A transformer connection is said to be in parallel operation if the primary winding of two transformers is wired to a common supply voltage and the secondary winding to a common load. 

To provide load over the transformer level, two or more transformers can be connected in parallel to the main transformer. The concurrent operation improves efficiency than a single larger unit. If two transformers are paired in parallel, the cost of maintaining the repair systems is lower. Instead of substituting the current transformer for a single larger unit, it is typically economical to add a further transformer in parallel. 

The transformer can be connected in cases: 

• If a load is increased and exceeds from the current transformer capacity, then the additional load can be supplied with another transformer parallel to the existing transformer.
• If there is no large transformer which can satisfy all load demands, two or more small transformers can concurrently be connected to increase capacity.
• If several transformers operate in parallel and a malfunction occurs in a transformer, the other parallel transformers still serve the load.

There are some conditions which need to be considered before connecting the transformer in parallel operation.

• The mandatory conditions are the same angle shift, the same polarity, the same phase sequence and the same frequency.
• Same voltage ratio and turns ratio, same impedances of percentage, the same rating KVA, the same location of tap changer are the most suitable conditions.

It is possible to function in parallel but not optimal when the suitable conditions are not met.