DEVICE AND METHOD FOR APPLYING AND MEASURING A TRANSIENT OVERVOLTAGE IN AN ELECTRICAL CABLE

Abstract

This device (10) for applying and measuring a transient overvoltage in an electrical cable comprises a test sample representative of the electrical cable, a first DC voltage generator (DC1) adapted to apply a predetermined DC voltage to the sample, and a second DC voltage generator (DC2) adapted to apply a predetermined overvoltage to the sample. It further comprises a high-voltage switch (12) connected to the second DC voltage generator (DC2) and adapted to convert the predetermined overvoltage into a square wave voltage, a control resistor (R2) adapted to control the voltage rise time of the sample, and a decoupling capacitor (C1) adapted to control the overvoltage decay time in the sample. Figure for the abstract: Fig. 1

Description

Title of the invention: DEVICE AND METHOD FOR APPLYING AND MEASURING A TRANSIENT OVERVOLTAGE IN AN ELECTRIC CABLE

[0001] The present invention relates to a device and a method for applying and measuring a transient overvoltage in an electrical cable.

[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 a transient overvoltage (TOV).

[0003] In the field of HVDC cables (High Voltage Direct Current cables), validation tests are required not only under direct current power supply, but also to verify the behavior of the electrical insulation system of the cable in the event of the occurrence of non-conventional waveforms, such as temporary overvoltages with a very slow rising edge.

[0004] Currently, these validation tests are usually performed on real insulation systems and on real cables. The process is time-consuming and expensive.

[0005] It is necessary to be able to carry out these tests in the laboratory, on samples of insulating plates and / or cables of reduced dimension representative of the insulation systems and real cables, in order to study more quickly and at lower cost the electrical properties of the insulation system when it undergoes such an unusual waveform.

[0006] The present invention aims to remedy the aforementioned drawbacks of the prior art.

[0007] For this purpose, the present invention proposes a device for applying and measuring a transient overvoltage in an electric cable, comprising: a first direct voltage generator adapted to generate a predetermined direct voltage; and a second direct voltage generator adapted to generate a predetermined overvoltage, the device being remarkable in that it also includes: a representative test sample of the electrical cable; in that the first DC voltage generator is adapted to apply the predetermined DC voltage to the test sample; in that the second direct voltage generator is adapted to apply the predetermined overvoltage to the test sample; and in that it further comprises: a high voltage switch connected to the second direct voltage generator and adapted to convert the predetermined overvoltage into a square wave voltage; a control resistor adapted to control the voltage rise time of the test sample; and a decoupling capacitance suitable for controlling the overvoltage decay time in the test sample.

[0008] Thus, the present invention makes it possible, thanks to a simple and inexpensive electrical circuit, to carry out validation tests in the laboratory on samples of insulation systems and / or on mini-cables, more quickly and more economically than in the field.

[0009] In a particular embodiment, the device further comprises at least one protective resistor.

[0010] This makes it possible not to damage the test sample during the study of its behavior when the overvoltage occurs.

[0011] In a particular embodiment, the high voltage switch comprises at least one high voltage fast transistor.

[0012] This allows a surge voltage to be quickly created and, in combination with the control resistor and the decoupling capacitance, to easily adjust the characteristics of the waveform associated with the surge voltage for the purposes of the sample validation test.

[0013] In a particular embodiment, the test sample is an electrically insulating plate having a thickness of between 500 μm and 1.5 mm.

[0014] This makes it possible to reconstruct at low cost, on the reduced scale of the test laboratory, the electrical properties of an insulation system equivalent to those of a real electrical insulation system of a cable.

[0015] Alternatively, the test sample is a high voltage DC mini-cable having 1.5 mm thick insulation.

[0016] This thickness of 1.5 mm is to be compared to a thickness of 20 mm of insulation for a real cable, which shows the saving made due to the use of a sample and not a real cable.

[0017] For the same purpose as that indicated above, the present invention also proposes a method for measuring a transient overvoltage in an electric cable by means of a device as succinctly described above, remarkable in that it comprises steps consisting of: applying the predetermined direct voltage to the test sample by means of the first direct voltage generator; applying to the test sample the predetermined overvoltage, in the form of the pulse voltage, by means of the second direct voltage generator and the high voltage switch; control the voltage rise time by means of the control resistor; control the overvoltage decay time by means of the decoupling capacitor.

[0018] In a particular embodiment, the overvoltage is equal to 0.7 Uo, where Uo denotes the predetermined direct voltage.

[0019] The particular characteristics and advantages of the method being similar to those of the device, they are not repeated here. Brief description of the drawings

[0020] 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:

[0021] [Fig-1] is a schematic representation of a device according to the present invention, in a particular embodiment.

[0022] [Fig.2] is a graph representing the overvoltage level as a function of time in a test sample included in a device according to the present invention, in a particular embodiment. Description of embodiment(s)

[0023] The electrical diagram of [Fig.l] represents a device 10 according to the present invention, for measuring a transient overvoltage in an 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.

[0024] The device 10 comprises a test sample, illustrated in dashed lines on the drawing and representative of the electric cable.

[0025] By way of non-limiting example, this test sample may be an electrically insulating plate having a thickness of between 500 μm and 1.5 mm, or a high-voltage direct current mini-cable having an insulator of length 1.5 mm.

[0026] The device 10 also comprises a first DC voltage generator DCi, adapted to generate a predetermined DC voltage Uo. One terminal of the first DC voltage generator DCi is connected to ground and its other terminal is connected to the test sample, which is also connected to ground. Thus, the first DC voltage generator DCi is adapted to apply the voltage Uo to the test sample. adapted to generate a predetermined overvoltage. In the particular embodiment of [Fig.l], this overvoltage is 0.7Uo. One terminal of the second DC2 direct voltage generator is connected to ground and its other terminal is connected to the test sample. Thus, the second DC2 direct voltage generator is adapted to apply the overvoltage 0.7Uo to the test sample.

[0028] In order for the overvoltage to be applied only temporarily, the device 10 further comprises a high-voltage switch 12. One terminal of the high-voltage switch 12 is connected to ground and its other terminal is connected to the second direct voltage generator DC2. The high-voltage switch 12 is adapted to convert the overvoltage into a voltage whose waveform is a square wave, for application to the test sample.

[0029] As a non-limiting example, the high voltage switch 12 can be closed for 130 ms, that is to say that during this time interval, the + and - terminals of the high voltage switch 12 are connected together. The overvoltage 0.7Uo is then applied to the test sample for 130 ms.

[0030] By way of non-limiting example, the high-voltage switch 12 may comprise at least one high-voltage fast transistor. In the particular embodiment illustrated, the high-voltage switch 12 is a high-voltage fast transistor.

[0031] To control the voltage rise time of the test sample when the overvoltage is applied to it, i.e. to measure this voltage rise time, the device 10 further comprises a control resistor R2, one terminal of which is connected to the second direct voltage generator DC2 and to the high voltage switch 12 and the other terminal is connected to the test sample and to ground.

[0032] Furthermore, to control the overvoltage decay time in the test sample, i.e. to measure this time when the overvoltage is no longer applied, the device 10 further comprises a decoupling capacitor C1, one terminal of which is connected to the second direct voltage generator DC2 and to the high voltage switch 12 and the other terminal is connected to the test sample and to ground.

[0033] Optionally, the device 10 may further comprise one or more protective resistors. In the particular embodiment illustrated, the device 10 comprises a first protective resistor RI connected between the second DC voltage generator DC2 and the high voltage switch 12 and a second protective resistor R3, one terminal of which is connected to the first DC voltage generator DCi and the other terminal of which is connected between, on the one hand, the test sample and, on the other hand, the second DC voltage generator DC2 and the high voltage switch 12. overvoltage as a function of time t.

[0035] The voltage at the start of the test is Uo. Then when the overvoltage O.7Uo is applied to the test sample, the voltage reaches a peak Ui whose value is therefore 1.7U0.

[0036] The control resistor R2 makes it possible to measure the time interval Tl between the start of the voltage rise and the voltage peak Up. Typically, it is found that Tl is between 1 and 3 ms.

[0037] The decoupling capacitance Cl makes it possible to measure the time interval T2 during which the voltage exceeds 90% of its maximum value (i.e. 0.9.(UrUo), as indicated on the graph). Typically, it is found that T2 is between 10 and 150 ms.

[0038] Thus, the method according to the present invention, for measuring a transient overvoltage of an electric cable by means of the device 10, comprises the following steps.

[0039] The first DC voltage generator DCi applies the predetermined DC voltage Uo to the test sample.

[0040] Then the second DC2 direct voltage generator applies to the test sample the predetermined overvoltage, 0.7Uo in the non-limiting example given here, in the form of a pulse provided by the high voltage switch 12.

[0041] The control resistor R2 then measures the time T1 of voltage rise of the test sample and the decoupling capacitor Cl then measures the time T2 of overvoltage decay.

[0042] Thus, the device 10 and the method according to the invention make it possible to precisely, quickly and inexpensively control the waveform associated with the applied overvoltage, in particular the voltage rise time T1 and the overvoltage decay time T2.

Claims

Claims

1. Device (10) for applying and measuring a transient overvoltage in an electric cable, comprising: a first direct voltage generator (DCi) adapted to generate a predetermined direct voltage (Uo); and a second direct voltage generator (DC2) adapted to generate a predetermined overvoltage, said device (10) being characterized in that it further comprises: a test sample representative of said electric cable; in that said first direct voltage generator (DCi) is adapted to apply said predetermined direct voltage (Uo) to said test sample; in that said second direct voltage generator (DC2) is adapted to apply said predetermined overvoltage to said test sample; and in that it further comprises: a high-voltage switch (12) connected to said second direct voltage generator (DC2) and adapted to convert said predetermined overvoltage into a square-wave voltage;a control resistor (R2) adapted to control the time (Tl) of voltage rise of said test sample; and a decoupling capacitor (Cl) adapted to control the time (T2) of overvoltage decay in said test sample.;

2. Device (10) according to claim 1, characterized in that it further comprises at least one protective resistor (RI, R3).

3. Device (10) according to claim 1 or 2, characterized in that said high voltage switch (12) comprises at least one high voltage fast transistor.

4. Device (10) according to claim 1, 2 or 3, characterized in that said test sample is an electrically insulating plate having a thickness of between 500 pm and 1.5 mm.

5. Device (10) according to claim 1, 2 or 3, characterized in that said test sample is a high voltage direct current mini-cable having an insulator of thickness 1.5 mm.

6. Method for applying and measuring a transient overvoltage in an electric cable by means of a device (10) according to any one of the preceding claims, characterized in that it comprises steps consisting of:

7. applying said predetermined direct voltage (Uo) to said test sample by means of said first direct voltage generator (DCi); applying said predetermined overvoltage, in the form of said square wave voltage, to said test sample by means of said second direct voltage generator (DC2) and said high voltage switch (12); controlling said voltage rise time (Tl) by means of said control resistor (R2); controlling said overvoltage decay time (T2) by means of said decoupling capacitor (Cl). Method according to claim 6, characterized in that said overvoltage is equal to 0.7 Uo, where Uo denotes said predetermined direct voltage.

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