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Will DryKeep extract the dissolved gases when it extracts the moisture?
DGA Fault analysis is widely used as a tool to carry out condition monitoring on transformers and has become a useful early warning system to eliminate catastrophic failure. We were very much aware of the value of the dissolved gas analysis (DGA) in determining the condition of the transformer and wanted to insure that DryKeep would not extract enough of the gasses to distort the results of the DGA.
Experiments have shown that the Acetylene gas content of the oil flowing directly out of the DryKeep system was reduced by 20 percent. Carbon monoxide was reduced by 25 percent. The moisture content was reduced by as much as 90 percent. However, the method of circulation of oil through the DryKeep system is on the basis of continuous partial circulation of the main oil system in the transformer. The result is that the DryKeep system can remove only a small percentage of acetylene gas from the oil. We must bear in mind that the maximum measured flow rate of the DryKeep system is only 90 liters (24 gallons) per hour. Thus, during a 24-hour period, the maximum volume of oil passing through the DryKeep is 2160 liters (570 gallons) per day. The following are the results of tests conducted by the Eskom Research Institution (TSI) and Rotek Engineering Division of Eskom, South Africa. The residual gas content of various size transformers was measured based on the total oil content of the transformers:
This high residual gas percentage means that, for a continuously gassing transformer, the action of the DryKeep can never mask the generation of gas and will not significantly affect a trending analysis of gas production rates. One big advantage of the DryKeep system is that it reduces the probability of flashovers and arcing, thereby reducing the generation of some of the gases. The power companies we spoke to indicated that they did not want our DryKeep system or any other system to remove a significant amount of the gasses as the gasses were needed to properly perform a reliable DGA. There are nine gases generated within a transformer. The gases should only be removed, using conventional methods, after samples had been obtained. The presence of acetylene is the key indicator of arcing. Arcing will breakdown the insulating oil, producing acetylene and hydrogen as the predominant fault gases. Partial discharges, which are low energy electrical discharges, produce mainly hydrogen and methane, with hydrogen being the key indicator. Some existing filtering systems employed today use a vacuum pump to extract the moisture but in doing so, they also extract the dissolved gases and, when used, will extract the blanket of nitrogen gas used in the sealed design transformers. We occasionally filter the oil by running it through a temporarily installed purifier to remove the moisture. Why should I install a DryKeep system? Moisture within the transformer is shared between the liquid insulation (Oil) and the solid insulation (Cellulose/Paper). Only about 2% of the total moisture is dissolved in the oil with the remaining 98% trapped within the paper. For many years it was assumed that removing the moisture from the oil by periodic filtering, took care of the moisture problem. In reality, it only removed a small percentage of the water. Depending on the design of filter unit used, the transformer may have to be de-energized while this filtering process is performed. As soon as you remove the filtering unit and re-energize the transformer, moisture will diffuse from the paper will attempt to dissolve in the oil. Eventually, the temperature will level off and remain somewhat constant. The diffusion of water from the insulation tends to slow as the transformer tries to reach a state of equilibrium. The problem occurs when the oil becomes saturated before equilibrium is reached. Water continues to flow from the paper but, because the oil is saturated, the water has nowhere to go. It can only flow as free water down between the interface of the outer layers of the solid insulation and the saturated oil. This free water is flowing in an energized field and the consequences can be disastrous. The installation of a DryKeep system is recommended to mitigate this problem. DryKeep is a permanently installed on-line process that continuously dries the oil, which in turn, slowly draws the moisture out of the solid insulation. Neither heat nor vacuum are involved. DryKeep is a passive system that presents no electrical risk to the transformer as it has only one small electric pump. Other systems have many electrical parts and vacuum pumps that can and do fail causing more electrical risk to the transformer than the moisture it is supposed to remove. Vacuum systems must never be used on an energized transformer. Excessive moisture in the insulation system and the resultant reduction of the dielectric strength causes partial discharges and treeing/surface-tracking effects within the moisture soaked cellulose paper. Moisture will also generate gas bubbles at high temperatures, which creates a significant threat to the dielectric integrity of the insulation. The dielectric strength is described as the ability of the material to withstand high electrical field stresses. Moisture in the insulation system together with operating temperature and oxygen constitute the most important insulating aging mechanism. It is, therefore, very important to manage the moisture content of wet power transformers. Installing a DryKeep system to a new or recently dried transformer is the best insurance you can buy for your transformer.
Moisture in an operating transformer can have a number of sources:
RESIDUAL MOISTURE: Moisture left in the transformer by the manufacturer. About 0.5% of the moisture, by weight, remains in the transformer when it leaves the factory. In some cases it has been as high as 2 or 3 %. INGRESS FROM THE ATMOSPHERE: Atmospheric moisture that comes into contact with the oil and paper insulation due to direct exposure to the air during manufacture or repair or ingress of moisture due to the differential between the atmospheric pressure and pressure in the tank. Leaks occur at poor sealing points such as bushings, explosion vents or points in the cooling loop. AGING OF THE CELLULOSE AND OIL: Moisture is generated internally to the transformer as a by-product of the Paper/Oil aging process caused by thermal and electrical stresses. The oil alone can generate significant amounts of moisture due to oxidation. This internal moisture build up occurs regardless of how "sealed" the transformer may be. For example, a 20-year-old transformer will have accumulated about 3% moisture in the insulation. Using a DryKeep Moisture Management System will, over a period of time, reduce the moisture content to a safe level of about 1.3% in order to avoid re-clamping of the windings. (It is not always possible to re-clamp windings in the field) Transformers fitted with springs or back-to-back spring washers can be dried down to factory levels. The amount of time required for dry out depends on the size of the transformer and how wet the transformer was before DryKeep was installed. It will safely dry out the windings while ON-LINE. There is neither loss of revenue nor interruption of service to the customers. In addition, the cartridges can be changed out under load and sent to our Savannah, GA regeneration facility.
The operating temperature certainly has a big influence on the migration of water within the transformer. With higher temperatures, more moisture will flow from the paper. However, even in transformers operated at 50% load, water will still be produced due to the normal aging of the cellulose and through migration of moisture into the oil from the atmosphere. Even a transformer sitting in storage will have moisture build-up.
Moisture build-up is more likely to occur in transformers that are used at pumped storage facilities as they are subjected to heating and cooling cycles. The units are loaded only during the pumping cycle causing an increase in the operating temperature. This is the most dangerous period as moisture driven from the paper causes electrical risk at the surface of the insulation. During the cooling cycle, the water flows out of the oil and back into the paper but it goes back into the paper at a much slower rate than it came out. Similar risks occur in units used as back-up transformers. Frequent changes in operating temperatures will create electrical risks. The ideal situation is to install DryKeep on a new transformer to control moisture build up. By doing so, you will extend the life of the solid insulation ten fold. You can also extend the life of older transformers by installing DryKeep before the Degree of Polymerization (DP) reaches a dangerously low level. Once the DP reaches 200, the insulation is at the end of life. Continuous on-line drying of the oil will eventually dry the solid insulation and will slow the aging process considerably. Only a permanently installed DryKeep System can effectively perform this function.
It is somewhat difficult to define the term "dry areas". The desert, for example, we would normally think of as being dry. In most cases this is not true as we find that the humidity of the air is fairly high and for that reason, the air that enters the transformer is still moist.
The big change in temperature between the day and night conditions affects the air above the oil, especially in a conservator tank, due to the expansion and contraction of this air mass. The oil in the transformer also expands and contracts and in doing so, assists the process of the air movement in and out of the transformer. This means that the oil in the transformer is well mixed with oxygen and enhances the process of moisture forming inside the transformer due to oxidation of the oil and paper. It is also true that moisture will be carried into the transformer when the flow rate of air through the silica gel breather is too fast to allow the silica gel to absorb all the moisture entering the breather. It must also be understood that, so far, we have been referring to moisture increase in the transformer from sources outside the transformer. The question above specifically referred to moisture increase "in the oil". The moisture content of the oil will increase because of the temperature increase. Any increase in temperature will upset the state of equilibrium, causing moisture to flow from the paper insulation into the oil and conversely, a decrease in temperature will cause moisture to flow from the oil back into the paper.
The majority of transformers installed in South Africa and Europe are fitted with conservators and are mostly free breathing into the atmosphere. Many of them use the DryKeep units. The conservators are fitted with a silica gel breather to prevent the ingress of moisture while the transformer is breathing. However, we know that the maintenance on these breathers is sometimes neglected and it does happen that moisture enters through the breather from atmosphere. This is not the main cause for the presence of moisture in transformers. One of the main contributing factors for the presence of moisture in transformers is due to the creation of moisture as a by-product of the aging of the oil and the paper insulation and this is the matter that we want to address. As the oil starts to age (increased acidity values) and the paper starts to age (decrease in DP value) moisture is naturally produced, and even though you believe you have a perfectly sealed transformer, there will still be an increase in the moisture content. The DryKeep will constantly remove the moisture that is formed in this situation and you never have the effect that the paper dimension expands due to an increase in moisture content.
Concerning air bags in the conservator, we must remember that the purpose of the bag is not to prevent moisture but to seal off the transformer from atmosphere and thus reduce the oxygen content, which is also a main aging component. The fact that the transformer conservator is fitted with a bag has nothing to do with the fact that moisture will still form due to the aging process. An oil leak high up on the transformer will sometimes result in the ingress of moisture from outside such as on a bushing turret. It is better to remove this moisture as it enters by typically having a DryKeep fitted, as the increase in moisture will have a negative effect on the insulation. If you do have a leak somewhere, oil may leak out and it is recommended to tend to these leaks as they are identified, and not to wait until you detect an increase in the moisture content as by this time the damage has already been done. If the bushing is hermetically sealed and there is a leak on the bushing, you will not detect this in the transformer oil volume, as the bushing oil is separate from the main tank oil volume. This is a totally separate issue and if you require more information, we can respond to it separately.
If you have any further questions about the DryKeep unit, please feel free to contact us. |