Undervoltage Load Shedding Research Articles

Intelligent Under Frequency and Under Voltage Load Shedding Method Based on the Active Participation of Smart Appliances

Under frequency load shedding (UFLS) and under voltage load shedding (UVLS) are effective methods to maintain the power balance of and prevent frequency or voltage collapse of power systems. With the construction of wide area measurement systems, the development of demand response technologies and the application of smart appliances, a new direction for load shedding method emerges under the background of smart grid. In this paper, an intelligent UFLS/UVLS method based on the active participation of smart appliances is proposed. First, this paper takes refrigerators and water heaters as research objects, establishes their practical models and proposes a suitable control method for intelligent load shedding. Then an intelligent UFLS/UVLS method is proposed and simulated in IEEE 39-bus system. Simulation results prove that this strategy can work well to suppress the frequency and voltage fall or even collapse, and reduce the amount of load shedding and economic losses.

A new undervoltage load shedding method to reduce active power curtailment

Summary This study presents a new undervoltage load shedding (UVLS) method to reduce the amount of power curtailment in emergency conditions. At first, the effect of temperature is considered in an existing multiport network model. Then, a new centralized method called 2-factor UVLS is proposed. The proposed method considers the load reactive power and the multiport network model to determine the effective location of the UVLS. So as to take the actual conditions into account, the main challenges related to the centralized applications are evaluated and then a solution is introduced to overcome these challenges. The proposed 2-factor method and the multiport network model are implemented in the IEEE 57-bus test system, IEEE 118-bus test system, and a real 341-bus power system. Moreover, a comparison is made between 2-factor and sensitivity-based methods. The results show that the proposed 2-factor UVLS method needs less active power shedding than the multiport network model and sensitivity-based method.

A Novel Load Shedding Scheme for Voltage Stability

Abstract Undervoltage load shedding serves to maintain voltage stability when a majority of loads are fast acting. An undervoltage load shedding scheme should address two tasks: the detection of voltage instability following a large disturbance and the determination of the amount of load to be shed. Additionally, in case of short-term voltage instability, the scheme should be fast. This paper proposes a method to predict voltage instability arising due to a large disturbance. The amount of load to be shed to maintain voltage stability is then determined from the Thevenin equivalent of the network as seen from the local bus. The proposed method uses local measurements of bus voltage and power, and does not require knowledge of the network. The method is validated by simulation of three test systems subjected to a large disturbance. The proposed scheme is fairly accurate in estimating the minimum amount of load to be shed to maintain stability. The method is also successful in maintaining stability in cases where voltage collapse is detected at multiple buses.

Optimal load shedding by a new binary PSO

Purpose The voltage stability is a basic principle in the power system operation. Different short circuits, load growth, generation shortage, and other faults which disturb the voltage stability are serious threats to the system security. The voltage instability causes dispersal of a power system into sub-systems, and leads to blackout as well as heavy damages of the system equipments. Optimum load shedding during contingency situations is one of the most important issues in power system security analysis. Design/methodology/approach In this paper, a New Binary Particle Swarm Optimization technique (NB-PSO) is proposed for solving the integer-valued modeling of under-voltage load shedding (UVLS) problem. The updating mechanisms for the position and velocity of binary particles are amended in the proposed NB-PSO by using a new velocity definition, which has an excellent efficiency for solving complex binary optimization problems. Findings The effectiveness and capability of the proposed NB-PSO optimization algorithm were illustrated according to the simulation results of applying it to the IEEE 118-bus test system. In addition, the performance of the proposed NB-PSO based method was compared with other optimization algorithms, such as the Binary Particle Swarm Optimization (BPSO) and Hybrid Discrete Particle Swarm Optimization (HDPSO) techniques. It was perceived that the NB-PSO performs superior than the BPSO and HDPSO for determining the best location and the minimum level of load shedding in order to prevent voltage instability. Originality/value The proposed NB-PSO has novel modifications and techniques to enhance both exploration and exploitation capabilities to find the optimal feasible solution. The simulation results confirmed the effectiveness of the proposed method in determining the best location and the minimum amount of load shedding for voltage collapse prevention.

Adaptive under‐voltage load shedding scheme for large interconnected smart grids based on wide area synchrophasor measurements

The existing local protection schemes for undervoltage protection in power systems are not engineered to protect the system against wide area disturbances. Therefore, system integrity protection schemes (SIPS) designed to preserve the integrity of the system are required. This study proposes a new response-based SIPS for adaptive undervoltage load shedding (UVLS) in large interconnected systems based on the IEEE C37.118.1-2011 synchrophasor measurements from widely dispersed phasor measurement units (PMUs) synchronised to the reference time of the global positioning system. The control signals to shed the feeder loads are based on high-speed IEC 61850 Generic Object Oriented Substation Event messages. The proposed adaptive UVLS-SIPS algorithm calculates in real-time the total amount of load to shed corresponding to the severity of the disturbance using the least squares algorithm. The amount of load to shed and its distribution across the interconnected system is based on a formulated reactive power mismatch and adaptive weighted summation real-time voltage stability assessment index, respectively. The proposed UVLS-SIPS algorithm was deployed onto a programmable logic controller (PLC), and was tested under various dynamic conditions using the New England 39-bus test system. Real-time simulations were conducted with an industrial-grade ‘proof-of-concept’ testbed.

A robust undervoltage load shedding scheme against voltage instability

Undervoltage load shedding (UVLS) is an effective measure for restoring voltage stability as a last resort in emergency conditions. In this paper, a UVLS scheme is proposed considering the uncertainties of load and line parameters. The loading margin is considered as the voltage stability index. The proposed scheme determines the optimal load shedding pattern subject to technical constraints. The info-gap decision theory uncertainty modeling technique is utilized to find a robust and cost-effective pattern of load shedding. The proposed scheme is formulated as a nonlinear programming problem, which is solved using the sequential quadratic programming technique. The performance of the proposed scheme is verified for the IEEE-14 bus and IEEE-118 bus test systems.

Techno-economic impacts of automatic undervoltage load shedding under emergency

Different schemes for voltage control under emergency are adopted in different jurisdictions around the world. While some features, such as Automatic Voltage Regulation (AVR), are common in all countries, for what concerns undervoltage load shedding (UVLS), to contrast voltage instability or collapse, different schemes are adopted. Most US transmission system operators (TSOs) adopt automatic UVLS schemes, with different capabilities and settings while TSOs in EU usually do not implement automatic UVLS but leave the decisions to the control room operators. The two options may lead to different impacts in terms of trajectory and final status of the transmission grid under emergency, with different unserved energy. In this paper we analyze the impacts from a technical and economic perspective, modeling the grid behavior with different UVLS schemes (none, manual and automatic). The comparison between the different schemes is done resorting to the Incident Response System (IRS), a software tool developed by the authors in the EU-FP7 SESAME project. An illustrative example to a realistic test case is presented and discussed. This paper shows that automatic UVLS is superior to Manual UVLS, from both technical and economic point of view, due to the fast evolution of voltage collapse phenomena and insufficient time for system operators’ manual reaction. The benefits of the scheme involving the automatic UVLS can be then compared with the investment costs of equipping the network with those devices.

Probabilistic undervoltage load shedding using point estimate method

Undervoltage load shedding (UVLS) is executed as a last resort to avoid voltage instability in critical situations. In this study, a robust UVLS scheme based on the point estimate method (PEM) is developed taking load uncertainties into consideration. The utilised PEM converts the continuous probability distribution functions (PDFs) of load uncertainty to discrete statistical moments. The extracted moments could represent the behaviours of original continuous PDFs. The proposed stochastic UVLS strategy is expressed as a non-linear programming problem, which is optimised using sequential quadratic programming technique. The fulfilment of the suggested scheme is compared with Monte Carlo simulation technique for two IEEE 14-bus and IEEE 118-bus test grids.

Voltage Multistability and Pulse Emergency Control for Distribution System With Power Flow Reversal

High levels of penetration of distributed generation and aggressive reactive power compensation may result in the reversal of power flows in future distribution grids. The voltage stability of these operating conditions may be very different from the more traditional power consumption regime. This paper focuses on the demonstration of multistability phenomenon in radial distribution systems with reversed power flow where multiple stable equilibria coexist for the given set of parameters. The system may experience transitions between different equilibria after being subjected to disturbances such as short-term losses of distributed generation or transient faults. Convergence to an undesirable equilibrium places the system in an emergency or in extremis state. Traditional emergency control schemes are not capable of restoring the system if it gets entrapped in one of the low voltage equilibria. Moreover, undervoltage load shedding may have a reverse action on the system and can induce voltage collapse. We propose a novel pulse emergency control strategy that restores the system to the normal state without any interruption of power delivery. The results are validated with dynamic simulations of IEEE 13-bus feeder performed with SystemModeler software. The dynamic models can also be used for characterization of the solution branches via a novel approach, so-called the admittance homotopy power flow method.

Adaptive Tuning of Frequency Thresholds Using Voltage Drop Data in Decentralized Load Shedding

Load shedding (LS) is the last firewall and the most expensive control action against power system blackout. In the conventional under frequency LS (UFLS) schemes, the load drop locations are already determined independently of the event location. Furthermore, the frequency thresholds of LS relays are prespecified and constant values which may not be a comprehensive solution for widespread range of possible events. This paper addresses the decentralized LS in which the instantaneous voltage deviation of load buses is used to determine the frequency thresholds of LS relays. The higher frequency thresholds are assigned to the loads with larger voltage decay which are often located in the vicinity of disturbance location. The proposed method simultaneously benefits from individual UFLS and under voltage LS (UVLS) features which operate in the power system without coordination. Numerical simulations in DigSilent PowerFactory software confirm the efficiency of proposed methodology in the stabilization of the power system after various severe contingencies.

전력계통 신뢰도 기준 분석을 통한 765kV 선로사고에 대한 부하차단 적정량 산정에 관한 연구

Load shedding is a last measure to avoid nationwide cascading collapses of power system by removing the pre-determined amount of loads from the main grid. In Korea, SPS(Special Protection System) is prepared to keep the power system stability from the extreme contingency of the critical transmission line losses. Among them, we need to pay attention to 765kV T/L’s because they have great influence on the total system stability. According to the present SPS operating guide, the total loads of 1,500MW should be removed through 2 step under-voltage load shedding(UVLS) scheme in case of 765kV T/L’s contingencies. However, it is not clear to defined how to determine the typical load reduction amounts for each case. This paper proposes a method to estimate appropriate amounts of load shed for 765kV T/L’s contingencies by analyzing the relevant national and international standards.

Frequency and Voltage Stability Assessment Applied to Load Shedding

Under voltage load shedding (UVLS) and under frequency load shedding (UFLS) is plays vital role in the power system operation. As large disturbances are occur frequently in previous days. Generally these two methods are work independently from each other, and are not designed to integrated way to use their combined effect on load shedding. The reactive power hardly consider in load shedding process. To fill this gap, we froth put in this paper a new concentration about adaptive load shedding algorithm. The phasor measurement unit can gives both voltage and frequency information. The main concept of the new method is consideration of reactive power combined with active power in load shedding strategy. This method is gives information about the combined voltage and frequency stability issues is better than the independent approaches. The new method is tested on the IEEE 39-Bus system, in order to compare with other methods. By applying this new method to the system, that system will be back to new stable steady state that is better from the perspective of voltage and frequency and loadability.

A new integer-value modeling of optimal load shedding to prevent voltage instability

Abstract This paper presents a novel integer-value modeling for optimal under voltage load shedding (UVLS) problem. The real-value modeling of UVLS is encountered to some drawbacks which lead to an ineffective practical solution. The proposed integer-value modeling overcomes these drawbacks by considering the load feeders of load buses in the modeling as well as assigning interruption penalty factors and participation penalty factors to feeders and buses respectively. Applying these factors to the proposed UVLS integer-value modeling result achieving objectives such as minimizing the total number of participating feeders in the load shedding, minimizing the total number of participating buses in the load shedding, minimizing the total amount of load to be shed, minimizing the total interruption cost and combination sets of the objectives. Without any limitation of applying the proposed modeling to large scale networks, it is implemented on IEEE 14-bus and 30-bus test systems and the results verify the effectiveness of the proposed method.

Under Voltage Load Shedding Clustering Control Based on PV Curve

In order to prevent voltage collapse while the voltages drops, adopts under voltage load shedding (UVLS) clustering control to recover voltage stability. First using the method of sensitivity sorting to cluster the load node, then using PV curve to calculate and determine the number of loads to be shed, finally the same cluster taking the same control parameters to shed the loads, the simulation validate it could recovery voltage stability and embodies the rationality and economical efficiency of this method.

The Existing Investigation and Prospect of Under-Frequency and Under-Voltage Load Shedding in Power Systems

Under-frequency load shedding (UFLS) and under-voltage load shedding (UVLS) as the last defense of the power system to maintain its stability are of great significance, especially for the current development of large-scale interconnected power system. At first, this paper presents the existing methods of load shedding and analyses their drawbacks. Then, a brief overview of the application of wide area measurement system (WAMS) in UFLS and UVLS is provided. Finally, a novel intelligent load shedding scheme which realizes load adjustment instead of load shedding based on the characteristics of smart grid and smart home is proposed.

An implementation of particle swarm optimization to evaluate optimal under-voltage load shedding in competitive electricity markets

Load shedding is a crucial issue in power systems especially under restructured electricity environment. Market-driven load shedding in reregulated power systems associated with security as well as reliability is investigated in this paper. A technoeconomic multi-objective function is introduced to reveal an optimal load shedding scheme considering maximum social welfare. The proposed optimization problem includes maximum GENCOs and loads' profits as well as maximum loadability limit under normal and contingency conditions. Particle swarm optimization (PSO) as a heuristic optimization technique, is utilized to find an optimal load shedding scheme. In a market-driven structure, generators offer their bidding blocks while the dispatchable loads will bid their price-responsive demands. An independent system operator (ISO) derives a market clearing price (MCP) while rescheduling the amount of generating power in both pre-contingency and post-contingency conditions. The proposed methodology is developed on a 3-bus system and then is applied to a modified IEEE 30-bus test system. The obtained results show the effectiveness of the proposed methodology in implementing the optimal load shedding satisfying social welfare by maintaining voltage stability margin (VSM) through technoeconomic analyses.

Adaptive load shedding based on combined frequency and voltage stability assessment using synchrophasor measurements

Under frequency load shedding (UFLS) and under voltage load shedding (UVLS) are attracting more attention, as large disturbances occur more frequently than in the past. Usually, these two schemes work independently from each other, and are not designed in an integrated way to exploit their combined effect on load shedding. Besides, reactive power is seldom considered in the load shedding process. To fill this gap, we propose in this paper a new centralized, adaptive load shedding algorithm, which uses both voltage and frequency information provided by phasor measurement units (PMUs). The main contribution of the new method is the consideration of reactive power together with active power in the load shedding strategy. Therefore, this method addresses the combined voltage and frequency stability issues better than the independent approaches. The new method is tested on the IEEE 39-Bus system, in order to compare it with other methods. Simulation results show that, after large disturbance, this method can bring the system back to a new stable steady state that is better from the point of view of frequency and voltage stability, and loadability.

Coordinated agent-based control for online voltage instability prevention

In this paper, a new method for online voltage instability control suitable for smart grid application is proposed. The method uses the Maximum Loadability Index to determine the amount of load relief necessary to restore stability. This measure can be determined based on local measurements. The load relief is obtained by: increase of local generation, virtual load shedding via adjusting the Load Tap Changers (LTCs) set-point, intelligent load control and the increase of reactive power compensation from Static Var Compensators. For the local generation, the control of two types is described: for micro Combined Heat and Power (micro-CHP) units and for large synchronous generators. In order to obtain constant power supply from a group of micro-CHPs, a Virtual Power Plant coordination method is proposed. The control of the actuators is outlined, and a method is described to coordinate their actions. Based on simulations it is demonstrated that the proposed method works properly: voltage collapse can be avoided, and voltage stability is restored. A sensitivity analysis is performed to determine the optimal settings of the coordinated controller. Furthermore, the merit of including reactive power control in the proposed control strategy is investigated. Compared to classical controls, such as Under-Voltage Load Shedding and LTCs tap blocking, the method performs better. Copyright © 2013 John Wiley & Sons, Ltd.

Adaptive under-voltage load shedding scheme using model predictive control

Recent system wide disturbances have demonstrated the need for automatic system protection schemes to prevent such cascading disturbances. In this paper, an adaptive under-voltage load shedding scheme using model predictive control is proposed to protect power system against voltage instability. The proposed scheme calculates the criticality of the system based on the measurements of voltage magnitudes and reactive power generations and then in case of voltage instability a model predictive based step-sized load shedding scheme will be triggered. The speed and amount of load shedding steps are adapted to the severity of contingency in the affected region. By devising the grid into the separate regions the proposed scheme acts as a distributed controller without having a central dispatching center. The performance of the proposed model predictive based scheme is verified over a 10-bus dynamic test case. The proposed model predictive control will be solved using sequential quadratic programming technique.

A Novel Online Load Shedding Strategy for Mitigating Fault-Induced Delayed Voltage Recovery

This paper develops a novel online fast load shedding strategy aimed at shedding the most effective load to mitigate fault-induced delayed voltage recovery (FIDVR). Induction motor kinetic energy deviation has been shown to be identical to the integral of power imbalance. Instead of using voltage as an indicator, the proposed strategy applies a centralized scheme making use of equivalent motor kinetic energy to identify the most effective loads to shed. In order to derive kinetic energy, the equivalent inertia and the speed of the induction motor are needed. A methodology is proposed to obtain these variables through online measurements. Simulations on a sample power system illustrate the novel load shedding strategy and its effectiveness. The proposed strategy is compared with conventional four-stage under-voltage load shedding (UVLS) scheme. The results have validated that the proposed strategy can effectively mitigate fault-induced delayed voltage recovery and require much less load to be shed.