A circuit breaker is a switching device capable of closing, carrying, and interrupting current under normal circuit conditions and closing, carrying, and interrupting current under abnormal circuit conditions within a specified time. Circuit breakers are classified into high-voltage circuit breakers and low-voltage circuit breakers according to their application range. The boundary between high and low voltage is somewhat blurred; generally, those above 3kV are considered high-voltage electrical appliances. Circuit breakers can be used to distribute electrical energy, infrequently start asynchronous motors, and protect power lines and motors. When serious overloads, short circuits, or undervoltage faults occur, they can automatically disconnect the circuit. Their function is equivalent to a combination of a fuse switch and over/under-temperature relays. Moreover, generally, no parts need to be replaced after interrupting a fault current. They have gained widespread application. In the generation, transmission, and use of electricity, power distribution is an extremely important link. The power distribution system includes transformers and various high and low voltage electrical equipment, and low-voltage circuit breakers are a widely used type of electrical appliance.
In power systems, circuit breakers play a crucial role as key electrical protection devices. They can quickly cut off power supply when a circuit fault occurs, protecting the circuit and related equipment from damage. This article will delve into the principles of circuit breakers, including their working principles, types, applications, and future development trends, aiming to provide readers with a more comprehensive and in-depth understanding.
I. Overview of Circuit Breakers
A circuit breaker is a protective electrical device used to connect or disconnect a circuit. When the current in a circuit exceeds its rated value, the circuit breaker can automatically disconnect the circuit to prevent equipment damage and fire accidents. Circuit breakers are widely used in low-voltage power distribution systems and motor protection circuits, and are an indispensable part of the power system.
II. Circuit Breaker Working Principle
The circuit breaker operates on the principle of thermal-magnetic tripping. When the current in the circuit exceeds a set value, the internal thermal element of the circuit breaker heats up and bends, pushing the tripping mechanism to activate and disconnect the circuit. Simultaneously, the circuit breaker is also equipped with a magnetic trip unit. When the short-circuit current in the circuit reaches a set value, the magnetic trip unit quickly activates and disconnects the circuit. This dual protection mechanism ensures that the circuit breaker can quickly and accurately disconnect the power supply in the event of a circuit fault.
III. Circuit Breaker Types
Depending on the application scenario and performance requirements, circuit breakers can be divided into several types. Here are some common types of circuit breakers:
Universal circuit breaker: Suitable for AC 50Hz power distribution networks with rated voltage of 380V or 660V and rated current of 200A to 4000A. It is mainly used to distribute electrical energy and protect lines and power equipment from overload, undervoltage and short circuit, as well as for infrequent switching of lines under normal conditions.
Molded case circuit breakers: can be manually operated (energy storage) or electrically opened and closed. They are mainly used in AC 50Hz power distribution networks with rated voltage up to 690V and rated current from 6A to 1250A. They also have overload, short circuit and undervoltage protection functions to protect lines and electrical equipment from damage caused by overload, undervoltage and short circuit current.
Residual current circuit breaker: In addition to all the protection functions of a molded case circuit breaker, it also has residual current protection function, which is mainly used to prevent electric shock accidents.
IV. Application of Circuit Breakers in Power Systems
Circuit breakers play a crucial role in power systems. Their main applications include the following:
Overload protection: When the load current in the circuit exceeds the rated value of the circuit breaker, the circuit breaker can automatically disconnect the circuit to prevent the equipment from being damaged due to overload.
Short circuit protection: When a short circuit occurs in a circuit, the short circuit current will increase rapidly. The circuit breaker can quickly disconnect the circuit to prevent the short circuit current from damaging the equipment and system.
Undervoltage protection: When the power supply voltage drops to a certain level, the circuit breaker can automatically disconnect the circuit to prevent the equipment from being damaged due to low voltage operation.
Leakage current protection: Leakage current circuit breakers can detect leakage current in the circuit and cut off the circuit when the leakage current reaches a certain value to prevent electric shock accidents.
V. Precautions for the selection and use of circuit breakers
When selecting and using circuit breakers, the following points should be noted:
Select the appropriate circuit breaker type and specifications based on parameters such as the circuit's rated voltage, rated current, and short-circuit capacity.
Ensure that the rated current and short-circuit capacity of the circuit breaker meet the needs of the circuit so that it can operate reliably in the event of a circuit fault.
Pay attention to the installation environment and conditions of the circuit breaker, and avoid its exposure to adverse factors such as humidity, high temperature, and vibration.
Regularly inspect and maintain circuit breakers to ensure they are in good working order.
VI. Development Trends of Circuit Breaker Technology
With the continuous development of power systems, the performance requirements for circuit breakers are becoming increasingly stringent. In the future, circuit breaker technology will develop in the following directions:
Intelligentization: By introducing intelligent control technology and sensor technology, the functions of remote monitoring, fault diagnosis and automatic adjustment of circuit breakers are realized, thereby improving the level of intelligence of the power system.
Modular design: The circuit breaker is designed with a modular structure, which allows users to combine and expand it as needed, improving the flexibility and maintainability of the equipment.
High performance: By optimizing the internal structure and materials of the circuit breaker, its breaking capacity, withstand voltage and short-circuit current resistance are improved, ensuring stable performance even under extreme conditions.
Environmental friendliness: By adopting environmentally friendly materials and energy-saving technologies, energy consumption and emissions during the production and use of circuit breakers are reduced, thereby achieving sustainable development.
The structure of a circuit breaker.
Figure 1 is a structural diagram of a thermal-magnetic circuit breaker. The text on the right side of the figure tells us that the circuit breaker has three main components: the contacts and arc extinguishing system, the operating mechanism, and the trip unit and control unit.
Key points of analysis in Figure 1:
First concept: Contact system
We can see the main contacts of phases A, B, and C, and they are already closed. Note that all three sets of contacts are single-contact systems.
It should be noted that the contacts of a circuit breaker are divided into two groups: the main circuit system and the auxiliary circuit system.
The main circuit refers to the circuit that controls the transmission of electrical energy. Its characteristic is a very high current; depending on the specifications and model of the circuit breaker, the current in the main circuit ranges from 10A to 6300A. See the yellow part in the diagram;
The auxiliary circuit refers to the circuit that performs control and signal transmission. Its characteristic is that it draws a relatively small current, typically below 5A. See the left side of Figure 2. Therefore, the auxiliary circuit does not have an arc-extinguishing device, while the main circuit must. The contacts in the auxiliary circuit are generally called contacts, and are the same as those in a regular relay.
The second concept: Holm repulsion and contact pressure at the contacts.
On the left is the moving and stationary contact system, where we can see the current lines. Note that due to the structure of the contacts, the contact point is actually a single point, so the current lines will slope towards the center.
The middle of Figure 3 shows the distribution of magnetic field lines generated by the current line I1X on the right side of the stationary contact. Using the right-hand screw rule, it is easy to determine that the magnetic field lines on the left side leave the paper and those on the right side enter the paper. Therefore, the entire current line I1s of the moving contact is within the range of magnetic field lines entering the paper.
The right side of Figure 3 shows the electrodynamic analysis of the moving contact current line I1s. Using the left-hand rule, we determine that the electrodynamic force acting on it is F, and the direction of F is perpendicular to the current line I1s, pointing towards the upper left corner. We decompose F into a horizontal component Fx and an upward component Fy. Since the horizontal component is canceled out by the rightward horizontal component generated by the symmetrically distributed current lines on the left side of the contact, there is no force acting on the contact in any direction. However, the upward electrodynamic forces are superimposed, forming an upward repulsive force Fh on the moving contact. The Fh repulsive force is also called the Holm repulsive force. Similarly, the stationary contact experiences a downward Holm repulsive force.
Therefore, when current flows through the contacts, Holmli attempts to repel the combination of moving and stationary contacts.
Note that the contact resistance of the contacts is related to the contact pressure: the greater the contact pressure, the smaller the contact resistance.
Therefore, it can be seen that circuit breakers must apply sufficient contact pressure to the moving and stationary contacts in order to achieve stable and reliable electrical contact.
Let's look at Figure 3 again: Figure 3 shows that by operating the handle, or by using the electric closing mechanism and the closing electromagnet, the moving and stationary contacts that were originally in the open state are closed. After closing, a set of springs applies pressure to the contacts to ensure that the contacts have sufficient contact pressure.
VII. Conclusion
As a critical piece of equipment in power systems, circuit breakers' principles, types, applications, and development trends are key areas that require our attention and research. By gaining a deeper understanding of the principles and working mechanisms of circuit breakers, we can better select and use them, protecting the safe and stable operation of circuits and equipment. Furthermore, with continuous technological advancements and innovations, we can expect future breakthroughs and advancements in circuit breaker performance, intelligence, and environmental friendliness.
