Name: protective devices in power systems
Uploaded: 2024-11-21T14:54:31.000Z
Duration: 1 min 23 s
Channel: JoVE

Um disjuntor interrompe as correntes de falha extinguindo o arco, que é alongado e resfriado para interromper o fluxo de eletricidade.
Os disjuntores são classificados com base em sua tensão de operação e no meio usado para extinguir o arco.
Os disjuntores de alta tensão fecham automaticamente de 15 a 50 ciclos após a interrupção e travam se a falha persistir, exigindo intervenção do operador.
As aplicações de baixa tensão usam disjuntores em caixa moldada com disparos magnéticos para grandes correntes de falha e disparos térmicos para sobrecargas menores e sustentadas.
O método simplificado E/X auxilia na seleção do disjuntor calculando a corrente máxima simétrica de curto-circuito usando tensão pré-falta e reatância do sistema, negligenciando outros fatores.
Para geradores, os disjuntores de dois ciclos são selecionados com base na corrente de falta subtransitória, calculada usando reatâncias subtransitórias. Para motores síncronos, aplicam-se reatâncias subtransitórias ou transitórias, dependendo da velocidade do disjuntor.
Fusíveis, dispositivos de sobrecorrente com um link fusível em um tubo cheio de areia, atuam como condutores em operação normal. A corrente de sobrecarga aumenta a temperatura do link, fazendo com que ele derreta e forme um arco.

seleção de disjuntor e fusível

Circuit Breaker and Fuse Selection

Um disjuntor é um dispositivo projetado para interromper correntes de falha e, às vezes, religar automaticamente. Quando uma corrente de falha é detectada, o disjuntor separa os contatos elétricos, o que gera um arco. Este arco é extinto por métodos como alongamento, resfriamento ou divisão, dependendo do projeto do disjuntor. Os disjuntores são categorizados com base na tensão em que operam e no meio usado para extinção do arco, como ar, óleo, gás SF6 ou vácuo.


 Em sistemas de alta tensão, os disjuntores normalmente apresentam mecanismos de religamento automático. Esses disjuntores religam dentro de 15 a 50 ciclos após uma interrupção. Se a falha persistir, eles travam, necessitando de intervenção manual por um operador. Em sistemas de baixa tensão, disjuntores de caixa moldada de disparo duplo são comuns. Esses disjuntores utilizam disparos instantâneos magnéticos para lidar com grandes falhas e disparos térmicos para lidar com falhas menores.


 O método E/X simplifica a seleção do disjuntor calculando a corrente máxima de curto-circuito simétrica. Esta abordagem usa as características de tensão pré-falha e reatância do sistema, ignorando outros fatores. Para geradores, disjuntores de dois ciclos são usados ​​para calcular a corrente de falha subtransitória, contando com reatâncias de máquina subtransitórias. Para motores síncronos, reatâncias subtransitórias ou transitórias são consideradas com base na velocidade operacional do disjuntor.


 Os fusíveis servem como dispositivos de proteção contra sobrecorrente, contendo um elo fusível dentro de um tubo cheio de areia. Em condições normais, o elo fusível conduz eletricidade. Quando ocorre uma corrente de sobrecarga, a temperatura do elo aumenta, fazendo com que ele derreta e forme um arco. Os fusíveis são especificados com base em quatro fatores principais: classificação de corrente, classificação de tensão, classificação de interrupção e características de retardo de tempo. Os fusíveis devem ser substituídos manualmente depois de derreterem.


 Tanto os disjuntores quanto os fusíveis desempenham papéis críticos na proteção de sistemas elétricos contra correntes de falha e condições de sobrecorrente. Os disjuntores, com sua capacidade de religar automaticamente e lidar com diferentes tipos de falhas, são essenciais em aplicações de alta e baixa tensão. Fusíveis, embora mais simples e que exijam substituição manual, fornecem proteção confiável ao interromper sobrecorrentes. Entender os requisitos e características específicas do sistema elétrico é crucial na seleção dos dispositivos de proteção apropriados para garantir a segurança e a continuidade operacional.

Seleção de Disjuntor e Fusível

Um disjuntor interrompe as correntes de falha extinguindo o arco, que é alongado e resfriado para interromper o fluxo de eletricidade.
Os disjuntores são ...

Education

JoVE Core

Electrical Engineering

Engenharia Elétrica

Unit 1

Unidade 1

Symmetrical and Unsymmetrical Faults

KEY TERMS AND CONCEPTS

TERMOS E CONCEITOS-CHAVE

protective devices in power systems

Circuit breakers and fuses protect electrical systems from fault current and overcurrent. They help stop damage when current rises above safe limits. Both devices are chosen based on the needs of the electrical system.

A circuit breaker interrupts fault current by separating its contacts. This creates an arc, which must be extinguished by elongation, cooling, or splitting, depending on the breaker design. Breakers are grouped by operating voltage and by the arc-extinguishing medium, such as air, oil, SF6 gas, or vacuum.

High-voltage circuit breakers often include automatic reclosing. They can reclose within 15 to 50 cycles after an interruption. If the fault remains, the breaker locks out and an operator must reset it manually. In low-voltage systems, dual-trip molded case breakers are common. These use magnetic instantaneous trips for large faults and thermal trips for smaller faults.

The E/X method is used to simplify breaker selection. It finds the maximum symmetrical short-circuit current from the prefault voltage and the system reactance. It ignores other factors. For generators, two-cycle breakers are used to calculate subtransient fault current from subtransient machine reactances. For synchronous motors, subtransient or transient reactances are used, depending on breaker operating speed.

Fuses are overcurrent devices with a fusible link inside a sand-filled tube. Under normal conditions, the fusible link carries current. During an overload, its temperature rises, the link melts, and an arc forms. Fuses are selected by current rating, voltage rating, interrupting rating, and time-delay characteristics. After they melt, they must be replaced by hand. Circuit breakers and fuses both provide important protection against fault conditions, but they do so in different ways.

Protective Devices in Power Systems

series r&#8212;l circuit transients

Fault Current in a Series R-L Circuit

A series resistor-inductor, or R-L, circuit can model a three-phase short circuit when the switch closes at the start of the time period. In this fault condition, all three phases of an unloaded synchronous machine are short-circuited. With no fault impedance and no initial current, the starting voltage depends on the source voltage phase angle. Kirchhoff's Voltage Law, or KVL, is used to analyze the circuit and find the total asymmetrical fault current. This current has two parts. The AC...

three-phase short circuit&#8212;unloaded synchronous machine

three-phase-short-circuitunloaded-synchronous-machine

Synchronous Machine Fault Current Decay

A three-phase short circuit on an unloaded synchronous machine shows how fault current changes over time. The AC fault current, with the DC offset removed, starts at a high value and then falls to a steady-state level in one phase of the machine. This change happens because of the magnetic flux created by the short-circuit armature currents. At first, these currents flow through high-reluctance paths. They later shift to lower-reluctance paths, which increases the armature inductance. This...

power system three-phase short circuits

Subtransient Fault Current in Power Systems

Subtransient fault current analysis in a power system uses simplified models for the main equipment. Transformers are represented by their leakage reactances. Transmission lines are modeled with equivalent series reactances, and synchronous machines are treated as constant voltage sources behind their subtransient reactances. Several elements are left out of the calculation because their effect on the immediate fault current is small or they make the model harder without changing the result...

bus impedance matrix

Three-Phase Fault Current Analysis

Three-phase fault current analysis in an N-bus power system uses the positive-sequence network and the bus impedance matrix. It helps estimate subtransient fault currents when a short circuit occurs at one bus. The method uses superposition, which means the problem is split into two circuits that are studied separately. In the first circuit, all machine voltage sources are short-circuited. Only the prefault voltage source at the faulted bus remains. The positive-sequence bus impedance matrix...

circuit breaker and fuse selection

Circuit breakers and fuses protect electrical systems from fault current and overcurrent. They help stop damage when current rises above safe limits. Both devices are chosen based on the needs of the electrical system. A circuit breaker interrupts fault current by separating its contacts. This creates an arc, which must be extinguished by elongation, cooling, or splitting, depending on the breaker design. Breakers are grouped by operating voltage and by the arc-extinguishing medium, such as...

sequence networks of rotating machines

Generator Sequence Currents and Impedances

Generator sequence currents and impedances explain how a Y-connected synchronous generator behaves under balanced and unbalanced conditions. When the generator is grounded through a neutral impedance, it is set up to produce balanced internal phase voltages with only positive-sequence components. The source voltage therefore appears only in the positive-sequence network, and the line-to-ground voltages at the generator terminals show the sequence components of this setup. Zero-sequence current...

per-unit sequence models

Transformer Sequence Networks in Per Unit

Transformer sequence networks in per unit show how three-phase transformers respond to positive-, negative-, and zero-sequence currents. For an ideal Y-Y transformer grounded through neutral impedances, the per-unit sequence networks act much like those of a single-phase ideal transformer when the current is balanced. Balanced positive-sequence and negative-sequence currents do not create neutral current, so they do not cause voltage drops across the neutral path. Zero-sequence current behaves...

fault types

Power System Faults in Sequence Networks

Power system faults can be analyzed by moving between the phase domain and the sequence domain. This approach helps relate fault currents and voltages for three-phase bus faults and keeps the fault impedance in the model. Fault impedance is zero for a bolted fault, equal to the arc impedance for an arcing fault, and it represents the total fault impedance for a transmission-line insulator flashover. For a single line-to-ground fault from phase A to ground, the phase-domain conditions are...

key terms and concepts

Learn how circuit breakers and fuses protect power systems from faults and overloads, including trip types, arc extinction, reclosing, and fuse ratings.

Protective Devices in Power Systems | JoVE K12

JoVE Core Electrical Engineering offers easy-to-understand animated videos that cover fundamental concepts of electric circuits along with circuit analysis techniques. It incorporates first-order and second-order circuits using operational amplifiers and circuit transformation concepts. The inclusion of practical design examples and real-world applications through scientist-in-action videos adds significant educational value, making complex concepts accessible.

electrical engineering

Protective Devices in Power Systems

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