Load Shedding Process 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.

A Centralized Stage-Wise Control Approach for Frequency and Voltage Regulation in PV–Wind–Storage Based Islanded Microgrid

In a microgrid, perturbations stemming from uncertain renewable sources and sudden load fluctuations can result in deviations in system voltage, frequency, and more, leading to instability. Thus, focusing on achieving stable microgrid operation, this paper proposes a novel stage-wise control approach for regulating the frequency and voltage of islanded microgrids. The approach employs an intelligent fuzzy logic controller with 64 rules. Specifically, the control approach is divided into three stages. In stage 1, the proposed approach manages the system under minor perturbations to regulate all system parameters within a feasible operating range, utilizing a battery as ancillary support. Furthermore, during significant perturbations, stages 2 and 3 of the proposed approach involve percentage-wise shedding non-critical loads to enhance stable microgrid operation. The load-shedding process continues until the system parameters stabilize within an acceptable operating range. The performance of the proposed approach is validated using MATLAB software and is compared against existing topologies. Notably, it has been observed that in the presence of significant small and large perturbations, the system demonstrates considerably lower deviations, following the recommended standards, i.e., 0.57 and 0.72% voltage deviation, including 0.14 and 0.17% frequency deviation. This observation emphasizes the superiority of the proposed control approach.

Backpropagation neural network based adaptive load scheduling method in an isolated power system

This work introduces an efficient load scheduling method for handling the day-to-day power supply needs. At peak load times, due to its instabilitythe power generation system fails and as a measure, the load shedding process is followed. The presented method overcomes this problem by scheduling the load based on necessity. For this load scheduling is handled with an artificial neural network (ANN). For the training purpose the backpropagation (BP) algorithm is used. The whole load essential is the input of the neural network (NN). The power generation of all resources and power losses at the instant of transmission is the NN output. The optimum scheduling of different power sources is important when considering all the available sources. Load scheduling shares the feasibility of entire load and losses. It is well-known as optimal scheduling if the constraints such as availability of power, load requirement, cost and power losses are considered. Training the system using a large number of parameters would be a difficult task. So, finest number of communally independent inputs is selected. The presented method aims to lower the power generation expenditure and formulate the power available on demand without alteration. The network is designed using MATLAB.

Experimental investigation on PEM fuel cell flooding mitigation under heavy loading condition

Water flooding is a crucial factor affecting the lifetime of a proton exchange membrane (PEM) fuel cell. In this paper, A laboratory-scale (25 cm2) single PEM fuel cell was tested to investigate the characteristics of fuel cell under a load shedding process with various air flow rates. The performance was assessed using output power and electrochemical impedance spectroscopy, while the cathode flooding was identified using voltage and cathode pressure drop. The results show that reducing the load within a specific range can lower the impedance and lessen floods. The results also demonstrate that there is an optimal inlet flow rate interval considering its effect on system efficiency, durability, and water flooding. In addition, a stepped load shedding pattern was proposed to solve the flooding problem of a fuel cell while taking the power demand into account. The effect of load shedding cycle and air inlet flow rate on flooding was evaluated by comparing the flooding severity coefficient and output power. The results prove that a longer current reduction cycle is more favorable in alleviating flooding problems. The optimal mode for the fuel cell in this paper was determined according to the flooding situation and power variation. Finally, the approach to obtaining the optimal load shedding pattern for other PEM fuel cells system was summarized. The flooding mitigation approach proposed in this paper can be utilized as a guide for developing strategies to extend the life of PEM fuel cells.

Low-Frequency Defense Strategy for Wind Turbine Participating in Power Grid Frequency Adjustment

With the continuous increase of wind power permeability, the dynamic behavior of the power system also changes, which also has a great impact on the setting of the low-frequency load-shedding scheme. The original low-frequency load-shedding scheme has no good adaptability to the system of high-permeability wind power. So, this article analyzes the principles of wind turbines in power grid frequency control. This paper expounds on the principle of low-frequency load-shedding strategy in the power system configuration. Connecting with the actual operation of the grid simulation, simulation results show that wind turbines in power grid frequency modulation can significantly reduce the low-frequency load shedding process of loads and wind power in frequency modulation of low-frequency with a positive impact.

Mitigation of Cyber-Attacks on Wide-Area Under-Frequency Load-Shedding Schemes

This paper investigates the problem of cyber-attacks on WAUFLS, as these schemes are critical to maintaining power systems stability. First, we perform a detailed analysis on existing WAUFLS schemes and show that an adversary can launch a FDI cyberattack by manipulating the frequency measurements or Power Flow Measurements (PFMs), which may lead to system losses, unnecessary shedding of important loads, and system-wide blackout. Second, to address this vulnerability, we propose a novel UF-RMMS scheme to protect against FDI cyber-attacks. The disturbance calculation and load shedding process in UF-RMMS are based on reliable system states, obtained using a proposed data-classification method on the PFMs that secures the state estimation operation. These reliable states are then used to perform the power flow in order to calculate the power mismatch. The calculated magnitude of disturbance, as well as the obtained system states, are used to decide on the amount and locations of the load-shedding. We validate the effectiveness and accuracy of UF-RMMS by conducting extensive simulation on the IEEE-39 bus New England system using PSCAD/EMTDC. The results confirm the proposed scheme’s capabilities in evaluating system disturbance and performing load-shedding, thus protecting the system during under-frequency conditions, and demonstrate its robustness against FDI attacks.

A Probabilistic Data-Driven Method for Response-based Load Shedding Against Fault-Induced Delayed Voltage Recovery in Power Systems

Aiming at timely and adaptive remedial control for the fault-induced voltage recovery (FIDVR) events in power systems, this paper develops a probabilistic data-driven method for response-based load shedding (RLS). In the proposed method, a scalable Gaussian process (SGP) model is developed to estimate the required load shedding (LS) amount and the confidence of the corresponding predictions. Based on the probabilistic information, a 2-stage LS process is designed to enhance the effectiveness and efficiency of the scheme, using the mean value for LS at the first stage and the upper-bound value for LS at the second stage. The 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> stage shed the amount of load with the largest likelihood, aiming to alleviate system stress with the least control cost. The 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> stage serves as a safety net to ensure system stability considering the possible prediction error. Compared with the conventional RLS schemes and other state-of-the-art approaches, simulation results verify that the proposed method can effectively mitigate FIDVR with a much less load shedding amount.

Control strategy of unintentional islanding transition with high adaptability for three/single-phase hybrid multimicrogrids

A seamless islanding transition from the grid-connected mode to the island mode can reduce system voltage and frequency drop and is the key technology for ensuring the safe and stable operation of a three/single-phase hybrid multimicrogrid (MMG). A three-phase unbalance problem and power shortage problem will occur during the unintentional islanding transition of a three/single-phase hybrid MMG. First, aiming to solve the three-phase unbalance problem caused by single-phase loads during the unintentional islanding period, a virtual combination method (VCM) of single-phase loads based on insertion sorting is proposed. Second, for the problem of power shortages during the unintentional islanding period, a load shedding control strategy based on a deep Q-learning network with prioritized experience replay (DQNPER) is proposed. The strategy not only eliminates the power shortage quickly but also considers the influence of the load level, frequency regulation factor, and three-phase unbalance factor on load shedding. Finally, a three/single-phase hybrid MMG with an IEEE 37-bus system is built to verify the effectiveness of the proposed strategy. The test results show that the proposed load shedding strategy has better adaptability than the model-based strategy in the unintentional islanding transition process. The proposed strategy can reduce the three-phase unbalance factor and frequency fluctuation range in the load shedding process and improve the power supply reliability.

Framework for probabilistic simulation of power transmission network performance under hurricanes

Structures in a power transmission network, such as towers and conductors, are vulnerable to hurricanes. Failures of these structures trip transmission lines and usually result in large scale power outages in the region. This paper presents a technique for the probabilistic simulation of power transmission systems under hurricane events and provides fundamental insights on the modeling and quantification of power system performance and resilience. The study models the power transmission system as a network of connected individual components, which are subjected to wind-induced mechanical failure and power flow constraints. A realistic power transmission network is developed for the study region. The geographical data are obtained for all components in the network based on a data collection and image processing campaign to reflect the realistic properties of the network serving the Lehigh Valley, PA. A hurricane simulator is utilized to generate a hurricane scenario providing time-varying wind intensities and wind directions for the component failure analysis. The spatio-temporal impact of the hurricane is investigated: a pool of component fragilities is generated to effectively incorporate the uncertainties in structural capacities into the analysis; the spatial correlation among structures is modeled efficiently by a random field based technique. At the system level, Monte Carlo simulation is adopted to determine the failure probability of transmission lines. The unmet demand of the system is computed probabilistically, based on the alternate current optimal power flow analysis, capacity constraints and load shedding process of the system. The simulation results can be used to quantify and visualize the power network performance, and help decision makers to identify critical components in the network to optimize the short-term pre-event preparation for an approaching hurricane and long-term retrofit strategy to enhance system resilience.

Under Frequency Load Shedding in Inverter Based Microgrids by Using Droop Characteristic

In this article, a new Under Frequency Load Shedding (UFLS) method for Inverter-Based Microgrids (IBMs) is presented. The proposed strategy uses frequency variations to estimate the power deficit in IBMs without using wide communication platform and calculating system inertia time constant. Variations of power generation are considered during the load shedding process. The shortage of power is also compensated by the load shedding during several stages. Estimated droop coefficient is updated in each load shedding stage, enabling the proposed method to have a reliable performance in the case of the microgrid's parameters changes which could happen during the load shedding process. The proposed method is simulated and implemented in an IBM by using MATLAB\Simulink software package; the obtained results show the effectiveness and the correctness of the proposed scheme to recognize power deficit and to compensate power shortage.

Optimal design of adaptive power scheduling using modified ant colony optimization algorithm

For generating and distributing an economic load scheduling approach, artificial neural network (ANN) has been introduced, because power generation and power consumption are economically non-identical. An efficient load scheduling method is suggested in this paper. Normally the power generation system fails due to its instability at peak load time. Traditionally, load shedding process is used in which low priority loads are disconnected from sources. The proposed method handles this problem by scheduling the load based on the power requirements. In many countries the power systems are facing limitations of energy. An efficient optimization algorithm is used to periodically schedule the load demand and the generation. Ant colony optimization (ACO) based ANN is used for this optimal load scheduling process. The present work analyse the technical economical and time-dependent limitations. Also the works meets the demanded load with minimum cost of energy. Inorder to train ANN back propagation (BP) technics is used. A hybrid training process is described in this work. Global optimization algorithms are used to provide back propagation with good initial connection weights.

Two-Stage Load Shedding for Secondary Control in Hierarchical Operation of Islanded Microgrids

A two-stage load shedding scheme is presented to cope with potential power deficits caused by microgrid islanding. Coordinated with the fast response of inverter-based grid-forming distributed energy resources, the proposed approach estimates the load shedding at each stage and the resulting power flow redistribution. The first stage of load shedding will cease any rapid frequency decline in which the locally measured frequency deviation is employed to guide the load shedding level and process. Once a new steady state is reached at the end of first stage, the second stage is activated, which performs load shedding according to load priorities. The effectiveness of the proposed scheme is verified through time-domain simulations in PSCAD/EMTDC based on a scaled-down microgrid system.

A method to study the intake consistency of the dual-stack polymer electrolyte membrane fuel cell system under dynamic operating conditions

The Polymer Electrolyte Membrane Fuel Cell (PEMFC) system is considered as one of the most potential power plant on vehicle. However, the efficiency of the high power fuel cell stack has restricted the high power system which is developed for the full power fuel cell vehicle. The multi-stack system integrated by several fuel cell stacks is a solution aimed for high power level system. However, the inconsistency of the stacks flow distribution restricts the performance uniformity of each stacks and the efficiency of the multi-stack fuel cell system. In this paper, a dual-stack polymer electrolyte membrane fuel cell system was setup, and its intake manifold was established. A porous medium was applied to replace the complex flow field structure inside the fuel cells to simulate the pressure drop of the gas through the stack. According to the operating conditions of the vehicle, variable conditions are designed to carry out simulation experiments for the first time. The results show that there is not a significant effect of the changing load’s range on the instantaneous uniformity. However, the initial value of the changing load has a great influence on the instantaneous uniformity. In addition, the settling time of the loading process is much shorter than that of the load shedding process. The equivalent model of porous media proposed provides a new method to model the fuel cell stack, which is very useful in the fuel cell pipe system design and system dynamic response analysis. The uniformity simulation result provides the directions for the fuel cell control strategy design and optimization, and the references for gas intake manifold design.

Nonlinear Modeling and Dynamic Analyses of the Hydro–Turbine Governing System in the Load Shedding Transient Regime

Load shedding processes are widespread in hydropower stations, which has great influence on the safe and stable operation of the hydro–turbine governing system. In order to study the dynamic characteristics of the hydro–turbine governing system during the load shedding process, a novel nonlinear mathematical model of the hydro–turbine governing system is established considering the hydro–turbine system, the generator system and the governor system. In particular, a novel nonlinear mathematical model of the six hydro–turbine transfer coefficients is presented based on the definitions and hydro–turbine internal characteristics. After that, from the viewpoint of nonlinear dynamics and the practical engineering, the dynamic characteristics of the hydro–turbine governing system are investigated utilizing bifurcation diagrams, time series, Poincare maps, power spectrums and phase planes. Some meaningful results are found. The advantages of the novel nonlinear mathematical model are illustrated and commented in detail in comparison with the previous model. Finally, these models and analysis results will provide some theoretical references for the operation of hydropower stations in the load shedding transient.

Extreme water-hammer pressure during one-after-another load shedding in pumped-storage stations

The intermittent and unpredictable wind and solar power leads to the frequent transient processing of pumped-storage stations, increasing the probability of load shedding. When one turbine sheds its load, the other turbines in the same hydraulic unit become overloaded and may shed their loads, which is referred to as a “one-after-another (OAA)” load-shedding process. An extremely high water-hammer pressure (WHP), namely, high spiral case pressure (SCP) or low draft tube pressure (DTP), may arise in this case, directly threatening the safety of the PSS. The objective of this study was to theoretically determine the hydraulic connections between the turbines and reveal the mechanism of the rapid rise in the WHP under the OAA load-shedding conditions. Theoretical derivations inferred that the drastic pressure changes in a trail shedding turbine (TST) are caused by the hydraulic connection with the lead shedding turbine (LST) in the S region. Furthermore, numerical simulations and model experiments were performed for the OAA load-shedding process, which confirmed the validity of the theoretical analysis. Finally, an analysis was conducted on the distribution of the water inertia in the upstream and downstream branch pipes, and engineering measures were proposed to guarantee the safe operation of PSS systems.

Real Time Congestion Management in Power Systems Considering Quasi-Dynamic Thermal Rating and Congestion Clearing Time

In a smart power system, smart measurement infrastructure brings so many advantages in monitoring and control of the power system. This paper proposes an optimal real time transmission congestion management algorithm in a competitive electricity market based on real time measurements in a smart power system. Instead of static and fix thermal rates, quasi-dynamic thermal rates of transmission lines are calculated and used to solve the real time congestion management optimization problem. The optimization problem includes an optimal generation rescheduling and load shedding scheme to minimize the congestion management costs in a specific time interval. This time interval is introduced and defined as congestion clearing time in this paper. Furthermore, in the proposed algorithm, the congestion clearing time is divided into few subsequent subintervals aimed at considering loads and power system variations during rescheduling procedure. The proposed method could be used by system operators (SOs) to adapt the real time congestion management procedure with regard to the power system loads and generations, or system configuration variations during rescheduling and load shedding process. Numerical results obtained from running the algorithm on the modified IEEE 30-bus test system and the transmission network of Esfahan Regional Power Network (ERPN), as a real size power system, demonstrate convincingly the ability of the proposed algorithm in the congestion management cost and load shedding reduction, while it increases the power system security margin in intensive congestion cases significantly.

IEC 61850-based islanding detection and load shedding in substation automation systems

Distributed generation (DG) systems are common within industrial plants and allow continuity of supply to critical loads. In cases when DG cannot support the entire system load, islanding detection (ID) and load shedding (LS) schemes are required. Such an automation scheme serves to monitor the connection to the grid and generation-load imbalance, and it controls the load shedding process. The design of such schemes has undergone significant revolution from electromechanical relays and PLC systems to the use of communications-enabled intelligent electronic devices. This paper discusses the design of an IEC 61850-based smart ID and LS scheme using a complete system design approach. Novel control algorithms are presented for the ID and initiation processes as well as the LS process. The study proposes an innovative system, which incorporates the generator governor gain factor in LS decisions, and assists in avoiding unnecessary amounts of LS.

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.

An Underfrequency Load Shedding Scheme for Hybrid and Multiarea Power Systems

The purpose of underfrequency load shedding (UFLS) is to maintain the frequency of an interconnected or island power system within permissible limits in case of severe power deficits. Thus, the proper performance of UFLS scheme is of great importance. In hybrid power systems, disconnecting and connecting the power generation sources and loads result in variations in the system parameters. Moreover, the output power of some distributed energy resources, such as photovoltaic (PV) or wind turbine generator (WTG), might change during the load shedding process. These factors may affect the performance of the load shedding scheme and decrease the reliability of power system. Hence, in this paper, a UFLS scheme is proposed that has two advantages: 1) it is independent of power system parameters; and 2) it considers power generation variations during the load shedding process. To verify the effectiveness of the proposed load shedding method, simulation studies are carried out on a hybrid power system including PV, WTG, fuel cell, diesel generator, and battery energy storage system. Finally, the performance of the proposed method on a multiarea power system is evaluated.

An underfrequency load shedding scheme for islanded microgrids

The underfrequency load shedding scheme is an important protective schemes, responsible for maintaining frequency of power systems within permissible limits in case of severe power deficits and as such, its proper performance is of great interest. In microgirds, disconnecting/connecting power generation sources and loads result in variations in the system parameters. Moreover, the output power of some distributed energy resources such as photovoltaic/wind turbine generators, might change during load shedding process and consequently affect the performance. Hence, an underfrequency load shedding scheme is proposed here which is not only independent of the microgrid parameters, but also considers power generation variations during the process. A microgrid power system with several distributed energy resources is adopted to demonstrate the effectiveness of the proposed method.