Abstract: This paper analyzes the generation of various interference signals during the operation of a KGPS (Silicon Controlled Gear) intermediate frequency power supply and their impact on the safe operation of the power supply. Effective measures are proposed to weaken interference, enhance anti-interference capabilities, and improve the reliability of the intermediate frequency power supply. Keywords: KGPS power supply, interference signals, anti-interference research 1 Introduction With the development of power electronics technology and the improvement of power electronic component manufacturing levels, intermediate frequency induction heating technology is becoming increasingly sophisticated. Intermediate frequency induction heating, with its convenient and efficient heating performance, is gradually replacing coal and oil fuel heating as the preferred method for industrial heating. As a power supply for induction heating devices, KGPS (Silicon Controlled Gear) has stronger load adaptability, higher efficiency, and is easier to form automatic heating lines compared to traditional BPS (medium frequency generator sets), and has been increasingly widely used in the industrial heating field. Whether in forging, casting, or heat treatment production lines, as a key piece of equipment in the front-end heating process, a failure of the KGPS power supply will directly cause the entire production line to shut down. Therefore, the reliability of KGPS power supply operation deserves special attention. In practical applications, the environmental conditions at the site of the thyristor power supply are often poor, with strong vibration, high temperature, and a lot of dust. At the same time, as a complex AC-DC-AC frequency conversion system, the power supply is easily affected by various interference signals during operation, especially the power supply triggering system and protection system. When the KGPS power supply triggering system circuit is interfered with, it often causes a decrease in the power supply's starting performance; when the protection system circuit is interfered with, it can cause the protection circuit (overcurrent, overvoltage, etc.) to malfunction, resulting in frequent power supply tripping, failure to work normally, or even protection failure, damage to power components, and equipment accidents. Due to the randomness of interference signals, the analysis and diagnosis of such faults are relatively difficult, and a slight oversight may even lead to the expansion of the fault. Therefore, a comprehensive analysis of the various interference signals involved in the operation of the thyristor intermediate frequency power supply and the research and determination of corresponding anti-interference measures are of great significance for improving the reliability of the intermediate frequency power supply operation. [b]2 Main Interference Signals in KGPS Power Supply Operation[/b] 2.1 Electric Field Interference In domestic and international electromagnetic compatibility standards, electric field interference is often referred to as radio frequency interference. The strength of electric field interference is primarily related to the electric field strength; the stronger the electric field in the environment where the control circuit is located, the stronger the interference. Electric field interference is also related to frequency; the lower the radio frequency, the weaker its transmission capability, and the smaller the effect of external interference. Specifically for induction heating devices, because the electric field frequency emitted by the frequency converter is far below 0.5MHz (radio frequency is generally above 0.5MHz), the intensity of electric field interference is very small compared to magnetic field interference. Only during the switching process of the inverter thyristor between conduction and cutoff, when the commutation frequency is higher, can the change in voltage du/dt between the cathode and anode potentially generate effective spatial electric field interference. 2.2 Magnetic Field Interference: Magnetic lines of force with the same frequency as the bus current exist around the energized bus. When these magnetic lines of force chain through the control lines, they induce current in the control lines, causing distortion of the control signal and thus generating magnetic field interference. Magnetic lines of force chaining through the control circuit board or signal detection circuit board can also generate effective magnetic field interference. In the frequency converter cabinet, both electric and magnetic field interference exist. In terms of interference intensity, the magnetic field interference is stronger than the electric field interference. In particular, all busbars of the inverter main circuit carry large intermediate frequency currents, and the surrounding magnetic field is very strong. Therefore, the interference generated cannot be ignored and should be suppressed. 2.3 Conducted Interference Conducted interference is divided into two parts. One is that if there are other electrical devices in the power grid supplying the induction heating frequency converter, especially high-power electrical devices, such as motors and capacitor cabinets, these devices will generate interference pulse voltages during start-up and shutdown operations, which will be conducted along the three-phase power lines into the induction heating frequency converter power supply. This part belongs to external conducted interference. The other part is the conducted interference generated internally by the frequency converter during normal operation. For example, the voltage spikes generated by the inverter phase conversion will also enter the main control board through various conduction paths to form interference. This part belongs to internal conducted interference. 3 Several Anti-interference Measures 3.1 Electromagnetic Shielding For electric and magnetic field interference, electromagnetic shielding is the most common and effective anti-interference measure. It belongs to passive anti-interference measures. While electromagnetic shielding cannot reduce the intensity of electric or magnetic field interference signals, it can weaken the impact of electromagnetic interference signals on normally operating circuits. Electric field shielding is mainly used to weaken electric field interference. In inverter cabinets, the main control board is best installed in an iron box, which is connected to the ground along with the casing, thus forming an effective electric field shield. Any control signal transmitted in a conductor generates current. Where there is current, there must be a loop, and a loop requires two conductors to form. If interfering magnetic lines pass through the gap between the two conductors, an interference current will be generated in the loop. To avoid this interference, the gap between the two conductors must be minimized. Using tightly twisted wire can minimize the gap between the two conductors, thus greatly reducing the intrusion of interfering magnetic lines. In KGPS inverter power supply cabinets, lines for thyristor trigger signals, intermediate frequency feedback signals, etc., should be laid out with shielded or twisted wires, and the wiring should be as short as possible. 3.2 Signal Filtering During the actual operation of a thyristor intermediate frequency power supply, it is impossible to eliminate external conducted interference from other electrical equipment in the same power supply network. Therefore, strengthening prevention is crucial, and signal filtering is the most commonly used preventative measure. In KGPs frequency converters, the RC absorption circuit at the three-phase power input is an effective anti-interference filter circuit. When other electrical equipment (especially high-capacity equipment) in the same power supply network starts or stops, current changes induce overvoltages in the power supply circuit (including leakage inductance of the power supply transformer and distributed inductance of the power supply line), forming interference signals and causing conducted interference to the intermediate frequency power supply. When passing through the RC signal filter circuit, capacitor C undergoes a charging process, which is the process of absorbing overvoltage energy. Usually, the duration of overvoltage is relatively short. If the capacitance of the capacitor is large enough, spikes will not be generated in the thyristor intermediate frequency power supply circuit, thus preventing interference. Similarly, RC filter circuits must be set in the input and output signals of the rectifier trigger synchronous transformer and all detection circuits. It is particularly important to note that the selection of R and C parameters in all signal filter circuits has a direct impact on the filtering effect and cannot be arbitrarily replaced. R is a damping resistor used to prevent resonance between the power supply circuit inductance and capacitor C. If R is chosen too small and resonance occurs, R and C will not only fail to absorb overvoltage but will also generate overvoltage, increasing the degree of interference to the line. For conducted interference generated within the intermediate frequency power supply itself, it is possible to limit its occurrence from the source of the interference. A proper arrangement of the main circuit busbars (buses) can reduce both electromagnetic interference and conducted interference. This is because the distributed inductance and capacitance of the busbars are the main causes of stray spikes and parasitic high-frequency oscillations in the inverter phase. These stray spikes and parasitic high-frequency oscillations can be mixed with the useful signal through detection elements such as intermediate frequency voltage or current transformers and enter the control circuit. 3.3 Busbar Layout A proper layout of the busbars (main circuit busbars) within the KGPS inverter power supply cabinet can minimize the intensity of the interfering electric and magnetic fields. This addresses the problem at its source and is the most proactive and effective method. In the main circuit busbars, whether DC, intermediate frequency, or power frequency, they are always paired, with one busbar for current input and the other for current output. If paired DC buses or intermediate frequency buses are placed as close as possible to each other, the magnetic field lines around the two buses will cancel each other out because they are equal in magnitude and opposite in direction, thus preventing electromagnetic interference. Furthermore, the distributed capacitance between two closely spaced buses can reduce "spikes" in the voltage waveform, which helps weaken electric field interference and reduces the burden on the RC absorption circuit. Simultaneously, close installation of related buses significantly reduces distributed inductance, which is beneficial for the startup of the intermediate frequency power supply and also helps reduce voltage drop on the bus, increasing the voltage across the inductor and thus improving effective heating power. Therefore, the layout of the main circuit busbar should be a key consideration for thyristor frequency converters. 3.4 Control Circuit Wiring The KGPS frequency converter cabinet contains numerous control signals, including both AC and DC signals, as well as power frequency and intermediate frequency signals. To reduce interference, various control signals should be laid out separately and kept as far away from the main circuit busbar as possible. Generally, the following two principles should be followed: ① Medium frequency control signal lines should be housed in separate cable trays. Control and signal lines related to the inverter are most susceptible to interference (leading to startup failure, inverter failure during normal operation, and overcurrent protection activation), and are also most likely to interfere with the normal operation of the rectifier section. To avoid this interference, control and signal lines related to the medium frequency should be housed in separate cable trays and kept as far away as possible from the main circuit bus. Some secondary lines (trigger signals, medium frequency feedback signals, etc.) must be tightly twisted together. ② Instrument leads should be housed in separate cable trays. The electromagnetic field around instrument leads is a significant source of interference. For example, DC voltmeter leads carry voltage waveforms at twice the inverter output frequency. Therefore, all instrument leads should not be mixed with other control lines in the same cable tray. 3.5 Relay Control Design The relay control circuit should prioritize the simplest design scheme. While meeting normal operation and maintenance requirements, the number of control buttons, switches, indicator lights, etc., should be minimized to facilitate operation and reduce interference. Control buttons, switches, indicator lights, etc., often require long leads to the device panel or even further to the operator, far from the device itself. These components sometimes connect to both the relay control system and the thyristor triggering or protection system. Longer leads tend to introduce interference, causing malfunctions in the triggering or protection system. Therefore, besides minimizing the number of buttons, switches, indicator lights, etc., any necessary long leads should use shielded insulated twisted-pair wires. Furthermore, the contacts in the thyristor triggering and protection systems can be isolated using miniature relays to improve interference immunity. Miniature relays can be directly installed within the triggering or protection system, resulting in extremely short contact leads that are less prone to interference. The relay coil can then be connected to the relay control system via a longer wire, significantly reducing the impact of interference introduced by long leads on the triggering or protection system. 3.6 Other Anti-interference Measures The points mentioned above are the most fundamental anti-interference measures. In addition, the following aspects should be noted: ① Typically, the operating environment of a thyristor intermediate frequency power supply is subject to strong vibrations. Loose connections in the main circuit wiring can easily cause overheating, and loose connections in the control circuit wiring can easily introduce interference signals, affecting the reliability of the power supply. Therefore, all wiring terminals (crimped, plugged, soldered, etc.) should be checked frequently to ensure reliability. ② When selecting a KGPS frequency converter, use control circuits with strong anti-interference capabilities whenever possible. For trigger pulse forming circuits, prioritize the use of digital integrated circuits. ③ Use a power frequency current transformer to collect intermediate frequency current signals, utilizing the slow response speed of the power frequency current transformer to filter out noise and high-order harmonics in the intermediate frequency current. 4 Conclusion During the operation of a thyristor frequency converter, the main circuit and control circuit work together. The interference of the electric and magnetic fields generated by the main circuit on the control circuit signals is, in principle, impossible to eliminate. External conducted interference from other electrical equipment in the same power supply network is also impossible to remove. Only by taking necessary measures to effectively reduce the strength of interference signals or improve the anti-interference capability of the control circuit can the interference signals be prevented from affecting the normal operation of the control circuit as much as possible. After taking the above anti-interference measures in a targeted manner, the reliability of the thyristor frequency converter power supply will be further improved. [b]References:[/b] 1. Electrical Engineer's Handbook. Machinery Industry Press, 1986 2. Huang Jun. Semiconductor Converter Technology. Machinery Industry Press, 1995.10 3. Xi'an Electric Furnace Research Institute, Technical Information Institute of the First Ministry of Machine Building, ed. Application of Heating Technology and Its Equipment Design Experience. Machinery Industry Press, 1975.12 Editor: Chen Dong