2.4 Performance improvement period at the end of the 20th century
After 1998, due to the quality of construction in China, the small steel mills were extensively reorganized. Many provinces have formulated policies to limit the use of medium-frequency power supplies with a capacity of less than 500 kW, prompting the development of medium-frequency power supplies with a single-machine capacity of more than 1000 kW, thus promoting the manufacture of fast thyristors. The level has been further improved. Nowadays, fast thyristor components with a single-tube current capacity of 2000A and 2500A can be produced in China, but the turn-off time is still difficult to drop below 20μs for thyristors above 1500A, which solves the heavy furnace start of large and medium frequency power supplies. The problem is that the domestic power electronics industry has developed the fifth-generation IF power control board. This is the self-phase-matching and phase-synchronized frequency-sweeping starter board of the synchronous transformer, which makes the performance and level of the thyristor IF power supply very high. The grade.

It should be mentioned that in order to solve the pollution problem of the power grid and improve the efficiency, the IGBT and MOSFET level increase, capacity expansion and cost reduction, the domestic induction heating medium frequency conversion power supply has been in the small capacity field from thyristor equipment to IGBT and MOSFET is the high-frequency power supply for the main power device (operating frequency is in the range of 20kHz~200kHz), and has been put into industrial production in batches. In this field, Baoding Hongxing High Frequency Equipment Factory and other enterprises are relatively large in this field, but due to IGBT Or device application technology such as MOSFET is not very mature in most domestic enterprises, so the production enterprises of high-frequency power supply are relatively few.

3. Status of medium and high frequency power supply technology for induction heating

China's medium and high frequency power supply for induction heating has grown from scratch. After four stages of development, it has formed a large scale in China and has been used in welding, quenching and melting in metallurgy, electric power, petroleum, chemical, electronics and other industries. In the fields of heat transfer and heat preservation, the development status can be summarized as follows:

(1) The induction heating intermediate frequency power supply with thyristor as the main power device has covered all fields with operating frequency below 8 kHz, and its single power capacity is divided into 50, 160, 250, 500, 1000, 2000, 2500, 3000 kW, working The frequency is 400Hz, 1kHz, 2.5kHz, 4kHz, 8kHz.
(2) The trigger of the three-phase full-controlled rectifier bridge in the intermediate frequency power supply has already bid farewell to the multi-plate structure composed of discrete components, and now it is a single large-plate structure integrating pulse formation, protection, power amplification and pulse shaping. Pulse generation with inverter bridge and amplifier and regulator).
(3) The thyristor trigger pulse generation of the three-phase rectifier bridge in the intermediate frequency power supply has been applied from the synchronous transformer. The on-site debugging needs to gradually transition the control mode of the phase sequence to the trigger with phase adaptation function without the synchronous transformer.
(4) The starting mode of thyristor intermediate frequency power supply has transitioned from impact start, zero pressure start, internal and external bridge conversion start to sweep start, and its control technology has been improved from voltage or current closed loop control to constant power control, thus making the intermediate frequency power supply The control effect is better and the efficiency of the user is improved.
(5) The single-tube capacity of the thyristor for the IF power supply has reached 2500A/2500V, and the shortest turn-off time has reached 15μs. The manufacturing technology of the non-inductive resistor high-frequency capacitor matched with the intermediate frequency power supply has made great progress, and it is the thyristor intermediate frequency. The production of the power supply brings great convenience.
(6) The components and accessories of thyristor intermediate frequency power supply such as radiators, fuses, reactors and control boards have been standardized, serialized, mass-produced, and brought great to manufacturers and maintenance personnel of thyristor intermediate frequency power supplies. Convenience.
(7) The capacity of fully controlled power semiconductor devices such as MOSFETs and IGBTs has been expanding, which has laid the foundation for the medium and high frequency power supply. The drivers and protection circuits for IGBTs and MOSFETs have been serialized and standardized, giving medium and high frequency and super The audio induction heating power supply lays the foundation and guarantee, which brings great convenience.
(8) The induction heating medium frequency power supply with a single unit capacity of more than 500 kW is basically a thyristor power supply with a clear color, but the working frequency is up to 8 kHz and the maximum capacity has reached 4000 kW. Some domestic enterprises are developing a thyristor intermediate frequency power supply with a single unit capacity of 6000 kW. The medium and high frequency power supply with IGBT and MOSFET as the main power devices has been mass-produced in China, but the production is still relatively small compared with the thyristor intermediate frequency power supply. The single unit capacity is less than 200 kW, and the operating frequency is basically at 20 kHz. In the range of ~200 kHz, medium and high frequency power supplies exceeding 20 kHz are basically applied MOSFETs. Since MOSFETs are still difficult to produce conditions that satisfy both high voltage and high current, it is necessary to adopt multiple MOSFETs in parallel. In actual situation, there are direct MOSFETs connected in parallel, and then inverters to obtain larger power output; there are also direct MOSFETs to form inverter bridges, and then multiple inverter bridges are connected in parallel; special attention should be paid to the two implementation methods have current sharing. Problem, the latter not only has several inverter parallel current sharing problems, but also several inverter bridge outputs in phase In parallel with the magnitude of the problem, but this solution causes the control system has a plurality of control units.
(9) The current medium-high frequency induction heating medium frequency power supply cooling method is a water-cooling method. Most of the water pressure relays are used. There is a problem that the water pipe is blocked and the water pressure is high, but it cannot be cooled, which easily causes overheating damage of the device. Therefore, there is a general deficiency in the protection scheme. The protection of the flow relay should be increased in conjunction with the water pressure relay.

4. Discussion on the development trend of medium and high frequency power supply for induction heating

(1) The intermediate frequency power supply with thyristor as the main power device will still not withdraw from the historical stage, and will still monopolize the field of high-power (several kilowatts or more) medium-frequency power supply, which will be 10 tons, 12 tons, 20 tons of steelmaking or insulation. Mainstream equipment with intermediate frequency power supply.
(2) The low-power thyristor intermediate frequency power supply (power capacity less than 1000kW) will gradually increase the usage as the requirements for efficiency and steelmaking quality continue to increase, but they will still be used for some time in the fields of quenching and bending. .
(3) The main power device is the IGCT and GTO induction heating medium frequency power supply will compete fiercely with the main power device for the thyristor intermediate frequency power supply and gradually reduce the former market share.
(4) The intermediate frequency power supply used in the medium and high frequency (frequency above 10kHz~30kHz) will use IGBT as the main device, and its single unit capacity will continue to expand with the continuous expansion of the IGBT's own capacity, and it will gain more and more use. .
(5) Inductive heating power supply in the field of high frequency (frequency above 100 kHz) will use MOSFET as the main device. With the continuous advancement of MOSFET manufacturing process and breakthrough, high frequency power supply with MOSFET as the main power device will be widely used. Capacity will continue to expand.
(6) The cooling technology of the intermediate frequency power supply for induction heating will achieve a big breakthrough, which will solve the water leakage caused by the water cooling method, and the water treatment is inconvenient, but it may take a long time.
(7) The components of the intermediate frequency power supply for induction heating will continue to improve, more standardized and more serialized, bringing greater convenience to the manufacture and maintenance of high-frequency power supplies.
(8) The single-unit power capacity of the intermediate frequency power supply for induction heating will continue to expand, and it is expected to break through 10MW, and its working frequency will be higher and higher.
(9) With the medium and high frequency power supply for induction heating, the EMI suppression technology that limits grid interference and ensures green grid power, power factor correction technology will be widely used, and the output waveform and efficiency of medium and high frequency induction heating power supply will be further improved.
(10) SIT and SITH devices will be used in China's medium and high frequency power supply field and fill the blank of medium and high frequency induction heating power sources that have not been developed and applied by these devices.
(11) The starting method of medium and high frequency induction heating power supply, the control technology will make breakthroughs, and further improve the performance of such power supply. The medium frequency power supply adopting the new control strategy will obtain a wide range of applications.

5 Conclusion

The medium and high frequency power supply for induction heating is a must-have equipment in China's industrial production. China has been engaged in the development and production of such power supplies for decades. Its development level still needs to be improved. The development status of China's medium and high frequency power sources involved in this paper and Trends can provide a reference for engineers and technicians engaged in research in this field.

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