Frequency Load Shedding Research Articles

Research on the Influence Factors and Coordinated Control Strategies between Unit and Grid for Isolated Power System

As the existing coordinated control strategies between grid and unit have limitations in isolated power system, this paper introduces new coordinated control strategies which can improve the stability of isolated system operation. This paper analyzes the power grid side and unit side influence factors on the isolated power system. The dynamic models which are suitable for islanding operation are applied to simulate and analyze the stability and dynamic characteristics of the isolated power system under the conditions of different load disturbances and governor parameters. With considering the differences of frequency characteristics between the interconnected and isolated power system, the adjusting and optimization methods of under frequency load shedding are proposed to meet the frequency stability requirements simultaneously in the two cases. Not only proper control strategies of the power plant but the settings of their parameters are suggested to improve the operation stability of the isolated power system. To confirm the correctness and effectiveness of the method mentioned above, the isolated system operation test was conducted under the real power system condition, and the results show that the proposed coordinated control strategies can greatly improve stability of the isolated power system.

An Intelligent Wide Area Synchrophasor Based System for Predicting and Mitigating Transient Instabilities

Increasing expansion of power systems and grids are accompanied nowadays by innovation in smart grid solutions to maintain systems stability. This paper proposes an intelligent wide area synchrophasor based system (IWAS) for predicting and mitigating transient instabilities. The IWAS incorporates artificial neural networks (ANN) for transient stability prediction. The ANN makes use of the advent of phasor measurements units (PMU) for real-time prediction. Coherent groups of generators-which swing together-is identified through an algorithm based on PMU measurements. A remedial action scheme (RAS) is applied to counteract the system instability by splitting the system into islands and initiate under frequency load shedding actions. The potential of the proposed approach is tested using New England 39 bus system.

Optimal design of adaptive under frequency load shedding using artificial neural networks in isolated power system

The frequency and voltage stability is a basic principle in the power system operation. Different short circuits, load growth, generation shortage, and other faults which disturb the voltage and frequency stability are serious threats to the system security. The frequency and voltage instability causes dispersal of a power system into sub-systems, and leads to blackout as well as heavy damages of the system equipments. Optimum load shedding during contingency situations is one of the most important issues in power system security analysis. This paper presents a fast and optimal adaptive load shedding method, for isolated power system using Artificial Neural Networks (ANNs). By creating an appropriate data-base of contingencies for training the neural network, the proposed method is able to perform correct load shedding in various loading scenario. In this regard, the total power generation, the total loads in power system, the existing spinning reserved capacity value in the network and frequency reduction rate were selected as the ANN inputs. This method has been tested on the New-England power system. The simulation results show that the proposed algorithm is very fast, robust and optimal values of load shedding in different loading scenarios, related to conventional method.

Effect of Load Shedding Strategy on Interconnected Power Systems Stability When a Blackout Occurs

 Abstract—From the long time ago properly and reliable operation of power systems were being the major portion of designer's and operators concernment. In the interconnected power systems voltage and frequency of system are most significant parameters for analysis the power system operation. In this paper sudden blackouts in a supposition interconnected power system will be simulate and effect of under frequency load shedding to restore the power system in stable condition will be study.

A Synchrophasor Assisted Frequency and Voltage Stability Based Load Shedding Scheme for Self-Healing of Power System

This paper presents a new scheme for load curtailment in the power system required for its self-healing under critical contingencies, which may pose a threat to frequency as well as the voltage stability of the system. The load shedding requirement in the system has been calculated based on the computed disturbance power as well as the voltage stability condition of the system, with the aid of real-time data, assumed to be available from the synchrophasor based wide area monitoring and control system (WAMCS). This scheme assesses voltage stability based on a dynamic voltage stability criterion, formulated using a Voltage Stability Risk Index (VSRI). Suitable locations for the load curtailments have been chosen according to the VSRI, calculated at each load bus. Performance of the proposed scheme has been tested on New England 39-bus system and a practical 246-bus Indian system. The results are compared with a conventional under frequency load shedding scheme and an adaptive load shedding scheme.

Determination of frequency stability border of power system to set the thresholds of under frequency load shedding relays

Frequency stability is a crucial consideration in power system operation and planning, particularly as a consequence of recent increase in load demand. There is an urgent need in each power system to consider load–generation balance to prevent system from breaking up into islands. In this paper, a system frequency response model has been presented and a new method has been offered to analytically calculate the border of frequency stability of power system. It can be realized, whether system is stable from frequency viewpoint or not. Moreover, the threshold of UFLS (under frequency load shedding) relays can more effectively be determined, compared to the previous works in the area. The proposed method is examined on a four unit test system. Obtained results confirm the validity of the developed approach.

Regional Coordination for under Frequency Load Shedding

Frequency deviation can be used as an indicator of imbalance between supply and demand. When generation is insufficient, it can cause frequency decline in a power system operation. Implementing under frequency load shedding (UFLS) is one of the common methods to overcome this problem. This paper proposes a novel approach for adaptive load shedding. The concept is an extension of shared and targeted load shedding using reserve margin. The optimal system configuration is then selected from those candidates to fulfill operational objectives. Operational constraints related to system parameters, threshold frequency, total of load shed and control area including line capacity are considered. An example using four sub-areas connected to an external system shows that the proposed regional coordination as an adaptive UFLS is feasible.

An unified external network equivalent in steady-state security assessment

The basic objective of this paper is to present the relevant methodological and practical aspects of forming the unified external network equivalent. This equivalent consistently respects the effects of primary voltage and frequency control of external power system, including the real possibilities and limitations, as well as the effects of possible action of under frequency load shedding or under frequency protection of generator. The first practical experience in the application of the equivalent has been gained on an example of the synchronous parallel operation of the 2nd UCTE Synchronous Zone. The results obtained show a very good accuracy of unified external network equivalent formed and also show the very high speed of steady-state security assessment in context of real interconnection.

Design of New Load Shedding Special Protection Schemes for a Double Area Power System

In this paper several Under Frequency Load Shedding (UFLS) schemes are designed as special protection schemes to preserve the integrity of islands, formed following the outage of tie lines connecting two areas in a double area power system. For the purpose of analysis, the Iranian interconnected grid is simulated in a suitable power system analysis software. The grid is connected to the Khorasan province network, the greatest province of the country, through a double circuit tie line. Following the outage of tie lines in a double area power system the area with great import of power may face large deficiency of energy and consequently blackout may occur in the island. The purpose of this research work is to propose a special protection scheme, being capable of preserving the balance of generation and consumption in order to prevent major outage. Special types of adaptive UFLS scheme are proposed in this work.

Current Electric System Operating Problems from a Cogenerator's Viewpoint

The power system operating problems from a cogenerator's viewpoint are presented considering steam power constraints of the cogeneration process, control of tie-lines to cogeneration plants, voltage support, emergency response, and maintenance. Also discussed is the technical planning problems associated with system protection, under frequency loadshedding and needs for long term operations planning. As the amount of cogeneration increases in the electric utility industry, additional concerns arise in various purchase and sale contracts and administration requirements for both the cogenerator and electric utility.

Maximum Frequency Decay Rate for Reactor Coolant Pump Motors

This paper presents methods and results of an analysis to determine the maximum expected frequency decay rate (FDR) at Pilgrim Station Unit 2, a nuclear power generating station utilizing a pressurized water reactor design. The maximum expected FDR would be of concern if it was greater than the FDR analogous to the freewheeling coast-down rate of the reactor coolant pump motors. This paper should benefit utilities and regulatory agencies evaluating pressurized water reactor station designs as it describes how to analyze the off-site power system. The paper includes discussions of the criteria for analytically creating overloads to force frequency excursions, load shedding and under-frequency relay effects on the FDR's, important assumptions made, a comparison of FDR's using different load models, and a comparison of the findings with those of other investigators.