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Assessing the insulation condition of high-voltage transformers is essential for repairing and maintaining based on condition of equipment. For the instant diagnostic examination of PD with oil-paper insulated transformers, basically physical method directly related to PD is currently used electric signals. Based on advanced sensor technology, PD detectors with enhanced options for discriminating disturbances were carried out on-site and on-line. PD values condition changes of the insulation system can early be identified and evaluated with simple trending. Consequently, PD monitoring and diagnosis systems allow conditioned based maintenance strategies, serve asset appraisements and permit continuous risk assessment. The reliability and safety of the HV equipment and the system operation of the power network can be increased. The application of on-line and on site PD diagnosis of power transformers in service is a powerful tool to detect defects in the coil insulation. The approach with long term monitoring with remote access through substation communications or the web is important for the lifetime management of ageing transformers The techniques are described, together with cased studies of their application on the engineering practice field.