Removal of moisture and impurities from insulation is one of the most important manufacturing processes. Removing moisture from the oil is relatively simple, which can be done by conventional methods of filtration or heat/vacuum cycles.
However, most of the moisture is in the solid insulation (it can be as high as 99%). Hence, a challenge is to make the solid insulation dry efficiently and quickly. With an increase in the size of transformers, the time taken for processing of their insulation also increases.
Processing of Transformer Insulation
The time taken by a conventional hot air vacuum process is considerably higher for large transformers with high voltage ratings. The conventional drying method may take more than 7 days for a 220 kV class transformer.
The method consists of heating the assembly of the core and windings in an air medium and applying vacuum for extracting moisture. Moisture in an insulating material can be reduced by raising its temperature and/or by reducing water vapor partial pressure, i.e., by applying vacuum. The application of a vacuum speeds the moisture extraction process; heating alone takes more time to remove a given amount of moisture from the solid insulation.
Depending on the rating and voltage class of the transformer, several cycles of alternate heating and vacuum are required until the transformer insulation is dried. A moisture content of less than 0.5% is usually taken as the acceptance criterion for ending the process.
Requirement of a faster and more efficient process along with the need for better insulation performance resulted in the development and use of the kerosene vapor phase drying (VPD) method.
Kerosene Vapor Phase Drying Method
It is a fast and efficient method in which kerosene vapor at a high temperature is used as the heating medium instead of hot air (used in the conventional method). A special grade of kerosene, heated to about 130°C in an evaporator and converted into vapor form, is injected into the autoclave or transformer tank housing the assembly of the core and windings. As the process of heating is done under vacuum, the moisture extraction starts during the heating period itself. When the insulation reaches a certain desired temperature, a fine vacuum is applied to remove the remaining moisture.
In the VPD process, windings and insulation systems are almost uniformly heated, whereas in the conventional process inner insulation components may not be heated adequately. The total process time in the VPD method is less than half of that for the conventional method in which, as heating is done through the medium of air in presence of oxygen, the temperature is limited to about 110°C.
On the contrary, in the VPD process, the drying is done in an oxygen-free atmosphere (virtually in vacuum), and hence there are no harmful effects or loss of insulation life even at 130°C. Thus, uniform heating at a higher temperature under vacuum results in faster removal of moisture.
The VPD process has another advantage that the vapor condensing on colder winding and insulation parts washes out impurities. This aspect is particularly useful when one wants to clean windings of transformers which come back to factory for repair. With uniform heating of all insulation parts, shrinkage is uniform. Furthermore, shrinkage is negligible during operation on site, ensuring the mechanical integrity of windings.
Low Frequency Heating Method
Low frequency heating method is also used for the drying purpose, particularly for distribution and medium range power transformers. A low frequency (few mHz) impedance voltage is applied to the HV winding with the LV winding short-circuited, which results in uniform heating of the windings of the transformer (since the eddy current loss in the winding conductors, a cause of uneven temperature distribution, is almost zero).
The windings are heated to 110°C and the moisture in the solid insulation is removed in less time as compared to the conventional technique, consisting of hot oil circulation and vacuum cycles, in which the operating temperature is about 60 to 80°C. The required impedance voltage is also low due to a very low frequency of operation. Generally, a current less than 50% of the rated value is required to reach a desired winding temperature.
Main advantages of the technique are reduction in the process time, energy savings, and on-site applicability. At the site, after taking the (wet) transformer out of service and with its oil removed, the method can be applied to remove the moisture in its solid insulation. This way, the problem is addressed directly since most of the moisture is in the solid insulation. The requirement of a lower voltage application goes well with the condition that the transformer is without oil.
Presence of small cellulose particles in oil has a pronounced effect on its dielectric strength. Due to the hygroscopic nature of cellulose, moisture in the oil plays a decisive role as it is absorbed into the cellulose. The cellulose particles have a deteriorating effect on the strength even when the ppm content of moisture in the oil is well within limits; the water content (saturation) in the cellulose insulation is the deciding factor.
Hence, it is absolutely essential to minimize content of suspended cellulose particles and fibers in the oil. These particles usually originate from the surface of paper/pressboard insulation. The major sources of these particles are the edges of radial spacers (pressboard insulation between disks) and axial spacers (pressboard strips in the major insulation between windings) if these components do not have a machined finish.
Punching operations required for making these components involve shearing of pressboards. Burrs, micro-delaminations and subsequent formation of cellulose fibers are unavoidable if punching tools are not of good quality or their maintenance is inadequate. Hence, it is always recommended to have an additional operation of milling so that the edges of the pressboard components are smooth and fiber free.
Once properly processed and impregnated with dry oil, it is important to prevent moisture from getting access to the oil impregnated solid/paper insulation. A simple and widely followed method for small and medium power transformers is to provide a breather with a dehydrating material (such as silica gel) and an oil seal. The oil seal provides isolation to the dehydrating material from the atmosphere. Thus, the transformer breathes through the dehydrating material, and hence the moisture from the atmosphere cannot get into the transformer oil.
Advanced breather systems (e.g., drycol breather) are also popular in some countries. Transformer oil deteriorates to some extent when exposed to the atmosphere. Air acts as an oxidizing agent forming sludge in the oil. Hence, many of the large transformers are equipped with an air bag fitted inside the conservator, so that the transformer oil does not come in direct contact with the outside atmosphere.
Changes in oil volume on account of temperature variations are absorbed by this flexible bag thus maintaining a constant pressure. One side of this bag is in contact with the atmosphere through a dehydrating agent so that if the bag ruptures, the oil is not exposed to the atmosphere since the dehydrating breather steps into the conventional mode of operation. Actions can be initiated to replace the ruptured bag in the meantime.
Once the transformer (insulation) is processed, it is filled with dry and degassed oil (having dielectric characteristics within acceptable limits) under vacuum. Immediately after the oil filling, some air bubbles are formed in the insulation which may lead to partial discharges. Hence, a certain hold/settling time needs to be provided before the commencement of high voltage tests.
During the settling period, heavier particles/impurities settle down at the bottom and the air bubbles move up or get absorbed. After the hold time, air bubbles should be released by bleeding (through air release plugs) at the top so that the transformer is free of (trapped) air. With this, dielectrically critical areas are cleared.
The hold time increases with the voltage rating of transformers. For 132 kV class transformers it should be minimum 24 to 36 hours, for 220 kV class it should be about 48 hours, and for 400-500 kV class transformers it is desirable to have the hold time of about 72 hours. A lower hold time may be adopted by the transformer manufacturers based on their experience.