Bringing an insulation to failure: the role of space charge

Abstract

Considering the title of this paper, one might think that everything has been said, shown, proven and discussed already on ageing and failure of electrical insulating materials, based on at least half century of research work and field feedbacks. However, perhaps there is still room to collect ideas and speculate on apparently proven or brand new concepts, such as the interrelation between ageing mechanisms and physically measurable quantities such as space charge, partial discharges and conduction current. The latter is, indeed, the scope of this paper, which is taken from the Whitehead Memorial Lecture given at the 2010 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP). On one hand we will revisit the concepts of space charge accumulation in insulating (polymeric) materials, considering also the latest achievements which deal with ultra-fast charge pulses (solitons), on the other hand we will discuss how space charge can contribute to ageing rate under dc, ac or impulsive voltage, directly through electric field alteration, or indirectly modifying a fundamental ageing factor such as partial discharges.The goal is, at the very end, to show how much such basic investigations can have a fundamental impact on insulation design and electrical apparatus long-term performance/ reliability. Index Terms -Space charge,degradation, charge packets,aging models, partial discharges, insulation breakdown. 2 2 4 ,where J ii is the impact ionization rate, I is the ionization energy, J DEA is the Dissociative Electron Attachment Gian Carlo Montanari (M'87-SM'90-F'00) was born on 8 November 1955. In 1979, he took the Master degree in electrical engineering at the University of Bologna. He is currently Full Professor of Electrical Technology at the Department of Electrical Engineering of the University of Bologna, and teaches courses of Technology and Reliability. He has worked since 1979 in the field of aging and endurance of solid insulating materials and systems, of diagnostics of electrical systems and innovative electrical materials (magnetics, nanomaterials, electrets, superconductors). He has been also engaged in the fields of power quality and energy market, power electronics, reliability and statistics of electrical systems. He is a member of AEI and Institute of Physics. He is a member of the AdCom of the IEEE DEIS. Since

On the nature of high field charge transport in reinforced silicone dielectrics: Experiment and simulation

“…The measured mobilities from both techniques fall into the range of 10 -11 to 10 -9 m 2 /Vs depending on the polymer type and do not exhibit negative field dependence. The transport was found to be thermally activated and was proposed to depend on the polymer side group relaxations 52 . It is difficult to imagine that the electronic charges in dielectrics can have two separate transport mechanisms that are entirely isolated from each other.…”

Physicochemical origin of space charge dynamics for aged XLPE cable insulation

“…At present, several mechanisms have been inferred for electro-thermal ageing of crystalline polymer [1,6]. The ageing is assumed to increase in regions of high space charge concentration (Dissado -Montanari -Mazzanti model) [14][15][16][17], high electro-mechanical stress (Lewis model) [18][19][20], or free volume which allow high local currents (Crine model) [21][22][23][24]. The Dissado -Montanari -Mazzanti model considers space charges as the driving force for ageing.…”

Charge packets modeling in insulating polymers based on transport description

“…The so-called "slow packets" -first observed in the 90's [2], have apparent mobility in the range 10 -14 -10 -16 m 2 V -1 s -1 and develop under high field 100 kV/mm. "Packets of charge" developing under lower field 20 kV/mm and having a much higher mobility (10 -10 m 2 V -1 s -1 ) -called "fast packets", have been reported more recently [3]. Our objective herein it to investigate the conditions for "slow packets" formation and propagation on the basis of a modeling approach.…”