Sources of moisture in the transformer system
Moisture is formed in the transformer through the degradation processes of both oil and cellulose paper. There can be an ingress of water through gaskets that does not seal properly and moisture is pulled in when the transformer cools down during the night or when the load is reduced. Silica gel breathers are not maintained by replacing the gel in the breather when they change color. The breather’s bottom seal might be leaking and this will cause the breather to be useless in its intended purpose. This will cause air to pass freely into the system without the proper barriers to filter out moisture. The neglect in this area will lead to a major ingress of moisture into the unit, which may prove detrimental to the transformer’s life as an increase in moisture and oxygen will lead to more degradation products to be formed in the insulation system, this will decrease the transformer’s reliable lifetime. Leading to higher maintenance costs in purification or other means to dry out the insulation system.
However, the main source of moisture is in the internal chemical reactions. It has been found that these reactions can contribute as much as 2ppm of moisture per year to the system.
So with all this – how can we manage and minimize the influence of moisture on the system?
Effective moisture management strategies
A conservator is a place of air exchange, it is advised to paint the conservator white to decrease the temperature and thereby decreasing the “breathing” or paint it with a layer of insulating material.
An air cell can be fitted. This will separate the atmosphere and the oil, this will lead to an isolated oil system that will decrease the ingress of oxygen. Oxygen is needed for the formation of moisture in the insulation system.
Regular oil sampling and analysis is required to ensure that the unit is reliable and optimized to ensure an extended lifetime. It is really important to emphasize the need for high-quality samples. The sample needs to be taken in such a manner that no contamination can take place. The sample needs to be clearly identified to ensure that no mix-ups of data are possible. The temperature of the oil should be indicated on the sampling container is important to determine the exact moisture content of the oil. The size of the unit kVA or MVA should be indicated as there are different thresholds of moisture that are acceptable for different transformer classes. If these criteria are not met, it is impossible to identify a problem. This, in turn, can lead to incorrect or unnecessary maintenance.
Why do we need to indicate the temperature of the oil when sampling?
Moisture in the transformer is distributed between the oil and the cellulose paper. Only a very small percentage of moisture is present in the oil at any time. But it should be noted that with changes in temperature the concentration equilibrium of the moisture in the paper and the oil will change. When the temperature increases a higher concentration of moisture will be present in the oil than under colder conditions or lower loading cycles. Research has been performed in this area and graphs are available to indicate the percentage of moisture in the oil at a specific temperature. The analyst will take this into consideration when reporting back to the customer the real value for the moisture that is present in the unit. When a transformer with a high concentration of moisture is allowed to cool down, the oil will become oversaturated if the paper is unable to absorb the large amount of moisture, moisture droplets will be formed. If the cool-down of the unit is rapid then the paper won’t be able to absorb the moisture as the equilibrium reaction for movement of moisture between the oil and paper needs time. It does not happen in an instant. This scenario can be catastrophic to the unit
Practical tips to ensure good quality samples
When a sample is taken it should not be exposed to extreme changes in temperature as this can have a large impact on the accuracy of the test results. Care should be taken by the analyst to ensure that the container is filled as per procedure requirements otherwise false positives may occur that would lead to unnecessary maintenance being performed. The sample should be carefully turned to ensure a uniform sample is used to perform analysis. When performing DGA analysis, some of the gases are prone to escape from the oil into the air pocket at the top of the sample container if the container is not filled to ensure no air pockets are present. No bubbles should be present in the sample while taking the sample when a DGA analysis needs to be done.
Moisture removal methods
Rapid moisture movement causes low dielectric strength. The aim of moisture removal is to prevent low dielectric strength and decreasing the long-term impact of high moisture levels on the insulating system. Currently, there are three methods that are widely used throughout the industry.
Heat and vacuum method
Caution should be taken when applying this method while the unit is still on-line. Starting the vacuum unit can draw oil from the transformer and if the oil levels are low, the core can be exposed and flashover can occur. Also, if the moisture levels are high then this method should not be used while the unit is on-line.
This process will take the moisture from the oil system and by decreasing the moisture in the oil, equilibrium will set in and more moisture will move from the core into the oil. The oil will be processed at a temperature only slightly higher than the operating temperature, the optimum temperature was found to be around 70 to 80 degrees Celsius. The oil cannot be heated too much as this will cause the breakdown of the chemical structure of the oil and an inefficient insulation system, with a low dielectric. The oil is pulled from the heating chamber into a vacuum chamber where the coalescers will disperse the oil and the moisture will be drawn off the oil. Very little of the moisture is drawn from the core during this process if applied for less than 24 hours. To be really beneficial this process needs to run consecutively for at least 72 hours if removal of moisture from the core is to be accomplished. If 12 passes are done by this method it should remove most moisture from the oil, but little from the core. The unit needs to operate for about two weeks and the process should be repeated. This routine can be followed until the moisture level is decreased to acceptable levels. Unfortunately, this is quite impractical for industry, unless the moisture content is not that high and the unit can process the oil while the transformer is still operational.
Vacuum chiller method
The oil will be taken from the transformer at the operating temperature, drawn into the chamber, a vacuum applied and the vapor will be cooled. This method can be used on-line. But it has proven to be not as efficient as the filtration type systems.
Filtration
The filtration system uses filters that operate on the moisture at a molecular level, trapping the moisture in the material structure of the filter. With the first technology, it was only possible to remove free water, but with new technology, this changed and dissolved moisture can be removed.
There are two types of filters available Paper filtration, membrane filter, ceramic filters, and “zeolite” bead filters.
The molecular bead filters (used in dry-keep) uses zeolite beads to target moisture. These small beads are manufactured with a specific pore size to effectively trap moisture.
Explotech’s MMS 1000 filtration systems are very effective in removing moisture. They will monitor the moisture in the oil, once the level has been determined the unit will start filtration. Once the desired moisture level is achieved the unit will stop filtration and the operator will be informed.
The amount of moisture is dependent on the concentration that moves into the oil during the equilibrium process. As stated previously the transformer will constantly form moisture as part of the normal inherent chemical processes going on in the system.
When a molecular filter is fitted to the system it will ensure continued moisture removal from the system, this will ensure low moisture levels in the unit, in turn, this will lead to slower degradation of the oil and paper system in the unit. Hereby ensuring lifetime optimization and cost reduction regarding maintenance.
References:
- Michel Belanger, Stephane Larouche, “Application of on-line dry-out process to power transformers”, Doble Engineering 2003.
- J Aubin, H Vogel, H Bennett, J Eltzel “Determination of moisture in transformer insulation from the long-duration measurement of moisture in oil” CIGRE paper 27, Brugge 2007
- VG Davydov, O Rotzman, “Moisture assessment in power transformers – lessons learned, Vaisala news.