The development and application of online monitoring technology has been carried out in my country for more than ten years. This work has played a positive role in improving the operation and maintenance level of equipment, timely discovering hidden faults, and reducing the occurrence of accidents and emissions. Since the 1950s, my country has been conducting electrical power outage tests, overhauls and maintenance for power equipment in accordance with the provisions of the preventive test regulations for power equipment for decades. Regular tests cannot find hidden faults inside the equipment in time, and the power failure test applies a test voltage lower than the operating voltage, which is not sensitive enough to reflect some defects. As the power system develops in the direction of high voltage and large capacity, it is more and more important to ensure the safe operation of power equipment, and the impact and loss of power outages on production and life are also increasing. Real-time or timed online monitoring of the status, timely reflect the degree of insulation quality and deterioration, so that preventive measures can be taken to avoid power outages.