Voltage flicker occurs when heavy loads are periodically turned on and off in a weak distribution system. If the distribution system’s short circuit capacity is not large enough, voltage fluctuations will occur. Starting large motors require an inrush of current, which causes a decrease in voltage. This voltage depression may cause a visible flicker on lighting circuits connected to the same power system.
Voltage flickering can be extremely harmful to sensitive electronic equipment. Computerized equipment requires stable voltage to perform properly. For this reason, voltage flicker is a major power quality problem.
The magnitude of the voltage flicker depends upon the size and type of the electrical load that is producing the disturbance.
A sag in voltage can also cause a voltage flicker, sudden voltage drops in the electrical distribution system can generate inrush current which can travel to sensitive equipment.
Voltage flicker can also be defined with respect to phase over-lapping. According to Kirchoff’s voltage law, the sum of voltages in a 3 phase system should always be the same irrespective of the load condition. If the voltage drops in one phase, it has to be shared by other two phases increasing the nominal voltage values of the other two.
In the above graphs, when the load is turned on it is seen that one phase is totally crushed to zero level, resulting in the increase of the other two phases to 470 volts. This is called voltage flicker in industrial terms. This is poor power quality.