US2212963A - Communication and power system - Google Patents

Description

Aug. 27, 1940. H. w. WAHLQUIST COMMUNICATION AND POWER SYSTEM Filed April 5, 1938 INVENTOR.

6 M Z 5 W w Z Z d 3 Y 41/3 ATTORNEY,

Patented Aug. 27, 1940 UNITED STATES amass PATENT @FFICE 19 Claims.

This invention relates to communication and power systems, such as rural distribution systems in which the power transmitted is small and in which telephone and power systems parallel each other so that the latter, by capacity and inductive effects, interferes with the transmission of speech over the telephone line. The invention is also applicable to systems in which power lines interfere with telegraph lines operating with alternating current.

This application is a continuation-in-part of rlny application Serial No. 152,899, filed July 10,

When a telephone line runs parallel to a power line, the magnetic and electric fields of the power line induce currents in the paralleling telephone line. These induced currents consist of the fundamental frequency (usually 60 cycles) and. the harmonics of the fundamental frequency, such as 180, 300, 420 cycles, etc.

The chief noise producing factors are frequently the higher odd harmonics, the odd harmonics being those with a frequency 3, 5, '7, etc., times that of the basic frequency of the power lines, which is usually 60 cycles. The ones that are chiefly objectionable are those from 9 upwards. Much can, therefore, be accomplished by reducing the higher harmonics in the power line.

However, there still remains induced current of 30 60 cycles and also of the lower harmonics which,

although they have frequencies below that required for the efficient transmission of the ordinary voice (500 upwards), are well within the audible range, and, if not largely eliminated, interfere with voice transmission. Although the telephone receiver has a relatively low response to the fundamental 60-cycle current, the presence of considerable (SO-cycle current in the receiver is disturbing. Such current tends to distort speech transmission rather than drown out the speech signals by the simple production of noise. It follows that substantially complete elimination of interference with voice transmission can be accomplished only by first reducing the higher harmonics in the power line and then eliminating the effect of low frequency current either by acoustical means or by by-passing the GO-cycle and lower harmonic currents induced in the telephone line, so that they do not flow through the receiver, or at least flow therethrough to a much lesser extent than they would do normally.

Telephone and power systems vary greatly as do the effects of one on the other, so that in 5'5 some cases it may be sufiicient either to reduce the higher harmonics in the power line or to reduce the fiow of induced GO-cycle and lower harmonic currents through the receiver.

Harmonics in a power line may originate from either the power supply system, such as a direct coupled generator or a transformer, or from particular types of loads, such as a rectifier, connected across the line. In the first case the harmonic currents fiow away from the supply system and in the second case toward the supply system as well as in the opposite direction, if the rectifier or the like is not at the end of the line.

The present application is chiefly concerned with power lines fed from a single supply source, as distinguished from a power network fed from a plurality of sources.

The application of my improvements with Which I am chiefly concerned is to rural power lines which have considerable capacitance and supply loads which are largely resistance, so that 20 the line and loads combined have a capacitance in excess of inductance for a frequency of about 500 cycles, usually considerably more. As the supply source is always inductive it follows that at some frequency the inductance of the supply 25 source and the capacitance of the line become equal and resonance occurs. If such resonant point is adjacent one of the important harmonics a large current may flow.

Under these conditions a large increase occurs in the magnitude of the voltage impressed on the rural line at the frequencies in the neighborhood of the resonant frequency, and there is a consequent large increase in the harmonic charging current in the line. In the majority of cases a substantial portion of the harmonic line current returns in the earth, and, due to the high coupling of this earth return circuit with a paralleling telephone, the induction may be severe. In situations of this character a suitable capacitor connected across the transformer on the rural line side will destroy the resonant condition at the higher harmonic frequency, and, in combination with the transformer and supply line impedances, reduce other high frequency voltages and currents in the rural line.

To reduce harmonics it has frequently been the practice to connect across the line a series of shunts tuned to 660, 780, 1020, 1140, 1380 and 1500 cycles, these being the principal harmonics to be taken care of in the majority of situations.

I have found that, instead of several tuned shunts, one shunt only is necessary, provided that its characteristics bear the proper relationship to the impedance of the power supply system.

ductance, a normal resonance at a frequency be- Where a plurality of tuned shunts have been used, the designers have considered merely the fact that to obtain resonance at, say, 1020 cycles, the product LC (inductancexcapacity) is fixed, and, hence, as the expensive part of the shunt is the condenser, they have made the condenser as' small as practicable in view of the current to be passed through the shunt.

My method of eliminating harmonics does not depend on tuning the shunt to any particular harmonic frequency, but rather on lowering the frequency at which the power supply system and shunt circuit resonates from a frequency above 500 cycles, say 1500, to a frequency below 5 preferably a non-harmonic frequency, such as 250. This latter relationship involvesa totally new ratio, viz: shunt capacity to power supply system inductance, instead of the shunt capacity to shunt inductance ratio, as has been used here to L20 as well as resonance at 250 cycles corresponding to (L1-FLz)C. Using the old rule of making the'condenser as small as practicable, therewould be two high resonance points. This follows from the fact thatthe product (L1+L2)C will be only a little greater than LzC and hence correspond to a resonance point at, say, 900 cycles,

instead ,of1'1020 for L1G, taken alone.

, It is better to have inductance as well as capacitance in the shunt to give lower impedance for the higher harmonics than is possible where Lg=0 and at the same time retain aresonance point around 250 cycles for (L1+L2)C.

Toobtain the desirable ratio between the inductance ,of the power supply system and the capacity of the shunt, the (L1+L2)C' product, where L1 is. the inductance of the supply system and L2 is the inductanceof the shunt in millihenries and C is the capaity of the shunt in microfarads, should preferably beabout 400 (resonance at 250) andnot greatly less than 125 (resonance at 450) ,In order to enable a single shuntto cut down all the higher harmonics L2 should be smallor than L1, preferably much smaller. That is a distinguishing feature of my invention, as heretoforeLz hasbeen larger than L1.

'The use. of a capacitor and reactor in combination sodesigned .and connected across a power line'supplied from a supply system of high inductance and having, by virtue of such intween 500 and 2 000 cycles, may reduce the reso. nant frequency of the power line below 500 cycles. A capacitor. designed as above described destroys the natural resonance between the line and power supply system at the higher harmonics. It is desirable to select a capacitor having such capacity with respect to the inductance of the power supply system that it forms a shunt circuit resonant to a frequency which does not corre spend with that of any important harmonic in the power circuit. Forexample, the important harmonies are 180, 300, .420, 'etc. By setting the capacitor to resonate at 250 there is no tendency toincrease .Inarkedly the 180 or 300 harmonics.

some increase occurs, but the incr ase is far less than it would he were the capacitor to be resonant to either 180 or 300 cycles. At all events, there is a great reduction in the telephone influence factor (which is an overall weighting of all harmonics present) although one or two of the lower harmonics may be increased.

One of the advantages of my invention is that not only will a single shunt take care of a plurality of harmonics but it will also take care of high frequency asynchronous currents, 'such'for example as 933 cycles, which'lies between the th and 16th harmonics.

Capacitors have been used for correcting power factor, and, when so used, are connected in shunt near the load and have their capacities adjusted so that at GO-cycle' current the impedance of the capacitors is substantially equal to the impedance due to the inductance of the load. Capacitors so located and havingv the impedances stated, frequently increase the undesired (from the point of View of noise) higher harmonic currents in the line'between the point at which power is supplied and the point of connection of the capacitor.

- When a capacitor is used in the present invention with a power supply source. of high induct.- ance it is usually located near the point of power supply. With capacitors connected near the load, the harmonics in the power line have to travel the full length of the line from the gener-v ator or transformer to the load, which often is beyond the section of line which parallels the telephone line.

The placing of a capacitor across the power linefwhile it reducesthehigher harmonics, may increase the lower harmonics. Hence, whilethe over-all npise reduction may be large, there may well-be such noise "remaining, due to the original low harmonic currents as well as those created'by the capacitor, as to make necessary the use of the receiver circuit shunt connection.

In a three-phase, three-wire power line, the usual arrangement involves three capacitors connectedbetweenthe various pairs of wires. In a three-phase, four-wire power line, usually a capacitor is connected between each of the phase wires and the neutral.

With a three-wire arrangement in which the line is suppliedv directly by a generator instead of through a transformer, and the generator is provided with a grounded neutral, the capacitors may beconnected between the wires and ground.

Inathree phase line having single phase extensions unequally distributed among the three phases the resonant point of each line does not occur at the same frequency and as a result, when 1 capacitorsare not used, there is'apt to be a large amount ofharrnonics flowing through the ground. Hence,. in such lines, in additionto reducing the higher harmonic voltages in the power line, the capacitors have a balancing effect on important harmonics so that a smaller proportion of them appear as ground return components. It the ground currents which have a particularly high influence onthe adjacent telephone line.

In isolated single phase power lines supplied with power througha single phase transformer it'is advantageous to .use two capacitors connected in series across the power line, with a connection toiground. from the conductor connecting the two capacitors. Theground connection may include a detector adapted to indicate when one of the conductors of the power line is grounded accidentally.

In the, caseof .multi-grounded lines'supplying loads which create harmonics a capacitive shunt adjacent the source of supply will not prevent earth currents of harmonics generated by such loads. To reduce the latter a second capacitive shunt may be placed near the load. I have found that in the case of a uni-grounded line the load harmonics act on the impedance of the supply system and cause harmonic voltages to appear and these voltages produce ground currents due to the capacity of the line to the ground. Where a capacitive shunt is used adjacent the supply system these load harmonics are prevented from building up harmonic voltages at such point and thereby are unable to create harmonic ground currents.

In one actual case the noise produced in a telephone line by a three-phase power line was reduced about 25 to 1 by the installation of a capacitor across the power line, the remaining noise being principally due to the lower harmonic components. In the above actual case, the line was three-phase, 12,000 Volts for 14 miles and had 20 miles of single-phase extensions. The specified noise reduction was accomplished by connecting a 30 kva. capacitor (1.74 microfarads) between each phase wire and neutral. As, however, in the above case, the telephone line was grounded, it was also necessary to apply a shunt receiver-condenser connection to the subscriber set or equivalent acoustical means to complete the noise reduction.

Heretofore, in the ordinary form of receiver of a local battery subscriber set commonly used in country areas, the receiver has been so connected that, except for the very small part which flows through the high-impedance ringer, all current flowing through the line, including all the induced (SO-cycle and lower. harmonic current, passes through the receiver. According to the present invention, the telephone line is connected to the return circuit independently of the receiver to permit low frequency current to enter the return circuit without passing through the receiver. This connection includes inductive means for transferring voice current potential to the receiver to cause voice current to flow therethrough. A condenser is connected in series with the receiver to filter out low frequency current. This has the result of by-passing more or less completely both the (SO-cycle induced current and the lower harmonic induced currents, while allowing the currents which are essential to the transmission of speech to pass through the receiver without substantial loss. This suppression of the induced currents is effective on both electrically induced and magnetically induced currents; and this new arrangement is also effective in suppressing in the receiver, currents resulting from differences in ground potential.

For the usual commercial subscriber set a condenser having a capacitance of around 1 microfarad is ordinarily satisfactory. This sized capacitance will reduce the GO-cycle current in the receiver about 200 or 300 to 1, the lBO-cycle current about 25 to 1, 300 cycles about 10 to 1, and 420 cycles about 4 to 1.

Various suitable applications of the invention to electrically coupled telephone and power systems are illustrated diagrammatically, by way of example, in the accompanying drawing, the usual ringing mechanism being omitted for sake of simplicity. In the drawing:

Fig. 1 illustrates a combination of power and local battery telephone lines embodying the present invention;

Fig. 2 shows an alternative form of power line and subscriber set circuit to that of Fig. 1;

Fig. 3 shows a power line capacitor circuit suitable for an isolated single-phase line; and

Fig. 4 shows a power line capacitor circuit suitable for an unbalanced three phase line.

As shown in Fig. 1, 10 represents a multigrounded power line supplying energy to a second power line H by a transformer 12. A telephone line running parallel to and in close proximity to the power line H is indicated by 13. Across the line H, preferably before it comes near the telephone line, is connected a capacitor 14 for reducing the higher harmonic currents in the line H beyond its point of connection.

While, as shown in Fig. 1, the capacitor shunt may be pure capacity, conditions often arise in which it may be desirable to place an inductance in the shunt, as indicated by 32 in Fig. 2. One case is where there is a prominent frequency between 500 and 2000 cycles and it is therefore desired to make the shunt alone resonant at such frequency, while making the shunt plus supply transformer resonant below 500 cycles.

Again, as the frequency goes up, there comes a point at which the distributed inductances and capacitances of the power line, in effect, neutralize each other, so that the line acts as if it had resistance only. For a line open at the far end, this point is known as the A; wave length (series resonant) frequency. This point may be reached, for example, when the frequency is around 1500 cycles for a line 27 miles long. As the pure resistance of the line is very low, this frequency is a resonant point at which currents of very considerable magnitude may flow. If, now, the capacitor shunt has inductance and capacity so proportioned that at the line resonant frequency they are equal and opposite, a substantial short circuit is created for such resonant currents.

A subscribers set is indicated diagrammatically at A and includes an induction coil having primary and secondary coils l8 and 19, respectively, a battery 28, transmitter 2!, hook switch 22 and ground connection 23. The usual signaling equipment, such as a magneto and ringer, are, for simplicity, omitted.

Around the primary 18 is a shunt circuit 25 in which the receiver 26 and a condenser 21 are connected in series. The condenser 21 is of such capacity that it offers great reactance towards (SO-cycle and lower harmonic currents and little reactance towards those currents by which speech is efficiently transmitted. As a result, low frequency current induced in the telephone line l3 flows to ground'chiefiy through the secondary l9, hook switch 22 and ground connection 23.

The capacitive reactance of the condenser 21 should be of the same order of magnitude as the inductive reactance of the receiver at frequencies around 1000 to 1500 cycles. When this relationship holds, the total impedance (capacitive and inductive reactances plus resistance) of the receiver and condenser in series is less than the impedance of the receiver alone. As a rule, the condenser should have a capacity between 0.5 and 1.5 microfarads.

In the form of construction shown in Fig. 2, an auto-transformer 30 is used; otherwise the set is the same as that of Fig. 1. For simplicity, the hook switch 22 has been omitted.

In the case of isolated, i. e., ungrounded, lines the circuit shown in Fig. 3 may be used to advantage. In that case two. capacitors 36 are placed in series and the conductor connecting them is connected to ground We lead 31. A ground detector D may be applied to the lead 3'? for the purpose of indicating accidental grounding of one or other of the conductors H of the power line.

As previously explained, where a single-phase extension is connected to a three-phase line, the system is unbalanced. The deleterious effects of this unbalancing is eliminated to a very large extent by means of capacitors connected as shown inFi-g. 4. In this case the three-phase conductors 4D and neutral 4! are fed from a suitable supply source by a delta-star transformer 52. A single phase extension isindicated by 43. Three capacitors 44 are connected between the conductors 4B and-the neutral 4!.

As already pointed out, a single-phase unigrounded line, as shown in Fig. 2, provided with a single shunt adjacent the source of supply does not ordinarily require another shunt near a load,

indicated at L out on the line which generates harmonics, to cut down the harmonics in the earth resulting from such load. The above is true even though the load is a rectifier generating very large amounts of harmonics, provided the line is of the single-phase uni-grounded type.

The invention is also applicable to telephone circuits which are metallicized, i. e., do not use a ground return for speech transmission. The susceptiveness of metallic telephone lines to induced noise is much lower than that of circuits operating over one wire with ground return whether or not the lines are transposed to reduce noise induction between the wires of the circuit. However, when transpositions are used, they do not change the noise voltages to ground of the line, and these latter voltages react on the unbalances' in the telephone line with resulting noise. Such unbalances may exist in the form of three contacts, high resistance joints in the line, or the presence of ringers connected from the line Wires to ground in cases where the ground return ringing system is used.

The capacitors may be used to prevent interferencewith A. C. telegraphic systems. To increase the number of telegraphic messages which may be sent over a single circuit, the telegraphic sending apparatus comprises a series of generators of varying frequencies and the receiving ap paratus comprises a corresponding series of circuits each tuned to one generator frequency. The dots and dashes are produced by interrupting the flow of current of the desired frequency. If currents of the same frequency are induced by a power line, there is a continuous, instead of an interrupted, flow of current which inter feres with the transmission'of the message.

Capacitors may also be used with single-wire ground-return power systems in the same way that they are used witha double-wire singlephase line. In the single-wire system the capacitors are connected to earth instead of to another wire.

What is claimed is:

1. A power line system With a power factor not greatly less than unity fed from a transformer having a single capacitive shunt having high capacity relative to inductance connected across the power line adjacent the transformer to reduces. {plurality of the higher harmonics in the power line.

25A power line system having a capacitive shunt connected across the. power dineito-cteducei 7.5, nl lialitypfqthe-higher harmonics illithfiiQOllLQIf line, the impedance of the shunt being substantially equal to that of the power supply source at a frequency below 500 and markedly different from the fundamental frequency and also the lower harmonics in the system.

3. A power line system having a capacitive shunt connected across the power line to reduce the higher harmonics in the system, the impedance of the shunt being such that the system as a whole is resonant at a frequency below 500 and markedly different from the fundamental frequency and also lower-harmonics in the systom.

4. In combination with a power line supplied from a supply system having a high inductance and having, by virtue of such inductance, a normal resonance at a frequency between 500 and 2000 cycles, a capacitive shunt across the power line having constants such that the resonant frequency of the power line and supply system is reduced to below 500 cycles to reduce the harmonics in the line above 500 cycles.

5. In combination with a power line grounded at the source of supply only and a load on such line generating harmonics, a single capacitive shunt having highcapacity relatively to inductance across the line adjacent the source of supply for said line for reducing a plurality of the higher harmonics in the ground arising from both th source of supply and the'lcad.

6. In combination with an isolated single phase power line supplied with power through a single phase transformer, two capacitors connected in series across the power line to reduce the higher harmonics therein and a connection to ground from theconductor connecting the twocapac itors.

7. The combination as in claim 6, in'which the ground connection includes a detector adapted to indicate when one of the conductors of the power line is grounded accidentally.

8. In combination with a power line and load having a combined capacitive reactance in" excess of inductive reactance at a frequency of about 500 cycles andvfed from a single source, a capacitor shunt having high capacity relatively to inductance connected across the power line to reduce a plurality of the higher harmonics in the power line.

- 9. In combinationwith a power line supplied from a supply system by a transformer, a shunt connected across the power line, such shunt having both capacitance and inductance, the capacitive and inductive reactances being equal at some frequency above 500 cycles and so proportioned with respect to the combined inductance of the transformer and supply system that the circuit through the transformer in series with the shunt has a resonant point-below 500 cycles.

10. A power line system having a capacitive shunt connected across the power line to reduce a plurality of the higher harmonics in the power line, the product (L1+L2)C, where L1 is the inductance. of the supply system and L2 is the inductance of the shunt in millihenries and C is the capacity of the shunt in'microfarads, being not greatly less than 125.

11. A power line system. having a capacitive shunt connected across the power line to reduce a plurality of the higher harmonics in the power line, the product (L1+L2)C', where L1 isthe inductance of the supply.v system ands; h'e inductance of theshun m mil-lihenries he irgalpaicityi ofcthes Shlll tein; ini'crofarad of the order of magnitude of 400. not

12. In combination with a power line, a supply system therefor having inductance, and a shunt connected across the power line to reduce the higher harmonics therein and having both capacitance and inductance, the capacitive and inductive reactances of the shunt alone being equal at some frequency well above 500 cycles, the capacitive reactance of the shunt being equal to the combined inductances of the supply system and shunt at a frequency well below'500 cycles, and the inductance of the shunt being much lower than that of the supply system.

13. The combination as in claim 12, in which the capacitive and inductive reactances of the shunt alone are equal at a frequency near that of one of the most prominent harmonics in the power line.

14. The combination as in claim 12, in which the capacitive reactance of the shunt is equal to the combined inductances of the supply system and shunt at a frequency well within the range of 180 to 300 cycles.

15. The combination as in claim 12, in which the capacitive and inductive reactances of the shunt alone are substantially equal at the A; wave length frequency of the power line.

16. In combination with a grounded threephase power line having single phase extensions unequally distributed among the three phases, a capacitive connection between each phase wire to ground to balance the harmonics and reduce the volume of harmonics flowing through the ground.

17. In combination with a three-phase fourwire power line system, a capacitive shunt between each line and the neutral, the impedance of the shunts being such that the system as a whole is resonant at a frequency below 500 and markedly difierent from the fundamental frequency and also the lower harmonics in the system.

18. In combination with a three-phase fourwire power line system with a power factor not greatly less than unity, a capacitive shunt between each line and the neutral to reduce a plurality of the higher harmonics in the system.

19. A power line system having a load thereon of a type creating harmonics, a capacitive shunt near the point of supply of power to the power line to reduce harmonics originating in the source of power supply, and a second capacitive shunt near the harmonic-generating load to reduce harmonics originating in such load.

HUGO W. WAHLQUIST.

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