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The use of frequency converters is becoming increasingly widespread, offering significant benefits in terms of production and process efficiency for industrial automation control in various industries. However, as the level of automation continues to increase, automated equipment causes greater pollution to the power supply, which in turn leads to more interference with automatic control systems. As a result, there is an increasing demand for power supply filtering and purification to obtain a relatively stable, clean power supply. Internationally, there are clear laws and regulations regarding the design and application of electromagnetic compatibility (EMC or EMI), with specific provisions for interference caused by and received by electronic devices, as well as for the harmonic content in power supplies. Due to the relatively slow development of automation in electronic equipment in our country, there is still a lack of understanding regarding the impact of their harmonic content on the power grid; as a result, regulations in this area have not yet been established. However, in some industrial settings where the level of automation is relatively high, the importance of electromagnetic compatibility becomes quite evident; some electronic devices are extremely sensitive to electromagnetic interference to the point that they cannot function properly. A pharmaceutical factory encountered such problems when installing frequency converter equipment; by using filtering measures and after several adjustments, the issue was successfully resolved. Here we will discuss the methods for solving problems, for everyone to explore together and learn from one another. Introduction to the Automatic Control System in Fermentation Workshops: A fully automatic DCS control system used in a pharmaceutical factory monitors the temperature, pressure, and pH value of each fermentation tank in detail. It also carries out all operations related to feeding and discharging materials during the fermentation process, including the addition of sugar, phenylacetic acid, ammonia, etc., in an automated manner. Three sensors are used for detection, sending electrical signals to the microcomputer control system. The microcomputer control system sends pulse signals (+5V) at the appropriate time, based on the detected voltage (or current) values, to control the opening and closing of the solenoid valve (whose operating voltage is +24V), thereby enabling the supply of material in and out. In this way, each fermentation tank is equipped with six solenoid valves for inlet and outlet control, three sensors, and two monitoring instruments; all of these are remotely monitored via a computer, with all data displayed comprehensively on a large screen wall. There are 18 fermenters in the entire workshop. It is thus evident that such systems have a large structure, complex control mechanisms, and dense wiring; careful and thorough consideration must be given during installation. Analysis and Solutions to Electromagnetic Interference Problems Since the user has already tried using an inverter, and has gained some understanding of energy savings achieved through the use of inverters by replacing pulleys and adjusting motor speeds, it was decided to install an inverter in order to achieve further energy savings and improved efficiency. During the first installation of the frequency converter, the severity of electromagnetic compatibility issues was not taken into account; as a result, once the frequency converter was turned on, it interfered with the original control system, causing alarm signals to be generated in multiple fermenters connected to the computer. Adding input and output reactors later did not resolve the problem either. Later, upon inspecting the original inverter, it was found that dedicated power supply filters were used at both the input and output ends of the inverter, and it has been operating normally for a year; therefore, the pharmaceutical factory suggested purchasing power supply filters of the same model. The power filter at the input side of the inverter uses ferrite cores and iron powder cores with high magnetic permeability, together with certain capacitors, to form an LC filter. This filter removes the higher harmonics generated by the inverter (within a specific frequency range), thereby preventing electrical devices operating on the same power grid from being disturbed and allowing them to function properly. The power filter at the output of the inverter uses an inductor (L) for filtering, thereby suppressing the conductive interference generated by the inverter’s output and reducing low-frequency radiation interference on the output wires. This helps to decrease the electromagnetic noise of the motor that is driven directly, as well as significantly reducing the copper loss and iron loss of the motor. After purchasing such filters, they were tuned on-site. Due to limited on-site experience with this type of situation, the technicians were not well prepared; although filters were added, the filtering effect was still inadequate. Interference persisted under heavy loads, preventing the DCS system from functioning properly and keeping the frequency converters from operating. So we conducted a detailed analysis of the problem. Reasons for interference generated by inverters: The main circuit of an inverter generally operates in an AC-DC-AC mode, as shown in Figure 3. The external 380V/50Hz mains power is rectified into a DC voltage signal through a three-phase bridge rectifier; this DC signal is then filtered by capacitors, and an inverted into an AC signal with a variable frequency using high-power transistor switching elements. In the rectification circuit, the waveform of the input current is an irregular rectangular wave. This waveform can be decomposed into a fundamental wave and various harmonics using Fourier series, among which the higher harmonics will interfere with the input power supply system. In the inverter output circuit, the output current signal is a pulse waveform modulated by a PWM carrier signal. For GTR high-power inverter components, the PWM carrier frequency ranges from 2 to 3 kHz, whereas for IGBT high-power inverter components, the maximum PWM carrier frequency can reach 15 kHz. Similarly, the output circuit current signal can also be decomposed into a fundamental wave consisting only of sine waves and various higher harmonics, with the higher harmonic currents directly interfering with the load. In addition, higher harmonic currents are also radiated into space through the cables, interfering with nearby electrical equipment. The main pathways of interference generated by frequency converters: When operating, a frequency converter acts as a strong source of interference, and its methods of causing interference generally include radiation, conduction, electromagnetic coupling, secondary radiation, and conduction combined with radiation. The radiation interference generated by inverters has a strong impact on surrounding radio receiving devices. Conductive interference causes electromagnetic noise in motors that are driven directly, leading to a significant increase in copper losses and iron losses. Meanwhile, both conductive and radiation interference have a considerable effect on electronic devices that are connected to or located near the power input terminal. In light of these two debugging sessions, as well as the interference generated by the frequency converter and the pathways through which such interference occurs, engineers from the power supply filter manufacturer were involved in analyzing the situation. Multiple telephone discussions were held with them to understand their working principles and wiring arrangements; it was concluded that the interference was mainly caused by high-frequency harmonics generated at the input side of the frequency converter. After installing the inverter, its input wires are located within the original power conduit, while the output wires are not in that conduit and are rather close to the motor. Furthermore, the original wiring system is not very reasonable: the distance between the power conduits and the control conduits is only 20 cm, whereas it should be at least 50 cm according to regulations. Additionally, the two types of conduits run side by side, which are both problematic practices. The ground wire of the inverter is also connected in an inappropriate manner – it is connected to the cable tray used for the power cables. The function of such a tray is, firstly, to support the power cables and, secondly, to act as a shield; the interference generated by the inverter then reaches the tray through the ground wire. The high-order harmonics generated by the inverter are radiated through the inverter’s input lines and ground wire to the power lines and signal lines of other devices (especially the signal lines of sensitive sensors). It should be emphasized here that our frequency converters and DCS control systems are not powered by the same transformer, which rules out the possibility of direct conductive interference disrupting the normal operation of the control system. Analyzing these issues, since the original wiring system is already fixed, making any changes is practically impossible; therefore, the idea of altering the layout of the power and signal wires should be discarded. The ground wire for the inverter can be routed separately, with a separate ground wire connected directly to the ground wire of the electrical control cabinet in the distribution room. Additional filtering measures can also be implemented at the input side of the inverter, and theoretically, this should resolve the problems. After the original fermentation tank was shut down on site, an additional set of common-mode and differential-mode magnetic rings was added to the existing filter; two differential-mode rings were installed on each phase line at the input and output ends, two common-mode magnetic rings were installed on the three input phase lines, and the ground wire was connected to the floor in the power distribution room. After such treatment and upon startup, it operates normally when the motor is unloaded, with no interference alarms occurring. During operation under load, interference alarms occurred in tanks 305 and 307. The ground wire was rerouted to the transformer’s ground wire connected to tank 307, which already uses an inverter – the output wires of that inverter run within the cable tray. As a result, tank 305 no longer causes interference-related alarms; however, tank 307 still experiences such alarms every few minutes. It is possible that this phenomenon is caused by the combined effect of common-mode noise generated by the two inverters, or it may be due to the long length of the wiring as the ground wire runs inside the power cable tray. A ground filter was installed on the ground wire, but its effectiveness was limited. Later, the ground wire was removed (after measurement, the leakage current of the entire inverter was very low, so it posed no hazard to humans; therefore, the ground wire could be removed). The performance improved somewhat, but the alarm still occurred intermittently. Based on this, it seems that the issue is not caused by the ground wire, but rather by insufficient filtering at the input side, which failed to eliminate high-frequency interference completely. Therefore, the system was shut down; two differential mode chokes were added to each of the input phases, and three common mode chokes were installed on the three input phases. Once it was restarted, it operated normally, with no interference occurring in the entire system anymore.