Reduced System Inertia Research Articles

Evaluation of false data injection attacks on automatic generation control considering wind power dynamics

The automatic generation control (AGC) system is a crucial control system based on information and communication technology in interconnected power systems. However, the dependence on cyber systems has increased the risk of false data injection attacks (FDIAs) against AGC systems. Furthermore, as the penetration level of wind power increases, an AGC dynamic framework incorporating wind stochastic modeling should be developed, and the impact of FDIAs on AGC considering wind power penetrations should also be evaluated. This paper proposes an evaluation method of the impact of FDIAs on AGC considering wind power penetrations. First, a linearized analysis framework is developed, in which the network model is considered due to its geographical smoothing effects. Then, stochastic differential equations (SDEs) are employed to model wind power and load temporal uncertain behaviors, and the overall system dynamics under different FDIA types are also proposed. Finally, the proposed methodology is illustrated via a 4-area practical AGC system. The simulation results show the effects of different types of attacks on the AGC system considering wind power penetrations and demonstrate that the existing evaluation frameworks may overestimate FDIA effects and wind power fluctuation and corresponding system inertia reduction may help the attacker to cause system instability.

Publisher’s Note

High-voltage direct current (HVDC) transmission systems are widely employed to transmit energy among systems with renewable energy integrated. However, the reduced system inertia and the risk of HVDC failure pose significant challenges to operators. To ensure system reliability especially frequency stability under extreme circumstances, a robust frequency-constrained unit commitment (FCUC) model is proposed in this paper. Unlike previous studies, an extra power-rescheduling procedure is embedded in our model, and consequently the FCUC model is formulated as a three-layer structure, where the day-ahead schedule model, emergency corrective model (primary frequency response), and power rescheduling (secondary frequency response) are co-optimized. Accordingly, an improved tri-level Benders Decomposition (BD) algorithm was developed to solve the problem. By leveraging the optimality cuts of the algorithm, the characteristics of fast power support and flexible regulation capabilities of HVDC system in real-time (RT) market are preemptively considered in DA market. Consequently, the model feasible region is more adaptive to the plausible contingencies The outperformance of our model was validated by the numerical experiments in an asynchronous HVDC interconnected system. The results demonstrate that by formulating the coordination of AC–DC dispatch framework, the global optimal operational cost is further captured when ensuring the system reliability.

Experimental assessment and validation of inertial behaviour of virtual synchronous machines

Increasing integration of converter-interfaced renewable generation has led to significant operational challenges for power systems. Such challenges are mainly caused by the different capabilities and dynamic responses of the converters compared with synchronous machines, for example, converters do not naturally provide inertia to the system. Virtual Synchronous Machine (VSM) is considered as a promising solution to address the challenges associated with reduced system inertia via the provision of emulated inertial response to support the operation of converter-dominated power systems. However, it has been observed that the dynamic behaviour of the VSM could differ significantly from that of a Synchronous Condenser (SC) and a Synchronous Generator (SG) in terms of inertial response provision, even when the VSM is configured with the same inertia constant. Furthermore, effective practical methods for evaluating the damping performance of VSMs are not presently available. To gain a better understanding and achieve a more accurate assessment of the dynamic inertial and damping performance of VSMs, this paper presents an experimental methodology for systematic evaluation of the dynamic response of the VSM in the frequency domain using the Network Frequency Perturbation (NFP) method. Experimental design and implementation of the NFP method are presented to assess VSM system's equivalent inertia and damping constants, where the VSM system under test can be treated as a black box without any knowledge of internal settings and control design. Case studies are conducted, where the proposed experimental design has been applied for testing and assessing the inertial and damping constants of a physical 246 kVA VSM prototype driven by a Battery Energy Storage System with comparison of the SC and SG with equivalent inertia constant. Power-Hardware-in-the-Loop (PHiL) testing is also conducted to demonstrate the VSM's inertia performance. The studies demonstrate that the developed experimental approach based on NFP method provides a valuable tool for network operators and manufacturers for evaluating the inertial and damping performance.

Adaptive Power Compensation-Based Frequency Regulation Strategy of Wind Turbine System

Given the increasing penetration of wind power generation, the power system faces great challenges in frequency regulation due to reduction in system inertia. This paper proposes an adaptive power compensation based frequency regulation (FR) strategy of the wind turbine system (WTS). In a frequency transient event, the proposed strategy exploits power excitation method to estimate the disturbance level, which enables the WTS to timely and accurately participate in FR. After the system frequency and the rate of change of frequency meet their requirements, the droop control is adopted to smooth the WTS out of the FR mode and further improve the frequency quality. By analyzing the operation mode and determining the switching logic, detailed implementation of the proposed strategy is developed. Its excellent frequency response performance is verified via results of hardware-in-the-loop experiment and MATLAB/Simulink-based simulation.

Adaptive LFC Incorporating Modified Virtual Rotor to Regulate Frequency and Tie-Line Power Flow in Multi-Area Microgrids

This research investigates a new coordination strategy for both isolated single-area and interconnected multi-area microgrids (MGs) using a modified virtual rotor-based derivative technique supported with Jaya optimizer based on balloon effect modulation (BE). Accordingly, the main concept of BE is to assist the classic Jaya to be more sensitive and trackable in the event of disturbances, as well as to provide optimum integral gain value on the secondary frequency controller adaptively for both suggested MGs. The proposed modified virtual rotor mechanism is consisting of virtual inertia and virtual damping that are added as a tertiary controller within proposed MGs considering full participation of the inverter-based energy storage systems. The proposed virtual rotor mechanism is consisting of virtual inertia and virtual damping that are added as a tertiary controller within proposed MGs to emulate the reduction in system inertia and the enhanced damping properties. Several nonlinearities were proposed in this work such as a dead band of governor, generation rate constraints, and communication time-delay are considered within the dynamic model of the suggested MGs. In addition, the proposed design of multi-area MGs takes the interval time-varying communication delays into account for stability conditions. In this study, A comparative study using unimodal (i.e., Sphere) and multimodal (i.e., Rastrigin) benchmark test functions are conducted to validate the proposed direct adaptive Jaya-based BE. Furthermore, Wilcoxon’s rank-signed non-parametric statistical test using a pairwise comparison was performed at a 5 % risk level to judge whether the proposed algorithm output varies from those of the other algorithms in a statistically significant manner. Thence, the superiority and effectiveness of the proposed method have also been verified against a variety of other metaheuristics optimization techniques, including classic electro-search, particle swarm, multi-objective seagull, and Jaya optimizers. In addition, an operative performance is assessed against the conventional integral controller, coefficient diagram method, and classic Jaya with/without virtual inertia. The final findings emphasize the superiority of the proposed direct adaptive Jaya-based BE supported by a modified virtual rotor and state better performance and stability compared to existing controllers.

RoCoF-Minimizing H₂ Norm Control Strategy for Multi-Wind Turbine Synthetic Inertia

The reduction in system inertia under the penetration of power electronic-interfaced generation has fostered various proposals on wind synthetic inertia. Several works in the state of the art propose control strategies to optimize synthetic inertia performance; however, most of the performance metrics indirectly represent frequency control adequacy indicators such as rate of change of frequency or frequency nadir. Also, control definitions assume that there is one variable speed wind turbine and one controller, which limits the applicability of the proposals as power systems normally count with various wind turbines/farms over extended geographical areas with different wind regimes. This work presents a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{2}$ </tex-math></inline-formula> optimal control approach for a variable speed wind turbine synthetic inertia controller. First, the objective of the formulation is to explicitly minimize the rate of change of power system frequency. Second, a stability proof for the multi-wind turbine case is proposed, allowing optimal controllers to be independently implemented for an arbitrary number of wind turbines and wind regimes. The effectiveness of the proposed control scheme is demonstrated with a numerical example, considering an empirically-validated, reduced-order model of the Electric Reliability Council of Texas frequency dynamics.

Determination of Control Requirements to Impose on CIG for Ensuring Frequency Stability of Low Inertia Power Systems

Power systems around the globe are undergoing a transformation characterized by a massive deployment of converter-interfaced generation (CIG) to effectively combat climate change. However, achieving a seamless transition from current power systems dominated by synchronous generators (SGs) to future ones with high levels of CIG requires overcoming several technical challenges. From a frequency stability perspective, reduced system inertia increases the frequency nadir after a loss of generation thereby endangering frequency stability. In this context, this paper proposes a novel methodology for determining control requirements to impose on CIG as their penetration in the network increases. Results of a case study based on the Chilean grid projected for the year 2046 show that, if only grid-following converters without frequency control capability are deployed, a maximum CIG penetration level of 75% can be achieved without threatening frequency stability. The Chilean system can reach a 99% CIG penetration, provided that the remaining CIGs are deployed in grid-following with frequency support capability. Finally, we show that if the last SG is replaced with a grid-forming converter, the system can still sustain frequency stability and exhibits a good dynamic performance. These results demonstrate that, at least from a frequency stability viewpoint, achieving a 100% based CIG system is technically possible. The proposed methodology can be used by energy regulators to define the control requirements necessary to impose on CIG for achieving renewable energy targets in a secure way. Although the obtained results are particular for the Chilean system, the proposed methodology can be applied to any power system.

Power reserve control of solar photovoltaic system for improved frequency response of a stand-alone power system

ABSTRACT The growing share of photovoltaic (PV) installations in power systems, with the consequent reduction in system inertia, has necessitated considering PV systems to participate in grid ancillary services such as fast frequency response (FFR). Power reserve control (PRC) of PV systems, to create the required reserve power, needs the PV systems to be operated at a point below the maximum-power point. This paper presents a novel power-command tracking algorithm for PV systems to realize PRC operation. Based on the single-diode model (SDM) and single measurement sample, the proposed PRC algorithm provides a straightforward estimation of the maximum-power point (MPP) quantities without requiring an irradiance sensor. A self-adaptive voltage-tracking method (inner control) is proposed which directly translates a reference power command into an equivalent voltage command for the interfacing DC-DC converter even under changing irradiance levels. Additionally, the strategy unifies the PRC mode and MPP mode into a single control algorithm, allowing an optimal transition between the two modes of operation. The proposed PRC scheme is implemented on a solar-diesel hybrid power system to provide frequency regulation. An appropriate power reference for the inner controller is generated via an optimally-tuned inertial and damping controller (ID-controller) with constraints on the power command. Various aspects of modeling and control are addressed, and simulation experiments are conducted to illustrate the effectiveness of the suggested approach. The results show that, with the proposed PRC, frequency deviation and rate-of-change of frequency (RoCoF) are reduced by 86% and 59%, respectively compared to MPP-mode operation.

Effective damping of local low frequency oscillations in power systems integrated with bulk PV generation

High penetration of renewable sources into conventional power systems results in reduction of system inertia and noticeable low-frequency oscillations (LFOs) in the rotor speed of synchronous generators. In this paper, we propose effective damping of LFOs by incorporating a supplementary damping controller with a photovoltaic (PV) generating station, where the parameters of this controller are coordinated optimally with those of a power system stabilizer (PSS). The proposed method is applied to damp local electromechanical modes by studying a system comprising a synchronous generator and a PV station connected to an infinite bus. The PV station is modeled following the instructions of the Western Electricity Coordinating Council. The problem is modeled as an optimization problem, where the damping ratio of the electromechanical modes is designed as the objective function. Constraints including upper and lower limits of decision parameters and damping ratio of other modes are considered by imposing penalties on the objective function. Different optimization algorithms are used to pursue the optimal design, such as political, improved gray wolves and equilibrium optimizers. The results validate the effectiveness of the proposed controller with PSS in damping local modes of oscillations.

Impact of Societal Events on Frequency Stability Considering LED TVs in Low Inertia Trending Power Systems

Due to environmental concerns, renewable energy sources (RESs) gradually replace conventional power generations. Over the past decade, wind and solar electricity generation have increased from 55 to 275 million MW·h and from 2 to 96 million MW·h, respectively, in the U.S. Recently, FERC order 2222 allows distributed energy resources, including distributed renewable generation and energy storage, to participate in the electricity wholesale market through aggregations. Limited rotating inertia of renewable generation resources results in reduced system inertia, which will increase the rate of change of frequency due to sudden mismatches of generation and demand. During the high visibility national social event, the brightness of LED TV display put simultaneous demand change in power systems. With the development of slim, tightly packaged, and yearly update features of smart TV, the penetration of different LED displays climbed rapidly from 9% to 60% in the U.S. during 2012–2019. In this article, the impact of societal events on frequency stability caused by large viewership on LED TVs are discussed. Increasing RESs penetration and FERC order 2222 are considered in this study. The power demand data of a 55-inch LED TV are collected and analyzed to address the problem. This article discusses the frequency stability issue caused by LED TVs in a low inertia power system. It can help system operators to develop frequency control practices accordingly.

Control Hardware-in-the-Loop Simulation on Fast Frequency Response of Battery Energy Storage System Equipped With Advanced Frequency Detection Algorithm

Fast frequency response (FFR) of battery energy storage system (BESS) is an effective way to mitigate the system frequency deviation induced by the fluctuation of power generation from the increased renewable energy resources and the impact of reduced system inertia caused by decreased synchronous generators. However, existing standards referring to technical requirements for the frequency measurement do not sufficiently consider abnormal grid conditions, such as voltage unbalance, frequency sweep, and dynamic voltage step. This article shows the impact of frequency measurement logic on the FFR performance of BESS in grid disturbance conditions by control hardware-in-the-loop (CHIL) testing and proposes improved frequency measurement logic based on a phase-locked loop (PLL). A benchmarking of the proposed PLL was conducted against existing PLL and discrete Fourier transformation-based frequency measurement logics. Finally, the FFR performance of BESS with each frequency measurement logic has been tested in CHIL simulation. This CHIL comprises the actual power plant controller, the actual bidirectional inverter control board, and a real-time simulator. In the CHIL experiments, several system disturbance situations likely to happen in existing power systems were considered as test cases and showed that the frequency measurement accuracy and latency were violated in some frequency measurement logics. Furthermore, this article reveals the effects of frequency measurement accuracy and latency on the FFR performance of BESS by a closed-loop power control simulation with a CHIL environment. In conclusion, considerations for the frequency measurement method used for FFR are proposed.

Application of VSG technology based on flexible parameter adjustment in PV unit grid-connected inverter

As a clean and effective renewable energy source, PV has been widely used in power systems. The application of VSG technology can effectively improve the system inertia reduction problem caused by the grid connection of PV and energy storage units. The virtual inertia and damping coefficient in VSG control have the unique advantages of being flexible and controllable. This paper designs a control strategy in which the virtual inertia and damping coefficient can be flexibly adjusted according to the system frequency, which further improves the operating performance of the PV and energy storage units based on VSG control. The frequency quality of the system is maintained. Finally, the effectiveness of the proposed flexible parameter adjustment strategy was verified through the simulation platform, which played a role in popularizing the application of the proposed strategy in engineering.

Addressing Frequency Control Challenges in Future Low-Inertia Power Systems: A Great Britain Perspective

The ambitious global targets on decarbonization present the need for massive integration of renewable generation in power systems, resulting in a significant decrease in the system inertia. In addition to the reduction in system inertia, the transmission system in Great Britain (GB) faces some unique challenges owing to its relatively small capacity, while being decoupled from other transmission systems and with the renewable resources largely non-uniformly distributed across the system. This paper presents opinions and insights on the challenges associated with frequency control in a low-inertia system and the potential solutions from a GB perspective. In this paper, we focus on three main techniques that act over different time scales: synchronous condensers, inertia emulation, and fast frequency response. We evaluate their relative advantages and limitations with learnings from recent research and development projects in GB, along with the opinions on their roles in addressing the frequency control challenges in future low-inertia systems.

Voltage control and virtual synchronous generator control of HVDC interconnection line for improving stability of power system including large‐scale wind farm

Large-scale integration of renewable energy sources (RESs) such as wind power can impact on the existing power system's stability. For example, there is a possibility that the wind farm (WF) cannot be maintained stable and then it is disconnected from the grid when the instantaneous voltage sag caused by a grid disturbance occurs. Moreover, the increasing integration of RESs in the power system can reduce the number of conventional synchronous generators (SGs) connected to the grid, and lead to a reduction of system inertia in the grid. The system inertia reduction can cause larger and faster frequency fluctuations during the transient period. Hence, this paper proposes a new control strategy composed of the voltage control for suppressing the voltage sag and the virtual synchronous generator (VSG) control based on the proportional-integral-differential (PID) fuzzy logic control (FLC) for maintaining the frequency stability which is performed by high voltage direct current (HVDC) interconnection line. The aim of the new control strategy is to improve the stability of the entire power system with large-scale integration of RESs. The effectiveness of the proposed strategy is confirmed through the simulation analyses by PSCAD/EMTDC.

Coordinated control among PSS, WTG and BESS for improving Small-Signal Stability

Abstract The goal towards attaining a sustainable future has led to the rapid increase in the integration of converter control based generators (CCBGs). The low inertia response characteristics of CCBGs and the weak tie lines in interconnected systems pose a huge threat to Small-Signal Stability (SSS). Adequate damping of low-frequency oscillations (LFO) is pivotal in ensuring the maximum power transfer through the critical transmission corridors. These operational issues become more serious with the significant reduction in system inertia as a result of the high penetration of CCBGs. Therefore, appropriate control techniques are an absolute requirement for preventing LFOs from limiting the penetration of CCBGs in interconnected networks. This may also eventually lead to revisions in grid codes mandating CCBGs to provide auxiliary damping control. But, the progressive addition of multiple damping controllers for specific target modes can lead to the drifting of eigenvalues (EVs) associated with other electromechanical modes (EMs) in the system. This is due to the adverse interactions between multiple damping controllers in the uncoordinated control approach and may result in deteriorating SSS. Therefore, this paper proposes a simultaneous coordinated control among Battery Energy Storage System (BESS), Wind Turbine Generators (WTG) and Power System Stabilizer (PSS) for enhancing SSS in networks with high wind penetration by considering both inter-area (IA) and local modes. The performance of the proposed coordinated control is corroborated using IEEE 68 bus system for multiple operating scenarios for which the critical modes in the system have the lowest damping index (DI). The effectiveness of modulating the active power, reactive power and simultaneous modulation of both active and reactive power injected by BESS along with a dual-channel Optimized WTG Damping Controller (DOWDC) and PSS is evaluated. The impact of the different coordinated control strategies on voltage dynamics is also investigated. The simulation results validate the better performance of the proposed coordinated control over uncoordinated control approaches.

Power System Transient Stability Assessment Using Stacked Autoencoder and Voting Ensemble

Increased integration of renewable energy sources brings new challenges to the secure and stable power system operation. Operational challenges emanating from the reduced system inertia, in particular, will have important repercussions on the power system transient stability assessment (TSA). At the same time, a rise of the “big data” in the power system, from the development of wide area monitoring systems, introduces new paradigms for dealing with these challenges. Transient stability concerns are drawing attention of various stakeholders as they can be the leading causes of major outages. The aim of this paper is to address the power system TSA problem from the perspective of data mining and machine learning (ML). A novel 3.8 GB open dataset of time-domain phasor measurements signals is built from dynamic simulations of the IEEE New England 39-bus test case power system. A data processing pipeline is developed for features engineering and statistical post-processing. A complete ML model is proposed for the TSA analysis, built from a denoising stacked autoencoder and a voting ensemble classifier. Ensemble consist of pooling predictions from a support vector machine and a random forest. Results from the classifier application on the test case power system are reported and discussed. The ML application to the TSA problem is promising, since it is able to ingest huge amounts of data while retaining the ability to generalize and support real-time decisions.

Power system frequency control enhancement by optimization of wind energy control system

Abstract Future trends are increasingly directed towards replacing traditional energy sources by new and renewable energy ones, with Wind Energy coming first in the row of these sources worldwide. The Egyptian electric power system still has a relatively low penetration level, around 3%, of Wind Energy to its generation mix. However, it is planned to increase this ratio to 14.6% by the year 2035. This paper studies the frequency dynamics associated with significant penetration levels of Wind Energy into the Egyptian grid, which leads to reducing the overall system inertia. The control strategy adopted depends on maximizing the wind energy contribution to frequency control, in order to compensate that reduction of system inertia. The Real data of the Egyptian power system model used is verified by the National Electricity Control Centre, updated to the latest values declared and tested under the lowest and highest loads recorded. A modified control loop is added to the control system in order to allow the Wind Energy Conversion System to significantly contribute to the Load Frequency Control. The PI controller gains used in that loop are tuned using three methods; Local Unimodal Sampling, Harmony Search Algorithm and Equilibrium Optimizer (EO). The Equilibrium Optimizer (EO) proved its superiority to the other methods, as it gave the best dynamic response. The simulated results are presented using MATLAB/SMULINK.

A Spectral Model of Grid Frequency for Assessing the Impact of Inertia Response on Wind Turbine Dynamics

The recent developments in renewable energy have led to a higher proportion of converter-connected power generation sources in the grid. Operating a high renewable energy penetration power system and ensuring the frequency stability could be challenging due to the reduced system inertia, which is usually provided by the conventional synchronous generators. Previous studies have shown the potential of wind turbines to provide an inertia response to the grid based on the measured rate of change of the grid frequency. This is achieved by controlling the kinetic energy extraction from the rotating parts by its converters. In this paper, we derive a spectral-based model of the grid frequency by analyzing historical measurements. The spectral model is then used to generate realistic, generic, and stochastic signals of the grid frequency for typical aero-elastic simulations of wind turbines. The spectral model enables the direct assessment of the additional impact of the inertia response control on wind turbines: the spectra of wind turbine output signals such as generator speed, tower base bending moment, and shaft torsional moment are calculated directly from the developed spectral model of the grid frequency and a commonly used spectral model of the turbulent wind. The calculation of output spectra is verified with non-linear time-domain simulations and spectral estimation. Based on this analysis, a notch filter is designed to significantly alleviate the negative impact on wind turbine’s structural loads due to the inertia response with only a small reduction on the grid support.

Local dynamic frequency response using domestic electric vehicles

There is an increasing interest to penetrate low carbon vehicles into the transport system. Take the Great Britain (GB) as an example, the number of electric and plug-in hybrid vehicles will make up to at least half of new vehicle sales. Electric vehicles (EVs) are expected to contribute to the ancillary services of the frequency response because EVs can provide immediate frequency response and sustain its response for considerable period of time. This paper addresses the design of a Dynamic Vehicle Grid Support (DVGS) control algorithm for the provision of local frequency response. The DVGS considers a dynamic relationship between the state of charge of EVs and frequency set-points. Thus, it can be installed locally avoiding the cost and the time delay associated with the communication system between EVs and the control centre. The DVGS control algorithm was demonstrated using the reduced GB transmission power system model with a reduced system inertia. The simulation results showed that the EVs are promising assets for the provision of frequency response and reducing the rate of change of frequency (RoCoF). Moreover, EVs can be controlled geographically to provide the zonal frequency response, reducing the dependency on the power from the spinning reserve, especially with a reduced system inertia. The financial benefits of using the aggregated DVGS for firm frequency response (FFR) service in the GB is calculated.

Enhancing Power System Frequency with a Novel Load Shedding Method Including Monitoring of Synchronous Condensers’ Power Injections

Under-frequency load shedding (UFLS) is a classic and a commonly accepted measure used to mitigate the frequency disturbances in case of loss-of-generation incidents in AC power grids. Triggering of UFLS is classically done at frequency thresholds when system frequency collapse is already close to happening. The renewed interest for synchronous condensers due to the global trends on massive commissioning of non-synchronous renewable power generation leading to reduction of system inertia gives an opportunity to rethink the approach used to trigger load-shedding activation. This question is especially relevant for the Baltic states facing a desynchronization from Russian power grid and a necessity to operate in an isolated island mode. The main goal of this paper is to introduce a predictive load shedding (LS) method without usage of either frequency or ROCOF measurements based on the monitoring of active power injections of synchronous condensers and to prove the efficiency of the concept through several sets of case study simulations. The paper shows that the proposed approach can provide a greatly improved frequency stability of the power system. The results are analyzed and discussed, the way forward for the practical implementation of the concept is sketched.