Investigation of electrical performance of impregnation resin material for superconducting magnet

1 Introduction

Advanced electric-power devices can be built by using the combination of superconducting technology and modern electrical technology,for example superconducting cable,superconducting transformer,superconducting current limiter,superconducting magnetic storage,etc.Superconducting electric devices have the advantages of large capacity,low dissipation,high current density,and self-control and self-healing functions.Adopting superconducting electric-power devices to build new power grids is one of the advanced technical solutions that facilitate robust and smart grids [1-3].

A superconducting magnet is a key part of superconducting electric devices.To ensure the safety of system structure,the pancake–pancake insulation,turn-to-turn insulation,and ground insulation are composed of impregnation resin material.The impregnation resin material acts as a connection,fastening,seal,filling,heat conduction,and insulation [4-7].Most conventional insulation materials can exhibit performance degradation at liquid nitrogen temperatures.Epoxy resins are the most frequently used impregnating materials in liquid nitrogen.For resin materials,the manufactures seldom carry out the relevant research and testing.Correlative theory and performance parameters are absent for engineering applications.In practice,research institutes develop the formulations according to their own needs and technical information is not disclosed,so that other researches cannot gain experience and reference.

In this paper,an impregnation resin material for a superconducting magnet is investigated at China Electric Power Research Institute.The power-frequency breakdown,lighting surges,relative dielectric constant,and loss angle tangent of the impregnation resin are tested at room temperature and liquid-nitrogen temperature.

2 Preparation of impregnation resin material

The impregnation resin material that we prepared comprised two epoxy resin materials and one solidified agent material [8].

The flowchart of sample preparation is shown in Fig.1.

Fig.1 Flowchart of sample preparation

2.1 Raw material

(1)Epoxy resin

Bisphenol F is used as the epoxy resin matrix because of its low viscosity.

(2)Curing agent

A low-viscosity liquid amine is used as the curing agent.

(3)Diluent agent

A long-chain epoxy resin is used as the plasticizer and diluting agent.

2.2 Configuration

2.2.1 Resin configuration

The three constituents mentioned above are poured together into a clear,dry,and colorless beaker under an epoxy resin matrix:solidified agent:diluent agent ratio of 50:17:30.The mixture is allowed to stand for some time,and the appearance is observed in scattered light until the air bubbles disappear,as shown in Fig.2.

Fig.2 Mixture of raw material

The mixture is whisked with a glass rod until it is smooth and creamy,and then poured into a flask with four necks.

Fig.3 shows the process that comprises three basic steps,viz.,whisking,heating,and vacuum pumping.The mixture is continuously whisked.at constant temperature until the air bubbles disappear completely.

Fig.3 Process of whisking,heating,and vacuum pumping

The mixture,which is the impregnation resin material we prepared,appears transparent in Fig.4.

Fig.4 Prepared Impregnation resin material

2.2.2 Resin curing

The curing curve is studied with a Differential Scanning Calorimeter (DSC).The test data is given in Table 1.

Table 1 DSC test data

Heating rate ℃/·minTs/℃TP/℃TE/℃4766169 105578171 156285192 206893201 5

Fig.5 Variation of TS,TP and TE with the heating rate β

TS is the starting temperature.TP is the peak temperature.TE is the end temperature.The variation of TS,TP,and TE with the heating rate is shown in Fig.5.The curing process is determined as 40 ℃/1 h + 60 ℃/24 h + 150 ℃/20 h by rheological analysis and practical experience.

2.3 Preparation of specimens

The specimen is prepared in accordance with the international standard ISO 10724:1994 via the injection molding method.The specimen is round with a diameter of no less than 100 mm,and the thickness is approximately 1 mm.Fig.4 shows the mold of the specimens.Fig.6 shows the sample that we prepared.Before casting the epoxy resin,we must ensure that the mold fits tightly to avoid leakage.

The thickness of the sample is determined as shown in Fig.7.

The thickness of the specimen is measured in each diagonal at both ends in order to remember (d1、d1′)、(d2、d2′)、(d3、d3′)、(d4、d4′).The average thickness of each diagonal is calculated at the in order to averaged4.The thickness of the central area is calculated by using formula (1).

Fig.6 Mold for specimen preparation

Fig.7 Prepared specimen

Fig.8 Schematic diagram for specimen thickness measurement

3 Electrical performance of impregnation resin material

3.1 Breakdown performance test

When the electric field intensity crosses a certain limit,the insulation material will break down and lose its insulation characteristics.

The ball electrode is used based on the standard GB/T 1408.1-2006.The diameter is approximately 20 mm,as shown in Fig.9 and Fig.10.

Fig.9 Ball electrodes

Fig.10 Electrode devices

3.1.1 Power-frequency breakdown test [9-14]

The transformer YDTW-125/250 without partial discharge is used as the power supply in Fig.11.A bubble chamber is used for holding the specimens and liquid nitrogen.

Fig.11 Power-frequency breakdown strength test

Table 2 shows the results of power-frequency breakdown test.The breakdown field strength at liquid nitrogen temperature is greater 13% than room temperature.

Table 2 Results of power-frequency breakdown test

12345 Liquid nitrogen temperature85.48684.283.884.484.84 Room Temperature7675.272.677.47475.04

3.1.2 Lighting surge test [15-18]

The 400 kV/20 kJ impulse voltage measuring system is used as the power supply,as shown in Fig.12.A bubble chamber is used for holding the specimens and liquid nitrogen,as shown in Fig.12.

Fig.12 Lighting surge strength test

Table 3 gives the results of the lighting surge test.The breakdown field strength at liquid nitrogen temperature is 20% greater than that at room temperature.

Table 3 Results of lighting surge test

3.2 The relative dielectric constant and loss angle tangent test

The relative dielectric constant and loss angle tangent test [7,8].

A high-precision LCR meter is used to measure the relative dielectric constant and loss angle tangent.The measurement principle is similar to that for a plate condenser,as shown in Fig.13.

The relative dielectric constant tanδ can be measured directly from Fig.13.

Fig.13 Relative dielectric constant

From the results,we can find that tan δ is 10-2 at room temperature and 10-3 at liquid nitrogen temperature.As the frequency increases,the tankinked line shows a slight increasing tendency.

The relative dielectric constant ε is de fined in (2).

where d is the thickness of the specimen,CP is the equivalent capacitance,A is the effective area of the electrodes,and 0εis the dielectric constant (= 8.85 ×10-12 F/m).

In the loss angle tangent test,CP can be measured directly.The relative dielectric constant can be calculated by using formula (2).In the frequency range,the chosen frequency samples are 100 Hz,200 Hz,300 Hz,400 Hz,500 Hz,600 Hz,700 Hz,800 Hz,900 Hz,1 kHz,2 kHz,3 kHz,4 kHz,5 kHz.The results of loss angle tangent are shown in Fig.14.

From the results,we can find that ε tends toward stability as the frequency increases.ε is slightly greater at liquid nitrogen temperature than at room temperature.

Fig.14 Loss angle tangent test

4 Comparison

As there are few reports on the study of low-temperature impregnated resins and almost no known performance parameters can be compared,the properties of some conventional impregnated resins in China are compared with those of the impregnated resins studied in this project,as shown in Table 4.R-1140-7 is a type of modified epoxy resin,heat-resistant unsaturated polyester resin,and curing agent manufactured by Jiaxing Rongterepai Insulation material Co.,Ltd.Et-90 is an epoxy modified unsaturated polyester impregnating paint manufactured by Wujiang Taihu Insulation material Factory.H9110 is an imine modified impregnating resin introduced by Harbin Qingyuan Insulation material Co.,Ltd.in the 1980s.

It can be seen from Table 4 that the impregnated resin studied in this project has high breakdown strength,which is more than three times that of conventional resin materials.The loss tangent of the impregnated resin studied in this project is stable at room temperature and liquid nitrogen.The impregnated resin studied in this project has good electrical properties and is suitable for impregnation and insulation treatment in low-temperature environments.

Table 4 Performance comparison

Index nameUnit Impregnation resin used in the project R-1140-7Et-90H9110 Appearance Amber transparent liquid Brownyellow transparent liquid Light brown transparent liquid Brownyellow transparent liquid Power frequency electrical strength MV/m75.04（20 ℃）23.8（20 ℃）MV/m84.84（-196 ℃）23（20 ℃）24（20 ℃）23.7（155 ℃）23.9（155 ℃）23.8（155 ℃）dielectric loss angle（50 Hz）%0.6（20 ℃）0.03（20 ℃）%0.05（-196 ℃）0.03（20 ℃）0.04（20 ℃）4.2（155 ℃）10.2（155 ℃）3.2（155 ℃）

5 Conclusion

This paper presents the preparation and performance testing of impregnation resin material.To confirm the suitability,the specimens are prepared to meet the testing requirements.Meanwhile the power-frequency breakdown,lighting surges,relative dielectric constant,and loss angle tangent of the impregnation resin are tested at room tempera-ture and liquid nitrogen temperature.The results show that the impregnation resin material can ensure the insulation of the superconducting magnet.

Acknowledgements

This work was supported by the State Grid Scientific and Technological Project (Research on New Cryogenic Insulating Material and Superconducting Magnet Application,No.DG71-16-001).