US-12625194-B2 - Device and method for measuring space charge in an electric cable specimen

Abstract

A device ( 10 ) for measuring space charge in a high-voltage direct current electric cable includes a DC voltage source ( 12 ), a voltage step generator ( 14 ), a first electrode ( 16 ) connected to the DC voltage source ( 12 ) and to the voltage step generator ( 14 ), a second electrode ( 18 ) that is grounded, a piezoelectric sensor ( 20 ) connected to the second electrode ( 18 ), an electrical-signal amplifier ( 22 ) connected to the piezoelectric sensor ( 20 ), and at least one specimen ( 24 ) of the electric cable placed between the first and second electrodes ( 16, 18 ) and including a cut of the cable made to a predetermined depth in the electrical insulation system of the cable. The amplitude of the signal measured at the output of the amplifier ( 22 ) is related to the charge density in the specimen ( 24 ) and the delay related to the distance of the charges from the piezoelectric sensor ( 20 ) giving the position of the charges.

Inventors

• Raphaël GUFFOND
• Baptiste MENEGUZZO

Assignees

• NEXANS

Dates

Publication Date

20260512

Application Date

20231030

Priority Date

20221102

Claims (8)

1. 1 . A device for measuring space charge in a high-voltage direct current electric cable, comprising: a DC voltage source; a voltage step generator; a first electrode connected to said DC voltage source and to said voltage step generator; a second electrode that is grounded; a piezoelectric sensor connected to said second electrode; and an electrical-signal amplifier connected to said piezoelectric sensor; wherein said device further comprises a plurality of specimens of said electric cable that are placed successively individually between said first and second electrodes and including cuts of said cable made to a plurality of predetermined depths in an electrical insulation system of said cable, an amplitude of the signal measured at the output of said amplifier being related to a charge density in said at least one specimen and a delay related to a distance of charges from said piezoelectric sensor giving the position of the charges in said at least one specimen.
2. 2 . The device according to claim 1 , wherein said device further comprises an impedance-matching resistor connected between said voltage step generator and said first electrode.
3. 3 . The device according to claim 1 , wherein said device further comprises a decoupling capacitor connected between said voltage step generator and said first electrode.
4. 4 . The device according to claim 1 , wherein said device further comprises a current-limiting resistor connected between said DC voltage source and said first electrode.
5. 5 . The device according to claim 1 , wherein said device further comprises a first carbon black-filled silicone rubber layer arranged between said first electrode and said at least one specimen and a second carbon black-filled silicone rubber layer arranged between said second electrode and said at least one specimen.
6. 6 . The device according to claim 1 , wherein said piezoelectric sensor is a polyvinylidene fluoride sensor.
7. 7 . The device according to claim 1 , wherein said at least one specimen comprises crosslinked polyethylene.
8. 8 . A method for measuring space charge in a high-voltage direct current electric cable by way of a device according to claim 1 , said method comprises the following steps: obtaining said plurality of specimens by removing cuts of the cable to a plurality of predetermined depths of said electric cable in an electrical insulation system of said cable and successively and individually placing said plurality of specimens between said first and second electrodes; applying a DC voltage to said first electrode by way of said DC voltage source; applying a voltage pulse to said first electrode by way of said voltage step generator; by way of said piezoelectric sensor, detecting an acoustic signal, converting said acoustic signal into an electrical signal and supplying said electrical signal to said amplifier, an amplitude of the signal measured at the output of said amplifier being related to a charge density in said plurality of specimens and a delay related to a distance of charges from said piezoelectric sensor giving the position of the charges in said plurality of specimens.

Description

RELATED APPLICATION This application claims the benefit of priority from French Patent Application No. 22 11410, filed on Nov. 2, 2022, the entirety of which is incorporated by reference. FIELD OF THE INVENTION The present invention relates to a device and a method for measuring space charge in an electric cable specimen. The invention belongs to the field of electric cables intended to transport energy and/or to transmit data. It is particularly applicable to the field of analysing the behaviour of the electrical insulation of high-voltage direct current cables in the event of space charge accumulation. DESCRIPTION OF THE RELATED ART In the field of HVDC (high-voltage direct current) cables, validation tests are required not only under DC power, but also to check the behaviour of the electrical insulation system of the cable in the event of space charge accumulation. Indeed, space charge accumulation is generally considered to be the main factor in speeding up the degradation of the electrical insulation of a cable, in particular the polymeric insulation for HVDC cables, compared to HVAC (high-voltage alternating current) cables. This is due to the amplification of the local electric field produced by the accumulated charge, which leads to an increase in the rate of degradation under direct current conditions. At present, these validation tests are generally carried out on real electrical insulation systems and on real life-size cables. For example, the pulsed electro-acoustic (PEA) method is known, which consists in 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 that propagate through the material. These waves are detected by a piezoelectric sensor attached to one of the electrodes. The piezoelectric sensor acts as a converter that converts the acoustic signal into an electrical signal. This electrical signal is fed into an amplifier and an oscilloscope in order 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. The thermal step method (TSM) is also known, which consists in applying a rapid temperature variation to the cable. Thermal diffusion leads to a local variation in permittivity and a local expansion of the cable. The space charge in response to these changes creates a measurable current. These known techniques require considerable time investments and are highly costly. Furthermore, during such measurements, it is often difficult to decorrelate the space charge signal from the noise coming from the environment of the cables. OBJECTS AND SUMMARY The present invention aims to rectify the abovementioned drawbacks of the prior art. To this end, the present invention proposes a device for measuring space charge in a high-voltage direct current electric cable, comprising: a DC voltage source;a voltage step generator;a first electrode connected to the DC voltage source and to the voltage step generator;a second electrode that is grounded;a piezoelectric sensor connected to the second electrode;an electrical-signal amplifier connected to the piezoelectric sensor;the device being noteworthy in that it furthermore comprises at least one specimen of the electric cable placed between the first and second electrodes and consisting of a cut of the cable made to a predetermined depth in the electrical insulation system of the cable, the amplitude of the signal measured at the output of the amplifier being related to the charge density in the at least one specimen and the delay related to the distance of the charges from the piezoelectric sensor giving the position of the charges in the at least one specimen. Thus, by virtue of a simple and inexpensive electrical circuit, the present invention makes it possible to carry out laboratory validation tests on cable cuts made to predetermined depths in the insulation system, more quickly and more economically than in the field. Moreover, knowing that the measurement is carried out not on the cable in its noisy environment but on a specimen, the problem of decorrelating the space charge signal from the noise coming from the environment of the cables does not arise. In one particular embodiment, the device comprises a plurality of specimens of the electric cable that are placed successively individually between the first and second electrodes and consisting of cuts of the cable that are made to a plurality of predetermined depths in the electrical insulation system of the cable. This makes it possible to take into consideration the influence of the temperature gradient and electric field gradient in the space charge accumulation of the electrical insulation system of the cable. In one particular embodiment, the device furthermore comprises an impedance-matching