The presence of water in the paper and oil system inside the transformer can be an extremely critical condition, especially when the criticality reaches an “extremely wet insulation” level (Table A1, IEC 60422, page 42), causing unreasonable risks of electrical failure.
Table A.1 – Guidelines for interpreting data expressed in per cent saturation – IEC 60422 ed. 4-2013
This condition cannot be resolved with physical treatment of the oil alone, which would reduce water in the oil but not in the papers. The phases of energisation of the transformer at low temperature with sudden load are particularly critical. In these cases, water bubbles are very likely to form.
The life cycle management strategy is to avoid the formation of water in the oil-paper system as much as possible. Although it is impossible to eliminate this criticality, it can be prevented or mitigated through appropriate operational practices (e. g. analytical control of oil and indirectly of papers, oil treatment, load profile management, cooling of the machine). If the transformer belongs to a family of equipment affected by failure for the same criticality, ad hoc operating practices can be defined that optimise the various critical factors (e. g. by installing dehydrating cartridges).
Prevention actions during the life cycle of the transformer
Monitoring symptomatic indicators (see symptoms above). If the first symptoms of criticality appear, such as a high content of water in the oil, it is recommended that the frequency of analyses be increased in order to monitor trends.
Perform appropriate oil treatments in order to keep moisture low in the papers and in any case in order to avoid reaching the “wet insulation” condition (Table A. 1 – IEC 60422 ed. 4-2013).
Suggested actions include:
Physical treatment
This is a process performed on site, keeping the transformer in service (and under load) without having to empty it. The operation is carried out using a Modular Decontamination Unit (MDU) specifically created by Sea Marconi. The transformer is connected to the DMU by flexible hoses; the oil contaminated with DBDS is sucked from the lower part of the transformer and transferred into the DMU, which heats it, filters it, degasses it and dehumidifies it before pumping it back into the upper part of the transformer. This creates a closed loop which, every time the oil is circulated, is able to restore the values of the main physical parameters of the oil (water, gas, particles). (read more)
Application of cartridges for dehumidifying the transformer
This activity is accomplished by means of an apparatus which is placed on the transformer and operates continuously as a loop circuit under load and has columns with molecular sieves for selective adsorption of moisture and other polar compounds.
For breathing machines with conservator, it is advisable to periodically check the drying salts (read more)
In the case of moist papers, the oil change is not a definitive option because the water absorbed from the papers would not be removed even by changing the oil. The oil change operation could also create air bubbles (and consequent partial discharges) that are trapped in the dead zones of the transformer: under the head or in the radiators.
It is recommended performing (and keeping updated) the dynamic inventory of transformers with indication of the markers (indicator) symptomatic of the “Water in the transformer (paper and oil)” criticality throughout all phases of the life cycle. It is also advisable to map the equipment in “wet” and “extremely wet” conditions.
Other than those using minerals, what are the preventive measures for electrical machines with insulating liquids?
With regard to natural ester oils and synthetic esters, the preventive actions are the same; however, it is advisable to choose countermeasures after careful assessment in terms of cost-benefit, cost-effectiveness and environmental impact (biodegradability and fire safety). For silicone oils in operation, the treatments recommended by the standard (IEC 60944:1988) are “vacuum treatment and filtration” and “molecular sieves and filtration”.
