Introduction
Bushings are one of the top failure modes of power transformers. 50% of bushing failures cause a major fire and total destruction of the transformer. Many times the failure modes of bushings are voltage or temperature dependent and may fail in a matter of weeks. This customer’s offline tests were performed approximately four months prior and all results were acceptable. Offline testing can provide misleading results since the full effect of operating conditions are not present. There are many cases where offline results show “good” readings, but in fact the bushing is defective.
50
50% of bushing failures end in fires!
The Challenge
During planning of an upcoming outage, the customer reviewed bushing data from Dynamic Ratings’ Diagnostic Transformer Monitor (DTM). The system was not connected to a SCADA system, so the customer regularly downloaded and reviewed data using the supplied software.
Upon review, polar diagrams and system graphs were generated. It was noticed that
the imbalance current and the C Phase Power Factor readings were increasing. The customer elected to perform preventive bushing service.
The Solution
Since the customer had an upcoming scheduled outage, they were able to build this bushing replacement into their outage schedule and avoid a last minute schedule adjustment or worse yet, an eventual and destructive failure.
The Result
Continuous monitoring of transformer bushings can provide early indication prior to a failure. By providing an early warning, asset managers and maintenance personnel can schedule a planned outage. Taking a planned outage is far more cost effective than having an unexpected outage. The customer estimated cost avoidances equal to several million dollars, noting the costs of a new transformer and lost production.
