Abstract
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.
Highlights
High-voltage direct current (HVDC) cable is an important way to carry out large-capacity, long-distance and low-loss power transmission [1,2]
DC voltage was applied first, pulse voltage was increased by 1 kV/step
Tree initiation and growth characteristics under combined voltage were investigated. It shows that electrical tree is still a threat to HVDC cable because the existence of this combined voltage
Summary
High-voltage direct current (HVDC) cable is an important way to carry out large-capacity, long-distance and low-loss power transmission [1,2]. Cable accessories are an important part of the HVDC cable system and the weakest link, where most cable insulation failure occurs [3]. The reason is that the internal insulation structure of cable accessories is composed of multilayer solid insulation, which contains cross-linked polyethylene (XLPE), silicone rubber (SIR) and so forth. Electrical tree is the main reason for insulation aging and breakdown [5,6,7,8,9,10]. In the study of alternating current (AC) trees, a number of models have been proposed, while there is no alternating electric field under DC voltage and most models are not suitable. Research on DC trees is mainly focused on the following aspects:
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