(1) The overvoltage of the breaking transformer is based on the theory of intercepting overvoltage. When the transformer is disconnected, the current in the transformer inductance cannot be abrupt, and the magnetic field energy stored therein oscillates between the transformer magnetizing inductance and the ground capacitance. An overvoltage has occurred.
(2) Overvoltage generated by transformer with load closing In the actual test, the combined no-load transformer has detected several times the overvoltage of the power supply voltage. The physical principle is that the no-load transformer can still be equivalent to a magnetizing inductance and Parallel connection of the equivalent capacitance of the transformer itself. If the neutral point of the transformer is not grounded, the switch is aperiodic closing (one phase or two phase first), due to the inductance of the feeder, the capacitance of the transformer to the ground, the longitudinal capacitance and the inductance of the transformer. As a result, a higher overvoltage is generated, especially in the transformer neutral point overvoltage. Although the transformer is basically loaded with a load, the transformer will also generate an overvoltage when it is loaded with a load, but it is smaller when it is relatively empty. There is a relatively large capacitance in the real load, because the storage of the capacitor does not suddenly increase, and the transmission cable has a distributed capacitance to the ground when transmitting a high frequency oscillating voltage, and these capacitors have an absorption effect on the overvoltage. The combination of the two causes the overvoltage of the transformer during the closing process to be suppressed, but sometimes the value is still high and may even be higher than the withstand voltage of the component, which is very dangerous.
(3) The commutation overvoltage rectifying element of the rectifying element is high in steering, and therefore the steering overvoltage is also high. This not only damages components, but also creates electromagnetic interference.
Frequency converter overvoltage processing method
(1) For the breaking overvoltage of the phase shifting transformer of the frequency converter, the overvoltage absorption circuit is formed by the RC absorption network and the zinc oxide arrester, and good results are obtained.
(2) For the overvoltage generated by the transformer with load closing, a switch with good cycle performance can be selected (the switch will have different periods after long-term operation); a good resistance-capacitance absorption circuit or active suppressor technical solution is adopted; The shielded transformer can also effectively suppress the closing overvoltage. However, the difficulty of making a high-power transformer in the formation of an electrostatic shielding layer will be considerable.
(3) For the overvoltage generated by the commutation of the rectifying element, the point of attention is: the reverse withstand voltage of the rectifying element is sufficient, and the second is that the absorption circuit and the freewheeling circuit must be properly controlled. Otherwise, the rectifier device may be broken down by an overvoltage. (4) Since the overvoltage during the operation of the inverter is basically generated when the transformer is opened, it is necessary to start from the transformer to find a way to suppress the overvoltage of the inverter. Can be used:
1 Increase the magnetizing inductance of the transformer and the capacitance to the ground, increase the excitation inductance to reduce the no-load current, which will increase the cost of the transformer.
2 increase the capacitance of the transformer to the ground: in principle, it is easy to analyze, but in fact, due to the structure and material limitations of the transformer itself, it is impossible to make a transformer with any insulation method or high insulation level, so it is necessary to increase it greatly. The capacitance to ground C of the transformer is also quite difficult.