Adaptive Underfrequency Load Shedding Research Articles

Response-driven adaptive under frequency load shedding scheme based on energy model

Developing reasonable and effective adaptive under frequency load shedding (AUFLS) schemes is crucial for preventing system frequency drops. In order to overcome the mismatching of event-driven load shedding scheme in the traditional second line of defense, this paper proposes a response-driven AUFLS scheme based on energy model. In this scheme, the energy model is established by the kinetic energy theorem, which represents the equivalent conversion relationship between rotor kinetic energy deviation (KED) and the unbalanced power does work (UPDW) of each component. Furthermore, a kinetic energy margin indicator (KEMI) reflecting the system frequency is introduced, and the minimum load shedding amount (MLSA) corresponding to the critical KEMI is calculated. The comprehensive load shedding allocation weight is established by combining the UPDW of load and shedding costs. The combination of MLSA and comprehensive load allocation weights enables precise control with minimal cost. Modified IEEE 10-generator 39-bus test system and CSEE-SSFS 197-bus real system are used to verify the performance of the proposed AUFLS scheme.

Enhancing power system stability with adaptive under frequency load shedding using synchrophasor measurements and empirical mode decomposition

This paper introduces an adaptive under frequency load shedding (AUFLS) scheme based on synchrophasor measurements and Empirical Mode Decomposition (EMD) to enhance power system stability during significant disturbances. The traditional UFLS methods, which rely on fixed frequency thresholds, often lead to unnecessary disconnections and a lack of flexibility. The proposed AUFLS scheme dynamically adapts to the magnitude of disturbances, frequency response, and voltage stability index, resulting in a more efficient and responsive system. Various UFLS techniques are reviewed, highlighting the advantages of adaptive strategies. The novelty of this research lies in the innovative application of the EMD algorithm within the AUFLS framework. This method identifies the center of inertia (CoI) of the rate of change of frequency (RoCoF) from frequency signals and calculates the total active power imbalance due to disturbances. Furthermore, EMD is applied to the voltage angle at each bus to identify and form coherent bus groups. These coefficients are subsequently employed to allocate the required active power shedding within each group to maintain system stability. Voltage stability indices are calculated for each group, and load shedding is distributed among the buses based on normalized voltage stability indices, thereby enhancing the precision of load shedding decisions and the overall stability of the power system. Results demonstrate that the proposed scheme provides satisfactory frequency response while requiring less load shedding compared to traditional methods, making it effective for modern power systems with high penetration of renewable energy sources (RES). The performance of the proposed scheme is assessed using the IEEE 39-bus test system, demonstrating its effectiveness in improving system resilience to frequency disturbances. The research concludes that the AUFLS scheme offers a promising approach for enhancing the resilience of power systems to frequency disturbances.

Adaptive Underfrequency Load Shedding And Voltage Stability In The Power System

Abstract Underfrequency load shedding is a common technique for maintaining the stability of the power system by removing the overload in a certain part of the system after a disturbance. The purpose of underfrequency load shedding is to balance output and load when a particular event causes a significant frequency drop in the power system. In conventional underfrequency load shedding schemes, the frequency thresholds of frequency relays are constant, this way it is difficult and sometimes impossible to control the frequency in various disturbances in the system. In this paper, an adaptive underfrequency load shedding (AUFLS) algorithm that is independent of communication between relays is presented. The relays are tuned to reduce loads taking into account local parameters such as voltage and frequency to prevent the occurrence of a cascade failure that can ultimately lead to the breakdown of the entire power system. In this paper, the rate of change of frequency (ROCOF) is obtained by applying the Hilbert-Huang transformation. Numerical simulations conducted on the New England 39 bus test system in the DIgSILENT PowerFactory and MATLAB software packages confirm the effectiveness of the proposed approach.

Adaptive Under-Frequency Load Shedding Control Strategy of Power Systems With Wind Turbines and UHVDC Participating in Frequency Regulation

Represented by wind turbines and ultra high-voltage DC (UHVDC), the power-electronic interfaced power sources participate in fast frequency control, which has a significant impact on the power system frequency. However, the conventional under-frequency load shedding (UFLS) scheme doesn’t take into account the impact above, resulting in unreasonable load shedding after a large loss of generation. To this end, this article proposes an adaptive UFLS control strategy of power systems with wind turbines and UHVDC participating in frequency regulation. Firstly, based on the virtual inertia control model and the primary frequency control model of wind turbines and UHVDC, the study establishes the simplified frequency response model of the power system considering the participation of wind turbines and UHVDC in frequency regulation. Furthermore, the impact of the active power response characteristics of wind turbines and UHVDC participating in frequency regulation on the magnitude of the active power deficiency is comprehensively analyzed. Thus the precise estimation of the magnitude of the power deficiency can be achieved, which provides technical guidance for multi-stage UFLS. Finally, simulation results demonstrate that the proposed UFLS strategy is capable of reflecting the power system frequency more objectively after a large loss of generation event. In addition, the proposed UFLS strategy outperforms the conventional UFLS strategy in terms of shedding less amount of load when the same desired effect of frequency recovery is achieved.

A New Adaptive Underfrequency Load Shedding Scheme to Improve Frequency Stability in Electric Power System

This paper proposes a new adaptive underfrequency load shedding scheme (UFLS) to avoid frequency instability in electrical power system during abnormal wide area disturbances. The developed scheme is based on online monitoring of the distance relay zone 3 decisions of some tie lines using (WAMS) and SCADA/EMS systems, to check rapidly and reliably the uncontrolled islanding conditions and, permit an automatic load shedding action to maintain frequency stability of power system. Simulation results on 400 kV Turkish transmission systems demonstrated the effectiveness of the proposed scheme compared to current frequency load shedding schemes which they cannot consider all possible circumstances because of their limited access to the power network data. Simulation results clearly indicate that large disturbances in power systems can be avoided and their propagation can also be stopped using the proposed scheme.

Adaptive Underfrequency Load Shedding design based on generation losing estimation using Artificial Neural Network method

Under Frequency Relay (UFR) is used to prevent the frequency instability of a generation outage by balancing the load with the supply. The approach of this paper is to design an Adaptive Under Frequency Load Shedding (AUFLS) model in power system simulation software by estimating the magnitude of losing generation using artificial neural network (ANN) based on the early response of frequency after the disturbance occurred then the shed loads will be obtained based on this value estimation. The New England IEEE 39 bus system is used for this dynamic simulation. There are three operation scenarios planned with different dispatches of the generators, each generator is simulated by removing it from the system then the frequency system response is used as training data for the ANN model. The analysis is performed by comparing the frequency response of the system using Conventional 4 Steps Under Frequency Relay Load Shedding (UFLS) and the Adaptive Under Frequency Load Shedding (AUFLS). The results show that the frequency response after disturbance using the Adaptive Under Frequency Load Shedding (AUFLS) revert to the initial frequency closer and faster than using Conventional 1 Steps Under Frequency Relay Load Shedding (UFLS).

Adaptive underfrequency load shedding in systems with renewable energy sources and storage capability

This work proposes an adaptive underfrequency load shedding (AUFLS) method suitable for power systems with high penetration of renewable energy sources (RES) and energy storage systems (ESS). The proposed AUFLS method takes into account the impact of these devices in the load shedding execution, considering important characteristics, such as the synthetic inertial response and the limited capacity of the ESS. In addition, the proposed approach combines into a single process different features of AUFLS methods already presented in the technical literature. Different scenarios of a modified version of the IEEE New England system is used to evaluate the proposed method and to compare its performance with other load shedding techniques. The results show that the proposed AUFLS is able to better estimate the generation deficit, resulting in a more precise and robust load shedding scheme.

An Adaptive Underfrequency Load Shedding Scheme in the Presence of Solar Photovoltaic Plants

Load shedding is performed, as last option, for emergency measure when system frequency drifts below the allowed limit. Utilities use underfrequency load shedding schemes to bring back frequency within limit. In this article, an adaptive underfrequency load shedding scheme is proposed for the power system in the presence of solar photovoltaic plants using wide-area measurements. Stepwise load shedding is performed at selected buses with consideration of any change in photovoltaic power output and load/generation at each step. Share of load to be shed at a bus is based on the voltage stability index of the buses at that instant considering predefined load priority. The voltage stability index at a bus is obtained using the Thevenin equivalent of the system. For a solar photovoltaic plant integrated system, the Thevenin equivalent changes with solar power variation, where available methods become inaccurate. A Thevenin equivalent calculation approach is proposed in this article using an equivalent model of synchronous generators and photovoltaic plants. The proposed load shedding method is tested on the New England 39-bus system and the 233-bus North Eastern Indian grid with integrated photovoltaic plants. It is compared with available methods and found to be a better technique in bringing system frequency to normalcy with adaptive load shedding.

Optimal design of an adaptive under-frequency load shedding scheme in smart grids considering operational uncertainties

In the light of the smart grid paradigm, power systems have recently brought about higher levels of adaptability and flexibility. The associated functions in energy management system blocks need to adopt robust strategies to provide greater levels of control and protective service. Under-frequency load shedding (UFLS) schemes can be regarded as a critical function, through which the system integrity and sustainability indices are protected reliably. Conventional UFLS schemes usually suffer from low adaptability and optimal setting in response to changes in the operational characteristics of the power system. This paper seeks to propose an hourly framework for the optimal design of an adaptive UFLS function based on advanced metering infrastructure (AMI) in smart grids. In this portfolio, the target is tailored to fit the main UFLS settings based on the system contingencies and associated operational uncertainties. The simulations are executed on the IEEE 39-bus test system, implemented on a 24-hour scheduling time horizon. The effectiveness of the proposed adaptive model is evaluated with a thorough analysis of the derived numerical results.

Adaptive Under-Frequency Load-Shedding Considering Load Dynamics and Post Corrective Actions to Prevent Voltage Instability

The conventional under frequency load shedding (UFLS) strategies disconnect the preset amount of the load from the power grid in specific steps. These approaches ignore the daily load variations and the total outage costs, leading to a non-optimal solution. This paper proposes an adaptive UFLS scheme that considers the daily profiles of the different load types to minimize the total outage cost. To obtain the exact amount of the load to be shed, the dynamic performance of the load clusters is modeled. Furthermore, a series of post load-shedding corrective actions (PLSA) are implemented after the UFLS to satisfy the voltage constraints, using the installed reactive power control apparatus and static VAR compensators (SVCs). Two static (frequency-domain) and transient (time-domain) indexes are defined to evaluate the feasibility of possible UFLS/PLSA scenarios. The promising solutions are ranked and the most favorable ones are embedded in a number of look-up tables. The proposed UFLS/PLSA approach is applied to the main grid as well as the potential islands. The performance of the proposed adaptive UFLS/PLSA approach is tested on the IEEE 39-bus system via numerous scenarios, and the results show feasibility, fast and optimal performance of the proposed method.

A Smart Under-Frequency Load Shedding Scheme based on Takagi-Sugeno Fuzzy Inference System and Flexible Load Priority

This paper proposes a new smart under frequency load shedding (UFLS) scheme, based on Takagi-Sugeno (TS) fuzzy inference system and flexible load priority. The proposed scheme consists of two parts. First part consists of fuzzy load shed amount estimation module (FLSAEM) which uses TS-fuzzy to estimate the amount of load shed and sends its value to accurate load shedding module (ALSM) to perform accurate load shedding using flexible load priority. The performance of the proposed scheme is tested for intentional islanding case and increment of sudden load in the system. Moreover, the response of the proposed scheme is compared with adaptive UFLS scheme to highlight its advantages. The simulation results show that the proposed UFLS scheme provides the accurate load shedding due to advantage of flexible priority whereas adaptive UFLS scheme due to fixed load priority does not succeed to achieve accurate load shedding.

Adaptive underfrequency load shedding using particle swarm optimization algorithm

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 properper formance 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, particles warm optimization algorithm is used to forecast the minimum frequency based on these samples. To verify the effectiveness of the proposed loads hedding scheme, its performance has been compared with a newly suggested method.

A New Centralized Adaptive Underfrequency Load Shedding Controller for Microgrids Based on a Distribution State Estimator

This paper presents a new centralized adaptive underfrequency load shedding controller that is integrated with a distribution state estimator to improve the frequency stability of islanded microgrids. While underfrequency load shedding is a well-established corrective emergency frequency-control action, it is not entirely suitable for use in microgrids with low inertia and a limited number of measurement devices. The novel centralized controller presented in this paper overcomes these challenges by incorporating a distribution state estimator to estimate the power consumption of the demand. In parallel to this, the actual active power imbalance in the microgrid is estimated by simultaneously monitoring the system frequency and its rate of change. The centralized controller uses these two variables to determine the correct amount of load to be shed. The performance of the proposed controller has been validated using computer simulations of the actual distribution network in Malaysia. These simulation results show the effectiveness of the proposed centralized controller for preventing power system instabilities, which might lead to cascading outages and even a complete blackout in microgrids that have been disconnected/islanded, from the main grid.

Enhanced state estimator incorporating adaptive underfrequency load shedding under contingencies via the alternating optimization method

An enhanced state estimator (SE) is proposed to incorporate the real-time underfrequency load shedding (UFLS) information in this paper. During contingencies, the historical SCADA measurements are not accurate enough while the PMU measurements may be inadequate for the SE. Considering that load shedding strategy is a common practice to prevent the system from collapse during contingencies, we can use the shedding information as a viable option to update/correct the SCADA measurements by employing proper techniques. The auxiliary binary variables and scheduled load shedding amount are introduced in the traditional weighted least square formulation so that a more accurate SE can be obtained. The alternating optimization method is utilized to solve the SE problem. Case studies are conducted on the IEEE 30- and 118-Bus system to illustrate the proposed methodology and demonstrate the evident improvements on the reconstructed states.

Predictive underfrequency load shedding scheme for islanded power systems with renewable generation

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.

Centralized Adaptive Under Frequency Load Shedding Schemes for Smart Grid Using Synchronous Phase Measurement Unit

Under frequency load shedding (UFLS) is an effective way to prevent system blackout after a serious disturbance occurs in a power system. A novel centralized adaptive under frequency load shedding (AUFLS) scheme using the synchronous phase measurement unit (PMU) is proposed in this paper. Two main stages are consisted of in the developed technique. In the first stage, the active power deficit is estimated by using the simplest expression of the generator swing equation and static load model since the frequency, voltages and their rate of change can be obtained by means of measurements in real-time from various devices such as phase measurement units. In the second stage, the UFLS schemes are adapted to the estimated magnitude based on the presented model. The effectiveness of the proposed AUFLS scheme is investigated simulating different disturbance in IEEE 10-generator 39-bus New England test system.

Regional Coordination for under Frequency Load Shedding

Frequency deviation can be used as an indicator of imbalance between supply and demand. When generation is insufficient, it can cause frequency decline in a power system operation. Implementing under frequency load shedding (UFLS) is one of the common methods to overcome this problem. This paper proposes a novel approach for adaptive load shedding. The concept is an extension of shared and targeted load shedding using reserve margin. The optimal system configuration is then selected from those candidates to fulfill operational objectives. Operational constraints related to system parameters, threshold frequency, total of load shed and control area including line capacity are considered. An example using four sub-areas connected to an external system shows that the proposed regional coordination as an adaptive UFLS is feasible.

Adaptive Underfrequency Load Shedding Based on the Magnitude of the Disturbance Estimation

In this paper, a new approach to adaptive underfrequency load shedding (UFLS), a procedure for protecting electric power systems from dynamic instability and frequency collapse, is presented. It consists of two main stages. In the first stage, the frequency and the rate of frequency change are estimated by the nonrecursive Newton-type algorithm. By using the simplest expression of the generator swing equation, in the second algorithm stage, the magnitude of the disturbance is determined. The UFLS plan is adapted to the magnitude estimated, obtaining in this way a more efficient system operation during emergency conditions. Results of procedure testing are demonstrated through the dynamic simulations by using: 1) a simple three-machine test system and 2) a ten-machine New England system

Adaptive underfrequency load shedding integrated with a frequency estimation numerical algorithm

An approach to adaptive underfrequency load shedding, a procedure for protecting electric power systems from dynamic instability and frequency collapse, is presented. It consists of two main stages. In the first stage, the frequency and the rate of frequency change are estimated by the nonrecursive Newton type algorithm. By using the simplest expression of the generator swing equation, in the second stage the magnitude of the disturbance is obtained. The UFLS plan is adapted to the magnitude estimated, to obtain a more efficient system for operation during emergency conditions. Results of procedure testing are demonstrated through computer simulations by using a 10-machine New England test system.