Calculation of DGA Limit Values and Sampling Intervals in Transformers in Service

Methods are presented in this paper allowing individual networks or transformer users to calculate their own specific DGA gas limits and required sampling intervals as a function of gas concentrations and rates of gas increase in service. This calculation can be done on the entire transformer population or on more specific segments of it. The purpose of this paper is only to provide a tool for calculating gas levels requiring more frequent sampling intervals for DGA. The actions to be taken on the equipment at any of these gas levels and sampling intervals (e.g., removal or not from service, additional testing, installation of on-line gas monitors) are outside the scope of this paper. These actions depend on a large number of other parameters (e.g., experience and maintenance practices of individual users, strategic importance and type of equipment used, type and location of the fault, fault active or not). The actual sampling intervals to be used also remain the decision of maintenance personnel, based on their best operational practices.