Low-inertia Grids Research Articles

Fast Frequency Response for Future Low-Carbon Indian Power System: Challenges and Solutions

India has transitioned from being an energy-deficient nation to an energy-surplus nation. As of July 2023, India’s power system has seen a rapid transformation, with renewable energy, including large hydro, accounting for 40% of the total installed capacity at 177 GW. The ongoing energy transition has not only changed the country’s energy mix but also brought new challenges for system operators. The primary challenge is frequency instability due to the reduction of system inertia and governor-based primary frequency response (PFR) availability by large-scale integration of uncertain renewable energy resources (RES) such as wind and solar photovoltaic generators. Furthermore, the absence of frequency control ancillary services market, inadequate penetration of electric vehicles (EVs) and uncertainty of RES generation enhance the complexity of the frequency response services in the Indian power system. In this context, this paper presents a comprehensive review of the frequency response challenges of the future low-carbon Indian power system and underlines the new potential fast frequency response (FFR) services from advanced energy storage and renewable generation technologies. Such FFR services can be swiftly deployed in low-inertia grids through the integration of energy storage systems and inverter-based resources in a shorter span. Modeling of FFR services in security constraint unit commitment (SCUC) can enhance grid resiliency and facilitate high penetration of RES in the Indian power system without loss of frequency stability.

Development of Virtual Inertia Control with State-of-Charge Recovery Strategy Using Coordinated Secondary Frequency Control for Optimized Battery Capacity in Isolated Low Inertia Grid

Integrating renewable energy through inverter-based generators has decreased the power system’s inertia. Reduced inertia may lead to frequency instability during power imbalance disturbances, particularly in an isolated power system with limited inertia. The Battery Energy Storage System (BESS) and a virtual inertia (VI) emulation control system have become popular to mitigate this issue. Nonetheless, the BESS utilization for VI emulation is highly dependent on the availability of BESS capacity, which may affect the energy cost. Therefore, developing a VI emulation control strategy that requires less energy and can recover the state of charge (SoC) to a desired level to optimize BESS utilization is required. This paper proposes a VI control with an SoC recovery strategy through coordination with the generators’ secondary frequency control. Instead of relying on the frequency, such as in the conventional approach, the controlled signal of the generators’ secondary frequency control also includes the VI power and BESS SoC. Hence, the generators can contribute to lowering the VI required energy and recovering the BESS SoC. The results show that the proposed method outperforms the conventional method by requiring around 36% lower energy and the ability to maintain the BESS SoC.

Stochastic unit commitment to determine frequency response ramp rate including wind turbines with synthetic inertia and virtual synchronous generator

In this paper, the dynamic frequency constraints are derived for power systems incorporating full-converter wind turbines. These wind turbines could be equipped with a virtual inertia by either synthetic inertia control or virtual synchronous generator concept. In the former scheme, deviations in the wind turbines rotor speed remain in synchronism with the system frequency. However, the wind turbines start to restore their rotor speed before the system frequency recovery initiation, in virtual synchronous generator scheme. The restriction on the virtual inertia are determined considering a limit variable on wind turbines rotor speed drop. The frequency response requirements are derived in terms of governor’s base power, ramp rate and reserve power. A linear frequency nadir constraint is also proposed for the synthetic inertia strategy. Then, the constraints are integrated into the stochastic unit commitment to be consistently and economically met over 24-hour period. As such, the contingency size management are coordinated with the virtual inertia provision. The unit commitment results of IEEE 118-bus system including multiple wind farms reveal that the virtual synchronous generator concept can be an optimal solution for future low-inertia grid’s challenges. Finally, accuracy of the unit commitment results is validated through time-domain simulations of the detailed system model.

Simulating the GB power system frequency during underfrequency events 2018–19

AbstractLightning hit a transmission power line outside London, England on 9 August 2019. There followed a loss of power from a cascade of generator outages that exceeded contingency reserves, leading to an exceptional fall in grid frequency causing widespread transport disruptions and the disconnection of over 1 m households. Simulating such events typically involves a system of differential equations representing the overall generation and load present at the time. A standard model based on the swing equation assumes unlimited capacity in aggregated resources, and the availability of these services throughout the duration of the frequency deviation. In simulating the effect of outages on the GB Grid frequency on 2019/8/9, the effect of limiting these services to the capacity of resources engaged during the event is examined. It is shown that by taking these refinements into account the timing and extent of the frequency nadir can be successfully estimated. Insight is gained into the responses of various grid characteristics and how they interact with unplanned generation imbalances. Using this adapted model, further events on the GB grid are examined to validate the influence of these features. With the model’s effectiveness validated, novel mitigating measures to preserve the stability of a low-inertia grid can be evaluated.

Grid-forming control strategy for PMSG wind turbines connected to the low-frequency AC transmission system

The inevitable consequence of the rapid transition to high-proportion renewable energy and high-proportion power electronics with the development of power systems is the loss of the traditional high-volume generation of synchronous generators, their inertia, and the accompanying frequency and voltage control mechanisms. This loss of moment of inertia changes the nature of the power system into a low inertia grid, which brings serious stability challenges. The new control technology of the converter promises to address these challenges and replicate the functionality provided so far by the synchronous generator. Firstly, the model of the low-frequency transmission wind turbine being integrated into the power grid is built. Secondly, a data-driven robust control strategy is designed for the machine-side inverter and the grid-side modular multilevel matrix converter (M3C), and the grid-forming control system of the wind turbine is established, so that the low-frequency transmission wind turbine can meet the grid-connection requirements. Finally, the real-time simulation experiment platform of the multi-fan collaborative control system is built. The experimental results verify the effectiveness of the grid-forming controller and the frequency stability of the grid-connected multiple fans.

Controlling the Frequency Response in AC/DC Microgrid using an Energy Storage Device

This paper deals with the issue of frequency regulation in a small insulated low inertia grid with a large participation of renewable energy sources (RESs). A strong decentralized control strategy is used, allowing various RESs such as batteries, supercapacitors, and fuel cells to provide additional frequency recovery service. A small grid with a synchronous machine, photovoltaic cells, and fuel cells was developed as generation units to test the effectiveness of the suggested technique. Furthermore, the battery and supercapacitors were added to the system to give additional service to suppliers. Simulating the system response to numerous uncertainty is used to evaluate the controller's performance.  the controller's efficiency is displayed in Graphical form. This paper will show how a decentralized method that allows all units to provide active power supports not only adjusts frequency nadir points but also minimizes the amount of active power required in the process. As a result, the electrical pressure on each element that supports the network against the heavy usage of RESs is reduced.

An improved adaptive hybrid controller for battery energy storage system to enhance frequency stability of a low inertia grid

Despite the promising dynamic characteristics of battery energy storage system (BESS) for efficient and reliable use in stability enhancement of a low inertia grid due to the large-scale integration of renewable energy sources (RESs), existing BESS controllers are found to be complex, inefficient and less responsive to adapt any changes in frequency of the system. This paper presents a simple improved adaptive Fuzzy-ANFIS hybrid algorithm-based BESS controller. The proposed controller can emulate virtual inertia by controlling the active power flow to improve frequency stability i.e. frequency nadir and rate of change of frequency (ROCOF). Necessary modeling of the controller was carried out and its performance has been analyzed to evaluate the performance of the controller. With different case studies and comparative analysis, the controller has also demonstrated its potentials to improve the frequency nadir by 0.43 % for sudden load variation and 0.61 % for transient short circuit fault. The promising performance of the controller suggests its utility for better grid inertial support to a low inertia grid. Additionally, due to the improvement in ROCOF and settling time, the proposed controller thus can increase the overall inertia in addition to the power balance at point of common coupling (PCC).

Transient Stability of Low-Inertia Power Systems With Inverter-Based Generation

This study examines the transient stability of low-inertia power systems with inverter-based generation (IBG) and proposes a sufficient stability criterion. In low-inertia grids, transient interactions are induced between the electromagnetic dynamics of the IBG and the electromechanical dynamics of the synchronous generator (SG) under a fault. For this, a hybrid IBG-SG system is established and a delta-power-frequency model is developed. Based on this model, new mechanisms of transient instability different from those of conventional power systems from the energy perspective are discovered. First, two loss-of-synchronization (LOS) types are identified based on the relative power imbalance owing to the mismatch between the inertia of the IBG and SG under a fault. Second, the relative angle and frequency will jump at the moment of a fault, thus affecting the system energy. Third, the cosine damping coefficient induces a positive energy dissipation, thereby contributing to the system stability. A unified criterion for identifying the two LOS types is proposed using the energy function method. This criterion is proved to be a sufficient stability condition for addressing the effects of the jumps and cosine damping coefficient on the system stability. The new mechanisms and effectiveness of the criterion are verified based on simulation results.

Unified Viewpoint of Soft Startup and Rate Limiter for SOGI-FLL

This letter demonstrates that the second-order generalized integrator-based frequency-locked loop (SOGI-FLL) with a soft startup/rate limiter essentially incorporates an adaptive feedforward relying on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</i> information of transient input. When a fair parameter is tuned, the soft startup and the limiter give a similar performance. The shortcoming is that they have a deceleration blind zone, depending on the frequency constraints and SOGIs gain. If the allowed rate of change of frequency (RoCoF) is fixed, the fast response FLL can weaken this shortcoming. Specifically, if the RoCoF of low-inertia grids is considered, they could produce a severely biased estimation under the harmonics and dc offsets. The experimental results are given to validate the claims.

Grid-Forming Converters for Stability Issues in Future Power Grids

Historically, the power system has relied on synchronous generators (SGs) to provide inertia and maintain grid stability. However, because of the increased integration of power-electronics-interfaced renewable energy sources, the grid’s stability has been challenged in the last decade due to a lack of inertia. Currently, the system predominantly uses grid-following (GFL) converters, built on the assumption that inertial sources regulate the system stability. Such an assumption does not hold for the low-inertia grids of the future. Grid-forming (GFM) converters, which mimic the traditional synchronous machinery’s functionalities, have been identified as a potential solution to support the low-inertia grids. The performance analysis of GFM converters for small-signal instability can be found in the literature, but large-signal instability is still an open research question. Moreover, various topologies and configurations of GFM converters have been proposed. Still, no comparative study combining all GFC configurations from the perspective of large-signal stability issues can be found. This paper combines and compares all the existing GFM control schemes from the perspective of large-signal stability issues to pave the way for future research and development of GFM converters for large-signal stability analysis and stabilization of the future low-inertia grids.

Modelling and utilisation of frequency responsive TCSC for enhancing the frequency response of a low inertia grid

The objective of this paper is to enhance the frequency resilience and wind penetration level of a low inertia power system by utilising frequency responsive Thyristor Controlled Series Capacitor (TCSC). In recent years, wind power integration has become prolific in numerous power grids around the world. As such, Synchronous Generators (SGs) are being substituted from the generation mix. Unlike conventional synchronous generators, variable speed Wind Turbine Generators (WTGs) generally do not participate in system frequency control without additional control strategy. When subjected to prolific wind generation and interconnection import from an adjacent grid, number of online SGs in a grid can be scant. During such a situation, a large contingency in the power importing zone may overload an AC interconnection. Consequently, the interconnection could trip due to loss of synchronism. It eventually may cause a system-wide blackout. In addition, due to the risk of network separation, non-synchronous wind penetration level is kept restricted in many power systems. To overcome these challenges, this paper proposes a frequency responsive TCSC model to increase interconnection transfer capability by reducing the line reactance. To this end, for determining optimal TCSC parameters, a strategy is outlined. Furthermore, a generalised expression is developed to find the maximum wind penetration level while maintaining the frequency response adequacy. The developed techniques are applied to a low inertia test system. Simulation results reveal that the risk of blackout is averted, and the maximum wind penetration level is significantly increased after deploying the frequency responsive TCSC.

Resilience of the Baltic Power System When Operating in Island Mode

In light of continuously growing penetration of nonsynchronous generation, the gas-fired plants along with hydro power will provide the major part of system inertia and frequency reserves. This article evaluates the resilience of the Baltic power grid in the event of outage of a major natural gas-fired power plant. An importance of such evaluation is justified by a set of additional challenges, the Baltic power grid will experience due to its disconnection from the unified power system of Russia and synchronization with the European Network of Transmission System Operators synchronous power system in 2025. A case study, highlighting the system resilience-related problems will be presented in the article. The results can be used in the planning activities of transmission system operators undergoing energy transition and preparing for operation in low-inertia grid modes.

Using PV systems and parking lots to provide virtual inertia and frequency regulation provision in low inertia grids

In this research, a novel formulation for smart frequency control and virtual inertia support in a low inertia grid is proposed. The approach takes advantage of EV parking lots and PV distributed resources abilities for this purpose directly or through aggregators. The model also considers governor control of the conventional plants in addition to the controllable loads, namely under-frequency load shedding devices. The model seeks to minimize the total amount of disconnected load in case of severe contingencies. Furthermore, the rate of change of frequency (ROCOF) is formulated and employed as another objective function. The control parameters are optimally tuned by Mixed Integer Linear Programming (MILP) model to minimize the amount of load shedding while satisfying the specified frequency constraints. Linearity of the model assures achieving the global optima and is suitable for large-scale real-life systems without convergence concerns. The proposed smart control scheme is implemented on the IEEE 39-bus system, where the simulation results demonstrate a dramatic decrease of load shedding from 0.1676 p.u to 0.1235 p.u for one of the cases by the incorporation of converter based frequency providers. The system ROCOF, moreover, is reduced from -2.94 Hz/s to -1.93 Hz/s which indicates the efficiency of the methodology.

A Fuzzy Logic-Based Emulated Inertia Control to a Supercapacitor System to Improve Inertia in a Low Inertia Grid with Renewables

The contribution of power generation from converter-dominated renewable energy sources (RES) has increased enormously. Consequently, the system inertia is decreasing, and it impacts the frequency of the system. With large-scale integration of power electronic inverter-based power generation from RES, inertia from energy storage devices would be unavoidable in future power grids. In this paper, the inertia emulator is formed with a supercapacitor (IE–SC) to improve inertia in a low inertia grid. To emulate the inertia in a low inertia grid, this paper proposes a fuzzy logic controller-based emulated inertia controller (FL-EIC) for an inverter attached to IE–SC. The proposed fuzzy logic controller estimates the inertial power required based on the frequency deviation and rate of change of frequency (ROCOF). The output of the fuzzy controller adds to the conventional emulated inertia control (EIC) technique to alter the load angle for the power electronic inverter of IE–SC. Specifically, the proposed FL-EIC achieves inertia emulation by proportionally linking the time derivative of the grid frequency and frequency deviation to active power references of IE–SC. A comparison of the conventional EIC and FL-EIC is carried out to prove the effectiveness of the proposed FL-EIC. Furthermore, real-time simulations with the help of the OPAL-RT real-time simulator (OP 5700) are presented to validate the advantage of the FL-EIC.

Load-Altering Attacks Against Power Grids Under COVID-19 Low-Inertia Conditions

The COVID-19 pandemic has impacted our society by forcing shutdowns and shifting the way people interacted worldwide. In relation to the impacts on the electric grid, it created a significant decrease in energy demands across the globe. Recent studies have shown that the low demand conditions caused by COVID-19 lockdowns combined with large renewable generation have resulted in extremely low-inertia grid conditions. In this work, we examine how an attacker could exploit these scenarios to cause unsafe grid operating conditions by executing load-altering attacks (LAAs) targeted at compromising hundreds of thousands of IoT-connected high-wattage loads in low-inertia power systems. Our study focuses on analyzing the impact of the COVID-19 mitigation measures on U.S. regional transmission operators (RTOs), formulating a plausible and realistic least-effort LAA targeted at transmission systems with low-inertia conditions, and evaluating the probability of these large-scale LAAs. Theoretical and simulation results are presented based on the WSCC 9-bus and IEEE 118-bus test systems. Results demonstrate how adversaries could provoke major frequency disturbances by targeting vulnerable load buses in low-inertia systems and offer insights into how the temporal fluctuations of renewable energy sources, considering generation scheduling, impact the grid&#x2019;s vulnerability to LAAs.

Under frequency load shedding for low inertia grids utilizing smart loads

Due to inertia deterioration which is caused by low inertia renewable resources, frequency-based protective schemes must be re-designed. This paper presents a multi-stage under frequency load shedding (UFLS) plan which is designed based on the yearly load duration curve (LDC) and activated according to the system frequency and the rate-of-change-of-frequency (RoCoF). In order to design a robust UFLS plan, a comprehensive list of credible operational and topological scenarios under different levels of renewables penetration are defined. For optimizing the UFLS performance over the constructed scenarios, a detailed system frequency response including the time response of governors, and natural load damping is developed. Then, the potential of frequency-dependent smart loads (SLs) as a remedy to postpone or reduce the total amount of load shedding in low inertia grids are investigated. Eventually, the proposed UFLS scheme is re-designed in the presence of SLs. The proposed method is formulated as a Mixed-Integer Linear Programming (MILP) model and solved using the CPLEX algorithm in GAMS and the effectiveness of the proposed method is investigated for the dynamic IEEE 39-bus test system.

Closed‐loop fast primary frequency‐response of type‐3 wind power plants in low inertia grids

Closed‐loop fast primary frequency‐response of type‐3 wind power plants in low inertia grids

Sizing of Hybrid Energy Storage Systems for Inertial and Primary Frequency Control

The exponential rise of renewable energy sources and microgrids brings about the challenge of guaranteeing frequency stability in low-inertia grids through the use of energy storage systems. This paper reviews the frequency response of an ac power system, highlighting its different time scales and control actions. Moreover, it pinpoints main distinctions among high-inertia interconnected systems relying on synchronous machines and low-inertia systems with high penetration of converter-interfaced generation. Grounded on these concepts and with a set of assumptions, it derives algebraic equations to rate an energy storage system providing inertial and primary control. The equations are independent of the energy storage technology, robust to system nonlinearities, and rely on parameters that are typically defined by system operators, industry standards, or network codes. Using these results, the authors provide a step-by-step procedure to size the main components of a converter-interfaced hybrid energy storage system. Finally, a case study of a wind-powered oil and gas platform in the North Sea demonstrates with numerical examples how the proposed methodology 1) can be applied in a practical problem and 2) allows the system designer to take advantage of different technologies and set specific requirements for each storage device and converter according to the type of frequency control provided.

A Frequency and Voltage Stability-Based Load Shedding Technique for Low Inertia Power Systems

The aim of this research work is to develop a load shedding methodology to improve the frequency response of low inertia grids by attaining satisfactory voltage stability. In recent times, wind energy integration has considerably increased in many power grids. Consequently, conventional synchronous machines are being replaced from dispatch. Unlike traditional synchronous machines, variable speed wind turbine generators usually do not take part in frequency regulation without supplementary control mechanism. During substantial wind penetration, a power system may have a small number of online synchronous machines. As a result, synchronous inertia and governor responsive reserve significantly reduce. Under such situation, a system has to rely on load shedding as a last line of defense to rescue the system frequency following a large contingency. However, the conventional Under-Frequency Load Shedding (UFLS) strategy may lead to larger frequency deviation and higher amount of load cut in certain cases. A new load shedding methodology is presented in this paper to overcome this challenge. Unlike conventional UFLS technique, higher proportion of load shedding is applied to relatively weaker buses in terms of voltage stability in the proposed mechanism. Based on reactive power margin, which is an index to specify voltage stability, a general expression to quantify load shedding is derived. Also, the adaptability of the proposed strategy to various load levels is ensured. Later on, performances of the developed strategy are explored in a low inertia wind dominated test network. Simulations are executed considering various penetration levels of wind power and for two severe contingencies - loss of 550 MW interconnection and loss of 650 MW interconnection. Investigations reveal that the proposed load shedding methodology ensures satisfactory frequency response in all simulation cases. Also, the developed technique yields less frequency deviation and load cut compared to the conventional UFLS mechanism. Therefore, the reported load shedding scheme is found to be more competent to concurrently maintain frequency and voltage stabilities in renewable dominated power systems.

Mitigation of Over-Frequency through Optimal Allocation of BESS in a Low-Inertia Power System

The primary objective of this paper is to alleviate the over-frequency problem in low-inertia power systems through optimal allocation of a Battery Energy Storage System (BESS). With prolific integration of wind power, conventional fossil-fuel driven synchronous generators are being replaced in the generation fleet. Variable speed wind machines are connected to the grid via power electronics converters. As such, these machines usually do not participate in frequency regulation. During high wind penetration, a generation-rich zone of an interconnected power system may face significant over-frequency following the loss of interconnection. If the frequency goes above a certain threshold, an Over-Frequency Generator Shedding (OFGS) scheme is activated. This may cause considerable amount of generation cut in a low-inertia power system. To address this challenge, this paper develops a siting and sizing methodology of frequency-responsive BESS to simultaneously maintain frequency and voltage stabilities. As such, BESS is placed at the most voltage-sensitive bus, determined by an index called reactive power margin. Furthermore, an optimization model is formulated to determine the BESS size to avoid generation shedding. The proposed technique is applied to a low-inertia power system, which resembles the equivalent high-voltage transmission network of South Australia. The simulation results reveal that the developed methodology successfully mitigates the over-frequency phenomenon. In addition, the proposed technique is found to be more effective than its counterpart (i.e., without BESS) to enhance the frequency resilience of a low-inertia grid.