Voltage Sags
They are also known as voltage dips. These are voltage drops which last for a short period, from 8milliseconds to 60 seconds. They are caused by momentary faults in the transmission lines which get cleared quickly. The drop in voltage may range from 10 to 90 percent.
Voltage Swells
These are momentary increases in the voltage exceeding 110% which last less than a minute. They can be caused by phase to ground faults.
Undervoltages
These are caused by a drop in the voltage to less than 90% of the voltage for more than 1 minute. They are also referred to as brownouts. Overloading of the distribution system can cause undervoltage.
Overvoltages
They are similar to voltage swells except that they can last for more than a minute. The voltage exceeds 110%. Overvoltages are caused by poor voltage regulation. They can also be caused by switching of capacitors or by sudden load reduction. Wrong setting of transformer taps can also lead to overvoltages.
Interruptions
Interruptions are defined as the drop of voltage to 10%. Interruptions are further classified into momentary interruptions lasting from 8 milliseconds to 3 seconds, temporary interruptions lasting from 3 seconds to 1 minute and sustained interruptions or long duration interruptions which last for more than a minute. Power interruptions can cause loss to production lines in industries. Some of the ways of mitigating the effects of interruptions are by installing a UPS, an emergency genset or by having power fed from two different feeders.
Unbalanced Voltages
When the voltages in all the three phases are not balanced, this situation occurs. This is caused by single phase overloading of one phase. Loads such as arc furnaces also lead to unbalanced loading. Unbalanced loading can be harmful for loads such as transformers and motors where the unbalanced currents which flow as a result of the unbalanced voltages can cause heating.
Voltage Fluctuation
These are caused by quick variations in the voltage between 90% to 105% of the rated value. Arc furnaces and welding transformers can cause voltage fluctuations. Voltage fluctuations can cause “flicker” in incandescent lamps