Power-system protection is a branch of electrical engineering that deals with the protection of power systems from errors through the isolation of damaged parts of other power grids. The purpose of the protection scheme is to keep the power system stable by isolating only components that are under cesarean, while allowing as many networks as possible to operate. Thus, the protection scheme should be applied with a very pragmatic and pessimistic approach to clean up system errors. The device used to protect the power system from errors is called protection devices .
Video Power-system protection
Components
The protection system usually consists of five components:
- Current and voltage transformers to lower the high voltage and power system flow to a convenient level for relays to handle
- Protective relays to detect faults and start travel, or disconnect, order;
- Circuit breakers for opening/closing systems based on relay and autorecloser commands;
- Battery to provide power in case of system power disconnection.
- The communication channel to allow analysis of currents and voltages at the remote terminal of a path and to allow the device to trip over long distances.
For parts of the distribution system, the fuses are able to detect and release errors.
Failure may occur in any part, such as failure of isolation, fall or breakage of transmission line, wrong circuit breaker operation, short circuit and open circuit. Protective devices are installed with the aim of protecting assets, and ensuring sustainable energy supply.
Switchgear is a combination of electric circuit breakers, fuses or circuit breakers used to control, protect and isolate electrical appliances. The switch is safe to open under normal load current, while the protective device is safe to be opened under fault current.
Maps Power-system protection
Protective devices
- The protective relay controls tripping circuit breakers that surround the part of the network being blamed
- Automatic operation, such as auto-reboot or system reinstatement
- Monitoring tools that collect data on the system for post event analysis
While the operating quality of these devices, and especially of the protective relays, is always important, different strategies are considered to protect the different parts of the system. A very important tool may have a completely redundant and independent protective system, while a small branch distribution line may have very low cost protection.
There are three parts of the protective device:
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- Instrument transformer: current or potential (CT or VT)
- Relays
- Circuit breaker
The advantages of these devices are protected with these three basic components including safety, economy, and accuracy.
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- Safety: The instrument transformer creates electrical isolation from the power system, and thus builds a safer environment for personnel working with relays.
- Economics: Relays can be simpler, smaller, and cheaper given low-level relay inputs.
- Accuracy: The power system voltage and current are accurately reproduced by the instrument transformer over a large operating range.
Type of protection
High voltage transmission network
Protection on transmission and distribution serves two functions: Crop protection and public protection (including employees). At the ground level, protection is seen to disconnect equipment that is overloaded or short to earth. Some items in substations such as transformers may require additional protection based on temperature or gas pressure, among others..
Genset
In power plants, protective relays are intended to prevent damage to the alternator or transformer in case of abnormal operating conditions, due to internal failure, as well as failure of isolation or regulatory malfunctions. Such failure is unusual, so the protective relay must operate very rarely. If a protective relay fails to detect an error, the damage caused by an alternator or transformer may require expensive replacement or replacement of equipment, as well as loss of income from the inability to produce and sell energy.
Overload and back-up for distance (overcurrent)
Excessive protection requires a current transformer that only measures current in the circuit. There are two types of overload protection: faster current flow and overtime (TOC). A faster current requires that the current exceeds a predetermined level for the circuit breaker to operate. TOC protection operates on the basis of the current vs. time curve. Based on this curve if the measured current exceeds the given level for a given amount of time, the circuit breaker or fuse will operate. The functions of both types are described in "More Non-directional Flow Protection" on YouTube.
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Back ground protection requires a current transformer and senses an imbalance in a three-phase circuit. Usually three phase currents are balanced, which is approximately equal. If one or two phases become connected to the earth through a low impedance path, their magnitude will increase dramatically, such as the current imbalance. If this imbalance exceeds a predetermined value, the circuit breaker must operate. Limited geothermal protection is a type of earth-damaging protection that searches for earth fault between two pairs of current transformers (hence limited to that zone).
Distance (impedance relay)
Distance protection detects voltage and current. Errors on the circuit will generally make a sag in the voltage level. If the ratio of voltage to the current measured at the relay terminal, which is equivalent to the impedance, landing at a predetermined rate, the circuit breaker will operate. This is useful for reasonable long lines, lines longer than 10 miles, because their operating characteristics are based on line characteristics. This means that when an error appears on the line of the impedance setting in the relay compared to the real line impedance from the relay terminal to the error. If the relay setting is determined to be below a real impedance, then it is determined that the fault is within the protection zone. When the transmission line length is too short, less than 10 miles, distance protection becomes more difficult to coordinate. In this case the best choice of protection is the current differential protection.
Backup
The purpose of protection is to remove only the affected parts of the plant and nothing else. Circuit breakers or protective relays may fail to operate. In important systems, failure of primary protection will usually result in the operation of backup protection. Remote back-up protection will generally remove both affected and unexposed plant items to clear errors. Local back-up protection removes the affected items from the plant to clear errors.
Low voltage network
Low voltage networks generally rely on fuses or low-voltage circuit breakers to remove overload and earth fault.
Coordinate
Co-ordination of protective equipment is the "most appropriate" timing process of the current disturbance when an abnormal electrical condition occurs. The goal is to minimize the blackout as much as possible. Historically, the coordination of protective devices was carried out on a translucent log-log paper. Modern methods typically include detailed computer-based analysis and reporting.
Coordination of protection is also handled by dividing the power system into a protective zone. If an error occurs in a particular zone, the necessary action will be taken to isolate the zone from the entire system. Account definition zones for generators, buses, transformers, transmission and distribution channels, and motors. Additionally, the zone has the following features: overlapping zones, overlapping areas showing circuit breakers, and all circuit breakers in a given zone with errors open to isolate errors. The overlapping area is made up of two sets of instrument and relay transformers for each circuit breaker. They are designed for redundancy to remove unprotected areas; however, overlapping areas are designed to remain as small as possible so that when errors occur in overlapping areas and two zones covering the fault are isolated, the power system sectors missing from the service are still small despite the two isolated zones.
Troubleshooting monitoring equipment
Monitor interference-monitoring equipment (DME) monitors and records system data related to errors. DME achieves three main goals:
- model validation,
- investigation of interruptions, and
- performance assessment of system protection.
The protection engineer defines dependency as the tendency of the protection system to operate correctly for errors within the zone. They define security as a tendency to not operate due to errors outside the zone. Dependence and security are both reliability issues. Error tree analysis is one tool that can be used by protection engineers to compare the relative reliability of the proposed protection schemes. Measuring the reliability of the protection is essential to making the best decision in improving the protection system, managing dependency versus security sacrifices, and getting the best results for the least amount of money. Quantitative understanding is essential in the competitive utility industry.
Performance and design criteria for system protection devices include reliability, selectivity, speed, cost, and simplicity.
- Reliability: The device must function consistently when a fault condition occurs, regardless of the possibility of unemployment for months or years. Without this reliability, the system can lead to costly damage.
- Selectivity: Devices should avoid unwarranted false trips.
- Speed: The device must work quickly to reduce equipment damage and the duration of interference, with only deliberate deliberate timing delays.
- Economy: Devices must provide maximum protection at minimum cost.
- Simplicity: Devices must minimize protection circuits and equipment.
See also
- Current fault limiter
- Network analyzer (AC power)
- Prospective short-circuit
Note
References
- http://perso.numericable.fr/michlami protection and monitoring of electric energy transmission networks
Source of the article : Wikipedia
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