The short term and long term behaviour of electrical insulation systems (EIS) and materials (EIM) under ac electrical stress has been widely investigated for decades. The concepts of electric strength, life line, voltage endurance coefficients, Weibull distribution of failure times, [1-5], have been employed to achieve a safe design of EIS for any electrical apparatus. Likewise, the design of EIS has been taking into account the stress coming from voltage impulses, considered as random events that can exceed the breakdown impulse strength of EIM. The recent evolution of power electronics and the consequent increased application for the control of electrical apparatus has brought about a new family of electrical stress, which consists of repetitive voltage impulses. As an example, the ac/dc/ac conversion used to adjust motor speed is generally carried out through pulse width modulation (PWM) which causes the presence of transient overvoltages, associated with commutations of the electronic switches, in the motor supply voltage. These repetitive voltage impulses can exceed largely the peak value of the fundamental voltage, thus subjecting the motor EIS to a new type of unconventional electrical stress [6-10]. Reasoning in terms of life consumption, or degradation rate, such overvoltages will provide a contribution to insulation system ageing that is additional to that attributed to the fundamental supply voltage, which might cause accelerated degradation and premature breakdown [11-15]. This is the reason of the large increase of failure rate observed in adjustable speed drives (ADS) in recent years, but it is also the potential source of premature failures of all the electrical apparatus associated with an AC/DC/AC or AC/DC conversion unit (such as cables and transformers). Once this background became clear, design of electrical apparatus began to be re-visited through appropriate actions, mainly involving two directives. One was to introduce system strategies to dump voltage impulses (e.g. filters, optimisation of the electrical apparatus assembly), the other to improve endurance of EIM and EIS to such kind of electrical stress. This paper will deal with the latter approach, that is, the effect of repetitive voltage impulses generated by power electronics on EIM and EIS and the relevant impact on electrical apparatus design, focusing on ageing mechanisms and modelling, diagnostic measurements and tests for quality control.
G.C. Montanari (2007). Power electronics and electrical apparatus: a threat?. LYNGBY : TAPIR ACADEMIC PRESS.
Power electronics and electrical apparatus: a threat?
MONTANARI, GIAN CARLO
2007
Abstract
The short term and long term behaviour of electrical insulation systems (EIS) and materials (EIM) under ac electrical stress has been widely investigated for decades. The concepts of electric strength, life line, voltage endurance coefficients, Weibull distribution of failure times, [1-5], have been employed to achieve a safe design of EIS for any electrical apparatus. Likewise, the design of EIS has been taking into account the stress coming from voltage impulses, considered as random events that can exceed the breakdown impulse strength of EIM. The recent evolution of power electronics and the consequent increased application for the control of electrical apparatus has brought about a new family of electrical stress, which consists of repetitive voltage impulses. As an example, the ac/dc/ac conversion used to adjust motor speed is generally carried out through pulse width modulation (PWM) which causes the presence of transient overvoltages, associated with commutations of the electronic switches, in the motor supply voltage. These repetitive voltage impulses can exceed largely the peak value of the fundamental voltage, thus subjecting the motor EIS to a new type of unconventional electrical stress [6-10]. Reasoning in terms of life consumption, or degradation rate, such overvoltages will provide a contribution to insulation system ageing that is additional to that attributed to the fundamental supply voltage, which might cause accelerated degradation and premature breakdown [11-15]. This is the reason of the large increase of failure rate observed in adjustable speed drives (ADS) in recent years, but it is also the potential source of premature failures of all the electrical apparatus associated with an AC/DC/AC or AC/DC conversion unit (such as cables and transformers). Once this background became clear, design of electrical apparatus began to be re-visited through appropriate actions, mainly involving two directives. One was to introduce system strategies to dump voltage impulses (e.g. filters, optimisation of the electrical apparatus assembly), the other to improve endurance of EIM and EIS to such kind of electrical stress. This paper will deal with the latter approach, that is, the effect of repetitive voltage impulses generated by power electronics on EIM and EIS and the relevant impact on electrical apparatus design, focusing on ageing mechanisms and modelling, diagnostic measurements and tests for quality control.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
