Signs (visual inspection)
Oil* affected by the “Water in the transformer (paper and oil)” criticality can have different characteristic aspects.
*here we mean the sample of oil representative of the total mass of fluid present in the transformer casing. To do this, it is necessary to take the sample at the bottom of the transformer casing. In the presence of accessories (isolators, switches, etc. ), the sampling is repeated at the bottom of each accessory. During the sampling phase, it is absolutely necessary to indicate the sampling point and the oil temperature at the time of the activity
- A. oil with clear water-oil separation (free water).
In this case the aqueous fraction is deposited at the bottom of the sample, while the overlying oily fraction appears in the form of whitish emulsion. Referred at room temperature (e. g. 20 °C)**. This is the worst condition - B. oil with an appearance of milky emulsion. Reported at room temperature (e. g. 20 °C)**.
- C. oil with an opaque, whitish appearance. Reported at room temperature (e. g. 20 °C)**.
- D. ooil which has no characteristic signs because the water contained is completely solubilised. Reported at room temperature (e. g. 20 °C)**.
** If the temperature decreases, the separation step is accentuated and the water on the bottom increases (the water has specific weight 1 and mineral oil 0. 0,875 mg/cm3).
If the temperature rises, the phenomenon of phase separation is reduced by the natural solubilisation of the water in oil.
Info
In cases of internal inspection of the transformer, signs of free water or drops of water can be observed mainly at the bottom of the casing and at the bottom of the transformer casing head. These are very dangerous because they are close to high voltage areas. Signs of rust or localised paint stripping can be observed in the same area. In the case of loss of casing seal, windings can sometimes also also show signs of water.
Representative sampling
During external inspection of the transformer it is necessary to take representative samples of insulating oil in accordance with the reference standard and the operating instructions attached to the sampling kits (read more).
It is important that suitable sampling protocols are used and that sampling kits be used that can ensure sample retention until the pre-analysis phase – IEC 60475 “Method of sampling insulating liquids”. Likewise, it is important that the kit offers suitable data collection tools (transformer plate data, progressive numbering for each sample, etc.).
Should it be decided to carry out an internal inspection of the transformer, following a failure or in order to carry out a thorough inspection, it is strongly recommended to take samples of the insulating papers in accordance with relevant protocols and procedures. In particular, it is advisable to take papers from the top, bottom and middle of the both individual primary and secondary windings for each phase, taking multiple paper samples in areas with greater signs of criticality (see below).
