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In recent years, with the rapid development of large-length ultra-high-pressure voltage extrusion insulated submarine cable technology, the cable system in the medium-length offshore power grid interconnection project has been gradually replaced by XLPE insulation by traditional oil-paper insulation. There are many large cross-section conductors used in long-distance power transmission cables, and according to the IEC 60287-1-1, it is recommended to use Milliken conductors to reduce the AC resistance of conductors, at the same time, in submarine cables, there is a high requirement on the water-blocking performance of conductors and the water resistance of Milliken conductors is poor, so it is not used in submarine cable conductors. This paper puts forward a layering enamelled stranded conductor and a calculation method of AC / DC resistance ratio for it. The validity of the calculation method is verified by experimental tests. At the same time, the calculation shows that the AC / DC resistance of the conductor at 1800 mm2 nominal cross section can be reduced by 16% compared with the conventional round compacted conductor in the prior art. The effectiveness of this conductor in reducing AC resistance is verified by finite element simulation.

Ethylene-propylene-diene terpolymer (EPDM) is widely used in HVDC cable accessories. However, the mismatch of DC conductivity between EPDM and cross-linked polyethylene (XLPE), and the effect of space charge accumulation at different insulating interfaces, will distort the electric field distribution and seriously affect the performance of insulating material. In this paper, EPDM was developed for ±535 kV HVDC cable accessories, and the conductivity, XLPE/EPDM space charge distribution and DC breakdown electric field were tested respectively. The electric field distribution of cable accessories was calculated by COMSOL analysis software. It is shown that electric field of the stress cone is far less than the DC breakdown electric field when the EPDM conductivity is close to XLPE; Furthermore, there is a small amount of space charge accumulation at XLPE/EPDM interface and in XLPE, but little space charge in EPDM. As a consequence, the research will make a potential application for ±535 kV HVDC cable accessories.

As DC transmission has been used widely, HVDC and EHVDC Flexible cables are the development direction of power grid in the future. The biggest existing problem of HVDC cable is accumulation of space charges in XLPE insulation and researchers tried to suppress space charges by improving shielding layers which are directly contact with XLPE. Four kinds of HVDC and HVAC cable shielding materials from different manufacturers were selected in this paper. Transmission electron microscope (TEM), thermogravimetric analysis (TGA), volume resistivity and mechanical properties of shielding materials were measured and analyzed. Commercially available insulation material of HVDC cable was selected to prepare samples for space charge tests and the pressure wave propagation (PWP) method was adopted to study space charge injecting effects of shielding materials on XLPE insulation. The results showed that addition of conductive carbon black in HVDC cable shielding materials were lower than that in HVAC cable shielding materials, although volume resistivity of these shielding materials were almost same. Tensile strength and hot set properties of these shielding materials were almost same, but elongation at break of HVDC cable shielding materials were better. HVDC cable shielding materials had a more positive effect on suppressing space charges in XLPE insulation on the basis of lower field enhancement factor (FEF) caused by space charges.

In this paper, the appropriate expression on resistivity and electric field for the DC cable insulation is proposed based on the available experimental data. It is indicated that there is e index relationship between resistivity and temperature, and there is power exponent relationship between resistivity and electric field. The effect of conductor temperature, the temperature difference of insulation etc. on the electric field distributio of DC cable insulation is analysed. It is shown that the high conductor temperature and the large temperature difference in insulation make not only the insulation property decline but also the electric field distortion. There is no effect of resistivity coefficient on the distribution of electric field, but the low coefficient leads to the decrease of DC cable ampacity. Based on the electric field expression, the life expression and effect of space charge, the method on design of DC cable insulation is proposed.

The space charges accumulation between HVDC cable and accessory insulation will accelerate degradation and breakdown of the cable system, endanger the safety of the power grid. As is known that conductivity mismatch of cross-linked polyethylene (XLPE) and ethylene-propylene-diene terpolymer (EPDM) is the main reason for the charges accumulation in the interface. To investigate the influence of EPDM conductivity on the interface behaviors of XLPE/EPDM, In this paper, high field conductivity of two kind of EPDM and XLPE were tested at −5 kV/mm, −10 kV/mm, −20 kV/mm, −30 kV/mm, −40 kV/mm, −50 kV/mm and −60 kV/mm, and the space charges distribution of XLPE/EPDM interface was tested by pressure wave propagation (PWP) at −10kV/mm, −20kV/mm and −30kV/mm. It is shown that, when the conductivity of XLPE is lower than EPDM, the interface charge is positive; On the contrary, when the conductivity of XLPE is higher than EPDM, the interface charge is negative, and the interface charges density increases as the electric field rise. The experimental results are consistent with the interface charge density calculated based on the Maxwell-Wagner theory.

As DC transmission has been used widely, HVDC cables are the development direction of power grid in the future. Shielding materials for HVDC cables were prepared by mixing EEA with super conductive carbon black at different filling rates in this paper. Scanning electron microscope (SEM) was used to observe the morphology of carbon black particles in EEA. Volume resistivity, tensile properties and melt mass flow rate (MFR) of shielding materials were measured and analyzed. The pressure wave propagation (PWP) method was adopted to study space charge properties of shielding materials on XLPE insulation. Finally, recommended addition of super conductive carbon black was given. The results showed that super conductive carbon black in EEA existed in the state of long chains just like grapes rather than single particle and these long chains were well distributed. As addition of super conductive carbon black increased, volume resistivity, tensile properties and MFR of shielding materials decreased to different degree. The field enhancement factor (FEF) caused by space charges decreased gradually and tended to be invariant with super conductive carbon black increasing. Take all these properties into account, shielding materials with this type of super conductive carbon black at filling rates varying from 12 % to 15 % were enough to meet requirements.

In this paper, a round tight pressure conductor with large cross-section based on layered insulating structure is proposed. Compared to the existing common conductors, the 1800mm2 new conductor with the same cross section can reduce the AC / DC resistance ratio 15% and meet the conductor appearance requirements, without changing the interface state and conductor shielded conductor. Insulated conductor of connection terminal crimping treatment effect on AC loss characteristics of the layered paint on the envelope of actual trial results showed that: 1) new conductor has a lower temperature in the final stable stage than the ordinary one and reduce the percentage of 3.75%; 2) removing the film of paint can effectively reduce the additional loss and heat caused by the uneven current at the crimping and connecting terminals, cut the temperature of conductor by 28% and connection terminal by 57%; 3) two different ways of removing paint including baking and solvent depainting were adopted to reduce the effect of contact resistance in two ways, but the first way is suggested considering the actual maneuverability of the field cable installation.

For the real HVDC cable, because of the limitation of technical level and experimental conditions, the measurement of space charge distribution is a difficult problem. The suitable method may be Laser PWP method which use a powerful laser to generate a pressure wave. In this paper, space charge of planar sample, HVDC Cable on external layer and from inner conductor are respectively measured. All the tests are kept in the same condition which includes the energy of laser, temperature, high voltage applied and so on. In order to reduce the spatial noise and improve the signal-noise ratio, the cable sample is set in a iron box with only one hole for the laser. Comparing the test results above, the amplitude and area of the current signal in the three tests are in correspondence with each other theoretically. The experiment results presented demonstrate the validity of the measurement system which picks signal from external layer, and also provide a good foundation of theory and experiment method for the measurement of real full-size HVDC cable.

The distribution of electric field in cable insulation under DC field is introduced, the non-uniform radial field and space charge accumulation lead to the distortion of electric field in cable insulation, so it is important to investigate the distribution of space charge in model cable insulation. It is indicated that the signal of space charge measurement is weak by pulsed electro-acoustic method (PEA method), the signal-to-noise ratio (SNR) is low, and it is difficult to measure space charge in model cable at high temperature. The further analysis revealed that the signal of space charge measurement is strong, i.e. the SNR is high by pressure wave propagation method (PWP method), and it is the appropriate method for the measuring space charge in thick cable insulation. In addition, PWP method is also the appropriate method for measuring space charge in model cable at high temperature.