I. Definition of Reactors

A reactor, also known as an inductor, is an electronic device whose core function is to generate an inductive effect in a circuit. By hindering the changes in current, reactors can control the voltage and current in a circuit. This kind of equipment is widely used in fields such as power, communication and electronics, playing a crucial role.
In a circuit, the hindrance effect that capacitors and inductors have on alternating current is called reactance, and its symbol is X. A reactor, often referred to as an inductor, works on the principle that when a conductor is electrified, a magnetic field is generated in the space around it. Although all current-carrying conductors possess this inductive capability, the inductance of long straight conductors is relatively small, and the magnetic field strength is limited. Therefore, in practical applications, reactors mostly adopt the design of winding wires into solenoids, that is, air-core reactors. To meet specific requirements, sometimes an iron core is inserted into a solenoid to enhance the inductive performance. This type of reactor is called an iron core reactor. To sum up, reactance can be divided into capacitive reactance and inductive reactance. From a professional perspective, inductors (i.e., inductive reactance devices) and capacitors (i.e., capacitive reactance devices) are collectively referred to as reactors.
Ii. The Role of Capacitive Reactance

Due to the diverse types and different functions of reactors, from the perspective of circuit principles, they can be classified into two major categories: series reactors and shunt reactors. The main functions of these two types of reactors are current limiting and filtering respectively. Next, we will delve into the specific functions of various types of reactors.
The function of shunt reactors: They can effectively change and improve the reactive power operation status of the power system and are often used in reactive power compensation. In short, reactors can improve the voltage distribution on long-distance transmission lines, absorb the charging capacitive reactive power in the cable lines, and prevent self-excitation resonance when generators are connected to long lines.

2. The function of series reactors: They are mainly used to limit short-circuit currents. In addition, in the filter, it can be connected in series or parallel with the capacitor, thereby limiting the higher harmonics in the power grid. Overall, the series reactor serves to limit the current.

3. The function of the DC reactor: It is usually placed between the DC rectification stage and the inverter stage of the frequency conversion system. Its main function is to limit the AC component superimposed on the DC current, ensure the continuity of the rectified current and reduce the current pulse value. Therefore, DC reactors can enhance the reliability of the inverter link and simultaneously improve the power factor of the frequency converter. (For specific functions, please refer to the figure below.)
4. The function of the input reactor: It is mainly used to prevent current surges caused by sudden changes in grid voltage or overvoltage during operation. In addition, when there are spike pulses in the power supply voltage or voltage defects caused by commutation in the bridge rectifier circuit, the input reactor can protect the frequency converter, improve the power factor, reduce the pollution of harmonic current to the power grid, and at the same time effectively reduce power grid interference.

5. The function of the output reactor: It can compensate for the influence of partial capacitance in the line, usually within the range of 50 to 200 meters. In addition, the output reactor can also suppress the output harmonic current, enhance the output high-frequency resistance, and reduce DV/DT to minimize high-frequency leakage, thereby protecting the frequency converter and reducing equipment noise. In the power compensation state, reactors may be impacted by harmonic voltages and currents, leading to capacitor damage and a decrease in power factor. Therefore, harmonic control is necessary.

6. The function of current-limiting reactors: They are mainly used for fault limiting in power systems. When a system fault occurs, they can effectively limit the overcurrent of feeders and ensure the stable operation of the power system.
7. The function of the smoothing reactor: In a rectifier circuit, the smoothing reactor plays a crucial role. It is mainly used to limit short-circuit current and ensure the safety of inverter thyristors during commutation. When the inverter thyristor is on and a short circuit fault occurs in the rectifier bridge load, the reactor can effectively prevent the direct short circuit from happening. In addition, it can also suppress the influence of the intermediate frequency component on the industrial frequency power grid, ensuring the stable operation of the power grid.

8. The function of the inverter reactor: The inverter reactor is usually installed on the input or output terminals of the DC speed regulator of the inverter. Its core function is to suppress the transmission of the third to fifth harmonics generated by the frequency converter to the power grid, thereby reducing the interference of harmonics on other components. At the same time, it can also improve the stability of the power grid, enhance the power factor, and limit abnormal fluctuations in grid voltage and inrush current. In addition, the inverter reactor can also alleviate the capacitive effect on no-load or light-load circuits, reducing the risk of power frequency transient overvoltage.

9. Function of filter reactors: Filter reactors are widely used in high and low voltage filter cabinets and are used in series with filter capacitors. When tuned to a certain resonant frequency range, it can absorb the harmonic current of the corresponding frequency in the power grid and eliminate the damage of high-order harmonics to the main transformer or other electrical equipment. Filter reactors play a significant role in enhancing the power factor of the system and ensuring the safety of the power grid.
In practical applications, reactors are typically made of hollow coils made of non-magnetic materials and can be assembled in vertical, horizontal or trapeze positions as required. When a short-circuit fault occurs in the power grid system, a powerful short-circuit current will be generated. If not restricted, it will seriously affect the stable operation of electrical equipment. Therefore, in order to meet the breaking capacity requirements of circuit breakers, reactors are usually connected in series at the position of the outgoing line circuit breaker to enhance short-circuit impedance and limit short-circuit current, ensuring the safe and stable operation of the power grid system. The working principle of the reactor is shown in the following figure.
Due to the application of reactors, when a short-circuit fault occurs, the voltage drop they generate is relatively large, which helps maintain the stable level of the bus voltage. Reactors can effectively control the fluctuation of bus voltage within a very small range, thereby ensuring that the equipment on those lines that have not failed can operate stably.