High voltage cables

The changes described above also allowed much easier access to the high voltage cable for routine (6-month) owner directed, service operations, and provided better upper and lower x-ray cabinet and control cabinet ventilation. With the exception of the x-ray tubes, all the individual manufactured components, on all four systems are identical. There are very subtle differences in the warm-up/start-up sequence on the x-ray controllers on the newer systems due to model/year and x-ray tube differences. The last three systems were supplied with environmental type key-boards for the image processors and base-mounted , rather than conduit-mounted exterior warning indicators. The first system was subsequently upgraded to include the better keyboard and the external warning appliances/features. 

Fig. 14-2 High-voltage cable grounded via dc decoupling devices (1) high-voltage installation, (2) insulating flange, (3) cable branching, (4) cable end sealing, (5) steel conduit, (6) dc decoupling devices.

Where pipelines and high-voltage cables cross, a distance of at least 0.2 m must be observed to prevent contact between cable and pipeline (this can be achieved by interposing insulating shells or plates). Such intermediate materials can be PVC or PE. Their disposition and shape must be determined by mutual agreement [2,6]. 

Where pipelines and high-voltage cables run parallel to each other, spacing of at least 0.4 m should be observed to ensure adequate working space. In bottlenecks, the spacing should not be less than 0.2 m [2]. 

Electronic conductivity Flexible conductor of electricity heating elements (resistance heating), shielding of electromagnetic radiation field flattening (high-voltage cables), materials with antistatic capability... 

A luminous discharge resulting from air breakdown in the vicinity of high voltage cables. Corona produces ozone (from the oxygen in the air) and may result in rapid deterioration of the cable insulation and other unsaturated mbber in the vicinity. 

High tenacity fibers High test molasses High touch fibers High voltage cables... 

Two additional xanthene analogues are termed fluorescent brighteneis. Fluorescent Brightener 74 (Cl 45550) and Fluorescent Brightener 155 (Cl 45555) are used in the formulation of solid dielectric compositions for application in high voltage cables to prevent conductive treeing. 

The so-called loss tangent, tan 8, is a very useful dimensionless parameter and is a measure of the ratio of the electric energy loss to energy stored in a periodic field. The product x tan 8 is directly proportional to the dielectric loss of energy, e.g. in a high-voltage cable. 

Multi-layer insulation constructions are used. With the high-voltage cable, three separate layers may cover the center conductor. The innermost layer will function as an electric screen, the intermediate layer is usually a PE insulation material, and the outer layer serves as an electric screen. The processing technique can be used for primary and secondary coating where the covering is on metallic conductors or coating previously insulated wires. 

Examples for cable entries suitable for high voltage cables are given in Figs 6.43 and 6.44 (for cast resin heads only). 

Figure 6.43 Cable entry for high voltage cables with sealing heads (cast resin). Type of protection EEx e I, EEx e II Certificates BVS 89.B.1098, BVS 89.C.2022 For cables with diameter 50... 80 mm.

Figure 6.49 Connecting terminals (single units) in a high voltage cable joint box (the following data are valid for the cable joint box in its entirety).

Tables 12.3 and 12.4 give a survey of the main technical data of low and high voltage cables for mining. The design of high voltage cables is shown in Figs 12.4(a) and (b).

Table 12.4 High voltage cables for coal mines - main technical data...

Figure 12.4 Cross-sectional drawings of high voltage cables for coal mining (according to [35]).

The degradation processes known as "water treeing" and "electrical treeing" are major problems causing premature failure of underground high voltage cables. 

Electrical trees are essentially breakdown channels whose size, typically 50 to 200 microns, together with the large variations in impurity concentrations in the surrounding polyethylene, makes the identification of the impurities associated with both kinds of trees very difficult by traditional techniques. The use of micro-PIXE for the location and analysis of trace elements in electrical and water trees found in the polyethylene insulation of high voltage cables will be described. 

To illustrate the use of PIXE and micro-PIXE in the study of breakdown phenomena in polyethylene high voltage cable insulation and other related topics we will describe a few typical measurements, first the study by standard PIXE of impurities in the organic semiconductor H2PC and in the carbon black semicon used in high voltage cables. Examples of the use of the microbeam to study some electrical and water trees as well as the diffusion of impurities from the semicon into polyethylene under typical electric field and humidity conditions will be given. 

In studies where a knowledge of the diffusion of metallic ions in polymers is important, one often wishes to measure a profile of the concentration as a function of depth. Neutron activation cannot be used to measure these profiles directly, but if the sample can be cut into thin slices with a microtome, these can be analysed individually to construct the profile. In our laboratory this technique is used extensively to study the migration of ions into the polyethylene insulation of high-voltage cables (10). These impurities contribute to the degradation with use of the electrical properties of the cable. 

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