Conventional Under-frequency Load Shedding Schemes Research Articles

Chance-constrained allocation of UFLS candidate feeders under high penetration of distributed generation

Under-Frequency Load Shedding (UFLS) schemes are the last resort to contain a frequency drop in the grid by disconnecting part of the demand. The allocation methods for selecting feeders that would contribute to the UFLS scheme have traditionally relied on the fact that electric demand followed fairly regular patterns, and could be forecast with high accuracy. However, recent integration of Distributed Generation (DG) increases the uncertainty in net consumption of feeders which, in turn, requires a reformulation of UFLS-allocation methods to account for this uncertainty. In this paper, a chance-constrained methodology for selecting feeders is proposed, with mathematical guarantees for the disconnection of the required amount of load with a certain pre-defined probability. The correlation in net-load forecasts among feeders is explicitly considered, given that uncertainty in DG power output is driven by meteorological conditions with high correlation across the network. Furthermore, this method is applicable either to systems with conventional UFLS schemes (where relays measure local frequency and trip if this magnitude falls below a certain threshold), or adaptive UFLS schemes (where relays are triggered by control signals sent in the few instants following a contingency). Relevant case studies demonstrate the applicability of the proposed method, and the need for explicit consideration of uncertainty in the UFLS-allocation process.

Adaptive Underfrequency Load Shedding And Voltage Stability In The Power System

Abstract Underfrequency load shedding is a common technique for maintaining the stability of the power system by removing the overload in a certain part of the system after a disturbance. The purpose of underfrequency load shedding is to balance output and load when a particular event causes a significant frequency drop in the power system. In conventional underfrequency load shedding schemes, the frequency thresholds of frequency relays are constant, this way it is difficult and sometimes impossible to control the frequency in various disturbances in the system. In this paper, an adaptive underfrequency load shedding (AUFLS) algorithm that is independent of communication between relays is presented. The relays are tuned to reduce loads taking into account local parameters such as voltage and frequency to prevent the occurrence of a cascade failure that can ultimately lead to the breakdown of the entire power system. In this paper, the rate of change of frequency (ROCOF) is obtained by applying the Hilbert-Huang transformation. Numerical simulations conducted on the New England 39 bus test system in the DIgSILENT PowerFactory and MATLAB software packages confirm the effectiveness of the proposed approach.

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.

Decentralized Adaptive Under Frequency Load Shedding Scheme Based on Load Information

The inertia of the power system changes when the generators in the system are disturbed or tripped. However, in some systems, there is no control center or communication device falling behind. It makes real-time inertia difficult to obtain and affects the adaptive under frequency load shedding (UFLS) scheme. This paper proposes an adaptive decentralized UFLS scheme based on load information to overcome this problem. First, a calculation method is derived according to the load information after the disturbance to identify the real-time of load equivalent virtual inertia. Then, the power deficit of load is estimated and the optimal adaptive load shedding scheme is obtained. Finally, compared with conventional UFLS schemes, the proposed scheme can be performed merely by load information. Hence, the requirement of communication conditions and the dependence on the control center are reduced. The simulation is carried out on the island utilizing the IEEE39-bus system. Furthermore, the accuracy of real-time identification of load equivalent virtual inertia and the validity of the load shedding scheme are verified.

Multi-layer fuzzy-based under-frequency load shedding in back-pressure smart industrial microgrids

In industrial microgrids (MGs), various operating conditions may lead to severe frequency fluctuations. If no any particular remedy is thought, these fluctuations may jeopardize the system stability against which load shedding is recognized as an effective countermeasure. This issue has not been properly addressed in back-pressure MGs where cogeneration of steam and electricity is required. If the permissible operating range is violated, under frequency load shedding (UFLS) scheme propagates suitable commands to disconnect a set of loads and hence assuring the frequency stability. This manuscript proposes an efficient multi-layer fuzzy-based load shedding (MLFLS) approach to enhance the MG overall performance. In comparison with conventional UFLS scheme, the amount of shed load is determined through a multi-layer mechanism. Frequency deviation and steam pressure are selected as two control inputs and the output signal is a control command to keep a specific load in connected or disconnected mode. To this end, suitable rule basis is embedded in MLFLS inference system to determine the amount of required load to be shed. The proposed approach is tested on a simulation model of a real-world autonomous MG and a suitable comparison is performed against the conventional UFLS scheme. The obtained results are discussed in depth.

An Under-frequency Load Shedding Scheme with Continuous Load Control Proportional to Frequency Deviation

Under frequency load shedding (UFLS) is an important measure to tackle with frequency drop caused by load-generation imbalance. In existing schemes, loads are shed by relays in a discontinuous way, which is the major reason leading to under-shedding and over-shedding problems. With the application of power electronics technology, some loads can be controlled continuously, and it is possible to improve the UFSL with continuous loads. This paper proposes an UFLS scheme by shedding loads continuously. The load shedding amount is proportional to frequency deviation before frequency reaches its minimum during transient process. The feasibility of the proposed scheme is analysed with analytical system frequency response model. The impacts of governor droop, system inertia, and frequency threshold on the performance of the proposed UFLS scheme are discussed. Cases are demonstrated to validate the proposed scheme by comparing it with conventional UFLS schemes.

Enhancement of Power System Stability Using Adaptive Combinational Load Shedding Methods

In the modern power systems operating at lower stability margins, conventional non-adaptive schemes cannot offer adequate protection for securing the power system, especially against combinational events. For major disturbances, the active power deficit is usually accompanied by reactive power deficit and frequency stability and voltage stability of the system are jeopardized simultaneously. In this paper, three adaptive combinational load shedding methods are proposed to improve operation of the conventional underfrequency load shedding scheme in order to enhance power system stability following severe disturbances. The proposed methods use locally measured frequency and voltage signals to counteract such events. In the proposed algorithms, load shedding is started from the locations which have higher voltage decay for longer period of time. The speed, location, and amount of load shedding are changed adaptively depending on the disturbance location, voltage status of the system, and the rate of frequency decline. Operation of the conventional and the proposed load shedding methods have been simulated in an actual large network. Obtained simulation results confirm that the proposed methods provide considerable enhancement in the power system voltage stability margin, and by using the proposed algorithms, various power system blackouts could be prevented.

Adaptive Scheme for Minimal Load Shedding Utilizing Synchrophasor Measurements to Ensure Frequency and Voltage Stability

This article presents a new scheme for load curtailment in the power system under critical contingencies that may pose a threat to the frequency and voltage stability of the system. The proposed scheme is based on a two-stage load-shedding strategy. In the first stage, disturbances have been classified based on the computed value of the disturbance power, and the load-shedding requirement in the system has been assessed based on the frequency stability, as well as dynamic voltage stability, conditions. The scheme assumes the availability of real-time data from a synchrophasor-based wide area monitoring and control system. A dynamic voltage stability criterion has been developed in this stage using a voltage stability risk index to assess the voltage stability in real time. Suitable locations for the load curtailments have been chosen according to the voltage stability risk index, which is calculated at each load bus. After the system attains transient stability, the second stage of the proposed scheme determines the optimal redispatching of generators for recovering the loads as much as possible, which had been curtailed in the first stage following the contingency. Performance of the proposed scheme has been tested on a 10-machine, New England 39-bus system, and the results are compared with a conventional under-frequency load-shedding scheme and an adaptive load-shedding scheme.

New centralised adaptive load-shedding algorithms to mitigate power system blackouts

Conventional under-frequency load-shedding scheme is designed to retrieve the balance of generation and consumption following a disturbance. In the conventional load-shedding method, frequency settings, time-delay settings and the amount of load to be shed in each step are constant values. The loads to be shed by this scheme are also constant load feeders and not selected adaptively. This constant non-adaptive load-shedding algorithm is not the most efficient scheme for all power system disturbances. Application of centralised load-shedding algorithms could enhance adaptability of the load-shedding schemes. Two centralised adaptive load-shedding algorithms are proposed. The first algorithm is response-based and the second one is a combination of event-based and response-based methods. The proposed methods are capable of preserving power system instability even for large disturbances and combinational events. They use both frequency and voltage variables to select appropriate amount of load shedding. Parameters of the suggested schemes are also selected adaptively based on the magnitude of the disturbance. Performances of the proposed algorithms are evaluated by the application of the adaptive algorithms to the distributed and dynamic simulated model of a real power system. Obtained simulation results confirm that by using these algorithms various power system blackouts may be prevented.