How to suppress high-order harmonics in the power grid after the reactor is connected in series with a capacitor?
Publish Time: 2025-08-28
In modern industrial power systems, with the widespread use of nonlinear devices such as inverters, rectifiers, and switching power supplies, high-order harmonics in the power grid are becoming increasingly prominent. These harmonic currents not only distort the voltage waveform and degrade power quality, but can also cause equipment overheating, false protection trips, and even frequent damage to capacitor banks. In low-voltage reactive power compensation cabinets, simply connecting capacitors in parallel can improve the power factor, but this can create parallel resonance with the system inductance, amplifying harmonics of specific frequencies and causing more serious consequences. To address this risk, reactors—series inductors—are introduced in series with capacitors, forming a composite branch that combines reactive power compensation and harmonic suppression, becoming a key measure for ensuring safe system operation.The core mechanism by which reactors suppress high-order harmonics lies in the positive correlation between their inductive reactance and frequency. A reactor is essentially a coil with a high-permeability iron core, and its inductive reactance increases with increasing current frequency. When connected in series with the capacitor, the entire branch forms an LC series circuit, whose impedance characteristics change with frequency. At power frequency, the capacitive reactance of the capacitor is much greater than the inductive reactance of the reactor, resulting in a capacitive compensation branch that can provide reactive power to the system and improve the power factor. However, when higher harmonic currents attempt to flow into this branch, the situation reverses: the harmonic frequency is much higher than the power frequency, the inductive reactance of the reactor increases significantly, while the capacitive reactance of the capacitor decreases significantly. At a specific frequency, the inductive and capacitive reactances may reach equilibrium, resulting in an extremely low impedance in the series branch, creating a "low-resistance path" for harmonics of that frequency. This diverts significant harmonic currents into this branch, preventing them from flowing into the main grid or main transformer.This design is not accidental; it is based on in-depth analysis of power grid harmonic characteristics. In most industrial scenarios, the fifth and seventh harmonics are the primary components. The reactance of the reactor is carefully selected so that, when combined with the capacitor, the resonant frequency of the series branch is slightly lower than the target harmonic frequency, ensuring an inductive impedance at that harmonic point and effectively absorbing harmonic energy. The absorbed harmonic current circulates between the reactor and capacitor, dissipating primarily as heat in the reactor. This reduces the harmonic content injected into the upstream grid and improves the sinusoidality of the overall voltage waveform.Furthermore, the series reactor changes the overall impedance characteristics of the compensation branch, preventing dangerous parallel resonance with the system impedance. Without a reactor, the capacitor may form parallel resonance with the transformer or line inductance at a certain harmonic frequency, causing severe amplification of that harmonic, leading to severe voltage distortion and even equipment insulation breakdown. With the addition of a reactor, the compensation branch becomes inductive at the harmonic frequency, disrupting the resonance conditions and fundamentally eliminating this potential risk.In addition to harmonic mitigation, this combination provides multiple additional protections. At the moment a capacitor is switched on, the difference between the system voltage and the capacitor's residual voltage generates a significant inrush current, potentially damaging contactor contacts or causing fuse malfunction. The reactor, by its ability to block sudden current changes, effectively limits the amplitude and rate of rise of this transient current, ensuring smooth switching. Similarly, transient overvoltages generated during operation are suppressed by the reactor, protecting the insulation life of the capacitors.Ultimately, the series combination of reactors and capacitors represents a masterful art of electrical balancing. While meeting reactive power compensation requirements, it proactively manages harmonics, transforming a potential "harmonic amplifier" into a "harmonic absorber." Rather than relying on complex electronic control, it utilizes fundamental electromagnetic principles to provide a silent and robust defense for modern power grids, safeguarding the safe operation of main transformers, cables, and precision equipment.
