POWER
Figure 2 TGP data from two new power transformers immediately after commissioning
dioxide, hydrogen, methane, acetylene, ethylene and ethane) represent a tiny fraction of the total pressure, whereas nitrogen and oxygen are dominant. An increase in TGP in a sealed transformer therefore signals an air leak, without the need for specialised expertise.
markers like furans or methanol, as well as gas concentrations such as oxygen, carbon dioxide and carbon monoxide. The assessment of solid insulation life expectancy is a complex task that demands a high level of expertise. This is because, in contrast with transformer faults that may develop rapidly, oxidation is a slow process, usually taking years to show significant progress. Therefore, real-time gas monitoring is better suited to the detection of quickly evolving faults, than it is for tracking oxidation. In reality, decision-makers in transformer maintenance do not need to know precise oxygen or nitrogen concentrations. What they really need to know is whether the insulation system of a sealed transformer is exposed to air, which allows them to make informed decisions on actions such as oil degassing or dry out or sealing system repair.
TOTAL GAS PRESSURE (TGP) – A STEP FORWARD IN AIR LEAK DETECTION A simpler way to detect air entry is by measuring the pressure of all dissolved gases. TGP sums up the partial pressures of all the gases, to produce a more useful, easy-to-understand parameter. The key fault gases (carbon monoxide, carbon
UKManufacturing Autumn 2024
Degassing of oil during commissioning should ensure that dissolved gas pressure is very low in newly installed sealed transformers. A noticeable increase in pressure therefore indicates a potential problem, and demonstrates the advantage of TGP over oxygen measurement, which may remain relatively unchanged due to oxygen-consuming reactions. A properly sealed transformer should maintain low gas pressure levels for its entire lifespan - well below atmospheric pressure 1000 hPa (14.5 psia). However, a rise in TGP values is a clear indication of air entry. If the data indicates a recent ingress of air marked by an increase in TGP, maintenance teams can decide whether they need to inspect and repair the sealing system during the next service break, to prevent further deterioration of the insulating paper due to oxygen.
TGP MEASUREMENT IN PRACTICE The TGP in operational membrane-sealed transformers usually ranges between 100 to 300 hPa (1.45-4.4 psia). Figure 1 illustrates TGP readings provided by the Vaisala OPT100 online DGA monitor in comparison with calculated partial pressures of oxygen and nitrogen at 50°C, based on gas concentrations determined by a laboratory from standard DGA oil samples with gas chromatography. The data is from a new transmission transformer with membrane sealing, with monitoring initiated a few weeks after commissioning.
Over an 11-month period, TGP increased by 23 hPa (0.33 psi) with a corresponding increase
in nitrogen (N2) of 1,500 ppm and no detected change in oxygen (O2) levels. The clear increase in nitrogen concentration without a rise in oxygen
indicates an active ageing process, which is likely to be consuming the oxygen. This highlights the sensitivity of pressure measurement to even minor levels of air ingress. An additional advantage of TGP measurement is that it is inherently free from air contamination, which is a constant risk when working with oil samples.
The GSU1 transformer maintains a consistently low TGP level in its initial two years. The TGP level rises markedly immediately after commissioning due to air from the transportation phase, present in the oil-impregnated paper. However, this gradually moves to the bulk oil, causing a slight increase in TGP, which then stabilizes at a low level. The average annual increase in TGP after stabilisation is only 7 hPa (0.1 psi), suggesting that it would take around 130 years to reach equilibrium with atmospheric pressure. The data therefore indicates that GSU1 is well-sealed and does not require any additional investigation or maintenance.
The TGP data for the GSU2 transformer presents a different, more concerning scenario. After degassing, a rapid TGP increase of f30 hPa (0.43 psi) is revealed, due to gas coming from the oil-impregnated paper. However, in contrast with GSU1, the readings do not then stabilise; instead, they continue to rise significantly. Over the next two years, the rate of change is approximately 60 hPa/year (0.87 psi/a), and if this were to continue, it will take just 15 years for the oil to become saturated with air. Clearly, oxygen and possibly moisture are entering the transformer continuously, potentially reducing its lifetime. With the benefit of the TGP monitoring data, it becomes possible to weigh the expected lifespan of the transformer against the costs of investigating and repairing the leak or implementing mitigation measures. TGP therefore represents the ideal monitoring technology for the reliable protection of transformers.
Vaisala
www.vaisala.com
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