Voids And Microvoids In Insulation

- Jun 11, 2019-

There is an important parameter in the mechanism of insulating material, that is, the voids and microvoids in the insulation. We should also pay attention to this parameter when purchasing heat shrinkable pipes. See the following analysis for the specific impact of these parameters.

Voids and micro-voids in insulation are holes filled with liquid or gas. The diameter of general void is over 0.05 mm. In chemical crosslinked polyethylene insulation, the main reasons for the voids are as follows:

(1) Dispersion of cross-linking agent or antioxidant is not uniform;

(2) Excessive humidity in insulators, conductors or semiconductor shielding materials.

(3) There are too many low molecular weight fractions in the polymer.

The allowable gap of cable insulation under low field strength can be determined by the starting or extinguishing voltage of partial discharge. Through the long-term aging test of XIPE cable samples, it is considered that the voids with a diameter of 0.08mm or less will not cause serious partial discharge which endangers the service life of XLPE cable even under high field strength. Generally, at 2.5 E0, 0.5 mm diameter gap will lead to 10 PC discharge, 0.125 mm diameter gap will lead to 0.1 PC discharge. When designing ultra-high voltage XLPE cables, no voids are allowed in the insulation where the average field strength of mining fields exceeds 5-6 kV/mm.

Microvoids are very fine groups of voids in the insulating layer or between the insulating layer and the shielding layer. When XLPE insulating wire core is crosslinked by steam, the insulator is under high pressure and high temperature, and the steam will infiltrate into the insulating layer. When the cable insulation core is cooled, the steam condenses and forms a group of micro-holes. The micropore is filled with liquid or gas, but it will not directly lead to partial discharge even at high field strength. In the slices of the insulating layer, the cloud-like rings are often visible to the naked eye, which is called micropore groups. The diameter of the micropore is about 1-2 um. Although microholes do not directly lead to partial discharge, they are the root of water tree phenomenon and affect cable life. In the production of high voltage XLPE cables, dry cross-linking method is required to prevent a large number of micro-holes.

XLPE cable (XLPE cable) n_6~9, the latest report: XLPE cable, n (> 10), XLPE cable with air gap (air gap discharge aging), n= 8.0~9.0; XLPE cable with air gap and impurities below 0.01 mm, n (> 10). Table 2-21 lists the requirements and production levels of air gap size in insulation for manufacturers in several countries. Because of the difference of n value, the slope of life curve is different. The larger the n value is, the longer the life is. According to the research, the size of air gap is related to the value, and if the air gap is large, the value decreases, and the breakdown field strength decreases, so that the insulation life decreases.

Because the breakdown strength of insulating layer of cable decreases with the increase of core radius, the average electric field strength is used to calculate the thickness of insulating layer of plastic cable at present.

We also know that Em < E'(power frequency breakdown strength) can make a certain margin of 120%~200% of the test voltage in order to keep the cable from breaking down under the test voltage.