One time testing cannot be based to a big decision towards any action on transformer. So periodic testing is Invariably required to monitor the trend in a test results and action can be initiated following the trend. If a Single transformer is saved due to action taken after monitoring the trend in result. It’s a big asset to the organization and on the other hand a huge data on different transformer is gathered on which future action can be based.

All the oil testing parameters can be used for trending purpose, but Dissolved Gas Analysis is the best tool which can be used for Trending, Monitoring and early fault detection purpose.

Information from the analysis of gasses dissolved in insulating oils is valuable in a preventative maintenance program. A number of samples must be taken over a period of time for developing trends. Data from DGA can:

-	Provide advance warning of developing faults
-	Provide a means for conveniently scheduling repairs
-	Monitor the rate of fault development

DGA reports have a diagnostic section where an analysis is performed on the dissolved gas results using IS:10593 (Ratio Method) & Roger Method. The information provided by the report is often sufficient to properly evaluate the operating status of the transformer but gassing history is also an extremely important aspect of fault detection.

At NDL, all DGA results are kept in a readily accessible database and historical data is checked every time a sample is analyzed. When a significant change in gas concentrations is observed, the client is contacted immediately. through Telephone, fax or e-mail.

At NDL Automation of Testing & Reporting led to better management of transformer oil tests. Re sampling schedule, gas history graphs and fault assessment algorithms based on stored DGA results are provided to offer a quick and simple way of deciding which transformers should be tested and in which priority.