Power and Communication Line Interferrence (2)

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` When a power and a communication line are operated in proximity, the

power circuits may produce certain conductive or inductive effects, which

may interfere with the normal operation of the communication circuit .

` Interference is an effect arising from the characteristics and interrelation of

power and communication systems of such character and magnitude aswould prevent the communication system from rendering service

satisfactorily and economically if methods of coordination were not applied.

` The electrical coordination problem arises principally because two distinct

types of circuits or systems are employed, namely

(1) Power System (2) Communication System

Introduction



` Another important consideration arises from the fact that the user of

electrical energy is generally also a user of electrical communication. for

example power lines for delivering electricity to homes and factories are

paralleled by telephone circuits required to give electrical communication for the same places.

` communication when subjected to excessive voltages, provide protection

but in doing so may render the circuits inoperative for communication

purposes.



` Extraneous voltages in communication circuits from power circuits are

caused by :

(i)Conduction is an important factor where circuits of the two types are

located to each other

` Under these conditions a

conductor failure or an

extraneous voltage wire

may impress extra voltage

on communication line



(ii)Electric induction ± A section of line with power conductor P energized

from a single phase grounded source and with communication conductors

c1 and c2.

as shown. if the communication circuit consists of two wires

separated even a short distance ,different potentials are induced onthem.



` 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 asa 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.

` 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:



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. 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 [cyclesper 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 onlyexceptionally is this remedy useful.



` This is a noise type that occurs in the AM radio reception and does not take

place in the FM band.

` The RI for both ac and dc EHV lines depends on the field strength Emax and

defined as

RI = CE2nmax dB

Where C is constant. Exponent n= 5 to 7 in a fair weather and between 1.5

and 3.5 in rain.

` For the same voltage gradient, RI level increases with the voltage as

RI = Kr 2 dB



The figure above shows a 3-phasse line with communication line conductors d

and e. If the balanced current through the power conductors are Ia, Ib, Ic then

Ia + Ib + Ic = 0

The flux linkage to conductor d due to a will be

ad = 2 * 10-7 Ia ln (D/Dad)

Where D is infinity. The flux linkage to conductor e due to current in conductor awill be ae = 2 * 10-7 Ia ln (D/Dae)

The mutual flux linkage between conductor d and e due to current Iaad - ae = 2 * 10-7 Ia ln (Dae/Dad)

The mutual inductance Ma between conductor a and loop de will be

Ma = (ad - ae ) / Ia = 2 * 10-7 ln (Dae/Dad) H/m



Similarly,

Mb = 2 * 10-7 ln (Dbe/Dbd) H/m

and

Mc = 2 * 10-7 ln (Dce/Dcd) H/m

Since the currents are displaced by 120o, these mutual inductances have also

displacement of 120o. The net mutual inductance M will be,

M = M a + M b + M c

If I is the current in the power conductor and frequency is f, voltage induced in

communication conductors d and e will be

V = jMI volt/m

The value of M will be small if the distance between power line and

communication line is large.

The maximum critical disruptive voltage occurs when the radius of the

conductor is (d/e).



Effects in Telecommunication Circuits Arising from Interference:Peculiarities and Sensitivity of these Circuits

Introduction` The purpose of any electrical telecommunication installation is to transmit

information, in the broadest sense, by means of fluctuating currents. Thedesign 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 byroom noise or electrically, for instance by induction from power lines. The

reduction of quality can obviously be compensated by an increase in thesignal level;

` Danger to InstallationsExchange and subscriber's equipment not separated by transformers fromthe line conductors must be protected, by fuses and/or protectors [(voltagearresters)], against induced voltages and, at least with open-wire lines,

atmospheric discharges.



` Danger to Persons

` When the internal installation is separated from the external circuit bymeans of a suitable transformer, the telephone operators are protected

against any induced 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

` Acoustic Shocks

` 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.



` Effects to be Considered for Different Types of Power Lines andTelecommunication Lines: Limits and Approximate Formulae

` One would expect the limiting values to be determined by technicalconsiderations 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 thesame 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 administrations and power organizations, and form a useful foundation for all interference

investigations, even if their technical validity is not always beyond doubt.` Reduction of Interference` The separation between a new power (or telecommunication) line and

existing telecommunication (or power) lines should always be as great aspossible. In planning new lines, even if there are no exposures immediatelyarising, 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 auseful reduction.

` 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 spacing and distances between the two circuits remainconstant, then a single transposition of the conductors of the telephone lineis theoretically sufficient, but with both circuits run on the same towers itmay 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 thesensitiveness of the receiving apparatus. In the case of a telephone linerunning parallel to a double-circuit power line, the problem is much moredifficult, and it is necessary to transpose the conductors of both power linesin addition to those of the telephone line. it will be seen that the scheme of transposition is a regular one for each individual circuit, and that it isarranged 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.



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Power and Communication Line Interferrence (2)

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