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In various industrial control systems, with the use of power electronic equipment such as frequency converters (hereinafter referred to as VFD), the electromagnetic interference of the system is also increasing. When it comes to the anti-interference problem of the inverter, it is necessary to understand the interference and the propagation mode, and then take different measures. Let's follow Qifeng to understand the inverter inElectrical control cabinetsHow to solve the problem of interference?
The inverter is inElectrical control cabinetsHow to solve the problem of interference?1. VFD interference sources.
Interference from the external power grid. Harmonic interference in the power grid is caused by the power supply interference of the frequency conversion governor, and there are a large number of harmonic sources in the power grid, such as various rectifier devices, AC and DC switchgear, electronic voltage regulators, nonlinear loads and lighting devices.
1.Interference of thyristor converters to VFD.
When there is a large-capacity thyristor converter in the power grid, it is easy to cause the grid voltage notch and serious waveform distortion because the thyristor will be turned on for part of the time of each phase and half a cycle. Excessive reverse recovery voltage may damage the rectifier circuit on the input side of the VFD, causing the input circuit to break down and burn out.
2.Power compensates for the interference of the capacitor on the inverter.
The power sector has requirements for the power factor of electrical equipment. More users use centralized capacitance compensation in substations to improve the power factor. During transient processes that compensate capacitors for turn-on or turn-off, the grid voltage may have high peaks, so the VFD's rectifier diodes may break down due to excessive reverse voltages.
VFD interferes with the ego of the outside world. The VFD rectifier bridge is a nonlinear load for the power grid, and the harmonics generated by it can cause harmonic interference to other electrical and electronic equipment in the same power grid. Most VFD inverters use PWM technology, which generates large coupling noise when operating in switching mode and high-speed switching. Therefore, the frequency converter is a source of electromagnetic interference to other electrical and electronic equipment in the system.
The input and output currents of a VFD contain a large number of higher harmonic components. In addition to the lower harmonics that can constitute reactive power loss in the power supply, there are also many higher frequency harmonic components. They will propagate their own components in various ways and form interference signals to the VFD itself and other devices.
2. Propagation method of interference signal.
The inverter generates higher power harmonics. It has a strong interference with other devices in the system. The interference method is the same as general electromagnetic interference, which first generates electromagnetic radiation to the surrounding electrical and electronic equipment. For the direct drive motor to generate electromagnetic noise, increase the iron and copper consumption of the motor. After interfering with the system's power supply and other equipment through the distribution network, the VFD is inductively coupled to other adjacent lines to sense the interfering voltage or current. Similarly, the interfering signal in the system can interfere with the normal operation of the VFD in the same way.
1) The circuit coupling pattern propagates in the power network. Since the input current is a non-sine wave, when the VFD capacity is large, the grid voltage will be distorted, affecting the work of other equipment workers. The conduction interference generated at the output will greatly increase the copper and iron loss of the direct drive motor, and affect the operating characteristics of the motor.
2) Inductive coupling. When the input circuit or output circuit of the inverter is close to the circuit of other equipment, the higher harmonic signal of the inverter is inductively coupled to the other equipment.
a.Electromagnetic induction is the way in which an electric current interferes with a signal.
b.Electrostatic induction is the way in which voltage interferes with signals.
3) Air radiation refers to the radiation into the air through electromagnetic waves, which is the propagation mode of high-frequency harmonic components.