Understanding Causes of Voltage Sags

A voltage sag, also known as a voltage dip, is a short-term reduction in the RMS voltage level. It's a common power quality issue that can affect sensitive electronic equipment. Sags typically last from 0.5 cycles to 1 minute and result in a voltage reduction to between 10% and 90% of the nominal voltage.

Analysis of Options Causing Voltage Sags

Let's analyze each option to determine which event causes voltage sags:

• Option 1 Analysis: Voltage increase of the unfaulted phases during a single line-to-ground fault

  During a single line-to-ground fault, the voltage on the unfaulted phases typically decreases, not increases. This reduction is caused by the flow of fault current through the system's impedance. An increase in voltage on the unfaulted phases would indicate a voltage swell, not a sag. Therefore, this statement is incorrect as a cause for voltage sags.

• Option 2 Analysis: Single line-to-ground fault

  A single line-to-ground fault occurs when one phase conductor accidentally contacts the ground. This creates a low-impedance path, causing a large fault current to flow. The flow of this current through the system's inherent impedance leads to a significant voltage drop, resulting in a voltage sag. The voltage level decreases considerably, especially on the faulted phase, and can impact adjacent phases depending on the fault characteristics and system grounding. This is a primary cause of voltage sags.

• Option 3 Analysis: Switching off of a large load

  When a large load is switched off, the total power demand on the system decreases. This reduction in demand causes less current to flow through the system impedances. Consequently, the voltage drop across these impedances decreases, leading to a rise in the overall system voltage. This phenomenon is known as a voltage swell, not a voltage sag.

• Option 4 Analysis: Energising a capacitor bank

  Capacitor banks are used to supply reactive power to the system. When a capacitor bank is switched on (energised), it injects reactive power, increasing the overall voltage level, particularly on lightly loaded systems. This injection of reactive power typically causes a voltage swell, similar to switching off a large load, rather than a voltage sag.

• Option 5 Analysis: Empty Option

  This option is empty and therefore cannot be evaluated as a cause for voltage sags.

Conclusion on Voltage Sag Causes

Based on the analysis, a single line-to-ground fault is a direct cause of voltage sags due to the significant voltage drop created by the large fault current flowing through the system impedance.