US20090250449A1 - System And Method For Deicing Of Power Line Cables - Google Patents

Abstract

A system and method for deicing power transmission cables divides the cable into sections. Switches are provided at each end of a section for coupling the conductors together in parallel in a normal mode, and at least some of the conductors in series in an anti-icing mode. When the switches couple the conductors in series, an electrical resistance of the cable section is effectively increased allowing self-heating of the cable by power-line current to deice the cable; the switches couple the conductors in parallel for less loss during normal operation. In an alternative embodiment, the system provides current through a steel strength core of each cable to provide deicing, while during normal operation current flows through low resistance conductor layers. Backup hardware is provided to return the system to low resistance operation should a cable overtemperature state occur.

Description

RELATED APPLICATIONS
• The present application claims priority to commonly owned U.S. Provisional Patent Application Ser. No. 61/041,875 filed Apr. 2, 2008, the disclosure of which is incorporated herein by reference.
FIELD
• The present document relates to the field of overhead power transmission lines. In particular, it relates to systems and methods for preventing or removing excessive ice accumulation on cables of such power transmission lines to prevent damage due to weight of the excessive ice.
BACKGROUND
• Ice storms are fairly common in some parts of the United States. These storms result in an accumulation of ice on structures, including overhead power transmission lines and associated poles and towers; this ice may reach thicknesses of several inches. Such ice storms fortunately represent only a small percentage of the total operating time of a power transmission line, and any one power transmission line typically encounters such conditions only a few times per year.
• The mass of ice accumulated causes significant problems by mechanically stressing cables and structures. For example, a cylinder of 2″-ice adds 5.7 ton/mile weight to a 1″-conductor. The altered profile of the cable will also increase wind-induced stress, further increasing the chance for it to break. Accumulated ice has caused power transmission lines and poles to break, and towers to collapse; either of which interfere with power transmission and can cause serious risk of harm to persons and property on the surface.
• Some power transmission lines are trolley wires used to transmit power to electric vehicles. Since ice is not a good conductor, ice on trolley wires can interfere with power transmission to the vehicles.
• Power transmission lines are normally designed to have a constant, low, overall resistance, so as to avoid excessive power losses and operation of wires at high temperatures. As wire reaches high temperatures, whether due to electrical self-heating, high ambient temperatures, or both, it tends to lengthen and weaken. This lengthening can cause the lines to sag between poles or towers, possibly causing hazard to persons or property on the surface. Further, low resistance during normal operation is desirable to avoid excessive power losses—every kilowatt lost to heating of lines is a kilowatt that must be generated but does not reach a customer. Finally, excessive voltage drops in the transmission line due to high resistance may cause instability of the power grid system.
• Many power transmission lines have cables that have several individual conductors, often spaced several inches apart and connected electrically in parallel for each phase. While allowing higher ampacity by improving cooling in high ambient temperatures over cables of conductors in thermal contact with each other, this design increases the amount of ice that may accumulate by providing additional surface for ice nucleation. For example, a system having two parallel transmission lines, each line having three cables with five conductors per cable separated by spacers, all coated with two inches of ice, could have over 172 tons of extra weight per mile. Further, such design is incompatible with single-switch deicing designs because only energized conductors, or conductors in thermal contact with energized conductors, get deiced.
• Not only can the high weight and increased wind drag of iced-over lines cause breakage of lines and collapse of a tower, but the sudden shift in forces on a tower resulting from an initial break in a line or collapse of a pole or tower can cause additional, adjacent, towers or poles to crumple like dominos—repair crews may find not just one flattened tower but wreckage of a dozen or more adjacent towers tangled among downed lines. Sudden collapse of a transmission line can also cause damage to switching equipment and power plants, and can lead to instability in power grids. In the worst case, sudden collapse of a transmission line can cause enough capacity loss and instability in power grids that resulting blackouts may extend over multiple states. It is therefore desirable to prevent, reduce, or remove ice accumulation on these lines.
• U.S. Pat. No. 6,396,132 to Couture and US Patent Applications 2003/0006652 and 2008/0061632 describe a system having load cells or other apparatus for detecting accumulated ice on a transmission line. In this system, when ice is detected one or more parallel conductors of a phase of a transmission line are disconnected by opening parallel mechanical and electronic switches, such that current flowing in the transmission line is diverted through and deices a selected one or a few of the parallel conductors. A pattern of open switches is then rearranged to divert current through a different one or a few of the parallel conductors.
• Other systems for deicing power transmission lines are known in the art. For example, U.S. Pat. No. 4,190,137 to Shimada, couples parallel lines of a trolley system into a loop, then superimposes a current around the loop upon power ordinarily transmitted through the loop to deice the lines. In an embodiment, Shimada discloses DC trolley lines, with a superimposed AC current around a loop of the trolley lines for inducing joule heating to deice the lines.
• Power transmission lines do not carry the same amount of current at all times. Current transmitted over a line varies with a wide variety of factors including load conditions—which in turn vary with time of day and weather, a particular selection of power plants operating at a moment in time, and other factors. For example, a power transmission line carrying power from a wind and solar farm into the power grid will carry current that may vary greatly with cloud, time of day, and wind conditions. Even conventional power plants, such as those having multiple units, may provide transmission line current that will change with time, for example one unit of a two unit plant may be shut down for repairs, Similarly, power transmission lines connecting energy storage systems, including pumped storage plants and battery storage plants to the power grid may conduct current intermittently.
SUMMARY
• A system for deicing of power transmission lines, the power transmission lines having cables (one for each phase of a 3-phase line, or one for each polarity of a DC line) having at least three mutually insulated conductors. The system has switches that when closed place all three conductors in parallel for normal, low resistance, operation; and when opened place all three conductors electrically in series to deice the cable. The system operates under control of a system controller.
• In a particular embodiment, a transmission line is a line providing electric power to an electric vehicle, such as a locomotive, a tramcar, or a trolley bus. One of several conductors is in direct electric contact with a sliding mechanical linkage such as a pantograph or trolley wire. In a particular embodiment, a conductor in contact with a pantograph is made of material having higher electrical resistivity but higher mechanical strength than the material of two other wires. For instance, a conductor for contacting pantographs can be made of steel, stainless steel, bronze, brass, or copper-clad or aluminum-clad steel while two parallel conductors are made of aluminum, aluminum alloy, or copper.
• In a particular embodiment, each cable has at least five mutually insulated conductors; with all five in parallel for normal operation and all five in series for deicing. Other embodiments are disclosed with three, seven, and other numbers of conductors.
• In another embodiment of a system for deicing cables of power transmission lines, each cable is divided into at least two sections. Each section has at least three conductors that are placed in parallel for normal operation and in series for deicing operation. A system controller is provided for sequentially deicing sections of the cables to prevent undue interference with power transmission by the transmission line.
• In a particular embodiment, an apparatus is provided for monitoring temperature of the cables, and for returning the conductors to parallel should overheating of a cable be detected.
• In another embodiment, a switchbox for switching conductors of a transmission line cable between a parallel configuration and a series configuration has an energy storage device with charger, a control signal receiver for receiving commands and at least one switch controlled by the control signal receiver for determining current flow through at least one conductor of the cable, and apparatus for overriding the control signal receiver and placing the cable conductors in a parallel configuration if a high temperature is detected on a conductor of the cable.
• In another embodiment, the cable need not have multiple conductors, but has an electrically resistive strength core—such as steel wire—and at least one conductor, this system has a switchbox for diverting sufficient current from the conductor through the resistive strength core to deice the cable in a first operating mode, and wherein substantially all current passes through the conductor in a second operating mode.
• In a particular embodiment, the switchbox diverts current through the strength core by placing or increasing an inductance in series with the conductor; the strength core is in parallel with the combined series inductance and conductor and takes an increased current because of the inductive reactance of the inductance.
• In another particular embodiment, the switchbox has a transformer and a switch, the transformer bypassed in normal operation and operating as a step up transformer to divert power into the strength core during a deicing mode.
• In another particular embodiment, the switchbox incorporates devices for monitoring a temperature of the cable and for reducing current in the strength core towards normal operating levels should high temperatures be encountered.
• A method is disclosed for deicing cables of a transmission line in which the cable has a section with several conductors between a first switchbox and a second switchbox. The section of cable has a normal operating mode where the conductors are electrically connected in parallel. When ice is detected and deicing is desired, the switchboxes are reconfigured to couple some of the conductors electrically in series thereby placing the section of cable in a high resistance deicing mode. Current flowing in the section of cable resistively heats and deices the section of cable. After deicing, the switches of the switchboxes are reconfigured to return the section of cable to the normal operating mode.
• In a particular embodiment of the method, current in the cable is monitored. In this embodiment, a controller selects between several deicing configurations of the switches according to the current in the cable. Further, if current is too low for deicing, the controller may request an increase of current in the cable.
BRIEF DESCRIPTION OF THE FIGURES
• FIG. 1 is a schematic diagram of a system for preventing or removing ice accumulation from a power transmission line.
• FIG. 2 illustrates an embodiment for preventing or removing ice accumulation from a trolley wire used to transmit power in a transportation system.
• FIG. 3 illustrates an alternative embodiment of a cable for use with the system ofFIG. 2.
• FIG. 4 is an electrical schematic of one section of one cable of an alternative embodiment of the system for preventing ice accumulation having five conductors per cable.
• FIG. 5 is an electrical schematic of an alternative method of operating one section of cable of an alternative embodiment of the system for preventing ice accumulation having five conductors per cable.
• FIG. 6 is an electrical schematic of an alternative method of operating one section of cable of an alternative embodiment of the system for preventing ice accumulation having five conductors per cable.
• FIG. 7 is an electrical schematic of one section of one cable of an alternative embodiment of the system for preventing ice accumulation having six conductors per cable.
• FIG. 8 is an electrical schematic of one section of one cable of an alternative embodiment having seven conductors per cable.
• FIG. 9 is a cross sectional diagram of a cable having seven conductors and a steel strength member in thermal contact with each other.
• FIG. 10 is a block diagram of a solar-battery-powered switchbox for use in the system.
• FIG. 11 is a block diagram of an alternative switchbox for use in the system.
• FIG. 12 illustrates a system having multiple cable sections each capable of independent or sequential deicing or anti-icing operation.
• FIG. 13 illustrates a cross-section of a first cable for use with the system ofFIG. 1.
• FIG. 14 illustrates a cross-section of a second cable for use with the system ofFIG. 4.
• FIG. 15 illustrates a cross-section of a third cable for use with the system ofFIG. 1.
• FIG. 16 illustrates an alternative embodiment having series connected switches.
• FIG. 17 illustrates a deicing system for power lines as proposed in PCT/US2004/27408.
• FIG. 18 illustrates a cross section of a cable having a steel strength core electrically insulated from an outer conductive layer.
• FIG. 19 illustrates a two-conductor, single-switch-per-section deicing system.
• FIG. 20 is a schematic diagram of an inductive switchbox suitable for use with the deicing system ofFIG. 19.
• FIG. 21 is a schematic diagram illustrating an alternative core for use with the inductive switchbox ofFIG. 20.
• FIG. 22 is a schematic diagram of an alternative single-switchbox-per-section deicing system having a step-up transformer to reduce voltage loss in the cable.
• FIG. 23 is a schematic diagram of an alternative embodiment having some features ofFIGS. 1 and 15.
DETAILED DESCRIPTION OF THE EMBODIMENTS
• A system for electrically removing accumulated, or preventing accumulation of, ice on apower transmission line100 is illustrated inFIG. 1. For simplicity, only one of the threecables102 or phases of a typical three-phase AC line is shown. In the embodiment ofFIG. 1, acable102 is constructed from threeparallel conductors104,106,108. The threeconductors104,106,108 are bundled together by insulatingspacers110 alongcable102.
• Cable102 is suspended byinsulators112 fromtowers114, or in an alternative embodiment from poles (not shown). At ends of a section ofcable102, afirst switch box116 andsecond switch box118 are suspended frominsulator112 along withcable102. Eachswitch box116,118 contains aswitch120 and a switch actuator-controller122.
• For a given section of transmission line, theswitch boxes116,118, are either in a first, switch-closed, state; or in a second switch-opened state. During normal operation, the switch boxes remain in the switch-closed state with allparallel conductors104,106,108 ofcable102 connected electrically in parallel. When ice accumulation along thepower transmission line100 is known or suspected, or when it is desired to prevent ice accumulation due to icing weather conditions, switches120 ofboxes116,118 are placed in the switch-opened state. This results in the threeconductors104,106,108 ofcable102 being connected electrically in series instead of in parallel, with oneconductor104 carrying power in the reverse direction, thereby increasing the effective resistance of the section ofcable102 by a factor of nine.
• With theswitches120 in switch-opened state, and the effective resistance ofcable102 increased to nine times the normal condition, an increase by a corresponding factor of nine in voltage along the segment gives a corresponding increase by a factor of nine in self-heating ofcable102 over the normal switch-closed state provides heating ofcable102 to melt accumulated ice and to retard accumulation of additional ice. For purposes of the present document, anti-icing operation is operation of a cable segment in a manner that provides heating ofcable102 to either melt accumulated ice or to retard accumulation of additional ice
• Theswitches120 ofswitch boxes116,118 operate under control of asystem controller124. In oneembodiment system controller124 is located at a network operations center. In anotherembodiment system controller124 is an automatic device capable of sensing local weather conditions including ice accumulation and attached to atower114 near a section ofcable102 subject to ice accumulation and havingswitchboxes116,118 under its control. In this way, switches of bothswitchboxes116 and118 can be opened or closed essentially simultaneously even ifswitchboxes116 and118 are located one or more miles apart.
• The embodiment ofFIG. 1 is also applicable to a cable or a polarity of a DC transmission line or trolley power line, as illustrated inFIG. 2. In the embodiment ofFIG. 2, there are threeparallel conductors150,152,154 coupled into a serpentine configuration between twoswitchboxes156,158. One of the three conductors, thecontact conductor154, is arranged so that it is accessible to contact withpantographs160 or other trolley-wire contacting apparatus of an electricallypowered vehicle162.
• Vehicle162 may be an electric locomotive, or a streetcar unit as illustrated, with return path for vehicle current through arail164. In an alternative embodiment, two sets ofparallel conductors154 andswitchboxes156,158 are provided with dual trolley-wire contacting apparatus160, one for each phase or polarity of a DC or AC trolley-wire system, such thatvehicle162 connects to both phases or polarities. In this alternative embodiment,vehicle162 may be a rubber-tired vehicle such as the electrically powered busses that have operated in San Francisco for many years.
• In the embodiment ofFIG. 2, switches168,166 may be opened to enter deicing mode, and closed for normal operating mode. Opening of theseswitches168,166, causes current flowing through theconductors154,152,150, such as current being drawn byvehicles162 in later sections of the system, to pass through all threeconductors154,152,150 in sequence rather than in parallel, increasing current density and heating the conductors.
• In the embodiment ofFIG. 2 thecontact conductor154, may, but need not, be fabricated from material different from that of the other ornon-contact conductors152,150. For example, the contact conductor may be a high-strength moderate-resistance bronze, brass, copper-clad steel, stainless steel, or aluminum-clad steel, withparallel conductors150,152 made of low resistance copper or aluminum. This embodiment has an advantage in that the high strength contact conductor may be better able to resist mechanical stresses due to contact with the pantograph or trolley-wire contacting apparatus160. Further, while it can be advisable to deice thenon-contact conductors152,150 to avoid weight and wind related damage, ice on thecontact conductor154 may interfere with power transfer from thecontact conductor154 to the pantograph or other trolleywire contacting apparatus160. Higher resistance of thecontact conductor154 may help to ensure prompt and rapid deicing of thecontact conductor154 to ensure continued operation of thevehicle162 during icing conditions. In this alternative embodiment, opening ofswitches166,168 for a brief time can deicecontact conductor154 to ensure continued operation, while opening ofswitches166,168 repeatedly or for a longer time can deice thenon-contact conductors152,150 when ice accumulation threatens weight or wind related damage.
• While in some embodiments of the trolley system ofFIG. 2non-contact conductors150,152 are separately strung from thecontact conductor154, or arenearby conductors150,152,154 separated by spacers; in an alternative embodiment, as illustrated inFIG. 3, acontact conductor154 may form a shell containing insulating material andnon-contact conductors150,152.
• Thissystem100 differs from that of Couture in that direction of current in oneconductor104 of thecable102 is reversed, and in that Couture deices only one or a few conductors at a time, while thesystem100 deices all three of the conductors of a segment simultaneously—in the case of spaced-conductor cables Couture requires several sequential deicing operations to clear all conductors of a cable. Thesystem100 also differs from that of Couture in number and position of the switches. Couture places one set of switches at one point between two ends of a section, while insystem100 the switches are placed at both section ends. Couture's system for a three-conductor line would have 3 switches, whilesystem100 has only 2 switches. One more difference is that if all of the system switches fail in open position, the current flow and, thus, electric-power transmission will be interrupted, whilesystem100 provides continuous current flow even with all the switches open, as may happen if the system fails or is damaged, for instance, by lightning. Similarly,system100 differs from that of Shimada because no loop is formed and no additional current is applied to a loop.
• The alternative embodiment of thesystem200 for removing or preventing ice accumulation ofFIG. 4 has five instead of three conductors percable202. In this embodiment, eachswitch box204,210 has two gangedswitches206,207,209 and actuator-controller208. In this embodiment, opening ofswitches206,207,209 has the consequence of increasing the effective resistance ofcable202 by a factor of twenty-five; thereby increasing self-heating ofcable102 to melt accumulated ice and retard accumulation of additional ice. In the embodiment ofFIG. 4, two conductors of the five carry current in a reverse direction while three conductors carry current in a forward direction.
• In the embodiment ofFIG. 4, the effective length of total conductor in a segment ofcable102 is increased by a factor of five. Phase shift introduced by this increase of length will not cause significant effect on power flow in the transmission line when operated in a power grid when segments of a few miles in length are deiced since the wavelength of sixty-cycle powerline AC current is approximately three thousand miles and this will not cause significant phase shift. Further, because the length (and conductor resistance) may be increased simultaneously in all three phase-lines by operating switches in all three phases simultaneously, there should not be a significant phase shift added by deicing operations between the different-phase conductors of the transmission line.
• The resistance and power dissipation increases stated above assume embodiments having equal resistance for each conductor of the cable, as is likely the case with open-air spacer-separated-conductor cables. In other embodiments, resistance of individual conductors in a cable may have differing resistances and resistance ratios achieved will vary with the actual resistances of the conductors.
• An increase in self-heating ofcable102 by a factor of twenty-five may be desirable when a cable is conducting low current, but may be excessive when the cable is operating at high currents and/or has several conductors bound together instead of being spaced apart by spacers. The switch arrangement illustrated inFIG. 4 may be operated in alternative ways to produce other effective power dissipation increases, such as is illustrated inFIG. 5.
• In the embodiment ofFIG. 5, switches206,209 are opened to enter a deicing mode, whileswitch207 is left closed. In this embodiment, assuming equal resistance per conductor, effective resistance of the cable segment is increased by a factor of five.
• Similarly, in theembodiment215 ofFIG. 6, switches206,207 are opened whileswitch209 is left closed. In this embodiment, assuming each conductor has resistance R, the effective resistance of the cable segment is increased from one-fifth R to three R, an increase of resistance by a factor of fifteen.
• Embodiments having cables with six or more conductors may have even numbers of conductors. In the six-conductor embodiment220 ofFIG. 7, resistance of the cable segment is increased from one-sixth R to three-halves R, an increase by a factor of nine when theswitches206,222 open. Other configurations of the system are possible, having other power increases in deice mode; for example ifswitch222 is left closed whileswitches206 open, resistance increases from one-sixth R to three-fourths R, an increase of four and a half.
• Similarly, analternative embodiment250 may have seven conductors in each cable and three or four (as illustrated inFIG. 8) switches252,254,256,258,260,262,264,266 in eachswitchbox268,270. In the embodiment ofFIG. 8, effective resistance of the cable is programmable according to which switches are open, as illustrated inFIG. 1, ranging up to forty-nine times the resistance of the cable with all switches closed. Note that there are additional alternatives and patterns not portrayed inFIG. 1. To a certain extent, the pattern of open switches can also select which conductors are heated and which are left unenergized in anti-icing operation. In an alternative embodiment, switches266 and252 are replaced by wire with minimal reduction in the resistance options provided. Operating modes between the minimum and maximum resistance configurations for a system are herein known as intermediate resistance modes; many of these are illustrated in Table 1. In an embodiment, a system controller monitors current through the transmission line and determines a resistance required for deicing, selecting a deicing mode from minimum resistance, maximum resistance, and intermediate resistance modes as appropriate for the current in the transmission line. In some embodiments, the system controller may also transmit a request to an energy storage system, generation system, or network operations center that current in the transmission line be increased to provide enough current for deicing.

• 	TABLE 1
  	Switches Open	Resistance Multiplier

  	None	1
  	254, 256, 262	2.33
  	254, 256, 264, 262	3.5
  	254, 256, 258	7
  	252, 254, 258, 262, 264	10.5
  	254, 256, 258, 260	21
  	254, 256, 258, 260, 262	35
  	254, 256, 258, 260, 262, 264	49

• Other alternative embodiments may exist having other numbers of conductors, for example an embodiment with nine conductors in each cable and four switches in each switchbox has effective resistance that increases by a factor of up to eighty-one when the switches open.
• In a particular embodiment, a transmission line system hasphase cables267 having multiple segments each of which corresponds to the schematic diagram ofFIG. 8. In this embodiment, thecables267 have sevenconductors253,255,257,259,261,263,265, made of aluminum or copper, that are bound in thermal and mechanical contact to each other and to a centralsteel strength member280, according to the cable cross section ofFIG. 9. The seven conductors correspond to the seven conductors ofFIG. 8. In this embodiment, the phase cables are suspended from towers and equipped with asystem controller124 in manner resembling that ofFIG. 1.
• In this embodiment,controller124 monitors current through the transmission line cables. When ice is detected, thecontroller124 determines a resistance increase that will provide adequate heating of thecable267 to deice the cable, while avoiding damage tocable267. The controller then automatically determines a configuration of open switches forswitches252,254,256,258,260,262,264,266 ofswitchboxes268,270, and transmits that configuration toswitchboxes268,270 to cause the system to enter deicing mode for aparticular cable267 segment. Upon completion of deicing of thecable267 segment, the switches are closed to return to normal operation.
• In the event that ice is detected and deicing is desired, butcable267 is carrying too little current to provide adequate heating for deicing even at a maximum resistance configuration ofswitches252,254,256,258,260,262,264,266 ofswitchboxes268,270,controller124 may transmit a request to a grid management system to reconfigure the power grid such that enough power is carried throughcable267 to deice thecable267. In the case of transmission lines connecting energy storage systems to the power grid, this may require that the storage system either store or release sufficient energy to deice the line.
• Resistance self-heating of a transmission line is proportional to current I through the transmission line squared, times the resistance R of the line (I²*R). The resistance increases of Table 1 are calculated based upon an assumption that each conductor of the cable has equal resistance. Since there may be times when current in a transmission line is quite low, there may be transmission line systems in which it is desirable to have conductors of differing resistance such that a maximum resistance increase can be significantly higher than would be accomplished with conductors of equal resistance. For example, in a variant embodiment of the embodiment ofFIG. 8,wires263 and265 have resistance ten times the resistance of the other, or low resistance,conductors253,255,257,259,261. During normal operation, theseconductors263,265 carry little current, and the effective resistance R is slightly less than one fifth that of the resistance of each of thelow resistance conductors253,255,257,259,261. Should all seven conductors be configured in series by closing only switches252 and256, the effective resistance is increased to one hundred twenty five R, with an intermediate increase to seventy R ifswitches258 and260 are closed. An assortment of other intermediate resistance increases is also available forcontroller124 to select from, and may be readily calculated.
• In yet another alternative embodiment,conductor263 has resistance ten times that of eachlow resistance conductor253,255,257,259,261, andconductor265 has resistance thirty times that of eachlow resistance conductor253,255,257,259,261. In this embodiment, an intermediate increase to seventy R is available, and a maximal increase to two hundred twenty five times R is available. In these embodiments, thecontroller124 selects a switch configuration appropriate to provide adequate heating for deicing based upon the amount of current available in the line. This configuration is then transmitted to switchboxes268,270 which set their switches accordingly. The controller continues to monitor current in the transmission line, and may reconfigure switches ofswitchboxes268,270 if current changes to provide appropriate heating for deicing while avoiding excessive heating that may damage the transmission line.Controller124 may be a separate controller or may be integrated into aswitchbox268,270.
• In an embodiment thetransmission line segment267 carries power from a solar or wind generation system having an energy storage subsystem. In this embodiment, upon entering anti-icing mode when the transmission line is carrying little or no current;controller124 may transmit a request to the energy storage subsystem requesting that some stored energy be released over the transmission line to provide current for deicing the line.
• Alternative embodiments may have additional wires, for illustration say N wires, each mutually insulated from each other in the cable. Each conductor of embodiments resembling that ofFIG. 8 may be assembled from one or more of the N wires. In an embodiment having M effective conductors seen by the switchboxes, with N insulated wires in the cable, M is less than or equal to N. The number of wires in each conductor may differ between conductors, conductors having greater resistance may have fewer wires than those conductors having lower resistance.
• While local power-distribution transmission lines often operate between 3,500 and 25,000 volts, many “high-tension” three-phase transmission lines operate at voltages between 60,000 and 1,200,000 volts. While embodiments having conventional construction may be suitable for use with some local distribution transmission lines, operation on high-tension transmission lines poses additional challenges.
• In an embodiment (FIG. 10) particularly suited for use with high-tension transmission lines, since all components of theswitchbox204,116,118 operate near power-line cable102,202 voltage,switchbox204,116,118 is attached at thecable202,102 end ofinsulators112 and is suspended with the cable. In such an embodiment, it is not practical to power theswitchbox204,116,118 from normal 115V AC power. In consequence,switchbox204,116,118,300 is powered by aninternal energy store302 such as an ultracapacitor or battery.
• In most embodiments,energy store302 is charged throughcharger310 by a device selected from devices such as aninductive pickup304 surrounding one or more conductors ofcable102,202, asolar panel306, or a small-value capacitor308 to ground.Energy store302 powers acontrol signal receiver312, which is normally the only component of theswitchbox300 to consume power.
• Whencontrol signal receiver312 receives a correctly encoded “deice” command fromsystem control124, which may be transmitted fromcontrol124 toreceiver312 via a high frequency carrier wave superimposed oncable102,202 along with the power being transmitted, optically over an optical fiber, or by radio, thereceiver312 activates electrically operatedswitch actuator314 that opens high current switch or switches316.Switch actuator314 may incorporate a solenoid, electromagnet, or an electric motor, and may incorporate additional springs for rapid opening and closing as known in the art of electrically-operated switching devices. In an alternative embodiment,switch316 is an electronic switch; yet another embodiment has electronic switches in parallel with electrically-operated mechanical switches.
• In an embodiment,actuator314 operates to oppose the force of aspring318 that tends to holdswitch316 closed.
• Because inadvertent opening ofswitches316 on a hot summer day while operating under full load can not only cause excessive power loss, and line heating, but can cause sufficient sag as to pose hazard to persons or property on the surface, or even cause damage tocable102;actuator314 pullsswitch316 open by acting not against a case ofswitchbox300, but through afusible link320 to aclamp322 that is attached to one conductor, such asconductor104, of cable102 a short distance fromswitchbox300.Fusible link320 is adjacent toconductor104, and is made of a low-melting metal or plastic such that it will break beforeconductor104 reaches a temperature at which excessive sag or damage tocable102 occurs and allowspring118 to closeswitch116. Therefore, should the system for ice removal or ice prevention fail,switches116 fail into the closed (low resistance) condition.
• An alternative embodiment, as illustrated inFIG. 11, has advantage that commercially available contactors and/or solid-state relays may be used for switching components of the switchboxes that may have to interrupt substantial currents. In this embodiment,control signal receiver312 normally switchescable102,202 between low and high resistance conditions by activating electrically actuatedcontactor modules340.Contactor modules340 may incorporate electromechanical switching devices or, since the maximum voltage seen across the switch is far less than the operating voltage of the transmission line, solid state relay devices, or both. An advantage in using solid state relay devices in parallel with properly timed electromechanical switching is that the electromechanical switching devices provide low switching resistance for transmission line currents that may be on the order of several hundred amperes and reduce self-heating of the solid state relay devices, while the solid state relay devices may suppress any contact arcing associated with opening and closing the electromechanical devices by being closed before the electromechanical devices close, and opened after the electromechanical devices open.
• In the embodiment ofFIG. 11,contactor modules340 are connected in parallel withsafety switches342 that are closed byspring344 wheneverfusible link320 melts due to excessive heating in theconductor346 to whichclamp322 is fastened. This effectively overrides both thecontrol signal receiver312 andswitches340 whenconductor346 reaches high temperatures. This will prevent excessive dip in, or overheat damage to,cable102,202 in the event of failed switchboxes, but poses some risk of ice damage tocable102,202 at a later time—especially if left unrepaired.
• In another embodiment,control signal receiver312 monitors temperature sensed bytemperature sensor324 and closesswitches340 to return all conductors to parallel operation at a temperature indicative of successful deicing but lower than a temperature required to meltfusible link320. In an embodiment, temperature/status transmitter326 transmits an indication of closing ofswitches340 due to high temperature tosystem controller124 so that the switchbox at the other end of the conductors can also return all conductors to parallel operation.Fusible link320 is preferably located on the conductor having the highest current when switchboxes of a line segment are in the inconsistent state of oneswitchbox having switches340 open and the otherswitchbox having switches340 closed.
• In order to provide feedback to thesystem control124, and encourage repair of failed switchboxes, a status of asensing switch347 ganged withsafety switches342, senses failure offusible link320 and transmits this information throughtransmitter326 tosystem control124.
• In order to assist with control of the system, a temperature sensor324 (FIGS. 10 and 11) may be attached to clamp322, temperature readings being transmitted bytemperature transmitter326 tosystem control124 to indicate when, for example, deicing of a section is expected to be complete because temperature of aconductor104 ofcable102 has significantly exceeding the freezing point of water.
• Alternatively,sensor324 can be used to maintain cable temperature at a pre-set value during de-icing or anti-icing operation, for instance, at +10° C. In doing so, the switches close when the temperature reaches the pre-set value and open when it falls below that value. That effectively reduces total energy consumed for de-icing/anti-icing, and also prevents cable overheating.
• In the embodiment ofFIG. 12, eachcable400 of the transmission line, which may be hundreds of miles long and traverse a variety of terrain and climate zones, is divided into sections, such assection402 andsection404, of from one tenth to ten miles length, for example. Each section has afirst switchbox406,410,414 and asecond switchbox408,412,416. In order to prevent excessive voltage drop in the transmission line, when it is determined that it is desirable to deicecable102, theswitchboxes406,408 of thefirst section402 are activated to open the switches. When that section is deiced, the switches of the first section are closed andswitchboxes410,412 of the second section are activated to open the switchboxes, and so on in sequence until all iced-over sections of thecable400 are deiced. Similarly, division ofcable400 into sections permits deicing of those sections of thecable400 that have been or are exposed to icing conditions, while allowing sections exposed to different weather to continue normal operation.
• Limiting voltage drop by sequentially deicing sections of the line helps maintain stability of the power grid and avoids voltage drops in the transmission line that may be noticed by customers.
• FIG. 13 illustrates a cross section of a cable suitable for use with a single-switch-per-switchbox, three-conductor cable of the present cable deicing system. Atriangular spacer502, which may be nonconductive plastic, ceramic, or metal with rubber insulators, is attached to eachconductor504 of the cable. Attachment ofspacer502 to theconductor504 may be by molding, gluing a cap over cable and base part of the insulator, with screws securing a cap to an insulator base, or such other methods as known in the art of spaced-conductor cables. Eachconductor504 may be a conductive copper or aluminum shell, over an optionalsteel supporting center506, or may be assembled from conductive copper or aluminum strands wrapped around a supporting center of multiple steel strands.Spacers502 are positioned at regular distances along the cable, spacer spacing is chosen to be small enough to prevent direct electric contact between the conductors of the cable.
• In the embodiment ofFIG. 14, fourconductors602 are positioned byspacers604 around acentral conductor606, each of the five conductors is of essentially equal ampacity. One, in the embodiment illustratedconductor606, or all fiveconductors602,606 may have asteel core608 to provide the strength needed for long spans between towers. Since all fiveconductors602 carry current during deicing, all five will be deiced even if these conductors are not in thermal contact with each other.
• In the embodiment ofFIG. 15, a cable700, for use ascable102 or202, has three (illustrated), five, seven, or nineconductors702,704,706 assembled around astrength core708 which may be stranded steel. Theconductors702,704,706, which may be stranded copper or aluminum, are insulated from each other and coated with an extrudedplastic insulation layer710.
• With reference toFIGS. 13,14, and15, it is anticipated that theconductors504,602,606,702,704,706 andsteel supporting cores506,608,708 need not be solid; in most embodiments these are of stranded construction for flexibility and ease of installation as known in the art of transmission line cabling. The conductors and steel cores may be merged—these may be stranded conductors having multiple individual strands of conductor-coated steel, such as stranded Copperweld® (copper clad steel) wire. Further, embodiments may have larger numbers of smaller insulated wires that are grouped into the conductors herein referenced; for example a transmission line cable may have six wires grouped into three groups of two wires each for purposes of deicing according to the present invention, and where each pair of wires are treated as a conductor for deicing as heretofore described.
• The principles described herein are applicable also to DC power transmission lines. While it is not possible to power switch boxes of a DC power transmission line by inductive pickup from current in the transmission line, or through a high-voltage capacitor, other switchbox powering arrangements may be used including but not limited to a solar cell and battery arrangement.
• The system herein described uses a control signal transmitted from asystem controller124 to switchboxes300. It is considered desirable that the control signals be transmitted in encrypted form, and encoded, to prevent accidental opening of switches of the switchboxes or sabotage of the system by unauthorized persons.
• In the embodiment ofFIG. 16, an alternative switch configuration provides similar effect. In this embodiment of aphase800, acable802 has an odd number ofconductors810,812,814,816,818 greater than three running between twoswitchboxes804,805. During normal operation, switches806 and807, connected in series, connectconductors812,814,816, and818 in parallel toconductor810 and theinput820 and to theoutput switchbox805, where corresponding switches are closed. When switchbox control andactuators808open switches806 and807, current is forced to flow through all fiveconductors810,812,814,816,818 in series thereby causing resistive self-heating of these conductors. This configuration has effect of reducing the voltage seen across any one switch, at the expense of increasing current in the first switches (e.g. switch806) in the series sequence.
• Deicing systems for transmission lines have been proposed where each of typically three phases is conducted over acable900, and that cable is divided into twoconductors904,906, as illustrated inFIG. 17 and as disclosed in PCT/US2004/27408. Aswitch908 at an end of asection910 of the cable transitions between normal operation with the twoconductors904,906 in parallel, and deicing operation with current flow in only one906 of the twoconductors904,906; the oneconductor906 used during deicing sized such that resistance of the cable is high enough to produce sufficient self-heating to deice the cable and prevent further ice accumulation, while theconductor904 that is placed in parallel during normal operation is sized to provide suitably low resistance for low losses during normal operation. At the opposite end ofsection910 from theswitch908, and ahead of theswitch914 of thenext section916, the twoconductors904,906 are electrically shorted912 together. Except atshorts912, theconductors904,906 are separated by a layer ofinsulation918. In the design disclosed in PCT/US2004/27408, deicingfirst conductor904 is an outer layer of the cable physically close to the ice to be removed, while normalsecond conductor906 is the central bulk of the cable, and may include any core of the cable.
• High-tension transmission line cables, including modified cable1000 (FIG. 18), generally havemany strands1002 of a conductor such as aluminum or copper surrounding a strengthcore having strands1004 of a stronger but more resistive material such as steel, the steel serves to help support the cable allowing greater tower or pole spacing than otherwise possible. In modifiedcable1000, there is an added layer ofinsulation1006 that prevents electrical contact betweenstrength core strands1004 andconductive strands1002.
• A modifieddeicing system1100 for power transmission cables, as illustrated inFIG. 19, has acable1102 having asteel core1104, aninsulation layer1106, and aconductive layer1108, theinsulation layer1106 preventing contact betweensteel core1104 andconductive layer1108; each orsteel core1104 andconductive layer1108 are typically formed of multiple strands. Additional layers, such as an outer insulation and weather protection layer, may exist.Cable1102 is separated intosections1110, at one end of asection1110 is aswitchbox1114, at the other end is ashort circuit connection1116 betweensteel core1104 andconductive layer1108.
• During normal operation,switchbox1114 maintains an electrical connection betweenconductive layer1108 of each section of thecable1102. In this normal mode, a majority of current throughcable102 pass throughconductive layer1108. To deice asection1110 ofcable1102, acontroller1118 of theswitchbox1114 associated with thatsection1110 ofcable1102 opens aswitch1120, thereby reducing or eliminating current inconductive layer1108 and, since the cable is part of a transmission line that is continuing to conduct power, correspondingly increasing current insteel core1104 of thatsection1110.
• In an alternative embodiment, as illustrated inFIG. 23 with reference toFIGS. 15 and 1, the conductive layer hasseveral conductors702,704,706, as illustrated inFIG. 15, coupled with switchboxes similar to those ofFIG. 1 orFIG. 4. Thestrength core708 is electrically connected betweenswitchboxes1401,1403 at each end of a segment of cable. When theswitches1402,1404 open, effective resistance of theconductive layer702,704,706 increases relative to that of the cable with closed switches, diverting more but not all current through thesteel strength core1104,708.
• In an embodiment,switchbox1114 contains aninductor1122. Whenswitch1120 opens, the inductor is placed electrically in series with the low resistance outerconductive layer1108 ofcable section1110, this series connection ofinductor1122 andconductive layer1108 is electrically in parallel withinner steel core1104 of that section; in consequence some but not all current incable1102 is diverted throughsteel core1104; the amount of this current being substantially greater than that throughsteel core1104 during normal operation withswitch1120 closed.
• Switchbox1114 has powering arrangements and high-temperature override apparatus as previously described with reference toFIG. 10 andFIG. 11.
• In an alternative embodiment, as illustrated inFIG. 20, aswitchbox1200 suitable for use in place ofswitchbox1114 has noswitch1120. In this embodiment,switchbox1200 has apower input connection1202 connected to both the outerconductive layer1108 andinner steel core1104 of a preceding cable section, and to apower output connection1204 for connection to theinner steel core1104 of thecable section1110; in some embodiments this connection may incorporate a locally baredsteel core1104 of the cable.
• The embodiment ofFIG. 20 also has acoil1206 having a few turns of high-ampacity wire,coil1206 connected betweenpower input connection1202 and a secondpower output connection1208 for connection to the outerconductive layer1108 of thecable section1110.Switchbox1200 has anenergy store1212 with charging arrangements as previously discussed with reference toFIGS. 10 and 11, and acontrol signal receiver1214. Whencontrol signal receiver1214 receives a command to deice thecable section1110,receiver1214 activates amotor actuator1216 that pulls on anonmagnetic cable1218.Nonmagnetic cable1218 runs overpulley1220 to amagnetic core element1222, activation ofmotor actuator1216 drawscore element1222 intocoil1206. Whencore element1222 is drawn intocoil1206, inductance ofcoil1206 is increased thereby diverting a portion of current incable1102 through resistiveinner steel core1104
• Pulley1220 is attached to a case ofswitchbox1200 through arelease catch1224, and aspring1226 having sufficient strength to overcome solenoid attraction ofcore element1222 intocoil1206 is connected to drawcore element1222 fromcoil1206. When switchbox1200control signal receiver1214 receives a command to discontinue deicing ofcable section1110,control signal1214 commandsmotor actuator1216 to unwindnonmagnetic cable1218. This permitsspring1226 to drawcore element1222 fromcoil1206 and returncable section1110 to normal operation.
• In the event that a fusible link, such as previously discussed with respect tofusible link320 ofFIG. 1, melts due to excessive heating ofcable section1110,safety actuator rod1230 is drawn intoswitchbox1200 by aspring1232.Actuator rod1230 being drawn intoswitchbox1200 triggersrelease catch1224 to releasepulley1220, which allowsspring1226 to drawcore element1222 fromcoil1206 and returncable section1110 to low-impedance operation; this effectively reduces current instrength core1104 and reduces self-heating of thecable1102.
• In an embodiment of the switchbox ofFIG. 20, the switchbox incorporates circuitry such as thesensor324 and temperature/status transmitter326 ofFIG. 11 such that system controller124 (FIG. 1) can determine when deicing is complete, whereuponsystem controller124 will commandswitchbox1200 to return to normal operation and commence deicing (if required) of the next cable section. In an embodiment, thecontrol signal receiver1214 also monitorssensor324 and attempts to return switchbox1200 to normal operation by extractingcore1222 at a temperature lower than that required to meltfusible link320.
• In an alternative embodiment resembling that ofFIG. 20, instead of a single-piecemovable core1222, a two-piece core is used as illustrated inFIG. 21. In this embodiment, a first L-shapedcore portion1240 is fixed to the switchbox. A second L-shapedcore portion1242 is arranged such that it may be extracted fromcoil1232 in a first position as shown as1242 inFIG. 21 to give a low-inductance setting, or drawn into thecoil1232 to asecond position1244 shown by dashed lines inFIG. 21 to give a high-inductance setting. In this embodiment, first and second L-shapedcore portions1242,1240 form a loop for magnetic flux when thesecond core portion1242 is in the high-inductance position.
• The embodiments ofFIGS. 19 and 20 operate under control of asystem controller124 as previously discussed with reference toFIG. 1; in an embodiment some sections of cable are deiced as described with reference toFIGS. 19 and 20, while some other sections are deiced as described with reference toFIGS. 1 and 4.
• The embodiment ofFIG. 22 also is asystem1300 for deicing transmission line cable, in this case by heating thecable1302 by diverting a portion of cable power through a step-up transformer (windings1304 and1306) and through thesteel supporting strands1308 ofcable1302. In this embodiment,steel strands1308 are surrounded byinsulation1310, and then surrounded by stranded aluminum orcopper conductive layer1312.Switchbox1313 has aswitch1314 open in normal operating mode, and1316 closed, to allow current to flow through theconductive layer1312 unimpeded.Switchbox1313 also has apower store1322 and command receiver1324 similar to and having equivalent charging circuitry to thepower store302 andcommand receiver312 described in reference toFIG. 11; as with other embodiments command receiver1324 is in communication withsystem controller124.
• When it is desired to deicecable1302, command receiver1324 receives a command and closes switch1314 first to establish a current path throughsteel supporting strands1308; then command receiver1324 opensswitch1316 to apply considerable current to transformer primary winding1306. Transformer secondary winding1304 thereupon provides power to supportingstrands1308.Transformer primary1306 has only a few turns, andtransformer core1318 is constructed of a saturable magnetic material, such that only a small proportion of the power available in the cable is applied to the supportingstrands1308; such as 100 to 300 watts per meter of cable—in a 600 kVtransmission line drawing 1000 amps, the 150 kW required to heat all three cables of one mile of line at 300 watts per meter is less than a tenth of a percent of the total power flowing through the transmission line, and voltage drop across the primary winding1306 may be held to a low level.
• As with other embodiments, the embodiment ofFIG. 22 has apparatus (not shown inFIG. 22) for sensing overheating of the cable such as a fusible link and temperature sensor. When the apparatus for sensing overheating of the cable detects an overheat condition of the cable,switch1316 or an auxiliary switch (not shown) is closed to reduce current incable core1308 by bypassing transformer primary1306; as in normal operating mode bypassing primary1306 greatly reduces current in thecable core1308 and reduces resistive heating of thecable1302.
• Switchboxes of all embodiments herein described, such as the switchboxes ofFIGS. 10,11,21, and22, sense overtemperature conditions of the cable and switchboxes, as for example throughtemperature sensor324, and attempt reversion from deicing to normal operation at temperatures below that required to melt fusible links such asfusible link320. Mechanical sensing and return to low-resistance operation provided byfusible links320 and associated apparatus is an overriding mechanical backup intended to prevent overheat damage to the transmission line and its cables should system control124,electrical switch actuators314, electrically actuatedswitches340,1120,1316,motor actuator1216,temperature sensors324, or other components fail into the deicing mode.
• While the forgoing has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and hereof. It is to be understood that various changes may be made in adapting the description to different embodiments without departing from the broader concepts disclosed herein and comprehended by the claims that follow.

Claims (24)

1. A system for deicing and anti-icing operations of AC and DC power transmission lines comprises:

at least one section of cable for transmission of power in the transmission line comprising at least a first, a second, and a third conductor; wherein the first, second, and third conductor are mutually electrically insulated along the length of the section but are connected at ends of the section to form a series serpentine path of at least three conductors connected in series; and

at least a first switch at a first end of the section of cable, and at least a second switch at a second end of the section of cable, the first and second switches operable such that the first, second, and third conductor are connected in parallel in a normal mode and operate in series in an anti-icing mode.

2. The system ofclaim 1 wherein connections for power transmission through the section of cable are made to the first conductor at the first switch, and to the third conductor at the second switch.

3. The system ofclaim 1 wherein at least one switch further comprises a switching device and a switch controller, wherein the switching device and controller are electrically isolated from ground, and wherein the switch is controlled by control signals from a anti-icing system controller at another location.

4. The system ofclaim 1, wherein the section of cable further comprises a fourth and a fifth conductor, and wherein there is a third switch at the first end of the section of cable for coupling the fourth and fifth conductors to the first conductor, and a fourth switch at the second end of the section of cable for coupling the third and fourth conductors to the fifth conductor, and wherein the third and fourth conductors are electrically coupled near the fourth switch, and the fourth and fifth conductors are electrically coupled near the third switch.

5. The system ofclaim 4 wherein connections for power transmission through the section of cable are made to the first conductor at the first switch, and to the fifth conductor at the second switch.

6. The system ofclaim 4 wherein the system further comprises a controller for monitoring current in the transmission line and determining when anti-icing is required, and for determining a switch configuration for anti-icing operation based upon the current in the transmission line.

7. The system ofclaim 4 wherein a conductor selected from the group consisting of the second conductor, the third conductor, the fourth conductor and the fifth conductor has a resistance substantially greater than a resistance of the first conductor.

8. The system ofclaim 4 wherein at least one switch further comprises a switching device and a controller, wherein the switching device and controller are electrically isolated from ground, wherein the switch is controlled by control signals from a system controller, and wherein the system controller is located at a location remote from the at least one switch.

9. A system for anti-icing operation of power transmission lines comprises:

at least one cable having at least two sections, wherein each section comprises:

at least a first, a second, and a third conductor; wherein the first, second, and third conductor are mutually insulated;

a first switch for coupling the second and third conductors to the first conductor at a first end of the cable, the second and third conductors being electrically coupled near the first switch;

a second switch for coupling the first and second conductors to the third conductor at a second end of the cable, the first and second conductors being electrically coupled near the second switch; and

wherein the third conductor of the first section is connected to the first conductor of the second section; and

a system controller for simultaneously opening the first and second switches of each section to increase resistance of the at least one cable by placing the first, second, and third conductors in series for anti-icing operation of the cable of that section, and wherein the system controller is capable of sequentially opening switches of sections.

10. The system ofclaim 1 further comprising apparatus for sensing overheating of at least one conductor of the cable, and for placing the first, second, and third conductors in parallel to reduce resistance of the cable upon sensing overheating.

11. A switchbox for switching conductors of a transmission line cable between a parallel configuration and a series configuration, the switchbox comprising:

an energy storage device for providing power to the switchbox;

apparatus for charging the energy storage device;

a control signal receiver for receiving switch operation commands, the control signal receiver powered from the energy storage device;

at least one switch for determining current flow through at least one conductor of the cable, the switch electrically actuated under control of the control signal receiver; and

apparatus for overriding the control signal receiver and placing the cable conductors in a parallel configuration if a high temperature is detected on a conductor of the cable.

12. A system for deicing of a cable of a power transmission line, the cable comprising N conductors, where N is an odd integer larger than one, where each of the N conductors are electrically insulated from the other conductors the system comprising:

a first and a second switchbox, wherein the first switchbox is coupled to a first end of the cable and the second switchbox is coupled to a second end of the cable;

wherein each switchbox has at least (N−1)/2 switches;

wherein in a first mode the switches of the switchboxes connect all N conductors of the cable in parallel, and in a second mode the switches of the switchboxes connect all N conductors in series to increase cable resistance for effective deicing operations; and

a system controller for placing the switchboxes in the first mode for normal operation and in the second mode for deicing the cable.

13. A system for anti-icing operation of a cable of a power transmission line, the cable comprising a resistive strength core and at least one conductor, the strength core being electrically insulated from the at least one conductor the system further comprising:

a switchbox for diverting sufficient current from the conductor through the resistive strength core for anti-icing operation of the cable in a first operating mode, and wherein a majority of the current passes through the conductor in a second operating mode.

14. The system ofclaim 13 wherein the switchbox places an inductance electrically in series with the conductor during the first operating mode, the strength core being electrically in parallel with the series combination of inductor and conductor.

15. The system ofclaim 14 wherein the switchbox places an inductance electrically in series with the conductor during the first operating mode by inserting a magnetic core material into a coil, and wherein the magnetic core material is removed from the coil during the second operating mode.

16. The system ofclaim 13, wherein the switchbox further comprises apparatus for switching between the first and the second operating mode under command of an external system controller, apparatus for sensing an overheat condition of the cable, and apparatus for reducing current in the resistive strength core when an overheat condition is detected.

17. The system ofclaim 16 wherein the switchbox comprises a transformer, the transformer having a secondary coupled to the strength core during deicing.

18. The system ofclaim 12, wherein the cable has a strength-reinforcement conductor of higher electrical resistance and mechanical strength than the N conductors of the cable; wherein

the strength-reinforcement conductor is electrically insulated from other conductors along the length of a section, but is connected at the first section end to a first conductor of the N conductors and at the second end of the section to an Nth conductor of the N conductors; and

wherein opening of switches in the switchboxes increases the effective electrical resistance of the N conductors between the switchboxes such that a larger current is diverted into the strength-reinforcement conductor to deice it.

19. A system for anti-icing of power transmission lines comprises:

at least one section of cable for transmission of power in the transmission line comprising at least a first, a second, and a third conductor; wherein the first, second, and third conductor are mutually electrically insulated;

at least a first switch at a first end of the section of cable, and at least a second switch at a second end of the section of cable, the first and second switches operable such that the first, second, and third conductor are capable of being connected in at least a low resistance configuration, an intermediate resistance configuration, and a high resistance configuration; and

a system controller for determining when anti-icing operation is required, for selecting an appropriate anti-icing configuration from the intermediate and high resistance configurations, and for setting the switches into the anti-icing configuration when anti-icing operation is required, and into the low resistance configuration when anti-icing operation is not required.

20. A method for deicing a section of a cable of a power transmission line, the cable comprising a plurality of conductors extending between a first switchbox and a second switchbox, the section of cable having a normal operating mode wherein the plurality of conductors are electrically in parallel, the method comprising:

detecting ice accumulation on the section of the cable;

configuring switches of the switchboxes to couple a plurality of the conductors of the cable electrically in series thereby placing the section of cable in a deicing mode having a deicing mode resistance greater than a resistance of the section of cable in the normal operating mode;

allowing a current flowing in the section of cable to resistively heat the section of cable to deice the section of cable; and

reconfiguring the switches of the switchboxes to return the section of cable to the normal operating mode.

21. The method ofclaim 20 wherein the switches in the switchboxes have at least a first configuration corresponding to the normal operating mode, a second configuration corresponding to the deicing mode having a first resistance between switchboxes, and a third configuration corresponding to a second deicing mode having a second resistance between switchboxes; the method further comprising:

monitoring the current flowing in the section of cable to determine a deicing mode appropriate for the current.

22. The method ofclaim 21 further comprising transmitting a message to request an increase in the current flowing in the section of cable when current in the cable is insufficient for deicing.

23. The method ofclaim 21 wherein a first conductor of the plurality of cables has a resistance substantially different from a resistance of a second conductor of the plurality of cables.

24. The method ofclaim 20 wherein the transmission line is configured to transmit power to electric vehicles.

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