Overvoltage limitation of the hottest vacuum circu

Overvoltage limit for vacuum circuit breaker to disconnect the no-load transformer

cut off overvoltage refers to the phenomenon that the current is forcibly disconnected before reaching the natural zero point due to the strong breaking capacity of the vacuum switch when the vacuum switch disconnects the small inductive current of the no-load transformer. This is mainly caused by the insufficient and stable metal vapor provided by the cathode spot when the arc current is small. Vacuum arc is mainly formed by metal evaporation caused by the heating of electrode at the moment of breaking. The arc makes some spots appear on the electrode surface, and the metal on these spots will continuously melt and evaporate to maintain the vacuum arc. When breaking a large current, the metal vapor is fully evaporated, and the arc is relatively stable. The arc is broken when the power frequency current naturally crosses zero. The test shows that when breaking hundreds to thousands of amps of AC current, the current cut-off phenomenon generally does not occur. When breaking small current, such as excitation current of no-load transformer (generally only 0.6% - 2% of rated current). Because the diffusion speed of the arc column is too fast, the pressure and temperature of metal vapor near the cathode spot drop sharply, so that the evaporation of metal particles can not maintain the diffusion of the arc column, resulting in forced arc extinction or current interception when the current reaches a certain instantaneous value ij before the point of zero fixture II. C is the equivalent capacitance of transformer to ground, and l is the excitation inductance of transformer. Assuming that the current cut-off occurs when I = ij, i.e. ij = imsina, due to the current cut-off, the current change rate di/dt in the circuit is very large, and the voltage drop U1 = LDI/DT on the inductance is very large, forming an overvoltage. From the perspective of energy, at the moment of current cutoff, the magnetic field energy li2j/2 is stored in the winding resistance, and the electric field energy cu20/2 is stored in the capacitor. These stored energy oscillates in the L-C circuit, and the oscillation frequency f0 = 1/2 π LC. Because the value of C is generally very small, when all the stored energy is converted into electric field energy, a very high overvoltage will appear in the capacitor C, that is, the cut-off voltage

2. Cut off the reignition in the flowing voltage without load

in fact, when switching off the no-load transformer, the contact separation distance is very short, the dielectric strength between the contacts is very limited, and gradually increases at a certain speed, while the recovery voltage increases rapidly due to high-frequency oscillation. When the recovery field strength is equal to the dielectric strength, the first reignition occurs. The charge on C is discharged at high frequency through the C ~ K ~ CS circuit, and its stored energy is quickly consumed, and the voltage on C is rapidly reduced to the power supply voltage U0. When the arc is broken again, part of the energy has been consumed during the high-frequency discharge time, so the inductive current and its total magnetic field energy at this time are smaller than the original, and the amplitude of the second recovery voltage is also smaller than the first time. Next, the multiple reignition of the arc makes the energy storage in the inductor with multistage seals smaller and smaller, limiting the maximum possible overvoltage, which is also confirmed by the actual measurement

when the no-load transformer opens from one side and causes overvoltage, because the main magnetic flux passes through the whole iron core, other windings will also induce overvoltage of the same multiple according to the transformation ratio relationship. Since the insulation margin of the high-voltage winding is smaller than that of the medium and low-voltage side, attention should be paid to the threat of the over-voltage to the high-voltage winding due to the impact of the APEC meeting

generally, the overvoltage of oil immersed transformer is not very high, but its electrical strength is high and its stray capacitance is large. Usually, overvoltage will not cause great harm; More attention should be paid to the adoption of new technologies and new modes to transform and upgrade traditional industries. However, for dry-type transformers with low impact strength or arc furnace transformers that need frequent operation, high harmful overvoltage will be generated, which should be specially protected. Generally, the smaller the transformer capacity is, the larger the characteristic impedance Z is, and the higher the overvoltage is