A 103 - Bell Tanks for Power Transformers

1. A typical transformer tank construction is a plain rectangular vessel with a top cover bolted or welded to the lower part after the active part is placed inside the tank. Sometimes, this tank joint is provided at bottom levels. Then, the upper tank will look like a bell placed over the active part to be bolted with the lower tank. This lower tank can be a plain plate, which, when assembled, will look like a conventional tank placed upside down.

2. Bell tanks are used for large-capacity transformers so that core coils or active parts can be inspected in the field by lifting the tank's relatively lighter bell or hood instead of lifting the heavy active part out of the tank. The perceived advantage from the end users' point of view is the reduced crane capacity required to access the active part for repair, say around 20-40 T max. for the bell compared to what could be 150 - 250 T for the active part.

3. Large transformers in India were imported from abroad until the middle of the last century. Along with the transformer, one complete limb of windings was also procured as spare winding. Substations used to have repair bays, just like power stations. In case of a fault in any transformer, it was not easy to get the unit repaired in OEM’s foreign factory. The failed transformer used to be taken to the repair bay, and the faulty winding was replaced with a spare winding. To limit the crane capacity in the repair bay, customers used to specify a bell tank with the tank band joint above 500 mm from the bottom. This way, crane capacity can be limited to the weight of the upper tank or spare coil. A minimum bell joint height of 500 mm was specified to accommodate the transformer in the well of the wagon used in rail transport. The tank joint had to be above the wagon well (pit) height to prevent the projecting tank band from fouling with the limited width of the rail wagon well.

4. Today, the scene has changed. All transformer ratings are made in the country. A repair bay is no longer provided in substations, and repair is usually done at the works of OEM or repair contractors. Transformers are transported by road by special low-bed trailers, and rail transport is rarely used. Hence, when a bell joint is specified, manufacturers prefer to provide the tank joint at the base plate level to shield the tank joint effectively from the winding leakage flux. This will also prevent the heating of tank bands and bolts from the leakage flux from windings. But still, many users insist on the tank band joint 500 mm above the ground without knowing why it was specified in the original case. I remember making transformer banks of 1000 MVA Generator transformers and 750 MVA auto-transformers (all at 420 kV level with bell tank construction specified) with an upper tank over the base plate, i.e. an inverted conventional tank. In those cases, I had difficulty convincing the user that this would not affect the reliability or performance of the transformer.

5. In the future, site repair may be necessary for large units to save the prohibitive transport costs and the long time needed for repair and to and from transport. In such cases, bell-type tank construction will be required to limit the crane size required for repair. China presented a paper in Paris at CIGRE 2018 on a repair bay built in the substation to repair and overhaul 1000 MVA 1100 kV single-phase auto-transformers. Repair may be tried even over the transformer plinth by putting up a temporary tent for disassembly and repairing the active part of the transformer.

6. About 45 years ago, I visited the Alsthom Savoisienne (forerunner of Alstom and now GE) transformer factory outside Paris. Their standard construction was a bell tank, with the windings resting on the bottom tank instead of the core frame, which is the regular practice of transformer manufacturers. There was a rail in the middle of the factory floor, running the length of the factory. Active part assembly was done in the lower tank and mounted on rollers. As clamped in the lower tank, the active part moved from one workstation to another, over the rail, using the overhead crane and pulley system. This construction is still followed by some Indian manufacturers who had technical collaboration with Alsthom.

7. In a nutshell, the advantages and disadvantages of bell-type tank construction can be summarised as follows:

Advantages: Lifting crane capacity is reduced for site repair/inspection, tanking becomes easy during site repair, and repair with the unit on the foundation block is possible.A significant advantage is that the active part is not lifted after the active part clamping, maintaining the integrity of winding clamping pressure. This is a plus point when the transformer is subjected to a dynamic short circuit test.

Disadvantages of bell tanks: The tank weight is higher due to the additional reinforcement. The tank band joint sees more oil head, so there are more chances for oil leakage. Effective magnetic shielding of the tank is complex due to a break in shield from the tank joint (this can result in excess current flow between the upper and lower tank—it can reach a couple of hundred amperes, causing additional stray losses, bolt heating, etc.) Of course, costly magnetic shielding solutions or joint welding are available to avoid such problems of tank band bolt heating. Another disadvantage of the bell tank is that all external accessories like bushings, conservators, and piping should be removed while lifting the active part. This creates a storage problem in the field. Also, complete oil from the tank should be drained and contained during lifting in the field. Construction with the base plate bottom may face difficulty with directed oil cooling as the cooling pipe has to be directed to the active.