Development of an on-line continuous tan(δ) monitoring system

The ageing and deterioration of oil-paper insulation in HV equipment is a matter of continuous concern. Explosions resulting in damage to property, loss of capital and human life, and the need for replacement has prompted the drive towards an improved transformer design and recognition of the need for insulation condition monitoring. Insulation testing is reasonably well defined for routine and type testing of HV equipment in laboratories. However diagnostic criteria and procedures for testing the insulation condition of equipment in service are not well established nor have they been standardised. Continuous online monitoring of insulation condition presents several advantages such as an increased probability of predicting insulation failure and increased safety. This paper reports on the development phase of an on-line continuous tan(δ) monitoring system. The criteria for such a system in terms of practical and conceptual ideas for in-service operation and future development are discussed. Results from laboratory tests and those from a system which has been installed in a substation demonstrate the concepts of absolute and relative tan(δ) measurement and the characteristic trends and patterns in the tan(δ) of oil-paper insulation. The approaches towards combining data such as type of equipment, age of equipment and load profiles, and methods of data evaluation are discussed with the objective of establishing a system to diagnose the condition of oil-paper insulation