Abstract : This article briefly introduces the electromagnetic environment of high-voltage inverters and the propagation path of electromagnetic interference on their signals. It also introduces several commonly used signal transmission technologies in high-voltage inverters. The main signal transmission technologies of high-voltage inverters include : digital signals, current signals, optical fiber communication, shielding, isolation, grounding, and twisted-pair cabling. Keywords : Electromagnetic environment, Electromagnetic interference, Signal transmission Generally , medium- and large-capacity inverters used to drive AC motors above 1kV are called high-voltage inverters. However, according to international practice and China's national standards, when the supply voltage is greater than or equal to 10kV, it is called high voltage, and when it is less than 10kV to 1kV, it should be called medium voltage [1]. Considering that when the voltage is greater than 6kV, most frequency converters in China adopt AC-DC-AC multi-level topology. The signal transmission technology discussed in this paper focuses on high voltage frequency converters with voltage greater than 6kV. I. Electromagnetic environment of high voltage frequency converters The main circuit of frequency converters, including high voltage frequency converters, is generally AC-DC-AC topology. The external power frequency input from the grid is rectified into a DC voltage signal by a three-phase bridge uncontrolled rectifier, and then converted into a variable frequency AC signal by capacitor filtering and high power transistor switching elements. The rectifier section of domestic high voltage frequency converters mostly uses phase-shifting transformers to keep the input current harmonics of the grid below 4%. In the output circuit of the inverter, the output voltage signal is modulated by the pulse waveform of the PWM carrier signal. For GTR high power inverter elements, the PWM carrier frequency is 2-3kHz, while the PWM carrier frequency of IGBT high power inverter elements can reach 15kHz. Although high-voltage frequency converters use phase-shifting transformers, multiplexing and other technical means to make the input and output waveforms of the frequency converters satisfactory, their power reaches more than megawatts, and the electromagnetic interference is proportional to its own power, so its electromagnetic interference is still serious. My colleague measured the common-mode interference voltage inside the frequency converter with a multimeter and found it to be AC200V, which means that the common-mode interference voltage on the signal line is above AC200V. II. Propagation path of electromagnetic interference on high-voltage frequency converter signals High-voltage frequency converters can generate very high-power harmonics, which are highly interfering with themselves and other equipment. Its interference propagation path is consistent with the general electromagnetic interference path, mainly divided into electromagnetic radiation, conduction and inductive coupling [2]. Its specific manifestations are: (1) generating electromagnetic radiation to surrounding electronic and electrical equipment. The inverter bridge of the high-voltage frequency converter adopts PWM technology. When the expected and repeatable switching mode is generated according to the given frequency and amplitude command, the power spectrum of its output voltage and current is discrete and has high-order harmonic groups corresponding to the switching frequency. The radiation interference caused by high carrier frequency and high-speed switching of field-controlled switching devices (du/dt can reach more than 1kV/us) is quite prominent. (2) In the strong electromagnetic interference environment, the inverter circuit generates inductive coupling, which induces interference voltage or current. When the magnetic field coupling interference of circuit 1 to circuit 2, the series interference voltage UN formed on circuit 2 is given by the formula where ω — angular frequency of the interference signal; B — magnetic flux density at the connection between the magnetic field formed by the interference source circuit 1 and circuit 2; A — the closed area of ​​the magnetic flux induced by the magnetic field in circuit 2; θ — the two appropriate angles between B and A. Electromagnetic coupling interference is mainly composed of two types: one is that the metal wire circuit in the harsh electromagnetic environment generates a very high common-mode voltage; the other is that the measured electrical signal itself contains rich noise. When the physical distance between the signal transmission line of the inverter and the power inverter device of the inverter is relatively close, the high-order harmonic signal of the inverter device will be coupled into the signal through induction. III. Signal Transmission Technology of High Voltage Inverters According to the basic principles of electromagnetics, electromagnetic interference (EMI) requires three elements: an electromagnetic interference source, an electromagnetic interference path, and a system sensitive to electromagnetic interference. The electromagnetic interference source is not within the scope of this paper. This paper mainly discusses the signal transmission technology of high voltage inverters from two aspects: cutting off the electromagnetic interference path and reducing the system's sensitivity to electromagnetic interference. High voltage inverters mainly ensure signal transmission technology from the following aspects. (1) When designing the system, select signal transmission technology with strong anti-interference capabilities, such as prioritizing digital and current-type signals. The anti-interference capability of digital signals is much stronger than that of analog signals. Except for analog signals used for measurement, digital signals should be given priority if the signal transmission speed is met. The main effect of a harsh electromagnetic environment on signal cables is the generation of common-mode voltage interference. Current-type signals do not have the problem of inconsistent current magnitudes at different points in the same circuit in the transmission path, and have a strong anti-inductive interference capability. It is recommended to use the 4-20mA signal commonly used in industry. (2) When transmitting digital signals or level signals, use optical fiber as the transmission medium to transmit the signal. Transmitting signals in the form of light waves can fundamentally eliminate interference during signal transmission; signals are transmitted at the speed of light in optical fibers, and optical fibers transmit signals quickly; using optical fibers can save non-ferrous metals - copper, making it more economical than transmitting signals via cables. (3) Isolation: Isolation is an effective method to cut off electromagnetic radiation and conducted interference, which can prevent interference signals from entering the next level of signal processing circuits. Isolation refers to cutting off the electrical propagation path of a signal and using light or magnetism to couple the input signal during signal transmission. Isolation can be divided into two signal isolation methods: isolation of analog signals and isolation of digital signals. Commonly used isolation measures for analog signals are isolation amplifiers and isolation transformers. Commonly used digital signal isolation methods are optocouplers, which are divided into ordinary and high-speed types. Figure 1(a) is the schematic diagram of the ISO124 model signal isolation amplifier, and (b) is the schematic diagram of a dual-channel high-speed digital signal optocoupler. Different types of signal lines are laid in isolation (not in the same cable tray or separated by metal partitions in the same cable tray), and they can be laid according to their anti-noise interference capabilities based on the different types of signals. (4) Shielding is to restrict the internal electromagnetic energy from going out of a certain area and to prevent external energy from entering a certain area. It is generally used to isolate and attenuate radiation interference. The essence of shielding is a fully enclosed shell made of metal material with good conductivity. When transmitting electrical signals in high voltage frequency converters, shielded cables must be used. The following points should be noted during use: ① The shield is grounded at one end, sometimes also called electrostatic shielding. The end of the shielding layer closest to the frequency converter should be connected to the common terminal (COM) of the control circuit, but not to the ground terminal (E) of the frequency converter or the earth, as shown in Figure 2 [3]. ② When the interference electric field is very strong and the circuit sensitivity is high, double shielding can be used. Note: The inner and outer shielding layers can only be connected at one point, and a filter circuit should be added. The distance between the two layers should be as large as possible. ③ To prevent spatial electromagnetic waves from interfering with the high-sensitivity receiver, a metal mesh shielding room can be used [4]. (5) Grounding. The function of grounding can be summarized into two types: protective grounding to protect people and equipment from damage and working grounding to suppress electromagnetic interference. This discussion focuses solely on operational grounding. Operational grounding is designed to ensure the reliable operation of the frequency converter control system and its connected instruments, guaranteeing measurement and control accuracy. In high-voltage frequency converter signal transmission, signal loop grounding and shielding grounding must be considered. For high-voltage frequency converters used in petrochemical or other explosion-proof systems, intrinsically safe grounding is also an issue. Signal loop grounding includes grounding the negative terminals of each frequency converter and the negative terminals of switch signals. In principle, transmitters and other sensors should not be grounded in the field; their negative terminals should be grounded at a single point on the frequency converter terminal. However, in some cases, field grounding is necessary. In such cases, it is crucial that the input terminals (upper double-ended) of the original signal are absolutely not electrically connected to the grounding wire of the frequency converter control system. When processing such signals, the frequency converter control system must employ effective isolation measures at the front end. Shielding grounding (simulation signal shielding grounding) is the most demanding type of grounding. High-voltage frequency converters require a grounding resistance of less than 0.1Ω, necessitating the installation of an analog ground busbar or other facilities inside the frequency converter cabinet. When wiring, the user connects the shielded wire to the analog ground busbar respectively. At the bottom of the cabinet, connect it to a point with insulated multi-strand copper wire. Then connect the busbars of each cabinet to the grounding point in a radial manner with insulated multi-strand copper wire or copper strip. Note that the connection resistance between each cabinet must be less than 1Ω. Intrinsic grounding is the grounding of intrinsically safe instruments or safety barriers. In addition to suppressing interference, this grounding is also one of the measures to make instruments and systems intrinsically safe. Intrinsic grounding will vary depending on the equipment used. The function of the safety barrier is to keep the dangerous field end always within the range of safe power supply and safe voltage. If the field end is short-circuited, due to the current limiting effect of the load resistance and the safety barrier resistance R, the current on the wire will be limited to the safe range, so that the field end will not generate a very high temperature and cause combustion. If a fault occurs at one end of the frequency converter, the high voltage signal is added to the signal circuit. Due to the effect of the Zener diode, the voltage is also kept within the safe range. (6) Use twisted pair cable. The twist pitch of the twisted pair divides the A loop in equation (1) into many small loops. If the twists of the twisted pair are consistent, then the areas of these small loops are equal and their directions are opposite, so their magnetic field interference can cancel each other out. However, it should be noted that the structure of the twisted pair does not have the ability to suppress electric field coupling interference. When a shielding layer is added to the twisted pair, its ability to suppress interference is stronger. Figure 3 shows several ways of using twisted pairs, which readers can compare. If there are 6 uniform twists every 2.54 cm, the reduction in dB of magnetic field interference is shown in the figure. Among them, Figure 3(a) uses a single-end grounding method, which has an attenuation capability of up to 55dB for magnetic field interference. However, in Figure 3(b), due to double-end grounding, the ground impedance is not symmetrical with the signal line impedance, and the ground loop current causes an imbalance in the twisted pair current, thus reducing the magnetic field interference resistance capability of the twisted pair to only 13dB. The shielding layer in Figure 3(c) is grounded at both ends, which has a certain ability to suppress magnetic field coupling interference. Combined with the effect of the twisted pair itself, it has an attenuation of 63dB. The shielding layer and twisted pair in Figure 3(d) are grounded at both ends, and the effect is only better than that in Figure 3(b), with only 63dB of attenuation [2]. IV. Conclusion As mentioned above, the electromagnetic environment of high-voltage frequency converters has its own characteristics in addition to the common characteristics of general electronic equipment. When considering its signal transmission, the signal transmission technologies mentioned in this article need to be used in combination. Our company's Fengguang brand frequency converters are reliable precisely because they adopt the above-mentioned signal transmission technologies. This is also one of the factors that our company's products have been rated as a famous Chinese brand. References: [1] Wu Zhongzhi, Application Manual of Medium (High) Voltage High Power Inverter [M], Machinery Industry Press, 2003 [2] Zhou Zhimin, Inverter - Engineering Application, Electromagnetic Compatibility, Fault Diagnosis [M], Electronic Industry Press, 2005 [3] Zhang Yanbin, SPWM Inverter Speed ​​Regulation Application Technology [M], Machinery Industry Press, April 2002, 2nd Edition [4] Sun Jiujun, Electrical Interference and Anti-interference Measures in Mechanical and Electronic Equipment [J], Experimental Technology and Testing Machine, 2006, 01. Author Biography : Sun Jiujun, male, born in July 1977, from Pingyin, Shandong, Han nationality, engineer, Shandong Xinfengguang Electronic Technology Development Co., Ltd., Master's degree, research direction: design of high voltage inverter.