GB2166305A

GB2166305A - Superimposing AC on HVDC cable to improve dielectric strength

Info

Publication number: GB2166305A
Application number: GB08526090A
Authority: GB
Inventors: Giandomenico Testi
Original assignee: Pirelli Cavi SpA; Cavi Pirelli SpA
Current assignee: Pirelli and C SpA
Priority date: 1984-10-26
Filing date: 1985-10-22
Publication date: 1986-04-30
Also published as: SE8505002L; ES547781A0; SE8505002D0; FI854194A0; IT8423347A0; ES8609844A1; GB8526090D0; BR8504738A; AU4694585A; DK493085D0; ES547782A0; NO854283L; DE3537900A1; FR2572598A1; GR852566B; JPS61109425A; IT1180238B; FI854194L; ES8609845A1; DK493085A

Abstract

In order to improve the transmissivity characteristics of an electric cable (1), particularly a submarine cable, transmitting power by means of a DC voltage, an AC voltage lower than the DC voltage (E1, E2) is superimposed on the DC voltage to improve dielectric strength. The AC voltage may be generated permanently, intermittently for periods of time, or prior to installation of the cable. Converter stations (4, 5) are provided at each end of the cable for transmitting and receiving power, and one station contains a first AC generator (E1) while the other contains a passive or active AC source (E2) which provides a voltage equal and in phase with the first generator (E1) to cancel out the alternating current flow in the loop comprising the cable and converter stations. The AC generators may be separate voltage generators or may be provided by modulation of the control system controlling the power conversion means in the generator. <IMAGE>

Description

SPECIFICATION Method and apparatus for improving the transmissivity characteristics of an electric power cable This invention relates to a method and apparatus for improving the transmissivity characteristics of an electric power cable carrying a high DC voltage for power supply.

Existing DC power transmission systems normally comprises at least one DC power transmission cable which is laid on or under the earth or is positioned overhead, and is connected to two or more converter stations. For the purposes of this specification, "converter station" means a terminal station of a power supply system where a voltage conversion takes place between AC and DC. The converter station may be an intermediate station in a complex distribution network, a branching-off point, a transition point from a buried cable to a submarine cable etc.

Such converter stations normally employ rectifiers or switches, preferably thyristors, whose firing angles are controlled by circuits which produce and distribute firing and cut-off pulses to the thyristors in order to control the thyristors so that the converter station operates at desired current and voltage values.

The transmission of power by high DC voltage has an important application in long sea-crossings since AC transmission techniques present considerable technical problems because of the high capacitance between the cable and earth. The costs of the cable in submarine applications are the most significant item where the cable has a length exceeding roughly ten kilometers and are a function of the rated current voltage of the cable, the depth of the seabed, and the conditions under which the cable is laid. The type of cable that is normally used in such applications has oil-impregnated paper insulation, and a nominal maximum working voltage of about + 300 kV DC.

Thus it is clearly highly desirable to increase the transmissivity of the cable in order to reduce cable costs by employing the minimum size of cable for the required amount of power to be supplied and it is an object of the invention to achieve such increase.

The present invention is based on the realisation that an AC voltage having an amplitude less than that of the DC voltage ad superimposed on the DC voltage can improve the transmissivity of the cable by improving the dielectric strength of the cable. It will be understood that such AC voltage is substantially larger than the normal AC voltage component present resulting from ripple from the converter stations, which ripple voltage normally has an amplitude of some tenths % of the DC voltage amplitude.

Accordingly in one aspect the invention provides a method for improving the transmissivity characteristics of an electric power cable, the method comprising connecting the power cable to power conversion means for applying a high DC voltage to the cable for the transmission of power and superimposing an AC voltage thereon having an am plitude less than that of the DC voltage wherein the AC current present in the cable resulting from the AC voltage is sufficiently small so as not to interfere with the operation of the power conversion means.

In a further aspect, the invention provides a method for improving the transmissivity characteristics of an electric power cable, the cable being connected between a first and a second converter station whereby to provide a high DC voltage to the cable for the supply of power between the stations, wherein the first converter station provides a first AC voltage superimposed on the DC voltage having an amplitude which is less than that of the DC voltage, and the second converter station provides a second compensating AC voltage which is such in relation to the first AC voltage that consequent AC current flow is reduced to such an extent that it is insufficient to interfere with the operation of the converter stations.

In a further aspect, the invention provides apparatus for improving the transmissivity characteristics of an electric power cable, comprising first and second converter stations connected to the cable for providing-a high DC voltage thereacross or the supply of power, wherein the first converter station includes means for providing a first AC voltage superimposed on the DC voltage having an amplitude which is less than the amplitude of the DC voltage, and the second converter station includes means for providing a second compensating AC voltage which is such in relation to the first AC voltage that AC current flow is reduced to such an extent that it is insufficient to interfere with the operation of the converter stations.

Thus in accordance with the invention, a conventional cable has its transmissivity characteristics improved and may be used to carry DC voltages and power levels even higher than the maximum rated voltage and power of the cable. Alternatively cables of a lower rating may be used for conveying voltages at levels which would otherwise require a cable of greater rating and higher cost.

The present invention has application to all types of high DC voltage cables, with the exception of those cables whose insulation is assisted by gas or liquid pressure.

Preferably the AC voltage has an amplitude between 0.5% and 10% of the amplitude of the DC voltage and a frequency theoretically between one Hz and 50Hz, although a practicable range is 1-20 Hz for waveforms of any shape. For sinusoidal waveforms, the range 2-10Hz is preferred. The voltage may be applied permanently during cable operation, or intermittently, each intermittent period lasting for a certain number of hours or days and the voltage being applied continuously throughout the period. Alternatively the cable may be subjected to the superposition of an AC voltage on a working DC voltage, prior to the use of the cable in a power supply system. In any event the present invention provides for reliable use of the cable at voltage, current and power values exceeding those of which the cable is rated under normal condi tions of use.

As preferred when employed in use in a power supply system the first AC voltage and second compensating AC voltage are of equal amplitude and of the same phase so as to render null the sum of the alternating electromotive forces in the closed loop comprising the cable and the converter stations. When the AC voltage is applied prior to installation, the DC voltage may not be the same as that employed in use and the AC voltage may be applied simply to the cable ends without the intermediary of converter stations.

In accordance with the invention, the AC current resulting from the AC voltage is minimized or reduced to such an extent as not to interfere with the regular working of the converter stations. Specifically in relation to presently used converter stations, the AC current should be less than 5% of the DC current present in the cable, preferably less than 4%. Thus the AC current is reduced to such an extent that it will not interfere with the operation of the control system of the power conversion means. The AC current circulating in the cable is a disturbance with respect to the regular working of the converter station and like any disturbance is taken care of by the control system which adjusts the operation of the station.Since the control system cannot cope with disturbances of any amplitude, the AC current must be kept within the limits specified above so that allowance can be made for real or external disturbances or load changes that might sum with the AC current and produce faulty operation of the control system.

As preferred, the AC source at one converter station and the compensating AC source at the other converter station are realised by modulating the time distribution of the pulses controlling the thyristors in the converter stations, whereby to produce in each station a synchronised AC voltage by means of the existing power conversion means.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings wherein: Figure 1 is a general circuit diagram of the major components of a known converter station; Figure 2 is a schematic diagram of a first embodiment of the invention constituting a single cable power supply system; Figure 3 is a general circuit diagram of a converter station for use in the embodiment of Figure 2; Figure 4 is a schematic diagram of a second embodiment of the invention employed in a two-cable power transmission system; and Figure 5 is a schematic diagram of a third embodiment of the invention employed in a two-cable power transmission system.

Referring to Figure 1, this shows an AC/DC converter station wherein a three phase AC voltage supply 10 is converted to a high value DC voltage for application to a cable 1.

The three phase power supply has filters 12 connected to each line of the power supply, which lines are connected to one or more transformers 13 (only one of which is shown in Figure 1) forming an input for the converter station. The three secondary windings of the transformer 13 are connected to respective arms of a bridge rectifying circuit 23.

Where more than one transformer 13 is provided, a corresponding number of bridge rectifying circuits are provided, which are connected in series as indicated by the broken line which terminates at A. The circuit or circuits 23 are also connected via a smoothing inductance 2 to a DC power cable 1.

Each bridge rectifying circuit includes in each arm a thyristor rectifying switch 32 having a control electrode 33. Each thyristor has a damping circuit 7 (only one of which is shown) connected in parallel with the main conducting path.

A control system 18 is provided comprising a control unit 20 and a protection unit 19. The system 18 receives feedback voltage and current signals on an AC line 15 and a DC line 16 in order to provide appropriate firing and cut-off pulses to the electrodes 33 of the thyristors.

The control system 18 controls the converter station to operate at desired current and voltage values by processing the feedback signals on lines 15 and 16 which are determined by the parameters of the network, and adjusting appropriately the timing of the firing and cut-off pulses which are generated and distributed to the thyristors.

It will be understood that at the other end of the cable is disposed a DC/AC converter station where the DC voltage on the cable is converted to an AC voltage and distributed to other sites. The DC/AC converter station at the other end of cable 1 is substantially similar, but operates in an inverse manner to receive the AC voltage, the bridge circuits operating as inverters.

Further details of such converter stations may be found in the book "DIRECT CURRENT TRANSMIS SION" Vol. No. 1. Edward W. Kimbark (Ed. Wiley Interscience, New York, 1971).

In the following Figures, the same reference numerals, are used to indicate the same of corresponding items.

Figure 2 schematically illustrates a preferred embodiment of the invention in a single cable transmission system - i.e. with ground return by sea and thence land.

An electric cable 1 is connected between a first AC/DC converter station 4 and a second DC/AC converter station 5 through smoothing inductances 2 at the ends of the cable. In accordance with the invention, as AC voltage is introduced at station 4 superimposed on the working DC voltage and having amplitude and frequency values as indicated above. The AC voltage is generated by a source El. Converter station 5 includes a compensating AC voltage source E2, which provides a voltage which is of equal amplitude and of the same phase as the AC voltage provided by source El, as indicated by the arrows shown at each source in Figure 2. These opposed voltages produce opposed currents whereby in the closed loop formed by the cable, the converter station and the ground return (however consituted, by earth, sea etc.) the sum of the AC electromotive forces is null whereby the net current is zero. In practice, a relatively small AC current component is present arising from the superimposed AC voltage due to the distributed capacitance of the cable with respect to ground, small phase differences between sources El and E2, etc. Nevertheless such alternating current is of a negligible value as compared with the direct current within the cable, e.g. less than 5% thereof.

Consider a specific working example. A 100 kV voltage is applied to the cable resulting in a direct current circulating of 1000A, and an AC voltage of 5.5 kV is applied through a source at the converter station 4. An equal voltage is applied at converter station 5. The amplitude of the resulting alternating current circulating in the cable is + 20 A which is small enough to ensure the control systems in each converter station will not be interfered with.

Refering now to the embodiment shown in Figure 3 of a converter station for use in the present invention, the converter station may for example constitute the converter station 5 of Figure 2.

An AC power supply is connected to filters 12 and input transformers 13, 14. The secondaries of the transformers are connected to respective bridge rectifier circuits 23, 24 which are connected in series between earth and a smoothing inductance 2 and DC power supply cable 1.

A control system 20 is provided connected to the control electrodes of the thyristor switches in the bridge rectifier circuits, indicated schematically as at 33, 34. Control system 20 generates firing and cut-off pulses for the thyristors and provides such pulses in accordance with the operating conditions (overload, etc) of the power supply system which are determined by a regulator 21. Regulator 21 is connected to system 20 via a summation circuit 26, and receives signals from voltage and current sensors responsive to the operating conditions of the system and indicated schematically by V1 block 22.

Thus depending on the currents and voltages present in the power supply system the timing and duration of the control pulses for the thyristors are modified in order to produce desired operating conditions.

A modulator unit 25 is provided, which is connected to control system 20 by way of summation circuit 26. Modulator unit 25 applies a modulating signal having a frequency between 1 and 50 Hz, preferably between 1 and 20 Hz in order to periodically vary the firing angle of the thyristors, in order to produce an AC voltage of the same frequency superimposed in the DC voltage at the output of the bridge rectifier circuits. Preferably the AC voltage is sinusoidal and has a frequency of between 2 and 10 Hz, so as to avoid those frequency values that are typical of electromechanical transients (below the lower limit) or of the sub-synchronous resonance (above the upper limit). The amplitude of the AC voltage is between 0.5% and 10% of the amplitude of the DC voltage.

Preferably the converter station is a DC/AC converter station, since it is easier to introduce the AC voltage in such a station by modulating the firing angle than at the AC/DC converter station. The DC/ AC converter station is thus arranged to produce a compensating voltage by modulating the firing angle or in other ways.

Although Figure 3 illustrates generation of an AC voltage by modulation of the power conversion means, other generation techniques can be employed such as for example a separate voltage generator. The advantage of separate voltage generators, is that the AC voltage source in one converter station can be interchanged with the AC voltage source in the other converter station.

Figure 4 illustrates a second embodiment of the invention employing separate voltage sources and comprises a two-cable power transmission system comprising cables 41, 51 connected between converter stations 44 and 45, station 44 including a first source of AC voltage El and station 45 including a second source of AC voltage E2. Both sources El and E2 are separate voltage generators consisting of active components, but are maintained in phase and at an equal amplitude by means of sensors (not shown) situated in the power supply system which provide feedback signals through control lines or along the cables themselves.

As regards the provision of an AC voltage source which provides a compensating voltage, this may be provided by a device consisting of passive elements. Although such a device is not interchangeable in the sense that it cannot be used as the source of the first AC voltage, it nevertheless has advantages in that it is unnecessary to synchronize operation of the two voltage sources.

One such passive device for producing a compensating AC voltage is shown in Figure 5 which represents a third embodiment of the invention.

This embodiment includes two cables 41, 51 connected between two converter stations, a first station including an AC voltage source El which may be a separate voltage generator or a source produced by modulation of the power conversion means and a second station including an LC circuit as shown, whose component values are selected to provide a circuit having a resonant frequency equal to the frequency of source El. The impedance offered to the voltage source El by the resonant circuit is very high and a voltage is thus present in the resonant circuit which is of equal amplitude and of the same phase as that provided by source El.

The resonant circuit may be situated either at the AC/DC converter station or at the DC/AC converter station, with the other station containing a voltage source generating an AC voltage.

Claims

1. A method for improving the transmissivity characteristics of an electric power cable, the method comprising connecting the power cable to power conversion means for applying a high DC voltage to the cable for the transmission of power, and superimposing an AC voltage thereon having an amplitude less than that of the DC voltage wherein the AC current present in the cable resulting from the AC voltage is sufficiently small so as not to interfere with the operation of the power conversion means.

2. A method as claimed in claim 1 wherein the method is carried out prior to use of the cable in a power supply system.

3. A method as claimed in claim 1 wherein the method is carried out with the cable installed and the AC voltage is applied either permanently or for intermittent periods of time, the AC voltage application being continuous in each such intermittent period of time.

4. A method as claimed in claim 1 wherein the AC current flow is less than 5% of the DC current flow.

5. A method as claimed in claim 1 wherein the first AC voltage has an amplitude which is between 0.5% and 10% of the DC voltage.

6. A method as claimed in claim 1 wherein the AC voltage has a frequency of between 2 and 10 Hz.

7. A method for improving the transmissivity characteristics of an electric power cable, the cable being connected between a first and a second converter station whereby to provide a high DC voltage to the cable for the supply of power between the stations, wherein the first converter station provides a first AC voltage superimposed on the DC voltage having an amplitude which is less than that of the DC voltage, and the second converter station provides a second compensating AC voltage which is such in relation to the first AC voltage that consequent AC current flow is reduced to such an extent that it is insufficient to interfere with the operation of the converter stations.

8. A method as claimed in claim 7 wherein the second compensating voltage is substantially equal in amplitude and phase to the first AC voltage.

9. Apparatus for improving the transmissivity characteristics of an electric power cable, comprising first and second converter stations connected to the cable for providing a high DC voltage thereacross for the supply of power, wherein the first converter station includes means for providing a first AC voltage superimposed on the DC voltage having an amplitude which is less than the amplitude of the DC voltage, and the second converter station includes means for providing a second compensating AC voltage which is such in relation to the first AC voltage that AC current flow is reduced to such as extent that it is insufficient to interfere with the bperation of the converter stations.

10. Apparatus as claimed in claim 9 wherein the means for providing said first AC voltage is a voltage generator.

11. Apparatus as claimed in claim 9 wherein said first AC voltage is provided by the AC/DC power conversion means present in the first converter station.

12. Apparatus as claimed in claim 9 wherein the second compensating AC voltage is provided by an AC voltage generator.

13. Apparatus as claimed in claim 9 wherein the second compensating AC voltage is provided by a passive LC circuit.

14. Apparatus as claimed in claim 9 wherein the second compensating AC voltage is provided by the AC/DC power conversion means present in the second converter station.

15. Apparatus as claimed in claim 9 wherein the means for providing first and second AC voltages are such that the first AC voltage is substantially equal in amplitude and phase to the second AC compensating voltage.

16. Apparatus as claimed in claim 11 or 14 wherein the respective AC/DC power conversion means includes a modulator means for modulating in a cyclic manner the time distribution of control pulses for the conversion means.

17. Apparatus for improving the transmissivity characteristics of an electric cable substantially as herein described with reference to Figure 2 of the accompanying drawings.

18. Apparatus for improving the transmissivity characteristics of an electric cable substantially as herein described with reference to Figure 4 of the accompanying drawings.

19. Apparatus for improving the transmissivity characteristics of an electric cable substantially as herein described with reference to Figure 5 of the accompanying drawings.

20. Apparatus as claimed in claim 17, 18 or 19 and employing a converter station shown in Figure 3 of the accompanying drawings.

21. A method as claimed in claim 7 and as specifically herein described with reference to the working example.