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The corona pattern from porcelain insulator strings on high voltage (HV) transmission lines has been investigated for inspecting faulty insulators on-line. Corona patterns from HV conductive line, metal accessory, and discharge patterns from bad connection, power stations, have also been studied for background pulse noise elimination. A pattern recognition method for inspecting faulty porcelain insulators is proposed, which consists of several criteria. It is found that the harmonics and carriers from the transmission lines influence the recognition seriously, which can be eliminated by choosing a high frequency band of sensors (2 MHz-20 MHz). Based on the above investigations, an instrument was developed and used on the field measurements in Lanzhou and Zhengzhou power utilities in China. The field results show a strong support for this method. On the measurement of 76 towers, the measuring accuracy of inspecting faulty insulators was about 87.5%.