Lead-covered Cables Research Articles

Analysis and evaluation of dielectric parameters for design verification and calibration of a newly developed diagnostic system for MV power cables

For the purpose of a condition-based asset management, in-depth knowledge of the equipment condition is needed. In medium-voltage power networks, paper-insulated lead-covered (PILC) cables are to see as special critical assets. In order to evaluate the most appropriate diagnostic parameters and their characteristic values, multiple PILC cables have been artificially aged at power frequency during a long-lasting laboratory ageing experiment. Over this process, the relevant parameters like dissipation factor and partial discharge behaviour have been monitored and recorded and a fundamental database was built up. As a next step, field measurements of these values need to follow. Therefore, a mobile measurement system has been furtherly developed and integrated in a cable test van. In order to get improved diagnostic criteria the mobile test system facilitates, that the dielectric properties of the PILC cables are determined at 0.1 and 50 Hz test voltage frequency. Here, the development, the calibration and the adjustment of the measurement system as well as influences on field measurements will be described and discussed.

Advanced signal processing and modeling for partial discharge diagnosis on mixed power cable systems

Power cable grids have been growing enormously over the years; due to their aging, there is a strong need for Partial Discharge (PD) measurement techniques in order to identify incorrectly installed or damaged cable joints or cavities in cross-linked polyethylene (XLPE) insulated and paper-insulated lead-covered (PILC) cables. Mixed power cable systems consist of a mixture of interconnected cables of various types. The Time Domain Reflectometry (TDR) is a standard technique for the location of PD in power cables. However, the localization process on-the-field is impeded by various interferences and noise. Additionally, propagation velocities of PD pulses traveling on the line differ depending on the cable type (XLPE or PILC cables), which leads to significant location errors. This paper provides automatable signal processing tools for increasing the detection ratio as well as location accuracy in the field of mixed cable systems. A mixed cable system model provides the possibility of analyzing PD data of even complex cable interconnections.

Artificial aging and diagnostic measurements on medium-voltage, paper-insulated, lead-covered cables

Present and future condition-based maintenance methods and preventive maintenance strategies will require knowledge of the present condition of the equipment. For cable networks in gen eral, and for PILC cables in particular, our understanding of the physical phenomena that occur inside cable insulation systems during operation, and their influence on the remaining service lifetime, is still fragmentary. The ICAAS system was developed primarily to facilitate an artificial aging program on MV PILC cables, based on tan δ, return voltage, and PD measurements. An extensive database was built up over a period of two years. Further results will be presented in due course. It should be noted that the ICAAS is fairly freely configu rable so that, with a little effort, it could be applied to other cable types, cable garnitures, sleeves, and terminations. It could also be used to study specific cable defects introduced during manu facture or for the general verification of production quality.

Life extension and condition assessment: techniques for an aging utility infrastructure

Many utilities are focusing their efforts on developing methods to assess the condition of equipment to extend the life of existing infrastructure. In many cases, these utilities focus their efforts on extending the life of power transformers and paper-insulated lead-covered (PILC) cables because these components offer the greatest payback. Extending the life of aging infrastructure requires the use of noninvasive condition assessment and online preventive techniques that can easily be implemented in the field. Such condition monitoring, assessment, and life extension techniques will help utilities decide whether an aging transformer or underground PILC distribution cable can be operated reliably beyond its design life.

通信用鉛皮ケーブルの腐食因子について

Some significant factors for chemical corrosion of lead-covered telegraphic cable were studied experimentally from the viewpoint of electrochemistry. Corrosion and polarization potential of the lead which is immersed in manhole water or other various corrosive media were measured.(1) The chemical species (especially anionic) contained in manhole water seem to have something to do with the potential behavioors, and some of them tend to form the protective films. Therefore, it seems more important that this factor should be considered chemically than electrolytically.(2) The heterogeneous potential distribution on the surface, caused by the ununiformities in micro or macro structures and in their surface differential aerations, were observed. The maximum difference of potential caused by local cell was about 100mV, which was enough to induce the local current leading to anodic corrosion. For example, it appears between the fresh surface and oxidized film. But no anodic acceleration occurred at the contact of lead-cover with the steel pipe used for protection of cables.(3) From the results above mentioned, some practical informations for preventing corrosion on the lead are proposed concerning the suitable conditions and materials of cable construction.

Coupled transmission lines as symmetrical directional couplers

If two similar unbalanced transmission lines are coupled together by sharing a common outer conductor for part of their length, so as to form a 3-conductor line, they constitute a symmetrical directional coupler. The output ratio of this varies sinusoidally with frequency, being greatest for lengths equal to an odd number of quarter-wavelengths; the characteristic impedance is independent of frequency, being determined by the cross-section of the 3-conductor line. Two forms of the directional coupler are described. One of these, intended for laboratory power measurement at frequencies up to 1 000Mc/s, consists of two parallel strips enclosed in a square-section outer conductor. The other, a very simple device for monitoring transmitter power, consists of two lead-covered coaxial cables grafted together. Among possible uses suggested are all-pass filters and applications to aerial systems consisting of two parts fed in phase quadrature.

Lepeth sheath for telephone cables

SINGE the introduction of lead-covered telephone cable, there have been many improvements in the methods of applying this protective sheath. However, until recently, the only significant changes in the basic design of the sheath had been in the composition of the alloy used.

Protective covering for lead-sheath power gables

THE MAJORITY of failures of impregnated-paper or varnished-cambric lead-covered power cable are due to the entrance of moisture through a break of some kind in the lead sheath. The sheath failures may be due to corrosion, electrolysis, mechanical damage, fatigue attributable to either vibration or cable movement, or high internal pressure. Corrosive action or electrolysis in some degree are fairly common to all lead-covered cables installed in ducts while damage due to mechanical failure is generally confined to those cables which are subjected to large daily variations in load. This latter type of failure has become of increasing importance with the higher loads which have been imposed on power cables during recent years.

Eddy-current losses in single-conductor paper-insulated lead-covered unarmoured cables of a single-phase system

Eddy-current losses in single-conductor paper-insulated lead-covered unarmoured cables of a single-phase system

Load Ratings or Cable-II

Since the author's 1939 paper, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> further data have been obtained regarding the loading of underground power systems having impregnated-paper-insulated lead-covered underground cable. The results of the studies may be summarized as follows: 1. The factors affecting the setting of maximum safe conductor temperatures are so numerous that no fixed value (or values) can be assumed as applicable to all installations of a given design and size of cable in this country. 2. For some cables, the present temperature limits for normal day-in and day-out operation may be safely exceeded, especially for wartime conditions. 3. For emergency operation safe temperatures may be even higher than listed in the previous paper, especially for wartime operation. 4. Operation in wartimes at special temperatures will mean in some cases substantial shortening of the life of underground circuits and an accompanying increase in service interruptions. 5. Wartime increases in usual maximum daily loading and in load factors may cause large increases in duct and copper temperatures, even if past current ratings are maintained. These temperatures will sometimes exceed those in the present standards. 6. The safe temperature for emergency operation, particularly for extra-high-voltage cables, may be limited by the joints. Also, for all kinds of cable sufficient room must be provided to avoid mechanical damage of cable or joints in manholes with cable movements incidental to emergency loads. 7.

Eddy-current losses in single-conductor paper-insulated lead-covered unarmoured cables of a single-phase system

An experimental investigation of the eddy-current losses in cables when carrying alternating current at power frequencies has been carried out at the National Physical Laboratory. Measurements were made with single-phase current on two parallel cables of equal length, and of 1.4 sq. in. conductor section, forming go and return. Formulae are put forward for estimating the a.c. resistance of a cable from the design data, and these formulae are shown to give good agreement with the experimental figures.

Load ratings of cable — II

Since the author's 1939 paper, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> further data have been obtained regarding the loading of underground power systems having impregnated-paper-insulated lead-covered underground cable. The results of the studies may be summarized as follows: 1. The factors affecting the setting of maximum safe conductor temperatures are so numerous that no fixed value (or values) can be assumed as applicable to all installations of a given design and size of cable in this country. 2. For some cables, the present temperature limits for normal day-in and day-out operation may be safely exceeded, especially for wartime conditions. 3. For emergency operation safe temperatures may be even higher than listed in the previous paper, especially for wartime operation. 4. Operation in wartimes at special temperatures will mean in some cases substantial shortening of the life of underground circuits and an accompanying increase in service interruptions. 5. Wartime increases in usual maximum daily loading and in load factors may cause large increases in duct and copper temperatures, even if past current ratings are maintained. These temperatures will sometimes exceed those in the present standards. 6. The safe temperature for emergency operation, particularly for extra-high-voltage cables, may be limited by the joints. Also, for all kinds of cable sufficient room must be provided to avoid mechanical damage of cable or joints in manholes with cable movements incidental to emergency loads. 7. Cracking of lead sheaths due to reciprocating cable movement into manholes may limit the temperature range for usual daily loading, but has little effect on the safe emergency loading. 8. For three-conductor solid-type cable, the insulation of the shielded type can safely withstand higher temperatures than the belted type, but the reverse is true as to the allowable daily temperature range with regard to its effect on the sheath in manholes. 9. Cable movement increases with length of conduit sections up to about 250 feet but shows little change with further increases in length up to 1,025 feet. 10. Changes in installation methods and the use of new types of repairs may help to mitigate troubles due to sheath cracking. 11. Copper shielding tape in three-conductor cable with relatively thin insulation has little effect in reducing the thermal drop from the conductor to the sheath. 12. In some cases considerable thermal advantage may be gained economically by making relatively deep installations of conduit. 13. In most cases, the maximum conduit temperature of 50 degrees centigrade given in the previous paper may be safely exceeded.

Eddy-current losses in multi-core paper-insulated lead-covered cables, armoured and unarmoured, carrying balanced 3-phase current

This report deals with an investigation of eddy-current losses in multi-core circular-conductor and shaped-conductor paper-insulated wire-armoured cables, and the development of formulae by means of which the losses can be computed. The experimental work covers tests with balanced three-phase currents at frequencies from 25 to 300 cycles per second.The eddy-current losses are divided into four parts, namely, skin effect, proximity effect, lead-sheath effect, and armouring effect. The ratio of a.c. to d.c. resistance of the conductors, calculated by the appropriate formula, is, with one exception, within 5% of the measured value under all the conditions covered by the investigation, the average difference between calculation and measurement being only 1%.

Eddy-current losses in multi-core paper-insulated lead-covered cables, armoured and unarmoured, carrying balanced 3-phase current

Eddy-current losses in multi-core paper-insulated lead-covered cables, armoured and unarmoured, carrying balanced 3-phase current

Load ratings of cable

Operating and test data concerning the maximum safe loading of impregnated-paper-insulated lead-covered cable are presented. The results of the study may be summarized as follows: 1. The occasional operation of cable at higher temperatures than are permitted by present temperature rules effects considerable economy. 2. During emergencies, temperatures of 5 to 35 degrees centigrade (depending on kind of cable) above those permitted by the rules are safe for the insulation. 3. For extra-high-voltage solid-type cable, void formation in insulation and expansion of lead sheaths may limit allowable temperatures and temperature ranges. 4. Cracking of lead sheaths due to reciprocating cable movement into manholes may limit the temperature range for usual daily loading. Limitation is more severe for longer conduit lengths up to 500 feet, but changes little with increase from 500-to 1,000-foot lengths. 5. Cracking of sheaths in manholes due to cable movement may be reduced by improving manhole conditions. 6. For many cables a balanced design requires a lead-alloy sheath that gives increased resistance to effects of cable movement and of internal pressures. 7. Continuous field temperature surveys are essential to efficient use of large conduit and cable systems. 8. Only a small fraction of the cable ever operates at the higher temperatures. 9. Data on center empty-duct temperatures and on average heat losses over 24-hour periods give satisfactory results in heat calculations. 10. Other practices which increase load ratings are the use of different ratings for various periods of the year, the replacement of poor soil in special cases, and the use of extra-large conductors in warmer conduits.

Load Ratings of Cable

Operating and test data concerning the maximum safe loading of impregnated-paper-insulated lead-covered cable are presented. The results of the study may be summarized as follows: 1. The occasional operation of cable at higher temperatures than are permitted by present temperature rules effects considerable economy. 2. During emergencies, temperatures of 5 to 35 degrees centigrade (depending on kind of cable) above those permitted by the rules are safe for the insulation. 3. For extra-high-voltage solid-type cable, void formation in insulation and expansion of lead sheaths may limit allowable temperatures and temperature ranges. 4. Cracking of lead sheaths due to reciprocating cable movement into manholes may limit the temperature range for usual daily loading. Limitation is more severe for longer conduit lengths up to 500 feet, but changes little with increase from 500-to 1,000-foot lengths. 5. Cracking of sheaths in manholes due to cable movement may be reduced by improving manhole conditions. 6. For many cables a balanced design requires a lead-alloy sheath that gives increased resistance to effects of cable movement and of internal pressures. 7. Continuous field temperature surveys are essential to efficient use of large conduit and cable systems. 8. Only a small fraction of the cable ever operates at the higher temperatures. 9. Data on center empty-duct temperatures and on average heat losses over 24-hour periods give satisfactory results in heat calculations. 10. Other practices which increase load ratings are the use of different ratings for various periods of the year, the replacement of poor soil in special cases, and the use of extra-large conductors in warmer conduits.

Current rating and impedance of cables in buildings and ships

An account is given of the investigations on which were based much of the revision and, in particular, the new rating and voltage-drop tables, with Appendix 5, of the Tenth Edition of the I.E.E. Regulations for the Electrical Equipment of Buildings. The results of these investigations will, subject to the approval of the I.E.E. Council, be embodied in the next edition of the Regulations for the Electrical Equipment of Ships. This work, carried out partly at the National Physical Laboratory and partly at the E.R.A. auxiliary laboratory, dealt mainly with the effect of alternating current instead of direct current; of new modes of installation such as heavy cables in cleats, large groups of cables in conduits, ducts, chases, and surface wiring systems; the use of bare copper rods, long vertical runs in chases or shafts, etc.; and with the effect of steel bulkheads or steel-reinforced concrete structures in proximity to a. c. systems. While the earlier figures of the Ninth Edition were largely confirmed as general averages for certain conditions, a large number of new circumstances were investigated in detail, and the values observed were theoretically substantiated to the accuracy permitted by the conditions. The main purpose of the paper is to indicate the more accurate figures on which the necessarily condensed average tables of the Regulations are based, but the detailed results have also other applications.The experimental work is classified broadly in three sections, as follows:—(a) The current-carrying capacities of large cables run in air in various configurations.(b) Effects due to proximity of large cables to steel structures. In this section of the research, cables were run parallel to a steel plate, through holes in a steel plate, and through a steel-rod structure (to simulate reinforced concrete). Tests were made on both singlephase and 3-phase systems.(c) The properties of small rubber-insulated cables (sizes up to 0.5 sq. in. were included in some conditions of test) installed in a variety of methods such as in conduits, ducts, etc.The results of the investigations show that the reduction in current-carrying capacity due to skin effect and sheath losses is only serious in cables of 0.5 sq. in. conductor section and above. The increase in current rating due to better cooling conditions when cables are spaced farther apart may be partially or wholly counterbalanced in lead-covered cables by increase in sheath losses. In the case of bare copper rods run vertically, the temperature of which may be higher than that of insulated cables, it was found that excessive heating of rubber-insulated cables connected to the rods occurred near the joints.When cables carrying alternating current are run close to a steel plate the impedance is increased, and in some cases the effect on the voltage-drop may be sufficient to necessitate reducing the current rating of the cable below that based on the permissible temperature-rise. Also, in the case of a reihforced-concrete building, steelwork may be so heated owing to heavy currents in large cables as to cause serious expansion.

Communal Aerials for Radio Reception

IN large towns and cities, the provision of a good aerial generally presents many difficulties, especially in houses divided up into self-contained flats. In the latter case, it would be necessary to erect on one roof a number of separate aerials which would satisfy the varying requirements of the listeners. In some places also there is an almost insuperable difficulty owing to interference from electric motors and other electric devices. Some years ago the Philips Research Laboratories at Eindhoven, Holland, evolved a communal aerial system which they call the ‘Antennaphil’ which surmounts most of the difficulties met with in practice. A description of it is given in the Journal of the Philips Research Laboratories of August last. In this system an aerial of suitable dimensions is erected at a spot where interference is limited, for example, at a height of twenty feet above the roof. Through a specially screened conductor made as short as possible, the aerial is connected to an aerial signal amplifier, from which the incoming signals are transmitted to a number of receiving sets through a lead-covered cable which acts as a screened distributing circuit. Up to fifty sets can be connected to an aerial fitted with this form of amplifier. If the distance of the spot from which reception, practically free from interference, can be received is a few hundred yards, then the number of sets is less. But even up to a distance of 1,000 yards, several sets can be supplied. An aerial of this type has been in use at the laboratory for several years at a distance of 200 yards, and has ensured reception free from all interference. It is proved that a concentric cable of suitable dimensions is the best to use for high-frequency distribution.

Burn-Off Characteristics of A-C. Low-Voltage Network Cables

This paper describes tests made to determine the characteristics of the clearance of faults in several types of a-c. low-voltage network copper-conductor cables installed in accordance with present standards of underground construction, and includes a discussion of test results together with other relative data studied. The investigation included the burning clear of various types of faults in buried non-magnetic sheath cables, and non-metallic sheath and lead-covered cables for duct installation. A comparison is made of the burn-off characteristics of the several cable constructions and their installations. The amount of short-circuit current and length of time required to clear the different types of faults in No. 4/0, 350,000-and 500,000-cir. mil cables have been determined by test and by calculations in some instances. Conclusive data were not obtained to clear up all present conflicting opinions related to the burn-off characteristics of a-c. low-voltage network cables. However, a study of the subject matter would seem to establish at least a partial basis in regard to burn-off characteristics for determining the conductor size of low-voltage network mains and the spacing of network transformers. Desirable developments for a-c. network cables indicated by the results of this investigation are emphasized.

Water penetration effects in lead-sheathed power cables

The immersion in water of lead-covered cables, with a view to the detection of faults in the lead sheathing, is discussed. Tests of the time taken to show up these defects in power cables at different water pressures are given for various cable samples. The conclusions drawn are that, except possibly in the case of low-voltage cables, no such defects will be discovered during factory routine, and that for high-voltage cables, at least, water immersion should be definitely discontinued.