Non-synchronous Generation Research Articles

Impact of current limiters and fast voltage boosters in grid-forming VSC-based generators on transient stability

Transient stability is a complex phenomenon presented in multi-machine and multi-converter systems, and it is still considered a key limiting factor for stressed power systems. The increasing integration of non-synchronous generation further emphasises the need to address the challenges of improving the transient stability faced by these power systems. Several studies have focused on developing control strategies for Grid-forming voltage source converter (GFM-VSC) to improve transient stability. These strategies include the use of current limiting algorithms and/or control of active/reactive power injections. This paper investigates the impact of fast voltage boosters (FVBs) and hybrid current limiters (HCLs) on transient stability of power systems with 100% grid-forming VSC-based generators. Short-circuit simulations and critical clearing time analysis are performed to evaluate the effectiveness of HCLs and FVBs in improving transient stability. The simulation results demonstrate the effectiveness of these approaches in avoiding the loss of synchronism. This research contributes to the current studies on transient stability in power systems and provides valuable insights into the potential of HCLs and FVBs as effective approaches to improve system stability.

Operation Constraints and Methodology for Stability Preservation in Power Systems with High Penetration of Non-Synchronous Generation

Nowadays, significant changes in the electricity generation mix raise questions about the secure operation of power systems. Most of the newly installed generation capacity is being connected to the grid via power electronic converters, thus can be seen as a non-synchronous generation. The theory of power system operation and control relies heavily on the characteristics of synchronous machines and conventional network structure. Therefore, the effective integration process of the new technologies must include the composition of new physical models for converter dominated large power systems as well as innovative solutions to ensure the secure operation in the future. The focus of this paper is a holistic analysis of the reducing power system inertia to frame up new constraints for system operators through simulation studies on the Institute of Electrical and Electronics Engineers standard 118 bus test system. The different system states and scenarios offer a comparison opportunity between stability preservation possibilities. Minimum inertia constraint calculation methodologies and various objective functions are being discussed. The utilization and effects of synthetic inertia from non-synchronous generators and energy storage systems is also considered to quantify the exact effects.

P-f Stability of 2030 Nordic Power System due to Non-Synchronous Generation

The Nordic Power System is undergoing significant transformation and development in the coming years. A driving factor for this is the increased dependency on renewable energy sources and high voltage direct current transmission (HVDC). Various stability challenges due to the dynamics of reactive power - voltage (Q-V) balancing and active power - frequency (P-f) balancing seep into the system with such rise of non-synchronous generation (NSG). This paper investigates the projected impact of NSGs on the P-f dynamic performance of Nordic power system in 2030. As NSGs are systems with low inertia, power discrepancies lead to larger oscillations, higher Rate of Change of Frequency (RoCoF) and bigger maximum frequency deviation (MFD) values. To address these issues, Emulated Inertia (EI) control for wind turbines and Synthetic Inertia (SI) control for VSC-HVDC links are proposed in this paper. The simulation is carried out using DIgSILENT PowerFactory 2022 SP1 simulation software. The results obtained confirm the Effectiveness of the proposed EI and SI methods to enhance frequency response and mitigate deviations.

A Bayesian Approach for Online Inertia Estimation of Synchronous and Nonsynchronous Generators

A Bayesian Approach for Online Inertia Estimation of Synchronous and Nonsynchronous Generators

Frequency Stability Analysis of a Low Inertia Power System with Interactions among Power Electronics Interfaced Generators with Frequency Response Capabilities

One of the main actions required to face and limit global warming is the substitution of conventional fossil-fueled electrical generators with renewable ones. Thus, it becomes fundamental to create non-dispatchable renewable generators able to provide services for power system stabilization that nowadays are delivered by conventional ones. Particularly, renewable generators are usually connected to the electrical power system through power electronic converters lacking natural responses to frequency variations. This challenges conventional frequency control methods that are based on synchronous generators’ capabilities, particularly in systems with high levels of non-synchronous generation. Solutions based on advanced controls that allow renewable generators to participate in frequency control are the subject of current research efforts worldwide. This paper contributes to these efforts by studying the benefits of introducing Power Reserve Control in photovoltaic generators and Extended Optimal Power Point Tracking control in wind generators to provide frequency control in low inertia power systems and the interactions between them. The tests and the simulations, prove that these kinds of controls help in stabilizing the system frequency thanks to the cooperative action of both types of renewable generators.

Radical change in the Spanish grid: Renewable energy generation profile and electric energy excess

The National Integrated Energy and Climate Plan 2021–2030 presented by Spain expected to increase the installed power of renewable energy in 62 GW by 2030. However, the electricity operator system (Red Eléctrica Española (REE)) foresees a different scenario, in which the new approved power generation plants (non-synchronous generation i.e.: wind and photovoltaic), for the peninsular system, will reach 84 GW. Moreover, the total power of the request forms, which have license to access the grid, rise up to 143 GW. This scenario leads to a new era of power generation in which the generation patterns differ from the current ones diverting the consumption profiles or making imperative new ways of energy storage to be able to reach the expected Renewable Energy share or prevent massive curtailments. The analysis presented in this work is based on the modelling and simulation of the wind and solar energy generation for several scenarios. The results of this analysis show a lack of energy balance (an excess of energy between 92 and 168 TWh with overgeneration during more than 3500 h after balancing mechanisms) due to the non-integrated technologies and due to the (over expected) future installed power derived from REE reports. In addition, the energy available for charging will be higher than the hypothetical discharge to the grid. This allows low charge–discharge efficiencies. Two energy storage technologies: thermal storage for thermal plants retrofitting and reversible hydrogen systems are proposed for analysis. These novel energy balancing technologies are economically modelled and assessed for this specific energy scenario and present moderate paybacks. Even with a high investment in storage facilities, and taking into account the exported energy, the in-excess energy will be very high. This will lead to massive curtailments of renewables (between 35% and 40%) or to a paradigm shift on the energy consumption behaviours.

Review of Stability Analysis in Power Systems with High Penetration of Non-synchronous Generation

Sustainability goals basically change the power generation portfolio, as the renewable energy resources developing rapidly. This means system integration challenges for system operators, as these new resources have different attributes. Most of the emerging generation technologies connect to the grid via power electronic converters, therefore does not contribute inherently to the system’s rotational inertia. However, advanced control methods became a progressive research area, where many practical pilots deliver valuable results and discussions nowadays. These stability enhancement techniques have great potential to enhance security of supply with synthetic inertia services. This paper aims to review generation-driven stability analysis techniques based on the modelling methods used for the nonsynchronous generators. Based on the stability phenomena (angle, frequency), system size, generation portfolio, computational time the adequate methods could be quite different. The proposed extension of the stability classification convers the need of different approaches with the integration of non-synchronous generation. Based on the review presented in this paper, the stability assessment needs for a specific system operator can be identified to choose the appropriate modelling assumptions. This can help system operators in long-term planning (stability adequacy) and in the dynamic security assessment tasks as well, both on the identification of possible issues and evaluate the countermeasures.

Probabilistic Day-Ahead Inertia Forecasting

Power system inertia is declining and is increasingly variable and uncertain in regions where the penetration of non-synchronous generation and interconnectors is growing. This presents a challenge to power system operators who must take appropriate actions to ensure the stability and security of power systems relying on short-term forecasts of the system’s inertial response. Existing models to forecast inertia fail to quantify uncertainty, which may prevent their utilization given the risk aversion of the system operators when handling stability issues. This paper is the first to develop a model to produce calibrated, data-driven probabilistic forecasts of the inertia contribution of transmission-connected synchronous generators. The model provides a necessary tool for system operators to quantify forecast uncertainty, allowing them to manage the risk of frequency instability cost-effectively. The paper demonstrates that the assumption of a Gaussian distribution of uncertainty applied in existing models is not acceptable to accurately forecast the inertial response and provides a satisfactory forecast model by combining non-parametric density forecasting with parametric tail distributions. Moreover, the paper shows that satisfactory predictive performance can only be achieved by adopting a rolling horizon forecast approach to deal with the rapidly changing characteristics of the inertial response in power systems.

Sequential feature selection for power system event classification utilizing wide-area PMU data

The increasing penetration of intermittent, non-synchronous generation has led to a reduction in total power system inertia. Low inertia systems are more sensitive to sudden changes and more susceptible to secondary issues that can result in large-scale events. Due to the short time frames involved, automatic methods for power system event detection and diagnosis are required. Wide-area monitoring systems (WAMS) can provide the data required to detect and diagnose events. However, due to the increasing quantity of data, it is almost impossible for power system operators to manually process raw data. The important information is required to be extracted and presented to system operators for real/near-time decision-making and control. This study demonstrates an approach for the wide-area classification of many power system events. A mixture of sequential feature selection and linear discriminant analysis (LAD) is adopted to reduce the dimensionality of PMU data. Successful event classification is obtained by employing quadratic discriminant analysis (QDA) on wide-area synchronized frequency, phase angle, and voltage measurements. The reliability of the proposed method is evaluated using simulated case studies and benchmarked against other classification methods.

Power System Oscillations with Different Prevalence of Grid-Following and Grid-Forming Converters

The oscillatory behaviour of the power system is an aspect that is significantly affected by the increasing integration of converter-based generation sources. Several works address the impact of non-synchronous generation on the operation of the system from different points of view, but only a few studies focus on power-frequency oscillations with a prevalence of generation sources interfaced through power electronics. A lack of research can be found in particular in the comparative analysis of the two main control strategies for power converters, namely grid-following and grid-forming. The article aims to contribute to this direction, starting from a theoretical analysis of the two control structures and then examining the case study of an existing transmission system. The research provides a specific insight into the fundamental aspects related to synchronisation mechanism and inertial capabilities of both grid-following with synthetic inertia and grid-forming controls. The difference in the relationship between synchronisation unit and inertial capability is recognised as the fundamental aspect determining the different impacts on the oscillatory characteristics of the system. The observation derived in the theoretical analysis is then applied to an actual power system with a high predominance of converter-based generation, considering the Colombian interconnected national system as a case study.

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.

Frequency Dynamics in Fully Non-Synchronous Electrical Grids: A Case Study of an Existing Island

The operation of a power system with 100% converter-interfaced generation poses several questions and challenges regarding various aspects of the design and the control of the system. Existing literature on the integration of renewable energy sources in isolated systems mainly focuses on energy aspects or steady-state issues, and only a few studies examine the dynamic issues of autonomous networks operated with fully non-synchronous generation. A lack of research can be found in particular in the determination of the required amount of grid-forming power, the selection of the number and rated power of the units which should implement the grid-forming controls, and the relative locations of the grid-forming converters. The paper aims to address those research gaps starting from a theoretical point of view and then by examining the actual electrical network of an existing island as a case study. The results obtained from the investigations indicate specific observations and design opportunities, which are essential for securing the synchronization and the stability of the grid. Possible solutions for a fully non-synchronous operation of autonomous systems, in terms of dynamic characteristics and frequency stability, are presented and discussed.

Power System Coherency Detection From Wide-Area Measurements by Typicality-Based Data Analysis

This paper presents a new data-driven methodology for power system coherency identification of generator and non-generator buses. This methodology is exclusively based on intrinsic statistical properties extracted directly from observations, without any prior assumption of the probability distribution function (PDF) for the data. The main advances of this proposal are: ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> ) gathering of statistical information from the data itself despite scenarios where the PDF may change (different inverter-based load and generation scenarios, load levels of the system, and changes in topology); and ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ii</i> ) assignment of buses into coherent areas without any tuning of parameters, nor manually labeling of huge amounts of training data. This new method, called typicality-based data analysis (TDA), is applied to the correlation metric of the distance between dynamic responses of buses, either voltage angles or frequencies. Simulated signals from a benchmark power system with cases considering the presence of non-synchronous generation and islanding conditions, and real data associated with generation trips in the U.S. Eastern Interconnection are used to corroborate the methodology effectiveness.

Wind energy is not sustainable when balanced by fossil energy

Ensuring access to affordable, reliable, sustainable and modern energy for all is one of the Sustainable Development Goals. Some countries have therefore invested significantly in wind energy, but emissions, which is a common measure for sustainability in this context, have not fallen significantly. Reductions between 20% and 40% are typical. We therefore test the hypothesis that wind energy reduces emissions compared to using gas turbines when life-cycle emissions are included. The Irish grid is studied due to its record-high wind penetration. The model is based on high resolution grid data covering four years and input from 828 Life-Cycle Assessment cases to allow detailed analysis of demand, supply, life-cycle emissions and their changes due to the increased ramping of gas turbines and increased grid reserves required to maintain grid reliability when wind is deployed. Indirect effects are included to some extent. The model is sampled 10,000 times using Monte Carlo simulations. The results show that emissions are reduced by 10–20%, which supports the hypothesis. However, with an average wind penetration of 34% in 2019, reaching many times the 65% limit for non-synchronous generation set by the system operator to maintain grid reliability, such modest reductions logically imply that achieving an affordable, low-carbon grid using wind together with fossil energy balancing is infeasible with today’s technology, emissions and costs. This key finding is transferable to other grids where wind has large penetration and requires fossil energy balancing. Thus, wind energy is not sustainable when balanced by fossil fuel generators.

Grid access of non-synchronous generation: Review of the Spanish regulation

The development of wind and solar photovoltaic generation depends critically on the access to the grid. In contrast to synchronous generation, the access to the grid of converter based generation (also called non-synchronous generation) is affected by a number of technical constraints. The Spanish regulation of the grid access of non-synchronous generation has been recently reformulated. This contribution will review the new regulation. The past regulation will be also discussed.

Frequency Stability Issues and Research Opportunities in Converter Dominated Power System

Stable power supply has become a crucial thing in the current era of technology and automation. Although the power system has multiple stability issues and causes, frequency fluctuation plays a vital role in normal operation, whereby a system with significant frequency deviation can lead to the needless blackouts of the whole power system. With the rapid growth in power electronic converter (PEC)-based technologies and the huge penetration of nonsynchronous generators, the modern power system is becoming more complex by the day. This paper provides a comprehensive study on the stability issues that occur in modern power systems, mainly due to PEC-based technology integration. The in-depth reasons and the impacts of unstable power systems, along with their controlling techniques, are discussed to generate a clear understanding. Furthermore, the importance of frequency stability in a power system is discussed with respect to some important events that occurred in the past. This paper also discusses some potential techniques that could be performed to overcome the existing and/or upcoming challenges in the upgrading power system.

A novel adaptive linear prediction-based parameter estimation method for monitoring sub-/inter-harmonics during SSI events

Sub-Synchronous Interaction (SSI), caused by the increasing penetration of inverter connected nonsynchronous generation, is posing a serious threat on power system stability. As the sub-/inter-harmonics emerging during SSI are featured by time-varying frequencies, concerns regarding the accuracy and the robustness of existing methods for monitoring oscillations of such phenomenon have been raised. This paper proposes a novel two-stage numerical method for monitoring the sub-harmonics, harmonics and inter-harmonics during SSI events. In the first stage, the model order of the estimation is selected using the proposed Adaptive Model Order Selection (AMOS) scheme, which is achieved through singular value decomposition. In the second stage, a linear prediction-based parameter estimation approach with high accuracy and strong robustness is used to estimate the signal parameters of the sub-harmonics, harmonics and inter-harmonics. By merging the above two stages, a novel and effective numerical method is obtained, namely the Adaptive Linear Prediction-based Parameter Estimation (ALPPE) method. In addition, an estimation calibration is implemented to increase the estimation accuracy. The proposed ALPPE method has then been tested using computer simulated signals and field measured data from an actual SSI event. The results of the tests confirmed the ALPPE method's high accuracy, adaptability, processing speed and immunity to noise.

Analysis of Control Strategies Based on Virtual Inertia for the Improvement of Frequency Stability in an Islanded Grid with Wind Generators and Battery Energy Storage Systems

Rising levels of non-synchronous generation in power systems are leading to increasing difficulties in primary frequency control. In response, there has been much research effort aimed at providing individual electronic interfaced generators with different frequency response capabilities. There is now a growing research interest in analyzing the interactions among different power system elements that include these features. This paper explores how the implementation of control strategies based on the concept of virtual inertia can help to improve frequency stability. More specifically, the work is focused on islanded systems with high share of wind generation interacting with battery energy storage systems. The paper presents a methodology for modeling a power system with virtual primary frequency control, as an aid to power system planning and operation. The methodology and its implementation are illustrated with a real case study.

Data-Driven Trajectory Prediction of Grid Power Frequency Based on Neural Models

Frequency in power systems is a real-time information that shows the balance between generation and demand. Good system frequency observation is vital for system security and protection. This paper analyses the system frequency response following disturbances and proposes a data-driven approach for predicting it by using machine learning techniques like Nonlinear Auto-regressive (NAR) Neural Networks (NN) and Long Short Term Memory (LSTM) networks from simulated and measured Phasor Measurement Unit (PMU) data. The proposed method uses a horizon-window that reconstructs the frequency input time-series data in order to predict the frequency features such as Nadir. Simulated scenarios are based on the gradual inertia reduction by including non-synchronous generation into the Nordic 32 test system, whereas the PMU collected data is taken from different locations in the Nordic Power System (NPS). Several horizon-windows are experimented in order to observe an adequate margin of prediction. Scenarios considering noisy signals are also evaluated in order to provide a robustness index of predictability. Results show the proper performance of the method and the adequate level of prediction based on the Root Mean Squared Error (RMSE) index.

Impact of the high share of converter‐interfaced generation on electromechanical oscillations in Continental Europe power system

This study investigates the effects of converter-interfaced generation integration on the dynamic response of the power system of Continental Europe. The system is analysed with a large-scale dynamic model of the entire synchronous area, considering different instantaneous integration percentages of converter-interfaced generation across the system. The study focuses on the reduction of the overall available kinetic energy and the impact on frequency dynamics and system oscillations. The dynamic model of the system originally provided by ENTSO-E is further developed according to a specific methodology, replacing a determined amount of synchronous generation and introducing a corresponding amount of converter-controlled current sources. The reference incident of a generation loss in Western Europe specified by ENTSO-E is considered in the analysis. The results of time-domain simulations and modal analysis show how the integration of non-synchronous generation affects the frequency dynamics of Western, Central and Eastern Europe, bringing to attention some relevant effects of the spatial distribution of different generation sources within an extensive system as the Continental Europe synchronous area.