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There are two hypotheses in the leader progression model to illustrate the insulator breakdown process. One is the streamer onset time is relatively small that can be ignored. The other is the leader velocity at both sides of the composite insulator strings are equal. These assumptions are mostly based on the experimental results in rod-rod air gaps. In this paper, experiments were performed to study the breakdown process of a 220-kV composite insulator string with grading rings under positive short tail lightning impulses. The discharge current at the high-voltage side and the high-speed photographs were recorded synchronously. The method of identifying four typical stages of the breakdown process from the current waveform was proposed. The experimental results show that the streamer onset time should not be ignored when calculating the breakdown time, especially for the composite insulator string with grading rings. By fitting the experimental results, the relationship between the streamer onset time andUP/U ${}_{\text{50}}$ has been given. The results also indicate that the velocity of the positive leader is nearly 1.3 times than that of the negative leader during the breakdown process when the positive impulse is applied to the composite insulator string. [ABSTRACT FROM AUTHOR]

The tracking wheel test specified in IEC/TR 62730:2012 provides a method that can be used to evaluate the resistance of composite insulators to tracking and erosion for rejecting inadequate materials or designs. Currently, most of the tests have been conducted on composite line insulators and some other insulator types with relatively small diameters. As the usage of large-diameter composite insulators, such as composite station insulators and composite cross-arms, increases rapidly, it is urgent to investigate if the tracking wheel test method is also suitable for their evaluation. In this study, a tracking wheel device for composite insulators with different diameters, designed and constructed in accordance with the IEC standard, is presented. The relationships between the leakage current and NaCl content as well as the average diameter of the specimen are investigated. It is shown that the test parameters specified in the IEC standard are not suitable for specimens of relatively large diameters. Accordingly, a relevant modification of test parameters has been introduced and verified, showing similar phenomena during the verified test as those observed during previous studies on composite line insulators. It is thus claimed that the modified test procedure is feasible for being applied on large-diameter insulators. [ABSTRACT FROM AUTHOR]