Optical waveguides – Optical transmission cable – With electrical conductor in the same cable
Patent
1992-10-23
1994-05-31
Ullah, Akm E.
Optical waveguides
Optical transmission cable
With electrical conductor in the same cable
174 99R, 174 34, G02B 600, G02B 636, G02B 644
Patent
active
053176650
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a cladding structure, in particular for optical cables, for use in high-voltage environments, comprising at least one elongated plastic outer sheath.
In practice there is an increasing trend to suspend cables for optical signal transmission on high-voltage power pylons to transmit, for example, measurement data and control commands for the purpose of electricity production and distribution. These cables may, of course, also be used for signal transmission over a long distance for the purpose of public communication systems. Railway companies may, for example, use the overhead wire supporting pylons for attaching optical telecommunication cables.
Although such solutions offer economic advantages because use can be made of an already existing infrastructure (pylons) there are, however, a number of technical disadvantages which are the consequence of the high-voltage environment to which the cables are exposed. The outer sheath of the cables concerned may be damaged, in particular, by corona discharges and by creepage path formation, termed "tracking" in the English-language specialist literature.
In practice, the cables are attached to a power pylon in an electrically conducting manner with helical suspension fixtures which are made, for example, of aluminum or steel plated with aluminum and which engages the plastic outer sheath. A cable suspended in this way is capacitively coupled to the high-voltage lines and earth and consequently forms a capacitive voltage divider. In addition, the cable is connected to earth in a conducting manner via the suspension fixtures, which are connected to the earthed pylons, and the plastic outer sheath, whose resistance is not infinitely high. As a consequence of this, a potential distribution which decreases to zero in the vicinity of the suspension points is produced over the cable.
In particular, corona discharges occur in the vicinity of the end, engaging the cable, of the helical suspension fixtures where an electrical field concentration arises as a consequence of sharp edges or points. Apart from the ends of the suspension fixtures, irregularities projecting from the outer sheath may also result in corona discharges.
Tracking occurs in the case of a wet and contaminated cable, under which circumstances an electrically conducting layer is formed on the outer sheath. As a consequence of the potential distribution described above, a current flows in the axial direction in the conducting layer formed by the moisture, from the centre of the cable to the earthed suspension points in the pylons. Because the contamination of the cable is never homogeneously distributed in practical situations, local regions having a higher current density will arise, partly as a consequence of irregularities in the cladding surface, where the moisture will evaporate and the conducting layer formed is interrupted. As a consequence of the relatively high conductivity of the wet or contaminated cable, there will be a relatively large voltage across these (narrow) interruptions, as a result of which discharge phenomena may arise. Under certain weather conditions, this process may occur virtually continuously and cause appreciable damage to the cable cladding.
In practice it is consequently desirable to protect optical cables for use in high-voltage environments as much as possible against the undesirable effects, described above, of corona discharge and tracking.
European Patent Application 0,214,480 discloses a metal-free, self-supporting optical cable for use in high-voltage environments, in which cable the core is rendered weakly electrically conducting. A conducting layer formed on the outer surface of the plastic cable cladding or jacket as a consequence of moisture or contamination is capacitively coupled to the weakly conducting core, under which circumstances, inter alia, the plastic outer sheath acts as a dielectric. As a result of this capacitive coupling, the voltage difference across an interruption of the conducting layer caused by moisture and/or contaminat
REFERENCES:
patent: 4772090 (1988-09-01), Atkins et al.
patent: 4776665 (1988-10-01), Oestreich
patent: 5181266 (1993-01-01), Brennecke et al.
patent: 5214733 (1993-05-01), Sadler
IEEE Standard Dictionary of Electrical and Electronics Terms, Fourth Addition, Jul. 8, 1988.
NKF Kabel B.V.
Ullah Akm E.
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