TECHNICAL FIELD

This invention relates to interfaces for transceivers and more particularly to interface architectures for coupling transceivers, or even transmitters or receivers only, to power lines.

BACKGROUND

Power line communication systems are already used in many countries for low voltage power network remote control and remote protection and for remote metering of user energy consumption.

The amount of transferred data for these purposes is limited. Therefore the CENELEC EN 50065-1 "A" band reserved for energy utilities [2], which corresponds to a frequency range from 5 to 95 kHz, can guarantee an adequate data transfer rate [3]. Likewise in Northern America and in Japan the same services can be supplied with PLC because the regulation is more permissive and frequencies up to 525 kHz can be used, i.e. up to the AM broadcast threshold. A further reference for PLC systems is the IEEE standard 643 - 2004 [4].

In recent years energy suppliers are showing an increasing interest for the use of power line communication also in the medium voltage network, instead of the wireless or GSM communication systems. These communication methods have weak reliability (particularly in bad weather conditions) and high intrinsic (additional) costs. On the other hand the actual solutions for PLC application in the MV network need a dedicated MV coupler, capacitive or inductive, to be installed near each MV/LV transformer substations. This solution is technically effective but present MV couplers are not so easy to be installed inside the existing MV switchboard. Moreover, the installation needs an interruption of the electrical power.

A problem for PLC application in the MV network is the design and engineering of the MV coupler for the PLC signal. The coupler must have low impedance for the PLC signal and high impedance for the main frequency voltage. Other problems for PLC application in the MV network are the high voltage level, the behaviour of the power transformers and of the power cables at the PLC signal frequency.

In the scientific literature there are few studies about medium voltage power line behaviour in the PLC frequency range [5], [6]. All the actual commercial solutions use a dedicated coupling network, capacitive or inductive, which has to be installed all over the MV network [7].

In 2008 L. Capetta and C. Tornelli, [8], studied the possibility to use the capacitive divider of a VDS as PLC coupler for the MV power network. They compared the performance of a dedicated MV coupler [7], with the performance of a capacitive divider of a VDS according to the prescriptions of the IEC 61243-5. They concluded that the capacitive divider can be used for the reception of the PLC signal but not for the transmission. Moreover in [8] the authors suggest realizing a high impedance receiver but do not explain how to adapt the receiver to the capacitive divider to obtain the higher received signal. Finally the authors concluded that the capacitive divider can not be used for bidirectional communication but only for the network characterization.

SUMMARY

Novel architectures of coupling interfaces that may be used for coupling transceivers, or even transmitters or receivers only, to power lines, have been found. The limitations of the known transmission/reception technique are brilliantly overcome by exploiting the capacitive voltage dividers that are already installed in the MV switchboards and are composed of the isolators directly connected to the power lines and a capacitor connected between the isolator and the ground. According to the novel method, an adjustable inductance is connected in parallel to a capacitor of a capacitive voltage divider or between the intermediate nodes of two voltage dividers, in order to constitute a resonant circuit tuned at the frequency of the transmitted/received signal, and signals are transmitted/received by connecting a transmitter/receiver in parallel to the adjustable inductance.

Novel architectures of coupling interfaces adapted to implement the novel method are also disclosed.

The invention is defined in the annexed claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a connection scheme of a three phase voltage detecting system with portable indicators.

Figure 2 is a voltage detecting system with portable indicator (separable VDS).

Figure 3 is a voltage detecting system with integrated indicator (integrated VDS).

Figure 4 is a connection scheme of the novel coupling interface between a medium voltage power line and the respective shield.

Figure 5 is a connection scheme of the novel coupling interface between two medium voltage power lines of a three-phase power line.

Figure 6 illustrates how to connect a novel coupling interface according to the scheme of FIG. 4.

Figure 7 illustrates how to connect a novel coupling interface according to the scheme of FIG. 5.

Figure 8 depicts an embodiment of a novel coupling interface adapted to be connected according to the scheme of FIG. 6.

Figure 9 depicts a L-C impedance matching circuit adapted for a novel interface connected according to the scheme of FIG. 6.

Figure 10 illustrates how to realize an adjustable inductance fixed by a controller.

Figure 11 illustrates how to realize an adjustable capacitance fixed by a controller.

Figure 12 depicts a R-C pass-band filter.

Figure 13 depicts an embodiment of a novel coupling interface adapted to be connected according to the scheme of FIG. 7.

Figure 14 depicts a L-C impedance matching circuit adapted for a novel interface connected according to the scheme of FIG. 7.

DESCRIPTION OF EXEMPLARY EMB ODFMENT S

The novel coupling interfaces use the already available Voltage Detecting Systems (VDS), usually installed in the MV switchboards, to inject or receive a PLC signal. Thus no dedicated MV coupler for power line communication has to be installed and thus a high reduction of the costs is achieved.

The capacitive dividers are a constitutive part of the voltage detecting system, typically installed in the medium voltage switchboards of the major electrical manufacturer all over the world. According to the standard IEC 61243-5 "Live working - Voltage detectors Part 5: Voltage detecting systems" [1], these devices have to detect the presence or the absence of the voltage on AC electrical systems for voltages from 1 kV to 52 kV and for frequencies from 162/3 Hz to 60 Hz. The standard IEC 61243-5 classifies Voltage Detecting Systems into: integrated systems and separable systems. The first ones are fixed and form an integral part of the equipment in which they are installed; the second ones have a separable indicator that can be connected to a fixed coupling system by means of an interface.

For each of the three phases of the MV power lines a capacitive divider and a voltage indicator are connected. The connection scheme of a three phase voltage detecting system with separable indicators is shown in FIG. 1. The socket panel 11 is composed of three sockets in each of which a plug with the voltage indicator 4 may be inserted. Each socket has a first terminal 106 connected to ground, and a second terminal 105 connected to one of the three capacitive dividers 103. The capacitance 104 is the equivalent capacitance seen from the terminals 105 and 106, when the plug is not connected.

FIG. 2 shows the electrical circuit for one phase of the separable system wherein the separable voltage indicator 4 is connected to the coupling system 2, whose elements will be described in detail in the following.

The standard IEC 61243-5 defines five different separable VDS systems. For each system the dimensional characteristic of the sockets and the plug arrangement are set. Moreover the electrical characteristic of the coupling system and the voltage indicator are also defined.

The novel coupling interface allows connecting a generic low voltage PLC transceiver to any of the different VDS systems. The plug and the separable indicator, usually built in a unique device 4, have to be substituted by a plug of the same size, through which the signal will be injected or received. Thus the PLC signal connection does not need any change inside the MV switchboard, but only the disconnection of the separable VDS indicator.

The novel coupling interface may be either connected in parallel to the capacitance 104, as the separable indicator of common VDS, or between the node 105 and the corresponding node 105' (or 105").

In the case of an integrated system, instead, the voltage indicator 12 is built in the MV switchboard, as shown in FIG. 3. The novel coupling interface may be used also in the case of integrated system. In this case the voltage indicator 12 has to be disconnected and signals to be transmitted may be injected through the test point terminal 13. The other terminal of the transmitting/receiving system may be either directly connected to ground of the transformer substation or, in case of a three-phase power line, to the test point terminal of a different phase line.

The capacitance 103 has a fixed value. One of its plates is connected to the MV bus-bar 1 and the other plate is available for the voltage detection. A voltage limiting device 7, the stray capacitances 8, the measuring circuit components 9 and a switch 10 for eventually short-circuiting them are usually connected between the second plate and ground. The capacitive behaviour at the mains frequency of all these elements may be modelled as an equivalent capacitance 104 (FIG. 1). The series of the capacitance 103 and the equivalent capacitance 104 constitute a voltage divider, thus the voltage measured at the sockets 105 and 106 is proportional to the mains amplitude voltage. The voltage reduction ratio is determined by the ratio between the values of the two capacitances.

Due to safety reason the measuring circuit component 9 is usually a capacitance of fixed value chosen in order to have a voltage lower than 100 V at the sockets 8. Therefore the equivalent capacitance 104 must have a larger value than the capacitive divider capacitance 103.

Since the equivalent capacitance 104 is greater than the capacitive divider capacitance 103, the use of the VDS socket panel 11 to inject the PLC signal is not so simple. If a generic low voltage transceiver were connected directly to the sockets, the signal would be short circuited by the capacitance 104, and only a small part of the signal would be driven through the capacitive divider 103 into the cable core.

In order to way around this limitation, according to this invention an adjustable inductance is connected between the socket terminals such to form a resonant circuit with the capacitances 103 and 104 tuned at the frequency of signals to be transmitted/received. In so doing the largest part of the energy of a signal to be transmitted will be sent on the power line and only a small part thereof will be dispersed to ground.

The adjustable inductance and other additional components, disclosed hereinafter, are embedded in an coupling interface, which has to be connected between the generic low voltage transceiver and the VDS (Voltage Detective System) sockets 105 and 106 or 105' in the switchboard front panel.

The PLC signal may be injected in two configurations: line-shield and line-line. A simple case of transmission in the line-shield configuration between two transformer substations 15 is shown in FIG. 4. For sake of simplicity the protection devices are not shown. Three cables 14 connect the two transformer substations. A MV/LV power transformer 16 is installed in each substation and a second MV line 17 is also connected to the MV bus-bars. The block 20 represents the coupling system for one phase realized as previously explained. The signal is injected between the core of one cable 101 and the shield 102 connected to ground at the ends of the line. In the case of line-line configuration of FIG. 5, instead, the signal is injected between the core of two cables, 101a and 101b. The coupling system in this case uses two capacitive dividers for each substation and a coupling interface including an adjustable inductance.

The two configurations can be also combined: for example transmitting the signal between two or three cable cores in parallel and using the third cable core and/or the shield as common conductor. The parallel will be realized only at the signal frequency thanks to a dedicated coupling interface.

The coupling interface will have a plug with the same arrangement suggested by the standard in order to be connected to the correspondent socket whose electrical and dimensional characteristic depends on the different systems of separable voltage indicator 4. In this way the coupling interface may be simply connected to the socket and no energy interruptions are necessary for the installation of the PLC system.

In the case of line-shield configuration, the coupling interface 200 will be connected between a phase terminal 105 and the ground terminal 106, thus in parallel with the capacitance 104, as shown in FIG. 6. The coupling interface 200 will be also connected by 108 to the transceiver 107, which will transmit or receive the PLC signal. A further connection 109 between the coupling interface 200 and the transceiver 107 is necessary to switch the coupling interface operation from transmission Tx to reception Rx and vice versa.

In the case of line-line configuration, the coupling interface 300 will be connected between two phase sockets 105, as shown in FIG. 7.

A further terminal 110 is used to ground the coupling interface.

Finally the transceiver signal will be connected by 108 to the coupling interface 300. The Tx/Rx switch signal generated by the transceiver will be also connected by 109 to the coupling interface.

An embodiment of the line-shield coupling interface 200 is shown in FIG. 7. The line terminals 205 and 206 are connected with a plug of standard size, such as the one of the separable indicator plug 4 of FIG. 1, to the sockets 105 and 106. A voltage limiting device 204 is connected to the terminals 205 and 206 to protect the electronic card from severe voltage transients. The voltage limiting device 204 must be chosen in order to have a low capacitance to avoid a short circuit for the communicating signal.

A different circuit was designed for the two cases: transmission and reception. The selection of the desired circuit is fulfilled by the switches 212 and 213. These switches are controlled by the Tx/Rx switch signal 109 from the transceiver. The Tx/Rx switch signal 109 coming from the transceiver has to be connected to the coupling interface terminal 209. In the case of line-shield configuration, the PLC signal is referred to ground, thus terminal 208b has to be connected to ground.

When the switches 212 and 213 are commuted to the Tx position, the signal transmitted by the transceiver is amplified by the amplifier 201 and then applied to the impedance matching circuit 202. The aim of the circuit 202 is to match the impedance seen by the amplifier to its output impedance. As an example in FIG. 9 a simple way to realize the impedance matching circuit, with passive components, is shown. The inductances 202c, 202b and the capacitance 202a shall be of adjustable value. A simpler version of the impedance matching circuit may be obtained without the inductance 202c.

The adjustable inductance 203 has to be adjusted in order to be in resonance at the signal frequency with the capacitance 104. The parallel group formed by the inductance 203 and the capacitance 104 becomes in this way a high impedance for the PLC signal, which will not be anymore short circuited by the capacitance 104 to ground. Thus a higher portion of the signal will cross the capacitive divider 103 and reach the cable core 101.

An example to obtain an inductance of variable value is to connect in series or in parallel several inductances of different value. The inductances which have not to be connected must be short circuited. A possible circuit to obtain an adjustable inductance, useful for the coupling interface, is shown in FIG. 10. Numerous inductances 401, 402, 403... 4xx are connected in series and may be shorted by the controlled switches 501, 502, 503... 5xx

In the same way it is possible to realize a capacitance of variable value by using capacitances of different values connected in series. Another possible way is to connect the inductances or the capacitances in parallel. In FIG. 11 the parallel configuration with capacitances of different value is shown. A combination of the two solutions may be easily obtained and for this reason it is not shown in the figures.

When the transceiver is in reception mode, the switches 212 and 213 are commuted to the Rx position and the adjustable inductance 214 is in parallel with the line terminals 205 and 206.

The inductance 214 as well as in transmission the inductance 203 have also a supplementary task: the voltage at mains frequency is short circuited at the input terminals 205 and 206 thanks to the very low value of the inductances 214 and 203 at the mains frequency and its harmonics. The pass band filter 215 filters both the disturbing high frequencies and the residual low frequency components.

As an example in FIG. 12 a simple pass-band filter realized with passive components is shown. The received and filtered signal is then buffered and amplified by the amplifier 216 and finally received by the transceiver, connected at the terminals 208a and 208b.

The electrical scheme of the line-line coupling interface 300 is shown in FIG. 13. The terminals 305a and 305b are connected to the two phase sockets 105, a further terminal 110 is connected to ground. A voltage limiting device 204 is connected between each of the two terminals 305a and 305b and the ground, to protect the electronic card from voltage transients. The switches 312, 313a and 313b select the transmission or reception circuit. The switches are controlled by the Tx/Rx switch signal received by the transceiver.

When the switches are commuted to the Tx position, the signal transmitted by the transceiver is amplified by the amplifier 301. The amplifier output is connected to an impedance matching circuit 302.

An embodiment of the impedance matching circuit with passive components is shown in FIG. 14. A simpler version of the impedance matching circuit may be obtained without the inductances 302c and 302e. An isolation transformer 302d is connected between the terminal a and the ground, in order to convert the signal from single-ended to differential. The signal generated from the transceiver, in fact, is referred to ground. The impedance matching circuit output signal (Vcb), instead, has to be differential, in order to be sent to the two line terminals 305a and 305b connected to the phase sockets 105. The adjustable inductances 303a and 303b are connected at the two terminals 305a and 305b. Each of these two inductances has to be adjusted in order to drive a larger part of the signal to the cable core and reduce the part of the signal short circuited to ground by constituting a resonant circuit together with the capacitance connected thereto.

When the transceiver is in reception, the switches 312, 313a and 313b are commuted to Rx. The inductances 314a and 314b are adjusted to be in resonance with the equivalent impedance seen from the terminals 305a and 305b and the ground. The received differential signal is filtered by a pass-band filter 315, that may be the one of FIG. 12. The filtered signal is then buffered and amplified by a differential amplifier 316 and finally received by the transceiver, connected at the terminals 208a and 208b.

The claims as filed are integral part of this description and are herein incorporated by reference.

REFERENCES

IEC 61243-5 "Live working - Voltage detectors Part 5: Voltage detecting systems (VDS)", 2002.

EN 50065-1 "Signalling on low-voltage electrical installations in the frequency range 3 to 148.5 kHz— Part 1 : General requirements, frequency bands and electromagnetic disturbances." EN 50065-1 : 1991, Amendment Al : 1992 to EN 50065-1 : 1991; Amendment A2: 1995 to EN 50065-1 : 1991; Amendment A3 : 1996 to EN 50065-1 : 1991.

P.A.A.F. Wouters, P.C.J.M. van der Wielen, J. Veen, P. Wagenaars, E.F. Steennis, "Effect of cable load impedance on coupling schemes for MV power line communication," IEEE Trans. Power Del., vol. 20, no. 2, pp. 638-645, Apr. 2005. IEEE Standards-643TM IEEE Guide for Power Line-Carrier Applications, IEEE Std. 643, Jun. 8, 2005.

A.Cataliotti, A.Daidone, G.Tine, "Power line communications in Medium Voltage system: Characterization of MV cables", IEEE Transactions on Power Delivery, vol. 23, n. 4, October 2008.

A.Cataliotti, A.Daidone, G.Tine, "A Medium Voltage Cable model for Power Line Communication", IEEE Transactions on Power Delivery, vol. 24, n. 1, pp. 129 - 135, January 2009.

R. Benato, R. Caldon, F. Cesena, "Application of Distribution Line Carrier-based protection to prevent DG islanding: an investigative procedure", Power Tech Conference Proceedings IEEE, Vol. 3, 23-26 June 2003 Bologna.

L. Capetta, C. Tornelli: "L'evoluzione del Sistema T&D - Metodiche di comunicazione per il monitoraggio, il controllo e le protezioni delle reti di distnbuzione" Febmary 2008 Cesi Ricerca.