U.S. patent application number 12/193650 was filed with the patent office on 2009-10-08 for system and method for deicing of power line cables.
This patent application is currently assigned to The Trustees of Dartmouth College. Invention is credited to Victor Petrenko, Charles R. Sullivan.
Application Number | 20090250449 12/193650 |
Document ID | / |
Family ID | 41132307 |
Filed Date | 2009-10-08 |
United States Patent
Application |
20090250449 |
Kind Code |
A1 |
Petrenko; Victor ; et
al. |
October 8, 2009 |
System And Method For Deicing Of Power Line Cables
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.
Inventors: |
Petrenko; Victor; (Lebanon,
NH) ; Sullivan; Charles R.; (West Lebanon,
NH) |
Correspondence
Address: |
LATHROP & GAGE LLP
4845 PEARL EAST CIRCLE, SUITE 201
BOULDER
CO
80301
US
|
Assignee: |
The Trustees of Dartmouth
College
Hanover
NH
|
Family ID: |
41132307 |
Appl. No.: |
12/193650 |
Filed: |
August 18, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61041875 |
Apr 2, 2008 |
|
|
|
Current U.S.
Class: |
219/262 |
Current CPC
Class: |
H02G 7/16 20130101; B60M
1/12 20130101 |
Class at
Publication: |
219/262 |
International
Class: |
F23Q 7/00 20060101
F23Q007/00 |
Claims
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.
Description
RELATED APPLICATIONS
[0001] 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
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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
[0024] FIG. 1 is a schematic diagram of a system for preventing or
removing ice accumulation from a power transmission line.
[0025] FIG. 2 illustrates an embodiment for preventing or removing
ice accumulation from a trolley wire used to transmit power in a
transportation system.
[0026] FIG. 3 illustrates an alternative embodiment of a cable for
use with the system of FIG. 2.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] FIG. 8 is an electrical schematic of one section of one
cable of an alternative embodiment having seven conductors per
cable.
[0032] FIG. 9 is a cross sectional diagram of a cable having seven
conductors and a steel strength member in thermal contact with each
other.
[0033] FIG. 10 is a block diagram of a solar-battery-powered
switchbox for use in the system.
[0034] FIG. 11 is a block diagram of an alternative switchbox for
use in the system.
[0035] FIG. 12 illustrates a system having multiple cable sections
each capable of independent or sequential deicing or anti-icing
operation.
[0036] FIG. 13 illustrates a cross-section of a first cable for use
with the system of FIG. 1.
[0037] FIG. 14 illustrates a cross-section of a second cable for
use with the system of FIG. 4.
[0038] FIG. 15 illustrates a cross-section of a third cable for use
with the system of FIG. 1.
[0039] FIG. 16 illustrates an alternative embodiment having series
connected switches.
[0040] FIG. 17 illustrates a deicing system for power lines as
proposed in PCT/US2004/27408.
[0041] FIG. 18 illustrates a cross section of a cable having a
steel strength core electrically insulated from an outer conductive
layer.
[0042] FIG. 19 illustrates a two-conductor,
single-switch-per-section deicing system.
[0043] FIG. 20 is a schematic diagram of an inductive switchbox
suitable for use with the deicing system of FIG. 19.
[0044] FIG. 21 is a schematic diagram illustrating an alternative
core for use with the inductive switchbox of FIG. 20.
[0045] 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.
[0046] FIG. 23 is a schematic diagram of an alternative embodiment
having some features of FIGS. 1 and 15.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0047] A system for electrically removing accumulated, or
preventing accumulation of, ice on a power transmission line 100 is
illustrated in FIG. 1. For simplicity, only one of the three cables
102 or phases of a typical three-phase AC line is shown. In the
embodiment of FIG. 1, a cable 102 is constructed from three
parallel conductors 104, 106, 108. The three conductors 104, 106,
108 are bundled together by insulating spacers 110 along cable
102.
[0048] 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.
[0049] 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.
[0050] 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
[0051] 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.
[0052] The embodiment of FIG. 1 is also applicable to a cable or a
polarity of a DC transmission line or trolley power line, as
illustrated in FIG. 2. In the embodiment of FIG. 2, there are three
parallel conductors 150, 152, 154 coupled into a serpentine
configuration between two switchboxes 156, 158. One of the three
conductors, the contact conductor 154, is arranged so that it is
accessible to contact with pantographs 160 or other trolley-wire
contacting apparatus of an electrically powered vehicle 162.
[0053] 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.
[0054] In the embodiment of FIG. 2, switches 168, 166 may be opened
to enter deicing mode, and closed for normal operating mode.
Opening of these switches 168, 166, causes current flowing through
the conductors 154, 152, 150, such as current being drawn by
vehicles 162 in later sections of the system, to pass through all
three conductors 154, 152, 150 in sequence rather than in parallel,
increasing current density and heating the conductors.
[0055] In the embodiment of FIG. 2 the contact conductor 154, may,
but need not, be fabricated from material different from that of
the other or non-contact conductors 152, 150. For example, the
contact conductor may be a high-strength moderate-resistance
bronze, brass, copper-clad steel, stainless steel, or aluminum-clad
steel, with parallel conductors 150, 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 apparatus 160. Further, while it can be
advisable to deice the non-contact conductors 152, 150 to avoid
weight and wind related damage, ice on the contact conductor 154
may interfere with power transfer from the contact conductor 154 to
the pantograph or other trolley wire contacting apparatus 160.
Higher resistance of the contact conductor 154 may help to ensure
prompt and rapid deicing of the contact conductor 154 to ensure
continued operation of the vehicle 162 during icing conditions. In
this alternative embodiment, opening of switches 166, 168 for a
brief time can deice contact conductor 154 to ensure continued
operation, while opening of switches 166, 168 repeatedly or for a
longer time can deice the non-contact conductors 152, 150 when ice
accumulation threatens weight or wind related damage.
[0056] While in some embodiments of the trolley system of FIG. 2
non-contact conductors 150, 152 are separately strung from the
contact conductor 154, or are nearby conductors 150, 152, 154
separated by spacers; in an alternative embodiment, as illustrated
in FIG. 3, a contact conductor 154 may form a shell containing
insulating material and non-contact conductors 150, 152.
[0057] 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.
[0058] The alternative embodiment of the system 200 for removing or
preventing ice accumulation of FIG. 4 has five instead of three
conductors per cable 202. In this embodiment, each switch box 204,
210 has two ganged switches 206, 207, 209 and actuator-controller
208. In this embodiment, opening of switches 206, 207, 209 has the
consequence of increasing the effective resistance of cable 202 by
a factor of twenty-five; thereby increasing self-heating of cable
102 to melt accumulated ice and retard accumulation of additional
ice. In the embodiment of FIG. 4, two conductors of the five carry
current in a reverse direction while three conductors carry current
in a forward direction.
[0059] In the embodiment of FIG. 4, the effective length of total
conductor in a segment of cable 102 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.
[0060] 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.
[0061] 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 FIG. 4 may be operated in alternative ways to produce other
effective power dissipation increases, such as is illustrated in
FIG. 5.
[0062] In the embodiment of FIG. 5, switches 206, 209 are opened to
enter a deicing mode, while switch 207 is left closed. In this
embodiment, assuming equal resistance per conductor, effective
resistance of the cable segment is increased by a factor of
five.
[0063] Similarly, in the embodiment 215 of FIG. 6, switches 206,
207 are opened while switch 209 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.
[0064] Embodiments having cables with six or more conductors may
have even numbers of conductors. In the six-conductor embodiment
220 of FIG. 7, resistance of the cable segment is increased from
one-sixth R to three-halves R, an increase by a factor of nine when
the switches 206, 222 open. Other configurations of the system are
possible, having other power increases in deice mode; for example
if switch 222 is left closed while switches 206 open, resistance
increases from one-sixth R to three-fourths R, an increase of four
and a half.
[0065] Similarly, an alternative embodiment 250 may have seven
conductors in each cable and three or four (as illustrated in FIG.
8) switches 252, 254, 256, 258, 260, 262, 264, 266 in each
switchbox 268, 270. In the embodiment of FIG. 8, effective
resistance of the cable is programmable according to which switches
are open, as illustrated in FIG. 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 in
FIG. 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, switches 266
and 252 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-US-00001 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
[0066] 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.
[0067] In a particular embodiment, a transmission line system has
phase cables 267 having multiple segments each of which corresponds
to the schematic diagram of FIG. 8. In this embodiment, the cables
267 have seven conductors 253, 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 central steel strength member 280,
according to the cable cross section of FIG. 9. The seven
conductors correspond to the seven conductors of FIG. 8. In this
embodiment, the phase cables are suspended from towers and equipped
with a system controller 124 in manner resembling that of FIG.
1.
[0068] 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.
[0069] 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.
[0070] Resistance self-heating of a transmission line is
proportional to current I through the transmission line squared,
times the resistance R of the line (I.sup.2*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 FIG. 8, wires 263 and 265 have
resistance ten times the resistance of the other, or low
resistance, conductors 253, 255, 257, 259, 261. During normal
operation, these conductors 263, 265 carry little current, and the
effective resistance R is slightly less than one fifth that of the
resistance of each of the low resistance conductors 253, 255, 257,
259, 261. Should all seven conductors be configured in series by
closing only switches 252 and 256, the effective resistance is
increased to one hundred twenty five R, with an intermediate
increase to seventy R if switches 258 and 260 are closed. An
assortment of other intermediate resistance increases is also
available for controller 124 to select from, and may be readily
calculated.
[0071] 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.
[0072] 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.
[0073] 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 FIG.
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.
[0074] 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.
[0075] In an embodiment (FIG. 10) particularly suited for use with
high-tension transmission lines, since all components of the
switchbox 204, 116, 118 operate near power-line cable 102, 202
voltage, switchbox 204, 116, 118 is attached at the cable 202, 102
end of insulators 112 and is suspended with the cable. In such an
embodiment, it is not practical to power the switchbox 204, 116,
118 from normal 115V AC power. In consequence, switchbox 204, 116,
118, 300 is powered by an internal energy store 302 such as an
ultracapacitor or battery.
[0076] 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.
[0077] 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.
[0078] In an embodiment, actuator 314 operates to oppose the force
of a spring 318 that tends to hold switch 316 closed.
[0079] 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.
[0080] An alternative embodiment, as illustrated in FIG. 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 receiver 312 normally switches cable 102, 202
between low and high resistance conditions by activating
electrically actuated contactor modules 340. Contactor modules 340
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.
[0081] In the embodiment of FIG. 11, contactor modules 340 are
connected in parallel with safety switches 342 that are closed by
spring 344 whenever fusible link 320 melts due to excessive heating
in the conductor 346 to which clamp 322 is fastened. This
effectively overrides both the control signal receiver 312 and
switches 340 when conductor 346 reaches high temperatures. This
will prevent excessive dip in, or overheat damage to, cable 102,
202 in the event of failed switchboxes, but poses some risk of ice
damage to cable 102, 202 at a later time--especially if left
unrepaired.
[0082] 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.
[0083] 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.
[0084] In order to assist with control of the system, a temperature
sensor 324 (FIGS. 10 and 11) may be attached to clamp 322,
temperature readings being transmitted by temperature transmitter
326 to system control 124 to indicate when, for example, deicing of
a section is expected to be complete because temperature of a
conductor 104 of cable 102 has significantly exceeding the freezing
point of water.
[0085] 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.degree. 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.
[0086] In the embodiment of FIG. 12, each cable 400 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 as section 402 and section 404, of from one tenth to ten miles
length, for example. Each section has a first switchbox 406, 410,
414 and a second switchbox 408, 412, 416. In order to prevent
excessive voltage drop in the transmission line, when it is
determined that it is desirable to deice cable 102, the switchboxes
406, 408 of the first section 402 are activated to open the
switches. When that section is deiced, the switches of the first
section are closed and switchboxes 410, 412 of the second section
are activated to open the switchboxes, and so on in sequence until
all iced-over sections of the cable 400 are deiced. Similarly,
division of cable 400 into sections permits deicing of those
sections of the cable 400 that have been or are exposed to icing
conditions, while allowing sections exposed to different weather to
continue normal operation.
[0087] 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.
[0088] 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. A triangular spacer 502, which
may be nonconductive plastic, ceramic, or metal with rubber
insulators, is attached to each conductor 504 of the cable.
Attachment of spacer 502 to the conductor 504 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. Each conductor 504 may be a
conductive copper or aluminum shell, over an optional steel
supporting center 506, or may be assembled from conductive copper
or aluminum strands wrapped around a supporting center of multiple
steel strands. Spacers 502 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.
[0089] In the embodiment of FIG. 14, four conductors 602 are
positioned by spacers 604 around a central conductor 606, each of
the five conductors is of essentially equal ampacity. One, in the
embodiment illustrated conductor 606, or all five conductors 602,
606 may have a steel core 608 to provide the strength needed for
long spans between towers. Since all five conductors 602 carry
current during deicing, all five will be deiced even if these
conductors are not in thermal contact with each other.
[0090] In the embodiment of FIG. 15, a cable 700, for use as cable
102 or 202, has three (illustrated), five, seven, or nine
conductors 702, 704, 706 assembled around a strength core 708 which
may be stranded steel. The conductors 702, 704, 706, which may be
stranded copper or aluminum, are insulated from each other and
coated with an extruded plastic insulation layer 710.
[0091] With reference to FIGS. 13, 14, and 15, it is anticipated
that the conductors 504, 602, 606, 702, 704, 706 and steel
supporting cores 506, 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.RTM. (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.
[0092] 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.
[0093] 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.
[0094] In the embodiment of FIG. 16, an alternative switch
configuration provides similar effect. In this embodiment of a
phase 800, a cable 802 has an odd number of conductors 810, 812,
814, 816, 818 greater than three running between two switchboxes
804, 805. During normal operation, switches 806 and 807, connected
in series, connect conductors 812, 814, 816, and 818 in parallel to
conductor 810 and the input 820 and to the output switchbox 805,
where corresponding switches are closed. When switchbox control and
actuators 808 open switches 806 and 807, current is forced to flow
through all five conductors 810, 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. switch 806) in the series sequence.
[0095] 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 FIG. 17 and as disclosed in PCT/US2004/27408. A
switch 908 at an end of a section 910 of the cable transitions
between normal operation with the two conductors 904, 906 in
parallel, and deicing operation with current flow in only one 906
of the two conductors 904, 906; the one conductor 906 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 the conductor 904 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 of section 910 from the switch 908, and ahead of the switch 914
of the next section 916, the two conductors 904, 906 are
electrically shorted 912 together. Except at shorts 912, the
conductors 904, 906 are separated by a layer of insulation 918. In
the design disclosed in PCT/US2004/27408, deicing first conductor
904 is an outer layer of the cable physically close to the ice to
be removed, while normal second conductor 906 is the central bulk
of the cable, and may include any core of the cable.
[0096] High-tension transmission line cables, including modified
cable 1000 (FIG. 18), generally have many strands 1002 of a
conductor such as aluminum or copper surrounding a strength core
having strands 1004 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 modified
cable 1000, there is an added layer of insulation 1006 that
prevents electrical contact between strength core strands 1004 and
conductive strands 1002.
[0097] A modified deicing system 1100 for power transmission
cables, as illustrated in FIG. 19, has a cable 1102 having a steel
core 1104, an insulation layer 1106, and a conductive layer 1108,
the insulation layer 1106 preventing contact between steel core
1104 and conductive layer 1108; each or steel core 1104 and
conductive layer 1108 are typically formed of multiple strands.
Additional layers, such as an outer insulation and weather
protection layer, may exist. Cable 1102 is separated into sections
1110, at one end of a section 1110 is a switchbox 1114, at the
other end is a short circuit connection 1116 between steel core
1104 and conductive layer 1108.
[0098] 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.
[0099] In an alternative embodiment, as illustrated in FIG. 23 with
reference to FIGS. 15 and 1, the conductive layer has several
conductors 702, 704, 706, as illustrated in FIG. 15, coupled with
switchboxes similar to those of FIG. 1 or FIG. 4. The strength core
708 is electrically connected between switchboxes 1401, 1403 at
each end of a segment of cable. When the switches 1402, 1404 open,
effective resistance of the conductive layer 702, 704, 706
increases relative to that of the cable with closed switches,
diverting more but not all current through the steel strength core
1104, 708.
[0100] 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.
[0101] Switchbox 1114 has powering arrangements and
high-temperature override apparatus as previously described with
reference to FIG. 10 and FIG. 11.
[0102] In an alternative embodiment, as illustrated in FIG. 20, a
switchbox 1200 suitable for use in place of switchbox 1114 has no
switch 1120. In this embodiment, switchbox 1200 has a power input
connection 1202 connected to both the outer conductive layer 1108
and inner steel core 1104 of a preceding cable section, and to a
power output connection 1204 for connection to the inner steel core
1104 of the cable section 1110; in some embodiments this connection
may incorporate a locally bared steel core 1104 of the cable.
[0103] The embodiment of FIG. 20 also has a coil 1206 having a few
turns of high-ampacity wire, coil 1206 connected between power
input connection 1202 and a second power output connection 1208 for
connection to the outer conductive layer 1108 of the cable section
1110. Switchbox 1200 has an energy store 1212 with charging
arrangements as previously discussed with reference to FIGS. 10 and
11, and a control signal receiver 1214. When control signal
receiver 1214 receives a command to deice the cable section 1110,
receiver 1214 activates a motor actuator 1216 that pulls on a
nonmagnetic cable 1218. Nonmagnetic cable 1218 runs over pulley
1220 to a magnetic core element 1222, activation of motor actuator
1216 draws core element 1222 into coil 1206. When core element 1222
is drawn into coil 1206, inductance of coil 1206 is increased
thereby diverting a portion of current in cable 1102 through
resistive inner steel core 1104
[0104] 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.
[0105] In the event that a fusible link, such as previously
discussed with respect to fusible link 320 of FIG. 1, melts due to
excessive heating of cable section 1110, safety actuator rod 1230
is drawn into switchbox 1200 by a spring 1232. Actuator rod 1230
being drawn into switchbox 1200 triggers release catch 1224 to
release pulley 1220, which allows spring 1226 to draw core element
1222 from coil 1206 and return cable section 1110 to low-impedance
operation; this effectively reduces current in strength core 1104
and reduces self-heating of the cable 1102.
[0106] In an embodiment of the switchbox of FIG. 20, the switchbox
incorporates circuitry such as the sensor 324 and
temperature/status transmitter 326 of FIG. 11 such that system
controller 124 (FIG. 1) can determine when deicing is complete,
whereupon system controller 124 will command switchbox 1200 to
return to normal operation and commence deicing (if required) of
the next cable section. In an embodiment, the control signal
receiver 1214 also monitors sensor 324 and attempts to return
switchbox 1200 to normal operation by extracting core 1222 at a
temperature lower than that required to melt fusible link 320.
[0107] In an alternative embodiment resembling that of FIG. 20,
instead of a single-piece movable core 1222, a two-piece core is
used as illustrated in FIG. 21. In this embodiment, a first
L-shaped core portion 1240 is fixed to the switchbox. A second
L-shaped core portion 1242 is arranged such that it may be
extracted from coil 1232 in a first position as shown as 1242 in
FIG. 21 to give a low-inductance setting, or drawn into the coil
1232 to a second position 1244 shown by dashed lines in FIG. 21 to
give a high-inductance setting. In this embodiment, first and
second L-shaped core portions 1242, 1240 form a loop for magnetic
flux when the second core portion 1242 is in the high-inductance
position.
[0108] The embodiments of FIGS. 19 and 20 operate under control of
a system controller 124 as previously discussed with reference to
FIG. 1; in an embodiment some sections of cable are deiced as
described with reference to FIGS. 19 and 20, while some other
sections are deiced as described with reference to FIGS. 1 and
4.
[0109] The embodiment of FIG. 22 also is a system 1300 for deicing
transmission line cable, in this case by heating the cable 1302 by
diverting a portion of cable power through a step-up transformer
(windings 1304 and 1306) and through the steel supporting strands
1308 of cable 1302. In this embodiment, steel strands 1308 are
surrounded by insulation 1310, and then surrounded by stranded
aluminum or copper conductive layer 1312. Switchbox 1313 has a
switch 1314 open in normal operating mode, and 1316 closed, to
allow current to flow through the conductive layer 1312 unimpeded.
Switchbox 1313 also has a power store 1322 and command receiver
1324 similar to and having equivalent charging circuitry to the
power store 302 and command receiver 312 described in reference to
FIG. 11; as with other embodiments command receiver 1324 is in
communication with system controller 124.
[0110] 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.
[0111] As with other embodiments, the embodiment of FIG. 22 has
apparatus (not shown in FIG. 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, switch 1316 or an auxiliary switch (not
shown) is closed to reduce current in cable core 1308 by bypassing
transformer primary 1306; as in normal operating mode bypassing
primary 1306 greatly reduces current in the cable core 1308 and
reduces resistive heating of the cable 1302.
[0112] Switchboxes of all embodiments herein described, such as the
switchboxes of FIGS. 10, 11, 21, and 22, sense overtemperature
conditions of the cable and switchboxes, as for example through
temperature sensor 324, and attempt reversion from deicing to
normal operation at temperatures below that required to melt
fusible links such as fusible link 320. Mechanical sensing and
return to low-resistance operation provided by fusible links 320
and associated apparatus is an overriding mechanical backup
intended to prevent overheat damage to the transmission line and
its cables should system control 124, electrical switch actuators
314, electrically actuated switches 340, 1120, 1316, motor actuator
1216, temperature sensors 324, or other components fail into the
deicing mode.
[0113] 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.
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