With the development of power electronics technology, inverters using sinusoidal pulse width modulation (SPWM) have been widely used, such as inverters and uninterruptible power supplies (UPS). In Tangshan Iron and Steel Co., a large number of AC variable frequency devices of different types and capacities are used in cold rolling, such as SIEMENS 6SE70 Series Masterdrives, MICROMASTER 430/440 Series, ABB ACS600/800 Series, and TMEIC TM70. The wide speed range, high precision, good energy-saving effect, stable and reliable system, bring significant economic benefits to the company. However, with the wide use of frequency converters, its harmonic interference issues have also attracted attention, and corresponding measures and supporting equipment have been taken to ensure that the frequency converter is safe, efficient, and with high power factor and environmental protection, in order to achieve electromagnetic compatibility.
1, electromagnetic interference and harmonic harm
1.1 Electromagnetic Interference (EMI)
The human understanding of electromagnetic compatibility (EMC) begins with the effect of electromagnetic interference (EMI): EMI may degrade the working performance of all electrical equipment, subsystems, or systems from expected ones or cause undesired performance. The deviation may even cause it to fail, and the performance will be permanently reduced until it is destroyed, and at the same time it will cause harm to human health. EMI is defined as "the degradation of equipment, transmission channels, or system performance caused by electromagnetic disturbance."
1.2 Harmonic Harm
The popularization of AC frequency converter speed regulation, especially the large-scale use of frequency converters, harmonics generated by the inverter will not be inhibited, and will have a great impact on the normal operation of other equipment and its own. Harmonics reduce the efficiency of the production, transmission, and utilization of electrical energy, causing electrical equipment to overheat, generate vibration and noise, and cause aging of insulation, reduced service life, and even failure or burn. Harmonics can also cause relay protection and automatic device malfunctions, causing confusion in the measurement of electrical energy. Harmonics can cause serious interference to communication equipment and electronic equipment outside the power system.
2. Main Reasons for Inverter Harmonic Generation
The harmonics of the frequency conversion device are generated in the main circuit, and the main circuit of the variable frequency speed control system is composed of a rectifier, an intermediate DC link and an inverter. The rectifying part is a diode three-phase bridge type rectifier. The middle filter part uses a large capacitor as a filter, and the inverter part is an IGBT three-phase bridge type inverter, and the input is a PWM waveform.
2.1 Harmonics generated on the input side of the inverter
The input circuit of the frequency converter usually consists of a diode three-phase bridge rectifier circuit and a filter circuit. Its circuit structure and input side voltage, harmonic current of the power supply side of the frequency converter are mainly produced by the rectifier circuit of AC to DC. From the basic theory of the rectifier circuit, it can be seen that while the rectifier circuit is outputting a DC current, the diode commutation causes the AC side to generate a characteristic harmonic current.
2.2 Harmonics generated at the output side of the inverter
From the basic composition and working principle of the inverter, it can be known that the inverter generally outputs several kilohertz high-voltage pulses under ideal conditions, but due to the current-torque characteristics of the asynchronous motor supplied by the inverter, in addition to the inverter output waveform, In addition to fundamental waves, there are several harmonics. The main causes of these harmonics are:
(1) The output voltage waveform of the AC drive is a rectified voltage waveform containing a large harmonic wave whose frequency is: f U1 = p kf i±(n-1)f 0 where p is the number of controlled rectifier pulse waves. ,p = 6 or 12;k is a natural number,k=1,2,3,...;n is the number of harmonics, n=1,2,3...;fi is the grid frequency, fi =50HZ;f 0 is the frequency converter Output frequency. After filtering by load inductance, most of the higher harmonics are filtered out, but low harmonics still exist.
(2) The inverter outputs high-voltage pulses. Because there is a distributed capacitance between the output line of the inverter and the ground and between the motor and the ground, a high-order harmonic current is generated. This current contains many odd-order harmonic components. When the frequency converter supplies power to the asynchronous motor, the inverter generates high-order harmonics from the rectifier section, and the odd-order harmonics generated by the inverter have large external disturbances. The inverter uses a high carrier frequency output to chop, so it is itself A source of harmonics. The higher harmonics generated by the inverter circuit are output to the power cable and the motor via the inverter. In the process of motor speed regulation, the frequency of these pulsating components changes because the fundamental frequency output from the inverter circuit changes constantly with the motor speed requirement. Output expression:
Among them: u0 is the inverter output voltage; ud is the DC side voltage of the inverter; α is the degree of modulation (O≤α≤l); ωr is the angular frequency of the modulation signal; φ is the initial phase angle of the sine wave; ωr is the carrier signal Bow angle frequency; n is the number of inverter output harmonics. The first item in the formula is the fundamental component in the output waveform, which is the sine wave required during modulation; the second item is the harmonic component in the output waveform.
3, harmonic suppression and interference measures
3.1 Inverter harmonic suppression
The harmonics entering the grid are mainly related to the rectifier circuit. In order to reduce the harmonics entering the grid, reactor filters are connected in series at the input end of the power supply. The reasonable choice of the size of the capacitor of the DC circuit is also very helpful for suppressing harmonics. Reduce the higher harmonics of the output loop, using a multi-level inverter and filtering scheme. To reduce electromagnetic noise and losses, an output filter can be set on the output side of the inverter so that the output current and voltage are close to a sine wave and the higher harmonics are further reduced. In the system anti-jamming and optimization of control performance, often used hardware and software anti-jamming measures, and the most basic and the most important anti-jamming measures are hardware anti-jamming. The main measures are isolation, filtering, shielding, grounding and other methods.
3.2 Inverter anti-jamming measures
To improve the anti-jamming ability of the frequency conversion control system, first ensure that all equipment in the control cabinet is well grounded, and use short, thick grounding wires (preferably flat conductors or metal meshes, because of their low impedance at high frequencies) to connect to public areas on-line. When installing and wiring, separate the power cable from the control cable. The control circuit cable must cross the power cable and should be routed at an angle of 90°. When using shielded conductors or twisted pair to connect the control circuit, make sure that the unshielded area is as short as possible and use cable bushings when conditions permit. There is also an arc extinguishing function in the control cabinet. The AC contactor uses a varistor suppressor. This is especially important if the contactor is controlled by the frequency converter relay. When shielded and armored cables are used as motor wiring, ground the shield at both ends. If the frequency converter is operating in a noise-sensitive environment, RFI filters can be used to reduce conducted and radiated interference from the frequency converter. For optimum results, there should be good conductivity between the filter and the mounting plate.
4 Conclusion
This article analyzes the causes of harmonics generated by the inverter and the characteristics of harmonics. It is believed that the harmonics generated by the input side of the inverter are mainly caused by the rectifier circuit, and mainly the low-frequency harmonics are mainly generated in the power supply system. influences. The harmonics generated at the output side of the inverter are caused by the inverter circuit. The harmonics are mainly high-order harmonics. The frequency of the harmonics is close to the modulation frequency. Harmonic on the output side will have an adverse effect on the drive system itself and adjacent devices.
1, electromagnetic interference and harmonic harm
1.1 Electromagnetic Interference (EMI)
The human understanding of electromagnetic compatibility (EMC) begins with the effect of electromagnetic interference (EMI): EMI may degrade the working performance of all electrical equipment, subsystems, or systems from expected ones or cause undesired performance. The deviation may even cause it to fail, and the performance will be permanently reduced until it is destroyed, and at the same time it will cause harm to human health. EMI is defined as "the degradation of equipment, transmission channels, or system performance caused by electromagnetic disturbance."
1.2 Harmonic Harm
The popularization of AC frequency converter speed regulation, especially the large-scale use of frequency converters, harmonics generated by the inverter will not be inhibited, and will have a great impact on the normal operation of other equipment and its own. Harmonics reduce the efficiency of the production, transmission, and utilization of electrical energy, causing electrical equipment to overheat, generate vibration and noise, and cause aging of insulation, reduced service life, and even failure or burn. Harmonics can also cause relay protection and automatic device malfunctions, causing confusion in the measurement of electrical energy. Harmonics can cause serious interference to communication equipment and electronic equipment outside the power system.
2. Main Reasons for Inverter Harmonic Generation
The harmonics of the frequency conversion device are generated in the main circuit, and the main circuit of the variable frequency speed control system is composed of a rectifier, an intermediate DC link and an inverter. The rectifying part is a diode three-phase bridge type rectifier. The middle filter part uses a large capacitor as a filter, and the inverter part is an IGBT three-phase bridge type inverter, and the input is a PWM waveform.
2.1 Harmonics generated on the input side of the inverter
The input circuit of the frequency converter usually consists of a diode three-phase bridge rectifier circuit and a filter circuit. Its circuit structure and input side voltage, harmonic current of the power supply side of the frequency converter are mainly produced by the rectifier circuit of AC to DC. From the basic theory of the rectifier circuit, it can be seen that while the rectifier circuit is outputting a DC current, the diode commutation causes the AC side to generate a characteristic harmonic current.
2.2 Harmonics generated at the output side of the inverter
From the basic composition and working principle of the inverter, it can be known that the inverter generally outputs several kilohertz high-voltage pulses under ideal conditions, but due to the current-torque characteristics of the asynchronous motor supplied by the inverter, in addition to the inverter output waveform, In addition to fundamental waves, there are several harmonics. The main causes of these harmonics are:
(1) The output voltage waveform of the AC drive is a rectified voltage waveform containing a large harmonic wave whose frequency is: f U1 = p kf i±(n-1)f 0 where p is the number of controlled rectifier pulse waves. ,p = 6 or 12;k is a natural number,k=1,2,3,...;n is the number of harmonics, n=1,2,3...;fi is the grid frequency, fi =50HZ;f 0 is the frequency converter Output frequency. After filtering by load inductance, most of the higher harmonics are filtered out, but low harmonics still exist.
(2) The inverter outputs high-voltage pulses. Because there is a distributed capacitance between the output line of the inverter and the ground and between the motor and the ground, a high-order harmonic current is generated. This current contains many odd-order harmonic components. When the frequency converter supplies power to the asynchronous motor, the inverter generates high-order harmonics from the rectifier section, and the odd-order harmonics generated by the inverter have large external disturbances. The inverter uses a high carrier frequency output to chop, so it is itself A source of harmonics. The higher harmonics generated by the inverter circuit are output to the power cable and the motor via the inverter. In the process of motor speed regulation, the frequency of these pulsating components changes because the fundamental frequency output from the inverter circuit changes constantly with the motor speed requirement. Output expression:
Among them: u0 is the inverter output voltage; ud is the DC side voltage of the inverter; α is the degree of modulation (O≤α≤l); ωr is the angular frequency of the modulation signal; φ is the initial phase angle of the sine wave; ωr is the carrier signal Bow angle frequency; n is the number of inverter output harmonics. The first item in the formula is the fundamental component in the output waveform, which is the sine wave required during modulation; the second item is the harmonic component in the output waveform.
3, harmonic suppression and interference measures
3.1 Inverter harmonic suppression
The harmonics entering the grid are mainly related to the rectifier circuit. In order to reduce the harmonics entering the grid, reactor filters are connected in series at the input end of the power supply. The reasonable choice of the size of the capacitor of the DC circuit is also very helpful for suppressing harmonics. Reduce the higher harmonics of the output loop, using a multi-level inverter and filtering scheme. To reduce electromagnetic noise and losses, an output filter can be set on the output side of the inverter so that the output current and voltage are close to a sine wave and the higher harmonics are further reduced. In the system anti-jamming and optimization of control performance, often used hardware and software anti-jamming measures, and the most basic and the most important anti-jamming measures are hardware anti-jamming. The main measures are isolation, filtering, shielding, grounding and other methods.
3.2 Inverter anti-jamming measures
To improve the anti-jamming ability of the frequency conversion control system, first ensure that all equipment in the control cabinet is well grounded, and use short, thick grounding wires (preferably flat conductors or metal meshes, because of their low impedance at high frequencies) to connect to public areas on-line. When installing and wiring, separate the power cable from the control cable. The control circuit cable must cross the power cable and should be routed at an angle of 90°. When using shielded conductors or twisted pair to connect the control circuit, make sure that the unshielded area is as short as possible and use cable bushings when conditions permit. There is also an arc extinguishing function in the control cabinet. The AC contactor uses a varistor suppressor. This is especially important if the contactor is controlled by the frequency converter relay. When shielded and armored cables are used as motor wiring, ground the shield at both ends. If the frequency converter is operating in a noise-sensitive environment, RFI filters can be used to reduce conducted and radiated interference from the frequency converter. For optimum results, there should be good conductivity between the filter and the mounting plate.
4 Conclusion
This article analyzes the causes of harmonics generated by the inverter and the characteristics of harmonics. It is believed that the harmonics generated by the input side of the inverter are mainly caused by the rectifier circuit, and mainly the low-frequency harmonics are mainly generated in the power supply system. influences. The harmonics generated at the output side of the inverter are caused by the inverter circuit. The harmonics are mainly high-order harmonics. The frequency of the harmonics is close to the modulation frequency. Harmonic on the output side will have an adverse effect on the drive system itself and adjacent devices.