f-

S.M. Rowland I.V. Nichols

Indexing terms: Dielectric cables, Dry-band arcing, Ageing

Abstract: All-dielectric self-supporting fibre optic cables which are hung on long-span, high voltage, overhead power lines are subjected to longitudinal electric fields. When the cable is polluted and wet, these fields can lead to dry- band arc activity and, ultimately, damage to the cable. The properties of the arcs are reviewed to explain why currents of a few milliamps are more damaging than either higher or lower currents. New measurements which investigate the importance of arc stability and arc compression to ageing processes are presented and discussed. It is suggested that a limit to the current of 1 mA should be set. If currents greater than this are present it is argued that a danger of catastrophic product failure exists. Such a current limit will be more restrictive in coastal conditions and areas of high pollution, where the resistance per unit length of the cable can fall significantly below typical values.

Introduction

Deregulation of electricity distribution companies and telecommunication networks has led to a worldwide increase in the use of fibre optic cables on long-span overhead power lines. In the UK, for example, there has been a tenfold increase in the rate of installation over the past five years. There are several commercially available cable types which allow the installation of optical fibres on existing overhead lines [l]. One such, is the all-dielectric self-supporting (ADSS) cable. This cable typically has a pultruded glass or aramid fibre strength member and a loose-tube optic fibre construc- tion [2].

Typically an ADSS cable is suspended from the lat- tice towers between the phase conductors, so clashing cannot occur. Distributed capacitances between the optical cable, the phase conductors, the earth wires and ground result in a voltage gradient along the length of the dielectric cable. If the cable is dry, the voltage gra- dient does not present a problem. However, if the cable surface becomes conductive, due to moisture, a current is drawn along its length. Because of the distributed 0 IEE, 1996 IEE Proceedings online no. 19960028 Paper first received 22nd June and in revised form 5th September 1995 The authors are with BICC Cables, Helsby Technology Centre, Helsby, Warrington, Cheshire WA6 ODJ, UK

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nature of the capacitive coupling the magnitude of this current is greatest at the towers where the cable enters a clamp at earth potential [3]. This current will tend to dry the cable surface inevitably leading to a first break (dry band) in the previously continuous conductive sur- face. Experiments and calculations have previously shown that sufficient potential difference can result across such a dry band to cause arcing [3,4]. Such arc- ing can degrade a cable sheath and the underlying strength member to an extent that the cable may fail mechanically.

The threat of dry-band arcing has been acknowl- edged for many years. The principal response of cable manufacturers has been to develop sheath materials which are resistant to ageing by arcs [5]. This has led to cables which age more slowly in the presence of arcs under specific test regimes. However, the ageing phe- nomena have not been sufficiently understood to allow guidelines to be agreed which define where a cable may, or may not, be safely installed. This paper presents new experimental results which are consistent with service experience and which may help in the defi- nition of such guidelines.

2 Service conditions

Using commercially available software, the midspan potential on the ADSS cable can be estimated for a given installation. This is also the maximum voltage which can be seen across a dry band. If assumptions are made concerning the conductivity of pollution on the ADSS cable, the maximum currents to earth can also be predicted. This has been reported extensively elsewhere [3] so details will not be given here. However, Table I shows how the available current may change with the resistance of the pollution on an optical cable on typical 132, 275 and 400kV symmetrical twin circuit towers. In practice the resistance per unit length of a wet cable will vary between over 10MB/m for a new cable in a clean rural environment and perhaps 2001tBl m for an older cable in an industrial or coastal loca- tion. The construction and location of towers varies considerably, so each span must be carefully consid- ered. The geometry of the towers considered in Table 1 is shown in Fig. 1. The suspension points for the ADSS cable have considerable influence on the currents induced. In the present case it was assumed that the ADSS cable was suspended midway between the bot- tom four phase conductors. Although this is not neces- sarily the optimum position electrically, it is often preferred mechanically. Towers with transposed phase arrangements are electrically more favourable for the

IEE Floc-Sei Meas Tcchnol, Vol 143, No 1, January 1996

Table 1: Calculated values of maximum current and voltage on ADSS cables as the environment changes their resistance per unit length

ADSS cable than

Midspan voltage and available current

Conditions Cable resistance 132kV system 275kV system 400kV system

Normal 5MWm 12kV, 0.2mA 24kV, 0.6mA 35kV, 0.6mA

Polluted 500MWm 12kV, 0.6mA 24kV, 2.0mA 35kV, 2.5mA

Severe 200 MWrn 12kV, 1.0mA 24kV, 3.2mA 35kV, 4.0mA coastal

Three tower constructions are considered, of 132kV, 275kV and 400kV system voltages

symmetrical ones. In the transposed case, conditions are worsened when one of the circuits is switched off for maintenance. More details on the effects of relative phases is given by Carter and Waldren [ 3 ] .

r i i e a r t h wire

phase

cable

conductors

Fig. 1 Tower geometries which have provided the results in Table 1

In summary, the currents seen on the optical cable are never likely to exceed 5mA when it is installed in a mechanically acceptable position. In most installations they will rarely reach 1mA. It is in this range of between 1 and 5mA that arcs cause the greatest dam- age to polymeric insulators. Thus an understanding of the ageing processes is important to assessing the risks associated with installation in the more extreme cases.

3 Effects of current on arc properties

It has long been known that the current in an arc is important in determining its behaviour, and the rate at which it does damage. However, only recently has a formal model been proposed for ageing of ADSS cables [3]. From that model it is evident that arcs are only stable if sufficient current is available, and that there is a threshold combination of induced voltage and cable resistance per unit length (determined by pol- lution and precipitation) below which stable arcing cannot occur. That work suggests that a threshold con- dition might be identified below which damage would not be sustained on the cable; this condition being when arcs become unstable. Carter and Waldren also suggested that higher currents cause less damage because these extend the dry band more quickly, thereby reducing the period of activity. It was sug- gested that these are the reasons that greatest damage results from currents in the range of a few milliamps.

One of the present authors has alternatively sug- gested that higher currents are less damaging because such arcs are not confined t o the minimum dry-band

IEE Proc -Sei Meas Technol, Vol 143, No I . January 1996

length: the arc roots may ‘wander’ over the electrolyte surface [4]. At low currents, of a few milliamps, the arc roots are confined to the edges of the dry band and thus localised heating and degradation of the polymeric sheath is increased. The exact value of current below which an arc is confined is determined by the conduc- tivity of the wet cable. Hampton’s criterion [7] states that the arc will only extend over the adjacent moisture if the field on the cable surface exceeds that in the arc. Further to this, a new mechanism of degradation has been identified [8]. Previously it was thought that all ageing was due to a gradual effect of long-term arcing. However, it has been observed that in conditions under which Hampton’s criterion is not met (i.e. when an arc is confined in length), arc compression can occur. In these circumstances, a dry band is reduced in length, for example, by moisture running along the cable, and an existing arc is continually shortened until it is extin- guished. Such an event may be rare, but only one such occurrence may be required for significant damage to be inflicted upon the cable [8]. Such ageing is superim- posed upon the more gradual arcing damage but is not continuous, rather it is dependent upon a particular set of conditions being met. Since the conditions for dry- band arc compression depend partly upon weather, they are very hard to predict in service; they may not be met for many years after installation or may not ever be met in some cases. The difficulty of forecasting events does not reduce the effect if they do occur.

Examples of damage to cables which have ‘dry-band arc resistant’ sheaths are shown in publications by Pea- cock and Wheeler [9] and Rowland and Easthope [SI. In these it is evident that low current damage is much more severe than high current effects. Damage by arcs of a few milliamps results in deep, localised troughs of erosion along the sheath. Damage by higher currents tends to take the form of surface roughening which is less localised.

The following set of experiments was carried out to determine whether dry-band arc compression always leads to sheath failure irrespective of arc current, and whether unstable arcs are able to damage the cable sur- face.

4 Experimental details

For the purposes of this experiment, copper electrodes were fixed directly onto the surface of the ADSS cable. The cable was sheathed in a typical arc-resistant ther- moplastic compound to a diameter of 14mm. The com- pound primarily comprised of a blend of polymers and LSF additives and was UV stabilised by the addition of carbon black. Other materials examined, including a crosslinked system based on EVA, behaved in a similar fashion but these results are not reported here. The

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nature of the electrodes did not affect results greatly but they were normally 0.2mm thick sheets of foil wrapped tightly around the cable (see Fig. 2). No attempt has been made to study the subtleties of elec- trodes, but using metals produced results similar to those seen with water [8]. Electrode separations between 10 and 30mm were used for a series of experi- ments.

copper foil electrodes ADSS cable J \

I H V transformer 2 ballast resistor

Fig.2 trodes

Electrode arrangement for. arcing experinients: starionarx elec-

ballast reslstor copper wire electrode i HV transformer

\ - - - )per foil \ I dielectric rod, CO I

clips to allow controlled i movement of electrode

Electrode arvangernent f o r arcing experiments: one inobile elec- Fig. 3 trode fo r compressing arcs

To reproduce arc compression events, a copper wire was fixed to a rod so that it could be pushed along the surface of the cable until it touched the other foil elec- trode (Fig. 3). The secondary coil of the high voltage transformer was connected across the electrodes in series with a ballast high voltage resistor, which limited the current. An oscilloscope was used to determine the stability of arcs, as detailed in [3,4]. Unstable arcs are those which have more than one breakdown per half cycle.

Experimental results are also presented from meas- urements taken from samples with electrodes up to l m apart, aged in salt fog chambers. Identical electrical test

circuits were employed, but now dry-band arcs formed naturally over a period of time. These experiments are as reported previously [4,8]. In these tests there was lit- tle control over whether or not dry-band arcs were compressed, although compression is not possible at higher currents since the arc can stray over the mois- ture on the cable and is not confined to the dry-band length. It has also been shown previously that compres- sion is encouraged when cable samples are sloping, since it is droplets running down the cable which are most likely to reduce the length of the arc [8].

5 Results

It was found that, as long as sufficient voltage was available to break down the gap, the magnitude of the voltage had no direct influence on the rate of ageing. Rather, it was the current which determined the effect of an arc. The size of the gap (and therefore arc length) had no great effect, as long as sufficient voltage was available to strike an arc. Unstable arcs were able to damage the sheath.

In the absence of compression, arcs of less than 2mA show little visible damage to the sheath surface; after several hours' exposure, the general form of damage is to produce a matt finish as might be expected from a low-level exposure to heat. Above 2mA, however, dam- age can occur quite rapidly (in 5min or so) the damage being visible to the naked eye and of deep localised erosion. Typical results are presented in Table 2. The definition of exact time of failure is rather arbitrary and ill-defined. However, because once started, damage progressed rapidly, different observers were in close agreement to its value. The time of failure was taken as the time at which damage had penetrated the sheath surface by 0.5mm.

If an arc of 2mA, which has been burning over a fixed length for one hour with little effect on the cable surface, is compressed in length just once (the gap reduced at around 5mm/s) severe damage can result. 0.5mm or more erosion along several millimetres of the sheath adjacent to the stationary electrode might result from one such compression. At lower currents several compressions may be required to produce equivalent

Table 2: Typical fixed arc length results, with fixed resistors limiting available current

~~~ ~ ~~

No. Gap, m m Voltage, kV Resistance, M a Current, mA Arcs Result*

1 10 15 7.5 2.0 unstable no failure after 2.5h

2 10 17 7.5 2.3 varied failed after 15min

3 10 25 10 2.5 stable failed after 10min

4 10 15 5 3.0 stable failed after 1 min

"'Failure' is defined as damage to the cable sheath penetrating more than 0.5mm

Table 3: Typical results from experiments in which arcs were compressed by moving metallic electrodes

No. Voltage, kV Resistor, MQ Current, mA Result"

1 20 19 1.1 no damage after 30min and 60 compressions

2 24 19 1.3 failed after 20min on 32nd compression

3 28 15 1.9 failed after 5min on 5th compression

4 15 5 3 failed after 5min on 2nd compression

5 15 2.5 6 failed after 5min on 2nd compression "'Failure' i s defined as damage to the cable sheath penetrating more than 0.5mm

12 IEE Proc.-Sci. Meas. Technol., Vol. 143, No. 1. January 1996

Table 4: Results from tests in salt fog chambers

No. Voltage, kV Resistor, MC2 Current, mA Result"

1 20 20 1 .o no damage at 300 h 2 25 10 2.5 failed at 30h: damage to sheath severe

3t 25 None 10 minimal damage to sheath at 300h

t In test 3 there was no limiting resistor but the current trip level was set at 25mA and the values of 10mA were measured. *Times to failure (erosion of the sheath by 1 mm) are typical values; there is always a spread in results and variation with sheath material

damage. Even at currents which result in unstable arcs, compression can result in failure. Below l.2mA no fail- ures were recorded after 60 compressions (over a 30min period). Results are summarised in Table 3.

Results from tests in salt fog chambers over consider- ably longer periods, of up to 1000h, also show a strong dependence on current. Table 4 summarises typical data. Some of these tests were carried out by an inde- pendent test house and they were based on IEC 507 (NaC1 solution of conductivity 1.6S/m at 20°C sprayed into a chamber at a rate of 0.41 m3/hP1).

6 Comparison with service experience

Experience of ADSS cables on towers carrying system voltages of 150kV and below extends over a decade. In that period, no failures due to electrical activity have been reported. However, some damage due to arcing has recently been reported on a cable's PVC vibration damping device [6] even though the cable sheath was undamaged. This suggests arcing is present even on 132kV systems but is sufficient only to threaten materi- als which form tracks. The physical presence of the vibration damping device may have also contributed to the occurrence of arcs. Calculations suggest that these installations will not see as much as lmA, even in severe pollution conditions (Table 1).

Cable failures tend not to be widely publicised but first reports of failure came from Europe where cables were hung on towers carrying system voltages greater than 150kV.

This trend has been confirmed by experiments at a coastal site in Scotland. In these experiments on 400kV lines, no failures occurred over a period of three years during which no currents larger than lmA were observed. However when conditions were such that currents rose to around 5mA, damage was sustained [61.

7 Discussion

The authors have found no evidence that failure can occur at currents less than lmA. Between 1 and 2mA some ageing occurred through gradual damage to the surface but significant damage only seems likely as a result of dry-band arc compression. This implies that the damage will not be a gradual process, rather time will elapse until conditions are reached whereby such a current can be drawn: if arc compression then occurs, damage will result. Higher currents will make single compression events more damaging. Around 2mA, even without compression, arcs will lead to gradual damage of the cable surface. The rate of this damage is unlikely to be constant and will depend upon sheath material. At currents in excess of around 3mA damage is rapid, a single occurrence of arcing may result in ero- siodburning through l.5mm of polymeric sheath [SI

IEE Proc -Sei Meas Tethnol, Vol 143, No I , January IY96

even without arc compression. The rate at which these higher currents of several milliamps erode a sheath is material dependent, and it is this respect of perform- ance which cable manufacturers have improved over recent years. Damage through arc compression is rapid in all the sheath materials investigated. Formal lifetime predictions from laboratory experiments [ 101 must be based on all the failure mechanisms which might be seen in service over all the possible conditions. The use of higher currents (tens of milliamps), does not acceler- ate ageing, rather it slows it down. In these conditions arcs are not confined to the dry-band length, so that arc compression is not possible, and the arc roots are not spatially confined to the dry-band edges.

Arc instability arises from insufficient voltage being available to drive the arc current. Arc stability is there- fore dependent upon the source impedance and arc length (since arc resistance is dependent upon its length and current). The source impedance is determined by the cable's resistance per unit length and the geometry of the towers and conductors. The issue of dry-band arc stability is secondary in this context since unstable arcs have been shown to be capable of damaging the cable sheath when compressed (indeed they can become stable as their length is decreased). However, in the absence of compression, unstable arcing damages at a slower rate than stable arcing. The relative severities of stable and unstable arcs have yet to be quantified.

These observations are consistent with experiments in salt-fog chambers. They are also consistent with the observations made in Scotland in full scale tests [6] and explain why no failures have been observed on installa- tions with system voltages less than 150kV.

It is proposed, therefore, that the limit to the applica- tion of ADSS cables should not be defined by system voltage but by the maximum current that the cable should be allowed to see in service. The details above suggest that 1mA might be a suitable upper limit at this stage in the understanding of the phenomena involved. The constraints of any such limit will depend upon the likely level of conductivity of pollutant on the cable as well as tower geometry, the cable surface qual- ity and the system voltage and phase arrangement. In coastai situations for example the use of ADSS cables may be precluded totally on 275 and 400kV systems.

8 Conclusions

Unstable arcs produce continuous ageing of polymeric insulation at a slower rate than stable arcs. Stable arcs are likely to occur if currents above 2mA are available.

Even unstable arcs can lead to rapid failure of poly- meric sheaths when they are repeatedly compressed in length, if the current exceeds 1 mA. But compression has most severe effects at higher currents, above 2mA, and is likely to occur when high current arcs are sus- tained for a long period of time (several hours).

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Whether this current is realised depends upon the geometry of the towers, the phase and voltage on the conductors, the location of the ADSS cable, the quality of the cable surface and the environmental conditions.

At much higher currents (tens of milliamps), arcs are not confined to the dry-band length, so that arc com- pression is not possible, and the arc roots are not spa- tially confined to the dry-band edges. For this reason higher currents are less damaging than those in the mil- liamp range. These currents are not seen in service but are sometimes used in laboratories for testing.

Care must be taken that, during unusual weather or pollution conditions, or single circuit outages on twin- circuit towers, currents greater than 1 mA cannot result. This requires careful consideration of the location of suspension points for the cable. It is likely that accepta- ble conditions can be achieved for the majority of installations considered but in some, particularly when system voltages exceed 150kV, it will not be possible. In these cases ADSS cables should not be used. Con- sideration of such transitory conditions as single circuit outages is important since the degradation mechanism of dry-band arc compression is rapid and not the grad- ual one usually associated with dry-band arcing. This 1 mA current limit may form the basis of an installation specification for all ADSS cables.

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3

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IO DISSADO, L.A., PARRY, M.J., WOLFE, s.v., SUMMERS, A.T. and CARTER, C.N.: ‘A new sheath evaluation technique for self-supporting optical fibre cables on overhead power lines’, International wire and cable symposium proceedings, 1990, pp. 743-751

References

MARTIN, J.R.: ‘Optical Fibres for Power Utilities’, CIGRE (1993) Australia, Paper 5.2 DAVIES, A.J., RADAGE, P., ROWLAND, S.M., and WALKER. D.J.: ‘The selection of materials for a lone soan. die- lectric, aenal, self-supporting cable for optical comm~nkations’, Plastics in Telecoms VI, London, Sept. 1992, pp. 2711-27110 CARTER, C.N., and WALDREN, M.A.: ‘Mathematical model of dry-band arcing on self-supporting, all-dielectric, optical cables, strung on overhead power lines’, IEE Proc. C, 1992, 139, pp. 185-196 ROWLAND, S.M.: ‘Sheathing materials for dielectric, aerial self- supporting cables for application on high voltage power lines’, Proc. 6th DMMA, 1992, IEE conference publication 363, pp, 53- 56 ROWLAND, S.M., and CARTER, C.N.: ‘The evaluation of sheathing materials for an all-dielectric, self-supporting, commu- nication cable for use on long span overhead power lines’, Proc. 5th DMMA, 1988, IEE conference publication 289, pp. 77-80 CARLTON, G., BARTLETT, A., CARTER, C., and PARKIN, A.: ‘UK power utilities experience with optical tele- communications’, Power Eng. J. 1995, Feb. 1995, pp. 7-14 HAMPTON, B.F.: ‘Flashover mechanism of polluted insulation’, Proc. IEE, 1964, 111, pp. 985-989 ROWLAND. S.M.. and EASTHOPE. F.: ‘Electrical aeeine and testinr of diklectric self-suooortinn cables for overhead ;lower lines’,”IEE Proc. A , 1993, 140, pp. 751-356 PEACOCK, A.J., and WHEELER, J.C.G.: ‘Development of aer- ial fibre optic cables for operation on 400 kV power lines’, IEE Proc. A . 1992. 139. nn. 304-313

14 IEE Proc.-Sei. Meas. Technol., Vol. 143, No. I , January 1996