Acceptable Gas-in-oil Levels In Instrument Transformers (current Vs Voltage Equipement)

Dissolved-gas analysis (DGA) has been used extensively over the last 20 years to detect incipient faults in high-voltage transformers. The early warning it provides often allows corrective measures to be taken before catastrophic failure occurs. Based on the relative proportions of the gases formed, several DG-4 interpretation methods are available for identification of the fault (arcing, corona or hot spots) [l-41. The concentration levels at which the identified faults can be considered acceptable or dangerous in service are more controversial, however. Recent studies show that the probability norms currently used for that purpose are not very reliable and an alternative method based on the actual probability of failure in service has been developed [5, 81. Acceptable and dangerous gas levels in transformers thus appear t o be significantly influenced by the type of fault involved as well as by the type and age of the equipment. The general trends are that safe levels are lower for arcing than for hot spots, in the early and late years of the equipment, and for sealed instrument transformers rather than power transformers. Cellulose participation in the fault, as indicated by the CO/COZ ratio, also reduces the safe level of dissolved CzH4 in power transformers. In the present work, the influence of instrument transformer type (current vs voltage equipment) is examined.