Submarine power feeding branching device for submarine power feeding system having submarine feeding cables arranged in mesh pattern

A submarine power feeding branching device comprises a constant current-constant current converter which isolates an input side for a trunk submarine cable from an output side for a branch submarine cable. The converter receives a first constant current and produces a second constant current by using the first constant current. The second constant current is supplied to the output side while the first constant current is returned to the input side. Because the input side and the output side are isolated, it is easy to add/remove the device to/from a submarine power feeding system. Intensity of the second constant current can be controlled by controlling duty ratios of switches included in the converter. Thus, it is possible that the intensity of the second constant current is equal to that of the first constant current. Therefore, a submarine repeater can be provided along either the trunk cable or the branch cable.

This application claims priority to prior application JP 2002-305918, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a submarine electric power feeding branching device and a submarine electric power feeding system using the submarine electric power feeding branching device. In particular, this invention relates to a submarine power feeding branching device suitable for arranging submarine cables in mesh pattern and constructing a submarine power feeding system having the submarine cables arranged in mesh pattern.

In fields of researches for submarine earthquakes, ocean environment or the like, there are demands for arranging a large number of submarine observation devices, such as seismometers, tsunami instruments, current meters, hydrometers or the like, in two dimensional arrangement (or a matrix) on the bottom of the sea to collect various data from the submarine observation devices.

To meet such demands, it is possible to construct a observation system that comprises submarine observation devices, which are provided on the bottom of the sea, and submarine cables, which are used for feeding electric power to the submarine observation devices and continuously collecting data from (or communicating with) the submarine observation devices.

However, it is impractical that the submarine cables individually connect the submarine observation devices to a land observation device(s). Furthermore, when an observation system has plural submarine observation devices which are connected to a submarine cable in series, it possesses low reliability. This is because the submarine observation devices located between a failure point and the end of the submarine cable can not receive electric power from a land observation device and communicate with the land observation device when the failure occurs in the submarine cable. Thus, a submarine cable system (or power feeding system) having submarine cables arranged in mesh or lattice pattern is necessary to construct an observation system having a large number of submarine observation devices arranged in two dimensional arrangement (or a matrix) and possessing high reliability.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a submarine electric power feeding branching device capable of constructing a submarine electric power feeding system having submarine cables arranged in mesh pattern and possessing high reliability.

Other objects of this invention will become clear as the description proceeds.

According to a first aspect of this invention, a power feeding branching device comprises a constant current-constant current converter having an input terminal, a first output terminal and a second output terminal which is electrically isolated from both the input terminal and the first output terminal. A controller is connected to the constant current-constant current converter and makes the constant current-constant current converter utilize a first constant current supplied to the input terminal to produce a second constant current and a restored first constant current. The second constant current and the restored first constant current are to be supplied to the second output terminal and the first output terminal, respectively.

The power feeding branching device may further comprise a bypass circuit connected between the input terminal and the first output terminal. The bypass circuit bypasses the constant current-constant current converter to allow the first constant current instead of the restored first constant current to lead from the input terminal to the first input terminal when the input terminal has an electrical potential higher than a predetermined potential.

Furthermore, the power feeding branching device may comprises a bypass diode connected between the second output terminal and a ground terminal.

According to a second aspect of this invention, a power feeding system includes a plurality of trunk cables connected to feeding devices, a plurality of branch cables each of which is provided between adjacent two of the trunk cables, and a plurality of power feeding branching devices for connecting the branch cables with the trunk cables. Each of the power feeding branching devices comprises a constant current-constant current converter having an input terminal, a first output terminal and a second output terminal which is electrically isolated from both the input terminal and the first output terminal. A controller is connected to the constant current-constant current converter and makes the constant current-constant current converter utilize a first constant current supplied to the input terminal to produce a second constant current and a restored first constant current. The second constant current and the restored first constant current are to be supplied to the second output terminal and the first output terminal, respectively.

In the power feeding system, the power feeding branching devices are classified into two types. One of the types leads the second constant current from the constant current-constant current converter to the second output terminal. The other of the types leads the second constant current from the second output terminal to constant current-constant current converter. Each of the branch cables is connected between two second output terminals of two of the power feeding branching devices different from each other in type.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Description will be at first directed to a conventional submarine cable system. As the conventional submarine cable system, a communication submarine cable system is well known.

The communication submarine cable system comprise a long communication submarine cable including a power feeder inside to feed electric power for repeaters provided along the communication submarine cable. The power feeder comprises a single conductor to reduce diameter and cost of the communication submarine cable. Seawater is used as a return circuit in the communication submarine cable system. The communication submarine cable system adopts a constant current feeding system to make easy insulation between circuits in each repeater and strengthen tolerance for failure of a short circuit of the submarine cable.

If the repeaters are fed with constant voltage, an electronic circuit in each repeater must have a grounded terminal. Accordingly, the electronic circuit has a higher voltage part and a lower voltage part. Therefore, the electronic circuit needs expensive electronic components which can withstand high voltage. Furthermore, dimensions of the electronic circuit tend to become large to maintain reliability regarding insulation between the electronic circuit and the seawater.

As it is, the electronic circuit in each repeater need not have the ground terminal because the repeaters are fed with constant current. Accordingly, differences of voltages are relatively small in the electronic circuit and the expensive electronic components which can withstand the high voltage are unnecessary for the electronic circuit. Furthermore, the electronic circuit is easily insulated from the seawater by covering it in whole with an insulator. In addition, the constant current feeding system can feed the constant current as far as a short circuit failure point from land feeding device through the submarine cable which is short circuited. In a case of the constant voltage feeding system, the communication submarine cable system is considerably affected by drop of electrical potential at the short circuit failure point.

Referring toFIG. 1, the description is made about an existing communication submarine cable system having submarine branching devices.

InFIG. 1, the communication submarine cable system comprises submarine branching devices11aand11b,submarine cables12a–12eand feeding devices13a–13d. While the submarine cables12a,12eand12dserve as a main submarine cable, each of the submarine cables12band12cserves as a branch submarine cable.

It may seem that the communication submarine cable system is applicable to a hydrographic observation system. However, such a system can comprise a comb submarine cable network but it can not comprise a submarine cable network having a lattice or mesh arrangement. This is because each branch cable12b,12cmust be connected to the feeding device13b,13cat one end and grounded to the seawater at the other end.

FIG. 2is a block diagram of a conventional submarine cable power feeding system designed with referring to the above mentioned communication submarine cable system.

The system ofFIG. 2comprises a land observation device21located on land, submarine branching devices22a,22blocated on the bottom of the sea, and submarine observation devices23a–23ealso located on the bottom of the sea. While the land observation device21comprises a land feeding device24, each of the submarine branching devices22a,22bcomprises a current limiter25a,25b.

The land feeding device24feeds a first constant current to a trunk feeder line26. Upon receiving the first constant current, each submarine branching device22a,22buses the current limiter24a,24bto feed a second constant current to a branch feeder line27a,27b(or the submarine observation device23a–23b,23c–23d).

The current limiter25ais composed, for example, as illustrated inFIG. 3. The current limiter25bis similar to the current limiter25ain structure. InFIG. 3, the feeding current on the trunk feeder line26is equal to 1 [A] while the feeding current on the branch feeder line27ais equal to 0.1 [A]. However, the feeding current on the branch feeder line27ais varied by adjusting a variable resistor RV. That is, the feeding current on the branch feeder line27ais decided by the Zener voltage of a Zener diode RC1and a resistance of the variable resistor RV. An emitter current of a transistor TR is held constant because the Zener voltage is stabilized. Such a submarine power feeding system is disclosed in Japanese Unexamined Patent Publication No. 2001-309553.

The submarine power feeding system has some problems as follow.

First, as understood fromFIG. 2, the submarine power feeding system has a comb shape. That is, use of the submarine branching device as shown inFIG. 3makes possible to contract a comb shaped submarine power feeding system. However, it is hard to construct a lattice or mesh arrangement of cables for a submarine power feeding system by the use of the submarine branching devices. This is because the submarine branching devices that are provided to adjacent branch feeding lines must form pairs to construct the lattice arrangement of the cables. In such a case, it is difficult to match a first current produced by one of each pair with a second current produced by the other of the pair though the first current must be matched with the second current.

Secondly, the submarine power feeding system ofFIG. 2has disadvantage of difficulty in adding and removing the submarine branching device(s). This is because each branching device (or the variable resistor thereof) must be adjusted in response to variation of the load though it is located on the bottom of the sea.

Thirdly, the submarine power feeding system can not operate when the trunk feeder line26is short circuited. This is because the trunk feeder line26has an electric potential of about 0 [V] and the submarine branching devices22aand22bare inoperative when the trunk feeder line26is short circuited.

Fourthly, the submarine branching device is an inefficient device. This is because the submarine branching device uses resistors to limits the feeding current on the branch feeder line. That is, the resistors waste electric power. Additionally, the submarine branching device must be designed with consideration of heat radiated by the resistors.

Fifthly, the submarine power feeding system needs plural submarine branching devices which have different specifications. This is because the second constant current on the branch feeder line is different from the first constant current on the trunk feeder line.

Sixthly, the submarine branching device is not insulated between an input aide and an output side. Electrical potential difference between the input side and the output side of the submarine branching device must be smaller than withstand voltage of electronic devices of the submarine branching device. In other words, the submarine branching device restricts freedom of design of the submarine power feeding system.

A proposal has been made about a submarine electric power feeding system having submarine cables arranged in mesh pattern. Such a submarine power feeding system is disclosed in Japanese Unexamined Patent Publication No. 2003-244032.

FIG. 4shows the proposed submarine electric power feeding system. The submarine power feeding system comprises main backbone cables41aand41bconnected to land constant voltage feeding devices42aand42brespectively. Sub backbone cables43a,43band43care connected to the main backbone cables41aand41bby the use of submarine power feeding branching devices44aand44b.Submarine repeaters45are connected to any one of the sub backbone cables43a,43band43cbetween any one pair of the submarine power feeding branching devices44aand44b.A submarine observation device46is connected to each of the submarine repeaters45.

The submarine power feeding system ofFIG. 4adopts a constant voltage feeding system for the main backbone cables and a constant current feeding system for the sub backbone cables.

Because of the constant voltage feeding system, each of the submarine power feeding branching devices44aand44bmust be grounded. This means that each of the submarine power feeding branching devices has a circuit including higher voltage circuitry and lower voltage circuit circuitry. Accordingly, expensive electronic components which can withstand high voltage are necessary for the power feeding branching devices. Furthermore, the circuit of the power feeding branching device tends to be large to maintain insulation between itself and seawater.

Referring toFIGS. 5 to 11, description will proceed to a submarine electric power feeding system according to a first embodiment of this invention.

FIG. 5shows a block diagram of the submarine electric power feeding system. The submarine electric power feeding system comprises a plurality of constant current feeding device51a,51band53cwhich are provided in land stations (not shown) located apart from one another.

The constant current feeding device51a,51band51cis connected to trunk submarine cables52a,52band52crespectively. The trunk submarine cables52a,52band52cgenerally extends offshore. The greater part of each of the trunk submarine cables52a,52band52cis placed on the bottom of the sea.

Two types of submarine power feeding branching devices53aand53bare provided along each of the trunk submarine cables52a,52band52c. In other words, the submarine power feeding branching devices53aand53bare interposed in each of the trunk submarine cables52a,52band52c.The submarine power feeding branching device53ahaving a first type and the submarine power feeding branching device53bhaving a second type are fundamentally similar to each other in structure. However, the first type53aproduces constant current flowing from the inside to an output terminal thereof while the second type53bproduces constant current flowing from an output terminal thereof to the inside.

Each of the submarine power feeding branching devices53ais a counterpart of any one of the submarine power feeding branching devices53b.In other words, the first type53aand the second type53bof the submarine power feeding branching devices make a pair. Companions of each pair of the submarine power feeding branching devices53aand53bare interposed in adjacent two of the trunk submarine cables52a–52c. For instance, the submarine power feeding branching device53ainterposed in the trunk submarine cable52ais a companion to the submarine power feeding branching device53binterposed in the trunk submarine cable52b.Besides, the submarine power feeding branching device53ainterposed in the trunk submarine cable52bis a companion to the submarine power feeding branching device53binterposed in the trunk submarine cable52c.

Branch submarine cables54a,54b,54cand54dare connected between companions of the pairs of the submarine power feeding branching devices53aand53b.The branch submarine cables are generally placed on the bottom of the sea. The branch submarine cables maybe approximately perpendicular to the trunk submarine cables52a,52band52c.The trunk submarine cables and the branch submarine cables are ideally arranged in mesh or lattice pattern. However, the configuration of the trunk and the branch submarine cables is not limited in the mesh pattern. The configuration is changed because of not only landform of the seabed but also the other factors. The branch submarine cables and the trunk submarine cables serve a net or mesh power feeding line.

Submarine repeaters55are interposed in (or provided along) the brunch submarine cables54a,54b,54cand54d.The repeaters55are connected to submarine observation devices56. The submarine observation devices56are placed on the bottom of the sea. The submarine observation devices56may be arranged in second dimensional arrangement (or a matrix).

With this structure, the land constant current feeding devices51a,51band51cfeed first constant currents to the submarine power feeding branching devices53aand53bthrough the trunk submarine cables52a,52band52c.

Each of the submarine power feeding branching devices53afeeds a second constant current to the branch submarine cable54a,54b,54cor54dwhen it receives the first constant current fed from the constant current feeding device51a,51bor51c.On the other hand, each of the submarine power feeding branching devices53babsorbs the second constant current fed from the submarine power feeding branching devices53awhich companions thereto. Here, the seawater is used as a return circuit for each of the trunk and the branch submarine cables.

The submarine repeaters55have a structure well known in the art. Each of the submarine repeaters55produces constant voltage from the second constant current fed from the submarine power feeding branching devices53ato feed it for the submarine observation device56connected thereto.

The submarine observation devices56also have a structure well known in the art. While each of the submarine observation devices56receives the constant voltage fed from the submarine repeater55connected thereto, it performs regular observation and produces observation data. The observation data produced by the submarine observation devices56are transmitted to a land observation device(s) provided in the land station(s) through the branch submarine cable(s) and trunk submarine cable(s).

Because the submarine power feeding system adopts the constant current feeding system for the trunk submarine cables52a,52band52c, a plurality of the submarine power feeding branching devices53aand53bcan be connected in series. Therefore, it is easy to extend the trunk submarine cables52a,52band52cand add additional submarine power feeding branching devices. In addition, it is easy to provide additional submarine repeaters along the trunk submarine cables52a,52band52ctogether with additional observation devices. Thus, the submarine power feeding system can be expanded over a wide area with a mesh pattern of submarine cables as illustrated inFIG. 5.

The submarine power feeding system also uses the constant current feeding system for feeding electric power to the branch submarine cables54a,54b,54cand54d.Therefore, it is easy to provide additional submarine repeaters along the branch submarine cables52a,52band52ctogether with additional observation devices.

Each of the submarine repeaters is fed with electrical power from two of the submarine feeding branching device53aand53b.Accordingly, the submarine power feeding system has tolerance for failure of a short circuit. Theoretically, even when only one short circuit occurs on the trunk and the branch submarine cables of the submarine power feeding system, all of the submarine repeaters provided to the branch submarine cables54a–54dcan be fed with electric power.

Next, the submarine power feeding branching device53awill be described in more detail with referring toFIG. 6.

FIG. 6shows an internal construction of the submarine power feeding branching device53a.As illustrated inFIG. 6, the submarine power feeding branching device53acomprises a constant current-constant current converter61, a switch controller62, a communication device63and a bypass circuit64. The submarine power feeding branching device53ais housed in a pressure-resistant case (not shown).

The constant current-constant current converter61has a pair of input (or primary) side terminals and a pair of output (or secondary) side terminals. The input side terminals are electrically isolated from the output side terminals. The input side terminals of the constant current-constant current converter61are connected to an input terminal65and a first output terminal66of the submarine power feeding branching device53a.The output side terminals of the constant current-constant current converter61serve as a second output terminal67and the ground terminal68of the submarine power feeding branching device53a.

The constant current-constant current converter61comprises a transformer TR1having primary and secondary windings. The primary winding is connected to the input terminal65at the midpoint thereof while the second winding is connected to the ground terminal68at the midpoint thereof. A first condenser C1is connected between the input terminal65and the first output terminal66. A first switch S1is connected between one end of the primary winding and the first output terminal66. A second switch S2is connected between the other end of the primary winding and the first output terminal66. Semiconductor switches, such as MOSFETs, bipolar transistors or the like, may be used for the first and the second switches S1and S2. A first diode D1is connected between one end of the secondary winding and the second output terminal67while a second diode D2is connected to the other end of the secondary winding and the second output terminal67. A second condenser C2is connected between the second output terminal67and the ground terminal68. A bypass diode611is connected between the second output terminal67and the ground terminal68.

While the first condenser C1and the first and the second switches S1and S2form a square waveform producing portion. The first and the second diode D1and D2and the second condenser C2forms a rectifying smoothing portion.

The switch controller62controls each of the switches S1and S2to make it an on state or an off state. When the first switch S1is in the on state and the second switch S2is in the off state, the current supplied to the input terminal65flows in the primary winding of the transformer TR1as shown by a solid line arrow N1-1. At this time, a secondary side current flows in the secondary winding of the transformer TR1as shown by a solid line arrow N2-1. On the other hand, when the first switch S1is in the off state and the second switch S2is in the on state, the current supplied to the input terminal65oppositely flows in the primary winding of the transformer TR1as shown by a broken line arrow N1-2. At this time, the secondary side current flows in the secondary winding of the transformer TR1as shown by a broken line arrow N2-2. Thus, the switch controller62produces square wave currents in the primary winding of the transformer TR1by controlling the switches S1and S2. The square wave currents are added to each other and returns to the first constant current. In other words, the square waveform producing portion produces a restored first constant current from the square wave currents to supply it to the first output terminal66.

The bypass circuit64comprises a switching circuit641and a voltage detecting circuit642for controlling the switch circuit641. The switching circuit641is normally in an off state. The voltage detecting circuit642detects a voltage difference between the input terminal65and the first output terminal66. The voltage detecting circuit642makes the switching circuit641an on state when it detects the voltage difference equal to or larger than a predetermined value.

The communication device63is connected to the land observation device or the like provided in the land station (not shown) through, for example, optical fibers provided in the submarine trunk cables and the submarine branch cables. In addition, the communication device63is connected to the switch controller62and the voltage detecting circuit642. The communication device63receives a control signal (or a command) transmitted from the land observation device to send it for the switch controller62and/or the voltage detecting circuit642. Furthermore, the communication device63transmits a measurement signal representing measured results concerning voltage and current at any points in the submarine power feeding branching device53a.

The description will be soon made about the operation of the submarine power feeding branching device53a. Hereinafter, it is assumed that the submarine power feeding branching device53ais connected to the trunk submarine cable52aand the branching submarine cable54a.

The first constant current fed to the input terminal65through the trunk submarine cable52ais supplied to both of the condenser C1and the midpoint of the primary winding of the transformer TR1. While the switches S1and S2are alternately repeatedly changed between the on and the off states, primary current with the square waves flows in the primary winding of the transformer TR1.

The switch controller62controls the switches S1and S2in a manner as described later in more detail to generate the square waves of the primary side current in the primary winding of the transformer TR1.

The condenser C1absorbs the first constant current flowing in the trunk submarine cable52ato prevent abnormal high voltage from occurring at the input terminal65in a case where both of the switches S1and S2are in the off state. The condenser C1further prevents noises produced by the operation of the switches S1and S2from being transmitted to the trunk submarine cable52a.

The transformer TR1isolates between the trunk submarine cable52aand the branch submarine cable54awhile it supplies power of the primary (or input) side thereof to the secondary (or output) side thereof. That is, the transformer TR1produces the secondary side current with square waves corresponding to the square waves of the primary side current.

A combination of the diodes D1and D2and the capacitor C2rectifies and smoothes the secondary side current and produces an output constant current. The output constant current is supplied to the branch submarine cable54athrough the second output terminal67as the second constant current.

The bypass diode611bypasses a surplus current fed from the submarine power feeding branching device53bwhich companions to the present submarine power feeding branching device53athrough the branch submarine cable54a, when the output current is smaller than that of the submarine power feeding branching device53b.The surplus current may occur when the submarine power feeding branching device53ahas a failure(s) anywhere. The bypass diode611has a cathode connected to the second output terminal67. In the submarine power feeding branching device53b,a bypass diode comprises an anode connected to a second output terminal (i.e. the branch submarine cable54a) differently from the bypass diode611.

The bypass circuit64detects overvoltage of the input terminal101to bypass the first constant current supplied to the input terminal65for the first output terminal66. The overvoltage may occur when the secondary wiring is opened or when each switch S1or S2and/or the switching controller62is out of order. In such a case, the first constant current supplied to the input terminal65bypasses the constant current-constant current converter61and thereby the bypass circuit64prevents excessive voltage from being given to the submarine power feeding branching device53a.

The voltage detecting circuit642detects the overvoltage of the input terminal101to control the switching circuit641. That is, the voltage detecting circuit642supplies a control signal for the switching circuit641when it detects the overvoltage of the input terminal101. The switching circuit641is normally in the off state as mentioned above. Upon receiving the control signal from the voltage detecting circuit642, the switching circuit641changes from the off state to the on state. Thus, the constant current-constant current converter is bypassed by the bypass circuit64.

After the overvoltage is detected once, the voltage detecting circuit642keeps the switching circuit641being in the on state until it receives a command signal transmitted from the land station through the communication device63. This is made to prevent the switching circuit641from chattering. When the voltage detecting circuit642receives the command signal from the land station, it returns the switching circuit to the original (i.e. off or open) state.

Next, the operation of the switches S1and S2will be described with referring toFIGS. 7 and 8.

FIG. 7is a timing chart in a case where the switch S1, S2has a duty ratio of 50% each. The duty ratio Rd is defined by:
Rd=(Ton/T)×100 [%]  (1).

In this case, the secondary current has the square waves with intensity Iout when the transformer TR1has a turns ratio of N1/N2. The intensity Iout is given by:
Iout=N1/N2×Iin  (2).

FIG. 8is a timing chart in a case where the switch S1and S2has a common duty ration smaller than 50%. As shown in the bottom ofFIG. 8, the input terminal65receives the first constant current with the intensity Iin regardless of the switches S1and S2. Accordingly, the condenser C1is charged with the first constant current Iin when both the switches S1and S2are in the off state. If either the switch S1or S2turns into the on state, electric charges charged in the condenser C1is supplied to the transformer TR1. At this time, the primary side current flowing in the primary winding of the transformer TR1has intensity I1given by:
I1=T/(2Ton)×Iin  (3).

Furthermore, the secondary side current flowing in the secondary winding of the transformer TR2has intensity I2given by:
I2=N1/N2×I1=N1/N2×T/(2Ton)×Iin  (4).

In addition, the output current has intensity Iout which is equal to average of the secondary side current I2. The intensity Iout of the output current is given by:
Iout=(2Ton)/T×I2=N1/N2×Iin  (5).

The formula (5) is identical to the formula (2). This shows that the output current supplied to the output terminal67is fixed even if the duty ratio is varied on condition that the duty ratio is smaller than 50%. Accordingly, it is possible to solve problems which occurs when the switches S1and S2are in the on state at the same time. When the switches S1and S2are in the on state at the same time, the primary winding short circuits and the electric charges charged in the condenser C1are suddenly discharged therefrom. The sudden discharge of the condenser C1is likely to bring any troubles to the transformer TR1and/or the condenser C1.

The submarine power feeding branching devices53aand53bwhich companion to each other substantially equally share feeding power fed to the branch submarine cable54a(54b,54c, or54d) in the submarine power feeding system ofFIG. 1. Thus, the submarine power feeding branching device53amust be adjusted and stabilized in the submarine power feeding system ofFIG. 1so that the output current thereof has the same intensity as that of the output current from the submarine power feeding branching device53bwhich companions thereto.

The description will be made about the adjustment for the submarine power feeding branching device53aand stability thereof in the following.

At first, the description is directed to the adjustment of the output current for the submarine power feeding branching device53a.

FIG. 9is a timing chart of the switches S1and S2. InFIG. 9, the switches S1and S2are different from each other in duty ratio.FIG. 9also shows a waveform of the secondary side current in the secondary winding of the transformer TR1.

Here, the duty ratio Rd1of the switch S1is represented by Ton1/T while the duty ratio Rd2of the switch S2is represented by Ton2/T. In addition, the period T is given by:
T=Ton1+Ton2  (6).

The formula (3) can be rewritten as follow.
Rd1+Rd2=1  (7).

The secondary current has intensity Iout1during a period of Ton1and intensity Iout2during a period Ton2. Therefore, the secondary current has peak to peak intensity Ioutp−p given by:
Ioutp−p=Iout1+Iout2  (8).

In addition, by the use of the turns ratio N2/N1, the peak to peak intensity Ioutp−p given by:
Ioutp−p=N2/N1×Iin  (9).

Furthermore, because a direct current is not transferred from the primary winding to the secondary winding, the following formula is valid.
Ton1×Iout1=Ton2×Iout2  (10).

The output current Iout, which is obtained by rectified, is represented by:
Iout=(Ton1×Iout1+Ton2×Iout2)/T(11).

By arranging the formulas (6) to (11), the following formula is obtained.
Iout=4Rd1(1·Rd1)×N2/N1×Iin  (12).

From the formula (12), a relation between the duty ratio Rd1of the switch S1and an output current-input current ratio Iout/Iin is derived.FIG. 10shows a graph representing the derived relation between Rd1and Iout/Iin.

As understood fromFIG. 10, when the duty ratio Rd1is equal to 50%, the secondary side current Iout has a maximum value of N2/N1×Iin. When the duty ratio Rd1is equal to 0% or 100%, the secondary side current Iout has a minimum value of zero. Thus, the secondary side current Iout varies according to the duty ratio Rd1(and Rd2). In other words, the secondary side current Iout can be controlled by the changing the duty ratios Rd1and Rd2.

Next, the description is directed to the stability of the pair of the submarine power feeding branching devices53aand53bwhich are adjusted to match the output currents produced by the pair.

FIG. 11is a graph representing a measured output voltage-current characteristics of a current-current converter which can be used for the constant current-constant current transformer T1. The output voltage-current characteristics has been measured by a measurement circuit as shown inFIG. 12. As shown inFIG. 12, the measurement circuit comprises a current generator connected to an input side of the current-current converter. A variable resister is connected to an output side of the current-current converter. The measurement has been made as resistance of the variable resister has been varied.

Returning toFIG. 11, the output current decreases from 609.8 [mA] to 602.5 [mA] as the output voltage increases from 0.3 [V] to 41.1 [V]. That is, the output voltage variation of 40.8 [V] (ΔV=41.1−0.3) is in conjunction with the output current variation of 7.3 [mA] (ΔI=609.8−602.5). This is because a transformer for the current-current converter has output impedance which decreases the output voltage with the increment of the output current. Here, a ratio of the output voltage variation ΔV to the output current variation ΔI is referred to as an inclined resistance Rout (=ΔV/ΔI=5.6 k Ω).

The pair of the submarine power feeding branching devices53aand53bconnected to each other through the branch submarine cable54a(54b,54c,or54d) have such inclined resistance Rout each. Accordingly, the pair of the submarine power feeding branching devices53aand53bare stabilized by the inclined resistance Rout when they are adjusted to match their output currents to each other.

FIG. 13shows output voltage-current characteristics of the submarine power feeding branching devices53aand53bconnected to each other through the branch submarine cable54atogether with their operating voltages and currents. For brevity's sake, load resistance R represents the total of conductor resistance of the branch submarine cable54aand electric resistance of the submarine repeater(s) interposed in the branch submarine cable54a.

InFIG. 13, two graphs of the output voltage-current characteristics of the submarine power feeding branching devices53aand53bare labeled “CH-1” and “CH-2” respectively. Each of the submarine power feeding branching devices53aand53bis restricted within a predetermined output voltage. The submarine power feeding branching devices53aand53balways have a common output current rout because they are connected in series.

As illustrated inFIG. 13, if the output current Iout is equal to a certain value I0, the submarine power feeding branching devices53aand53bmust produce the output voltages Vout1and Vout2, respectively. Here, assuming that the submarine power feeding branching devices53aand53bforms a combined device, an output voltage Vout of the combined device is given by:
Vout=Vout1+Vout2.

Accordingly, output voltage-current characteristics of the combined device can be obtained by the use of the output current Iout as a parameter. A graph of the output voltage-current characteristics of the combined device are depicted inFIG. 13and labeled “COMBINED CHARACTERISTICS”.

In Consideration of the load resistance R, a graph of load characteristics can be depicted inFIG. 13. The load characteristics are given by:
Vout=R×Iout.

The combination device has an operation point which corresponds to an intersection between the graph of the output voltage-current characteristics of the combined device and the graph of the load characteristics. At the operation point, the output voltage has a value of Voutop while the output current has a value of Ioutop. Operation voltages Voutop1and Voutop2of the submarine power feeding branching devices53aand53bare obtained by the use of the output current Ioutop. Because the operation point is the intersection of the two graphs, it is stable.

The inclined resistance described previously makes possible to find the stable operation point for the combined device. If each of the submarine power feeding branching device of the combined device is an ideal current generator, the inclined resistance is infinite and the combined device can not share feeding power. That is, one of ideal current generators is charged with the feeding power while the other has the output voltage of 0 [V] in such a case.

As mentioned above, by the use of the submarine power feeding branching device53ahaving the structure ofFIG. 6and the power feeding branching device53bhaving the same structure as shown inFIG. 6, the submarine power feeding system as illustrated inFIG. 5can be constructed.

In the submarine power feeding system ofFIG. 5, the constant current feeding devices51a,51band51cprovided on land feed first constant currents to the submarine power feeding branching devices53aand53bthrough the trunk submarine cables52a,52band52c.The submarine power feeding branching devices53aand53buses the first constant currents from the constant current feeding devices51a,51band51cas power sources to feed second constant currents for the submarine repeaters55through the branch submarine cables54a,54b,54cand54d. The submarine power feeding branching devices53aand53bwhich make a pair produce the second constant currents flowing in opposite direction and having identical intensity. The pair of the submarine power feeding branching devices53aand53babout equally share feeding power between them. A plurality of the submarine power feeding branching devices53aand/or53bcan be provided along each trunk submarine cables52a,52bor52c.Therefore, the trunk submarine cables52a,52band52cand the branch submarine cables54ato54dcan be widely spread in mesh or lattice pattern. In consequence, the submarine repeaters (and the submarine observation devices) can be widely arranged in second dimensional arrangement (or matrix).

As understood fromFIG. 6, each of the submarine power feeding branching devices53aand53bcan efficiently produce the second constant current because it includes no element which wastes electric power.

Furthermore, each submarine power feeding branching device53aor53ballows electrical potential difference between the trunk submarine cable and the branch submarine cable which are connected thereto. This is because the submarine power feeding branching device is isolated between the input side and the output side thereof. Therefore, the submarine power feeding branching device make possible to construct various power feeding system flexibly.

In addition, the submarine power feeding branching device can produce the second constant current having equal intensity with the first constant current supplied thereto. Accordingly, the submarine power feeding branching device makes possible to construct the power feeding system that the first constant current on each trunk submarine cable is equivalent to the second constant current on each branch submarine cable. In such a system, the submarine repeater can be interposed in either the trunk submarine cable or the branch submarine cable.

Referring toFIG. 14, the description will be mad about a submarine power feeding branching device according to a second embodiment of this invention. The submarine power feeding branching device530is similar to that (53a) ofFIG. 6but has an additional resistor612.

The additional resistor612is connected between the second output terminal67and the ground terminal68. The additional resistor612reduces an inclined resistance R in comparison with that of the submarine power feeding branching device53aofFIG. 6. Hereby, a combined device comprising the submarine power feeding branching device530and a similar device has a wide range of variable output current.

Referring toFIG. 15, the description will be made about a submarine power feeding branching device according to a third embodiment of this invention. The submarine power feeding branching device531comprises two submarine power feeding branching devices53aofFIG. 6.

As shown inFIG. 15, the first output terminal66of the submarine power feeding branching device53a-1is connected to the input terminal65of the submarine power feeding branching device53a-2. Furthermore, the first ground terminal68of the submarine power feeding branching device53a-1is connected to the second output terminal67of the submarine power feeding branching device53a-2.

For normal operation, the submarine power feeding branching devices53a-1and53a-2must produce identical output currents. This can be made by controlling the duty ratio of the switches S1and S2in each constant current-constant current converter61. That is, in each of the submarine power feeding branching devices53a-1and53a-2, the output current is adjusted by controlling the duty ratio of the switches S1and S2. Both of the submarine power feeding branching devices53a-1and53a-2are stabilized by the effect of the inclined resistors of them. Thus, the output currents of the submarine power feeding branching devices53a-1and53a-2matches with each other.

The submarine power feeding branching devices531can feed larger power for the branch submarine cable54abecause it can produce higher output voltage in comparison with that (53a) ofFIG. 6.

Three or more submarine power feeding branching devices may be connected in serial to produce further larger output voltage.

Referring toFIG. 16, the description will be made about a submarine power feeding branching device according to a fourth embodiment of this invention. The submarine power feeding branching device532is similar to that (53a) ofFIG. 6but has a transformer T2instead of the transformer T1. The transformer T2differs from the transformer T1in that it has taps613and614provided along the primary winding. The taps613and614are equidistant from the midpoint of the primary winding.

The submarine power feeding branching device532further comprises third to sixth switches S3–S6. The third switch S3is connected between the first switch S1and one end of the primary winding. The forth switch S4is connected between the first switch S1and the tap613nearer the end of the primary winding that is connected to the third switch S3. The fifth switch S5is connected between the second switch S2and the tap614nearer the other end of the primary winding. The sixth switch S4is connected between the second switch S2and the other end of the primary winding.

The switch controller62performs a different operation to control not only the switches S1and S2but also the switches S3–S6. The switch controller62receives a control signal transmitted from the land station through the communication device63and controls the switches S1–S6according to the control signal.

While the switch controller62turns switches S3and S6on and turns switches S4and S5off, the submarine power feeding branching device532can operate in the same manner as the submarine power feeding branching device53aofFIG. 6. On the other hand, while the switch controller62turns switches S3and S6off and turns switches S4and S5on, active length of the primary winding is shorter than that ofFIG. 6. That is, the turns ration N2/N1is larger than that ofFIG. 6in this case. Thus, the submarine power feeding branching device532can have a wide range of variable output current in comparison with the submarine power feeding branching device532.

While this invention has thus far been described in conjunction with the few embodiments thereof, it will readily be possible for those skilled in the art to put this invention into practice in various other manners. For example, the submarine repeater may be provided along the trunk submarine cable. Furthermore, a device such as the submarine power feeding branching device ofFIG. 15may be made by using the submarine power feeding branching device ofFIG. 14or16. In addition, the submarine power feeding branching device ofFIG. 16may comprise a resistor612as shown inFIG. 14.