We know that the very basic purpose of Transformer is to transmit power at two different voltage levels. For example, if a generating station is generating electrical power at 21 kV and evacuates it power at 400 kV then a three phase transformer of voltage rating 21 kV / 400 kV is used. But the question which arises here is whether we should opt for a single three phase Transformer or bank of three single phase Transformer? Well, mostly you might have seen a single three phase transformer due to economy. But this does not mean that bank of three single phase transformer is never used. If ever you get a chance to visit 700 MW or 1000 MW power plant, you will see the use of bank of three single phase transformer instead of a single three phase transformer.
Generally for a large power generating units, bank of 3 single phase transformer is preferred. The main reason is the size and transportation. Let us take an example for better understanding. Consider a 700 MW generating station where we need to choose a generator transformer (GT) to evacuate power to 400 kV Grid. The size of a single three phase transformer of 823 MVA (700 / 0.85) will be quite high and hence will not be easily transportable from the Transformer factory to the site. Thus for this case, bank of three single phase transformer is used. There are many other advantages of using bank of three single phase transformer like ease in maintenance, less cost of spare inventory requirement etc.
Let us have detailed comparison assuming same rating for both type of transformers.
| Bank of 3 Single Phase Transformer | Single Three Phase Transformer |
| It is more expensive due to requirement of 3 single phase transformer. 3 single phase transformer means requirement of more iron for core, oil and accessories. | It is quite economic due to the use of less iron core, less volume of tank and hence less volume of Transformer Oil. |
| In such case star or delta connection on HV side requires six different HV Bushings to bring out the HV terminals of 3 single phase transformers. | This only requires three / four HV Bushings as the delta / star connection is done inside the tank of transformer. |
| Space requirement is more for installation. | Less space requirement for installation. |
| It offers greater flexibility in erection and installation. | Due to single unit, it doesn’t have any flexibility in erection and installation. |
| The spare inventory cost is less. Only one single phase transformer is required as spare which is less costly. Suppose a generating station has 2 generating units. For both the generating units (2xbank of three single phase transformer), only one single phase transformer is required as spare. | The cost of single three phase transformer as spare is quite high when compared with a single unit of single phase transformer. |
| The maintenance becomes easier due to separate units. Replacement of single unit is also easy. | It is quite difficult to repair and replace. |
| This is less efficient due to losses in the three units. The losses are more due to use of more iron core. | It is more efficient and losses are less due to lesser requirement of iron core. |
From the above comparison, it is clear that use of a single three phase transformer offers many advantages and above all it is economic. This is the reason, mostly three phase transformers are used instead of bank of three single phase transformer. However, bank of three single phase transformer is used in large generating units for the reason discussed earlier in the post.