Low Inertia Power Systems Research Articles

A power-hardware-in-the-loop testbed for intelligent operation and control of low-inertia power systems

Abstract Low-inertia power systems, i.e., power-electronics-dominated power systems, possess significantly different dynamics to conventional power systems, both on a component and a system-wide level. A direct implication of these substantial changes is that a pure simulation-based assessment of novel control and operational schemes for such systems is insufficient. Instead, flexible and easily reconfigurable experimental testing facilities are required. A prominent concept to enable such capabilities is power-hardware-in-the-loop (PHiL) testing. We present a PHiL testbed facility (230/400 VAC, 750 VDC, 100 kW) specifically designed for experimentally testing and validating control and operational schemes for low-inertia power systems. The main features of the testbed are its flexibility to rapidly implement and test advanced control algorithms, ranging from low-level controls of individual components to distributed and system-wide controls, its ability to be configured with different network topologies, and the efficient emulation of commonly observed parameter uncertainties as well as disturbances. The detailed description of the PHiL testbed is complemented by a performance demonstration via a case study.

Active power oscillation suppression and dynamic performance improvement for multi-VSG grids based on consensus control via COI frequency

Virtual synchronous generator (VSG) is deemed to be the key foundation for the future low inertia power system for its superiority to provide flexible inertia support. However, complicated dynamic frequency interaction, active power oscillation, and inappropriate transient active power-sharing will occur when VSGs are widely applied. Center of Inertia (COI) is introduced for solving these problems. But the calculation of COI frequency relies on global communication and a centralized controller, which results in high communication costs and complexity, and reduces the system reliability. This paper proposes a distributed control method via COI frequency for multi-VSG grids. The COI frequency estimator is designed based on dynamic consensus control. Communication between adjacent neighbors is required, which improves reliability and reduces communication costs. The stability of the proposed control method is proved and the feasible range of the control parameter is presented in this paper. Simulations and experiments are carried out to verify the effectiveness and feasibility.

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.

Provision of Virtual Inertia Support Using Battery Energy Storage System

The paper presents the importance of the grid inertia constant for the frequency stability of the future high-res low-inertia power systems. Since more and more renewable energy sources (RES) are being connected to the power system via frequency converters, the grid inertia constant is reduced. This issue can be mitigated by applying appropriate control mechanisms through which RES can provide virtual inertia and provide rotating reserves for primary frequency control. The concept of a virtual synchronous generator for providing virtual inertia is elaborated, as a solution to the presented problems. By applying virtual synchronous generators, RES can provide support for frequency control during disturbances almost like conventional synchronous generators. The influence of virtual inertia on the stability of the Croatian power system was analyzed using a battery energy storage systems (BESS) with a control mechanism that enables its participation in frequency control.

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.

A Comprehensive Review on a Virtual-Synchronous Generator: Topologies, Control Orders and Techniques, Energy Storages, and Applications

In recent years, the penetration of renewable power generations into the electrical grid has substantially increased. Continuous deployment of power electronic-based distributed generations and the reduction of traditional synchronous machines with their essential dynamics in modern power networks are very critical in this change. The use of power electronic inverters leads to the dissociation of sources and loads and lowering the power system inertia. Under power imbalance, this drop causes an elevated rate of change in frequency and frequency divergences, which has a notable impact on the system’s frequency stability. As a result, enhanced control techniques for grid-tied electronic converters are required to secure the power system’s stability and support. The virtual-synchronous generator (VSG) control is used to mimic the dynamics of a rotating synchronous generator and improve the power system’s stability. In this article, the problems of such low-inertia power systems, as well as the VSG technologies, are explored. This research also looks at different control orders and strategies for virtual-synchronous generators (VSG). In addition, the utilization of energy storage and critical matters in VSG and further research recommendations are explained.

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.

Performance analysis of grid-forming inverters in existing conformance testing

Grid-forming (GFM) inverters are promising technologies in future power systems. Although the voltage-source characteristic of the GFM inverter has been validated to enhance the stabilities in low-inertia power systems, modifying protective function mechanisms is needed from grid-following (GFL) inverters with the current-source characteristic. Therefore, the protective functions that coexist with grid stabilization capabilities in GFM inverters and their verification methods should be studied for practical deployment. This paper applies the existing Japanese conformance testing on prototypes of GFL and GFM inverters to reveal issues in the existing conformance testing framework, which was not designed to consider the voltage-source characteristics of GFM inverters. The test results identify the three main issues of GFM inverters in the existing test framework. The discussion based on the results will provide helpful knowledge to design future provisions on the functional requirements of GFM inverters and their verification methods.

Comparison of Phase-Locked Loops Used for Frequency Measurements in a Low-Inertia Power Grid with Wind Generation

The need for more precise frequency measurements in electric power systems is increasing because of the growing penetration of renewable energy and the subsequent reduction in inertia, which gives rise to more intense frequency fluctuations. Phase-locked loops (PLLs) are now more accurate and present faster dynamic responses, helping to implement primary frequency controllers for renewable generators connected to the grid through power electronics. However, there are differences among PLL implementations that affect their behavior when estimating frequency, depending on factors as location, voltage level, or penetration of renewable energy, among other things. How this affects the ability of PLL-based frequency controllers to adequately estimate the instantaneous mismatch between generation and demand, especially in low-inertia power systems, is still unclear, and constitutes an open research topic. This paper contributes to this research effort by presenting a comparative study of different PLL configurations in order to gain insight into their performance in different scenarios in a low-inertia power system with a high share of wind energy. Constant and variable wind speed scenarios are considered, and PLL behavior is assessed in terms of the absolute mean value and the root-mean-square value of the frequency error, and the absolute value of the frequency derivative error. The results show that the PLL behavior depends on the voltage level and the location, and it was found that the further away the PLL is from the source that controls the frequency, the more noise is produced, which impacts the quality of the frequency measurement.

Control of Low-Inertia Power Systems

Electric power systems are undergoing an unprecedented transition from fossil fuel–based power plants to low-inertia systems that rely predominantly on power electronics and renewable energy resources. This article reviews the resulting control challenges and modeling fallacies, at both the device and system level, and focuses on novel aspects or classical concepts that need to be revised in light of the transition to low-inertia systems. To this end, we survey the literature on modeling of low-inertia systems, review research on the control of grid-connected power converters, and discuss the frequency dynamics of low-inertia systems. Moreover, we discuss system-level services from a control perspective. Overall, we conclude that the system-theoretic mindset is essential to bridge different research communities and understand the complex interactions of power electronics, electric machines, and their controls in large-scale low-inertia power systems.

Model Predictive Control–Based Load-Frequency Regulation of Grid-Forming Inverter–Based Power Systems

The high-penetrated renewables enable flexible and versatile dispatchable resources in power systems. As the digitalization process improves with the help of information and communication technology (ICT), a real-time online control framework is essential to regulate such a low-inertia power system dominated by renewables. To enable control practices, a control-oriented model is developed linking load increments to frequency dynamics. Based on such a control-oriented model, a model predictive control (MPC)–based online feedback algorithm is well designed by the virtue of rapid data transmission enabled by digitalization. The MPC optimization is a convex quadratic programming, where an objective function that balances frequency deviations and load increments is formulated and operational constraints are integrated into a matrix-type inequality. In the end, the proposed MPC-based load-frequency control framework is illustrated by several case studies, where validations and analyses of constraints inside the model are also included.

Adaptive droop control strategy for Flywheel Energy Storage Systems: A Power Hardware-in-the-Loop validation

Low-inertia power systems can suffer from high rates of change of frequency during imbalances between the generation and the demand. Fast-reacting storage systems such as a Flywheel Energy Storage System (FESS) can help maintain the frequency by quickly reacting to frequency disturbances, with no concern over its lifetime. While a modern high-speed FESS has a significantly higher energy density than the conventional low-speed ones, the capacity of this storage technology is still limited. Therefore, this paper proposes a new adaptive droop controller for a FESS, considering the practical advantages and also limitations of this storage technology. The proposed controller increases the contribution of the FESS for frequency support during the first instances of a disturbance, while it reduces its output when the frequency is recovering. To verify the advantages of the proposed control strategy, the controller is implemented on a real 60 kW high-speed FESS using the concept of rapid control prototyping. Next, the performance of the FESS with the new controller is tested using Power Hardware-in-the-Loop simulations in a low-voltage microgrid. The PHIL simulation results show that the proposed adaptive controller improves the performance of the FESS in terms of limiting the frequency deviations, while preserving more energy in the FESS.

Quantifying the uncertainty imposed by inaccurate modeling of active distribution grids

Replacing conventional generation with inverter-interfaced units has turned distribution networks (DNs) from consumers to active and responding intelligent DNs. These modern DNs contain several devices that can support the transmission network (TN) and system stability. Typically, deterministic and aggregated models for inverter-interfaced generation and conventional loads are used to include entire DNs in bulk system stability studies, and contributions from smart loads are neglected. This approach introduces errors to the dynamic modeling that can lead to instabilities. In this paper, we first present a full detailed model of a modern DN, enhancing existing thermal load and distributed generation models to include frequency and voltage support and protection functions required in low-inertia systems. Then, we incorporate the uncertainty that stems from the parameterization of such units using a Monte-Carlo method. Finally, we assess the impact of neglecting specific protection and support functions against frequency disturbances. The results show the crucial importance of accurately modeling protection and support functions to analyze the impact of modern DNs on bulk system stability. In addition, the findings highlight the increased relevance of considering uncertainty in stability studies of weak and low-inertia power systems.

Small-Signal Distributed Frequency Modeling and Analysis for Grid-Forming Inverter-Based Power Systems

More and more renewable energy sources are integrated into power grids, leading to a power electronic-based low-inertia power system. The grid-forming (GFM) inverter is an effective method for improving the inertia of the system. However, with the increased GFM inverters in the system, how the multiple control parameters affect the frequency response is still not clear. In this study, first, the power-phase model of the power grid is established; then, a small-signal distributed frequency model of the GFM inverter-based power system is established associating with the power-phase model of the power grid and the power-frequency model of the GFM inverter. Based on the proposed model, the influence of the multiple parameters to the frequency response is analyzed. It is concluded that both the inertia and damping coefficient affect the settling time, overshoot, and oscillation of the frequency. Finally, the simulation results verify the proposed model and the conclusion.

Guest Editorial: Special issue from 9th IET Renewable Power Generation Conference

The 9th IET Renewable Power Generation Conference took place online in March 2021, having originally been intended to take place in Dublin, Ireland some months earlier. A major motivation for the conference was the recognition of the challenges and possibilities arising from increasing renewable shares in many countries, and the critical need for cost-effective, reliable, and robust solutions as part of the ongoing transformation of our power and energy systems. Despite the restrictions of the pandemic, approximately 70 papers were successfully presented at the conference across 15 oral and poster sessions, covering a wide range of themes including low inertia power systems, wind power plant modelling and control, renewable energy forecasting, distribution network operation and long-term planning, power system protection, electricity markets and system services etc. Subsequent to the conference, a number of the authors of the presented papers were invited to submit an extended version of their papers to a special issue of the IET RPG journal. Ultimately, after a thorough peer review process, 16 papers were accepted for the special issue, which have been grouped here under five main themes: electrical network operation and planning; power electronic converter control systems; flexibility/frequency support measures and electricity markets; power system security, protection and monitoring; and tidal stream energy.

Comparative small-signal evaluation of advanced grid-forming control techniques

This paper presents the small-signal modelling and analysis of the most recent grid-forming control techniques, namely the matching control (MC) and the dispatchable virtual oscillator (dVOC). These are compared to more classical SME techniques such as synchronverters (SV) and voltage-controlled virtual synchronous machines (VCVSM) under different grid conditions. In addition to studying the time-domain response (inertial behaviour, steady-state operation point, etc.), a thorough evaluation of the dominant eigenvalues of each control is carried out by obtaining the participation factors and the sensitivity to physical and control parameter variations. Simulation results are obtained for a grid-forming converter connected to a dynamic grid model that emulates different properties of low-inertia power systems—e.g. primary reserve, inertial strength, coupling strength, etc. Besides, hardware-in-the-loop experimental results are presented to validate the analysis.

A Novel Approach for the Implementation of Fast Frequency Control in Low-Inertia Power Systems Based on Local Measurements and Provision Costs

Transitioning towards carbon-free energy has brought severe difficulties related to reduced inertia in electric power systems. Regarding frequency stability, low-inertia systems are more sensitive to disturbance, and traditional frequency control is becoming insufficient to maintain frequency within acceptable limits. Consequently, there is a necessity for faster frequency support that can be activated before the primary frequency control and that can decelerate further frequency decay. This paper proposes a local control strategy for a multi-stage fast frequency response (FFR) provided as an ancillary service that considers the location of the disturbance and the distribution of system inertia. The novelty of the presented control strategy is the ranking of FFR resources by price, which takes the economic component into consideration. The proposed control is simple, based only on RoCoF measurements that trigger the activation of FFR resources. Its advantage over other methods is the ability to adapt the FFR resource response to the disturbance without complex calculations and the ability to ensure a bigger response closer to the disturbance, as well as in low-inertia parts of the system. In that way, there is a bigger activation of resources in the parts of the system that are more endangered by disturbances, which, as a result, minimizes the propagation of the disturbance’s impact on system stability. The applicability of the presented method is demonstrated in a simple 3-area power system and IEEE 68-bus system implemented in MATLAB/Simulink. The results show that the proposed control enables the largest response closer to the disturbance, thus mitigating the propagation of the disturbance. Furthermore, the results confirm that the proposed control enables lower provision costs and more support in low-inertia areas that are more vulnerable to disturbances.

Verifying Machine Learning conclusions for securing Low Inertia systems

Machine Learning (ML) for real-time Dynamic Security Assessment (DSA) promises a probabilistic approach to secure lower safety margins and costs. However, future systems with a high share of renewables have low inertia and converter-interfaced devices resulting in faster dynamics. Past research on ML-based DSA used high inertia systems to study ‘the best’ ML data, features, and models building upon each other’s work for decades. Seldom has ML-based research for DSA questioned whether the underlying assumptions for (and the conclusions of) these studies are still valid for low inertia systems. This work studies exemplary changes in assumptions (and conclusions) for ML-based DSA when moving from High Inertia (HI) to Low Inertia (LI) systems. The dynamical system of the LI system is brought in perspective with the most typical ML-based approaches, which are organised in sequential steps. The steps consider the generation of the training database, the data pre-processing and feature selection, the model training and validation. This work analyses each step individually for the changed assumptions in the dynamical LI system, and subsequently, a case study provides the evidence that considering a LI system to identify the ‘best’ ML approaches is important. The case studies on IEEE 14 and 68 bus systems confirm that LI systems must be optimised for security (otherwise, they result in 80% less security than HI systems). The key findings, however, are that using ML makes significantly more sense in LI systems than in HI systems as the LI dynamics are in shorter timescales (and the advantage of ML is to predict security in milliseconds) and that secure/insecure operations can be separated more straightforwardly in LI systems as ML increases the accuracy by up-to 40% towards close to 100% when using neural networks. • A dynamic model for low inertia power systems. • Impact analysis of the newly introduced low-inertia dynamics on ML-based real-time dynamic security assessment. • Case studies on the IEEE 14 and 68 bus system to compare the performance of ML-based approaches in high and low inertia systems. • High performance of ML-based security in low inertia systems.

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.

Control of Distributed Photovoltaic Inverters for Frequency Support and System Recovery

Replacing conventional synchronous generator-based power plants with inverter-based renewable energy resources results in a reduction of the inertia in power systems. To sustain the security and reliability of these low-inertia power systems, frequency support is increasingly required in new standards for grid-connected renewable energy resources, especially distributed photovoltaic (DPV) systems. This article proposes a frequency droop-based control in DPV inverters to improve frequency response in power grids with high penetration of renewable energy resources. A predefined power reserve is kept in the DPV inverter, using flexible power point tracking. The proposed algorithm uses this available power reserve to support the grid frequency. Furthermore, a recovery process is proposed to continue injecting the maximum power after the disturbance, until frequency steady-state conditions are met. The positive inertia using the proposed recovery mode, leads to recovery of the frequency in a shorter period. A composite load model of a distribution feeder, including DPV, is developed to assess the effectiveness of the proposed frequency support algorithm in power systems with high penetration of DPV inverters. The dynamic performance of the proposed frequency support algorithm with over frequency and under frequency events is also evaluated experimentally.