A brief discussion on the anti-interference and interference performance of AC inverter systems

With the development of China's economy and the progress of science and technology, the application of AC variable frequency speed regulation is becoming more and more extensive. Among various speed regulation methods, AC variable frequency speed regulation technology has been recognized at home and abroad as the most ideal and most promising speed regulation method. When factories and equipment use AC speed regulation, harmonic interference will be generated on the power supply side and the motor side of the inverter. On the one hand, when the inverter is running, it is necessary to prevent electromagnetic interference from the outside world; on the other hand, it is necessary to prevent the generation of high-order harmonics to interfere with other external devices, which is the so-called "EMC".
1. What is EMC?

EMC is "electromagnetic compatibility". It refers to the good working ability of electrical equipment in an electromagnetic environment and cannot generate electromagnetic interference that other equipment working in this environment cannot accept.

The definition of electromagnetic compatibility by the International Electrotechnical Commission (IEC) is: "Electromagnetic compatibility is a function of electronic equipment. Electronic equipment can complete its function in an electromagnetic environment without generating intolerable interference."

In the national standard of "electromagnetic compatibility" promulgated by China, electromagnetic compatibility is defined as follows: "Equipment or system can work normally in its electromagnetic environment and does not constitute unbearable electromagnetic interference to anything in the environment."
Obviously, electromagnetic compatibility has a dual meaning: anti-interference and interference.
2. Frequency converter and electromagnetic compatibility

Generally speaking, electrical equipment must have the ability to suppress both high-frequency and low-frequency interference. High-frequency interference mainly includes electrostatic discharge, pulse interference and electromagnetic field of radio frequency, etc., while low-frequency interference mainly refers to power supply voltage fluctuation, undervoltage and frequency instability. Usually, the frequency converter can operate in an industrial environment where high electromagnetic interference (EM1) may exist. It is both a noise emission source and a noise receiver.
1. External interference to the frequency converter
1) Interference of transistor commutation equipment on the frequency converter:
When there is a large-capacity thyristor commutation equipment in the power supply network, since the thyristor is always turned on for part of the time in each half cycle of each phase, it is easy to cause the network voltage to be uneven (as shown in Figure 1-1). It may cause the rectifier circuit on the input side of the frequency converter to have a large reverse recovery voltage and be damaged.

2) Interference of the inverter caused by the input and output of compensation capacitors:
When the centralized capacitor compensation method is used in the substation of the power supply line to improve the power factor, the network voltage may have a very high peak value during the transient process of input and output of compensation capacitors (as shown in Figure 1-2). As a result, the inverter's rectifier diode may be subjected to excessive reverse voltage and break down.

2. Inverter interference to the outside
1)
Inverter current waveform: The input current and output current of the inverter both have strong high-order harmonic components, which will interfere with other control devices and affect the normal operation of other devices.
Input current waveform:
For example, the input side of the "AC-DC-AC" voltage inverter is a rectifier and filter circuit (as shown in Figure 2-2). Only when the line voltage U2 of the power supply is greater than the voltage
UD across the capacitor, there is a charging current in the rectifier bridge. The charging current always appears near the amplitude value of the power supply voltage in the form of a discontinuous shock wave (as shown in Figure 2-1). It has a very high odd harmonic component, especially the 5th and 7th harmonics, as shown in Table 1 (taking Siemens MM3 inverter as an example).

The parasitic capacitance Cp exists in the motor cable and inside the motor, so the switch wing of the PWM output voltage waveform of the inverter generates a high-frequency pulse current Is through the parasitic capacitance, making the inverter a harmonic interference source. Since the source of the harmonic current Is is the inverter, it must flow back to the inverter. In the figure, Ze is the
earth impedance, and Zn is the impedance between the power cable and the ground. The voltage drop caused by the harmonic current flowing through these two impedances will affect other equipment on the same power grid and cause interference.

DC reactor (smoothing reactor): connected in series between the rectifier bridge and the filter capacitor (as shown in Ld in Figure 4-1), its main function is to weaken the high-order harmonic components in the inverter input current, and can improve the power factor by suppressing the harmonic current.
Filter:
In the input and output circuits of the inverter, in addition to the above-mentioned lower-order harmonic components, there are also many higher-frequency harmonic currents, which will form interference signals to other equipment in various ways. The filter is a method used to weaken the higher-frequency harmonic components (as shown in Figure 4-2).
Input filter: There are usually two types
(1) Line filter: mainly composed of inductor coils (as shown in F11 in Figure 4-2). It mainly weakens the higher-frequency harmonic current by increasing the impedance of the line at high frequencies.
(2)
Radiation filter: mainly composed of high-frequency capacitors (as shown in F12 in Figure 4-2, it can also be connected in a △ connection). It will absorb high-frequency harmonic components with radiation energy. The high-frequency current flowing back to the power supply is greatly reduced. (You can also use the special "radio anti-interference filter" provided by the inverter manufacturer).

Output filter: It is also mainly composed of an inductor coil (as shown in F0 in Figure 4-2). It can effectively weaken the high-order harmonic components in the output current, not only playing an anti-interference role, but also weakening the additional torque caused by the high-order harmonic current in the motor.
Note: When the output filter is composed of an LC circuit, the side of the filter connected to the capacitor must be connected to the motor.

3) Use shielded cables and reasonable wiring: Interference signals transmitted by induction can be eliminated by the following methods.

Shielded signal cables increase anti-interference (as shown in Figure 4-4):
When the inverter is the object of interference, high-order harmonic interference current Is can enter the inverter through the potential and coupling capacitor and
generate a voltage drop on the impedance Zi, causing noise interference. The most effective way to do this is to strictly isolate high-frequency interference and signal cables, and the signal cable shield must be grounded at both ends.

It is best to use shielded cables for control cables. Generally speaking, the shielding layer of the control cable should be directly grounded inside the inverter, and the other side should be grounded through a high-frequency small capacitor (such as 3.3nf/3000V). When the differential mode voltage at both ends of the shielding layer is not high and connected to the same ground wire, both ends of the shielding layer can also be directly grounded. The signal line and its return line are twisted together to reduce the interference caused by inductive coupling. The closer the twisting is to the terminal, the better. The transmission line of analog signals should use double-shielded twisted pair. Different analog signal lines should be routed independently and have their own shielding layers to reduce coupling between lines. Do not place different analog signals on the same common return line. It is best to use double-shielded twisted pair cables for low-voltage digital signals, or single-shielded twisted pair cables can also be used.

Good grounding and reasonable wiring: Ensure that all equipment in the cabinet is well grounded, and use short and thick grounding wires to connect to the common grounding point or grounding busbar. It is particularly important that any control equipment connected to the inverter should share the same ground with it, and also use short and thick wires to ground it. It is best to use flat conductors (such as metal mesh) because they have low impedance at high frequencies.

Reasonable wiring is also very important. The transmission cables for analog signals and digital signals should be routed separately. Try not to share the same cable with the low-voltage line and the 220VAC power line. Try to avoid long-distance parallel routing of motor cables and other cables.
When designing the control cabinet, pay attention to the regional principle of EMC, and try to plan different equipment in different areas.
III. Conclusion

The anti-interference and interference of equipment is a very important issue. At present, EMC has become a major cause of system failure. One of the principles of EMC is "prevention is the most effective and economical solution." Therefore, EMC has become an important issue that cannot be ignored to ensure the reliable and normal operation of frequency conversion equipment.