Fault Phenomenon

The inverter thyristor of the medium frequency power supply often burns out during use. After replacing with a new thyristor, the power supply works normally when powered on, but the thyristor burns out after working for a period of time.

Fault Analysis and Troubleshooting

After replacing the thyristor of a medium frequency power supply, do not power it on immediately. You should conduct a systematic inspection of the medium frequency power supply to eliminate faults, and start the medium frequency power supply only after confirming that there is no fault. Otherwise, the phenomenon that the power supply works normally when powered on but the thyristor burns out after a period of operation will occur. The main reasons for frequent burning of inverter thyristors during the use of medium frequency power supply are as follows.

(1) Electrical components in the control section have poor thermal characteristics, such as poor insulation of pulse transformers and poor contact of thyristor control electrode wiring, etc.

(2) Repeated use of heat sinks, or the heat sink is matched with a thyristor with a smaller mesa (compared with planar thyristors, its mounting surface presents a mesa structure), will cause the central depression of the heat sink mesa, resulting in poor contact between the heat sink mesa and the thyristor mesa. The temperature of the thyristor rises due to poor heat dissipation, and its turn-off time prolongs with the increase of temperature, which eventually leads to the failure of the thyristor to turn off, causes inverter commutation failure and burns out the thyristor.

When reusing the water-cooled heat sink of thyristors, special attention should be paid to checking whether the mesa is smooth and flat, whether there is scale and blockage in the water cavity, and especially whether the mesa is sunken. If any of the above situations occur, the heat sink shall be replaced.

The mesa of the heat sink must match the mesa size of the thyristor to prevent offset and skewed pressing, which will damage the thyristor. The heat sink mesa must have high flatness and finish. The roughness of the heat sink mesa shall be less than or equal to 1.6mm, and the flatness shall be less than or equal to 30mm. During installation, the thyristor mesa and the heat sink mesa shall be kept clean and free of oil and other dirt.

Keep the mesas of the thyristor and the heat sink completely parallel during installation. During the installation process, pressure shall be applied through the center line of the component to make the pressure evenly distributed over the entire contact area. For manual installation, a torque wrench shall be used. Apply uniform force alternately to all tightening nuts, and the pressure shall meet the specified requirements. The correct installation pressure of the thyristor is 150~200kgf/cm2.

When pressing a new thyristor, be sure to maintain uniform pressure, otherwise it will cause mechanical damage to the internal chip of the thyristor, leading to a significant drop in the voltage withstand value of the thyristor, and the thyristor will burn out immediately after power-on. If the pressure does not meet the requirement, the thyristor will burn out due to poor heat dissipation.

(3) When the inside of the thyristor heat sink (commonly known as water jacket) is severely corroded or the scale is too thick, poor thermal conductivity will cause the temperature of the thyristor to rise. The turn-off time of the thyristor prolongs with the increase of temperature, which eventually leads to the failure of the thyristor to turn off, causes inverter commutation failure and burns out the thyristor. The method to remove internal scale of the thyristor heat sink is flushing with hydrochloric acid, but multiple flushing with hydrochloric acid will corrode the interior of the water jacket. Inlet water will not circulate inside the water jacket and flow directly out of the outlet, creating a false impression of good water flow and causing repeated burning of thyristors. In this case, the water jacket should be replaced.

The method to judge whether thyristor damage is caused by scaling of the water-cooled jacket is: let the medium frequency power supply operate at a value close to the overcurrent value for about 10 minutes, after shutdown, quickly touch the core of the thyristor by hand. If you feel it is burning hot, it can be judged that scaling has formed on the wall of the water-cooled jacket.

(4) When there is poor contact or broken wire in the tank circuit connecting conductor, arcing will occur after the power rises to a certain level, which affects the normal operation of the medium frequency power supply and thus triggers the protection action of the medium frequency power supply. Sometimes arcing will generate instantaneous overvoltage at both ends of the thyristor. If the overvoltage protection fails to act in time, the thyristor will be burned out, and the overvoltage and overcurrent protection of the medium frequency power supply will act at the same time.

(5) If the operating temperature of the thyristor is too high, the anti-interference ability of the gate will decrease, which will further cause false triggering of the thyristor, leading to arm straight-through and burning of the thyristor. You can use an oscilloscope to detect whether there is any interference signal. If interference exists, take the following measures: increase the capacitance of the capacitor connected in parallel between the thyristor's control electrode and cathode, generally increase the capacitance to 0.47-1μF.

(6) When the thyristor of a medium frequency power supply is turned off in reverse, it may be damaged due to excessively high instantaneous reverse glitch voltage. For this problem, the resistance-capacitance (RC) absorption circuit should be inspected. In the main circuit of the medium frequency power supply, instantaneous reverse glitch voltage is absorbed by the RC absorption circuit. If the resistor or capacitor in the absorption circuit is open-circuited, or the capacitor dries out and fails, the instantaneous reverse glitch voltage will become too high and burn the thyristor. When the power is cut off, use a multimeter to measure the resistance value and capacitance to judge whether the RC absorption circuit is faulty. Loose connection wires of the RC absorption circuit can also generate high voltage. After shutting down the operating medium frequency power supply (you need to turn off the internal main air switch of the power supply), touch by hand (it may be very hot) the non-inductive resistors for inversion RC absorption (their temperatures are basically the same under normal conditions). If you find that the temperature of some non-inductive resistors is much higher than that of other non-inductive resistors, it indicates that the capacitor connected in series with this non-inductive resistor has leakage.

If the resistance temperature is very low, it indicates that the capacitance of the capacitor connected in series with the resistor has decreased or the capacitor is open-circuited; if the resistor is burned out, the capacitor connected in series with it has been broken down. If you have spare parts, you can use the replacement method to judge whether the RC absorption circuit is faulty.

(7) Fault in the pulse trigger circuit. When the medium frequency power supply is in operation, if the trigger pulse is suddenly lost, it will cause open circuit in the inversion stage, generate high voltage at the output end of the medium frequency power supply, and burn the thyristor. Loose connecting wires of the current transformer will cause phase change of the composite signal of the inversion pulse forming circuit. If pre-triggering occurs, it will cause inversion commutation failure and burn the thyristor. Check whether the internal windings of the current transformer and medium frequency voltage transformer are open-circuited, and whether the wires are broken due to corrosion by harmful environmental gases because of their small diameter, which will invalidate the overcurrent limiting and overvoltage limiting protection. Once sudden instantaneous mutation of current and voltage occurs, exceeding the rated current and withstand voltage of the thyristor, the thyristor will be burned. Such faults usually occur in the inversion pulse forming circuit and output circuit, and can be checked with an oscilloscope. The fault may also be caused by poor contact of the inversion pulse lead, so you should check the joints of the inversion pulse lead.

(8) The fast fuse is improperly selected or has poor quality, so it cannot perform the protection function.

(9) Load open circuit during operation of the medium frequency power supply. When the medium frequency power supply is operating at high power, if the load suddenly becomes open-circuited, high voltage will be formed at the output end and burn the thyristor. For example, poor contact of main circuit connections, such as broken water-cooled cable, will cause the load circuit to be disconnected suddenly under high current working conditions, making the smoothing reactor generate very high self-induced electromotive force, and break down the inversion and rectifier thyristors.

(10) Load short circuit during operation of the medium frequency power supply. When the medium frequency power supply is operating at high power, if the load is suddenly short-circuited, there will be a large short-circuit current impact on the thyristor. If the overcurrent protection does not act in time, the thyristor will be burned.

(11) Reactor fault. Internal arcing of the reactor causes current interruption on the inversion side, which will also generate high voltage on the inversion input side and burn the thyristor. In addition, if the reactor is replaced during maintenance, and the inductance and core area of the new reactor are smaller than the required values, the reactor will be saturated and lose its current limiting function when working under high current, resulting in burning of the thyristor.

(12) Protection circuit fault (protection failure). Whether a thyristor can work safely is mainly guaranteed by the protection circuit. If the protection circuit fails, the safety of the thyristor will be endangered when a fault occurs in the medium-frequency power supply. Therefore, when the thyristor is burned out, inspection of the protection circuit is indispensable.

(13) Poor quality of the thyristor. The thyristor will be broken down at the moment the medium-frequency power supply starts, or broken down when the load increases. You should check whether the electrical parameters of the thyristor meet the requirements.