Name: synchronous machine fault current decay
Uploaded: 2024-11-21T14:54:30.000Z
Duration: 1 min 21 s
Channel: JoVE

Um teste de curto-circuito trifásico em uma máquina síncrona sem carga é realizado para estudar seu impacto.
O oscilograma da corrente de falta CA com seu deslocamento CC removido, correspondendo a uma fase de uma máquina síncrona sem carga, mostra uma amplitude decrescente da forma de onda de um alto inicial para um valor de estado estacionário.
Isso é atribuído ao fluxo magnético causado por correntes de curto-circuito da armadura, mudando de caminhos de alta relutância para caminhos mais baixos, aumentando a indutância da armadura. Isso pode ser modelado com um circuito RL em série variável no tempo.
A teoria padrão da máquina fornece reatâncias específicas usadas para calcular a corrente de falta CA instantânea que compreende a tensão terminal de pré-falta linha-neutro RMS.
A corrente de falta subtransitória RMS é determinada em tempo igual a zero pela constante de tempo subtransitória de curto-circuito do eixo direto e, à medida que o tempo avança, a corrente de falta RMS CA se aproxima de seu valor de estado estacionário.
Cada fase tem um deslocamento DC diferente, atingindo o pico quando alfa é igual a zero.
As reatâncias da máquina e as constantes de tempo, fornecidas pelos fabricantes ou obtidas a partir de testes, auxiliam na previsão do comportamento do sistema de potência durante as faltas, permitindo o controle de danos e a estabilidade do sistema.

curto-circuito trifásico - máquina síncrona descarregada

Three-Phase Short Circuit&#8212;Unloaded Synchronous Machine

Realizar um teste de curto-circuito trifásico em uma máquina síncrona descarregada ajuda a entender seu impacto no sistema. O oscilograma da corrente de falha CA, com o deslocamento CC removido, revela que a amplitude da forma de onda diminui de um valor inicialmente alto para um nível de estado estável para uma fase da máquina.


 Esse comportamento ocorre devido ao fluxo magnético produzido pelas correntes de armadura de curto-circuito. Inicialmente, essas correntes seguem caminhos de alta relutância, mas eventualmente mudam para caminhos de menor relutância, aumentando assim a indutância da armadura. Essa dinâmica pode ser representada por um circuito resistor-indutor em série (R-L) variável no tempo.


 De acordo com a teoria da máquina padrão, reatâncias específicas, como reatâncias subtransitórias, transitórias e de estado estável, são usadas para calcular a corrente de falha CA instantânea. Esse cálculo é baseado na tensão do terminal de pré-falha de linha para neutro RMS. A corrente de falha subtransitória RMS no momento da falha é determinada pela reatância subtransitória de curto-circuito do eixo direto e pela constante de tempo associada. Com o tempo, a corrente de falha CA RMS se estabiliza em seu valor de estado estável.


 Cada fase da máquina experimenta um deslocamento CC diferente, com o deslocamento de pico ocorrendo quando o ângulo de corrente inicial é zero. As reatâncias e constantes de tempo da máquina, fornecidas pelos fabricantes ou derivadas de testes, são cruciais para prever o comportamento da máquina durante falhas. Essas informações auxiliam no gerenciamento de danos potenciais e na manutenção da estabilidade do sistema durante falhas.


 A compreensão dessas características é essencial para projetar sistemas de energia robustos. Os dados obtidos desses testes permitem que os engenheiros prevejam como o sistema se comportará em condições de falha, facilitando o desenvolvimento de estratégias para controlar os danos e garantir a estabilidade do sistema. Essa abordagem garante que os sistemas de energia possam suportar falhas e continuar operando de forma confiável, mantendo assim a integridade e a segurança das redes elétricas.

Curto-Circuito Trifásico - Máquina Síncrona Descarregada

Um teste de curto-circuito trifásico em uma máquina síncrona sem carga é realizado para estudar seu impacto.
O oscilograma da corrente de falha CA com seu ...

three-phase-short-circuitunloaded-synchronous-machine

Education

JoVE Core

Electrical Engineering

Engenharia Elétrica

Unit 1

Unidade 1

Symmetrical and Unsymmetrical Faults

KEY TERMS AND CONCEPTS

TERMOS E CONCEITOS-CHAVE

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 behavior can be modeled as a time-varying series resistor-inductor, or R-L, circuit.

Standard machine theory uses sub-transient, transient, and steady-state reactances to calculate the instantaneous AC fault current. The calculation starts from the RMS line-to-neutral prefault terminal voltage. The RMS sub-transient fault current at the moment of the fault depends on the direct axis short-circuit sub-transient reactance and its time constant. As time passes, the RMS AC fault current settles to its steady-state value.

Each phase also has a different DC offset. The peak offset occurs when the initial current angle is zero. The reactances and time constants, which may come from the manufacturer or from tests, help predict machine behavior during faults. This information supports damage control and system stability, which are important for designing strong power systems and keeping electrical networks reliable and safe.

Synchronous Machine Fault Current Decay

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

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

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...

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

Study how a three-phase short circuit changes fault current in an unloaded synchronous machine, from sub-transient response to steady-state behavior.

Synchronous Machine Fault Current Decay | 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

Synchronous Machine Fault Current Decay

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