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

Un disyuntor interrumpe las corrientes de falla extinguiendo el arco, que se alarga y se enfría para detener el flujo de electricidad.
Los interruptores se clasifican en función de su voltaje de funcionamiento y el medio utilizado para extinguir el arco.
Los interruptores de alto voltaje se reconectan automáticamente de 15 a 50 ciclos después de la interrupción y se bloquean si la falla persiste, lo que requiere la intervención del operador.
Las aplicaciones de bajo voltaje utilizan disyuntores de caja moldeada con disparos magnéticos para grandes corrientes de falla y disparos térmicos para sobrecargas más pequeñas y sostenidas.
El método simplificado E/X ayuda a la selección del interruptor mediante el cálculo de la corriente de cortocircuito simétrica máxima utilizando el voltaje previo a la falla y la reactancia del sistema, descuidando otros factores.
Para los generadores, los interruptores de dos ciclos se seleccionan en función de la corriente de falla subtransitoria, calculada mediante reactancias subtransitorias. En el caso de los motores síncronos, se aplican reactancias subtransitorias o transitorias, en función de la velocidad del interruptor.
Los fusibles, dispositivos de sobrecorriente con un enlace fusible en un tubo lleno de arena, actúan como conductores en condiciones normales de funcionamiento. La corriente de sobrecarga eleva la temperatura del enlace, lo que hace que se derrita y forme un arco.

selección de disyuntores y fusibles

Circuit Breaker and Fuse Selection

Un disyuntor es un dispositivo diseñado para interrumpir corrientes de falla y, a veces, volver a cerrarse automáticamente. Cuando se detecta una corriente de falla, el disyuntor separa los contactos eléctricos, lo que genera un arco. Este arco se extingue mediante métodos como elongación, enfriamiento o división, según el diseño del disyuntor. Los disyuntores se clasifican según la tensión al que funcionan y el medio utilizado para la extinción del arco, como aire, aceite, gas SF6 o vacío.

En los sistemas de alta tensión, los disyuntores suelen contar con mecanismos de reconexión automática. Estos disyuntores se reconectan entre 15 y 50 ciclos después de una interrupción. Si la falla persiste, se bloquean, lo que requiere la intervención manual de un operador. En los sistemas de baja tensión, son comunes los disyuntores de caja moldeada de doble disparo. Estos disyuntores utilizan disparos instantáneos magnéticos para manejar fallas grandes y disparos térmicos para manejar fallas más pequeñas.

El método E/X simplifica la selección del disyuntor al calcular la corriente máxima de cortocircuito simétrica. Este enfoque utiliza las características de voltaje y reactancia del sistema antes de la falla, ignorando otros factores. Para los generadores, se utilizan disyuntores de dos ciclos para calcular la corriente de falla subtransitoria, basándose en las reactancias subtransitorias de la máquina. Para los motores sincrónicos, las reactancias subtransitorias o transitorias se consideran en función de la velocidad de operación del disyuntor.

Los fusibles sirven como dispositivos de protección contra sobrecorrientes y contienen un fusible dentro de un tubo lleno de arena. En condiciones normales, el fusible conduce electricidad. Cuando se produce una corriente de sobrecarga, la temperatura del fusible aumenta, lo que hace que se derrita y forme un arco. Los fusibles se especifican en función de cuatro factores clave: capacidad nominal de corriente, capacidad nominal de tensión, capacidad nominal de interrupción y características de retardo de tiempo. Los fusibles deben reemplazarse manualmente una vez que se hayan derretido.

Tanto los disyuntores como los fusibles desempeñan un papel fundamental en la protección de los sistemas eléctricos contra corrientes de falla y sobrecorrientes. Los disyuntores, con su capacidad de volver a cerrarse automáticamente y manejar diferentes tipos de fallas, son esenciales en aplicaciones de alto y bajo voltaje. Los fusibles, si bien son más simples y requieren reemplazo manual, brindan protección confiable al interrumpir las sobrecorrientes. Comprender los requisitos y las características específicas del sistema eléctrico es crucial para seleccionar los dispositivos de protección adecuados para garantizar la seguridad y la continuidad operativa.

Selección de disyuntores y fusibles

Un disyuntor interrumpe las corrientes de falla extinguiendo el arco, que se alarga y se enfría para detener el flujo de electricidad.
Los interruptores son ...

Education

Educación

JoVE Core

Electrical Engineering

Ingeniería Eléctrica

Unit 1

Unidad 1

Symmetrical and Unsymmetrical Faults

Fallas Simétricas y Asimétricas

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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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