Harmonic Analysis and Suppression of Arc Welding Inverter Power Supply

　　Preface

　　Since the 1970s, with the continuous development of power electronics technology, inverter technology has been gradually introduced into the welding field. In the 1980s, high-performance high-power electronic components such as power transistors, field effect tubes, IGBTs, etc. have appeared one after another, promoting the further development of arc welding . The inverter power supply is developed by using these advanced power electronic components and inverter technology. It saves 80% to 90% of materials and 20% to 30 % of energy compared with the traditional power frequency rectifier power supply, and the dynamic response speed is increased by 2 to 3 orders of magnitude. Due to its many advantages, the inverter power supply has become one of the main development directions of arc welding power supply. However, there are still many problems in the development of inverter power supply, such as reliability and market management, among which the most important is the electromagnetic compatibility (EMC) problem of harmonic interference . Harmonic suppression technology is a new research direction. Many experts and scholars at home and abroad have studied and explored the theory and suppression scheme of harmonics. Restricted by various conditions, domestic welding machine developers often rarely consider the electromagnetic compatibility of products. Since 1996, the European Community market has put forward stricter requirements on the electromagnetic compatibility of electronic products, and solving the harmonic problem has become more urgent. Although my country started late, it has also promulgated corresponding standards and stipulated that they will be enforced from August 2003.

　　1 Harmonic analysis of arc welding inverter power supply

　　1.1 Causes of harmonic generation

　　Since the first 300A thyristor arc welding inverter power supply was developed in the United States in 1972, arc welding inverter power supply has made great progress. It has gone through thyristor inverter, high-power transistor inverter, field effect inverter and IGBT inverter. Its capacity and performance have been greatly improved. At present, arc welding inverter power supply has become the mainstream product of welding equipment in industrially developed countries [1]. As a typical power electronic device, arc welding inverter power supply has the advantages of small size, light weight and good control performance, but there are rectification and inversion links in its circuit, which leads to current waveform distortion and generates a large number of high-order harmonics. There is a serious phase shift between the high-order voltage and current harmonics, resulting in a very low power factor of the welding machine. The main reasons for the generation of harmonics are the following two factors:

　　(1) Internal interference sources of the inverter power supply The inverter power supply is a system that combines strong and weak currents. During the welding process, the welding current can reach hundreds or even thousands of amperes. Because the current will generate a large electromagnetic field, especially in the welding power supply system with a high inverter frequency in the inverter main circuit, the rectifier tube rectification, high-frequency transformer leakage, control system oscillation, high-frequency arc ignition, power tube switching, etc. will all generate strong harmonic interference.

　　Secondly, if the tungsten inert gas arc welding machine uses high-frequency arc initiation, since the welding machine uses a high-frequency high voltage with a frequency of hundreds of thousands of hertz and a voltage of thousands of volts to break through the air gap to form an arc, high-frequency arc initiation is also a strong source of harmonic interference. For computer-controlled intelligent arc welding inverter power supply, since the computer control system used is running at an increasingly high speed, the control board itself has become a source of harmonic interference, and higher requirements are also placed on the wiring of the control board.

　　(2) External interference sources of inverter power supply Pollution on the power grid is a more serious interference to the power supply system. Since the loads added to the power grid are ever-changing, these loads will more or less generate harmonic interference to the power grid. For example, the use of high-power equipment will cause distortion of the grid voltage waveform, accidental factors will cause instantaneous power outages, and the opening of high-frequency equipment will cause the grid voltage waveform to have high-frequency pulses and spike pulse components. In addition, in the welding workshop, since the grounding wires of different welding power supplies may be connected to each other when in use, if appropriate measures are not taken, the high-frequency harmonic signals will easily penetrate into the control system, causing the power supply to fail to work normally or even be damaged.

　　1.2 Characteristics and hazards of harmonics

　　Arc welding inverter power supply is famous for its high-efficiency power conversion. As power control devices develop towards practicality and large capacity, arc welding inverter power supply will also enter the era of high frequency and large capacity. For the power grid, arc welding inverter power supply is essentially a large rectifier power supply. Since power electronic devices generate pulses with steep leading and trailing edges during the commutation process, serious harmonic interference is caused. The input current of the inverter power supply is a sharp-angle wave, which causes a large number of high-order harmonics in the power grid. There is a serious phase shift between high-order voltage and current harmonics, resulting in a very low power factor of the welding machine. The low-frequency distortion problem is a common problem of current power electronic equipment, and it has attracted considerable attention in the communications industry and home appliance industry. In addition, most inverter welding machines currently use hard switching, which inevitably generates harmonic interference to the space during the switching process of power components. These interferences form conducted interference through near-field and far-field coupling, seriously polluting the surrounding electromagnetic environment and power supply environment, which will not only reduce the reliability of the inverter circuit itself, but also seriously affect the operation quality of the power grid and adjacent equipment.

　　2 Commonly used harmonic suppression measures for arc welding inverter power supply

　　Harmonic interference is an important issue that affects the normal operation of arc welding inverter power supply and should be given sufficient attention. In order to suppress the harmonic level and ensure the normal operation of arc welding inverter power supply, filtering methods can usually be used. According to the different devices used and their filtering principles, they can be divided into passive filters and active filters.

　　2.1 Passive Filter (PF)

　　The traditional method of harmonic suppression and reactive power compensation is the power passive filtering technology, also known as the indirect filtering method, which is to use passive devices such as power capacitors to form a passive filter, which is connected in parallel with the nonlinear load that needs to be compensated to provide a low-impedance path for the harmonics and provide the reactive power required by the load. Specifically, the distorted 50Hz sine wave is decomposed into the fundamental wave and the related main harmonic components, and then the series resonance principle is used to tune (or de-tune) the filter branches composed of L, C (or R) to the main harmonic frequencies to form a low-impedance channel to filter them out [2-3]. It is a passive defense to reduce the harm of harmonics to electrical equipment when harmonics have already been generated.

　　Passive filtering solutions are low-cost and mature in technology, but they also have the following shortcomings: (1) The filtering effect is affected by the system impedance; (2) Since its resonant frequency is fixed, it is not effective in the case of frequency offset; (3) It may cause series or parallel resonance with the system impedance, causing overload.

　　In small and medium power applications, it is gradually being replaced by active filters.

　　2.2 Active Filter (AF)

　　As early as the early 1970s, some scholars proposed the basic principle of active power filter. However, due to the lack of high-power switching elements and corresponding control technology at that time, only linear amplifiers and other methods could be used to generate compensation current, which had fatal weaknesses such as low efficiency, high cost, and difficulty in large capacity and failed to be put into practical use. With the improvement of the performance of power semiconductor switching elements and the development of corresponding PWM technology, it has become possible to develop large-capacity and low-loss harmonic current generators, thus making active filtering technology practical. When a harmonic source appears in the system, a compensation current with equal magnitude and opposite phase to the harmonic current is generated by some method, and it is connected in parallel with the circuit that becomes the harmonic source to offset the harmonics of the harmonic source, so that the current on the DC side is only the fundamental component without harmonic components. When the harmonics generated by the harmonic source cannot be predicted as what kind of high-order harmonic current they are, and they change at any time, it is necessary to detect the harmonic current ih signal from the load current il. The harmonic current ih signal after detection is modulated by a modulator and converted into a switching mode to control the working mode of the current inverter according to a prescribed method, so that the current inverter generates a compensation current iFM and injects it into the circuit to offset the harmonic current ih. The inverter main circuit generally adopts a DC/AC full-bridge inverter circuit, in which the switching elements can be high-power controllable power semiconductor elements such as GTO, GTR, SIT or IGBT. With the help of the on and off of the switching elements, the output current waveform is controlled to generate the required compensation current. Active power filters are the most promising power devices for suppressing grid harmonics and compensating reactive power to improve grid power supply quality. Compared with passive power filters, they have the following advantages [5]: (1) They realize dynamic compensation and can compensate for harmonics with changing frequencies and magnitudes as well as changing reactive power, and respond very quickly to changes in the compensation object; (2) They can compensate for harmonics and reactive power at the same time, and the magnitude of compensated reactive power can be adjusted continuously; (3) No energy storage elements are required for reactive power compensation, and the capacity of energy storage elements 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 greatly affected by the grid impedance and is not easy to resonate with the grid impedance; (6) It can track changes in grid frequency, so the compensation performance is not affected by frequency changes; (7) It can compensate for one harmonic and reactive power separately, or it can compensate for multiple harmonics and reactive power collectively.

　　3 Soft switching technology

　　As power electronics technology develops towards high frequency and high power density, the switching loss and harmonic interference problems of hard switching working mode are becoming increasingly prominent. From the perspective of improving conversion efficiency, device utilization, enhancing electromagnetic compatibility and device reliability, soft switching technology is beneficial to any switching power converter. It is especially necessary in some special cases (such as occasions with power density requirements or heat dissipation conditions). Among the two major types of soft switching technologies, passive and active, the passive method that does not use additional switching elements, detection methods and control strategies has many advantages such as low additional cost, reliability, conversion efficiency and high performance-price ratio, and has basically established a mainstream position in the field of single-ended converter manufacturing in the industry. For topological structures, the method of series inductance and parallel capacitance is the only passive soft switching method. The so-called passive soft switching technology evolved from this is actually a lossless absorption technology. As far as the bridge inverter circuit is concerned, from the early energy-consuming absorption to the later proposed partial energy-feeding and lossless solutions, there are problems such as strong load dependence, narrow operating frequency range, high additional stress, and overly complex networks, and the practicality is poor. At the same time, under the trend of modularization of switching power devices, the space available for placing absorption components is getting smaller and smaller, and lossless absorption technology suitable for inverter modules is rarely seen in the literature. In general, passive absorption technology suitable for inverter modularization is still under further research and development due to its special structure and difficulty.

　　4 Conclusion

　　There are a lot of harmonics in the arc welding inverter power supply, which are seriously harmful. In order to suppress harmonics and improve the power factor, corresponding suppression measures must be taken. The traditional PF method has obvious shortcomings, which limits its application. The AF method can make up for the shortcomings of PF and effectively suppress the harmonics of the arc welding inverter power supply. It has been more and more widely used. Soft switching technology can also achieve good filtering effects to a certain extent.