Causes of Voltage Sag

You should not mix voltage sags (caused by transmission/distribution faults) and voltage fluctuations (starting large motors), voltage transients, switching disturbances or fault disturbances (resulting in sags) ect. are different events.

Every time you speak about voltage sags, we are starting a discussion on power quality and reliability and the definitions, events and for identifying voltage profiles (profiles, durations...) the causes and for providing safe and reliable power systems, you need to refer to standards since the solutions dealing with sags or voltage fluctuations or power quality and reliability are different.

For example a new index to describe voltage sag performance was developed. It is called the System Average RMS (Variation) Frequency Index (SARFI). This index represents the average number of voltage sags experienced by a customer each year with a specified characteristic. For example, SARFI70 represents the expected number of voltage sags where the minimum voltage is less than 70% of nominal. These voltage sags are different than other voltage transients or fluctuations caused by different events. Refer to standards for definitions and further clarifications.

The voltage sags are different than other voltage transients or fluctuations caused by different events. Refer to standards for definitions and further clarifications. You can find these definitions and guidelines in IEC or IEEE.

Based on IEEE definitions: voltage fluctuations are defined as caused by motor starting, large loads, ect... and power system or utility disturbances (fault disturbances causing sags) and you have also transients (switching capacitors, reactors, ect..). These events (voltage fluctuations, transient and disturbances) have different causes and you may even have some interactions. So it is better not to mix or group everything under "sags" since this may have an impact or other issues doing other work.

Voltage issues resulting from high peak demands or equipment failure (contingencies) is known as low voltage or sometimes referred as under voltage. The time frame is longer in duration (five minutes or more).

Where voltage issues due to motor starting, switching (line isolators, capacitor banks, and etc.), and sudden increase or decrease of loads are known as voltage sags or swells. Voltage sags/swells usually last anywhere from 1 cycle (sub-cycle event) to 1 second.

In some IEEE guidelines, resulting voltages caused by motor starting or switching are not even included or define as "sags" but as voltage fluctuations (under voltage) or transients for a good reason since the resulting voltage profiles are different. However, we always include voltage drop caused by motor starting as "sag". On the other hand, dealing with all these voltage events (transients, disturbances, voltage fluctuations, sags...) to make analysis or trying to resolve the problems preparing studies, monitoring and using tools such as power quality and reliability methods, it maybe necessary to make some difference or keep defining "sags" as abnormal voltages resulting from any events?