Abstract:
There is provided a method for protecting loads associated with power distribution system inductive signal couplers. The method includes (a) providing an inductive signal coupler having a first winding in series with a line conductor of a power distribution system, a second winding having a terminal, and a magnetic core for coupling a signal between the first winding and the second winding, (b) situating a layer of electrical insulation between the second winding and the magnetic core, and (c) connecting the terminal to an electrical ground so that a voltage between the magnetic core and the electrical ground is placed across the layer of electrical insulation. There is also provided an inductive signal coupler having (a) a winding in series with a line conductor of a power distribution system, and (b) a conductive plate at an end of the coupler distal from the first winding, for routing a flashover current to an electrical ground.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is claiming priority of U.S. Provisional Patent Application Ser. No. 60/364,321, filed on Mar. 14, 2002, and U.S. Provisional Patent Application Ser. No. 60/376,377, filed on Apr. 29, 2002. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to coupling communication signals to electrical power distribution systems. 
   2. Background of the Related Art 
   Radio frequency (rf) modulated data signals can be coupled to and communicated over medium and low voltage power distribution networks. Use of inductive couplers for this purpose is described in U.S. Pat. No. 6,452,482, entitled “Inductive Coupling of a Data Signal to a Power Transmission Cable”, and U.S. patent application Ser. No. 10/082,063, filed Feb. 25, 2002, entitled, “Coupling Broadband Modems to Power Lines”, both of which are assigned to the assignee of the present application, and the contents of which are incorporated herein by reference. 
   Power distribution networks are occasionally subject to significant transients in voltage and current. For example, a strong current pulse of fast rise time is created when a power line device such as a distribution transformer short circuits, or when power lines fall and touch each other. Similarly, a lightning strike to a nearby point on the power line generates a traveling wave on the power line. A standard method of simulating a lightning strike is the Basic Impulse Loading (BIL) pulse, used for testing power line devices that would be connected to power lines, and it has a rise time of 1.2 microseconds, with a much longer fall time. The amplitude of such test pulses can vary between 90 and 200 kV peak. 
   A power line inductive coupler is basically a transformer whose primary is connected to the power line and whose secondary is connected to a communications apparatus such as a modem. The primary winding has one or just a few turns and presents a very low impedance at a power frequency. However, the coupler is capable of coupling the high frequency energy represented by the fast onset of a lightning pulse or other transient, and substantial voltage would be induced in the coupler secondary circuit. 
   Coupler flashover of medium voltage from a primary power wire to ground occurs when the wire&#39;s voltage exceeds the insulation capability of the coupler, whether during normal operation or during transient voltage pulses originating in lightning strikes or switching transients. Flashover can occur on the outer surface of the coupler or internally between parts of the coupler. Flashover may be considered a very rare event for suitably insulated devices attached to a medium voltage power line. For example, current and potential transformers commonly used by utilities often do not carry special protective circuitry. But in the case of a data coupler, which is intended to be used ubiquitously for a large customer base, it is considered prudent to protect against rare events, to prevent injury or damage. 
   In addition, since the modem is connected to lines leading to customer equipment, the modem is grounded. Therefore, the distribution power voltage must be insulated from the modem. If the inductive coupler&#39;s secondary were insulated from ground, then the voltage difference between the power line and ground would be divided across (a) the coupler&#39;s primary to secondary insulation and (b) the insulation of other devices in the chain of devices leading to the modem. The voltage drops would be proportional to the impedances across each insulation interface, and thus inversely proportional to the stray capacitance across each such interface. 
   When dealing with medium voltage ac power lines, with voltages in excess of 2,000 volts rms relative to neutral or ground, this capacitive voltage division would be difficult to make deterministic, as the coupler capacitance would depend on the position and diameter of the power line within the coupler. Therefore, any other insulating devices would need to be capable of insulating the full power line voltage, and thus be large and expensive. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention are directed to techniques for protecting an inductive coupler of data signals to a power distribution network from electrical transients such as transient over-voltage and over-current conditions. More specifically, embodiments of the present invention enable an inductive coupler to withstand voltage spikes and provides protection against surges from flashover, i.e., the sudden breakdown of electrical insulation in the coupler, with optimal coupling of the rf data signal between an rf data modem and the power line. Embodiments also protect against transient current pulses that may develop on the power distribution line from such causes as a lightning strike or short-circuiting of the line to electrical ground. 
   A method for protecting loads associated with power distribution system inductive signal couplers includes (a) providing an inductive signal coupler having a first winding in series with a line conductor of a power distribution system, and a second winding having first and second connection terminals, (b) connecting a first terminal of a first fuse to the first connection terminal, and a first terminal of a second fuse to the second connection terminal, a second terminal of each fuse being connected to a communication device, and (c) connecting a first terminal of a first choke to the second terminal of the first fuse, and a first terminal of a second choke to the second of the second fuse, a second terminal of each choke being connected to an electrical ground. 
   Another method for protecting loads associated with power distribution system inductive signal couplers includes (a) providing an inductive signal coupler having a first winding in series with a line conductor of a power distribution system, and a second winding having first and second connection terminals, (b) encapsulating the second winding inside a layer of electrical insulation, and (c) connecting the second winding to an electrical ground using protection circuits so as to place any high voltage field across the layer of electrical insulation. 
   Another method for protecting loads associated with power distribution system inductive signal couplers includes providing an inductive signal coupler having a first winding in series with a line conductor of a power distribution system, and a second winding having first and second connection terminals, in which the coupler has a body including sheds providing a leakage path to avoid external flashover during an electrical transient. 
   Another method for protecting loads associated with power distribution system inductive signal couplers includes (a) providing an inductive signal coupler having a first winding in series with a line conductor of a power distribution system, and a second winding having first and second connection terminals, the coupler having a body including a conductive plate at an end of the coupler distal from the first winding, and (b) connecting the conductive plate to an electrical ground so as to route a flashover current directly to the electrical ground. 
   Another method for protecting loads associated with power distribution system inductive signal couplers includes (a) providing an inductive signal coupler having a first winding in series with a line conductor of a power distribution system, and a second winding having first and second connection terminals, and (b) connecting each terminal of the second winding to an electrical ground via a choke, the choke presenting a high impedance to signal frequencies and a low impedance to current from an electrical fault signal. 
   An arrangement of components includes (a) an inductive signal coupler having a first winding in series with a line conductor of a power distribution system, and a second winding having a first connection terminal and a second connection terminal, (b) a first fuse having a first terminal connected to the first connection terminal, and a second terminal for coupling a signal to a first terminal of a communication device, (c) a second fuse having a first terminal connected to the second connection terminal, and a second terminal for coupling a signal to a second terminal of the communication device, (d) a first choke having a first terminal connected to the second terminal of the first fuse, and a second terminal connected to an electrical ground, and (e) a second choke having a first terminal connected to the second terminal of the second fuse, and a second terminal connected to the electrical ground. 
   Another arrangement of components includes (a) an inductive signal coupler having a first winding in series with a line conductor of a power distribution system, and a second winding encapsulated inside a layer of electrical insulation, and (b) a circuit between the second winding and an electrical ground for placing a high voltage field across the layer of electrical insulation. 
   Another arrangement of components includes (a) an inductive signal coupler having a first winding in series with a line conductor of a power distribution system, and a second winding having a first connection terminal and a second connection terminal, (b) a first choke between the first connection terminal and an electrical ground, and (c) a second choke between the second connection terminal and the electrical ground. Each of the first choke and the second choke present a high impedance to a signal frequency and a low impedance to current from an electrical fault signal. 
   An inductive signal coupler for coupling a signal to a power distribution system includes a first winding in series with a line conductor of the power distribution system, and sheds for providing a leakage path to avoid external flashover during an electrical transient. 
   Another inductive signal coupler for coupling a signal to a power distribution system includes a winding in series with a line conductor of the power distribution system, and a conductive plate at an end of the coupler distal from the first winding, for routing a flashover current to an electrical ground. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more readily understood by reference to the following detailed description taken with the accompanying drawings, in which: 
       FIG. 1  shows an inductive coupler circuit according to one embodiment of the present invention, which is protected against over-voltage transients. 
       FIGS. 2A ,  2 B and  2 C show embodiments of the present invention in which an arrangement of capacitors and surge suppressors protect against electrical transients. 
       FIG. 3  shows the equivalent circuit for a voltage transient created by flashover. 
       FIG. 4  shows a cross-section of one particular physical implementation of an inductive coupler according to an embodiment of the present invention. 
       FIG. 4A  shows a section  4 A— 4 A of the inductive coupler of  FIG. 4 . 
       FIGS. 5A and 5B  shows particular specific implementations of a dual fuse according to an embodiment of the present invention. 
   

   DESCRIPTION OF THE INVENTION 
   One embodiment of the present invention grounds the secondary winding of an inductive coupler by appropriate rf devices. This protects against over-voltage transients and takes full advantage of the principle of magnetic coupling, which is not affected by the thickness of the winding insulation. Consequently, the medium voltage of the power line is insulated from the modem solely by the insulation of the secondary winding. This approach prevents flashover current from propagating to low voltage lines and loads, and so prevents damage to the modem and other equipment to which the coupler may be connected. 
     FIG. 1  shows an inductive coupler circuit according to one embodiment of the present invention, which is protected against over-voltage transients. Power distribution line  100  forms a primary winding  105  of inductive coupler  110 , which in turn is connected to an rf data signal modem (not shown) via output terminals  160  and  165 . Secondary winding  115  has terminals  120  and  125 , which connect to the upper terminals of transient protection fuses  130  and  135  respectively. Rf chokes  140  and  145  connect the lower terminals of the fuses to ground  150 , typically via a wire  155  connected to the “pole ground,” a ground wire running from a ground rod at the base of the electric pole, up to the top of the pole. This pole ground  150  will generally be readily available in typical applications of a power line inductance coupler  110  used to bypass a distribution transformer on electric power poles. 
   The coupler  110  physically bridges a space between the power line  100  and the ground  150  connected to the coupler secondary winding  115 . Thus, a leakage path is needed that is long enough to preclude external flashover. A typical embodiment provides “sheds.” In case external flashover occurs anyway, the coupler  110  may include an exposed metal base connected to the ground  150  to which an external flashover arc may jump without harm. 
   The rf chokes  140  and  145  are provided to ground any potential internal flashover current within the coupler  110 . The secondary winding  115  is typically embedded in insulating material body of the coupler  110 , which should be thick enough to provide a sufficient insulation rating for both steady state (“withstand”) voltage and for fast high voltage BIL pulses. The rf chokes  140  and  145  provide an rf impedance substantially greater than the rf impedance of the coupler secondary winding  115 , while providing a low impedance to ground  150  after a few microseconds of a fault pulse. The connection of chokes  140  and  145  in shunt with the signal voltage provides a high pass filtering effect, as low frequencies are effectively shorted to ground  150 . For modem frequencies above 1 MHz, chokes  140  and  145  might typically have an inductance of 10 uH each, providing a reactance across the coupler secondary winding  115  in excess of 124 ohms and rising with frequency. The chokes  140  and  145  should have a self-resonant frequency above the highest frequency of interest. 
   Flashover current is limited only by the capacity of the power line  100 , typically up to 10,000 amps rms or about 14,000 amps peak. This flashover current is interrupted by and divided roughly equally between the fuses  130  and  135 . Until the fuses  130  and  135  blow open, the rf chokes  140  and  145  need to carry the short circuit current without failing. Thus, rf chokes  140  and  145  should be wound with wire capable of withstanding the flashover current pulse that might flow. 
   The speed and size of a possible flashover current pulse suggests use of suitably rated expulsion fuses or current limiting fuses for the transient protection fuses  130  and  135 . An expulsion fuse can interrupt current up to 8 milliseconds after the onset of a flashover transient. A current limiting fuse may interrupt faster, estimated not to exceed 4 milliseconds after the onset of a flashover transient. To maintain compliance with electromagnetic radiation standards, data signal current is expected to be much less than one ampere, so a 1 amp current rating for the fuses  130  and  135  would be suitable for minimizing the duration of any flashover current after an internal insulation failure. 
   Both current limiting and expulsion fuses have considerable length and breadth, as needed to extinguish the high energy arc initiated and maintained by the kilo-ampere short circuit current of power distribution lines. Placement of two such individually packaged fuses  130  and  135  next to each other creates a substantial enclosed area in the plane of the fuse pair, producing a substantial inductance in series with the high frequency signal. It may be noticed that during normal operation, only the small signal voltage is applied between the fuses  130  and  135 , and that during an internal flashover, they would both be clearing essentially the same fault. Therefore it may be advantageous to combine the two fuses  130  and  135  into a single housing, and share the arc extinguishing mechanism. By placing the two fuses  130  and  135  in parallel with each other with a spacing and thickness commensurate with the characteristic impedance seen from the coupler secondary winding  115 , the effect of spurious inductance and capacitance would be minimized, to the extent that the coupler secondary impedance was constant over frequency and known. 
   In the case of a current limiting fuse where wires would be wound in a double helix on a “spider” coil form, in preparation for filling the volume with sand, there is a further technique to reduce the spurious effects of fuse reactances. A magnetic core stick may be inserted inside the helix, transforming it into a common mode choke. Such a choke has minimal differential mode attenuation, even when the coupling coefficient between the windings is much less than unity. 
   One inherent mechanism that limits the transfer of fault energy is the saturation of the coupler cores. Once a fault current causes core saturation, magnetomotive force and induced secondary voltage are basically clamped. Power line fault transients and surges possess a waveform containing energies over a broad spectrum of frequencies. Only frequencies relevant to the modem communications should reach the modem. To that end, series capacitors can be used as high pass filters that limit the transient energy reaching the modem. 
   Another side-effect of attaching an inductive coupler to a power line is the flow of circulation current. The inductive coupler may be viewed as a current transformer (CT), and in the choke circuits described below, the CT secondary is short circuited by the series combination of the two chokes. 
   Flashover can be treated as an instantaneous short-circuiting of the secondary circuit to the primary circuit, and since the choke inductors  140  and  145  initially act as an open circuit, the entire primary voltage would appear across each choke  140  and  145 , for an initial few tens of nanoseconds. This can be addressed as shown in  FIG. 2A  by adding high frequency coupling capacitors  200  and surge suppressors  205 , which lower the initial instantaneous voltage impressed on both the chokes  140  and  145  and capacitors  200 , by acting as a temporary short circuit for the critical first tens of nanoseconds. This allow use of chokes  140  and  145  and capacitors  200  whose voltage rating is 10 to 100 times less than the peak primary voltage. 
   In an alternative embodiment shown in  FIG. 2B , a spark gap or gas tube arrestor  220  is connected across secondary winding  115 , to absorb at least part of the energy coupled to the secondary by a fast risetime surge current. The addition of this device in any of the embodiments shown in  FIGS. 1 and 2  would reduce the surge energy that subsequent surge protectors need to safely absorb. 
   In an alternative embodiment shown in  FIG. 2C , an additional surge suppressor  210  may be placed in parallel with the surge suppressors  205 . The surge suppressors  205  and  210  act as a low impedance when a current fault generates voltages exceeding their clamping voltage. If the devices are identical, suppressor  210  would act as the primary voltage limiter for the differential mode, while the series pair of suppressors  205  would act as a backup limiter in case the primary suppressor  210  failed in the open circuit condition. 
   The high pass filtering of the shunt chokes and series capacitors limits the duration of fault pulses, and allows the use of relatively low power surge suppressors. Only such low power devices are available with the low terminal capacitance necessary to avoid high frequency loading of the signal by the surge suppressors. The very small power-frequency impedance of a high frequency coupler reduces the electromotive force (emf) generated in the inductor secondary  115 , and the existence of sufficient fuse resistance, or optionally the addition of a small value resistor  215  in series with each secondary lead (typically, one half to one ohm), typically can reduce the resultant current flow to less than one ampere per thousand amperes flowing on the power line  100 . 
   We can consider an internal flashover of coupler  110 , from primary winding  105  to the secondary winding  115 , simplified here to one terminal  120  of that winding (see  FIG. 1 ).  FIG. 3  shows the equivalent circuit as seen by the flashover voltage transient. A 10 kV dc source  300  represents the instantaneous peak voltage of a 15 kV class distribution transformer having a typical phase to neutral voltage of 7–8 kV rms. Source resistance  305  limits current to a 10 kA short circuit value. Transmission lines  310  and  315  represent a single phase of overhead distribution lines. The closing of switch  320  represents an instantaneous short circuit due to internal flashover. Resistor  325  represents the resistance of a fuse such as  130  and  135 , and choke  330  (equivalent to choke  140  of  FIG. 1 ) closes the circuit to pole ground  335 . Capacitor  340  high pass couples the communications signals to modem terminals  345 , and surge suppressor  350  (equivalent to suppressor  205  in  FIG. 2 ) protects the modem against over-voltage transients. The shunt capacitive loading of capacitor  340  and suppressor  350  (the latter acting as nearly a short circuit during transient events) lowers the initial transient voltage at node  355 , and therefore across capacitor  340 , allowing use of a lower cost capacitor. 
     FIG. 4  illustrates in cross-section one particular physical implementation of an inductive coupler according to an embodiment of the present invention.  FIG. 4A  shows a section  4 A— 4 A of the inductive coupler of  FIG. 4 . Coupler  410  includes a magnetic core having an upper portion  405  and a lower portion  406  that form an aperture therebetween. Coupler  410  is situated on a primary power line  400  such that primary power wire  400  passes through the aperture, in the vicinity of upper portion  405 . Coupler  410  also includes a secondary wire  415  that passes through the aperture, in the vicinity of lower portion  406 . Secondary wire  415  and lower portion  406  are encapsulated in an insulating material  417  so that a layer of insulating material  417  is situated between secondary winding  415  and lower portion  406 . The layer of insulating material  417  between secondary winding  415  and lower portion  406  has a thickness  420  appropriate for the line&#39;s withstand voltage and BIL voltage. Sheds  425  provide the appropriate leakage path. Conductive plate  430  is attached to the base of the coupler body, and connected via wire  435  to the pole ground  440 . 
   If the coupler body does not provide a sufficient leakage path or insulation commensurate with the steady state or transient voltage on the power line conductor  400 , then a flashover could occur. Flashover current external to the coupler  410  would jump to the conductive plate  430 , and be routed harmlessly to the pole ground  440 . 
     FIG. 5   a  illustrates a dual fuse  500 , as implemented in an expulsion fuse. Wires  505  connect the fuse elements  510  to two-terminal headers  515 . The fuse elements  510  are tensed by springs  520 , and the entire volume is encased in an arc-quenching material, with ports (not shown) through which any arc gasses are expelled. 
     FIG. 5   b  illustrates a dual fuse  550 , as implemented in a current-limiting fuse. Fuse elements  555  are wound on spider form  560  and terminate on two-terminal headers  565 . Optionally, the spider may have a hollow core in which magnetic core  570  optionally may be inserted. The entire volume is filled with sand (not shown). 
   Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the true scope of the invention.