By hvhipot 
HV Hipot Electric Co., Ltd. specializes in producing series resonance (also known as power frequency withstand voltage tester). Next, we will share with you an introduction to the resonance phenomenon.
Capacitive and resistive components in power systems can generally be considered linear parameters, but inductive components are not. Due to the presence of inductance elements with different characteristics in the oscillation circuit, there are three different types of resonance:
(1) Linear resonance
The resonant circuit is composed of inductive elements without iron cores or inductive elements with iron cores that have excitation characteristics close to linearity, and capacitive elements in the system. Under the action of a sine power supply, linear resonance may occur when the system’s natural frequency is equal to or close to the power supply frequency.
(2) Parameter resonance
The resonant circuit is composed of inductive elements and system capacitive elements with periodic changes in inductance parameters. When the parameters are matched appropriately, the periodic variation of inductance continuously delivers energy to the resonant system, which will cause parameter resonance.
(3) Ferromagnetic resonance
The resonant circuit is composed of inductive components with iron cores and capacitive components in the system. Due to the saturation of the iron core, the inductance parameters of the iron core inductance element are nonlinear. This type of circuit containing nonlinear inductance elements will generate ferromagnetic resonance when certain resonance conditions are met.
Due to improper coordination of inductance and capacitance parameters, various long-term resonance phenomena and resonance overvoltage caused by voltage rise occur in the system. Common types include linear resonance overvoltage, ferromagnetic resonance overvoltage, and parametric resonance overvoltage. So why is ferromagnetic resonance the most difficult to prevent, and what are its characteristics and hazards?
Characteristics of ferromagnetic resonance
(1) The occurrence of ferromagnetic resonance requires that the natural frequency composed of the starting value of the iron core inductance and the equivalent capacitance at both ends of the inductance must be less than and close to the resonance frequency.
(2) When the circuit parameters change smoothly, there will be a jump in the resonant voltage and current.
(3) Reverse tilting phenomenon occurs during resonance
(4) The resonant frequency must be composed of the fundamental frequency of the power supply and its simple fractional or integer multiples of the high frequency.
(5) After resonance, it can maintain a stable state on its own.
(6) Resonance is generally generated by external excitation when subjected to sufficiently strong disturbances, and can also be self excited under certain conditions.
When switching or malfunctioning, these inductive and capacitive components may form oscillation circuits with different natural frequencies, causing resonance under the action of an external power series resonance and resulting in ferromagnetic resonance overvoltage. Common ferromagnetic resonances include PT disconnection resonance, overvoltage transmission, ferromagnetic resonance caused by electromagnetic voltage transformers, and ferromagnetic resonance in series capacitor compensation lines. Ferroresonance can not only damage the insulation and burn out electrical equipment, but may also affect the normal operation of overvoltage protection devices. Therefore, ferromagnetic resonance overvoltage is the most common and difficult to prevent resonance overvoltage.