Design of Cable Termination for AC Breakdown Voltage Tests

In engineering, an ideal solution or project must be established due to economic, technical, practical, and environmental limitations. In the design and construction of power transmission or distribution lines, two types of cable can be used, namely overhead or underground cables [1]. High-voltage underground cables have been widely used as electrical conductors in different applications such as medium-voltage industrial facilities, underground and submarine transmission connections, renewable energy plants [1], the feeding of residential buildings and urban centers, as well as in facilities where environmental and visual aspects require the use of underground cables. However, the installation of underground cables represents a significant financial expenditure when compared with the application of overhead cables [1,2].

 In addition, the complex manufacturing process of underground cables and the diversity of products and manufacturers can lead to the commercialization of low-performance cables. The use of good performance cables is essential for underground applications, since the cables are subjected to several stresses during their lifetime, such as electrical (due to operation voltages, overvoltage surges, and others), thermal (since the cables are subjected to abnormal temperature rises, thermal expansion, and contraction), mechanical (such as external damages, lateral impact, and pressure abnormalities), and environmental (due to humidity, oxidation, solar radiation, and other phenomena)

Therefore, in order to ensure operation under the aforementioned stresses and improve the power supply reliability and continuity, cables must be exposed to routine and type electrical tests to guarantee, essentially, the dielectric performance of the insulation material and reduce, consequently, the financial losses for power utilities and industries. One of the most widely used materials is cross-linked polyethylene (XLPE)

Although most failures in a power cable occur at its junctions and terminations, the evaluation of the cable insulation material is extremely necessary. One of the tests required for this evaluation is the cable breakdown voltage determination. International standards IEC 60229 and IEC 60520-2 [6,7] establish the test requirements for high-voltage cables. In Brazil, the standards NBR 10299 and NBR 16132 [8,9] provide the specifications in regard to the tests for the statistical distribution of puncture electric field strength in cables for systems with a voltage above 15 kV. NBR 10299 aims to set the minimum failure rate based on the installed cable length. A generally accepted value is 6.7 × 10−4 failure/(year × km)

The prescribed tests should be performed using a sample of at least 3 m effective length, that is, without considering the terminations on both sides. The external shielding is grounded and a rising AC voltage is applied over the cable until the internal breakdown is reached. The experimental setup should ensure the breakdown occurrence on the effective length cable. Considering that the tests can subject the cable to overvoltages with values of 5 to 10 times greater than the normal operation voltage, the main problem faced during the tests is the electric field distortion at the cable ends, which causes external rupture and prevents the evaluation of the inner insulating material. To carry out the installation in field or breakdown voltage tests in insulated cables, it is necessary to remove a part of the cable insulation. While the electric field inside the effective length has a predictable distribution, with a radial direction and logarithmical behavior [1,2,10], there is a great field intensification at the cable ends. The electric field in the vicinity of the shield end is illustrated in Figure 1. Thus, such a predictable distribution should be guaranteed to allow successful tests.

The cable end consists of a conductor, semiconductor layers, insulating layer, and conductive tape for the shielding, in addition to air. The diversity of materials with different electrical characteristics, dielectric strength, and relative permittivity provides highly non-uniform electric fields with both axial and tangential field components. The tangential electric field is one of the main reasons for failures in terminals [12,13]. The field enhancement at the cable end produces surface and external discharges in the air, which can be prevented by using properly designed terminations [1,9,10,14]. In this sense, arrangements with different characteristics were studied by [12–19].

Most of the studies that have dealt with the electric field distribution in high-voltage cable terminations apply commercial software based on the finite element method (FEM) for the purpose of analysis and/or design of terminations, joints, or stress relief cones. In [12,15,16], simulations were used to assist in the design of stress cones based on high-temperature superconductors (HTS). The authors of [16] proposed the use of an epoxy/ZnO conductive layer to improve the electric field distribution. Other studies compared different materials and field grading options for cables. In [13], for example, different types of field grading options for 36 kV paper-insulated lead cables (PILC) and cross-linked polyethylene (XLPE) cables were compared. In [17], the impact of defects on cable joints was analyzed,and [18] studied the electric field in a cable termination undergoing transient stresses. During the project stage of a termination, another possible objective is to estimate areas more susceptible to defects and thus improve prototypes. In this regard, [19] calculated the electric field in a sleeve for a 110 kV cable using FEM, in order to estimate zones more susceptible to dielectric breakdown and thus improve a sleeve prototype.

Some of the aforementioned studies presented termination projects based on the stress cone concept, and analyzed electric field distribution or the influence of different materials. However, there is a lack of studies related to termination performance during overvoltage tests or breakdown voltage tests. In addition, some proposed prototypes require expensive materials.Therefore, a methodology for the conception, drawing, and electrostatic simulation of a feasible termination is reported in this paper. The termination must be able to ensure the completion of overvoltage tests on cables. The proposed procedure can also be used for the design of optimized cable terminations. Such terminations are usually responsible, for example, for the linking between different transmission lines, functioning as connectors. For the analyses of the electrical stresses at the termination, computational simulations were performed using a commercial software based on the finite element method and a single-phase 35 kV cable model, which was used as a sample. Conventional materials were considered in the project, which represents a potential cost reduction.