Together with water and temperature, Oxygen is one of the three key elements responsible for accelerating the transformer aging process.

Oxygen (O2) enters into the transformer from the atmosphere, caused by continuous breathing of the transformer due to oil volume changes with load and temperature variations. At the same time O2 is also naturally consumed in the transformer due to the aging and degradation process. The rate of ingress and increase of O2 depends on the oil protection system type and integrity, while the rate of consumption that results in O2 decrease depends on the speed of degradation.

Where there is a failure of the sealing system, for example caused by a membrane rupture, O2 will increase. By comparing O2 readings and their trend with the sealing characteristics, this can easily be detected.

On the other hand, if the insulation is degrading over time and consuming O2, O2 levels will decrease and by comparing it with CO and CO2 data, this can also easily be detected.

Monitoring O2 online is important because:

• It detects insulation aging and allows us to calculate aging speed and an estimate of remaining life, combined with temperature and moisture effect (ref IEC60076)
• It detects failures in the sealing system
• It is more reliable than a laboratory test, where results are strongly affected by manual sampling mistakes, leading to non-repeatable trending.
• O2/N2 is now used to calculate DGA condition according to IEEEc57.104:2019

There are no Alternatives

There are monitors available that, due to technical limitations, cannot measure O2, and their suppliers propose alternative methodologies. When looking at monitor suppliers, it’s important to consider that insulation aging and sealing failures both involve O2. Detecting O2 will directly measure these two phenomena. Any alternative method will be an indirect estimation. The total gas pressure will be able to detect large leaks but it has no advantage over the direct measurement of O2 which is the main gas of interest when there is a leak. Additionally, total gas pressure will not be sensitive enough to detect small leaks or aging consumption when compared with O2 measurements, because O2 is only a portion of the total gas pressure.

In all cases, if O2 is the key parameter, measuring O2 is the best choice.

Furthermore, insulation aging is typically assessed by measuring O2, CO, CO2 and then comparing this with oil quality data, mainly furans and acidity. It is important to note that gases such as C3Hx propane and propylene are not directly linked with insulation aging and are only occasionally used to confirm low temperature overheating (not a sign of ageing).

CIGRE TB771 states that C3 gases “do not really provide additional information”

IEEE C57.104 states that C3 gases are “not used in this guide”

And IEC60599 states that “diagnostics can be made without taking into account these hydrocarbons”.

Click here to find out more about the TOTUS Transformer Monitor from Camlin Energy.