In substations ,when we operate 2 Power transformers in parallel ,what will be the net load? pl. give formula.

Answer 1

It all depends on your contingency conditions and your ability to shed load to other substations. The following is a hypothetical example.

Assumptions:
Two 40 MVA transformers (nameplate rating)
Normal loading capability is 50 MVA each
Emergency (5-day) capability is 60 MVA each

In theory you could load the substation to 100 MVA, however what if you were to lose one of the transformers, or have to take it out of service for maintenance repairs? In that case you would either have to shed load to other substations, or install a portable (temporary) transformer to handle the excess load.

Under the situation described above, it is not uncommon for a utility to have an "allowable" substation load of around 66 MVA. This assumes that you can shift 10% of the load (6 MVA) to other substations in a rather short period of time (less than two hours). This also assumes that within five days you can install, test, and put the replacement transformer into service.

The Capacity Planning department will forecast the substation loads and project when the load will exceed 66 MVA. At that point they will initiate projects to increase the capacity of the substation, or transfer load to other stations.

Final Notes:
When paralleling transformers, you need to match the impedance of the units and keep LTC's in synch with one another to avoid circulating currents. Depending on the size of the transformers, you will also need to be conscious of the available fault currents due to the paralleled transformers.