Recently, high-voltage power cables from 6 to 500 kV of modern designs have been increasingly used for transmission and distribution of electricity, especially in large cities and industrial enterprises, where the level of electricity consumption and load density are quite significant. The most widespread are single-phase power cables with insulation made of cross-linked polyethylene.
The high voltage level of the core of a single-phase cable in networks of classes 6 kV and more leads to the need to use a metal screen made in the form of wires and/or tape. Its main purpose is to ensure the uniformity of the electric field acting on the main insulation of the cable (insulation «core-screen»), which is achieved only in the case of grounding of the screen. It is convenient to ground the screen at the ends of the cable, since there are usually grounding systems there.
The screens of modern single-phase 6-500 kV cables are made of a well-conducting material (copper); their grounding simultaneously at both ends of the cable leads to the appearance of significant AC currents in the screens comparable to the current of the cable core (both in normal symmetrical mode and in short circuits).
Measurements made on many cable lines of various voltage classes of 6-500 kV indicate that when using single-phase cables, it is necessary to pay increased attention to the choice of the method of bonding and grounding the screens and to carry out appropriate justifying calculations.
To limit the currents in the screens that cause dangerous overheating of the cable, in practice it is necessary to abandon the simple grounding of the screens at both ends of the cable and instead use alternative schemes for connecting and grounding the screens: either one-end grounding of the screens, or cross-bonding (transposition) of the screens.
The annual cost of power losses in the screens of a 6-500 kV cable line of medium length, which has a simple grounding of the screens on both ends, can reach 10 000 euro or even more. Therefore, any capital investments in alternative grounding schemes of screens pay for themselves already in the first years of operation of the line!
ZEU-cable offers equipment for organizing in practice bonding and grounding schemes for screens of single-phase 6-500 kV cables, and also performs all necessary calculations of currents, voltages, and power losses induced in the screens.