[已经解决 ] 1.5PAPER INSULATED CABLES
P: 2016-11-09 15:39:21
1.5PAPER INSULATED CABLES
The firstimportant line insulated with paper was installed by Sebastian de Ferrantiin 1890 between Deptford (on the south side of the River Thames) andthe City of London, for single-phase operation at 10,000 volts . Some ofthese mains were still in use at the original voltage after more than 50 years.The cables consisted of two concentric copper conductors insulated with widestrips of paper applied helically around the conductor and saturated with arosin-based oil. The insulated conductors were forced into an iron pipe filledwith bitumen and installed in 20-foot lengths inside train tunnels under theriver. This system operated successfully for 43 years and may be the source ofthe “40 year life” of power cables .
In theperiod between 1885 and 1887, cables insulated with helically applied nar-rowpaper strips saturated with paraffin and later in a rosin compound and cov-eredwith a lead sheath (very similar in design to those used at the present time)were manufactured in the United States by the Norwich Wire Company. Thesewere the first flexible paper-insulated cables, and all subsequent progress hasbeen made through improvements in the general design.
Paper-insulatedcables were improved through the following years by:
Theintroduction of the shielded design of multiple conductor cables by MartinHochstadter in 1914. This cable is still known as Type H.
LuigiEmanueli’sdemonstration in 1920 that voids due to expansion and contraction could becontrolled by the use of a thin oil impregnating fluid and reservoirs. Thispermitted the voltages to be raised to 69 kV and higher.
The1927 patent by H. W. Fisher and R. W. Atkinson revealed that the dielectricstrength of impregnated paper-insulated cable could be greatly increased bymaintaining the insulating system under pressure. This system was not usedcommercially until the 1932 installation of a 200 psi pressur-ized cable inLondon.
Impregnatedpaper became the most common form of insulation for cables used for bulktransmission and distribution of electrical power, particularly for operatingvoltages of 12.5 kV and above, where low dielectric loss,low dissipation factor, and high ionization level are important factors indetermining the cable life.
Impregnatedpaper insulation consists of multiple layers of paper tapes, each tape from 2.5to 7.5 mils in thickness, wrapped helically around the conductor to beinsulated. The entire wall of paper tapes is then heated, vacuum dried, andimpregnated with an insulat-ing fluid. The quality of the impregnated paperinsulation depends not only on the proper-ties and characteristics of the paperand impregnating fluid, but also on the mechanical application of the papertapes over the conductor, the thoroughness of the vacuum drying, and thecontrol of the saturating and cooling cycles during the manufacturing.
Originally,most of the paper used was made from Manila-rope fiber. This was erratic in itsphysical properties and not always susceptible to adequate oil penetra-tion.Increased knowledge of the chemical treatment of the wood (in order to obtainpure cellulose by the adjustment of the fiber content and removal oflignin), the con-trol of tear resistance, and the availability of long fiberstock resulted in the almost universal use of wood pulp paper in cables after1900.
Theimpregnating compound was changed from a rosin-based compound to a pure mineraloilcirca 1925, or oil blended to obtain higher viscosity, untilpolybutene replaced oil circa 1983.
Paper-insulated,lead-covered cables were the predominant power cables of all the large,metropolitan transmission and distribution systems in the United States, andthe rest of the world, throughout the twentieth century. Their reliability wasexcellent. It was, however, necessary to have a high degree of skill for propersplic-ing and terminating. A shift toward extruded dielectric cables beganabout 1975 in those metropolitan areas, but the majority of the distributioncables of the large cities remained paper-insulated, lead-covered cables as thecentury ended.
Considerableresearch has been carried out by the utilities, technical organizations, andmanufacturers of cables to obtain improved paper and laminatedpolypropylene-paper-polypropylene (PPP, now used in transmission cables) tapesand insulating fluids that are able to withstand high, continuous operatingtemperatures.
Impregnatedpaper insulation has excellent electrical properties, such as high dielectricstrength, low dissipation factor, and dielectric loss. Because of these prop-erties,the thickness of impregnated paper insulation was considerably less than forrubber or varnished cambric insulations for the same working voltages.Polyethylene and cross-linked polyethylene cables in the distribution classesare frequently made with the same wall thickness as today’s impregnated papercables.