Thermal Stability in HVDC Cables: Whether it is Internal or External?

The breakdown in HVDC Cable insulation is more often traced to thermal origins. Possibility of two kinds of instability has been suggested in the literature; internal (or intrinsic) thermal instability and external (or interactive) thermal instability. The internal thermal stability is believed to be due to the limitations of thermal and dc resistance of insulation. The interactive thermal stability, on the other hand, is believed to be due to escalation of sheath temperature with voltage under the influence of the thermal resistance of the surroundings. In other words, an interactive instability is caused by an imbalance between heat produced in the insulation and that which is transferred to surroundings from sheath boundary leading to failure of equilibrium (resulting in the escalation or runaway in sheath temperature). Until now, in the literature, the latter type (interactive) received much attention and was almost taken synonymous to the thermal stability. In the present paper the authors investigate for the differences between these two types of thermal failures. Tangible explanations relating to thermal instability in dc cables are presented in the paper.