Water in oil and cellulose; dehydration techniques by Sea Marconi on electric transformers

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The topic of water (moisture) in oil is one of the most debated questions in the domain of management of electrical equipment with insulating fluids. A lot has been written on this topic and a lot of debates took place during technical committees and conferences in this sector. The My Transfo Meeting, a conference organised by Sea Marconi, has always dedicated ample space to the problem of water in oil, thanks also to the privilege of having hosted the greatest worldwide expert on this topic: Viktor Sokolov (deceased in January 2008).The objective of this article is to illustrate the technical capability by Sea Marconi to provide a solution or to mitigate the problem of water in oil and cellulose through on-site dehydration processes (at the Customer’s location) with the transformer in operation or de-energised. The activities are carried out by mobile systems developed and manufactured internally: DMU (decontamination mobile unit).We divide the dehydration techniques that Sea Marconi performs with the transformer in operation (on-line) and those with the transformer off-line.

A. On-line dehydration techniques (Transfo in operation)

The key parameters are temperature and duration (time). The application requires several weeks or even months, as a function of the mass of insulation and the level of hydration, and is relatively less effective with respect to the off-line techniques. The interruption of the operation of the transformer is very short 8a few hours).

With respect to the off-line techniques using vacuum, they are less costly and applicable also for aged transformers and/or not accepting vacuum.

The most popular are:

A1. Molecular sieves

Molecular sieves are porous materials that have the property of capturing water dissolved in the oil, providing a progressive dehydration. The material is contained by columns. Installed on-board the transformer, through which the oil of the tank is re-circulated ad a very low flow (60 – 100 l/h). The absorption capacity of the system is about 9 litres of water.

The saturation time depends on the level of hydration (4 – 24 months). Sea Marconi is the exclusive distributor in Italy and France of the system produced by TRANSEC ltd (see photo here below).

A2. Hot oil circulation and drying

The oil in the main tank is heated with the scope of favouring the diffusion of water and is re-circulated in continuous mode through an under vacuum dehydration system.

Process parameters:

Oil temperature (top side of tank): 70-80°C

Circulation flow: 500 – 5.000 l/h

Water extracted: 3- 0,5 litres week

Duration: 4 -10 weeks

Step-up transformer

Capacity: 70 MVA

Maximum voltage: 225 kV

Oil mass: 33 ton

Duration: 10 weeks

Water extracted: 20 litres

B. Off-line de-hydration techniques (Transfo out of service)

The key parameters are temperature and vacuum.

They offer higher dehydration effectiveness in a relatively short time (days/weeks).

They are more expensive, technically demanding and with difficult application on aged transformers (the tank must be vacuum proof).

B1. Hot oil + vacuum

The oil is heated at a temperature exceeding 80° C to heat the active parts.

Once the temperature is reached, the oil is rapidly drained to expose the active parts and, at the same time, vacuum is applied:

Once the temperature of the winding reaches 40-50 °C, the oil is reintroduced and the heating and vacuum cycle is repeated.

B2. Heating, vacuum and re-circulation of hot oil (Hot oil + vacuum + hot oil spray)

It provides the same operational conditions of the preceding technique. In order to maintain longer the temperature of the windings, an aliquot of hot oil is continuously re-circulated and injected in the topside of the transformer, if possible at the top of the windings.

Activity carried out in France.

The pictures show the use of a non-conductive tarpaulin used to reduce the dispersion of heat and to increment the temperature inside the transformer during the process.

Step-up transformer

Capacity 660 MVA

Maximum voltage 225 kV

Oil mass 61 ton

Duration 1 week

Water extracted 25 litres

B3. Low Frequency Heating (LFH)

Not carried out by Sea Marconi

B4.Vapour phase

Not carried out by Sea Marconi

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