**1. Introduction**

Cross-linked polyethylene (XLPE) is wildly used in HVDC cables as the insulation material. Electrical tree degradation is an important issue to deteriorate overall insulation level, eventually leading to the electrical failure of cables [1–3]. Researchers have tried many ways to improve the electrical tree breakdown resistance of high voltage (HV) cables, including material blending modification, nanoparticle modification, and polycyclic compound modification [4–6]. Results showed that polycyclic compound had an excellent function of suppressing electrical treeing in polymeric insulating material [7,8]. The America Dow Chemical Company studied the effects of a series of siloxane polycyclic compounds containing pendant aromatic groups on the growth characteristics of electrical tree. It was found that the aromatic ring side groups are indispensable groups for suppressing electrical tree [9]. It was also found that aromatic ketones and diketones had grea<sup>t</sup> effects on inhibiting electrical treeing growth in XLPE and polyethylene (PE) [5,10]. Although much research has been carried out on the suppression effect of polycyclic compounds on electrical tree growth, the relationship between polycyclic compounds, charges, trapping levels, and electrical treeing is still unclear.

Research presented a lot of methods to analyze the electrical treeing process. The acoustic emission method and artificial neural networks were used in the detection of the treeing process [11,12]. In addition, imaging techniques such as transmission electron microscope (TEM), scanning electron microscope (SEM), or X-ray computed tomography (XCT) were used to reveal the more complete

representation of the electrical treeing phenomenon [13]. Electrical treeing is a kind of cumulative breakdown which is also associated with the charge movement and trap distribution [14,15]. In the process of electron trapping and recombination, the hot electrons gain energy, impact, and destroy the molecular chains of polymer, forming the new electrical tree channel [16,17]. The deep trap level can capture mobile hot electrons, resulting in the decrease of the internal free charge [18]. However, little research has been done on the relationship between electrical tree and trap distribution in the XLPE/polycyclic compound composite.

In the DC transmission system, due to the on and o ff operations of power electronic devices, impulse overvoltage occurs. The lightning events and operating conditions may also cause lightning and operating overvoltage [19–21]. These impulse voltages are superimposed on the rated DC voltage to produce the DC-impulse voltage that a ffects the insulation electrical tree degradation process [22,23]. The electrical initiation and treeing characteristics in epoxy resin and polypropylene (PP) with DC-impulse voltage were studied, respectively [24,25]. Results revealed that the electrical treeing characteristics with DC-impulse voltage was di fferent from those with DC voltage, and the polarity of impulse voltage had a significant e ffect on the electrical initiation and growth characteristics. However, at present, research on the e ffect of polycyclic compound on electrical tree suppression mostly focuses on AC voltage, and the electrical tree dependence on polycyclic compounds with DC-impulse voltage still needs to be studied. In addition, due to the heat generated by the large current, during the operation of HVDC cables, cables operate under high temperature conditions for a long time [26]. The maximum long-term design temperature of XLPE cables is generally 90 ◦C, and the operating temperature is usually around 50–60 ◦ C [27]. It was found that high temperature a ffected the charge movement and partial discharge characteristics in the insulation, thus a ffecting the electrical treeing process [28,29]. However, most studies on polycyclic compounds are carried out at room temperature, and the e ffect of high temperature on polycyclic compounds still needs to be further studied.

In this paper, XLPE is employed as the polymer matrix, and three types of polycyclic compounds with the content of 0.5% are added into the XLPE. The electrical treeing properties of XLPE/ polycyclic compound composites are researched with DC-impulse voltage at 30, 60 and 90 ◦C. In addition, the trap distribution and carrier mobility behaviors are also studied to further reveal the mechanism of polycyclic compounds fillers on electrical treeing growth.
