Harmonic interference and suppression of VFD (Variable-frequency Drive)

VFD (Variable-frequency Drive) requires high-power diode rectification and high-power transistor inversion. As a result, high-order current harmonics are generated in the input and output circuits, which interfere with the power supply system, loads and other nearby electrical equipment. In the actual use process, VFD (Variable-frequency Drive) harmonic interference problems are often encountered. The following is a brief introduction to the mechanism of harmonic generation, propagation channels and effective methods of suppressing interference.

1. VFD (Variable-frequency Drive) harmonic generation mechanism

The main circuit of VFD (Variable-frequency Drive) is generally composed of AC-DC-AC. The external input 380V/50Hz power frequency power supply is uncontrollably rectified into a DC voltage signal by a three-phase bridge circuit, filtered by a filter capacitor and switched by a high-power transistor. The element inverts to an AC signal with variable frequency. 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 various harmonics according to the Fourier series, and the high-order harmonics will interfere with the input power supply system. In the inverter output circuit, the output current signal is a pulse waveform modulated by the PWM carrier signal. For the GTR high-power inverter component, the PWM carrier frequency is 2-3kHz, and the PWM maximum carrier frequency of the IGBT high-power inverter component is Up to 15kHz. Similarly, the output loop current signal can also be decomposed into the fundamental wave containing only sine waves and other harmonics, while the high-order harmonic current directly interferes with the load. In addition, the high-order harmonic current radiates to the space through the cable and interferes with adjacent electrical equipment.

2. Commonly used methods to suppress harmonic interference

The propagation path of harmonics is conduction and radiation. To solve the conduction interference is to filter out or isolate the conducted high-frequency current in the circuit; to solve the radiation interference is to shield the radiation source or the disturbed line. Specific common methods: (1) The power supply of the frequency conversion system is independent from the power supply of other equipment, or an isolation transformer is installed on the input side of the VFD (Variable-frequency Drive) and other electrical equipment to cut off the harmonic current. (2) Connect a suitable reactor in series with the input side and output side of the VFD (Variable-frequency Drive), or install a harmonic filter. The composition of the filter must be LC type, absorbing harmonics and increasing the impedance of the power supply or load , to achieve the purpose of suppressing harmonics. (3) The cables between the motor and VFD (Variable-frequency Drive) should be laid through steel pipes or armored cables, and laid separately from other weak current signals in different cable trenches to avoid radiation interference. (4) The signal line adopts shielded line, and the wiring is staggered with a certain distance (at least 20cm) from the VFD (Variable-frequency Drive) main circuit control line to cut off the radiation interference. (5) VFD (Variable-frequency Drive) uses a dedicated grounding wire, and it is grounded with thick and short wires. The ground wires adjacent to other electrical equipment must be separated from the VFD (Variable-frequency Drive) wiring, and short wires are used. This can effectively suppress the radiation interference of current harmonics to adjacent equipment.

3. Examples of harmonic interference suppression

Example 1: In a frequency conversion switching control system, the VFD (Variable-frequency Drive) starts and runs normally, but the reading of the adjacent liquid level gauge is too high. When the primary meter inputs 4mA, the liquid level display is not the lower limit value; the liquid level is not up to the set value When the limit value is reached, the liquid level gauge displays the upper limit, causing the VFD (Variable-frequency Drive) to receive a shutdown command, forcing the VFD (Variable-frequency Drive) to stop running.

This is obviously VFD (Variable-frequency Drive) high-order harmonics that interfere with the liquid level gauge, and the interference propagation path is the power circuit or signal line of the liquid level gauge. Solution: Take the power supply of the liquid level gauge from another power supply transformer to weaken the harmonic interference, and then lay the signal line through the steel pipe, and separate it from the VFD (Variable-frequency Drive) main circuit line by a certain distance. After treatment, the harmonic 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 VFD (Variable-frequency Drive) are installed in the same cabinet, the VFD (Variable-frequency Drive) works normally, but the liquid level gauge displays inaccurate and unstable, At first we had problems with the primary meter, secondary meter, signal line and fluid medium. After replacing all these meters and signal cables and improving the fluid characteristics, the fault still exists, and this fault is the high-order of VFD (Variable-frequency Drive) Harmonic current radiates outward through the output loop cable and is transmitted to the signal cable, causing interference.

Solution: The liquid level gauge signal line and its control line are separated from the VFD (Variable-frequency Drive) control line and main circuit line by a certain distance, and the signal line outside the cabinet is laid through the steel pipe, the shell is well grounded, and the fault is eliminated. Example 3, a certain frequency conversion control system is composed of two VFD (Variable-frequency Drive), and in the same cabinet, the VFD (Variable-frequency Drive) frequency modulation mode is the potentiometer manual adjustment mode, running a VFD (Variable-frequency Drive) -frequency Drive), it works normally. When the two are running at the same time, the frequency interferes with each other, that is, adjusting the potentiometer of one VFD (Variable-frequency Drive) has an impact on the frequency of the other VFD (Variable-frequency Drive). The same goes for coming over. At first we thought it was the fault of the potentiometer and the control line. After excluding this possibility, we concluded that it was caused by harmonic interference.

Solution: Move one of the potentiometers to other cabinets for fixing, and use shielded signal wires for the lead wires. As a result, the interference is weakened. In order to completely suppress interference, rework an electric control cabinet and place it at a certain distance from the original cabinet, move one of the VFDs (Variable-frequency Drive) to the electric control cabinet, and make necessary changes to the corresponding wiring and leads. After this treatment, the interference is basically eliminated, and the fault is eliminated. Example 4: In a frequency conversion control system, two sets of motor pumps were switched. The motor pumps used to rely on auto-coupling step-down to start the normal operation of power frequency, but now they are changed to frequency conversion operation. Although the function of frequency modulation and deceleration can be realized, VFD (Variable-frequency Drive) The output line between the output terminal and the motor is seriously heated, the temperature rise of the motor shell is aggravated, and protection trips often occur. This is because the VFD (Variable-frequency Drive) output voltage and current signals contain PWM high-order harmonics, and the harmonic currents form additional power losses on the output wires and motor windings.