Inertial Response Research Articles

Verification of power hardware-in-the-loop environment for testing grid-forming inverter

Grid-forming inverters (GFMIs) are promising technologies that can replace some of the capabilities traditionally provided by synchronous generators (SGs), such as inertial response. While there have been many simulation-based studies on GFMIs, performance testing has not been adequately discussed. Power hardware-in-the-loop (PHIL) simulation is an attractive option for testing GFMIs. The interaction between GFMI and power systems can be observed under a variety of conditions, including low-inertia and contingency conditions. Since PHIL testing was primarily used for grid-following inverters (GFLIs), the PHIL configuration needs to be adjusted to test GFMIs, which respond faster to changes in grid voltage than GFLIs. This paper proposes the PHIL setup for testing GFMIs. It utilizes the PHIL interface developed for testing GFLIs and adjusts it for testing GFMIs. The stability and accuracy of the PHIL testing are evaluated in terms of frequency stability in low-inertia power systems by comparing test and simulation results.

Wind Turbine Load Optimization Control Strategy Based on LIDAR Feed-Forward Control for Primary Frequency Modulation Process with Pitch Angle Reservation

Because wind power is connected to the grid on a large scale, frequency fluctuation in the power grid, which is defined as a system safety risk to the power grid, occurs from time to time. According to the grid code rules of China, wind turbines are required to be equipped with primary frequency modulation or inertia response control capability, which are used to support the safe and stable operation of the power grid. During the traditional frequency modulation process of the wind turbine, power limiting operation or pitch angle reservation is generally adopted to ensure that the reserved energy can be released at any time to support the frequency change in the power grid. However, the frequency support method leads to a large loss of power generation, and does not consider the coordination between mechanical load characteristics control and primary frequency modulation. In this paper, a mechanical load optimization control strategy for a wind turbine during the primary frequency modulation process, based on LIDAR (light detection and ranging) feed forward control technology, is proposed and verified. Through LIDAR feed forward control, the characteristics of incoming wind speed can be sensed in advance, with the consequence that the wind turbine can participate in, and actively control, the primary frequency modulation procedure. According to the characteristics of incoming wind, for instance the amplitude and turbulence, simultaneously, the size of the reserved pitch angle can be adjusted in real time. This kind of method, coordinating with the mechanical load of the wind turbine, can be used to reduce both the ultimate load and fatigue damage as much as possible. Finally, the mechanical load characteristics of the wind turbine with and without the control strategy are compared and studied through simulation. The research results show that the load optimization control strategy based on LIDAR feed-forward control technology can effectively reduce the fatigue and ultimate loads of the wind turbine while supporting the frequency change in the power grid; especially for the fatigue load of tower base tilt and roll bending moments, the reducing proportion will be about 4.3% and 6.3%, respectively.

Bottleneck Generator Identification and the Corresponding N-1 Frequency Security Constrained Intraday Generator Dispatch

Generators are typically dispatched in a preventive way to guarantee frequency stability in any single generator trip contingency. The largest generator outage is regarded as the severest by the conventional N-1 frequency security rule, since the contingency size is often the dominating factor in traditional systems. However, with the continuous integration of renewable energy sources and fast response devices (e.g., battery), the inertia and frequency response characteristics of generators become diversified. In such situations, the largest generator cannot be simply identified as the bottleneck generator for preventive dispatch, since the post-contingency frequency dynamic is determined by both the contingency size and the frequency response ability of the remaining system. In this paper, a system's frequency response ability (FRA) is quantitatively defined and derived as a function of system intrinsic parameters (e.g., system inertia, ramp rates of generators, load frequency relief coefficient, etc.). Then, the bottleneck generator can be identified through comparing the capacity of a unit with the maximum FRA of its corresponding remaining system. On this basis, an innovative N-1 frequency security constrained optimal intraday generator dispatch scheme is correspondingly established. Simulation results verify the superiority of the proposed method in both frequency security improvement and operation cost saving.

Assigning Shadow Prices to Synthetic Inertia and Frequency Response Reserves From Renewable Energy Sources

Modern electricity grids throughout the world, particularly in islands such as Great Britain, face a major problem on the road to decarbonisation: the significantly reduced level of system inertia due to integration of Renewable Energy Sources (RES). Given that most RES such as wind and solar are decoupled from the grid through power electronics converters, they do not naturally contribute to system inertia. However, RES could support grid stability through appropriately controlling the converters, but currently no market incentives exist for RES to provide this support. In this paper we develop a methodology to optimally clear a market of ancillary services for frequency control, while explicitly considering the participation of grid-forming and grid-following inverter-based technologies. We propose a mathematical framework that allows to compute shadow prices for ancillary services offered by a pool of diverse providers: synchronous and synthetic inertia, enhanced frequency response (e.g. from curtailed RES) and traditional primary frequency response (e.g. by thermal generators). Several case studies are run on a simplified Great Britain system, to illustrate the applicability and benefits of this pricing scheme.

Determining the Required Frequency Control Reserve and Capacity and Location of Synchronous and Virtual Inertial Resources

Replacing conventional generating units with inverter-based generation has lowered the natural inertia of power grids and the primary frequency control reserve provided by synchronous generators. Some of the most important consequences include large frequency deviations and large rates of change of frequency (RoCoF) during sudden load and generation loss. Increasing the inertia of the grid through virtual inertia and synchronous machines, and fast primary frequency response are possible solutions for these problems. This paper proposes an optimization problem to determine the capacity of ultracapacitors and synchronous condensers installed at each bus for inertial response and also the optimal amount of fast primary frequency response while considering technical and economic metrics. A combinational model is proposed to calculate the values of these technical metrics, which results in acceptable accuracy and computational cost. Finally, a modified low-inertia version of the IEEE 39-bus test system is used to verify the effectiveness of the proposed method. The results show that the method is successful in keeping the RoCoF and frequency deviations within acceptable limits while reaching a compromise between technical and economic aspects.

Observation of parallel intersystem crossing and charge transfer-state dynamics in [Fe(bpy)3]2+ from ultrafast 2D electronic spectroscopy.

Transition metal-based charge-transfer complexes represent a broad class of inorganic compounds with diverse photochemical applications. Charge-transfer complexes based on earth-abundant elements have been of increasing interest, particularly the canonical [Fe(bpy)3]2+. Photoexcitation into the singlet metal-ligand charge transfer (1MLCT) state is followed by relaxation first to the ligand-field manifold and then to the ground state. While these dynamics have been well-studied, processes within the MLCT manifold that facilitate and/or compete with relaxation have been more elusive. We applied ultrafast two-dimensional electronic spectroscopy (2DES) to disentangle the dynamics immediately following MLCT excitation of this compound. First, dynamics ascribed to relaxation out of the initially formed 1MLCT state was found to correlate with the inertial response time of the solvent. Second, the additional dimension of the 2D spectra revealed a peak consistent with a ∼20 fs 1MLCT → 3MLCT intersystem crossing process. These two observations indicate that the complex simultaneously undergoes intersystem crossing and direct conversion to ligand-field state(s). Resolution of these parallel pathways in this prototypical earth-abundant complex highlights the ability of 2DES to deconvolve the otherwise obscured excited-state dynamics of charge-transfer complexes.

Automatic Generation Control with Virtual Synchronous Renewables

As synchronous generators (SGs) are gradually displaced by renewable energy sources (RESs), the frequency stability of power systems deteriorates because RESs, represented by utility-scale solar and wind power sources, do not provide the inertial response, primary frequency response, secondary frequency response, and tertiary frequency regulation. As a result, the remaining SGs may not be sufficient to maintain the power balance and frequency stability. The concept and control strategies of virtual synchronous generators (VSGs) enable the inverter-based wind and solar power sources to emulate the outer characteristics of traditional SGs and participate in the active power and frequency control of power systems. This paper focuses on the automatic generation control (AGC) with virtual synchronous renewables (VSRs). First, the VSR strategy that enables the RESs to participate in AGC is introduced. Second, based on the interval representation of uncertainty, the output of RES is transformed into two portions, i.e., the dispatchable portion and the stochastic portion. In the dispatchable portion, the RESs can participate in AGC jointly with SGs. Accordingly, a security-constrained economic dispatch (SCED) model is built considering the RESs operating in VSR mode. Third, the solution strategy that employs the slack variables to acquire deterministic constraints is introduced. Finally, the proposed SCED model is solved based on the 6-bus and 39-bus systems. The results show that, compared with the maximum power point tracking (MPPT) mode, VSRs can participate in the active power and frequency control jointly with SGs, increase the maximum penetration level of RESs, and decrease the operating cost.

Rotating spoke frequency in E × B penning source with two-ion-species plasma

In E B sources with different degrees of magnetization between electrons and ions, collisionless Simon–Hoh instability is prone to be induced, which may result in long-wavelength and low-frequency plasma oscillations along the azimuthal direction known as the rotating spoke. Recent studies have shown that the inertial response of unmagnetized ions is a key factor that affects the rotating spoke frequency, and in-depth studies on the correlation between the ion mass and spoke frequency have been conducted for various types of E B sources (e.g. Hall thruster, magnetron and Penning sources). In the case of the E B Penning source where the generation of desired ion species is maximized through molecular gas discharge, the intrinsic characteristics of the instability dependent on the ion mass inevitably require consideration of the coexistence of various ion species. In this work, we experimentally studied the spoke in the E B Penning source with two-ion-species plasma. By adjusting the mixture ratio of argon and krypton gases, we investigated the changes in instability frequency with electrical diagnostics. Two frequency peaks were clearly observed, with the peaks of lower and higher frequencies corresponding to mode numbers 2 and 3, respectively. The frequencies of each peak shifted to higher values when the argon content in the gas mixture was increased. Finally, the theoretical scaling of the spoke suggests that the changes in the frequencies are proportional to the inverse square root of the effective ion mass.

Comparative small-signal stability analysis of voltage-controlled and enhanced current-controlled virtual synchronous generators under weak and stiff grid conditions

Currently, the academic and industrial community have considerable expectations for the modern power converter control strategy – a grid-forming control. Within this strategy, the most promising and effective is considered one based on the voltage-controlled virtual synchronous generator (VC-VSG). However, such control system is characterized by challenges in ensuring small-signal stability under continuously changing conditions of the grid strength. The major cause is associated with the emergence of the power coupling problem. Therefore, this paper proposes an enhanced control structure based on a current-controlled virtual synchronous generator (CC-VSG). Additionally, in this paper theoretical analysis, analysis of small-signal models in Matlab and experimental studies of detailed nonlinear models via PSCAD™/EMTDC™ for the conventional structure of VC-VSG and the proposed CC-VSG were performed. As a result, it has been proved that in the enhanced CC-VSG model the power coupling problem is practically excluded due to which small-signal stability is ensured in a wide range of short-circuit current ratio changes from 1 to 100. At the same time, a fast dynamic performance and a high damping level that provide an appropriate inertial response have been achieved. It is also proved that the improvement of damping by tuning the virtual damper winding allows to reduce the rise time and has a favorable impact on the inertial response of the CC-VSG model over the entire considered range of grid strength variation, which cannot be obtained for the conventional VC-VSG.

Coordinated Frequency Control of an Energy Storage System with a Generator for Frequency Regulation in a Power Plant

Considering the controllability and high responsiveness of an energy storage system (ESS) to changes in frequency, the inertial response (IR) and primary frequency response (PFR) enable its application in frequency regulation (FR) when system contingency occurs. This paper presents a coordinated control of an ESS with a generator for analyzing and stabilizing a power plant by controlling the grid frequency deviation, ESS output power response, equipment active power, and state of charge (SoC) limitation of the ESS in a power plant. The conventional generator and FR-ESS controllers were investigated and compared. To obtain the optimal frequency and power response, an ESS-based adaptive droop control method was proposed. The proposed control strategy was developed and implemented considering the changes and limitations of the dynamic characteristics of the system, FR requirements, and an ESS using the PSCAD/EMTDC software. The simulation results showed that the proposed method was more effective than the conventional droop-control-based FR-ESS, and the effectiveness of this method was validated.

Study on Quantitative Evaluation Index of Power System Frequency Response Capability

Frequency stability is an important factor for the safety and stability of the power system operation. In a traditional power system, the operation stability is ensured by the inertia response, primary frequency modulation, and secondary frequency modulation. In recent years, in order to achieve the goal of carbon neutralization and carbon peaking, China has made great efforts in new energy development. With large-scale new energy connected to the power grid, the proportion of traditional conventional synchronous units has gradually declined. At the same time, a large number of power electronic devices have been used in the power grid, which led to the capability decline of the inertia response and primary frequency modulation. For example, the East China Power Grid has experienced a sharp frequency drop in such an environment. In order to solve the above problems, the operation principle and control mode of various new energy resources are analyzed in this paper. Moreover, the process and principle of power grid frequency response are studied and the evaluation index of frequency response capability is proposed. The research results can quantitatively evaluate the system inertia response and primary frequency modulation level and provides a judgment tool for dispatching operators and system planners.

Adaptive integrated control strategy for MMC-MTDC transmission system considering dynamic frequency response and power sharing

Due to the lack of grid inertia caused by the increasing penetration of renewable energy sources, the ability of system to withstand disturbance is reduced. In attempt to address this challenge, an adaptive virtual inertial frequency control (AVIFC) strategy for multi-terminal dc (MMC-MTDC) transmission system considering dynamic frequency response is proposed. Based on the frequency variation characteristics in inertia response and primary response stages, the virtual inertia coefficient can be automatically adjusted. In the inertia response stage, considering the limitation of DC voltage, the virtual inertia coefficient adaptively decreases according to the frequency deviation to reduce the maximum frequency deviation. In the primary response stage, the virtual inertia coefficient decreases to 0 rapidly following the frequency deviation to speed up the frequency recovery. Furthermore, aiming at the problem of coordinated power sharing between converter stations of weak AC systems after disturbance, an adaptive droop control (ADC) strategy based on DC voltage deviation and converter station power margin is studied to realize the optimal distribution of unbalanced power so that DC voltage deviation is reduced, DC voltage regulation time is shortened and the stability of DC voltage is maintained at the same time. Finally, based on PSCAD / EMTDC simulation platform, the control model of a five-terminal MMC-MTDC system is established. Three working conditions, namely the load reduction, the load increment and the converter station out of operation, are selected to verify the effectiveness and feasibility of the proposed adaptive integrated control (AIC) strategy including AVIFC and ADC.

Effect Quantification of BESS Providing Frequency Response on Penetration Limit of VER in Power Systems

Increasing the penetration of variable energy resources (VER) can reduce the inertia and frequency response performance of power systems supported by replacement synchronous power generation. Therefore, it is necessary to manage the VER penetration limit in power systems for stable operation and to increase the operability to the desired level. This study proposes a method to evaluate and quantify the effect of increasing the penetration limit of VER by controlling a battery energy storage system (BESS). The BESS can provide a fast response, but frequency response performance varies depending on the operating conditions. In the proposed quantification method, various control methods of a BESS, operating conditions of the power system, and penetration conditions of additional VER were analyzed, and the effect of the BESS on increasing the penetration limit of VER was evaluated. This evaluation and analysis enabled the selection of the BESS operating conditions to achieve the target VER capacity in the power system. The proposed quantification method was analyzed through simulations based on the Korean power system model. Therefore, it can contribute to estimating the required performance of the BESS for each system operating condition required to achieve the VER target.

Machine Learning Approaches to Predict Electricity Production from Renewable Energy Sources

Bearing in mind European Green Deal assumptions regarding a significant reduction of green house emissions, electricity generation from Renewable Energy Sources (RES) is more and more important nowadays. Besides this, accurate and reliable electricity generation forecasts from RES are needed for capacity planning, scheduling, managing inertia and frequency response during contingency events. The recent three years have proved that Machine Learning (ML) models are a promising solution for forecasting electricity generation from RES. In this review, the 8-step methodology was used to find and analyze 262 relevant research articles from the Scopus database. Statistic analysis based on eight criteria (ML method used, renewable energy source involved, affiliation location, hybrid model proposed, short term prediction, author name, number of citations, and journal title) was shown. The results indicate that (1) Extreme Learning Machine and ensemble methods were the most popular methods used for electricity generation forecasting from RES in the last three years (2020–2022), (2) most of the research was carried out for wind systems, (3) the hybrid models accounted for about a third of the analyzed works, (4) most of the articles concerned short-term models, (5) the most researchers came from China, (6) and the journal which published the most papers in the analyzed field was Energies. Moreover, strengths, weaknesses, opportunities, and threats for the analyzed ML forecasting models were identified and presented in this paper.

Sardinia as a 100% renewable power system: A frequency stability study

This paper investigates a real complex power system supplied by a 100% renewable generation, focusing on the primary frequency regulation. The grid and the power production park are represented by means of an electro-mechanical model in a free open-source environment. The lack of synchronous generators in the grid, due to the conventional power plants shutdowns, requires higher performances from renewable power plants (RPPs) in terms of synchronization and inertial response. Doubly fed induction generators (DFIGs) are identified as the promising power plants able to lessen inertial issues. DFIG system control is investigated in the present paper, according to the recent studies in the literature, implementing a new strategy of control in case of avoiding maximum power point tracker (MPPT) operation. The 100% renewable 112-bus Sardinia islanded power system is the case study of this paper. Four scenarios were selected to show the primary frequency response in the most critical under and upper frequency events among the year. The simulations revealed that the most severe frequency nadir in the simulation reached a decrease of 0.37 Hz. DFIGs control system appeared robust in terms of inertial response and capability of reaching new stable operating point.

Approach to Inertial Compensation of HVdc Offshore Wind Farms by MMC With Ultracapacitor Energy Storage Integration

Recent studies have shown that the grid connection of offshore wind farms through modular multilevel converter–high voltage direct current (MMC-HVdc) transmission system has brought low inertia issue. It is an urgent requirement for offshore wind farms to compensate for the inertia. After analysis and evaluation in this article, it is found that integrating an ultracapacitor energy storage system (UESS) into an MMC has an advantage comprehensively considering cost, size, and dynamic. Based on the evaluation, this article proposes an approach to inertial compensation of HVdc offshore wind farms by MMC-UESS integration. An ultracapacitor is selected as an energy storage element (ESE) for its high power density since the inertial response is a short-term response and it is hoped that added ESE does not significantly increase the size of an MMC submodule. Batteries with high energy density but low power density are not suitable for such applications. The optimal operating voltage of a UESS and the control scheme between MMC loops and inertial emulation are proposed in this article. Experimental results from real-time simulation and a downscaled prototype are presented to verify the effectiveness of the proposed method.

Multiarea Inertia Estimation Using Convolutional Neural Networks and Federated Learning

With the increase in penetration of renewable energy sources (RES), traditional inertia estimation techniques based purely on the number of online synchronous generators are increasingly unsuitable, ultimately leading towards suboptimal frequency control in the electric power grid. The stochastic nature of RES additionally makes the system inertia a time-varying quantity. Furthermore, the frequency and inertial response of power systems change drastically in multiarea power systems with interconnected tie-lines. Hence, it is important for state/parameter estimation (e.g., inertia) in multiarea systems, while ensuring communication between each of the areas. In this article, a client–server-based federated learning framework is used to estimate power system inertia in a multiarea system. Federated learning is a machine learning technique where multiple decentralized devices are trained with local data, and a global model is updated and redistributed by a central server by aggregating the trained weights of the decentralized devices, without exchanging the local data. Using local frequency measurements, obtained from the phase-locked loop of an energy storage system, the inertia at each of the areas can be estimated locally via offline training using convolutional neural networks (CNNs), whereas the CNN weights update in an online fashion. The framework, tested on a two-area power system, accurately estimated the inertia constant for both independent and identically distributed (IID) and non-IID data. Furthermore, the CNN-based method outperformed conventional neural network-based estimation techniques in terms of number of communication rounds and estimation accuracy.

A new protection algorithm for tackling the impact of fault‐resistance and cloud energy storage on coordination of recloser‐fuse protection

AbstractPresenting cloud energy storage system (CESS) in the landscape of storage devices exposes microgrids (MGs) to a substantial change. Employing a specific type of inverter namely synchronverter as the connecting interface for CESS gains high inertial response in MGs with high penetration level of inverter‐based distributed generations (IBDGs). However, high fault contribution of synchronverters can lead to challenging issues. Nevertheless, presence of fault resistance which results in malfunctioning of the recloser‐fuse pair intensifies the cruciality of the dilemma. In this paper, at the outset, the effects of contribution of synchronverter‐based CESS and also fault resistance on recloser‐fuse protection are explored and then, a new algorithm is proposed for tackling these bottlenecks. To overcome these hurdles, the conventional recloser‐fuse scheme is applied at first. Then, by using the obtained schemes for fast and slow operations of the recloser, a specific part of them is set in a manner that the fuse‐saving task is performed properly even when the synchronverter‐based CESS is deployed. On the other hand, the other part of the recloser scheme is altered in a way that the maloperation of the recloser‐fuse pair owing to the fault resistance is alleviated. Adequate numerical analyses are conducted to evaluate the proposed scheme.

Impact of inertia response control strategy based doubly fed induction generator on frequency stability of power system

While the doubly fed induction generator (DFIG) is widely applied in the new power system, its decoupled control and fluctuation of output power have led to serious problems in frequency stability. Inertia response control (IRC) of DFIG has attracted significant attention, while its impact has not been clearly researched. In this study, a low-order model of DFIG focusing on frequency stability is proposed. Firstly, the simplified model of DFIG considering the IRC is introduced through Taylor series expansion. The frequency response model considering the energy storage system (ESS) is proposed. Taking the rate of change of frequency (RoCoF) and steady-state frequency deviation (SFD) as the constraint, an analytical solution for the maximum proportion of DFIGs is deduced to reflect the impact of IRC on the system frequency stability. Finally, the accuracy of the proposed method and the effectiveness of IRC is verified on the modified IEEE-9 system.

Performance evaluation of grid-following and grid-forming inverters on frequency stability in low-inertia power systems by power hardware-in-the-loop testing

Developing advanced control functions for inverter-based resources (IBRs) to replace the inertial response of retiring synchronous generators (SGs) is progressing rapidly. Although various control mechanisms have been proposed for grid-following (GFL) inverters and grid-forming (GFM) inverters, the comprehensive comparison of their performance in contributing to grid stabilization based on hardware testings has not been studied well. This paper performs power hardware-in-the-loop (PHIL) testing for multiple inverter prototypes from different manufacturers and compares their performance concerning the difference in inverter types, control methods, and IBR penetration conditions in the power system. It also tests the possible interference between islanding detection methods (IDMs) and frequency control capabilities of GFL and GFM inverters.