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In this paper the sequence impedances of an existing HVAC insulated cable, protected by a steel reinforced concrete structure, are estimated. The paper aims at investigating if the presence of metallic protective structures significantly affects the sequence impedances of the line. The present analyses are carried out by applying both the multiconductor cell analysis and the finite element method in order to model the HVAC line with and without protective structure. The results arising from the use of the different modelling approaches are compared. [ABSTRACT FROM AUTHOR]

With an impetuous growth of DC links and future grids that will be often hybrid, providing AC and DC supply to nearby or the same customer, an important element is often neglected, that is, electrical insulation. There are no doubts that overhead lines will step back compared to insulated cables, for known reasons of environmental impact, right of way and reliability, especially in an urban or harsh environment. On the other hand, while AC insulation systems have been investigated and used for decades, there is no analogue experience for DC polymeric insulation, nor for insulation systems subjected to power electronics supply. This study aims at enhancing the attention on this topic, focusing on insulation ageing mechanisms and models of relevance for hybrid AC/DC assets and grids. Ageing and life models for AC, DC and power electronics supply are discussed and compared to experimental data, focusing on partial discharge and space charge. It is shown how these two phenomena affect lifetime for different AC, DC and transient operating conditions. The main drivers for insulation design and material selection are discussed, showing that insulation reliability can be achieved only if operating stresses, ageing mechanisms and life models are known and accounted for properly by insulation designers.

The paper deals with the sequence impedances (positive/negative and zero sequences) of high- and extra-high-voltage land single-core insulated cables. In particular, it presents the comparisons between sequence impedance measurements and computations. The computations of the sequence impedances are carried out by means of the most important international normative and council references (IEC/Cigré) and of multiconductor cell analysis which is a consolidated and powerful tool developed by University of Padova in order to analyse power frequency regimes of multiconductor asymmetric power systems. The comparisons are presented with reference to four high- and extra-high voltage insulated cables, even if the available ones are much higher: however, the conclusions derived from these four reference cases are general and can be useful for transmission system operators and for power electric system engineers involved in insulated cables. The paper demonstrates, for the first time in technical literature, that direct formulae cannot correctly evaluate the sequence impedances of installed single-core land cable systems. Extensive on-field measurement campaigns have served to this purpose.

Nowadays, the analysis of the lightning performance of high-voltage (HV) underground sections has become increasingly important, thanks to their increasing usage with overhead transmission lines. In this way, the objective of this paper is to investigate the sensitivity of the overvoltages, at the terminal of underground cables (UCs), due to the incidence of negative lightning flashes at the shield wire or the tower top. It is done by means of different representations of the grounding behavior, different points of lightning incidence, different values of soil resistivity, and considering or not the variation of the soil electrical parameters with frequency. It is observed that a reliable investigation on lightning overvoltages is important and it must consider a suitable representation of the grounding behavior. An improper modeling can lead to nonrepresentative results. In this way, it is highlighted how the frequency dependence of the soil parameters influences the results in many different situations. For long distances from the UCs and low soil resistivity regions, all the grounding models produce similar results. On the other hand, for regions with high soil resistivities, the use of a proper model for the tower grounding is important, since the discrepancies between the results produced considering or not the dependence of the soil parameters with frequency become substantial. The inclusion of the grounding behavior with frequency allows to obtain reliable simulations of the transient behavior of the insulated cables, which constitute the underground section under study. [ABSTRACT FROM AUTHOR]

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The knowledge of ground return current in fault occurrence plays a key role in the dimensioning of the earthing grid of substations and of cable sealing end compounds, in the computation of rise of earth potential at substation sites and in electromagnetic interference (EMI) on neighbouring parallel metallic conductors (pipes, handrails, etc.). Moreover, the ground return current evaluation is also important in steady-state regime since this stray current can be responsible for EMI and also for alternating current (AC) corrosion. In fault situations and under some assumptions, the ground return current value at a substation site can be computed by means of k-factors. The paper shows that these simplified and approximated approaches have a lot of limitations and only multiconductor analysis can show the ground return current behaviour along the cable (not only the two end values) both in steady-state regime and in short circuit occurrence (e.g., phase-to-ground and phase-to-phase-to-ground). Multiconductor cell analysis (MCA) considers the cable system in its real asymmetry without simplified and approximated hypotheses. The sensitivity of ground return current on circuit parameters (cross-bonding box resistances, substation earthing resistances, soil resistivity) is presented in the paper.

The author has already presented some papers which allow studying cable systems by means of the multiconductor cell analysis (MCA). This method considers the cable system in its real asymmetry without simplified and approximated hypotheses. The multiconductor matrix procedure based on the use of admittance matrices, which account for the line cells (with earth return currents), different types of screen bonding, possible multiple circuits (single and double circuit or more), allows predicting the steady-state regime of any cable system. In the previous papers, these matrix algorithms have been presented with reference to a short extra-high voltage (EHV) double-circuit cross-bonded (CB) underground cable (UGC) system. Since the cable link was short, the shunt reactive compensation was not necessary and consequently not considered. In this paper the procedure is generalized in order to take into account three single-phase (or also one three-phase) reactors installed at the cable ends or also at intermediate locations.

Current rating computations for HV- and EHV-insulated cables play a crucial role in the planning phase of a new line for both an entirely underground cable link and a mixed or hybrid (cascade connection of overhead and cable line) one. The paper gives a wide overview of the technical solutions which allow very high current ratings in undergrounded insulated cables. These solutions are examined from a theoretical standpoint and practical applications are shown and explained. The paper demonstrates that in underground insulated cables, current rating up to 2500 A can be reached. [ABSTRACT FROM AUTHOR]

The Laplace domain is a well-known tool in order to study circuit transients. When dealing with power transmission lines with uniformly distributed parameters the usual transmission matrix or ABCD matrix can be also written in Laplace domain. This involves voltage and current dependence upon space and time independent variables. In case of an insulated cable line (ICL) energization, the voltage at no-load end represents one of the most important switching overvoltages. Therefore the possibility of having a reliable, simple, fast and self-implemented tool to determine the transient behaviour can be extremely important for power electric engineers both for validation of commercial software (e.g. Electromagnetic Transient Program - Reconstructed Version, with acronym EMTP-RV) and for understanding the different roles played by each parameter in the transient. [ABSTRACT FROM AUTHOR]

The knowledge of ground return current in fault occurrence plays a key role in the dimensioning of the earthing grid of substations and of cable sealing end compounds, in the computation of rise of earth potential at substation sites and in electromagnetic interference (EMI) on neighbouring parallel metallic conductors (pipes, handrails, etc.). Moreover, the ground return current evaluation is also important in steady-state regime since this stray current can be responsible for EMI and also for alternating current (AC) corrosion. In fault situations and under some assumptions, the ground return current value at a substation site can be computed by means of k-factors. The paper shows that these simplified and approximated approaches have a lot of limitations and only multiconductor analysis can show the ground return current behaviour along the cable (not only the two end values) both in steady-state regime and in short circuit occurrence (e.g., phase-to-ground and phase-to-phase-to-ground). Multiconductor cell analysis (MCA) considers the cable system in its real asymmetry without simplified and approximated hypotheses. The sensitivity of ground return current on circuit parameters (cross-bonding box resistances, substation earthing resistances, soil resistivity) is presented in the paper. [ABSTRACT FROM AUTHOR]

This paper deals with the comparison between the overall whole-of-life costs of overhead lines (OHL) and those of underground insulated cables (UGC). Almost all the investigations published so far, when analyzing maintainability issues, take into account only the costs of planned/periodical maintenance; here, a method for assessing also the expenses sustained for repair after random failures is proposed. The number of random failure events for each kind of component over the entire service life of a transmission line can only be predicted on a probabilistic basis: its expected value is estimated by making use of the relevant mean failure rates from recent statistical surveys. The entire procedure is shown by carrying out a particular case study as an example; the break-even point between the OHL-UGC overall costs, corresponding to a typical rural land Italian market value wx=\ 26.9\ \rm Euros/m^2, is identified. Nonetheless, the method may be widely applied to any type of OHL–UGC comparison. [ABSTRACT FROM AUTHOR]

The author has already presented some papers which allow studying cable systems by means of the multiconductor cell analysis (MCA). This method considers the cable system in its real asymmetry without simplified and approximated hypotheses. The multiconductor matrix procedure based on the use of admittance matrices, which account for the line cells (with earth return currents), different types of screen bonding, possible multiple circuits (single and double circuit or more), allows predicting the steady-state regime of any cable system. In the previous papers, these matrix algorithms have been presented with reference to a short extra-high voltage (EHV) double-circuit cross-bonded (CB) underground cable (UGC) system. Since the cable link was short, the shunt reactive compensation was not necessary and consequently not considered. In this paper the procedure is generalized in order to take into account three single-phase (or also one three-phase) reactors installed at the cable ends or also at intermediate locations. [ABSTRACT FROM AUTHOR]

This paper discusses cable modeling for long high-voltage ac underground cables. In investigating the possibility of using long cables instead of overhead lines, the simulation results must be trustworthy. Therefore, model validation is of great importance. This paper gives a benchmark case for measurements on a 400-kV cable system with cross-bonded sheaths. This paper describes in detail the modeling procedure for the cable system and compares simulation results with the transient field test results. It is shown that although the main characteristics of the waveforms are well reproduced in the initial transient, there are significant deviations between the simulation and measurement results. An analysis indicates that the main cause for the deviation is inadequate representation of the current distribution on conductors since the modeling approach does not take proximity effects into account. The measurement results can be received by contacting the first author of this paper. [ABSTRACT FROM AUTHOR]

EMTP-type programs include dedicated support routines (cable constants) for calculating an electric representation of cable systems in terms of a series impedance matrix Z and a shunt admittance matrix Y, based on cable data defined by geometry and material properties. Z and Y are the basic input of the various cable models that are used in time-domain transient simulations. This paper discusses the modeling of high-voltage cables: single-core, three-phase, and pipe-type cables. Material properties are given for commonly used conductive and insulating materials, and how to represent semiconductjve screens, lossy insulation materials, and magnetic armors is shown. The significance of the grounding condition of sheaths and armors is discussed. In transient calculations, it is always important to accurately represent the core conductor, insulation, semiconductive layers, and the metallic sheath. Frequency-dependent losses of paper-oil insulation need to be taken into account for very-high-frequency transients. The significance of conductors external to the cable depends on the shielding effect of the cable sheath, which depends on the sheath design and the frequency content of the transient. The conclusions are supported by numerical simulation results. [ABSTRACT FROM AUTHOR]

The paper deals with the sequence impedances (positive/negative and zero sequences) of high- and extra-high-voltage land single-core insulated cables. In particular, it presents the comparisons between sequence impedance measurements and computations. The computations of the sequence impedances are carried out by means of the most important international normative and council references (IEC/Cigré) and of multiconductor cell analysis which is a consolidated and powerful tool developed by University of Padova in order to analyse power frequency regimes of multiconductor asymmetric power systems. The comparisons are presented with reference to four high- and extra-high voltage insulated cables, even if the available ones are much higher: however, the conclusions derived from these four reference cases are general and can be useful for transmission system operators and for power electric system engineers involved in insulated cables. The paper demonstrates, for the first time in technical literature, that direct formulae cannot correctly evaluate the sequence impedances of installed single-core land cable systems. Extensive on-field measurement campaigns have served to this purpose. [ABSTRACT FROM AUTHOR]

It has been discussed in various papers, how lack of proximity effect in modelling affects simulations and gives inaccurate results at high frequencies. This paper describes a method for how to include the proximity effect calculations in EMT¹-based simulations. A subdivision of conductors method is used to calculate conductor (core and sheath) impedances. A full phase impedance matrix is then constructed, including semiconductive layers, insulation layers and a special screen of two conducting materials. Finally the method is tested using the universal line model and is verified against field measurements. It is shown in the paper how including the proximity effect, it is possible to obtain good comparison between measurements and simulations, even for the intersheath mode. [ABSTRACT FROM AUTHOR]