Oil storage tanks are divided into two main types: open type and sealed type. Sealed types can be further categorized into rubber bladder or rubber diaphragm sealed types and metal bellows sealed types.
Open type oil tanks consist of a single cylinder made from rolled steel plates. The insulating oil communicates with the atmosphere through a simple desiccant. Due to the limited effectiveness of the desiccant, the insulating oil is prone to moisture absorption and oxidation during operation. Currently, open type oil tanks are not allowed for transformers rated above 35kV.
To address the issue of isolating insulating oil from the atmosphere, rubber bladder and rubber diaphragm oil tanks have been developed. The bladder type oil tank is based on the open type, with an internal cloth-reinforced rubber bladder installed to isolate the oil from air, forming a sealed structure with the outer shell. The bladder compensates for the oil volume through expansion and contraction, and its cavity connects to the atmosphere through a breather. This type of tank initially achieves isolation of the insulating oil from air, reducing moisture absorption and oxidation to some extent, and mitigating the effects of oil cracking on equipment. However, several problems have emerged after years of use:
(1) During transformer installation, operation, and maintenance, the bladder is prone to rupture due to insufficient mechanical strength, causing oil leaks that are difficult to detect once damaged.
(2) Rubber material itself is prone to aging and has a short lifespan, requiring periodic replacement. Replacing and installing the bladder requires draining and refilling the transformer oil, which is labor-intensive. Domestic bladders typically last 3–5 years, while imported bladders last 8–10 years. Even with imported bladders, they need to be replaced 3–4 times during the expected life of a transformer.
(3) The interior of the bladder has heat-absorbing surfaces, which can lead to condensation, posing significant potential risks. Installing a desiccant cannot completely solve this problem.
(4) Poor bladder expansion can cause breathing blockage, leading to accidents.
(5) Due to the low mechanical strength of the rubber bladder, during transformer vacuum operations, internal and external pressure must be balanced. Improper installation or operation can easily damage the bladder, resulting in unnecessary losses and affecting normal equipment operation.
