Sep 05, 2020
Inverter interference and suppression
In the frequency converter, high-power diode rectification and high-power transistor inversion are required. As a result, high-order harmonics of current are generated in the input and output circuits, which interfere with the power supply system, load and other nearby electrical equipment. In the actual use process, we often encounter the problem of harmonic interference from the inverter. The following briefly introduces the mechanism of harmonic generation, the propagation path and the effective method of suppressing interference.
1. Frequency converter harmonic generation mechanism
The main circuit of the inverter is generally composed of AC-DC-AC. The external input 380V/50Hz industrial frequency power is uncontrollably rectified into a DC voltage signal by a three-phase bridge, and it is inverted to frequency by a filter capacitor and a high-power transistor switching element. Variable AC signal. In the rectifier circuit, the waveform of the input current is an irregular rectangular wave, and the waveform is decomposed into the fundamental wave and each harmonic according to the Fourier series, and the higher harmonics will interfere with the input power supply system. In the inverter output circuit, the output current signal is a pulse waveform modulated by a PWM carrier signal. For GTR high-power inverter components, the PWM carrier frequency is 2～3kHz, and the PWM highest carrier frequency of IGBT high-power inverter components Up to 15kHz. Similarly, the output loop current signal can also be decomposed into the fundamental wave containing only the sine wave and other harmonics, and the higher harmonic current directly interferes with the load. In addition, high-order harmonic currents radiate to space through cables and interfere with nearby electrical equipment.
2. Common methods to suppress harmonic interference
The propagation path of harmonics is conduction and radiation. The solution to conduction interference is mainly to filter or isolate the conducted high-frequency current in the circuit; to solve the radiation interference is to shield the radiation source or the interfered line. Specific common methods:
(1) The power supply of the variable frequency system is independent of the power supply of other equipment, or install an isolation transformer on the input side of the inverter and other electrical equipment to cut off the harmonic current.
(2) Connect a suitable reactor in series on the input side and output side of the inverter, or install a harmonic filter. The composition of the filter must be LC type to absorb harmonics and increase the impedance of the power supply or load to suppress harmonics the goal of.
(3) The cables between the motor and the inverter should be laid through steel pipes or armored cables, and should be laid in separate cable trenches with other weak current signals to avoid radiation interference.
(4) The signal wire is shielded wire, and the wiring is staggered from the main circuit control wire of the inverter by a certain distance (at least 20cm or more) to cut off the radiation interference.
(5) The inverter uses a dedicated ground wire, and a thick and short wire is used for grounding. The ground wire adjacent to other electrical equipment must be separated from the inverter wiring and use a short wire. This can effectively suppress the radiation interference of current harmonics to adjacent equipment.
3. Examples of suppressing harmonic interference
Example 1: In a frequency conversion control system, the frequency converter starts and runs normally, and the adjacent level gauge reads high. When the meter inputs 4mA, the liquid level display is not the lower limit; when the liquid level does not reach the set upper limit, the liquid The position meter displays the upper limit, causing the inverter to receive a stop command, forcing the inverter to stop running.
This is obviously the high-order harmonics of the frequency converter interfering with the level gauge, and the interference propagation path is the power circuit or signal line of the level gauge.
Solution: Take the power supply of the level gauge from another power supply transformer, reduce the harmonic interference, and then lay the signal line through the steel pipe, and separate it from the main circuit line of the frequency converter by a certain distance. The interference is basically suppressed, and the level gauge works back to normal.
Example 2: In a frequency conversion control liquid level display system, the liquid level gauge and the frequency converter are installed in the same cabinet. The frequency converter works normally, but the liquid level gauge display is inaccurate and unstable. At first, we had a primary meter and a secondary meter. , The signal line and the fluid medium have problems. Replace all these meters, signal cables, and improve the fluid characteristics. The fault still exists, and this fault is that the high-order harmonic current of the inverter radiates through the output circuit cable and is transmitted to the signal cable. , Cause interference.
Solution: The signal line of the level gauge and its control line are separated from the control line and main circuit line of the inverter by a certain distance, and the signal line outside the cabinet is laid through the steel pipe, and the shell is well grounded. Troubleshooting.
Example 3: A frequency conversion control system is composed of two frequency converters, and in the same cabinet, the frequency adjustment mode of the frequency converter is the potentiometer manual adjustment mode. When one frequency converter is running, it works normally, and when two units are running at the same time, The frequencies interfere with each other, that is, adjusting the potentiometer of one inverter has an effect on the frequency of another inverter, and vice versa. At first, we thought it was a fault of the potentiometer and control line. After eliminating this possibility, we concluded that it was caused by harmonic interference.
Solution: Move one of the potentiometers to other cabinets and fix it, and use shielded signal wires for the leads, resulting in reduced interference. In order to completely suppress the interference, reprocess an electric control cabinet and place it at a certain distance from the original cabinet. Move one of the inverters to the electric control cabinet, and make necessary changes to the corresponding wiring and leads. After this treatment, the interference Basically eliminated, troubleshooting. Example 4, a variable frequency control system, switching between two sets of pumps, the original pump is * auto-coupling step-down start, power frequency operation is normal, now it is changed to frequency conversion operation, although the frequency modulation deceleration function can be achieved, but the output of the frequency converter and the motor The output line of the motor is seriously heated, and the temperature rise of the motor shell is aggravated, and the protection trips often occur. This is because the output voltage and current signals of the inverter contain PWM high-order harmonics, and the harmonic currents form additional power losses on the output wires and motor windings.
Example 4: A frequency conversion control system switches between two sets of pumps. The original pumps were started by auto-coupling and stepped down. The power frequency is running normally. Now it is changed to frequency conversion. Although the frequency modulation and deceleration function can be realized, the output of the frequency converter and the motor are connected. The output line of the motor is seriously heated, and the temperature rise of the motor shell is aggravated, and the protection trips often occur. This is because the output voltage and current signals of the inverter contain PWM high-order harmonics, and the harmonic currents form additional power losses on the output wires and motor windings.
Solution: Separate the input wire and output wire of the inverter, and go through their respective cable trenches. Use a cable with a larger cross-section to replace the original cable. The length of the cable between the output end and the motor should be as short as possible. After this treatment, the heating fault is eliminated. Basically, it can be smoothly suppressed according to the above-mentioned methods for the high-order harmonic interference of various inverters that appear on the scene. However, it is very difficult to completely suppress the high-order harmonic interference for equipment with strict requirements on the harmonic components and amplitude. Further solve the problem.