Breaker Technology Research Articles

CVD diamond coated cemented carbide cutting tools

The development of CVD diamond coated cutting tools has faced several challenges since the commencement of the low pressure synthesis of diamond coatings from a vapor mixture of hydrogen and a carbon containing gas based on the original work by Derjaguin and Fedoseev. The reason for the fairly slow progress can be attributed to a number of problems. For example, obtaining a satisfactory and consistent adhesion between the diamond coating and the substrate has proven to be difficult. Further, the problems faced in expanding any of the existing CVD diamond laboratory processes into a large scale production technique have also turned out to be greater than exprected. The recent development of a scaled-up process with a high current density plasma beam will be described. This new CVD diamond process, the high current DC-arc process (HCDCA), which is based on a high current DC discharge arc with a long discharge column, has been successful and high quality diamond coated cemented carbide inserts can be manufactured with good reproducibility and large productivity. Diamond coated tools are compared with both PCD tools and uncoated carbide tools. The main advantage compared to PCD tools are the possibilities of using multiple edges per tool insert and advanced chip breaker technology. Compared to an uncoated carbide tool, the CVD diamond coated insert shows, in addition to a much larger abrasive wear resistance, less built-up edge and a lower cutting forces resulting in a much improved surface finish to the workpiece material. Several results from field tests at end user machine shops in the turning of Al-Si alloys, particularly in wheel turning, are presented. In the tests diamond coated carbide inserts perform typically 3–5 times better than uncoated tools but peak values of 10–20 times improvement can be obtained. An example of machining of Cu is reported where the diamond coated tool is found to outperform a PCD tool.

Statistical processing of chopped currents and overvoltages in two types of sulphur hexafluoride breaker technology

Tests on breaking three-phase low inductive currents were carried out with two medium-voltage SF6 equipments using different breaking techniques; the puffer method and the rotating arc. The statistical interpretation of the chopped currents and overvoltages produced by the two types of equipment showed that the two technologies performed satisfactorily, that is without excessive stresses for the system in service (in particular no successive restrikes on breaking) and with acceptable overvoltages caused by chopping. The typical values of the chopped currents measured were less than 1 A for rotating arc, and 3 to 8 A for puffer type circuit breaker. The overvoltages generated by the rotating arc circuit breaker (typically 2 p.u.) remain perceptibly lower than those caused by the other circuit breaker (typically 3 p.u.), even when using the former equipment at a voltage higher than its rated voltage. The results obtained are well in line with the theory and various experimental results published so far. Moreover, the investigation undertaken revealed that the graphs of cumulative probabilities in Gaussian scale can be an efficient tool for presentation and interpretation of the statistical results obtained.

Phase-Phase and Phase-Neutral Switching Surges on 500-kV Open-Ended Lines

System insulation requirements on EHV systems, especially above 345 kV, are predominantly a function of switching surge overvoltages. Modem lightning arrester and circuit breaker technology and increased knowledge and understanding of insulation systems relative to switching surges have allowed substantial economics at higher transmission levels.