What causes the frequent fusing of high-voltage fuses? This question has been haunting us, and there is no answer. We almost all encounter this problem when using fuses. Today let’s talk about it.
Any loop of the power system can be simplified into a series-parallel loop of resistance R, inductance wL, and capacitive reactance 1/wC. Regardless of whether it is a series or parallel circuit, when the capacitive reactance 1/wC and the inductive reactance wL are equal, the circuit will resonate. The inductance and capacitance elements in the loop will generate overvoltage and overcurrent. At this time, the electric field energy (capacitance) and the magnetic field energy exchange reach the maximum value. In the high-voltage circuit, due to the distributed capacitance of electrical equipment such as lines to the ground, coupled with the existence of non-linear ferromagnetic element inductance such as voltage transformers, high-voltage wall bushings have the necessary conditions to form resonance. Once the system voltage is disturbed, it may excite resonance, due to the nonlinearity of ferromagnetic elements (for example, the inductive reactance will become smaller when the iron core is saturated), this resonance will further increase, when wL=1/wC appears, this resonance is called ferromagnetic resonance. The ferromagnetic resonance generates a very high overvoltage to the ground, which may be several to several tens of times the rated voltage, It causes the porcelain insulation to discharge, the iron parts of the high-voltage load switch, bushing, etc. appear corona, and the fuse of the voltage transformer is blown, which will damage the equipment in serious cases.
The specific reasons for ferromagnetic resonance in actual operation may include the following aspects:
①Single-phase grounding, single-phase disconnection or trip occurs in the neutral point ungrounded system, and the three-phase load is seriously asymmetrical.
② It is related to the saturation of the iron core of the voltage transformer. When using a neutral-grounded voltage transformer in a neutral-point ungrounded system, if its iron core saturates prematurely, it is easier to produce ferromagnetic resonance.
③During the switching operation, the operation mode happens to constitute a resonance condition, such as when the three-phase circuit breaker is divided into different stages, it will cause voltage and current fluctuations and cause ferromagnetic resonance.