Introduction to Various Cables Used in Practice

Introduction

Transmission system operators (TSOs) throughout the world have been seeing growing numbers of transmission line projects in recent years for different reasons including the increase of cross-border trade, renewable energy sources, smart grid projects, the replacement of aging facilities, and in some countries due to growing demand.

Until recently, TSOs have responded to these necessary transmission upgrades mostly by the introduction of overhead lines (OHLs). HVAC underground cable systems have been used, but their applications have been mainly limited to densely populated areas. As such, HVAC underground cable systems are limited both in length and number to date.

This tendency has been changing over the past 10 years as the service experience of HVAC, especially EHV AC, cable systems has become satisfactory. The applications of HVAC cable systems are proposed more often in order to protect the landscape and also public health (e.g., EMF). Hence, HVAC cable systems recently planned or installed are longer than those installed previously.

For example, in Denmark, after receiving public and political pressures to underground its OHLs, Danish TSO, Energinet.dk, published a report on the future expansion and undergrounding of its transmission grid on the 3rd of April 2008. The report proposed and compared five principles (A–E in Figure 1.1). From the five principles, the Danish government has selected Principle C, as shown in Figure 1.2, in which all new 400 kV lines will basically be undergrounded.

A similar tendency can be observed on HVDC, especially EHV DC, submarine cable systems. The Nor Ned cable, which connects Norway and the Netherlands, and the Brit Ned cable, which connects the Netherlands and the UK, are symbolic examples of such a trend. These cable lines, mainly for cross-border trades, have a total length of 580 and 260 km, respectively.

Five principles for the future grid expansion

The scale of these projects is beyond the level many people expected at the beginning of this century.

As these cable projects increase, there is an increased need to study cable system transients. In particular, the introduction of long cable systems may cause peculiar phenomena, such as resonance over voltages, which require careful attention. Severe temporary over voltages in the power system with long cable systems which can be caused in specific network conditions or configurations have been reported.

Cable modeling for studies on cable system transients, as discussed in Chapter 4, requires the understanding of cable systems. This chapter first discusses the cable itself and then introduces the laying configuration and the sheath bonding, that is, the cable as the cable system. Various cables used in practice are explained in the following two sections – land cables in Section “Land Cables” and submarine cables in Section “Submarine Cables”. Section “Laying Configurations” discusses the laying configuration including the sheath bonding. The main focus of this chapter is on how these physical characteristics of cable systems affect their electrical characteristics.

Land cables in Section “Land Cables” cover three major cable types, that is, XLPE (cross-linked polyethylene, PE) cables, SCOF/SCFF (self-contained oil-filled/self-contained fluid-filled) cables and HPOF/HPFF (high-pressure oil-filled/high-pressure fluid-filled) cables. The term “fluid-filled” is used to include both oil-filled cables and gas-filled cables, but most fluid-filled cables are oil-filled cables in actual installations. Even though XLPE cables are increasingly selected for new cable lines in many countries, SCOF cables and HPOF cables are still a popular choice in some countries. HPOF cables are selected, in particular, for the replacement of old HPOF cables since it is often possible to continue using their steel pipes even after the cable replacement.

The laying configuration and the sheath bonding affect cable system transients as the cable itself does. They need to be modeled correctly in order to obtain accurate impedance/admittance of the cable system or reasonable simulation results. Section 1.4 discusses different laying conditions and sheath bonding methods together with their impact on the cable system transients.

Grid expansion plan based on Principle C

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