WO1998028858A1

WO1998028858A1 - A termination circuit

Info

Publication number: WO1998028858A1
Application number: PCT/AU1997/000873
Authority: WO
Inventors: Geoffrey Ormiston Stone
Original assignee: United Energy Ltd.
Priority date: 1996-12-24
Filing date: 1997-12-22
Publication date: 1998-07-02
Also published as: EA199900594A1; ID22229A; NZ336192A; CA2275986A1; EA002170B1; AUPO440796A0; EA200100599A1; EP0951757A1

Abstract

A termination circuit for N power distribution lines, having resistance rij connected between points Pi on N-1 of the lines and between the points Pi and a ground, which is connected to the remaining one of the lines. A circuit for use in delivering a communication signal on a power distribution system which distributes power signals, the communication signal having a high frequency relative to a frequency of the power signals, the circuit including a transformer which has windings for each line of the distribution system, and has substantially no net flux for the power signals, and has a net flux for the communication signal.

Description

A TERMINATION CIRCUIT

The present invention relates to a termination circuit for power distribution lines, and a method for determining values of components of the circuit. The present invention also relates to an isolation or conditioning circuit.

Power distribution systems provide an existing infrastructure which can advantageously be used for the transmission of communication signals. A difficulty associated with establishing a communication system using a power distribution system is providing an effective termination circuit for the lines of the distribution system. A further difficulty is also posed by a need to provide sufficient isolation between power signals traditionally distributed on the lines, and the communications signals. Isolation may be required both at a customer's premises, and at connection points on lines of the system.

The configuration of multi-line power distribution systems varies considerably depending on local factors, such as regulations, stage of development and physical characteristics. In Australia, power distribution lines are used to distribute to customers a high voltage and low frequency power signal, which typically has a frequency of 50 hertz and a distribution voltage which may be 415 volt phase to phase (LV), 6.5, 11 , 22 or 66 kilovolt phase to phase (HV) on each of the N lines of the system. The number of lines N usually ranges from 2 to 5 but may be higher where both LV and HV systems are together in close proximity. Typically N=4 for a three phase 415 volt AC service, with one line being designated neutral. The wires of the lines may be open in that they are strung overhead with no insulation and are separated by air. The wires may also be bundled, such as in an aerial bundle or for underground cables, where they are separated by insulation and covered.

When the lines reach an end point where they no longer need to be continued, such as at the end of a street, they are not electrically terminated. Other open circuit points are also present where physical tension points are included in the lines or where power delivered from opposite directions meets an open point. Step down transformers, on the other hand, present an impedance discontinuity which may be a short circuit for low frequency signals and a relatively high impedance for high frequency signals. These open and short circuit characteristics inhibit efficient transmission of a low voltage radiofrequency (RF) communication signal. The problem could be addressed by including a termination circuit for the communication signal which impedance matches the lines at the open and short circuit points so as to eliminate unwanted reflections of the communication signal. Yet it has proved particularly difficult to provide and correctly configure an impedance matched termination circuit. The difficulties arise primarily because any RF pulse transmitted on the lines causes coupling between the lines, thereby rendering it difficult to make effective impedance measurements to determine component values for a termination circuit, particularly when a wide carrier frequency band needs to be catered for.

The capacity of the distribution lines to transmit high frequency communications signals is also inhibited by impedance discontinuities which occur at different points in the distribution system, and in particular occur at the connection points made at most supporting power poles of an overhead system. Communication services to customer premises would normally be delivered on service wires to the premises and are typically connected to the rest of the distribution system at the poles using junction boxes. Due to a shunting affect introduced by the impedance discontinuities at the junction boxes, a considerable amount of the communication signal power can be directed down the service wires, leaving little power for transmission further down the rest of the distribution system for other connection points. If not attended to, this can result in rapid communication signal attenuation as it propagates down the lines of the system to other customers.

In accordance with the present invention there is provided a termination circuit for N power distribution lines, having resistances r_tJ connected between points P, on N-1 of said lines and between said points P, and a ground, which is connected to the remaining one of said lines

The present invention also provides a method of determining values of the components of the termination circuit, including determining matched termination values t_tJ between said lines when at least one of the lines is connected to a communication signal source and the remaining lines are connected to said ground, setting said resistances r,_j to nominal values and measuring the resistance between said points P, to obtain measured point impedances Pl determining, on the basis of said t, , final point impedances FPI_ιr and determining, on the basis of said t_(J and PI_M, sequence point impedances SPI, which need to be set in sequence and measured to set the point impedances PI_M in the termination circuit to the final point impedances FPi._j

The present invention also provides a circuit for use in delivering a communication signal on a power distribution system which distributes power signals, said communication signal having a high frequency relative to a frequency of said power signals, said circuit including a transformer which has windings for each phase of the distribution system, and has no net flux for the power signals, and has a net flux for the communication signal

Preferred embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein Figure 1 is a circuit diagram of a preferred embodiment of a termination circuit,

Figure 2 is a flow diagram of an impedance determination program, Figure 3 is a circuit diagram of bridged power distribution lines, Figure 4 is a circuit diagram of a preferred embodiment of an isolation circuit connected in a customer's premises, Figure 5 is a circuit diagram of a preferred embodiment of a conditioning circuit;

Figure 6 is a circuit diagram of a first equivalent circuit of the circuit of Figure 5; Figure 7 is a circuit diagram of a second equivalent circuit of the circuit of

Figure 5; and

Figure 8 is a block diagram of a junction box incorporating the conditioning circuit.

A termination circuit 2 for N lines 4, 6, 8 and 10 of a power distribution system includes three parts 12, 14 and 16, as shown in Figure 1.

The first part 12 is an isolation part which is used to isolate the high voltage level low frequency power signal on the lines 4 to 10 from the low voltage level radiofrequency (RF) signal handled by the second and third parts 14 and 16 of the termination circuit 2. The power signal typically has a frequency of 50 hertz and one of the distribution voltages, such as 415 volt phase to phase, 6.5, 11 , 22 or 66 kilovolt phase to phase on each of the lines 4 to 10. The radiofrequency signal is typically less than 1 volt rms with a frequency in a range of 2 to 100 megahertz. Accordingly, effective isolation can be achieved by placing isolation capacitors 18 in the lines 4 to 10.

The second part 14 of the termination circuit 2 is a driving point network which includes a drive transformer 22 having its secondary coil 24 connected to input/output points P., P₂ and P₃ of N-1 of the lines 4, 6 and 8. Drive resistances 26, 28 and 30, having values r_{1 (} r₂ and r₃ are connected in parallel to the secondary coil 24 between the coil 24 and respective points P_{1 ?} P₂ and P₃, as shown in Figure 1. The primary coil 32 of the transformer 22 is connected to an RF input/output coaxial termination 20 for inputting and outputting the RF communication signal, - which is either placed on the points P.,, P₂ and P₃ or received from the points. The remaining line 10 is connected to RF ground 34, together with the opposite terminals of the coils 32 and 24 of the transformer and the outer sheath of the coaxial termination 20. The line 10 which is connected to ground 34 would normally be the neutral line.

The third part of the circuit 2 is a termination network 16 which is able to terminate the N-1 lines 4 to 8 so as to present a matched impedance to any RF communication signal received on the lines 4 to 8. Assuming the lines 4 to 10 exhibit low losses, to absorb all RF signals incident on the lines 4 to 8, the termination network 16 comprises N(N-1 )/2 resistors connected to the lines 4 to 10 in all combinations to cater for coupling between the lines. This involves connecting resistors with appropriate resistance values between all possible pairs of the drive points P.,, P₂ and P₃ and between each of the drive points P_{1 (} P₂ and P₃ and the RF ground 34. As shown in Figure 1 for N=4, resistances ^ provided by potentiometers 36, 38, 40, 42, 44 and 46 are connected between respective pairs of points P_{1 t} P₂ and P₃ and between respective ones of the drive points P.,, P₂ and P₃ and the RF ground 34.

The effectiveness of the termination network 16 can be shown by considering an arbitrary voltage travelling wave on the power distribution system, together with its corresponding current travelling wave, linked by the inductance per unit length and capacitance per unit length matrices of the distribution lines 4 to 10. To provide a matched termination, the termination network 16 needs to maintain and appear to continue the relationship between the voltage and the current travelling waves when the waves arrive at the network 16. The waves can be shown to be in phase so the termination network 16 has an admittance matrix which represents a network of positive resistances interconnecting every line, which is the form of the termination network 16 described above. Correct determination and setting of the resistance values r-,, is described hereinafter.

A unique set of resistance values r,, (ij=0 to N-1 ) need to be established for each particular configuration of N lines 4 to 10 being terminated. Existing configurations of power distribution lines 4 to 10 vary not only in the number N of lines but also the size and spacing of the conductors for the lines 4 to 10 The lines 4 to 10 may also be, as described above, closely bundled and twisted metal cables which are covered with insulating material

Initially a set of matched termination values t„ is determined by conducting a series of N(N-2)/2 experiments on the N lines 4 to 10, which may be an actual section of the lines to be used or a simulation which constitutes a scale model If a scale model is used, the dimensional proportions of a cross-section of the lines 4 to 10 of the distribution system needs to be preserved Each experiment involves the determination of a matched termination value t_tJ for a particular bridge configuration on each end of the line 4 to 10 In this context "bridging" means connecting an RF short circuit between certain combinations of lines, at both source and load ends Each bridging combination provides an RF "ground" line or set of lines and an RF "active" line or set of lines Each bridging combination is also independent of the others and identical at the source and load ends in a particular experiment For these experiments a suitable pulse generator is connected at the source end, via an impedance transformer if impedance mismatches justify, to the RF ground and RF active lines or sets of lines The source end is monitored with an oscilloscope A single adjustable termination resistor is connected at the load end between the RF ground and RF active lines or sets of lines Each experiment then consists of setting the variable termination resistor whilst monitoring the reflected signal at the source end, such that no reflected signal is seen coming back from the load end The value of this resistance is measured and comprises the matched termination value t„ for that experiment The procedure is repeated for the other experiments An example of the bridging conditions and symbols representing the resulting matched terminations is shown in Table 1 Table 1 : Example of Bridge Conditions <RF activexRF ground> [Matched Termination Symbol]

Εxperiment not applicable for the value of N For example for N=4, referring to Table 1 , in the first three experiments 1A, 1 B and 1 C one of the lines 4, 6 and 8 including the corresponding drive point P^ P₂ and P₃ is respectively connected to the RF active, whilst the remaining lines, including the neutral line 10 are connected to RF ground 34 In each experiment the variable resistance is connected between RF ground and the line which is selected to be the RF active In the three remaining experiments, 2A, 2B and 2C the neutral line 10 and one of the other lines 4, 6 or 8 are connected to RF ground with the remaining two lines being used as RF active The remaining two lines are connected to one other and connected to RF active, and again the variable resistor is connected between the RF active lines and the RF ground lines A set of matched termination values obtained for a set of open wire powerlines for N=4 is shown in Table 2

Table 2: Example of Matched Termination values for N=4 obtained from experiments defined in Table 1

Once the matched termination values t,, have been determined, they can be used in a procedure, which can be executed by a computer program 50 as shown in Figure 2, to determine the final point impedances FPI,, which need to be seen between the points P, and also between the points P, and the RF ground 34, to render the termination network 16 effective The matched termination values t,_. are inputted at step 52 of the program 50 and at step 54 are transformed to termination network admittance elements g„ for the network 16 using a linear transform LT1 An admittance matrix [Y_TN] is then obtained using a second linear transform LT2, in step 55, from the matched termination values t,_r Next, at step 56, the admittance matrix [Y_TN] is used to obtain the final point impedances FPI_fJ using a third transform LT3. The final point impedances FPI, are the ultimate desired measured resistance values across the points P, (i=0 to N-1 ) for the termination network 16. Given a set of termination values t,, expressed as admittances with the end conditions set in turn as described in Table 1 , the values of the final point impedances FPI,, can be determined. For example, for N=4 in experiment 1A the second and third lines 6 and 8 bridge to the RF ground 34 with the neutral line 10 at the source end, and the remaining first line 4 is used as the RF active to receive the RF test signal. At the opposite load end, the test signal will arrive in the same form that it left source end, and therefore bridging the load end will not affect the reflected wave. Hence t_1a can be expressed in terms of the resistances r_(J1 or the corresponding admittances g,,, which are not short circuited by bridging for this experiment, as shown in Figure 3. Hence it follows that t_1a = 1/r₀₁ + 1/r₁₂ + 1/r₁₃ = g₀₁ + g₁₂ + g₁₃ . Other expressions follow similarly. Solving the g, in terms of the t_u defines the transform LT1 . For example for N=4, the linear transform LT1 is defined by the equations

9oι = ^1/2 ( _2b ⁺ t_2c - *ib - t_1c)

90₂ = ^{1 z} (t_2c ⁺ t_2a - t_{1 c} - t_1a)

90₃ = ^{1 2} (t_2a ⁺ t_2b - t_{1 a} - t_{1 b})

which are the values for g„ used in step 66, as described hereinafter. The transform LT2 used in step 55 can be obtained by standard circuit analysis techniques. The linear transform LT2 for N=4 is defined by equations relating the terminating network admittance matrix elements Y_TN|J to the t, as follows Y ' TN11 = t ^l1a

Y ¹ TN22 = t ^l1 b

Y ^T TN33 = t c

Y ' TN12 = Y ' TN21 = ^{1 2} (tac - t_{1 a} - t_{1 b})

Y ¹ TN₂3 = Y ' TN3₂ = ^{1 2} (t_2a ^" ib ^" tic)

^T 31 "N13 ' (t₂b _{1 c} t_{1 a})

These nine numbers form the admittance matrix [Y_TN] relating the three voltages at the points P„ i=1 to 3 relative to RF ground P₀ 34 to the currents into these points. Inverting gives the impedance matrix elements Z_TN|J, which for the values of Table 2 is as follows:

440.3 155.5 169.7

^TNJ - L ^TNJ 155.5 443.0 214.1 169.7 214.1 496.7

The final point impedances FPI,, are related to the Z_TNlJ, as determined in step 56, are as follows

F ^ΓP^ΓI^I01 = Z-TN11

F ^rP ^Γ I 'θ2 = Z ^-TN22

F ^ΓP^ΓI^I03 = Z ^"TN33

FPI₁₂ = Z_{TN1 1} + Z_TN22 - 2Z_TN12

FPI₂₃ = Z_TN22 + Z_TN33 - 2Z_TN23

FP'31 ⁼ ^TN33 ⁺ ^TN11 ^" 2Z_TN31

which give the final values in Table 3 described below. The last two sets of relationships connecting the Y_TN|J with the FPI,, define the transform LT3.

The driving point network 14 may not be included if all that is required is to terminate the lines 4 to 10. However if the driving point network 14 is present it will affect the impedances seen from the N-1 lines 4 to 8. For the N=4 configuration as shown in Figure 1 , the driving point network 14 appears as an equivalent termination network in parallel with the actual termination network 16 as seen from the points P.,, P₂ and P₃. The driving point network 14 can be represented by an admittance matrix [Y_DPN] and therefore the effective termination network has an admittance matrix given by [Y_TNeff] equal to [Y_TN] + [Y_DPN]. With or without the driving point network 14 the final point impedances FPI,, will remain the same, although the settings of the potentiometers 36 to 46 will be different. Under DC measurement conditions, a resistor 49 is placed in parallel to the transformer 22 with a resistance value r₀ to represent the input impedance presented by the secondary coil 24 of the transformer 22 in use, because in DC measurement conditions the coil 24 represents a short circuit. The transformer 22 is wound accordingly as an impedance transformer with an RF source impedance, which is typically 50 ohms, being on the primary coil 32. The impedance transformation is only an approximation so the actual value of r₀ is chosen to be the resistance measured looking into the secondary coil 24 at the RF frequency of interest, i.e. the frequency of the carrier of the communication signal. For realisability, i.e. to produce positive resistance values, elements y_{TNeff rj} and y_{DPN lJ} of the last two matrices must therefore satisfy y_{DPN lJ} < y_{TNeff lJ} for every ij =1 ... 3 for N=4 because y_{DPN lJ} + y_{TN lj} = y_TNβff„. This is used as an aid in determining the values for the drive resistances 26, 28 and 30. The values of the drive resistances 26 to 30 are chosen to maximise power delivery to the lines 4 to 8 for a given RF input. The drive resistances 26 to 30 can be given the same resistance value for similar signal levels on all lines 4 to 8, and the sum of the drive resistance values r + r₂ + r₃ is chosen to be greater than the maximum resistance r_(J for realisability because a star-to-delta transformation of the drive resistances 26 to 30 puts r, + r₂ + r₃ in parallel with each of the interline resistances r,_r The r_{J are set, as described above, to give a matching termination network 16 and if the drive resistances are too high insertion loss will be excessive so the selection procedure should be repeated with smaller drive resistance values until the r_u are just realisable according to the program 50.

The final part of the program 50 involves determining a sequence of point impedances SPI_fJ which can be measured and set in sequence to finally arrive at the desired final point impedances FPI,_r The sequence is important because adjusting any of the resistance values r„ will affect the point impedances Pl,_r At step 58, the resistances r„ are initially set by placing the potentiometers 36 to 46 in a centre position and initial point impedances P\_tj are measured As discussed previously, during the measurement conditions the transformer 22 is out of circuit, but is represented by the resistor 49 having a value r₀, which allows DC measurements to be taken simulating actual impedances prevailing at operating radiofrequencies At step 60, the measured point impedances PI,, are transformed to actual or measured element admittances g_(J A sequence determination loop 62 is then entered at step 64 for k iterations The number of iterations k is the number of impedances between pairs of the points P, and the points P, and the RF ground 34 For N=4, k=6 In the first step 66 of the loop 62 one of the actual admittances g,, is replaced by the desired admittance g,, determined in step 54 All of the admittances are then transformed to point impedances PI,, at step 68 using the relationship between g,, and t,, described previously and the transforms LT2 and LT3 At step 70, the kth point impedance in the sequence to be set SPI,, is taken to have the value PI,, corresponding to the impedance obtained in the matrix derived by step 68 and corresponding to the element g„ chosen in step 66 For example if g₁₀ had been set to its desired value in step 66, then SPI₁₀ is the first impedance in the sequence and is taken to have the value Pl₁₀ in step 70 which is derived in step 68 At the decision step 72 a determination is made as to whether all iterations of the loop 62 have been completed Once all iterations of the loop 62 have been completed the sequence values SPI,, are recorded in order together with the final FPI,, values at step 74 and the program 50 completed Table 3 below sets out the results produced by the program 50 for N=4, where the first column specifies the point impedances between pairs of lines, with Neutral corresponding to the RF ground line 34, Red corresponding to the first line 4, White corresponding to the second line 6, and Blue corresponding to the third line 8 The initial measured point impedances PI,, inputted in step 58 are set out in the first column, the sequence point impedances SPI,, determined by the loop 62 are set out in the second column and the final point impedances FPI,, are set out in the third column Therefore to achieve the final point impedances FPI,,, firstly the impedance between the red line 4 at point P₁ and ground needs to be set and measured at 381 ohms Next the impedance between the white line 6 point P₂ and ground needs to be set and measured at 411 ohms, etc. until the impedance between the blue line 8 point P₃ and the red line 4 point P^ is finally measured and set at 598 ohms.

Table 3 Termination Network Impedances

The termination circuit 2 can be used in a customer's premises, as shown in Figure 4, to receive signals inputted on the distribution lines 4 to 10 from a source 103. The distribution lines 4 to 10 also provide, according to their normal function, power to the customer's premises which constitutes a power load 100. In order to isolate the termination circuit 2 from the load 100, an isolation circuit 102 is used which includes a toroidal core 104 placed in series with the lines 4 to 10 connected to the load 100, and capacitors C_{1 t} C₂ and C₃ connected between the neutral line 10 and the red, white and blue lines 4, 6 and 8, respectively. An impedance Z₀ is also connected across the coil of the toroidal core 104 for the neutral line 10, 160. The isolation circuit 102 provides isolation for the termination circuit 2 from spurious noise and impedance effects of the load 100 at RF frequencies whilst allowing maximum demand current to pass to the load 100 at the mains frequency of the power distribution system. The toroidal core 104 has different characteristics for the mains frequency and the RF frequencies. At the mains frequency the coils for each phase are wound on the ring 104 such that the magnetic fluxes add. A coi| for the neutral line 10 is also wound but in such a way that it cancels the flux produced by the phases of the other lines 4 to 8 so that net flux at mains frequency in the toroidal core 104 is 0, guaranteeing that the ring will not saturate due to the mains current This ensures the isolation circuit 102 presents a very low impedance to the source 104 at mains frequency At RF frequencies, the neutral line 10 is substantially bypassed by the impedance Z₀ to produce a net RF flux in the ring and hence introduce an inductance which is used as part of an RF filter of the circuit 102 The impedance Z₀ includes a capacitor C₀ and resistor R₀ in series R₀ is included to prevent a magnetic short circuit for the active phases The capacitors C,, C₂ and C₃ form the remainder of the RF filter of the circuit 102 This ensures the circuit 102 presents a high impedance for the termination circuit 2 at RF frequencies The capacitors C_{1 :} C₂ and C₃ shunt any RF signals, such as RF noise output by the load 100

The toroidal core 104 can also advantageously be used as part of a conditioning circuit 150, as shown in Figure 5, for use in connecting service cables 152 from a customer's premises to the overhead distribution lines 4 to 10 The customer service lines 152 which run from the overhead lines 4 to 10 to the customer's premises include red, white, blue and neutral lines 154, 156, 158 and 160 for a three phase service The red, white, blue and neutral distribution lines 4 to 10 are connected by respective series windings about the core 104 to the red, white, blue and neutral customer service lines 154 to 160, respectively An impedance Z₀ is again connected across the winding for the neutral lines 10 and 160, whereas respective conditioning impedances Z_R, Z_w and Z_B are connected across the windings for the remaining lines The core 104 is again wound so that for the mains frequency, the magnetic fluxes add around the core for the red and white and blue phases, and the winding for the neutral line is such that it cancels the flux produced by the remaining phases, so that the net flux at the mains frequency in the core 104 is zero The value of the capacitance C₀ of the impedance Z₀ is also selected such that at the RF frequencies, the neutral line 10, 160, substantially bypasses the core 104, to thereby produce a net RF flux in the core 104 R₀ is again used to prevent a magnetic short circuit for the active phases The impedance value presented to the overhead lines 4 to 10 can therefore be varied between the mains frequency and the RF frequency, to provide impedance conditions which are unchanged for power distribution on the service cables 152, and which also prevent rapid signal attenuation of the communication signal at the RF frequency. The difference between the mains frequency and the RF frequency used for the communication signal enables the conditioning circuit 150 to present an inductance value to the lines 4 to 10 which has a very low reactance at 50 hertz but a high reactance at RF. The circuit 150 also does not present any problems with saturation due to the potentially large currents at the mains frequency, as the sum of the net flux in the core 104 will be zero.

With regard to the behaviour of the conditioning circuit 150 at the RF frequencies, an equivalent circuit 170 is shown in Figure 6. At the RF frequencies, the inductances of the windings for each phase L_R, L_w and L_B will all have significant reactances. The leakage inductances L_LR, L_LW and L_LB will also have significant reactances as the transformer 104 is not tightly coupled and only a small number of turns is used. There is also a falling off of magnetic permeability at the RF frequencies. It can be seen from the equivalent circuit 170, that the impedances presented at each input for each active phase to the customer premises, i.e. R-R_c, B-B_c and W-W_c will be high for the RF frequencies, thereby preventing significant loss or rapid attenuation of the communication signal for each set of service cables 152 along the distribution system. It can be shown that the combined input impedances for the active phases is the sum of the impedances of the overhead distribution lines 4 to 10 and the service lines 152 all sharing a common neutral line 10, 160, and accordingly will be high. The impedance seen at the input to red phase of the core 104 due to the white and blue phases connected to the core 104, will be of the same order as X_R, being the reactance of L_R.

It may however occur that the signal arriving on the service cables 152 for each of the active phases 154 to 158 may be unfavourably out of balance. This can be addressed by including, within the circuit 150, the conditioning impedances Z_R, Z_w and Z_B, which comprise resistors and capacitors connected in series across the phase windings of the core 104, as shown in the RF equivalent circuit 172 of Figure 7. The capacitors of Z_R, Z_w and Z_B are chosen so that they present a short circuit at the RF frequencies, but provide a blocking impedance for power signals at the mains frequency. This allows the resistors of Z_R, Z_w and Z_B to be adjusted and selected so as to balance the RF signals on the phases submitted to the customer's premises. The circuit 150 also inherently acts as a signal equalising device via the coupled windings of the core 104. The circuit therefore produces a reactive isolation and conditioning effect which can be adjusted as desired depending on the number of the turns of the windings, the size and the material used in the core 104, and the values chosen for the conditioning components Z_R, Z_w, Z_B and Z₀.

The circuit 150 can be incorporated into a junction box 180, as shown in Figure 8, mounted on the supporting poles of an overhead distribution system to connect the overhead distribution lines 4 to 10 to the customer service lines 152 to 160.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as hereinbefore described with reference to the accompanying drawings.

Claims

1 A termination circuit for N power distribution lines, having resistances r,, connected between points P, on N-1 of said lines and between said points P, and a ground, which is connected to the remaining one of said lines

2 A termination circuit as claimed in claim 1 , including isolation capacitors in said lines for isolating power signals distributed on said lines from a communication signal having a high frequency relative to a frequency of said power signals

3 A termination circuit as claimed in claim 2, including a drive circuit for inputting said communication signal at said points for transmission on said lines, said communication signal having a low voltage relative to said power signals

4 A termination circuit as claimed in claim 3, wherein said drive circuit includes a transformer having a secondary coil connected by drive resistances to said points PΓÇ₧ respectively, said drive resistances being in parallel

5 A termination circuit as claimed in claim 2, wherein said communication signal is a radiofrequency (RF) signal and said ground is an RF ground

6 A method of determining values of the components of a termination circuit as claimed in any one of the preceding claims, including determining matched termination values t,, between said lines when at least one of the lines is connected to a communication signal source and the remaining lines are connected to said ground, setting said resistances r,, to nominal values and measuring the resistance between said points P, to obtain measured point impedances PI,,, determining, on the basis of said t,,, final point impedances FPI,,, and determining, on the basis of said t,, and PI,,, sequence point impedances SPI,, which need to be set in sequence and measured to set the point impedances PI,, in the termination circuit to the final point impedances FPI,,

7 A method as claimed in claim 6, wherein said final point impedances FPI,, are obtained by transforming said matched termination values tΓÇ₧ based on the structure of said termination circuit

8 A method as claimed in claim 7, wherein the matched termination values t,, are transformed into an admittance matrix, which is transformed into an impedance matrix from which the final point impedances FP,, are obtained

9 A method as claimed in claim 6, wherein said sequence point impedances SP_|, are obtained by transforming the matched termination values t,,to matched admittances, transforming the measured point impedances PI,, to measured admittances, and for each resistance value r,,, iteratively substituting one of the measured admittances with one of the matched admittances to produce a transform matrix, transforming the transform matrix to a point impedance matrix, and setting a sequence point impedance to an impedance in the point impedance matrix which corresponds to said one of the measured admittances

10 A termination circuit as claimed in any of claims 2 to 5, including an isolation circuit for isolating the power signals from the communication signal at a customer's premises

11 A circuit for use in delivering a communication signal on a power distribution system which distributes power signals, said communication signal having a high frequency relative to a frequency of said power signals, said circuit including a transformer which has windings for each line of the distribution system, and has substantially no net flux for the power signals, and has a net flux for the communication signal

12 A circuit as claimed in claim 11 , wherein the transformer includes a closed single magnetic path core.

13. A circuit as claimed in claim 11 , wherein a capacitor is connected across the winding for one of the lines.

5

14. A circuit as claimed in claim 13, wherein the windings have first terminals connected to a load for the power signals, and second terminals connected to a termination circuit for the communication signal.

10 15. A circuit as claimed in claim 14, wherein isolation capacitors are connected from the first terminal of the winding for said one of the lines to the first terminals of the windings for the other lines.

16. A circuit as claimed in claim 15, wherein said second terminals are 15 connected to distribution lines of the distribution system.

17. A circuit as claimed in claim 14, wherein said termination circuit is as claimed in any one of claims 1 to 5.

20 18. A circuit as claimed in claim 14, wherein first terminals of the windings are connected to service lines for a customer's premises, and second terminals of the windings are connected to distribution lines of the distribution system.

19. A circuit as claimed in claim 18, wherein conditioning impedances are 25 connected across the windings of the other lines.

20. A circuit as claimed in claim 19, wherein the conditioning impedances include a resistance and capacitance in series.

30 21. A circuit as claimed in claim 13 or 15, wherein said one of the lines is a neutral line.