0 Introduction Instantaneous voltage drop in mains power is caused by lightning strikes, short-circuit faults, or the operation of special high-current equipment. This phenomenon is difficult to avoid and poses a significant threat to the reliable operation of equipment such as computers and modern speed control systems. Since the 1980s, with the popularization of computer and power electronics technologies, the economic losses caused by instantaneous voltage drop in mains power have become increasingly significant, prompting people to pay attention to the problem and make many efforts to reduce its impact. However, few economically effective solutions have been found. Over the past decade, instantaneous voltage drop has had a significant impact on the production of units such as the Jiangxi Washing Mill and a factory in Fukui Prefecture, Japan. We have conducted an analysis and research on this issue, and the results are detailed below. 1 Statistical Data on Instantaneous Voltage Drop 1.1 Instantaneous Voltage Drop and Unplanned Power Outages Instantaneous voltage drop in mains power is an unavoidable power quality problem, even in developed countries. Unplanned power outages, on the other hand, have gradually decreased with technological advancements and the development of the power industry. The ratio of the number of occurrences of the two each year is difficult to determine, but the former is usually much more frequent than the latter (related to the power consumption level and the user's power grid). 1.2 Probability Distribution of Instantaneous Voltage Drop Amplitude and Technical Time Based on the records of 195 instantaneous voltage drops and sudden power outages at a factory in Fukui Prefecture, Japan, from January 13, 1991 to January 21, 1998, we plotted a probability distribution histogram, as shown in Figure 1. The voltage drop rate represents the percentage of voltage drop below the rated value. Figure (1) shows that the probability of instantaneous voltage drops is much greater than that of unplanned power outages. More than 98% of the instantaneous voltage drops are above 60%, and the duration is less than 0.35 seconds, which is basically consistent with the literature [sup]［1］[/sup] . 1.3 Monthly Distribution of Instantaneous Voltage Drop (Literature [sup]［2］[/sup]) The monthly distribution of instantaneous voltage drops is shown in Figure 2. From the causes of instantaneous voltage drops and in conjunction with Figure 2, we can know that the number of instantaneous voltage drops is related to the number of days with thunderstorms, and of course, it is also related to the technical condition of the power grid. 1.4 Example of Instantaneous Voltage Drop at Jiangxi Diesel Wheel Factory Table 1 lists excerpts from the general dispatching operation log from November 16, 1990 to October 10, 1991, regarding shutdown records caused by instantaneous voltage drops (Note: original data from July 1991 is missing). 2. Hazards of Instantaneous Voltage Drop 2.1 The Withstand Capacity of Common Electrical Equipment to Instantaneous Voltage Drops from Mains Figure 3 shows the withstand capacity of common electrical equipment to instantaneous voltage drops from mains. 2.1.1 Computers—When the voltage drop exceeds 10% and lasts for more than approximately 10ms, it will affect normal operation (see Figure 3). For ordinary computers, a sudden power outage may result in the loss of a large amount of data or even damage to hardware. For some microcontroller control systems, it means loss of control and a standby state. Instantaneous voltage drops can interfere with computer systems, and in severe cases, cause the system to malfunction. For example, when the voltage of a computer's switching power supply drops to a certain value, the exchanger stops oscillating, the high-frequency transformer stops transmitting energy, and the output voltage of the switching power supply drops rapidly, causing a "false power outage" phenomenon. Even if the duration is very short, the system may malfunction due to the loss of programs, flags, and critical field data. Some computers may restart, and some may even crash. In industrial control computer systems, it can cause actuators to remain in unsafe positions or states. 2.1.2 Electromagnetic Switch—When the voltage drop exceeds 50% and lasts for more than approximately 10ms, it will automatically disengage. After the instantaneous voltage drop occurs, the armature returns to its original position under the action of gravity and the elastic force of the energy storage spring, and the contacts return to normal. 2.1.3 Electrical Speed ​​Control System—When the voltage drop exceeds approximately 15% and lasts for more than approximately 10ms, it will damage components or automatically shut down. Electrical speed control systems typically employ multiple power electronic devices as their power conversion units, systematically switching the on and off states of these devices as needed during operation. For non-self-turn-off devices such as thyristors, turn-off requires a reverse voltage. While a mains sine wave can contribute to this reverse voltage under certain conditions, a momentary voltage drop can lead to insufficient reverse voltage, preventing the conducting thyristor from turning off in time. Consequently, a thyristor in the same bridge arm that is supposed to be off may turn on, resulting in a short circuit and damage. For controllable devices such as IGBTs and thyristors, turn-on requires a trigger pulse. Since this trigger pulse is sometimes formed under the combined action of a mains sine wave and a control voltage, and requires a strict phase relationship with the main circuit (mains), a momentary voltage drop in the mains can cause the trigger pulse to be lost or the conduction angle to change, leading to converter failure. To avoid damage to power electronic devices caused by instantaneous voltage drops, manufacturers often install protective circuits in the equipment. These circuits quickly shut down the machine upon detecting an instantaneous voltage drop. While this protects the power electronic speed control device from damage, the driven machinery stops, inevitably causing significant losses to equipment with stringent requirements for continuous production. 2.1.4 Gas Discharge Lamps—When the voltage drop exceeds 20% and lasts for more than approximately 100ms, it will automatically extinguish. For example, once a high-pressure mercury lamp extinguishes, it takes several minutes to regain its rated brightness after relighting. 2.1.5 Undervoltage Protection—In devices or systems equipped with undervoltage protection, a malfunction will occur when the mains voltage momentarily drops below the set value. 2.2 Hazards of Instantaneous Voltage Drops a. Equipment and systems susceptible to instantaneous voltage drops include: computers, elevators, memory displays, electrical drives, medical equipment, critical lighting, doping furnaces, and assembly lines, etc. b. Industries or sectors susceptible to instantaneous voltage drops include: high-rise buildings, chemical fibers, semiconductors, steel, papermaking, airports, hospitals, banks, post and telecommunications, government agencies, and research institutes. c. Since its commissioning in 1984, the Jiangxi Polyester Factory has been affected by instantaneous voltage drops in the power grid. According to records from the factory's dispatch office regarding the regular spinning workshop, the number of shutdowns caused by instantaneous voltage drops in the second-line metering pump frequency converter was: 16 times from March to December 1996, and 18 times from January to August 1997. Extrapolating from this, it is approximately 20 times per year, with each shutdown lasting about 20-30 seconds. The average loss per line is approximately 1000 yuan (including 315 yuan for scrapped spinning yarn, 140 yuan for normal production loss, 140 yuan for component loss, 200 yuan for labor costs, and 200 yuan for other expenses). The average annual loss per line is 20,000 yuan; based on the factory's seven lines, the annual loss reaches as high as 140,000 yuan. 3. Countermeasures and Characteristics 3.1 Countermeasures To reduce the harm caused by instantaneous voltage drop, the main countermeasures that can be taken are: 3.1.1 Electric-generator set principle: The electric motor drives the generator, and then the generator supplies power to the load. Due to the large rotational inertia of the rotor, the effect of instantaneous voltage drop can be overcome to a certain extent. 3.1.2 Flywheel energy storage system principle: The electric motor drives the flywheel and the generator. The flywheel with a large rotational inertia is placed in a vacuum to reduce energy loss. It can overcome the influence of many power supply quality problems on the load over a large range. Problem: High cost, difficult to manufacture, and difficult to maintain rotating machinery. 3.1.3 Uninterruptible power supply (UPS) [4] Principle: The AC power is rectified and stored in the battery. Then, the DC power is converted into AC power by the inverter to supply power to the load. Its main function is to prevent sudden power outages. Problem: High cost, large size, short battery life, difficult maintenance and high maintenance cost. 3.1.4 New Type of Instantaneous Voltage Drop Compensator [5] Principle: Instantaneous voltage drop in mains power can also be described as an instantaneous energy shortage. It is noted that more than 98% of instantaneous voltage drops are above 60% and last for less than 0.35 seconds. The load voltage is maintained constant by compensating for part of the voltage at the moment of voltage drop, which is to say, by using the method of instantaneous energy compensation. Therefore, electrolytic capacitors are used instead of batteries for energy storage, thereby reducing the cost of the equipment and extending its service life. Problem: Like UPS, the online standby time is too long. 3.1.5 AC Electronic Voltage Regulator Advantages: Fast response speed and better AC output waveform. Disadvantages: Small capacity. 3.1.6 Other Methods: Make some minor improvements to commercially available equipment Problems: a. Instantaneous voltage drop in mains power is essentially an instantaneous energy shortage. Not providing stable energy can only treat the symptoms, not the root cause; b. The integration of electronic circuits is getting higher and higher, and the designs of different companies are also different. Most manufacturers do not provide detailed drawings and programs. Therefore, apart from having a certain effect on electromagnetic devices, the scope of operation is extremely limited. 3.2 Performance comparison of several countermeasures: Electric generator sets and flywheel energy storage systems are expensive, difficult to manufacture, and difficult to maintain rotating machinery. The capacity of AC electronic voltage regulators is too small. The main function of UPS is to prevent the load from being cut off due to a sudden power outage. Although it can also eliminate the harm caused by instantaneous voltage drop, if it is only used to deal with instantaneous voltage drop, the energy storage capacity is more than enough (energy storage must have a certain capacity to ensure sufficient current output), which leads to high cost. Instantaneous voltage compensators are cheaper than UPS because they only compensate for part of the energy instantaneously. They have a longer life and are easier to maintain. References: [1] Takahashi Isao, et al. The opening of freewheeled UPS (1) - Advantages of freewheeled UPS, Proceedings of the National Congress of the Electrical Society, 1990. [2] (Electrical Society Research Report, Figure 2-4-4) [3] (Electrical Society Research Report, Figure 1-2-3) [4] Li Chengzhang. Working principle and practical maintenance technology of small and medium-sized UPS uninterruptible power supply for microcomputers, Electronic Industry Press, 1992. [5] Wu Jiaju et al. "Development of instantaneous voltage drop compensator". Power Electronics Technology, 1999, No. 6.