Load Shedding System Research Articles

Hydro-Quebec's defence plan against extreme contingencies

In recent years, Hydro-Quebec has undertaken a major program to upgrade the reliability of its transmission system. Much effort has focused on increasing the system's ability to withstand extreme contingencies, usually caused by multiple incidents or the successive tripping of transmission lines. In order to counteract such events on its system, Hydro-Quebec has adopted a number of special defensive measures, globally called the "defence plan against extreme contingencies". The goal of this plan, which will be fully operational by 1998, is to detect and confine incidents that exceed the system's strength. This paper summarizes Hydro-Quebec's current orientation for safeguarding its transmission system after extreme contingencies and describes the various automatic measures adopted and, in particular, the RPTC system, one of the main components of the defence plan. The RPTC system will detect multiple line losses directly from fifteen 735 kV substations, include four generation rejection sites and a centralized remote load shedding system.

Optimization of a load shedding scheme

An integrated process facility recently went through an update of its existing load shedding scheme in the context of process changes planned for the coming years. Transient simulations indicated that by changing the load shedding philosophy, the frequency profile during power shortages could be significantly improved. This article discusses the plant's electrical/steam load balance issues and the development of a high speed load shedding scheme. Shortage of power in a network is invariably reflected in collapse of frequency. Traditionally, frequency decay patterns have been calculated using simplified methods. Simplifications have included the assumption of constant deceleration constant voltages, and constant generator power. The results obtained using the simplified methods were generally adequate for simple, slow-acting load shedding systems. In the case at hand, the loads assigned for shedding and those to be saved by load shedding are part of an integrated process plant sequence, where speed of operation and selectivity of the load shedding is crucial to the continuity of the oil-production process. In the search for ways to optimize the load shedding scheme, transient stability simulations were used to calculate frequency profile, taking into account generators' transient responses and those of the excitation and speed governor systems. This article describes the outcome of the analysis in terms of possible improvements in load shedding performance through the use of rate of change of frequency as the shedding indicator.

Co-ordination of emergency secondary-voltage control and load shedding to prevent voltage instability

The authors investigate emergency action in terms of secondary-voltage control and the system load shedding for prevention of voltage instability. Two modes of co-ordination of these control methods with corresponding models are proposed, and these are summarised into developed emergency-control algorithms. The emergency actions of the individual controls and of the proposed co-ordinated control modes are then simulated on a real multimachine power system. The results so obtaned show the superiority of the proposed co-ordinated controls over a single control action in preventing voltage instability and system breakdown.

Neural-networks for predicting the operation of an under-frequency load shedding system

Dynamic security assessment is of special importance to island power systems. The CPU time required in order to apply conventional methods for those calculations does not allow real-time application. The fast calculation time is, therefore, an important advantage of artificial neural networks compared to other methods. This paper presents two neural network models that were designed to calculate the minimal frequency and the load shedding system operation during a forced outage of a generating unit. The minimal frequency and the extent of the load shedding are strong indications of the severity of the fault. Hence, it is a significant part of the dynamic security assessment procedure.

A 735 kV shunt reactors automatic switching system for Hydro-Quebec network

Hydro-Quebec has undertaken a major program to upgrade the reliability of its transmission system. Much effort has been directed toward increasing the system's capacity to withstand extreme contingencies, usually caused by multiple incidents or the successive tripping of transmission lines. In order to counter such events, Hydro-Quebec has adopted a defensive scheme. Based entirely on automatic action, this scheme will mainly rely on: a 735 kV shunt reactor switching system (called MAIS); a generation rejection and/or remote load-shedding system (called RPTC); and an underfrequency load-shedding system. The MAIS system, which is the subject of this paper, will be implemented in 22 substations and is required to control voltage on the system after a severe event. Each MAIS system, acting locally, is entirely independent and will close or trip shunt reactors in response to local conditions.

DYNAMIC PHENOMENA IN ISLAND POWER SYSTEMS

ABSTRACT The paper deals with dynamic phenomena in island power systems. Most of the large disturbances that initiate the dynamic processes in the power system are caused by forced outages of generating units. The paper presents two large disturbances in the Israeli island power system: simultaneous forced outage of 3 large generating units and a large scale load rejection. An island power system must preserve sufficient measures to cope with disturbances of the kind mentioned above. The main measures are the load shedding system and the operational spinning reserve margin. The paper demonstrates the relations between the spinning reserve margin and the dynamic response of the power system. The paper also deals with the economic value of preserving a certain spinning reserve level.

Optimization of load shedding system

An optimization algorithm is presented for an underfrequency load shedding system which is composed of several stages that are tripped at preset frequencies. The optimization of this system is considered with respect to a cost function that includes a dynamic part, which is the integral of the deviation from nominal frequency, and a static part which is the total load shedding. The optimization is constrained by the requirement of minimum allowed frequency and limitation on the total load of the shedding system. A projected gradient method is used for the solution, and analytic expressions for the partial derivatives are used to simplify the computation. Results of applying the optimization to a model of the Israeli power system are given together with a study of the cost parameters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Microprocessor-based load shedding keeps industry systems in balance

The use of a computer-based load-shedding system to save critical loads, reduce process restart time, minimize system disturbance, and provide flexibility is discussed. The components of the system and how it operates are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement of power system frequency deviation using a microprocessor

The measurement of power system frequency deviation is useful for the design of power system stabilizers, automatic load shedding systems and the monitoring of power station frequency. Here, a simple scheme using a 90° phase shifter is suggested. The scheme is suitable for implementation using a microprocessor.

A Survey of Current Operational Problems

This paper is one of a series prepared for use in the Working Group on Current Operational Problems (COPS) forums with the goal of focusing attention of the industry on problems faced by those who are involved in actual power system operation. The questionnaire was sent to System Operation Subcommittee members and to North American Electric Reliability Council (NERC) key operating members. One hundred thirty responses were received. The high priority topics identified as problems by the survey can be divided up into three categories of people, analysis and control, and computer system. Some of the items, for example, Dispatcher Training Simulator could be assigned to any of these new categories. We arranged them as follows: People Problems-Dispatcher Training Simulators Management Problems due to Shift Work Evaluation of Training Programs Simulators to Practice System Restoration Job Desirability Dispatcher Selection Relations with Dispatchers Analysis and Control Problems - Key Monitoring Indicators Load Shedding System Condition Assessment Contingency Voltage Levels Transmission Access and Compensation Generation Cycling Resource Scheduling Computer System Problems - Alarms (present techniques need improvement) Alarming (Artificial Intelligence systems are needed) Performance Considerations (Computer systems handling inadequately during emergencies)

Quelques opérations non-linéaires automatiques sur un grand réseau électrique

Due to the abrupt nature of the major disturbances that may endanger the security of an electrical network, it is but natural to conclude that some of the corrective actions should also be rapidly applied automatically for early completion, in a bulk, non-linear fashion; the linear controls can then take over and use up the design margin of the system to optimize its subsequent security. These techniques are often almost equivalent to reducing the absolute contingency level of the disturbance by one, that is from the complete network, by using reliable and well coordinated automatic schemes, in place of some overdesign to be sacrificed at the first event. Amongst the many non-linear automatic operations available to the designer <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> , one finds generator dropping, load shedding, reactive elements switching, line switching, etc. This paper outlines the main aspects of three such schemes designed to meet some of Hydro-Quebec's particular system problems in order to improve its stability and security; these are: Automatic removal of 735 kV line compensation inductors for stabilization purposes, automatic generator dropping system for the Churchill Falls generation and transmission network and, automatic load shedding system for the interconnected network.