFIG-SMES more adaptive. Meanwhile a state of capacity (SOC) control is utilized in battery energy storage (BES) to extend battery life. Simulation results indicate that the proposed frequency control approach can maintain the frequency stability of islanded microgrids even in emergency conditions.

Stability Assessment of Isolated Micro-grids Powered by Distributed Combined Heat and Power Micro-units

AbstractIt is expected that combined heat and power (CHP) micro-units will soon play a significant role in the energy supply of private households. To increase the reliability of the system the isolated operation of this micro-grid should be possible. It is assumed that the electrical load is the reference input variable of the local grid to reduce the necessity of electrical energy storages. The produced thermal energy will be stored. The paper presents a study of the voltage and frequency stability in micro-grids powered by CHP micro-units. The simulation model of the micro-co-generation plant is based on a combustion engine and brush-less synchronous generator. The example network represents an urban residential neighbourhood. The electrical household loads are based on probabilistic modelling, with high time resolution. Several scenarios for normal and disturbed operation are defined, which vary in electrical load, number and size of the CHP units and the generation schedule. For these scenarios the stability and voltage limits during normal operation and for selected three phase faults are investigated. The voltage and frequency stability of the micro-grid is determined by dynamic simulations.