The main concern of consumer side of electricity was the reliability of supply. Reliability refers to continuity of electric supply, even though the power generation in most countries is fairly reliable, the distribution is not always so. It is however not only reliability matters for the consumers, but also quality too is very important to them. For example, a consumer connected to the same bus that supplies a large motor load may have to face a severe dip in his supply voltage every time the motor load is switched on. In some extreme cases, consumers have to bear with blackouts. This can be quite unacceptable to most customers. There are also very sensitive loads in the system such as hospitals (life support, operation theatre, and patient database system), processing plants (semiconductor, food, rayon and fabrics), air traffic control, financial institutions and numerous others that require clean and uninterrupted power. In several processes such as semiconductor manufacturing or food processing plants, a batch of product can be ruined by a voltage dip of very short duration. Such customers are very largely affected by such dips since each such interruption cost them a substantial amount of money. Even short dips are sufficient to cause contactors on motor drives to fail. Stoppage in a portion of a process can destroy the conditions for quality control of the product and require restarting of production again. Thus in this changed scenario in which the customers increasingly demand quality power, the term power quality (PQ) attains increased significant important.
Transmission lines are exposed to the forces of nature. Furthermore, each transmission line has its load ability limit that is often determined by either stability considerations or by thermal limits. Even though the power quality problem is a distribution side problem, transmission lines often have an impact on the quality of power supplied. It is however to be noted that while most problems associated with transmission systems arise due to the forces of nature or due to the interconnection of power systems, individual customers are responsible for a more substantial fraction of the problems of power distribution systems