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Voltage Sag and Its Impact on Performance

Voltage sag, also known as voltage dip, is a brief but significant drop in the voltage level of an electric power system. It occurs when the RMS voltage decreases between 10 and 90 percent of the nominal voltage for a duration ranging from half a cycle to one minute. This phenomenon can be triggered by high current demands, such as the inrush current from starting electric motors, transformers, or heaters, or by fault currents due to overloads or short circuits.

Figure (1)

Internal resistance model of a source of voltage, where ε is the electromotive force of the source, R is the load resistance, V is the voltage drop across the load, I is the current delivered by the source, and r is the internal resistance.

Voltage sags occur when a sudden increase in current demand causes a drop in the RMS voltage of a power system. This happens because the internal resistance of the power source causes a voltage drop when current flows through it. The greater the current drawn, the larger the voltage drop across the internal resistance, reducing the voltage available to the load.

This relationship is governed by Ohm’s law, which states that voltage equals current multiplied by resistance. The magnitude of a voltage sag depends on the distance between the fault or load and the affected equipment. A fault closer to the load causes a more severe sag because the voltage drop is higher.

System configuration, such as the impedance of transformers and transmission lines, also influences how much the voltage decreases. These factors determine how much of the system’s voltage is lost during a sudden current surge. Since power is the product of voltage and current, a lower voltage means less power is delivered to the load, even if the current remains high.

The sag’s duration and depth determine how likely this is to happen, with longer or deeper sags having more severe effects. Monitoring and mitigating voltage sags is important for maintaining power quality. Understanding the relationship between current, resistance, and voltage is key to analyzing and preventing the impact of sags on electrical systems.

Key Points

  • A voltage sag is a short-duration reduction in the voltage of an electric power distribution system, defined by a decrease in RMS voltage between 10 and 90 percent of nominal voltage for one-half cycle to one minute.
  • Battery internal resistance is the impedance in series with an ideal voltage source that represents the voltage drop when the battery delivers current, affecting the measured voltage output.

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