When an oscillating system is subjected to periodic external force, the amplitude of the oscillation increases sharply when the frequency of the external force is the same as or very close to the natural oscillation frequency of the system. The frequency at which resonance occurs is called "resonance frequency". In electrical engineering, the resonance phenomenon of an oscillating circuit. The resonance of an inductor and a capacitor in series is called "series resonance" or "voltage resonance"; the resonance of a parallel circuit is called "parallel resonance" or "current resonance".

How to judge whether it is in a resonant state, there are several ways to judge whether the circuit is in a resonant state:
1. Check the circuit components: First, check the components in the circuit, especially the inductor (L) and the capacitor (C). Resonance usually occurs in circuits containing these components.
2. Calculate the resonant frequency: For an LC circuit, its resonant frequency can be calculated by the formula. If the signal frequency in the circuit is close to this calculated resonant frequency, then the circuit is likely to resonate.
3. Observe the voltage and current waveforms: When resonance occurs, the voltage and current waveforms will show a specific shape. Usually, the voltage and current will reach the maximum value at the same time, and the waveform will show a sine wave.
4. Measure the impedance: At the resonant frequency, the impedance of the circuit will reach the minimum. By measuring the impedance of the circuit, you can determine whether the circuit is in a resonant state.
5. Use an oscilloscope: By observing the voltage and current waveforms in the circuit through an oscilloscope, you can more intuitively determine whether the circuit is resonant.
6. Change the values of L and C: In the experiment, you can change the values of inductance (L) and capacitance (C) and measure the current, voltage and other parameters in the circuit at the same time. When the maximum value is reached, it is the resonance point.
7. Determine whether the reactance of the capacitor and inductor is equal: In theory, you can determine whether the circuit is in a resonant state by determining whether the reactance of the capacitor and inductor is equal.
8. Determine whether the waveforms of the loop current and voltage are in phase: In the experiment, you can determine whether the circuit is in a resonant state by determining whether the waveforms of the loop current and voltage are in phase. This can be achieved by using a dual-trace oscilloscope.

In short, determining whether the circuit is in a resonant state requires a combination of theoretical knowledge and practical experience. By mastering the above steps and methods, we can more accurately determine whether the circuit is in a resonant state, and thus take corresponding measures to optimize the circuit performance.

As the requirements for power quality become more standardized, reactive power compensation and AHF (active harmonic filter) are more and more widely installed and used. However, because there is no reactor configured in the early capacitor cabinet, the system is prone to resonance after the installation of AHF, so we usually recommend that no matter what kind of switch the capacitor chooses, it is necessary to add a reactor.