What is voltage sag? How can it be managed?

Voltage sag refers to a situation in a power system where the voltage suddenly drops significantly for various reasons. This condition usually recovers in a very short time but can last for several seconds or longer. Causes of voltage sag may include lightning strikes, the sudden start of large loads, short circuit faults, etc. In daily electrical work, we may encounter voltage sags, but what exactly are they? What harm can they bring to our lives and work? And how can they be effectively managed?

Basic overview of voltage sags

Voltage sag, also known as voltage dip, refers to a sudden decrease or almost complete loss of the effective value of supply voltage, followed by a recovery to near-normal values. Voltage sags are generally described by retained voltage and duration. The definition of voltage sag by the Institute of Electrical and Electronics Engineers (IEEE) is a sudden drop in the effective value of power supply voltage to 10%-90% of its rated value in the power supply system, which returns to normal operating conditions within a subsequent short period of 1 minute.

Hazards of voltage sags

    Voltage sags have little impact on many users, especially public and residential building users, some of whom may not even notice their occurrence. Due to the very short duration, it’s difficult to detect voltage sags without installing a dedicated monitoring instrument on the power grid.

    1. Significant impact on the IT and semiconductor industries

    For users and facilities sensitive to voltage sags (such as the semiconductor industry, electronic numerical control equipment, variable-speed motor devices, IT industry equipment enterprises, etc.), the losses caused by voltage sags can be enormous.

    2. Huge impact on the information industry:

    Misoperation of automatic control devices, computer system malfunctions, etc.

    Harm to sensitive mechanical equipment: Harm to DC generators is likely when the voltage falls below 80% of normal, potentially causing circuit trip accidents.

    For PLC controllers:

    Voltage sags or circuit short-circuits can cause control program disorders.

    For variable frequency drives:

    Operation is ceased if the voltage continuously stays below 70% for 120s.

    3. Recognized as the most significant power quality issue

    Voltage sags and fluctuations can cause economic losses. For sensitive users, over 90% of power quality issues are attributed to problems caused by voltage sags, making it a major issue for the normal and safe operation of such electrical equipment. According to statistics, the attention to voltage sags by the European and American power sectors and users is much stronger compared to other power quality issues. An important factor is that complaints caused by voltage sags account for over 80% of all power quality complaints, while those caused by harmonics, switch operation over-voltages, etc., account for less than 20%. Experts believe that voltage sags have become the most important power quality issue, posing new challenges to the quality of power supply in the information society.

    Can voltage sags be avoided?

    The answer is that they cannot be avoided even with high power supply reliability. Many reasons can cause voltage sags, such as unexpected lightning strikes, accidental contact by animals or foreign objects, line faults caused by strong winds and other natural environmental factors, and phenomena produced by power fault switching. All these faults causing momentary power outages or voltage sags are currently unavoidable in power systems, with the occurrence of voltage sags far exceeding complete power outages. It is understood that foreign power users, especially some enterprises with higher quality electricity demands, initially demanded the power companies to improve power supply reliability. This approach significantly increased initial investment costs, yet the problem of voltage sags was never effectively solved. Even with a power supply reliability of 99.99999% (the ultimate goal of power companies, which is difficult to achieve in practice), calculated over 365 days a year, the total annual power outage time is only a few seconds. However, despite such high reliability, voltage sags can still occur up to about ten times a year. Despite the significant investment costs, the suppression effect on voltage sags is not ideal.

    How are voltage sags managed?

    Studies have shown that the cost and effectiveness of managing voltage sags from the power supply source to the entire production line, equipment level, and device control level decrease exponentially. Therefore, from the perspective of management costs and effectiveness, it is advocated to conduct management at the device endpoint, even deep inside the electrical control components of the device. The closer to the endpoint the management is conducted, the less the cost and the same management effect can be achieved.

    The most typical voltage compensation devices installed in circuits are Dynamic Voltage Restorers (DVR) and Uninterruptible Power Supplies (UPS).


    The grid voltage is converted to direct current voltage by an AC-DC inverter, supplied to a DC-AC inverter, and stable AC voltage is provided to the load. Meanwhile, the grid voltage charges the energy storage battery. When the grid voltage is low or suddenly drops, the UPS power starts to work, supplying the required power to the load from the energy storage battery to maintain normal production. When the load is severely overloaded, the grid voltage supplies power directly to the load through rectification. UPS can effectively solve problems such as voltage sags and short-term power supply interruptions. However, the entire power of the load must be transformed through UPS, increasing system losses and reducing efficiency.


    When a voltage sag occurs, a DC-AC inverter with pulse width modulation function synthesizes a controlled amplitude, frequency, and waveform voltage, which is added to the line voltage through a series-boost transformer, reacting to the voltage sag within 1/4 cycle, raising the output voltage to the level required by the system. The PWM modulation wave of the inverter is based on a standard sine wave, and by comparing the collected voltage waveform with the standard wave, voltage harmonics can be effectively compensated. The energy for voltage boost is provided by a direct current capacitor.

    DVR is currently the most attention-getting device for solving voltage sags domestically and internationally. Although the DVR operates in series on the line, since it only needs to compensate for the portion of the energy during a voltage sag, its design power is only 1/5 to 1/3 of the total load capacity, making its price more favorable than that of the same capacity UPS and its loss far less than the latter, demonstrating significant technical advancement.

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