Name: subtransient fault current in power systems
Uploaded: 2024-11-21T14:54:30.000Z
Duration: 1 min 21 s
Channel: JoVE

在故障电流分析中，变压器采用漏电抗进行建模，而输电线路则用等效串联电抗表示。
同步电机被建模为次暂态电抗后的恒定电压源。
该分析排除了绕组和串联电阻、并联导纳、Δ-Y 相位偏移、电枢电阻、饱和效应、凸极性、非旋转阻抗负载以及功率小于 50 马力的小型电机。
考虑一个同步发电机通过两台变压器和一条输电线路为电动机供电的单线图。使用正序等效电路分析三相短路。
同步电机的故障前内部电压由其瞬变电抗后的电压表示。
当开关闭合时，在故障点发生电压降，从而产生故障。
利用叠加法计算故障电流。如果相量与故障前电压匹配，则第二个电路表示故障前的系统状态。
该方法可预测系统在故障期间的行为，确保稳定性并减少损坏。

电力系统三相短路

Power System Three-Phase Short Circuits

确定电力系统中的亚瞬态故障电流需要用漏电抗表示变压器，用等效串联电抗表示输电线，用亚瞬态电抗后面的恒压源表示同步电机。在这种分析中，某些元素被排除在外，例如绕组电阻、串联电阻、并联导纳、Δ-Y 相移、电枢电阻、饱和、凸极效应、非旋转阻抗负载和额定功率低于 50 马力的小型电机，因为它们对亚瞬态故障电流的影响可以忽略不计或使分析复杂化，但对结果没有显著影响。

当发生三相短路时，使用两个具有相同相量值的反向电压源对故障进行建模。应用叠加原理计算故障电流。这涉及使用两个具有相同相量值的反向电压源，可以通过移除相量值来简化系统，从而隔离故障电流分量以便于计算。

确定亚瞬态故障电流的过程需要考虑发电机和电动机的贡献。通过使用初始电路条件并应用适当的电抗，可以准确计算出亚瞬态故障电流。这种方法确保分析考虑到故障对电力系统的直接影响。

该方法对了解电力系统在故障期间的行为提供了重要依据，使工程师能够设计出可以承受此类事件的系统。预测亚瞬态故障电流的能力有助于保持系统稳定性并最大限度地减少潜在损害。了解故障期间不同组件的贡献并准确建模它们的电抗对于有效的故障管理至关重要。

通过关注关键电抗并采用叠加，该分析有助于设计和管理强大的电力系统。这确保了可靠性和安全性，从而可以在故障期间采取有效的缓解策略。预测故障行为并了解其直接影响对于维护电网的完整性至关重要，确保电力系统在故障条件下能够继续可靠运行。

在短路电流分析中，变压器采用漏电抗建模，而输电线路则用等效串联电抗表示。

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

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 much. These include winding resistances, series resistances, shunt admittances, delta-Y phase shifts, armature resistance, saturation, saliency, non-rotating impedance loads, and small motors rated below 50 horsepower. Removing these parts keeps the fault study focused on the most important reactances.

A three-phase short circuit is modeled with two opposing voltage sources that have equal phasor values. Phasors are rotating values used to describe AC voltage and current. The superposition principle is then used to find the fault current by removing the equal phasor values and isolating the fault component.

The calculation also includes the contributions from both the generator and the motor. Using the initial circuit conditions and the correct reactances, the subtransient fault current can be found accurately. This method helps engineers understand the first effects of a fault, predict system behavior, and design power systems that can better handle short-circuit events.

Subtransient Fault Current in Power Systems

subtransient-fault-current-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 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

Learn how transformers, lines, generators, and motors are modeled to calculate subtransient fault current during a three-phase short circuit.

Subtransient Fault Current 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

Subtransient Fault Current in Power Systems

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