System And Method For Deicing Of Power Line Cables
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.
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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.
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.
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.
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
DETAILED DESCRIPTION OF THE EMBODIMENTS
A system for electrically removing accumulated, or preventing accumulation of, ice on a power transmission line 100 is illustrated in
Cable 102 is suspended by insulators 112 from towers 114, or in an alternative embodiment from poles (not shown). At ends of a section of cable 102, a first switch box 116 and second switch box 118 are suspended from insulator 112 along with cable 102. Each switch box 116, 118 contains a switch 120 and a switch actuator-controller 122.
For a given section of transmission line, the switch boxes 116, 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 all parallel conductors 104, 106, 108 of cable 102 connected electrically in parallel. When ice accumulation along the power transmission line 100 is known or suspected, or when it is desired to prevent ice accumulation due to icing weather conditions, switches 120 of boxes 116, 118 are placed in the switch-opened state. This results in the three conductors 104, 106, 108 of cable 102 being connected electrically in series instead of in parallel, with one conductor 104 carrying power in the reverse direction, thereby increasing the effective resistance of the section of cable 102 by a factor of nine.
With the switches 120 in switch-opened state, and the effective resistance of cable 102 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 of cable 102 over the normal switch-closed state provides heating of cable 102 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 of cable 102 to either melt accumulated ice or to retard accumulation of additional ice
The switches 120 of switch boxes 116, 118 operate under control of a system controller 124. In one embodiment system controller 124 is located at a network operations center. In another embodiment system controller 124 is an automatic device capable of sensing local weather conditions including ice accumulation and attached to a tower 114 near a section of cable 102 subject to ice accumulation and having switchboxes 116, 118 under its control. In this way, switches of both switchboxes 116 and 118 can be opened or closed essentially simultaneously even if switchboxes 116 and 118 are located one or more miles apart.
The embodiment of
Vehicle 162 may be an electric locomotive, or a streetcar unit as illustrated, with return path for vehicle current through a rail 164. In an alternative embodiment, two sets of parallel conductors 154 and switchboxes 156, 158 are provided with dual trolley-wire contacting apparatus 160, one for each phase or polarity of a DC or AC trolley-wire system, such that vehicle 162 connects to both phases or polarities. In this alternative embodiment, vehicle 162 may be a rubber-tired vehicle such as the electrically powered busses that have operated in San Francisco for many years.
In the embodiment of
In the embodiment of
While in some embodiments of the trolley system of
This system 100 differs from that of Couture in that direction of current in one conductor 104 of the cable 102 is reversed, and in that Couture deices only one or a few conductors at a time, while the system 100 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. The system 100 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 in system 100 the switches are placed at both section ends. Couture's system for a three-conductor line would have 3 switches, while system 100 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, while system 100 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, system 100 differs from that of Shimada because no loop is formed and no additional current is applied to a loop.
The alternative embodiment of the system 200 for removing or preventing ice accumulation of
In the embodiment of
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 of cable 102 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 in
In the embodiment of
Similarly, in the embodiment 215 of
Embodiments having cables with six or more conductors may have even numbers of conductors. In the six-conductor embodiment 220 of
Similarly, an alternative embodiment 250 may have seven conductors in each cable and three or four (as illustrated in
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 has phase cables 267 having multiple segments each of which corresponds to the schematic diagram of
In this embodiment, controller 124 monitors current through the transmission line cables. When ice is detected, the controller 124 determines a resistance increase that will provide adequate heating of the cable 267 to deice the cable, while avoiding damage to cable 267. The controller then automatically determines a configuration of open switches for switches 252, 254, 256, 258, 260, 262, 264, 266 of switchboxes 268, 270, and transmits that configuration to switchboxes 268, 270 to cause the system to enter deicing mode for a particular cable 267 segment. Upon completion of deicing of the cable 267 segment, the switches are closed to return to normal operation.
In the event that ice is detected and deicing is desired, but cable 267 is carrying too little current to provide adequate heating for deicing even at a maximum resistance configuration of switches 252, 254, 256, 258, 260, 262, 264, 266 of switchboxes 268, 270, controller 124 may transmit a request to a grid management system to reconfigure the power grid such that enough power is carried through cable 267 to deice the cable 267. 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 of
In yet another alternative embodiment, conductor 263 has resistance ten times that of each low resistance conductor 253, 255, 257, 259, 261, and conductor 265 has resistance thirty times that of each low resistance conductor 253, 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, the controller 124 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 switchboxes 268, 270 which set their switches accordingly. The controller continues to monitor current in the transmission line, and may reconfigure switches of switchboxes 268, 270 if current changes to provide appropriate heating for deicing while avoiding excessive heating that may damage the transmission line. Controller 124 may be a separate controller or may be integrated into a switchbox 268, 270.
In an embodiment the transmission line segment 267 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; controller 124 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 of
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 (
In most embodiments, energy store 302 is charged through charger 310 by a device selected from devices such as an inductive pickup 304 surrounding one or more conductors of cable 102, 202, a solar panel 306, or a small-value capacitor 308 to ground. Energy store 302 powers a control signal receiver 312, which is normally the only component of the switchbox 300 to consume power.
When control signal receiver 312 receives a correctly encoded “deice” command from system control 124, which may be transmitted from control 124 to receiver 312 via a high frequency carrier wave superimposed on cable 102, 202 along with the power being transmitted, optically over an optical fiber, or by radio, the receiver 312 activates electrically operated switch actuator 314 that opens high current switch or switches 316. Switch actuator 314 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, switch 316 is an electronic switch; yet another embodiment has electronic switches in parallel with electrically-operated mechanical switches.
In an embodiment, actuator 314 operates to oppose the force of a spring 318 that tends to hold switch 316 closed.
Because inadvertent opening of switches 316 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 to cable 102; actuator 314 pulls switch 316 open by acting not against a case of switchbox 300, but through a fusible link 320 to a clamp 322 that is attached to one conductor, such as conductor 104, of cable 102 a short distance from switchbox 300. Fusible link 320 is adjacent to conductor 104, and is made of a low-melting metal or plastic such that it will break before conductor 104 reaches a temperature at which excessive sag or damage to cable 102 occurs and allow spring 118 to close switch 116. Therefore, should the system for ice removal or ice prevention fail, switches 116 fail into the closed (low resistance) condition.
An alternative embodiment, as illustrated in
In the embodiment of
In another embodiment, control signal receiver 312 monitors temperature sensed by temperature sensor 324 and closes switches 340 to return all conductors to parallel operation at a temperature indicative of successful deicing but lower than a temperature required to melt fusible link 320. In an embodiment, temperature/status transmitter 326 transmits an indication of closing of switches 340 due to high temperature to system controller 124 so that the switchbox at the other end of the conductors can also return all conductors to parallel operation. Fusible link 320 is preferably located on the conductor having the highest current when switchboxes of a line segment are in the inconsistent state of one switchbox having switches 340 open and the other switchbox having switches 340 closed.
In order to provide feedback to the system control 124, and encourage repair of failed switchboxes, a status of a sensing switch 347 ganged with safety switches 342, senses failure of fusible link 320 and transmits this information through transmitter 326 to system control 124.
In order to assist with control of the system, a temperature sensor 324 (
Alternatively, sensor 324 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 of
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.
In the embodiment of
In the embodiment of
With reference to
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 a system controller 124 to switchboxes 300. 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 of
Deicing systems for transmission lines have been proposed where each of typically three phases is conducted over a cable 900, and that cable is divided into two conductors 904, 906, as illustrated in
High-tension transmission line cables, including modified cable 1000 (
A modified deicing system 1100 for power transmission cables, as illustrated in
During normal operation, switchbox 1114 maintains an electrical connection between conductive layer 1108 of each section of the cable 1102. In this normal mode, a majority of current through cable 102 pass through conductive layer 1108. To deice a section 1110 of cable 1102, a controller 1118 of the switchbox 1114 associated with that section 1110 of cable 1102 opens a switch 1120, thereby reducing or eliminating current in conductive layer 1108 and, since the cable is part of a transmission line that is continuing to conduct power, correspondingly increasing current in steel core 1104 of that section 1110.
In an alternative embodiment, as illustrated in
In an embodiment, switchbox 1114 contains an inductor 1122. When switch 1120 opens, the inductor is placed electrically in series with the low resistance outer conductive layer 1108 of cable section 1110, this series connection of inductor 1122 and conductive layer 1108 is electrically in parallel with inner steel core 1104 of that section; in consequence some but not all current in cable 1102 is diverted through steel core 1104; the amount of this current being substantially greater than that through steel core 1104 during normal operation with switch 1120 closed.
Switchbox 1114 has powering arrangements and high-temperature override apparatus as previously described with reference to
In an alternative embodiment, as illustrated in
The embodiment of
Pulley 1220 is attached to a case of switchbox 1200 through a release catch 1224, and a spring 1226 having sufficient strength to overcome solenoid attraction of core element 1222 into coil 1206 is connected to draw core element 1222 from coil 1206. When switchbox 1200 control signal receiver 1214 receives a command to discontinue deicing of cable section 1110, control signal 1214 commands motor actuator 1216 to unwind nonmagnetic cable 1218. This permits spring 1226 to draw core element 1222 from coil 1206 and return cable section 1110 to normal operation.
In the event that a fusible link, such as previously discussed with respect to fusible link 320 of
In an embodiment of the switchbox of
In an alternative embodiment resembling that of
The embodiments of
The embodiment of
When it is desired to deice cable 1302, command receiver 1324 receives a command and closes switch 1314 first to establish a current path through steel supporting strands 1308; then command receiver 1324 opens switch 1316 to apply considerable current to transformer primary winding 1306. Transformer secondary winding 1304 thereupon provides power to supporting strands 1308. Transformer primary 1306 has only a few turns, and transformer core 1318 is constructed of a saturable magnetic material, such that only a small proportion of the power available in the cable is applied to the supporting strands 1308; such as 100 to 300 watts per meter of cable—in a 600 kV transmission 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 winding 1306 may be held to a low level.
As with other embodiments, the embodiment of
Switchboxes of all embodiments herein described, such as the switchboxes of
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.
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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 16 wherein the switchbox comprises a transformer, the transformer having a secondary coupled to the strength core during deicing.
18. The system of claim 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 of claim 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 of claim 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 of claim 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 of claim 20 wherein the transmission line is configured to transmit power to electric vehicles.
International Classification: F23Q 7/00 (20060101);