ient disturbances (voltage sags, voltage swells, impulses, etc.) and steady-state disturbances (harmonic distortion, unbalance, and flicker). Each of these power quality problems has a different cause. Some problems are strictly related to the shared nature of the electrical grid. For example, a fault on the network may cause a dip that will affect some customers connected to the grid, and the more severe the fault, the larger the number of users involved. Other problems, such as harmonics, arise from the customer loads and may or may not propagate onto the network and so affect other customers. In most of the cases, industrial customers complain about evident power defects such as interruptions (which range from a few seconds to several hours) and voltage dips or sags where the voltage drops to a lower value for a short duration. In fact, long power interruptions are a problem for all users, but many operations are very sensitive to very short interruptions. Examples of such sensitive operations are the continuous processes, where even short interruptions can lead to the loss of synchronization among different machineries and then stop all production processes.
Although majority of the customers are more sensitive to transient disturbances, since immediate economical losses are involved, there are more underhand problems affecting electrical power such as harmonics and unbalance. In fact, the effects of harmonics on power system apparatus include resonance, reduced operating life of rotating machines, malfunctioning of power system protection devices, errors in power measurements, additional losses, etc. Moreover, unbalance phenomena should also be well monitored, detected and corrected. A machine operating under an unbalanced supply will draw a current with a degree of unbalance. As a result, the three-phase currents might differ considerably and a rise in temperature would take place in the machine. Motors and generators, particularly the large and more expensive ones, may be fitted with protection to detect extreme unbalance and to trip the machine. The behavior of multiphase converters is also affected by an unbalanced supply; this causes an undesirable ripple component on the DC side and non-characteristic harmonics on the AC side. Moreover, the presence of an unbalanced load creates unbalanced current components that cause voltage drops across the source impedance and hence generate harmonic powers flowing backward from the load to the network.
Taking into consideration the above-mentioned aspects, the following section is focused mainly on harmonics whose effects could become more evident in the future.
2 Different kinds of disturbances
It is possible to define some characteristic parameters in order to assess the quality of power delivered from the supply:
• Frequency;
• Amplitude;
• Waveform;
• Symmetry of the voltage system.
