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The traditional load shedding schemes––––under frequency load shedding (UFLS) and under voltage load shedding (UVLS), are designed independently and they constitute the last line of defense against frequency and voltage instability. However, the traditional load shedding schemes are not capable of dealing with combined instabilities. Moreover, a new Adaptive load shedding under load shedding method was proposed based on wide area measurement information just because the traditional load shedding schemes was not optimization algorithm. The main purpose of the method goal is to set up an emergency load shedding strategy, considering the recovery system of frequency stability at the same time, improved the voltage stability. This process has two steps, first of all, when considering the system disturbance amplitude and classifying the degree of interference according to system disturbance, Furthermore, the voltage sensitivity analysis was considered to determine the shedding location and corresponding shedding value. Finally, this algorithm was applied in IEEE 10-generator 39-bus test system. The implementation emergency of traditional and new shedding load two methods Under the system by different levels of power disturbances, comparing and analysand the feasibility and effectiveness of the new project. The results showed that the new method avoiding shedding over, more shedding part of the reactive power load, and then improving the system voltage, also avoiding damage to the voltage stability in the process of the system restore.

In this paper, we have proposed some under-frequency load shedding (UFLS) schemes for implementation in the Khorasan Network. In this regard, we have suggested a new relation to determine the number of substations required for load shedding at each frequency step, using which, the guideline of IGMC is well observed for the UFLS schemes. In the analysis of the proposed schemes, after reviewing the required constraints, their features have been analyzed in terms of three basic aspects: 1.The amount of load shedding at each frequency step; 2.The frequency response and its stability after disturbance (generator or tie-line outage or sudden load increase); 3.The geographical distribution of substations in each scheme; The Khorasan grid was modeled in the DigSilent Power Factory software with all the actual information of loads and power plants as well as the required dynamic parameters to analyze the frequency of the system in the contingency. The results of this study will be highly effective and useful in the quantitative evaluation of UFLS and selecting the optimal schemes.

This paper discusses why voltage as well as frequency load shedding may be necessary to prevent major system blackouts. This paper explores the nature of recent power system blackouts and explains why voltage collapse is the leading edge indicator of impending power system problems. It also discusses the design and security issues that need to be addressed in the design of an undervoltage load shedding (UVLS) scheme and why relying on underfrequency load shedding (UFLS) may be "too little, too late." The paper addresses the current level of UVLS on utility systems as well as current NERC (North American Electric Reliability Council) pronouncements on the subject

In recent years several catastrophic power systems blackouts have occurred worldwide. Various reasons have been declared for these failures. Economical limitations due to power system restructuring restrictions, inadvertent operation of protective relays and inefficient design of conventional load shedding schemes are of the most important reasons causing these blackouts. In fact, due to both economical and technological restrictions, it is not possible to completely prevent these blackouts. However, with the aid of some protection and control strategies, frequency and intensity of these blackouts may be reduced. One of the important protection strategies used for this purpose is a class of protection schemes known as ‘System Protection Schemes’ or ‘Wide Area Protection Schemes’. One of the most commonly used types of system protection schemes, generally accepted after the north-eastern blackout of 1965, is Under Frequency Load Shedding (UFLS) scheme. Conventional under frequency load shedding scheme is designed to retrieve the balance of generation and consumption following a disturbance. In the conventional load shedding method frequency settings, time delay settings and the amount of load to be shed in each step are constant values. The loads to be shed by this scheme are also constant load feeders and are not selected adaptively. Using this constant non-adaptive load shedding algorithm is not the most efficient scheme for all power system disturbances. In some combinational disturbances, events causing frequency to drop are followed by other events causing voltage drop. In these cases, since loads are voltage dependent, total system load is reduced and system frequency might not decrease so much to activate UFLS relays. However, the system could eventually collapse due to voltage instability. In many such cases the system would survive if the load shedding relays operate adaptively and appropriately. For example if for large disturbances, higher frequency settings and lower time delays are used adaptively, a faster load shedding response is obtained and as a result system collapses may be prevented. In this paper a new UFLS algorithm is proposed. The purpose of this algorithm is to adaptively adjust speed of load shedding based on the magnitude of disturbance. In this method rate of frequency decline is used as a criterion to determine intensity of disturbance. Thereby, for large disturbances higher frequency settings and lower time delays are used. Application of the proposed algorithm to the simulated model of Khorasan network in Iran confirms its satisfactory performance. As the results of simulations show, several voltage collapse instabilities may be prevented by using the proposed adaptive UFLS method.

This chapter contains sections titled: Introduction Conventional Load Shedding Intelligent Load Shedding Formulation of Optimal Load Shedding Optimal Load Shedding with Network Constraints Optimal Load Shedding without Network Constraints Distributed Interruptible Load Shedding Undervoltage Load Shedding Congestion Management References

A novel adaptive control strategy for under frequency protection and load shedding associated with synchronous phase measurement unit (PMU) is proposed in this paper. With the help of the PMU, the magnitude of 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. Then, a robust control strategy with consideration of the uncertainties in system is presented. In the proposed robust control strategy, the under frequency load shedding (UFLS) schemes are used to prevent system frequency collapse. And minimal amounts of remote information are required. The effectiveness and robustness of the proposed control strategy investigated carrying out on 17-generator 69-bus test system under various disturbance scenarios

Modified centralized ROCOF based load shedding scheme in an islanded distribution network

Emergency conditions arising due to generating power deficiency and the consequent drop in power system frequency can lead to system collapse and a large scale loss of load. Underfrequency load shedding (UFLS) is a globally accepted practice to handle this situation. Most UFLS schemes use pre-specified step sizes based upon frequency measurements. This paper presents an adaptive scheme which uses both frequency and rate-of-change of frequency measurements to dynamically set UFLS relays if more than one generation outage is experienced. It then proposes a technique for coordinating UFLS and the activation of spinning reserves through localized governor control. Recent developments in computer based frequency and rate-of-change of frequency relays makes this scheme both feasible and attractive.

A new comprehensive framework for cascading outage simulation in power systems

With high penetration of distributed generation (DG), the conventional underfrequency load shedding (UFLS) faces many challenges and may not perform as expected. This paper proposes new UFLS schemes, which are designed to overcome the shortcomings of the traditional load-shedding scheme. These schemes utilize directional relays, power flow through feeders, wind, and photovoltaic (PV) measurements to optimally select the feeders to be disconnected during load shedding such that DG disconnection is minimized while disconnecting the required amount of consumption. These different UFLS schemes are compared in terms of frequency response, amount of consumption, and DG disconnected during load shedding.

Under frequency load shedding is implemented to restore power system frequency stability if system frequency drops below the operational set point during major disturbance such as lost of generation. Different countries/utility companies have their own philosophies in implementing the under frequency load shedding scheme. Generally, it is based on country/utility requirements, e.g. the overall power system network and the country's demographic. This paper presents the principles and implementation of the under frequency load shedding (UFLS) and presented using simulations of 56 test bus-system. The performance of the developed schemes under various conditions of disturbance were compared and analyzed. All the simulation works were performed using Siemens PTI software PSS®E.

Under-frequency load shedding (UFLS) schemes are the latest safety measures applied for safeguarding the integrity of the grid against abrupt frequency imbalances. The overall inertia of electrical power systems is expected to decrease with an increased penetration of renewable energy as well as elements connected through power electronic interfaces. However, voltage source converter-based high voltage direct current (VSC-HVDC) links can provide virtual inertia through a control loop that allows for a reaction to occur at certain frequency fluctuations. This paper evaluates a UFLS scheme that considers the injection of virtual inertia through a VSC-HVDC link. A genetic algorithm (GA) is used to determine the location of the UFLS relays, the activation threshold of each stage, the delay time and the percentage of load shedding at each stage. It was found that the virtual inertia causes the nadir to delay and sometimes reach a greater depth. Furthermore, the implemented GA approximates the frequency response to the limits set with the constraints, reducing the load shedding but achieving a steeper nadir and a lower steady-state frequency level than traditional UFLS. The simulations were performed using the IEEE 39-bus test system.

With the rapid development of microgrids, the types of users and loads in microgrids are increasing. Comprehensively reflecting the value of loads in load shedding decisions is one of the main challenges for the intelligent control of an islanded microgrid. However, the existing load shedding schemes ignore the user’s subjective preference for multiple attributes of loads, which are insufficient for reflecting the comprehensive value of loads. To address the aforementioned problem, an underfrequency load shedding (UFLS) scheme based on the comprehensive weight of loads is proposed. This scheme evaluates the multiple attributes of loads and the user’s preference for load attributes based on the entropy method with high adaptability (EMHA) and the analytical hierarchy process (AHP) to obtain the objective and subjective weights of loads. Then, the comprehensive weight of loads is calculated based on the relative entropy to determine the sequence of load shedding. Furthermore, the optimal objective function of the comprehensive cost of load shedding is constructed, and gray wolf optimization (GWO) is used to solve the objective function and obtain the optimal load shedding scheme. Ultimately, the effectiveness of the proposed scheme is verified based on a modified standard IEEE 37-bus system.

A smart load management system based on the grasshopper optimization algorithm using the under-frequency load shedding approach

This paper presents a data-driven methodology for estimating under frequency load shedding (UFLS) in small power systems. UFLS plays a vital role in maintaining system stability by shedding load when the frequency drops below a specified threshold following loss of generation. Using a dynamic system frequency response (SFR) model we generate different values of UFLS (i.e., labels) predicated on a set of carefully selected operating conditions (i.e., features). Machine learning (ML) algorithms are then applied to learn the relationship between chosen features and the UFLS labels. A novel regression tree and the Tobit model are suggested for this purpose and we show how the resulting non-linear model can be directly incorporated into a mixed integer linear programming (MILP) problem. The trained model can be used to estimate UFLS in security-constrained operational planning problems, improving frequency response, optimizing reserve allocation, and reducing costs. The methodology is applied to the La Palma island power system, demonstrating its accuracy and effectiveness. The results confirm that the amount of UFLS can be estimated with the mean absolute error (MAE) as small as 0.179 MW for the whole process, with a model that is representable as a MILP for use in scheduling problems such as unit commitment among others.