Discussion on Suppressing Harmonic Interference of Frequency Converter

1. Frequency converter harmonic generation mechanism

The main circuit of the frequency converter is generally composed of AC-DC-AC. The external input 380V/50Hz industrial frequency power supply is uncontrollably rectified into a DC voltage signal through a three-phase bridge circuit, filtered by a filter capacitor and inverted into a frequency-variable AC signal by a high-power transistor switch element. In the rectifier circuit, the waveform of the input current is an irregular rectangular wave, which is decomposed into a fundamental wave and harmonics according to the Fourier series. 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 a PWM carrier signal. For GTR high-power inverter elements, the PWM carrier frequency is 2 to 3kHz, while the PWM maximum carrier frequency of IGBT high-power inverter elements can reach 15kHz. Similarly, the output circuit current signal can also be decomposed into a fundamental wave containing only a sine wave and other harmonics, and the high-order harmonic current directly interferes with the load. In addition, the high-order harmonic current also radiates into space through cables, interfering with nearby electrical equipment.

2. Common methods for suppressing harmonic interference

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

3. Example of suppressing harmonic interference

Example 1: In a certain frequency conversion switching control system, the inverter starts and runs normally, but the reading of the adjacent liquid level meter is too high. When the meter inputs 4mA, the liquid level display is not the lower limit value; when the liquid level does not reach the set upper limit value, the liquid level meter displays the upper limit, causing the inverter to receive a shutdown command, forcing the inverter to stop running.

This is obviously the high-order harmonic interference of the frequency converter on the level meter, and the interference propagation path is the power supply circuit or signal line of the level meter. Solution: Take the power supply of the level meter from another power supply transformer to 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. After this treatment, the harmonic interference is basically suppressed and the level meter returns to normal operation.

Example 2: In a frequency conversion controlled liquid level display system, the liquid level meter and the frequency converter are installed in the same cabinet. The frequency converter works normally, but the liquid level meter displays inaccurate and unstable. At first, we suspected that there were problems with the primary meter, secondary meter, signal line and fluid medium. We replaced all these instruments and signal cables and improved the fluid characteristics, but the fault still existed. The fault was that the high-order harmonic current of the frequency converter radiated outward through the output loop cable and was transmitted to the signal cable, causing interference.

Solution: The signal line of the level meter 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 in a steel pipe, and the shell is well grounded, and the fault is eliminated. Example 3: A certain frequency conversion control system consists of two inverters, and they are in the same cabinet. The frequency adjustment method of the inverter is the manual adjustment method of the potentiometer. When running a certain inverter, it works normally. When the two are running at the same time, the frequencies interfere with each other, that is, adjusting the potentiometer of one inverter affects the frequency of the other inverter, and vice versa. At first, we thought that the potentiometer and control line were faulty. After excluding this possibility, we concluded that it was caused by harmonic interference.

Solution: Move one of the potentiometers to another cabinet and fix it, and use shielded signal wires for the leads, which reduces the 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 is basically eliminated and the fault is eliminated. Example 4: In a certain frequency conversion control system, two sets of pumps are switched. The original pumps were running normally at the industrial frequency by auto-coupling step-down starting, but now they are changed to variable frequency operation. Although the frequency modulation and deceleration function can be realized, the output line between the output end of the inverter and the motor is seriously heated, the temperature rise of the motor casing increases, and the protection tripping often occurs. This is because the output voltage and current signals of the inverter contain PWM high-order harmonics, and the harmonic current forms additional power loss on the output wires and motor windings.

Solution: Separate the input and output lines of the inverter, put them in their own cable trenches, use cables with a larger cross-section to replace the original cables, and keep the cable length between the output end and the motor as short as possible. After this treatment, the heating fault is eliminated. The various inverter high-order harmonic interferences that appear on the scene can basically be suppressed smoothly according to the methods introduced above, but it is very difficult to completely suppress high-order harmonic interference for equipment with strict requirements on harmonic components and amplitudes, which needs further research and solution.