WO2020207513A2 - Testing apparatus and evaluation method for evaluating emission performance of semi-conductive blocking material - Google Patents

Abstract

The present invention relates to a testing apparatus for evaluating the emission performance of a semi-conductive blocking material, comprising a vacuum control system and a performance testing system. The performance testing system comprises a testing chamber, a testing sample, an object placement platform, an anode, an ammeter, a protective resistor, and a high-voltage direct current power supply. The testing sample is fixed on the object placement platform and functions as a cathode. The anode is connected to a positive electrode terminal of the high-voltage direct current power supply. The testing sample is connected to a cathode terminal of the high-voltage direct current power supply. The cathode terminal of the high-voltage direct current power supply is grounded. The protective resistor and the ammeter are serially connected in a circuit. The anode and the cathode are both disposed inside of the testing chamber. The vacuum control system controls the testing chamber to be in a vacuum state. The present apparatus can directly test the charge carrier emission state of a semi-conductive blocking material in a high electric field, which represents the performance of the semi-conductive blocking material with respect to the insulating layer electric charge implantation when used as a high-voltage direct current electrical cable. The present invention has significance in the research of high-voltage direct current transmission cables.

Description

Test device and evaluation method for evaluating emission performance of semi-conductive shielding material Technical field

The invention relates to a test device and an evaluation method for evaluating the emission performance of a semiconductive shielding material.

Background technique

Because of its unique advantages in long-distance and large-capacity transmission points and grid interconnection, HVDC power transmission is the main trend of future power grid development. It is suitable for power supply to islands, capacity expansion of urban load centers, and grid connection of wind power, especially urban DC The development of power distribution systems requires high-voltage direct current transmission. Flexible DC transmission using high-voltage DC plastic cables is the mainstream direction advocated by large international power grids. But currently only Japan and a few Western European countries can produce high-voltage and ultra-high voltage plastic insulated DC cables. Although a small number of cable companies in China have developed 320kV DC cables, the insulating materials and semi-conductive shielding materials used to manufacture the cables are completely dependent on imports, forming a technical barrier.

High-voltage DC cables are generally composed of conductive cores, semi-conductive shielding layers, insulating layers and other protective layers from inside to outside. There are space charges in high-voltage DC cables, which can cause local electric field distortion and electrical performance degradation in the insulation layer. Especially in actual operation, the interface effect will aggravate the electric field distortion of the outer insulation layer of the cable and reduce the service life of the cable. Therefore, in high-voltage DC cables, a semi-conductive shielding layer is often set between the conductive core and the insulating layer to achieve good contact between the conductor and the insulating layer, improve the electric field distribution on the conductor surface, and reduce the emission to the insulating layer. The carrier suppresses the accumulation of space charge in the insulating layer, so the selection of the semi-conductive shielding material in the semi-conductive shielding layer is very important. As a semi-conductive shielding material for high-voltage DC cables, its basic requirement is to have low carrier emission under the condition of an ultra-smooth interface. Therefore, how to reduce the carrier emission of semi-conductive shielding materials and reduce the accumulation of space charge in the insulating layer is the current research hotspot of HVDC cable materials.

At present, the most widely used method for measuring the space charge of solid dielectrics is the electro-acoustic pulse method (PEA), which tests the space charge that enters the insulating layer from the conductive core through the semi-conductive shielding layer, and is measured by the amount of charge in the insulating layer. It reflects the carrier emission performance of semi-conductive shielding materials to determine the quality of semi-conductive shielding materials. The determination of carrier emission of semi-conductive shielding materials is an indirect method. In addition, the electroacoustic pulse method also has obvious shortcomings: "It cannot remove the wrong information, lack of understanding of the nature of the signal, poor measurement repeatability, and cannot be unified or obtain a recognized result" [1]. Therefore, it is of great theoretical significance and engineering value to seek a method that can directly test and evaluate the carrier emission performance of the semi-conductive shielding material of the HVDC cable.

References: [1] How to understand the two basic physical processes of polarization and conductance in engineering dielectrics and the scientific principles and methods of measurement, Proceedings of the Chinese Society of Electrical Engineering.

Summary of the invention

In order to solve the above problems, the present invention proposes a test device and an evaluation method for evaluating the emission performance of a semi-conductive shielding material, which can directly test the carrier emission performance of the semi-conductive shielding material.

The present invention solves the above technical problems through the following technical solutions:

A test device for evaluating the emission performance of a semi-conductive shielding material, including a vacuum control system and a performance test system. The performance test system includes a test chamber, a test sample, a stage, an anode, an ammeter, a protection resistor, a high-voltage DC power supply, and The test sample is fixed on the stage as the cathode, the anode is connected to the positive terminal of the high-voltage DC power supply, the test sample is connected to the cathode terminal of the high-voltage DC power supply, the cathode terminal of the high-voltage DC power supply is grounded, and the protection resistor and the ammeter are connected in series in the circuit. Both the anode and the cathode are located in the test chamber, and the vacuum control system controls the test chamber to be in a vacuum state.

Further, the vacuum control system includes a temperature control device and a molecular pump.

Further, the material of the anode is a copper rod.

Further, the temperature control device includes a water circulation cooling system and a resistance heating system.

Further, the ammeter can be replaced with a multimeter.

Further, the protection resistance is 1MΩ.

A method for evaluating the emission performance of semi-conductive shielding materials, which specifically includes the following steps:

S1. Adjust the gap value between the test sample and the anode to 10μm～80μm;

S2. Adjust the vacuum degree of the test chamber to above 3.5×10 ^-5 Pa, and close the vacuum control system;

S3. Turn on the high-voltage DC power supply and measure the I-U curve;

S4. Convert the I-U curve to the Fowler-Nordheim (F-N) curve.

Further, a detection step ST is added between steps S2 and S3. The detection step ST is: apply a voltage to the test sample, observe the reading of the ammeter, if the current indicator is always zero, adjust the voltage back to zero, and continue the step S3; otherwise, go back to step S1.

Further, the step S2 specifically includes: S21, turning on the temperature control device, adjusting the pressure of the test chamber below 2.5 Pa, and turning off the temperature control device; S22, turning on the molecular pump, and adjusting the vacuum degree of the test chamber to 3.5×10 ^-5 Pa or more, turn off the molecular pump.

Further, the rotational speed of the molecular pump is 200 r/s.

Further, the voltage applied in the detecting step ST does not exceed 20V.

On the basis of conforming to the common knowledge in the field, the above-mentioned preferred conditions can be arbitrarily combined to form an embodiment of the present invention.

The positive effect of the present invention is that compared with the currently widely used PEA testing methods, the method of first conducting field emission testing and then using PEA verification can largely find effective means that complement or replace PEA and ensure that The accuracy of the measurement results has better repeatability. It can directly test the carrier emission of the semi-conductive shielding material under high electric field, and can be tested by the FN equation to ensure that the current is the field emission current, thereby characterizing the charge injection of the semi-conductive shielding material used as a high-voltage DC cable to the insulating layer The pros and cons of performance expand a new direction for the performance characterization of semi-conductive shielding materials, and are of great significance for the development of high-voltage DC transmission cables.

Description of the drawings

Figure 1 is a structural device diagram of the present invention;

Figure 2 is the I-U curve corresponding to the cathode material of sample No. 1;

Figure 3 is the F-N curve corresponding to the cathode material of sample 1;

Figure 4 is the I-U curve corresponding to the cathode material of sample 2;

Figure 5 is the F-N curve corresponding to the cathode material of sample 2;

Figure 6 is the I-U curve corresponding to the cathode material of sample 3;

Figure 7 is the F-N curve corresponding to the cathode material of sample 3;

Figure 8 is the I-U curve corresponding to the cathode material of sample No. 4;

Figure 9 is the F-N curve corresponding to the cathode material of sample No. 4;

Figure 10 shows the PEA pressure curves of four samples when the field strength is 20kV/mm;

Figure 11 shows the integral of the charge injected into the LDPE by the four samples as semi-conductive shielding materials when the field strength is 20kV/mm.

Among them, 1-anode, 2-test sample, 3-test chamber, 4-molecular pump, 5-resistance heating system, 6-stage, 7-protection resistance.

detailed description

In order to make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be described in further detail below.

Example 1

A test device for evaluating the emission performance of a semi-conductive shielding material, including a vacuum control system and a performance test system. The performance test system includes a test chamber, a test sample, a stage, an anode, an ammeter, a protection resistor, a high-voltage DC power supply, and The test sample is fixed on the stage as the cathode, the anode is connected to the positive terminal of the high-voltage DC power supply, the test sample is connected to the cathode terminal of the high-voltage DC power supply, the cathode terminal of the high-voltage DC power supply is grounded, and the protection resistor and the ammeter are connected in series in the circuit. The anode and the cathode are both located in the test chamber, and the vacuum control system controls the test chamber to be in a vacuum state; the vacuum control system includes a temperature control device and a molecular pump; the temperature control device includes a water circulation cooling system and a resistance heating system; The material of the anode is a copper rod.

Example 2

The test device in Example 1 was used to evaluate samples 1, 2, 3, and 4 respectively, and the only variable that distinguished the four samples was the different proportions of carbon black during preparation, as shown in Table 1 for details.

A method for evaluating the emission performance of semi-conductive shielding materials, which specifically includes the following steps:

S1. Adjust the gap value between the test sample and the anode to 20μm;

S21. Turn on the temperature control device, adjust the pressure of the test chamber to 2.3Pa, and close the temperature control device;

S22. Turn on the molecular pump, the rotational speed of the molecular pump is 200r/s, adjust the vacuum of the test chamber to 3.5×10 ^-5 Pa or more, and turn off the molecular pump;

ST. Apply voltage to the test sample and observe the reading of the ammeter. If the current indicator is always zero, adjust the voltage back to zero and continue to step S3; otherwise, return to step S1.

S3. Turn on the high-voltage DC power supply and measure the I-U curve;

S4. Convert the I-U curve to the Fowler-Nordheim (F-N) curve.

It can be seen from Fig. 2-9 that the IU curves of the four samples are basically exponential curves, and the corresponding FN curves are close to linear, and the slope is negative, which confirms that the current generated by the four samples is indeed derived from Field launch. An important index is often used to characterize the field emission performance, that is, the threshold electric field. Its size indicates how easy it is for the material to emit electrons under the action of an external electric field. The smaller the threshold electric field, the easier it is for the cathode material to emit field emission electrons.

Table 1

sample	CB concentration (%)	Threshold electric field (V/mm)
1	20	637
2	25	928
3	30	1557
4	40	683

It can be seen from Table 1 that the size relationship of the threshold electric field corresponding to the four different CB concentrations of the cathode materials is: No. 1 <No. 4 <No. 2 <No. 3, and the largest threshold electric field is the No. 3 sample. The larger the threshold electric field, the more difficult it is for the cathode material to generate field emission electrons. Therefore, sample No. 3 is the most difficult to generate field emission electrons. Combining the structure of the high-voltage DC cable and the principle of charge injection, when the electrode injects charges into the insulating layer through the semi-conductive shielding material, the more difficult it is for the semi-conductive shielding material to emit electrons, the less charge is injected into the insulating layer, which is not conducive to space charge. Accumulation can effectively prevent the insulation from aging and help increase the service life of the cable. Therefore, from the perspective of several current CB doping concentrations, the semi-conductive shielding material prepared when the CB doping concentration is 30% theoretically has better electrical properties.

In order to further verify the conclusions drawn by the test device and evaluation method of the present invention, PEA tests were performed on four samples, uniformly under the condition of a field strength of 20kV/mm, and the insulating layer was made of LDPE with a thickness of 0.3mm, and they were measured. The pressure curve is shown in Figure 10.

It can be seen from Figure 10 that the four samples all have the same polarity charges near the anode. The highest peak value of the same polarity is the No. 4 sample, followed by the No. 2 sample. There are more negative charges injected into the LDPE.

In order to further quantitatively study the amount of charge injected into the medium, we integrate the charge inside the PEA pressure curve medium, and the charge amount calculation formula is derived from the curve shown in Figure 11. The comparison shows that when the field strength is 20kV/mm, the four samples are used as semiconducting shielding materials to inject charges into the LDPE respectively. The magnitude of the charge relationship is: No. 3 <No. 2 <No. 4 <No. 1 and No. 3 The amount of charge injected into the sample is the least, which is completely consistent with the conclusion drawn from the previous calculation of the threshold electric field.

In addition, it should be understood that although this specification is described in accordance with the implementation manners, not each implementation manner only contains an independent technical solution. This narration in the specification is only for clarity, and those skilled in the art should regard the specification as a whole The technical solutions in each embodiment can also be appropriately combined to form other implementations that can be understood by those skilled in the art.

Claims (6)

1. A test device for evaluating the emission performance of a semi-conductive shielding material, which is characterized by comprising a vacuum control system and a performance test system. The performance test system includes a test chamber, a test sample, a stage, an anode, an ammeter, a protection resistor, and a high voltage The test sample is fixed on the stage as the cathode, the anode is connected to the positive terminal of the high-voltage DC power supply, the test sample is connected to the cathode terminal of the high-voltage DC power supply, the cathode terminal of the high-voltage DC power supply is grounded, and the protection resistor and the ammeter are connected in series in the circuit Wherein, the anode and the cathode are both located in the test chamber, and the vacuum control system controls the test chamber to be in a vacuum state.
2. The testing device for evaluating the emission performance of semi-conductive shielding materials according to claim 1, wherein the vacuum control system includes a temperature control device and a molecular pump.
3. A method for evaluating the emission performance of a semi-conductive shielding material, characterized in that using the test device for evaluating a semi-conductive shielding material according to claim 2 specifically includes the following steps:

   S1. Adjust the gap value between the test sample and the anode to 10μm～80μm;

   S2. Adjust the vacuum degree of the test chamber to below 3.5×10 ^-5 Pa and close the vacuum control system;

   S3. Turn on the high-voltage DC power supply and measure the I-U curve;

   S4. Convert the I-U curve to the Fowler-Nordheim (F-N) curve.
4. A method for evaluating the emission performance of a semi-conductive shielding material according to claim 3, wherein a detection step ST is added between steps S2 and S3, and the detection step ST is: applying a voltage to the test sample and observing The ammeter reading, if the current indication is always zero, set the voltage back to zero and continue to step S3; otherwise, return to step S1.
5. The method for evaluating the emission performance of a semi-conductive shielding material according to claim 4, wherein the step S2 specifically includes:

   S21. Turn on the temperature control device, adjust the pressure of the test chamber to below 2.5 Pa, and close the temperature control device;

   S22. Turn on the molecular pump, adjust the vacuum of the test chamber to below 3.5×10 ^-5 Pa, and turn off the molecular pump.
6. The method for evaluating the emission performance of a semi-conductive shielding material according to claim 4, wherein the voltage applied in the detection step ST does not exceed 20V.

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