Method and apparatus for preventing dry band arcing in a combined overhead electrical power and optical transmission system

An overhead electrical power and optical transmission system, comprising overhead electrical phase conductors (2) extending between and supported by towers (10), and at least one optical cable (1) that extends between and is supported by, the towers. Each optical cable has a resistive element (12) that is removably supported thereby and which extends from a tower (10) where it is earthed part of the way along the span of the optical cable, and the resistive element has the necessary length and conductivity such that if a dry band (6) occurs on the cable at the end of the element, the potential difference (VG') across the band is insufficient to form an arc, such that any induced current is insufficient to sustain any arc that may occur across the dry band.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to optical cables that are supported along the route 
of electrical power transmission system by means of towers, masts or other 
upstanding supports that are also employed to support electrical power 
cables. 
2. Discussion of the Prior Art 
In systems of this kind it is the general practice to earth the optical 
cable or cables at the towers, masts or other supports hereinafter simply 
referred to as towers. When the electrical power lines are under load, 
electrical currents may be capacitively induced on the optical cable due 
to the distributed capacitance between the cable and the power lines. The 
voltage induced on the optical cable will reach a maximum at mid-span 
between the towers, while the current flowing along the cable will be 
greatest in the region of the towers. Under dry conditions the induced 
currents will be relatively small due to the relatively high longitudinal 
resistance of the cable, e.g. in the region of 10.sup.12 ohm m.sup.-1 but 
under wet conditions when the surface resistance of the cable is much 
lower, e.g. in the region of 10 Mohm m.sup.-1, much higher currents will 
be induced. Joule heating of the cable surface by the induced currents can 
cause a short length of the cable surface to become dry, usually in the 
region of a tower where the current is highest. When this happens the 
major part of the induced voltage on the cable is dropped across the short 
dry band due to its high longitudinal resistance, and so called "dry-band 
arcing" may occur which can cause severe damage to the cable. 
It is possible to overcome the problem of dry-band-arcing in an optical 
cable by providing the cable with a longitudinally extending electrically 
conductive path. However, an optical cable having such an electrically 
conductive path has the disadvantage that there are considerable safety 
issues to be taken into account if it is to be installed between towers of 
an overhead electric power transmissions line that is on load in view of 
the danger of it touching one of the transmission lines; moreover, it is 
not always possible or desirable to interrupt the electrical power 
transmitted by the overhead electric power transmission line for a time 
sufficient to enable such an optical cable to be installed. 
It has been proposed, for example in European Patent Application No: 
214,480, to employ a cable having a resistive element and a linear 
resistance of 10.sup.7 to 10.sup.12 ohm m.sup.-1. However, such systems 
have the disadvantages (among others) that the electrical properties of 
the resistive element may vary with time due to aging, pollution, cable 
strain and the like and as a result lose its efficacy. 
It has also been proposed, for example in European Patent Application No. 
403,285 to include a resistive flitting on the optical cable adjacent to 
the tower in order to reduce arcing on the cable and joule heating. 
However, such fittings do not eliminate the occurrence of stable dry-band 
arcing. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a combined overhead 
electrical power and optical transmission system which comprises overhead 
electrical phase conductors extending between and supported by, towers, 
and at least one optical cable that extends between, and is supported by, 
the towers, the or each optical cable having a resistive element that is 
removably supported thereby and which extends from a tower where it is 
earthed part of the way along the span of the optical cable, the resistive 
element having a length and conductivity such that if a dry-band occurs on 
the cable at the end of the element, the potential difference across the 
band is insufficient to form an arc, and/or such that the induced current 
is insufficient to sustain any arc that may occur across the dry band. 
The system according to the invention has the advantage that it is possible 
for the phenomenon of stable dry-band arcing to be substantially 
eliminated. This is achieved partly by virtue of the fact that the element 
moves the point at which any dry band arc could be formed away from the 
tower to a position in which capacitively induced currents are relatively 
low. In addition, the voltage at the end of the resistive element can be 
raised (by capacitive coupling with the phase conductors and due to the 
induced currents flowing through it) sufficiently to prevent an are being 
formed across any dry band that may occur, and the resistive element will 
act as a resistor between the are and ground, and so limit the current of 
any are to a value that cannot sustain the are. In addition, the resistive 
element can be removed when necessary, for example when its electrical 
properties have altered due to aging and/or pollutants, and can be 
replaced. Preferably, the resistive element can be installed on the 
optical cable by locating one end thereof on the cable and sliding the 
element along the cable from the tower. Such an element and method of 
installation enables the operations on the element during its removal and 
replacement to be performed from the tower in relative safety even when 
the electric power lines are on load. Thus, for example, the element may 
be sufficiently flexible to be rolled up when being transported up the 
tower, but be sufficiently stiff that once its end, and any intermediate 
position thereon is slidably attached to the cable, it can simply be 
pushed along the cable to its full extent. As an example a rod of 2 to 10 
mm diameter, preferably 4 to 6 mm diameter and especially 5 mm diameter 
(e.g. glass reinforced plastics) with a flexural modulus of 20 to 50 GPa, 
preferably 30 to 45 GPa and especially A40 GPa would be appropriate. 
Preferably the resistive element has a sufficient length that, when a dry 
band is formed on the cable at the end of the element, the voltage at the 
end of the element is raised (by virtue of capacitive coupling of the 
element to the phase conductors and by virtue of the current flowing 
through the element) sufficiently to prevent formation of a stable arc 
across the dry band. Thus, by appropriate choice of conductivity of the 
resistive element and its length, it is possible to reduce the voltage 
occurring across any dry band to a value that is insufficient to form a 
stable arc, and to reduce the current that can flow through any arc to a 
value that is insufficient to sustain it. In practice, the element will 
have a length of at least 20 m, preferably at least 40 m, but not more 
than 100 m, and especially not more than 60 m. 
When the resistive elements have a length of this order of magnitude, they 
will each extend along the cable over a significant fraction of the span 
of the cable, for example from 10 to 30% of the span, but will not extend 
over the mid-span part of the cable. 
Because optical cables tend to have a lower modulus and weight than that of 
the phase conductor they tend to be displaced laterally to a greater 
extent than the phase conductors in high winds and so can move into 
regions of high electric fields. If the cable is provided with a 
conductor, or semiconductor along its entire length, its potential will 
differ substantially from that of the phase conductors throughout its 
entire span between towers, with the result that corona discharge may 
occur during high winds. There is even the possibility that the cable may 
become so close to the phase conductor that flashover between the phase 
conductor and the cable may occur which can trip out the power supply. 
However, because the mid-span region of the cable is dielectric in the 
system according to the invention, and so its induced voltage is allowed 
to rise toward that of the phase conductors, the occurrence of corona 
discharge at the mid-span region of the cable is reduced. Also, any 
clashing of the cable with any phase conductors in the mid-span region 
will not produce damaging currents. 
The resistive element should have a linear conductivity that is 
significantly greater than the longitudinal conductivity of the optical 
cable under dry conditions, preferably a conductivity at least 100 times 
that of the cable, so that any capacitively induced current will be 
conducted to ground by the resistive element instead of the cable jacket. 
Normally the element will have a linear resistance of not more than 2 Mohm 
m.sup.-1 and more preferably not more than 500 kohms m.sup.-1, but a 
linear resistance of at least 200 and especially at least 300 kohms 
m.sup.-1. 
The resistive element may be foraged from any materials that conventionally 
are employed to manufacture such semiconducting articles, for example from 
carbon loaded plastics materials or carbon loaded non-woven tapes. 
Advantageously the element is formed from a plastics material that 
incorporates electrically conductive carbonaceous fibres. Such fibres may 
be formed by partial pyrolysis of a polymer for example polyacrylonitrile 
or acrylonitrile copolymers having an acrylonitrile content of at least 85 
mole percent and up to 15 mole percent of copolymers (PAN). Such fibres 
may have a carbon content of 65% to 92%, preferably less than 85% and a 
nitrogen content in the range of 5 to 20%, preferably 16 to 20%. 
Carbonaceous fibre tows that are suitable for use in the present invention 
are commercially available for example from R. K. Technololgies Ltd of 
Heaton Norris, Stockport, Cheshire, United Kingdom. 
According to another aspect, the invention provides a resistive element 
which can be removably installed on an optical cable that extends freely 
between, and is supported by, towers of a combined electrical power and 
optical transmission system, which element includes a plurality of means 
for supporting the element on the optical cable, that enables sliding of 
the element along the cable, and has a length and conductivity such that, 
in use, if a dry band occurs on the cable at the end of the element the 
potential difference across the band is insufficient to form an are, 
and/or such that the induced current is insufficient to sustain any are 
that may occur across the dry band.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the accompanying drawings, FIG. 1 illustrates a conventional 
"all dielectric self-supporting" (ADSS) optical cable 1 that is supported 
between a pair of towers that are also employed to support an electric 
power cable 2. The ADSS cable 1 is supported at the tower by means of a 
metallic clamp or fitting 4 which is earthed by means of the tower. In 
this system there is a distributed capacitance between the optical cable 1 
and the phase conductors of which one conductor 2 is shown, indicated by 
lumped capacitances C.sub.1, and a distributed capacitance between the 
optical cable 1 and ground, indicated by lumped capacitances C.sub.2. In 
addition the cable has a large but finite longitudinal resistance 
indicated by lumped resistance R. 
Under dry conditions induced voltages (V.sub.d) and currents (I.sub.d) 
occur on the cable as shown in FIG. 2. The induced voltage is highest at 
mid-span, typically reaching a value of up to 30 kV on a 400 kV line, and 
is clearly at earth potential at the tower, while the current will be at a 
maximum at the tower, for example having a value of up to to 100 
microamps. Under wet conditions the longitudinal resistance of the optical 
cable is considerably lower, with the result that the maximum voltage 
(V.sub.w) on the optical cable is lower but the induced current (I.sub.w) 
has risen considerably to a typical value of 0.1 to 10 mA, as shown in 
FIG. 3. 
Under these conditions, as shown in FIG. 4, a dry band 6 of typical length 
50 mm may form on the cable 1 in the region of the clamp 4 at the tower, 
due to joule heating of the surface water on the cable. This has the 
result that almost the entire induced voltage is dropped across this 
length of cable, and arcing may occur at this point with consequent damage 
to the cable jacket. If sufficient potential difference exists to strike 
an arc, this will only be stable if sufficient current is available to 
maintain the are (of the order of 0.5 mA). 
FIG. 5 shows schematically part of a tower 10 of a combined electrical 
power and optical transmission system according to the present invention 
which includes an all dielectric optical cable 1 extending from a cable 
clamp 4 at the tower. A resistive element 12 in the form of a semi-rigid 
rod that has a number of clips 14 extending along its length in the order 
of every 400 mm. The rod is passed up the tower 10 in the vertical 
direction, bent at the tower into a direction parallel to the optical 
cable 1, and secured to the cable by means of the end clip 14. The rod is 
sufficiently flexible to allow it to be bent at the tower, but is 
sufficiently rigid that it can be pushed along the cable from the tower in 
the direction of the arrow to deploy it fully along the cable. As each of 
the clips 14 comes into proximity with the cable it is clipped onto the 
cable and the element pushed further along it. The clips may be 
electrically conductive or semiconductive, or even electrically insulating 
since the element 12 will capacitively couple to the optical cable 1 to a 
much greater extent than to the phase conductors in view of the proximity 
of the element and the cable. When the element 12 is deployed to its full 
extent it is connected to the cable clamp 4 in order to ground the 
proximal end thereof. In order to remove the resistive element, the steps 
are simply reversed. 
As the element is fed into the span a current will flow to earth along its 
length. For this reason it is desirable to provide an earth path between 
the element and ground located at a point on the element in the region of 
the tower but beyond which installation personnel located in the tower 
will not touch. 
FIG. 6 shows schematically a tower 10 of the system and that part of an 
optical cable 1 extending from the tower to the mid point of the span. 
Other elements of the system such as the phase conductors of the system 
have been omitted for the sake of clarity. In addition, the capacitively 
induced voltages and currents are shown graphically on the same horizontal 
scale, both for the system according to the invention and for a 
conventional system. 
In wet conditions the induced voltage V.sub.w falls, and the induced 
current I.sub.w increases toward the tower in the same way as shown in 
FIG. 3, which causes joule heating and dry-band formation at the part of 
the cable adjacent to the tower in the conventional system (point A). As 
soon as a dry band is formed, the entire induced voltage V.sub.b is 
dropped across the dry band so that the induced voltage has the form 
V.sub.w band, with the result that an are can be formed. As soon as an arc 
is formed the voltage distribution returns to the curve V.sub.w, and the 
arc is sustained by the relatively high value of the induced current 
(curve I.sub.w) at point A. 
In the system according to the present invention under wet conditions the 
induced voltage and current will have the same form (V.sub.w and I.sub.w). 
If a dry band is formed on the part of the cable adjacent to the tower, 
the resistive element will limit the voltage drop across the dry band to a 
value well below that required for arc formation (1 mA through 500 kohms 
m.sup.-1 giving only 25V across 50 mm). However, a dry band may still form 
beyond the end of the resistive element (point B), whereupon the induced 
voltage distribution will change to that shown by curve V.sub.w band'. In 
this case the voltage V.sub.b ' that is dropped across that part of the 
band beyond the end of the resistive element, is significantly smaller 
than V.sub.b due to the fact that the voltage at the end of the resistive 
element, V.sub.SE, has risen significantly (for example by 10 kV) due 
partly to capacitive coupling between the resistive element and the phase 
conductors and partly to the induced current flowing through the resistive 
element. Not only is the voltage drop across that part of the dry band 
beyond the end of the resistive element reduced, but also the induced 
current I.sub.w at point B is significantly lower than at point A, with 
the result that an arc cannot be sustained. If the resistance per unit 
length of the element is suitably chosen then joule heating can be 
avoided. For example 500 kohms m.sup.-1 and 1 mA yields a power of 0.5W 
m.sup.-1 which is insufficient to warm the element or the moisture on the 
cable. Thus, formation of a single dry band through positive feedback, by 
its resistance increasing as it dries out is also avoided. This is an 
additional benefit of the invention. 
The fact that the element is separate from the cable, and not trader the 
cable sheath, increases its ability to dissipate heat. This reduces the 
effect of Joule heating and allows greater currents to be dram without 
detrimental effects of heating. 
In a typical 400 kV power distribution system using an L6 tower with 
circuits with phase arranged symmetrically ABCABC in the place normally 
preferred for hanging ADSS cables, midway between the bottom four phase 
conductors, the ADSS cable may have 35 kV imposed at mid-span, which is 
available for dry-band arc formation. In conditions where the pollution is 
such that the surface resistance of the cable is 500 kohm m.sup.-1 an 
induced current of 2.5 mA may flow, enough to allow the formation of a 
stable dry-band arc and cause cable degradation. If, however, the system 
includes a 50 meter long resistive element of linear resistance 300 kohm 
m.sup.-1 according to the invention, the voltage (V.sub.b ') available for 
dry-band arcing at the end of the element is reduced to 19 kV and the 
current to 0.8 mA. If the linear resistance of the element is 400 kohm 
m.sup.-1, the voltage drop V.sub.b ' becomes 16 kV and the induced current 
is 0.6 mA, while if the linear resistance of the element is 500 kohm 
m.sup.-1, the voltage drop V.sub.b ' becomes 13 kV and the induced current 
is 0.5 mA.