Examples of harmonic interference prevention of th

Examples of harmonic interference prevention and control of frequency converter

this paper analyzes the harmonic generation mechanism of frequency converter, the ways of harmonic interference, the harm of harmonic interference and the common methods of restraining harmonic interference, and gives several typical examples of restraining harmonic interference

1. Introduction

in recent years, due to the need of speed regulation and energy saving, frequency conversion speed regulation technology has been used in more and more occasions. The core part of the frequency converter is power electronic devices, including electronic components and computer chips, which are vulnerable to some external electrical interference. Therefore, when the frequency converter is put into operation, it is necessary to consider whether the electric voltage is symmetrical, the size of the transformer capacity and whether there are nonlinear devices connected to the distribution bus; On the other hand, the input side of the frequency converter itself is a nonlinear rectifier circuit, which will have an impact on the waveform of the power supply. The output side of the frequency converter adopts Rockwell hardness tester voltage, current, non sinusoidal or incomplete sinusoidal wave, which contains rich harmonics. As high-power diode rectification and high-power transistor inversion are required in the frequency converter, the result is that high-order current harmonics are generated in the input and output circuit, interfering with the power supply system, load and other adjacent electrical equipment. In the actual use process, we often encounter the problem of harmonic interference of frequency converter. The following briefly introduces the mechanism of harmonic generation of frequency converter, interference ways, hazards, and Countermeasures to effectively prevent or suppress interference when the operators are operating groups in various positions

2. Harmonic generation mechanism of the frequency converter

the main circuit of the frequency converter is generally composed of ac-dc-ac. the external input 380V/50Hz power frequency power supply is uncontrollably rectified into a DC voltage signal through a three-phase bridge, which is filtered by a filter capacitor and inverted into a frequency variable AC signal by a high-power transistor switching element

harmonic generation mechanism at the input side: not limited to general-purpose frequency converters, DC motors powered by thyristors, commutatorless motors, etc. all those with rectifier circuits at the power side will produce harmonics caused by their nonlinearity. In the three-phase bridge rectifier circuit, the waveform of the input current is rectangular wave, which is decomposed into fundamental wave and harmonics according to Fourier series, and usually contains 6n+1 (n=l, 2, 3...) harmonics. The higher harmonics will interfere with the power supply system

harmonic generation mechanism on the output side: in the inverter output circuit, the output voltage and current have harmonics. For PWM controlled frequency converter, as long as it is a voltage type frequency converter, no matter what kind of PWM control, its output voltage waveform is rectangular wave. Among them, the harmonic frequency is related to the modulation frequency of the frequency converter. The modulation frequency is low (such as 1 ~ 2KHz), and human ears can hear the electromagnetic noise (scream) generated by the higher harmonic frequency. If the modulation frequency is high (for example, the IGBT frequency converter can reach 20kHz), human ears cannot hear it, but the high-frequency signal exists objectively. From the voltage square wave and current sinusoidal sawtooth wave, it is not difficult to analyze the content of each harmonic with Fourier series. Therefore, the output loop current signal can also be decomposed into fundamental wave and other harmonics containing only sine wave, and the higher harmonic current directly interferes with the load. In addition, higher harmonic currents also radiate into space through cables, interfering with adjacent electrical equipment

3. Harmonic interference path

the harmonic interference path of the frequency converter is divided into conduction and radiation as the general radio interference. In the process of conducting the increasing demand of the transmission from the information and communication machinery and other manufacturing industries, the wires laid parallel to the output line of the frequency converter will produce electromagnetic coupling to form inductive interference; The harmonic at the output side of the frequency converter will radiate again, causing interference to the nearby radio equipment. Its interference path is shown in Figure 1

4. Harm of harmonic interference

(1) transformer: current harmonic will increase copper loss, and voltage harmonic will increase iron loss. The comprehensive effect is to increase the temperature of the transformer, affect its insulation capacity, and reduce the capacity margin. Harmonics may also cause resonance between transformer windings and line to line capacitance, and cause core flux saturation or skew, resulting in noise

(2) motor: the influence of output harmonic on the motor is mainly caused by additional heating of the motor, resulting in additional temperature rise of the motor. The motor is often derated for use. Due to the distortion of output waveform, the repetitive peak voltage of the motor is increased, affecting the insulation of the motor. Harmonic will also cause torque ripple of the motor and increase in noise

(3) power capacitor bank: the standard specification of general capacitors stipulates that only 35% overload is allowed for its maximum current, but in actual operation, serious overload often occurs due to the influence of harmonics. As the impedance of the capacitor decreases with the increase of frequency, when harmonics are generated, the capacitor will become a trap point and flow a large amount of current, which will lead to overheating, increase the dielectric stress, and even damage the power capacitor. When the capacitor and line impedance reach resonance conditions, vibration short circuit, overcurrent and noise will occur

(4) switchgear: due to the existence of harmonic current, the switchgear produces a high di/dt current change rate at the moment of starting, which increases the peak value of transient recovery voltage and destroys the insulation

(5) protect electrical appliances: the current contains harmonics, which will produce additional torque, change the action characteristics of electrical appliances, and cause malfunction

(6) measuring instruments: electric energy meters and other measuring instruments, due to harmonics, will cause the induction turntable to produce additional electromagnetic torque, cause errors, and reduce accuracy

(7) power electronic equipment: in many occasions, electronic equipment often produces harmonic current sources, and it is easy to feel harmonic distortion and malfunction

(8) other devices such as lighting equipment, communication equipment, television and audio equipment, computer equipment, carrier frequency remote control equipment, etc. are vulnerable to harmonic interference, affecting their normal work or reducing their service life

5. Countermeasures to suppress harmonic interference

the transmission path of harmonic is conduction and radiation, and 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 disturbed line. Specific common methods are:

5. 1 Countermeasures on the input side of the frequency converter:

(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) set the AC reactor

(3) set the AC filter

(4) the multiplex technology of rectifier. For large capacity thyristor frequency converter, this method can be adopted. The rectifier on the power side is divided into two, and the transformer connected with y, Y-D or D, Y-D windings is installed on its input side, so as to use the multiplex to suppress the high-order harmonics flowing to the power side. Because the rectifier needs to be separated, it is not used in the general frequency converter [1]

5, 2 Countermeasures on the output side of the frequency converter

countermeasures to prevent harmonic interference on the output side are roughly divided into two categories. The first category is traditional, that is, to reduce the noise; The second category is a new attempt. Its basic concept and practice is to try to turn meaningless noise into optional consultation. This method has proved its effectiveness in experiments [2]. The first method can be divided into:

(1) use power electronic devices with higher switching frequency than human ears can't hear, such as MOSFET, IGBT, etc

(2) add a filter after the output end of the frequency converter to make the power waveform sent to the front of the power equipment sine wave

(3) improve PWM modulation method and reduce harmonic content

(4) use closed-loop control methods, such as ADSM and DSMC, to improve the harmonic phenomenon of general traditional PWM

in addition, the cables between the motor and the frequency converter should be laid through steel pipes or armored cables, and laid separately with other weak current signals in different cable trenches to avoid radiation interference

the signal line adopts shielded wire, and the wiring is staggered from the main circuit control line of the frequency converter by a certain distance (at least 20cm) to cut off the radiated interference

the frequency converter uses a special grounding wire, and it is grounded with thick and short wires. The grounding wire adjacent to other electrical equipment must be separated from the frequency converter wiring, and short wires must be used. This can effectively suppress the radiation interference of current harmonics to adjacent equipment [3]

6. Examples of harmonic interference suppression

example 1: in a frequency conversion switching control system, the frequency converter starts and operates normally, while the reading of the adjacent liquid level gauge is high. When the primary 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 gauge displays the upper limit, causing the frequency converter to receive the shutdown command, forcing the frequency converter to stop running

this is obviously the high-order harmonic interference of the frequency converter to the liquid level gauge, and the interference is transmitted through the power circuit or signal line of the liquid level gauge. Solution: take the power supply of the liquid level meter from another power supply transformer, reduce the harmonic interference, and then thread the signal line into the steel pipe for laying, and separate it from the main circuit of the frequency converter by a certain distance. After this treatment, the harmonic interference is basically suppressed, and the operation of the liquid level meter returns 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 suspected that there are problems with the primary meter, secondary meter, signal wire and fluid medium. Replace all these meters and signal cables, and improve the fluid characteristics. The fault still exists, The fault is that the high-order harmonic current of the frequency converter radiates outward through the output circuit cable and is transmitted to the signal cable, causing interference

solution: the signal line and its control line of the liquid level gauge are separated from the control line and main circuit line of the frequency converter by a certain distance, and the signal line outside the cabinet is laid through the steel pipe, and the shell is well grounded, so the fault is eliminated

example 3, a frequency conversion control system is composed of two frequency converters, and in the same cabinet, the frequency modulation mode of the frequency converter is potentiometer manual adjustment mode. When running a frequency converter, it works normally, and when the two operate at the same time, the frequency interferes with each other, that is, adjusting the potentiometer of one frequency converter has an impact on the frequency of the other frequency converter, and vice versa. At first, we thought it was the potentiometer and control line fault. After eliminating this possibility, we concluded that it was caused by harmonic interference

solution: move one of the potentiometers to other cabinets for fixation, and use shielded signal wires for leads, resulting in reduced interference. In order to completely suppress interference, reprocess an electric control cabinet and place it at a certain distance from the original cabinet. Move one of the frequency converters to the electric control cabinet, and make necessary changes to the corresponding wiring and leads. After such treatment, the interference is basically eliminated and the fault is eliminated

example 4, a frequency conversion control system switches two sets of pumps. The original pump was started by self coupling voltage reduction, and the power frequency operation is normal. Now it is changed to frequency conversion operation. Although the frequency modulation and deceleration function can be realized, the output from the output end of the frequency converter to the motor. Now the college students studying in Colleges and universities are about 20 years old, the outgoing line is seriously heated, the temperature rise of the motor shell is aggravated, and protection tripping often occurs. This is because the output voltage and current signals of the frequency converter contain PWM high-order harmonics, and the harmonic current forms additional power loss in the output wire and motor winding

solution: separate the input line and output line of the frequency converter, go through their respective cable trenches, select the cable with a larger cross-section to replace the original cable, and the length of the cable between the output end and the motor shall be as short as possible. After this treatment, the heating fault is eliminated. It can basically eliminate the high-order harmonic interference of various frequency converters on site