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Understanding Voltage Fluctuations and Steady-State

April 14, 2020
Why a nominal voltage must be standardized to troubleshoot long-duration voltage variations

Power quality primarily encompasses short-duration events on the electrical system that cause equipment to misoperate or shutdown. The time delineation between a power quality event and a steady-state condition is 1 min. In terms of power quality, 1 min. is a long time; most events are cycles to seconds. However, long-duration voltage variations fall within the scope of power quality ― especially when equipment misoperations are concerned. A nominal voltage must be standardized to troubleshoot the long-duration voltage variations.

ANSI C84.1-2016 provides nominal voltages and specifies an acceptable range of fluctuation. At the electrical service, the electric utility should attempt to keep the voltage ±5% of nominal. The electrician needs to be aware of the acceptable electric utility fluctuation and plan to keep the voltage at the point of utilization between +5/-10% of nominal. (See A History Lesson in Standardizing Voltage and Frequency in the Electrical Industry for more information on ANSI C84.1.) The 1-min. time delineation plays a big role in troubleshooting overvoltage and undervoltage conditions.

When the voltage increases more than 110% of nominal RMS voltage, it is considered an overvoltage. An overvoltage can be caused by large loads turning off, incorrect transformer tap settings, and misoperating electric utility voltage regulation apparatus (e.g., capacitor banks, voltage regulators, and load tap changers). Voltage decreases of more than 90% of nominal RMS voltage define undervoltage conditions. The cause of undervoltage is similar to that of overvoltage with the exception of undersized conductor. Turning off capacitor banks, incorrectly tapped transformers, and misoperating electric utility voltage regulation apparatus all lead to undervoltage conditions.

It is always recommended to work with the electric utility when attempting to resolve a long-duration voltage variation problem. The electric utility will provide the voltage delivery thresholds they are governed, which should not be much different than what’s outlined in ANSI C84.1. When the voltage at the electric service is established to be within the allowable fluctuation of ±5%, further troubleshooting in the facility needs to be performed. It is important to check the voltage drop of the conductor servicing the equipment. Additionally, if a transformer is used, check the output voltage and tap setting. In some cases — depending on the severity of the voltage issue and sensitivity of the equipment — customer side voltage regulation equipment may need to be installed. Being aware of nominal voltage, electric utility fluctuations, and steady-state voltage issues will help to resolve problems more quickly.

About the Author

Bryan Glenn | Power Quality Consultant

Bryan Glenn is a power quality consultant for EC&M magazine. He can be reached at [email protected].

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