ANALYSIS OF THE RELIABILITY AND OPERATIONAL SAFETY OF FLEXIBLE HIGH-VOLTAGE EXCAVATOR CABLES

V. G. Kulemzin, V. F. Kushpalenko

(Presented by the Department of Occupational Safety)

Flexible high-voltage cables used in open-pit mining operations to supply electric power to excavators are operated under severe conditions. They are subjected to repeated and complex deformations: bending, twisting, stretching, impacts from falling pieces of coal and rock; they are moved over rocky ground and are exposed to climatic factors. Under these conditions, frequent cable damage occurs, which increases the danger of electric shock to maintenance personnel and causes prolonged downtime of high-performance equipment.

Analysis shows that damage to excavator cables is often the cause of short circuits in high-voltage networks of open pits. According to the literature [1, 2, 3], the share of this cause among others is 45–52%, while in the total number of accidents of this type, single-phase ground faults account for 60–80%.

If, in addition to the above, one takes into account the relatively great length of excavator cables (about 200 m per unit), the high operating voltage (6 kV), and the systematic contact of maintenance personnel with these cables, then they should be classified as highly hazardous elements of the electrical equipment of open pits.

At present, domestic enterprises of the mining, coal, and peat industries use flexible cables with rubber insulation and sheath of the GTSh and KShVG grades to supply electric power to excavators and other powerful mobile machines. Their designs are shown schematically in the figure, where they are designated respectively by the letters “a” and “b.”

Fig. 1. Designs of excavator cables of types GTSh (a) and KShVG (b):
1 — current-carrying conductor; 2 — conductor insulation; 3 — cable core; 4 — insulating rubber sheath; 5 — rubberized fabric; 6 — metal shield; 7 — hose sheath; 8 — sheath of ozone-resistant rubber; 9 — grounding conductor.

The design of excavator cables provides for a metallic shield, consisting of a braid of tinned copper wires laid over the twisted insulated conductors—in GTP cables—or over each insulated power conductor—in KShVG cables. The braid covers 40–60% of the surface of the insulated power conductors.

During operation, the cable shields are grounded. In a GTP cable, which has no grounding conductor, the shield is grounded only from the switching-station side; at the other end of the cable the shield is connected to the excavator frame and serves as the grounding conductor for the latter.

The shield is an important element that reduces electrical hazard in cases of damage to the insulation of the cable’s current-carrying conductors. It forms the section through which the electrical circuit is closed for current to flow from the damaged phase to ground. As a result, the touch voltage is substantially reduced and it becomes possible to disconnect the cable from the network by means of protective equipment.

However, if the shield has suffered a break at some point along its length, the continuity of the cable grounding circuit is disrupted and the possibility of performing the above-mentioned protective functions is lost. Cases of damage to the shielding braid in operating cables are a frequent and rapidly developing phenomenon.

A study of the experience of enterprises mining coal and peat by the open-pit method established that the shields of GTP cables fail after 1.5–4 months, and those of KShVG cables after 4–8 months of operation. Certain characteristic cases of failures and the electrical injuries to people caused by them are also indicated in the work of engineer A. M. Mavritsyn [4].

The low reliability of shield operation is explained by its unsatisfactory design and by the severe operating conditions of the cable, during which the latter undergoes cyclic alternating deformations of torsion, bending, and tension. Let us analyze the phenomena that occur in this process.

Torsion of the cable in the direction coinciding with the lay direction of the shield wires, or opposite to it, will respectively cause an increase or decrease in the length of the wire helix within the pitch of the braid lay. In the first case the wires are stretched; in the second, excess wire length appears, owing to which bulges arise in the form of Z-shaped deformations.

When the cable is bent, the following occurs: the shield wires forming the upper section of the helix (the upper fibers) are subjected to tension; in the same wires, in the lower sections, excess length appears, contributing to the formation of Z-shaped deformations.

The combined action of the phenomena considered causes a progressive accumulation of fatigue in the metal of the shield wires and their intensive failure. The previously cited data on shield service lives make it possible to note their especially rapid failure in GTP cables. This indicates the irrationality of the geometrical parameters of the shields of these cables.

The power conductors of cables are subject to failure to a significantly lesser extent than the shielding braids. The comparatively high endurance of the power conductors is explained by the fact that they are more flexible in operation, owing to the advantages of their design and their more rational arrangement in the cross-section of the cable.

Attempts to raise the durability of the shielding braid to the level of the power conductors by finding a better design for it (changing the number and

the diameter of the wires, the angle and direction of the braid) did not lead to positive results.

A cable with a destroyed shield, under electrical-safety conditions, must be withdrawn from service. Consequently, the low quality of the shield limits the service life of the cable and reduces the economic efficiency of its operation. Taking this circumstance into account and considering the irreparability of the shortcomings in the shields of GTSh cables, at our suggestion the latter have been discontinued, and in KShVG cables the shield design has been improved by replacing the braid with a winding of the cores with wires.

To verify the conclusions obtained by analyzing the phenomena of destruction of the metallic elements of the cable structure, and also to assess the advisability of the proposals mentioned, laboratory investigations were carried out, the results of which are presented in Tables 1 and 2.

Table 1

Results of investigations of cables for mechanical endurance under different types of deformation

Table 2

Change in the length of shield wires during tests of cables for twisting

In Table 1 the following notation is used: \(d\) is the diameter of the rollers around which the cable is bent during bending (mm); \(P\) is the magnitude of the tensile load (kg). As can be seen from this table, under all types of mechanical action—bending, twisting, testing on the traversing machine—and under identical experimental conditions

the shielding braid is completely destroyed in GTSh cables and, to a lesser degree, in KShVG cables with an improved shield design. Under the same conditions, no damage is found in the power conductors of any of the experimental cables. Only under a more severe test regime (with a significantly increased number of cycles) on the running machine does the process of destruction of the power conductors become noticeable in KShVG cables with a shielding winding.

A study of the axial deformations of the shield wires that arise when the cable is twisted further reveals the unsatisfactory quality of the shield of the GTSh cable. The magnitude of these deformations, over a base length of 1 m, is about 15% of the base length (Table 2). In Table 2, the minus sign denotes a decrease in the length of the braid in the direction of the cable axis, and the plus sign denotes an increase in this length.

At the same time, it should be noted that the indicated improvement of the braid in KShVG cables does not solve the problem of sharply increasing the electrical safety of the cable: the durability of the shield remains below that of the conductors; only part of the surface of the conductor insulation is shielded; during cable operation, the strands may shift around the perimeter of the conductor and the distance between them may increase, etc.

To ensure safe, reliable, and economical operation of cables, a comprehensive solution is required, based on combining improved external cable protection (protection against short-circuit currents) with the cable’s own high operational qualities.

At present, modern protective equipment is being developed and introduced in open-pit mining operations, guaranteeing disconnection of a damaged cable from the electric-power supply source. In order to improve the quality of the cable itself, it is necessary, above all, to change fundamentally the design of its shielding device. One effective solution to this problem may be the use of electrically conductive rubber as a shielding coating over the insulation of the power conductors.

REFERENCES

1. B. P. Belykh, I. S. Sverdel’, F. M. Usov. Electrical Protection against Faults in Quarry Networks. “Nedra,” Moscow, 1967.

2. A. M. Mavritsyn, V. F. BuktoyaroV. Protection against single-phase ground faults in quarries. “Nedra,” Moscow, 1968.

3. L. V. Gladilin, V. I. Shchutskii, I. E. Vasil’ev. Comparative assessment of certain factors affecting the safety and accident-free operation of electrical networks with voltages above 1000 V. “Gornyi zhurnal,” 1968, No. 12.

4. A. M. Mavritsyn. On design defects of flexible cables used in quarries. “Promyshlennaya energetika,” 1968, No. 1.