Total gas pressure for automated transformer condition assessment
A transformer’s lifetime is primarily determined by the condition of its solid insulation.
bySenja Leivo
Introduction to maximizing a transformer’s lifespan
A transformer’s lifetime is primarily determined by the condition of its solid insulation. Presence of oxygen in the transformer accelerates the ageing of its solid insulation, among which the most significant is the paper within the windings.
Monitoring and confirming proper sealing against ambient air ingress is crucial to maximizing a transformers’ operational life. With the industry increasingly turning to sealed transformer designs, the issue of air leaks as a source of oxygen has only grown in importance. At the same time, unmanned substations are becoming more and more common, the need for automation and online monitoring with electronic sensors increases. And this is where traditional methods are failing.
The easiest way to reliably detect air leaks is to monitor the total pressure of all dissolved gases in the oil. It is a completely new method to detect any air leaks into a sealed power transformer’s tank in real-time and in a very early phase. This new parameter, in addition to the online DGA monitoring, is a valuable tool for health indexes, automated transformer condition assessment and even operative loading guidance, just to give a few examples.
The Vaisala OPT100 DGA monitor uses partial vacuum to extract gases from the transformer oil, and it also includes a pressure sensor, which makes it possible to determine the air leaks by measuring total gas pressure
Shortcomings of traditional methods
Traditionally, air leaks into the transformer have been detected by measuring oxygen and nitrogen from standard DGA oil samples. However, the challenge with sampling at the transformer site, followed by transportation and handling of the samples, is a risk of sample contamination from ambient air. This kind of contamination can be identified by observing nitrogen level fluctuations in DGA results over extended periods of time (Fig. 1).
Some online monitors have integrated oxygen measurement. In some of them, nitrogen levels are calculated from the oxygen values. The problem is that such calculations always assume that the oil is saturated to ambient air with a constant oxygen-nitrogen ratio. And here is where it gets interesting: such calculated nitrogen values are valid for free-breathing transformers only — and not at all valid for sealed transformers.
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