Positive third mode streamers in insulating oil

Third mode streamers have been studied in the mineral oil Nytro 10 XN, gas-to-liquid oil Diala S4 ZX-1 and the high viscosity white oil Primol 352, in an 80 mm point-plane gap under step impulse voltage. Third mode streamer velocities were in the range 4-60 km/s. The "acceleration" of the average velocity with increasing voltage is not due to third mode, and third mode existed in a wide voltage range in both Nytro 10 XN and Primol 352. The voltage where third mode becomes common is almost the same in Nytro 10 XN and Diala S4 ZX-1, indicating that the presence of aromatics may have little influence over the initiation voltage for third mode. Onset voltage for third mode was very low in Primol 352.