Electrical network protection
Selection criteria Examples of architectures
Presentation Power-system architecture Neutral earthing
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Five neutral earthing systems Isolated neutralResistance earthing Low reactance earthing Compensation reactance earthing Solidly earthed neutral Introduction to short-circuits Types of short-circuit Short-circuit across generator terminals Calculation of short-circuit currents Equipment behaviour during short-circuits Phase-current sensors (CT) Phase-current sensors (LPCT) Residual-current sensors Voltage transformers (VT) Generalcharacteristics List of functions Associated functions
Time-based discrimination Current-based discrimination Logic discrimination Directional protection discrimination Differential protection discrimination Combined discrimination
Single-incomer power systems Dual-incomer power systems Open looppower systems Closed loop power systems Types of faults and protection functions Types of faults and protection functions Types of faults Protection functions Recommended settings Examples of applications
Link (line and cable) protection Transformer protection
Types of faults Protection functions Recommended settings Examples of applications Types offaults Protection functions Recommended settings Examples of applications
Types of faults Protection functions Recommended settings and examples of applications Glossary - Key words and definitions Bibliography Definitions of symbols Index of technical terms
Protection units continuouslymonitor the electrical status of power system components and de-energize them (for instance by tripping a circuit breaker) when they are the site of a serious disturbance such as a short-circuit, insulation fault, etc. The choice of a protection device is not the result of an isolated study, but rather one of the most important steps in the design of the power system. Based on an analysis of thebehaviour of electrical equipment (motors, transformers, etc.) during faults and the phenomena produced, this guide is intended to facilitate your choice of the most suitable protective devices.
Among their multiple purposes, protection devices: b contribute to protecting people against electrical hazards, b avoid damage to equipment (a three-phase short-circuit onmedium-voltage busbars can melt up to 50 kg of copper in one second and the temperature at the centre of the arc can exceed 10000 °C), b limit thermal, dielectric and mechanical stress on equipment, b maintain stability and service continuity in the power system, b protect adjacent installations (for example, by reducing induced voltage in adjacent circuits). In order to attain these objectives, aprotection system must be fast, reliable and ensure discrimination. Protection, however, has its limits because faults must first occur before the protection system can react. Protection therefore cannot prevent disturbances; it can only limit their effects and their duration. Furthermore, the choice of a protection system is often a technical and economic compromise between the availability and safety ofthe electrical power supply.
Designing power system protection
The design of protection for a power system can be broken down into two distinct steps: b definition of the protection system, also called the protection-system study, b determination of the settings for each protection unit, also called protection coordination or discrimination. This step includes selection of the protection...
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