Mechanism Analysis of Transformer Oil Under Lightning Impulse Voltage

作者： GOZ Electric 时间：2024-08-30 09:40:59 阅读：10

In the past, in the analysis of the breakdown mechanism of liquid dielectrics, a single physical process was often used to explain the development process of streamers under certain discharge conditions or in a certain type of liquid. However, more and more evidence shows that the development process of streamers in liquids involves multiple mechanisms at the same time. The development and propagation mechanisms of streamers are different within different pulse durations and at different time stages of the same pulse voltage. The traditional bubble theory or liquid direct ionization theory cannot fully explain the entire process of streamer development in liquids, especially in gas-based transformer oil.

Based on the aforementioned quantitative analysis of the change law of streamer morphological and structural parameters in gas-based oil and mineral oil under lightning impulse, the specific role of Joule heating effect and spatial photoionization in the initial, development and final stages of streamers is discussed, thereby establishing a complete physical model of slow and fast streamers in gas-based oil in the entire discharge process.

1. The physical nature and formation mechanism of fast and slow streamers in the initial stage

In the initial stage of streamers, the physical nature of fast and slow streamers is a low-density gas channel formed by local gas-liquid phase change. The three physical processes that may lead to the formation of the initial bubble cavity of the streamer are shown in the figure. As can be seen from the figure, under the action of lightning impulse voltage, there are three possible physical processes that lead to the formation of the initial bubble cavity of the streamer: local liquid vaporization caused by the conduction current and field emission current of the streamer head under the action of external voltage, the stretching and rupture of the microbubbles, holes, gaps and invisible microbubbles originally existing in the liquid under strong field strength, and the cavities and low-density areas in the liquid caused by the local liquid rupture due to the electrostrictive effect under the action of the strong electric field when the pulse duration reaches the sub-nanosecond level. For slow streamers, the evaporation of liquid caused by the Joule heating effect of the current and the formation of microbubbles are the main reasons for the formation of the initial streamer cavity. For fast streamers, the original microbubbles in the liquid and the electrostrictive effect under strong field strength are the main reasons for the formation of the initial streamer cavity.

Figure 1

The mechanism of microbubbles on the formation of the initial discharge channel of fast streamers is as follows: Theoretically, the mean free path of liquid molecules is very short, and it is difficult for electrons to obtain enough kinetic energy to collide with liquid molecules under the electric field to ionize, triggering an avalanche ionization process. In addition, free electrons can be captured by liquid molecules within picoseconds, which increases the difficulty of maintaining free electrons and makes breakdown difficult to occur. In order to trigger the avalanche ionization process, the mean free path of liquid molecules needs to be increased. The low-density gas channels that already exist in the oil can promote the occurrence of streamer discharge. These low-density channels, such as microbubbles, can generate high-energy electrons during discharge through cavitation, shock waves and chemical reactions, breaking the valence bonds of oil molecules, and then stretching out a series of new cavities, which further undergo chemical changes under the action of strong field strength to produce initial streamer cavities.

The mechanism of electrostriction on the formation of initial discharge channels for fast streamers is as follows: When a rapidly changing strong electric field is applied to the electrode, the liquid near the electrode will be affected by the mass force pointing in the direction of the electrode, but the liquid cannot move due to inertia, which will cause the local liquid to be subjected to huge negative pressure in an instant, causing the local liquid to rupture and produce cavities. Under the action of a strong electric field, the cavity will deform along the direction of the electric field line, which provides sufficient acceleration distance for free electrons and creates conditions for the occurrence of an avalanche ionization process.

At the beginning of the streamer, for the slow streamer, the initial bubble cavity is first formed due to the Joule heat, and then the bubble explodes under the action of the strong field strength, generating shock waves and micro-discharges. The shock wave diffusion process promotes the development of the ionization process, generating more bubbles, and finally leading to the formation of the gas channel and the streamer discharge.

For the fast streamer, the initial arc channel has extremely high temperature and pressure, so it will expand outward rapidly and generate shock waves. After a short initial expansion process, the wavefront of the shock wave separates from the edge of the arc channel. After that, the propagation speed of the shock wave is greater than the expansion speed of the arc channel. When the arc expands to a certain stage, a gas transition sheath with lower brightness appears between the outer wall of the arc channel and the liquid, which also marks the formation of outer bubbles. The greater the energy deposited by the arc, the higher the intensity of the shock wave.

2. The physical nature and formation mechanism of fast and slow streamers in the intermediate and final stages

In the streamer propagation stage, the physical nature of fast and slow streamers is the photoionization process caused by the high field strength at the streamer head, and the continuous generation of gas-liquid interface during the direct ionization of the liquid. The development and breakdown of the streamer under lightning is shown in Figure 2. Under negative polarity, after the initial streamer cavity is formed, strong spatial photoionization is formed at its head with a strong electric field. The main streamer collapse head expands outward from the cathode. The free electrons formed by photoionization at the top of the head will further excite atoms to form new photons. These photons continue to diffuse in all directions, promoting the formation of secondary electron collapse. After a period of time, the secondary electron collapse merges with the main collapse head and quickly develops to the anode as a negative streamer that penetrates the electrode gap. The propagation speed of the negative streamer is much greater than the expansion speed of the electron collapse.

Figure 2

Under positive polarity, due to the effect of the external electric field, the cathode emits electrons to move toward the anode to form an initial electron avalanche, and the initial streamer cavity is formed through processes such as electrostriction and stretching of microbubbles. As electrons are continuously injected and accelerated, strong photoionization occurs in the initial cavity of the streamer, resulting in an increase in the charge density of the head space and strengthening the electric field of the tail of the streamer. At this time, the streamer releases a large number of photons and causes a secondary electron avalanche. The secondary electron avalanche will converge with the main avalanche head and release photoelectrons to the streamer head, causing the streamer head to become a plasma region composed of a large number of positive and negative charged particles. The electric field of the streamer head is further strengthened and continues to advance toward the cathode, eventually running through the entire discharge channel.

From the above studies, it can be seen that the slow streamer mainly presents a dark channel morphology with a weaker degree of photoionization in the development and final stages, and the fast streamer mainly presents a bright channel morphology with a stronger degree of photoionization in the development and final stages. The change of the streamer morphology from dark channel to bright channel is the mechanism reason for the sudden change of the streamer propagation mode. The author believes that the ionization of a small amount of low ionization energy compounds such as aromatic hydrocarbons in the oil leads to the formation of dark channels. The ionization of high ionization energy compounds such as alkanes leads to the formation of bright streamer channels. After the initial streamer channel is formed in the insulating oil, due to the action of the shock wave, during the impact process, the particles in the discharge channel may be affected by the particles at the electrode or the electric field force, generating an interactive dislocation force or energy, causing the surrounding liquid to undergo a chemical reaction to produce new gas, resulting in the continued formation of the gas-liquid interface.

Conclusion

The discharge and breakdown phenomena and mechanisms of streamers in gas-made oil and common mineral oils in a 25 mm needle-plate electrode system were observed and analyzed in this paper. The results show that compared with mineral oil, gas-made oil has higher positive and negative polarity lightning impulse breakdown voltages. Under positive polarity, the acceleration voltage of the gas-made oil streamer propagation from a slow streamer to a fast streamer is significantly lower than that of mineral oil. Under negative polarity, the acceleration voltage of the gas-made oil streamer is significantly higher than that of mineral oil.

At the beginning of the streamer, the physical essence of both the fast and slow streamers in transformer oil is a low-density gas channel formed by local gas-liquid phase change. For the slow streamer, the evaporation of the liquid caused by the Joule heating effect of the current and the formation of microbubbles are the main reasons for the formation of the initial streamer cavity. For the fast streamer, the microbubbles originally existing in the liquid and the electrostrictive effect under strong field strength are the main reasons for the formation of the initial streamer cavity. At the stage of streamer propagation, the physical essence of both the fast and slow streamers is the photoionization process caused by the high field strength of the streamer head, and the continuous generation of the gas-liquid interface during the direct ionization of the liquid, and finally the continuous advancement of the streamer head through the entire electrode gap, resulting in breakdown.

Based on the analysis of the propagation characteristics of streamers in gas-to-liquid and mineral oil, it is inferred that the transformation of the streamer morphology from dark channel to bright channel is the mechanism causing the sudden change of the streamer propagation mode. The ionization of low ionization energy compounds such as aromatic hydrocarbons in gas-to-liquid leads to the formation of dark channels. The ionization of high ionization energy compounds such as alkanes leads to the formation of bright streamer channels.