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An exclusive insulation coordination study has been conducted for the 500 kV DC cables that will be installed in a tunnel under the Saint Lawrence river. These cables will be part of the Radisson-Nicolet section of the Quebec-New England HVDC line. The authors provide an overview of the studies and the phenomena that will cause decisive overvoltage stresses on the cable insulation. The basic assumptions and the simulations of these phenomena with the Electromagnetic Transients Program (EMTP) are also discussed. Finally, the results of the overvoltage studies and the specified insulation levels for the DC cables are presented. >

Thirty-four porous-coated anatomical (PCA) total knee arthroplasties in twenty-eight patients with rheumatoid arthritis were followed for an average of seventeen months. In these thirty-four knees, twenty-three tibial and fifteen femoral components were cemented. The rest were uncemented. At follow-up, more than half of the knees (nineteen of thirty-four) showed radiographic evidence of displacement of some of the beads from the porous coating. In fourteen knees the beads loosened more than three months postoperatively, and all of these knees had a radiolucent zone. Six knees had beads within the joint. Eight of nine non-cemented tibial components (excluding one tibial component) and five of the nineteen femoral components that were not cemented showed late loosening of beads (after three months). This study indicates that the metallurgical technique for porous coating with heat-sintered cobalt-chromium beads does not produce a coating that is sufficiently strong to prevent loosening in the case of the porous-coated anatomical prosthesis. The long-term clinical relevance of our findings is unclear. The mechanical strength of the porous coating on the prosthetic surface should be improved and the prosthesis should be tested in limited clinical studies before cementless fixation can be recommended.

Thirty-four porous-coated anatomical (PCA) total knee arthroplasties in twenty-eight patients with rheumatoid arthritis were followed for an average of seventeen months. In these thirty-four knees, twenty-three tibial and fifteen femoral components were cemented. The rest were uncemented. At follow-up, more than half of the knees (nineteen of thirty-four) showed radiographic evidence of displacement of some of the beads from the porous coating. In fourteen knees the beads loosened more than three months postoperatively, and all of these knees had a radiolucent zone. Six knees had beads within the joint. Eight of nine non-cemented tibial components (excluding one tibial component) and five of the nineteen femoral components that were not cemented showed late loosening of beads (after three months). This study indicates that the metallurgical technique for porous coating with heat-sintered cobalt-chromium beads does not produce a coating that is sufficiently strong to prevent loosening in the case of the porous-coated anatomical prosthesis. The long-term clinical relevance of our findings is unclear. The mechanical strength of the porous coating on the prosthetic surface should be improved and the prosthesis should be tested in limited clinical studies before cementless fixation can be recommended.

Twenty patients were examined by standard radiography and roentgen stereophotogrammetric analysis (RSA) during a two-year period after total hip arthroplasty. Eleven of the acetabular components migrated cranially and three femoral components migrated distally. This migration was most rapid during the first four months after operation. Our findings support the possibility that mechanical loosening is initiated by thermal injury during polymerisation of the cement; the less frequent migration of the metallic femoral component compared with the polyethylene acetabular component may be because the metal acts as a heat sink. Standard radiographs were inadequate for assessment of early mechanical loosening, whereas RSA could reveal migration within four months of the arthroplasty.

The torsional data of hydro generators are examined and it is found that the inertia ratio between the generator and the turbine lies in the range of 10 to 40 and that the torsional frequency lies between 10 Hz and 24 Hz. It is shown that the high inherent generator to turbine inertia ratio of a hydro unit increases the modal damping at the torsional frequency significantly. This is substantiated by digital simulations for a generator connected to a series compensated AC network and by simulations on an HVDC-simulator for a generator feeding an HVDC-link. It is shown that with reasonable values of the turbine damping no torsional interactions problem would occur for the cases studied.