Showing papers by "Roberto Candela published in 2020"

A Model for Assessing the Magnitude and Distribution of Sheath Currents in Medium and High-Voltage Cable Lines

Abstract: In this article, the authors discuss a simulation model to study the effect of cross-bonding of metallic sheaths, and/or nonmagnetic armors, of single-core medium- and high-voltage cables in the same circuit. In single-core cables, the resistive losses due to the induced circulating currents in cable sheaths or armors cause an increase of the cable temperature, which therefore reduces its ampacity. This is a serious issue affecting the distribution and transmission lines. In addition, the risk of electric shock due to induced voltages may be present if a person is in contact with the armor/sheath at its unbounded end. For these reasons, special bonding techniques of metal sheaths are employed to reduce these currents. The simulation model to assess the magnitude and distribution of induced armor/sheath currents of medium- and high-voltage cables that is herein proposed may be used to optimize the cross-bonding configuration of single-core cables employed in high-current industrial applications or in transmission/distribution power grids. The model has been experimentally validated by means of actual data from a high-voltage underground line and field measurements performed by Prysmian Electronics.

A Model for the Study of Sheath Currents in Medium Voltage Cables for Industrial Application

Abstract: In this paper, the implementation of a simulation model for studying the effect of cross-bonding of metallic sheaths and/or non-magnetic armor of single-core medium voltage cables in the same circuit is discussed. With the use of single-core cables, the resistive losses due to the induced circulating currents in cable sheaths or armors causes an increase of cable temperature that reduces its ampacity. In addition, the risk of electric shock due to induced voltages may be present if a person is exposed to the armor/sheath at the unbounded end. For this reason, special bonding techniques are used to significantly reduce these currents. The authors have implemented a model that could be used to help optimize the cross-bonding configuration for single-core cables employed in high-current industrial applications. The model has been experimentally validated thanks to actual data from a medium-voltage underground line.

A Comparison of Special Bonding Techniques for Transmission and Distribution Cables

Abstract: In this paper, a review of the existing special bonding techniques for medium voltage (MV) and high-voltage (HV) cables is presented. Special bonding techniques have the purpose of reducing sheath currents, thereby limiting copper losses and the reduction of the ampacity of cables. The authors present a literature review to identify various bonding techniques and how these have evolved over the years thanks to new technologies. Simulations of the most used techniques have been performed in Matlab/Simulink, to compare their strengths and drawbacks.

Polarity reversal in HVDC joints - the effect of the axial thermal conduction

Abstract: It has been shown that the establishment of a thermal gradient over the radius of HVDC cables involves the accumulation of space charge within the dielectric layer. High thermal gradients over the insulation thickness of loaded cables can lead to the inversion of the radial electric field pattern. In this scenarios, transient overvoltages and polarity reversal can lead to local and transitory peaks of electric field. Since the temperature distribution plays an important role in reaching critical values of the electric field, it has been considered interesting to have a more in-depth view of the thermal behavior of HVDC systems close the discontinuities of the geometry along the cable axis. The main goal of this research is to investigate the effect of the axial thermal conduction on space charge phenomena occurring on joints between two HVDC cable segments. In particular, the values of the electric field have been calculated in several probe points in a joint area during a polarity reversal event. The calculations are carried out by means of Comsol Multiphysics® on a 2D axial symmetric geometric model. The results demonstrate that the role of the axial heat transfer in the behavior of the electric field during a voltage transient strongly depends on the thermal conductivity of the joint insulation.

Investigation on partial discharges in HVDC cables after polarity reversal events

Abstract: Due to the accumulation of space charge inside the insulating layer of HVDC cables, the electric field under load conditions may be altered compared to what is established in HVAC cables. For example, a high thermal gradient leads to the inversion of the electric field pattern until the maximum value is reached in proximity of the dielectric-semicon interfaces. These maximum values can be further increased due to transient overvoltages and polarity reversal events until reaching electric field values higher than the rated ones. The main goal of this research is to investigate the possibility that, during these transient phenomena, conditions are created that favor the occurrence of partial discharges (PDs). In order to calculate the effect of a polarity reversal on the occurrence of partial discharge in HVDC cables a 3D thermal and electric model of a typical HVDC cable is coupled with a conductance model for the evaluation of partial discharges. A void volume is inserted in the dielectric layer close to the outer surface. The main characteristics of partial discharges occurring during and immediately after a polarity reversal event are evaluated and discussed in this paper. The findings of this research offer in-depth view of the partial discharge phenomena in HVDC cables under load during a polarity reversal in terms of energy released around defects.