Electrical network protection guide
ntroduction to Electrical network protection guide
Among their multiple purposes, protection devices:
- Contribute to protecting people against electrical hazards,
- Avoid damage to equipment (a three-phase short-circuit on medium-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),
- Limit thermal, dielectric and mechanical stress on equipment,
- Maintain stability and service continuity in the power system,
- Protect adjacent installations (for example, by reducing induced voltage in adjacent circuits).
In order to attain these objectives, a protection 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 of the electrical power supply.
Scope
The process for identifying the need for an UPS system, selecting, installing, and maintaining the UPS system are covered.
Covered are – theory and principles of static and rotary UPS systems, design and selection of UPS, installation and testing of UPS, maintenance and operation of UPS systems, principles of static and rotary UPS, UPS system rating and sizing selection, operations/maintenance, batteries, troubleshooting, harmonic distortions, grounding, checklists, and acceptance testing.
Protection units continuously monitor 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 the behaviour 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.
Designing power system protection
The design of protection for a power system can be broken down into two distinct steps:
- Definition of the protection system, also called the protection-system study,
- Determination of the settings for each protection unit, also called protection coordination or discrimination.
Definition of the protection system
This step includes selection of the protection components and a consistent, overall structure suited to the power system. The protection system is made up of a string of devices including the following (refer to Figure 1):
- Measurement sensors (current and voltage) supplying the data required to detect faults,
- Protection relays in charge of continuously monitoring the electrical status of the power system up to and including the formulation and emission of orders to the trip circuit to clear the faulty parts,
- Switchgear in charge of clearing faults, such as circuit breakers or combinations of switches or contactors and fuses.
The protection-system study determines the devices to be used to protect against the main faults affecting the power system and the machines:
- Phase-to-phase and phase-to-earth short-circuits,
- Overloads,
- Faults specific to rotating-machines.
The protection-system study must take the following parameters into account:
- Power system architecture and size, as well as the various operating modes,
- The neutral-earthing systems,
- The characteristics of current sources and their contributions in the event of a fault,
- The types of loads,
- The need for continuity of service.
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