Insulators Home > Book Reference Info > Interference Between Power and Telecom Lines
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The 'Foreword' (by Sir Gordon Radley) states:
The Electrical Research Association has been actively interested in problems of interference between power and communication circuits since 1922, when a committee was formed to strudy the subject. It very soon became one of great practical and economic importance, as proposals were made for the construction of the first 132 kV grid lines in the United Kingdom. The Post Office were apprehensive of the possible effects of large fault currents flowing through the earthed neutral points of the system. Comparatively little was understood at the time about the distribution of fault currents in the earth which gave rise to the interference phenomena, and I was first brought into contact with Dr. Klewe, the author of this book, when he gave us assistance in a series of field experiments. He was already an authority on the subject. He had made important contributions to the work of the International Consultative Committee on long-distance telephony, and helped it to formulate directives dealing with power circuit interference. [...] Now all the information that is available has been brought together by Dr. Klewe in this book. IT should be of immense benefit to all who are concerned with the construction of power and communication lines. To a certain extent the problems posed by the co-existence of the two have been eased by the modern use of high-frequency cables for long-distance telephone conversations. But in many countries, power and communication lines must be built side by side in conditions favourable to interference. [...] Many of the effects are complex, and their implications difficult to understand. All the basic information required by the engineer is, however, contained in this useful book.
The following information is excerpted from this book.
After a development of more than a century, it is often forgotten that the techniques of telecommunication are about five decades older than those of power transmission, since the transport of electric energy began about 50 years later than the transmission of information by means of electrical impulses over the same distances. Telegraphy started about 1830, after many previous attempts using even static electricity. Morse's patent is dated 1838; the same year saw a railway telegraph line near London; the first submarine cable was laid in 1850; and a worldwide network of telegraph lines and cables, as well as some telephone lines of considerable length (New York - Pittsburgh), existed in 1886. It is characteristic, too, that the present associations of electrical engineers were founded in the main by telegraph engineers; the Institution of Electrical Engineers was founded in 1871 as the Society of Telegraph Engineers.
It is true that the beginnings of power utilization are as old as, or older than, telegraphy (Davy's arc lamp; electrolysis; Jacobi's electric boat in Petersburg), but even after the invention of the dynamo about 1870, electrical energy was always generated near the points of use, without long connections between power station and consumer. The transmission of power did not commence until about 1890.
The techniques of telecommunications were thus free to develop without any consideration of other transmission lines. When the use of the earth as return conductor was re-invented about 1840, practically all telegraph connections and, at first, all telephone connections used the earth as return for technical and economic reasons. The mutual interference between these lines was at first not critical; later, the coupling between parallel telegraph lines was often compensated by coupling transformers or capacitors. The use of earth returns permits simplification of the exchange circuit and economy in the connecting lines; in fact, the physical limitations of space available prevented the use of two-wire circuits in urban telephone networks before the introduction of cables.
It is likely that when power transmission began, no interference was expected. The first power lines were short, and operated without earth connections. Trouble arose on the introduction of three-wire systems with multiple earthed neutral, tramways, and asymmetric or long high voltage lines. As far as telegraphy was concerned, interference by tramways or d.c. three-wire systems could generally be cured by resiting the earth used for the telegraph circuits. In telephony, interference accelerated the transition, even today [in the 1950s] not completed, from single-wire to double-wire circuits.
At the beginning of this century [the 1900s], the problems of interference with local circuits seemed to be eliminated by the use of two-wire circuits, often cabled, for urban telephone systems. For some years, it was even regarded as possible, with power systems p to 15 kV, 3-phase, to erect without risk quite long power lines and telephone lines close together, e.g., on the two sides of a street or a railway, or even on the same poles. Disappointment followed quicky: noise, acoustic shock, and dangerously high voltages in the case of joint use of poles, compelled detailed study of the effect of the electric field of high voltage lines, which is relatively strong at short distances. [...]
The situation became worse with the increasing voltage of power lines; a radical remedy was therefore applied generally, in Europe at least, namely to separate the lines as far as possible and in particular to avoid parallelism on the same road. National regulations and international agreements to enforce this step resulted in the almost complete elimination of interference from the electric field. In the United States, an opposite course was followed. Joint use of poles for telephone lines and for power lines up to 10 kV was adopted. It reduced costs of distribution in sparsely populated areas, and interference was minimised by proper arrangement of the power and telephone conductors, and by careful maintenance. Close proximity of power and telephone conductors always, however, introduces an element of risk.
About 1920, certain events occurred nearly simultaneously which were important from the point of view of interference. It is true that on the introduction of amplifiers, telephone trunk lines could be cabled, and became thus nearly immune from interference. But the urban and suburban telephone systems became more and more complicated. The two wires of a circuit were used to transmit not only the speech and ringing current, but also microphone current, dialling impulses, and other special signals used in suburban circuits. [...] Harmonics from rotary converters and especially from the newly introduced mercury arc rectifiers supplying tramways, produced rather important magnetic fields at audio-frequencies, and some bitter conflicts arose because of lack of co-operation between power and telephone engineers. Main-line railways supplied by single-phase alternating current, and short-circuits to earth in the high voltage power lines, now widely interconnected and supplied by large power stations, produced important fundamental frequency magnetic fields, interfering with even distant telephone lines. [...]
A newcomer to questions of interference is often astonished by the fact that interference occurs even with considerable separation between power lines and telecommunications lines, i.e. with very weak coupling between the circuits. The following table of rounded values of the power used in the two systems will perhaps indicate the reason:
Power Used in Power Lines and in Telecommunication Lines
400 kV, double circuit
100 kV, single circuit
15 kV, single circuit
240/400 V mains
Telegraph line: sending end, receiving end
2 W, 10-1 W
Telephone line: sending end, receiving end
10-3 W, 10-5 W
In addition, the table shows a characteristic difference between power and telecommunication circuits, namely that for telegraph and telephone lines, power levels are different at the two ends. The power system transmits a kind of raw material, electric energy; the efficiency must be high, but purity of waveform is not of primary importance. The telecommunication line transmits a finished product, a message concealed in a complicated waveform. [...] The waveform must not be distorted. An additional or suppressed impule in a telegram may falsify a letter and thus the meaning of the message. Harmonics may reduce or destroy the intelligibility of the speech, or distort music transmitted by land line for radio. The more perfect the transmission, the more sensitive it is to disturbance.
The receiving apparatus used in telephony, the combination of receiver and human ear, is extremely sensitive. A power of 100 watts, at 1000 c/s [cycles per second], would be enough to interfere simultaneously with all the telephone receivers on earth, and an energy of 1 kWh could produce uncomfortable acoustic shocks in each of those receivers. It has often been proposed that the sensitivity of the receiver should be reduced and the power at the transmitting end increased; but only exceptionally is this remedy useful. Univerally applied, it would entail intolerable waste of conducting materials and of power, and the telephone lines would become power lines.
Power interference may thus be manifest at very different power levels: as hardly perceptible parasitic current (causing noise or small distortion of telegraphic symbols), as grave disturbance of the service, as dangerous acoustic shock, or even as overvoltage endangering life or installations. A distinction is, therefore, generally made between disturbance and danger. Normal operation of the telecommunication system is only possible if, during normal operating conditions in neighbouring power installations, danger is non-existent and disturbance is sufficiently low. During a fault of short duration in the power system, a disturbance is usually tolerable, more so in a telephone than in a telegraph system because of the difficulty of recognizing an erroneous symbol in the latter. The question of what, if any, degree of danger is tolerable for a short time is one of very great difficulty. [...]
It is desirable in every case that the designers of new apparatus and new isntallations should at an early stage take into account the possibility of interference. The line engineers are immediately concerned and will probably consider the interference, but engineers developing new apparatus and new systems often forget the risk of interference. [...]
A satisfactory solution of all these problems requires a friendly co-operation between the two parties. Telephone and power lines often cross frontiers. It is obviously impossible to allow, in an international telephone line, more noise in one country than in another, and equally so to have different rules concerning the limitation of short circuit currents or purity of waveform for an international high voltage line. In consequence, it is useful to regulate internationally at least the most important questions. [...]
It is not surprising that the Comité Consultatif International Téléphonique (C.C.I.F.), soon after its formation, took up the study of protective measures against disturbance in telephone lines and against corrosion. It also established, in conjunction with the interested power organizations, a special Commission Mixte Internationale (C.M.I.) for the study of technical problems of common interest. [...]
[...] Even normal operating conditions, capacitive currents (charging currents) exist in a.c. systems along with the normal service currents; at high voltages, they may be quite large. Moreover, faults of different kinds may occur: break of a conductor, short-circuit between two or even three conductors, fault to earth of one conductor, faults to earth of several conductors at the same place or at different places, or a combination of any of these conditions. [...]
The overwhelming majority of power lines today consists of three-phase lines, with or without neutral conductor. The characteristic feature of this system is that three complete circuits are formed by three conductors only; a fourth conductor, often of smaller section, is required only when the load is out of balance. [...]
Three-phase power is produced in an alternator with three windings displaced in phase by 120° ('electrical degrees', also in the case only of a two pole alternator angular degrees). These windings are generally 'star' or 'Y', sometimes 'delta'. In the transformer, any combination of Y and delta is possible, for example, Y/Y; Y/delta; delta/delta. The transformers at the two ends of a line may have different connections. In the case of a star connection, the starpoint may be insulated or earthed directly or through an impedance of any value. [...]
[...] The advantage of transpositions is unfortunately not as large as might be expected. [...] The obvious condition that both the lines must be exactly parallel is only seldom fulfilled. It would be possible, but not easy, to adapt the distance between transpositions to variations of the separation, but only for electric or magnetic induction, not for both together. Further, [...] voltages to earth and balancing currents at certain points of the telephone line, particularly opposite the transpositions, may produce noise in combination with earth unablances of the telephone circuit. This undesirable effect decreases with the distance between transpositions, but transpositions are rather expensive. [...]
Effects of Recent Developments
Power networks, originally designed only for the transmission of electrical energy, are often used nowadays for purposes belonging properly to the sphere of telecommunication. As regards the transmission of information or signals by means of carrier frequencies above the range of audio frequencies (telephony, telemetering, telecommand, broadcasting along power wires), possible interference with other telecommunication services in the same frequency range is outside the scope of this book. But often harmonics in the audio frequency range are superimposed in order to control, simultaneously, switches at several points of a network ('ripple control' for street lighting, air raid warning, connection or disconnection of groups of consumers, etc.), the receivers being, for example, tuned relays. [...]
In rural medium-voltage networks, sometimes only two phase conductors, or one phase conductor and the neutral, are carried to distant consumers, for economic reasons. Exposures to these spurs may give rise to severe interference; moreover, the main three-phase line becomes unbalanced. Single-phase loads of this kind should as far as possible be distributed equally between the three phases, not only with regard to the load, but also to the length of the single-phase lines. [...]
The transmission of electrical energy from distant hydro-electric stations to the load centre often requires lines of considerable length with a very high voltage; in Sweden, a 380 kV line was commissioned in 1952. With lines of this kind, undesirable effects due to capacitance may occur. [...]
The purpose of any electrical telecommunication installation is to transmit information, in the broadest sense, by means of fluctuating currents. [...] The design of a transmission system is always a compromise between quality of the transmission and cost, as both increase with the bandwidth.
The quality will be impaired if there is interference of any kind, directly by room noise or electrically, for instance by induction from power lines. The reduction of quality can obviously be compensated by an increase in the signal level; byt a less obvious cure is the use of a broader band, or the transmission of more information than is essential. If a telegram is transmitted simultaneously over several frequency bands, the average of the received messages will have much less faults than a single telegram; with telephony, the correct meaning of a distorted message can often be guessed and restored from the redundant information. There is thus a close relation between the noise level and the width of the frequency band required for a certain quality of transmission, and any reduction of the noise allows better use of the telecommunication circuits.
The design of these circuits is, moreover, often affected, or even dictated, by the consideration of relatively high, and thus dangerous, voltages induced, by neighbouring power lines, during faults and sometimes even during normal operation. The induction may endanger installations and/or persons; in consequence, the conditions and limitations to be imposed will be different. Protection against this kind of interference often requires additional equipment.
Danger to Installations
[...] Exchange and subscriber's equipment not separated by transformers from the line conductors must be protected, by fuses and/or protectors [(voltage arresters)], against induced voltages and, at least with open-wire lines, atmospheric discharges. Such protection, previously required mainly for short cable circuits (especially if direct current must be transmitted) and open-wire lines, is nowadays often used in longer cable circuits, too, because of the extension of operating methods requiring d.c. transmission (through dialling; signalling over the phantom circuit). For long distance communications, carrier frequency circuits are now generally used. [...]
Danger to Persons
[...] When the internal installation is separated from the external circuit by means of a suitable transformer, the telephone operators are protected against any indueced voltage between lines and earth. [...]
It is more difficult to protect those working on the outside circuits, especially against high voltages induced only during infrequent faults on power lines. Repairs and maintenance are usually done without interruption of the telephone service; thus, protection by earthing the circuit, as practised on high voltage lines, can rarely be used. Relatively high voltages may exist permanently in cables exposed to induction by single-phase railways. [...]
In spite of the small probability of an accident with power lines, some protection for the linesmen may be thought necessary. There exists no perfect technical solution, and any kind of safety regulations can be enforced only by close supervision, because of the rare occurrence of the fault against which protection is required. [...]
Effects of Electric Currents on Human Beings
The understanding of the question of the danger of electricity for human beings is difficult for technical and physiological reasons. The effect upon the body depends, obviously, mostly upon the intensity of the current flowing through it. Where data is available, however, from accidents, only the voltage is generally known, and calculation of the current is impossible because of the unknown resistances in the circuit, particularly that of the body. [...]
A very common case is flow of the current between limbs (hand to hand, or hand to foot), so that an important part of the current passes the heart. For this case, the following scale gives an idea of the effects produced by alternating current (~50 c/s). Direct current is a little less dangerous; high frequency alternating current is practically harmless, apart from the possibility of burns.
1 mA can be perceived.
10 mA are painful; with a somewhat greater current, it becomes impossible to let go [of] a firmly grasped electrode.
100 mA may give rise, after a short time (~1 sec), to a fatal accident. The regular operation of the heart is disturbed, a 'fibrillation' starts, and the heart does not recover by itself. The fibrillation is sometimes remedied by a mechanical or electrical shock, applied quickly enough. A current of shorter duration produces fibrillation only if it coincides with an appropriate phase of the heart cycle.
One or more amperes are less dangerous if applied for a short period only; the fibrillation is perhaps cured immediately after it starts. But with a longer duration, external and even internal burns may occur. Thus currents arising from a contact with a high voltage line may produce extensive burns with death as an after-effect, but not immediate death.
If the current passes the centre of respiration, at the base of the brain (e.g. when entering the head), the respiration may be disturbed. This kind of accident is often cured by artificial respiration, sometimes maintained over a very long period. In the case of an accident, the kind of damage is seldom known; thus, with an unconscious victim, artificial respiration should always be applied immediately and maintained for hours if necessary.
Indirect injury may result from quite a trivial shock causing the victim to fall from a pole or other high structure. Even a few milliamperes, if suffered without warning, may throw the victim off balance and prove dangerous in this way.
The actual resistance of the human body is of the order of 1000 ohms, sometimes less; dry skin is a good insulator, but it does not withstand high voltages. With low contact resistances (wet hands, wet shoes, or, e.g., contact with a faulty appliance during a bath), less than 100 volts may cause death. But if the current passes only between fingers of one hand, much higher voltages are less harmful.
Telecommunication linesmen are rarely exposed to contact with a conductor at high voltage. The main possibilities of danger are: contact with a 240/400 volt circuit, [or] contact with a telecommunication circuit exposed to magnetic or electric induction. [...]
High voltage lines have often two circuits on one line of towers. For maintenance, usually only one circuit is disconnected, the other one remaining alive. With such a long and close exposure, even electrostatic induction can produce a current of more than 0.1 amperes, and thus, fatal accidents.
Telephone operators, and to some degree even telephone subscribers, are exposed to a particular kind of danger, namely 'acoustic shock'. Because of the high sensitivity of the telephone receiver, quite small currents can produce a loud impulsive noise, particularly if the diaphragm of the receiver strikes the magnetic system. The consequence is a shock, and in severe cases, nervous disturbance may follow.
During the first decades of the [twentieth] century, acoustic shocks gave rise to rather disagreeable trouble, even in Germany to strikes of telephone operators. Such shocks are much rarer to-day, for several reasons: fewer faults in the power lines, fewer exposures with small separation, and use of protective devices. [...] To-day, in the U.K. as in most countries, acoustic shock absorbers are normally provided only for the operator's receiver. But a new subscriber's set developed in the U.S.A. contains a varistor element; in Switzerland, the sets in telephone kiosks have been similarly equipped for a number of years, as well as a desk set introduced in 1954. [...]
The current induced, under normal conditions, in the telephone loop is insufficient to produce an acoustic shock; coincidence is required of a voltage between line and earth and of an abnormally large imbalance. Such an imbalance, however, arises easily when the protectors at the end of a circuit do not strike simultaneously and/or uniformly. [...]
Indirect Types of Danger
Before discussing disturbances in the usual sense, a particular type of interference must be mentioned which, although technically only disturbance, in practice constitutes a danger; i.e., interference such that a faulty message (or signal) may have dangerous consequences. Such may arise with, for instance, the train signalling circuits used by railways, circuits for traffic regulation, or fire-alarm circuits. [...]
In a certain continental manual railway signalling system, the indicators showing at the beginning of each section, are set in the 'engaged' position when the train enters the section, and in the 'free' position by an alternating current with ~25 c/s and ~50 volts, sent by the signalman at the end of the section by means of a manually operated magneto generator as soon as the train leaves the section. [...]
In the modern, more or less automatic, signalling systems, track circuits are employed to a great extent. By placing insulators between rail ends at suitable locations, the track can be broken up electrically into sections of any length desired. In any section thus formed, a circuit is established from a low voltage source (direct current or alternating current, several volts) at one end, and a relay at the other end, the insulation between the rails being good enough to ensure that a sufficient current reaches the relay to operate it and to indicate the section as 'free'. But as soon as a train enters the section, a short-circuit is made between the rails, the relay opens, and the section is indicated as 'engaged', until the last axle of the train leaves the section. [...]
Special consideration should be given to the effects of interference to railway signalling equipment in localities where automatic signalling is employed. [...]
Disturbances in Telegraph Circuits
The early inventors of telegraph apparatus did remarkably well. The morse apparatus (1837/40) is still extensively used, and even Hughes' printing telegraph (1855) is not completely obsolete. [...]
Acceleration or delay of the arriving signal may produce a misprint. [...]
Telegraph apparatus of these types [with impulses indicating the start and stop of each symbol] are known under several names: teleprinter, telex, Fernschreiber, etc. They are used today to a large extent, not only in the public communication network, but in private networks as well (railways, banks, commercial undertakings). Moreover, in many countries a service similar to the telephone service is provided, using such equipment with transmitters and receivers at the subscriber's premises, and connections via exchanges. The transmitters are worked like typewriters, and the receivers take a message automatically.
It may be mentiond that the working speed of telegraph systems is measured today by a unit called 'Baud', after Baudot. [...]
Open-wire telegraph circuits even today often use earth returns, with a single conductor or a phantom circuit superposed on a 2-wire telephone circuit. [...]
Disturbance in Telephone Circuits
[...] From the point of view of interference, the best audio-frequency circuit is a perfectly balanced cabled two-wire line terminated at both ends by balanced and screened transformers; the worst, a single open-wire line with earth return. There are numerous intermediate stages, but even the worst is not yet obsolete, and serves quite well under appropriate conditions.
Moreover, telephony is in practice not restricted to the transmission of speech; a number of singlas must also be transmitted (ringing, dialling, engaged, terminating, counting), and in subscribers' lines even the microphone current. With important trunk circuits, it may be economical to have operators permanently at each end and to transmit all this additional information orally, but in general this information is transmitted more or less automatically, very often by means of superimposed d.c. circuits. All such additional equipment tends to impair the balance of the circuit, and to increase the amount of noise. [...]
The audio-frequency currents which transmit speech in telephone circuits are often accompanied by other currents in the same frequency range coming from external sources, such as cross-talk, harmonics in the telephone power plant, induction from power lines, [and] non-linearity of circuit elements. If strong enough, these parasitic currents may impair or even destroy the intelligibility of the speech. Their ability to interfere depends on the frequency reponse of the telephone receiver and of the human ear, and in consequence, current or voltage measurements by normal meters are not a satisfactory indication of their subjective effects. [...]
[...] One would expect the limiting values to be determined by technical considerations only. Unfortunately, the limits are very much a matter of judgement (e.g. permissible noise, danger voltage) and sometimes even of bargain. Thus, different sets of regulations often give different limits for the same effect. We shall use, in the following, the limits recommended by the 'Directives' of the 'Comité Consultatif International Telephonique' (C.C.I.F.). These limits are agreed by telephone adminsitrations and power organizations, and form a useful foundation for all interference investigations, even if their technical validity is not always beyond doubt. [...]
In the case of a sustained voltage in an open aerial line due to magnetic induction, danger arises mainly when a person in contact with the general mass of the earth comes at the same time into contact with the induced wire. A voltage of 60 volts (r.m.s.) is taken as dangerous by the Directives; other rules permit 100 volts. [...]
The voltage induced by a short-circuit current may be much higher; even if it exists only during some tenths of a second, it may be dangerous for personnel and for installations; moreover, acoustic shocks may occur. The Directives allow a longitudinal induced voltage of 430 volts before the induced current is thought to be dangerous. This limit is the result of a compromise, and really not very satisfactory: to avoid danger for a person in contact with the wire the voltage should be as low as 60 volts. [...] The probability of an accident for linesmen is extremely small; in consequence, difficulties are not experienced even with 430 volts. [...]
More recently ([in] October 1954), the C.C.I.F. has admitted that for a particular group of high voltage lines, the 'high security electric lines', a longitudinal induced e.m.f. of 650 volts may be tolerated without any particular study. The main characteristics of these lines are low probability of faults, due to appropriate mechanical and electrical design and favourable geographical situation; and quick clearance of earth faults, both characteristics reducing the time during which an induced e.m.f. may occur.
These voltage limits are valid, in principle, for cable circuits as well as for open aerial lines. [...]
During normal operating conditions of the power system, single-wire telegraph and signalling circuits will be seldom disturbed, but trouble by magnetic induction due to stray zero sequence currents at fundamental frequency is not impossible. [...]
Double-wire telegraph and signalling circuits are normally exempt from disturbances, provided that the balance to earth is not too bad.
Telephone circuits, like telegraph lines, may become disturbed by fundamental frequency induction if the unbalance to earth is large, thus particularly if single-wire circuits with earth returns are concerned. The effect would be interference with ringing, dialling, or signalling. [...]
The examination of plans for an exposure may show that some of the limits set out [earlier] are infringed. With an existing exposure, a change in the power supply network, or in the equipment or mode of operation of the power or telecommunication system, may cause a great increase in what has previously been considered a tolerable interference. [...]
The separation between a new power (or telecommunication) line and existing telecommunication (or power) lines should always be as great as possible. In planning new lines, even if there are no exposures immediately arising, it is desirable, in order to avoid future difficulties, to select the route judiciously so as to leave as much space as possible for other lines which may be erected in [the] future.
Some interference effects are inversely proportional to the square of the separation, so that even a small increase is helpful; with others, for example magnetic induction, a proportionately larger increase is required to obtain a useful reduction. The additional costs involved in a change of route may be well balanced by savings on equipment required solely to reduce interference. [...]
Archiv für Elektrotechnik
Archiv der elektrischen Übertragung
Annales des Postes, Télégraphes et Téléphones
Archiv für Post und Telegraphie
Bulletin Association Suisse des Electriciens
Archiv für Technisches Messen
Bulletin Société Français des Electriciens
Bell System Technical Journal
Conférence Internationale des Grands Réseaux Electriques
C. & T.
Câbles & Transmission
Engineering Report of the Joint Subcommittee on Development
and Research: Edison Electric Institude and Bell Telephone System
The British Electrical and Allied Industries Research
Association, Technical Report
Elektrotechnik und Maschinenbau
General Electric Review
Journal of the Institution of Electrical Engineers
Journal Télégraphique; later Journal des Télécommunications
Engineering Report of the Joint Subcommittee on Development
and Research: National Electric Light Association and Bell
Österreichische Zeitschrift für Telegraphen-, Telephon-,
Funk-, und Fernsehtechnik
Österreichische Zeitschrift für Elektrizitätswirtschaft
Post Office Electrical Engineers' Journal
Proceedings [of the] Instition [of] Electrical Engineers
Revue Générale de l'Electricité
Telegraphen und Fernsprechtechnik
Technische Mitteilungen PTT
Transactions of the American Institute of Electrical Engineers
Wissenschaftliche Veröffentlichunden aus dem Siemens Konzern
Wissenschaftliche Veröffentlichunden aus den Siemens Werken
This information comes from
(Additional information from this book is located on the Power Systems page, the Overvoltage and Flashovers page, the Poles and Towers page, and the Insulator Usage page.)
The following information is excerpted from this book.
It is very common for a telephone line to be run along the same route as a power line, possibly for a few miles only or, in a few cases, for many miles. In the case of a communication line which is the property of a power company, this line may be run on the same towers as the power line. Interference with such communication circuits may be due to both electromagnetic and electrostatic action, the former producing currents which are superposed on the true speech currents, thereby setting up distortion, and the latter raising the potential of the communication circuit as a whole. In extreme cases, this raising of the potential above that of the ground may be sufficiently high to render the handling of the telephone receiver extremely dangerous, and in such cases elaborate precautions have to be taken to avoid this danger. [...]
In some cases, the electromagnetically induced current in the communication circuit may be so great as to render speech impossible. The disturbances can be kept down by means of a thorough transposition of the conductors of both the power line and the telephone line. This transposition has the effect of splitting the induced E.M.F. into a series of mutually opposing E.M.F.s, the principle being identical with that underlying the transposition of heavy laminated conductors in large alternators and transformers. In the case of a telephone line running parallel to a single-circuit power line, if the power line has no branch lines, i.e. the current is constant throughout its length, and the spacings and distances between the two circuits remain constant, then a single transposition of the conductors of the telephone line is theoretically sufficient, but with both circuits run on the same towers it may be necessary to transpose the power conductors every three or four miles, and the telephone conductors about every 500 feet. The number of transpositions necessary is governed largely by the sensitiveness of the receiving apparatus. In the case of a telephone line running parallel to a double-circuit power line, the problem is much more difficult, and it is necessary to transpose the conductors of both pwer lines in addition to those of the telephone line. A possible scheme is shown in Fig. 9.4, from which it will be seen that the scheme of tranposition is a regular one for each individual circuit, and that it is arranged that not more than one transposition will take place at any one point in the line.
Each transposition of a telephone line consists of a complete cross-over of the two conductors, while each transposition of a three-phase line consists of a twist, in a plane at right angles to the run of the line, of one-third of a revolution. Thus three transpositions are necessary to bring the phases back to their original positions. Various methods of carrying out the transpositions on both telephone and power lines are shown in Fig. 9.5.
It is to be noted that the electrostatic charging of the telephone line will also result in the flow of current, and this current also will tend to interfere with the clarity of the speech, an effect which cannot be eliminated entirely by transposition.
In extreme cases of electrostatic charging, as in single-phase electric railway systems with overhead trolley wires, it may be necessary completely to isolate the telephone apparatus from the telephone line by means of highly insulated transformers, and also to ensure the dissipation of the induced charges by means of such devices as earthed 'drainage' coils and lightning arresters. [...]
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(Additional information from this book is located on the Power Systems page, the Poles and Towers page, and the Insulator Usage page.)
The following information is excerpted from this book.
A corona is a partial discharge, and takes place at the surface of a transmission line conductor when the electrical stress -- that is, the electric field intensity (or surface potential gradient) -- of a conductor exceeds the breakdown strength of the surrounding air. [...] These manifestations are called coronas due to the similarity between them and the glow, or corona, surrounding the sun (which can only be observed during a total eclipse of the sun). [...]
Corona on transmission lines causes power loss, radio and television interference, and audible noise (in terms of buzzing, hissing, or frying sounds) in the vicinity of the line. At extra-high-voltage levels (i.e., at 345 kV and higher), the conductor itself is the major source of audible noise, radio interference, television interference, and corona loss. The audible noise is a relatively new environmental concern, and is becoming more important with increasing voltage levels. For example, for transmission lines up to 800 kV, audible noise and electric field effects have become major design factors, and have received considerable testing and study. It had been observed that the audible noise from the corona process mainly takes place in foul weather. In dry conditions, the conductors normally operate below the corona detection level, and therefore very few corona sources exist. In wet conditions, however, water drops on the conductors cause large numbers of corona discharges and a resulting burst of noise. At ultra-high-voltage levels (1000 kV and higher), such audible noise is the limiting environmental design factor.
Manifestations of Corona
[...] Perhaps [the] most serious manifestation of the corona is the electrical effect that causes radio interference (RI) and/or television interference (TVI). The [corona] avalanches, being electrons in motion, actually constitute electric currents, and therefore produce both magnetic and electrostatic fields in the vicinity. Since they are formed very suddenly and have short duration, these magnetic and electrostatic fields can induce high-frequency voltage pulses in nearby radio (or television) antennas, and thus may cause RI (or TVI). [...]
[...] The frequency spectrum of such [corona] pulses is so large that it can include a significant portion of the radio frequency band, which extends from 3 kHz to 30,000 MHz. Therefore, the term radio noise is a general term that includes the terms radio interference and television interference.
The radio interference (also called the radio influence) is a noise type that occurs in the AM radio reception, including the standard broadcast band from 0.5 to 1.6 MHz. It does not take place in the FM band. [...]
In general, power line RN sources disturbing television reception are due to noncorona sources. Souch power line interference in the VHF (30 - 300 MHz) and UHF (300 - 3000 MHz) bands is almost always caused by sparking.
[...] It has been shown that the broadband component of random noise generated by corona may extend to frequencies well beyond the sonic range. The noise manifests itself as a sizzle, crackle, or hiss. Additionally, corona creates low-frequency pure tones (hum), basically 120 and 240 Hz, which are cause by the movement of the space charge surrounding the conductor. [...]
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