WO1999058992A2

WO1999058992A2 - A power cable monitoring system

Info

Publication number: WO1999058992A2
Application number: PCT/DK1999/000254
Authority: WO
Inventors: Neils H. Skovgaard
Original assignee: Dsc Communications A/S
Priority date: 1998-05-07
Filing date: 1999-05-06
Publication date: 1999-11-18
Also published as: WO1999058992A3; AU3596099A

Abstract

Power cable monitoring system comprising one or more transducers distributed along the cable. Each of the transducers is operationally connected to the cable for measurement of operational parameters of the cable, such as temperature, pressure, voltage, current, etc., and provides an output signal in response to actual parameter values. A data cable, such as a coax cable, a twisted a pair cable, an optical fibre cable, etc., positioned adjacent to and extending substantially in parallel is connected to the transducers.

Description

A POWER CABLE MONITORING SYSTEM

FIELD OF THE INVENTION

The present invention relates to a method and a system for monitoring power cables and in particular for monitoring buried cables and sea cables.

BACKGROUND OF THE INVENTION

It is well known in the art to transmit electrical energy from a power generating plant through high voltage cables such as buried cables, sea cables or submarine cables, etc.

The amount of energy that can be transmitted through a specific cable is limited by the temperature increase caused by losses in the cable. The operating temperature of the cable must be kept below a safe limit below which the cable does not deteriorate.

In some cables, oil is used as an isolator.

SUMMARY OF THE INVENTION

During operation of a power cable, it is desirable to be able to monitor various parameters, such as temperature, pressure, voltage, current, etc, of the power cable so that a failure in the cable can be detected and, preferably, located soon after occurrence.

Further, monitoring of a power cable may allow transmission of an increased amount of power through the cable as parameter safe guard ranges may be decreased. A safe guard range is the range between a maximum value of a parameter, above which the cable will deteriorate, and the specified maximum operating value of the parameter, at which value it is considered safe to operate the power cable. For example, if the temperature of a sea cable is measured at its end points at the sea shore, a safe guard interval for an accepted operating temperature should take into account that the sea cable may be buried in various types of sea beds having different heat conductive characteristics. Thus, in order to ensure safe operation, the upper limit of acceptable operating temperatures at the end points may be relatively low. If, however, the operating temperatures are monitored along the entire length of the cable, the amount of power which can be safely transmitted through the cable may be raised, as the temperature of the cable is known along the entire cable.

For power cables containing oil, it is desirable to be able to detect and preferably, locate possible oil leakages. Oil leakages pollute the environment and further increase the risk of a occurrence of a short circuit of the power cable due to loss of the isolating effect of the oil.

According to the present invention, a power cable monitoring system is provided for monitoring operational parameters of a power cable. The system comprises one transducer or a plurality of transducers distributed along the power cable. Each of the transducers is operationally connected to the power cable for measurement of operational parameters of the power cable, such as temperature, pressure, voltage, current, etc, and provides an output signal in response to actual parameter values. The system further comprises a data cable, such as a coax cable, a twisted pair cable, an optical fibre cable, etc, positioned adjacent to and extending substantially in parallel with the power cable. The data cable is operationally connected to the transducers for transmission of the output signals. The operational connection may comprise an electrical cable, infrared transmission means, radio signal transmission means, etc. A controller may be connected to the data cable for reception of the output signals being adapted to process the output signals. The power cable monitoring system may be used for monitoring the operation of any power cable and it is particularly suitable for monitoring of buried cables and sea cables, such as armoured 400 - 2000 mm² copper power cables.

The transducers may be any transducer suited to measure the desired parameters, such as piezo- electric pressure transducers, NTC resistor temperature sensors, thermocouple temperature sensors, opto- electric field element voltage sensors, Hall effect current sensors, etc.

A pressure transducer may be substituted with a position transducer detecting changes in a thickness of the cable, since thickness indicates the internal pressure of the power cable.

Each transducer may be operationally connected to the power cable in any suitable way for measurement of the parameter in question. However, it is preferred that each transducer is connected to the outer surface of the power cable, so that any risk of damaging the power cable is minimized. Alternatively, the outer most layer of the power cable may be penetrated by a transducer without decreasing the reliability or deteriorating the functioning of the power cable.

At each transducer position along the power cable, the transducers may be housed in a housing, preferably made of metal. The housing may be clamped around the power cable so that the power cable extends through the housing. A temperature transducer could be located in direct contact with the outer most layer of the power cable and the housing. A transducer could be mounted directly on the outer most layer of the power cable by gluing, welding, bolting, by double adhesive tape, etc.

The data cable may be laid out along the power cable when the power cable is laid out so that lay out of the power cable and the data cable is performed as one operation or, the data cable may be laid out along an existing power cable that has been laid out previously, or vice versa.

Data signals may enter the data cable at predetermined positions along the cable.

When the data cable comprises electrical conductors, electrical signals may enter the electrical data cable at predetermined positions along the electrical data cable, e.g. through electrical taps electrically connected to conductors of the data cable at the predetermined positions before lay out of the cable or, the data cable may comprise a set of electrical conductors of different lengths so that data signals may enter the data cable at predetermined positions along the cable at the corresponding end points of the conductors .

When the data cable comprises an optical fibre, optical signals may enter the optical fibre cable at predetermined positions along the optical fibre cable, e.g. through directional couplers connected to the fibre cable at the predetermined positions before lay out of the cable or, the optical fibre cable may comprise a set of optical fibres of different lengths so that optical signals may enter the fibre cable at predetermined positions along the cable at the corresponding end points of the fibres.

Data signal entry points are preferably distributed equidistantly along the power cable to be monitored with extra data entry points positioned at locations of special interest, such as locations in a sea bed with low heat conduction, location of an oil-blocking joint, location of a power cable joint, etc.

It is preferred to use an optical fibre cable for monitoring of long power cables, such as sea cables, as data may be transmitted through optical fibres for long distances, e.g. more than 600 kilometres at a suitable data rate, without repeaters thus facilitating utilisation of a system without repeaters along the data cable.

In another embodiment of the invention, the optical fibre cable is also used to transmit high speed data signals for telecommunication. The embodiment may comprise repeaters positioned along the optical fibre cable as is well known in the field of telecommunication.

Each transducer typically generates an output signal, e.g. a current or a voltage, that corresponds to the parameter value currently sensed by the transducer. The output signal is converted into a data signal, such as an optical signal, for transmission through the data cable by a transducer signal converter which is operationally connected between the transducer and the data cable. The transducer signal converter may be positioned at the transducer location at the power cable, e.g. in the transducer housing. However, it is presently preferred to position the converter at the data cable, since the environment e.g. temperature and magnetic field strength adjacent to the power cable may be demanding and thus, may require special precautions to be taken in the design of electronic circuits in the transducer signal converter.

Preferably, power for the transducers and transducer signal converters is supplied through electrical conductors positioned in the data cable. Preferably, a low voltage, such as a voltage in the range from 50 V to 500 V, preferably from 50 V to 150 V, even more preferred approximately 100 V, is supplied to the transducers and transducer signal converters. A high voltage e.g. in the range of 500 to 20000 Volt could also be used for the supply but a low voltage is preferred. When a low voltage supply is used the electronic circuits in the transducer signal converters need not be protected against high voltages, and voltage isolation requirements between conductors in the data cable are low. Local grounding may be established by connections to the transducer housing or, preferably, to the power cable sheath.

The transducer signals may be encoded into electrical or optical communication signals according to well known methods. The signals are transmitted to a controller which is adapted to receive and decode the signals into parameter values and, preferably, the controller is further adapted to display received parameter values according to well known methods to operators of the power cable. The controller may be adapted to compare received parameter values with reference values and to control the amount of power transmitted through the power cable according to predetermined rules and based on such comparisons . The controller may be a part of a power plant control system.

The power cable monitoring system may be installed for monitoring of a sea cable by a diver after lay out of the data cable along the power cable. If the sea cable is buried in the sea bed, the diver may dig out a suitable length of the power cable at a position of the data cable, e.g. the optical fibre cable, at which data signals, e.g. optical signals, may be entered into the data cable. Then, the diver may clamp a transducer housing around the power cable, may connect the transducers to the power cable and may interconnect the transducer outputs with corresponding inputs of the transducer signal converters which are located at the data cable.

The controller, the transducers, and the data cable may form a local area network, and data may be communicated between the controller and the transducers according to standards for local area networks, such as Ethernet, Token Ring, etc. Each transducer may have a dedicated address so that the controller may identify each transducer. The bit rate in the local area network can be quite low, e.g in the range from 10 bit/s to 1 Mbit/s, due to the limited amount of data transmitted between each transducer and the controller. Alternatively, each transducer may be connected to the controller via a separate communication channel, such as a separate optical fibre, the optical fibre cable comprising optical fibres of different lengths and one fibre for each transducer housing. Each transducer may transmit parameter values to the controller at regular intervals or upon interrogation from the controller.

In a power cable monitoring system according to the invention wherein the transducers and associated electronic circuits receive their power supply from batteries, considerable power savings may be obtained by designing the electronic circuits to operate in a power down mode and in an active mode. In the power down mode, the circuits are in an idle mode and no data is transmitted to the data cable, and in the active mode transducer parameter values are converted into data signals for transmission through the data cable.

Operation of the circuits may be controlled by means of a timer circuit comprised in the transducer signal converter. A timer in the present context is a circuit which at predetermined time intervals produces a control signal. The control signal is used to control circuit operation, such as operation of the transducer signal converter. The timer may be programmed to interrogate the transducer signal converter one time every minute, hour, day or at any other suitable time interval. Upon timer interrogation, the transducer signal converter changes operation mode from its power down mode to its active mode. In the active operation mode the transducer output signals are converted into a data signal and the data signal is transmitted to the controller. The transducer signal converter returns to its power down mode after the data signal has been transmitted. The low power consumption of the power down mode may be obtained according to well known design methods in the art, such as by removing the power supply from a part of the electronic circuits, lowering clock frequency in the electronic circuits, etc. A system according to the present invention with temperature transducers for measurement of operating temperature of the power cable positioned at a number of positions along the power cable makes it possible to increase the amount of power which can be transmitted through the power cable above a level that is considered safe for transmission through the power cable without the monitoring system since the operating temperatures along the power cable may be monitored substantially continuously whereby safe guard intervals may be decreased.

A sea cable may for example extend below sea level for 500 - 800 kilometres and may be buried 1 meter below the sea bed. Typically, the maximum operating temperature of a sea cable is 70 - 80°C. Various types of sea bed conduct different amounts of heat away from the sea cable and to cope with such differences a power safe guard of typically 20 % of the maximum power rating of the sea cable has previously been applied since only cable temperatures proximate to the sea shore may be known. Thus, without a monitoring system according to the present invention, the sea cable has been used for transmission of up to 80 % of its maximum power rating. Utilizing the system of the present invention, the cable may transmit power up to a power level at which the maximum allowable operating temperature is reached at the sea bed location with the lowest heat conduction.

A system according to the present invention with pressure transducers for measurement of pressure in the power cable at a number of positions along the power cable may be utilized for monitoring of power cables containing oil whereby possible leakage of oil from the cable may be detected very early after occurrence of the leakage and further, the approximate position of the leakage may be detected.

The pressure transducer may comprise a position transducer for sensing the position of an area of the surface of the power cable whereby the pressure of the power cable is indicated as the shape of the cross section of the power cable is dependent upon the internal pressure of the cable.

It is an advantage of the present invention that the power cable monitoring system operates independently of the power cable and vice versa, i.e. operation of the power cable may continue during a breakdown of the power monitoring system.

It is another advantage of the present invention that the power cable monitoring system may be installed and connected to the power cable while the power cable is operating.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings, wherein

Fig. 1 is a block diagram of the power cable monitoring system according to a preferred embodiment of the invention,

Fig. 2 shows a sea cable with a monitoring system according to the invention,

Fig. 3 shows a transducer housing that is clamped on to a sea cable and which has a position transducer positioned in physical contact with the sea cable, and

Fig. 4 shows a branching unit according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Fig. 1 diagrammatically illustrates a power cable monitoring system according to the invention. The power cable monitoring system shown comprises several temperature and pressure transducers 1 that are operationally connected to the sea cable 10 for measurement of operational parameters of the sea cable 10. Each transducer 1 provides an output signal in response to actual parameter values . A data cable 8 extends substantially in parallel with the sea cable 10 and the data cable 8 is operationally connected to the transducers 1, respectively, for transmission of transducer output signals to a controller 9 that is connected to the data cable for reception of the output signals and adapted to process the output signals. The power cable monitoring system shown comprises two controllers 9 positioned at each end of the sea cable 10 whereby the maximum transmission distance between transducers 1 and corresponding controllers 9 can be halved compared to a system comprising a single controller 9. The controllers 9 may be located on land and be part of a power plant control system. The control system may be adapted to adjust the transmitted power through the sea cable 10 in response to substantially continuously monitored operational parameters of the sea cable 10.

The controllers 9 may be interconnected through known telecommunication means, such as telephone lines, data lines, local area network, wide area network, etc.

The data cable 8 comprises an optical fibre cable comprising several optical fibres 7 of different lengths. Each of the transducers 1 is connected to a corresponding branching unit 20, which is connected to one of the controllers 9 by a separate optical fibre 7. Each individual optical fibre 7 runs between the corresponding branching unit 20 and the end of the data cable 8 that is closest to the branching unit 20 in question in order to minimize the transmission distance of the transducer data signals from the branching unit 20.

The transducers 1 are connected to the data cable via corresponding drop cables 3 and the branching units 20 (also shown in Fig. 4) comprising a transducer signal converter with an amplifier 2 for amplifying the transducer signals from the corresponding transducer 1 and supplying the amplified signal to an A/D- converter 4. Each of the signals from the temperature transducer and the position transducer, respectively, may be amplified by a separate amplifier 2 for conversion into digital values by a separate A/D converter however, in the power cable monitoring system shown an analog input multiplexer (not shown) alternatingly connects the transducer outputs to the amplifier 2 so that each set of position and temperature transducers 1 has a common amplifier 2 and A/D converter 4. The transducer signal converter further comprises a communication handler 5 for encoding the digitized transducer signals from the A/D- converter 4 according to a data transmission protocol of the system and transmission of the encoded signal to the corresponding controller 9 through an electro-optical converter 6 and the corresponding optical fibre 7.

It may be advantageous to position the amplifier 2 close to the transducer 1 to minimize noise pick up in the drop cable 3.

Communication between the branching units 20 and the corresponding controller 9 is preferably initiated by the controller 9 in question in order to minimize power consumption of the transducer signal converter. Upon interrogation, the communication handler circuit 5 transmits an encoded signal comprising the current transducer parameter values. Each of the controllers 9 may receive data signals from the corresponding transducers 1 sequentially. When data signals from all the transducers 1 have been received, the controllers 9 may repeat the interrogation procedure thereby providing a substantially continuous monitoring of the operational parameters of the sea cable, such as temperature and pressure of the sea cable 10.

Fig. 2 (not to scale) shows a part of a sea cable monitoring system and a sea cable as laid out on the sea bed. Operational connections between the data cable 8 and each of the transducers 1 are constituted by corresponding drop cables 3 and transducer signal converters comprised in the corresponding branching units 20. The drop cables 3 are shielded coaxial cables having a plurality of internal conductors some of which supply electrical power to the corresponding transducer 1, and some of which transmit the electrical output signals from the corresponding transducer 1 to the corresponding amplifier 2.

The distance between the cables is, preferably, 50 - 100 metres which is sufficiently large for individually repairing each cable without affecting the other cable. The distance is further sufficient for laying out of the data cable 8 with a cable plough substantially in parallel to an existing sea cable 10 without damaging the sea cable 10. A distance of 50 - 100 metres between the cables is required since the accuracy by which a cable laying vessel can locate the position of an existing sea cable that has been laid out previously is limited to approximately 50 metres.

The distance between the branching units 20 is approximately 50 kilometres for a sea cable with a total length of 500 - 800 kilometres. It is preferred that transducers 1 are located at joints on the sea cable 10, and it is further preferred that transducers 1 are more closely spaced in sea beds of low heat conduction. The transducers 1 may be mounted in an iron transducer housing 30 that is clamped on the sea cable 10.

Fig. 3 shows a transducer housing 30 that is clamped on the sea cable 10. A position transducer 31 is mounted on the transducer housing 30 between the housing 30 and the exterior of the sea cable 10. A temperature transducer of the NTC resistor type (not shown) is provided in the transducer housing 30 preferably abutting the exterior of the sea cable 10. The position transducer 31 measures a thickness of the sea cable 10, the thickness indicating the internal pressure of the sea cable 10. A branching unit 20 according to the invention is shown in Fig. 4. The optical data cable 8 extends through the unit 20. A coaxial drop cable 3 is also connected to the branching unit. The transducer signal converter comprises electronic circuits 2, 4, 5, 6 shown in Fig. 1, which circuits, preferably, are contained within the branching unit 20.

Claims

1. A power cable monitoring system for monitoring operational parameters of a power cable, comprising

at least one transducer operationally connected to the power cable for measurement of the operational parameters of the power cable and providing an output signal in response to actual parameter values,

a data cable extending substantially in parallel with the power cable and being operationally connected to the at least one transducer for transmission of the output signals, and

a controller connected to the data cable for reception of the output signals and adapted to process the output signals.

2. A system according to claim 1, wherein the at least one transducer is connected to the exterior of the power cable without penetration of the sheath of the power cable.

3. A system according to any of the preceding claims, wherein the at least one transducer comprise pressure sensing means for measurement of pressure in the power cable.

4. A system according to any of the preceding claims, wherein the pressure sensing means senses a thickness of the power cable.

5. A system according to any of the preceding claims, wherein the at least one transducer comprises temperature sensing means for sensing an outer temperature of the power cable.

6. A system according to any of the preceding claims, wherein the at least one transducer comprises current sensing means for measurement of current in the power cable.

7. A system according to any of the preceding claims, wherein the at least one transducer comprises voltage sensing means for measurement of voltage in the power cable.

8. A system according to any of the preceding claims, wherein the power cable is longer than 200 kilometres.

9. A system according to any of the preceding claims, wherein the data cable comprises an optical fibre cable.

10. A system according to claim 9, further comprising at least one electrical -optical converter for conversion of an electrical output signal from the at least one transducer to an optical signal for transmission through the optical fibre cable to the controller.

11. A system according to claim 9 or 10, wherein the optical fibre cable is also used to transmit telecommunication data signals.

12. A system according to claim 11, further comprising at least one repeater mounted on the optical fibre cable for repeating the telecommunication data signals and/or the transducer data signals.

13. A system according to any of the preceding claims, wherein each of the transducers is connected to the controller by a separate electrical conductor or optical fibre.

14. A system according to any of claims 1-12, wherein the transducers and the controller are interconnected with a network multiplexing output signals from a plurality of transducers on a single electrical conductor or optical fibre .

15. A system according any of the preceding claims, wherein the distance between the power cable and the data cable is sufficiently large for repairing each of the cables without affecting the other cable.

16. A system according to any of claims 1-14, wherein the data cable is positioned next to a sea cable.

17. A system according to any of claims 1-14, wherein the data cable is positioned next to a buried cable.

18. A method of manufacturing a power cable monitoring system comprising the steps of

producing a data cable with one branching unit or a plurality of branching units positioned along the data cable at predetermined positions,

laying out the data cable along a power cable to be monitored, and

at each branching unit operationally connecting a transducer to the power cable for measurement of operational parameters of the power cable and provision of output signals in response to actual parameter values and connecting the output signals to the branching unit in question

whereby a power cable monitoring system can be provided and maintained without affecting operation of the power cable to be monitored.

19. A method according to claim 18, the step of laying out the data cable further comprises simultaneous lay out of the power cable to be monitored.

20. A method according to claim 18, wherein the data cable is laid out next to a power cable having been laid out previously.

21. A method of monitoring a power cable comprising the steps of

measuring the operational parameters of the power cable at predetermined positions along the power cable utilizing at least one transducer that is operationally connected to the power cable at the at least one position, each of the at least one transducers providing an output signal in response to actual parameter values at the corresponding position,

transmitting the output signal or signals through a data cable positioned adjacent to and extending substantially in parallel with the power cable and being operationally connected to the at least one transducer, and

reception of the output signal or signals from the data cable at a monitoring site.