Calculation of Electrical Stresses in DC Cable Insulation

IEEE Trans. Dielect. Electr. Insul.

2007

The insulation in a dc cable is subjected to both thermal and electric stress at the same time. While the electric stress is generic to the cable, the temperature rise in the insulation is, by and large, due to the Ohmic losses in the conductor. The consequence of this synergic effect is to reduce the maximum operating voltage and causes a premature failure of the cable. The authors examine this subject in some detail and propose a comprehensive theoretical formulation relating the maximum thermal voltage (MTV) to the physical and geometrical parameters of the insulation. The heat flow patterns and boundary conditions considered by the authors here and those found in earlier literature are provided. The MTV of a dc cable is shown to be a function of the load current apart from the resistance of the insulation. The results obtained using the expressions, developed by the authors, are compared with relevant results published in the literature and found to be in close conformity.Index Terms -Maximum thermal voltage, thick slab, dc cable, breakdown, insulation, heat flow patterns, external heat injection.

Section: Computation Of Mtv Of a DC Cablementioning

confidence: 76%

Estimation of thermal breakdown voltage of HVDC cables - A theoretical framework

IEEE Trans. Dielect. Electr. Insul.

2007

“…Several phenomenological formulations are available for dc conductivity, σ(r), in the literature [6][7][8][9][10]. Authors have examined, at length, many empirical models for dc conductivity.…”

Section: Phenomenological Modelmentioning

confidence: 99%

“…Ignoring, for a moment, the insulation leakage current losses, the conductor boundary temperature can be shown to be: ln 2π (7) where T con is purely the contribution of conductor losses to the T m .…”

Section: The Critical Temperature (Mitb)mentioning

confidence: 99%

On the intrinsic thermal stability in HVDC cables

IEEE Trans. Dielect. Electr. Insul.

2007

The paper deals with some important aspects of steady state thermal stability in the insulation of HVDC cables. The resistively graded electric field distribution and the temperature distribution, under steady state, in dc cables, are interrelated. The dc conductivity is known to be a function of stress as well as of temperature. Reasonably convincing theories of electric stress and temperature distribution are now available in the literature. Until recently, the thermal runaway was believed, primarily, to be a consequence of an inordinate run away in the sheath temperature due to dynamic interaction with the ambient. While this premise is partly true, there are other overriding factors controlling the stability. This paper presents a detailed investigation on this aspect and it is shown that the multi-factor dependent dc conductivity alone causes the thermal instability, boundary conditions at sheath having a marginal role.

“…Radial variation of the temperature dependent dc conductivity, forces the stress inversion under certain conditions. A large body of literature exists on the theory of the stress distribution in the insulation [1][2][3][4][5][6][7]. Significant contributions reported in the recent past indicate that the stress distribution in the stable state is nearly a linear function of the radius.…”

Section: DC Conductivity Modelsmentioning

confidence: 99%

“…The following semi-empirical dc conductivity models are of frequent use in the literature of the dc cables [1][2][3][4][5][6][7]: b aE a= ux e T xe a (2) Cu= A x e SXbl Sinh(a' ) The Authors have worked on all the models above. A critical analysis is presented later in this work.…”

Section: DC Conductivity Modelsmentioning

confidence: 99%

On the DC Conductivity of HV DC Cable Insulation - Cautions in using the Empirical Models

Conference Record of the 2008 IEEE International Symposium on Electrical Insulation

2008

The electrical conduction in insulating materials is a complex process and several theories have been suggested in the literature. Many phenomenological empirical models are in use in the dc cable literature. However, the impact of using different models for cable insulation has not been investigated until now, but for the claims of relative accuracy. The steady state electric field in the dc cable insulation is known to be a strong function of dc conductivity. The dc conductivity, in turn, is a complex function of electric field and temperature. As a result, under certain conditions, the stress at cable screen is higher than that at the conductor boundary. The paper presents detailed investigations on using different empirical conductivity models suggested in the literature for HV DC cable applications. It has been expressly shown that certain models give rise to erroneous results in electric field and temperature computations. It is pointed out that the use of these models in the design or evaluation of cables will lead to errors.