FR2572598A1 - METHOD AND INSTALLATION FOR POWERING AN ELECTRIC CABLE WITH A CONTINUOUS VOLTAGE WHICH IS SUPERPOSED TO AN ALTERNATIVE VOLTAGE - Google Patents

Abstract

THE INVENTION RELATES TO THE TRANSPORT OF ELECTRIC ENERGY. THE ALTERNATIVE VOLTAGE IS GENERATED IN ONE OF THE TERMINAL STATIONS 4 AND COMPENSATED AT THE OTHER TERMINAL STATION 5 SO AS TO MAKE NULL THE SUM OF THE ALTERNATIVE ELECTROMOTOR FORCES IN THE MESH WHICH INCLUDES THE CABLE 1. THE ALTERNATIVE VOLTAGE SOURCE AND THE DEVICE. COMPENSATOR ARE ACHIEVED BY ACTING ON THE CONTROL PULSES OF THE ACDC AND DCAC CONVERSION CIRCUITS, OR THEY ARE CONSTITUTED BY VOLTAGE GENERATORS. MAIN APPLICATIONS: UNDERWATER CABLES.

Description

The present invention relates to a method and an installation for

  power an electrical cable with

  a DC voltage which is superimposed on a voltage

  alternating voltage of a value lower than this DC voltage and a low frequency. Specifically, the process and the installation

  which constitute the object of the present invention

  attempt to continuously supply an electrical cable connected between two terminal stations with a DC voltage which may be of the order of a few hundred

  kV, which is superimposed on a voltage component

  alternative having a peak-to-peak value

  between 0.5% and 10% of the value of the

  and a frequency below 50 Hz, and preferably

between 1 and 20 Hz.

  In the present description, it is understood that

  generally speaking, by the term "terminal station" a head station of the cable in which it is carried out

  converting a DC voltage into an alternating voltage

  or vice versa. This station can therefore be a

  intermediate station of a shared distribution network

  plex, a point of branching, the point of transition between a buried or submerged cable and an overhead line, etc.

  - The DC voltage may be the continuous voltage

  bare of service provided for the cable or it possesses

  of a higher or lower value but in any case

  This voltage is at least an order of magnitude higher than the AC voltage. In addition, the term "continuously" means a food which is maintained for a period of time

measure at least in hours.

  Current DC power transmission facilities that utilize

  High Voltage (HVDC) cables include global-

  two or more conversion stations (

  trainers and inverters) that transmit the

  power of the AC network to the mains network

  continuous and vice versa, and at least one DC transmission line which may be aerial

and / or cable.

  The converters make use of valves, preferably thyristors, whose conduction is controlled by control devices which provide the functions of regulation and generation / distribution of pulses.

  control valves and operate in such a way that the

  bucket operates at the values of intensity and voltage de-

sirées.

  In particular, the use of

  high voltage direct current cable

  Until now, it has been used in sea crossings

  times of a length greater than a few kilometers, since the traditional AC transmission technique poses significant technical problems

  which are due to the great value of the capac-

  in relation to the earth itself.

  In such installations, especially if they are longer than a few tens of kilometers, the cost of the cables can become the post

  by far the most important of the economy of the world.

project.

  The cost of these cables depends on the intensity

  Nominal tension and tension as well as depth

  the sea to cross and, more generally, conditions

laying procedures.

  The type of cable that best meets the technical and economic requirements of use includes an insulation composed of paper impregnated with oil and the maximum rated voltage used to date is

about + 300 kV DC.

  In the operation of DC transmission installations, under steady-state conditions, the voltage applied to the cable is in a substantially continuous form, except for a very low ripple ("ripple") whose

  peak to peak is of the order of a few points per thousand.

  It has been observed that, if one superimposes

  continuous voltage applied to the cable continuously and over the entire length of the cable, an alternating voltage having an amplitude of between 0.5 and 10% of the value of the DC voltage and a frequency

  between 1 Hz and a few tens of Hz, the characteristics

  cable transmission are significantly improved.

  The DC voltage may be the nominal voltage of

  vice or possess a different value than this one but

  of the same order of magnitude. In addition, this type of food

  may be used permanently during the service of the cable, or in an irregular manner (but continuously for at least a certain number of hours or days) or, finally, the cable may be subjected to

  treatment of this kind before its implementation. In part-

  In this way, an improvement in the dielectric strength characteristics of the cable is obtained.

  a cable under conditions of reliability even

  their voltage and / or intensity greater than those

  which are provided under normal conditions of service

this.

  This has the effect that a cable made from the

  traditional way (ie without any research

  additional values) for the maximum voltage usable with a given cable structure, may be used

  for the transport of electrical energy to a higher

  laughing. Alternatively, an undersize cable can be used.

  to transmit power to voltages

  which would otherwise require the use of a cable

  with superior benefits and cost.

  For the simplicity of the exposition, the invention is

  described below in relation to certain specific cases, but it is applicable generally to all

  dc high voltage cables which have

  unassisted isolation by pressure or

gas, or liquid.

  According to the invention, an alternating voltage pos-

  sedenting the characteristics indicated above is

  connected to the continuous service voltage at a terminal station by at least one source having the appropriate characteristics while

  at the other end there is provision for a

  thinker (active or passive) capable of producing a

  the same amplitude, but in the opposite direction, than that

  of the source, so as to nullify the sum of the

  these alternative electromotive devices in the mesh which

  take the cable.and the two terminal stations. It is

  out of what has been stated above that the tension

  tinue may also not be the actual voltage to which the cable will be subjected in service and that the points of application of the alternating voltage may

  reduce to the simple ends of the cable.

  The subject of the invention is a process for feeding

  continuously connect an electrical cable connected between

  two terminal stations with a continuous voltage at

  superimposed on a low-frequency alternating voltage which has a peak-to-peak value less than the value of the DC voltage, characterized in that such an alternating voltage is generated at one of the terminal stations, by compensating for it, at the other station

  terminal, by an identical alternating voltage,

  to minimize the current flowing through the cable

  which is due to the AC voltage component.

  The term "minimal" means a value of

  alternating current calculated in order not to interfere with

  regular operation of the control devices mentioned above. More precisely and for the installations

  currently known, this current is considered half

  when it is less than 5% of the

naked flowing in the cable.

  The invention furthermore consists of an installation

  to continuously power an electrical cable.

  connnected between two terminal stations, with a DC voltage on which is superimposed a low frequency AC voltage and a peak value at

  peak less than the value of the DC voltage,

  formally to the process described above and characterized in

  it includes at least one alternate voltage source

  at one of the stations and at the other station at least one compensating device

  find a voltage equal to said AC voltage.

  According to a preferred embodiment of the invention,

  the source and the compensating device are

  made by modulating over time and in a coordinated way

  the distribution of the control pulses of the

  in the conversion stations.

  The figures of the appended drawing, given solely by way of non-limiting example, will make it clear how the invention can be implemented. On this drawing,

  Figure 1 is a diagram illustrating the arrangement

  general assembly of a known conversion station;

  Figure 2 schematically illustrates the invention

  in the case of a single transmission installation

  polar; Fig. 3 shows a terminal station in more detail; Figure 4 schematically illustrates a shape

  of the invention relating to an installation

bipolar transmission; and

  FIG. 9 illustrates a variant of embodiment

  invention, also for a two-way installation

polar.

  Figure 1 shows in a detailed manner the dis-

  general position of an AC / DC converter station (

  ternative / continuous) in which the AC voltage from an electrical network 10 (three-phase in the case shown) is converted into a DC voltage and

transmitted along the cable 1.

  The three-phase line 10 comprises filters 12 and is connected to the primary windings of one or more transformers, one of which is represented

  in the figure by the reference 13. The windings se-

  Conductors are connected to bridge rectifier circuits, one of which is shown in FIG. 23, and which includes thyristor valves 32 whose conduction is controlled by electrodes 33 which receive the ignition pulses. from a control apparatus 18, which comprises

  the control unit 20 and the protection unit 19. The

  This control unit receives voltage and current signals from the AC voltage line.

  and the continuous voltage line and it produces the

  pulses 16 sent to the bridge control electrodes. The latter are connected in series with each other (as indicated by the dashed line) and with the cable 1. In the figure, there is furthermore shown a leveling inductor 2 and one of the damping circuits 7 arranged in parallel with the different thyristors. The other bridges connected in series are schematized by a broken line which ends at the electrode line A.

  At the other end of the cable, there is a

  conversion ratio DC / AC in which the voltage

  is converted into an alternating and trans-

  to distribution stations and / or other

  conversion requirements for transmission to other

bedded.

  In addition to producing and distributing

  the control units of the valves, the control unit

  also the regulation of an installation so that

  the latter operates at desired intensity and voltage values, by elaborating the feedback signals related to the parameters of the networks and modifying at the same time

time delay.

  The DC / AC conversion station is substantially

  analogous, with reverse operation, which

  Kills AC voltage thanks to bridge circuits

  connected in inverters. Other details of these

  are found in the literature and, in particular,

  link, reference is made to the text "Direct Current Transmis-

  Edward W. Kimbark, Wiley-Interscience, Vol.

New York 1971.

  In the following figures, and for the simplicity

  of the drawing, use schematizations

  current which can easily be reported to

  position illustrated in Figure 1, and the same reference numerals have been used to indicate the elements

identical or corresponding.

  Figure 2 schematically represents the invention

  in the case of a single transmission installation

  polar, that is to say, in which the return is

  killed by the sea or possibly by the land.

  An electric cable 1 is connected between two terminal stations 4 and 5, which are respectively

  AC / DC and DC / AC conversion stations, through

  leveling duct 2 at the ends. According to the invention

  In one of these stations, a voltage is injected

  alternative with amplitude and frequency values

  the limits which have been indicated above and which is superimposed on the DC voltage of

service that powers the cable.

  The production of this alternating voltage is provided by a source schematized by E1 in FIG. 2. As will be more clearly described hereinafter, it is not necessary for the source E1 to be a physically distinct active component contained in FIG.

terminal station.

  In the other terminal station, there is provided a compensating device schematized in the form of a source E2, which is able to present at its terminals

  a voltage equal to the said alternating voltage and in phase

  with this one. Arrows respectively associated-

  the two sources indicate the directions of the tensions which are opposite for the flow of the current. In this case also, the source E2 is not necessarily a separate component and / or of active type but it can be achieved by modifying the operation of existing circuits, or can be achieved using

added to the installation.

  With the features described above, it is observed that, in the mesh formed by the cable, the

  two terminal stations and the return (through

  re, the sea or another specially designed driver),

  there is hardly any movement of cou-

  rant due to the superimposed alternating voltage. In fact, the sum of the alternating electromotive forces in the

mesh is zero.

  In reality, there appears in the target a

  relatively moderate health of alternating current,

  the distributed capacity of the cable (towards the sea or towards the return conductor), small differences in

  phases between E1 and E2 and other minor phenomena.

  However, this current is of negligible value.

  compared to the values of the direct current which

  circulates in the cable and the invention proposes just-

  minimizing the alternating current flowing

in the cable.

  As an indication, an installation will be considered

  transmission voltage at 100 kV DC, in which circulates a DC current of about 1000 A and

  to which, according to the invention, an alternating voltage

  value of 5.5 kV by means of a source and a compensating device placed respectively in

  the two terminal stations. Oscillation or excur-

  current flow through the cable is included

  between + 20 A, that is, sufficiently

  to enable the control device to function

  correctly. In general and with the control devices currently used, the superimposed alternating current component is considered to be minimal

  when limited to a band of lower amplitude

  at 5% of the DC component flowing

  in the cable and preferably less than 4%.

  This gives two important advantages.

  The first is that not appreciable dissipation of power occurs, and this fact, besides that it does not alter the transmission characteristics of the cable, implies that, to maintain the

  cable subjected to AC voltage over its entire length

  it is not necessary to plan for great powers.

  The second is that this current does not

  not affect the regularity of the functioning of

  the conversion station. This command is

  the intensity values present in the cable and is therefore not appreciably altered by

  the superposition of the alternating current.

  According to a preferred embodiment of the invention, the alternating voltage is injected into

  one of the conversion stations and superimposed on the

  continuous service, by direct intervention on the control, as shown in FIG. 3,

  on which the same numerical references were used

  than in the preceding figures to indicate the

corresponding elements.

  The terminal station shown in the figure

  3 comprises rectifier bridges 23, 24 which are connected

  on one side, to the cable 1 through a reactance of

  2 and, on the other hand, to the alternative network

  10, through transformers 13, 14. This network comprises

  also takes filtering devices.

  may be the one providing the AC / DC conversion or the

  DC / AC conversion, depending on the type of operation.

  Control electrodes 33, 34 of the valves

  bridges are connected to the outputs of the communication unit

  20 which generates the ignition and ex-

  tinction of the valves. The unit is already programmed for? 2598

  adapt to foreseeable service situations, overload

  etc., thanks to a regulator 21 which receives

  signals from plant sensors (voltage, current, etc. sensors) represented by block 22, for the purpose of changing times and times

  pulses transmitted to the valves.

  According to this embodiment of the invention

  tion is applied to the unit of production and

  tribution of pulses a signal modulating a frequency

  quence between 1 and 50 Hz, preferably between 1

  and 20 Hz, and which is adapted to vary periodically-

  the ignition angle of the valves. The modulating signal is generated by a modulator circuit 25 and applied to the unit 20 through a summing node 55. The voltage present at the output of the bridges now includes a

  alternative component of a frequency equal to the frequency

  quence of the modulating signal and an amplitude

  between 0.5% and 10% of the DC voltage.

The station shown in FIG. 3 is preferably the DC / AC conversion station, because it

  It is easier to modulate the ignition angle to introduce

  the alternative component in the

  and to perform the compensation (by acting on the ignition angle, or by acting in other modes that will be described later) in the station

AC DC.

  According to another embodiment, the AC voltage source is a low frequency voltage generator itself. Figure 4 illustrates the

  case of a bipolar transmission installation which includes

  Cables 41 and 51 are connected between two end stations 44 and 45 which respectively comprise two alternating voltage generators E1 and E2. The voltage produced by said voltage generators or external sources is kept identical in amplitude and in phase by the central controller by means of the sensors placed along the network and at return signals (transmitted by means of specially designed conductors,

  or along the cables themselves).

  Also in the embodiment of FIG. 4, the voltage generators can be replaced by a modulation in the conversion device, such as

  the one shown in Figure 3, since in practice

  tick, the origin of the superimposed alternating

is irrelevant.

  Interchangeability between the means

  material source or compensating device

  originates from the fact that these means include

  active ingredients, that is, able to produce energy. An alternative embodiment for the device

  compensator which is not interchangeable is

  5 and comprises a resonant circuit

  of inductances and capaci-

  schematically by the components L and C, and which is added to one of the terminal stations. The resonance frequency of this circuit, which constitutes an LC network, is equal to that of the applied AC voltage, so that it has a very high impedance for this voltage, thus achieving the goal of producing an equal voltage to that applied to

the other terminal.

  In this case also, the source El can be

  of the type obtained by controlling the modulation

  bridge, or it can be a generator

  well said. The resonant circuit can also be placed

  so much at the AC / DC conversion station as at the station

  DC / AC conversion, the corresponding source is

  in the other terminal station.

  Because of its "passive" nature, the os-

  can not be used as a source of tension

  alternative but only as a compensatory

tor.

  The alternating voltage is preferably of the type

  sinusoidal and with a frequency between 2 and 10

  Hz, so as to avoid the typical frequency values

  electromechanical transients (below

  the lower limit) and sub-synchronous resonance (

  above the upper limit). A degree of phase difference between the source and the compensating device can also be assumed, provided that the value of the circulating alternating current can in any case be limited.

  at less than 5% of that of the conti-

naked flowing in the cable.

  It goes without saying that various modifications

  The embodiments described above can be made by those skilled in the art, in particular by substituting equivalent technical means,

  without departing from the scope of the invention.

Claims (10)

R E V E N D I C A T IO N S   1 - A method for continuously supplying an electric cable connected between two terminal stations with a DC voltage on which is superimposed a low frequency AC voltage which has a peak-to-peak value lower than the value of the DC voltage, characterized in that that creates a tension   alternative of this kind in one of the final stations   by compensating for this, at the other end station, by an identical alternating voltage, so as to make the current flowing in the cable which is due to   the alternating voltage component.   2 - Installation to feed continuously   an electrical cable connected between two   with a continuous voltage at which is super-   a low-frequency alternating voltage and a peak-to-peak value lower than the value of the DC voltage, according to the method of claim 1, characterized in that it comprises at least one source   alternating voltage at one (4; 44) of these stations.   while she understands, at the other station (5; 45) at least one compensating device at the terminals of which   find a voltage equal to said AC voltage.   3 - Installation according to claim 2,   characterized in that one (4; 44) of said stations   is an AC / DC converter station with cir-   cooked in bridges mounted in rectifiers while the other   station (5; 45) is a DC / AC converter   using UPS-mounted bridging circuits, a   control unit (20) being provided in each of the sta-   to generate corresponding impulses of lighting and extinguishing bridges.   4 - Installation according to claim 3,   characterized in that said source is realized by means of a modulator connected to the control unit (20) of one of the conversion stations and which modulates in the L4 time the distribution of the control pulses transmitted by said unit.   Installation according to claim 3, characterized in that said source is provided in the DC / AC conversion station.   6 - Installation according to claim 3,   characterized in that said source is provided in the AC / DC conversion station.   7 - Installation according to claim 3,   characterized in that the compensating device is real-   by means of a modulator connected to the communication unit   one of the conversion stations, which modulates the distribution of the control pulses over time issued by said unit.   8 - Installation according to claim 7,   characterized in that said compensating device is   provided in the DC / AC conversion station.   9 - Installation according to claim 7,   characterized in that said compensating device is   provided in the AC / DC conversion station.   - Installation according to one of the claims   2 and 3, characterized in that said source of   AC voltage is a voltage generator.   11 - Installation according to one of the claims   2 and 3, characterized in that said device for   compensation is a voltage generator.   12 - Installation according to one of the claims   2 and 3, characterized in that said device for   compensation is an LC network whose frequency of   nance is equal to the frequency of the said alternating voltage tive.