Effects of mechanical stretching, produced after the expansion of cable accessories, on the electrical tree growth characteristics of EPDM (Ethylene Propylene Diene Monomer) are reported in the present work. FFV (fractional free volume) and CED (cohesion energy density), which were respectively related to free path of electrons and molecular chains splitting during electrical tree propagation process, were calculated through molecular dynamics simulations under different stretching ratios. A needle-plane electrode system with the stretching ratio ranging from 0 to 30% was used to investigate the electrical treeing characteristics in the experimental study. The simulation results show that the FFV becomes larger at higher stretching ratio, which enhances the collision between the hot electron and the molecular chain during the treeing process by extending the free path of the hot electron. While CED tends to become smaller at higher stretching ratio, reducing the cohesion strength for the tree channel crack to grow. Moreover, FFV and CED are investigated during the stretching recovery process, where FFV tends to decrease and CED turns to be larger. The experimental results indicate that mechanical stretching affects the electrical tree inception and propagation characteristics. The inception time of electrical tree becomes shorter as stretching ratio increases from 0 to 30%. Tree length and accumulated damage become larger with increasing the stretching ratio. Electrical tree propagation mechanism considering chemical and physical traps, which are generated under mechanical stretching, are also discussed and analyzed.

Polypropylene (PP) has been considered as an alternation to cross-linked polyethylene (XLPE) using as the high voltage direct current (HVDC) cable insulation material due to its no crosslinking process and environment friendly characteristics. During the operation, repetitive pulse voltage may generate in HVDC cable system and the challenge of the low temperature environment may occur. This paper compares the tree characteristics of PP and XLPE with the pulse voltage of 12 kV and 400 Hz under low temperature. Furthermore, the influence of pulse amplitude and frequency on the electrical trees of PP under low temperature is investigated. The frequencies of pulse voltage were 400, 800 and 1200 Hz and the pulse amplitudes were 12 and 15 kV. The experimental temperature from 30 to -196 °C was applied in the test. The results show that compared with the XLPE, the electrical tree is more difficult to be generated in the PP. The growth rate and fractal dimension of tree in the PP are smaller than these in the XLPE with the same treeing time. The low temperature, pulse frequency and amplitude have an influence on the treeing process. It is obtained that the growth of electrical tree is resisted under the low temperature. The higher pulse frequency accelerates the tree propagation process. The higher amplitude accelerates the tree growth rate and fractal dimension at -30 and -196 °C. However, the tree propagation process is not accelerated by the higher amplitude at -90 °C.

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.

HVDC converters can generate harmonic voltage, which distorts the DC voltage quality and threatens epoxy resin insulation reliability. In this paper, the electrical tree growth was investigated in epoxy resin under harmonic superimposed DC voltage. The DC voltage ranged from −20 to +20 kV and harmonic frequency ranged from 50 to 450 Hz. This experiment was carried out with needle-plate electrodes system. The effects of DC amplitude and harmonic order were characterized by electrical tree length and accumulated damage. Results show that the electrical tree varies with different combinations of DC amplitudes and harmonic orders. The tree length and accumulated damage experience a non-linear trend with the rise of harmonic order. The low-order harmonics bring more damage to epoxy resin under superimposed voltage. The time to breakdown shows a minimum value at 3 ^rd harmonic superimposed voltage. The DC amplitude has an acceleration on the electrical treeing process. With higher DC amplitude, the electrical tree breakdown happens immediately when the electrical tree reaches the plate electrode. The charge transport process accounts for the tree initiation characteristics under different superimposed DC amplitudes and harmonic orders. Meanwhile, “reverse tree” has been found in epoxy resin sample. The field-driven tree growth model has been employed to interpret reverse tree growth from the perspective of dynamic electric field distribution.

In this letter, a room-temperature synthesized amorphous indium-zinc oxide (InZnxOy) sensing membrane on flexible polyimide is employed as an extended-gate field-effect transistor (EGFET) pH sensor for the first time. The InZnxOy-based EGFET sensor exhibited a high pH sensitivity of 56.29 mV/pH with an excellent linearity of 0.999. Furthermore, it also demonstrated good stability in terms of a low hysteresis voltage of 4.6 mV as well as a small drift rate 2.08 mV/h, presumably suggesting the formation of uniform, dense, small grains, and stoichiometric InZnxOy thin film, which facilitates a high conductivity and provides a large number of surface binding sites. The InZnxOy-based EGFET pH sensor showed an excellent mechanical bendability after more than 500 bending cycles.

Cross-linked polyethylene (XLPE) is an important insulating material in warm dielectric superconducting cable. When it is operated in a high electric field, an electrical tree may occur and cause insulation breakdown. In this paper, the XLPE samples were prepared and a needle-plate electrode was employed. The treeing process at 0, -30, and -60 °C was recorded. The magnetic field applied on the samples was 0, 200, 400, and 600 mT. An electrical tree was initiated by an impulse-superimposed dc voltage. Two types of tree structures were found and their growth processes were recorded. In addition, the fractal dimension and accumulated damage were both calculated and analyzed. The result indicates that there is a magnetic field threshold to accelerate the tree growth, which increases with the decrease in temperature. Above -60 °C, it is below 600 mT and turns out to be higher than 600 mT at -60 °C. The magnetic field influences the tree-growth process by affecting the space-charge distribution.