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Dynamically adaptive method for under frequency load shedding protection scheme reconfiguration
Abstract
The increased integration of the new generation technologies into the electric power system (EPS) which are mainly inverter-based changes its characteristics. The main issue that arises is the appearance of faster frequency dynamics that are caused by production connected through inverters. This leads to the need for changing conventional under-frequency load shedding (UFLS) schemes because, in cases of high dynamics of frequency change, conventional UFLS algorithms are not able to stop initial frequency drop and prevent power system collapse. This paper proposes reconfiguration method for conventional strategies transforming their response from static and inflexible to a dynamic and adaptive one. The focus of the paper is the proposed method for dynamical change of UFLS stage thresholds to minimize the number of activated UFLS stages during a particular disturbance event and therefore enable more efficient response of the system to further disturbances. The method effectiveness was demonstrated on a multi-machine power system model consisting of various generation technologies.
... Various strategies for implementing under frequency load shedding have been described in the literature [1][2][3][4][5][6][7][8][9]. Consequently, different types of UFLS techniques are categorized into five groups (Fig. 1). ...
... Another innovative strategy integrates disturbance localization into adaptive UFLS, focusing on efficiency but dependent on accurate frequency data [7]. A dynamic adaptation of UFLS schemes is suggested to minimize operational stages, heavily reliant on the precision of real-time information [8]. Monte Carlo simulations are employed for UFLS relay settings, facilitating strategy evaluation despite the complexity and data accuracy challenges [9]. ...
... Such feature makes the dynamics and response to frequency imbalances faster and more prone to instabilities. This raises the need for new UFLS schemes since fast frequency dynamics make traditional UFLS unable to detect initial imbalances between generation and demand to prevent the collapse of the power system [41]. In [42], the development of an UFLS that considers the variation of inertia levels in the power grid is considered. ...
... Additionally, a novel technique for detecting the inflection point is presented to eliminate the effect of local frequency oscillations. On the other hand, [41] proposes a method for reconfiguring conventional UFLS to adapt to the changing dynamics of the power system. ...
The frequency of an electrical power system can be interpreted as an indicator of the equilibrium between generation and demand. Under fault conditions where this balance is disrupted, the Under-Frequency Load Shedding scheme is typically the ultimate resource employed to prevent the frequency from dropping to undesirable thresholds that could lead to a blackout. Smart loads have the capability to dynamically adjust their consumption in response to frequency variations, which has the potential to enhance frequency regulation and mitigate the need for abrupt load disconnection through the Under-Frequency Load Shedding scheme. The objective of this paper is to present a literature review of Under-Frequency Load Shedding schemes, an innovative approach to their classifications, and the introduction of a new category within these schemes that incorporates smart loads. Additionally, an overview of smart loads is included to be potentially employed in the design of new schemes and contribute to a shift in the operation philosophy, considering that the primary mission of electric power systems is to provide energy to loads rather than disconnecting them.
... Both types of analysis focus primarily on the lowest frequency value during the disturbance, known as the frequency nadir. Maintaining the frequency nadir above the specified value is crucial to prevent under-frequency load shedding [1]. ...
... Both types of analysis focus primarily on the lowest frequency value during the disturbance, known as the frequency nadir. Maintaining the frequency nadir above the specified value is crucial to prevent under-frequency load shedding [1]. ...
... In addition to frequency amplitude, the WAMS also allows monitoring the rate of change of frequency (ROCOF) df/dt. It monitors the rates of change of frequency from 0 Hz/s to 10 Hz/s, which basically occur as result of large imbalance [21]. • monitoring of power oscillations on lines. ...
The most commonly used method for including photovoltaic system in the control of system frequency is to operate a PV power plant at reduced power while maintaining a specific amount of available power reserve. Such operating regime, often referred to as de-loaded, can cause additional costs and, consequently, determining a required power reduction becomes vitally important. This paper introduces a method for determining the required minimum de-loaded reserve of a photovoltaic system. To achieve the desired result a parabolic approximation of the system frequency response is developed to derive an explicit function of the frequency nadir. Taking this approximation into consideration, a method for determining the required minimum de-loaded margin was developed and tested in a case-study which observed a small power system consisting of several power plants. To demonstrate the accuracy of the proposed parabolic approximation, the calculated approximate values of the frequency nadir were compared to the values of the frequency nadir obtained from the frequency response simulations performed on the power system model developed in MATLAB/Simulink.
The conventional Under Frequency Load Shedding (UFLS) scheme could result in unacceptably low frequency nadirs or overshedding in power systems with volatile inertia. This paper proposes a novel UFLS scheme for modern power systems whose inertia may vary in a wide range due to high penetration of renewable energy sources (RESs). The proposed scheme estimates the rate of change of frequency (RoCoF) of the center of inertia (CoI), and consequently, the loss of generation (LoG) size, using local frequency measurements only. An innovative inflection point detector technique is presented to remove the effect of local frequency oscillations. This enables fast and accurate LoG size calculation, thereby more effective load shedding. The proposed UFLS scheme also accounts for the effect of the inertia change resulting from LoG events. The performance of the proposed scheme is validated by conducting extensive dynamic simulations on the IEEE 39-bus test system using Real Time Digital Simulator (RTDS). Simulation results confirm that the proposed UFLS scheme outperforms the conventional UFLS scheme in terms of both arresting frequency deviations and the amount of load shed. Index Terms-Center of inertia (CoI), Underfrequency load shedding (UFLS), Rate of change of frequency (RoCoF).
The paper describes a centralized Under Frequency Load Shedding (UFLS) method, where load shedding decisions are based on the solution of an optimization problem. The proposed approach anticipates the evolution of the grid frequency trajectory by means of a system dynamic model. Moreover, the method is augmented by the equations derived from the Optimal Power Flow (OPF) problem allowing to constrain asymptotic values of node voltages and line currents. The proposed OPF-based method differs from traditional UFLS methods as it enables the user to compute the minimum amount of load to be shed and, at the same time, provides a feasible grid trajectory. The trajectory of the system frequency due to the contingency, and the subsequent load shedding, is predicted over the entire time horizon by means of a second-order dynamic model. The feasibility and applicability of the proposed method are assessed by means of numerical simulations carried out using a real-time simulator, where the time-domain full-replica model of the IEEE 39-bus system has been implemented. Two contingency scenarios are investigated and the performance of the proposed method is compared against the UFLS strategy recommended by the European Network of Transmission System Operators (ENTSO-E). The metrics used for such a comparison are the amount of energy not served, the frequency variation and the violation of the grid safety constraints.
Increased wind energy penetration influences the power system dynamic response to transient disturbances. Replacement of conventional production units with converter-connected wind turbines reduces natural power system inertia contained in rotational masses of synchronously connected turbine-generator units, therefore creating low-inertia power systems. Such a transition has an adverse effect on system resilience to disturbances and on the capability to maintain stable operation. This research examines the impact of high regional wind power production on system transient stability in the case of island operation of the Croatian power system. The system is divided into four geographical areas modeled as four centers of inertia with aggregated parameters. The study investigated initial transient RoCoF values in different areas for current and future wind capacity share scenarios, loading data, and primary frequency regulation settings. The modeling and scenario analysis have been performed on a detailed phasor power system model in the MATLAB/Simulink environment.
This paper investigates the impact of synchrophasor estimation algorithms in Under-Frequency Load-Shedding (UFLS) and Load-Restoration (LR) schemes, relying on frequency and Rate-of-Change-of-Frequency (ROCOF) measurements produced by Phasor Measurement Units (PMUs). We compare two consolidated window-based synchrophasor estimation algorithms, as representative approaches based on static and dynamic signal models, with a focus on the appropriateness of utilizing PMU-based ROCOF measurements. The performance of the proposed relaying scheme is assessed by means of a Real-Time Digital Simulator implementing the time-domain full-replica dynamic model of the IEEE 39-Bus power system.
This letter presents an innovative and easy-to-implement approach for improvement of conventional under-frequency load shedding, where frequency thresholds are adjusted locally in real-time. The proposed method, that would be implemented locally in each under frequency relay, estimates the forthcoming frequency evolution and adjusts the predefined frequency thresholds values according to the estimated frequency nadir. As a result, significantly improved frequency response by reducing overshoots can be observed. Concept verification was performed by means of Slovenian power-system model simulations, where the comparison with conventional constant-threshold setting showed high level of efficiency.
Severe generation outages, may affect the power system frequency stability and cause abnormal frequency declines especially under the high penetration of low inertia generation technologies. Under frequency load shedding (UFLS), as a special protection scheme, is utilized to restore the power balance and maintain system frequency in an acceptable range. This paper develops a semi-adaptive UFLS plan in which the multistage load shedding is triggered based on the rate of change of frequency (RoCoF) value. The proposed semi adaptive UFLS plan activates predetermined later stage as early stage under the severe generation deficiencies. Most credible power imbalance scenarios including conventional generation outages and islanding scenarios are considered in semi-adaptive setting of the proposed UFLS plan. In order to achieve the optimal settings, the proposed UFLS plan is formulated as a mixed integer linear programming (MILP) model. The dynamic of governors and load damping are considered in the discretized system frequency response. A scoring method is developed to select the UFLS settings with the most desired system frequency response. The performance of the proposed UFLS plan is verified over the IEEE 39-bus test system under credible and most likely scenarios of generation outages.
For the developed dynamic angle instability simulation framework a reference IEEE 9 bus model was adapted and created in Matlab simulation environment. Model was validated and verified with available real operation historical data and fine tuning process was conducted. With the reference model, flexible platform for study work in the field of transmission network behavior during disturbances was created. Series of simulation scenarios were done analyzing dynamic angle instability: from a wide range of small active power oscillations to larger active power oscillations and out-of-step conditions occurrence. The paper is not oriented towards operations analyses of generators rather observes and enhances protection functions of the transmission lines in the transmission system. Simulation data that is generated is used to create setting parameter sets for protection functions in the proposed Wide Area Monitoring Protection and Control (WAMPAC) system, utilizing simulated synchrophasor measurements.
Over the last decade, many power systems have significantly changed with the proliferation of renewable generation sources such as wind and solar photovoltaic (PV). Due to their variability and non-synchronous nature, new challenges and complexities have emerged regarding operational security of modern power systems. The 2016 South Australia (SA) blackout exactly occurred under such a high renewable situation. An official report has been recently published to review the causes and provide the corresponding recommendations for improvement of network operation, control and security. However, there are still a number of critical issues and debates which remain unsolved, such as network bottleneck identification, over-voltage explanation, pole slip concern, frequency dip mystery, and frequency/voltage instability debate. In this paper, based on the reconstruction of the event, these unsettled issues are prudently analyzed to unveil their root causes. In addition, an innovative scheme is proposed to prevent the blackout by identifying the network separation at an early stage. This research will not only further advance the understanding of the 2016 SA blackout but also will provide valuable guidelines for the management of future renewable-rich networks.
The activation of Under Frequency Load Shedding (UFLS) is the last automated action against the severe frequency drops in order to re-balance the system. In this paper, the setting parameters of a multistage load shedding plan are obtained and optimized using a discretized model of dynamic system frequency response. The uncertainties of system parameters including inertia time constant, load damping and generation deficiency are taken into account. The proposed UFLS model is formulated as a mixed integer linear programming optimization problem to minimize the expected amount of load shedding. The activation of Rate-of-Change-of-Frequency (RoCoF) relays as the anti-islanding protection of Distributed Generators (DGs) are considered. The MCS method is utilized for modeling the uncertainties of system parameters. The results of probabilistic UFLS are then utilized to design four different UFLS strategies. The proposed dynamic UFLS plans are simulated over the IEEE 39-bus and the large-scale practical Iranian national grid. IEEE
Underfrequency load shedding plays an important role in prevention of the power system blackout. The common load shedding methods are based on measuring the frequency first derivative; therefore, an error in measurement process can highly affect their performance. Also, for proper performance of these load shedding schemes, the exact value of some parameters of power system is needed. Any error in estimation of these parameters reduces the reliability of these load shedding schemes. In this paper, an underfrequency load shedding method based on the forecast minimum frequency of system is proposed. In this method, the samples of the power system frequency are taken after disturbance; then, particle swarm optimization algorithm is used to forecast the minimum frequency based on these samples. To verify the effectiveness of the proposed load shedding scheme, its performance has been compared with a newly suggested method.
To diminish damaging consequences of power system major disturbances, under frequency load shedding (UFLS) plans are devised so as to prevent power networks from collapse. Since load shedding scenarios in conventional UFLS plans are specified regardless to voltage stability criteria, they cannot bring desired outcomes at the event of severe and combinational contingencies. In these circumstances, the location of load drops, in addition to their amounts, should be determined as well. This paper presents a two-unit wide-area adaptive load shedding scheme based on synchrophasors. In the first unit, an adaptive precise system frequency response (SFR) model is developed, by the help of which, the appropriate amount of load rejections is determined such that the dynamic and steady state frequency limitations are fulfilled. Incorporation of both static and dynamic loads into the adaptive SFR model results in a negligible error in calculations. Specifying the best location of load drops based on suitable voltage stability criteria is the main task of the second unit. These two units are executed in a parallel way; the computational burden of the proposed approach is thus tractable for real world applications. The new method is successfully testified in the IEEE 39-bus system and Khorasan network.
Wide-area phasor measurements provide a dynamic view of the power system in short-term intervals. The time synchronization and concentration of these measurements opens up a range for wide area applications for protection and control. For each kind of instability, related detection and action schemes can be defined to limit or prevent the system from severe disturbances or even collapses. This paper proposes a novel predictive method for emergency frequency control to be used on such occasions. The proposed method avoids the drawbacks of local underfrequency load shedding schemes, such as delayed response and overshedding, by basing control on synchronized phasor measurements. A single machine equivalent for each power system island including the frequency sensitivity of loads, is computed online using phasor measurements. Using this model the progression of frequency stability after a contingency is predicted and stabilizing load shedding is determined in a co-ordinated way for the supervised area of the power system.
The problem of frequency load shedding in an isolated power system
is considered. A proper design of the load shedding scheme should
minimize consumer disruption in the case of the load excess and help
balance the load to the remaining generation in a system. An adaptive
strategy, which can reduce total amount of shed close to the theoretical
minimum, is proposed and compared to conventional load shedding scheme
Frequency stability is one of the major concerns for secured operation of a power system. A severe disturbance causes rapid frequency decline, which may instigate power system blackout in an extreme case. In emergency situations, load shedding is applied to arrest frequency excursion. Therefore, a load shedding scheme needs to be sufficiently adaptive to tackle sudden generation loss. Note that conventional load shedding schemes may fail to provide satisfactory frequency response in certain cases. To address this challenge, a new dynamic-adaptive load shedding mechanism is proposed in this paper. The proposed methodology considers the actual contingency size, frequency sensitivity due to load damping, frequency response and ranking of load buses according to voltage stability index. The method ensures that the actual load cut is sufficiently close to the required load shedding to keep system frequency within the given acceptable limit. To do so, it ranks the load buses within a zone and dynamically selects appropriate feeders for load shedding. It also proportionally applies higher amount of load shedding to relatively weaker buses for taking care of the voltage stability. To examine the performance of the proposed scheme, it is applied to the IEEE 39 bus test system that is divided into six zones. It is found from simulations that for 600 MW, 700 MW and 900 MW generation loss events in zone-3, the required amounts of load shedding are 430 MW, 531 MW and 730 MW respectively to keep system frequency above 49.1 Hz. However, these quantities increase to 450 MW, 550 MW and 749 MW respectively to retain system frequency above 49.2 Hz. Furthermore, the impact of 400 MW generation loss in zone-1 is explored. It is found that after shedding additional loads in zone-1 and zone-2, the risk of tie line overloading is mitigated that eventually assures tie line security. In addition, performances of the proposed algorithm are compared to that of an adaptive technique and a semi-adaptive technique. It is noticed that the proposed methodology provides better results than the existing methods. Therefore, it can be articulated that the developed scheme ensures the correct amount of load shedding to maintain satisfactory frequency response and voltage stability.
Frequency drop due to loss of massive generation is a threat to power system frequency stability. Under-frequency load shedding (UFLS) is the principal measure to prevent successive frequency declination and blackouts. Based on traditional stage-by-stage UFLS scheme, a new continuous UFLS scheme is proposed in this paper to shed loads proportional to frequency deviation. The characteristic of the proposed scheme is analyzed with a closed-form solution of frequency dynamics. Frequency threshold and time delay are added to make the proposed scheme practical. A line-by-line scheme based on precise load control is introduced to implement the continuous scheme for systems without enough continuously controllable loads. The load shedding scale factor of the proposed scheme is tuned with an analytical method to achieve adaptability to different operating conditions. The adaptability of the proposed scheme is validated with 39-bus New England model and simplified Shandong Power Grid of China.
Automatic under frequency load shedding schemes need to be carefully designed in order to reduce the risk of widespread system collapse. This paper proposes a centralized hierarchical multi-agent system that coordinates various stages of monitoring and decision making processes. The main contribution is to improve traditional contingency response algorithms such as load shedding schemes, taking advantage of the future smart grid infrastructure. The multi-agent system seeks for a minimum amount of load disconnection in a short period of time, causing the least possible disturbance in the system frequency. A hardware-in-the-loop simulation of a full electric power system using a real time digital simulator was utilized. The solution was embedded in a real time system, consisting of hardware and software, to test and validate the proposed methodology. In addition, the studied methodology was compared with two other load shedding philosophies through a load shedding metric score. Shedding was carried out in a single step and the amount of disconnected load was close to the dynamic power unbalance. The results show that it is possible to improve the traditional load shedding philosophy schemes and use advanced communication infrastructure, monitoring and embedded processing capabilities to provide better stability and reduce unnecessary load disconnections from the system.
A description of the importance of rate of change of frequency (ROCOF) measurements to the operation of electricity networks is given. The susceptibility of ROCOF measurements to common power system disturbances such as phase steps is described. A measurement campaign to observe ROCOF at multiple locations in an island grid dominated by renewable generation is described and some results are given. These captured ROCOF events are dominated by those associated with phase steps, which occur without significant change to underlying power system frequency. It is concluded that they constitute a "false" ROCOF event. A new algorithm is presented that attempts to remove the influence of the phase step and reduce the associated ROCOF error such that the reliability of ROCOF measurements can be improved in the presence of phase steps. The algorithm is then applied to some recorded waveform sequences from the island that contains phase steps, and the results are presented. In one example, it is shown that a false ROCOF spike in excess of 100 Hz/s was reduced to less than 5 Hz/s.
Power system inertia is gradually being reduced due to the ongoing replacement of conventional synchronous power plants by intermittent generation. This affects the frequency response of the system and necessitates the estimation of power system inertia, so that sufficient power reserves are retained. This paper contributes with a novel disturbance-based inertia estimation method, that simultaneously estimates the power change after a disturbance. The proposed method accommodates the frequency and voltage dynamics, which significantly affect the system's power change, and hence the inertia estimation. Two separate approaches – that are also capable of standing alone – are combined, in order to accommodate the dynamics. An extended version of the Nordic32 test system is used for the application of the method, where several case studies and a comparison are investigated, so as to examine the method's accuracy and robustness.
This study presents a method of estimating the effective inertia of a power system from ambient frequency and active power signals measured by PMUs. Most importantly, we demonstrate that inertia can be estimated from ambient measurement data, not only from disturbances. This leads to the possibility of monitoring inertia in a close to continuous manner in the time scale of minutes or tens of minutes. The method allows the system to be divided into a number of areas and the effective inertia of each area to be estimated as a separate quantity. In principle, inertia is estimated by observing the dynamics between changes in active power and resulting frequency deviations during normal operation of the system. The method is based on applying system identification on these measurements and extracting inertia values from identified models. Efficacy of the method is demonstrated on results of real measurements from the Icelandic power system.
The imbalance between the generated power and the load demand is the major factor that is usually responsible for frequency instability in power systems, most especially islanded microgrids. To determine the size of the loads that should be shed and their appropriate locations in the power system, to maintain the system frequency within the permissible limits, this paper presents an effective adaptive control scheme. In the proposed controller, a stepwise load-shedding approach is designed in the islanded MGs to regulate the grid frequency while providing the amount of power shortage. To this achieve, it locally measures the system parameters most especially voltage and frequency signals. Thereafter, a stepwise load-shedding will take place in locations where the highest voltage drop and frequency variation are experienced. The load-shedding step changes according to certain factors such as shedding speed, location and value, and the rate of frequency change. The proposed approach eliminates the adjustable loads to return the frequency back to the desired value. Simulation results of the proposed method under different practical scenarios, when compared with the conventional PID controller, provide considerable enhancement in the power system frequency stability.
This letter presents a centralized underfrequency load-shedding (UFLS) scheme using the initial rate-of-change of frequency (ROCOF) to decide whether and how much load needs to be shed. The applicability of the centralized UFLS scheme is shown for a Spanish small isolated power system.
Underfrequency load shedding is one of the most important protection mechanisms in a power system. However, in the majority of power systems it has remained unchanged for decades, despite the advances in computer and communication technologies. Wide Area Monitoring System proved to be very useful and reliable in the last few years and this is why much effort is given into development of Wide Area Monitoring, Protection and Control concepts. Adaptive underfrequency load shedding is certainly a very suitable candidate for the task. In order to accelerate changes in this area, a proposal for an adaptive scheme has been developed, which is a good option from both the technical and economic points of view. In contrast to majority of adaptive schemes, it handles problems of active-power deficit estimation and its variations during the transient due to voltage dependent loads differently, i.e., by on-line forecasting the operating point trajectory in a phase space. The scheme has been tested on previously proven (by comparison to WAMS measurements) dynamic model of a part of a Slovenian power system and the results indicate a large improvement compared to the traditional scheme.
This paper provides a tutorial introduction to phasor measurement units (PMU) when applied in a power system environment, provides an overview of communication alternatives for wide area measurement systems (WAMS), and computes the delay budget for each type of communication link. The goal of this study is to provide data regarding the communication delay that can be incorporated into the analysis and simulation of WAMS.
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