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

A circuit breaker interrupts fault currents by extinguishing the arc, which is elongated and cooled to stop the flow of electricity.
Breakers are classified based on their operating voltage and the medium used to extinguish the arc.
High-voltage breakers automatically reclose 15 to 50 cycles post-interruption and lock out if the fault persists, requiring operator intervention.
Low-voltage applications use molded-case circuit breakers with magnetic trips for large fault currents and thermal trips for smaller, sustained overloads.
The E/X simplified method aids breaker selection by calculating the maximum symmetrical short-circuit current using pre-fault voltage and system reactance, neglecting other factors.
For generators, two-cycle breakers are selected based on sub-transient fault current, calculated using sub-transient reactances. For synchronous motors, sub-transient or transient reactances apply, depending on breaker speed.
Fuses, overcurrent devices with a fusible link in a sand-filled tube, act as conductors under normal operation. Overload current raises the link temperature, causing it to melt and form an arc.

circuit breaker and fuse selection

Circuit Breaker and Fuse Selection

A circuit breaker is a device engineered to interrupt fault currents and sometimes reclose automatically. When a fault current is detected, the breaker separates the electrical contacts, which generates an arc. This arc is extinguished by methods such as elongation, cooling, or splitting, depending on the breaker&#39;s design. Breakers are categorized based on the voltage they operate at and the medium used for arc extinction, such as air, oil, SF6 gas, or vacuum.
In high-voltage systems, circuit breakers typically feature automatic reclosing mechanisms. These breakers reclose within 15 to 50 cycles after an interruption. If the fault persists, they lock out, necessitating manual intervention by an operator. In low-voltage systems, dual-trip molded case breakers are common. These breakers utilize magnetic instantaneous trips to handle large faults and thermal trips to handle smaller faults.
The E/X method simplifies breaker selection by calculating the maximum symmetrical short-circuit current. This approach uses the prefault voltage and system reactance characteristics, ignoring other factors. For generators, two-cycle breakers are used to calculate subtransient fault current, relying on subtransient machine reactances. For synchronous motors, subtransient or transient reactances are considered based on the breaker&#39;s operating speed.
Fuses serve as overcurrent protection devices, containing a fusible link within a sand-filled tube. Under normal conditions, the fusible link conducts electricity. When an overload current occurs, the link&#39;s temperature rises, causing it to melt and form an arc. Fuses are specified based on four key factors: current rating, voltage rating, interrupting rating, and time-delay characteristics. Fuses must be replaced manually once they have melted.
Both circuit breakers and fuses play critical roles in protecting electrical systems from fault currents and overcurrent conditions. Circuit breakers, with their ability to automatically reclose and handle different types of faults, are essential in high and low-voltage applications. Fuses, while simpler and requiring manual replacement, provide reliable protection by interrupting overcurrents. Understanding the specific requirements and characteristics of the electrical system is crucial in selecting the appropriate protective devices to ensure safety and operational continuity.

A circuit breaker interrupts fault currents by extinguishing the arc, which is elongated and cooled to stop the flow of electricity.
Breakers are ...

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JoVE Core

Electrical Engineering

Unit 1

Symmetrical and Unsymmetrical Faults

KEY TERMS AND CONCEPTS

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 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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