Since 2012 three cross-linked polyethylene (XLPE) insulated HVDC cables with rated voltage level of ±160 kV, ±200 kV and ±320 kV have been tested or successfully commissioned in HVDC transmission projects. They are Nan’ao three-terminal VSC project for island wind farm connection to China Southern Power Grid, with a total length of 37 km submarine and land cable of rated ±160 kV, which has been put into operation in December 2013; Zhoushan five-terminal VSC project with 294 km ±200 kV DC submarine power cable and has been in operation since June 2014; and another VSC project at the voltage level of ±320 kV is under construction in Xiamen, with a cable in length of 21 km deployed in cable tunnel. On the completion of these three projects within three years of time, China has realized triple leap in its HVDC submarine and land cable development. This paper presents the technical achievement of XLPE insulated HVDC cable development in detailed material characterization, space charge behavior, degassing processing and testing considerations. Their application in the three projects are introduced as well to illustrate the insulation and coordination design by considering each individual transmission system.

To achieve the localization of HVDC cable insulating materials and guarantee the long-term operation reliability of DC cable system, test samples are parepared by two typical formulations of DC cable XLPE insulating and semiconductive shielding materials. The elementary component and molecular structure characteristics of the test samples are analysed by using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) in order to evaluate the performance of cable insulating material. Furthermore, the thermal and mechanical properties are experimentally investigated to study the changes of thermal conductivity and mechanical performance before and after thermal aging test. The comparison and analysis of the experimental results may provide basic experimental data support for future HVDC cable system engineering design and material selection, and forming a fundamental DC insulating material quality evaluation system.

With the rise of XLPE insulated DC cable voltage level, the requirements on the quality and reliability of semi-conductive shielding material are increasing. In this paper, two imported and one domestic high quality HVDC semi-conductive shielding materials in combination with one domestic XLPE DC cable insulation material are selected for testing. Their thermal, electrical properties, shielded and insulated composite performance are tested. The effects of DC cables shielding materials on space charge in XLPE insulation are analyzed. On the basis of these results and analysis, the filling amount and particle size of carbon black, the matrix resinare and interface bonding are proposed to be the key factors which affect the volume resistivity and space charge of semi-conductive shielding materials.

Grounded DC tree experiments and space charge measurement based on the pulsed electro-acoustic (PEA) method are performed on XLPE and 0.5% by weight MgO/XLPE nanocomposite. The tree experiment shows that MgO/XLPE nanocomposite exhibits higher 50% tree inception voltage under positive voltages. The space charge measurement shows that more homocharge is injected in XLPE, which indicates that nanoparticles impede the homocharge injection or heterocharge extration at needle tip, thus impedes the generation of electrical tree. And MgO/XLPE nanocomposite shows smaller average tree length and width when under positive voltages, which may be attributed to the carries generated from high stress region colliding with nanoparticles and slowing the tree propagation. Moreover, the average tree length-width ratio in MgO/XLPE nanocomposite decreases faster with the positive voltage increasing. This can be explained by the fact that electrical tree is hard to penetrate nanoparticles, but can only develop around their surfaces, which results in the change of tree channel direction and emergence of many thin lateral branches.

Space charge phenomena seriously restrict the development of high voltage direct current plastic cables. At present, pulse electric-acoustic (PEA) method is one of the common non-destructive methods for measurement of space charge distribution in solid dielectrics in the world. Several space charge measurement technologies based on PEA for coaxial plastic cables are reviewed in consideration of the high voltage pulse injection modes. An improved space charge measurement device based on a high voltage pulse injecting into cables sample from the measuring electrode is also proposed, in which, to improve the bandwidth of the system measurement signal, the oscilloscope for data collection is insulated against ground through storage battery, epoxy base, and photoelectric converter, etc. Furthermore, the wave recovery method of space charge for coaxial plastic cable is introduced. Finally, the space charge measurement technology applied to evaluating the aging degree of power cable and the feasibility of space charge measurement on online in the future are summarized.

Pulsed electro-acoustic method (PEA) based space charge measurement is one of the main technical means for cross-linked polyethylene (XLPE) insulated DC cable material selection and evaluation, which is widely used in the DC cable film specimen. However, when PEA is applied to a thick flat sample or a large scale coaxial cable sample to evaluate the influence of the sample itself on space charge, the attenuation and dispersion of the acoustic signal propagation in the sample affect directly the accuracy and resolution of space charge measurement. In this paper, the charge densities inside and outside the electrode surface are determined by calculating DC electric field using theoretical method during space charge calibration process. Then, the transfer function, which includes the attenuation and dispersion coefficients, of pulse acoustic signal transfering in the insulating medium is calculated, and the system response is removed from the original signal with deconvolution operation. In the frequency domain, the pulse acoustic signal after attenuation and dispersion correction is obtained by a product of the measurement signal and the transfer function to ensure the accuracy measurement of charge density and spatial resolution of the full-size DC cable sample. Meanwhile, a PEA method based full-size DC cable space charge measurement system is designed, and a data acquisition and processing system is developed under LabVIEW.

Extruded insulated DC cable has been widely used in VSC-HVDC transmission systems. The reliability of DC cable system is challenged by the transient over-voltage caused by switching operation, internal fault and lightning stroke. Based on Nan’ao demonstration project, a simulation model of HVDC power cable system is established with PSCAD/EMTDC. Different types of faults including AC single-phase grounding fault, two-phase short circuit fault and arm reactor grounding fault are set up to study the transient characteristics of the DC cable system. Research results show that the maximum overvoltage peak at the DC cable terminal can run up to about 2 p.u.;and the over-voltage at DC cable terminal is affected by the fault resistance values.

To improve the electric field distribution inside the intermediate joint of the cross-linked polyethylene (XLPE) insulated high voltage direct current (HVDC) cable, nonlinear silicone rubber composite materials for cable joint stress control body are prepared by adding nano-fillers. Based on the tested conductivity characteristics of the composite materials, an HVDC cable joint simulation model is established to calculate the electric field distribution inside the joint. The results indicate that the conductivities of the unmodified silicone rubber under different electric field strength are all smaller than conventional HVDC cable XLPE insulation at 70℃. So the maximum field strength in the cable joint is located in the silicone rubber reinforced insulation and is much larger than the average electric field strength of the cable insulation. When silicone rubber with suitable non-linear conductivity characteristic is used as the reinforced insulation, its conductivity obviously exceeds the conductivity of cable XLPE insulation after the electric field strength exceeds a certain value, thus ensured the maximum field strength is always located in the cable XLPE insulation and close to the average electric field strength.

The interface of DC accessory insulation and cable insulation suffers both the bulk electric field along cable radial direction and the interfacial electric field along cable axial direction. The distribution and concentration of the two orthogonal electric fields make the interfacial space charge property complex. Aiming at the problem, the cross-linked polyethylene (XLPE) and silicone rubber (SIR) are used to compose a double layer sample to imitate the compound insulation of cable accessory, and the interfacial space charge at XLPE/SIR interface are measured by pulse electro-acoustic (PEA) method under bulk electric field and three different orthogonal electric stresses, namely bulk electric field from electrodes of PEA and interfacial electric field provided by aluminum foil needle-plate electrodes (the polarity of needle electrode is positive/negative) or plate-plate electrodes. Results show that the XLPE/SIR interface space charge changes obviously under orthogonal electric stress (both bulk and interface electric fields). After twenty minutes’ applying of orthogonal electric stress, the interface charge increases first and then decreases with electric stress under the interfacial electric field from negative polarity need-plate electrodes, which leads to an injection of electrons to XLPE from negative electrode; interface charge decreases with electric stress under the interfacial electric field from positive polarity need-plate electrodes, which increases the injection of positive charge to SIR from positive electrode; under the interfacial electric field from plate-plate electrodes, the interfacial space charge decreases a little.

XLPE (cross-linked polyethylene) insulated cable is an important component of power transmission lines. In allusion to the operating status of HVAC and HVDC cables, the paper introduces the electrical ageing model of XLPE cable insulation based on the typical V-t characteristics (the relationship between breakdown voltage and breakdown time) and the research method of V-t characteristics both of HVAC and HVDC XLPE insulated cables. Previous research show that the electrical aging lifetime index n of AC XLPE cable is between 9 and 25, while the electrical aging lifetime index n of DC XLPE cable from very limted research shows that it’s between 13 and 20. And there is no research on V-t characteristics of DC cables having been reported yet in China.

The accuracy evaluation of cable products is directly depended on the test technical speifications for HVDC cable system, and the cable structure and operation designs are affected indirectly by them. An overview on technical specifications and standards for the test of HVDC extruded insulation cable systems is given in this paper. The recommended test items and requirements of type tests and prequalification tests in TB 496 of CIGRE and TICW 7.1-7.4 of China are in focus. The research works and application cases on DC cable system tests in some developed countries are also introduced. Advices on related subjects to be further studied are presented.

The partial discharge detection method has been widely used in evaluating the insulating property of AC electiric equipments. However, the research on partial discharge detection under DC voltage is still at the primary stage. Some useful analysis methods under AC voltage cannot be applied to the DC voltage situation because of lack of the phase angle conception. The research status domestic and abroad is summarized in this paper, the physical model and the influence of temperature and some other factors on partial discharge under DC voltage are introduced. Some common ways to analyze partial discharge under DC voltage are also summarized, and typical test cases of partial discharge of DC cables are analyzed.

Aiming at axial temperature distribution of XLPE DC cable load cycle testing system, a simulation study about the coupling calculation of fluid flow, fluid heat transfer and solid heat transfer is carried out in this paper. The COMSOL Multiphysics software is used for simulation and a 2D axisymmetric model is set up. The distribution correlations among water temperature, core conductor temperature and temperature difference of insulation are obtained, finding that the axial uniformity of core conductor temperature and temperature difference of insulation will reach pre-set values if the temperature difference between water inlet and outlet can be precisely controlled.

Defects may probably appear during the lifetime of HVDC cable, and possibly distort the electric field distribution of HVDC cable and further accelerates insulation degradation. Two-dimension and three-dimension finite element analysis (FEA) are applied to study the influence of single spherical defect on the surrounding dielectric under differernt conditions including different temperature difference level across insulation, different defect location and size. A general equation for describing field distortion by spherical defect in HVDC cable is given in the paper, and it is concluded that three-dimension analysis is advantageous in researching the defect distortion, electric conductivity is the key factor in determining the electric field enhancement by defect and the influence of defect on surrounding insulation material is limited in a small region.

It is so easy for polyethylene to accumulate space charge that there is a problem of insulation deterioration and service life reduction for polyethylene insulated HVDC cable. In order to research the properties of space charge and conduction current of insulation materials under DC electric field, and taking the fact into account that for the same piece of samples, separate measurement results cannot be used for combined analysis strictly because of the displacement current component in external current, a new type of equipment based on pulse electro acoustic method (PEA) and the “same region time division” technique is designed to perform simultaneous measurement on plate specimen. The measurement results of space charge distribution and high electric field conduction in low density polyethylene (LDPE) plate sample are presented in this paper. The experimental results show that with the increase of electric field, increasing heterocharges around anode is accumulated, and the transition from heterocharges to homocharges around cathode can be observed. The phenomenon has relationship with impurity dissociation and different electrode materials. Displacement current density at room temperature under the condition of low electric field among the external current ratio is greater than that of the conduction current. The relationship between conduction current density and the electric field intensity also conforms to the Space Charge Limited Current theory.