1. 1 causes of harmonics.
Since the first 300A thyristor arc welding inverter power supply, arc welding inverter power supply has made great development, including thyristor inverter power supply, high-power transistor inverter power supply, field effect inverter power supply and IGBT inverter power supply, and its capacity and performance have been greatly improved. At present, arc welding inverter power supply has become the mainstream product of welding equipment in industrialized countries [1]. As a typical power electronic device, arc welding inverter power supply has the advantages of small volume, light weight and good control performance, but there are rectification and inverter links in its circuit, which leads to the distortion of current waveform and produces a large number of higher harmonics. There is a serious phase shift between high-order voltage harmonics and current harmonics, which leads to low power factor of welding machine. There are two main reasons for harmonic generation:
(1) The internal interference source of inverter power supply is a combination of strong current and weak current. In the welding process, the welding current can reach hundreds or even thousands of amperes. Because the current will produce a large electromagnetic field, especially in the welding power supply system with high inverter frequency, rectification, high-frequency transformer leakage, control system oscillation, high-frequency arc discharge, power tube switch and so on will produce strong harmonic interference.
Secondly, if the argon tungsten arc welding machine adopts high-frequency arc initiation, the high-frequency arc initiation is also a strong harmonic interference source because the welding machine uses high-frequency and high-voltage with frequency of several hundred thousand hertz and voltage of several thousand volts to break through the air gap to form an arc. For the computer-controlled intelligent arc welding inverter power supply, because the computer control system is running faster and faster, the control panel itself has become a source of harmonic interference, which also puts forward higher requirements for the wiring of the control panel.
(2) The pollution of external interference sources of inverter power supply is a serious interference to the power supply system. As changing loads are added to the power grid, these loads have more or less harmonic interference to the power grid. For example, the use of high-power equipment distorts the voltage waveform of the power grid, and accidental factors cause instantaneous power failure. The opening of high-frequency equipment makes the voltage waveform of power grid appear high-frequency pulse and spike pulse.
In addition, in the welding workshop, because the grounding wires of different welding power supplies may be connected with each other when in use, if corresponding measures are not taken, the harmonic signals of high frequency components can easily enter the control system, making the power supplies unable to work normally or even damaged.
Characteristics and Harm of 1.2 Harmonic
Arc welding inverter power supply is famous for its high efficiency of electric energy conversion. With the development of power control devices towards practicality and large capacity, arc welding inverter power supply will also enter the era of high frequency and large capacity. Arc welding inverter power supply is essentially a large rectifier power supply for power grid. Due to the steep pulse at the leading edge and trailing edge in the commutation process of power electronic devices, serious harmonic interference is caused. The input current of inverter power supply is a sharp wave, which makes the power grid contain a lot of higher harmonics. There is a serious phase shift between high-order voltage harmonics and current harmonics, which leads to low power factor of welding machine. Low-frequency distortion is a problem of power electronic equipment at present, which has aroused widespread concern in communication industry and home appliance industry. In addition, at present, inverter welding machines mostly adopt hard switching mode, and power components inevitably produce harmonic interference to the ground during switching. These interferences form conducted interference through the coupling of near field and far field, which seriously pollutes the surrounding electromagnetic environment and power supply environment, not only reduces the reliability of inverter circuit itself, but also seriously affects the operation quality of power grid and adjacent equipment.
Harmonic suppression measures commonly used in arc welding inverter power supply
2. 1 passive filter (PF)
The traditional method of harmonic suppression and reactive power compensation is power passive filtering technology, also known as indirect filtering division, that is, using passive devices such as power capacitors to form a passive filter, which is connected in parallel with the nonlinear load to be compensated, providing a low-resistance path for harmonics and providing reactive power required by the load. Specifically, the distorted 50Hz sine wave is decomposed into fundamental wave and related main harmonic components, and then each filtering branch composed of L, C (or R) is tuned (or biased) to each main harmonic frequency to form a low-resistance channel and filter it [2-3]. When harmonics have been generated, it passively defends and reduces the harm of harmonics to electrical equipment.
Passive filtering scheme has low cost and mature technology, but it also has the following shortcomings: (1) the filtering effect is affected by the system impedance; (2) Because its resonant frequency is fixed, it is invalid for frequency shift; (3) Series or parallel resonance of system impedance may occur, resulting in overload.
In medium and small power situations, it is gradually replaced by active filters.
2.2 Active Power Filter (AF for short) As early as the early 1970s, some scholars put forward the basic principle of active power filter. However, due to the lack of high-power switching elements and corresponding control technology at that time, compensation current can only be generated by linear amplifier and other methods, which has fatal weaknesses such as low efficiency, high cost and difficulty in large capacity. With the improvement of the performance of power semiconductor switching elements and the development of corresponding PWM technology, it is possible to develop a harmonic current generator with large capacity and low loss, thus making active filtering technology practical. When there is a harmonic source in the system, a compensation current with equal magnitude and opposite phase is generated by some method, and it is connected in parallel with the circuit that becomes the harmonic source to cancel the harmonic of the harmonic source, so that the current on the DC side is only the fundamental component and does not contain the harmonic component. When the harmonics generated by the harmonic generator can't be predicted and change at any time, it is necessary to detect the harmonic current ih signal from the load current il. The detected harmonic current ih signal is modulated by the modulator and converted into the switching mode. The working mode of the current inverter is controlled according to the established method, so that the current inverter can generate the compensation current ifm and inject it into the circuit. In order to eliminate harmonic current ih, DC/ AC full-bridge inverter circuit is usually used as the main inverter circuit, in which the switching element can be a high-power controllable power semiconductor element, such as GTO, GTR, SIT or IGBT, and the output current waveform is controlled by the on-off of the switching element to generate the required compensation current.
As the most promising power device to suppress harmonics and compensate reactive power and improve power supply quality, active power filter has the following advantages compared with passive power filter [5]:( 1) It can realize dynamic compensation, compensate harmonics and reactive power with frequency and amplitude changes, and has extremely fast response to the changes of compensation objects; (2) Harmonic and reactive power can be compensated at the same time, and the magnitude of compensated reactive power can be continuously adjusted; (3) Reactive power compensation does not need energy storage components, and the capacity of energy storage components required for harmonic compensation is not large; (4) Even if the current of the compensation object is too large, the active power filter will not be overloaded and can play a normal compensation role; (5) it is not affected by the power grid impedance, and it is not easy to resonate with the power grid impedance; (6) It can track the change of power grid frequency, so the compensation performance is not affected by frequency change; (7) First harmonic and reactive power can be compensated separately, and multiple harmonics and reactive power can be compensated collectively. There are a lot of harmonics in arc welding inverter power supply, which is very harmful. In order to suppress harmonics and improve power factor, corresponding suppression measures must be taken. The traditional PF mode has obvious shortcomings, which limits its application, while AF mode can make up for the shortcomings of PF and effectively suppress the harmonics of arc welding inverter power supply, and has been more and more widely used. Soft switching technology can also achieve good filtering effect to a certain extent.