DEVICE AND METHOD FOR MEASURING THE SPACE CHARGE IN A SAMPLE OF ELECTRICAL CABLE

Abstract

This device (10) for measuring the space charge in a high-voltage DC electrical cable comprises a DC voltage source (12); a voltage step generator (14); a first electrode (16) connected to the DC voltage source (12) and the voltage step generator (14); a second electrode (18) connected to ground; a piezoelectric sensor (20) connected to the second electrode (18); an electrical signal amplifier (22) connected to the piezoelectric sensor (20);at least one sample (24) of the electrical cable placed between the first and second electrodes (16, 18) and consisting of a section of the cable made at a predetermined depth in the electrical insulation system of the cable, the amplitude of the signal measured at the output of the amplifier (22) being related to the charge density in the sample (24) and the delay related to the distance of the charges from the piezoelectric sensor (20) giving the position of the charges. Figure for the abstract: Fig. 1;

Description

Title of the invention: DEVICE AND METHOD FOR MEASURING THE SPACE CHARGE IN AN ELECTRICAL CABLE SAMPLE

[0001] The present invention relates to a device and a method for measuring the space charge in an electrical cable sample.

[0002] The invention belongs to the field of electrical cables intended for the transport of energy and / or the transmission of data. It finds application in particular in the field of analysis of the behavior of the electrical insulation of high-voltage direct current cables in the event of space charge accumulation.

[0003] In the field of HVDC cables (High Voltage Direct Current cables), validation tests are required not only under direct current power, but also to verify the behavior of the cable's electrical insulation system in the event of space charge accumulation.

[0004] Indeed, space charge accumulation is generally considered to be the main factor accelerating the degradation of the electrical insulation of a cable, in particular polymeric insulation for HVDC cables, compared to HVAC cables (High Voltage Alternating Current). This is due to the amplification of the local electric field produced by the accumulated charge, which leads to an increase in the degradation rate under direct current conditions.

[0005] Currently, these validation tests are generally carried out on real electrical insulation systems and on real full-scale cables.

[0006] For example, the pulsed electro-acoustic (PEA) method is known, which consists of applying a voltage pulse to a material placed between two electrodes. The space charge and the charges at the electrodes in response to this electrical pulse launch acoustic waves which propagate in the material. These waves are detected by a piezoelectric sensor attached to one of the electrodes. The piezoelectric sensor acts as a converter which converts the acoustic signal into an electrical signal. This electrical signal is fed into an amplifier and an oscilloscope to acquire an easily measurable signal. The amplitude of the signal measured from the amplifier is related to the charge density and the delay related to the distance of the charges from the sensor gives the position of the charges.

[0007] The thermal wave method (TSM, in English "Thermal Step Method"), which involves applying a rapid temperature variation to the cable. Thermal diffusion induces a local variation in permittivity and a local expansion of the cable. The space charge in response to these changes creates a measurable current.

[0008] These known techniques require considerable time investments and are very expensive. Furthermore, during such measurements, it is often difficult to decorrelate the space charge signal from noise coming from the cable environment.

[0009] The present invention aims to remedy the aforementioned drawbacks of the prior art.

[0010] For this purpose, the present invention provides a device for measuring the space charge in a high voltage direct current electric cable, comprising: a direct current voltage source; a voltage step generator; a first electrode connected to the direct current voltage source and to the voltage step generator; a second electrode connected to ground; a piezoelectric sensor connected to the second electrode; an electrical signal amplifier connected to the piezoelectric sensor; the device being remarkable in that it further comprises at least one sample of the electric cable placed between the first and second electrodes and consisting of a cut of the cable made at a predetermined depth in the electrical insulation system of the cable, the amplitude of the signal measured at the output of the amplifier being linked to the charge density in the at least one sample and the delay linked to the distance of the charges relative to the piezoelectric sensor giving the position of the charges in the at least one sample.

[0011] Thus, the present invention makes it possible, thanks to a simple and inexpensive electrical circuit, to carry out laboratory validation tests on cable cuts made at predetermined depths in the insulation system, more quickly and more economically than in the field.

[0012] Furthermore, knowing that the measurement is made, not on the cable in its noisy environment, but on a sample, the problem of decorrelating the space charge signal from the noise coming from the cable environment does not arise.

[0013] In a particular embodiment, the device comprises a plurality of samples of the electrical cable placed successively individually between the first and second electrodes and consisting of sections of the cable made at a plurality of predetermined depths in the electrical insulation system of the cable.

[0014] This makes it possible to take into consideration the influence of the temperature gradient and electric field in the space charge accumulation of the cable's electrical insulation system.

[0015] In a particular embodiment, the device further comprises an impedance matching resistor connected between the voltage step generator and the first electrode.

[0016] This makes it possible to optimize the transmission of the voltage signal between the voltage step generator and the first electrode.

[0017] In a particular embodiment, the device further comprises a decoupling capacitor connected between the voltage step generator and the first electrode.

[0018] This makes it possible to electrically protect the voltage step generator from the DC voltage applied in parallel to the sample.

[0019] In a particular embodiment, the device further comprises a current limiting resistor connected between the direct current voltage source and the first electrode.

[0020] This resistance provides electrical protection to the first electrode.

[0021] In a particular embodiment, the device further comprises a first layer of silicone rubber loaded with carbon black disposed between the first electrode and the at least one sample and a second layer of silicone rubber loaded with carbon black disposed between the second electrode and the at least one sample.

[0022] Thus, the electrodes have the advantage of adhering well to the sample and having good electrical conductivity over the entire measurement temperature range.

[0023] In a particular embodiment, the piezoelectric sensor is a polyvinylidene fluoride sensor.

[0024] This polymer having ferroelectric properties is in fact capable of creating an electrical signal in response to an acoustic wave.

[0025] In a particular embodiment, the at least one sample comprises crosslinked polyethylene.

[0026] Crosslinked polyethylene is an insulating substance in which the device is in fact particularly suitable for measuring space charge.

[0027] For the same purpose as that indicated above, the present invention also proposes a method for measuring the space charge in a high voltage direct current electric cable by means of a device as succinctly described above, remarkable in that it comprises steps consisting of: obtaining the at least one sample by taking a cable section at a predetermined depth of the electrical cable in the electrical insulation system of the cable and placing the at least one sample between the first and second electrodes; applying a direct current voltage to the first electrode by means of the direct current voltage source; applying a voltage pulse to the first electrode by means of the voltage step generator; by means of the piezoelectric sensor, detecting an acoustic signal, converting this acoustic signal into an electrical signal and supplying this electrical signal to the amplifier, the amplitude of the signal measured at the output of the amplifier being linked to the charge density in the at least one sample and the delay linked to the distance of the charges from the piezoelectric sensor giving the position of the charges in the at least one sample.

[0028] Since the particular characteristics and advantages of the method are similar to those of the device, they are not repeated here. Brief description of the drawings

[0029] Other aspects and advantages of the invention will appear on reading the detailed description below of particular embodiments, given as non-limiting examples, with reference to the appended drawings, in which:

[0030] [Fig-1] is a schematic representation of a device according to the present invention, in a particular embodiment.

[0031] [Fig.2] is a partial schematic representation of a sample of cable included in a device according to the present invention, in a particular embodiment.

[0032] [Fig.3] is a schematic representation of a step of taking a section of cable included in a method according to the present invention, in a particular embodiment. Description of embodiment(s)

[0033] The electrical diagram of [Fig.l] represents a device 10 according to the present invention, for measuring the space charge in a high voltage direct current electric cable, in a particular embodiment. The invention applies in particular, but not exclusively, to HVDC cables subjected to voltages of up to 800 kV.

[0034] The device 10 comprises a direct current voltage source 12, one terminal of which is connected to ground. By way of non-limiting example, the direct current voltage source 12 can deliver a direct current voltage Uo of ± 65 kV.

[0035] The device 10 also comprises a voltage step generator 14, one terminal of which is connected to ground and which delivers a voltage step Ustep.

[0036] The device 10 also comprises a first electrode 16, which is a high voltage electrode connected to the direct current voltage source 12 and to the generator 14. voltage step. The device 10 also comprises a second electrode 18 connected to ground.

[0037] By way of non-limiting example, the first and second electrodes 16, 18 may comprise aluminum.

[0038] The device 10 further comprises a piezoelectric sensor 20 connected to the second electrode 18.

[0039] By way of non-limiting example, the piezoelectric sensor 20 may be a polyvinylidene fluoride sensor and may have a thickness of 28 μm.

[0040] Optionally, the device 10 may further comprise, on the side of the piezoelectric sensor 20 opposite the second electrode 18, a layer 36 of conductive material arranged between, on the one hand, the piezoelectric sensor 20 and, on the other hand, a layer 38 of elastomer, itself placed on a support 40 connected to ground.

[0041] The device 10 further comprises an amplifier 22 of electrical signals connected to the piezoelectric sensor 20 as well as to ground.

[0042] Optionally, in order to facilitate the visualization of the measured signals, the device may further comprise an oscilloscope 42 connected to the output of the electrical signal amplifier 22.

[0043] According to the present invention, the device 10 further comprises at least one sample 24 of the electrical cable placed between the first electrode 16 and the second electrode 18. The at least one sample 24 consists of a section of the cable made at a predetermined depth in the electrical insulation system of the cable.

[0044] By way of non-limiting example, the at least one sample 24 may comprise crosslinked polyethylene.

[0045] By way of non-limiting example, the at least one sample 24 may have a length of 70 mm.

[0046] The amplitude of the signal measured at the output of the amplifier 22 is linked to the charge density in the at least one sample and the delay linked to the distance of the charges relative to the piezoelectric sensor 20 gives the position of the charges in the at least one sample. Thus, the device 10 makes it possible to measure the space charge in the at least one sample 24 of the electric cable.

[0047] The device 10 may comprise a single sample 24, or a plurality of samples 24 of the electrical cable, which will be placed successively individually between the first electrode 16 and the second electrode 18 and which each consist of a cut of the cable made at a distinct predetermined depth in the electrical insulation system of the cable.

[0048] Optionally, the device 10 may further comprise an impedance matching resistor 26 connected between the voltage step generator 14 and the first electrode 16 and also connected to ground. By way of non-limiting example, the impedance matching resistor 26 can have a value of 50 Q.

[0049] Optionally, the device 10 may further comprise a decoupling capacitor 28 connected between the voltage step generator 14 and the first electrode 16.

[0050] Optionally, the device 10 may further comprise a current limiting resistor 30 connected between the direct current voltage source 12 and the first electrode 16.

[0051] By way of non-limiting example, the current limiting resistor 30 may have a value of 100 MΩ.

[0052] Optionally, the device 10 may further comprise a first layer 32 of silicone rubber loaded with carbon black disposed between the first electrode 16 and the at least one sample 24 and a second layer 34 of silicone rubber loaded with carbon black disposed between the second electrode 18 and the at least one sample 24.

[0053] By way of non-limiting example, when the at least one sample 24 has a length of 70 mm, the layers 32 and 34 of silicone rubber loaded with carbon black can extend over a length of 20 mm.

[0054] The partial view of [Fig. 2] shows a portion of the at least one sample 24, as well as the x-axis of the depth in this sample. xcath denotes the depth of the sample 24 in contact with the cathode, i.e. the first electrode 16, possibly via the first layer 32 of silicone rubber loaded with carbon black and xan denotes the depth of the sample 24 in contact with the anode, i.e. the second electrode 18, possibly via the second layer 34 of silicone rubber loaded with carbon black.

[0055] If it is desired to measure only the space charge in the sample 24, without the charge at the first and second electrodes 16, 18, a distance d is chosen so as to measure the space charge between the depths xcath-d and xan+d. As a non-limiting example, d = 75 pm can be chosen.

[0056] As shown in [Fig. 3], the method according to the present invention for measuring the space charge in a high voltage direct current electric cable by means of the device 12, comprises a first step consisting in obtaining the at least one sample 24 by taking a cable section at a predetermined depth of the electric cable 44 in the electrical insulation system 46 of the cable. The arrow F indicates the direction of the section in a particular embodiment. The sample 24 taken is placed between the first and second electrodes 16 and 18.

[0057] The next step consists of applying the direct current voltage Uo to the first electrode 16 by means of the direct current voltage source 12.

[0058] Then the voltage pulse Ustep is applied to the first electrode 16 by means of the 14 voltage step generator.

[0059] Then, when the piezoelectric sensor 20 detects an acoustic signal, it converts this acoustic signal into an electrical signal and provides this electrical signal to the amplifier 22. The amplitude of the signal measured at the output of the amplifier 22 and possibly displayed on the oscilloscope 42 is linked to the charge density in the at least one sample 24 and the delay linked to the distance of the charges relative to the piezoelectric sensor 20 gives the position of the charges in the at least one sample 24.

[0060] Before a cable system is placed on the market, so-called prequalification tests are carried out to demonstrate the long-term performance of the cable system, as well as so-called type tests to demonstrate its good performance specific to a given use. Following the CIGRE (International Council on Large Electrical Networks) recommendations, the prequalification tests consist in particular of subjecting the cable system to electrical and thermal stress for one year and the type tests consist in particular of subjecting the cable system to electrical and thermal stress for one month.

[0061] The measurement of the space charge by means of the device 10 can be carried out before and after the prequalification test and / or the type test, in order to monitor the evolution of the space charge caused by these tests and to note the impact of the aging of the cable system. Furthermore, the space charge can be measured at different temperatures and for different values ​​of the electric field.

Claims

Claims

1. Device (10) for measuring the space charge in a high-voltage direct current electric cable, comprising: a direct current voltage source (12); a voltage step generator (14); a first electrode (16) connected to said direct current voltage source (12) and to said voltage step generator (14); a second electrode (18) connected to ground; a piezoelectric sensor (20) connected to said second electrode (18); an electrical signal amplifier (22) connected to said piezoelectric sensor (20);said device (10) being characterized in that it further comprises at least one sample (24) of said electrical cable placed between said first and second electrodes (16, 18) and consisting of a cut of said cable made at a predetermined depth in the electrical insulation system of said cable, the amplitude of the signal measured at the output of said amplifier (22) being linked to the charge density in said at least one sample (24) and the delay linked to the distance of the charges relative to said piezoelectric sensor (20) giving the position of the charges in said at least one sample (24).;

2. Device (10) according to claim 1, characterized in that it comprises a plurality of samples (24) of said electrical cable placed successively individually between said first and second electrodes (16, 18) and consisting of sections of said cable made at a plurality of predetermined depths in the electrical insulation system of said cable.

3. Device (10) according to claim 1 or 2, characterized in that it further comprises an impedance matching resistor (26) connected between said voltage step generator (14) and said first electrode (16).

4. Device (10) according to claim 1, 2 or 3, characterized in that it further comprises a decoupling capacitor (28) connected between said voltage step generator (14) and said first electrode (16).

5. Device (10) according to any one of the preceding claims, characterized in that it further comprises a current limiting resistor (30) connected between said direct current voltage source (12) and said first electrode (16).

6. Device (10) according to any one of the preceding claims, characterized in that it further comprises a first layer (32) of silicone rubber loaded with carbon black disposed between said first electrode (16) and said at least one sample (24) and a second layer (34) of silicone rubber loaded with carbon black disposed between said second electrode (18) and said at least one sample (24).

7. Device (10) according to any one of the preceding claims, characterized in that said piezoelectric sensor (20) is a polyvinylidene fluoride sensor.

8. Device (10) according to any one of the preceding claims, characterized in that said at least one sample (24) comprises crosslinked polyethylene.

9. A method of measuring the space charge in a high voltage direct current electrical cable by means of a device (10) according to any one of the preceding claims, characterized in that it comprises steps consisting in: obtaining said at least one sample (24) by taking a cable section at a predetermined depth of said electrical cable in the electrical insulation system of said cable and placing said at least one sample (24) between said first and second electrodes (16, 18); applying to said first electrode (16) a direct current voltage by means of said direct current voltage source (12); applying to said first electrode (16) a voltage pulse by means of said voltage step generator (14);by means of said piezoelectric sensor (20), detecting an acoustic signal, converting said acoustic signal into an electrical signal and supplying said electrical signal to said amplifier (22), the amplitude of the signal measured at the output of said amplifier (22) being linked to the charge density in said at least one sample (24) and the delay linked to the distance of the charges from said piezoelectric sensor (20) giving the position of the charges in said at least one sample (24).;

Images