Patent Publication Number: US-2002002040-A1

Title: Method and apparatus for interfacing RF signals to medium voltage power lines

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims priority under 35 U.S.C. § 119(e) from provisional application No. 60/198,306, filed Apr. 19, 2000. The 60/198,306 provisional application is incorporated by reference herein, in its entirety, for all purposes. 
    
    
     
       INTRODUCTION  
       [0002] The present invention relates generally to the field of digital communications. More particularly, the present invention relates to communication of digital signals, including information and control signals, via power lines.  
       BACKGROUND OF THE INVENTION  
       [0003] Referring to FIG. 1, a typical electric power distribution architecture is illustrated. The configuration shown has a transformer  10  that steps medium voltage (tens of thousands of volts) power down to low voltage power (a few hundred volts AC, typically 100 to 240 VAC). The low voltage (LV) power is fed to several homes  20 . Using this architecture for a power line communications system, the medium voltage (MV) line can be used to transfer data to and from the LV lines.  
       [0004] Since the MV line is typically rated at a few tens of kilovolts, interfacing to the MV line can prove to be costly, size prohibitive, and pose safety hazards. For example, an RF (radio frequency) signal is typically used in a powerline communications system as a carrier for digital information or control signals. In order to couple such an RF signal into a wire with large voltage at low frequencies (50-60 Hz), the solution would appear to call for a high pass filter that can withstand high voltage (i.e., the voltage level of the MV line). The high pass filter is used to prevent the low frequency (typically in the range of 50-60 Hz) power line voltage from being coupled into the RF transmitter and receiver circuitry, referred to below as the “communications device”. A capacitor could provide such a filter, however such a filter capacitor would need to be able to withstand tens of kilovolts carried on the MV line. A filter capacitor according such requirements is expensive and physically large.  
       [0005] Thus, what is needed is a low cost, small form factor, safe scheme for interfacing RF signals to a MV power line.  
       SUMMARY OF THE INVENTION  
       [0006] The present invention provides a method and apparatus of interfacing a high frequency signal in a power line communications system. The present invention is especially useful in, although by no means limited to, applications in which an RF signal is coupled to and from a medium voltage power line, such as a power distribution or transmission line. There are two types of MV distribution systems that power utilities use: (1) aerial and (2) underground. Presently preferred embodiments of the invention work for both distribution system types.  
       [0007] A presently preferred implementation of the invention employs a metal oxide varistor (MOV) as an element of a high pass filter to substantially attenuate the low frequency power line voltage and current waveforms, so as to prevent them from damaging the communications device used to transmit and receive the digital signal. Other aspects of presently preferred embodiments of the invention are described below. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008] Additional objects and advantages of the present invention will be apparent in the following detailed description read in conjunction with the accompanying drawing figures.  
     [0009]FIG. 1 illustrates a conceptual view of a typical electric power distribution topology.  
     [0010]FIG. 2 illustrates a schematic block diagram of an interface circuit for a medium voltage power line according to an embodiment of the present invention.  
     [0011]FIG. 3 illustrates a graph of the transfer function for a front-end circuit embodied according to one embodiment of the present invention.  
     [0012]FIG. 4 illustrates a schematic diagram of an interface circuit front-end portion, according to an alternate embodiment of the present invention, with a ferrite bead to ground.  
     [0013]FIG. 5 illustrates a schematic diagram of an interface circuit front-end portion, according to another alternate embodiment of the present invention, with a quarter wave length tap. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0014] The present invention may be advantageously embodied as an interface circuit for interfacing transmitted and received radio frequency communications signals with a medium voltage power line. A preferred embodiment of the interface circuit has a medium voltage node that is adapted for connection to the medium voltage power line, a reactive element adapted to be connected to ground, and a metal oxide varistor connected between the medium voltage node and the reactive element. A first opto coupler couples into the interface circuit the transmitted radio frequency communications signals to be interfaced via the medium voltage node, and a second opto coupler couples out of the interface circuit the received radio frequency communications signals interfaced via the medium voltage node. A radio frequency combiner is connected to the reactive element. The radio frequency combiner is also connected to the first opto coupler so as to make the transmitted radio frequency communication signals available to the medium voltage node via the metal oxide varistor. The radio frequency combiner, which is optional and may be used in situations in which the transmit and receive signals are coupled to/from the MV line via a common interface, is further connected to the second opto coupler so as to make the received radio frequency communications signals available from the medium voltage node via the metal oxide varistor.  
     [0015] In order to couple an RF signal into a wire with high voltage at low frequencies (50-60 Hz), essentially a high pass filter that can withstand high voltage may be used. As mentioned above, a capacitor that can operate as such a filter and withstand tens of kilovolts is prohibitively expensive and physically large.  
     [0016] One device that provides capacitance (other than a capacitor) in a communications application is a MOV. A column of MOVs can withstand up to 80 kV peak, and provide about 10 pF of capacitance. The physical size is acceptable as well. An additional feature of using MOVs is that they provide surge protection. As MV lines are subject to lightning strikes, high energy surges, switching transients, etc., this increased safety feature is important. MOV products fitting these requirements (e.g., MOV gapped elbow arrester  235 - 55 ) are available from Cooper Power Systems, Inc. of Pittsburgh, Pa.  
     [0017] An opto-coupler is used for additional safety, thus electrically isolating the MV circuitry from the communications equipment via light.  
     [0018] Referring to FIG. 2, an exemplary embodiment for constructing such an interface circuit is illustrated. The MOV  202  is connected to the MV line and to the primary windings of a transformer  212 . An RF signal on the MV line will flow through the MOV  202  and will be present on the secondary of the transformer  212 . The MOV  202  is an open circuit for the medium voltage power (since the power line voltage is less than the MOV&#39;s conduction voltage). Preferably, the transformer  212  is a toroidal transformer with a few windings of the MOV  202  ground wire on the primary and a few windings of magnetic wire on the secondary. The RF combiner  204  combines the receive and transmit signals from the communications device. The amplifier  206  on the receive side increases receiver sensitivity but is an optional feature that is not necessary for all applications. The opto-couplers  208 ,  210  decouple the communications device from the medium voltage line circuitry (a desired safety feature); this too is an optional feature that is not necessary for all applications.  
     [0019] Referring to FIG. 3, the transfer function for an exemplary front end circuit is illustrated. Frequency (in MHz) is represented along the horizontal-axis, and attenuation (in dB) is represented along the vertical-axis. A broad band of relatively low attenuation is evident from about 15 MHz to about 115 MHz. Low (i.e., power distribution) frequencies, such as those in the 50-60 Hz band, are heavily attenuated.  
     [0020] Referring to FIG. 4, an interface circuit front-end portion according to an alternate embodiment is illustrated. In this embodiment, a ferrite bead  420  is placed between the MOV  410  and ground to choke the RF signal from being grounded. The ferrite bead  420  is connected in parallel with the transformer  430 .  
     [0021] Referring to FIG. 5, an interface circuit front-end portion according to another alternate embodiment is illustrated. The transformer shown in the previously described embodiments is omitted and its functionality is replaced by feeding the ground line  502  between the MOV  510  and ground at an interval L that is a quarter wavelength (λ/4), measured at the carrier frequency, from the ground point. Installation according to this alternate embodiment may be tricky, but has the advantage of being particularly elegant.  
     [0022] The present invention has been described in terms of preferred embodiments, however, it will be appreciated that various modifications and improvements may be made to the described embodiments without departing from the scope of the invention.