Wednesday, 14 November 2012

Transformer Maintenance - Silicone Oils

This entry is an extension of Transformer Maintenance - Mineral Oils to some extend. Again, this article is not a complete guide. It gives you an overview of how complicated it is. A lot is to be considered in making an informed decision in regards to a transformer especially if it is a highly critical asset.

Silicone oil when new contains a saturated amount of oxygen. In the initial years of operations, carbon monoxide and carbon dioxide will be generated. As the transformer ages, oxygen is depleted, generation of these gasses slows and plateau off after a few years of operations assuming without any faults. The generation rates of these gases should be relatively constant from normal aging after that. Which is why it is very important to start DGA immediately and start plotting the graph curves and track these changes. Without a graph, it is almost impossible to make a judgement.

Comparison of Silicone Oil and Mineral Oil.

  1. Silicone oil-filled transformer will have a great deal more CO than normal mineral oil-filled transformers. CO comes from the oil itself and from degradation of paper insulation. It is therefore if DGA indicated little other fault in gas generation besides CO, the only way to tell for certain if CO is coming from paper degradation is through furan analysis. If other gasses are involved, there obviously is a fault and paper degradation was accelerated.
  2. Hydrogen level is generally higher comparatively to Mineral oil filled transformer.
  3. Due to "fault masking" environment with Silicone oil, DGA lost many of its fault finding capabilities. One exception is acetylene that points to an active arcing. It is then very important to continually track the gas generation rates and operating history. Records, records, records!
  4. Oxygen level will be high during new and consumed over its life by the generation of CO and CO2.
  5. Any spike in O2, CO2 and N2 after a few plateau reading would very likely indicate a leak to atmosphere.
Due to the rather infancy stage of Silicone Oil usage, these gas limit extracts are use as a reference and will change over time as the world gain more experience dealing with them. Use it with care. This is a Doble 95% Norm limits of 299 operating transformers, which are more conservative in some way than IEEE limits.

Hydrogen                              511ppm
Methane                                134ppm
Ethane                                     26ppm
Ethylene                                  17ppm
Acetylene                                  1ppm
CO                                     1750ppm
CO2                                 15480ppm
Total Combustibles              2000ppm

Keep in mind that the amount of gas is not the key. The key is the generation rate of the gasses. Refer to IEC 60599 for the generation rates. G1 rates should raised concerns along with sampling rates increased and expert opinions seeked. G2 rates should be an immediate extreme concern that the reaching the L3 - high limit of IEEE will happen very quickly. Consideration should be taken to take it offline.

A reference of physical test limits for service-aged silicone fluid
Test                                     Acceptable limits      Unacceptable values indicated         ASTM method
Visual                                  Clear free of particles        Particulates, free water                     D1524, D2129
Dielectric breakdown                     30kV                     Particulates, dissolved water                      D877
Water content max.        70ppm(Doble) 100ppm (IEEE)  Dissolved water contamination              D1533
Power Factor max@25degC         0.2                           Polar/ionic contamination                         D924
Viscosity at 25degC, cSt          47.5-52.5                    Fluid degradation contamination                  D44
Acid number                        0.1(Doble) 0.2(IEEE)  Degradation of cellulose or contamination        D974

Reference of this article:
  1. Transformers: Basics, Maintenance, and Diagnostics - Reclamation, US Department of the Interior Bureau of Reclamation, April 2005.
  2. Trial-Use Guide for the Interpretation of Gases Generated in Silicone-Immersed Transformers, IEEE P1258, 1999.

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