Power cables with impregnated paper insulation for voltage 1-10 kV

Power cords with harness

The majority of power cables for voltages up to 10 kV are three-core with sector cores, so-called belt-insulated cables. These cables are available with copper and aluminum conductors with cross-sections from 6 to 240 mm2. Aluminum conductors can be single-core in the entire range of cross-sections, in addition, in the range 70-240 mm2, cables with multi-core sealed conductors are also produced. Copper conductors are mainly manufactured with multi-core, but in the range of cross-sections from 6 to 50 mm2, single-core conductors are used.

It is known that the traditional methods for conductive wires are copper and aluminum. In recent years, copper has become extremely scarce, which is why aluminum is most widely used in the cable industry, both for conductors and sheaths.

The electrical conductivity of aluminum is 1.65 times less than that of copper, and its density is 3.3 times less than that of copper, which makes it possible to obtain aluminum wires with the same electrical resistance 2 times lighter than the copper ones. The production of single stranded aluminum conductors in the form of a dense sector gives a great economic effect in the cable industry. The use of such wires makes it possible to reduce the diameter of the cable, in addition, in the production of such wires, labor productivity increases, because compared to the production of multi-wire wires, the volume of pulling operations is reduced and the operation of twisting wires is excluded . Solid sector wires have greater rigidity than twisted ones; in addition, the complexity of installing cables with such wires increases to some extent. However, as studies show, the stiffness of the cable is mainly determined not by the current-carrying conductors, but primarily by the material and structure of the sheath.

Cable insulation consists of strips of cable paper impregnated with a rosin composition. In cables for a voltage of 1-10 kV, each phase is insulated separately, and then a common belt insulation is applied to the twisted insulated wires. The thicknesses of the phase and strip insulation are selected from the conditions of the cable in working mode (in the Republic of Belarus 6, 10 kV networks are implemented with an isolated neutral), ensuring its reliable operation in emergency mode.

In household cables, the thickness of the insulation between the phases is approximately 36% greater than the thickness of the insulation between the core and the sheath.So, for cables with a voltage of 6 kV, the thickness of the phase insulation is 2 mm, and the thickness of the insulation of the belt is 0.95 mm, for cables with a voltage of 10 kV — 2.75 t 1.25 mm, respectively.

For cables with a voltage of 1 and 3 kV, the thickness of the insulation is selected mainly on the basis of its mechanical strength (without damage during bending). The gaps between the insulated wires are filled with bundles of sulfate paper.

The main disadvantage of impregnated paper insulation is its high hygroscopicity, therefore, in order to protect the insulation from moisture during storage, laying and operation, the cables are enclosed in a metal sheath.

Power cables are available in lead and aluminum sheath. Aluminum sheaths are sufficiently tight and mechanically stronger than lead sheaths. The high electrical conductivity of aluminum makes it possible to use aluminum sheaths as the fourth conductor of the cable, which provides a significant saving on aluminum, insulating and protective covers. However, cables with aluminum sheaths cannot be used in conditions of exposure to aggressive environments (alkaline vapors, concentrated alkaline solutions). In such conditions it is necessary to use cables with a lead sheath.

Experience in the manufacture and installation of cables with an aluminum sheath with a diameter of more than 40 mm revealed their excessive rigidity, therefore cables for voltage 1 kV with a cross section of 3 × 240 mm2, 6 kV with a cross section of 3 × 150 mm2 and more, 10 kV with a cross section 3 × 120 mm2 and above must be made with a corrugated aluminum sheath.

The use of a corrugated sheath increases the flexibility of the cables, but when such cables are laid on inclined routes, the impregnating compound can run down the corrugation and form air inclusions in the cable insulation. In this regard, corrugated sheaths can only be used in cables whose insulation is impregnated with non-flowing compounds.

Riser cables

When laying cables with impregnated paper insulation on routes with a large difference in levels, there is a danger that the impregnating mixture will descend to the lower part of the route. The composition flows mainly along the gaps between the conductors in the twisted multiwire conductors, as well as in the gap between the metal sheath and the insulation, and to a lesser extent inside the paper insulation itself.

Thus, in the upper sections of the track, the dielectric strength of the cable decreases due to the appearance of air gaps in the insulation. In the lower sections of the route, due to the increased pressure of the rigid joint, the cable may be under pressure. Therefore, cables with impregnated paper insulation of conventional design can be laid on routes with a difference in level between the highest and lowest point of the cable location no more than 15-25 m. A reduction in the effect of leakage can be achieved impregnating composition by the following measures: the use of closing connectors.