Abstract: Soft start technology is a new technology that has developed in recent years, organically combining power electronics, microprocessor, and automatic control technologies. Compared with traditional reduced voltage starting control technology, it has many advantages. This article starts with the concept of soft starter, analyzes the causes and hazards of harmonic generation in soft starters, and proposes methods to suppress harmonics. Keywords: soft starter; harmonics; hazard; suppression Three-phase squirrel-cage induction motors are widely used, but their high starting current is a significant drawback. To improve starting performance, reduce starting current, and increase starting torque, traditional methods employ series reactors and autotransformers for voltage reduction starting. While these methods reduce starting current, they also reduce starting torque, and the discontinuous starting current leads to increased maintenance, increasing costs and reducing reliability. A soft starter, also known internationally, is a new type of motor control equipment that integrates soft starting, soft stopping, light-load energy saving, and multi-functional protection. It not only enables smooth, shock-free motor starting throughout the entire process but also allows for adjustment of starting parameters, such as current limiting, starting time, and starting duration, based on the motor's load characteristics. Furthermore, it offers various motor protection functions, fundamentally solving many of the shortcomings of traditional voltage reduction starting equipment. Major international companies such as AB, ABB, Schneider Electric, and Siemens all offer related products. 1. Soft Starter Principle and Performance Characteristics The main components of a soft starter are three anti-parallel thyristors connected in series between the power supply and the controlled motor, along with their electronic control circuit. Modern soft starters generally employ power electronics and microcomputer control technologies, using a microcontroller as the central controller to perform measurements and various control algorithms. Therefore, soft starters possess strong functionality and flexibility. [align=center]Figure 1 Main Circuit of Thyristor Soft Starter[/align] The entire starting process is an automatic operation under the control of digital program software. Utilizing the electronic switching characteristics of the three pairs of thyristors, the microcontroller in the starter controls their trigger pulses to change the trigger angle, thereby changing the conduction time of the thyristors. The output voltage of the device rises according to a certain pattern, causing the voltage of the controlled motor to rise from zero to full voltage, and the speed to smoothly accelerate from zero to the rated speed accordingly. It is a synthesis of power electronics and automation control technologies, combining high-voltage and low-voltage control technologies. Performance characteristics of soft starters: ① Adjustable starting voltage ensures minimum starting torque for motor startup, avoiding motor overheating and energy waste; ② Smooth motor startup reduces starting current surges; ③ Starter current can be adjusted according to load conditions, reducing starting losses and generating optimal torque with minimal current; ④ Adjustable starting time allows the motor speed to gradually increase within this time range, avoiding speed surges; ⑤ Protects transmission machinery, eliminates torque surges, and reduces inrush current; ⑥ Constant acceleration and deceleration eliminates the need for a tachometer, even when the motor load changes; ⑦ Free stop and soft stop are selectable, with adjustable soft stop speed; ⑧ Detection and protection against phase sequence, phase loss, overheating, overcurrent during startup, overcurrent during operation, and overload, with adjustable overcurrent and overload values. 2. Starting methods of soft starters Currently, soft starters offer the following starting methods: ① Constant current soft start. ① **Motor starting current:** During startup, the motor starting current remains constant (i.e., the starting current is limited), and its current limit is usually selected between 1.5 and 4.5 times the motor's rated current. ② **Ramp voltage soft start:** As the name suggests, the voltage increases linearly in a ramp from low to high, transforming the traditional stepped reduction starting into a stepless process. This starting method is the simplest, lacking current closed-loop control; it only controls the thyristor's conduction angle to increase the starting voltage at a set rate before returning to the rated voltage. Its disadvantages are low initial torque, a parabolic torque characteristic that is detrimental to the drive system, and a long starting time that is harmful to the motor. ③ **Ramp constant current soft start:** In the initial stage of startup, the starting current gradually increases at a set rate (slope). After reaching the set value, it remains constant until startup is complete. The rate of increase of the starting current can be adjusted according to load requirements. A higher rate of current increase results in a larger starting torque and a shorter starting time. The starting torque can generally be adjusted between 5% and 90% of the rated torque. The starting time can be adjusted between 2 and 30 seconds. This starting method is most suitable for fan and pump loads and is the most commonly used soft starting method in practice. ④ Pulse constant current soft start. This starting method has a large starting impact current in the initial stage of starting. This current value is greater than the set constant current value, thus generating a large starting impact torque to overcome the large static friction torque, enabling the equipment to start. Then it enters the constant current starting stage until the start-up ends. Obviously, this starting method has a large starting torque and is suitable for heavy-load starting, such as the load starting of belt conveyors and coal mills. 3. Generation of harmonics in soft starters Soft starters use three pairs of anti-parallel thyristors to achieve AC voltage regulation. Since thyristors are typical nonlinear devices, they generate a large number of harmonics during use, which will have an adverse effect on the stable operation of the equipment and the power grid. The root cause of harmonic generation is the nonlinear load. With the widespread application of power electronic devices, it has become the primary source of harmonics. Harmonics are all caused by devices with nonlinear voltage-current characteristics. An ideal sinusoidal voltage applied to a nonlinear device will generate a non-sinusoidal current, becoming a harmonic source. For example, the power frequency voltage is given by the formula, where U is the effective value of the power frequency voltage, i.e., the power frequency. When applied to the two ends of a nonlinear time-invariant resistor with current-voltage characteristics, the generated current is: It can be seen that the input voltage is a sinusoidal wave with frequency ω_sub, while the output current is a non-sinusoidal wave containing both ω_sub and 3ω_sub frequency components. A harmonic is a sinusoidal component of a periodic electrical quantity, and its frequency is an integer multiple of the fundamental frequency. Since the harmonic frequency is always higher than the fundamental frequency, it is called a higher harmonic. Higher harmonics superimposed on the fundamental frequency will distort the waveform, turning a sine wave into a non-sinusoidal wave. With the development of power electronics technology, various new electrical devices are constantly being put into use, and various nonlinear loads are increasing, resulting in a large amount of harmonic current being injected into the power grid. The presence of harmonics in the power grid can damage various electrical equipment and, in some places, even directly threaten the safe operation of the power grid. 4. Harmful Effects of Harmonics For small-capacity systems, the interference generated by harmonics cannot be ignored. The occurrence of harmonic currents and harmonic voltages is a form of pollution to the public power grid. It deteriorates the environment in which electrical equipment is located and causes harm to surrounding communication systems and equipment outside the public power grid. The severity of the harm caused by harmonic pollution to the power system is mainly manifested in the following aspects: (1) Harmonics cause additional harmonic losses to the power supply lines. (2) Harmonics affect the normal operation of various electrical equipment. (3) Harmonics cause harmonics to be generated in the capacitors in the power grid. At the power frequency, the circuit of the capacitors installed in the system for various purposes is much larger than the inductive reactance in the system and will not resonate. However, at the harmonic frequency, the inductive reactance increases by a factor of two and the capacitive reactance decreases by a factor of two, which may lead to resonance. Resonance will amplify the harmonic current, causing the capacitors and other equipment to burn out. (4) Harmonics cause local parallel resonance and series resonance in the public power grid, thereby amplifying the harm, which greatly increases the harm and may even cause serious accidents. (5) Harmonics will cause relay protection and automatic devices to malfunction, and will cause large errors in instruments and electricity metering. Harmonics also have great harm to other systems and power users. (6) The main effect of harmonics on asynchronous motors is to increase the additional losses of the motor, reduce efficiency, and in severe cases, cause the motor to overheat. In particular, negative sequence harmonics generate a negative sequence rotating magnetic field in the motor, forming a torque opposite to the direction of motor rotation, which acts as a brake, thereby reducing the output of the motor. In addition, when the frequency of harmonic current in the motor is close to the natural frequency of a certain component, it will also cause the motor to produce mechanical vibration and generate a lot of noise. 5. Significance of Harmonic Research First, harmonics are very harmful. Harmonics generated by various harmonic sources cause huge pollution to the power system and affect the operating environment of the entire power system. Second, it also has an impact on the development of power electronics technology itself. Power electronics technology is an important pillar of future scientific and technological development, but the harmonic pollution generated by power electronic devices has become a major obstacle to the development of electronic technology. However, it is difficult to completely eliminate the high-order harmonics generated with the current level of technology and economic conditions. 6. Harmonic Suppression In order to suppress the proliferation of harmonics, purify the waveform of the power system, and improve the quality and efficiency of the system's power, the State Bureau of Technical Supervision issued the national standard GB-T14549-93 "Power Quality - Harmonics in Public Power Grids" in 1993. The appendix extracts the harmonic voltage (phase voltage) limits of the public power grid (common voltage levels in low-voltage distribution systems) in this standard, as shown in the table below. Harmonic suppression means reducing or even eliminating the harmonic components generated by the harmonic source, thereby reducing the harmonics injected into the power grid to below the values ​​specified in the national standard. Harmonic suppression measures are generally divided into two types: compensation and elimination. Compensation involves setting up devices to absorb harmonics; elimination involves changing the working mode and characteristics of the harmonic source to reduce or even eliminate the production of harmonics. Commonly used methods and measures include: (1) Installing LC filters. LC filters are traditional harmonic compensation devices and are still widely used in engineering. They have a simple structure, low investment, high reliability, and relatively low operating costs. An LC filter, also known as a frequency-modulated filter, is a series resonant circuit composed of inductors, capacitors, and resistors of appropriate values. Its working principle is essentially to provide a release path for harmonics in the circuit, that is, to retain the fundamental frequency while short-circuiting the harmonics, preventing them from being injected into the power grid. It is generally connected in a three-phase star configuration. However, its disadvantage is that it can only compensate for harmonics of a fixed frequency. LC filters are further divided into single-tuned filters, high-pass filters, and double-tuned filters. In practical applications, several sets of single-tuned filters and one set of high-pass filters are commonly used to form a filtering device. 1. The circuit diagram of a single-tuned filter is shown in Figure 2. The impedance of the filter to the nth harmonic is given by the formula, where fn represents the nth single-tuned filter, and ωs represents the fundamental frequency. The single-tuned filter is constructed using the series L-C resonance principle, and the harmonic order is given by ωs. At the resonance point, ωs is very small, and the filter exhibits low impedance characteristics. The nth harmonic current is mainly shunt and rarely flows into the power grid. For other harmonic orders, the impedance is much greater, and the filter exhibits high impedance with minimal current shunting. Therefore, as long as the resonant order of the filter is set to be the same as the harmonic order to be filtered, most of that harmonic will be filtered out. 2. High-pass filter A high-pass filter, also known as an amplitude reduction filter, presents low impedance for all frequencies after a certain frequency, forming a low-impedance path for higher-order harmonics, allowing most of these harmonic currents to flow into the filter, thus achieving the purpose of filtering. High-pass filters are divided into four types: first-order, second-order, third-order, and C-type. [align=center] Figure 3 High-pass filter[/align] (a) First-order (b) Second-order (c) Third-order (d) C-order First-order high-pass filters require too large a capacitor and have too large a fundamental frequency loss, so they are generally not used. Second-order high-pass filters have the best filtering performance, but compared with third-order filters, their fundamental frequency loss is higher. Third-order high-pass filters have one more capacitor than second-order filters, which increases the impedance of the filter to the fundamental frequency, thereby greatly reducing the fundamental frequency loss. This is the main advantage of third-order high-pass filters. The performance of a C-type high-pass filter is between that of a second-order and a third-order filter. The disadvantage is that it is sensitive to fundamental frequency detuning and component parameter drift. [align=center] Figure 4 Double-tuned filter[/align] Of the four types of high-pass filters mentioned above, the most commonly used is the second-order high-pass filter. 3. Double-tuned filter As shown in the figure, it has two resonant frequencies and absorbs harmonics of two frequencies at the same time. Its function is equivalent to two single-tuned filters in parallel. Compared with two single-tuned filters, the fundamental loss of the double-tuned filter is smaller. During normal operation, the fundamental impedance of the series circuit is much greater than that of the parallel circuit. Therefore, the power frequency voltage that the parallel circuit bears is much lower than that of the series circuit. However, the double-tuned filter has a complex structure and is difficult to tune, so its application is still relatively limited. (2) Active Power Filter (APF) Active power filter (APF) is a new technology that has developed rapidly in recent years. It is connected in series or parallel in the main circuit, adopting a closed-loop operation mode with real-time detection. It detects high-order harmonics in the current in real time, and inputs a current with equal amplitude but opposite phase to the high-order harmonic components based on the detection results, achieving real-time compensation of harmonic current, thus ensuring that the grid current contains only the fundamental current. Compared with traditional passive filters, APF has the following characteristics: ① It can not only compensate for various harmonics, but also suppress flicker and compensate for reactive power, possessing multi-functionality; ② Its green wave performance is not affected by system impedance, eliminating the danger of resonance with system impedance; ③ It has an adaptive function, automatically tracking and compensating for changing harmonics, exhibiting high controllability and fast response. However, the high cost of APF limits its widespread use. 7. Conclusion In summary, compared with traditional reduced-voltage starting equipment, motor soft starters can better solve the problems generated during motor starting and stopping, and have broad application prospects. However, the harmonic interference generated by soft starters also poses a potential threat to the stable operation of equipment. How to suppress the harmonics generated during the operation of soft starters to the greatest extent remains a research topic. References: [1] Wang Zhaoan, Yang Jun, Liu Jinjun. Harmonic Suppression and Reactive Power Compensation [M]. Beijing: Machinery Industry Press, 2004 [2] Luo An, Power Grid Harmonic Control and Reactive Power Compensation Technology and Equipment [M]. Beijing: China Electric Power Press, 2006, (4) [3] Xu Honggang, Xu Fangyi. Soft Starter Principle and Application [J]. Energy Technology, 2002, 23 (3) [4] Gan Shihong, Wu Yanxiang, Yang Chen. Soft Starter for High Voltage Asynchronous Motor [J]. Electrical Engineering, 2007, 26 (7) [5] Chen Boshi, Automatic Control System for Electric Drive [M]. Shanghai: Shanghai University of Technology, 1991 [6] Liu Hongwei, Wang Yi, Research on Soft Starter for Asynchronous Motor [J]. Small and Medium-sized Motors, 2002, 29 (2) [7] George J. Wakeleh, Power System Harmonics - Basic Principles, Analysis Methods and Filter Design [M]. 2005