The Invisible Grid Killer And New Paths To Mitigation-UPS Uninterruptible Power Supply

In the modern industrial society, there is a type of power quality issue that often goes unnoticed, yet it can cause huge losses in the blink of an eye. Unlike a total blackout, it does not completely cut off the power supply, nor is it as easy to detect as general voltage fluctuations. It comes and goes without a trace, yet it can instantly shut down automated production lines and paralyze precision equipment. This phenomenon is commonly known as Voltage Sag.

What is a Voltage Sag?

A voltage sag, also referred to as a voltage dip or momentary voltage drop, occurs when the RMS voltage decreases to between 10% and 90% of the nominal voltage for a duration ranging from 10 milliseconds to 1 minute. It is primarily defined by two key indicators: remaining voltage​ and duration.

According to standards set by the International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE), a voltage sag typically lasts anywhere from a fraction of a second to one minute. Because it happens so quickly, ordinary residential appliances hardly notice it, and specialized power quality monitors are often required to capture its occurrence.

Why is it the Most Challenging Power Quality Issue?

As modern industries shift toward digitalization and automation, equipment on production lines has become increasingly sophisticated. Programmable Logic Controllers (PLCs), Variable Frequency Drives (VFDs), industrial robots, and data center servers all demand extremely high voltage stability.

When a voltage sag strikes, even if it lasts only tens of milliseconds, it can lead to:

Malfunction of automated devices: Relays trip and contactors drop out;

Data loss and system reboots: Computers crash and control systems reset;

Production line shutdowns: Scrap semiconductor wafers and interrupted chemical reactions can cause economic losses amounting to hundreds of thousands or even millions of dollars.

Statistics show that complaints related to voltage sags account for over 80% of all power quality complaints. It is fair to say that voltage sags pose the greatest challenge to power supply in today's information age.

Can It Be Completely Eliminated?

Unfortunately, voltage sags are unavoidable. Natural factors such as lightning strikes and strong winds, as well as human-induced external contacts and equipment switching, can trigger line faults and subsequently cause voltage sags. Even if the power grid achieves a reliability rate of 99.99999% (commonly known as "six nines"), approximately 10 voltage sags may still occur annually.

Therefore, relying solely on power supply companies to improve grid reliability is not only enormously costly but also treats the symptoms rather than the root cause. The industry has reached a consensus: the mitigation of voltage sags requires collaborative efforts from the grid side, equipment manufacturers, and the user side.

 Multi-Stakeholder Mitigation Pathways

1. Optimization on the Power Supply Side (Grid Side)

Power grid companies can minimize the duration and scope of voltage sags by optimizing the grid structure, shortening fault-clearing times, and installing solid-state circuit breakers.

2. Improved Immunity of Equipment

Equipment manufacturers can optimize circuit designs and enhance anti-interference performance during the production of sensitive equipment, allowing the devices to withstand short-term voltage drops without immediately shutting down.

3. End-User Mitigation

This is currently recognized as the most cost-effective level of governance. By installing compensation devices at the front end of sensitive equipment, external voltage disturbances can be effectively isolated. Common devices include:

UPS (Uninterruptible Power Supply): Provides independent backup power, but comes with high costs and energy consumption;

DVR (Dynamic Voltage Restorer): Only needs to compensate for the missing portion of the voltage sag. Its capacity is merely 1/5 to 1/3 of the total load, making it low-cost and highly efficient. It is currently the most focused-upon mitigation technology.

Conclusion

Although a voltage sag is a brief event, it can trigger widespread consequences. Reducing its impact on production and daily life cannot rely solely on the efforts of power supply enterprises; it requires joint participation from equipment manufacturers and users. In the future, power quality will become a crucial benchmark for measuring power supply service levels and a key factor for users when choosing their electricity suppliers.