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A Framework to Assess the Effect of Reduction in Inertia on System Frequency Response
Abstract
This paper presents a framework to analyze the impact of different penetration levels of renewable generation sources (RES) and consequent reduction in inertia on grid frequency. The developed methodology is demonstrated using three operating conditions of the network. For each operating condition, the decrease in the network loading is balanced by disconnecting a part of synchronous generation. To establish the critical penetration levels of renewables and inertia limits for the grid frequency, the uncertainty of loads, intermittent and stochastic patterns of RES generation around each operating condition are simulated. The results clearly identify the critical penetration levels of RES and reduction in inertia limits of the system for frequency stability. In addition, the performed analysis quantifies the effect of primary frequency response and reduction in inertia on frequency nadir. The proposed framework is applied to the modified 16 machine and 68 bus network.
... Wind speed [120][121][122][123][124], system loads [120,124,125], and PV generation [120,125]. ...
... Wind speed [120][121][122][123][124], system loads [120,124,125], and PV generation [120,125]. ...
... Rate of change of frequency (ROCOF) [120,123,124,126], frequency nadir [120,[123][124][125][126], frequency excursion [123,126], and frequency response inadequacy (FRI) [120][121][122][123]. ...
In pursuit of identifying the most accurate and efficient uncertainty modelling (UM) techniques, this paper provides an extensive review and classification of the available UM techniques for probabilistic power system stability analysis. The increased penetration of system uncertainties related to renewable energy sources, new types of loads and their fluctuations, and deregulation of the electricity markets necessitates probabilistic power system analysis. The abovementioned factors significantly affect the power system stability, which requires computationally intensive simulation, including frequency, voltage, transient, and small disturbance stability. Altogether 40 UM techniques are collated with their characteristics, advantages, disadvantages, and application areas, particularly highlighting their accuracy and efficiency (as both are crucial for power system stability applications). This review recommends the most accurate and efficient UM techniques that could be used for probabilistic stability analysis of renewable-rich power systems.
... The increasing number of inverter-based Distributed Energy Resources (DERs) and High Voltage Direct Current (HVDC) tielines reduces the inertia of the power system and thereby its ability to withstand disturbances [1], [2]. Established Load Frequency Control (LFC) schemes, notably Automatic Generation Control (AGC), have difficulties coping with the entailed changing dynamics and the volatility of renewable energy sources because of their rigid tuning requirements [3], [4]. ...
... with the inertia J and the (angular) frequency ω = 2πf [26]. Within a synchronous area A, we define a neighborhood N H i = {i, AC i } ⊆ A to describe its dynamic behavior using (2). The Cell under consideration is denoted by i, and j ∈ AC i are the adjacent Cells coupled over tie-lines with the breaker state δ ij (n). ...
... The simulation model of SYSLAB in Section IV-A was developed in MATLAB/Simulink. Each Cell is modeled using one swing equation (2) and the physical resources in Section III-A, incorporating the following constraints: Output saturations, ramping rate P ramp limiting the change of primary and secondary power, and the reaction time T sec delay of secondary resources. Tielines are modeled as serial admittances as described in [31] without the parallel susceptances. ...
The increasing share of volatile and inverter-based energy sources render electric power grids increasingly susceptible to disturbances. Established Load Frequency Control (LFC) schemes are rigid and require careful tuning, making them unsuitable for dynamically changing environments. In this paper, we present a fast and tuningless frequency control approach that tackles these shortcomings by means of modern grid monitoring and communications infrastructures in a two-fold concurrent process. First, direct observation of supply and demand enables fast power balancing decoupled from the total system dynamics. Second, primary resources are actively involved in frequency restoration by systematic adjustment of their frequency reference setpoints. In contrast to the commonly used Automatic Generation Control (AGC), the proposed Direct Load Frequency Control (DLFC) does not require an integrator for frequency control in the closed loop even under partial grid observability. The approach is Lyapunov-stable for a wide range of system parameters, including ramping limits of controlled resources. A performance study against AGC has been conducted on a three-area power system in simulations as well as in a real laboratory grid with an installed generation capacity of 110 kW.
... II. SYSTEM MODELING Reference [13] demonstrates that the frequency stability will deteriorate if online SG generation is less than 50% of the total generation. Therefore, a scenario in which the generation of SGs is responsible for approximately 50% of the total load demand is studied here, with the remaining demand being provided by IBGs. ...
... Based on the singular perturbation method, the root of the synchronization model should be calculated first. The solutions of the differential equation (16) Then the reduced "slow" submodule is derived, which is called the quasi-steady model of (13). ...
... Renewable energy sources have emerged as a viable option for meeting rising energy demand, mitigating climate change and contributing to sustainable growth [2,3]. e introduction of such systems is primarily accomplished through the use of wellorganized microgrid systems; this provides a collection of technical explanations that allow the sharing of knowledge between customers and distributed generation centers, implying that they must be handled optimally [4]. ...
... More research is being done on virtual inertia (VI) control and the effect of decreasing system rotation inertia on system frequency stability after increasing renewable energy permeability. Reference [4] examines the relationship between the frequency change rate, the lowest point of frequency, and the inertia of system to reveal the effect of system inertia reduction on frequency stability. Instead of synchronous generators, system reserve capacity and battery energy storage are used to provide inertia energy for smoothing renewable energy generation fluctuations [5]. ...
Power plant emissions are a major cause of pollution in the environment. This necessitates the progressive replacement of conventional power plants with renewable energy sources. Changes in the quotas for conventional generating and renewable energy sources present new issues for modern power networks for example photovoltaic and wind turbines are replacing conventional power plants, which do not add to system inertia and due to the earth’s diurnal cycle and weather conditions. Solar radiations are not consistent all through the day, and photovoltaic (PV) generation is sometimes insufficient to meet the power requirement of the shifting local load. The amount of inertia in the power system, as well as the action of adjustable frequency reserves and the amount of power imbalance, all have an impact on frequency stability. As a result, estimating power system inertia and assessing frequency response are required so that necessary actions can be taken to assure frequency stability. In this way, the system frequency, power, and voltage stability are the major issues when high proportion of renewables are added. In this paper, we explained estimating power system inertia-related frequency problems. The approach account for the frequency and voltage fluctuations that occur after a disturbance and estimate the system’s total inertia constant as well as its overall power imbalance. The anticipated technique based on computational intelligence is used to analyze frequency responses from simulations of a test system under various circumstances on SIMULINK and focuses on the standalone PV system is critical for controlling it. As a result, the modelling of a PV system, battery, and generator using analogous circuits is discussed. As a matter of fact, maximum power should be harvested from a PV array to increase its efficiency that is depicted from the result outcomes of this research.
... The authors proposed a framework to analyze the impact of increased penetration of RES on grid frequency taking into account the stochastic and intermittent patterns of RES [13]. The proposed methodology successfully identified the critical penetration levels and critical inertia of the system. ...
... As the inertia of the system reduces due to increased penetration of RES, small variations in active power disturbance and changes in headroom of SG cause bigger variation in frequency nadir. In these studies, a threshold of 500 mHz for frequency nadir variation is considered [13] as indicative of a critical inertia level given the same amount of PFR that is scheduled in the system for the same disturbance. ...
... The authors proposed a framework to analyze the impact of increased penetration of RES on grid frequency taking into account the stochastic and intermittent patterns of RES [13]. The proposed methodology successfully identified the critical penetration levels and critical inertia of the system. ...
... As the inertia of the system reduces due to increased penetration of RES, small variations in active power disturbance and changes in headroom of SG cause bigger variation in frequency nadir. In these studies, a threshold of 500 mHz for frequency nadir variation is considered [13] as indicative of a critical inertia level given the same amount of PFR that is scheduled in the system for the same disturbance. ...
This study presents a thorough analysis of the effect of load models on frequency response, small and large disturbance stability of the power system, in order to identify the type of stability exhibiting most sensitivity to load models, and for each type of studied stability to pinpoint the load model that has the worst effect. The presented analysis shows clearly that transient stability is the most sensitive to load models. The number of unstable cases varies considerably with each type of studied load model. The effect of the load model magnifies with the reduction in headroom of synchronous generators. The results of frequency response of the system following an active power disturbance demonstrate that the influence of constant power loads can be significant if the system is operating with reduced primary frequency response at high load. High integration of renewable energy sources (RES) increases the variation in the damping of electromechanical modes due to increased uncertainties, and a high proportion of induction machines can reduce the damping of inter‐area modes considerably, making a well stable mode unstable for certain operating points. The influence of load models has been illustrated using a 68‐bus system with 30% and 52% penetration of RES.
... The increasing penetration level of VER results in the degradation of the inertia energy of the system, and therefore, the importance of inertia management increases. Consequently, it becomes imperative to institute a dedicated evaluation framework for power system operation, as proposed in [17]. The framework would enable practical assessments of both VER penetration and frequency operation simultaneously, ensuring the preservation of frequency stability in adherence to operational rules. ...
While aiming to increase the dispatch of variable energy resources (VER) during the operation of a power system, the stability of the power system should be maintained, which can be realized by evaluating the penetration limit of the VER. This paper proposes a method for evaluating the penetration limit of VER. When a disturbance such as a generator trip occurs, the frequency stability depends on the inertia energy secured by generators, the characteristics of the primary frequency response, and the load response, which would change owing to an increase in the VER of the power system. In the proposed evaluation method, the frequency response performance of the power system is analyzed based on the historical operation data, and the penetration limit of VER is evaluated by applying the frequency criterion to maintain the frequency stability of the power system. Therefore, the potential penetration limit of VER can be estimated depending on the operating conditions. In addition, the effectiveness of the proposed evaluation method is verified by comparing the simulation results based on the Korean power system model-based simulation, and the online application is evaluated by applying it based on the real system operation data.
... Moreover, the replacement of the conventional synchronous generator providing inertia and frequency response by VER would degrade the frequency control performances of the power system. Therefore, the power system operator would need to secure the operation method for frequency stability [4]- [6]. As one of the measures to maintain frequency stability, the power system operates the BESS (Battery Energy Storage System) as a reserve power resource [7]- [10]. ...
Although the variable energy resources (VER) are expected to increase rapidly according to its expansion plan, the power system might have to limit the penetration level of VER for its frequency stability since VER has limited inertia and frequency response performance. The battery energy storage system (BESS) has been considered to be a solution for increasing the frequency stability limit of VER by using its fast frequency response capability. Since the performance of the BESS-based frequency response would depend on the control strategies, this paper analyzes its effects on increasing the penetration limit of VER in the power system by considering various control algorithms of BESS for providing the frequency response.
... First, the integration of converter-based generation, phase-out of thermal units lead to the reduction of system inertia [49,50]. The system inertia and rotating load-damping effect are further challenged by the growth of high voltage direct current connections [51] and the uptake of load frequency drivers [52], respectively. Second, the increased penetration of RESs and the phase-out campaigns of polluting thermal units reduce the number of generators able to provide reserve K. Sevdari et al. power for primary (containment reserve) and secondary (restoration reserve) frequency control [53]. ...
The transformation towards a sustainable power system calls for new ways of operating the network. In that regard, electric vehicles (EVs) with their charging infrastructure qualify as a flexible resource. This paper interconnects ancillary services and EV flexibility to help system operators (SOs) and flexibility providers understand the role and localize EV-chargers in the power system. First, the focus is on SOs. The manuscript reviews ancillary services based on power system operational challenges. The ancillary services are differentiated between 8 frequency and 32 flexibility services. These are then subdivided depending on the management control: the first group includes inertia, primary, and secondary/tertiary frequency control, while the second includes congestion management, voltage regulation, power quality, grid stability, and emission management. Of all the different services, the ones that can be provided by EV-charger are highlighted and classified into 12 geo-electrical charging clusters. Second, the focus is moved to the flexibility providers. Independently from location, to provide ancillary services with EVs, multiple actors are recognized: the end-user, the charging site operator (CSO), the charging point operator (CPO), the aggregator, the energy community, the distribution system operator (DSO), and the transmission system operator (TSO). The collaboration between the actors is today carried out by making alliances, to help exchange knowledge and gain confidence in ancillary services provision. In conclusion, the literature review presents the characteristics of 27 slow (up to 50 kW) smart chargers, the common flexibility features being scheduling (100%), modulation (89%), and phase switching (10%).
... The RESs exhibit no or very weak inertia support to the disturbances in the interconnected power system. Further, the available inertia in the power system with RESs is lower to support instant dynamic power mismatch due to high short-circuit capacity or low source-to-load impedance [3], [4]. Moreover, inertia supported by the synchronous machines is considered as base reference to design the frequency stability controllers and frequency protection relays in the conventional power system [5]. ...
... This simple, yet effective, principle is challenged by the increase of uncontrollable sources connected to the grid. Together with the growth of high voltage direct current connections and converter-connected resources, the ability of the power system to maintain a stable operation is highly reduced due to the reduction in inertia [1]. ...
Converter-connected resources, such as electric vehicles and stationary batteries, are being recognized by grid operators as contributors to frequency stability. Their fast-response potential makes them ideal candidates to provide fast frequency control.
In this paper we discuss and present evidence on what kind of dynamics a power system experiences when using electric vehicles to provide frequency control. Initially, we present a method to assess a system balance by deriving so called system residuals, given historical frequency time series and components characteristics. The system residuals consist of the non-controllable, and not explicitly measured, part of generation and load. Afterwards, the Danish island of Bornholm, where vehicles are used on a commercial basis to provide primary frequency control, is used as the test-bed for the investigation. The analysis is supported by validating the described components and system models against real measurements from the islanded power system.
The key outcome is that the overall response time needs to be within 1 second. For the considered system scenarios, the share of reserve provided by electric vehicles should be up to 30% to improve the system stability. We finally generalize the results by highlighting key factors needed to replicate the findings in other power systems.
... Wind turbines (WTs) are one of the most common displacing technologies [2]. The displacement of SGs reduces the energy storage available to rapidly balance power and risks severe frequency variations [3]. Low inertia levels and frequency instability are affecting systems with high penetration of power converters such as Ireland and Great Britain, which have to curtail power converter interfaced generation to maintain system security [4], [5]. ...
Wind turbine (WT) control is being adapted to enable inertia provision that supports network frequency on short timescales. Measuring the inertia contribution from wind turbines is critical to asses the provision of the service as well as understand the WT operation. However, inertia measurement methods disagree on the impact of the wind and how to approximate its effects. This paper uses data from a ScottishPower Renewables (SPR) test of a grid connected wind farm to highlight that wind can impact inertia provision and that external network power measurements are unable to measure the inertia. Two proposals are made to improve inertia measurements. First, the International Electrotechnical Commission (IEC) industrial standard for WT inertia measurement is adapted, and secondly, an alternate method using system identification is proposed that considers characteristics of the WT’s dynamic response. The measurement methods from the literature and the proposals are assessed using the output of time-domain WT models to find the sensitivity of their accuracies to variations in the wind, frequency, and control-setting conditions. The methods from the literature are inaccurate during variable wind conditions but the proposed approaches improve the accuracy. The findings of the sensitivity study are then validated by applying the measurement methods to the SPR wind farm experimental data and confirm that the proposed system identification method is the most accurate measurement approach.
... Concurrently, renewable energy sources (RES) are seeing rapid growth in recent years due to the increasing emission awareness from governments worldwide. Increased penetration of power electronic interfaced generation and subsequent retirement of large synchronous generators effectively reduces the inertia in power systems and thus exhibit a higher risk of instability [1]. Future system operators will face the difficult task of controlling and maintaining the system's stability [2]. ...
This paper presents a framework for defining multi-stability operational boundaries of power systems under varying penetration levels of power electronic interfaced generation and uncertain system’s loading. Boundaries are established considering the small-disturbance and large-disturbance rotor angle stability, as well as the frequency and voltage stability of the system. Estimating system’s operational boundary from multi-stability perspectives will provide invaluable information on how to best operate the system and how to mitigate certain condition that leads to instability especially in the current deregulated market and high penetration of power electronic interfaced generations. A singular stability assessment and its subsequent operational boundary may be valid only for a specific stability study and those may not be true for other types of stability. This paper illustrates the proposed framework in a modified New England Test System using a probabilistic approach. The impact of these varying loading conditions and renewable energy sources penetration levels are quantified on a per-stability-type basis and then aggregated into the multi-stability perspective. The established multi-stability operational boundary is heavily influenced by the frequency and large-signal disturbance rotor angle stability. A significant improvement in the system’s operational stability boundaries, particularly for voltage stability and small disturbance rotor angle stability, occur when the voltage source converter high voltage direct current’s ability to independently control active and reactive power via VDC-Q control is fully utilized.
... Tertiary frequency regulation (TFR) consists of an economic dispatching of energy aimed at adapting the outputs of the generators to minimize operating costs [4,5]. Nevertheless, the increased penetration of RESs leads to a deterioration in the power system inertia [6]. Moreover, the variations caused by these sources increase the frequency deviations. ...
Nowadays, due to the decreasing use of traditional generators in favor of renewable energy sources, power grids are facing a reduction of system inertia and primary frequency regulation capability. Such an issue is exacerbated by the continuously increasing number of electric vehicles (EVs), which results in enforcing novel approaches in the grid operations management. However, from being an issue, the increase of EVs may turn to be a solution to several power system challenges. In this context, a crucial role is played by the so-called vehicle-to-grid (V2G) mode of operation, which has the potential to provide ancillary services to the power grid, such as peak clipping, load shifting, and frequency regulation. More in detail, EVs have recently started to be effectively used for one of the most traditional frequency regulation approaches: the so-called frequency droop control (FDC). This is a primary frequency regulation, currently obtained by adjusting the active power of generators in the main grid. Because to the decommissioning of traditional power plants, EVs are thus recognized as particularly valuable solutions since they can respond to frequency deviation signals by charging or discharging their batteries. Against this background, we address frequency regulation of a power grid model including loads, traditional generators, and several EVs. The latter independently participate in the grid optimization process providing the grid with ancillary services, namely the FDC. We propose two novel control strategies for the optimal control of the batteries of EVs during the frequency regulation service. On the one hand, the control strategies ensure re-balancing the power and stabilizing the frequency of the main grid. On the other hand, the approaches are able to satisfy different types of needs of EVs during the charging process. Differently from the related literature, where the EVs perspective is generally oriented to achieve the optimal charge level, the proposed approaches aim at minimizing the degradation of battery devices. Finally, the proposed strategies are compared with other state-of-the-art V2G control approaches. The results of numerical experiments using a realistic power grid model show the effectiveness of the proposed strategies under the actual operating conditions.
... With the increasing share of the power electronic inverter interfaced renewable energy sources (RESs), some changes have taken place in power system inertia. Many synchronous generators are displaced by RESs, thus leading to a persistent decrease in the conventionally available inertia resources [2]- [4]. Meanwhile, with different control strategies and parameters, various inertia suppliers such as virtual inertia control and energy storage systems are employed to improve the power system inertia [5]- [7]. ...
An online estimation method for the power system inertia constant under normal operating conditions is proposed. First of all, a dynamic model relating the active power to the bus frequency at the generation node is identified in the frequency domain using ambient data measured with the phasor measurement units (PMUs). Then, the inertia constant at the generation node is extracted from the unit step response of the identified model in the time domain using the swing equation. Finally, with the sliding window method and the exponential smoothing method, the estimated inertia constant is updated in real-time. Compared to the conventional methods using large disturbance data or field test data, the proposed method can estimate the inertia constant under normal operating conditions, and therefore, can provide the tracking trajectory of the power system inertia constant in real-time. The effectiveness of the proposed method is validated in the IEEE 39-bus system. The results show that the relative error of the identified inertia constant is below 5% and the identified inertia constant can be updated within 1s.
... A framework for assessing renewable integration limits with respect to frequency adequacy, using a simplified four-area system of Australia, was proposed in [8]. The authors proposed a framework to analyze the impact of increased penetration of RES on grid frequency taking into account the stochastic and intermittent patterns of RES [9]. The proposed methodology successfully identified the critical penetration levels and critical inertia of the system. ...
... A few studies though have been performed with dynamic models to investigate the frequency response of a low-inertia power system, e.g., on the U.S Eastern and Western interconnection [18,19], on the power system of Continental Europe (CE) [20]. Detailed dynamic models were also used in [21,22] to estimate the critical inertia and the effect of the mode of deployment of energy storage on frequency support. A framework for assessing renewable integration limits with respect to frequency adequacy using a simplified four-area system of Australia was proposed in [23]. ...
Based on fundamental dynamic simulation principles and illustrative case studies, this study aims to identify key changes in the dynamic behaviour of the system due to the high penetration of renewable energy sources (RES) affecting frequency dynamics and provide guidelines for performing frequency stability analysis in low‐inertia power systems with a large penetration of RES. First, a critical comparison of the frequency responses obtained using a system equivalent model and a detailed dynamic model following an active power disturbance are presented. It is then demonstrated, using detailed dynamic models of two multi‐area systems, such that when the proportion of RES prevails over the share of synchronous generator (SG), the dynamic coupling among areas (even separated by short‐medium length lines) reduces and the inertia of the system should be considered as heterogeneous instead of a global parameter. Following this, the study discusses how frequency dynamics may be strongly and complexly affected by the physical characteristics of converter‐connected generation leading to increased frequency and voltage interactions when the proportion of converter connected RES becomes greater than SG. The resulting changes in the system's dynamic behaviour due to frequency and voltage interactions can increase the actual active power imbalance size following a credible contingency so that careful considerations to protect the system against an excessively high rate of change of frequency should be made. In order to evaluate the extent of this change in system dynamic behaviour, an appropriate dynamic model of the network should be used with adequate multi‐area representation of RES and loads.
... The connection of renewable generation replaces the conventional large power plants which results a reduction in system inertia [24]. In the presence of a large number of renewable generators, the number of rotating machines in the system will be less. ...
The analysis of power systems with a significant share of renewable generation using probabilistic tools is essential to appropriately consider the impact that the variability and intermittency of the renewable generation has on the grid. This paper provides a critical assessment and classification of the available probabilistic computational methods that have been applied to power system stability assessment (including small and large disturbance angular stability, voltage and frequency stability). A probabilistic analysis framework with a state-of-the-art review of the existing literature in the area is presented comprising of a review of (i) input variable modelling, (ii) computational methods and (iii) presentation techniques for the output/results. The most widely used probabilistic methods in power system studies are presented with their specific application areas, advantages, and disadvantages. The purpose of this overview, classification, and assessment of the existing methods is to identify the most appropriate probabilistic methods to be used in their current forms, or suitably modified, for different types of stability studies of power systems containing renewable generation.
... Energies 2018, 11, 3320 2 of 24 in the presence of a large amount of RESs has become a key aspect of large power systems [3][4][5][6][7][8]. Two main solutions have been highlighted in the recent literature: ...
An increasing share of renewable energy sources in power systems requires ad-hoc tools to guarantee the closeness of the system’s frequency to its rated value. At present, the use of new technologies, such as battery energy storage systems, is widely debated for its participation in the service of frequency containment. Since battery installation costs are still high, the estimation of their lifetime appears crucial in both the planning and operations of power systems’ regulation service. As the frequency response of batteries is strongly dependent on the stochastic nature of the various contingencies which can occur on power systems, the estimation of the battery lifetime is a very complex issue. In the present paper, the stochastic process which better represents the power system frequency is analyzed first; then the battery lifetime is properly estimated on the basis of realistic dynamic modeling including the state of the charge control strategy. The dynamic evolution of the state of charge is then used in combination with the celebrated rain-flow procedure with the aim of evaluating the number of charging/discharging cycles whose knowledge allows estimating the battery damage. Numerical simulations are carried out in the last part of the paper, highlighting the resulting lifetime probabilistic expectation and the impact of the state of the charge control strategy on the battery lifetime. The main findings of the present work are the proposed autoregressive model, which allows creating accurate pseudo-samples of frequency patterns and the analysis of the incidence of the control law on the battery lifetime. The numerical applications clearly show the prominent importance of this last aspect since it has an opposing impact on the economic issue by influencing the battery lifetime and technical effects by modifying the availability of the frequency regulation service.
... The rising share of inverter-coupled distributed energy resources (DER) raises new challenges in maintaining stable grid operation. One of the main issues is the reduction of the system inertia due to the replacement of rotating generators by converter-connected resources, as well as the expansion of highvoltage direct current (HVDC) connections, which decouples the inertial response between the interconnected areas [1]. Thus, the system's ability to withstand frequency changes by releasing or absorbing the energy stored in the rotating masses is notably reduced, leading to faster frequency dynamics [2]. ...
The increasing share of distributed and inertia-less resources entails an upsurge in balancing and system stabilisation services. In particular, the displacement of conventional generation reduces the available rotational inertia in the power system, leading to high interest in synthetic inertia solutions. The objective of this paper is twofold: first, it aims to implement and validate fast frequency control and synthetic (virtual) inertia control, employing single phase electric vehicles as flexibility resources. Second, it proposes a trade-off analysis between the two controllers. The interdependency between frequency containment and synthetic inertia control on the transient frequency variation is shown analytically. The capabilities and limits of series produced EVs in providing such services are investigated, first on a simulation based approach and subsequently by using real hardware. The results show that fast frequency control can improve the transient frequency behaviour. However, both on the simulation and on the experimental level, the implementation of synthetic inertia control is more challenging. In fact, due its derivative nature and the system dynamics, its performance is limited. Furthermore, the crucial importance of the EVs’ response time for both controllers is highlighted.
As one of the efforts to overcome the problem of climate change, increasing the share of renewable energy (RE) in the national energy mix has become intensive in many countries, especially after the ratification of the Paris Agreement in 2015. Although this effort can effectively reduce carbon emissions, challenges to the security of power systems with increasing RE penetration are also emerging. This paper aims to provide an overview of several security issues on power systems, along with challenges arising from the impact of inertial reduction, RE fluctuations, RE prediction errors, and fault response, addressed to researchers as a reference for further studies. Case studies of security issues experienced by several system operators (SOs) when RE penetration is high in their electrical grids are discussed as a lesson for modern power systems operations. Moreover, measures to prevent and overcome these problems are proposed, including the need for changes and development in security assessment, protection and control schemes, and more relevant services for facing system security challenges in the future.
Increasing the penetration of variable energy resources (VER) can reduce the inertia and frequency response performance of power systems supported by replacement synchronous power generation. Therefore, it is necessary to manage the VER penetration limit in power systems for stable operation and to increase the operability to the desired level. This study proposes a method to evaluate and quantify the effect of increasing the penetration limit of VER by controlling a battery energy storage system (BESS). The BESS can provide a fast response, but frequency response performance varies depending on the operating conditions. In the proposed quantification method, various control methods of a BESS, operating conditions of the power system, and penetration conditions of additional VER were analyzed, and the effect of the BESS on increasing the penetration limit of VER was evaluated. This evaluation and analysis enabled the selection of the BESS operating conditions to achieve the target VER capacity in the power system. The proposed quantification method was analyzed through simulations based on the Korean power system model. Therefore, it can contribute to estimating the required performance of the BESS for each system operating condition required to achieve the VER target.
Wind speed characteristics may have a significant impact on the power system operation, stability and dynamics. Hence, appropriate modelling and quantification of wind probability distribution function (PDF) are critical for probabilistic power system analysis. Weibull distribution is the most widely used PDF for power system (particularly power system stability) studies, which might not be a valid PDF for different wind regimes having high, average, and low wind speed data. In this study, twenty different wind speed datasets (each dataset is different based on the location, duration, and resolution) have been collected and analysed with eight PDFs to model the wind speed to replicate different wind speed characteristics. The PDFs include Rayleigh, Weibull, Gamma, Lognormal, Normal, Inverse Gaussian, Generalized Extreme Value, and Exponential distributions. Root mean square error (RMSE) and the coefficient of determination (R²) are applied as the measure of accuracy. The obtained results show that the locations, duration, and resolution have a notable impact on the selection of suitable PDFs. It was found that the Gamma distribution is the best-suited PDF for representing low wind speed data. In comparison, the Weibull distribution represents the best PDF for high wind speed data. Also, the selection of the Gamma PDF for low wind speed data and Weibull distribution for high wind speed data is further validated by presenting voltage profile and voltage stability analysis. Study outcomes would assist power system planners to select the appropriate PDF for power system stability studies based on the characteristics of the available wind speed dataset.
With the rise in utilisation level of renewables, the effect of small inertia changes the dynamics and stability of the system network. A possible solution with regard to stabilization of such a grid is to provide virtual inertia in the system. This chapter discusses the concept of virtual inertia to improve the frequency control in a grid with a large share of renewable energy sources. The impact of low system inertia on power system voltage and operation and control due to a large share of renewables has been highlighted in detail. Virtual inertia emulation methods for wind and solar PV systems without and with energy storage systems have been elaborated in this chapter. The utilisation of various energy storage technologies for emulating virtual inertia has been discussed.
This paper presents a novel approach for the optimal design of the control system for battery energy storage systems adopted in power systems in the framework of the primary frequency control. The challenge issued by the use of batteries for frequency control in modern power systems is related to the need of the repeatedly deep charge and discharge phases. The presence of intermittent loads and generators, in fact, imposes very stressful power profiles to the battery which imply high rate of degradation to its lifetime. Based on the stress model of the battery, the proposed approach aims to design a control system which allows for containing the state of charge time variation within intervals able to match the requirements of frequency profile improvements and lifetime preservation. At this aim, the typical control scheme for batteries adopted for primary frequency control is analyzed and elaborated to address stable dynamic behavior of the regulator. Moreover, starting from the observation of the statistical nature of the power request for the pure frequency regulation service, a statistical study is conducted to perform an optimal design of the regulator in terms of both lifetime duration and primary frequency regulation service.
This paper presents a risk-based probabilistic small-disturbance security analysis (PSSA) methodology for use with power systems with uncertainties. This novel addition to existing dynamic security assessment (DSA) techniques can be used to quantify the small-disturbance stability risks associated with forecasted operating conditions. This approach first establishes the probability density functions (PDFs) for the damping of the critical oscillatory electromechanical modes by modeling the stochastic variation of system uncertainties, such as loading levels, intermittent generation sources, and power flows through voltage-source converter-high-voltage direct current (VSC-HVDC) lines. The produced PDFs are then combined with severity measures (either simple risk matrices or continuous functions) in order to quantify the risk of stability issues for the system associated with the forecasted operating scenario. In addition, the PSSA is used to establish risk-based operational limits and the concept of a probabilistic security margin is introduced to more accurately represent the probabilistic operation of uncertain power systems. The proposed techniques are demonstrated using a multiarea meshed power system incorporating two VSC-HVDC systems, one of which is connected to a large wind farm.
Wind power fluctuations cause frequency deviation from the 60-Hz standard. Using the composite transfer function G ( fw ) of a small system of thermal power plants, it is estimated that power fluctuation of 5% of the total thermal plant capacity can be tolerated without exceeding 1% frequency deviation. The technology to filter out the power fluctuations by wind turbine generators for increasing wind power penetration already exists. However, perfect filtering sacrifices as much as 27.6% of the wind power that can otherwise be utilized. The paper presents a method of quantifying wind penetration based on the amount of fluctuating power that can be filtered by wind turbine generators and thermal plants. For optimal wind power acquisition, the penetration level is conservatively estimated to be 50%.
Present renewable portfolio standards are changing power systems by replacing conventional generation with alternate energy resources such as photovoltaic (PV) systems. With the increase in penetration of PV resources, power systems are expected to experience a change in dynamic and operational characteristics. This paper studies the impact of increased penetration of PV systems on static performance as well as transient stability of a large power system, in particular the transmission system. Utility scale and residential rooftop PVs are added to the aforementioned system to replace a portion of conventional generation resources. While steady state voltages are observed under various PV penetration levels, the impact of reduced inertia on transient stability performance is also examined. The studied system is a large test system representing a portion of the Western U.S. interconnection. The simulation results obtained effectively identify both detrimental and beneficial impacts of increased PV penetration both for steady state stability and transient stability performance.
This paper presents the modular structure of wind turbine generator models to be published in a new standard IEC 61400-27 for “Electrical simulation models for wind power generation”. The purpose of this standardization work is to define generic simulation models for wind turbines (Part 1) and wind power plants (Part 2), which are intended for short-term power system stability analyses. Part 1 has passed the first committee draft stage, whereas Part 2 is in an early stage of development. Initially, the paper describes the interfaces between wind turbine, wind power plant and grid models, and then gives a more detailed description of the modular structure of the types of wind turbines that are included in Part 1.
Several detailed technical investigations of grid ancillary service impacts of wind power plants in the United States have recently been performed. These studies were applied to Xcel Energy (in Minnesota) and PacifiCorp and the Bonneville Power Administration (both in the northwestern United States). Although the approaches vary, three utility time frames appear to be most at issue: regulation, load following and unit commitment. This article describes and compares the analytic frameworks from recent analysis and discusses the implications and cost estimates of wind integration. The findings of these studies indicate that relatively large-scale wind generation will have an impact on power system operation and costs, but these impacts and costs are relatively low at penetration rates that are expected over the next several years. Published in 2004 by John Wiley & Sons, Ltd.
This paper explores the available control options for enabling wind power generation plants to participate on the maintenance of system frequency following a major power imbalance. Taking the currently employed control structures for wind generators as the baseline case, possible expansions and additional features have been discussed. The options include voltage or alternatively frequency dependent active power control. The responses of these control schemes vis-a-vis their frequency supporting capability in a power system contingency situation have been simulated and with one another compared. It was found out that at the conceptual level there are indeed a range of options which would place wind generating plants in a position to support system frequency in an emergency situation
The authors have systematically kept track of Eastern interconnection frequency governing characteristic, called beta, for a period of more than 10 years. While there are some errors inherent to the process followed, we believe that the errors do not obscure the overall trend. We have found that Beta has been declining, and has reached a level of 31 MW/mHz. The true beta is probably somewhat lower. This has important implications in system behavior and simulation, particularly in simulations involving islanding.
Since the deregulation of the electrical energy market, the technical realisation of power transactions based on energy market contracts often effects large stepwise power programme changes - especially at the change of the hour. Originating from the current market boundaries and system rules, these stepwise power programme changes lead to remarkable power imbalances within the European Power System causing large unintended frequency deviations with a negative impact on the control performance of power plants and power system. Within the framework of this paper, a cause study to establish an understanding of the origins of the power imbalances is presented. Subsequently, counteractive measures for an improvement of the performance of scheduled stepwise power programme changes are proposed.
With an increased number of wind turbine generators (WTGs) connected to an electricity network the system operator may request that they participate in frequency control in the event of a sudden unbalancing of power generated and consumed on the system. In this paper the frequency response capability of the full converter variable speed wind turbine generator (FCWTG) with permanent magnet synchronous generator (PMSG) is investigated. A control scheme is developed that improves the frequency control performance, illustrating the importance of the initial active power output of the FCWTG. A method of carefully ending the frequency support of a wind farm is proposed and simulated. The resulting frequency control performance compares favorably to that of a conventional synchronous generator plant.
Robust Control in Power Systems
- P Pal
- B Chauduri
P. Pal and B. Chauduri, Robust Control in Power Systems. New York:
Springer Science & Business Media, 2005.