Vaisala has demonstrated the role of online monitoring in keeping tabs on developing gas trends in a transformer and assessing the efficacy of repairs. As part of a study conducted alongside Fingrid, Finland's national transmission system operator, Vaisala deployed its Optimus online Dissolved Gas Analysis (DGA) device to monitor the removal of dissolved gases from a 400 MVA transformer. The study demonstrates that, for transformers with a fault history, online measurement of dissolved gas levels is crucial, even after repair and maintenance work.
Managing a Recurring Problem
Once significant levels of fault-generated gases are detected in its oil, a transformer must be taken offline and have its fault diagnosed. Often, the transformer oil is also treated during or after the actual repair of the fault. This process is known as degassing. When the gases are removed from the oil, a new baseline reference is formed to detect any increasing gas trends in the future; this in turn is essential to reveal internal thermal activity or fault, which generates gases. It is difficult to achieve full removal of all dissolved gases, meaning that gas levels may begin to rise again shortly after degassing, even without any fault being involved.
As a result, online DGA monitoring is required, since a transformer continues in operation after the fault has been repaired. In conjunction with Fingrid, Vaisala equipped a 400 MVA power transformer in Finland with a multi-gas Optimus DGA monitor prototype, which remained in place before, during and after scheduled degassing work. The monitor tracked changes in dissolved gas levels in real time, demonstrating how, even after the transformer in question had undergone repair and initial degassing, an underlying thermal issue remained. Thus, utilizing a multi-gas monitoring device is essential also going forward, to monitor this transformer continuously and with enough accuracy and good repeatability to illustrate gas trends consistently.