To enhance the capacity of a substation, we need to connect more than one power transformer in parallel. In the parallel operation of transformers, we connect the HV side of the transformers to the same HV bus of the substation. At the same time, we connect the LV side of the transformers to the same LV bus of the substation. Otherwise, we cannot say that it is a parallel connection.
The parallel operation of transformers increases the capacity, reliability, and flexibility of the substation.
Capacity
It is not always feasible to install a big transformer to enhance the capacity of the existing substation. For a new greenfield substation, it is not economical to have a single large power transformer for the total capacity. So, we use multiple small transformers in parallel to achieve the required capacity. Nowadays, we use the (N – 1) concept for parallel operation. Here, we install N numbers of power transformers in the substation; however, (N – 1) number of transformers are enough to achieve the required capacity of the substation.
Continuity of supply and Maintenance without load-shading
If we perform maintenance on one of the transformers, we can easily disconnect it from the system without disrupting the continuity of supply. This is undoubtedly possible if we have configured the substation with an (N-1) scheme. Even if (N-1) configuration is not available, we can take out a transformer for maintenance. However, for that, we may have to impose a partial load shading on the consumer end.
Future expansion
It is not practical to replace an existing transformer with a larger one when the demand of a substation increases. Instead, it is more practical to increase the number of transformers step by step according to the growing load demand.
Better efficiency
A transformer operates more efficiently when it runs close to full-load conditions. So, by controlling the number of transformers operating in parallel, we can keep each transformer near its maximum efficiency.
Conditions for Parallel Operation of Transformers
1. Same polarity
If we connect the polarity of two single-phase transformers incorrectly during paralleling, the short-circuit current starts circulating immediately through both of the transformers.
2. Same phase sequence and phase displacement
For 3-phase transformers, the phase sequence of all transformers in parallel must be identical. Otherwise, for the same reason, short – circuit currents start circulating between the transformers. Also, their vector group must be compatible. For example, a Dyn11 transformer runs parallel with another Dyn11 transformer. On the other hand, when we connect a Dyn11 transformer with a Dyn1 transformer, a circulating current starts circulating between the transformers. The phase shift of 60° between Dyn11 and Dyn1 transformers causes the circulating current.
3. Same turns ratio (same tap setting)
If we connect two or more transformers with different turn ratios to a common primary bus, the secondary voltages will become unequal. So, circulating current flows even at no-load conditions. This causes unnecessary heating in the windings.
4. Same percentage impedance (or very close)
The percentage impedance decides how much load each transformer will take. If the percentage impedance of all the transformers is the same, they will share the total load according to their capacities. It implies that the load shared by each transformer is inversely proportional to its impedance. It is good practice to use impedance tolerance within ±10%.
5. Similar X/R ratio (impedance angle)
If X/R ratio of transformer windings differs greatly, load sharing becomes unbalanced for real power and reactive power.