There are three main aspects to consider when selecting a reactor: the selection of the reactor's reactance K value, the reactor structure (air core, iron core), and the reactor's installation location (power supply side, neutral point side).
I. Determination of the reactance rate K value of the reactor:
1. If the harmonic content in the system is very low and only the limiting of inrush current is considered, then K=(0.5~1)% can meet the standard requirements. However, this type of reactor has a severe amplification of the 5th harmonic current and a slight amplification of the 3rd harmonic current.
2. If the harmonics present in the system are not negligible, the background harmonic content of the power supply system should be determined before reasonably determining the K value. To suppress harmonics, the reactance should be configured such that the overall harmonic impedance at the capacitor connection point is inductive.
When the background harmonics of the power grid in the system are 5th order or higher, the reactance should be configured to be (4.5~6)%. Generally, the 5th harmonic is the most significant, followed by the 7th, and the 3rd is the least significant. Therefore, in engineering, reactors with K=4.5%~6% are commonly used, and this is also the international standard.
A 6% reactor effectively suppresses the 5th harmonic, but it significantly amplifies the 3rd harmonic. Its resonant point (204Hz) is far from the frequency of the 5th harmonic (250Hz), providing a large margin.
A 4.5% reactor provides only slight amplification of the third harmonic, making it suitable for suppressing fifth and higher harmonics while minimizing the amplification of the third harmonic. However, its resonant point (235Hz) has a relatively small frequency gap with the fifth harmonic.
When the background harmonics in the system are third order or higher, a reactor with a reactance of 12% should be configured. Due to the increasing number of electrical devices with third harmonic sources in recent years, the third harmonics in the system are constantly increasing, a phenomenon that cannot be ignored, especially in the metallurgical industry.
In summary, the principle for configuring reactors is that the configuration must be determined based on a comprehensive consideration of the system's background harmonic content.
II. Reactor Structure Selection:
Reactors mainly come in two structural forms: air core and iron core.
The main advantages of iron-core reactors are low loss, good electromagnetic compatibility, and small size. The disadvantages are noise and loss of current-limiting ability due to core saturation under high fault current. Dry-type iron-core reactors using resin-coated coils exhibit excellent dynamic and thermal stability, making them suitable for cabinet installation. Oil-immersed iron-core reactors, although larger, are quieter, have better heat dissipation, are easier to install, and are suitable for outdoor use.
The main advantages of air-core reactors are: good linearity, strong ability to limit short-circuit current, and low noise. The disadvantages are: high losses and large size. These reactors are suitable for both indoor and outdoor use, but not for cabinet installation. Outdoor installation makes it easier to prevent electromagnetic induction. The best arrangement is a three-phase "品" or "一" (three-dimensional) pattern. This spacing between phases helps prevent phase-to-phase short circuits and reduces the scope of an accident. Therefore, this arrangement is the preferred choice. When space constraints prevent a three-phase arrangement, stacked products can be used. In three-phase stacked products, the B-phase coil winding direction is reversed, causing the support insulation to bear pressure; therefore, installation must be performed according to the manufacturer's specifications.
III. Installation location of the reactor:
Whether a series reactor is installed on the power supply side or the neutral point side of a capacitor, it serves the same purpose in limiting inrush current and suppressing harmonics.
When a reactor is installed on the power supply side, the operating conditions are harsh. It is subjected to the impact of short-circuit currents, and the reactor's voltage to ground is high (relative to the neutral point side). Therefore, high dynamic and thermal stability requirements are necessary. Based on these requirements, epoxy-encapsulated air-core reactors are more suitable, while iron-core reactors pose a risk of core saturation.
When the reactor is installed on the neutral point side, the requirements for the reactor are relatively low, and it is generally not affected by short-circuit current. Therefore, there are no special requirements for dynamic and thermal stability, and the voltage to ground that the reactor withstands is low, so air-core, iron-core dry-type, and iron-core oil-immersed types can all be used.
A reactor installed on the neutral point side lacks the ability to withstand short-circuit current surges compared to one installed on the power supply side.
