Failure analysis of high voltage XLPE cable buffer layer

introduction

As the overhead line undergrounding project is gradually advanced, cross-linked polyethylene (XLPE) cables are more widely used in urban main grid transmission lines, and the stable and reliable operation of cable lines is more closely related to the safety of the power grid. Taking Beijing as an example, as of 2021, the main grid cable lines under the jurisdiction of State Grid Beijing Electric Power Company are nearly 3,000 km.

The buffer layer is an important component of high-voltage XLPE cables, and its quality will affect the overall electrical, thermal, water-blocking and mechanical properties of the cable. However, in recent years, a type of fault located between the metal sheath and the insulation shield has appeared in many 110k6 and above corrugated aluminum sheathed high-voltage cables. After analyzing the nature of the cable body faults, it was found that a large number of body faults were caused by the ablation of the semi-conductive water layer in the cable structure, and the number accounted for about 30V of the body faults. The faulty cable was disassembled, and white powder and ablation (or corrosion) marks were found on the inner side of the cable metal sheath, the buffer layer, and the main insulation shield of the cable. The existence of this phenomenon will cause damage to the main insulation of the cable and lead to cable breakdown. Domestic and foreign scholars have conducted a lot of research on the degradation and failure causes of the buffer layer of high-voltage XLPE cables. For the operating cable lines, the ablation phenomenon on the buffer layer will affect the safe operation of the high-voltage power cable, and must attract the attention of the operating personnel.

1 Buffer layer structure analysis

The buffer layer is located between the inner sheath and the insulating shielding layer, as shown in Figure 1 (9). The inner sheath of domestic cables is mostly a corrugated aluminum sheath, which has the functions of radial water blocking, mechanical protection, and good conductivity.

The buffer layer usually includes water-blocking tape, gold cloth and air gap. The water-blocking tape is tightly wrapped on the surface of the insulating shielding layer, and the gold cloth is wrapped on the surface of the water-blocking tape. The water-blocking tape is in close contact with the trough of the corrugated aluminum sheath, and there is an air gap between the water-blocking tape and the peak of the aluminum sheath, as shown in Figure 1(b). The water-blocking tape is usually composed of fluffy cotton, water-blocking powder and non-woven fabric, as shown in Figure 1(c). It is used to buffer the interaction between the insulating shielding layer and the inner sheath, and reduce the extrusion stress between the insulating shielding layer and the inner sheath caused by the thermal expansion inside the cable. The water-blocking powder in the water-blocking tape absorbs water and expands to block water radially. The main component of the water-blocking powder is sodium polyacrylate, which is weakly alkaline. In addition, it also contains monomer CM (chlorinated polyethylene), cross-linking agent, initiator and deionized water.

2 Experiments

2.1 Fault conditions

Due to a defect in the buffer layer, a 110k6 high-voltage cross-linked polyethylene cable line A phase body fault occurred. The cable was manufactured by a domestic manufacturer, model ZR-YJLw02-64/110kV-1×800mm2. By the time the fault occurred, the line had been in operation for 18 years.

The outer sheath of the cable fell off at the breakdown point, and the outer sheath at the adjacent position was explosively cracked. The aluminum sheath, outer semi-conductive layer, and main insulation layer were obviously ablated, and the breakdown channel of the fault point was clear, with an inverted trapezoidal structure with a large outside and a small inside (Figure 2).

2.2 Disassembly inspection

2.2.1 Disintegration of fault point

The fault point was disassembled layer by layer from the outside to the inside on site. It can be seen that in addition to the metal sheath being completely burned through at the fault point, there are also common traces of discharge ablation on the inner side of the metal sheath at the adjacent position. The ablation point is generally located at the trough of the corrugated aluminum sheath (Figure 3), that is, the position where the aluminum sheath contacts the semi-conductive water hose. White powdery substances are widely present on the semi-conductive water hose at the corresponding position (Figure 4). The surface ablation phenomenon of the inner outer semi-conductive layer is common (Figure 5). Similar to the fault point penetration channel, the damage points on the outer semi-conductive layer generally present an inverted trapezoidal structure with a large outer area and a small inner area.

Figure 3 Ablation of the inner side of the aluminum sheath

Figure 4 White powdery substance exists on the surface of the semi-conductive water hose

Figure 5: Ablation discharge marks are seen on the outer semi-conductive layer

2.2.2 Disintegration at other locations on the same cable section

In the cable section near the joint of phase A of the line, four other locations were evenly selected to disconnect and disassemble the cable except the fault point. The disassembly results showed that other locations of the cable had similar conditions as those near the fault point, with white powder appearing on the semi-conductive water hose (Figure 6), and the semi-conductive surface of the cable was mostly ablated and damaged (Figure 7).

2.3X-ray inspection

2.3.1 Detection principle

The basic principle of x-ray detection is that x-rays interact with matter when penetrating different objects, causing industrial intensity changes due to absorption and scattering. After the photosensitive material (film, IP board, DR board) receives the signal of the intensity change, it forms a common image through signal processing.

2.3.2 Detection situation

Before and after the dissection, the selected cable segments were subjected to X-ray inspection, and typical results are shown in Figures 8, 9, and 10.

Figure 8 Actual cable section corresponding to the test results

Figure 9 Suspected defective cable segment

Figure 10 Normal cable segment

From the detection effect, the x-ray is effective for larger ablation points, but the imaging effect for medium-sized and small ablation points is not obvious. The aluminum sheath has a greater impact on the x-ray detection results. After stripping off the aluminum sheath, the defect points are clearly imaged, but after adding the aluminum sheath, the suspected ablation points detected by the x-ray cannot completely correspond to the ablation points of the buffer layer inside the cable, and the defect points are not easy to identify.

X-ray detection shows that ablation of cut cable segments can be detected, but the detection effect is affected by the aluminum sheath.

2.4 Strip material testing

Combined with the abnormal situation of the water-blocking tape found during the disassembly of the faulty phase cable, the three-phase cable of the line was tested for insulation thickness, eccentricity, mechanical properties, thermal extension, and resistivity of the semi-conductive water-blocking tape. The test results are shown in Table 1. The resistivity of the three-phase semi-conductive water-blocking tape, including the non-faulty phase, is higher than the standard value. According to the requirements of "Water-blocking Tape for Cables and Optical Cables" (1B/T10259-2014), the surface resistance should not be greater than 1500Ω, and the volume resistivity should not be greater than 1×1050·cm.

3. Failure Analysis

3.1 Summary of disassembly, testing and inspection

(1) The degree of ablation has no obvious positional correlation. After the fault occurred, a large number of cables were cut off for disassembly analysis. In different sections of the cable, it was found that the outer semi-conductive layer, the semi-conductive water layer, and the inner wall of the corrugated aluminum sheath had different degrees of burning. There was no obvious increase or decrease in the burning phenomenon, and no obvious correlation was found between the burning phenomenon and the cable position.

(2) The degree of burn damage is related to the degree of contact of the corrugated aluminum sheath. The burn damage is concentrated at the trough of the corrugated aluminum sheath: the phenomenon is non-circular uniform burn damage. The tighter the contact between the corrugated aluminum sheath and the semi-conductive water hose, the more serious the burn damage. There are few obvious burn points on the circumferential surface that is not in close contact with the metal sheath. According to on-site observations, the cable does not have eccentricity or obvious deformation caused by transportation and laying. The main reason for the difference in contact degree at different angles may be the cable's own gravity, which makes the contact between the aluminum sheath on the upper and lower sides of the cable and the semi-conductive water hose tighter than the left and right sides. The fault point is also a typical close contact point, and the vertical downward direction of the breakdown channel is consistent with the direction of most defective points on the entire line.

(3) According to the fault point and the strip and outer semi-conductive ablation found during disassembly, it can be judged that the ablation process occurs from the outside to the inside.

3.2 Cause Analysis

Through the disassembly analysis of the cable, it was found that there were a large number of unevenly distributed and unevenly sized discharge ablation marks on the outer semi-conductive layer of the cable. After testing, it was confirmed that the volume resistivity and surface resistance of the cable buffer layer did not meet the standard requirements. In addition, the cable metal sheath was in uneven contact with the semi-conductive buffer layer, resulting in a higher density of induced current in the close contact area between the metal sheath and the buffer layer (trough position). Local overheating caused the semi-conductive buffer layer of the cable to burn, and gradually burned inward the outer semi-conductive layer of the cable and the cable insulation, causing the main insulation of the cable to break down, leading to cable failure.