A novel optical detection method for partial discharge in HV/EHV cable terminations has been proposed. Optical sensor fibres integrated into the HV equipment provide high sensitivity as well as immunity to electromagnetic interference and enable therefore on-line monitoring in electromagnetically noisy environment. The availability of optically transparent silicone rubbers that meet strict dielectric and mechanical criteria is a crucial prerequisite for the implementation of this method. The optically transparent silicone rubbers can be applied for the fabrication of a modern rubber stress cone as well as for the development of a new optical sensing element sensitive to PD activities. In this thesis, AC dielectric strength behaviour and mechanical properties of three types of commercially available silicone rubbers were investigated. One of the characterized silicone rubbers was a translucent type whereas the two others were optically transparent types, however with different chemical curing reactions.
The measurements of tensile strength and elongation at break were carried out according to the ISO 37 standard. For investigation of the dielectric strength behaviour of the virgin and modified silicone rubbers, a new methodology was developed. It is, at the same time, highly reliable and efficient, saves time and reduces material consumption in comparison to previously reported methodologies. The key component of this methodology is a specifically developed test facility. Furthermore, the methodology comprises determinations for easy preparation and handling of high-quality test specimens. This test method provides various advantages over other methods that have previously been used for measurement of the fundamental quantity value of silicone rubbers. Both technical and economic demands are satisfied. The new facility also enables cost-effective routine tests in material research laboratories. The high quality of the obtained test results was verified by statistical analysis based on the 2-parameter Weibull distribution function.
The investigations revealed that the virgin translucent silicone rubber has a large elastic region with an acceptable plastic deformation and also provides an AC 50 Hz dielectric strength of approximately 24 kV/mm for 0.5 mm thickness. These values enable considering the tested translucent silicone as replacement material for an opaque elastomer that is currently used for a rubber stress cone of HV cable accessories. Unfortunately, its optical transmittance is poor compared to optically clear transparent silicone rubbers. On the other hand, the mechanical properties of virgin transparent silicone rubbers do not comply with those demanded from push-on stress cones. In particular, their elongation at break is considered too low for that application. However they provide the AC dielectric strength values in either 28 kV/mm or 29 kV/mm for 0.5 mm thickness, which are higher than those of the translucent type. Moreover, it was found that the post-curing process does not provide a positive impact on the ultimate elongation of silicone rubbers. Hence, the elongation at break of virgin transparent silicone rubbers must be improved before they can be used as insulating material for a rubber stress cone. In addition, the influence of mechanical tensile stress on the dielectric strength of the virgin translucent silicone rubber was investigated. The results show that mechanical tensile stress does not negatively influence on dielectric strength of such silicone rubber, so it can be well-operated under combined electrical and mechanical stresses.
Beside the improvement of optical PD detection performance in the translucent silicone insulation materials, the influence of fluorescent dye’s modification was investigated. The results indicate that the commercially available fluorescent dyes of 0.02 wt. % mixed into the translucent silicone polymer do not negatively influence on the value of such silicone material. So an optically compatible silicone rubber is perfectly suitable for the fabrication of novel fluorescent silicone optical fibres, which can be integrated into the modified transparent rubber stress cones of HV cable terminations.
The final outcomes of this investigation are experimentally substantiated recommendations for future revision of IEC 60243-1, especially the chapter dealing with the determination of AC dielectric strength of silicone rubbers. Recommendations and suggestions for further investigations are addressed in the final chapter of this thesis.

1 Introduction 1

1.1 Innovative research for PD on-line monitoring in HV cable termination 1

1.2 The thesis work motivation 4

2 State-of-the-art silicone rubbers for HV applications 7

2.1 Silicone rubber 7

      2.1.1 History 7

      2.1.2 Products and processing technology for silicone elastomers 8

      2.1.3 Chemical structure 10

      2.1.4 Curing or cross-linking reaction 10

      2.1.5 Fillers 13

2.2 General properties of silicone rubbers 15

      2.2.1 Physicochemical properties 15

      2.2.2 Hydrophobic recovery property 15

      2.2.3 Heat and cool resistance 16

      2.2.4 Electrical properties 16

      2.2.5 Weatherability 18

      2.2.6 Mechanical properties 18

      2.2.7 Optical properties 18

      2.2.8 Flame Retardancy 18

2.3 Silicone rubbers in medium- and high-voltage applications 19

2.4 Silicone rubbers as power cable insulation 21

2.5 Silicone rubbers for optical partial discharge (PD) detection 24

      2.5.1 The influence of embedded polymeric-optical sensor element into the rubber stress cone of HV cable accessories 25

      2.5.2 Fluorescent silicone optical fibre as sensor element 27

      2.5.3 Modification of siloxane insulation material 28

2.6 Conclusions 29

3 Theoretical background 31

3.1 Electrical field distribution and breakdown strength of insulating materials 31

3.2 Fields in homogeneous, isotropic materials 34

      3.2.1 Coaxial cylindrical fields 35

      3.2.2 Sphere-to-plane electrode configuration 36

3.3 Breakdown in solid dielectrics 37

      3.3.1 Intrinsic breakdown 39

      3.3.2 Streamer breakdown 42

      3.3.3 Electromechanical breakdown 42

      3.3.4 Edge breakdown and treeing 43

      3.3.5 Thermal breakdown 45

      3.3.6 Erosion and electrochemical breakdown 49

      3.3.7 Tracking 52

3.4 Mechanism of electrical degradation and breakdown in polymers 53

      3.4.1 Low level degradation in polymers 53

      3.4.2 Electrical treeing 56

      3.4.3 Electroluminescence under electric field 58

      3.4.4 Deterministic models of breakdown in polymeric materials 60

3.5 Partial discharges 63

3.6 Dielectric polarization and permittivity 67

      3.6.1 Polarization mechanisms 67

      3.6.2 Dielectric permittivity 68

      3.6.3 Complex permittivity and dielectric loss (tan) 70

      3.6.4 Factors influencing dielectric properties 73

4 Development of the breakdown test facility for silicone rubbers 75

4.1 State-of-the-art review of dielectric breakdown testing methods for silicone rubbers 75

4.2 Steps in test cell construction for dielectric breakdown test of silicone rubbers 81

      4.2.1 The test cell configuration with five embedded sphere electrodes 81

      4.2.2 The test cell configuration with single embedded sphere electrode 82

      4.2.3 Summary 83

4.3 Development of a new efficient methodology to measure dielectric strength of silicone rubbers 84

4.4 Conclusions 88

5 Experimental details 91

5.1 Description of the materials used 91

5.2 Preparation of the test specimen 94

      5.2.1 A silicone sheet specimen 94

      5.2.2 A small test cell with embedded sphere electrode 95

5.3 Experimental setup 96

      5.3.1 Calibration of partial discharge measuring system 98

      5.3.2 Method of voltage application 98

5.4 Methodology for statistical analysis of dielectric breakdown results 99

      5.4.1 The Weibull distribution for dielectric breakdown data 99

      5.4.2 Plotting of the Weibull function 101

      5.4.3 Plotting the experimental data into the Weibull probability diagram 102

      5.4.4 Parameter estimation for the Weibull distributed data 103

      5.4.5 Estimation of confidence intervals for the Weibull function 105

      5.4.6 Tests with increasing voltage 105

6 Experimental results and discussions 107

6.1 Mechanical properties of the optically compatible silicone rubbers 107

      6.1.1 Mechanical properties of ESA 7250 silicone rubber 109

      6.1.2 Mechanical properties of LSR 7665 silicone rubber 111

      6.1.3 Mechanical properties of LSR 3003/30 silicone rubber 113

      6.1.4 Discussion and conclusion 115

6.2 Dielectric strength of silicone rubbers 118

      6.2.1 The reliability of measurements 118

      6.2.2 The dielectric strength value of optically compatible silicone rubbers 121

      6.2.3 The influence of specimen thickness on dielectric breakdown measurements 124

      6.2.4 The influence of voltage increase rate on breakdown test results of silicone rubber 126

6.3 Dielectric strength behaviour of silicone rubber under mechanical tensile stress 130

6.4 Dielectric strength behaviour of fluorescent silicone rubbers 132

6.5 Dielectric strength behaviour of silicone rubber with embedded sphere electrode 135

7 Conclusions 139

7.1 A novel methodology for dielectric breakdown test of silicone rubbers 139

7.2 Mechanical properties and dielectric strength behaviour of optically compatible silicone rubbers 140

      7.2.1 Mechanical properties 140

      7.2.2 AC 50 Hz dielectric strength behaviour 141

7.3 Observations 142

References 143

List of publications 157

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