Load Shedding Procedure Research Articles

Effect of Stress Ratio, Constraint, and Load History on the Intrinsic and Extrinsic Components of Threshold Stress Intensity

Abstract Threshold intensity range, ΔKth, was experimentally investigated as the sum of two components, both of which are sensitive to applied loading conditions. The intrinsic component, ΔKth,i, is characterized as a sole function of a near-tip stress parameter, σ*, in much the same way as fatigue limit depends on mean stress or stress ratio. The extrinsic component, ΔKcl, is the attenuated fraction of crack tip response because of crack wake contact, whose exact value is not readily measured at threshold. The ΔKth,i versus σ* relationship is established using a specially designed load shedding process under constant baseline maximum load, with σ* modulated through periodic overload-underload combinations whose periodicity is regulated to avoid undue crack extension at threshold. This relationship is then used to extract the ΔKcl component from ΔKth readouts obtained on the same batch of material using conventional load shedding procedure. The effect of constraint on ΔKth and its two components was studied using specimens with sharp side grooves to maximize constraint across the crack front. The study was performed on a medium strength low-alloy steel.

Adaptive Wide-Area Load Shedding Scheme Based on the Sink and Source Concept to Preserve Power System Stability

In this article, a novel wide-area scheme is proposed based on a new sink and source area (SSA) concept. The SSA concept identifies the sink and source regions using a limited number of system voltage phasors. These voltage phasors can be easily transmitted to the control center using a few number of phasor measurement units (PMUs). The SSA idea is used to determine the priority of load buses or regions in the load shedding procedure. Since the SSAs are directly dependent to the generation and load distributions among the resource and demands, they vary over the time. The load shedding locations identified by SSA will accordingly change, which makes more technical sense versus selection of fixed candidates regardless of the real-time operation status. Furthermore, the amount of load shedding is adaptively determined based on the comprehensive analysis of interaction characteristics between load buses. The performance of the proposed approach is evaluated on two networks under various realistic scenarios. The simulation studies’ results reveal promising operation of the proposed approach comparing to that of other load shedding techniques. Moreover, the required number of PMUs and communication infrastructure are presented and the impact of communication link delay on the proposed algorithm is investigated.

A novel congestion management method through power system partitioning

• Presenting a novel real time partition based CM method through power tracing and sensitivity analysis. • Introducing a new congestion index (CI) that simultaneously considers the contributions of each element in the congested line(s) and the effectiveness of the elements in alleviating the congestion. • Developing a new zonal CM based rescheduling and load shedding procedure. Congestion management (CM) is inevitable in today's competitive power markets. A CM method should be fast, fair, effective, and motivational. Moreover, in critical congestions, the system simplification and congestion clearing time are also of considerable importance. All the mentioned features can be found in an intelligent zonal CM. In this paper, using sensitivity analysis and power tracing techniques, a new CM model is developed based on power system partitioning. By the proposed model, a congestion index (CI) is introduced by which a candidate zone(s) is specified including some elements (generators and loads) with the highest CIs which means that they have high participation in congestion creation and simultaneously high effectiveness in congestion alleviation. Using the proposed method, power system operators can put their focus on the candidate zone(s) and alleviate the congestion effectively by some remedial measures such as generation rescheduling and load shedding. The proposed methodology is applied on IEEE 39-bus New England test system including 46 transmission lines. Results show the effectiveness and practicality of the proposed model, so that the congestion at the critical lines is alleviated well and their available transfer capabilities (ATCs) are increased to the reliable amounts.

Under-frequency load shedding in isolated multi-microgrids

In this paper, a new under-frequency load shedding (UFLS) scheme is proposed for multi-microgrid (MMG) application. MMG, consists of multiple micro-grids, perhaps with different owners, connected together. Multiple owners make load shedding policies more complex in MMG compared to the policies employed in micro-grids and large power systems. Accordingly, an appropriate UFLS scheme is required, in order to ensure the frequency stability of the MMG, while considering its economic aspects. Therefore, in this paper, an optimization problem has been introduced which minimizes the cost imposed on the owners for mandatory load shedding alongside the cost of MMGs operation after the load shedding procedure. By solving the proposed optimization problem, the optimal locations of loads to be shed are determined. Two optimization algorithms – genetic algorithm (GA) and exchange market algorithm (EMA) – and GAMS software have been used for solving this optimization problem. A new index is introduced in this paper in order to distribute load shedding among MGs based on their total power generation and load demand. Simulation results confirm the efficiency of the proposed methodology. • An exclusive under-frequency load shedding scheme is proposed for multi-microgrids. • The UFLS scheme provides a fair distribution of load shedding among microgrids. • The location of removable loads is determined by solving the optimization problem. • A mixed integer non-linear programming optimization problem is proposed. • A new index is introduced in order to distribute load shedding among MGs.

A Remedial Action Scheme to Prevent Mid/Long-Term Voltage Instabilities

The main objective of this article is to propose a special protection system (SPS) to execute efficient remedial actions to prevent mid-term and long-term voltage instabilities. In this method, when the operating point (OP) leaves the normal operation state, the proposed SPS is initiated to execute the required corrective remedial actions to bring the OP back to a normal state and maintain bus voltages above prespecified thresholds. Considering the local nature of Volt/Var control and in order to execute fast remedial actions, in this approach, the required generation rescheduling or load shedding procedures were selected based on the electrical distance concept. This allows for remedial actions with the highest impact on volt/var control. In addition, in generation rescheduling procedures, the proposed method uses the ability of utility-scale photovoltaic resources, which can change their generation quickly. This plays a major role in maintaining stability of power systems. Efficiency of the proposed algorithm was validated through several scenarios performed in IEEE 39-bus, Nordic32, and PST 16 test systems using DIgSILENT PowerFactory software. Results of these dynamic simulations and their comparison with some previously published methods show the effectiveness of this method in timely executing appropriate remedial actions to maintain system stability.

Analysis of Under-frequency Load Shedding (UFLS) Relay Setting during Disturbances

Load changes on the system will affect the system stability itself. Load demand that exceeds the generated power will cause the system frequency to decline. Therefore, a load shedding procedure is required to improve the frequency. This research focuses on how to design a load shedding scheme that is activated by the operation of under frequency relay. There are two scenarios to analyze the system performance with a simulation, which are losing power on line about 128 MVA and generator loss power about 192 MVA. Those scenarios result in a decrease in the system frequency to 47.48 Hz and 47.90, respectively. After the load shedding scenario is performed, the frequency became an increase in the range of 51.54 Hz and 49 Hz within a few seconds.

Integrating Model-Driven and Data-Driven Methods for Power System Frequency Stability Assessment and Control

With increase of practical power system complexity, power system online stability assessment and control is more and more important. Application of the traditional model-driven methods is always limited by contradiction between accuracy and efficiency, while data-driven methods demonstrate strong abilities for the online decision-making support with advancement of various data mining techniques. Instead of direct application of data-driven methods in the power system, this paper first discusses feasible integration approaches for the model-driven and data-driven methods based on the existing achievements, and then, proposes to integrate both methods for the power system online frequency stability assessment and control. The integrated method consists of frequency dynamics prediction and load shedding procedure. In frequency dynamics prediction procedure, integration of system frequency response (SFR) model and the extreme learning machine (ELM)-based learning model is applied, where basic physical causality is kept in the SFR model and ELM is used to fit and correct error of the SFR. The ELM also plays a part in load shedding prediction model construction by digging out mapping relationship from samples. Finally, the proposed prediction and control scheme for the frequency stability is verified by simulations on WSCC 9-bus, New England 39-bus, and NPCC 140-bus system. Results show that the reliability, time efficiency, and accuracy are enhanced with the proposed method.

Event‐driven frequency and voltage stability predictive assessment and unified load shedding

A well-designed scheme is essential for assessment and protection of the stability of power system after an unanticipated event. This paper proposes a strategy for regulating three-phase faults and other events that endanger frequency or voltage stability of the system. The aim is to promptly identify such events utilizing supervised learning-based classification modules, and initiate suitable emergency control. The imbalances of active and reactive power after an event have been utilized to predict the stability of system. A new optimization-based formulation has been presented for estimation of reactive power imbalance. An event predicted as unstable is counteracted by prompt load shedding. Conventionally, load shedding is based on frequency or voltage information independently which reduces the effect of load shedding. This work proposes a new load shedding procedure considering both active and reactive power. The proposed scheme is tested on New England (NE) 39-bus system and Northern Regional Power Grid (NRPG) 246-bus system, and results of load shedding have been compared with two existing load shedding schemes. The results indicate the accuracy of classification modules in identifying faults and unstable events, and the proposed load shedding restores stability of the system with less amount of load shed and better voltage profile.

Decentralised self‐healing model for gas and electricity distribution network

With regard to the global environmental challenges, low cost of the natural gas and fast developing of new gas-fired power generation technologies, conduce to unwanted integration of gas and electricity network. Moreover, one of the benefits of emerging distribution network as microgrids (MGs) would be self-healing capability. So, in this research a model has been formulated as Mixed Integer Linear Programming (MILP) problem which determines optimally the power generation of micro turbines (MTs), Energy Storages (ESs) and Combined Heat and Power (CHP) units during the normal operation of MGs with consideration of combined gas and electricity network. In case of fault occurrence in MG/MGs and need to compensate the power generation shortage the model determines the power transferring rate of all MGs with normal operation mode, adjacent to the faulty MG/MGs, according to the cost-based optimization. In critical situations load shedding procedure will be applied to the faulty MG/MGs. It would be considered that CHPs supply both heat and electricity loads and the remaining heat demands will be supplied by natural gas network. The proposed model has been applied to the test system with six MGs as three scenarios and also verified by applying to the network with 30 MGs.

Hybrid intelligent technique for voltage/VAR control in power systems

Modern power systems are far from immune from voltage collapse, and examples abound over the past decade. This is due to the fact the current generation of automatic emergency and operational Volt/VAr control systems can be ineffective and unreliable for some cases. The artificial intelligence (AI) techniques allow developing new automatic intelligent Volt/VAr control systems, which will mitigate negative consequences caused by the human factor and the shortcomings of traditional control systems. This study presents a hybrid Volt/VAr control system using an AI-based technology. The system consists of two components: a centralised deep learning control system (DeepCS) for preventive security control and a decentralised multi-agent control system (MACS) for emergency control. The DeepCS includes two deep neural networks – a deep multilayer perceptron to recognise reactive power injections and a long short-term memory network to predict system state after control actions. The MACS is a decentralised automatic Volt/VAr control that involves determining the time of critical overload and switching to the load shedding procedure. The proposed approach is highly efficient for various scenarios of two (IEEE 6, IEEE 118) test systems and one real 33-bus Bodaibo subsystem.

Microscopic analysis of non-propagating fatigue crack tips

Herein, the phenomena around a non-propagating fatigue crack tip in polycrystalline interstitial-free steel were investigated on a microscopic scale. A non-propagating fatigue crack was introduced in a standard compact tension specimen using a load-shedding procedure. The near-tip area was subsequently investigated via scanning electron microscopy and transmission electron microscopy. The wake in the tip area was covered with an oxide layer. Discrete dislocations, whose densities were relatively low, were only found in front of the crack. These results may suggest that an increase of the opening point was attributable to the oxide-induced crack closure rather than the plasticity-induced one.

Local Power Controller Based Load Shedding Scheme in Islanded Microgrids

To decrease the blackout risk due to the large disturbances, it is recommended to intentionally create islanded conditions in power systems. In this event, system elements will be insulated and the supply for creating the islands can be preserved. To do so, resources and generations in the islands are discharged and loads should be shed in case it is required. In this condition, voltage and frequency stay in their limits. In this paper, a priority-based load shedding technique is proposed using a Local Power Controller (LPC) for islanded microgrids. The suggested method utilizes frequency and rate of change of frequency (ROCOF) based approaches to activate the shredding procedure. Moreover, the proposed approach controls the islanded microgrids for developing support for the main grid in the case of frequency deviation events whereas supplying the local critical loads is mandatory. The advantages of the method are the minimum equipment requirement and suitability for end-users with critical loads. In the proposed methodology, a load addition mechanism is also considered. This mechanism automatically switches the shed loads when the system frequency recovers to a certain level. The proposed method scheme is verified and validated in a laboratory microgrid test bench under two operation modes, specifically allowing load shedding procedure in advance of and consequent to the islanded situation. In both cases, the results demonstrate that the scheme is able to sustain the islands.

Fatigue crack tip damaging micromechanisms in a ferritic-pearlitic ductile cast iron

Due to the peculiar graphite elements shape, obtained by means of a chemical composition control (mainly small addition of elements like Mg, Ca or Ce), Ductile Cast Irons (DCIs) are able to offer the good castability of gray irons with the high mechanical properties of irons (first of all, toughness). This interesting properties combination can be improved both by means of the chemical composition control and by means of different heat treatments(e.g. annealing, normalizing, quenching, austempering etc). In this work, fatigue crack tip damaging micromechanisms in a ferritic-pearlitic DCI were investigated by means of scanning electron microscope observations performed on a lateral surface of Compact Type (CT) specimens during the fatigue crack propagation test (step by step procedure), performed according to the “load shedding procedure”. On the basis of the experimental results, different fatigue damaging micromechanisms were identified, both in the graphite nodules and in the ferritic – pearlitic matrix.

Effect of load ratio on fatigue crack growth in the near-threshold regime: A literature review, and a combined crack closure and driving force approach

The current state of influence of load ratio, R, on fatigue crack growth (FCG) behavior is thoroughly reviewed. Two main approaches, including crack closure concept and driving force parameter method, when used independently, are found to be short of accuracy during load-shedding procedure in the near-threshold regime. By combining an equivalent driving force model and crack closure approach, a new crack opening equation and a general FCG law is established for the load-shedding technique. This merging method does not involve crack closure measurement, and is capable to predict R dependence of FCG curves and fatigue thresholds fairly well.

Fatigue Crack Propagation in Thin Wires of Ultra-High Strength Steel<sup></sup>

Fatigue crack propagation is studied in thin wires of about 1 mm in diameter of an ultrahigh strength steel (σmax> 2400MPa) in ambient air and in air with controlled residual humidity. A specific equipment is developed based on an electro-dynamic testing machine equipped with an environmental chamber for air humidity control. Threshold tests are run using a load shedding procedure specifically adapted to the specimen size. The relation between load ratio and crack closure is evaluated from constant Kmax tests. The results are discussed on the basis of fracture surface observations and of existing modelling for environmentally assisted fatigue crack propagation.

Thresholds of time dependent intergranular crack growth in a nickel disc alloy Alloy 720Li

At high temperatures in air, introducing a dwell period at the peak stress of fatigue cycles promotes time dependent intergranular crack growth which can increase crack growth rates by upto a few orders of magnitude from the rates of transgranular fatigue crack growth in superalloys. It is expected that time dependent intergranular crack growth in nickel-based superalloys may not occur below a critical mechanical driving force, Δ K th−IG , analogous to a fatigue threshold (Δ K th ) and a critical temperature, T th . In this study, dwell fatigue crack growth tests have been carefully designed and conducted on Alloy 720Li to examine such thresholds. Unlike a fatigue threshold, the threshold stress intensity factor range for intergranular crack growth is observed to be highly sensitive to microstructure, dwell time and test procedure. The near threshold crack growth behaviour is made complex by the interactions between grain boundary oxidation embrittlement and crack tip stress relaxation. In general, lower Δ K th−IG values are associated with finer grain size and/or shorter dwell times. Often a load increasing procedure promotes stress relaxation and tends to lead to higher Δ K th−IG . When there is limited stress relaxation at the crack tip, similar Δ K th−IG values are measured with load increasing and load shedding procedures. They are generally higher than the fatigue threshold (Δ K th ) despite faster crack growth rates (d a /d N ) in the stable crack growth regime. Time dependent intergranular crack growth cannot be activated below a temperature of 500 ∘ C.

Game-theoretic approach to cooperative control of distributed energy resources in islanded microgrid considering voltage and frequency stability

A microgrid (MG) comprises a low-voltage network with several microsources, critical and noncritical loads, and energy storage systems (ESSs). It can operate in the grid-connected or islanded modes. In islanded mode, the voltage and frequency of the MG should be controlled by different distributed energy resources (DERs). This paper focuses on the analysis of frequency stability in an autonomous MG with renewable energy sources. In this paper, colonial competitive algorithms are used to design the DERs controllers in a cooperative manner that these controllers can keep the MG stable. Artificial neural network tool trained by Levenberg---Marquardt algorithm is used to generate controlling signal for every controller to keep the frequency and voltage in a desired range. This paper investigates a new optimal control solution for maintaining the frequency stability in the MG, by using a combination of an ESS and load-shedding procedure. The cooperative game theory is used in this paper to model the interaction among different DERs, ESSs, and loads.

Controlling Stress Intensity Factors During a Fatigue Crack Propagation Using Digital Image Correlation and a Load Shedding Procedure

Digital Image Correlation is used for controlling load shedding fatigue crack propagation. A specific algorithm is used to perform Stress Intensity Factors (SIFs) and crack length estimation in real time. Crack length measurements are validated by comparison with potential drop technique. SIFs results are compared with more common techniques using standard analytical formula considering confined plasticity at the crack tip. The proposed non-contact method is shown to be a powerful tool to control crack propagation.

A novel approach to frequency control in an islanded microgrid by load shedding scheduling

A microgrid is composed of distributed power generation systems (DGs), distributed energy storage devices (DSs), and loads. To maintain a specific frequency in the island mode as an important requirement, the control of DGs output and charge action of DGs are used in supply surplus conditions and load-shedding and discharge action of DGs are used in supply shortage conditions. Recently, multiagent systems (MAS) for autonomous microgrid operation have been studied. Especially, load-shedding, which is intentional reduction of electricity use, is a critical problem in islanded microgrid operation based on the MAS. Therefore, effective schemes for load-shedding are required. A novel optimization algorithm called ICA has been used to optimize this load shedding procedure.

Analysis of Underfrequency Load Shedding Using a Frequency Gradient

Underfrequency load shedding is one of the most important protection systems, as in many cases it represents the last chance to prevent a system blackout after a serious disturbance occurs in a power system. In order to improve traditional schemes, many efforts have been concentrated on attempts to use the frequency gradient as an indicator for determining the lack of active power in a system. This paper concentrates on analyzing the factors that influence the gradient. Analyses have shown that the gradient can give misleading information about the active-power deficit, as long as certain factors are ignored or assumed to be constant. A draft of a gradient-based underfrequency load-shedding scheme is presented, which also highlights the problems associated with the use of gradient. On the one hand, a frequency gradient alone does not appear to be sufficient for the active-power deficit estimation. Nevertheless, for the actual load-shedding procedure it is found to be very useful.