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

With the development of my country'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 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 that interfere with other external equipment, which is the so-called "EMC".

1. What is EMC?

EMC stands for "electromagnetic compatibility". It refers to the ability of electrical equipment to work well in an electromagnetic environment and not to generate electromagnetic interference that is unacceptable to other equipment working in this environment.

The International Electrotechnical Commission (IEC) defines electromagnetic compatibility as: "Electromagnetic compatibility is a function of electronic equipment that can perform its functions in an electromagnetic environment without causing intolerable interference."

In the national standard of "Electromagnetic Compatibility" promulgated by my country, electromagnetic compatibility is defined as follows: "The equipment or system can work normally in its electromagnetic environment and does not cause unacceptable 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 be able to suppress both high-frequency and low-frequency interference. High-frequency interference mainly includes electrostatic discharge, pulse interference and electromagnetic fields of radio frequency, while low-frequency interference mainly refers to power supply voltage fluctuation, undervoltage and frequency instability. Usually, the inverter can operate in an industrial environment where high electromagnetic interference (EM1) may exist. It is both a noise emitter and a noise receiver.

1. External interference to the inverter

1) Interference of transistor commutation equipment on inverter:

When there are large-capacity thyristor commutation devices in the power supply network, the thyristor is always turned on for part of the time in each half cycle, which can easily cause the network voltage to be uneven (as shown in Figure 1-1). This may cause the rectifier circuit on the input side of the inverter to have a large reverse recovery voltage and be damaged.

2) Interference of the input and output of compensation capacitors on the inverter:

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 the compensation capacitor being switched in and out (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's interference to the outside world

1) Inverter current waveform: Both the input current and output current of the inverter have strong high-order harmonic components, which will interfere with other control devices and affect the normal operation of other devices.

Input current waveform:

If the input side of the "AC-DC-AC" voltage inverter is a rectifier and filter circuit (as shown in Figure 2-2), only the line voltage U2 of the power supply is greater than the voltage across the capacitor.

When UD, there is charging current in the rectifier bridge. The charging current always appears near the amplitude value of the power supply voltage, in the form of discontinuous shock waves (as shown in Figure 2-1). It has very high odd harmonic components, 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 (such as Ld in Figure 4-1). Its main function is to weaken the high-order harmonic components in the inverter input current and improve the power factor by suppressing harmonic current.

filter;

In the input and output circuits of the inverter, in addition to the lower-order harmonic components mentioned above, 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: It is mainly composed of an inductor coil (as shown in F11 in Figure 4-2). It mainly weakens the high-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 △ way). It absorbs high-frequency harmonic components with radiation energy. This greatly reduces the high-frequency current flowing back to the power supply. (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 capacitor connected to the filter 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 interfered object, the 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, resulting in noise interference. The most effective way to do this is to strictly isolate the 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 shield 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 shield layer is not high and connected to the same ground wire, both ends of the shield 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 for low-voltage digital signals, or single-shielded twisted pair.

Good grounding and reasonable wiring: Make sure 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 be grounded with it, and also grounded with short and thick wires. 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 of analog signals and digital signals should be routed separately. Try not to use the low-voltage line and the 220VAC power line in the same cable. Try to avoid running the motor cable parallel to other cables for a long distance.

When designing the control cabinet, pay attention to the EMC regional principles and try to plan different equipment in different areas.

3. Conclusion

The anti-interference and interference resistance 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.