همانطور که می دانید روغن در برگیرنده هسته و سیم پیچی ترانس باید قابلیت لازم برای تغییر حجم به ازای تغییرات دما ناشی از بار گذاری ترانس را داشته باشد. به همین منظور است که تانک حاوی روغن در ترانسهای روغنی با هوای آزاد مرتبط است. در انواع کنسرواتوری برای به حداقل رساندن این سطح تماس از مخزن انبساط یا کنسرواتور استفاده می شود زیرا زیادی سطح تماس منجر به آلودگی روغن یا حل شدن بیش از حد آب و اکسیژن در آن می شود که به نوبه خود سبب کاهش کیفیت روغن و مشکلات ناشی از آن می شود. یک گزینه دیگر برای جدا سازی سطح روغن از هوا، استفاده از کیسه هوا می باشد که به واسطه آن تقریباً روغن ترانس seal شده و تبعات ناشی از تماس مستقیم روغن با هوا کاهش می یابد.
The use of a conservator allows the main tank to be filled to the cover, thus permitting cover-mounted bushings, where required, and it also makes possible the use of a Buchholz relay (see below). However, the most important feature of a conservator is that it reduces the surface area of the oil exposed to atmospheric air. This reduces the rate of oxidisation of the oil and also reduces the level of dissolved oxygen, which would otherwise tend to shorten insulation life.
Alternative oil preservation systems
Refrigeration breathers are usually considered too costly to be used on any but the larger more expensive transformers operating at 132 kV or higher for which a high level of oil dryness is necessary. In very humid climates such as those prevailing in many tropical countries the task of maintaining a satisfactory level of dryness of the drying agent in a silica gel-type breather can be too demanding so that alternative forms of breathing arrangements must be adopted. The most common is the air-bag system shown diagrammatically in Figure below.
With this arrangement the transformer has what is basically a normal conservator except that the space above the oil is filled with a synthetic rubber bag. The interior of the bag is then connected to atmosphere so that it can breathe in air when the transformer cools and the oil volume is reduced and breathe this out when the transformer heats up. With this arrangement the oil is prevented from coming into direct contact with the air and thereby lies its disadvantage.
If the air space within the conservator is maintained in a dry condition, either by means of a well maintained silica gel breather or by a refrigeration breather, this will allow moisture to migrate from the oil, and ultimately from the paper insulation to maintain this in a dry condition and minimise ageing. If this moisture remains trapped in the transformer by the presence of a synthetic rubber diaphragm or by other means, the rate of ageing will be increased.
A better arrangement than that just described is again to use basically a normal conservator but to arrange that the space above the oil is filled with dry nitrogen. This can be provided from a cylinder of compressed gas via a pressure reducing valve. When the transformer breathes in due to a reduction in load or ambient temperature the pressure reducing valve allows more nitrogen to be released. When the oil volume increases nitrogen is vented to atmosphere by means of a vent valve. Because the nitrogen is always maintained in a dry state, this arrangement has the great advantage that it maintains the oil and insulation in as dry a condition as possible. The only disadvantage is the supply and cost of the nitrogen needed to maintain a constant supply thus adding to the routine maintenance activities.
It is now common practice, not only in climates having high humidity, for smaller oil-filled distribution transformers to be permanently hermetically sealed. This has the great advantage of being cheap and of requiring virtually no maintenance. Since transformer oil is incompressible, with a sealed arrangement it is necessary to provide space above the oil, filled with either dry air or nitrogen, to act as a cushion for expansion and contraction of the oil.
Without this cushion the tank internals would experience very large changes of pressure between the no load and the loaded condition. (To some extent, this problem is reduced if the transformer has a corrugated tank, see below.)
These pressure variations can cause joints to leak so that external air is drawn in at light load conditions, usually bringing in with it moisture or even water, or they can cause dissolved gas in the oil to be brought out of solution and thus form voids leading to internal electrical discharges and ultimate failure. The more sophisticated or strategically important sealed transformers are provided with a pressure gauge which shows an internal positive pressure when the transformer is loaded, thus indicating that the seal remains sound.