Voltage sag, also known as voltage dip or voltage interruption, refers to a sudden decrease or nearly complete loss of the effective value of supplied voltage, followed by a recovery to near-normal levels. Voltage sag is typically characterized by the retained voltage and duration. In the electrical and electronic engineering community, voltage sag is defined as a sudden drop in the RMS voltage of the power supply system to between 10% and 90% of its rated value, followed by a return to normal operating conditions within a brief period of 1 minute.

Hazards of voltage sag:

1. Minimal impact on public users: Voltage sag has minimal impact on many users, especially those in public and residential buildings, with some users not even noticing its occurrence. Due to its brief duration, specialized monitoring equipment is required on the power grid to determine if voltage sag has occurred.

2. Significant impact on IT and semiconductor industries: Users and facilities sensitive to voltage sag (e.g., semiconductor industry, electronic CNC equipment, variable speed motor drives, IT equipment manufacturers, etc.) can suffer significant losses when voltage sag occurs.

According to statistics, power utilities and users in Europe and the United States pay much more attention to voltage sag than to other power quality issues. One important factor is that complaints related to voltage sag account for over 80% of all power quality complaints, while complaints related to issues such as harmonics and overvoltage caused by switch operations make up less than 20%. Experts believe that voltage sag has emerged as the most important power quality issue, posing a new challenge to supply quality in the information society.

Remediation of voltage sag:

Measures taken by power supply systems:

1. Reduce the frequency of voltage sag occurrences and decrease the time for circuit fault troubleshooting. Reducing the time for circuit fault troubleshooting also reduces the duration of voltage sag, minimizing its impact on machinery and equipment and thus reducing economic losses. To reduce fault troubleshooting time, circuits commonly use static short-circuit devices and current-limiting fuses.

2. Change the system design of the circuit network to avoid or reduce voltage disturbance. This measure mainly involves changing the wiring, using special transformers, such as K-type transformers, to improve the performance of the equipment, and thickening the wires. Use sensitive anti-interference protection measures for sensitive equipment, use independent loops, reduce grounding resistance, improve arrester configuration and parameters, etc.

Measures taken on the user side:

1. Enhance equipment resistance to voltage sag.

Use motor-generator sets (MG) to utilize the inertia of motors to keep generator voltage stable when voltage sag occurs. Circuit transformers use magnetic resonance transformers (CVT), which can provide stable voltage when the circuit voltage drops to 70% of normal voltage, with the only drawback being a slightly larger volume compared to regular transformers.

2. Install voltage compensation devices in circuits.

The most typical voltage compensation devices installed in circuits are dynamic voltage restorers (DVRs) and uninterruptible power supplies (UPS).