As a general rule, in the event of a temporary voltage fluctuation or interruption, automatic restart of the process, as soon as the power supply is restored, can minimize its impact, namely in the cleaning and calibration procedure of the machinery. On the other hand, the automatic restart of a process is not always safe and simple to apply. The number of incidents on a system for any category of client increases with the range of the distribution system and the number of clients. This is a direct consequence of the actual trend to increase the distribution voltage level to gain on the power transfer capability and to increase the line length to serve more clients. A wide variety of electric phenomena falls into the power quality issue, so it is useful to classify the different types of power supply disturbances in different categories.
Some disturbances come from the supply network, while others are produced by the load itself.
Long Duration Voltage Variations Long duration voltage variations include undervoltage or overvoltage lasting more than 1 minute. An undervoltage is a decrease of more than 10 per cent in the supply voltage whereas an overvoltage is an increase of more than 10 per cent. Overvoltage and undervoltage are voltage regulation problems usually solved by better planning of the distribution system. Voltage regulation problems occur when the power system is not strong enough to supply the load properly and there is too much voltage drop in the power system impedance under heavy load. When the voltage variation becomes greater than 3 per cent of the system rated voltage, switching the capacitor bank in steps instead of in one block is normally considered.