AU2011101421A4 - Complementary capacitive-inductive data coupler for power line communications - Google Patents

Abstract

A complementary data coupler for power-line communications, combining the impedance adaptation strengths of inductive couplers and capacitive couplers, is disclosed. Unpredictable power-line impedance fluctuations is a fundamental problem for power-line communications, as it makes impedance adaptation almost impossible. Used separately, capacitive couplers are well suited to higher power-line impedance levels, while inductive couplers are well suited to lower power-line impedance levels. The performance of both types of couplers deteriorate drastically, as power-line impedance levels move away from one type of coupler's optimum level toward the other type of coupler's optimum level. However, if the received signal from both are combined, these weaknesses are minimised, yielding a complementary coupler that is reasonably immune against fluctuations in power line impedance. SkQ T1 C2 T2 R4 OA E _pl 52 k Figure 3 R1_eqv + E R_pl R2eqv Figure 4

Description

1 COMPLEMENTARY CAPACITIVE-INDUCTIVE DATA COUPLER FOR POWER LINE COMMUNICATIONS. TECHNICAL FIELD [0001] This invention relates to power-line communications, and more particularly, to data couplers for power-line communications. BACKGROUND ART [0002] Power-line communications deploy communication signals through power cables and/or networks, thus utilising the power grid for a communication network without having to pay for cabling. However, couplers are required to superimpose and / or extract this communication signal to- and from the power waveform. Thus a power-line communications coupler protects sensitive communication equipment (modems) from being damaged by the power voltage, whilst facilitating the flow of the communication signal to- and from the power cable. Traditionally, either capacitive or inductive couplers have been used for this filtering task, each having unique advantages and disadvantages resulting in a typical type of deployment. [0003] Power-line communications couplers may be broadly classified into two groups, namely inductive couplers and capacitive couplers. Inductive couplers are electrically insulated from the power cables, and therefore no direct electrical connection is made. However, in most cases thyroidal (also called ring-shaped or doughnut-shaped) magnetic core-halves have to be clamped around the power cable, to be coupled to. See Figure 1. In essence, inductive couplers are specialized transformers with a modem-side primary winding, and the power cable acting as a half-turn secondary winding. Signals are therefore directly induced through the coupler, to and from the power cable (that extends through the hole in the core). In exceptional cases, the power cable may be wound around the one half (leg) of the open core to increase the coupling coefficient. These inductive couplers are well suited to substation installations, as well as other central points where power cables are insulated and/or tampering with cables is not advisable. [0004] Capacitive couplers however, make up the bulk of power-line couplers. This is especially true in low-voltage applications where numerous transmitter and receiver nodes have to be implemented economically in order to compete against other network technologies such as 2 wireless networks as well as cable networks. As the majority of low-voltage power-line communications modems require dedicated ac power for their dc power supplies, the direct electrical connection to the power grid is already a given. Therefore the choice of a low-cost capacitive coupler, with a direct electrical connection to the power network, is obvious. All capacitive couplers have four terminals (two modem connections as well as two power-line connections to live and neutral). These capacitive couplers may be as simple as a single series capacitor, or a simple passive L-C filter. However, the majority of capacitive couplers make use of an added coupling transformer for impedance adaptation as well as protection. See Figure 2. [0005] Impedance adaptation is a crucial function of all power-line communications couplers. This task is an almost impossible one, for a typical power-line impedance (as experienced by a modem at the communication frequencies), varies drastically with time and location. This in turn, is caused by loads/appliances that are switched 'on' and 'off, into and out of the network. Conflicting impedance adaptation requirements are further imposed by cable branches (series impedances) and loads (parallel impedances). Reference 1 shows that improved transmission and reception results may be obtained with capacitive couplers, by means of varying the transformer winding ratio. This in turn, determines the voltage/current balance of the signal to be optimally transmitted or received. [0006] For effective transmission of a power-line communications signal, the effect of (relatively large) series impedances needs to be minimized by transmitting the signal largely as a voltage using low current levels. Conversely, the negative effect of (relatively small) parallel impedances may be countered by transmitting the signal largely as a current - using low voltage levels. This trade-off between satisfying cable impedances versus load impedances, is a challenging problem for power-line communications. As mentioned above, this challenge is further complicated by the unpredictable switching of parallel loads, making it almost impossible to implement impedance adaptation effectively. SUMMARY OF INVENTION [0007] This invention describes a method to harness both inductive and capacitive power-line coupling in a complementary manner, thus yielding the inherent impedance adaptation benefits of both kinds of couplers at a single transmitter / receiver node.

3 [0008] Capacitive couplers, in principle, operate as voltage couplers, and are connected in parallel with loads, similar to a voltmeter. Therefore, these couplers also need to exhibit high internal or input impedances in order to not drain and compromise the signal to be measured. Capacitive couplers are thus well suited to send or receive power-line communications signals as voltage signals, having a small current. Examples where this is applicable, include power-line topologies where cable distances are long, compared to the severity of the loads currently connected. [0009] Inductive couplers further operate (in principle) as current couplers, and although most are clamped around the power-line conductor, the effective impedance experienced by the circuit, is in series with the load. Therefore inductive couplers should be designed and used similar to ammeters and exhibit a small effective internal impedance. Inductive couplers are thus well suited to transmit or receive a power-line communications signal as a current signal, having a small voltage. This is true of power-line networks where many loads are in near vicinity, and/or when the effect of load severity, dominates the effect of cable distance. [0010] Although Reference 1 makes use of a capacitive coupler only, to optimally receive or transmit a power-line communications signal in voltage or current mode, the inherent qualities of a capacitive coupler cause unwanted side-effects such as power losses, when it is leveraged by a high winding ratio to receive a current signal - especially when power-line impedance levels deviate from the optimum, design point. This invention alleviates these power losses, and provides a more constant performance across a wide range of power-line impedances as will be discussed below. [0011] Figures 1 and 2 show typical inductive and capacitive couplers respectively. This invention requires that both outputs from these couplers are utilised in order to guarantee better performance under fluctuating power-line impedance conditions. For example, these two signals may be fed into a summing device, such as a summing operational amplifier, to superimpose the two received signals before they are interpreted by the receiver as a combined signal. DESCRIPTION OF INVENTION [0012] Figure 3 shows a circuit diagram of one possible application of the invention for a receiver setup. In Figure 3, E and R-pl form the power circuit, E being the voltage source and 4 R_pl representing a (simplified) power-line impedance, lumped as a parallel load resistance for clarity. Take note that the value of Rpl fluctuates, making impedance adaptation almost impossible. TI and RI represent the inductive coupler with a 50-Q terminating resistor respectively. The capacitive coupler comprises of C2 with T2 (optional) and R2, also a 50-Q terminating resistor. [0013] Both these stages are fed to an ordinary inverting summing amplifier OA, with suggested series input resistors of 5 kQ each, R3 and R4. R5 is an adjustable feedback gain resistor, with suggested maximum value of 50 kQ. R5 may be adjusted to boost the received signal to a suitable level just below the dynamic range of the modem input. The gain achieved should also combat the ~ 3 dB of losses incurred by R6, the series output resistor for further matching with a 50-Q modem input. Although other impedance levels may be chosen for a specific application, the results and knowledge obtained in Reference 1 are based on a 50-Q system, and therefore the circuit in Figure 3, as well as the following examples will be based on 50-Q terminating impedances. [0014] In order to understand the complementary operation of the two kinds of couplers, a simplified, equivalent circuit is shown in Figure 4. Here, only reflected values, as experienced by the power circuit, is considered, to show how received power varies as the power-line impedance fluctuates. Take note that impedance values are the equivalent values at communication frequencies, and are assumed to be resistive for the sake of clarity. [0015] In Figure 3, an inductive coupler with winding ratio of 1:7 is assumed, reflecting the 50 Q terminating resistor by the square of its winding ratio. This reflected terminating resistor of approximately 1 Q, is effectively in series with the power line (see Figure 4). The capacitive coupler is assumed to have a 'transparent' winding ratio of 1:1, and the 50-Q terminating resistor is effectively in parallel with the power-line impedance (shown in Figure 4). For this example, it is further assumed that the power-line impedance would vary or fluctuate between the extremes of 1 Q and 50 Q. Thus for this example, the two winding ratios of the inductive and capacitive couplers have been chosen, to yield reflected terminating impedances equal to the expected limits of power-line impedance.

5 [0016] The differing performance of the two couplers can now easily be understood by considering Figure 4, and for the purpose of comparison, assuming that a constant power of say 7 W is received by the combined, effective load. In Figure 4, it can be seen that i) the 1-Q equivalent resistor of the inductive coupler, is in series with a parallel branch of ii) the current power-line impedance Rpl and iii) the 50-Q equivalent resistor of the capacitive coupler. Three cases will be considered below. First, a power-line impedance of 1 Q, then a power-line impedance of 50 Q, and finally a power-line impedance of 7 Q. [0017] If the current power-line impedance is 1 Q, the parallel branch of 1 Q ||50 Q has an equivalent value of~ 0.98 Q. This value, added to the 1-Q series resistor RIeqv, yields a total load of 1.98 Q and thus a total received r.m.s. voltage of V = sqrt(P*R) = sqrt(7* 1.98) ~ 3.72 V, shared almost equally as ~ 1.88 V across RIeqv and ~ 1.84 V across the parallel branch. These values predict a received power of 3.53 W for the inductive coupler (1 Q) and only 68 mW for the capacitive coupler (50 Q resistor). The remaining ~ 3.4 W is wasted in the equivalent power line impedance, currently 1 Q. Thus the inductive coupler would perform almost optimally at ~ 3 dB for this power-line impedance level, whilst the capacitive coupler performs dismally. [0018] Similarly, if the current power-line impedance is 50 Q, the parallel branch of 50 Q ||50 Q yields a value of~ 25 Q. This value, added to the 1-Q series resistor R1 eqv, yields a total load of 26 Q and thus a total received voltage of V = sqrt(P*R) = sqrt(7*26) ~ 13.5 V, shared by the two series branches as ~ 0.52 V across RIeqv and the remaining ~ 12.98 V across the parallel branch. These values now predict a received power of only ~ 270 mW for the inductive coupler (1 Q) while 3.37 W is received by the capacitive coupler (50 Q resistor). Again, the remaining ~ 3.37 W is wasted in the equivalent power-line impedance, currently 50 Q. Thus now the inductive coupler performs poorly for this high power-line impedance level, whilst the capacitive coupler performs almost optimally at ~ -3 dB. [0019] As a final example, consider a current power-line impedance of 7 Q, where the parallel branch of 7 Q ||50 Q yields an equivalent value of~ 6.14 Q. This value, added to the 1-Q series resistor R1_eqv, yields a total load ofz 7.14 Q and thus a total received voltage of V = sqrt(P*R) ~ 7.07 V, shared by the two series branches as ~ 0.99 V across RIeqv and the remaining ~ 6.08 V across the parallel branch. These values now predict a received power of~ 980 mW for the inductive coupler (1 Q resistor) while ~ 740 mW is received by the capacitive coupler (50 Q 6 resistor). Thus a large portion of the total power, ~ 5.28 W is wasted in the equivalent power-line impedance, currently 7 Q. In this case, performance of both the inductive coupler and the capacitive coupler is not ideal, but acceptable. [0020] From the above 3 examples, the complementary functioning of a combined inductive and capacitive coupler can be understood. If the signal received by both couplers are combined as suggested in Figure 3, the combined, received power levels, fluctuate from ~ 3.6 W to ~ 1.72 W and back to ~ 3.64 W, as the power-line impedance fluctuates from 1 Q to 7 Q and finally 50 Q. This implies that the combined, received power of the complementary coupler, would theoretically never drop below 1.72 W. Compared to the minimum received power of 68 mW and 72 mW for separate inductive and capacitive couplers, the factor ~ 25 (or ~ 14 dB) gain in power during worst-case impedance levels, clearly illustrates the advantage of this invention, even if coupler costs double. [0021] A side-effect of the above implementation, that has to be taken into consideration, is phase distortion. The two signals from the two different couplers, will be out of phase when they are summed. Thus the resulting waveform would show distortion, and also harmonic content associated with this change in waveform shape. Some modulation techniques may be more suitable and more immune against phase distortion, such as FSK and to a lesser degree OFDM. Other types of modulation, where the phase angle of the signal carries information, such as QPSK and others, may be unsuitable for this complementary coupler scheme. [0022] The techniques described in this specification are for illustrative purposes, and should not be interpreted to imply or acknowledge any particular limitation on the current invention. Diverse variances of the current invention could be developed by persons skilled in the art. This specification is intended to encompass all such alternatives, modifications and combinations that fall within the scope of the applicable claims. CITATION LIST 1. P. A. Janse van Rensburg, H. C. Ferreira, "Coupler winding ratio selection for effective narrow-band power-line communications," IEEE Transactions on Power Delivery, vol. 23, no. 1, January 2008, ISSN 0885-8977, pp. 140-149.

Claims (5)

1. A method to combine capacitive and inductive power-line communications coupling for a single receiver node, in order to harness the unique impedance adaptation strengths of each type of coupler, resulting in a complementary system for improved signal power reception across a wide range of fluctuating power-line impedance values as experienced at communication frequencies.

2. A complementary transmitter coupler according to claim 1, wherein the signal to be transmitted, is sent to both the inductive as well as the capacitive coupler, in order that the transmitted power is shared between the couplers, and a more guaranteed performance is obtained for transmission during power-line impedance fluctuations.

3. A combined bidirectional complementary coupler according to the above two claims, wherein the same inductive and capacitive couplers are used for both transmission and reception.

4. A complementary coupler according to any of the above claims, wherein the ideal impedance adaptation ratio of each of the inductive and capacitive couplers are chosen or designed such that they will not necessarily share power equally at expected power-line impedance extremes as to cater for certain power-line impedance levels that are expected to be more prevalent.

5. A complementary coupler according to any of the above claims, wherein the received / transmitted signal from / to each of the inductive and capacitive couplers are processed electronically in a different fashion in order to combine / split the two said signals for improved performance under fluctuating power-line impedance conditions. P.A. JANSE VAN RENSBURG 3 NOVEMBER 2011 A.J. SNYDERS H.C. FERREIRA