Surge arresters for cable sheath preventing power losses in MV networks

Abstract

Cable sheaths of power cables in the distribution networks are generally earthed on both sides. The disadvantage is an additional power loss, caused by the current flowing in the circuit sheath to earth. However, the power losses can be avoided, if only one side of the cable sheath is earthed, and a surge arrester is inserted between the sheath and earth at the other side of the cable. Additional the surge arrester limits overvoltages between sheath and earth, and in this way the arrester protects the cable-sheath insulation. The field-control electrode of the sealing end and the cable sheath have always a similarly defined voltage, if both ends of the sheath are earthed. This kind of protective circuit is not new. It is usually already applied to cable connections with high currents in high voltage systems. The arresters are standardized for typical lengths of cable-connections, and typical values of short-circuit currents. Its protective level should be as low as possible, because the voltage withstand strength of the sheath insulation is not well defined, and is not assured by any standardized test. On the other hand the arresters should withstand the induces sheath-to-earth voltage at maximum fault current. Beyond this, the paper specifies the additional cable-power loss for different kinds of cable laying and the economical advantage of the solution. It also describes the effects of the grounding arrangement of the whole network.