LOCATION OF A CABLE IN A DC CONNECTION IN A BIPOLAR 12-PULSE SYSTEM FOR TRANSMISSION OF ELECTRICAL ENERGY BY MEANS OF HIGH-VOLTAGE DIRECT CURRENT

Abstract: A dc connection (CAB1, CAB2) in a bipolar 12-pulse system for transmission of electrical energy by means of high-voltage direct current interconnects two converters, each one having two 6-pulse valve bridges (BR11, BR12, BR21, BR22) connected in series at a connection point (J1, J2). In a method for location of the cable, said cable is located by sensing of a magnetic field associated with a current flowing through the cable. The dc connection is temporarily supplied with a ground-mode current, intended for the location, and a magnetic field associated with said ground-mode current is sensed.

Description: Location of a cable in a dc connection in a bipolar 12- pulse system for transmission of electrical energy by means of high-voltage direct current TECHNICAL FIELD The present invention relates to a method for location of a cable in a dc connection in a bipolar 12-pulse system for transmission of electrical energy by means of high- voltage direct current, whereby the cable is located by sensing a magnetic field associated with a current flowing through the cable, and a device for carrying out the method. BACKGROUND ART A system for transmission of electrical energy by means of high-voltage direct current (HVDC) comprises two line- com utated converters, each one via a separate converter transformer connected to a separate three-phase ac network. Each one of the converters comprises two 6-pulse valve bridges, mutually series-connected at a connection point, each valve bridge being connected to a respective ac network via a respective secondary winding on the converter transformer. This is adapted such that the voltages from the respective secondary winding exhibits a mutual phase shift of 30 electrical degrees such that the two valve bridges of the converter in a known manner form a 12-pulse unit. Via their dc voltage terminals, the converters are connected to each other by means of a dc connection with two dc conductors such that the transmission system will form a bipolar 12-pulse system. For a general description of HVDC systems, reference is made to Ake Ekstrδm: High Power Electronics HVDC and SVC. Stockholm 1990, in particular pages 3-10 to 3-11 with Figures 3-11, and pages 7-1 to 7-4 and pages 7-8 to 7-13 with Figure 7-2. The dc conductors may, at least in part, be made in the form of cables . When the energy is to be transmitted over a region with water, most often a sea region, the cables are usually buried beneath the sea bed to protect them from, for example, trawls from fishing boats and from anchors lowered from ships. For location of the position of the cables during the burial stage, an electric current is fed through the cable where the position of the cable is determined in a known manner by means of magnetic sensors which sense the magnetic field generated by the current. Since, particularly in shallower waters, the sea bed may be subjected to movements, for example because of storms, it is also necessary, after a certain time of operation of the energy transmission, to check, at recurrent intervals, the position and depth of trench of the cables below the sea bed. In those cases where the cables are laid close to each other on the sea bed or are arranged in the form of a common double cable, however, a circulating current in the dc connection, fed in via the converters, does not give rise to a reliably detectable external magnetic field. To carry out a check of the position of the cable, that part of the dc connection which consists of cable may, for example, be disconnected from the other parts of the dc connection and a current be supplied to either of the cables by means of an external current source. However, this implies relatively extensive work and during the time needed for this work and for restoration of the dc connection, the transmission system is not available for transmission of electrical energy. SUMMARY OF THE INVENTION The object of the invention is to achieve an improved method of the kind described in the introduction, which permits a location of the position of the cable while the transmission system is in normal operation, whereby the above-mentioned disadvantages concerning the availability of the transmission system for transmission of electrical energy can be avoided, and a device for carrying out the method. According to the invention, this object is achieved by temporarily supplying to the dc connection a ground-mode current intended for the location, and by sensing a magnetic field associated with this ground-mode current. According to an advantageous improvement of the invention, one of the connection points is connected to ground via an impedance circuit with an impedance, capable of being influenced, of a substantially inductive nature, whereby the ground-mode current is supplied to the dc connection by the inductance of the impedance circuit being temporarily reduced. According to another advantageous improvement of the invention, the device comprises a current source connected between one of the connection points and ground. According to still another advantageous improvement of the invention, the ground-mode current is supplied to the dc connection by bringing at least one of the converters to generate a ground-mode current. Further improvements of the invention will become clear from the following description and claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in greater detail by description of embodiments with reference to the accompanying drawings, which are all schematic and in the form of single-line diagrams and block diagrams, respectively, and wherein Figure 1 shows a known design of a bipolar 12-pulse system for transmission of electrical energy by means of high-voltage direct current, Figure 2 shows an embodiment of the invention comprising means for influencing the inductance of an impedance circuit connected to ground, in a system according to Figure 1, Figure 3 shows an embodiment of the invention comprising a current source in a system according to Figure 1, Figure 4 shows an embodiment of the invention comprising means for influencing a control angle for a converter in a system according to Figure 1, and Figure 5 shows an embodiment of the invention comprising means for influencing the switching position of a tap changer in a system according to Figure 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS The following description relates to the method as well as to the device . The block diagrams may thus be regarded both as signal- flow diagrams and block diagrams of a device. The functions performed by the blocks shown in the block diagrams may, in applicable parts, be performed by means of analog and/or digital technique, or as computer programs in a microprocessor. Thus, where appropriate, the expression "signal" may also be understood as a value formed in a computer program and occurring as such only. Further, it is to be understood that, although the blocks shown in the figures are mentioned as units, members, etc., these are to be understood as means for achieving the desired function, especially in those cases where their functions are implemented as software in, for example, microprocessors. Figure 1 shows a known embodiment of a bipolar 12-pulse system for transmission of electrical energy by means of high-voltage direct current. A first converter with two six-pulse valve bridges BRll and BR12 , respectively, is connected to a three-phase electric ac network Nl via a converter transformer Tl . This comprises two secondary windings, which are adapted such that the voltages from the respective secondary winding exhibit a mutual phase shift of 30 electrical degrees, for example, as indicated in the figure, by the secondary winding for feeding the bridge BRll being star-connected whereas the secondary winding for feeding the bridge BR12 is delta-connected. The valve bridges are series-connected, from a dc voltage point of view, in that they are connected to a common connection point Jl such that the converter in a known manner forms a 12-pulse unit. A second converter with two six-pulse valve bridges BR21 och BR22, respectively, is connected to a three-phase electric ac network N2 via a converter station T2 of the same kind as the converter transformer Tl . From the dc voltage point of view, the valve bridges BR21 and BR22 are series-connected in that they are connected to a common connection point J2. A dc connection between the converters described comprises two cables CABl and CAB2 , which are connected to the first converter via smoothing inductors Lll and L12 and to the second converter via smoothing inductors L21 and L22. The cables CABl and CAB2 are each separately insulated and arranged in a double cable CAB with a common external screening and reinforcement. In addition thereto, the dc connection may, depending on local conditions, comprise parts which are designed as cables and/or overhead lines. In this embodiment, the double cable is assumed to be laid under a sea bed, usually of the order of magnitude of 0.5 - 1 m under the sea bed . The valve bridges BRll and BR12 are controlled in a known manner in dependence on common control equipment (not shown in the figure) and generate dc voltages Udll and Udl2 , respectively, in dependence on a common control angle order α generated by the control equipment . The bridge BRll supplies a current Idll to the cable CABl and the bridge BR12 supplies a current Idl2 to the cable CAB2. The connection point Jl is connected via an inductor LE to a ground terminal, symbolically shown in the figure, for the purpose of obtaining a good connection to ground of the dc connection, from the dc voltage point of view. The ac network Nl is assumed to have a nominal mains frequency F, which is usually equal to 50 or 60 Hz. When the valve bridges BRll and BR12 are connected to the ac network, a voltage, which contains a component of a frequency equal to six times the nominal mains frequency, occurs at the connection point. This voltage component generates a current Iel flowing through the inductor EL. The dc connection interconnects two converters via the cables CABl and CAB2 , which converters, as mentioned above, form a 12-pulse unit each. The transmission system is thus advantageous in those cases where it is desirable to transmit the electrical energy via the above-mentioned cables only whereas a direct current through ground is not accepted. The inductor LE must thus limit the amplitude of the current Iel such that the converters, from a practical point of view, behave as 12-pulse units. Unbalance in the direct current is suitably avoided by connecting only one of the converters, from the point of view of dc voltage, to ground via an inductor. To describe the invention, it is assumed in the following that the inductor LE has an inductance which limits the amplitude of the current Iel to a level which is negligible for this purpose (but hence also to a level which does not permit location of the double cable by sensing the magnetic field associated with the deviation in amplitude, depending on the current Iel, between the current through the cable CABl and the current through the cable CAB2 ) . Location of cables under the sea bed under the circumstances described in the introductory part of the description is performed, in a manner known per se, by conducting a current through the cable and moving magnetic sensor equipment MS to the area where the cable is assumed to be located. The sensor equipment is connected to locating equipment LOC, which usually, at least in part, is placed onboard a ship. The sensor equipment supplies to the locating equipment measured values of the magnetic field associated with the current, and by evaluation of these values while moving the sensor equipment, the position of the cable, including its depth of trench, can be located. In the case described here, where the cables CABl and CAB2 are arranged in a double cable CAB, the magnetic fields from the cable CABl and from the cable CAB2 , which are associated with a current circulating through the dc connection, counteract each other. The resultant magnetic field occurring outside the double cable thus becomes very weak also in the vicinity of the cable. According to the invention, the dc connection is tempora- rily supplied with a ground-mode current intended for the location. The occurrence of a ground-mode current implies that the sum of the currents in the cables CABl and CAB2 , with directions defined as in Figure 1, is different from zero, that is, the current through the cable CAB2 does not balance the current through the cable CABl . This results in a magnetic field around the double cable which may be utilized for location thereof by means of the sensor equipment MS. The amplitude of a ground-mode current which leads to a deviation between the currents through the cables CABl and CAB2 , which deviation is appropriate for practical purposes and detectable with the sensor equipment, may be calculated in advance, in a manner well-known to the person skilled in the art, with knowledge of data for the cables, of the soil/sand layer in which the double cable is buried and other conditions of burial, such as the depth of trench under the sea bed, for the double cable . Since the ground-mode current, supplied according to the invention, is an alternating current, the reference directions for current and voltage indicated in Figure 1 are preferred in the following, which thus imply that, during normal operation, the current Idl2 and the voltage Udl2 assume negative values. By definition, the following then applies to the ground-mode current Ig and the pole- mode current Ip : Ig = (Idll + Idl2)/2 and Ip = (Idll - Idl2)/2. Inversely, the current Idll in cable CABl and the current Idl2 in cable CAB2 , respectively, are then obtained as Idll = Ig + Ip and Idl2 = Ig - Ip . In similar manner, for a ground-mode voltage Ug and a pole-mode voltage Up the following definitions apply: Ug - (Udll + Udl2)/2 + UJ1 och Up = (Udll - Udl2)/2, where the voltage UJ1 is the voltage between the connection point Jl and ground. Inversely, the voltage Udll across the bridge BRll and the voltage Udl2 across the bridge BR12 , respectively, are obtained as Udll = Ug + Up - UJ1 and Udl2 = Ug - Up - UJ1. From the above definitions follows that, for the current Iel from the connection point Jl to ground, the following applies: Iel = -2lg and Idll + Idl2 = 2Ig = -Iel. Figure 2 shows an embodiment of the invention in an energy transmission system of the kind described with reference to Figure 1. Those parts of the transmission system which have been described with reference to Figure 1 are shown in Figure 2 with the same reference numerals . The design of the double cable is usually such that the cables CABl and CAB2 exhibit capacitances towards the external screen surrounding the double cable, which screen is indicated in the figure as a dash-dotted cylinder. The screen is in electrically conducting contact with the surrounding soil/sand layer, in the figure symbolically marked by ground terminals at both ends of the screen. The capacitance of the cable CABl towards the screen, although of course distributed along the length of the screen, is illustrated in the figure by discrete capacitor symbols Csll and Cs21 and the capacitance of the cable CAB2 is similarly illustrated by capacitor symbols Csl2 and Cs22. When a location of the cable is to be carried out, a location order SORD is given for supplying a ground-mode current to the dc connection. An inductor LES may, via a switching member SWD, in dependence on the above-mentioned location order, be connected between the connection point Jl and ground. The current which will thus flow through the inductor, in the figure designated Is, is an alternating current with a dominating component of a frequency equal to six times the mains frequency. As will be clear from the above, the dc connection is supplied with a ground-mode current which is equal to -Is/2 (as mentioned above, the current Iel through the inductor LE is negligible in this connection) . This results in the formation of a magnetic field around the double cable, which field, at least in the vicinity of that end of the double cable which is connected to the smoothing inductors Lll, L12 , is substantially dependent on the sum of the currents Idll and Idl2 , a sum current which according to the above is equal to Idll + Idl2 = 2Ig = - Is. However, the ground- mode current is successively dissipated through the capacitances to ground of the cables CABl and CAB2 , whereby the sum current mentioned, and hence the sensable magnetic field, decrease with the distance from the connection point Jl . The amplitude of the current Is is determined, besides by the voltage UJl at the connection point Jl , by the inductance values for the inductor LES and the smoothing inductors Lll and L12 and by the capacitance of the cables, whereas the smoothing inductors L21 and L22 from a practical point of view prevent any part of the current Is from flowing through the valve bridges BR21, BR22. In a first approximation, the current Is is determined by the inductance values of the inductor LES and the smoothing inductors Lll and L12 , in that the impedance associated with the capacitance of the double cable, at least in the case of long cables, may be omitted. A dimensioning of the inductor LES which leads to a deviation between the currents through the cables CABl and CAB2 , which deviation is suitable for practical purposes and detectable by means of the sensor equipment, may thus be pre-calculated with knowledge of the other quantities mentioned. Typically, the circuits for the ground-mode current may be dimensioned such that the amplitude of the current Is, when location of the cable is to be carried out, amounts to the order of magnitude of 1 to 5 % of the rated current of the installation, with the higher value for long cables . A typical value for the current Iel through the inductor LE is then of the order of magnitude of 0.1 % . Figure 3 shows another embodiment of the invention in an energy transmission system of the kind described with reference to Figure 1. Those parts of the transmission system which have been described with reference to Figure 1 are illustrated in Figure 3 with corresponding reference numerals . Between the connection point Jl and ground, a current source in the form of a filter unit FU1 , in series connection with a voltage generator G, is connected. The voltage generator, which is of a design known per se, is brought, in dependence on the location order, to generate a generator voltage with a chosen signal frequency. The signal frequency is suitably chosen such that it differs from six times the nominal mains frequency but advantageously constitutes an even multiple of the nominal mains frequency, for example twice or four times that frequency. The filter unit comprises a series connection of an inductor LI and a capacitor Cl, and the inductance value for the inductor and the capacitance value for the capacitor are then chosen such that the filter unit advantageously exhibits series resonance at the signal frequency, that is, has a pass band with a low impedance for this frequency. The filter unit is further dimensioned such that it exhibits a high impedance for a frequency equal to six times the nominal mains frequency. Between the connection point J2 and ground, a filter unit FU2 is connected, which filter unit comprises a series connection of an inductor L2 and a capacitor C2. The inductance value for the inductor LF2 and the capacitance value for the capacitor C2 are chosen such that the filter unit FU2 will have frequency properties similar to those of FUl . As is clear from the above-mentioned expressions and assumptions, and with definitions as illustrated in Figure 1, the current Is generated by the current source is also equal to the sum of the current through the cable CABl and the current through the cable CAB2. The amplitude of the generator voltage is chosen such that the current Is is given an amplitude which, for practical purposes, may be detected by the detector equipment in a manner previously stated in the description. The voltage generator may, for example, comprise a tone generator and a power-amplifica- tion step with power transistors. Typically, the power requirement from the voltage generator may amount to 2.5 kV and 5 A at 100 Hz. In this embodiment, the filter unit FU2 is included in the current path of the supplied ground-mode current. Especially in cables which are short and/or have a low capacitance and with a chosen low signal frequency, for example equal to twice the nominal mains frequency, the cable may be located along all of its length. Alterna- tively, of course, a voltage generator may be connected in series with the filter unit FU2. When applying a ground- mode current from this voltage generator, the voltage generator is bypassed at the filter unit FUl. Figure 4 shows another embodiment of the invention in an energy transmission system of the kind described with reference to Figure 1. Those parts of the transmission system which have been described with reference to Figure 1 are illustrated in Figure 4 with corresponding reference numerals. In this embodiment, the ground-mode current intended for the location is generated by supplying a ground-mode current to the dc connection via the valve bridges BRll and BR12. The figure schematically shows parts of a common piece of control equipment for the valve bridges. Control equipment comprises, in a known manner, a current controller CC, the output signal of which constitutes a control angle order α, common to both valve bridges, for phase-angle control of the bridges. A summator Dl is supplied with a reference value Idr, formed in some manner known per se, for the current through the dc connection and with an instantaneous value Ida for this current, and forms therefrom a difference which is supplied to the current controller (a ring at the symbol for the summator indicates that the input is a negating input) . Control pulse devices SP11 and SP12 generate, in a known manner, in dependence on control-angle orders supplied thereto, firing signals Fll and F12 , respectively, which are supplied to the valve bridges BRll and BR12 , respectively, for firing of semiconductor elements comprised therein, normally thyristors. A signal generator SG generates, in dependence on the location order SORD, a signal voltage Us of a chosen signal frequency. This signal voltage and the control- angle order α are supplied to a summator Dll as well as to a summator D12 , the signal voltage being supplied to a negating input of the summator D12. The output signal all from the summator Dll, which thus constitutes a sum of the control-angle order α and of the signal voltage Us, is supplied to the control-pulse device SP11, which, in dependence on this sum, generates the firing signals Fll. The output signal αl2 from the summator D12 , which thus constitutes a difference of the control-angle order α and of the signal voltage Us, is supplied to the control-pulse device SP12, which, in dependence on this difference, generates the firing signals F12. It is realized from this that the converters are brought to generate a ground-mode voltage Ug of the chosen signal frequency, which in turn leads to ground-mode current being supplied to the dc transmission. The currents ID11 and Idl2 are sensed by means of the current-measuring devices IM11 and IM12 , respectively. To make possible location of the double cable during normal operation of the dc transmission, the measured values of the currents Idll and Idl2 (which measured values, in order not to burden the description with distinctions self-evident to the person skilled in the art, are similarly designated Idll and Idl2, respectively, in the figure) to a mean- value forming unit MVI (with the reference directions chosen, the measured value of the current Idl2 is supplied to a negating input of the mean-value forming unit) . The mean-value forming unit forms, in dependence on these measured value, an output signal Ida = (Idll - Idl2)/2, which, as is clear from the above, corresponds to the pole-mode current Ip . The output signal Ida is supplied to the summator Dl as the instantaneous value for the current controller. The frequency of the signal voltage and the associated dimensioning of the filter units FUl and FU2 may, for example, be chosen in a manner described above with reference to Figure 3. Figure 5 shows a further embodiment of the invention in which the ground-mode current intended for the location is generated by supplying a ground-mode voltage to the dc connection via the valve bridges BRll and BR12. The energy transmission system is of the kind described with reference to Figure 1 and those parts of the transmission system which have been described with reference to Figure 1 are illustrated in Figure 5 with the same reference numerals . The secondary windings of the converter transformer Tl are each, in a manner known per se, provided with a tap changer. The tap changer of the star-connected secondary winding is schematically marked in the figure by an arrow TCY and the tap changer of the delta-connected secondary winding is marked by an arrow TCD. The positions of the tap changers may be adjusted individually via operating mechanisms OPY and OPD, respectively, which are arranged such that the switch position for each one of the phases of the three-phase ac network may be adjusted indivi- dually. This is easily done, for example, for those cases where the converter transformer in a known manner is designed as three separate single-phase units. In this embodiment of the invention, an initiating unit CPI for changing the tap-changer position, in dependence on the location order SORD, generates a position-change order CHO, which is supplied to the operating mechanism OPY. The position-change order is also supplied to a sign- reversing unit SCH, which forms, as output signal, a position-change order CHO' signifying a position change in a direction opposite to that of the position-change order CHO. The position-change order CHO' is supplied to the operating mechanism OPD. The two position-change orders CHO and CHO' are then adapted, in some known manner, to influence only one and the same of the three phases U, V, W of the respective secondary windings, for example the U- phase. The location order SORD thus initiates, for example, a reduction of the position of the tap changer TCY for the U-phase by one step and hence at the same time an increase of the position of the tap changer TCD for the U-phase by one step. The result of this change of the switch positions is that the dc transmission is supplied with a ground-mode voltage of twice the nominal mains frequency, that is, at a 50 Hz ac network, a ground-mode voltage and a ground-mode current, associated therewith, of the frequency 100 Hz. The inductance values for the inductors LI and L2 , and the capacitance values of the capacitors Cl and C2 are then advantageously chosen such that the filter units FUl and FU2 exhibit series resonance at a frequency equal to twice the nominal mains frequency, but exhibit a high impedance to a frequency equal to six times the nominal mains frequency. In the embodiments of the invention which have been described above with reference to Figures 3-4, a signal frequency is preferably used which is lower than that which is given in an embodiment according to Figure 2. In this way, also the circuits for the ground-mode current may be typically dimensioned for a lower unbalance current, typically such that the amplitude of the current Is in these embodiments amounts to the order of magnitude of 0.05 to 1 % of the rated current of the installation. The invention is not limited to the embodiments shown but a plurality of modi ications are feasible for the inven- tion defined by the claims. Although described in detail only with reference to the embodiment according to Figure 4, it is to be understood that the instantaneous value Ida of the current controller which is normally included in this kind of energy transmission system is formed in similar manner in all the embodiments of the invention. The means shown in Figures 2-5 for supplying the dc connection with a ground-mode current and a ground-mode voltage, respectively, are arranged at the converter formed by the valve bridges BRll, BR12. These means may, of course, also be arranged at the converter formed by the valve bridges BR21, BR22, or at each one of the converters. In the latter case, the ground-mode current and the ground-mode voltage, respectively, may be supplied to the dc connection alternatively from the first or the second of the converters to locate, in particular, that part of the cable which is positioned closest to the respective converter. To avoid oscillation phenomena in the ground- mode current, however, ground-mode current should not be supplied to the dc transmission simultaneously from both converters . To facilitate the location of the cable at that end of the double cable which is connected to the smoothing inductors L21, L22, in the embodiment of the invention described with reference to Figure 2, an inductor which is connect- able via a switching member may also be arranged for connection between the connection point J2 and ground, in dependence on the location order. To avoid the formation of a dc circuit through ground, it must thus be ensured that only one of the connection points Jl and J2 is simultaneously dc-connected to ground. This may, for example, be achieved by operating the switching members mentioned or by means of connection of a capacitor in series with the respective inductor. As an alternative to connecting an inductor LES between the connection point Jl and ground, the inductor LE may be designed with switchable inductance, for example by being provided with end terminals 1 and 2 and a third terminal 3 arranged between the terminals 1 and 2. The switching member SWD may, in this embodiment, be arranged, in dependence on the location order SORD, to short-circuit that part of the inductor LE which is arranged between the terminals 2 and 3, resulting in its inductance being reduced. In the event that such a switchable inductor is arranged at both connection points, preferably only one of them should be simultaneously connected to ground. Although the signal frequency for the ground-mode current and the ground-mode voltage, respectively, is exemplified as constituting an even multiple of the mains frequency, other signal frequencies may, of course, also be used. A limiting factor for the signal frequency is the capaci- tance to ground exhibited by the cable. Also signal frequencies considerably lower than the mains frequency, for example of the order of magnitude of 5 to 10 Hz, may in certain applications, particularly in the case of long cables, be advantageous. In the above embodiments, the cables CABl and CAB2 have been described as being arranged in a double cable CAB, but the invention is, of course, also applicable also to those cases where these cables consist of two separate cables laid close to each other. To sum up, the invention always offers a possibility of locating a cable of the kind mentioned at least in the vicinity of that converter where the ground-mode current and the ground-mode voltage, respectively, are supplied to the dc connection. In the embodiment described with reference to Figure 2, that part of the cable which may be reliably located may be extended by increasing the amplitude of the supplied current Is. To locate the cable along the whole of its length, however, a ground-mode current should be supplied to each of the converters in the manner described above. With the embodiments described with reference to Figures 3-5, with a filter unit FUl connected to that converter where ground-mode current and ground-mode voltage, res- pectively, are supplied to the dc connection, a cable may also be located at least in the vicinity of this converter. An exception to this may be very short cables whose capacitance is so low that a ground-mode current of sufficient magnitude is not formed at the signal frequency in question. For this case and in order to locate a cable along the whole of its length, in any of the embodiments according to Figures 3-5, filter units FUl, FU2 are preferably arranged at both converters, as illustrated in Figures 3-5. In the case of very long cables, however, it may be preferable, also in these embodiments, to supply the dc connection with ground-mode current or ground-mode voltage at both converters, however, preferably only at one of them simultaneously.

Claims: CLAIMS 1. A method for location of a cable (CAB, CABl, CAB2 ) in a dc connection (CABl, CAB2 ) in a bipolar 12-pulse system for transmission of electrical energy by means of high- voltage direct current, the dc connection interconnecting two converters, each one having two 6-pulse valve bridges (BRll, BR12, BR21, BR22) connected in series at a connection point (Jl, J2 ) , wherein the cable is located by sensing of a magnetic field associated with a current flowing through the cable, characterized in that a ground- mode current, intended for the location, is temporarily supplied to the dc connection, and that a magnetic field associated with said ground-mode current is sensed. 2. A method according to claim 1, wherein at least one of the connection points is connected to ground via an impedance circuit (LE, LES) with an impedance of a substantially inductive character, and with means (SWD) for influencing the impedance of the impedance circuit, characterized in that said ground-mode current is supplied to the dc connection by the inductance of the impedance circuit being reduced. 3. A method according to claim 1, characterized in that a current source (G, FUl) is connected between one of the connection points and ground and that said ground-mode current is supplied to the dc connection by means of said current source. 4. A method according to claim 1, characterized in that the ground-mode current is supplied to the dc connection in that one of the converters is brought to generate a ground-mode voltage. 5. A method according to claim 4, characterized in that said ground-mode voltage is generated by influencing a control-angle order (α) for at least one of the valve bridges . 6. A method according to claim 4, wherein the valve bridges are each connected to an ac network (Nl, N2 ) via respective secondary windings on a converter transformer (Tl, T2 ) , at least one of the secondary windings with a tap changer (TCY, TCD) , characterized in that said ground- mode voltage is generated by influencing the switch position of the tap changer. 7. A method according to any of claims 3-6, the converters being connected to an ac network with a known nominal mains frequency (F) , and wherein the supplied ground-mode current has a given signal frequency which deviates from a frequency which is six times the mains frequency, characterized in that at least one of the connection points is connected to ground via a filter unit (FUl, FU2 ) with pass band for the signal frequency. 8. A method according to claim 7 , characterized in that each one of the connection points is connected to ground via a separate filter unit (FUl, FU2 ) with a pass band for the signal frequency. 9. A method according to any of claims 7-8, characterized in that the given signal frequency is chosen equal to twice the nominal mains frequency. 10. A device for location of a cable (CAB, CABl, CAB2 ) in a dc connection (CABl, CAB2 ) in a bipolar 12-pulse system for transmission of electrical energy by means of high- voltage direct current, the dc connection interconnecting two converters, each one having two 6-pulse valve bridges (BRll, BR12, BR21, BR22) connected in series at a connection point (Jl, J2 ) , wherein the cable is located by sensing of a magnetic field associated with a current flowing through the cable, characterized in that the device comprises means (G, FUl, SG, Dll, D12, CPI, SCH, LE, LES, SWD) for temporarily supplying a ground-mode current, intended for the location, to the dc connection, whereby a magnetic field associated with said ground-mode current is sensed. 11. A device according to claim 10, wherein at least one of the connection points is connected to ground via an impedance circuit (LE, LES) with an impedance of a substantially inductive character, characterized in that the device comprises means (SWD) for influencing the impedance of the impedance circuit and that said ground-mode current is supplied to the dc connection by the inductance of the impedance circuit being reduced. 12. A device according to claim 10, characterized in that it comprises a current source (G, FUl) for connection between one of the connection points and ground. 13. A device according to claim 10, characterized in that the device comprises means (SG, Dll, D12 , CPI, SCH) for generating a ground-mode voltage via one of the converters . 14. A device according to claim 13 , wherein the valve bridges generate dc voltages (Udll, Udl2) in dependence on a control-angle order (α) generated by control equipment (CC) , characterized in that the control equipment comprises means (SG, Dll, D12) for generating said ground-mode voltage by influencing the control-angle order for at least one of the valve bridges. 15. A device according to claim 13, wherein the valve bridges are each connected to an ac network (Nl, N2 ) via respective secondary windings on a converter transformer (Tl, T2 ) , at least one of the secondary windings with a tap changer (TCY, TCD) , characterized in that the device comprises means (CPI, SCH) for generating said ground-mode voltage by influencing the switch position of the tap changer . 16. A device according to any of claims 12-15, wherein the converters are connected to an ac network with a known nominal mains frequency (F) , and wherein the supplied ground-mode current has a given signal frequency which deviates from a frequency which is six times the mains frequency, characterized in that the device for at least one of the connection points comprises a filter unit (FUl, FU2 ) with a pass band for the signal frequency and that said at least one connection point is connected to ground via said filter unit. 17. A device according to claim 16, characterized in that, for each of the connection points, it comprises a filter unit (FUl, FU2 ) with a pass band for the signal frequency and that the respective connection points are connected to ground via a respective one of said filter units. 18. A device according to any of claims 16-17, characterized in that said current source and said means (SG, Dll, D12, CPI, SCH) for generating a ground-mode voltage via one of the converters, respectively, generate a signal frequency which is equal to twice the nominal mains frequency. AMENDED CLAIMS [received by the International Bureau on 13 June 2000 (13.06.00); original claims 1-18 replaced by new claims 1-18 (5 pages)] 1. A method for location of a cable (CAB, CABl, CAB2 ) in a dc connection (CABl, CAB2 ) in a bipolar 12-pulse system for transmission of electrical energy by means of high- voltage direct current, the dc connection interconnecting a first and a second converter, each one having two 6- pulse valve bridges (BRll, BR12 and BR21, BR22, respectively) connected in series at a respective connection point (Jl, J2 ) , wherein at least the connection point at the first converter is coupled to ground via a reactive impedance element (LE, FUl, FU2 ) , and the cable is located by sensing of a magnetic field associated with a current flowing through the cable, characterized in that a ground- mode current, intended for the location, is supplied to the dc connection flowing through the connection point at the first converter, ground, and the valve bridges of the first converter, and that a magnetic field associated with said ground-mode current is sensed. 2. A method according to claim 1, wherein at least the connection point at the first converter is connected to ground via an impedance circuit (LE, LES) with an impedance of a substantially inductive character, and with means (SWD) for influencing the impedance of the impedance circuit, characterized in that said ground-mode current is supplied to the dc connection by at least temporarily reducing the inductance of the impedance circuit. 3. A method according to claim 1, characterized in that a current source (G, FUl) is connected between the connection point at the first converter and ground and that said ground-mode current is supplied to the dc connection by means of said current source. 4. A method according to claim 1, characterized in that the ground-mode current is supplied to the dc connection by bringing at least the first converter to generate a ground-mode voltage. 5. A method according to claim 4, characterized in that said ground-mode voltage is generated by influencing a control-angle order (oc) for at least one of the valve bridges . 6. A method according to claim 4, wherein the valve bridges are each connected to an ac network (Nl, N2 ) via respective secondary windings on a converter transformer (Tl, T2), at least one of the secondary windings with a tap changer (TCY, TCD) , characterized in that said ground- mode voltage is generated by influencing the switch position of the tap changer. 7. A method according to any of claims 3-6, the converters being connected to an ac network with a known nominal mains frequency (F) , and wherein the supplied ground-mode current has a given signal frequency which deviates from a frequency which is six times the mains frequency, characterized in that at least the connection point at the first converter is connected to ground via a filter unit (FUl, FU2 ) with a pass band for the signal frequency. 8. A method according to claim 7, characterized in that each one of the connection points is connected to ground via a separate filter unit (FUl, FU2 ) with a pass band for the signal frequency. 9. A method according to any of claims 7-8, characterized in that the given signal frequency is chosen equal to twice the nominal mains frequency. 10. A device for location of a cable (CAB, CABl, CAB2 ) in a dc connection (CABl, CAB2 ) in a bipolar 12-pulse system for transmission of electrical energy by means of high- voltage direct current, the dc connection interconnecting a first and a second converter, each one having two 6- pulse valve bridges (BRll, BR12 and BR21, BR22, respectively) connected in series at a respective connection point (Jl, J2 ) , wherein at least the connection point at the first converter is coupled to ground via a reactive impedance element (LE, FUl, FU2 ) , and the cable is located by sensing of a magnetic field associated with a current flowing through the cable, characterized in that the device comprises means (G, FUl, SG, Dll, D12 , CPI, SCH, LE, LES, SWD) for supplying a ground-mode current, intended for the location, to the dc connection, said means generating said ground-mode current to flow through the connection point at the first converter, ground, and the valve bridges of the first converter, whereby a magnetic field associated with said ground-mode current is sensed. 11. A device according to claim 10, wherein at least the connection point at the first converter is connected to ground via an impedance circuit (LE, LES) with an impedance of a substantially inductive character, characterized in that the device comprises means (SWD) for influencing the impedance of the impedance circuit and that said ground-mode current is supplied to the dc connection by at least temporarily reducing the inductance of the impedance circuit. 12. A device according to claim 10, characterized in that it comprises a current source (G, FUl) for connection between the connection point at the first converter and ground . 13. A device according to claim 10, characterized in that the device comprises means (SG, Dll, D12, CPI, SCH) for generating a ground-mode voltage via at least the first converter. 14. A device according to claim 13, wherein the valve bridges generate dc voltages (Udll, Udl2) in dependence on a control-angle order (α) generated by control equipment (CC) , characterized in that the control equipment compri- ses means (SG, Dll, D12 ) for generating said ground-mode voltage by influencing the control-angle order for at least one of the valve bridges. 15. A device according to claim 13, wherein the valve bridges are each connected to an ac network (Nl, N2 ) via respective secondary windings on a converter transformer (Tl, T2 ) , at least one of the secondary windings with a tap changer (TCY, TCD) , characterized in that the device comprises means (CPI, SCH) for generating said ground-mode voltage by influencing the switch position of the tap changer . 16. A device according to any of claims 12-15, wherein the converters are connected to an ac network with a known nominal mains frequency (F) , and wherein the supplied ground-mode current has a given signal frequency which deviates from a frequency which is six times the mains frequency, characterized in that the device for at least the connection point at the first converter comprises a filter unit (FUl, FU2 ) with a pass band for the signal frequency and that said connection point is connected to ground via said filter unit. 17. A device according to claim 16, characterized in that, for each of the connection points, it comprises a filter unit (FUl, FU2 ) with a pass band for the signal frequency and that the respective connection points are connected to ground via a respective one of said filter units. 18. A device according to any of claims 16-17, characterized in that said current source and said means (SG, Dll, D12, CPI, SCH) for generating a ground-mode voltage via at least the first converter, respectively, generate a ground-mode current with a signal frequency which is equal to twice the nominal mains frequency.

Inventors: JUHLIN LARS-ERIK, JUHLIN, LARS-ERIK
Applicants: ABB AB, JUHLIN LARS ERIK, JUHLIN, LARS-ERIK
Application year: 1999
Main classification: G01V3
IPC codes: G01V3