Frequency Response Of Microgrid Research Articles

Stabilized frequency response of a microgrid using a two-degree-of-freedom controller with African vultures optimization algorithm

Nowadays, there is extensive penetration of renewable energy sources (RESs) in microgrids such as solar power stations (SPS) and wind power stations (WPS). The RESs are power electronic converter-dominated systems that have zero inertia making the microgrid to have very low inertia. Low inertia microgrid has a high rate of change of frequency (RoCoF), and the frequency response is highly volatile. To cope with this issue virtual inertia and damping are emulated into the microgrid. Virtual inertia and damping, i.e., converter with short-term energy storage device (ESD), which delivers and absorbs electrical power depending on the frequency response of microgrid and minimizes the power variation between power generation and power consumption. In this paper virtual inertia and damping are emulated based on a novel two-degree of freedom PID (2DOFPID) controller optimized with African vultures optimization algorithm (AVOA) technique. The meta-heuristic technique, AVOA, tunes the gains of the 2DOFPID controller and also the inertia and damping gain of the virtual inertia and damping control (VIADC) loop. AVOA comes out to be superior to other optimization techniques when compared in terms of convergence rate and quality. The performance of the proposed controller is compared to other conventional control methodology that has demonstrated its better performance. The dynamic response of such a proposed methodology in a microgrid model is verified in an OPAL-RT real-time environmental simulator, i.e., OP4510.

On frequency response modeling of interconnected microgrids: Two different perspectives

This paper deals with the frequency response modeling of interconnected microgrids. The frequency response model is used for frequency and tie-line power control analysis and synthesis. The proposed frequency response model is obtained using two different perspectives: (1) based on frequency sensitivity analysis, (2) based on the inertia concept. Besides, the dynamic of back-to-back converters and controllable active power sources are also included to obtain a generalized frequency response model with desirable performance. Indeed, it can be observed that in the first method, some circuit parameters can be neglected in the frequency response model. This finding is effective in providing a more simplified model. In the second method, the frequency response model has been presented as an innovative approach. In a way, we have selected the nature of distributed generations as an indicator of source classification in the model. This model is also enhanced by considering the inertia dynamics and controllable power source and has a better performance to show the network status. Since the microgrid frequency response model only covers the low-frequency dynamics, the fast dynamics of inner control loops as well as the modulation units and sensors are not considered. The proposed frequency response model can be used for frequency stability analysis studies as well as frequency and tie-line power control analysis and synthesis. Comprehensive simulation studies are carried out using the MATLAB simulation environment to compare and evaluate the developed models. Further, the proposed models are verified via PSCAD/EMTDC, considering two microgrids interconnected through back-to-back converters. The results showed that the proposed models are suitable and efficient for frequency response studies.

Frequency response enhancement of an AC micro-grid has renewable energy resources based generators using inertia controller

Since low inertia has drawbacks on the frequency response and stability of AC micro-grids, availability of sufficient inertia with high penetration of renewable energy sources, has recently become a topic of wide concern. To tackle the problems of low inertia operation, most research is focused on using of an energy storage system (ESS) or de-loading of the renewable generation units. However, these techniques have economic problems due to investment in extra elements or reduction of the system productivity. A new inertia controller that depends on the inherent system's elements, such as the kinetic energy stored in wind turbine and the high bandwidth of interface converter, is proposed. The controller is applied on an AC micro-grid consisting of a thermal power plant and a doubly-fed induction generator based wind turbine. To ensure the validity of the proposed technique, different simulation cases such as load disturbance, wind speed variation, and three-phase fault are performed using MATLAB/ Simulink platform. The results are compared with the responses of two other AC micro-grids operate under the same conditions. The suggested controller successfully enhances the micro-grid frequency response with less frequency nadir and less rate of change of frequency. Eventually, the mathematical description of the proposed controller effect on the inertia of the WECS and the inertia of the micro-grid is deduced.

Distributed Energy Resources Based Microgrid: Review of Architecture, Control, and Reliability

To accomplish feasible large-scale integration of distributed energy resources (DER) into the existing grid system, microgrid implementation has proven to be the most effective. This article reviews the vital aspects of DER based microgrid and presents simulations to investigate the impacts of DER sources, electric vehicles (EV), and energy storage system (ESS) on practicable architectures' resilient operation. The focus is primarily on the concept and definition of microgrid, comparison of control strategies (primary, secondary, and tertiary strategies), energy management strategies, power quality (PQ) issues associated with DER based microgrid, and state-of-the-art entities such as ESS and EV's applications toward microgrid reliability. Following discussion on the different attributes of DER sources-based microgrid, simulations are performed to verify the results of the past works on the effects of solar, wind energy sources, ESS, and EVs on the microgrid frequency response. Additional simulations are conducted to assess the influences of DERs, ESS, EVs, and their operational strategies on the microgrid reliability aspects.

Modified Deloading Strategy of Wind Turbine Generators for Primary Frequency Regulation in Micro-Grid

In this work, the contribution of wind turbine generator (WTG) to support micro-grid (MG) during depressed frequency condition has been studied with a modified strategy for improving the primary load frequency response of MG. The majority of existing deloading techniques to estimate the reference power are based on the linear relationship. Hence they are not so accurate to mimic the actual speed versus power dynamics of turbine rotor during deloading operation which is inherently nonlinear in nature. In this work, the existing methods have been analyzed in detail with an aim to improve the performance. Based on the analysis a modified deloading method assuming non-linear relation between rotor speed and power of WTG system has been proposed which shows the improved load frequency response in MG. The proposed deloading technique is simulated and validated using real-time digital simulator (OP4510) for the variation in load and generation inside an islanded MG. The results obtained using modified deloading method are compared with existing deloading method and it is found that proposed deloading method handles the non-linearity of the system during deloading operation and also it contributes more power to MG in response to load demand at the cost of slight increase in the speed of turbine rotor.

Primary Frequency Response of Microgrid Using Doubly Fed Induction Generator With Finite Control Set Model Predictive Control Plus Droop Control and Storage System

This paper presents a new methodology for primary frequency response (PFR) in a microgrid through the finite control set-model predictive control (FCS-MPC) plus droop control applied to the grid side converter (GSC) of a doubly fed induction generator (DFIG). In this configuration, the rotor side converter (RSC) is responsible for maintaining wind turbine operation at the maximum power point (MPP) extraction, even at the time of a disturbance, while the GSC is responsible for processing the power required to reestablish the microgrid frequency at its rated value. The power required for frequency control comes from a battery energy storage system (BESS) connected to the DC-link, and its value is selected via the FSC-MPC by continuously adjusting the droop gain value. This control configuration has considerable benefits such as continuous operation at the MPP extraction, injection of power proportional to the frequency imbalance, the capability to impose restrictions through the control and it does not use any type of communication between the storage system and the control. Through the FCS-MPC, the gain of the droop controller is selected, which maximizes the power needed to control the frequency of the microgrid. To verify the performance of the proposed control strategy, simulations are performed for an unexpected islanding of the microgrid under different wind speed scenarios. The results show that the DFIG equipped with the proposed control strategy is able to provide ancillary services such as PFR in all DFIG operating modes.

Improvement of Transient Response in Microgrids Using Virtual Inertia

Generation is shifting from a centralized power generating facility having large synchronous generators to distributed generation involving sources of smaller capacity. Most of these sources require inverters on the front end while being connected to the grid. Lower available kinetic energy, coupled with less short-circuit current ratio compared to large synchronous generators, compromises the transient stability of the microgrid when isolated from the main grid. Sources in the microgrid use droop control to share power according to their capacity without any form of communication. This paper proposes a novel controller for inverters to improve the frequency response of microgrid under disturbances involving large frequency deviations. It also discusses design of various parameters defined for the proposed control. The microgrid, which has two inverters and two synchronous generators, is simulated using Simulink/MATLAB software to test the proposed control strategy.

Biogeography Based Optimal State Feedback Controller for Frequency Regulation of a Smart Microgrid

Development of “Q” and “R” matrices for designing a Linear Quadratic Regulator (LQR) is still a research challenge. The theory says they should belong to the group of positive definite matrices, so we need to find out the most suitable amongst them in order to obtain the desired response. In this paper biogeography based optimization (BBO) technique has been applied to come up with the best “Q” and “R” matrices such that the frequency excursion following a disturbance in a microgrid is minimized. As all the states in a practical system may not be measurable hence, we have used Kalman estimator to estimate them. These estimated states along with other measured states are used by the LQR to produce the desired control signal. The microgrid is made smarter by using the agent based scheme integrated with a master controller and a proper communication protocol. The simulation results show that the proposed approach improves the microgrid frequency response and also gives a new alternative method for frequency control of a smart microgrid.