High voltage electrical power cables
High voltage (HV) electrical power cable is used for electric power transmission at high voltage. Like other electrical power cables, high voltage cables have the structural elements of one or more conductors, insulation and a protective jacket. High voltage cables differ from lower voltage cables in that they have additional internal layers in the insulation jacket to control the electric field around the conductor. For circuits operating at or above 2.000 V between conductors, a conductive shield may surround each insulated conductor. This equalizes electrical stress on the cable insulation.
High voltage is defined as any voltage over 1000 volts. Modern high voltage electrical power cables have a simple design consisting of few parts. A conductor of copper or aluminum wires transports the current (Figure 1).
Figure 1 – cross section of a AC high voltage electrical power cable: (1) conductor, (3) insulation, (2) and (4) semiconducting layers, (5) outer conductor and outer coat
The individual strands are often preshaped to provide a smoother overall circumference. The insulation may consist of crosslinked polyethylene, also called XLPE. It is reasonably flexible and tolerates operating temperatures up to 120°C. EPDM (ethylene propylene diene monomer (M-class) rubber) is also used as an insulation.
At the inner and outer sides of this insulation, semi-conducting layers are fused to the insulation. The function of these layers is to prevent air filled cavities between the metal conductors and the dielectric so that little electric discharges can not arise and endanger the insulation material. The outer conductor or sheath serves as an earthed layer and will conduct leakage currents if needed.
Most high voltage electrical power cables for power transmission currently used are insulated by a sheath of crosslinked polyethylene (XLPE). Some cables may have a lead or aluminium jacket in conjunction with XLPE insulation to allow for fiber optics. 50-60 years ago, underground power cables were insulated with oil and paper and ran in a rigid steel pipe, or a semi rigid aluminium or lead jacket or sheath. The oil was kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. There are still many of these oil and paper insulated cables in use worldwide. Between 1960 and 1990, polymers became more widely used at distribution voltages, mostly EPDM (ethylene propylene diene M-class); however, their relative unreliability, particularly early XLPE, resulted in a slow uptake at transmission voltages.
A high voltage electrical power cable for DC transmission (HVDC) has the same construction as the AC cable shown in Figure 1. The physics and the test requirements are different. In this case the smoothness of the semiconducting layers is most important. Cleanliness of the insulation remains imperative. Many HVDC cables are used for DC submarine connections, because at distances over 30 km AC can no longer be used. Most of these long deep sea cables are made in an older construction, using oil impregnated paper as an insulator.
Figure 2 – A cross-section through a 400 kV cable, showing the stranded segmented copper conductor in the center, semiconducting and insulating layers, copper shield conductors, aluminum sheath and plastic outer jacket