After the intermediate frequency power supply is replaced with a new thyristor, do not start it immediately, but carry out system inspection on the intermediate frequency power supply to eliminate the fault. Start the intermediate frequency power supply after confirming that there is no fault in the intermediate frequency power supply. Otherwise, the thyristor will be burned after normal startup and operation for a period of time. The main reasons why inverter thyristors are often burned in the use of intermediate frequency power supply are as follows.
(1) The thermal characteristics of electrical components in the control part are not good, such as poor insulation of pulse transformer, poor contact of thyristor control electrode wiring, etc.
(2) The radiator is reused, or the radiator is installed with too small mesa transistors (relative to the planar thyristor, its mounting surface presents a mesa structure), which causes the center of the radiator mesa to be concave, resulting in poor contact between the radiator mesa and the thyristor mesa. The temperature of thyristor rises due to poor heat dissipation, and its shutdown time extends with the increase of temperature, which eventually leads to the failure of thyristor shutdown, causing inverter overturn and burning thyristor.
When reusing the water-cooled radiator of thyristor, pay special attention to check whether its table surface is smooth and flat, whether there is scale and blockage in the water cavity, and pay special attention to whether the table surface is sunken. If the above conditions occur, it should be replaced.
The table top of the radiator must match the size of the thyristor table top to prevent the thyristor from being damaged by pressure deviation and deflection. The radiator table must have high flatness and smoothness. The roughness of the radiator table should be less than or equal to 1.6mm, and the flatness should be less than or equal to 30mm. During installation, the thyristor table top and radiator table top should be kept clean and free of dirt such as oil dirt.
During installation, keep the table top of thyristor completely parallel to the table top of radiator. During installation, pressure is required to be applied through the center line of components to make the pressure evenly distributed in the whole contact area. During manual installation, a torque wrench should be used. All fastening nuts should be alternately and evenly applied, and the pressure should meet the specified requirements. The correct installation pressure of thyristor is 150~200kgf/cm2.
When pressing a new thyristor, we must pay attention to the pressure balance, otherwise it will cause mechanical damage to the chip inside the thyristor, resulting in a sharp decline in the voltage withstand value of the thyristor, and the phenomenon of burning the thyristor as soon as it is started. If the pressure cannot be reached, the transistor will be burned due to poor heat dissipation.
(3) The internal corrosion of the thyristor radiator (commonly known as the water jacket) is serious or the scale is too thick, and the poor heat conduction leads to the rise of the thyristor temperature. The thyristor shutdown time is extended with the rise of the temperature, which eventually leads to the thyristor cannot be closed, causing the inverter to overturn and burn the thyristor. The method to deal with the scale inside the thyristor radiator is to wash it with hydrochloric acid, but more times of hydrochloric acid washing will corrode the inside of the water jacket, and the water will not circulate inside the water jacket and directly flow out of the water outlet, causing the illusion of good water supply and repeatedly burning the thyristor. At this time, the water jacket should be replaced with a new one.
The method to judge whether the damage of the thyristor is caused by the scaling of the water-cooled jacket is to let the medium frequency power supply run at a value close to the overcurrent value for about 10min. After the shutdown, quickly touch the core of the thyristor with your hand. If you feel hot, you can judge that the scaling has occurred on the wall of the water-cooled jacket.
(4) When the groove connecting wire has poor contact or broken wire, the ignition phenomenon will occur when the power rises to a certain value, affecting the normal operation of the intermediate frequency power supply, resulting in the protection action of the intermediate frequency power supply. Sometimes, transient overvoltage will be generated at both ends of the thyristor due to ignition. If the overvoltage protection action is not timely, the thyristor will be burned out, and the overvoltage and overcurrent protection of the intermediate frequency power supply will act at the same time.
(5) If the working temperature of the thyristor is too high, the anti-interference ability of the grid will be reduced, which will trigger the thyristor by mistake, causing the bridge arm to pass through and burn the thyristor. The oscilloscope can be used to measure whether there is interference signal. If there is, take the following measures: increase the capacitance of the shunt capacitor between the thyristor control electrode and the cathode, which can generally be increased by 0.47-1uf.
(6) When the intermediate frequency power supply thyristor is turned off in reverse, it is damaged due to the high instantaneous reverse burr voltage. For this, the resistance capacitance absorption circuit should be checked. In the main circuit of the intermediate frequency power supply, the instantaneous reverse burr voltage is absorbed by the resistance capacitance absorption circuit. If the resistance or capacitor in the absorption circuit is open and the capacitor is dry and invalid, the instantaneous reverse burr voltage will be too high and the thyristor will be burnt out. In case of power failure, use a multimeter to measure the resistance and capacitance, and judge whether the resistance capacitance absorption circuit is faulty. The loose connection wire of the resistance capacitance absorption circuit will also produce high voltage. After the operating medium frequency power supply is shut down (it is necessary to turn off the main air switch inside the power supply), touch (it may be hot) the non inductive resistance absorbed by the inverter resistance capacitance by hand (the temperature is basically the same under normal conditions). If it is found that the temperature of some non inductive resistance is much higher than that of other non inductive resistance, it indicates that the capacitor in series with the non inductive resistance is leaking electricity.
If the resistance temperature is very low, it indicates that the capacity of the capacitor in series with the resistance is reduced or open circuit; If the resistance burns out, the capacitor connected in series with it has broken down. If there are spare parts, the replacement method can be used to judge whether the resistance capacitance absorption circuit is faulty.
(7) Pulse trigger circuit failure. When the intermediate frequency power supply is running, if the trigger pulse is suddenly lost, the inverter will be open circuit, and the output end of the intermediate frequency power supply will generate high voltage and burn out the thyristor. The loose connecting line of the current transformer will cause the phase change of the synthetic signal of the inverter pulse forming circuit. If there is an early trigger phenomenon, the inverter will fail to convert current and burn the thyristor. Check whether the internal winding of current transformer and intermediate frequency voltage transformer is open circuit, and whether the wire is broken due to the corrosion of fine wire diameter by adverse environmental gases, resulting in the failure of overcurrent and overvoltage protection. Once the current and voltage suddenly change, the transistor will be burned out if it exceeds the rated current and withstand voltage of thyristor. This kind of fault usually occurs in the inverter pulse forming circuit and output circuit, which can be checked by oscilloscope. It may also be that the inverter pulse lead is in poor contact. Check the inverter pulse lead connector.
(8) The selection of fast fuse is inappropriate or of poor quality, which does not play a protective role.
(9) The load of intermediate frequency power supply is open during operation. When the medium frequency power supply is running at high power, if the load is suddenly in an open circuit state, a high voltage will be formed at the output end to burn out the thyristor. For example, the poor contact of the main circuit connector, such as the breaking of the water-cooled cable, causes the sudden disconnection of the load circuit under the high current working condition, so that the smoothing reactor generates a high self induced potential, and makes the inverter and rectifier thyristor breakdown.
(10) If power supply is short circuited to load during operation. When the medium frequency power supply is running at high power, if the load is suddenly in a short-circuit state, there will be a great short-circuit current impact on the thyristor. If the overcurrent protection is too late to act, the thyristor will be burned out.
(11) Reactor failure. The internal ignition of the reactor will cause the interruption of the current at the inverter side, and will also generate high voltage at the inverter input side to burn out the thyristor. In addition, if the reactor is replaced during maintenance, and the inductance and iron core area of the reactor are less than the required value, it will also cause the reactor to lose the current limiting effect due to saturation and burn out the thyristor during high current operation.
(12) Protection circuit failure (protection failure). Whether the thyristor can work safely depends mainly on the protection circuit. If the protection circuit fails, when the intermediate frequency power supply fails, it will endanger the safety of the thyristor. Therefore, when the thyristor is burnt out, it is essential to check the protection circuit.
(13) The quality of thyristor is poor, and it is broken down at the moment when the medium frequency power supply is started, or when the load increases, the electrical parameters of thyristor should be checked to see whether they meet the requirements.