Optimal Load Shedding

Power system blackouts have become a serious problem for electric utilities especially in recent years. Different forms of system instability have emerged in recent blackouts, such as voltage instability and frequency instability. To counteract each form of system instability, special algorithms have been designed in the protection system, e.g. Under Frequency Load Shedding (UFLS) and Under Voltage Load Shedding (UVLS) schemes. One of the major weaknesses of these algorithms is that combination of different forms of instability is not considered in their design, while any one form of instability may not occur in its pure form. This is particularly true in highly stressed systems and for cascading events. This paper presents an adaptive algorithm to combine UVLS and UFLS schemes. The purpose of this method is to enhance the flexibility of under frequency relays and increase the security of power system during large disturbances by improving system voltage stability margins. In the proposed algorithm, loads with greater voltage decay are shed sooner. In this way, locations of load shedding become dependent to the location of disturbance and voltage stability margins of the system are enhanced. Indeed, load shedding is performed faster for severe events accompanying large voltage or frequency declines. Using this load shedding method, faster reactions could be obtained for major system failures. This way, system blackouts could be better controlled. Performance of the proposed method is compared with the conventional UFLS method. In this paper, dynamic model of the Khorasan HV network of Iran national grid is used for simulation. This network includes 75 high voltage 400 kV and 132 kV buses with about 2700 MW of generation. Performance of the schemes has been evaluated using dynamic simulations as well as the P-V curves, and reactive power margins for a number of events. Simulation results show that conventional UFLS algorithm will not necessarily result in acceptable voltage stability margins for the system following a disturbance. Meanwhile, the proposed algorithm improves voltage stability margins. The proposed algorithm is suitable for saving system stability after occurrence of major system disturbances.

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.

This article presents a new approach to studying the optimal load-shedding problem in power systems. An analytic hierarchical process (AHP) and network flow programming (NFP) are adopted for implementing this approach. The optimal load-shedding scheme should be minimum load curtailment and maximum system benefits, while satisfying the network security. Therefore, the mathematical model of load shedding, in which the objective is a benefit function, is devised. This leads to a network flow programming (NFP) problem with upper and lower limits. So a special solution in NFP, out-of-kilter algorithm (OKA), is employed. A test example of an IEEE 30 bus system is presented for the proposed model and method.

A new comprehensive framework for cascading outage simulation in power systems

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 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

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

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.

Under-Frequency Load Shedding (UFLS) can increase power system reliability by preventing total system collapse during emergency operating situations when load exceeds generation. UFLS is required by The North American Electric Reliability Council (NERC) for large consumers and has been implemented by utilities across the country. When the frequency drops to some predefined set points, feeder breakers are opened to shed loads in favor of maintaining generation/load balance. With increasing pressure to improve system reliability through more sophisticated controls and SCADA (Supervisory Control And Data Acquisition) equipments, electric utilities are encouraged to apply more localized frequency sensing and load shedding schemes on the distribution system. This research work aims at studying the effects of voluntary load shedding for different load shedding scenarios using the simulation package PSS/E. In addition, a consumer- based load shedding controller is proposed, designed and developed.

Objectives: This article shows the application of the Analytical Hierarchical Process to select conveyance in a company that distributes Cement Blocks. Methodology: The Analytical Hierarchical Process was applied taking into account the criteria, Variable Cost, Service, Capacity and Reliability, and conveyance that fit the characteristics of the municipality and the company. Findings: The Process, which allowed to evaluate these criteria and based on these determine the prioritization of the alternatives and the selection of the most efficient method according to the selected criteria. Application / Improvements: The Analytical Hierarchical Process (AHP) is a tool which allows supporting Decision Making, by analysing multiple criteria and depending on their relationship, obtaining a hierarchy in the alternatives, being the one with the greatest impact or hierarchy the decision to choose. Keywords: Alternatives, Analytical Hierarchical Analysis Process, Criteria, Hierarchy, Transportation MeansAHP

The aim of this research work is to develop a load shedding methodology to improve the frequency response of low inertia grids by attaining satisfactory voltage stability. In recent times, wind energy integration has considerably increased in many power grids. Consequently, conventional synchronous machines are being replaced from dispatch. Unlike traditional synchronous machines, variable speed wind turbine generators usually do not take part in frequency regulation without supplementary control mechanism. During substantial wind penetration, a power system may have a small number of online synchronous machines. As a result, synchronous inertia and governor responsive reserve significantly reduce. Under such situation, a system has to rely on load shedding as a last line of defense to rescue the system frequency following a large contingency. However, the conventional Under-Frequency Load Shedding (UFLS) strategy may lead to larger frequency deviation and higher amount of load cut in certain cases. A new load shedding methodology is presented in this paper to overcome this challenge. Unlike conventional UFLS technique, higher proportion of load shedding is applied to relatively weaker buses in terms of voltage stability in the proposed mechanism. Based on reactive power margin, which is an index to specify voltage stability, a general expression to quantify load shedding is derived. Also, the adaptability of the proposed strategy to various load levels is ensured. Later on, performances of the developed strategy are explored in a low inertia wind dominated test network. Simulations are executed considering various penetration levels of wind power and for two severe contingencies - loss of 550 MW interconnection and loss of 650 MW interconnection. Investigations reveal that the proposed load shedding methodology ensures satisfactory frequency response in all simulation cases. Also, the developed technique yields less frequency deviation and load cut compared to the conventional UFLS mechanism. Therefore, the reported load shedding scheme is found to be more competent to concurrently maintain frequency and voltage stabilities in renewable dominated power systems.

AC-Quasi-Steady-State (AC-QSS) models for cascading failure often suffer from divergence issues. Presently, this is handled by uniform load shedding (ULS) or its variants till convergence is achieved. In reality, a pre-defined undervoltage load shedding (UVLS) scheme sheds loads in blocks below a voltage threshold and the system reaches a post-UVLS equilibrium, if it exists. This may not be captured by the post-ULS equilibrium, which in turn can lead to a different path of cascade propagation. We use the ULS equilibrium as the initial state and pose the problem of reaching post-UVLS equilibrium as an optimization problem. Due to the nature of the problem, an exhaustive search can be used that has exponential complexity in inputs. To solve this, we propose a heuristic that reaches a post-UVLS candidate equilibrium. The proposed algorithm is tested on a 2383-bus Polish system.

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

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.

This study aims to analyze the performance of the load shedding system using Under Frequency Relay (UFR) at the Bolangi Substation as an effort to maintain the stability of the power system frequency. A supply disruption from Bayline Daya Baru (#2) occurred, namely a disturbance due to a lightning strike, which is an important background in this study. A decrease in frequency due to power imbalance can cause a total power outage (blackout), so a responsive UFR protection system is needed. Simulations were carried out using ETAP 19.0 software with a bayline release scenario to analyze the frequency response and effectiveness of automatic load shedding. The simulation results show that a frequency decrease of up to 48.60 Hz can be restored to 50 Hz in 2.47 seconds through gradual load shedding of 13.34 MW by UFR stages 2 and 4. This study provides an overview of the load shedding stages based on the frequency setting values ​​and periodic UFR working times as well as the use of ETAP for continuous evaluation of the protection system at the substation.