DEVICE FOR COMPENSATION OF AN ALTERNATING VOLTAGE WHICH OCCURS BETWEEN A MEDIUM AND A METALLIC PIPELINE DISPOSED IN THE MEDIUM

Abstract: In a metallic pipeline (1) disposed in the ground, voltages may be induced from adjacent transmission lines and cause corrosion. To reduce the risk of corrosion, voltages are induced in the pipeline with the aid of a transformer (8), which voltages counteract the induced voltages. A measuring conductor (2) provides a signal which is a measure of the induced voltage. In a suitable way, this signal controls the amplitude and phase position of a voltage (U1) which is generated by a converter coupling (7) and is applied to the primary winding of the transformer (8).

Description: Device for compensation of an alternating voltage which occurs between a medium and a metallic pipeline disposed in the medium TECHNICAL FIELD The invention relates to a device for compensation of an alternating voltage which occurs between a medium and a metallic pipeline disposed in the medium, the pipeline being surrounded by a layer (mantle) of electrically insulating material. BACKGROUND ART In case of parallelism between a.c. transmission lines and metal pipes for, for example, natural gas, the normal operating current of the transmission line induces a voltage in the metal pipe. For example, from a 400 kv line with an operating current of 1000 A at a distance of 50 m from the pipeline, an induced voltage of about 20 V/km can be obtained. A metal pipe of the above kind may, for example, constitute part of a long gas conduit, which is disposed in the ground and possibly partially also in water. A conduit of this kind is usually divided into sections with the aid of electri¬ cally insulating joints. The length of one section may vary from several kilometres up to several tens of kilometres. If a transmission line runs parallel to such a line for a distance of some length, induced voltages of a considerable magnitude may therefore occur. When the alternating voltage between the pipe and the surrounding ground (water) exceeds a few tens of volts, this may entail an increased risk of corrosion damage to the pipeline because of electrolytic corrosion. Metal pipes of the kind in question are provided with a protective coating of an electrically insulating material. However, damage unavoidably arises in this coating, whereby the metal pipe is brought into electrical contact with the surrounding medium. At these points the above-mentioned risk of corro- sion occurs. Different types of measures for protection against corrosion are previously known. However, these do not provide any protection against the risk of corrosion which is caused by alternating voltages induced in a pipeline. SUMMARY OF THE INVENTION The invention aims to provide a device which, in a simple and advantageous manner, provides good protection against the risks of corrosion which, in pipelines of the kind mentioned in the introduction, are caused by alternating voltages induced in the pipelines. What characterizes a device according to the invention will become clear from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in greater detail with reference to the accompanying Figures 1-5, wherein Figure 1 shows an example of a device according to the invention, wherein Figure la shows a general diagram of the device, Figure lb shows the transformer included in the device and the location of the transformer around the pipeline, and Figure lc illustrates the reduction of the voltage between the pipeline and the surrounding medium which can be obtained with the aid of the device shown in Figure la and Figure lb, Figure 2a shows how, in equipment according to the inven¬ tion, several transformers can be arranged along a section of the pipeline, and Figure 2b shows the reduction of the voltage between the pipeline and ground which can be obtained in this way, Figure 3 shows an alternative embodiment in which the trans¬ former included in the device is supplied from a power amplifier, Figure 4 shows how a controllable transformer coupling can be used as an alternative for supply of the transformer of the equipment, and Figures 5a and 5b show an alternative method for sensing the voltage induced in the pipeline and for controlling the supply voltage to the transformer included in the equipment. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure la shows an elementary diagram of a piece of equip- ment according to the invention. The figure shows a section 1 of a metallic natural gas conduit 1 disposed in the ground, the conduit being provided with an electrically insulating coating and being electrically insulated from adjoining pipe sections with the aid of electrically insu- lating joints 11 and 12. To provide a measure of the alternating voltage which can be induced in the section 1 by electric transmission lines, which extend in the vicinity of and wholly or partially parallel to the pipe section, a measuring conductor 2 insulated from ground is arranged. This conductor may be arranged in the ground, on the ground or above the ground. The measuring conductor 2 is suitably arranged parallel to the pipeline and close to this. The length of the measuring conductor may be small in relation to the length of the section 1, but if desirable for obtaining a sufficient magnitude of the measured signal from the conductor, the length of the conductor may constitute a considerable part of the length of the section. The conduc¬ tor 2 may be grounded at a suitable point. The voltage u s induced in the conductor 2 is supplied to an instrumentation amplifier 3, the output signal of which is designated u' g . Due to the location of the measuring conductor 2 parallel to and close to the pipe section 1, the signals u g and u' become a good measure of the voltage induced in the pipe section by the operating current of the transmission line. The signal u' from the instrumentation amplifier 3 is supplied to an absolute value generator 4 and a phase detector 5. The absolute value generator 4 delivers a signal U which is proportional to the amplitude of the voltage u„ induced in the measuring conductor 2. The phase detector 5 delivers a signal φ which is proportional to the phase difference between the signal u' and a reference voltage u ref * The reference signal is an alternating signal with the same frequency as the frequency in the transmission line which causes the voltages induced in the pipeline. As shown in the figure, the reference voltage can be obtained in the simplest manner from a local network 6, which belongs to the same power network as the above-mentioned transmission line and therefore has the same frequency as this. The signals U and φ are supplied to a controller 7, which is adapted to supply an alternating voltage U^ with controll¬ able amplitude and with controllable phase position. As shown in Figure la, the controller 7 may consist of an alternating voltage converter, for example an intermediate link converter with a controllable rectifier supplied from the network 6, a direct voltage intermediate link, and a self-commutated inverter adapted to supply an alternating voltage with controllable frequency and hence with controll¬ able phase position. With this embodiment of the controller 7 Λ the voltage U is adapted to control the intermediate link direct voltage and hence the amplitude of the voltage U-, , and the signal φ is adapted to control the inverter such that the voltage U-i assumes a phase position φ-, = φ + π in relation to the reference voltage, that is, the voltage U-i is in phase opposition to the measured signal u . The voltage U-, generated by the controller 7 is supplied to a transformer 8. As shown in more detail in Figure lb, this transformer has an iron core 81 with an annular or rectan¬ gular cross section, which surrounds the pipeline 1. The iron core is suitably made of oriented sheet metal and can be made wound from one single coherent strip of sheet. Alternatively, the core may consist of a number of composite sheets with their planes perpendicular to the longitudinal axis of the pipeline. As schematically shown in Figure lb, a primary winding 82 is applied on the core, the voltage U-^ from the controller 7 being connected to this primary win¬ ding. The winding 82 and the controller 7 are designed such that suitable current and voltage levels are obtained. For example, the winding 82 may consist of ten turns, the vol- tage U^ have a root-mean square (RMS) value of the order of magnitude of 100 V, and the current through the primary winding of the transformer have an RMS of about 13 A. The pipeline 1 functions as a single-turn secondary winding to the transformer 8, and an EMF E 2 = is induced in the pipeline. As will be clear from the fore¬ going, this EMF is in phase opposition to the EMF induced in the pipeline by the transmission line. Thus, these two EMFs will counteract each other, and if the equipment according to the invention is correctly designed and adjusted, an almost complete suppression of the voltages induced in the pipeline 1 by the transmission line current can be obtained. The constant k in the expression above is chosen and adjus¬ ted in the control system such that the desired degree of suppression is obtained of the voltage induced in the pipe¬ line. The constant k can be determined by calculation, measurement or by practical tests. If considered necessary, the signal from the measuring conductor 2 can be filtered in a band-pass filter tuned to the frequency of the transmission line, this in order to eliminate the effect of voltages occurring in the measuring conductor and emanating from other sources than the trans¬ mission line. Figure lc shows the voltage in the pipeline in relation to ground plotted against the distance x from one end of the line section. The section is assumed to have the length 1 and be grounded at its centre, for example through damage to the electrical insulation of the line. The curve designated a in the figure shows the voltage which would be caused by a transmission line extending in parallel with the line section along the whole of its length. The voltage assumes a maximum value ± u at the end points of the section. If a transformer 8 according to the invention is arranged at the centre of the line section and adapted to induce in the pipeline an EMF of the magnitude u , the voltage will have an appearance as shown by the curve b. As will be clear from the figure, the maximum voltage between the pipeline and ground is reduced by a factor 2. If this reduction is not considered sufficient, a further reduction can be obtained by arranging a number of trans¬ formers according to the invention along the line section. Figure 2a shows such an example where three transformers 8a, 8b and 8c are arranged evenly distributed along the length of the section. The primary windings of the transformers are connected in parallel to the controller 7 and are thus supplied with the voltage U-, . In Figure 2b, the curve c shows the voltage which is obtained between the pipeline and ground. As is clear, in this case a reduction of the maximum voltage by a factor of 4 is obtained. Figure 3 shows an alternative embodiment of the equipment according to the invention. The signal u' from the instru¬ mentation amplifier 3 is supplied to a sign-reversing power amplifier 9, the output signal U-. of which is supplied to the transformer 8. By the sign reversal in the amplifier 9, the signal U-, will be in phase opposition to the signal u" and by a suitable adjustment of the amplification factor of the amplifier, in principle a complete suppression of the voltages induced in the pipeline 1 can be obtained. The amplifier 9 may, for example, be a switched power amplifier of a kind known per se. Figure 4 shows how, as an alternative, a transformer coupling can be used for generating the supply voltage to the transformer 8. The coupling comprises two single-phase transformers 22 and 23. The transformer 22 has its primary winding connected to the phases S and T of the local network 6, and the transformer 23 has its primary winding connected between the phase R and the neutral line 0 of the network. The amplitude of the output voltage of each transformer is controllable, continuously or in steps. The transformers may, for example, consist of servo-motor operated adjustable transformers or of transformers which are provided with tap changers. In the coupling shown, the output voltage U A from the transformer 23 will have a phase shift of 90° in rela¬ tion to the output voltage U β from the transformer 22. Since the secondary windings of the two transformers are connected in series, their output voltages will be added vectorially, and their vector sum constitutes the supply voltage U^ to the transformer 8. If the output voltage of each transformer can be varied from maximum amplitude in one phase position to maximum amplitude in the opposite phase position, the output voltage U-. may in a known manner be controlled arbi¬ trarily both with respect to amplitude and phase position within all four quadrants. For control of the transformers, the signals U and φ (see Fig. la) are supplied to a control unit 21, which delivers control signals si and s2 to the actuators of the transformers. The control device may, for example, deliver such control signals si and s2 to the transformers that the output voltages thereof become: U A = Usin(φ + π) U A = Ucos(φ + π) In this way, the supply voltage to the transformer 8 will have the amplitude U and a phase position which is in oppo¬ sition to the alternating voltage induced in the pipeline 1. The control of the equipment according to the invention can be carried out in other ways than the one described above. For example, as shown in Figure 5a, the voltage between the pipeline and ground may be sensed at one or a plurality of points distributed along the pipeline. In the example of Figure 5a, this is done by connecting instrumentation ampli¬ fiers 31, 32, 33 between ground and the points P12, P2, P3 on the pipeline. The output signals u' s ^, U ' S 2' u 's3 °"^ t * ιe instrumentation amplifier are supplied to an optimization unit 34 (Fig. 5b) . This, in turn, delivers a control signal s3 to the controller 7. The control signal s3 influences the amplitude and phase position of the voltage U-> generated by the controller, which voltage is supplied to the transformer 8. The optimization unit 34 may, for example, consist of a suitably programmed computer adapted to influence the vol- tage U-^ via the control signal s3 in such a way in depen¬ dence on the measured signals that the risk of corrosion of the pipeline is minimized. The optimization unit may, for example, form the mean value of the measured signals and by successive attempts vary the amplitude and phase position of the voltage U until this mean value reaches a minimum. Instead of minimizing the mean value of the measured signals as described above, it is, of course, possible to form and minimize some other quantity representative of the risk of corrosion. As an alternative example, the quantity which is minimized can consist of that of the measured signals which has the greatest absolute value. The input signal or signals to the optimization unit 34 in Figure 5b need not, of course, be formed in the manner shown in Figure 5a. In an alternative embodiment, for example, the input signals to the optimization unit may consist of the measured signal or signals from one or more measuring con¬ ductors 2 of the kind shown in Figure 1. Figure 2a shows how several transformers, supplied from a common voltage source, can be disposed along the pipeline section in question to achieve a greater reduction of the induced voltages. Alternatively, the same effect can be attained by placing several complete pieces of equipment of the kind shown in Figure la along the pipeline section. The measuring conductors 2 shown in Figures 1 and 3 con¬ stitute one way of forming a quantity which is a measure of the voltage induced in the pipeline. Also other ways are feasible. As mentioned, the voltage induced in the pipeline is, with respect to magnitude and phase position, directly dependent on the load current of the transmission line. Where it is possible and suitable to measure this current, it can be used directly as a measure of the voltage induced in the pipeline. In the above description, it has been implicitly assumed that the load current in the transmission line, and hence the voltage induced in the pipeline, is a pure sine wave current without harmonics. In practice, harmonics may occur in the load current and induce alternating voltages of corresponding frequencies in the pipeline, which voltages, in the same way as the fundamental component, may cause risks of corrosion. The embodiment of the invention shown in Figure 3 will automatically entail a compensation also of induced harmonics, since the voltage U-, applied to the transformer constitutes a sign-reversed reproduction of the measured signal u g obtained from the measuring conductor 2. Harmonics in the induced voltage may, of course, be compen¬ sated for also in other ways. Thus, for example, both the fundamental component and the harmonics in question may be separated out of the measured signal with the aid of the band-pass filter and be determined individually in amplitude and phase position, whereupon the desired voltage U-. for suppressing all the sensed components are synthetized in a suitable way with the aid of suitable electronic circuits. As an alternative to the converter coupling 7 shown in Figure 1 and to the transformer coupling shown in Figure 4, a cascade connection of an induction regulator and an adjustable transformer can be used, the induction regulator being used for controlling the phase position of the supply voltage to the transformer 8 and the adjustable transformer being used for controlling the amplitude of the voltage.

Claims: CLAIMS 1. A device for compensation of an alternating voltage which occurs between a medium and a metallic pipeline (1) disposed in the medium, said pipeline being surrounded by a layer or a mantle of electrically insulating material, characterized in that the device comprises a) a transformer (8) with a core (81) surrounding the pipeline and with a winding (82) applied on the core, b) first members (2, 3, 4, 5) adapted to form a quantity (U, φ) which corresponds to the alternating voltage occurring in the pipeline, and c) voltage-generating members (7) adapted to be supplied with said quantity, to generate, in dependence thereon, an alternating voltage (U-i ) and to supply this voltage to the transformer winding in such a way that the vol- tage between the medium and the pipeline is influenced to reduce the risk of corrosion of the pipeline. 2. A device according to claim 1, characterized in that said first members comprise a measuring conductor (2) disposed substantially parallel to the pipeline, and that said quantity is formed from the voltage (u_) induced in the measuring conductor. 3. A device according to claim 2, characterized in that said first members comprise amplitude-sensing members (4) adapted to form an amplitude signal (U) corresponding to the amplitude of the induced voltage, as well as phase-angle sensing members (5) adapted to form a phase position signal (φ) corresponding to the phase position of the induced voltage, which signals are adapted to be supplied to said voltage-generating member (7), which in turn is adapted to generate and supply the transformer (8) with a voltage (U-^) with an amplitude corresponding to the amplitude signal and a phase position corresponding to the phase position signal. 4. A device according to claim 3, characterized in that said phase-angle sensing members (5) are adapted to form the phase position signal in dependence on the phase position of the voltage induced in the measuring conductor in relation to a reference alternating voltage (u re f) • a- n d- that said voltage-generating member (7) is adapted to generate an alternating voltage (U^) with the same frequency as the reference voltage and with a phase position, in relation to the reference signal (u re f) , which is dependent on the phase position signal (φ) . 5. A device according to claim 1, characterized in that said first members comprise members (31, 32, 33, 34) adapted to sense the voltage between the pipeline and the surroun¬ ding medium at one or at a plurality of points (Pi, P2, P3) along the pipeline and, in dependence on the sensed voltage values ( u, • ,, u 's2' u s3^' to ° rm a control quantity ( s3 ) for controlling the voltage-generating member (7). 6. A device according to any of the preceding claims, characterized in that it comprises a plurality of transformers (8a, 8b, 8c) distributed along the pipeline. 7. A device according to claim 6, characterized in that the transformers are supplied from a common voltage source (7) . 8. A device according to any of the preceding claims, characterized in that the voltage-generating member (9) consist of a power amplifier. 9. A device according to any of claims 1-7 ' , characterized in that the voltage-generating member (7) consists o-f a static converter coupling. 10. A device according to any of claims 1-7, characterized in that the voltage-generating member (21, 22, 23) consists of a controllable transformer coupling (21, 22, 23).

Inventors: JONSSON UNO, KARLSSON DAN, JONSSON, UNO, KARLSSON, DAN
Applicants: STRI AB
Application year: 1993
Main classification: C23F13
IPC codes: C23F13