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This paper has the aim of providing a view of a lively debated topic which has broad impact on the design of electrical apparatus and new insulating materials, that is, the interaction between space charge and aging processes of polymeric insulation. Aging models developed in recent decades that consider explicitly or implicitly the contribution of space charge to insulation degradation, under both dc and ac voltage, are dealt with, with the intention to point out their range of validity. Some conventional phenomenological models that have been used for much more than two decades without referring to space charges can be exploited to account for electrical field and activation energy modification due to space charge. These, together with models conceived considering space charges as the driving force for aging, are especially examined. In addition, recent models that disregard the action of space charge as an ageing factor, but consider space charge as the consequence of degradation processes are also discussed.

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The mechanisms of charge injection, transport and trapping in low-density, high-density and cross-linked polyethylene (LDPE, HDPE and XLPE) are investigated in this paper through charging-discharging current measurements and space-charge observations. The conductivity of LDPE is much larger than that of XLPE and HDPE. The threshold for space-charge accumulation and that for a space-charge-limited current mechanism, coinciding for the same material, are almost identical for LDPE and HDPE, while the threshold of XLPE is higher. However, HDPE accumulates more charge than the other two materials. The depolarization space-charge curves and the conduction current versus field characteristics indicate that the mobility of LDPE is larger than that of XLPE and HDPE, which supports the significant difference in conductivity. The lower mobility, as well as the nature, depth and density of trap sites, can explain the difference in space-charge accumulation and thresholds.

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This paper has the purpose of highlighting the achievements during the last decades in life modeling under electrothermal stress. Phenomenological and physical models are examined, considering also a statistical approach, with applications to experimental results..

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