122 / 2018-08-24 22:39:15

Investigations of DC PD Characteristics induced by Metal Particles in Flowing Transformer Oil

Convert transformer, DC voltage, partial discharge, flowing transformer oil, metal particles, computational fluid dynamics model

Power converter transformer is one of the critical components in high-voltage direct current (HVDC) transmission systems. Compared to the insulation in alternate current (AC) power transformer, its insulation, i.e., transformer oil and insulation paper, will withstand AC and DC electric field. Moreover, as same as AC power transformer, a converter transformer still has particle contaminations in transformer oil, such as metal particles from manufacture, installation and other processes. Those particles will damage the internal insulation performance of converter transformer by means of partial discharge (PD) and even ultimately lead to insulation failure. Therefore, many researchers have deeply studied the PD characteristics of metal particle in stationary transformer oil under AC, DC and composite voltage, and made much progress. However, it is well-known that the transformer oil in practical power transformers is forced to circulate in order to reduce the temperature rise of the winding and thus keep flowing. Besides, motion characteristics of the particles affects significantly the PD characteristics according to much previous researches. Under this circumstance, the PD characteristics in flowing oil must be different from that in the case of stationary due to the change of the motion characteristics in flowing oil. Recently, it has been shown that under AC voltage, increasing flowing velocity of transformer oil could suppress the occurrence of PDs based on the PD measurements of ultra-high frequency (UHF) antenna. However, the related research on the PD characteristics in flowing transformer oil under DC voltage is quite scarce.
Therefore, this paper studied the characteristics by constructing a transformer oil flow circulation platform with a standard PD measurement circuit of IEC 60270. The circuit could measure PD current pulses generated by micron-order metal particles in flowing oil between parallel plate electrodes under DC high voltage. The results showed that as the flowing velocity increased, the discharge number sharply decreased. In order to explain the results, we established a computational fluid dynamics (CFD) model based on a Euler-Lagrange method and coupled DC electric field to obtain the motion characteristics, such as trajectories of metal particles. The simulated trajectories showed that, in early stage, the uncharged particles would move horizontally with the flowing oil and fall gradually toward the grounded electrode at the same time due to the gravitational force. After obtaining initial charge by contacting the electrode, they continued to move horizontally and simultaneously oscillated up and down between parallel electrodes due to the electric field force, and collided with the electrodes. On one hand, according to the counting results of the particles falling toward the grounded electrode under different flowing velocities, the particles in the oil under a low flowing velocity more easily fell toward the grounded electrode than that under a high flowing velocity. Therefore, more uncharged particles in slow transformer oil flow could obtain initial charge, which leads to more collisions with the electrodes. On the other hand, for a single particle, both the time to fall toward the grounded electrode and the time interval between two consecutive collisions with the electrodes increased with the increase of the flowing velocity, thereby also increasing the number of the collision. Furthermore, the counting results of the collision showed that as the velocity increased, the number of the collision sharply decreased. If one collision with the electrode was considered as an occurrence of PD, it was found that the counting results were in accordance with the experimental results in terms of the descending trend. Consequently, we concluded that under DC voltage, increasing the flowing velocity of transformer oil suppressed the occurrence of PDs.