Patent Description:
A partial discharge test is performed when testing the quality of the insulation of electric cables.

<CIT> describes how the insulation of a high voltage cable is tested during manufacturing by a test in which short high voltage pulses are used to provoke partial discharges by ionisation. A cable from an extruder travels at high speed through processing areas which include cooling. An electrode conducts to ground. A group of electrodes form a test station where test pulses from a high voltage generator are applied for detection of faults. Detected signals are applied via differential input amplifier to coupling circuit which isolates high voltages from detected signal. The signal is amplified and applied to oscilloscope and recording instruments. Faults are indicated by marking on cable or by an audible warning indicator.

One weakness of the above method is that the sensitivity of the partial discharge test is low due to the moving cable. Therefore, a high sensitivity partial discharge test is typically performed by energizing the entire length of cable over a period of time.

In some applications, high voltage cables are manufactured in cable sections and each cable section is tested individually by a high sensitivity partial discharge test. Then, the cable sections are joined to form a desired length of cable. The cable sections may be joined by means of so-called factory joints, where the purpose of the factory joint is to maintain electrical and mechanical properties, while avoiding considerable increase in the outer diameter of the joint relative to the outer diameter of the cable sections. The reason for the use of the factory joint is to be able to spool the joined high voltage cable onto a larger turntable before transportation to the installation site.

Alternatively, the respective cable sections are transported to the installation site individually and then joined at the installation site, typically by using larger joints. However, this alternative is not a viable option for some applications, in particular for sea cables, where the operation of joining cables must be performed on a ship, which is costly, and which is vulnerable to abruptions caused by unfavourable weather conditions.

The insulation of a factory joint is typically tested together with the testing of the insulation of the cable sections, i.e. and entire length of joined cable (comprising cable sections and the factory joints) is tested in one operation. Today, it is considered possible to perform a partial discharge test of a <NUM> kV rated joined cable having a length up to <NUM>. However, it is not considered possible to perform a partial discharge test of longer lengths of joined cables at this voltage level and/or to perform a partial discharge test at higher voltage levels at this length. One reason is that it is not possible to energize the entire length of joined high voltage cable.

Documents <CIT>, <CIT>, <CIT> and <CIT> are also directed to methods for testing a cable comprising a cable joint.

Hence, one object of the present invention is to provide a system and method for performing a partial discharge test on longer lengths of joined cables and/or for performing a partial discharge test on higher voltage levels.

The present invention relates to a system for performing a partial discharge test of a factory joint joining a first high voltage cable section and a second high voltage cable section;.

In one aspect, the first cable section comprises a proximal end proximal to the factory joint and a distal end and the second cable section comprises a proximal end proximal to the factory joint and a distal end.

The joint conductor is joining the proximal end of the first conductor and the proximal end of the second conductor. The joint semiconductive sheath is joining the proximal end of the first semiconductive sheath and the proximal end of the second semiconductive sheath.

In one aspect, the third earthing conductor is grounding the first conductor at its distal end and/or is grounding the second conductor at its distal end. By grounding either the first conductor at its distal end or grounding the second conductor at its distal end, the entire conductor, i.e. the first conductor, the joint conductor and the second conductor is considered to be grounded.

In one aspect, the joint side part of the first semiconductive sheath is provided at a distance from the remaining part of the first semiconductive sheath by removal of the first semiconductive sheath from a section of the first cable between the joint side part of the first semiconductive sheath and the remaining part of the first semiconductive sheath.

In one aspect, the section of the first cable from which the first semiconducting sheath has been removed is provided in the proximal end of the first cable section.

In one aspect, the first insulation system comprises:.

In one aspect, the joint side part of the second semiconductive sheath is provided at a distance from the remaining part of the second semiconductive sheath by removal of the second semiconductive sheath from a section of the second cable between the joint side part of the second semiconductive sheath and the remaining part of the second semiconductive sheath.

In one aspect, the section of the second cable from which the second semiconducting sheath has been removed is provided in the proximal end of the second cable section.

In one aspect, the second insulation system comprises:.

In one aspect, the third earthing conductor is grounding the first conductor at its distal end; and wherein the system comprises a fourth earthing conductor for grounding the second conductor at its distal end.

In one aspect, the partial discharge sensing system comprises a balanced partial discharge detector and an analysing system provided in communication with the balanced partial discharge detector by means of a fibre optic cable.

Alternatively, the cable sections, the high voltage transformer and the partial discharge sensing system may be located within a Faraday cage, and the partial discharge detector may be an unbalanced partial discharge detector.

In one aspect, the high voltage transformer comprises a <NUM>/<NUM> transformer, a variable inductance series resonance high voltage generator or a variable frequency series resonance high voltage generator.

In one aspect, the first cable section is at least partially spooled onto a first drum and wherein the second cable section is at least partially spooled onto a second drum.

The present invention also relates to a method for performing a test of a joined cable, wherein the method comprises:.

In one aspect, the step of testing the first cable section comprises:.

In one aspect, the step of testing the second cable section comprises:.

In one aspect, as the high voltage transformer is supplying the test voltage to the semiconducting sheath of the factory joint, the test voltage will also be supplied to the joint side part of the first semiconductive sheath and to the joint side part of the second semiconductive sheath, due to the conducting properties of the semiconducting sheaths.

The joined cable is defined with a longitudinal direction. The term "joint side part" is here used to refer to a part which is relatively closer to the joint than other parts. The "joint side part" will be closer to the joint in the longitudinal direction of the cable than such other parts.

The term "factory joint" is here used to describe a joint used to join two cable sections, wherein the joined cable, i.e. the two cable sections together with the joint, has substantially the same mechanical/electrical properties as the respective cable sections. In particular, the outer diameter of the factory joint is equal to, or only slightly larger than, the outer diameter of the two cable sections. Consequently, the joined cable can be handled as a single cable without any consideration related to the location of the joint. According to the CIGRE publication "Recommendations for Testing of Long AC Submarine Cables with Extruded Insulation for System Voltage above <NUM> (<NUM>) to <NUM> (<NUM>) kV" TB <NUM> chapter <NUM>, a factory joint is manufactured prior to the armouring operation, so that the section of cable containing the joint is continuously armoured without any discontinuity or appreciable distortion of the armour wires in the vicinity of the joint. One main feature of a factory joint is that it shall not impose any restrictions on further cable handling or installation operations, nor imply a variation in the cable mechanical and electrical performance. This generally implies that the factory joints are fully flexible, with same bending radius, pulling force limit and coiling performance with respect to the original cable.

Moreover, the above CIGRE publication states that "The defined length of a factory joint is the removed length of the metal sheath/outer semi-conducting screen plus <NUM> metre on each side of that length (. For three-core cables, the length of the factory joint is defined from the beginning of first core joint to the end of the last core joint, plus <NUM> metre in each end of the core joints.

The opposite of a factory joint is typically referred to as an "on site joint", which is used to join two cable sections on the installation site. Such joints typically have a size considerably larger than the outer diameter of the cable sections, and the joined cable cannot be handled as one single cable.

The term "partial discharge test" is here used to describe a test used to test the properties, in particular the electrical insulation properties, of the insulation layer between the conductor and the semiconductive sheath.

The term "high voltage" is here used for nominal voltages above <NUM> kV. As an example, the term "high voltage" may be in the range <NUM> kV - <NUM> kV, in the range <NUM> kV - <NUM> kV or in the range <NUM> kV and up to even higher voltages.

In one aspect, the test voltage is a test voltage according to CIGRE TB <NUM> cl <NUM> and IEC <NUM> & <NUM> cl <NUM>. Hence, one test criteria indicating a successful test is that measured partial discharges above a predetermined amount of picoColoumbs in the factory joint is less than a predetermined number during the test period.

According to the invention above, it is achieved a system and method where the factory joint can be tested with high sensitivity. The term "high sensitivity" here refers to being able to measure partial discharges of <NUM> - <NUM> picoColoumbs.

According to the above, by first testing each cable section individually, and then by testing each factory joint separately, the entire joined high voltage cable can be considered tested.

Embodiments of the invention will now be described in detail with reference to the enclosed drawings, wherein:.

<FIG> shows a cross section of a first high voltage cable <NUM> which in the present example is joined to a second high voltage cable <NUM> identical to the first high voltage cable <NUM> by means of a factory joint <NUM>. As the first high voltage cable <NUM> is identical to the second high voltage cable <NUM>, <FIG> is considered to illustrate the cross section of both such cables. In the present example, the cable sections are <NUM> kV XLPE (cross-linked polyethylene) cable sections.

The high voltage cable <NUM>, <NUM> comprises a central conductor 2C, 4C and a semiconducting sheath <NUM>, <NUM> separated from the central conductor 2C, 4C by means of an insulation layer 2I, 4I. Outside of the semiconducting sheath <NUM>, <NUM> further insulation layers and protective layers may be provided. These outer layers are indicated as 2out, 4out.

It should further be noted that some high voltage cables comprise an inner semiconductive sheath (not shown in <FIG>) outside of the central conductor 2C, 4C but inside of the insulation layer <NUM>, 4I. Hence, the semiconducting sheath <NUM>, <NUM> outside of the insulation layer <NUM>, 4I are in some cases referred to as the outer semiconductive sheath.

<FIG> shows the factory joint <NUM>. The factory joint <NUM> comprises a joint conductor 1C joining the first conductor 2C with the second conductor 4C, a joint semiconductor sheath <NUM> joining the first semiconductor sheath <NUM> with the second semiconductor sheath <NUM> and an insulation layer 1I joining the insulation layers 2I, 4I. Outside of the semiconducting sheath <NUM> further insulation layers and protective layers lout may be provided.

It should be noted that <FIG> and the description of these drawings above are schematically only - factory joints <NUM> may have various designs depending on the type of cable, the electrical and mechanical requirements for the joined cable etc. Factory joints may also be used to join different high voltage cables of different types.

According to the factory joint definition above, stating that the defined length of a factory joint is the removed length of the metal sheath/outer semi-conducting screen plus <NUM> metre on each side of that length, there is a clear boundary at which the cables <NUM>, <NUM> end and at which the joint <NUM> starts. The boundary between the first cable <NUM> and the joint <NUM> is indicated by a dashed line B1 (indicated at the end of the leftmost <NUM> curly bracket) and the boundary between the second cable <NUM> and the joint <NUM> is indicated as a dashed line B2 (indicated at the end of the rightmost <NUM> curly bracket) in <FIG>. These boundaries B1, B2 are also indicated in <FIG>, <FIG>.

It is now referred to <FIG>. Here it is shown a first cable drum <NUM>, typically in the form of a basket, on which the first cable section <NUM> is spooled onto. One end of the first cable section <NUM> is referred to as a proximal end 2a as it is proximal to the factory joint <NUM>. The opposite end is hence referred to as a distal end 2b. It is also shown a second cable drum <NUM>, typically in the form of a turntable, on which the second cable section <NUM> is spooled onto. One end of the second cable section <NUM> is referred to as a proximal end 4a as it is proximal to the factory joint <NUM>. The opposite end is hence referred to as a distal end 4b.

It should be noted that in the present embodiment, the first cable section <NUM> is one length of manufactured cable (which typically may have a length of more than a few kilometers for the <NUM> kV XLPE (cross-linked polyethylene) type of cable, while the second cable section <NUM> may comprise several such joined lengths of manufactured cable. Hence, the present invention is at least partially integrated with the joining process and the spooling process. According to the above CIGRE publication, a "manufactured length" is defined as a complete extrusion run or a part thereof, which normally does not contain any factory joints.

As shown in <FIG>, the proximal end 2a of the first cable section <NUM>, the joint <NUM> and the proximal end 4a of the second cable section <NUM> are provided within the dashed box <NUM>, indicating a test system for performing a partial discharge test of the factory joint <NUM>.

Initially, it should be mentioned that in <FIG>, the joint <NUM> is shown with a thicker line than the respective cable sections, which may be interpreted as the joint <NUM> having a larger cross section than the respective cables. However, as discussed above, the factory joint <NUM> preferably has the same outer diameter as the respective cable sections <NUM>, <NUM>. A longitudinally direction LD of the joint, the proximal end 2a of the first cable section <NUM> and the proximal end 4a of the second cable section <NUM> has also been indicated.

The system <NUM> comprises a first insulating system <NUM>, a second insulating system <NUM>, a high voltage transformer <NUM> and a partial discharge sensing system <NUM>. These parts of the system <NUM> will be described in detail below.

The first insulating system <NUM> is shown in <FIG>, and comprises a non-metallic housing <NUM> in which a compartment <NUM> is provided. The housing <NUM> may for example be made of an epoxy pipe. The non-metallic housing <NUM> has end openings 41a, 41b in each end. The first insulating system <NUM> further comprises a fluid-circulating system <NUM> for circulating de-ionized water through the compartment <NUM> during the test. The housing <NUM> may again be located within a gas insulated enclosure <NUM>, filled with SF6 or an equivalent electrically insulating gas.

It is also shown in <FIG> that the first semiconducting sheath <NUM> (together with layers/sheaths outside of the first semiconducting sheath <NUM>) has been removed from a section 2AS of the first cable section <NUM>. This section 2AS, from which the first semiconducting sheath <NUM> has been removed, is provided in the compartment <NUM>. Hence, a joint side part 2SJ of the first semiconductive sheath <NUM> has been electrically insulated from a remaining part 2SR of the first semiconductive sheath <NUM>. The length of the section 2AS may be approximately <NUM> for the above <NUM> kV cable.

It should be noted that the joint side part 2SJ of the first semiconducting sheath <NUM> is sealingly engaged in the end opening 41a of the non-metallic housing <NUM>, while the remaining part 2SR of the first semiconducting sheath <NUM> is sealingly engaged in the end opening 41b of the non-metallic housing <NUM>, to prevent leakage of de-ionized water from the compartment <NUM>.

It should further be noted that the joint side part 2SJ of the first cable section <NUM> is relatively short, as the distance between the first insulating system <NUM> and the first boundary B1 is relatively short, typically less than <NUM> meters.

The second insulating system <NUM> is shown in <FIG>, and is in the present embodiment similar to the first insulating system <NUM>. The second insulating system50 comprises a non-metallic housing <NUM> in which a compartment <NUM> is provided. The non-metallic housing <NUM> has end openings 51a, 51b in each end. The second insulating system <NUM> further comprises a fluid-circulating system <NUM> for circulating de-ionized water through the compartment <NUM> during the test. The housing <NUM> may again be located within a gas insulated enclosure <NUM>, filled with SF6 or an equivalent electrically insulating gas.

It is also shown in <FIG> that the second semiconducting sheath <NUM> (together with layers/sheaths outside of the second semiconducting sheath <NUM>) has been removed from a section 4AS of the second cable section <NUM>. This section 4AS, from which the second semiconducting sheath <NUM> has been removed, is provided in the compartment <NUM>. Hence, a joint side part 4SJ of the second semiconductive sheath <NUM> has been electrically insulated from a remaining part 4SR of the second semiconductive sheath <NUM>.

It should be noted that the joint side part 4SJ of the second semiconducting sheath <NUM> is sealingly engaged in the end opening51a of the non-metallic housing <NUM>, while the remaining part 4SR of the second semiconducting sheath <NUM> is sealingly engaged in the end opening 51b of the non-metallic housing <NUM>, to prevent leakage of de-ionized water from the compartment <NUM>.

The high voltage transformer <NUM> may be a <NUM>/<NUM> transformer, a variable inductance series resonance high voltage generator or a variable frequency series resonance high voltage generator. These types of high voltage transformers are known for a person skilled in the art, and will not be described further in detail herein.

The partial discharge sensing system <NUM> is a system for sensing partial discharges in the factory joint <NUM> during the test period. The partial discharge sensing system <NUM> comprises a balanced partial discharge detector <NUM> and an analyzing system <NUM> provided in communication with the balanced partial discharge detector <NUM> by means of a fiber optic cable <NUM>. The analyzing system <NUM> is typically an application running on a computer. One commercially available partial discharge sensing system <NUM> is the Omicron MBB1 measurement balanced bridge used together with the Omicron MPD600 measurement and analysis system, connected to a computer via a fiber optic bus controller, such as the Omicron MCU <NUM>.

The operation of the test system <NUM> will now be described.

Initially, it should be noted that the system <NUM> comprises a first earthing conductor 12a for grounding the remaining part 2SR of the first semiconductive sheath <NUM>. Hence, almost the entire semiconductive sheath <NUM> is connected to ground, except for the relatively short joint side part 2SJ of the semiconductive sheath <NUM>.

Similarly, the system <NUM> comprises a second earthing conductor 14a for grounding the remaining part 4SR of the second semiconductive sheath <NUM>. Again, almost the entire semiconductive sheath <NUM> is connected to ground, except for the relatively short joint side part 4SJ of the semiconductive sheath <NUM>.

Finally, the system <NUM> comprises a third earthing conductor 12b for grounding the first conductor 2C. This will typically connect the first conductor 2c, the joint conductor 1C and the second conductor 4C to ground. However, the system <NUM> may also comprise a fourth earthing conductor 14b for grounding the second conductor 4C at its distal end 4b.

Preferably, the first cable section <NUM> and the second cable section <NUM> have been tested separately before the joining process. If possible, the first cable section <NUM> and the second cable section <NUM> have been tested by means of a partial discharge test. This is today only possible for <NUM> kV XLPE cables shorter than <NUM>. Hence, for longer cables, a high voltage withstand test is considered sufficient in order to test the insulation layers 2I, 4I.

Then the joining operation is performed, in which the proximal end 2a of the first cable section is joined to the proximal end 4a of the second cable section <NUM>.

Then, the joint <NUM> is moved to the location of the partial discharge test, wherein the a joint side part 2SJ of the first semiconductive sheath <NUM> is electrically isolated from the remaining part 2SR of the first semiconductive sheath <NUM> by removing the semiconductive sheath of the section 2AS and by enclosing the section 2AS in the first insulating system <NUM> as described above.

Similarly, the joint side part 4SJ of the second semiconductive sheath <NUM> is electrically isolated from the remaining part 4SR of the second semiconductive sheath <NUM> by removing the semiconductive sheath of the section 4AS and by enclosing the section 4AS in the second insulating system <NUM> as described above.

Now, a test voltage is applied to the semiconducting sheath <NUM> of the factory joint <NUM> by means of the high voltage transformer <NUM> during a test period. Due to the conducting properties of the semiconducting sheaths, the test voltage will also be applied to the joint side part 2SJ of the first semiconductive sheath <NUM> and to the joint side part 4SJ of the second semiconductive sheath <NUM>. However, as the conductors 1C, 2C and 4C are grounded, and as the remaining parts 2SR, 4SR of the semiconductive sheaths <NUM>, <NUM> are grounded, partial discharges will only occur in the joint insulation layer 1I and possibly also in the insulation layers of the relatively short joint side parts 2SJ, 4SJ.

Any such partial discharges will be sensed by means of the partial discharge sensing system <NUM> during the test period.

If the test of the joint is satisfying, the entire joined cable comprising the first cable section, the joint and the second cable section is considered to be tested sufficiently.

Then, the sections 2AS, 4AS are moved out from the insulation systems <NUM>, <NUM> and semiconductive sheaths are applied outside of the insulation layer again. Hence, there is one continuous layer of semiconductive sheath from the distal end 2b of the first cable section <NUM> to the distal end 4b of the second cable section <NUM> again. Then, further outer layers are applied to the sections 2AS, 4AS, and the entire joined cable is considered to be complete again.

In a final step, the first cable section <NUM> is spooled out from the basket <NUM> and onto the turntable <NUM>.

It is now referred to <FIG>. Here, a third cable section <NUM> on a basket 3a is joined to the previously joined cable (as indicated in <FIG> to comprise the first cable section <NUM>, the joint <NUM> and the second cable section <NUM>) by means of a further factory joint 1b. Also this third cable section <NUM> has been tested before the joining operation. This further factory joint 1b is also tested by the above partial discharge test before the third cable section <NUM> and the further joint 1b is spooled onto the turntable <NUM>. In this way, the further cable sections can be joined to the joined cable and each joint can be tested separately.

As discussed in the introduction above, the first cable and the second cable may be different from each other. Such different cables may still be joined by a factory joint and may be tested by the above method and system.

Claim 1:
System (<NUM>) for performing a partial discharge test of a factory joint (<NUM>) joining a first high voltage cable section (<NUM>) and a second high voltage cable section (<NUM>);
wherein the first cable section (<NUM>) comprises a first conductor (2C) and a first semiconductive sheath (<NUM>);
wherein the second cable section (<NUM>) comprises a second conductor (4C) and a second semiconductive sheath (<NUM>);
wherein the factory joint (<NUM>) comprises a joint conductor (1C) joining the first conductor (2C) and the second conductor (4C) and a joint semiconductive sheath (<NUM>) joining the first semiconductive sheath (<NUM>) and the second semiconductive sheath (<NUM>);
wherein the system (<NUM>) comprises:
- a first insulation system (<NUM>) for electrically insulating a joint side part (2SJ) of the first semiconductive sheath (<NUM>) from a remaining part (2SR) of the first semiconductive sheath (<NUM>);
- a second insulation system (<NUM>) for electrically insulating a joint side part (4SJ) of the second semiconductive sheath (<NUM>) from a remaining part (4SR) of the second semiconductive sheath (<NUM>);
- a first earthing conductor (12a) for grounding the remaining part (2SR) of the first semiconductive sheath (<NUM>);
- a second earthing conductor (14a) for grounding the remaining part (4SR) of the second semiconductive sheath (<NUM>);
- a third earthing conductor (12b, 14b) for grounding the first conductor (2C) and/or the second conductor (4C);
- a high voltage transformer (<NUM>) for supplying a test voltage to the semiconducting sheath (<NUM>) of the factory joint (<NUM>) during a test period;
- a partial discharge sensing system (<NUM>) for sensing partial discharges in the factory joint (<NUM>) during the test period.