What are the functions of series and parallel reactors?
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A reactor is an inductive component, mainly used for limiting short-circuit current, reactive power compensation, and so on. In the power system, reactors can be connected in series or in parallel to the circuit, and their functions are somewhat different. The following is a detailed introduction:
### Functions of Series Reactors - **Limiting Short-Circuit Current**: When a short-circuit fault occurs in the power system, the short-circuit current will increase sharply, which may cause serious damage to electrical equipment. Series reactors can increase the reactance value of the circuit, thereby limiting the magnitude of the short-circuit current and keeping it within the tolerance range of electrical equipment, thus playing a role in protecting the equipment.
- **Suppressing Harmonic Currents**: With the wide application of power electronic equipment, more and more harmonic currents are generated in the power system. Harmonic currents will lead to distortion of the voltage waveform and affect the power quality of the power system. A filter composed of series reactors and capacitors can generate a relatively large reactance to the harmonic currents of specific frequencies, thereby suppressing the passage of harmonic currents and improving the voltage waveform.
- **Limiting Inrush Currents**: At the moment when transformers, motors and other equipment are switched on, relatively large inrush currents may be generated. Inrush currents will impact the insulation of the equipment and affect its service life. Series reactors can limit the magnitude and rising speed of inrush currents and protect the equipment from the damage caused by inrush currents.
- **Phase-Shifting Function**: In some special power systems, such as high-voltage transmission lines, series reactors can be used to achieve the phase-shifting function. By adjusting the reactance value of the reactor, the voltage phase of the line can be changed, thus realizing the control and optimization of the power flow in the power system.
### Functions of Parallel Reactors - **Reactive Power Compensation**: Many devices in the power system, such as motors and transformers, need to consume reactive power during operation. The lack of reactive power will lead to a decrease in the system voltage and affect the power quality. Parallel reactors can absorb the inductive reactive power in the system, play a role in reactive power compensation, improve the power factor of the system, and stabilize the system voltage.
- **Limiting Overvoltage**: In the power system, overvoltage may be generated due to switching operations, faults and other reasons. Overvoltage will damage the insulation of electrical equipment and threaten the safe operation of the equipment. Parallel reactors can limit the amplitude and rising speed of overvoltage by absorbing the excess charges in the system and protect the equipment from being damaged by overvoltage.
- **Reducing System Losses**: When there are a large number of inductive loads in the system, the reactive current of the system will increase, which will increase the losses of the system. Parallel reactors reduce the flow of reactive current in the system by compensating reactive power, reduce the losses of the system, and improve the operation efficiency of the system.
- **Improving System Stability**: In the power system, parallel reactors can improve the voltage stability and power angle stability of the system by adjusting the distribution of reactive power in the system. When the system fails or is disturbed, parallel reactors can respond quickly, provide necessary reactive support, and help the system restore stable operation. The selection of series and parallel reactors needs to consider multiple factors comprehensively. The following are the specific selection methods:
### Selection Methods for Series Reactors
1. **Determining Rated Voltage**: Determine the rated voltage of the series reactor according to the rated voltage level of the power system to which it is connected. Ensure that its rated voltage matches the system voltage to ensure that the reactor can work normally and its insulation performance meets the requirements.
2. **Calculating Reactance Rate**: The reactance rate is an important parameter of the series reactor, which is closely related to the harmonic situation of the system. It is necessary to calculate an appropriate reactance rate according to the possible harmonic orders and contents in the system. Generally speaking, for the 5th harmonic, a reactance rate of 4.5% - 7% can be selected; for the 3rd harmonic, a reactance rate of 12% - 13% is often adopted.
3. **Calculating Rated Current**: Determine the rated current of the series reactor according to the maximum operating current of the system. The rated current should be greater than the maximum long-term working current that may occur in the system. Meanwhile, the overload capacity of the system and the future development margin should be considered. Generally, it is recommended to select the rated current at about 1.3 times the maximum operating current of the system.
4. **Considering Dynamic and Thermal Stability Performance**: Series reactors will bear a relatively large current impact when a short-circuit fault occurs in the system, so they need to have sufficient dynamic and thermal stability performance. It is necessary to check whether the dynamic and thermal stability current of the reactor meets the requirements according to the short-circuit current level of the system to ensure that the reactor will not fail due to overheating or mechanical damage when a short-circuit fault occurs.
5. **Selecting Appropriate Insulation Level**: The insulation level is directly related to the safe operation and service life of the reactor. Select an appropriate insulation grade according to the insulation requirements and operating environment of the system. For example, dry-type reactors usually adopt F-level or H-level insulation, and the insulation level of oil-immersed reactors should also meet the corresponding standards.
6. **Paying Attention to Losses and Efficiency**: The losses of series reactors of different models and manufacturers may vary. Products with lower losses should be selected to improve the operation efficiency and economy of the system. Meanwhile, the heat dissipation conditions of the reactors should be considered to ensure that the heat generated by losses can be dissipated in a timely manner during long-term operation.
7. **Considering Installation Environment**: Select an appropriate type of reactor and protection level according to the installation location and environmental conditions of the reactor. For example, reactors installed outdoors need to have good waterproof, dustproof and sun-proof properties; for reactors installed indoors, factors such as space limitations and ventilation conditions should be considered.
Selection Methods for Parallel Reactors
1. **Defining Rated Capacity**: Determine the rated capacity of the parallel reactor according to the reactive power compensation requirements of the power system. It is necessary to calculate the reactive power shortage of the system, combine the operation mode and load characteristics of the system, and select an appropriate rated capacity to achieve effective compensation for the reactive power of the system, improve the power factor and voltage stability of the system.
2. **Determining Rated Voltage**: Similar to the series reactor, the rated voltage of the parallel reactor should also match the rated voltage level of the power system to which it is connected to ensure that the reactor can withstand the system voltage during normal operation and also have a certain insulation margin under overvoltage conditions.
3. **Calculating Reactance Value**: Determine the required reactance value of the parallel reactor through power system calculations according to parameters such as the short-circuit capacity, line length, and operating voltage of the system. An appropriate reactance value helps optimize the distribution of reactive power in the system, reduce line losses, and improve the stability and transmission capacity of the system.
4. **Considering Overload Capacity**: Parallel reactors may encounter load fluctuations or special operating conditions during the operation of the system, so they need to have a certain overload capacity. Generally, it is recommended to select reactors with an overload capacity of more than 1.3 times the rated capacity to cope with possible short-term overload situations.
5. **Checking Dynamic and Thermal Stability**: Although the current of parallel reactors is relatively small during normal operation, they may also bear a relatively large current impact when the system fails or is in abnormal situations. Therefore, it is necessary to check the dynamic and thermal stability performance of the reactors to ensure that they can operate safely and reliably under fault conditions such as short circuits.
6. **Selecting Insulation Type and Protection Level**: Select an appropriate insulation type and protection level according to the installation position and operating environment of the reactor. Dry-type parallel reactors have the advantages of convenient maintenance and no oil pollution, and are suitable for indoor or places with high environmental requirements; oil-immersed reactors have good insulation performance and strong heat dissipation performance, and are often used in outdoor or systems with high insulation requirements.
7. **Considering Harmonic Characteristics**: If there are harmonic currents in the system, it is necessary to consider the amplification or suppression effect of parallel reactors on harmonics. Select reactors with appropriate harmonic characteristics to avoid harmonic amplification caused by resonance between the reactor and the system and affecting the power quality of the system.
8. **Evaluating Control Methods**: According to the automation degree and control requirements of the system, select parallel reactors with appropriate control methods. Some reactors are equipped with intelligent control devices that can realize functions such as automatic switching and reactive power regulation, improving the operation flexibility and automation level of the system.
