Induction from Power Parallels

Case A. Voltages Induced by Balanced Voltages. A grounded three-phase power line, paralleled by a metallic telephone circuit, is shown in Fig. 18. Since the power line is grounded, it is apparent that the phase voltages E₁, E₂, and E₃ exist between the corresponding line wire and ground. If the power line is not grounded, voltages will still exist between the power wires and ground because of the distributed capacitances as explained on page 555.

All three voltages existing to ground on the power line of Fig. 18 are assumed to be equal in magnitude and 120 degrees out of phase. If the power wires were closely grouped together, then these three voltages would produce no resultant electric field to ground because of the cancellation which would occur. To provide insulation, the power wires must be spaced some distance apart, and hence a resultant field is produced in the vicinity of the telephone wires, and this resultant field will accordingly raise each telephone wire to a voltage above ground. For the condition of Fig. 18, these voltages E_g1 and E_g2 to ground which are induced on the telephone wires are unequal.

Figure 18. The phase voltages E1, E2 and E3 exist between the corresponding power wires and ground. Since these power wires are some distance apart, they will raise the telephone wires to voltages Eg1 and Eg2 above ground.

Or, from the standpoint of the distributed capacitances as discussed on page 546, each power wire will tend to raise each telephone wire to a voltage above ground. If the power wires were close together so that the distances involved were equal, then the three voltages induced between the telephone wires and ground would be equal and 120 degrees out of phase, and they would therefore cancel, leaving no resultant voltage between the telephone wires and ground. Since the power wires are not grouped together, each telephone wire is raised to a voltage above ground as represented by the generators E_g1 and E_g2 of Fig. 18. Furthermore, the voltage on wire 1 will exceed that on wire 2.

As was explained on page 548, raising two telephone wires to unequal voltages to ground may cause noise in connected telephone sets in two ways: First, because of the fact that the two voltages to ground are unequal, a difference of potential will exist between the two wires, and noise currents will flow through the two connected telephone sets

(page 547). Second, the voltages to ground will act through any series unbalances, or through unbalances to ground, and cause noise.

From these facts, two remedies are apparent. First, if the two telephone wires are transposed, as explained on page 550, the noise-producing effects of the unequal voltages to ground will be lessened, and, if the telephone circuit is also well balanced, both in series and to ground, little noise will result. Second, if the power wires are transposed,³¹ then each power transposition section will tend to raise each portion of the telephone line parallel to it to a resultant potential above ground. These resultant induced voltages will be of substantially equal magnitude, 120 degrees out of phase, and their effects will cancel, giving a low overall resultant noise on the telephone line for each power-line 360-degree transposition "barrel" as shown in Fig. 19.

Figure 19. Showing a complete 360-degree rotation or "barrel" of the power line, and coordinated transpositions in the paralleling telephone line.

The power-line transpositions must be reasonably close together if the effects of phase differences, due to the finite velocity of propagation, and line attenuation are to be negligible. For usual conditions at roadside separation, the power-line transpositions are often about one mile apart. The telephone transpositions must be coordinated with those in the power line.

Case B. Voltages Induced by Residual Voltages. As was explained on page 554, the residual voltage on a three-phase power line acts between the three power wires as one side of the circuit and ground as shown in Fig. 20. That is, the residual voltage acts like the power system consisting of one wire and a ground return.

Figure 20. A residual voltage acts between the power wires in parallel (as a single conductor) and ground.

The residual voltage will raise each telephone wire to a voltage above ground (page 546). These voltages will be unequal, and, if the telephone line is not transposed, a large difference of potential will exist between the telephone wires (page 551), and excessive noise currents will flow through the connected telephone sets. If the telephone wires are transposed, the difference of potential and the noise will be reduced. However, voltages to ground will still exist and may act through unbalances in series and to ground to cause noise (page 548).

The reason residual voltages are so likely to cause noise is now apparent. Transposing the power line will not reduce the induction due to residuals unless the residuals are caused by the fact that the power line was formerly unbalanced with respect to ground. Since most power lines are transposed and since in most instances the residuals are caused by the connected power equipment, it can be stated that in general transposing the power line has no effect on the induction due to residuals because they act between the power wires in parallel and ground. Usually, with properly constructed and maintained lines, the only remedy from induction due to residuals is to correct the connected equipment causing them.

Case C. Voltages Induced by Balanced Currents. The balanced currents are the useful currents, and in each wire of a three-phase power system they are equal in magnitude and 120 degrees out of phase. The paths of the balanced components are confined entirely to the line wires. The balanced currents in each of the three line wires tend to produce a magnetic field. If the three power wires were very close together, no appreciable magnetic field would be produced. Since the wires are not close together, a resultant magnetic field will exist in the vicinity of the paralleling telephone line.

This resultant magnetic field will induce voltages in series with each of the telephone-line wires. As was shown in Fig. 1, these two induced voltages will act in the same direction. If the telephone line is not transposed the voltage e₁ induced in wire 1 will exceed e₂ in wire 2. A resultant voltage will accordingly exist, and a noise current will flow through the connected telephone sets. If the telephone wires are transposed, then, as in Fig. 9, the two induced voltages will be more nearly the same, and the noise will be reduced.

But, as was shown on page 549, even equal voltages in series with the telephone wires may cause noise, and it is important to keep these voltages as low as possible. Transpositions in the power line will tend to do this. Within one complete 360-degree rotation or "barrel," as shown in Fig. 19, each power wire is closest to the telephone line for one-third the distance. Thus each portion of the corresponding section of the telephone line will have an induced voltage corresponding to a particular power wire. The result is that, instead of one power wire having a predominating effect for a given length, with power transpositions each power wire will affect the telephone line more equally. Thus, if the power transpositions are reasonably close together, so that the phase shift and attenuation effects are negligible, substantially equal voltages which are approximately 120 degrees out of phase will be induced, and these will tend to cancel, greatly reducing the resultant voltage existing in series in each telephone wire.

Case D. Voltages Induced by Residual Currents. Because residual currents flow along the power wires in parallel and the ground as the other side of the circuit, the method of induction is illustrated by Fig. 1 and the accompanying theory.

Transpositions in the telephone circuits will tend to equalize the voltages induced in series in each line wire, but they will not eliminate these voltages. As explained on page 549, these may cause noise by acting through unbalances. Power-line transpositions will not be effective in reducing these induced series voltages unless such transpositions cause a reduction in the residual currents themselves.

It is possible to calculate the voltages and currents that will be induced in telephone circuits by a power-line exposure.^18,19,20 Also, it is possible to predetermine the amount of telephone-circuit noise that an exposure will cause.^19,20

The rapid extension of rural power systems has necessitated much investigational work.³² The discussions of the preceding pages has been largely concerned with induction to telephone circuits. In general, what has been said for telephone circuits applies also to telegraph circuits.³³