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INTERFERENCE PROBLEMS WITH 25 KV AC TRACTION 8.0 INTRODUCTION : Interference to line-side cables from an adjacent ac traction system is a particular case of the more general problem of interference between power and telecommunication lines. This interference may arise either from the ele...

INTERFERENCE PROBLEMS WITH 25 KV AC TRACTION 8.0 INTRODUCTION : Interference to line-side cables from an adjacent ac traction system is a particular case of the more general problem of interference between power and telecommunication lines. This interference may arise either from the electric field or from the magnetic field. To appreciate how even with considerable separation between power lines and telecommunication lines i.e with very weak coupling between the circuits interference occurs, one has to realize the power used in the two systems. Power used in transmission lines and telecommunications lines : 400 kV 109 W 132 kV lines 108 W 33 kV lines 5 x 106 W 11 kV lines 106 W 400/240 V mains 104 W Telephone line - Sending end 10-3 W - Receiving end 10-5 W In a power system which transmits a kind of raw material i.e electric energy, the efficiency must be high but purity of wave form is not of primary importance. The telecom line transmits a finished product, a message concealed in a complicated wave form, therefore, the waveform must not get distorted, power losses in telecom line are not important. Additional or suppressed impulse in a telegram may falsify a letter and then the meaning of a message. Harmonies 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 does it become to disturbance. Power interference may thus manifest at very different power levels – as hardly perceptible noise or as grave disturbance of the service, as dangerous acoustic shock or even as over voltage endangering life or installations. Hence, a distinction is generally made between disturbance and danger. Normal operation of telecommunication system is possible if danger is non existent and disturbance is sufficiently low with normal operating conditions obtainable in the neighbouring power installations. During a fault of short duration in the power system the disturbance in a telephone system is usually tolerable. The question arises as to what should be the tolerable limit of danger. 141  To combat interference, co-ordinated action is called for from power and telephone engineers because both installations have to serve the same people who have to pay for the protective measures. Hence, technically and economically best remedies are to be adopted even when the two installations are under the control of separate administrations. General rules are laid down for all new installations so as to exclude the possibility of interference. 8.1 CCIT DIRECTIVES: The International Telegraph and Telephone consultative committee has recommended the limits of permissible induced voltage under different conditions of operation. 8.1.1 Permissible Voltage Levels in the Case of Normal Operation of the Inducing Line. To avoid danger, it is recommended that the permissible continuous induced voltage be limited to 60 volts rms. This applies to screened or unscreened cables or open wire line to which access is required for work operations by staff. Under conditions of particular difficulty, the permissible voltage limit may be raised to 150 volts rms induced on conductors of a cable or an open wire line, provided special precautions are taken. These special precautions may include. · The issue of special instructions to personnel likely to have access to circuits exposed to voltage in excess of 60 volts rms so that special work measures can be applied. · The marking of accessible parts of the installations or equipment with warnings. 8.1.2 Permissible voltage levels in the case of a fault on the inducing line. Except for the cases described in the following paragraphs of this section, it is recommended that the permissible voltage induced on cable conductors or open wire telecommunications lines be limited to : a) 430 volts rms during a fault on a nearby inducing line that is constructed to usually accepted technical standards. b) 650 volts rms during a fault on a nearby high-reliability power line. c) 1000 volts peak during a contact to earth of one wire of a nearby dc power or electrified railway line. The permissible induced voltage may be increased for conductors in cables with earthed metallic sheath or screen and that are terminated in isolating transformers at both ends, or at one end with the other end connected through a low resistance to earth or, to a metallic cable sheath or screen, or if all the cable conductors re fitted with lightning protectors at their ends. In such cases, the permissible values are i) For cables tested for breakdown strength between conductors and sheath or screen after installation: an rms value equal to 60% of the test voltage if tested with dc or, 85% of the test voltage if tested with ac. 142 ii) For cables where the above tests are not made, an rms value equal to 60% of the lowest dc voltage to 85% of the lowest ac voltage used in factory tests to ensure the breakdown strength between conductors and sheath or screen unless there is reason to fear that the laying and jointing operations have caused any appreciable reduction in the breakdown point. In such cases, special studies should be made to select the method for determining the permissible limit. Where only some of the cable pairs are terminated satisfying the above conditions, the voltage limit indicated in (i) or (ii) is permitted on such cable pairs provided that the dielectric strength from other parts in the cable is sufficient to avoid breakdown. The isolating transformers and other line apparatus should have a dielectric strength equivalent to or greater than available on the cable conductors, unless lightning protectors are used. Experience shows that dangerous levels of induced voltage are unlikely on cable conductors where the above conditions are met and where faults on medium voltage inducing lines are involved and where protective devices are used. Where the permissible induced voltage on cable conductors is increased above the permissible levels for open wire lines, it is desirable to consider safety precautions when work is carried out on these cables and to ensure that equipment connected to the line can withstand the resultant common mode voltages and currents. Allowance may need to be made when considering permitted induced voltages induced into Cables carrying significant telecommunications voltages (e.g. power feeding systems). 8.1.3 Permissible capacitively – coupled current : In cases of capacitive coupling, a resulting current through a contact between a conductor and earth of other metallic structure, upto 10 mA is permissible. 8.2 COUPLING BETWEEN CIRCUITS: The coupling between two circuits may be conductive or alternately due to electric or the magnetic field. These are distinguished as conduction, electrostatic induction and electromagnetic induction. Even when all the three kinds of couplings occur simultaneously, usually one of them will usually prevail. 8.3 CONDUCTIVE COUPLING: Conductive coupling is present when two circuits I and II have a common branch (See Fig.8.1). If the common branch is sufficiently well defined the distribution of the current and the effect produced in II may be calculated. If they are coupled by a common resistance “r”, the parasitic current I in circuit II is given by I = Er_____ RW + Rr + Wr Generally, Wr >> R >> r because I is a power circuit and II a telecommunication circuit. It is approximately equal to I = Fr/RW. The effect in II is same as if a parasitic voltage 143 e = Er / R were introduced in the voltage produced on r without the circuit II. This means that reaction produced on I by II is negligible. Fig. 8.1 Conductive coupling occurs often between two circuits, using to some extent, the earth as conductor. Very weak couplings of this kind exists, obviously between all circuits because of imperfect insulation. In practice, conductive coupling exists when electrified railways use rails as a return conductor. In telephony, coupling of this kind may arise in all circuits in which earth is used as an auxiliary or third conductor. Especially all circuits using common batteries. In all such cases disturbances may occur, if the earth connection of a telecommunication circuit is near enough to an earthing point on the power system. In practice interference by conductive coupling between lines can be neglected. Conductive coupling is present if the interference can be suppressed by resting the earthing connections or by replacing the earth return by a metallic return conductor well away from the existing one. Electric or magnetic coupling is present if the interference can be suppressed by a displacement of the line well away from the inducing line without any alteration of the earthing connections. 8.4 ELECTROSTATIC INDUCTION : Electric induction occurs due to capacitive coupling. Discussion hereunder is confined to the systems of lines which are long in relation to all dimensions perpendicular to the length (distance, diameter), the lines are assumed to be parallel with the surface of the earth and with each other. Consider line I at voltage U1 with the frequency f Line4 is insulated (see fig. 8.2). The mutual capacitance K14 and the earth capacitance K40 are in the series. The voltage U 1 produces a charging current. U1jw - ___K14K40__ = U1 JWK14 as K40 >>K14 K14 + K40 Voltage on line 4 is U4 = U1 K14 = U1 K14/K40 K14 + K40 144  If line 4 is earthed the charging current from 1 to 4 and thence to earth is given by U1jw K14. This is proportional to the frequency and like K14 to the length of parallelism. Thus the voltage electrostatically induced in an insulated line does not depend on frequency and length of exposure whereas the current to earth from an earthed line is proportional both to frequency and length. 1 Fig. 8.2 The earth and mutual capacitance are calculated from the dimensions of the lines and the relations between voltage and charge of conductors established. The effect of earth is taken into account by means of Kelvins method of electrostatic images. The image of each conductor in the earth’s surface has the same charge as the actual conductor but with opposite sign. The induced voltage on overhead bare conductors running parallel to a 25 kV contact wire and insulated throughout can be calculated by the formula. V = E x ___bc_____ 4 a + b + c where V = induced voltage to earth E = contact wire voltage i.e 25 kV. a = horizontal spacing between the contact wire and the overhead conductor. b = height of inducing line above ground. c = height of overhead conductor. For the usual heights of contact wire and the overhead communication lines, the approximate induced voltages in the latter for different spacings are indicated below. Separation (m) Induced voltage with 25 kV system 3 4600 6 2600 10 1440 145 It is seen that even if the lines are within 10 metre of contact wire they are subjected to induced voltages exceeding 1000. This would lead to a continuous discharge across the spark gaps with which telephone circuits are normally equipped and which have a nominal break down voltage of about 100V dc. When such bare conductors situated in the electric field are earthed through a person’s body the resulting discharge current is proportional to the inducing voltage and the capacitance or length of parallelism. If the parallelism reaches around 10 km the current could reach dangerous proportions. Hence, it is not possible to contemplate normal operation of circuits with bare overhead conductors over any significant length alongside an ac electrified railway. Electrostatic effects decrease very rapidly when the separation between the inducing line and the line receiving induced emf is increased. If separation is increased to 40m the voltage in conductors placed parallel to 25 kV contact wire hardly exceeds 150V rms and the drawback of continuous discharge across the spark gaps is immediately removed. The CCITT gives the following formula (which is more conservative than the one cited above) to arrive at the minimum separation between contact wire and the communication line to limit the induced voltage to 300 Volts. The minimum spacing is given by a = 1/3 E Where E is the contact wire voltage. For 25 kV system this works out to 53 m. In order to calculate electric induction due to an oblique exposure the distance a is replaced by the geometrical mean a1a2 between the distances at the ends in the formula. The calculation takes into account ideal conditions i.e lines parallel to earth’s surface and mostly to one another, free from additional capacitances and pure sinusoidal alternating current. In practice these conditions are never fulfilled. Line sags reduces average height and additional capacitances occur between wires and poles including capacitance of insulators. Further, roughness of earth’s surface, vegetation, buildings, etc. result in reducing the effective height of conductors. The combined effect of all these is to increase capacitances to earth by 20% and mutual capacitances between conductors get reduced. Hence, measured values of electric induced open circuit voltages are usually smaller than the calculated values. However, high harmonics, even with a small amplitude may increase considerably, the electric induced short circuit current. 146 8.5 ELECTROMAGNETIC INDUCTION The emf induced in an overhead line or a cable running parallel to an electrified railway is given by the formula :- e = 2π fm L 1 Kr Kc Km where f = Supply frequency M = Mutual inductance per unit length L = Length of parallelism I = Catenary current Kr = a reduction factor which takes into account the presence of a current flown in the rails the effects of which partly compensate for those due to current flowing in the contact system. Kc = a reduction factor when the line in which currents are induced is in a metal sheathed cable. Km = a reduction factor to take into account the existence near the railway or in the vicinity of the line receiving induced currents, earthed metallic pipework carrying an appreciable induced current. It is seen that an appreciable reduction in interference is obtained from the screening effect of earthed conductors such as cable sheaths, metal pipes, earth wires and finally the rails themselves. These reduction factors are also known as screening factors. The mutual inductance M per unit length is a complex factor which depends on the separation of the inducing and induced line (a) and the soil conductively (s) and the frequencies of inducing current. The function M is in the form M = f (a s f) 147  Fig. 8.3 Fig. 8.3 contains a family of curves, for the mutual impedance M per kilometer Vs distance of separation of the lines, for various values of parameter. At a little distance from the track Kt can be approximated to I – It /Ic’ where It is the total induced current flowing in the rails and Ic the catenary current. 148 Reduction factor Kc represents the relationship of the emf developed between a cable conductor and its sheath to the emf which appears between an insulated conductor and earth in the absence of sheath. This reduction factor depends on character and dimensions of sheath and armouring and magnetic properties of the metal used for armouring. This improves as the frequency of the inducing current increases. For cables located at a little distance from the track Kc is smaller under heavy inductive conditions like short circuits than under less severe conditions like normal operation. Screening of cable sheaths can be improved by reducing the dc resistance of sheath and by increasing the mutual inductance between sheath and wires. Aluminium sheath has resistance of about 1/7th of similar sized lead sheath. As against screening factor of 0.8 for lead sheathed cable for A1 sheathed cable it is 0.16. If the inductance is increased by steel tape armouring. Screening factor gets reduced to 0.025 and 0.2 respectively. The presence of metal work electrically connected to earth near the track, or the circuit subjected to induction exerts a reducing effect, which may be considerable especially when several cables are buried in same trench as they shield each other to certain extent. 8.6 RAIL CURRENTS The rails form a conductor with rather uncommon qualities. The resistance is very small and the leakance very large. The attenuation is so large that the return current is diverted completely to earth after about few kilometers and with higher frequencies even sooner. If the distance between feeding and loading points is large enough and if the track is homogenous, the rail current divides equally in both directions at both points without any preference for the ‘inside’ direction. Part of the current penetrates deep into the earth and some leaves the earth to find a path in cable sheaths, metal pipes and other similar conductor parallel to the track. Near the feeding point the whole of the current returns to the earthed end of the traction transformer winding through the rail/earth mat of the feeding point. If the return current could be wholly retained in the rails the inducing effect on an adjacent telephone line would be that from a comparatively narrow loop formed y the overhead wires and rails and would be relatively small. In practice the load current rapidly leaves the rails for earth as shown in fig. 8.4 (b) which is applicable to an electrically long section where the rails continue for some distance on either side of the section. The rails will however themselves be subjected to an induced voltage from the overhead wire which will cause a current to flow in the rails virtually in the opposite direction to the contact wire current as shown in fig. 8.4 (c). In the centre of a long section the value of this current will be uniform and is equal to the induced voltage divided by the series impedence of the rails. 149  Typical Rail Current Distribution Fig. 8.4 It is usually about 0.4 to 0.7 of the overhead line current. Combining this induced current with the load current in rails, the total distribution of current in rails is shown in fig.8.4(d). The resistivity of steel rails is rather high but the cross section is so large that the resistance mainly depends upon the resistance of the joints between rails. The dc resistance of welded rails is about 0.03 ohm/km. The screening factor becomes less than 0.6 if the track is well maintained. With imperfect fish plates there may be no screening at all at fundamental frequency even though certain amount of screening may exists even at this condition. Experience shows even bad fish plates are welded together by a coherer effect (high resistance at low voltages and negligible resistance at higher voltages) and thus the resistance across bad fish plates falls as soon as the induced voltages in rails becomes high enough. The screening factor then becomes dependent upon the primary current and may be good enough at short circuit currents even if bad at normal service current. This behaviour of the screening effect often makes special bonds unnecessary. 150 8.7 PROTECTIVE MEASURES: Protective measures can be applied either in the low current communication circuits affected or at source (in the traction supply) or in both systems. Protective measures in telecommunication circuits consists of insertion of isolating transformers at intervals to limit the longitudinal build up of emf. balancing of the circuits and equipment and increasing the signal to noise ratio. Additional protective devices such as discharge tubes, drainage coils are also used. The existing line side open – wire aerial communication lines will have to be abandoned because of intolerable induced voltages at fundamental frequency. The induced harmonic voltage is also dangerous as sophomoric voltages will be high. Hence, by use of cables the induced voltages can be reduced by the appropriate screening factors of different types of cables. A cable with a break down test voltage of 2000 volt should be able to withstand the voltages induced by a short circuit. All the ac circuits must be terminated by transformers. The terminating transformers only allow currents produced by differences of voltage (transverse voltage) to pass out to the external apparatus. As a result, in general the noise so produced is hardly noticeable and is not troublesome if the cable is only subjected to moderate induction and if there are not too many harmonics in the traction current. Exchange and subscriber’s equipment not separated by transformers from the line conductors must be protected by fuses and voltage arrestors or protectors against induced voltages. DC circuits should be replaced by ac or impulse circuits. Special measures are required for the protection of the operators, the main precaution being to avoid any possibility of a simultaneous contact with the apparatus and with earth. Anti induction measures include periodic transposition of the positions of conductors in a circuit at their supports, to produce compensation along the length of the line between the emf’s induced between the conductor themselves. Even under favourable conditions (almost perfect parallelism between the inducing circuit and the circuits subjected to induction and regular spacing of the supports at which transpositions are made) and even if the distances between transposing points are small (less than 1 km) perfect compensation is not obtained. In an overhead line at a short distance from the track the effects of electrostatic induction from the contact wire are added to the inductive effects due to traction current. Even if a bare overhead line is located at 300 m from a railway line with a parallelism of 4 to 5 km. it is likely to be affected by noise which creates difficulties for conversation. Cabling the overhead communication circuits is an effective means of reducing interference. The screening is improved by reducing the resistance of cable sheath i.e by conductivity screening. This is achieved either by use of aluminium sheath or by addition of copper wires under the lead sheath. The cable sheath is also effectively earthed at intervals of about a km. Secondly, the magnetic coupling between sheath and conductors is increased by provision of steel tape armouring over the conducting sheath. The screening factor with such cables coupled with that provided by the rails and buried metal pipes, etc. can be around 0.06. Further improvement would be possible if the cable circuits are laid far away from the electrified sections. 151 8.8 SUPPRESSION OF INTERFERENCE AT SOURCE: Though the above measures are generally adequate for protection of communication circuits, there might be special cases where highly sensitive long distance communication circuits or dense urban communication networks exists either parallel to the track or in its vicinity. When such a railway line is taken up for electrification, the above remedial measures might prove either too expensive or inadequate. In such cases suppression of interference at source may have to be resorted to. Considerable reduction in the interference effects of electrified railways can be obtained by the use of booster transformers. These transformers have a 1.1 ratio with the primary winding connected in series with the contact wire and the secondary is connected either to the rails (as in Scandinavian countries) or to a return conductor as per general practice followed elsewhere. The return conductor arrangement is more favourable for reducing telephone interference. 8.9 RAIL CONNECTED BOOSTER TRANSFORMERS: In this system the secondary winding is connected to the rails on either side of insulated rail joints and the current in the rails is thereby increased to an extent that for a booster transformer spacing of 2.66 km, only less than 5% of return current flows in the earth. The effective area of the inducing loop is much reduced and the interference effects are correspondingly reduced. The screening effect of this system depends on the spacing of the booster transformers and the propagation coefficient of rail-earth return circuit which in turn depends on frequency and on the insulation of rails to earth. With a spacing of 2.66km the theoretical screening factor is taken as 0.05 at 50 hz. As the rail screening factor without boosters would be 0.5, the improvement ratio due to the provision of boosters is 10:1. The reduction at higher frequencies is less. The disadvantage of rail-connected booster system is that a considerable voltage can exist across the insulated rail joints, endangering the safety of maintenance personnel quite apart from the difficulty in proper maintenance of insulated joints. The screening factor for harmonic currents is lower than for fundamental current as the series impedance of rails is greater for harmonics than for fundamental and therefore larger proportion of harmonics escape into earth. Hence, this method is not satisfactory for the elimination of noise due to harmonics. 8.10 BOOSTER TRANSFORMERS WITH RETURN CONDUCTOR: In this system the secondary windings are connected in series with a return conductor which is connected to the rails midway between booster transformers. The return current flows almost entirely in the return conductor and very little in the earth or rails except in section where the load current is being taken. The return conductor is erected on the overhead masts carrying the catenary and the inducing loop formed by the traction and return currents is therefore of small width. 152  Fig. 8.5 With return conductor system two effects need to be considered. The first is induction from through currents i.e those currents taken by trains well beyond the parallelism and confined wholly to the contact wire and return conductor. The second is induction due to train in section ffect i.e where the train is in a BT cell within the parallelism and the current is flowing alongwith the rails. For telecommunication lines well removed from railways the first effect i.e direct induction from the contact wire return conductor loop (which are rarely equidistant from the cable) can be ignored However, the rails are not and cannot be symmetrically disposed with respect to contact wire and return conductors and hence an induced current flows in rails which causes induction in telecom lines. Rail screening factor of remote cables would be about 0.025 and is independent of frequency. This represents an improvement of 20:1. Considering the second effect, maximum induced voltage occurs when a train is close to a booster transformer in which case the length between the train and rail return conductor connection may be treated as being equivalent to a normal feeder section without booster transformer for which a screening factor of 0.5 at all frequencies would be appropriate provided the parallelism extends for about 3 km on either side of the equivalent section. 8.11 SALIENT FEATURES OF BOOSTER TRANSFORMER SYSTEM: As the primaries of boosters are connected in series with the contact system with voltages of 336 V (for 100k VA boosters spaced at 2.66 km) they have to be designed to withstand 25kV. Since they are in series with the OHE they must be capable of withstanding the mechanical and thermal stresses caused by system short circuits. 153 Magnetizing current is required to flow in the primary to induce secondary voltage to enable secondary current to flow in the loop. This magnetizing current which flows in primary and (not in secondary) is superimposed on the load current. To limit the uncompensated current in the OHE to the minimum the exciting current has to be kept as low as possible. The harmonic current of the exciting current is to be minimum as this uncompensated current would create noise in telecommunication lines. To reduce the harmonic component of excitation current, the flux density in the core has to be kept low so that it lies on the linear portion of BH curve for the maximum voltage that may develop across the primary/secondary winding of the BT at 700 A (assumed maximum catenary current) Cold rolled grain oriented steel is used with maximum flux density of 0.7 T to contain the exciting current to 0.2 to 3% of full load current and the harmonics at 10 to 15% of the exciting current. A high exciting impedance at harmonic frequencies is required to obtain compensation of harmonics induced voltages by currents in the return conductor. The exciting impedance at 800 Hz should not be less than 450 Ohms. Since several booster transformers are in series and they tend to add to the OHE impedance, the leakage impedance of boosters is to be kept as low as possible. 8.12 LIMITATIONS OF BOOSTER TRANSFORMERS: There is always a residual induced voltage in communication conductors due to proximity of other conductors including rails carrying induced currents etc. For a train in section only partial compensation is obtained. Whenever the booster primary is shorted by the pantograph of the locomotive while negotiating the BT overlap span, there will be no compensation in that cell even for through currents for that duration, though it is very small. During system short circuits due to saturation of core, compensation tends to be less than normal. Both even and odd harmonics are introduced in the exciting current flowing in the OHE. 8.13 DRAWBACKS OF BOOSTER TRANSFORMERS: The initial cost of the system of booster transformers and return conductors is substantial. The impedance of the OHE is increased by more than 50% thereby increasing the voltage drop and decreasing the permissible loading of the section, necessitating closer spacing of substations. There will also be additional loss of energy due to additional impedance of the booster transformers and return conductors. 8.14 AT SYSTEM : With increased system loading due to introduction of high speed passenger trains and heavy freight trains, the 2 x 25kV AT system has certain advantages over the conventional system apart from suppression of induction at source. BT system will be found wanting due to problems like heavy arcing at overlaps and higher voltage drops due to heavier currents. In the 2 x 25 kV AT system, electric power from traction sub station at single phase 50 kV is transmitted along the track between the OHE and a separate feeder wire supported on suitable insulators. At intervals of about 10 to 15kms along the track, centre tapped single phase auto-transformers are installed and connected between the OHE and the feeder wire, with the midpoint of these ATs being connected to the rails. Both the OHE and feeder wire will be at 25kV with respect to the rail but the actual voltage of transmission will be 50kV. The current from the substation flows between the OHE and the feeder wire to the two ATs on either side of the load. 154 These two ATs feed the current required for the load connected between the OHE and the rail. Due to equal and opposite currents flowing in the OHE and the feeder the magnetic fields produced will neutralize causing no interference except the AT cell effect. Voltage regulation on the traction system is also better with the AT system. ********* 155