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

对未加载的同步电机进行三相短路试验，以研究其影响。
去除直流偏移后的交流故障电流波形图，对应于空载同步电机的一相，显示波形幅值从初始较高值逐渐衰减至稳态值。
这是由于电枢短路电流引起的磁通从高磁阻路径转移到低磁阻路径，导致电枢电感增加。该现象可用一个随时间变化的串联RL电路进行建模。
标准电机理论提供了用于计算瞬时交流短路电流的特定电抗，该电流由故障前线对中性点电压的有效值（RMS）组成。
在时间等于零时，RMS次暂态故障电流由直轴短路次暂态时间常数确定；随着时间推移，RMS交流故障电流逐渐接近其稳态值。
每个相位具有不同的直流偏移，当 alpha 等于零时达到峰值。
由制造商提供或通过测试获得的电机电抗和时间常数，有助于预测电力系统在故障期间的行为，从而实现损害控制和系统稳定性。

三相短路 - 空载同步电机

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

对空载同步电机进行三相短路测试有助于了解其对系统的影响。交流故障电流的波形图（消除了直流偏移）显示，电机某一相的波形幅度从最初的高值下降到稳定状态水平。

这种现象是由于短路电枢电流产生的磁通而引起的。最初，这些电流沿着高磁阻路径流动，但最终转向低磁阻路径，从而增加了电枢电感。这种动态可以用时变串联电阻电感 (R-L) 电路来表示。

根据标准电机理论，特定电抗（例如亚瞬态、瞬态和稳态电抗）用于计算瞬时交流故障电流。此计算基于线路对中性点故障前端电压有效值。故障时刻的亚瞬态故障电流有效值由直轴短路亚瞬态电抗和相关时间常数决定。随着时间的推移，交流故障电流有效值稳定到其稳态值。

电机的每个相都会经历不同的直流偏移，峰值偏移发生在初始电流角为零时。由制造商提供或通过测试得出的电机的电抗和时间常数对于预测电机在故障期间的行为至关重要。这些信息有助于管理潜在损坏并在故障期间保持系统稳定性。

了解这些特性对于设计强大的电力系统至关重要。从这些测试中获得的数据使工程师能够预测系统在故障条件下的行为方式，从而有助于制定控制损害和确保系统稳定性的策略。这种方法确保电力系统能够承受故障并继续可靠运行，从而保持电网的完整性和安全性。

对空载同步电机进行三相短路试验以研究其影响。  
交流故障电流的示波图及其直流分量……

three-phase-short-circuitunloaded-synchronous-machine

Education

教育

JoVE Core

Electrical Engineering

电气工程

Unit 1

第一单元

Symmetrical and Unsymmetrical Faults

对称与不对称故障

KEY TERMS AND CONCEPTS

关键术语与概念

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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