Under-frequency Load Shedding Scheme Research Articles

Economic optimal load management control of microgrid system using energy storage system

The microgrid (MG) implementation has changed the power system's configuration, and it has optimally permitted the integration of renewable energy resources (RERs). This paper addresses an optimal load management control method application for an economic load shedding problem. The control scheme is based on coordination of the diesel generator (DG) system combined with a standalone MG that contains RER (wind and photovoltaic (PV), two biomass generation systems and an energy storage system (ESS). The control model minimises the DG unit's fuel cost incorporated into an MG where a milling plant is part of the loads. The optimal control scheme has been executed to minimise the DG's operating cost, maximise the energy from RERs and finally increase the MG's profit during the load shedding operation. This strategy develops an under-frequency load shedding (UFLS) scheme to control the MG based-multiobjective optimally. An adaptive UFLS strategy is designed to coordinate the system behaviour by adjusting the distributed generation in the function of variation in the demand side. It is observed that it is economically reliable to solve the load shedding problem and save the fuel cost of the diesel generator in the MG power system by optimally fashioning the control scheme. The profitability of this adaptive strategy is impressive, which is about 100%. Besides, the application of different distributed energy structures in an MG impacts the demand side. It implies adjusting the total power supply fluctuations, which remarkably improves the MG power system's lifecycle cost (LC) and stabilise the system frequency and voltage.

Experimental and HIL investigation of under-frequency and rate of change of frequency load shedding schemes in interconnected networks with high penetration of renewable generation

Under-frequency Load Shedding Schemes (UFLS) are a well-established part of any under-frequency defence plan. Their future effectiveness in systems with increased penetration of renewable energy sources without rotating mass should however be questioned: lower inertia implies higher frequency gradients; more power electronics implies higher harmonics. Both factors will concur to make frequency measurements and short relay operation times more challenging: In this contribution, the impact of the penetration of renewable sources on the rate of change of frequency (RoCoF) in continental Europe has been simulated with the dynamic ENTSO-E model for different penetration levels, confirming the higher RoCoF and lower frequency nadir to be expected in case of large disturbances. The operation of the current UFLS scheme is included in the model and has been compared with a scheme incorporating a RoCoF criterion. The authors have also performed laboratory tests in order to assess the use of RoCoF-based load shedding as an improvement to frequency-only based load shedding schemes. Central European transmission and distribution system operators have established joint bodies for agreeing on the best evolution of underfrequency defence plans: challenges include frequency measurement and correct coordination of several control schemes and approaches within the CE transmission system community.

Event-Based Under-Frequency Load Shedding Scheme in a Standalone Power System

Under-frequency load shedding (UFLS) prevents a power grid from a blackout when a severe contingency occurs. UFLS schemes can be classified into two categories—event-based and response-driven. A response-driven scheme utilizes 81L relays with pre-determined settings while an event-based scheme develops a pre-specified look-up table. In this work, an event-based UFLS scheme is presented for use in an offshore standalone power grid with renewables to avoid cascading outages due to low frequency protection of wind power generators and photovoltaic arrays. Possible “N-1” and “N-2” forced outages for peak and off-peak load scenarios in summer and winter are investigated. For each forced outage event, the total shed load is minimized and the frequency nadir is maximized using particle swarm optimization (PSO). In order to reduce the computation time, initialization and parallel computing are implemented using MATLAB/Simulink because all forced outage events and all particles in PSO are mutually independent. A standalone 38-bus power grid with two wind turbines of 2 × 2 MW and photovoltaics of 7.563 MW was studied. For each event, the proposed method generally obtains a result with a smaller shed load and a smaller overshoot frequency than the utility and existing methods. These simulation results verify that the proposed method is practically applicable in a standalone power system with penetration of renewables.

Simplified Event-Based Load Shedding Scheme for Frequency Stability in an Isolated Power System With High Renewable Penetration. El Hierro: A Case Study

This study presents a novel approach to calculate the load to be shed in El Hierro isolated power system in generation tripping events. The proposed shedding law is based on a linear regression model. The regression model is obtained by means of offline dynamic simulations in a set of representative test cases. The proposed shedding law has been compared to the under-frequency load shedding scheme currently utilized in El Hierro, considering three different implementation schemes. The results of this study demonstrate that the proposed shedding law can contribute to reduce the average amount of load shed in generation tripping events and meet the requirements of the system’s frequency with a moderate investment in smart power management devices.

Application of polynomial regression and MILP for under-frequency load shedding scheme in islanded distribution system

Distributed Generation (DG) integration, especially based on renewable energy resources, has gained great attention by power utilities and frequently utilized in the electrical distribution systems. However, DG integration imposes some risks towards system stability which may lead to system blackouts. This mainly occurs when the grid is decoupled from a portion of the distribution system consisting DGs while the total load demand is greater than total DGs output power. In order to overcome this problem, load shedding technique can be adopted to stabilize the system frequency. However, existing load shedding techniques were unable to accurately estimate the power imbalance due the variation in system loading. This results in excessive/inadequate load shedding to stabilize the system frequency. Moreover, random selection of the loads without load prioritization might cause vital loads to be shed. Therefore, in this paper, a new load shedding strategy for islanded distribution system is proposed. Polynomial regression analysis estimates the power mismatch while MILP optimization estimates optimal load combination for shedding. Furthermore, load priority (i.e., vital, non-vital, and semi-vital) is also considered to avoid disconnecting vital loads. Efficiency of the proposed scheme is evaluated on three different test systems. Validation is performed by modelling the proposed load shedding on PSCAD/EMTDC software for dynamic analysis. From the results, it can be analyzed that the proposed technique is superior compared to other techniques proposed in the literature.

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 New Adaptive Underfrequency Load Shedding Scheme to Improve Frequency Stability in Electric Power System

This paper proposes a new adaptive underfrequency load shedding scheme (UFLS) to avoid frequency instability in electrical power system during abnormal wide area disturbances. The developed scheme is based on online monitoring of the distance relay zone 3 decisions of some tie lines using (WAMS) and SCADA/EMS systems, to check rapidly and reliably the uncontrolled islanding conditions and, permit an automatic load shedding action to maintain frequency stability of power system. Simulation results on 400 kV Turkish transmission systems demonstrated the effectiveness of the proposed scheme compared to current frequency load shedding schemes which they cannot consider all possible circumstances because of their limited access to the power network data. Simulation results clearly indicate that large disturbances in power systems can be avoided and their propagation can also be stopped using the proposed scheme.

An underfrequency load shedding scheme for high dependability and security tolerant to induction motors dynamics✰

The load shedding scheme is a last resort to prevent power system collapse when the available generation is less than the load demand. The traditional load shedding schemes monitors the frequency and voltage by underfrequency and undervoltage relays and provides load cutoff until the frequency returns to a stable point of operation. The reduction of rotating inertia demands faster load shedding to meet the stability requirements. In some electrical power systems in Brazil, it was found that, in the event of opening a transmission line upstream of the load shedding substation, some under frequency relay may operate improperly. This paper proposes a new method to avoid the unwanted operation of load shedding schemes when the utility substation (source) is out of service. The proposed method monitors negative sequence voltage, frequency and its rate of change, enabling faster load shedding (high dependability) without unwanted (high security) operation when compared with traditional methods. Events were classified by linear discriminant analysis and tested in the IEEE 9 bus test system. The tests consisted of several disturbances such as loss of generation and line outages at different levels of feeder loading, reactive compensation and distributed generation. The proposed method reached a 100% rate of correct classification.

Under Frequency Protection Enhancement of an Islanded Active Distribution Network Using a Virtual Inertia-Controlled-Battery Energy Storage System

When an islanding condition caused by an unintentional single-line to ground fault occurs in an active distribution network with distributed generation, the frequency stability and protection issues remain challenging. Therefore, this paper presents the under frequency protection enhancement of the active distribution network using a virtual inertia-controlled-battery energy storage system to improve the frequency stability under the islanding condition caused by unintentional faults. The virtual inertia control is designed based on the direct and quadrature axis-controlled battery energy storage system to generate the virtual inertia power, compensating the system’s inertia to enhance the stability margin. The proposed method is verified by the simulation results that reveal the frequency stability performance and the under-frequency load shedding enhancement of the study active distribution network in Thailand. The study is divided into two cases: the normal control parameters and the parameter uncertainty scenarios, compared with a power-frequency droop control. The simulation results demonstrate that the proposed virtual inertia control can effectively improve the frequency and transient stabilities in the islanding condition, diminishing the number of loads disconnected by the proposed under-frequency load shedding scheme.

An approach for a multi-stage under-frequency based load shedding scheme for a power system network

The integration of load shedding schemes with mainstream protection in power system networks is vital. The traditional power system network incorporates different protection schemes to protect its components. Once the power network reaches its maximum limits, and the load demand continue to increase the whole system will experience power system instability. The system frequency usually drops due to the loss of substantial generation creating imbalance. The best method to recover the system from instability is by introducing an under-frequency load shedding (UFLS) scheme in parallel with the protection schemes. This paper proposed a new UFLS scheme used in power systems and industry to maintain stability. Three case studies were implemented in this paper. Multi-stage decision-making algorithms load shedding in the environment of the DIgSILENT power factory platform is developed. The proposed algorithm speeds-up the operation of the UFLS scheme. The load shedding algorithm of the proposed scheme is implemented as a systematic process to achieve stability of the power network which is exposed to different operating conditions. The flexibility of the proposed scheme is validated with the modified IEEE 39-bus New England model. The application of the proposed novel UFLS schemes will contribute further to the development of new types of engineers.

Optimal Under-Frequency Load Shedding Setting at Altai-Uliastai Regional Power System, Mongolia

The Altai-Uliastai regional power system (AURPS) is a regional power system radially interconnected to the power system of Mongolia. The 110 kV interconnection is exceptionally long and susceptible to frequent trips because of weather conditions. The load-rich and low-inertia AURPS must be islanded during interconnection outages, and the under-frequency load shedding (UFLS) scheme must act to ensure secure operation. Traditional UFLS over-sheds local demand, negatively affecting the local population, especially during the cold Mongolian winter season. This research paper proposes a novel methodology to optimally calculate the settings of the UFLS scheme, where each parameter of the scheme is individually adjusted to minimise the total amount of disconnected load. This paper presents a computationally efficient methodology that is illustrated in a specially created co-simulation environment (DIgSILENT® PowerFactoryTM + Python). The results demonstrate an outstanding performance of the proposed approach when compared with the traditional one.

An Adaptive Underfrequency Load Shedding Scheme in the Presence of Solar Photovoltaic Plants

Load shedding is performed, as last option, for emergency measure when system frequency drifts below the allowed limit. Utilities use underfrequency load shedding schemes to bring back frequency within limit. In this article, an adaptive underfrequency load shedding scheme is proposed for the power system in the presence of solar photovoltaic plants using wide-area measurements. Stepwise load shedding is performed at selected buses with consideration of any change in photovoltaic power output and load/generation at each step. Share of load to be shed at a bus is based on the voltage stability index of the buses at that instant considering predefined load priority. The voltage stability index at a bus is obtained using the Thevenin equivalent of the system. For a solar photovoltaic plant integrated system, the Thevenin equivalent changes with solar power variation, where available methods become inaccurate. A Thevenin equivalent calculation approach is proposed in this article using an equivalent model of synchronous generators and photovoltaic plants. The proposed load shedding method is tested on the New England 39-bus system and the 233-bus North Eastern Indian grid with integrated photovoltaic plants. It is compared with available methods and found to be a better technique in bringing system frequency to normalcy with adaptive load shedding.

Optimal design of an adaptive under-frequency load shedding scheme in smart grids considering operational uncertainties

In the light of the smart grid paradigm, power systems have recently brought about higher levels of adaptability and flexibility. The associated functions in energy management system blocks need to adopt robust strategies to provide greater levels of control and protective service. Under-frequency load shedding (UFLS) schemes can be regarded as a critical function, through which the system integrity and sustainability indices are protected reliably. Conventional UFLS schemes usually suffer from low adaptability and optimal setting in response to changes in the operational characteristics of the power system. This paper seeks to propose an hourly framework for the optimal design of an adaptive UFLS function based on advanced metering infrastructure (AMI) in smart grids. In this portfolio, the target is tailored to fit the main UFLS settings based on the system contingencies and associated operational uncertainties. The simulations are executed on the IEEE 39-bus test system, implemented on a 24-hour scheduling time horizon. The effectiveness of the proposed adaptive model is evaluated with a thorough analysis of the derived numerical results.

Towards an enhanced power system sustainability: An MILP under-frequency load shedding scheme considering demand response resources

Renewable energy sources (RESs) in the power system have led to the continuation of electricity supply to the consumer and to the sustainable development of the society. Due to the increased penetration of RESs, the uncertainties of electrical energy systems have been intensified, and smart power system operators are thus faced with growing levels of frequency security risks. For preservation of frequency in a power system, a multi-stage automated Under-Frequency Load Shedding (UFLS) program can be utilized to shed the minimum amount of load. In the light of the smart grid paradigm, Demand Response (DR) can be considered as an appropriate strategy to face the adverse effects of the uncertainties of the RESs and increase the balance between generation and consumption. Consequently, it is regarded as a necessity in a smart grid environment to design an innovative UFLS scheme that includes the uncertainties of the RESs and DR programs. This paper addressed a new optimal UFLS intended to minimize load shedding, after accurate tracking of frequency deviations. To resolve the problem of setting UFLS system parameters stochastically, a Mixed-Integer Linear Programming (MILP) formulation is used in an optimization framework. The proposed model is implemented on the IEEE 39-bus test system.

Underfrequency Load Shedding: An Innovative Algorithm Based on Fuzzy Logic

In contemporary power systems, the load shedding schemes are typically based on disconnecting a pre-specified amount of load after the frequency drops below a predetermined value. The actual conditions at the time of disturbance may largely differ from the assumptions, which can lead to non-optimal or ineffective operation of the load shedding scheme. For many years, increasing the effectiveness of the underfrequency load shedding (UFLS) schemes has been the subject of research around the world. Unfortunately, the proposed solutions often require costly technical resources and/or large amounts of real-time data monitoring. This paper puts forth an UFLS scheme characterized by increased effectiveness in the case of large disturbances and reduced disconnected power in the case of small and medium disturbances compared to the conventional load-shedding solutions. These advantages are achieved by replacing time-consuming consecutive load dropping with the simultaneous load dropping mechanism and by replacing ineffective fixed-frequency activation thresholds independent of the state of the system with implicit adaptive thresholds based on fuzzy logic computations. The proposed algorithm does not require complex and costly technical solutions. The performance of the proposed scheme was validated using multivariate computer simulations. Selected test results are included in this paper.

Vulnerability assessment of distributed load shedding algorithm for active distribution power system under denial of service attack

In order to deal with frequency deviation and supply-demand imbalance in active distribution power system, in this paper a distributed under frequency load shedding (UFLS) strategy is proposed. Different from conventional centralized UFLS schemes, no centralized master station gathering all the buses' information is required. Instead, each bus decides its own load shedding amount by only relying on limited peer-to-peer communication. However, such UFLS strategy may suffer from some unexpected cyber-attacks such as integrity attacks and denial of service (DoS) attack. The latter DoS attack aims to degrade the system performance by jamming or breaking the communication, which is of high probability to happen in practical power system. To assess the vulnerability of proposed distributed UFLS algorithm, the effect of DoS attack on distributed average consensus algorithm is theoretically derived, which indicates that the final consensus value can be estimated by a given attack probability. It is also investigated that such DoS attack does harm to the load shedding amount and finally affects the system frequency performance in the active distribution power system. Several case studies implemented on an IEEE 33-bus active distribution power system are conducted to verify the effectiveness of the theoretical findings and investigate the vulnerability of the considered power system.

Stochastic optimal robust design of a new multi-stage under-frequency load shedding system considering renewable energy sources

The ever-increasing penetration of Renewable Energy Sources (RESs) into the power system has faced system operators with higher risks subject to a growing level of the associated uncertainties. To preserve the system frequency security, an under-frequency load shedding (UFLS) scheme can usually be utilized as a final remedial action, which is aimed at removing the excessive load. UFLS can be managed in a multi-stage portfolio based on the priority and sensitivity of the loads under control to cope optimally with the occurring power imbalances. Design of an optimal, robust UFLS scheme is a vital procedure. To that end, the present paper proposes a new UFLS protective system which is conducted to shed the minimum optimal load after precise detection of the frequency excursions. The problem is transferred into a Mixed-Integer Linear Programming (MILP) based optimization framework, and it is to be solved in several stochastic scenarios for setting the UFLS system parameters. The analysis of the UFLS setting results, which are extracted through implementation of the model on the IEEE 39-bus test system, demonstrates the effectiveness of the proposed MILP-based methodology in dealing with the severe uncertainties resulting from RES output variations and load fluctuations.

Multi-Objective Particle Swarm for Optimal Load Shedding Remedy Strategies of Power System

Load shedding is an emergency strategy that mitigates substantial mismatch between the generation and the loads. It generally sustains system stability during/after severe disturbances. This article proposes robust, simple, and innovative Under-Frequency Load Shedding (UFLS) technique based on Multi-Objective Particle Swarm Optimization (MOPSO). MOPSO has the objectives of: minimizing the amount of the dropped load and maximizing the lowest swing frequency. The functionality and feasibility of the proposed MOPSO are corroborated via comprehensive comparison with traditional, adaptive, Single Objective PSO (SOPSO), and Genetic Algorithm (GA) UFLS schemes. IEEE 9-bus and 39-bus systems are used cases for examining the reliability, applicability, and viability of the proposed MOPSO. Different scenarios as: outage of single, multiple generating plants and load increase are applied in the test systems, while load shedding is executed via MOPSO, SOPOS, GA, traditional, and adaptive UFLS approaches. The DigSilent power factor software is used for simulating the test systems while subjected to the different disturbance levels. MATLAB is used for coding SOPSO, MOPSO, adaptive, and GA algorithms. The results show that MOPSO-based UFLS relay produces higher lowest swing frequency and lower amount of dropped load than SOPSO and GA. MOPSO requires less computation requirements than GA approach.

Continuous Under-Frequency Load Shedding Scheme for Power System Adaptive Frequency Control

Frequency drop due to loss of massive generation is a threat to power system frequency stability. Under-frequency load shedding (UFLS) is the principal measure to prevent successive frequency declination and blackouts. Based on traditional stage-by-stage UFLS scheme, a new continuous UFLS scheme is proposed in this paper to shed loads proportional to frequency deviation. The characteristic of the proposed scheme is analyzed with a closed-form solution of frequency dynamics. Frequency threshold and time delay are added to make the proposed scheme practical. A line-by-line scheme based on precise load control is introduced to implement the continuous scheme for systems without enough continuously controllable loads. The load shedding scale factor of the proposed scheme is tuned with an analytical method to achieve adaptability to different operating conditions. The adaptability of the proposed scheme is validated with 39-bus New England model and simplified Shandong Power Grid of China.

Clustering‐based method for the feeder selection to improve the characteristics of load shedding

Under-frequency load shedding (UFLS) schemes are designed by specifying a given amount of load to shed at various frequency thresholds to prevent the collapse of the electrical power system in the event of a large generation-load imbalance. An UFLS step is constituted of a group of medium-voltage feeders that trip when a given frequency threshold is reached. This study focuses on the method to be used when allocating a given feeder to a given step. First, the authors introduce performance metrics to quantify the accuracy level with which the UFLS target is met. Second, they model: the allocation method currently used in France; a variant of that method; and a new method introduced in this study, based on an automated clustering technique. Third, based on real consumption patterns measured from a vast area in France, and using the introduced performance metrics, they compare the efficiency of the three described methods. This study is conducted for the current state of loading of the considered distribution network and for a hypothetical situation with an increased share of distribution-side photovoltaic generation. For the chosen performance metrics, they demonstrate that the first two methods provide similar results while the clustering-based method performs remarkably better.