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With economic development and the widespread use of high-tech equipment, users are placing increasing demands on the quality of electrical power (voltage). In the past, the power quality issues that people were concerned about mainly included frequency deviation, voltage compliance rate, harmonics, voltage imbalance, and flicker. In recent years, there has been increasing attention to the issue of instantaneous voltage drops caused by factors such as grid failures, namely Voltage Dips or Voltage Sags. According to relevant information, the primary cause of abnormal operation or shutdown of users’ electrical equipment is voltage drops. I. Power Quality—Basic Concepts of Voltage Sags. Voltage quality disturbances can be divided into steady-state and transient types. Steady-state disturbances include voltage deviations, imbalance, flicker, and harmonic issues ； Transient disturbances include transient overvoltages, voltage sags, voltage swells, and instantaneous power supply interruptions. In terms of their frequency of occurrence and the degree of harm they cause to electrical equipment, the main modern power quality issues are voltage sags, harmonics, voltage imbalance, and flicker. 1. Definition and causes of voltage dips According to the European standard EN50160 and the IEEE Standard 159-1992 recommended by the Institute of Electrical and Electronics Engineers in the United States, a voltage dip (or Voltage Sag) is defined as a sudden drop in the effective value of the supply voltage to between 90% and 10% of the rated voltage (0.9–0.1Pu), after which it returns to normal levels; this process lasts for 10 ms to 60 seconds. The main cause of voltage drops is a short circuit in the power grid or electrical equipment. The startup of some electrical equipment (such as motors) or sudden load application can also cause an instantaneous drop in grid voltage. Figure 1 shows the waveform of a voltage dip caused by a typical power grid fault. The graph shows that the fault caused the RMS value of the supply voltage to drop to 55% of its normal value, and this condition persisted for 6 cycle times. Compared to long-term power outages, voltage sags are characterized by a higher frequency of occurrence and less obvious causes, making them more difficult to handle as well. 2. Hazards of voltage sags Similar to voltage interruptions, voltage sags can cause failures in electrical devices that are sensitive to voltage changes. These include: (1) Data loss due to sudden power outages in computer systems ； (2) Contactor trips due to loss of voltage or low-voltage protection activation ； (3) Sudden failure of automatic controllers, such as programmable logic controllers and variable frequency drives ； (4) The high-temperature light source, such as an iodine-tungsten lamp, suddenly goes out. The hazards caused by voltage drops to users include: (1) causing interruptions in automated manufacturing production lines, damaging production equipment, and resulting in defective products ； (2) Cause the automated systems of business users to fail, resulting in the loss of customer data ； (3) Causing sudden shutdown of public facilities or failure of lighting systems, thereby endangering public order and personal safety. It is evident that the hazards of voltage sags cannot be underestimated; a fault at any point in the power grid can lead to widespread voltage sags. As a result, the frequency of voltage sags is quite high; some users experience hundreds of such sags per year, and the issue of voltage sags is attracting increasing attention. In Europe, after realizing the impact of power quality on their production, companies stipulate in their power supply agreements with utility providers that voltage drops caused by the provider shall not occur more than a certain number of times per year (such as 5 times per year); otherwise, the customers have the right to claim compensation from the supplier. In Singapore, it is stipulated that each user shall experience no more than 3 voltage drops per year caused by the power supplier; otherwise, fines will be imposed or even the right to receive power supply may be revoked. II. Control strategies for voltage sags. Improving voltage sags can be approached from three aspects: the power supply system, the equipment itself, and the user’s own system. In the short term, taking preventive measures and making improvements within the user’s own system is the most effective approach ； In the long run, a thorough solution should still be found from the perspective of the equipment’s own durability. The improvement of the power supply system has wide-ranging implications, and a comprehensive evaluation as well as strategic consideration are required to determine the most effective and cost-efficient way to make improvements. This article provides a brief introduction to voltage sags control and improvement devices on the user side. 1. Battery-powered uninterruptible power supply UPS. 2. Static Transfer Switch STS (Static Transfer Switch). When the user is powered by two separate feeders, STS can be used to enable rapid isolation and switching to an alternative power source in the event of a failure in one of the feeders, with a switching time of less than half a cycle. 3. Dynamic Voltage Restorer (DVR): DVR is a series compensation device that is highly effective in ensuring the quality of power supply for sensitive loads. It can restore the voltage to its normal level within milliseconds, making it an effective means of suppressing dynamic voltage disturbances. 4. Static Voltage Regulator SVR: The SVR is capable of quickly selecting the appropriate transformer tap within 1/4 of a cycle, thereby keeping the transformer’s output voltage within a range acceptable for the load. 5. Dynamic diesel engine uninterruptible power supply system: Diesel UPS.