Thyristor aging

Thyristor is the work horse of power electronics today and its reliability has a major effect on the reliability of the equipment which employs it. This is especially true in case of HVDC, SVC and other FACTS valves because they use large numbers of thyristors connected in series. It is generally believed that thyristors are not subject to aging because they have no moving parts to wear and because they are designed to operate at temperatures well below the levels where serious decrease in lifetime expectancy could be expected. The intention of this paper is to call the attention of electric utilities, who operate the high voltage valves, to the fact that cases of systemic deterioration of thyristor characteristics have been observed in some equipments operated by Hydro-Quebec (Canada). It is not important whether the cause of the thyristor deterioration is classified as aging or materializes as a consequence of inadequate design, manufacturing defects or even user´s own negligence. It is important that if the phenomenon exists, more serious problems than randomly failed thyristors are likely to develop. If it is recognized early, preventive maintenance should be considered