Space charges tend to accumulate on the surface and at the interface of ethylene–propylene–diene terpolymer (EPDM), serving as high voltage direct current (HVDC) cable accessory insulation, which likely induces electrical field distortion and dielectric breakdown. Direct fluorination is an effective method to modify the surface characteristics of the EPDM without altering the bulk properties too much. In this paper, the surface morphology, hydrophobic properties, relative permittivity, and DC conductivity of the EPDM before and after fluorination treatment were tested. Furthermore, the surface and interface charge behaviors in the HVDC cable accessory were investigated by the pulsed electroacoustic (PEA) method, and explained from the point of view of trap distribution. The results show that fluorination helps the EPDM polymer obtain lower surface energy and relative permittivity, which is beneficial to the interface match in composite insulation systems. The lowest degree of space charge accumulation occurs in EPDM with 30 min of fluorination. After analyzing the results of the 3D potentials and the density of states (DOS) behaviors in EPDM before and after fluorination, it can be found that fluorination treatment introduces shallower electron traps, and the special electrostatic potential after fluorination can significantly suppress the space charge accumulation at the interface in the HVDC cable accessory.

Electrical tree is a serious threat to silicone rubber (SIR) insulation and can even cause breakdown. Electrical trees under alternating current (AC) and direct current (DC) voltage have been widely researched. While there are pulses in high-voltage direct current (HVDC) cables under operating conditions caused by lightning and operating overvoltage in the power system, little research has been reported about trees under combined DC-pulse voltage. Their inception and growth mechanism is still not clear. In this paper, electrical trees are studied under several types of combined DC-pulse voltage. The initiation and growth process was recorded by a digital microscope system. The experimental results indicate that the inception pulse voltage is different under each voltage type and is influenced by the combined DC. The initial tree has two structures, determined by the pulse polarity. With increased DC prestressing time, tree inception pulse voltage with the same polarity is clearly decreased. Moreover, a special initial bubble tree was observed after the prestressing DC.

Chosen as an important insulation material of high temperature superconducting DC cable terminals, epoxy resin needs to address the challenge of the liquid nitrogen temperature and various kinds of voltage due to the special operating environment. The switching over-voltage, lightning strike and current impact will introduce the pulse voltage into the operation of the DC equipments, which makes it necessary to study the insulating characteristics of epoxy resin under DC superimposed pulse voltage condition. This paper investigates the electrical tree growth characteristics of epoxy resin with application of DC superimposed pulse voltage in liquid nitrogen (LN2). The tested samples were stressed with combined DC-pulse voltage with different amplitudes and polarities in a needle-plate geometry electrode system. Tree length and accumulated damage are employed to characterize the electrical tree. The experimental results indicate that when the DC and pulse voltage are of the same polarity, the rate of tree growth increases with the amplitude of DC voltage rising but would slow down after a period time of treeing. When the polarity of DC and pulse voltage is opposite, the growth rate decreases with the DC voltage amplitude increasing, while it could maintain a relatively high level during the whole treeing time.

Nonuniform electric field distribution is one of the key issues for the superconducting cable terminals. This paper tries to modify the electric field distribution in the stress cone of HTS dc cable terminals by the nonlinear conductive epoxy/ZnO insulation, in-stead of the insulation with a fixed conductivity. The nonlinear conductivity of epoxy/ZnO composites is measured, and a finite element simulation model of the +100 kV HTS dc cable terminal is established to analyze the electric field distribution at the steady and transient state. The results show that the field-dependent conductivity has a great effect on the dc electric field distribution. If the nonlinear coefficient is excessively large, the electric field in the polypropylene laminated paper (PPLP) will be extremely strong; otherwise, the electric field in the terminal insulation will be extremely strong. The epoxy/ZnO composites with an appropriate nonlinear conductivity can effectively modi-fy the dc electric field distribution at the stress cone surface, es-pecially at the triple junction. As the impulse superimposed on the dc voltage is applied, the maximum field is always located in the PPLP layer. The nonlinear conductive epoxy/ZnO compo-sites could play an important role in promoting the development of stress cone of HTS dc cable terminals.

Silicone rubber (SIR) is widely used as an insulation material in high voltage cable accessories. Electrical tree is a typical electrical degradation and is easily initiated because of the distorted electric field. In this study, graphene nanoplatelets at contents of 0.001–0.010 wt % (0.00044–0.00436 vol %) were added into SIR to improve the electrical tree inhibiting ability. Scanning electron microscopy, conductivity and surface potential decay tests were conducted to analyze the characteristics of graphene/SIR nanocomposites. The typical electrical treeing experiment was employed to observe the electrical tree inhibition of graphene in SIR. The results show that graphene nanoplatelets were well dispersed in SIR. The conductivity was higher after the addition of graphene nanoplatelets, and the trap distribution was affected by graphene nanoplatelets. The tree was changed from a bush-branch structure to a bush structure after the addition of graphene. Tree inception voltage improved and reached the highest mean value at 0.003 wt %. The tree length was inhibited at 0.001 to 0.007 wt % and the lowest tree length occurred at 0.005 wt %.

To investigate liquid nitrogen (LN2) effects on insulation materials in high temperature superconducting (HTS) system, the electrical tree was studied in ethylene propylene diene monomer (EPDM)/antioxidant specimens by using a needle-plane electrode system. The pulse amplitude and frequency were respectively 25 kV and 200 Hz. In LN2, the free volume was reduced and the nonband energy was got larger, the charge carriers were difficult to migrate in the samples. The charge trapping center was decreased from 0.925 to 0.195 eV as the temperature was reduced from 293 to 77 K, there were less charges to collide with molecular chains in LN2. The electrical tree growth was restrained in EPDM specimens under LN2. When the antioxidant was filled into EPDM, the charge trapping center was increased from 0.195 to 0.21 eV, the nonband energy was improved to reach a more stable state. Thus, the antioxidant could be served as the degradation inhibitor in EPDM/antioxidant specimens by regulating the charge transportation to restrain the electrical treeing process. This research has substantial commercial significance for insulation design and pre-shipment inspections in HTS insulation areas.