**4. Discussion**

#### *4.1. Mechanism of Polycyclic Compounds Fillers Inhibiting Electrical Treeing Growth*

The growth process of the electrical tree with the DC-impulse voltage is closely related to the hot electrons motion and trap distribution [40]. After the addition of the polycyclic compound, the trap depth and density of samples increase significantly. The traps enhance the charge trapping ability of the needle tip and easily forms the same polarity charge accumulation to weaken the external electric field, resulting in a decrease in the injected charge amount [41]. At the same time, with the deep trap level and the trap density increasing, the internal charge trapping of the sample enhances, resulting in a decrease in the internal free charge of the sample. For the growth of the electrical tree, the charge injection process is suppressed and the free charge inside the sample is reduced. Therefore, the molecular chain breaking process is suppressed, and the electrical tree deterioration resistance of the XLPE/ polycyclic compounds composites is improved. In addition, in the process of electrical tree growth, some of the electrical tree channels are carbonized, and the carbonization channel delivers charges as a conductive current to the end of the channel, enhancing the electric field strength [42]. The deeper trap captures the transport charges in the carbonized electrical tree channels, reducing the electric field strength at the end of the channel, thereby decreasing the growth of the electrical tree [29]. The carrier migration process is closely related to the trap distribution of the composite material. The free charge may be captured by the local state (trap) during the extended state migration process. The charge transfer and exchange process between the extended state and the local state affects the migration characteristics of carriers [41]. After the addition of the polycyclic compound, the carrier mobility is reduced, and the carrier mobility of XLPE-A is the smallest. The deep trap energy level increases, leading to the increase of trap barrier to overcome for charge trapping. The deep trap density increases, leading to a decrease of the charge average free travel and energy obtained from migration, increasing the probability of charge trapping, and leading to a decrease in mobility. Therefore, the internal collision ionization probability of the composite material is reduced, leading to the improvement in the resistance to electrical tree.

#### *4.2. Electrical Tree Dependence on Polycyclic Compounds with DC-Impulse Voltage*

It can be seen from the results in Section 3.1 that the e ffect of the polycyclic compound is related to the polarity of the DC-impulse voltage, and the suppression e ffect of the polycyclic compound with the opposite polarity is better than with the same polarity. When −35 kV impulse is superimposed on −25 kV DC voltage, after the DC voltage is applied, electrons are accumulated at the needle tip. After the impulse voltage is applied, a large number of electrons are injected into polymer over a short period of time to push accumulated charges to move. During the process of charge trapping, as the state of charge changes from high energy to low energy state, excess energy is transferred to other charges, making them hot electrons, destroying the XLPE molecular chain, and causing growth of the electrical tree. When +35 kV impulse is superimposed on −25 kV DC voltage, and after the polarity of the voltage changes, part of the positive charge is injected and the positive and negative charges neutralize to generate energy, which accelerates the growth of the electrical tree [43]. In addition, as the polarity of the voltage changes from negative to positive, the electrons change from the same-polar space charge to the hetero-polar space charge, causing electric field distortion, aggravating partial discharge, causing molecular chain breakage, and accelerating electrical tree growth [44]. The mechanism of electrical tree growth with di fferent polarities is di fferent, which results in di fferent e ffects of polycyclic compounds with di fferent polarities. With the opposite DC-impulse polarity voltage, and after the polycyclic compound is added, on the one hand, the electric field distortion caused by the polarity change is reduced due to the decrease of the injected charges; on the other hand, since free charges inside the sample are reduced, the energy generated by the positive and negative charges is reduced. Therefore, the polycyclic compound e ffect with the opposite polarity is better.

#### *4.3. Electrical Tree Dependence on Polycyclic Compounds at Di*ff*erent Temperatures*

Electrical treeing is an electro-thermal aging phenomenon, which is a comprehensive process including charge motion, partial discharge, local high pressure, and local high temperature. The hot electrons accelerate in the free volume, which impacts the molecular chain of the polymer and accelerates the formation of low-density regions. Charges collide with ionization in low-density regions, releasing energy to destroy more molecular chains and forming micropores. Subsequent discharges then occur in the micropores. Partial discharge produces local temperature rise in a short time. When the ambient temperature plus local temperature rise is greater than the local softening temperature of the insulating material ( Δ *T ambient temperature* + Δ *T discharge temperature*<sup>&</sup>gt; *T softening temperature*), local thermal breakdown will occur, and the air gaps form cracks along the applied field direction [45]. Under the action of charged particles, the XLPE molecular chains are broken rapidly and proceed to decompose and gasify. With these gases generated in a short time, the gas pressure in the micropores and cracks increases rapidly, resulting in the material around the micropores or cracks to undergo an expansion stress. Under the action of this stress, the micropores and cracks rapidly expand toward the amorphous region where the mechanical strength is weak, resulting in the growth of the electrical tree. As the temperature increases, on the one hand, the carrier mobility increases, the ionization probability increases, and the polycyclic compound's ability to capture hot electrons is relatively reduced; on the other hand, the local thermal breakdown increases, and the influence of hot electrons on the growth characteristics of the electrical tree is relatively reduced. Therefore, the e ffect of the polycyclic compound decreases with temperature increase.
