Moisture

The life of a transformer is dependent upon three crucial parameters; temperature, oxygen and moisture.  This tutorial deals with moisture.  Information about the detrimental effects of oxygen is given elsewhere.

Most power transformers use paper and oil as the main form of insulation and during manufacture stringent efforts are made to ensure that both are as dry as possible when new plant leaves the factory.  Once in service the moisture content begins to increase.  Excessive moisture can put the life of a transformer at risk.  It is important to understand the source of this moisture, its effect and the preventive measures that can be taken.

Source of Moisture
Once in service a transformer is subjected to the following sources of moisture:

External moisture is repeatedly drawn into a free-breathing transformer as the working temperature rises and falls with load.

Internal moisture is generated as cellulose insulation ages.  The process by which this occurs is extremely complex but can be summarised as follows.  Cellulose consists of long chains of hydro-carbons.  When these chains are broken the carbon and hydrogen combine with any oxygen present to form carbon monoxide, carbon dioxide and water.  The gases are dissolved in the oil - see Tutorial T3 DGA.  The water remains largely in the paper where its presence contributes to further ageing as a vicious cycle is established.

Effect of Moisture
Moisture reduces the dielectric strength of paper and oil which in turn increases the risk of electrical failure.  It also reduces the mechanical strength of the paper.

Since hot oil is able to support more moisture than cold oil, percentage saturation is temperature dependent.  If hot oil is cooled the relative saturation will increase and an emulsion of water and oil may form.  Free water may also be produced which will have a serious effect on dielectric strength and can lead to the formation of rust where it collects, typically in the bottom of main tanks and conservators.

Furthermore, when wet paper gets very hot, for example during a period of sustained over-loading, the possibility of bubble formation arises.  When this occurs moisture in the paper is boiled off as water vapour introducing a risk of partial discharge and electrical breakdown.

Moisture Dynamics
Moisture in a transformer is very dynamic, moving with temperature between the paper and the oil.  By far the greatest quantity is contained in the paper which acts like a sponge.  The ratio of water in paper to oil is about 1000:1 (reducing to 500:1 if the oil is old).  As the temperature rises moisture moves relatively rapidly from the paper to the oil.  As the temperature falls the moisture moves back into the paper, but more slowly.

After a period of prolonged steady temperature, the water in the paper and oil reaches equilibrium.  At that point by measuring the moisture content of the oil it is possible to estimate the quantity of water in the paper from standard look-up tables.  In a large transformer we are talking about tonnes of paper, thousands of litres of oil and hundreds of litres of water.

Asset Management
Dealing with the problem of moisture in a transformer is a very important part of effective asset management.  For example:

Always record the temperature when an oil sample is taken for moisture analysis.

Never change oil because it is wet.  The new oil will very quickly degrade to the same condition as the old as moisture leaves the paper to re-establish a condition of equilibrium.

Consider:
(a)    Careful oil sampling and testing
(b)    The use of on-line moisture sensors
(c)    Silica gel breather design, application and maintenance
(d)    On-line conditioning and monitoring systems
(e)    Devices that continuously remove moisture from the air-space in the conservator
(f)     Continuous on-line moisture removal systems connected into the oil system
(g)    Specialist field drying services using heat and vacuum.

For further information visit our Catalogue of products and services or contact us

End

Tutorial Index