From the previous posts, it can be inferred that, ultimately at a Substation, either a Voltage level is stepped up or stepped down aiding to the transmission or distribution of power. Hence, we can say that the Transformer is the core of a substation which is supported by other equipment for protection.

Primary function of a Power Transformer is to transform voltage form a nominal level to another. In most cases, the use of two smaller size transformer, to operated in parallel in one circuit in lieu of  one full size transformer is not recommended. The reason being, the two transformers would cost more than a single full transformer, their combined loss will be higher and obviously, an expensive and bigger structure of Substation is required. 

The 1 x 100 % Approach:

In this approach a single transformer with a capacity of 20% more than the Maximum Demand at the Main Substation (A Main Substation is where the EHV level voltage is stepped down to 11 or 6.6 or 3.3 kV) is used. In case of an outage of the transformer, the captive site-generation is adequate to maintain in service, the critical and major portion of the plant. Since the restoration of the failed transformer back into service, takes several months, this approach is not recommended.

Even though the parallel operation has the disadvantages as mentioned earlier in this post, when situation exists for parallel operation where the continuity of at least parallel service in event of failure of one of the transformer is of greater importance, any two or more transformers with impedance in same order of magnitude can be used. The impedance has to be in proportionate order so that the load is equally shared between the transformers.

In the case of a parallel operation, in order to meet the HV bus arrangement has to match the transformer capacity provided on the basis of concepts labelled as follows: 

The 2 x 100 % Approach:
 
In this concept, two power transformers each capable of meeting 100 % load of the project demand is used to be operated in parallel. The substations will be having four number of circuit breakers in an 'H' type bus arrangement. Two circuit breakers are for line side control and two other for transformer. A fifth circuit breaker is used to parallel the buses by connecting it between the two buses. Generally, an off load isolator is used for this purpose. If 20 MVA is the project demand, two transformers of 25 MVA is used so that, even at the outage of one of the transformer, the other will carry 100% of the 20 MVA project demand with a margin of 5 MVA left for meeting the MVA requirements while starting of the HV motor of largest rating.

The 3 x 50 % Approach: 

In this approach, three power transformers each with a capacity of 50% more than maximum demand of the project is used, so that at anytime one of the three transformers can be released for maintenance purpose. With only two transformers in service out of the three, each transformer has to cater for 50% of the maximum demand. But, the capacity of the transformer has to be greater than 50% of Maximum Demand so that there is some margin for the start of largest HV motor. For instance, if 50% of MD is 15 MVA, then three transformers of 18 MVA each is to be used. 

The 4 x 33 % Approach:

This arrangement calls for four power transformers each rated to more than 33% of the maximum demand and generally has a six bus arrangement. for instance, if the maximum demand of the project is 60 MVA, four transformers of 25 MVA each will be needed such that at the outage of one transformer, the other three can meet the maximum demand.