Due to the rapid development of power electronics technology, high-voltage variable frequency drives (HVVFDs) have been widely used in industries such as petrochemicals, power, and metallurgy, playing a significant role in energy saving and speed regulation of high-voltage motor equipment. At the same time, these applications place higher demands on the reliability and stability of HVVFDs. This paper aims to analyze the impact and solutions of electromagnetic interference (EMI) from the perspective of the controller section of HVVFDs.
High-voltage frequency converters are advanced automation systems that integrate microcontrollers, high-power devices, magnetic materials, sensors, and other strong and weak current components. Their control systems generally consist of a control box, PLC, touch screen, and related control components. Some also include a host computer and DSC system. Therefore, electromagnetic interference (EMI) problems are becoming increasingly complex. EMI can cause signal corruption in the core of the transmission system—the computer control system—and can also damage or reduce the performance of other electronic devices, leading to serious consequences.
1. Control system structure and components generating electromagnetic interference
The functional block diagram of the main controller is shown in Figure 1. Its structure is modular, with a core dual-DSP CPU unit that communicates with the interface board and phase control boards A, B, and C via a bus. The interface submodules DI and AI receive operation commands, given signals, motor current and voltage, etc. The CPU board calculates control and status information based on the operation commands, given signals, and other input signals. Phase control boards A, B, and C receive control information from the CPU board, generate PWM control signals, and send control optical signals to the power unit via an electro-optical converter. Response signals from the power unit are converted into electrical signals in the phase control boards A, B, and C, pre-processed, and then sent to the CPU board for further processing. Status information can be sent out through the interface board and interface submodules.
Electromagnetic interference (EMI) generally comprises three components: the EMI source, the EMI transmission path (conduction, radiation, coupling), and the receiver of the EMI. These three components are quite complex and manifest differently in different situations. Based on fundamental physical laws such as electromagnetic induction, the skin effect, electromagnetic oscillation, and electromagnetic wave propagation, it is known that the faster an electromagnetic physical quantity changes over time, the easier it is to induce EMI; the higher the frequency, the easier it is to generate radiation; electromagnetic field strength is inversely proportional to the square of the distance; and some highly sensitive unshielded circuits are prone to coupling, etc.
Electromagnetic interference in high-voltage frequency converter control systems is classified into two main categories based on its propagation mode: conducted interference and radiated interference. Conducted interference refers to interference caused by electromagnetic interference propagating through power lines, grounding wires, and signal lines to reach the target device; radiated interference refers to interference propagating through space to sensitive devices. Capacitive coupling and inductive coupling between signal transmission lines and other electrical equipment are significant sources of interference in the control system.
2 Electromagnetic compatibility analysis
Control systems are composed of multiple units, making it impossible to completely avoid electromagnetic interference. Therefore, anti-interference measures must be taken on sensitive equipment in the controller. Shielding, filtering, proper grounding, and rational layout are all effective methods for suppressing interference. Based on the three elements of electromagnetic interference, control methods such as shielding, grounding, interconnection, and rational wiring can be adopted. In addition, avoidance and diversion techniques can be used, such as spatial separation, filtering, absorption, and bypassing. These are control methods frequently used by experienced engineers. Solving electromagnetic interference problems should be carried out simultaneously during the entire electrical system design, wiring, installation, and commissioning phases, and should not be addressed only during the commissioning stage.
2.1 Shielding
Shielding is generally divided into two types: electrostatic shielding and electromagnetic shielding. Electrostatic shielding is mainly used to prevent the effects of electrostatic fields and constant magnetic fields. Electrostatic shielding should have a complete shielding body and good grounding. Electromagnetic shielding is mainly used to prevent the effects of alternating electric fields, alternating magnetic fields, and alternating electromagnetic fields. Electromagnetic shielding not only requires good grounding but also requires the shielding body to have good conductivity continuity. The conductivity requirements for the shielding body are much higher than those for electrostatic shielding. The electromagnetic interference immunity principle of using shielded signal cables is shown in Figure 2.
If the shielding layer of a shielded cable is not properly grounded, it will not function as a shield against interference sources and may even become a source of interference itself (the cable's shielding layer will absorb external electromagnetic interference). The cable's shielding layer should be connected to the grounding terminal PE at only one end.
2.2 Grounding
Grounding, seemingly simple, is actually a difficult problem to master and handle because a systematic theory or model has yet to be developed. In reality, a solution that works well in one situation may not be suitable for another. Grounding design largely depends on engineers' understanding of the concept of "grounding" and their practical experience.
There are many methods for grounding, and the specific method used depends on the structure and function of the system. There are three commonly used methods.
1) Single-point grounding provides a common potential reference point for many circuits connected together, allowing signals to be transmitted between different circuits. This point is often referenced to the earth. Since there is only one reference point, it can be assumed that there are no ground loops, and therefore no interference problems.
2) In multi-point grounding systems, the internal circuitry uses the chassis as a reference point, while each chassis uses ground as its reference point. This grounding structure provides lower grounding impedance because each ground wire can be very short, and the parallel connection of multiple wires reduces the total inductance of the grounding conductor. Multi-point grounding is essential in high-frequency circuits, and the length of each ground wire must be less than 1/200 of the signal wavelength.
3) Hybrid grounding combines the characteristics of both single-point grounding and multi-point grounding. For example, the power supply in the system requires single-point grounding, while the radio frequency signal requires multi-point grounding. In this case, hybrid grounding can be used.
According to grounding requirements, grounding can be divided into the following types.
1) Safety grounding Equipment using AC power must be grounded through a yellow-green safety ground wire. Otherwise, when the insulation resistance between the power supply inside the equipment and the casing decreases, it may cause electric shock.
2) Electromagnetic compatibility grounding: Grounding required by electromagnetic compatibility design, including:
(1) Shielding and grounding In order to prevent mutual interference between circuits due to the existence of parasitic capacitance, the circuit radiates electric fields or is sensitive to external electric fields, necessary isolation and shielding must be carried out. The metals of these isolation and shielding must be grounded.
(2) Filter grounding Filters generally contain bypass capacitors from signal lines or power lines to ground. When the filter is not grounded, these capacitors are in a floating state and cannot play the role of bypass.
(3) Noise and interference suppression Controlling internal noise and external interference requires many points on the equipment or system to be connected to ground, thereby providing a “lowest impedance” path for interference signals.
(4) For signals to be transmitted correctly between circuits, there must be a common potential reference point, which is ground. Therefore, all interconnected circuits must be grounded.
2.3 Filtering
Filtering is an effective method for compressing the interference spectrum. When the interference spectrum differs from the frequency band of the useful signal, a filter can be used to remove the interference. Therefore, proper selection and correct use of filters are crucial for suppressing conducted interference.
Filtering divides the signal spectrum into two segments: useful frequency components and interference components, eliminating the interference. Filters are generally classified as low-pass filters, high-pass filters, band-pass filters, and band-stop filters. Filters on the AC side of the main circuit are primarily used to filter out electromagnetic interference from the power grid; Figure 3 shows common spike interference on the power grid. Filters in the DC circuit mainly reduce interference caused by the inductive effect of the line.
When using a power supply filter, it should be installed as close as possible to the power input, and the input and output terminals of the filter should be shielded and isolated to prevent electromagnetic interference from directly coupling from the input to the output. Furthermore, the filter's grounding point should be as close as possible to the equipment's grounding point. Figure 4 shows the circuit diagram of a power supply filter.
2.4 Isolation
Isolation is an effective measure to eliminate common impedance interference caused by ground loops. Common methods include isolation transformers, optocouplers, and optical fibers. Opto-isolation offers advantages such as unidirectional signal transmission with a wide bandwidth, strong anti-interference capability, high insulation voltage, small size, low cost, and shock resistance, making it widely used in control systems. Furthermore, differential circuits and balanced circuits can reduce ground loop currents, thus suppressing interference.
3. Hardware measures to solve electromagnetic interference
High-voltage frequency converters generate numerous high-frequency and low-frequency electromagnetic interference waves during operation due to rectification and inversion. These electromagnetic waves can interfere with system controllers, PLCs, touch screens, digital instruments, and sensors. To suppress interference from high-voltage frequency converters to other low-voltage equipment and instruments, all components should be reliably grounded. Shielded control cables should be used for connections between electrical components, instruments, and meters, and the shielding layer should be grounded. Filtering measures should be implemented for input and output analog and switching signals, and opto-isolation methods should be used when necessary.
1) Shielded cables are used for both power lines and control signal lines in the system. The high-voltage cables used in the high-voltage motors are shielded, which partially suppresses the high-frequency components of noise current. Shielded cables consist of an unshielded ordinary conductor with a metal shielding layer added outside. The metal shielding layer's reflection, absorption, and skin effect prevent electromagnetic interference and radiation. Shielded cables combine the balance principle of twisted pairs with the shielding effect of the shielding layer, thus possessing excellent electromagnetic compatibility characteristics. Fiber optic communication is used between the controller and the power unit to ensure effective isolation between high-voltage and low-voltage circuits.
2) The entire system must be properly grounded. The grounding of the high-voltage section and the control section should be handled separately. Proper grounding of the frequency converter is crucial for improving the sensitivity of the control system and suppressing noise. The grounding resistance of the inverter's PE terminal should be as low as possible, and the cross-sectional area of the grounding conductor should be ≥2mm². The grounding of the high-voltage system should ideally be separate from the grounding point of the control equipment to prevent signal crosstalk. The shielding layer of the signal input line should have one end connected to the PE terminal, while the other end should be left floating; otherwise, it will cause signal fluctuations and system oscillations. All equipment in the control cabinet should be electrically connected, which can be achieved using copper core wires for bridging. The PE terminals of each frequency converter should be connected to form an equipotential connection. The zero potential of the control section should be connected separately to the grounding electrode.
3) Install an AC filter and isolation transformer on the AC input side of the frequency converter controller to improve the input power quality. Figure 5 shows the shielding and grounding method of the isolation transformer. To ensure uninterrupted operation of the controller and prevent voltage drops, a UPS can be installed.
4) Adding filters to the inverter control circuit and network circuit can suppress low- and medium-frequency electromagnetic interference. Adding a du/dt filter or differential mode filter will have a better effect.
5) Control cables should be routed as far away from power cables as possible (minimum interval of 20cm), preferably using a separate cable tray. If the same cable tray is used, a partition plate must be installed in the middle, and the partition plate must have multiple grounding points along its length. When control cables and power cables must cross, they should cross at a 90° angle to minimize electromagnetic interference.
6) Install an opto-isolation card between the PLC and the frequency converter to prevent the high-voltage frequency converter from transmitting electromagnetic interference to the control network through the PLC.
7) Suppression components, such as RC, diodes, and varistors, must be used on the coils of contactors, relays, etc. in the control cabinet.
8) The spare wires of the cable are grounded at both ends to increase the shielding effect.
9) Using anti-interference components, such as ferrite beads, ferrite rings, power filters, and shielding covers, in critical locations such as DSP I/O ports, power lines, and circuit board connections can significantly improve the circuit's anti-interference performance.
4. Software anti-interference design
1) Use polling instead of interrupts to minimize interrupts and avoid false triggering and sensor-triggered triggering.
2) Digital filtering is used in A/D conversion to prevent sudden interference. This can be achieved using methods such as averaging or comparison averaging.
3) Set up watchdogs and software traps in key parts of the software so that the system can remain under control even if the software crashes.
4) Debounce the input switch signal by delay.
5) For I/O ports to operate correctly, it is necessary to check the execution status of I/O port commands to prevent external faults from causing control commands to fail to execute.
6) Parity checks or other methods such as querying, voting, and comparison should be used in communication to prevent errors. If necessary, the communication register settings should be reset to prevent communication failures or other malfunctions caused by errors.
5 Conclusion
In high-voltage variable frequency speed control electrical systems, the presence of high-voltage converters generates significant electromagnetic interference (EMI). If left unchecked, this interference will affect the normal operation of the entire control system. However, completely eliminating EMI is unrealistic. EMI suppression should be tailored to different components and electromagnetic environments, with the system's ability to operate normally serving as the benchmark. It is unnecessary to employ complex measures solely to achieve high EMI suppression targets. Generally, the strength of EMI suppression capability is directly proportional to investment. Electromagnetic compatibility (EMC) in variable frequency speed control electrical systems is a highly complex systems engineering project, requiring the summarization of practical experience and the exploration of numerous theoretical aspects.
